chapter 2 - structural design

Rules for the Certification and Construction IV Industrial Services

7 Offshore Substations

2 Structural Design

Edition 2013

The following Rules come into force on 1 October 2013.

Germanischer Lloyd SE

Head Office Brooktorkai 18, 20457 Hamburg, Germany

Phone: +49 40 36149-0 Fax: +49 40 36149-200

[email protected]

www.gl-group.com

"General Terms and Conditions" of the respective latest edition will be applicable (see Rules for Classification and Construction, I - Ship Technology, Part 0 - Classification and Surveys).

Reproduction by printing or photostatic means is only permissible with the consent of Germanischer Lloyd SE.

Published by: Germanischer Lloyd SE, Hamburg

Table of Contents

Section 1 Environmental Conditions A Basic Considerations ........................................................................................................ 1- 1 B Wind.................................................................................................................................. 1- 3 C Sea Currents..................................................................................................................... 1- 5 D Sea Waves........................................................................................................................ 1- 8 E Sea Level .......................................................................................................................... 1- 15 F Tsunamis .......................................................................................................................... 1- 15 G Climatic Conditions, Temperature and Marine Growth..................................................... 1- 15 H Sea Ice and Icebergs ........................................................................................................ 1- 16 I Sea Bed ............................................................................................................................ 1- 17

Section 2 Design Loads A Basic Considerations ........................................................................................................ 2- 1 B Environmental Loads ........................................................................................................ 2- 1 C Permanent Loads.............................................................................................................. 2- 14 D Functional Loads............................................................................................................... 2- 14 E Accidental Loads............................................................................................................... 2- 15 F Transportation and Installation Loads............................................................................... 2- 17 G Earthquake Loads............................................................................................................. 2- 18

Section 3 Principles for Structural Design A General ............................................................................................................................. 3- 1 B Design Methods and Criteria ............................................................................................ 3- 2 C Loading Conditions ........................................................................................................... 3- 5 D Dynamic Analysis.............................................................................................................. 3- 11 E Effective Width of Plating .................................................................................................. 3- 13 F Beam Member Design ...................................................................................................... 3- 14 G Tubular Joint Design ......................................................................................................... 3- 15 H Buckling of Plates ............................................................................................................. 3- 18 I Fatigue Strength ............................................................................................................... 3- 18 J Application of BSH Requirements .................................................................................... 3- 24

Section 4 Steel Structures A Materials ........................................................................................................................... 4- 1 B Fabrication ........................................................................................................................ 4- 20 C Welding ............................................................................................................................. 4- 39 D Survey, Inspection and Testing ........................................................................................ 4- 66 E Survey and Inspection Scheme........................................................................................ 4- 68 F Non-Destructive Testing ................................................................................................... 4- 69 G Visual Inspection............................................................................................................... 4- 74 H Magnetic Particle Inspection............................................................................................. 4- 75 I Penetrant Testing.............................................................................................................. 4- 77 J Radiographic Testing ........................................................................................................ 4- 78 K Ultrasonic Testing ............................................................................................................. 4- 80

Rules IV Industrial Services Part 7 Offshore Substations Chapter 2 Structural Design

Table of Contents

Edition 2013 Germanischer Lloyd

Section 5 Corrosion Protection A General ............................................................................................................................ 5- 1 B Material Selection............................................................................................................. 5- 6 C Coating Selection ............................................................................................................. 5- 11 D Cathodic Protection.......................................................................................................... 5- 14

Section 6 Foundations A General ............................................................................................................................ 6- 1 B Geotechnical Investigations ............................................................................................. 6- 1 C General Design Considerations ....................................................................................... 6- 2 D Steel Pile Foundations ..................................................................................................... 6- 4 E Shallow Foundations........................................................................................................ 6- 8

Section 7 Crane and Crane Support Structures A Cranes.............................................................................................................................. 7- 1

Section 8 Helicopter Deck Structures A General ............................................................................................................................ 8- 1 B Structure of the Helicopter Deck ...................................................................................... 8- 2

Section 9 Marine Operations A General ............................................................................................................................ 9- 1 B Standards and Guidelines................................................................................................ 9- 1 C Lifting Operations ............................................................................................................. 9- 2

Section 10 Containers A Scope ............................................................................................................................... 10- 1 B Certification for Offshore Service Containers .................................................................. 10- 2 C Structural Parts ................................................................................................................ 10- 6 D Materials and Production ................................................................................................. 10- 7 E Electrical Requirements ................................................................................................... 10- 8 F Heating, Ventilation and Air Conditioning ........................................................................ 10- 10 G Public Address and Main Alarm Systems........................................................................ 10- 10 H Portable Offshore Unit ..................................................................................................... 10- 11 I Certification Procedure .................................................................................................... 10- 12


Table of Contents

Edition 2013 Germanischer Lloyd

Section 1 Environmental Conditions A Basic Considerations .................................................................................................... 1- 1 B Wind .............................................................................................................................. 1- 3 C Sea Currents ................................................................................................................. 1- 5 D Sea Waves .................................................................................................................... 1- 8 E Sea Level ...................................................................................................................... 1- 15 F Tsunamis ...................................................................................................................... 1- 15 G Climatic Conditions, Temperature and Marine Growth ................................................. 1- 15 H Sea Ice and Icebergs .................................................................................................... 1- 16 I Sea Bed ........................................................................................................................ 1- 17

A Basic Considerations Environmental conditions give rise to loads imposed on the structure by natural phenomena including wind, current, waves, earthquake, snow, ice, and earth movement. Environmental loads also include the variation in hydrostatic pressure and buoyancy on structural components caused by changes in the water level due to waves and tides. Environmental loads shall be anticipated from any direction unless knowl-edge of specific conditions makes a different assumption more reasonable.

The unit/installation shall be designed for the appropriate loading conditions that will produce the most severe effects on the structure. Environmental loads, with the exception of earthquake load, shall be combined in a manner consistent with the probability of their simultaneous occurrence.

A.1 Determination

Pertinent meteorological and oceanographic conditions affecting a unit/installation’s operating site shall be defined. The corresponding environmental design data should be prepared to develop the descriptions of normal and extreme environmental conditions. Environmental conditions may be determined by wind, sea currents, sea waves, sea level, climatic conditions, temperature and fouling, sea ice, sea bed condi-tions, and other influences as applicable.

A.2 Normal environmental design conditions

Normal environmental conditions are those conditions that are expected to occur frequently during the life of the unit. Normal environmental conditions, important both during the construction and the service life of a unit/installation, consider the most adverse possible effect during the erection and the operation of the unit/installation.

A.3 Extreme environmental design conditions

A.3.1 Extreme environmental conditions occur rarely during the life of the unit/installation. Extreme design conditions are important in formulating design loads for the unit when out of operation, while rest-ing on its site with all equipment secured in a seaworthy condition. Design loads may be specified on the basis of statistical observations if available. Probabilistic estimations, if not specified in the Rules, are to be approved by Germanischer Lloyd.

A.3.2 Fixed offshore installations shall be designed to meet limiting environmental conditions valid for the designated operating area.

A.3.3 Mobile offshore units shall be designed and operated to meet limiting environmental conditions as specified in the operating manual.


Section 1 Environmental Conditions

Edition 2013 Germanischer Lloyd Page 1–1

A.4 Estimation of design parameters

Estimation of design parameters on the basis of environmental design conditions, e.g., estimation of wave particle velocity or acceleration on the basis of wave height and period, if not carried out as specified in the Rules, is to be approved by Germanischer Lloyd.

A.5 Superposition of parameters

Superposition of different environmental design parameters is to be based on physical relations or on stochastic correlations between these parameters or between the environmental phenomena to which they belong.

A.6 Simplifying assumptions

Simplifying assumptions in these Rules are made in accordance with the specification of related safety factors for structural design. If estimates of environmental design parameters are based on assumptions that are more appropriate for the design case than those specified in the Rules, a reduction of the safety factor may be approved by Germanischer Lloyd, see also Section 3, C.

A.7 General definitions

In the following paragraphs the general designations used are (see also Fig. 1.1):

x : horizontal coordinate axis in the direction of the wave propagation [m]

z : vertical coordinate axis above still water level [m]

d : water depth [m]

H : wave height, crest to trough [m]

T : wave period [s]

λ : wave length [m]

Fig. 1.1 General definitions




B Wind Although wind loads are dynamic in nature, offshore structures respond to them in a nearly static fashion. However, a dynamic analysis of the unit/installation is indicated when the wind field contains energy at frequencies near the natural frequencies of the unit/installation (be it fixed or moored to the ocean bot-tom). Sustained wind speeds should be used to determine global loads acting on the unit/installation, and gust speeds should be used for the design of individual structural elements.

B.1 Wind properties

Wind speed changes with both time and height above sea level. Therefore, the averaging time and height shall be specified. Common reference times are one minute, ten minutes, or one hour. The common ref-erence height is ten (10) meters. Wind forces should be computed using the one (1) minute mean wind speed, and appropriate formulas and coefficients may be derived from applicable wind tunnel tests.

B.2 Mean wind speed

The mean wind speed at the reference height of 10 m, averaged over time t, may be estimated by the formula

u10(t) = Ct ⋅ u10(tr)

where

u10(t) : mean wind speed at a reference height of 10 m [m/s]

Ct : wind speed averaging time factor

: [1 – 0.047 ln (t / tr)]

u10(tr) : mean wind speed at a reference height of 10 m for the reference averaging time [m/s]

t : averaging time [minutes]

tr : reference averaging time

: 10 [minutes]

Wind speed averaging time factors Ct for selected averaging times t are given in Table 1.1:

Table 1.1 Wind speed averaging time factors Ct related to reference time tr = 10min

Averaging time t Ct

3 seconds 1.249

5 seconds 1.225

15 seconds 1.173

1 minute 1.108

10 minutes 1.000

1 hour 0.916

B.3 Minimum wind speed for stability calculations of mobile offshore units

For the requirements for stability calculations of mobile offshore units please refer to GL Rules for Mobile Offshore Units (IV-6-2), Section 7, B.4.




B.4 Sustained wind speed

The greatest one (1) minute mean wind speed, expected to occur over a return period of 100 years and related to a reference level of 10 m above sea level, is generally referred to as the sustained wind speed, uS. This sustained wind speed is to be used for the determination of global loads.

B.5 Gust wind speed

The greatest three (3) second mean wind speed, expected to occur over a return period of 100 years, is referred to as the gust wind speed, uG. This gust wind speed is to be used for the determination of local loads. The gust wind speed is related to the sustained wind speed as follows:

uG = 1.137 ⋅ uS

The gust wind component may be considered as a zero mean random wind component which, when su-perimposed on the constant, average wind component yields the short-term wind speed.

The wind in a 3 second gust is appropriate to determine the maximum static wind load on individual members; 5 second gusts are appropriate for maximum total loads on structures whose maximum hori-zontal dimension is less than 50 m; and 15 second gusts are appropriate for the maximum total static wind load on larger structures. The one minute sustained wind is appropriate for total static superstructure wind loads associated with maximum wave forces for structures.

B.6 Design wind speeds

A design value of wind speed at height z above still-water level may be calculated according to the follow-ing formula:

110

10zu(z, t) u (t) [m / s]

10m⎛ ⎞= ⋅ ⎜ ⎟⎝ ⎠

where

u10(t) : mean wind speed, averaged over time t for the reference height of 10 m see(3.) [m/s]

z : vertical coordinate axis above mean still water level [m]

Fig. 1.2 Wind profile




B.7 Statistics of wind speed

The Weibull distribution may be used to describe the statistical behavior of the average wind speed u(z,t).

C Sea Currents

C.1 Sea currents are characterized as: • near-surface currents, i.e., wind/wave generated currents, see C.2 • sub-surface currents, i.e., tidal currents and thermosaline currents, see C.3 • near-shore currents, i.e., wave induced surf currents, see C.4

C.2 Near-surface currents

Near-surface currents are typically wind induced.

For near-surface currents, the design velocity may be estimated as follows (see Fig. 1.3):

uw(z) = k(z) ⋅ uS

where

uw(z) : near surface current velocity [m/s]

k(z) : factor depending linearly on the vertical coordinate z

: 0.01 for z = 0 m

: 0 for z ≤ –15 m

: to be obtained by linear interpolation for 0 > z > –15 m

z : vertical coordinate axis above mean sea level [m]

uS : sustained wind speed used for design [m/s]

: one (1) minute mean at z = 10 m as defined under B.5 [m/s]

C.3 Sub-surface currents

For sub-surface currents the design velocity is to be based on the current velocity at sea level (z = 0), which shall be based on observed values provided by competent institutions. The data are subject to approval by Germanischer Lloyd. The vertical velocity distribution for 0 ≥ z ≥ – d may be determined as follows (see Fig. 1.3):

uSS(z) = [(z + d) / d] 1/7 ⋅ uS0

where

uSS(z) : sub-surface current velocity [m/s]

d : water depth [m]

z : vertical coordinate axis [m]

uS0 : current velocity at sea level [m/s]

Linear superposition of uSS(z) and uw(z) is applicable.




Fig. 1.3 Current profiles

C.4 Near-shore currents

For near-shore currents, which have a direction parallel to the shore line, the design velocity at the loca-tion of breaking waves may be estimated as follows:

nS Bu 2 s g H= ⋅ ⋅ ⋅

where

unS : near-shore current velocity [m/s]

s : beach slope

: tan α

α : inclination of beach

g : 9.81 m/s2

HB : breaking wave height [m]

: b / [1 / dB + a / (g ⋅ TB2)]

a : 44 ⋅ [1 – exp(–19 ⋅ s)]

b : 1.6 / [1 + exp(–19 ⋅ s)]

dB : water depth at the location of the breaking wave [m]

TB : period of the breaking wave [s]

For very small beach slopes, HB may be estimated from

HB = 0.8 ⋅ dB [m]

C.5 Combined wave and current kinematics

Wave kinematics, adjusted for directional spreading and irregularity, should be combined vectorial with the current profile, adjusted for blockage.

C.5.1 Wave kinematics

C.5.1.1 Jackets

For deterministic/regular wave load calculations on jackets a directional spreading factor Ø of 0.95 down to 0.87 can be applied on wave horizontal particle velocities and accelerations. Guidelines can be found in ISO 19901-1. The applied factor shall be agreed upon case by case early in the design phase.




C.5.1.2 Jack-ups

For deterministic/regular wave load calculations on jack-ups a kinematic reduction factor of 0.86 on max. wave height HD can be applied. This factor considers wave spreading and the irregular nature of the Sea. No further reduction on wave particle velocities or accelerations is permitted.

For stochastic or random time-domain analysis a wave kinematics factor Ø according to C.5.1.1 can be applied.

C.5.2 Current blockage

For structures that are more or less transparent, the current velocity in the vicinity of a structure is re-duced by blockage. The presence of the structure causes the incident flow to diverge, with some of the incident flow going around the structure rather than through it.

The degree of blockage depends on the kind of structure. For dense fixed space frame structures it will be large, while for some kinds of transparent floaters it will be small. Approximate current blockage fac-tors for typical jacket-type structures are given in Table 1.2.

For structures with other configurations, a current factor can be calculated according to C.2.3.1 b4 of API RP 2A-WSD. Factors less than 0.7 should not be used unless empirical evidence supports them.

Current blockage for lattice legs of self-elevating units can be considered according to SNAME, Rev. 3, C4.5.3 by formula (4.5.2).

Table 1.2 Current blockage factors

Number of legs

Current heading

Blockage factor

3 All 0.90

4 End-on 0.80

Diagonal 0.85

Broadside 0.80

6 End-on 0.75

Diagonal 0.85

Broadside 0.80

8 End-on 0.70

Diagonal 0.85

Broadside 0.80

C.5.3 Stretching of current profile and linear wave kinematics

As the current profile is specified only to storm mean water level, it shall be stretched or compressed to the local wave elevation. For slab current profiles, simple vertical extension of the current profile from storm mean water level to the wave elevation is acceptable. For other current profiles, linear stretching is acceptable. In linear stretching (Wheeler), the current at a point with elevation z, above which the wave surface elevation is η, is obtained from the specified current profile at elevation z’.

The elevations z and z’ are related as follows:

(z z ') d(z z ')(d )+ ⋅

+ =+ η

where d is the water depth. Points z and η are both positive above and negative below storm mean water level.




In case linear wave theory is applied the wave particle velocity and acceleration needs also to be stretched to the instantaneous free surface elevation. Wheeler stretching (see above) or delta stretching (see ISO 19901-1) can be used.

For the calculation of wave kinematics the intrinsic wave period (or frequency) should be applied. The intrinsic wave period is based on a reference coordinate system that travels with the current speed in its direction. Please refer to ISO 19901-1.

D Sea Waves

D.1 Design criteria

The design environmental criteria shall be developed from the environmental information. These criteria may be based on risk analysis where prior experience is limited. For new and relocated structures that are manned during the design event, or where the loss of or severe damage to the structure could result in severe damage, a 100-year recurrence interval should be used for oceanographic design criteria. Con-sideration may be given to reduced design requirements for the design or relocation of other structures that are unmanned or evacuated during the design event, that have either a shorter design life than the typical 20 years, or where the loss of or severe damage to the structure would not result in a high conse-quence of failure.

D.2 Design conditions

Two alternative methods may be used to specify design conditions for sea waves, namely, the determi-nistic method based on the use of an equivalent design wave or the stochastic method based on the ap-plication of wave spectra.

The deterministic method describes the seaway as regular periodic waves, characterized by wave period (or wave length), wave height, wave direction, and possible shape parameters.

The deterministic wave parameters may be based on statistical methods.

D.3 Two-dimensional wave kinematics of regular waves

Analytical or numerical wave theories may describe the wave kinematics. For a specified wave period T, wave height H, and water depth d, two-dimensional regular wave kinematics can be calculated using ap-propriate wave theory. The following theories are mentioned: • linear (Airy) wave theory, where a sine function describes the wave profile • Stokes 5th order wave theory, appropriate for high waves • stream function theory, where wave kinematics are accurately described over a broad range of wa-

ter depths • solitary wave theory for shallow waters

Other wave theories, such as Chappelear, may be used, provided an adequate order of the solution is selected.

In the (H/gT2, d/gT2) plane, regions of applicability of various theories are shown in Fig. 1.4 as functions of wave period T, wave height H, and storm water depth d (g = acceleration of gravity).

D.3.1 Equivalent design wave

Wave loads on a platform are dynamic in nature. For most design water depths presently encountered, these loads may be adequately represented by their static equivalents. For deeper waters or where plat-forms tend to be more flexible, a load analysis involving the dynamic action of the structure is required.

D.3.2 Design wave period and design wave height

The design wave period TD, is associated with the highest wave. In any given storm, there is only one maximum wave, and the associated period is the most probable period of the maximum wave. For this reason it is common to give a range of wave periods for TD that are independent of the peak period Tp.




The peak period is the value associated with the “peak” of the wave spectrum. A range of design wave periods is specified as follows:

1/3 D 1/33.5 H T 4.5 H⋅ < < ⋅

where

TD : design wave period [s]

H1/3 : significant wave height [m]

HD : design wave height [m]

The design wave period TD shall not exceed 25 s.

The design wave height HD, may be estimated with the following relations:

TD HD / H1/3

0 – 9 s 10 – 15 s 16 – 25 s

1.86 1.95 2.23

This method generally applies to deep water waves, i.e. waves of periods T that satisfy the following con-dition:

( )2d / g T 0.06⋅ >

where

d : water depth [m]

Design wave parameters for wind-induced transitional water waves, i.e. water waves of periods T that satisfy the condition

( )20.002 d / g T 0.06< ⋅ <

may be defined from information on extreme wind speed and fetch, using relevant theories subject to approval by Germanischer Lloyd.




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Fig. 1.4 Regions of applicability of wave theories

D.4 Short-term wave conditions

The principal natural sea state parameters are as follows: • The significant wave height H1/3 is defined as the average of the one-third highest wave heights in a

record of stationary sea surface elevations. • The characteristic wave period T1 is defined as the mean wave period in a record of stationary sea

surface elevations.

The visually observed wave height Hv and the visually observed wave period Tv may be considered ap-proximately equal to H1/3 and T1, respectively.

Short-term stationary natural sea states may be described by a wave spectrum, i.e. by the power spectral density function of the vertical sea surface elevation. Short-term wave conditions are defined as seaways




whose representative spectrum does not change for a brief but not closely specified duration. Wave spec-tra may be given in tabular form, as measured data, or in analytic form.

The two parameter modified Pierson-Moskowitz spectrum and the JONSWAP spectrum are most fre-quently applied. Both spectra describe sea conditions relevant for the most severe sea states. Application of other wave spectrum formulations is to be approved by Germanischer Lloyd.

D.4.1 Spectral density

The following expression describes the wave energy spectral density:

( ) ( ) ⎥⎥⎦

⎤

⎢⎢⎣

⎡

⋅

−−

⎥⎥

⎦

⎤

⎢⎢

⎣

⎡

⎟⎟⎠

⎞⎜⎜⎝

⎛−

⋅⋅=

2p

2

2p

ωσ2

)ω(ωexp4

p5

4p

21/3 γ

ωω

45exp

ω

ωHγfωS

where

S(ω) : spectral density [m2s]

ω : circular wave frequency [s1]

: 2π/T

T : wave period [s]

f(γ) : 0.06240.1850.23 0.0336 γ1.9 γ

+ ⋅ −+

γ : peakedness parameter (see D.4.4)

ωp : circular spectral peak frequency [s-1]

: 2π/Tp

Tp : modal period or peak period [s]

: period at which the spectrum is a maximum

σ : spectral shape parameter (see D.4.4)

D.4.2 Spectral moments

The spectral moments, mn, of order n are defined as follows:

( ) ( )n

n 0m ω ω S ω dω

∞= ∫ for n = 0, 1, 2, …

where

S(ω) : wave energy spectral density [m2s], see D.4.1

The following quantities may be defined in terms of the spectral moments: • Significant wave height:

01/3 m4H ⋅= [m]

• Mean wave period (center of gravity of the wave spectrum):

1

01 m

m2T π= [s]

• Zero-up-crossing period (average period between successive up- crossings):

2

0z m

m2πT = [s]

• Crest period (average period between successive wave crests):




2C

4

mT 2

m= π [s]

• Significant wave slope:

0

2

mm

gπ2s ⋅⋅

=

D.4.3 Spectral density and moments in terms of wave frequency

The wave spectral density may also be given in terms of wave frequency f in [Hz]. The relationship is

S(f ) 2 S( )= π⋅ ω

The moments of the wave energy spectral density may also be given in terms of the wave frequency f [Hz]. The relationship is

n nn n0m (f ) f S(f ) df (2 ) m∞= ⋅ ⋅ = π ⋅∫

D.4.4 Wave spectra

D.4.4.1 The Pierson-Moskowitz spectrum

The parameters for the spectral density of the Pierson-Moskowitz spectrum are defined as follows:

γ : peakedness parameter

: 1.0 for a mean spectrum

σ : spectral shape parameter is not defined as it is not relevant if γ = 1.0

D.4.4.2 The JONSWAP spectrum

The JONSWAP spectrum is also frequently applied. In addition to the significant wave height H1/3 pa-rameters are needed to describe the spectral shape, namely, the peakedness parameter γ and the spec-tral shape parameter σ:

γ : peakedness parameter

: 3.30 for a mean spectrum

σ : spectral shape parameter

: 0.07 if ω ≤ 2⋅π/Tp

: 0.09 if ω ≤ 2⋅π/Tp

D.4.5 Wave spreading and short-crested sea states

The wave spectra given above are uni-directional. However, real waves travel in many different direc-tions. Directional short-crested wave spectra may be derived from a uni-directional wave spectrum as follows:

)(W).(S),(S αω=αω

where

S(ω,γ) : directional short-crested wave energy spectral density

α : angle between direction of elementary wave trains and primary wave direction

W(α) : directionality function

: const ⋅ coss α

The value of the power constant s shall reflect an accurate correlation to the actual sea state. For typically occurring conditions in the open ocean s = 2 is appropriate.




For design purposes it is usual to assume that the secondary wave directions are spread over –90° < α < + 90°. Energy conservation requires that the directionality function fulfils the requirement

1)(Wmax

min

=α∫α

α

D.4.6 Wave period relationships

The following wave periods are in use:

Tp is the period at which the spectrum is a maximum (modal period or peak period)

Tz is the average period between successive up- crossings (Zero-up-crossing period)

T1/3 is the average period of the 1/3 highest wave periods

Tmax is the associated period of the wave with maximum height

D.4.6.1 For the Pierson-Moskowitz spectrum, wave periods are related to each other as follows:

Tp : 1.40 ⋅ Tz

T1 : 0.77 ⋅ Tp

T1/3 : 0.88 ⋅ Tp

Tmax : 1.04 ⋅ Tp

D.4.6.2 For the JONSWAP spectrum, wave periods are related to each other as follows:

Tp : 1.28 ⋅ Tz

T1 : 0.83 ⋅ Tp

D.5 Response in natural seas

D.5.1 Response from different wave components

A useful consequence of linearly predicted response is that results from regular waves of different ampli-tudes, wave lengths and propagation directions can be superimposed to obtain the response in natural seaways made up of a large number N of regular waves of different length and height. Thus, the re-sponse R from different wave components can be written as

( ) ( )[ ]∑=

++⋅⋅⋅=N

1nnnnnan εωδtωsinωHςR

where

⎟H(ωn)⎜ : response amplitude per unit wave amplitude (transfer function)

δ(ωn) : a phase angle associated with the response

Both⎟H(ωn)⎜and δ(ωn) are functions of the frequency of oscillation ωn. The response can be any linear wave-induced motion of or load on the structure.

D.5.2 Variance of the response

In the limit, as N approaches infinity, the variance of the response, σr, is found as follows:

2 2r

0S( ) H ( ) d

∞σ = ω ⋅ ω ⋅ ω∫

The Rayleigh probability function may be used to approximate the probability density function for the maxima (peak values) of the response, p(R):




2 2r2

r

Rp(R) exp ( 0.5 R / )= − ⋅ σσ

D.5.3 Most probable largest value during a short-term seastate

During a short-term seastate of significant wave height H1/3 and mean wave period of the sea spectrum T1 the most probable largest value Rmax over time t is then

)ln(t/Tσ2R 12rmax ⋅⋅=

Strictly speaking, the mean period of the response variable should be used instead of the average mean wave period T1. However, for linear wave-induced motions and loads the difference when estimating Rmax is small and can be neglected.

D.5.4 Long-term probability of the response

Short-term response predictions are sometimes considered inadequate for the design and reliability analysis of offshore structures, because the assumption of stationarity (of a statistically invariant record) restricts the validity of the analysis. A long-term probability defines events and extreme value statistics for a period on the order of 20 to 100 years, as opposed to a few hours for the short-term probability.

The long-term prediction considers all sea states the structure is expected to encounter during its design lifetime. This may be accomplished in the form of the frequency of occurrences of all possible sea states or the long-term wave data over the entire life of the structure, i.e. in the form of a wave scatter diagram that conveniently describes a family of sea states, each characterized by the wave spectrum parameters (H1/3, T1) or (H1/3, T1, γ) as defined in D.4.1 and D.4.2, respectively.

The long-term method provides a rational criterion of acceptability in the form of a uniform likelihood that a given response level will be exceeded during the lifetime of the structure.

The long-term prediction can be performed in the frequency domain for a linear system or in the time domain for a nonlinear system.

Combining the Rayleigh distribution and a joint probability (H1/3, T1) from a wave scatter diagram listing M significant wave height intervals and K spectral peak periods yields the long-term probability of the re-sponse:

2M Kjk2j 1k 1 rjk

RP(R) 1 exp p2= =

⎛ ⎞⎜ ⎟= − − ⋅∑ ∑ ⎜ ⎟⋅σ⎝ ⎠

where

M : number of significant wave height intervals

K : number of spectral peak periods listed in the wave scatter diagram

P(R) : long-term probability that the peak value of the response does not exceed R

σrjk : standard deviation of the response in the seastate (H1/3, T1) of interval j and mean period k

pjk : probability of occurrence of seastate (H1/3, T1) of interval numbers j and k

D.6 Derived wave parameters

Derived wave parameters, such as wave particle velocities and accelerations, may be used to determine drag and inertial forces on underwater portions of offshore drilling units or structures that are operating or situated in locations where the water depth is considered deep. Subject to approval by Germanischer Lloyd, other appropriate methods to determine these forces will be considered, provided they are refer-enced and supported by calculations




E Sea Level

E.1 The highest still water level to be used for design is defined by the water depth d at the highest astronomical tide plus the water level elevations due to storm surge.

E.2 The smallest still water level to be used for design is defined by the smaller value of either the water depth d due to the lowest astronomical tide minus the water level decline due to storm surge or the chart datum, whichever is applicable.

E.3 The highest wave elevation above still water level ζ* defined in E.1. or E.2. is given as

ζ* = δ ⋅ HD [m]

where

HD : wave height (see D.3.2) [m]

δ : wave elevation coefficient

The wave elevation coefficient δ is a function of the wave period TD (see D.3.2) and the water depth d as given in Table 1.3.

Table 1.3 Wave elevation coefficient δ

HD/(gTD2) d/(gTD2)

0.02 0.01 0.005 0.001 0.0005 0.0001

≥ 0.2 0.60 0.55 0.50 0.50 0.50 0.50

0.02 - 0.68 0.58 0.52 0.50 0.50 0.002 - - - 0.87 0.80 0.68

E.4 Water depth and statistical wave data shall be provided by a competent Authority. The appli-cation of the data is subject to approval by Germanischer Lloyd.

F Tsunamis Tsunamis are generated by large and sometimes distant earthquakes or undersea fault movements or by large sea floor slides triggered by earthquakes or by volcanic eruptions. Traveling through deep water, tsunamis are long surface waves with low height and pose little hazard to floating or fixed offshore struc-tures. When they reach shallow water, the wave form pushes upward from the bottom to make a rise and fall of water that can break in shallow water and wash inland with great power. Tsunamis travel great distances very quickly and can affect regions not normally associated with such events. The likelihood of tsunamis affecting the location of the structure shall be considered.

The greatest hazard to offshore structures results from inflow and outflow of water in the form of waves and currents. These waves can be significant in shallow water, causing substantial actions on structures. Currents caused by the inflow and outflow of water can cause excessive scour problems.

G Climatic Conditions, Temperature and Marine Growth

G.1 Design temperature

G.1.1 Estimates of design temperatures of air and water shall be based on relevant data that are provided by competent institutions. Preferably, 1 percent and 99 percent fractiles shall be used as lowest




and highest design temperatures, respectively. Least square fits of the data may be used to obtain a sto-chastic model.

G.1.2 If design air temperatures are less than 0 °C, the amount of snow and rain turning to ice on exposed surfaces is to be specified.

G.1.3 If design air temperatures are less than -2 °C, the amount of ice accretion on free surfaces, e.g., lattice structures, as result of seawater spray is to be specified.

G.2 Other environmental factors

Depending on circumstances, other environmental factors can effect operations and can consequently influence the design of structures. Appropriate data shall be compiled, including, where appropriate, re-cords and or predictions of • precipitation • humidity • fog • wind chill • salinity of sea water • oxygen content of sea water

G.3 Marine growth

G.3.1 The thickness and kind of marine growth depends on location, age of structure, and the main-tenance regime. Experience in one area of the world cannot necessarily be applied to another. Site-specific studies shall be conducted where necessary to establish the likely thickness and the water depth dependence of marine growth.

G.3.2 The influence of marine growth on hydrodynamic actions is caused by increased dimensions and increased drag coefficients of rough surfaces. Components with circular cross sections shall be classed as either “smooth” or “rough,” depending on the thickness and kind of marine growth expected to accumulate on them.

Structural components can be considered hydrodynamically smooth if they are either above the highest still-water level or sufficiently deep, such that marine growth is sparse enough to ignore the effect of roughness. Site-specific data should be used to establish the extent of the hydrodynamically rough zones.

H Sea Ice and Icebergs

H.1 The probability of occurrence of sea ice and icebergs in the operating area shall be estab-lished.

H.2 If seasonal sea ice is probable in the operating area, the modes of occurrence and the me-chanical properties of the ice shall be determined by a competent Authority upon agreement with Ger-manischer Lloyd.

H.3 If permanent sea ice is probable in the operating area, thickness and extension as well as mechanical properties and drifting characteristics shall be determined by a competent Authority upon agreement with Germanischer Lloyd.

H.4 This data shall be used to determine design characteristics of the installation as well as possi-ble evacuation procedures.




I Sea Bed

I.1 Structures founded on the sea bed may be affected by gradual or transient changes of the sea bed. These changes shall be accounted for because they influence the validity of design parameters (compare E) and affect the structural integrity.

I.2 Sea bed stability

As outlined in Section 6, it will have to be determined whether, owing to topography and soil configura-tion, the possibility of slope failure or slides, cavity failure, or erosion phenomena have to be considered. Settlement and soil liquefaction generally have to be taken into account for gravity foundations.

I.3 Seismic activities

Where seismic activity is to be expected, two different earthquake levels have to be distinguished: • A strength level earthquake (SLE) represents a level not unlikely to happen in the region of the

planned installation. For this level the strength of the structure is used up to the minimum nominal upper yield stress ReH, but the main functions of the offshore installation can still be fulfilled and, af-ter a sufficient checking operation, can be continued.

• A ductility level earthquake (DLE) represents an extreme level that normally puts the offshore instal-lation out of operation, but still allows the main elements of the installation to keep their position, and the structure is thus able to survive. This should make it possible for the crew to save themselves or to be evacuated and should avoid extreme damage of the installation and to the environment.

The characteristics of these two earthquake levels (acceleration intensity, duration) shall be established by a competent Authority and agreed upon with Germanischer Lloyd, compare Section 2, G and Section 3, C.




Section 2 Design Loads A Basic Considerations .................................................................................................. 2-1 B Environmental Loads .................................................................................................. 2-1 C Permanent Loads...................................................................................................... 2-14 D Functional Loads....................................................................................................... 2-14 E Accidental Loads....................................................................................................... 2-15 F Transportation and Installation Loads....................................................................... 2-17 G Earthquake Loads..................................................................................................... 2-18

A Basic Considerations

A.1.1 Design loads for marine structures include environmental loads, permanent loads, functional loads, and accidental loads.

A.1.2 Loads, or load effects such as stresses, may result from direct action (imposed forces) or indi-rect action (imposed or constrained deformations, such as settling or temperature gradients). Loads of the latter category are usually termed deformation loads.

In the context of these Rules (Loading Conditions, Section 3, C), such loads may generally be grouped into one of the categories listed in A.1.1.

A.2 These Rules specify - explicitly or implicitly - nominal or design loads to be used in structural analysis.

A.3 Design values of permanent loads shall be vectorially added to the design values of other loads as listed in A.1.1.

B Environmental Loads

B.1 Environmental loads include all loads attributable to environmental conditions as specified in Section 1: • wind loads, see B.2. • sea current loads, see B.3 • wave loads, see B.5 • vibratory loads caused by vortex shedding, see B.6 • loads caused by climatic conditions, see B.7 • loads caused by sea ice, see B.8 • mooring loads, see D.2 • Other environmental loads, such as loads caused by sea bed movements, earthquakes, etc., as ap-

plicable.

B.2 Wind loads

B.2.1 Wind pressure

A design value of wind pressure on structural elements at height z above still-water level may be calcu-lated according to the following formula:


Section 2 Design Loads


q = ½ ρ ⋅ u2 ⋅ CS

where

q : wind pressure [kPa]

u : design wind speed at the height z, averaged over time t [m/s]

: u(z,t), see Section 1, B.2 and Section 1, B.6

ρ : density of air

: 0.001224 for dry air [kN · s2/m4]

z : coordinate for height above sea level [m]

CS : shape coefficient from Table 2.1

In the absence of data indicating otherwise, the following shape coefficients (CS) are recommended for perpendicular wind approach angles with respect to each projected area.

Table 2.1 Wind pressure shape coefficients CS

Shape CS

Spherical shapes 0.4

Cylindrical shapes (all sizes) 0.5

Large flat surfaces (hull, deckhouses, smooth deck areas) 1.0

Drilling derrick 1.25

Wires 1.2

Exposed beams and girders under deck 1.3

Small parts 1.4

Isolated shapes (cranes, beams, etc.) 1.5

Clustered deckhouses and similar structures 1.1

Shape coefficients for shapes or combination of shapes that do not readily fall into the specified catego-ries are subject to special consideration or may be provided by competent institutions. These values are to be approved by Germanischer Lloyd.

In calculating the wind overturning moment for fixed platforms and jack-up rigs in elevated mode, the lever for the wind overturning force is to be taken vertically from the seabed to the centre of pressure of all wind exposed surfaces.

In calculating the wind overturning (heeling) moment for mobile offshore units, the lever for the wind over-turning force is to be taken vertically from the centre of lateral resistance of the underwater body of the unit to the centre of pressure of all wind exposed surfaces. (The unit shall be assumed floating free of mooring restraints.)

Wind heeling moments derived from wind tunnel tests on a representative scale model of the structure may be considered as alternatives. The determination of such heeling moments shall include lift and drag effects.

B.2.2 Shielding

Shielding may be considered when the object lies close enough behind another object. In such a case, shielding factors listed in Table 2.2 may be used.

B.2.3 Wind force

The design wind load may be calculated according to the following formula:

F = q ⋅ A




where

F : wind force [kN]

q : wind pressure [kPa]

A : wind projected area of all exposed surfaces in either upright or heeled condition [m2]

The exposed surfaces are to be taken in either the upright or heeled condition.

The wind load calculation for stability calculations for floating units is to be done according to GL Rules for Mobile Offshore Units (IV-6-2), Section 7, Table 7.2

B.2.4 Wind force directions

Wind forces shall be considered from any direction relative to the structure.

The wind direction shall generally be assumed to be identical with the dominant direction of wave propa-gation.

B.2.5 Recommendations • For units with columns, the projected areas of all columns are to be included without shielding. • Areas exposed because of heel, such as underdecks, are to be included using the appropriate

shape coefficients. • Cranes, isolated houses, structural shapes, etc., are to be calculated individually, using the appro-

priate shape coefficients of Table 2.1. • Open truss structural components, such as lattice structures, derrick towers, crane booms, and cer-

tain kinds of masts may be approximated by taking 60 % of the projected area of one side, using the shape coefficient of 1.25.

B.2.6 Design wind loads on the whole structure may be estimated by superposition of all design wind loads on individual structural elements, or by wind tunnel tests with a model of the entire structure.

B.3 Sea current loads

B.3.1 Where current is acting alone (i.e., no waves), a design value of sea current pressure on struc-tural elements at depth z below the still-water level is defined as follows:

qD(z) = ½ ρ uD2 (z) for 0 ≥ z ≥ –d

where

qD(z) : sea current pressure [kPa]

uD(z) : design sea current speed [m/s]

uD(z) is either uW(z), uSS(z) or unS(z), as defined in Section 1, C.2, C.3 or C.4, or a superposition of these values, as applicable.

ρ : density of seawater

: 1.025 [kN ⋅ s2/m4]

z : coordinate for height above sea level [m]

d : depth from still-water to bottom [m]

B.3.2 Using qD(z) as defined above, design sea current loads F(z) may be calculated according to the following formula:

F = qD(z) ⋅ A

where

F : current force [kN]

qD(z) : current pressure [kPa]




A : current projected area [m2]

Values of the current projected area A may need to be corrected for effects of marine growth, as applica-ble, compare Section 1, F.3

B.3.3 Consideration shall be given to the possible superposition of current and waves. In those cases where this superposition is deemed necessary, the current velocity shall be added vectorially to the wave particle velocity. The resultant velocity is to be used to compute the total force.

B.4 Conductor shielding

The hydrodynamic forces on conductor arrays can be reduced by applying a conductor shielding factor KS.

The shielding depends on the spacing of the conductors and the amplitude of oscillation.

A shielding factor for the steady flow or oscillating flow with high amplitudes KSS can be estimated accord-ing to Table 2.2, in which:

S : center-to-center spacing of the conductors in the current direction [m]

D : diameter of the conductors, including marine growth [m]

Table 2.2 Shielding factor KSS

S/D KSS

2.00 0.50

4.00 and larger 1.00

For other values of S/D, linear interpolation may be applied.

The shielding factor KS can be calculated as follows:

KS : KSS ⋅ KA

where

KA : factor for adjusting the amplitude of oscillatory flow according to Table 2.3.

Table 2.3 Adjustment factor KA

Amp/S KSS

≥2.5 1

0.5 ≤ S

Amp < 2.5 SS

1 1 Amp1 2.5 12 K S⎛ ⎞ ⎛ ⎞⋅ − ⋅ − +⎜ ⎟ ⎜ ⎟

⎝ ⎠⎝ ⎠

< 0.5 SSK1

Amp : Amplitude of oscillation

: m0 iU T[m]

2π⋅

Um0 : max. wave induced orbital velocity [m/s]

Ti : intrinsic wave period [s], please refer to Section 1, C.5.3




B.5 Wave loads

B.5.1 General

B.5.1.1 The design wave criteria have to be specified by the Owner / Designer. A design value of wave load on a structural element (local wave load) or on the entire structure (global wave load) may be estimated by either using a design wave method, as outlined in B.5.2 to B.5.6, or a spectral method, as outlined in B.5.7.

B.5.1.2 Consideration is to be given to waves of less than maximum height where, due to their period, the effects on various structural elements may be greater.

B.5.1.3 Forces caused by the action of waves on the installation/unit are to be taken into account in the structural design, with regard to forces produced directly on the immersed elements and forces result-ing from heeled positions or accelerations due to motions. Theories used to calculate wave forces and to select relevant force coefficients are to be acceptable to Germanischer Lloyd.

B.5.1.4 Forces on appurtenances such as boat landings, fenders or bumpers, J-tubes, walkways, stairways, grout lines, anodes, etc. shall be considered for inclusion in the hydrodynamic model of the structure. Forces on some appurtenances may be important for local member design. Appurtenances are generally modeled by non-structural members that contribute equivalent wave forces.

B.5.2 Design wave methods

Methods to estimate design wave loads depend on the hydrodynamic characteristics of the structure and on the way the structure is held on location, as outlined in Table 2.4.

B.5.3 Method I.1: Rigidly positioned, hydrodynamically transparent structures

B.5.3.1 The Morison formula may be used to calculate the force exerted by waves on a cylindrical object, such as a structural member. The wave force can be considered as the sum of a drag force and an inertia force, as follows:

F = FD + FI = ½ ρ ⋅ CD ⋅ A ⋅ U ⋅ |U | + CM ρ V (δU/δt)

Table 2.4 Summary of design wave methods

Hydrodynamic characteristics Kind of positioning

I. Transparent 1 II. Compact 2

1. Rigid 3 Method I.1 (see 4.3) Method II.1 (see 4.5)

2. Flexible 4 Method I.2 (see 4.4) Method II.2 (see 4.6)

1 A structure is considered hydrodynamically transparent if wave scattering can be neglected, i.e., if the structure does not significantly modify the incident wave. This condition is satisfied if D/L < 0.2, where D is a characteristic dimension of the structure (e.g., diameter of structural members, such as platform legs) in the direction of wave propagation, and L is the wave length.

2 A structure is considered hydrodynamically compact if scattering of sea waves cannot be neglected. This condition occurs if D/L > 0.2. If D/L is about 0.2, a design value may be defined as the smaller one of the design values obtained from the al-ternative application of methods I and II.

3 A structure is considered rigidly positioned if the wave-induced deviation from its equilibrium position in still-water is negligi-bly small when compared to the wave height.

4 A structure is considered flexibly positioned if the wave-induced deviation from its still-water equilibrium position is of the order of the wave height or more.




where

F : hydrodynamic force vector per unit length acting normal to the axis of the member [kN/m]

FD : drag force vector per unit length acting on the member in the plane of the member axis and U [kN/m]

FI : inertia force vector per unit length acting normal to the axis of the member in the plane of the member axis and δU/δt [kN/m]

CD : drag coefficient

ρ : density of water [kNs2/m4]

A : projected area normal to the cylinder axis per unit length

: D for circular cylinders [m]

V : displaced volume of the cylinder per unit length

: π D2/4 for circular cylinders [m2]

D : effective diameter of circular cylindrical member, including marine growth [m]

U : component of the velocity vector (due to wave and/or current) of the water normal to the axis of the member [m/s]

| U | : absolute value of U [m/s]

CM : inertia coefficient

δU/δt : component of the local acceleration vector of the water normal to the axis of the member [m/s2]

Note that the Morison equation, as stated here, ignores the convective acceleration component in the inertia force calculation as well as lift forces, impact related slam forces, and axial Froude-Krylov forces.

B.5.3.2 The vectorial superposition of local wave loads in a phase correct manner defines global wave loads on the structure. Total base shear and overturning moment are calculated by a vector summation of local drag and inertia forces due to waves and currents, dynamic amplification of wave and current forces (see Section 3, D), and wind forces on the exposed portions of the structure (see B.2).

Slam forces can be neglected because they are nearly vertical. Lift forces can be neglected for jacket-type structures because they are not correlated from member to member. Axial Froude-Krylov forces can also be neglected.

The wave crest shall be positioned relative to the structure so that the total base shear and overturning moment have their maximum values. The following aspects need to be considered: • The maximum base shear may not occur at the same wave position as the maximum overturning

moment. • In special cases of waves with low steepness and an opposing current, maximum global structure

force may occur near the wave trough rather than near the wave crest. • Maximum local member stresses may occur for a wave position other than that causing the maxi-

mum global structure force.

B.5.3.3 If the Morison equation is used to calculate hydrodynamic loads on a structure, the variation of CD as a function of Reynolds number, Keulegan-Carpender number, and roughness shall be taken into account in addition to the variation of the cross-sectional geometry:

Reynolds number Rn : UmaxD/ν

Keulegan-Carpenter number KC : UmaxT/D

Relative roughness : k/D

where

D : diameter [m]

T : wave period [s]




U : maximum of velocity U defined in 4.3.1 [m/s]

ν : kinematic viscosity of water

≈ 1.4 ⋅ 10-6 at 10° C [m2/s]

k : roughness height [m]

B.5.3.4 As guidance in determining drag coefficients, surface roughness values listed in Table 2.5 may be used.

Table 2.5 Surface roughness

Surface k

Steel, new uncoated 5 ⋅ 10-6

Steel, painted 5 ⋅ 10-6

Steel, highly rusted 3 ⋅ 10-3

Concrete 3 ⋅ 10-3

Marine growth 5 ⋅ 10-3 to 5 ⋅ 10-2

B.5.3.5 For smooth circular cylinders in steady flow, where the cylinder length is significantly larger than the cylinder diameter, Table 2.6 lists design values of hydrodynamic coefficients CD and CM for two classes of Reynolds number.

Table 2.6 Hydrodynamic coefficients CD and CM for circular cylinders in steady flow conditions

Reynolds number (Rn) CD CM

< 10-5 1.2 2.0

> 10-5 0.7 1.5

1. Corrections due to marine growth have to be made, as applicable.

2. CD and CM values for more complicated structural elements shall be provided by competent institutions. These values are to be approved by Germanischer Lloyd.

B.5.3.6 For circular cylinders in supercritical oscillatory flow at high values of KC, with in-service ma-rine roughness, Table 2.7 lists design values of hydrodynamic coefficients CD and CM.

Table 2.7 Hydrodynamic coefficients CD and CM for circular cylinders in oscillatory flow condi-tions

Surface condition CD CM

Multiyear roughness, k/D > 1/100 1.05 1.8

Mobile unit (cleaned), k/D < 1/100 1.0 1.8

Smooth member, k/D < 1/10000 0.65 2.0

The smooth values will normally apply 2 m above the mean water level and the rough values 2 m below the mean wa-ter level.

The roughness for a mobile unit applies when marine growth is removed between submersions of members.




B.5.3.7 For several cylinders close together, group effects may be taken into account. If no adequate documentation of group effects for the specific case is available, the drag coefficients for the individual cylinder may be used.

B.5.3.8 The hydrodynamic modeling of jack-up legs may be carried out by utilizing the ‘detailed’ or the ‘equivalent’ technique. The ‘detailed’ technique models all relevant members with their own unique de-scriptions for the Morison term values and with the correct orientation to determine relative normal veloci-ties and accelerations and the corresponding drag and inertia coefficients, as defined in B.5.3.6. The hy-drodynamic model of the ‘equivalent’ technique comprises one ‘equivalent’ vertical tubular located at the geometric center of the actual leg. The corresponding (horizontal) relative velocities and accelerations are applied together with equivalent hydrodynamic coefficients (CD and CM) and equivalent diameters (D). The model shall be varied with elevation, as necessary, to account for changes in dimensions, marine growth thickness, etc.

For non-tubular geometries, such as split tube and triangular leg chords, the appropriate hydrodynamic coefficients may, in lieu of more detailed information, be assessed from relevant literature and/or appro-priate interpretation of (model) tests. The SNAME (2002) Technical & Research Bulletin 5-5A “Guidelines for Site Specific Assessment of Mobile Jack-Up Platforms” is an example of a document that provides such information.

B.5.4 Method I.2: Flexibly positioned, hydrodynamically transparent structures

B.5.4.1 Depending on the kind of structural elements considered in a strength analysis, this method requires calculation of wave-induced first order motions (see B.5.4.2) as well as second order motions of the moored structure (see B.5.4.3).

B.5.4.2 Loads on structural elements not belonging to the mooring system

Wave-induced design loads used for strength analysis of structural elements that do not belong to the mooring system may be estimated by a phase correct vectorial superposition of all local wave loads act-ing on the rigidly positioned (fixed) structure according to B.5.3, Method I.1. The corresponding design loads for all motion-induced loads may be calculated on the basis of a linear motion analysis. Such an analysis shall account for all relevant degrees of freedom of the structure being considered. Typical off-shore structures may be classified according to the number of degrees of freedom of rigid-body motions as listed in Table 2.8:

Table 2.8 Degrees of freedom of different kinds of offshore structures

Degrees of free-

dom Kind of structure

6

Drill ships, supply vessels, tugs Anchored or dynamically positioned semisubmersibles Loading buoys, storage tanks Floating cranes, pipelaying vessels

3 Tension-leg platforms Guyed towers

2 Articulated towers

For a six degree-of-freedom system the motion analysis may generally be based on the following linear motion equations:

6 2

D jk jk D jk jk Dk jj 1

.(M A ) i. .B C s F=⎡ ⎤−ω + + ω + ⋅ =∑ ⎣ ⎦

for j 1, 2 ... 6=




where

j : index denoting the direction of the force

k : index denoting the direction of the motion

ωD : circular frequency of motion

: 2π / TD

TD : period of the design wave

Mjk : generalized mass matrix

Ajk : generalized added mass matrix

Bjk : generalized damping matrix

Cjk : generalized restoring force matrix

sDk : generalized vector of motion amplitudes

Fj : generalized global wave-induced load vector, determined according to B.5.4.3

To solve these equations for the motion components sDk, the generalized masses Mjk of the structure (including mass moments of inertia for i, j > 3), the respective global hydrodynamic added masses Ajk, the global damping coefficients Bjk (viscous damping for hydrodynamically transparent structures), and the global hydrostatic (restoring) coefficients Cjk need to be defined. These global hydromechanic coeffi-cients may be obtained from calculation of local loads on all structural elements below the still-water line. Linear superposition of all local hydromechanic coefficients determines the required global hydrome-chanic coefficients. Details can be found in related text books. Appropriate computer codes or a compati-ble procedure may be applied upon agreement with Germanischer Lloyd.

After determination of the motion components, motion induced local loads on all structural elements can be estimated and added to the local wave loads as determined for the rigidly positioned (fixed) structure, as applicable, finally yielding the resulting wave induced (first order) local design loads on structural ele-ments.

B.5.4.3 Loads on structural elements belonging to the mooring system

Wave-induced design loads used for strength analyses of structural elements that belong to the mooring system shall be estimated by superposition of wave loads due to first and second order (drift) motions. The second order motion analyses shall include restoring forces of the mooring system, which may be linearized upon agreement with Germanischer Lloyd. Simulation techniques with statistical evaluation or equivalent spectral analysis methods are acceptable. Details are subject to approval by Germanischer Lloyd.

Selected model tests are recommended to determine (validate) the influence of damping.

B.5.5 Method II.1: Rigidly positioned, hydrodynamically compact structures

B.5.5.1 Linear wave-induced loads on large body, hydrodynamically compact structure shall be de-termined by diffraction theory. Diffraction theories may be based on sink-source methods or finite fluid volume methods. For simple geometric shapes, analytical solutions may be used. For surface piercing bodies, results from sink-source methods shall be checked to avoid unreliable predictions in the neighborhood of irregular frequencies. Details can be found in related text books. Appropriate computer codes or a compatible procedure may be applied upon agreement with Germanischer Lloyd.

Model experiments are recommended if a new structural concept is to be analyzed and the loads cannot be adequately predicted by state of the art methods.

B.5.5.2 Wave-induced loads on structures comprising large volume parts and slender members may be obtained by combining the use of the wave diffraction theory and the Morrison equation. The modified velocities and accelerations caused by the large volume parts, however, should be accounted for when using the Morrison equation.

B.5.5.3 Hydrodynamic interaction between large volume parts should be accounted for.




B.5.5.4 In the vicinity of large bodies the free surface elevation (i.e., wave height) may be increased. This increase shall be accounted for not only in the wave-induced load calculation, but also when estimat-ing the under deck clearance.

B.5.5.5 In many cases second order wave-induced loads may be important for the design of hydrody-namically compact structures. • In addition to first order (linear) loads, mean (nonlinear) second order wave (drift) loads and non-

linear loads varying in time with twice the first order (wave) frequency act on the structure. Wave-induced load effects of higher order than two are usually neglected.

• In a natural (irregular) seaway, composed of an infinite number of elementary regular wave compo-nents, the resulting second order wave-induced loads contain three components, namely, the mean (drift) loads, loads varying in time with the difference frequencies of component waves (slowly vary-ing drift loads), and loads varying in time with the sum frequencies of component waves (high fre-quency loads).

• The sum frequency loads may be important in considering wave load effects on certain offshore structures, such as deep water gravity structures.

• Second order loads shall be determined by a consistent second order theory or by appropriate model tests.

B.5.6 Method II.2: Flexibly positioned, hydrodynamically compact structures

B.5.6.1 Depending on the kind of structural elements considered in a strength analysis, this method requires calculation of wave-induced first order motions (see B.5.5.1) as well as second order motions of the moored structure (see B.5.5.5).

B.5.6.2 Wave induced local design loads used for strength analysis of structural elements that do not belong to the mooring system may be estimated on the basis of the linear motion analysis as outlined in B.5.4.2, with Fj representing the components of the global wave load vector determined according to B.5.5.

Contrary to B.5.4.2, global hydrodynamic coefficients Ajk and Bjk may be determined directly by diffraction theory based methods. Details can be found in related text books. Appropriate computer programs or compatible procedures may be used upon agreement with Germanischer Lloyd.

B.5.6.3 The second order (slowly varying) difference frequency wave-induced loads (see B.5.5.5) may be important for the design of mooring or dynamic positioning systems as well as for offshore loading systems. For hydrodynamically compact structures with a small waterplane area, the slow drift forces may cause large vertical motions.

Second order loads shall be determined by a consistent second order theory or by appropriate model tests.

B.5.7 Spectral methods may be used to obtain the response in natural (irregular) seas by linearly superposing results from regular wave components of different amplitudes, wave lengths, and propaga-tion directions, assuming the natural seaway is made up of a large number of regular waves of different length and height. The procedure to follow is outlined in Section 1, D.5.

B.5.8 Wave impact loads

Structural members in the splash zone are susceptible to loads caused by wave-induced impact (slam-ming) when the member is submerged. These loads are difficult to estimate accurately and should gener-ally be avoided by structural design measures. Long horizontal members in the splash zone are particu-larly susceptible to impact related loads.

B.5.9 For a cylindrical shaped structural member the slamming force per unit length may be calcu-lated as follows:

FS = ½ ⋅ ρ ⋅ CS ⋅ D ⋅ v2

where

FS : slamming force per unit length in the direction of the velocity v [kN/m]




ρ : density of water [kNs2/m4]

CS : slamming coefficient

D : member diameter [m]

v : relative velocity normal to member surface [m/s]

For a smooth circular cylinder the slamming coefficient shall be at least CS = 3.0.

B.5.9.1 The slamming pressure space averaged over a broader area (several plate fields) may be obtained as follows:

pS = ½ ⋅ ρ ⋅ Cps ⋅ v2

where

pS : space averaged slamming pressure [kPa]

ρ : density of water [kNs²/m4]

Cps : space averaged slamming pressure coefficient

v : relative velocity normal to member surface [m/s]

For a smooth circular cylinder the space averaged slamming pressure coefficient shall be at least Cps = 3.0. For a flat bottom the slamming pressure coefficient shall be at least Cps = 6.3.

B.5.9.2 Space averaged slamming pressure coefficients for other shapes shall be determined using recognized numerical and/or experimental methods, taking into account three-dimensional effects. Inter-face capturing techniques of the volume-of-fluid (VOF) type, for example, are appropriate numerical methods as they take into account viscosity, flow turbulence, and deformation of the free surface.

B.5.9.3 Fatigue damage caused by wave slamming shall be taken into account. The fatigue damage caused by slamming has to be added to the fatigue contribution from other viable loads.

B.5.10 Impact related pressure from breaking waves

Breaking waves causing impact pressures on vertical surfaces shall be considered. In the absence of more reliable methods, the procedure described in B.5.8 may be used to calculate this impact pressure. The coefficient CS depends on the configuration of the area exposed to this impact related pressure. The impact velocity v shall correspond to the most probable largest wave height.

B.5.11 Air gap

B.5.11.1 Air gap for units

The air gap is defined as the minimum clearance between the underside of the unit in the elevated posi-tion and the calculated crest of the design wave with adequate allowance for safety. It is usually not prac-tical to change the air gap in preparation for a storm and, therefore, the air gap for an intended operation shall be determined based on a suitable return period. Omnidirectional guideline wave heights with a re-turn period of 100 years are recommended to compute wave crest elevations above storm water level, even if a lower return period is used for other purposes.

For jack-up platforms, the air gap shall be 1.20 m or 10 percent of the combined storm tide, astronomical tide, and height of the maximum wave crest above mean low water level, whichever is less. An additional air gap shall be provided for any known or predicted long-term seafloor subsidence.

B.5.11.2 Air gap for installations

The air gap is the clearance between the highest water surface that occurs during the extreme environ-mental conditions and the lowest part not designed to withstand wave impingement. For fixed platforms, the air gap to be maintained shall be at least 1.5 m.

B.6 Vibratory loads due to vortex shedding

Flow past a structural member may cause unsteady flow patterns from vortex shedding, leading to vibra-tions of the member normal or in line to its longitudinal axis. Such vibrations shall be investigated.




At certain critical flow conditions, the vortex shedding frequency may coincide with or be a multiple of the natural frequency of motion of the member, resulting in harmonic or subharmonic resonance vibrations.

The vortex shedding frequency in steady flow with Keulegan-Carpender number KC (see B.5.3.3) greater than 40, such as in a current, may be obtained as follows:

f = Sn ⋅ (U / D)

where

f : vortex shedding frequency [HZ] Sn : Strouhal number

U : local flow velocity normal to the member axis [m/s] D : diameter of the member [m]

At certain critical velocity ranges, the vortex shedding will be such as to lock-in to the natural frequency of the member. This lock-in to the natural frequency can occur for flows in line as well as transverse to the member.

Vortex shedding on structural members may also be generated in waves. Vortex shedding occurs in that part of the wave motion where the acceleration is small. Vortex shedding in waves falls into two catego-ries. For KC > 40, the vortex shedding is of the same kind as in a steady current. For 6 < KC < 40, vortex shedding frequency depends on the type of wave motion. One distinguishes between two limiting cases: • If the wave motion can be considered regular, the vortex shedding frequency is a multiple of the

wave frequency. • If the wave motion is irregular, the vortex shedding frequency is the same as the in a steady current,

and the frequency depends on Sn.

For irregular waves characterized by a narrow band spectrum, the vortex shedding frequencies are a combination of these two limiting cases.

To reduce the severity of flow-induced oscillations caused by vortex shedding, either the member’s shape or its structural properties should be changed. This can be achieved by altering the member’s mass or the member’s damping characteristics. As a result, its natural frequency will be changed so as to lie outside the range for the onset of resonant vortex shedding.

Spoiling devices are often used to suppress vortex shedding lock-on. The underlying principle is to either reduce drag on the member or to cause the shed vortices to become uneven over the length of the mem-ber. Drag can be reduced by adding streamlined fins and splitter plates that break the oscillating flow pattern. Uneven vortex shedding can be induced by making the member irregular, i.e., by adding spoiling devices, such twisted fins, helical strakes, or ropes wrapped around the member. Tests should determine the efficiency of the spoiling device to be used.

B.7 Loads due to climatic conditions

B.7.1 Snow and ice accretion

B.7.1.1 If icing is possible, and ice or snow covers parts of the structure, the weight of ice or snow shall be added to the permanent load under any loading conditions.

B.7.1.2 Snow can settle on both horizontal surfaces and, if the snow is sufficiently wet, on non-horizontal windward parts of installations or units. On horizontal surfaces, dry snow is blown off as soon as any thickness accumulates, while wet snow can remain in position for several hours. Snow covering on surfaces inclined more than 60° to the horizontal can be disregarded. Snow covering on surfaces in-clined between 0 and 60° can be reduced linearly. Snow can freeze and remain as ice only on vertical surfaces.

B.7.1.3 Ice on the topsides of installations or units can accumulate through a number of mechanisms: • freezing of old wet snow • freezing sea spray • freezing fog and super cooled cloud droplets • freezing rain




Where ice covering is caused mainly by sea water spray, it may be taken to decrease linearly to zero from a level corresponding to the highest wave elevation to a level corresponding to 60 m above the high-est wave elevation.

The effects of topside icing on the stability of floating structures and on the operation of emergency equip-ment are particular aspects that shall be considered when designing for operations in cold climates.

B.7.1.4 Loads caused by ice and/or snow on open decks and externally exposed walls are to be specified according to recommendations of competent independent authorities/institutions. In absence of specific information, new snow may be assumed to have a density of 100 kg/m3, and the average density of ice formed on the structure may be taken as having a density of 900 kg/m3.

B.7.2 Temperature influences

Stresses in and deformations of the structure or parts of it, which are induced by extreme temperature gradients in the structure, shall be added to the permanent load induced stresses and deformations under operating conditions, where they are deemed to be relevant.

B.7.3 Marine growth

B.7.3.1 Marine growth may be considerable in some areas and shall be taken into account, e.g., when assessing the wave and current loads acting on submerged parts of the structure. Relevant information shall be submitted to Germanischer Lloyd for verification, see also Section 1, G.3.

B.7.3.2 Thickness of marine growth shall be assessed according to local experience. If no relevant data are available, a thickness of 50 mm may be chosen for normal climatic conditions.

B.8 Loads due to sea ice

B.8.1 Forces exerted on a structure by ice are to be evaluated for their effect on local structural ele-ments and for global effects on the structure as a whole.

B.8.2 Ice loads are to be evaluated for a range of ice-structure interactions. The range of interac-tions is determined by the ice environment of the area of operations, see Section 1, G, and may include the following aspects: • ice pressure loads from continuous first or multi-year level ice • loads from collision with first and/or multi-year ridges within the ice field • impact loads caused by drifting ice floes (sea ice or glacial ice) • impact loads caused by icebergs

B.8.3 Ice load evaluations are to include the forces exerted by ice on rubble ice or other ice pieces that are in firm contact with or held by the structure. This is of particular concern for multi-legged struc-tures and for structures designed to cause ice failure in modes other than crushing.

B.8.4 The brittle nature of ice can lead to periodic dynamic loading, even during ice-structure inter-actions where this is not initially apparent. Dynamic amplification of the structure’s response in its natural vibratory modes shall be evaluated.

B.8.5 When calculating ice loads, the following parameters specifying the condition of the ice have to be considered: • thickness of a level ice sheet • temperature or temperature gradient in the ice • orientation of the ice crystals • bulk salinity • total porosity of the ice (brine volume, gas pockets, voids) • density • ice strength




• loading rate • scale effects (size of structure/ice thickness)

B.8.6 Any attempt to determine ice loads on offshore structures shall be based on probabilistic methods. Ice parameters and corresponding forces shall be analyzed over the entire range of possible parameter values by using simulation techniques, such as the Monte Carlo simulation.

B.8.7 Exclusively using deterministic methods for ice loads, only considering a maximum ice thick-ness, introduces uncertainties of the following kind: • randomness of every natural process • incomplete ability to describe natural processes in engineering models • human participation in engineering processes

In case of ice-structure interaction, the most significant uncertainties arise from the available models. It should always be considered that none of the publicly available methods delivers reliable results.

B.8.8 Model tests are recommended for evaluation of global loads and confirmation of expected ice failure modes.

B.8.9 Further details are described in GL Guideline for the Construction of Fixed Offshore Installa-tions in Ice Infested Waters (IV-6-7).

C Permanent Loads

C.1 Permanent loads are loads acting throughout the lifetime of the unit/installation or during pro-longed operating periods. Such loads may comprise the weight of structures (dead load), equipment, permanent ballast, and effects of hydrostatic pressure exerted on parts of the submerged structure (buoy-ancy).

C.2 Permanent Loads shall be clearly stated and accounted for in the design documents and cal-culations.

C.3 In cases where loads/weights may be acting over longer periods, but not necessarily at all times (e.g., when using certain kinds of equipment), these cases may have to be investigated to account for the most unfavorable condition.

D Functional Loads

D.1 Functional loads are variable loads caused by normal operations in a variable manner as fol-lows: • weight of tools and mobile equipment • stores, frequently varying ballast, fuel, wastes • loads from operations of cranes and other conveyance equipment • loads from transport operations, e.g., helicopters • mooring/fendering loads from vessels serving the unit/installation • operational boat impact on the boat landing structure

D.2 Loads on windlasses, etc., exerted by mooring lines on mobile units, shall generally be re-garded as functional loads, although they are mainly due to environmental influence. Regarding permissi-ble stresses, these mooring loads shall be attributed to loading conditions 2 or 3 as applicable, see Section 3, C and GL Rules for Mobile Offshore Units (IV-6-2), Section 8.




D.3 Deck loads, weight of equipment, etc. shall be specified by the Owner/Designer. The specifi-cation shall also contain indications of permissible load combinations and limitations regarding the overall weight of the structure or installation.

D.4 If not otherwise specified, the following minimum loading on deck surfaces is to be taken: • crew spaces, walkways, etc.: 4.5 kN/m2 • access platforms: 3 kN/m2 • working areas: 9 kN/m2 • storage areas: 13 kN/m2 • helicopter platform: 2 kN/m2

For railings and foot rails, a horizontal load in form of a moving single load of 0.3 kN/m has to be as-sumed.

The above loads on walkways, stairs, access platforms, and similar areas shall be fully considered for the local design; but for the overall design of the installation/unit, a minimum of 50 % of the live loads on these areas shall be considered.

D.5 Layout plan for loads

A layout plot plan, including weights, shall be prepared for each design. This plan is to show the perma-nent design loads, see C, and functional maximum design loads (uniform loads as well as concentrated loads) for all areas and for each mode of operation. Compare also GL Rules for Certification and Surveys (IV-7-1), Section 3, B.2.6.1.

E Accidental Loads

E.1 General

E.1.1 Accidental loads are loads not normally occurring during the installation and operating phases, but which shall be taken into account, depending on location, type of operations, and possible conse-quences of failure. Causes for accidental loads within this context may be the following: • collisions (other than normal mooring impacts referred to under D.1) • falling or dropped objects • failing or inadequate crane operations • explosions, fire • strength level earthquakes according to Section 1, I.3

E.1.2 The installation/unit shall be so designed and, if necessary, protected that the consequences of damage are acceptable and that an adequate margin of safety against collapse is maintained.

E.2 Collision of vessels with offshore installations

E.2.1 Collision zone

Accidental damage shall be considered for all exposed elements of an installation in the collision zone. The vertical extent of the collision zone shall be assessed on the basis of visiting vessel draft, maximum operational wave height, and tidal elevation.

E.2.2 Accidental impact energy

E.2.2.1 Total kinetic energy

The total kinetic energy involved in accidental collisions can be expressed as follows:

E = ½ ⋅ a ⋅ Δ ⋅ v2




where

E : total kinetic energy [kJ]

Δ : displacement of the vessel [t]

a : vessel added mass coefficient

: 1.4 for sideways collision

: 1.1 for bow or stern collision

v : impact speed [m/s]

The minimum value for E shall be as follows: • 14 MJ for sideways collisions and • 11 MJ for bow or stern collisions

These minimum values correspond to a vessel of 5000 t displacement with an impact speed of 2 m/s. A reduced impact energy may be acceptable if the size of the visiting vessels and/or their operations near the offshore installation are restricted. In this instance, a reduced vessel size and/or a reduced impact speed may be considered.

E.2.2.2 Impact speed

The reduced impact speed may be estimated from the following empirical relationship:

v = ½ ⋅ HS

where


HS : significant wave height [m]

Here the significant wave height HS is to be taken equal to the maximum permissible significant wave height for vessel operations near to the installation.

E.2.2.3 Energy absorbed by the structure

The energy absorbed by the installation during the collision impact will be less than or equal to the impact kinetic energy, depending on the relative stiffness of the relevant parts of the installation and the impact-ing vessel that the installation comes in contact with, the mode shape of the collision, and the vessel’s operation. These factors may be taken into account when considering the energy absorbed by the instal-lation.

For a fixed steel jacket structure, it is recommended that the energy absorbed by the installation should not be taken less than 4 MJ unless a study of the collision hazards and consequences specific to the installation demonstrates that a lower value is appropriate. Examples of such structures are small, occa-sionally manned and infrequently supplied installations, or structures where there is a restriction on the size of the visiting vessel.

For vessels of less than 5000 t displacement, the minimum energy to be absorbed by the installation may be reduced to a value given by the following relationship:

Ea = 0.5 + Δ2 (4.2 × 10-7 – 5.6 x 10-11 Δ)

Ea : energy absorbed by the installation [MJ]


The energy absorbed by the installation may be taken less than this value if the stiffness of the impacted part of the installation is large compared to the stiffness of the impacting part of the vessel, as for exam-ple in collisions involving concrete installations or fully grouted installation elements. In such cases the effects of impact loading shall be considered in detail as in the following.

E.2.3 Accidental impact loading

In cases where the stiffness of the impacted part of the installation is large compared to stiffness of the impacting part of the vessel, as for example in collisions involving concrete installations or fully grouted




elements, the impact energy absorbed locally by the installation may be low. In such cases, it is important to examine damage caused by the impact force.

The impact force may be calculated as follows:

F = P0

or

F v c a= ⋅ ⋅ ⋅Δ

whichever is the less, where

F : impact force [kN]

P0 : minimum crushing strength or punching shear, as appropriate, of the impacting part of the vessel and the impacted part of the installation [kN]


c : total spring stiffness of the vessel and installation related to the impact point [kN/m]

a : vessel added mass coefficient, see E.2.2.1


E.3 The regulations of competent Authorities and/or Administrations have to be observed, particu-larly regarding collisions.

E.4 According to Section 3, C.2.2.1, loading condition 4 – Accidental loads, the choice of accident cases and the determination of loads will be considered on a case to case basis.

E.5 Consequences of damage

The primary structure shall be designed to ensure that accidental damage does not cause complete col-lapse. Damaged members shall be considered to be totally ineffective unless it can be shown by analysis or tests that they retain residual load-carrying capacity.

F Transportation and Installation Loads

F.1 General

Marine operations for transportation and installation are routine operations of limited duration. Such op-erations depend on the type of the offshore installation and shall include all the transient movements and other activities where the structure or operation is at risk in the marine environment. Samples of such operations are the following: • load-out from shore to barge or vessel • float-out from drydock • construction and outfitting afloat • wet or dry towage and other marine transportation • installation • jacket launching • float-over of topsides • piling, grouting, connection to permanent moorings • modifications to an existing structure • total or partial decommissioning and removal of the structure




F.2 Loads

The loads for these phases have to be defined on a case by case basis according to the type of installa-tion and the situation on site. In determining these loads in an actual case, the following aspects shall be considered: • The required equipment, vessels, and other means involved are to be designed and checked for

adequate performance with respect to their intended use. • Redundancy in the equipment shall cover possible breakdown situations. • Operating weather conditions, chosen at values smaller than the specified design criteria, are to be

forecasted for a period long enough to complete the required marine operations. • The marine operations are planned, in nature and in duration, such that accidental situations, break-

downs, or delays have a low probability of occurrence and are covered by detailed contingency ac-tions.

• Adequate documentation has been prepared for a safe step-by-step execution of the operation, with clear indications of the organization and adopted chain of command.

• Marine operations are to be conducted by suitably experienced and qualified personnel.

G Earthquake Loads

G.1 Seismic actions

The characteristics of earthquake levels (acceleration intensity, duration, etc.) shall be established by a competent Authority and agreed upon with Germanischer Lloyd.

G.2 Seismic analysis

G.2.1 General

Two levels of seismic loading on a structure shall be considered in accordance with G.1:

G.2.1.1 Strength level earthquake

Strength level earthquakes (SLE) shall be assessed as an ultimate strength condition (ULS). In zones with low to moderate seismic activity, the action effects obtained from an analysis in which the structure is modeled as linear and elastic will normally be such that the structural design can be performed based on conventional linear elastic strength analyses, employing normal design and detailing rules for the rein-forcement design.

G.2.1.2 Ductility level earthquake

Provided the structure survives, it is permissible to assess ductility level earthquakes (DLE) with ductile behavior of the structure, assuming extensive plastic deformation occurs.

If, under the DLE event, ductile response of specific components of the structure is predicted or considered in the analysis, such components shall be carefully detailed to ensure appropriate ductility characteristics and structural constraints. Expected best estimate of stress/strain parameters associated with ductile be-havior may be adopted in the analysis. Due consideration shall be given to the effects of excessive strength with respect to the transfer of forces acting at adjoining members as well as to the design of those failure modes of such members that are not ductile, such as shear failure. For those cases where the structure can be designed to the DLE event by applying normal ULS criteria, no special detailing for ductility is required.

G.2.1.3 Seismic response

Seismic events may be represented by input response spectra or by time histories of significant ground motion. Where the global response of the structure is essentially linear, a dynamic spectral analysis shall normally be appropriate. Where nonlinear response of the structure is significant, a transient dynamic analysis shall be performed.




Section 3 Principles for Structural Design A General ....................................................................................................................... 3-1 B Design Methods and Criteria ...................................................................................... 3-2 C Loading Conditions ..................................................................................................... 3-5 D Dynamic Analysis...................................................................................................... 3-11 E Effective Width of Plating .......................................................................................... 3-13 F Beam Member Design .............................................................................................. 3-14 G Tubular Joint Design ................................................................................................. 3-15 H Buckling of Plates ..................................................................................................... 3-18 I Fatigue Strength ....................................................................................................... 3-18 J Application of BSH Requirements ............................................................................ 3-24

A General

A.1 Scope

This Section defines the principles for structural design of fixed steel offshore installations. Subsections A - C are applicable to structures in general. Subsections D - I refer to steel structures. If structures made of aluminium alloys shall be provided, the design should follow the GL Rules for Hull Structures (I-1-1) or Eurocode. Fixed Concrete Structures shall be designed according to ISO 19903. The design principles for mobile offshore units are defined in GL Rules for Mobile Offshore Units (IV-6-2)

The following rules are based on and mostly refer to:

API Recommended Practice 2A-WSD (RP 2A-WSD) for jacket and topside

ISO 19902:2007 (same as DIN EN ISO 19902:2008-07) for jacket

ISO 19901-3:2010 (same as DIN EN ISO 19903:2010) for topside

The requirements in the following paragraphs B – I of this Section are to be read together with the cited ISO standards for analysis performed based on the Load and Resistance Factor Design method (LRFD) and together with the cited API standard for analysis performed based on the Allowable Stress Design method (ASD)

J provides recommendations on how to use Eurocode 3 to fulfil BSH regulations for offshore installations in the German Exclusive Economic Zone.

Eurocode 3 consists of the following documents:

DIN EN 1990:2002 + A1:2005 + A12005/AC:2010 (including German National Annex)

DIN EN 1993-1-1:2005 + AC:2009 (including German National Annex)

DIN EN 1993-1-8:2005 + AC:2009 (including German National Annex)

A.2 General design considerations

A.2.1 Structural integrity throughout the lifetime of an offshore installation depends on the thorough-ness of design investigations, on the quality of materials and manufacture, and on the in-service inspec-tion and maintenance. This Section deals with the required level of design investigations.

A.2.2 Serviceability and functional considerations may have influence on the structural design. They will generally be specified by the Owner/Operator of the installation, but will be taken into account in the design review by GL where relevant.


Section 3 Principles for Structural Design


A.2.3 Loads and loading conditions should generally be treated in accordance with Sections 1 and 2. More specific or especially adapted load criteria/ values shall be well documented and agreed upon. The load cases and combinations to be considered in the design calculations shall cover the most unfa-vourable conditions likely to occur.

A.2.4 The dynamic nature of some loads acting on the structure has to be taken into account by suitable calculation methods where these loads are considered important, e.g. because they produce considerable accelerations of (parts of) the structure. In some cases dynamic forces can be accounted for quasi-statically, i.e. using adequate increments on static forces acting simultaneously, see D.

A.2.5 Strength/stress analysis may be carried through according to different, recognized methods and standards, see B, and GL Rules for Certification and Surveys (IV-7-1), Section 1, E. However, it shall be ascertained in every case that the design fundamentals and standards used are consistent and com-patible for one structure or installation, regarding the considerations stated in A.2.1.

A.2.6 Structural redundancy shall be considered at an early stage. As redundancy is not explicitly taken account of in the all design methods currently in use, with the exception of the categories grouping of structural elements (special, primary, secondary - see Section 4, A), the consequence of a failure of individual structural elements will have to be specially considered. Aspects of reparability, especially of in-water structures should be taken into account.

A.3 Avoidance of wave impact

Where structures are not or cannot be suitably designed to resist wave impact, the latter shall be avoided by providing sufficient distance between the probable highest wave elevation and the lower edge of the structure (e.g. lowest closed deck). A clearance - "air gap" - of 1.5 m is recommended. In case of struc-tures not connected rigidly to the sea floor, their vertical motions shall be accounted for. For mobile units, see also GL Rules for Mobile Offshore Units (IV-6-2), Section 2 and 3.

A.4 Corrosion and wear allowances

Determination of scantlings, using the following indications or equivalent design methods, is based on the assumption that an accepted corrosion protection system is provided for, see Section 5. Otherwise ade-quate corrosion allowances are to be agreed upon, depending on the environmental and operating condi-tions as well as the design lifetime. The same applies to structural elements prone to wear, e.g. by chaf-ing of cables or chains.

A.5 Loading conditions

Loading conditions occurring during construction, transport or sea installation may have to be considered in the design, see also C and Section 9.

B Design Methods and Criteria

B.1 Choice of design concept/methods

B.1.1 Structural strength calculations is generally to be based on linear elastic theory. However, non-linear relations between loads and load effects shall be properly accounted for, where they are found to be important

B.1.2 ASD ("allowable stress design")

Where deterministic methods incorporating global safety factors are used, they shall comply with the re-quirements given in this section. This method is also known as WSD (“working stress design”)

B.1.3 LRFD ("load and resistance factor design")

A semi-probabilistic design method in association with limit state and partial safety factor design, see B.2 and B.3, may be applied alternatively.




B.2 Design criteria, limit states

B.2.1 A structure may become unsafe or unfit for use by damage or other changes of state accord-ing to different criteria. They may be defined by "Limit States", i.e. states of loading, straining (deforma-tion) or other impairment, at which a structure, or structural component, looses its planned operability or function. The limit states are usually classified as follows.

B.2.2 Ultimate limit states (ULS)

Ultimate limit states are related to the maximum load carrying capacity of a structure or structural compo-nent, which may be reached by:

• loss of equilibrium (e.g. toppling) of the structure, considered as a rigid body • yielding and/or fracture of material, see F. and G • local and global buckling (structural instability) • fatigue (formation of cracks after repeated loading), see I

B.2.3 Serviceability limit states (SLS)

Serviceability limit states refer to the impairment of normal use or operation by changes or effects other than structural failure, such as

• excessive deformation or displacement • unacceptable vibrations • leakage (loss of tightness)

B.2.4 Accidental limit state (ALS)

The limit state of progressive collapse is related to a collapse of a structural system after accidental dam-age such as:

• dropped objects • collisions • explosions

This limit state may also be considered for the evaluation of the "strength level earthquake (SLE)", see Section 1, I.3.

B.2.5 Fatigue limit state (FLS)

Fatigue limit states address structural failure due to permanent cyclic loading, see I.

B.2.6 Corrosion allowance

Corrosion allowance should be defined by the designer/operator of the platform and agreed upon with GL and should considered in member design (reduced wall thickness and diameter).

B.3 Definition of safety format

B.3.1 In connection with deterministic as well as semi-probabilistic design methods the safety condi-tion is based on statistically representative "characteristic values" of the loading effects and of the resis-tance of the structure, as well as on safety factors as defined below.

B.3.2 In deterministic design (ASD) the safety condition may be expressed as follows:

γg ⋅ SC / RC ≤ 1

SC : characteristic loading effect (e.g. a stress) resulting from loads/load combinations as defined in these Rules, see C and Section 2.

RC : characteristic resistance (for instance, the yield strength, the ultimate strength or the critical buckling force or stress)

γg : global safety factor (taking account of all uncertainties in loads and resistance estimation)




: numerical values are given in F and G

B.3.3 In semi-probabilistic design methods (LRFD) the safety condition may be expressed as fol-lows:

Sd / Rd ≤ 1

Sd : S(F)

: function, e.g. stress, of the total design loading effect

F : ∑ (Fi ⋅ γfi)

: total design loading effect consisting of various characteristic loads Fi, multiplied by individu-ally defined load factors γfi which take account of the uncertainties of the loads and the prob-ability of their simultaneous occurrence

Rd : RC / γm

RC : characteristic resistance, see B.3.2

γm : partial safety factor for materials (covering the possible deviations of material properties for characteristic, statistically determined values)

B.4 Modelling of the structure

B.4.1 The calculation model ("idealization") used has to take account of all main load bearing and stiffening components, and of the relevant supporting and constraining effects.

The degree of subdivision (detailing) should take account of the geometry of the structure and its influ-ence on the load distribution and introduction, of the distribution of external loads, and of the expected stress pattern.

Elements or members considered as being of "secondary" importance may nevertheless have to be ac-counted for where they have an influence on stress distribution or dynamical properties of the structure.

Appurtenances need to be taken into account if they contribute to the overall weight or attract aerody-namic or hydrodynamic forces as boatlandings, conductors or risers do.

B.4.2 Where modelling is made by means of beam elements, the actual rigidities are to be ac-counted for as precisely as practicable, particularly in way of connections (joints). The effective width of associated plating may be chosen according to accepted standards, see also E.

The nodes of the model shall be the intersection points (working points) of the centre lines of the braces with the centre line of the chord. The offset is the distance between brace working points. For global analyses, only offsets larger than chord diameter D/4 shall be included in the model.

Depending on the diameter of the chord, the length between the physical end of the brace stub and the centre line of the chord can be significant. In such cases the length of braces between the outer surface of the chord and its centre line should be modelled as rigid connections with a stiffness at least one order of magnitude (10 times) greater than that of the brace. Rigid links shall not contribute to the mass and shall not attract direct hydrodynamic or aerodynamic forces.




Fig. 3.1 Definition of member rigid links

B.4.3 Stress concentration effects shall be carefully investigated, either by using adequate calcula-tion methods (e.g. finite elements), or by applying stress concentration factors where proven and applica-ble design data exist.

B.4.4 The soil stiffness is to be represented as precisely as practicable when designing structures resting on or penetrating into the sea floor, see also Section 6.

B.4.5 For calculations analysing the dynamic behaviour of the whole structure (global behaviour), the mass distribution may generally be simulated in a simplified form, using equivalent or lumped masses under consideration of its center of gravity and moments of inertia. Details may have to be agreed upon, see also D.

B.5 Model tests

Model investigations (tests) may be accepted as a supplement to structural analysis provided that a rec-ognized institution is involved and the particulars have been agreed. In special cases model tests may be required.

B.6 Superposition of load effect components

Stresses or other load effects resulting from relevant global and local effects shall be added according to their degree of simultaneousness, see also C and Section 2, A.3 Stresses of different type (e.g. tension, shear) and different direction shall be combined according to recognized criteria (equivalent stress, v. Mises).

C Loading Conditions

C.1 General

C.1.1 Global or partial safety factors as defined in B and F to G. are related to specified loading con-ditions and load combinations, see Table 3.1. For kinds and definition of loads see Section 2.

C.1.2 In case of predominant compression stresses, the possibility of buckling is to be investigated, see F. and H

C.1.3 Where calculations are carried through, e.g. using finite element methods, in order to deter-mine stress concentrations, see B.4, values up to the yield strength may be admitted for loading condition 3, provided that the material employed at such points corresponds to category "special steel member" (see Section 4) and that fatigue considerations (see I) are explicitly proven or not essential.




C.1.4 In case of loading repeated with great frequency (fatigue) it must be ensured that no damage ratio exceeds 1.0, see I.

C.1.5 A reduction of admissible stresses may be necessary (ASD only) • in case of unfavourable inspection and testing conditions • in case of not sufficiently well known loads • where the consequences of a possible failure are deemed to be extremely negative

(lack of redundancy, A.2.6)

For LRFD checks instead of reducing the design load the relevant partial load factor will be increased.

C.1.6 A higher stress level may be admitted in exceptional cases where the calculations have a high degree of accuracy and the load prediction is extremely conservative.

C.1.7 The design check for elements of special purposes such as cranes or drilling rigs may be taken according to acknowledged regulations relating to such equipment (GL Rules for Loading Gear on Seagoing Ships and Offshore Installations (VI-2-2)). The applicability of relevant loading conditions, in comparison with those defined in this subsection, shall be carefully checked.

C.1.8 Limiting operating conditions

The limiting operating conditions, i.e. environmental conditions tolerable during specified operations, and the kind of (limited) operations to continue during extreme environmental conditions, will generally be defined by the Owner or Operator, checked by GL within the design review, and established in the operat-ing instructions.

C.2 2 ASD - Definition of loading conditions

C.2.1 Six loading conditions as summarized in Table 3.1 and defined in the following are generally to be considered:

C.2.1.1 Loading condition 1: Permanent loads

This (still water) loading condition covers all gravity, buoyancy and hydrostatic loads which would nor-mally act continuously, and shall also take care of variable gravity loads which may be assumed to be in existence during a prolonged period of time, such as ballast and weight of equipment.

For fixed structures this loading condition is generally not relevant and needs to be investigated only in combination with variable design loads.




Table 3.1 Loading conditions, load combinations

Type of load

Loading condition

Dead-load/

Equip-ment

Variable / funct. Loads

Buo-yancy

Envi-ron-

mental loads

(red.) 5

Envi-ron-

mental loads(extr.)

Impact / colli-sion

Strength level earth-quake

Ductil-ity

level earth-quake

Dyn. Load

(see D)

1. Perma-nent loads × × -- 1 -- -- -- -- -- --

2. Operat-ing loads × × × × × -- × 6 -- -- ×

3. Extreme environ-mental loads

× × 2 × -- × -- -- -- ×

4. Acci-dental loads

× × ×8 × -- -- × 3 × 3 -- --

5. Marine Operations × × 1 1 4 -- -- -- -- 7

6. Ductility level earth-quake

× × × × -- -- -- -- × ×

1 if applicable 2 functional loads may be reduced 3 alternatively impact collision or strength level earthquake 4 maximum permissible environmental conditions to be defined 5 recurrence period to be defined individually, typically 1 year 6 impact during normal operations, e.g. mooring of attending vessels 7 dynamic amplification for lifting see Section 9, C.2.2 8 in case of earthquake analysis 75 % of supply and storage loads to be considered

C.2.1.2 Loading condition 2: Operating loads

This condition includes the loads as specified in C.2.1.1, plus other variable functional loads such as from crane or drilling operations, supply vessels operating around the installation and those environmental loads which, according to C.1.8, are to be taken into account during normal, unrestricted use of the instal-lation. If not specified individually, a 1 year recurrence period is considered to be acceptable for the envi-ronmental loads.

C.2.1.3 Loading condition 3: Extreme environmental loads

C.2.2 This loading condition includes the loads as specified in C.2.1.1, the most unfavourable (100 years recurrence period) environmental loads and those functional loads which, according to C.1.8, are assumed to be also acting in such environment. According to the principle mentioned in Section 1, A.3, several possible load combinations or cases may have to be considered.

C.2.2.1 Loading condition 4: Accidental loads

This loading condition is intended to cover unusual loads and damage resulting from accidents, such as collisions or fire, in addition to permanent loads, to be defined in the individual case. The kinds of acci-dents and extent of damages will also have to be agreed upon, possibly together with the competent Ad-ministration(s), depending on local conditions. The general philosophy is that immediate total failure of the structure is prevented so that emergency and rescue operations are possible.

An acceptable supposition as to the assessment of associated environmental loads prevailing after the accident is a recurrence period of one year.




In earthquake regions, a "strength level earthquake" (see Section 1, I.3) shall be considered under this loading condition as an alternative to an impact or collision. The associated environmental and functional loads shall be agreed upon in the individual case.

C.2.2.2 Loading condition 5: Marine operations

This loading condition is only temporary and covers operations associated with moving or transporting an offshore structure or part thereof during the construction, installation or abandonment process, compare Section 9.

The situation during transport and installation or de-installation has to be investigated individually case by case.

C.2.2.3 Loading condition 6: Ductility level earthquake

This loading condition covers an extreme level of earthquake, defined in Section 1, I.3 The earthquake forms the excitation for the dynamic loads to be investigated according to D. This loading condition re-quires suitable (nonlinear) analysis methods. See ISO 19902 “Recommendations for ductile design”.

C.2.3 2.3 Code Checks

The code checks for the jacket are to be carried out according to formula stipulated in API RP 2A-WSD, latest edition. For the topside structure API RP 2A is also applicable in conjunction with ANSI/AISC 360-10 (“Specification for Structural Steel Buildings”)

C.3 LRFD - Definition of loading conditions

Several load cases are multiplied by an individual partial load factor and then added up to yield load com-binations that are to be investigated. The load combinations and the factors are given in Table 3.2.

Table 3.2 Load Factors for LRFD Design

loading condi-tion

γf,G1 γf,G2 γf,Q1 γf,Q2 γf,Eo γf,Ee γac γf,GT γf,QT γf,T γeq

permanent and variable Loads

1.3 1.3 1.5 1.5

operating loads 1.3 1.3 1.5 1.5 0.9 γfE

extreme environmental additive loads

1.1 1.1 1.1 γfE

extreme envi-ronmental opposing loads

0.9 0.9 0.8 γfE

accidental 1.0 1.0 1.0 1.0 1.0

marine opera-tions (*1) 1.3 1.3 1.0

marine opera-tions (*2) 1.1 1.35

marine opera-tions (*3) 0.9 0.9 1.35

earthquake adding to gravity

1.1 1.1 0.9

earthquake opposing gravity

0.9 0.9 0.8 0.9




C.3.1 Permanent and variable loads

Four kinds of permanent loads are distinguished depending on the time the structure is exposed to them. If any of them are movable, their position causing the most severe effect has to be chosen.

The loads are comprised of: (ISO 19902)

C.3.1.1 Permanent loads G1

These are the loads that never change over time, e.g. structural weight, ballast or weight of equipment that will never be moved during the operation time. Hydrostatic loads (buoyancy and water enclosed in the structure) are also part of G1.

C.3.1.2 Permanent loads G2

Loads that remain constant over a long period and vary only with the mode of operation, e.g. if the plat-form is refitted (end of commissioning phase with additional living quarters, begin of production)

C.3.1.3 Variable Loads Q1

Q1 includes varying loads that are moved frequently over free spaces on the platform like personal ef-fects, containers, etc., but also ice loads and fluids in pipes and tanks.

C.3.1.4 Variable Loads Q2

Q2 covers all loads that happen during operation over a short period, like crane lifts, boat impacts or heli-copter landings.

C.3.2 Environmental Loads due to wind and waves

Wind and wave forces acting on all sides of a structure have to be considered. If not specified otherwise a recurrence period of

• 1 year is acceptable for operating conditions Eo • 100 years is acceptable for extreme conditions Ee

Spacing of wave directions to be considered must not be larger than 45°. For fixed platforms a more ac-curate distribution of wave parameters (height, period, etc.) depending on the different directions may be used as site specific topographic features might be of influence.

Calculation of wave forces under consideration of marine growth, current blockage, conductor shielding, etc. is shown in Section 2.

Forces due to wind, wave and current have to be combined in a way that maximal loading is caused. Two separate cases, maximum shear force and maximum overturning moment, shall be investigated. Both depend on water depth (highest or lowest tide) and wave crest position.

For long and slender structural appurtenances (e.g. risers or flare booms) dynamic effects due to vortex shedding need to be considered.

Equivalent quasi-static loads D from dynamic response are added to the static loads S

fE fDE (S D)= γ ⋅ + γ

γfE and γfD are partial factors for the environmental loads that have to be agreed between GL and the owner. Both are discussed in ISO 19902 annex 9.9.

The value of γfE depends principally on:

• the target safety level represented by the exposure levels L1, L2 and L3, see 6.6, • the characteristics of the long-term environment at the offshore location of the structure, specifically

the local climate of waves, current and wind at the geographical location (the wave climate usually being the most influential, in particular its sensitivity to return period)

• the geometrical and structural properties of the structure considered.

Additionally, the value will generally be affected by the method(s) used to derive the extreme wind, wave and current parameters and the corresponding extreme environmental action.




The partial load factor for environmental actions γfE shall be 1.35, unless rational analysis demonstrates an alternative value is appropriate.

C.3.3 Accidental loads / Temporary loading conditions, ac

This loading condition is intended to cover unusual loads and damage resulting from accidents, such as • collisions • fire, or explosion • dropped objects • abnormal environmental conditions (“10000 years wave”, abnormal level earthquake)

The kinds of accidents and extent of damages will have to be agreed upon, possibly together with the competent Administration(s), depending on local conditions. The general philosophy is that immediate total failure of the structure is prevented so that emergency and rescue operations are possible.

All partial load factors are set to 1.0 when establishing accidental load combinations.

C.3.4 Marine operations, GT, QT, T

These loads comprise all loadings during • loadout • transportation • launching and uprighting • relocation after a certain service time • final removal

Marine operations (*1) governs for components in which permanent and variable load effects are domi-nant. Marine operations (*2) governs for components in which transient loads are dominant and in which the permanent and variable loads increase the magnitudes of the internal forces. (*3) governs for compo-nents in which transient load effects are dominant but in which the permanent and variable actions de-crease the magnitudes of the internal forces.

d f ,GT T f ,GT T f ,TF G Q T= γ + γ + γ

Where:

GT is the load imposed either by the weight of the structure in air, or by the submerged weight of the structure in water, during the transient situation being considered, including any permanent equipment or other objects and any piles or conductors installed on the structure, as well as any ballast installed in or carried by the structure.

QT is the load imposed by the weight of the temporary equipment or other objects, including any rigging installed or carried by the structure, during the transient situation being considered.

T represents the loads from the transient situation being considered, including: • when appropriate, environmental loads, • when appropriate, a suitable representation of dynamic effects • for lifting, the effects of fabrication tolerances and variances in sling length • for loadout, allowances for misalignment • for transportation, any hydrostatic and hydrodynamic loads on the structure, including any inertial

loads resulting from accelerations of the structure • for installation, the lifting loads and hydrostatic pressure on the structure

For more information refer to ISO 19902 Chapter 8, for lifting operations see also Section 9.

During launching and upending every relevant time step of the procedure has to be calculated individu-ally.

C.3.5 Seismic Loads

Seismic loads are determined by a dynamic analysis (see D).




Earthquake analyses are outlined in Subsection D.6.

C.3.6 Code Checks

The code checks for the jacket are to be carried out according to formula stipulated in ISO 19902, latest edition.

In principle the design of the topside has to be checked against the standard ISO 19901-3, latest edition. This standard does not define explicitly code checks but references instead to any national or interna-tional civil or building code. However, a so called correspondence factor Kc has to be applied to account for any differences between the ISO 19902 standard and the applied civil or building standard. This factor is to be multiplied on the design value of resistance calculated according to the applied building and civil code to obtain the final design value of resistance according to ISO 19901-3. The design value of the actions is to be taken from the ISO 19900 series and is not to be modified.

Please refer to ISO 19901-3 for details and requirements on how to apply and calculate the correspon-dence factor Kc.

D Dynamic Analysis

D.1 General

An investigation of the dynamic behaviour of the structure is required in case of risk of resonance of global or local structural vibration modes with energyrich dynamic loads (periodical excitation forces).

D.2 Sources of excitation

Dynamic loads may be imposed by • environmental loads such as waves, wind, currents, earthquake • by variable functional loads from drilling, process or other rotating equipment • from the propulsion system in case of mobile units

D.3 Global design

D.3.1 Fixed structures

For fixed structures a dynamic analysis will be necessary in case of a platform natural frequency close to the frequency of energy-rich design sea states.

The global dynamic behaviour will generally have to be accounted for in case of eigenperiods larger than 3 seconds.

D.3.2 Special structures

Dynamic analyses will generally be necessary for structures with large motions, e.g. articulated towers or guyed ("compliant") structures.

Also for structures installed in seismically active areas a dynamic investigation is required to know the inertia induced forces.

D.4 Local design

D.4.1 Larger structural components

The design of larger structural components, e.g. flareboom, helicopter deck, and of local structural mem-bers shall take account of possible dynamic loads, e.g. due to wind, wave, current, ice-flow or wave slamming.




D.4.2 Slender structures

Due regard shall be given to dynamic loads on slender structures such as radio towers and flare booms. Wind induced vortex shedding and also variable wind speeds (gusts) may cause dynamic stress amplifi-cations and reduce the fatigue life.

D.4.3 Rotating equipment

For structural components supporting rotating equipment with free excitation forces and moments reso-nance shall be avoided. If this is not possible, the dynamic stress level shall be proven in a forced vibra-tion investigation.

D.4.4 Earthquake loading

For structures which are subject to earthquake loading the eigen-frequencies of appurtenances or equip-ment supports should be substantially larger than the frequencies of the basic structure modes, see also D.6

D.5 Structural analysis

D.5.1 Structural modelling

The structural modeling shall take account of the requirements outlined in B.4

D.5.2 Structure/foundation interaction

Special regard shall be given to the structure / foundation interaction. Compared to dynamic fatigue inves-tigation for extreme environmental response a possible reduction of the foundation stiffness has to be considered. When an equivalent linear foundation model is used, the stiffness of the pile foundation should be compatible with the expected level of non-linearity in the axial and lateral foundation response. See ISO 19901-4 for guidance.

D.5.3 Hydrodynamic masses

Besides all effective structural masses, the hydrodynamic masses, accounting for increased member thickness due to marine growth, and water enclosed in submerged members are to be considered.

D.5.4 Viscous damping

Equivalent viscous damping may be used for dynamic investigations of fixed steel structures. As an esti-mate two to three percent of critical damping may be applicable.

D.5.5 Method of analysis

D.5.5.1 The analysis method to be used will depend on the type of loading and the structural re-sponse.

D.5.5.2 Response spectral analysis will be used for structures with a linear elastic response to random loading, e.g. due to non-deterministic wave loads, provided a linearization of the non-linear load effects is possible. The method may be appropriate to determine dynamic amplification effects due to extreme wave loads on fixed jacket structures and also to perform fatigue damage accumulation calculations.

D.5.5.3 Time history analysis, e.g. by direct integration, will have to be used for dynamic problems with non-linear nature.




D.6 Earthquake analysis

D.6.1 Strength and stiffness of structure

For seismically active areas the strength and stiffness of offshore structures shall be adequate to ensure that no significant damage will occur due to the expected design earthquake. 1

D.6.2 Resistance against earthquake loading

An earthquake analysis is not required for areas where the design horizontal ground acceleration (strength level) is less than 0.05 · g, provided other significant environmental loads are to be accounted for, so that sufficient resistance against earthquake loading is ensured.

D.6.3 Dynamic response

The analysis of the dynamic response should be performed using recognized procedures such as • response spectrum analysis • time history analysis, see D.5.5

Generally a three-dimensional structure model shall be used for the analysis.

D.6.4 Modal maxima

When the response spectrum analysis is applied for the combination of the modal maxima, the use of the "Complete Quadratic combination" (CQC) method is recommended.

D.6.5 Earthquake induced response

The earthquake induced structural response has to be combined with other load components acting per-manently on the structure, see C.

E Effective Width of Plating

E.1 Frames and stiffeners

Generally, the spacing of frames and stiffeners may be taken as effective width of plating.

E.2 Girders

E.2.1 The effective width of plating em of frames and girders may be determined according to Table 3.3 considering the type of loading.

Special calculations may be required for determining the effective width of one-sided or non-symmetrical flanges.

E.2.2 The effective cross sectional area of plates is not to be less than the cross sectional area of the face plate.

Cantilevers

Where cantilevers are fitted at a greater spacing the effective width of plating at the respective cross sec-tion may approximately be taken as the distance of the cross section from the point on which the load is acting, however, not greater than the spacing of the cantilevers.

––––––––––––––

1 As a guideline for the design of a platform for earthquake ground motion the use of American Petroleum Institute API RP2A

“Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms" is recommended.




Table 3.3 Effective width of plating em for frames and girders

l / e 0 1 2 3 4 5 6 7 ≥ 8 em1 / e 0 0.36 0.64 0.82 0.91 0.96 0.98 1.00 1.0 em2 / e 0 0.20 0.37 0.52 0.65 0.75 0.84 0.89 0.9

em1 is to be applied where girders are loaded by uniformly distributed loads or else by not less than 6 equally spaced single loads.

em2 is to be applied where girders are loaded by 3 or less single loads. Intermediate values may be obtained by direct interpolation. l = length between zero-points of bending moment curve, i.e. unsupported span in case of simply supported girders and 0.6

x unsupported span in case of constrained of both ends of girder e = width of plating supported, measured from centre to centre of the adjacent unsupported fields

F Beam Member Design All structural beam members shall fulfil the requirements of this Section. It covers strength against ten-sion, compression, shear, bending, torsion and buckling.

Buckling of plates is covered in H.

F.1 Tubular “wet” members

This section covers all unstiffened and ring stiffened cylindrical tubulars that may be subjected to hydro-static pressure. These are typically part of jackets, towers or jack-ups.

The tubes have a thickness of more than 6 mm and a diameter to thickness ratio of D/t ≤ 120. In addition, yield strengths shall be less than 500 MPa.

Nontubular members shall be designed according to F.2.

F.2 Hydrostatic pressure

For each element the hydrostatic pressure has to be evaluated:

w zp f g H= ⋅ ρ

[ ]wz

cos h k(d z)HH z

2 cos h (k d)+

= − +

with

f : 1.0 for allowable stress design (ASD)

: γG1 for load resistance factor design (LRFD), load factor for permanent loads G1

ρw : density of the sea water [kg/m3]

g : acceleration due to gravity [m/s2]

z : depth of the member relative to still water level, measured positive upwards [m]

d : still water depth to the sea floor [m]

Hw : wave height [m]

K : wave number, k = 2π / λ [1/m]

λ : wave length [m]




F.2.1 Member checks using ASD

For each load combination as defined in Table 3.1 the design has to be checked according to API RP 2A WSD Section 3. For extreme environmental conditions (C, loading condition 3) the allowable stresses may be increased by 33 %.

F.2.2 Member checks using LRFD

For each load combination as defined in Table 3.2 the design has to be checked according to ISO 19902 Section 13

F.3 “Dry” members

Dry members are those that are not subjected to hydrodynamic loading, generally topside beams.

The design has to be checked according to ANSI/AISC Specifications for Structural Steel Buildings that provide rules for ASD as well as LRFD. The relevant load combinations have to be chosen accordingly as defined in Table 3.1 (ASD) or Table 3.2 (LRFD)

G Tubular Joint Design

G.1 General

Tubular joints are of primary importance for offshore steel structures.

The requirements given in this section apply to tubular connections within the following validity ranges:

yield strength less than 500 MPa

θ brace angle between 30° and 90°

β : d/D between 0.2 and 1.0

γ : D/2T between 10 and 50

τ : t/T smaller than 1.0

Fig. 3.2 Brace connection; definitions

G.2 Classification of Joints

A plane is defined by a chord and a brace. A joint is made up of this and possibly other braces that are on the same plane or tilted less than 15° to it. Are there more braces lying on different sides (planes) con-nected to the chord they comprise other joints that are treated differently.




There are three basic joint types: Y, K and X. Although their names are derived from their respective geometric shapes, the classification almost entirely depends on the distribution of the axial loads.

The classification is done like this: 1. a brace is a K-joint if the component of axial force in the brace perpendicular to the chord is bal-

anced to within 10 % by force components (perpendicular to the chord) in other braces in the same plane and on the same side of the joint;

2. a brace is a Y-joint if it does not meet the criteria a) and if the component of axial force in the brace perpendicular to the chord is reacted as beam shear in the chord. This is always true for single braces.

3. a brace should be considered as an X-joint if it does not meet the criteria a) and b); in this classifica-tion the axial force in the brace is transferred through the chord to the opposite side (e.g. to other braces, to padeyes, launch rails or similar structural components).

The classification of each individual brace-chord combination for a given load case shall be as a Y-, K- or X-joint. If the brace-chord combination carries part of the axial brace force as a K-joint, and part as a Y-joint or X-joint, it shall be classified as a proportion of each relevant type, e.g. 70 % as a K-joint and 30 % as an X-joint.




!

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Fig. 3.3 Shows some simple examples of the brace joint classification scheme

G.3 Tubular joint design using ASD

For each load combination as defined in Table 3.1 the design has to be checked according to API RP 2A WSD Section 4.

G.4 Tubular joint design using LRFD

For each load combination as defined in Table 3.2 the design has to be checked according to ISO 19902 Section 14.




G.5 Complex joints

Complex joints not covered here shall be designed on the basis of appropriate theoretical or experimental investigations. These include overlapping joints, conical transitions, grouted joints, etc. See API RP 2A WSD or ISO 19902 for further details.

H Buckling of Plates

H.1 General indications

H.1.1 Structural elements subjected to predominantly compressive or shear stresses, which may also result from bending or torsion, are to be investigated for overall and/or local buckling failure.

H.1.2 Deformations due to loads (e.g. lateral load producing bending) or manufacture (fabrication tolerances, welding distortion) shall be taken into account where they may be important, using e.g. sec-ond order theory. On the other hand, buckling calculations may result in special manufacturing instruc-tions regarding tolerable imperfections.

H.1.3 Local buckling may be admitted where the structure is designed with sufficient redundancy, i.e. where adjoining elements are designed to take over the load originally carried by the buckled part with sufficient safety. Serviceability considerations have to be kept in mind.

H.1.4 Detailed buckling analysis may be carried through according to recognized codes and stan-dards such as

• GL Rules for Hull Structures (I-1-1), Section 3

Eurocode 3 • AISC (Specification for the Design, Fabrication and Erection of Structural Steel for Buildings, and

Manual of Steel Construction).

I Fatigue Strength

I.1 General

A full fatigue analysis of the foundation of a fixed offshore structure is to be carried out according to API RP 2A WSD or ISO 19902. For a jacket type structure the tubular joints and for a monopole structure the girth welds as well as the welds of attachments are to be checked for fatigue damage.

A full fatigue analysis of all joints can be dispensed with in case that for these joints it can be shown that the acceptance criteria of the simplified method are fulfilled. The simplified method is described in para-graph I.2.

Fatigue critical details other than tubular joints and girth welds (e.g. brackets, welds of plates with differ-ent thicknesses, etc.) are to be assessed as described in GL Rules for Hull Structures (I-1-1), Section 20

I.2 Simplified method

I.2.1 This method can be used to check if a full fatigue analysis is required




I.2.2 Definitions

&

'

(

Fig. 3.4 Stress cycle

Δ : applied stress range (σmax - σmin) [N/mm2]

σmax : maximum upper stress of a stress cycle [N/mm2]

σmin : maximum lower stress of a stress cycle [N/mm2]

Δmax : applied peak stress range within a stress range spectrum [N/mm2]

σm : mean stress (σmax / 2 + σmin / 2) [N/mm2]

Δp : permissible stress range [N/mm2]

Δ : corresponding range for shear stress [N/mm2] n : number of applied stress cycles

N : number of endured stress cycles according to S-N curve ( = endured stress cycles under constant amplitude loading)

ΔR : fatigue strength reference value of S-N curve at 2 cycles of stress range [N/mm2]

ft : correction factor for thickness effect

fc : correction factor for corrosion effects

fm : correction factor for material effects

fR : correction factor for mean stress effect

fW : correction factor for weld shape effect

fi : correction factor for importance of structural element

fS : additional correction factor for hot spot stress analysis

fn : factor considering stress spectrum and number of cycles for calculation of permissible stress range

ΔRc : corrected fatigue strength reference value of S-N curve at 2 ⋅ 106 stress cycles [N/mm2]

γfat : safety factor for fatigue

D : cumulative damage ratio

I.2.3 Scope

A fatigue strength analysis is to be performed for all structures which are predominantly subjected to cy-clic loading, in order to ensure structural integrity and to assess the relative importance of possible fatigue damages for the establishment of an efficient inspection programme. The extent of the analysis is influ-enced by the local stress range and/or the number of cycles due to alternating loads on the structure. Wave loading is the main source of potential fatigue cracking in jacket structures. However, any other cyclic loading may contribute to fatigue failure and should therefore be considered.




The notched details, i.e. the welded connections at tubular joints and girth welds, plates and beams as well as notches at free plate edges are to be considered individually. The fatigue strength assessment is to be carried out on the basis of a cumulative damage ratio, according to the Minor Rule. Preliminary as-sessments can be done on the basis of a permissible peak stress range for standard stress spectra.

I.2.4 Application

The rules are applicable to structural steels up to a specified minimum yield strength of 690 N/mm2. Bolts are acceptable up to a tensile yield strength of 1000 N/mm2. Other materials such as cast steel, and hy-brid joints, can be treated in an analogous manner by adopting appropriate stress concentration factors and design S-N curves. These must be agreed upon with GL prior to application.

I.2.5 Low cycle fatigue

Low cycle fatigue problems in connection with extensive cyclic yielding have to be specially considered. When applying the following rules, the calculated nominal stress range should not exceed 1.5 times the minimum nominal upper yield point. In special cases the fatigue strength analysis may be performed by considering the local elasto-plastic stresses.

I.2.6 Limitation of fatigue assessment

No fatigue assessment is required if one of the following conditions is satisfied:

I.2.6.1 The maximum stress range Δσmax due to wave loads satisfies the following criterion and ade-quate protection exists in the case of corrosive environment:

γfat ⋅ Δmax ≤ ΔR ⋅ ft ⋅ fW ⋅ fm ⋅ fR ⋅ fi ⋅ fS

The above criterion is based on the conservative assumption that the long term distribution of stress range resembles a Weibull distribution with a shape parameter h ≤ 2 and that the total number of cycles is below 109.

I.2.6.2 The expected number of cycles during the life-time of the structure is less than

2 ⋅ 106 ⋅ [ΔR ⋅ ft ⋅ fC ⋅ fW ⋅ fm ⋅ fR ⋅ fi ⋅ fS / (γfat ⋅ Δmax) ]3

Δmax : the maximum nominal stress range or, for tubular joints, the maximum hot spot stress range, [N/mm2] see I.2.8

γfat : safety factor according to Table 3.4

ΔR : the fatigue strength reference value (for tubular joints, ΔR = 100 N/mm2

ft ⋅ fC ⋅ fW ⋅ fm ⋅ fR ⋅ fi ⋅ fS see I.2.9

I.2.7 Fatigue assessment procedure

The fatigue assessment shall verify that within an acceptable probability level the performance of the structure during its design life is satisfactory so that fatigue failure is unlikely to occur. An assessment should be made for every potential crack location.

I.2.7.1 Influence of stress range

The number of stress cycles which may be endured by the structural element depends mainly on the magnitude of the stress ranges Δ. The influence of the mean stress σm is generally small and may be taken care of by the correction factor fR defined in I.2.9.5

I.2.7.2 Safety factor

For the fatigue assessment procedure the stress ranges shall be multiplied by a safety factor γfat which covers the accessibility of the detail considered and the consequences of failure. Concerning the conse-quences of failure, the following two types of structures are to be considered:

• "fail-safe" structures, with reduced consequences of failure, i.e. local failure of a component does not result in a catastrophic failure of the structure




• "non fail-safe" structures, where local failure of a component leads rapidly to a catastrophic failure of the structure

The safety factors to be applied are given in Table 3.4.

Table 3.4 Safety factors on stress range for the fatigue assessment

Structural detail is part of a “fail-safe” structure part of a non “fail-safe” structure

easily accessible γfat = 1.00 γfat = 1.25

not easily accessible (e.g. un-der water) γfat = 1.15 γfat = 1.35

I.2.8 Hot spot stress definition

I.2.8.1 General

As mentioned above, tubular joints shall be assessed on the basis of the hot spot stress range at the weldtoe. This is the largest stress value at the intersection of the brace and chord and is defined as the extrapolation of the structural or geometric stress distribution to the weld toe, see Fig. 3.5. With this defini-tion the hot spot stress incorporates the effects of the overall geometry (structural or geometric stress concentration) but omits the influence of the notch at the weld toe (local stress concentration). The hot spot stress has to be considered in connection with a particular design S-N curve which reflects the mi-croscale effect occuring at the weld toe.

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* -2

+ 4*+5+

/ 01 2

+-

Fig. 3.5 Example of hot spot stresses in a tubular joint




I.2.8.2 Stress concentration factors

The hot spot stress can be calculated as the sum of nominal stresses due to the individual load compo-nents (i.e. axial load, in-plane and out-of-plane bending moment), each multiplied by a corresponding stress concentration factor (SCF). Stress concentration factors for the individual load components may be derived from finite element analyses, model tests or semi-empirical parametric equations based on such methods. Parametric equations should be used with caution in view of their accuracy and inherent limita-tions.

At least four different locations around the chord/brace intersection shall be considered because the indi-vidual load components produce different stress peaks at the crown and saddle points, see Fig. 3.5. Both the chord and the brace side of the weld shall be checked in the fatigue analysis.

I.2.8.3 Alternative stress concentration factors

For tubular joints other stress concentration factors are also applicable: • M. Efthymiou (“Development of SCF formulae and generalised influence functions for use in fatigue

analysis” Recent Developments in Tubular Joint Technology, OTJ ’88, London, plus updates”) • A. Almar-Næss (“Fatigue Handbook – Offshore Steel Structures”, Tapir Akademisk Forlag, Trond-

heim”)

I.2.8.4 Complex geometry

Tubular joints with complex geometry (braces overlapping or stiffened by gusset plates or ring stiffeners) have to be investigated by a special analysis.

I.2.8.5 Root failure

In addition to the assessment of the hot spot stress at the weld toe, the fatigue strength with regard to root failure has to be considered for partial penetration welds by application of the respective detail classi-fication, see GL Rules for Hull Structures (I-1-1), Section 20.

I.2.9 Correction of the reference value of the design S-N curve

I.2.9.1 A correction of the reference value of the S-N curve (or detail category) is required to account for additional influence factors on fatigue strength as follows:

ΔσRc = ft ⋅ fC ⋅ fW ⋅ fm ⋅ fR ⋅ fi ⋅ fS ⋅ ΔR

ft, fC, fW, fm, fR, fi, fS as defined in I.2.9.2 – I.2.9.8

For the description of the corrected design S-N curve, the formulae may be used, replacing ΔσR by ΔσRc.

I.2.9.2 Thickness effect (ft)

For welded connections oriented transversely to the direction of the applied stress with plate thickness t [mm], exceeding 25 mm at the potential crack locations:

ft = (25 / t) 0.25

For all other cases:

ft = 1.0

I.2.9.3 Corrosion effect (fC)

For parts of a structure in corrosive environment without adequate corrosion protection:

fC = 0.7

For all other cases:

fC = 1.0




I.2.9.4 Material effect (fm)

For welded joints it is generally assumed that the fatigue strength is independent of steel strength i.e.:

fm = 1.0

I.2.9.5 Effect of mean stress (fR)

The correction factor is calculated as follows: • in the range of tensile pulsating stresses:

fR = 1.0

σm ≥ Δmax / 2

• in the range of alternating stresses: fR = 1 + c ⋅ (1 – 2 ⋅ σm / Δmax)

– Δmax / 2 ≤ σm ≤ Δmax / 2

• in the range of compressive pulsating stresses: fR = 1 + 2 ⋅ c

σm ≤ – Δmax / 2

c : 0 for welded joints subjected to constant stress cycles

: 0.15 for welded joints subjected to variable stress cycles

I.2.9.6 Effect of weld shape (fW)

In normal cases:

fW = 1.0

A factor fW > 1.0 applies for welds treated e.g. by grinding. By this surface defects such as slag inclusions, porosity and crack-like undercuts shall be removed and a smooth transition from the weld to the base material shall be achieved. Final grinding shall be performed transversely to the weld direction. The depth should be approx. 0.5 mm larger than that of visible undercuts. For ground weld toes of fillet and K-butt welds:

fW = 1.15

For butt welds ground flush a weld shape factor

fW = 1.25

may be applied in case of effective protection from sea water corrosion.

The assessment of a local post-weld treatment of the weld surface and the weld toe by other methods has to be agreed on in each case.

I.2.9.7 Influence of importance of structural element (fi)

The influence of importance is usually covered by the safety factor γfat, see Table 3.4, i.e.:

fi = 1.0

I.2.9.8 Hot spot stress analysis (fS)

For tubular joints the factor fS is always 1.




J Application of BSH Requirements

J.1 Scope

The „Bundesamt fur Seeschifffahrt und Hydrographie” (BSH) requires design checks according to Euro-code 3 that uses an LRFD algorithm. The rules are found in:

• DIN EN 1990:2010-12 + DIN EN 1990/NA:2010-12 (National Annex for Germany) • DIN EN 1993-1-1:2010-12 + DIN EN 1993-1-1/NA:2010-12 (National Annex for Germany) • DIN EN 1993-1-8:2010-12 + DIN EN 1993-1-8/NA:2010-12 (National Annex for Germany)

The following describes GL’s interpretation of how to apply these rules for offshore installations. However, the final decision of using them correctly lies with the BSH.

Note

GL advices to perform the structural design according to the API or ISO standards as cited in this Section and to do the required EURO CODE checks in addition. Scope of these additional checks has to be de-fined case by case.

J.2 Basic Loads

J.2.1 Permanent Loads G

These are loads that vary never or only little over time. Examples are self weight, weight of embedded equipment, etc. but also buoyancy.

J.2.2 Variable Loads Q

Q summarizes all other loads that occur during operation. This includes live loads on decks, movable equipment and environmental loading like effects of ice drift, aero- and hydrodynamic forces. These loads are to be calculated as outlined in Section 1 and 2.

J.2.3 Pretension P

Generally pretension is negligible for offshore structures and need not to be accounted for.

J.2.4 Accidental Loads A

These are extraordinary loads such as explosions, collisions, earthquakes.

J.3 Combination of Basic Loads

J.3.1 Applicable Formula

All design checks have to be done for the following kinds of possible failure modes: • STR: failure of any structural element • EQU: loss of onbottom stability • GEO: insufficient soil stiffness

DIN EN 1990/NA (German national annex to DIN EN 1990:2010-12) states that only the sum of the fol-lowing equation yields the loading. The use of equationss. 6.10 a, b of DIN EN 1990:2010-12 is not al-lowed.

d G, j Gk, j P Pk Q,1 Qk,1 Q,i 0,i Qk,ij 1 i 1

E E E E E≥ >

= γ ⋅ + γ ⋅ + γ ⋅ + γ ⋅ ψ ⋅∑ ∑

EQk,1 is the leading action. To limit the number of calculations the leading action will be defined in J.3.2 and J.3.3

γ … are the partial load factors, see Table 3.5

ψ … are the combination factors applicable when adding different loads, see Table 3.6




Table 3.5 Partial Load Factors γ

Action γ

max 1.35 STR / GEO

min 1.00

max 1.1

permanent loads G

EQU

min 0.9

pretension P Refer to DIN EN 1992 until 1999

max 1.5 STR / GEO

min 0.0

max 1.5

environmental / variable loads Q

EQU

min 0.0

Table 3.6 Combination Factors ψ0

Action ψ0,i

walkways 0.7

laydown areas, helicopter deck 1.0

ice loads on deck 0.5

aerodynamic loads 0.6

hydrodynamic loads 0.6

J.3.2 Leading action for Jacket Structures

As leading action for jackets hydrodynamic loads are recommend (wave and current with a return period of 100 years). Sustained wind (1 min) is an accompanying action.

Forces due to environmental loading should be calculated as outlined in Section 2. Characteristic pa-rameters such as wave heights, periods, currents and wind speeds are site specific and should be dis-cussed with GL.

J.3.3 Leading action for Topsides

For topsides the leading action is the variable gravitational loads (equipment, live loads, etc.). There may be special cases where other loads might be dominant, so project specific exceptions may be discussed with GL.

J.4 Member Checks (DIN EN 1993-1-1:2010-12 + DIN EN 1993-1-1/NA:2010-12)

Member checks must be done as described in F as the methods in F includes hydrodynamic pressure. In addition Eurocode 3 can be used.

The design resistance Rd is defined by the representative resistance Rk divided by the safety factor γMi :

Rd = Rk / γMi with

Table 3.7 Safety Factors γMi

i γMi Applicable for

0 1.00 Strength check of member and joints

1 1.10 Stability checks

2 1.25 Rupture (tension)




J.5 Tubular Joint Design (DIN EN 1993-1-8:2010-12 + DIN EN 1993-1-8/NA:2010-12)

Partial safety factor γM = γM5 = 1.0

Joints are classified solely by their geometric shapes • Y or T joint (single brace) • X joint • K or N joint

Depending on the axial force in the brace(s) different formulae have to be used to check against punching shear.

DIN 1993 further requires the consideration of three dimensional effects if a node consists of several joints (only for KK and XX joints). It is done by reducing the resistance.

J.6 Fatigue

The proof of fatigue resistance has to be done as outlined in I.




Section 4 Steel Structures A Materials ..................................................................................................................... 4-1 B Fabrication ................................................................................................................ 4-20 C Welding ..................................................................................................................... 4-39 D Survey, Inspection and Testing ................................................................................ 4-66 E Survey and Inspection Scheme................................................................................ 4-68 F Non-Destructive Testing ........................................................................................... 4-69 G Visual Inspection....................................................................................................... 4-74 H Magnetic Particle Inspection..................................................................................... 4-75 I Penetrant Testing...................................................................................................... 4-77 J Radiographic Testing ................................................................................................ 4-78 K Ultrasonic Testing ..................................................................................................... 4-80

A Materials

A.1 General

A.1.1 The material requirements for fixed offshore installations and mobile offshore units shall reflect the overall philosophy regarding design life time, cost profile, inspection and maintenance philosophy, safety and environmental profile, failure risk evaluation and other project requirements.

A.1.2 Structures shall be categorized by various levels to determine criteria that are appropriate for the intended service of the structure.

The levels are determined by consideration of life-safety and of environmental and economic conse-quences.

The life-safety category, see Table 4.1, addresses personnel on the platform and the likelihood of suc-cessful evacuation before a design environmental event occurs:

Table 4.1 Life safety category

Category Explanation Further details

S1 Manned non-evacuated

ISO 19902 Chapter 6.6.2, GL Rules for Certification and Surveys (IV-7-1), Section 1, B.4.3.1

S2 Manned evac-uated

ISO 19902 Chapter 6.6.2 GL Rules for Procedure (VI-7-1), Section 1, B.4.3.2

S3 Unmanned ISO 19902 Chapter 6.6.2 GL Rules for Certification and Surveys (IV-7-1), Section 1, B.4.3.3

The consequence category, see Table 4.2, considers the potential risk to life of personnel brought in, to react to any incident the potential risk of environmental damage and the potential risk of economic loss:


Section 4 Steel Structures


Table 4.2 Consequence category

Category Explanation Further details

C1 High consequence ISO 19902, Chapter 6.6.3

C2 Medium consequence ISO 19902, Chapter 6.6.3

C3 Low consequence ISO 19902, Chapter 6.6.3

The three categories for both life-safety and consequence shall be combined into nine Exposure Levels. However, the level to be used for structure categorization is the more restrictive level for either life-safety or consequence. The results are illustrated in Table 4.3:

Table 4.3 Exposure levels

Consequence category Life-safety category

C1 High C2 Medium C3 Low

S1 Manned non-evacuated L1 L1 L1

S2 Manned evacuated L1 L2 L2

S3 Unmanned L1 L2 L3

The Exposure Levels applicable to a structure, see Table 4.4, shall be determined by the Owner prior to the design of a new structure and be agreed upon by the Certifying Body where applicable in addition to the selection in the oil and gas industries the following selection shall apply:

Table 4.4 Classification of typical constructions

Type of construction Exposure Level

Transformer stations L1

Converter station L1

Accommodation platform L1

Mobile offshore units L2

Offshore meteo station L2

Handling tools for MWS (Marine Warranty Survey)

L3

Based on this selection the requirements for the structural material (e.g. minimum Charpy V-notch impact energy) and welding procedure qualification (WPQ) is specified in both Tables 4.10 and 4.11

A.1.3 The materials for fixed offshore installations shall fulfil the minimum requirements detailed in this Section. This Section defines typical structural elements used in offshore steel structures, where or-dinary building codes or Shipbuilding Rules lack relevant recommendations. Such elements are tubular members, tubular joints, conical transitions and some load situations for plates and stiffened plates.

The material requirements of this Section shall apply to following parts: • all sub-sea structures • all parts below the atmospheric zone • all parts of the foundation structure including deck-to leg connections




The battery limits related to the application of this Section and Shipbuilding Rules or ordinary building codes have to be defined in the specific project considering A.1.1.

The minimum requirements for parts outside of this Section shall be as for exposure level L2. In the event that Shipbuilding Rules shall be used the material class III applies for special members and class II applies for primary members for Design Temperature > - 20 °C and GL Rules for Hull Structures (I-1-1), Section 2, Table 2.10 for Design Temperature ≤ -20 °C, see GL Rules for Hull Structures (I-1-1), Section 2, Table 2.7.

A.1.4 Materials for the steel structure shall be supplied by manufacturers approved by GL.

Approvals by other recognized Certifying Bodies may be accepted.

A.1.5 As far as possible, steels suitable for welding are to be employed in accordance with recog-nized standards. If other steels are intended to be used, the relevant specifications with all details re-quired for appraisal are to be submitted to GL for approval.

A.2 Categories of structural members

A.2.1 General

For the selection of materials for the different members of the steel structure the criteria explained below are to be observed, see also Section 3, A.2.6: • importance of the member within the structure (consequence of failure, redundancy) • character of load and stress level (static or dynamic loads, residual stresses, stress concentrations,

direction of stresses in relation to the rolling direction of the material, etc.) • design temperatures • chemical composition (suitability for welding) • yield and tensile strength of the material (dimensioning criteria) • ductility of the material (resistance to brittle fracture at given design temperature) • through-thickness properties (resistance to lamellar tearing)

Additional properties, such as corrosion resistance, may have to be considered.

A.2.2 Structural members

Depending on the importance of the structural members and on the type of load and the stress level, a structure can be subdivided into the following component categories:

A.2.2.1 Special structural members

These are members essential to the overall integrity of the structure and which, apart from a high calcu-lated stress level, are exposed to particularly arduous conditions (e.g. stress concentrations or multi-axial stresses due to the geometric shape of the structural member and/or weld connections, or stresses acting in the through-thickness direction due to large-volume weld connections on the plate surface).

Note

This applies especially to: cans of tubular nodes, thick-walled deck-to leg and column connections thick-walled points of introduction or reversal of forces, etc.

A.2.2.2 Primary structural members

These are members participating in the overall integrity of the structure or which are important for opera-tional safety and exposed to calculated load stresses comparable to the special structural members, but not to additional straining as mentioned above.

Note:

These are, e.g. all other tubes of tubular truss work structures (jacket legs, bracings other than cans, piles), riser support structures, boat landing structures, girders and beams in the deck, self-supporting plating, helicopter decks, module support structures, crane columns and cranes, etc.




A.2.2.3 Secondary structural members

These are all structural members of minor significance, exposed to minor stresses only, and not coming under the above categories of both “special” and “primary”.

Note:

These are e.g. non-structural walls, stairs, pedestals, rails, mountings for piping and cables, etc.

A.2.2.4 Identification of structural members

The categories of structural members as per A.2.2 are to be fixed in the design stage and indicated in the construction documentation.

A.3 Selection criteria of steels

A.3.1 Structural steels are defined depending on their specified minimum yield strength (SMYS) in the following strength classes: • normal strength (mild) steels with SMYS up to 275 MPa • higher strength steels with SMYS over 275 MPa up to 360 MPa • high strength steels with SMYS over 360 MPa

Note

Fine grained structural steels, applied for special and primary structure in case of resistance to fatigue, are recommended with a nominal yield strength not exceeding 360 MPa.

High strength steels having SMYS exceeding 500 MPa may be employed in technically motivated excep-tional cases only with GL’s consent. Application of steels with SMYS higher than 690MPa however is not covered by this Rule.

A.3.2 The strength class chosen for the respective structural member and/or the kind of material corresponding to it are to be indicated in the construction documentation. The same applies to the special requirements possibly to be met by the material.

A.3.3 Steels with improved through-thickness properties are to be employed where structural mem-bers are rigid and/or particularly thick and where high residual welding stresses are to be expected, e.g. due to large-volume single-bevel butt joints or double-bevel butt joints with full root penetration, simulta-neously implying high stresses acting in the through-thickness direction of the materials.

This will normally be the case where special structural members are concerned.

The same applies for members subject to high external loads acting in thickness direction.

A.3.4 Depending on the structural member category, the design temperature and the material thick-ness of all steels to be employed for the structure, have to meet the requirements listed in A.4, in particu-lar those for impact energy.

A.3.5 Owing to the risk of hydrogen induced stress cracking (HISC), steels with a SMYS of > 500 MPa shall not be used for critical cathodically protected components without special considerations, i.e. in the submerged zone of fixed offshore structures or mobile offshore units, see Section 5. This imposes some restrictions: • any welding or other construction method affecting ductility or tensile properties shall be carried out

according to a qualified procedure which limits hardness to 325 HV10 (300 HB for base material) • this restricts the use of welded structural steels with SMYS of 690 MPa.

A.4 Requirements for steels

A.4.1 Manufacturing procedures

All steels shall be made by generally both the basic oxygen process and basic electric arc process, or by other methods approved by GL. The open hearth furnace process shall not be used.




Steel shall be as follows: • steels with an Charpy V-notch impact test temperature of > 0 °C, see Table 4.11, shall be semi-killed

as a minimum, i.e. equivalent to the deoxidation method FN of EN 10025 • steels with an Charpy V-notch impact test temperature of < 0 °C, see Table 4.11, shall be fully killed,

i.e. equivalent to the deoxidation method FF of EN 10025 • all Z35 steels according to EN 10164 and Z30 steels according to API Specification 2H, S4 shall be

fully killed and fine grained

A.4.2 Supply condition and heat treatment

Steels intended for special and primary structural members shall be supplied as follows: • steels with minimum yield strength up to 275 MPa: normalized/normalized rolled (N) • steels with minimum yield strength over 275 and up to 360 MPa: normalized/normalized rolled (N),

thermomechanical rolled (M) or quenched and tempered (QT) • steels with minimum yield strength over 360 MPa: thermomechanical rolled (M) or quenched and

tempered (QT) as defined in EN 10225.

The following thickness limits shall apply: • normalized/normalized rolled (N):

up to 250 mm • thermomechanical rolled (M):

up to 150 mm for normal and higher strength steels up to 100 mm for high strength steels

• quenched and tempered (QT): up to 150 mm for higher strength steels up to 100 mm for high strength steels

The minimum rolling reduction ratio of concast material for plates shall be 4 : 1, except for piling, where it shall be 3 : 1.

A.4.3 Chemical composition and suitability for welding

The steels shall conform to the requirements for chemical composition, as determined by heat analysis, prescribed in the both Tables 4.5, 4.7, 4.8 and 4.9 or equal.

The Carbon Equivalent CEV and Pcm of the heat analysis shall be calculated by both of the following equations:

CEV = C + Mn/6+ (Cr + Mo + V) / 5 + (Ni + Cu) / 15

Pcm = C + Si/30 + (Mn + Cu + Cr) / 20 + Ni/60 + Mo/15 + V/10 + 5B

The maximum Carbon Equivalent CEV and Pcm shall not exceed the values prescribed in the relevant standards or the values of Table 4.5, the smaller value shall apply. CEV and Pcm computed on product shall not exceed the CEV and Pcm computed on heat by more than 0.02 % and 0.01 % respectively.




Table 4.5 Maximum carbon equivalent

CEV 1 Pcm1 Steel strength class

N M QT N M QT

Normal strength class 0.43 0.38 - - - -

Higher strength class 0.45 0.43 0.43 0.24 0.24 0.24

High strength class 0.45 0.43 0.43 0.25 0.25 0.25

1 Valid for steel classes with maximum yield strengths up to 500 MPa

A.4.4 Mechanical properties

A.4.4.1 Tensile test properties

The requirements for tensile strength Rm, specified minimum yield strength (SMYS or ReH) and for elonga-tion at break, as stated in the GL Rules II – Materials and Welding, relevant standards and/or approved material specifications, are to be observed; see also A.3.1.

Beyond this, for steels with required notch bar impact tests at test temperature < -20 °C, see Table 4.11, the yield strength ratio ReH / Rm respectively Rp0.2 / Rm as defined in Table 4.6 shall not be exceeded.

Table 4.6 Yield strength ratios ReH / Rm (Rp0.2 / Rm respectively)

Material quality N Material quality M, QT Steel strength class

t ≤ 16 mm t > 16 mm t ≤ 16 mm t > 16 mm

High strength 1 0.87 0.85 0.93 0.90

Higher strength 0.87 0.85 0.93 0.90

Normal strength 0.80 0.80 0.80 0.80

1 Valid for steel classes with maximum yield strengths up to 500 MPa

A.4.4.2 Impact energy

The minimum values listed in Table 4.10 apply, proof of which to be furnished at the test temperature TT indicated in Table 4.11.

A.4.4.3 Through-thickness properties

Through thickness testing is required for special structural members, if stresses acting in through-thickness direction. The through-thickness test shall be carried out in the final heat treatment condition in accordance to EN 10164 or API Specification 2H, Supplementary Requirement S4 and the test results as follows: Z35 for EN 10164 and Z30 for API Spec. 2H, S4. Through-thickness testing is not required for a product thickness below 25 mm or if the sulphur content is below 0.007 %. For 6 mm ≤ t < 25 mm UT in accordance to EN 10160 “Class S1/E2” is required. Plates and rolled section of special and primary struc-tural members without through thickness stresses shall be inspected by UT per ”Class S0/E1” of EN 10160.




Table 4.7a Appropriate steels for plates (special structural members/high strength steel)

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Table 4.7b Appropriate steels for plates (special structural members/ higher strength steel)

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Table 4.10 Minimum Charpy V-notch impact energy requirements 1, 2, 3

Table 4.11 Charpy V-notch impact test temperatures TT [°C] for exposure levels L1, L2 and L3 1, 2,

3

Thickness t of product [mm] Category of sea (Temp. range)

Steel category t ≤ 12,5 12,5 < t ≤ 25 25 < t ≤ 50 t > 50

TD1 –20 °C –20 °C –20 °C –20 °C

Exp. level L1 L2 L3 L1 L2 L3 L1 L2 L3 L1 L2 L3

Special –30 –20 –20 –50 –35 –20 –50 –35 –20 –50 –35 –20

Primary –20 –20 –20 –30 –20 –20 –50 –35 –20 –50 –35 –20

Polar sea e.g. Alaska Sea, Bering Sea (TD 1 ≤ –20 °C)

Secon-dary –10 –10 –10 –10 –10 –10 –30 –20 –20 – – –

TD1 –10 °C –10 °C –10 °C –10 °C

Exp. level

L1 L2 L3 L1 L2 L3 L1 L2 L3 L1 L2 L3

Special –20 –10 –10 –40 –25 –10 –40 –25 –10 –40 –25 –10

Primary –10 –10 –10 –20 –10 –10 –40 –25 –10 –40 –25 –10

Cold sea e.g. North sea, Baltic Sea, Irish Sea (–15 °C ≤ TD

1 ≤ 0 °C)

Secon-dary 0 0 0 0 0 0 –20 –10 –10 – – –

Minimum nominal yield strength 4 [MPa] 235 275 290 355 420

Minimum average 27 30 34 40 48 Transversal: Special category

Minimum single 19 21 24 28 34

Minimum average 27 30 34 40 47

Charpy V-Energy [Joule] Longitudinal:

primary + secondary category Minimum single 19 21 24 28 33

Minimum nominal yield strength 4 [MPa] 460 500 550 620 690

Minimum average 50 50 55 62 69 Transversal: Special category

Minimum single 35 35 39 44 49

Minimum average 50 50 55 62 69

Charpy V-Energy [Joule] Longitudinal:

primary + secondary category Minimum single 35 35 39 44 49

1 average value from 3 specimens, one single value may be below minimum average 2 steels with Charpy values of 27 J may be used as 40 J at 20 °C higher test temperature or 30 J at 10 °C higher test temperature 3 e.g. 27 J(T) ≅ 40 J(L) (see Table 6, EN 10028-3) 4 intermediate values may be interpolated




TD

1 0 °C 0 °C 0 °C 0 °C

Exp. level

L1 L2 L3 L1 L2 L3 L1 L2 L3 L1 L2 L3

Special –10 0 0 –30 –15 0 –30 –15 0 –30 –15 0

Primary 0 0 0 –10 0 0 –30 –15 0 –30 –15 0

Temperate sea e.g. English Chan-nel, Bay of Biscay, West Mediterranean (0 °C ≤ TD

1 ≤ +15 °C) Secon-

dary +10 +10 +10 +10 +10 +10 –10 0 0 – – –

TD1 +20 °C +20 °C +20 °C +20 °C

Exp. level L1 L2 L3 L1 L2 L3 L1 L2 L3 L1 L2 L3

Special +10 +20 +20 –10 +5 +20 –10 +5 +20 –10 +5 +20

Primary +20 +20 +20 +10 +20 +20 –10 +5 +20 –10 +5 +20

Warm sea e.g. Gulf of Guinea, Arabian and Persian Gulf, Indian Sea, East Mediterranean (TD

1 > +15 °C) Secon-dary +30 +30 +30 +30 +30 +30 +10 +20 +20 – – –

1 TD = design temperature to be determined in accordance with Section 1, F.1A

2 For intermediate temperatures TD the Charpy V-notch impact test temperatures TT may be interpolated.

3 The Charpy V-notch test temperature TT stated in Table 4.11 may differ from the values stated in the various material standards because they are depending on the design temperature TD and the exposure level. For ordering of materials it is possible to either specify the Charpy V-notch values according to Table 4.10 and the test temperatures according to Table 4.11 or to use comparable materials (see examples in the following describing the way to find comparable complying materials).

A.4.4.4 Ordering of material

For ordering materials it is not necessary to specify the Charpy V-notch values (Table 4.10) and TT (Table 4.11) in accordance to these Tables; comparable materials can be used. The following examples shall explain one possible way to find comparable materials in accordance to this Section. 1. Use the Tables 4.7 to 4.9 for pre-selection

These Tables provide materials listed with the most important properties for material selection. a) Selection of different structural members, e.g. sulphur values (≤ 0.007 % or Z35/Z30, see A.4.4.3)

and Charpy V-notch values in transversal direction for special members.

b) Selection of material suitable for welding, e.g. all listed material have CEV- and PCM -values in accordance to Table 4.5. The thickness limitations of some steel grades are based on these re-quirements.

2. Use tables of standards showing the requirements of Charpy V-notch values depending on TT like EN 10025 Part 3,4 and 6, both Tables 6 and 7 , EN 10028 Part 3, 5 and 6, Tables 4, 5 and 6 or standards with Charpy V-notch values at two different test temperatures as additional options , like API Specification 2 H, 2 W and 2Y.

A.4.4.5 Examples for steel selection

Table 4.12 Example no. 1

SMYS

[MPa]

TD

[°C]

t

[mm] Exposure Level

Material type 355 -15 60 L1

Application Plate for can with diameter of 950 mm




Explanation:

1. We preselect the Grade S355G8 + M (EN 10225), because cans of tubular nodes are exposed to stress acting in the through-thickness direction, i.e. special members with through-thickness properties are required, see both A.4.4.3and Table 4.7b. This is fulfilled with a maximum sulphur value of 0.007 %, i.e. no supplementary requirements no. 13 with through-thickness testing are required. Grade S355G8 + M has a CEVmax. = 0.43 % and Pcmmax. = 0.24 %, i.e. the requirements for suitability for welding are ful-filled, see Table 4.5. The required safety factor ”Yield Strength Ratio of maximum 0.90, see Table 4.6, will also be reached, see EN 10225, Table 10.

In accordance to both Table 4.10 and 4.11 the material shall provide minimum average Charpy V-notch values of 40 J at -45 °C in transversal direction (T). In EN 10225 and Table 4.7b S355G8 + M the mini-mum average Charpy V-notch value is 50 J (transversal)) at -40 °C.

2. The comparability of 40 J (T) at -45 °C and 50 J (T) at -40 °C can be verified by comparison of Grade 50 (+M) of API Specification 2 W with S355G8 + M.

Table 4.13 Material properties example no. 1

CEV max. [%]

Pcm max. [%]

S max. [%]

Charpy V-notch value min. average

Grade 50 (+M) 0.43 0.24 0.010 41 J at -60 °C

S355G8 + M 0.43 0.24 0.007 50 J at -40 °C

3. Therefore is 50 J (T) at -40 °C comparable with 41 J (T) at -60 °C and this includes 40 J (T) at -45 °C. This comparison shows, that S355G8 + M is acceptable down to a Design Temperature TD of -30 °C for the Exposure Level 1. 4. In the same manner the comparability of all Grades of EN 10225 listed in the Table 4.7 to 4.9 can be verified by comparison with API Specification 2H, 2W and 2Y.

Table 4.14 Example no. 2

SMYS [MPa]

TD [°C]

t [mm] Exposure Level

Material type 355 -10 60 L1

Application Plate for legs with diameter of 950 mm

Explanation:

1. We pre-select Grade P355ML1 (EN 10028-5). Grade P355ML1 specify only the CEV max. value with 0.40 %. This is acceptable in accordance to both A.4.3 and Table 4.5. The required safety factor “Yield Strength Ratio” can be calculated by:

ReH (min.)/Rm(min.) = 345/450MPa = 0.766 This value is below 0.90, i.e. is acceptable, see both A.4.4.1 and Table 4.6. In accordance to both Tables 4.10 and 4.11 the material shall provide minimum average Charpy V-notch values of 40 J at - 40 °C in longitudinal (L) direction. In EN 10028-5 Table 5 P355ML1 provides 27 J at -40 °C in transversal (T) di-rection. 2. The comparability of 40 J (L) at -40 °C and 27 J (T) at -40 °C can be verified by comparison with Grade S355 ML (EN 10025-4).




Table 4.15 Material properties example no. 2

CEV max. [%]

Charpy V-notch value min. average (L)

Charpy V-notch value min. average (T)

S355ML 0.4 31 J at - 40 °C ≙ 155 %

20 J at - 40 °C ≙ 100 %

P355ML1 0.4 41.8 at - 40 °C ≙ 155 %

27 J at - 40 °C ≙ 100 %

3. Therefore 27 J (transversal) at - 40 °C is comparable with 41.8 J (longitudinal) at - 40 °C for P355ML1; i.e. the charpy properties are acceptable.

A.4.5 Stress relieving treatment

A.4.5.1 General

Where the thickness, complexity or high level of stresses of the welded assemblies will require, or at least indicate, the probable requirement of stress relieving treatment after welding, the corresponding steels shall be ordered with tests on their physical properties (at least for tensile and impact properties) after such a treatment or after welding without such a treatment, see also CTOD, A.4.5.5.

A.4.5.2 Reference thickness and thickness limits for stress relieving treatment

Stress relieving treatment is required for materials of a Reference Thickness exceeding 50 mm for Spe-cial Category and high strength steels of Primary Category members joined by full penetration welds.

For mobile offshore units and structure of exposure level L3, see Table 4.4, the stress relieving treatment may be waved, if inspection intervals are shortened and inspections are performed based on specific test plans. These test plans shall be based on the stress history since the last inspection.

In a full penetration weld (e.g. butt welds, T-, K-, Y- welds, etc.) the Reference Thickness is that thickness which governs the thickness of the weld. For example: • thickness of thinner member in a butt weld • thickness of stub in stub/can junction in a tubular node • i.e. stress relieving treatment is not generally required for Special Structural Members with a thick-

ness of t > 50 mm. It is only required, if the stress acting on Special Structural Members are as specified in A.2.2.1. In the case of stub/can conjunction stress relieving treatment is only required, if the thickness of the stub is thicker than 50 mm, but not if only the can is thicker than 50 mm, see Fig. 4.33.

• stress relieving treatment can be waived by CTOD tests, see A.4.5.5.

A.4.5.3 Post-weld heat treatment (PWHT)

This heat treatment shall be defined by the steel producing mill and shall be submitted to GL for informa-tion, see EN 10225, Chapter 8.3.2. The holding time shall be 2 minutes per mm of thickness. Acceptance tests shall then be performed on samples having undergone the heat treatment under simulated condi-tions. The results shall comply with requirements stated in this specification or other contractual docu-ments with upgrading values concerning steel qualities and grades.

A.4.5.4 Weld improvement techniques

A.4.5.4.1 General

Weld improvement techniques may be considered as a last resort measure that only can be justified when the project is too far advanced to still enable design changes to be introduced in a timely and prac-tical manner.

A.4.5.4.2 Peening

Peening of local areas of weld toes or weld transitions to improve fatigue life of structures is to be speci-fied in a detailed procedure about performance, tools, measurements and documentation.




Application has to be specifically agreed with GL case by case.

A.4.5.4.3 TIG dressing

Due to uncertainties regarding quality assurance of the welding process, this method may not be recom-mendable for general use in the design stage.

In case of performance on a later stage a detailed procedure to be submitted and approved. Specific ar-eas are to be mentioned.

A.4.5.4.4 Weld toe grinding

This method shall apply only for high fatigue utilization, see also B.10.15. A detailed procedure is to be prepared by the manufacturer and to be approved by GL.

In case of removal of undercuts in the toe area a smooth transition to be ground and toe defects to be removed. The minimum radii of the weld profiles shall not less than 10 mm. Grinding marks shall run per-pendicular to the weld axis and under no circumstances parallel. After grinding both 100 % visual exami-nation and 100 % MT shall be performed. The ground surface shall be free of any cracks and crack-like indications.

A.4.5.5 Crack tip opening displacement (CTOD)

A.4.5.5.1 General

The CTOD test may wave the stress relieving treatment in accordance to A.4.5 and shall meet a require-ment of minimum 0.25 mm (as-welded) at -10 °C for Design Temperature down to -15 °C, see Table 4.11: Cold sea, and above submerged zone (0 °C for submerged zone). CTOD-testing of welds shall be carried out with the fatigue notch tip positioned in the coarse grained region of the heat affected zone (HAZ) and in the weld.

Other test temperatures, especially for a design temperature < -15 °C, see Table 4.11: Polar sea, shall be prescribed by the Designer and approved by GL. (For CTOD-testing, see both EN 10225 Attachment E and BS 7448 – 3).

A.4.5.5.2 Pre-production qualification

The CTOD testing of the heat affected zone (HAZ) may be omitted if the following requirements of the pre-qualification by the manufacturer are qualified: • CTOD tests shall be performed for each condition of material supply from each manufacturer. • CTOD data for the HAZ is generally provided by the steel supplier's weldability data. • CTOD testing of the HAZ of the test weld may be specified in the welding procedure qualification re-

quirements. • Charpy V-notch tests results on the PQR should not be substantially lower than those obtained by

the steel supplier. • Qualification welding shall be conducted at both the highest heat input/interpass temperature and

lowest heat input/interpass temperature to be allowed for production welding, as governed by the welding procedure specification (WPS).

• The test welding positions shall be selected as those representing the highest and lowest heat input of the procedure(s) to be qualified by such tests.

A.4.5.5.3 Additional essential variables

For welding procedures that are CTOD-tested, the following changes shall be considered essential vari-ables, in addition to the essential variables listed in the selected welding standard, see also C.3.7.2, etc.: • any increase in the maximum interpass temperature over that qualified in the high heat input CTOD

procedure qualification test • an increase in joint thickness greater than that used for the CTOD procedure qualification tests • a decrease in heat input from the heat input qualified during low heat input CTOD procedure pre-

qualification test for the HAZ • any increase in the heat input from the average heat input qualified of the fill passes during the high

heat input CTOD procedure qualification tests • any change in electrode or electrode/flux brand name or the place of manufacturer




• a reduction in the depth of back-gouging as measured during the CTOD qualification testing • a reduction in the width of back-gouging as measured during the CTOD qualification testing • an increase in the layer thickness (as measured in the fusion line) greater than 10 %, versus that

measured during CTOD pre-qualification testing

A.4.6 Impact energy after strain ageing

Steels which are to be subjected to a cold (below 250 °C) or a warm forming process of more than 5 % strain, either heat treatment shall be performed, or strain ageing tests shall be carried out according to the following procedure:

material shall be strained locally to the design deformation • material shall be heated at 250 °C for 1 hour • set of 3 impact test specimens shall be tested from the base material in the strained plus heated

condition; the notch shall be located within the strained portion, where the part has received the highest strain

• the impact test temperature shall be in accordance to Table 4.11 • the impact energy shall comply with Table 4.10 and not be more than 25 % lower than the impact

value for the material before deformation and strain ageing

If straining is performed at a temperature above 250 °C, it shall be documented that the material proper-ties, weldability, weld and HAZ properties satisfy the material Certificate and this Section.

A.4.7 Non-destructive testing

A.4.7.1 Plates and rolled sections

Plates and rolled sections of H and I shaped type (or equivalent) shall be ultrasonic inspected in accor-dance with EN 10160 for plates and EN 10306 for H and I shaped type. UT scanning shall be carried out over 100 % of webs and flanges of rolled sections and edges and body of plates.

Each plate and rolled section of “Special members with Through Thickness Properties” shall be inspected as per “Class S1/E2” of EN 10160 or equivalent.

Plates and rolled sections of “Special and primary members without Through Thickness Properties” shall be inspected as per “Class S0/E1” of EN 10160 or equivalent.

A.4.7.2 Welded and seamless tubes

This Section applies to structural tubulars manufactured by pipe mills or tubular workshops which are qualified to produce tubulars according to API Specification 2B or EN 10225.

A.4.7.2.1 Welded tubes

Steel plates shall be ultrasonically tested in accordance with the requirements of A.4.7.1. No weld repairs shall be permitted on plates.

A.4.7.2.2 Seamless tubes

All tubulars shall be NDT tested over their full length. Ultrasonic testing shall apply for wall thickness of 10 mm and above, details see EN 10225, Chapter 8.5.3.2.

A.4.8 Inspections

Inspections of materials shall be documented by the following Inspection documents according to EN 10204: • Secondary structural members: 2.2 Test report • Primary structural members 3.1 Inspection Certificate • Special structural members 3.2 Inspection Certificate

A.4.9 Identification and marking

A.4.9.1 All products are to be marked according to the standard such as to enable their clear identifi-cation and correlation to the inspection documentation.




A.4.9.2 Products which cannot be clearly identified shall be rejected, unless the properties stipulated can be determined by both re-inspection and re-testing, see also B.5.2.

A.5 Steel forgings and castings

A.5.1 General

Where steel forgings or castings are intended to be employed for special and primary structural members, relevant material specifications containing all details required for assessment are to be submitted to GL. For secondary structural members appropriate materials in accordance with the standards may be em-ployed.

A.5.2 Approval test

Steel forgings and castings for special and primary structural members are to be subjected to an approval test by GL.

The scope of testing will be fixed from case to case. An approval test may be dispensed with, where stan-dardized forgings and castings with nominal yield points up to 360 MPa are concerned, which do not have to meet any special requirements.

A.5.3 Supply conditions

All products are to be supplied in heat-treated condition. Heat treatment may comprise: • normalizing • normalizing and tempering • quenching and tempering

A.5.4 Chemical composition and suitability for welding

The composition of forged and cast carbon and carbon manganese steel grades suitable for welding has to meet the minimum requirements defined in both Table 4.16 and 4.17. The values apply to both ladle and product analysis.

Table 4.16 Forged structural steel

Min. Yield strength SMYS

Max. S content

Max. P content

Max. CEV

Max. Pcm

[MPa] [%] [%] [%] [%]

355 0.005 0.015 0.45 0.22

420 0.007 0.015 0.47 0.22

460 0.007 0.015 0.55 0.27

Table 4.17 Cast structural steel

Min. Yield strength SMYS

Max. S content

Max. P content

Max. CEV

Max. Pcm

[MPa] [%] [%] [%] [%]

310 0.008 0.012 0.41 0.24

420 0.008 0.012 0.48 0.26

460 0.005 0.012 0.55 0.28




Weldability data are required for a weld bevel thickness above 25 mm. The documentation of weldability shall be in accordance with EN 10225, Annex E. The heat input shall be 3.5 ± 0.2 KJ/mm, see EN 10225, Table E.3.

A.5.5 Material properties

Product testing shall be carried out for each cast and forged item above 1 t weight. For smaller forgings or large series another test frequency may be agreed upon. The results obtained shall comply with Tables 4.6, 4.10 and 4.11.

A.5.6 Inspections

Unless otherwise fixed or agreed, plates for special structural members are to be tested individually per rolled length. For all other products the inspection lots as indicated in the standards and/or GL Rules II – Materials and Welding apply.

Inspections of materials shall be documented by the following Inspection documents according to EN 10204: • Secondary structural members: 2.2 Test report • Primary structural members: 3.1 Inspection Certificate • Special structural members: 3.2 Inspection Certificate

A.5.7 Identification and marking

A.5.7.1 All products are to be marked according to the standard such as to enable their clear identifi-cation and correlation to the inspection documentation.

A.5.7.2 Products which cannot be clearly identified shall be rejected, unless the properties stipulated can be determined by both re-inspection and re-testing, see also B.5.2.

A.6 Non-welded steels and bolts

The properties of non-welded steels are determined by the categories of structural members. Depending on the structural member category and the design temperature to be employed for the structure, have to meet the following requirements listed in A.4 and A.5: • Owing to the risk of hydrogen induced stress cracking (HISC), steels with SMYS of > 500 MPa shall

not be used for critical cathodically protected components without special considerations, i.e. in the submerged zone of fixed offshore structures or mobile offshore units, see Section 5. This imposes some restrictions: a) any construction method affecting ductility or tensile properties shall be carried out according to

a qualified procedure which limits hardness to 300 HB (or 32 HRC), b) this restricts the use of structural steels with SMYS of 690 MPa and bolts with class 8.8 accord-

ing to ISO 898. • The minimum Charpy V-notch impact properties in accordance to both Tables 4.10 and 4.11, but in-

dependent of the material thickness at Charpy V-notch test temperature for ≤ 12.5 mm. • Bolts with a diameter above 25 mm shall be impact tested to the same requirements as for non-

welded steels be bolted, see also B.10.11.

B Fabrication

B.1 General

B.1.1 Fabrication, other than welding, shall be in accordance with a national or regional fabrication specification that complements the design code used. Fabrication tolerances shall be compatible with design assumptions, except where specific service requirements dictate the use of more severe control over the deviations assumed in the design.




B.1.2 The minimum requirements for the fabrication and construction of steel structures are defined herein. These requirements analogously apply to work carried out on the steel structure prior to or during transport, installation and operation and for repairs. For welding, see C and for inspection and testing, see D.

B.2 Standards and specifications

The application of relevant codes, standards, guidelines, specifications, etc. (e.g. AWS D 1.1, NORSOK M-101 or EEMUA No. 158) may be agreed by GL. Should such codes, standards, guidelines, specifica-tions, etc. contradict the present rules then the latter shall take precedence over all others.

B.3 Fabrication specifications, schedule

Prior to commencement of the work the contractor shall submit specifications containing the main aspects of fabrication and construction to GL for approval.

Additionally following items are required: • welding procedure specifications (WPSs) • NDT specifications • related drawings and/or descriptive lists/tables with weld details and NDT requirements • inspection and testing plans (ITPs)

B.4 Quality assurance and control

B.4.1 Manufacturers will be responsible for compliance with these Rules, with the approved draw-ings and specifications as well as with any other conditions which may be agreed or stated in the approv-als. The inspections performed by GL do not relieve manufacturers of their responsibility.

B.4.2 Manufacturers are to introduce a quality assurance system which ensures that the materials, fabrication and construction are in accordance with these Rules.

The manufacturer shall have an implemented and documented quality system with minimum require-ments of ISO 3834-2 for special and primary members and of ISO 3834-3 for secondary members.

B.4.3 The quality control shall be performed by the competent personnel with defined responsibilities covering all aspects of quality control including welding and NDT. The quality assurance department shall be independent from the manufacturing department.

An organization chart, including proof of qualification of the personnel in charge of control, is to be submit-ted to GL.

B.4.4 During fabrication and construction adequate controls shall be carried out by the contractor, inter alia, as to: • the storage and marking of materials • the preparation and assembly of components • the accuracy to size of the components and the overall structure • faults and deficiencies shall be corrected before painting or other means of permanent covers have

been applied

B.4.5 The quality assurance is also to cover all work which may affect the integrity and strength of the structure, including the fitting of auxiliary structures and equipment to the structure, fairing and repair work, as well as corrosion protection measures.

B.4.6 All quality assurance measures are to be recorded, thus proving the kind, scope and results of the checks. A summary analysis of results is to offer a clear picture of the progress of work, including possible rework and controls. The records and analysis are to be presented to the GL Surveyor for ap-proval at reasonable intervals.




B.5 Identification and storage of materials

B.5.1 All materials (plates, girders, shapes, tubes, etc.) or other types of shape for main structural parts (special and primary) are to be clearly marked as to enable them to be definitely identified on the basis of the pertinent Certificates.

B.5.2 Non-identified materials are to be rejected, unless compliance with these Rules and/or the approved specifications has been verified by renewed testing. The number and type of tests shall be in accordance with A.4.9.2 or A.5.7.2, respectively.

B.5.3 Manufactures shall have a system, to trace the material within the structure. During each stage of manufacture correlation of the pertinent material Certificates to each individual component shall be possible.

B.5.4 All materials and structural members are to be stored and handled such as to exclude their surface finish and properties being unduly affected. For the storage of welding consumables and auxiliary materials, see C.2.3.

B.6 Deviations, defects and repair work

B.6.1 Any deviation from the Rules, approved drawings and specifications or conditions agreed or stated in the approvals require prior approval by GL. The same applies analogously to major repairs. A proposal for intended repair work is to be prepared by the manufacturer and to be presented to GL for approval together with a report on defects. Repair work shall be started only following approval of the repair procedure by GL. For admissible tolerances, see B.15.

B.6.2 For any such substitutions and revisions made during fabrication, suitable records shall be documented by the fabricator and listed as corrections to the fabrication drawings.

B.6.3 Following types of documents are required: • Non-conformity Report (NCR) • Technical Query (TQ) • Site Query (SQ)

Whenever such a document was raised by the contractor the approval of the Owner/Purchaser and GL Surveyor shall be mandatory. In certain major cases, i.e. involvement in special and/or primary structural elements, the approval of GL HO is required.

B.6.4 The responsibility for the compilation of these records with other documentation related to the construction and inspection of the structure and the retention of these permanent records shall be speci-fied by the Owner/Purchaser.

B.7 Surface and edge preparation of both materials and structural members

B.7.1 When applying thermal (cutting) procedures, the possibility of hardening of the basic material and – depending on the materials employed – variation of the strength and/or toughness values and are to be taken into account.

B.7.2 Excessive flame cutting drag lines, (e.g. due to burner failure) are to be ground notch-free with smooth transitions. Where possible, weld repairs are to be avoided and require approval by the GL Sur-veyor.

B.7.3 Plates or tubes shall be prepared by machining or by thermal cutting (flame cutting, plasma cutting, etc.) followed by mechanical cleaning where appropriate in accordance with an internationally accepted standard. The same applies analogously to shear cuts, the fins of which are to be rough-planed in all cases.

Surfaces to be welded shall be visually examined and to be free of any kind of defects, details see C.6.3.10.




B.7.4 Flame cut edges that are not subsequently welded shall be ground to remove nicks and burrs. The use of flame gouging or “washing” is not permitted.

B.7.5 Where practicable, penetrations through main members (e.g. for piping or cables) are to be drilled. Flame-cut penetrations shall, under all circumstances, be completely ground notch-free. The edges are to be chamfered or rounded off.

B.7.6 The surface of piles and followers shall be untreated bare metal, free from mill varnish, oil and paint, except for specific markings. The area of the surface used for markings shall be minimum required consistent with adequate identifications.

B.8 Cold and hot forming

B.8.1 Cold forming (i.e. forming below 250 °C) resulting in permanent deformation (elongation) by more than 5 % requires strain ageing test of the base material with retesting of mechanical properties, see A.4.6.

B.8.2 The forming of T-sections into flanged ring stiffeners of any diameter may require mechanical tests to demonstrate that the properties required by the application are maintained after forming and welding.

Acceptance of wedges or other techniques to assist rolling will be subject to the visual acceptability of the product and to acceptable mechanical test results from regions of maximum strain.

B.8.3 In opposite to TMCP steels it is possible to recover the original properties by normalizing or quenching and tempering in case of N or QT steels. The mechanical tests in the final condition shall be carried out. Hot forming is not allowed for TMPC steels.

B.9 Splices

B.9.1 General

Splices shall be fabricated using full penetration welds and subject to the following restrictions. The loca-tion of all splices shall be shown in both design and as-built drawings.

B.9.2 Pipes

Pipe splices should be in accordance with the requirements of API Specification 2B, Section 5.4 to 5.6. Pipes used as beams shall comply with the requirements of B.9.5.

B.9.3 Ring stiffeners

Ring stiffeners shall be spaced at least 100 mm from circumferential seams. Where this is not possible, the welds shall overlap by at least 10 mm. Exceptions are rings used to stiffen cone-cylinder junctions which shall not be offset and the welds shall be overlapped by at least 10 mm to avoid coincident location of weld toes.

B.9.4 Tubular joints

In tubular joints, circumferential and longitudinal weld joints should not be placed in the shaded areas, see in both Fig. 4.1, unless otherwise shown on design drawings.

Circumferential welds are not permitted within cones and node stubs unless specified on design or ap-proved shop drawings.




Fig. 4.1 Prohibited locations of welds in tubular joints

B.9.5 Beams

Segments of beams with the same cross-sections may be spliced. Either straight or staggered splices may be used for beam or plate splices.

Splices in plate girders and sections shall not be less than one meter or twice the depth of the beam apart whichever is greater.

Longitudinal stiffeners shall be spaced at least 100 mm from longitudinal seams.

For staggered splices, splices in girder and column flanges shall be spaced at least 100 mm from the web splice.

B.9.6 Piles

B.9.6.1 General

The surface of piles and followers shall be bare metal, free from mill varnish, oil and paint, except for spe-cific markings. The area of the surface used for markings shall be the minimum required consistent with adequate identification.

B.9.6.2 Pile-to-sleeve connections

If forces to be transferred from steel member to steel member across the steel to grout interfaces and through the grout, specific preparations of sleeves and piles to be performed and certain shear keys to be




welded outside of the piles and inside of the sleeves. The spacing between both shall be about 40 mm, depending of the type of grout used; details see Figures 4.2a and 4.2b.

Welding and testing requirements see C.3.9.

!

!

!

"

" #$ %! ! ! #

Fig. 4.2a Grouted connections

& # #

Fig. 4.2b Type of shear keys




B.10 Fitting and assembly

B.10.1 General

Excessive jamming is to be avoided during fitting and assembly. Major deformations of individual mem-bers are to be faired prior to further assembly. The welding-on of assembly aids is to be restricted to a minimum. For weld preparation, assembly and tack welding, see C.6.3.

B.10.2 Spliced components

Components which are to be spliced shall be aligned as accurately as possible. Special attention shall be paid to the alignment of (butting) members, which are interrupted by transverse members. If necessary, such alignment shall be facilitated by drilling check holes in the transverse members which are later seal-welded. The fit-up is to be checked before welding.

For allowable tolerances, see B.15.

B.10.3 Slotted members

When members are slotted to receive gusset plates, the slot should be 305 mm or twelve times the mem-ber wall thickness, whichever is greater, from any circumferential weld. To avoid notches the slotted member should be drilled or cut out and ground smooth at the end of the slot with a diameter of at least 3 mm greater than the width of the slot. Where the gusset plate passes through the slot, the edge of the gusset plate should be ground to an approximately half round shape to provide a better fit-up and welding condition.

B.10.4 Trial fitting, maximum root gaps

All components should be trial fitted before welding and grinding or buttering of the weld preparation at the installation site should be avoided for special and primary steel fabrication. Where root gaps are out of tolerance, the joint shall be dismantled to permit local grinding or buttering following by grinding, back to the required profile see also C.6.8. For special and primary members the maximum build-up shall be 8 mm and the maximum root gap 10 mm. For parts outside of the areas specified in A.1.3 the maximum root gap may be applied in accordance to IACS REC. No.47, i.e. 1.5·t or max. 25 mm for butt joints and 16 mm for T-joints. Alternatively the requirements of B.6.1 may be applied to establish an appropriate repair procedure.

B.10.5 Joining of different thicknesses

Where butt welds are to be made between members of different thickness, a transition using a taper of 1 : 4 shall be provided. This taper may be wholly or partially within the width of the weld. When neces-sary, the thicker member shall be tapered, details to be given in relevant drawings and/or standards, see also both Figs. 4.30 and 4.31.

B.10.6 Joints in the deck plating

Joints in grating and mechanical joints in deck plates shall be at support points unless the design includes other appropriate details. Welded joints in deck plate should preferably offset from welds to the supporting structures. Mismatches greater than 2 mm at grating and deck plate joints shall be ground down and re-coated.

B.10.7 Cope-holes (rat-holes), penetrations and other cut-outs

Cope-holes, penetrations and other cut-outs shall be avoided wherever possible. Cope-holes may be permitted provided they are approved on shop drawings and the following criteria shall apply: • Permanent cope-holes shall have a radius of 50 mm or twice the plate thickness, whichever is lar-

ger, and shall be ground smooth. Where cope-holes of these dimensions would jeopardize structural integrity cope-holes less than this radius may be considered. Fillet welds shall be returned through the cope-holes. The cope-hole corners to be sealed by continuous fillet weld.

• Temporary cope-holes (those approved subject to their being reinstated or sealed) shall have a ra-dius not less than twice the plate thickness and shall be reinstated to an approved procedure. Filling with weld metal is not permitted.

• The radius of any approved cut-outs other than cope-holes shall not be less than 100 mm. Where necessary, the area shall be checked by ultrasonic testing before cutting to ensure that no cut is




made within 300 mm of internal stiffeners. The original plate shall be clearly marked and shall be welded back into the cut-out, unless other proposals are agreed.

B.10.8 Tubular structural members

Tubular members framing into joints shall be carefully contoured to obtain accurate alignment. The bevel shall be formed providing a continuous transition from maximum to minimum bevel angel around the cir-cumference. Generally, the fabrication shall be planned in such a manner that back welding can be per-formed to the largest extent possible.

B.10.9 Alignment and tack welds

Members to be welded shall be brought into correct alignment and held in position by clamps, other suit-able devices or by tack welds until welding has been completed or progressed to a stage where the hold-ing devices or tack welds can be removed without danger of distortion, shrinkage or cracking. Suitable allowances shall be made for distortion and shrinkage, where appropriate.

Tack welds shall have a minimum length of 50 to 75 mm and number of tacks is to be sufficient to avoid cracking. Temporary tack welds using bridging or bullets, shall only be performed using materials equiva-lent to the base material and using a WPS based on a qualified welding procedure. All such tack welds and any spacer wedges shall be removed from the final weldment, see also C.6.3.

Tack welds to be fused into the weld shall be made in the weld groove only and the ends of the tack welds shall have their ends ground.

B.10.10 Check of fit-up and before welding

The fit-up shall be checked for dimensional accuracy before welding. Surfaces to be welded shall be free from mill scale, slag, rust, grease, paint etc. Edges are to have smooth and uniform surface. No welding shall be performed when the surfaces are damp. Suitable protection shall be arranged when welding is performed during inclement weather conditions. The groove shall be dry at the time of welding.

B.10.11 Bolting connection

Bolting material shall comply with the requirements in A.6 and GL Rules for Steel and Iron Materials (II-1-2), Section 6, C. Following requirements shall apply additionally to A.6: • all details including material specification and drawing to be approved by GL • holes shall be made by machine drilling • bolts and nuts shall not be welded under any circumstances • the hardness shall be verified by spot hardness testing of each delivery, batch and size of bolts used • for submerged applications phosphating shall be used • methods to prevent loosening of the fasteners shall be considered • threaded fasteners should normally be pre-tensioned

B.10.12 Seal/blind compartments

Crevices and areas which become inaccessible after fabrication or assembly shall be sealed-off from the outside atmosphere. Seal welds have a throat thickness of at least 3 mm. Where steel items shall be hot dip galvanized, hollow sections shall be ventilated.

B.10.13 Temporary cut-outs

Temporary cut-outs shall not be located in restricted areas as shown on design drawings. Temporary cut-outs shall have a corner radius not less than 100 mm. Temporary cut-outs shall be closed by refitting the same or an equivalent plate and employing the same welding, inspection and documentation procedures and requirements that govern the structural part in question.

B.10.14 Doubler plates

All temporary attachments which shall be flame-cut or welded under water shall be attached to the struc-ture by using doubler plates.

All attachments in the extended tidal zone shall be attached to the structure by using doubler plates.




B.10.15 Grinding details for High Fatigue Utilisation

When grinding is specified on design drawings or instructed as a corrective action, the grinding shall be performed according to a detailed procedure, see also A.4.5.4.4. Grinding tools, direction, surface rough-ness and final profile shall be specified.

B.11 Weather protection

Prefabrication is to be performed as far as possible in weather-protected workshops. For assembly the components and in particular the areas, in which welding is performed, are to be protected by provisional coverings against the effects of weather (e.g. rain and wind). For welding under low ambient tempera-tures and pre-heating, see C.6.4.

B.12 Removal of temporary attachments and auxiliary materials

B.12.1 Clamping plates, aligning ties, scaffolding and other auxiliary materials temporarily welded on structural members, shall be limited as much as practicable.

B.12.2 Where attachments are necessary, the following requirements shall apply: • Temporary attachments shall not be removed by the hammering or arc-air gouging. Attachments to

leg joint cans, skirt sleeve joint cans, brace joint cans, brace stub ends and joint stiffening rings shall be flame-cut to 3 mm above parent metal and mechanically ground smooth flush to the parent metal.

• The ground area shall be visually inspected and MT shall be performed in accordance with the in-spection category in question. If any further welding is required, the testing has to be executed be-fore the permanent weld is executed, see also C.6.3.6.

• Attachments to all other areas, not defined above, shall be removed by flame cutting just above the attachment weld (maximum 6 mm above weld); the remaining attachment steel shall be completely seal welded.

• Attachments to aid in the splicing of legs, braces, sleeves, piling, conductors, etc. shall be removed to a smooth and flush finish.

• Attachments on all areas that will be painted shall be removed as above, prior to any painting.

B.12.3 If during rough-planning the surface of the structural members is damaged, the areas con-cerned are to be ground both flat and notch-free and subsequently checked for cracks by MT. Where possible, repair by welding is to be avoided; otherwise, approval by the GL Surveyor is required.

B.13 Flame straightening

B.13.1 Flame straightening shall neither impair the properties of the materials nor of the welded joints and shall be avoided completely, where practicable. In case of doubt, proof of satisfactory performance of thermal treatment may be demanded.

B.13.2 Members distorted by welding shall be straightened according to a detailed work instruction.

B.13.3 The following flame-straightening temperatures for normalized, thermo-mechanical rolled and quenched and tempered delivery conditions are recommended maximum values, see Table 4.18.

Table 4.18 Flame-straightening temperatures

Short superficial heating

Short full section heating

Full section heating with longer holding time Delivery condition

[°C] [°C] [°C]

Normalized ≤ 900 ≤ 700 ≤ 650




Thermo-mechanical rolled up to S460 ≤ 900 ≤ 700 ≤ 650

Thermo-mechanical rolled S500 to S690 ≤ 900 ≤ 600 ≤ 550

Quenched and tem-pered

≤ Tempering temperature applied to the original product – 20 °C (generally below 550 °C)

B.14 Heat treatment

If heat treatment is required, it is to be performed in accordance with a written procedure specification, listing in detail: • structural member, dimensions, material and supply condition • heating facilities and insulation • control devices and recording equipment • heating and cooling rates (temperature gradients) • holding range of temperature and time

The specification is to be submitted to GL for approval. For post-weld heat treatment, see C.6.9.

B.15 Fabrication tolerances

B.15.1 General

B.15.1.1 Allowable fabrication and construction tolerances shall be specified depending on the signifi-cance of the structural members and the loads acting on them. They shall be submitted to the Owner/Purchaser for acceptance. The resulting specification is to be submitted to GL for approval. A gen-eral guidance for allowable tolerances is given in B.15.2 to B.15.19.

For weld imperfections, see C.6.6 and Fig. 4.1. In case of fatigue assessment the detail classification of the different welded joints as given in Table 3.14 is based on the assumption that the fabrication of the structural detail or welded joint, respectively, corresponds in regard to external defects at least to quality group B according to ISO 5817 and in regard to internal defects at least to quality group C.

B.15.2 Length tolerances

The length tolerances are defined in Table 4.19.

Table 4.19 Length tolerances according to ISO 13920

Nominal length [mm]

Special and primary structures [mm]

Secondary structures [mm]

2 – 30 ±1 ±2

120 - 400 ±1 ±2

401 – 1000 ±2 ±3

1001 – 2000 ±3 ±4

2001 – 4000 ±4 ±6

4001 – 8000 ±5 ±8

8001 – 12000 ±6 ±10




12001 – 16000 ±7 ±12

16001 – 20000 ±8 ±14

Above 20000 ±9 ±16

B.15.3 Misalignment of butt joints

The misalignment e shall not exceed the following, whichever is less, see Fig. 4.3:

e ≤ 0.10 × t1 for special structures, max. 3 mm

e ≤ 0.15 × t1 for primary structures, max. 4 mm

e ≤ 0.30 × t1 for secondary structures, max. 6 mm, tapering 1 : 4 required

e, t : Dimensions [mm]

Fig. 4.3 Definition of misalignment at butt joints

B.15.4 Misalignment of cruciform joints

The misalignment e shall not exceed the following, see Fig. 4.4

e ≤ 0.15 × t for special structures

e ≤ 0.30 × t for primary structures

e ≤ 0.50 × t for secondary structures

t is the smallest thickness of t1, t2 and t3.

e, t1, t2, t3 : Dimension [mm]

Fig. 4.4 Definition of misalignment at cruciform joints

B.15.5 Roundness of tubular members

B.15.5.1 The external circumference shall not differ from the design or calculated circumference by more than +0.3 % of the specified external diameter.

B.15.5.2 The difference between the measured maximum and minimum diameters (out of roundness) shall not exceed 1 % of the specified diameter or 6 mm, whichever is less.

B.15.5.3 The maximum deviation from the nominal diameter, measured at a stiffener or a bulkhead shall not exceed 0.5 %.

B.15.5.4 Lower values and/or additional limitations may be required for masts, crane pillars and similar components of lifting appliances.




B.15.5.5 The global and local tolerances for circular cylinder shells (e.g. of flotation tanks, buoyancy structures, etc.) exposed to outer pressure are to be calculated and specified in each single case depend-ing on maximum pressure, diameter to wall thickness ratio and stiffener distance to diameter ratio.

B.15.5.6 For limitations regarding buckling, see Section 3, H

B.15.5.7 For deviations of longitudinal stiffeners, see both B.15.10 and B.15.11.

B.15.5.8 Local deviations out of roundness and out of straightness

The measuring length m of a straight rod or of a curved template shall be according to both Fig. 4.5 and 4.6.

The deviation out of the straightness or out of roundness shall be e ≤ 0.01 · m. Where for straight rods:

m 4 r t= ⋅ ⋅

for curved template m is the smallest of:

m s=

m2.3 r

r rt

⋅=

⋅

or: m r2= π ⋅

with:

r : radius of shell curvature

: unsupported length (i.e. length between bulkheads or ring stiffeners)

s : stiffener spacing of longitudinal stiffeners

t : plate thickness of shell

s, e, , m, r, t : Dimensions [mm]

'

(

Fig. 4.5 Measuring length m of straight rod




Fig. 4.6 Measuring length m of curved template

B.15.6 Straightness of (prefabricated) tubular members, girders or beams

The deviation of straightness e shall be ≤ 0.001 · L or 3 mm, whichever is greater, for compression mem-bers, e.g. columns, primary truss members and girders or beams with non-hollow cross sections, see Fig. 4.7.

The deviation of straightness shall be e ≤ 0.002 · L or 5 mm, whichever is greater, for girders or beams with hollow cross sections and for all other applications.

L : unsupported length [mm]

)

Fig. 4.7 Definition of straightness

B.15.7 Deviation from the theoretical axis in tubular nodes

If the specified (designed) eccentricity x is less D/4, the deviations are defined as follows, see Fig. 4.8: • x as built < x designed (x1 in Fig. 4.8):

e1 ≤ x1, but separation requirement of 50 mm minimum to be observed, see Fig. 4.8

• x as built > x designed (x2 in Fig. 4.8):

e2 ≤ 0.1 · x2 [mm], maximum 50 mm

If the specified (designed) eccentricity x > D/4, which has to be considered in the strength calculations, the deviations are:

e1,2 < 0.1 · x1,2 [mm], maximum 50 mm

separation requirement of 50 mm minimum to be observed, see Fig. 4.8.

*

+

+

!

++

Fig. 4.8 Deviations at tubular nodes




B.15.8 Bulges in plating

The deviation e of bulges in plating from straight line are defined as follows, see Fig. 4.9:

e ≤ 0.004 · s

s = stiffener distance

e, s : Dimensions [mm]

Fig. 4.9 Definition of bulges in plating

B.15.9 Deformation of plane, stiffened plating

The permissible deviation e of deformations in plane, stiffened plating from straight line is defined as fol-lows, see Fig. 4.10:

e ≤ 0.0015 · L

e, L : Dimensions [mm]

)

Fig. 4.10 Definition of plating deformation

B.15.10 Lateral miss-positioning or deflection of stiffeners between supporting members

The permissible lateral miss-positioning or deflection of stiffeners between two supporting members is defined as follows, see Fig. 4.11:

e1 ≤ 1 + 0.015 · L [mm], maximum 5 mm

e2 ≤ 1 + 0.010 · h [mm], maximum 3 mm

e1, e2, L, h : Dimensions [mm]

Smaller values may be required for reasons of buckling strength.

)

Fig. 4.11 Definition of deviations of stiffeners between two supports




B.15.11 Deviation and/or distortion of girders

The permissible deviation and/or distortion of girders is defined as follows, see Fig. 4.12:

e1 ≤ 1 + 0.01 · h [mm], maximum 3 mm

e2 ≤ 0.01 · h [mm], maximum 2 mm

e1, e2, h : Dimensions [mm]

The permissible deflection of the face plate to the girder plane is defined as follows, see Fig. 4.12:

e3 ≤ 1 + 0.01 · b [mm], maximum 3 mm

e4 ≤ 0.01 · b [mm], maximum 2 mm

e3, e4, b : Dimensions [mm]

Smaller values may be required for reasons of buckling strength.

"

% %

Fig. 4.12 Definition of deviations of girders

B.15.12 Lateral deflection and inclination of (as built) columns

The lateral deflection e1, see Fig. 4.13, shall be in accordance with the requirements defined in B.15.6, if L = H [mm].

The permissible deflection e2 shall be less or equal 0.0015 · H.




-

$

-

$

-

$

.

Fig. 4.13 Definition of column deviations

B.15.13 Deviation of box girders

The tolerances for both box girders and box girder stiffeners shall comply with the following additional requirements, see Fig. 4.14 a) to d): • Maximum deviation: e1 = + 3 mm (see Fig. 4.14a))

• Out of straightness: e2 = + 0.0015 · L; max. 10 mm (see Fig. 4.14b))

• Buckling of plates: e3 = + 0.005 · W or (0.005 · H); max. 10 mm (see Fig.4.14 c))

• Twisting of girder: e4 = + 0.005 × H; max. 6 mm (see Fig.4.14 d))

e1, e2, e3, e4, H, L, W : Dimensions [mm]

/

0

%

)

%

(

.

.

"% %

% 1 % % 2 % 3 Fig. 4.14 Deviation of box girders




B.15.14 Global horizontal tolerances

Following main tolerances for leg spacing at plan bracing levels shall apply, see Fig. 4.15 a) and b): • the horizontal centre to centre distance between adjacent legs at the top of a structure where a deck

or other structure is to be placed (stab-in-nodes) shall be within ± 10 mm of the design values • the horizontal centre-to-centre distance between legs at other locations shall be within ± 20 mm of

the design values • the horizontal centre-to-centre diagonal distances between legs at the top of a structure where a

deck or other structure is to be placed (stab-in-nodes) shall be within ± 10 mm of the design values • the horizontal centre-to-centre diagonal distance between legs at other locations shall be within ±

20 mm of the design values

L : Dimension in [mm]

)4

)4

)4)4

% 3 5 % 2 Fig. 4.15 Global horizontal tolerances

B.15.15 Global vertical tolerances

Following vertical tolerances for plan bracings shall apply, see Fig. 4.16: • the elevation of the top of legs shall be within ± 13 mm of the design values • the vertical level of braces within a horizontal plane shall be within ± 13 mm of the design values • the vertical distance between plan bracing elevation shall be within ± 13 mm of the design values • each leg shall be straight to within ± 10 mm over the entire length

all Dimensions [mm]




"

)4 4

4

4

"

Fig. 4.16 Vertical tolerances

B.15.16 Tolerances on straightness of tubular members assembled into structure

Following tolerances for the straightness of tubular members, when assembled into the structures, shall apply, see Fig. 4.17: • brace members shall be straight to within e ≤ 0.12 % of the nominal length between work points L1

• chord members shall be straight to within e ≤ 0.10 % of the nominal length measured between the work points of major framing members L2

L1, L2: Dimensions [mm]

(6

(6

)

)

(6

(6

(67(

% 8 % 9 Fig. 4.17 Tolerances on straightness of tubular members assembled in to structure




B.15.17 Leg alignment tolerances at plan bracing level (structures with more than 4 legs)

Following tolerances for the alignment and straightness of legs shall apply, see Fig. 4.18: • at each plan bracing level the legs shall be aligned horizontally within ± 10 mm of the design location

each leg shall be straight to within ± 10 mm over the entire length, see both B.15.15 and Fig. 4.16

all Dimensions [mm]

4

Fig. 4.18 Leg alignment tolerances at plan bracing level (structures with more than 4 legs)

B.15.18 Conductor guide alignment tolerances

Following tolerances for conductors, pile guides, pile sleeves and other appurtenance supports shall ap-ply, see Fig. 4.19: • the distance e1 to the best-fit line shall be less than 10 mm at the top guide

• for vertical spacing between guides of less and equal than 12 m, the distance e2 to the best-fit shall be less than 10 mm

• for vertical spacing between the guides of more than 12 m, the distance e3 to the best-fit shall be less than 13 mm

e1, e2, e3 : Dimensions [mm]




"

" 5

Fig. 4.19 Conductor guide alignment tolerances

B.15.19 Tolerances for trusses

Tolerances for trusses are to be specified analogously to those given in B.15.7 (deviation from the theo-retical axis in tubular nodes), B.15.11 (deviation and/or distortion of girders) and B.15.12 (lateral deflec-tion and inclination of as built columns). Special consideration is to be given to areas of introduction or deviation of forces, e.g. at stabbing points or connections of crane pedestals.

C Welding

C.1 General requirements

C.1.1 Scope, welding rules

These Rules define the general conditions for the preparation and performance of all welding work includ-ing quality assurance measures. It is taken for granted that all further details of welding (not listed here) will be laid down in the welding specifications analogously to the GL Rules II – Materials and Welding, Part 3 – Welding, in the following referred to as “GL Welding Rules”, or to other codes, standards, etc. recognized by GL.




C.1.2 Codes and standards

C.1.2.1 The codes, standards, etc. mentioned below or in the GL Welding Rules constitute an integral part of these Rules and require no special consent. The version current at the time when these Rules are issued shall apply. New editions of the codes, standards, etc. may be used in agreement with GL.

C.1.2.2 The application of other codes, standards, etc. may be agreed by GL. They are to be listed in the construction particulars, e.g. in the welding specifications, and presented to GL on request. The actual version of these standards shall apply; otherwise the year of publication shall be indicated.

C.1.2.3 If any of these codes, standards, etc. contradict these Rules, the latter shall take precedence over all the others. Any deviations require approval by GL from case to case.

C.1.3 Terms and definitions

The terms and definitions as contained in recognized codes, standards, etc. are to be employed. The code or standard used is to be indicated in the design and/or construction documentation and presented to GL on request. Any other deviating terms and definitions are to be separately explained in the welding specifications.

C.1.4 Welding works equipment

The manufacturers shall have suitable equipment and plant for the satisfactory performance of welding work. Apart from perfectly functioning cutting and welding machines and tools, this equipment shall in-clude dry, heatable storage facilities, baking ovens and heated quivers for the welding consumables, and equipment for preheating, heat treatment, calibration and temperature control.

C.1.5 Welding supervision

The preparation and performance of welding operations shall be supervised by specially trained and ex-perienced welding supervisors. Proof of their professional qualifications is to be furnished to GL; their tasks and responsibilities are to be defined and made known to GL. For more details, see GL Welding Rules.

C.1.6 Welding processes

The welding processes in accordance to ISO 15614-1 shall apply unless otherwise specified.

C.1.7 Performance of qualification tests

Welding qualification tests are to be performed under the supervision of GL on the manufacturers’ prem-ises. Fabrication conditions shall be simulated as far as possible. Non-destructive and destructive tests may – in agreement with GL – be performed on the premises of another independent, neutral testing body.

C.1.8 Documentation

The manufacturer shall both produce and submit to GL prior to production a documentation covering the main aspects of welding work as follows: • drawings showing all weld details • welding procedure specifications (WPS) • welding consumables handling procedures • non-destructive testing specifications • welding procedure qualification records (WPQR) • welding supervisors’ qualification in acc. to ISO 3834-2 • welder's qualification Certificates acc. to to EN 287-1 and/or ISO 9606-4 • welding operator's Certificate acc. to ISO 14732 • welders’ identification system




C.2 Welding consumables

C.2.1 Approval of welding consumables

C.2.1.1 All welding consumables and auxiliary materials (covered electrodes, wire electrodes, wire-gas combinations or wire-flux combinations) intended to be used on offshore steel structures shall be approved by GL in accordance with GL Welding Rules for the respective range of application (base mate-rials, welding positions, heat treatment condition, etc.).

C.2.1.2 Welding consumables and auxiliary materials not approved by GL according to GL Welding Rules may be qualified – with special agreement by GL – in the course of the welding procedure qualifica-tion tests. In this case, one round tensile test specimen is to be additionally taken lengthwise, mainly from the weld metal of the butt welds.

Such qualifications are, however, limited to the user’s work in the range of application, according to the approved welding procedure specifications, and remain valid for not longer than one year, unless repeat tests are carried out in accordance with GL Welding Rules.

C.2.2 Selection of welding consumables

C.2.2.1 The welding consumables and auxiliary materials are to be chosen such that in the weld con-nection, including heat affected zone (HAZ) at least the same mechanical properties as those specified for the base materials are achieved. When different steels are to be joined, the welding consumables and auxiliary materials are to be chosen based on the lower yield strength.

C.2.2.2 Welding consumables and auxiliary materials with a controlled low hydrogen content in the weld metal, e.g. H5 according to ISO 3690, shall be used for welding of higher and high strength steels (yield strength > 275 MPa), for all steel having a carbon equivalent > 0.42 % and are recommended for all highly stressed and thick walled components exposed to low temperatures as well as for steel forgings and castings.

Welding consumables and auxiliary materials other than low hydrogen types may only be used with spe-cial agreement by GL, except for secondary structures.

C.2.3 Storage and re-drying of welding consumables

C.2.3.1 All welding consumables and auxiliary materials (except shielding gases) are to be stored, as far as possible in the following way: • in original packages • in a storage room, where a minimum temperature of 18 °C and a humidity of the atmosphere of less

than 60 % is maintained • air circulation of the room is required • storage of packages preferably in shelves or on pallets, but not directly on the bottom • the welding consumables and the auxiliary materials shall be properly identifiable up to their final

use • general storage time not longer than 2 years • special packages (tin or vacuum) are without any time limitation • the manufacturer’s recommendations are to be prevailed

Prior to use, low hydrogen covered electrodes, folded flux core wire electrodes and fluxes are to be re-dried (baked) in accordance with the manufacturer’s instructions (observing the maximum baking time specified).

Unless otherwise specified, the baking temperature should be 250 °C to 350 °C, the baking time > 2 hours, but not longer than 12 hours in total. Repeated baking is permissible, as long as the maximum drying period of 12 hours is not exceeded for basic type.

C.2.3.2 At the working area, the welding consumables and auxiliary materials are to be protected against the effects of weather (moisture) and contamination. Low hydrogen covered electrodes and fluxes are to be kept in heated quivers at temperatures between 100 °C and 150 °C. All unidentifiable, dam-




aged, wet, rusty or otherwise contaminated welding consumables and auxiliary materials are to be dis-carded.

C.3 Welding procedure specification and qualification

C.3.1 General

Detailed welding procedures shall be prepared for all welding covered by these Rules. The welding pro-cedure may be based on previously qualified welding procedures provided that all the specified require-ments can be fulfilled.

C.3.2 Welding procedure specification (WPS)

A welding procedure specification (WPS) is a specification based on a welding procedure qualification record (WPQR) and accepted in accordance with those requirements. The WPS is the preliminary weld-ing procedure specification (pWPS) revised to reflect the welding and variables qualified by the welding procedure tests. All production welding shall be performed in accordance with a WPS.

WPS are to be prepared and submitted to GL for review. The WPS are to contain all essential variables on the preparation and performance of welds, such as: • applicable standards • materials (grade, standard, delivery condition, CEV-ranges, thickness range, outside diameter) • which range of structural categories shall apply (above or below LAT) • joint and grove design (joint type, single or multiple, root opening, root face, groove angle) • joint preparation (cutting method) and tacking • cleaning method • welding process and parameters (for root, hot, fill and cap layers) • welding consumables and auxiliary materials (manufacturer, trade name, classification) • gas shielding, flow rate • welding positions and progressions • preheating, interpass temperatures • heat input • weld build-up (welding sequence) • root treatment (back gouging) • post-weld (heat) treatment • if the WPS shall be used for submerged parts, that may be exposed to cathodic protection (below

LAT), than this fact shall be noted, see A.3.5.

It is recommended to prepare a list with all welding procedure specifications used in the project and all procedures to be attached with detailed information for new as well as for repair welds. Inspection re-quirements are subject to the relevant specifications and/or ITPs, see also F.4.2.

C.3.3 Preliminary welding procedure specification (pWPS)

A preliminary welding procedure specification (pWPS) shall be prepared for each new welding procedure qualification. The pWPS shall specify the ranges for all relevant parameters.

The pWPS shall be submitted for review and acceptance by the Owner/Purchaser prior to commencing the welding procedure qualification, see also C.3.10.3.

C.3.4 Welding procedure qualification (WPQ)

C.3.4.1 The manufacturers shall prove their ability to apply the specified welding procedures in a suffi-cient manner and with satisfactory results, in conjunction with the materials and welding consumables actually used. Such proof is to be provided prior to start of fabrication.




C.3.4.2 Unless otherwise agreed, a welding procedure qualification (WPQ) shall be proved by a weld-ing procedure qualification test (WPQ-Test) under actual or simulated fabrication conditions and wit-nessed by GL.

C.3.4.3 In exceptional cases, GL may recognize in part or in whole existing welding procedure qualifi-cations based on tests witnessed and approved by another competent and independent Authority. There-fore, adequate documentation, including complete test reports, to be submitted to GL.

C.3.4.4 The approval for a welding procedure qualification is restricted to the manufacturer and to the welding works or working unit (e.g. platform, jacket, barge, etc.), where the test welds were performed. The approval is valid within the limitations of the essential variables according to C.3.7.

C.3.4.5 The approval for the use of the qualified procedure is normally valid for the fabrication period of the structure. Where the fabrication has been interrupted for more than 3 months, GL may require pro-duction tests or repeated welding procedure qualification tests.

C.3.4.6 Production tests or repeated welding procedure qualification tests may also be required if the conditions under which the approval was granted (e.g. personnel premises) do not longer apply, or in the event of doubts as to satisfactory execution of the welding.

C.3.5 Welding procedure qualification tests

Welding procedure qualification tests are to be carried out in accordance with ISO 15614-1 and the addi-tional requirements of this Section, specified in both Tables 4.20 and 4.21, or– with the consent of GL – subject to other recognized codes or standards (e.g. AWS D 1.1, NORSOK M-101 or EEMUA No.158).

Table 4.20 Welding procedure qualification tests - types and number of tests

Mechanical-technological tests

Joint type Thick-ness [mm]

Visual 100 %

+ specified

NDT

Trans-verse tensile

test speci-men

Trans-verse bend test

speci-men

Charpy impact

test spec-imen

Macro etchings/ hardness test speci-

men

Others

t ≤ 30 100 % RT or UT 9 + MT or PT 10

2 1 2 face 22 root 4 × 3 3,5 1/2 rows 6

Butt welds (plates and tubes)

t > 30 100 % UT or RT + MT or PT10

2 1 4 side 6 × 3 3,4,5 1/3 rows 6,7 PWHT (CTOD) see A.4.5

t ≤ 30 100 % UT 9 + MT or PT 10

- - 4 × 3 3,5,8 2/2 rows 6 T-joints (full penetration)

t > 30 100 % UT + MT or PT 10 - - 6 × 3 3,4,5,8 2/3 rows 6,7

PWHT (CTOD see A.4.5

T-joints (fillet welds) all 100 % MT or

PT 10 - - - 2/2 rows 6

t ≤ 30 100 % UT 9 + MT or PT 10

- - 4 × 3 3,5,8 1/2 rows 6 Tubular joints (nodes) t > 30 100 % UT + MT

or PT 10 - - 6 × 3 3,4,5,8 1/3 rows 6,7 PWHT (CTOD) see A.4.5




1 Additional round tensile test specimen to be taken longitudinally from weld metal, if welding consumables are not approved by GL, see C.2.1.2.

2 For t > 12 mm, 4 side bend test specimens may be chosen instead of 2 face and 2 root bend test specimens.

3 Charpy impact test specimen to be sampled approximately 1 mm below base material surface at the capping side, notch positioned perpendicular to the surface:

• in the centre of the welding

• on the fusion line (50 % weld metal, 50 % heat affected zone)

• 2 mm from fusion line in the heat affected zone

• 5 mm from fusion line in the heat affected zone

• see sketch below, both T- and tubular joints analogously, see Fig. 4.20

4 Additional Charpy impact test specimens to be sampled in the root region with the notch positioned perpendicular to the surface:

• in the centre of the welding

• on the fusion line (50 % weld metal, 50 % heat affected zone,see sketch below, both T- and tubular joints analogously, see Fig. 4.20)

5 If different welding consumables or welding processes are used for the same joint, impact testing shall be performed for all related areas of the weld.

6 One hardness measurement row 2 mm below each surface, measuring points concentrated near fusion line in the heat affected zone with some spots in the weld metal and base material, details see ISO 9015-1 Figures 3, 4 and 5.

7 Additionally 1 row in the root area.

8 If the Charpy test specimens can't be taken out of the T-joints the qualification shall be done on butt welds.

9 UT shall not be used for t < 8mm.

10 PT shall be selected according to F.3

77

Fig. 4.20 Location of charpy test specimens

Table 4.21 Welding procedure qualification tests - acceptance requirements

Type of test Acceptance requirements

Non-destructive testing The soundness of the welds is to comply with the acceptance requirements laid down in ISO 15614-1, Chapter 7.5.

All weld metal tensile tests 1

Minimum yield strength and tensile strength as specified for the base material to be welded. 2

Transverse tensile tests In accordance to ISO 15614-1.

Face/root and side bend tests Bending angle 180°. Mandrel diameter 4 · t in accordance to ISO 15614-1.

Charpy V-notch impact tests

Average and minimum absorbed energy for each notch position (centre of weld, fusion line and heat affected zone) shall meet the base material re-quirements, see A.4.4.2, Table 4.10. Test temperature shall be one exposure level lower than for the base material, see A.4.4.2, Table 4.11.




Hardness tests The maximum hardness shall not exceed 350 HV10 above the submerged zone, i.e. LAT (see Section 5) and in the submerged zone that may be ex-posed to cathodic protection the maximum hardness shall be 325 HV10.

Macro etchings According to ISO 15614-1, and the hardness indents shall be clearly shown and a scale shall be added to the macro.

CTOD or PWHT See A.4.5

1 If required according to C.2.1.2

2 The tensile strength of all weld metal for welding high strength steels may be up to 10 % below the minimum tensile strength of the base material, provided that the results obtained with the transverse test specimen across the weld meet the require-ments. Special consideration is to be given for material/wall thickness ≥ 50 mm.

C.3.6 Welding procedure qualification record (WPQR)

The welding procedure qualification record (WPQR) shall be a record of all welding data, such as: • manufacturer, place and date • applicable standard • joint preparation (root opening, root face, groove angle drawn in a sketch) • welding sequences with sketch and pass-nos. • welding process(es) • welding position and progression • plate/tubular – wall thickness/diameter • base material specification and heat-no., material Certificate • welding consumable(s), code designation, trade name, lot-no., consumable(s) Certificate(s) • welding parameters for root, hot, fill and cap layers (pass-no., diameter, gas composition, gas flow

[l/min], current polarity, amperage, voltage, travel speed, heat input) • preheat temperature • interpass temperature for each layer • PWHT, if required • other information like weaving, backing, gouging, grinding, etc. • manufacturer's signature • Third Party's signature

The WPQR shall be submitted for review and agreement prior to start of production. The complete WPQR with satisfactory test results qualifies a pWPS to a WPS.

C.3.7 Limit applications of qualified welding procedures

C.3.7.1 General

Changes in the variables as specified in C.3.7.3 to C.3.7.14, respectively, are to be considered as being essential and in general require performance of a new or additional procedure qualification test unless otherwise agreed. When a combination of welding processes is used, the variables applicable to each process shall apply, see also AWS D 1.1 and ISO 15614-1.

C.3.7.2 Essential variables

Approved welding procedures may be used within the limitations of these essential variables regarding: • details of base materials • thickness • tube diameter (if applicable) • joint configuration (butt, fillet, etc.) • bevel configuration incl. backing (if required) and/or back gouging




• welding process and/or multiple processes • welding positions and progression • type of welding consumables (classification acc. to code and trade name) • welding parameters • preheating and working temperatures • number and sequence of layers • post-weld heat treatment, if required

as laid down in the following for welding above water.

For CTOD, see also A.4.5.5.3 and for welding under water see C.3.10.5. GL may specify other limits if the peculiarities of a welding procedure, the material, the joint configuration and thicknesses or other parame-ters have a considerable effect on the properties of a weld.

C.3.7.3 Details of base material

Changes beyond the ranges given in ISO 15614-1, Chapter 8.3. Additional items shall apply: • A change from wrought (rolled, forged) steel to cast steel or vice versa shall require a new welding

procedure qualification. • For special and primary members an increase of the carbon equivalent value (CEV) greater than

0,03 % or an increase in the carbon equivalent parameter for cracks (PCM) greater than 0,02 %, with respect to the values specified in the procedure qualification test.

• For special and primary members a change in the delivery condition (normalized, thermo-mechanical controlled process or quenched and tempered).

Note:

General classification of material with its groups is mentioned in CEN ISO/TR 20172.

C.3.7.4 Thickness

Changes in plate/wall thicknesses beyond the ranges given in ISO 15614-1, Chapter 8.3.2.

C.3.7.5 Tube diameter

Changes from diameters beyond the ranges given in ISO 15614-1, Chapter 8.3.2.

C.3.7.6 Joint configuration

Changes beyond the ranges given in ISO 15614-1, Chapter 8.4.3.

C.3.7.7 Groove design

A decrease in groove angle more than 10°:

For groove angles less than 30° the limitation is + 20°/- 0°.

C.3.7.8 Welding process

Any changes beyond the ranges given in ISO 15614-1, Chapter 8.4.1 and 8.5.

C.3.7.9 Welding position and progression

Change from one principal welding position and progression to another, unless covered by the inclusions given in Table 4.22.

Furthermore additional changes shall apply: • change from weaving to stringer bead technique or vice versa • change of stringer to weave ratio outside the tolerances specified in the agreed WPS • changes beyond the ranges given in ISO 15614-1, Chapter 8.4.2.

Welding procedures may be qualified in two positions which represent the highest heat input and the low-est heat input. Then all positions are qualified.




Table 4.22 Qualified welding positions

Test weld positions 1 Qualified positions 1

Joint con-figuration

Plate or tube posi-tion Butt welds Fillet welds, plates

or tubes

ISO 6947 AWS Plates Tubes

Plates, butt welds

PA PC

PF PG PE

1G 2G

3G↑ 3G↓ 4G

PA/1G PC/2G

PA/1G and PF/3G↑

PG/3G↓ PA/1G, PC/2G and PE/4G

PA/1G 2 PA/1G and PC/2G 2

- - -

- - - - -

Tubes, butt welds 3

PA PC PH PJ

H-L045 J-L045 H-L045

1G 2G 5G↑ 5G↓ 6G↑ 6G↓

6GR 4

PA/1G PA/1G and PC/2G

PA/1G, PE/4G and PH/5G PA/1G, PE/4G and PJ/5G

All except PG/3↓ All except PF/3↑

-

- - - - - - -

Fillet welds

PA PA PB PB PC

PF

PG PD

PH PJ

1F 1FR 2F

2FR 2F

3F↑

3F↓ 4F

5F↑ 5F↓

Plate Pipe rotating

Plate and pipe Pipe rotating

Plate and pipe

Plate

Plate Plate

Pipe fixed Pipe fixed

- - - - - - - - - - -

PA/1F PA/1FR

PA/1F and PB/2F PA/1F and PB/2F PA/1F,PB/2F and

PC/2F PA/1FPB/2F and

PF/3F PG/3F

All except PF/3F↑ and PG/3F↓

All except PG/5F↓ All except PC/2F

and PH/5F↑

1 Welding positions according to ISO 6947 and AWS A3.0 2 Qualifies also for butt welding tubes with diameter > 500 mm 3 Tubular joints (node sections) are to be qualified separately 4 To be applied for welders’ qualification tests only

C.3.7.10 Welding consumables

Changes beyond the ranges given in ISO 15614-1, Chapters 8.4.4, 8.4.5 and 8.5.

C.3.7.11 Welding parameters

Changes beyond the ranges given in ISO 15614-1, Chapter 8.4.8. Additional item shall apply:

A change greater than 25 % in the heat input of the welding passes from the average value calculated in the supporting procedure qualification test. Heat input average shall be calculated separately for root, hot, fill and cap passes.

C.3.7.12 Preheating and interpass temperatures

Changes beyond the ranges given in ISO 15614-1, Chapter 8.4.9 and 8.4.10.




C.3.7.13 Sequence of layers

Change from welding with temper beads to welding without temper beads. Sequence of deposition of different consumables required.

C.3.7.14 Post-weld heat treatment

Change from as welded condition to post-weld heat treated condition and vice versa. Change beyond specified temperature range, holding time and heating/cooling rates (temperature gradients).

C.3.8 Qualification for tubular node connections

C.3.8.1 For full penetration groove welds between tubes and/or plates or girders (T-, K- or Y-joints) forming node connections, welding procedure qualification tests are to be performed analogously to the requirements stated above and following the indications given in C.3.8.2 to C.3.8.4. A detailed test pro-gramme is to be prepared by the manufacturers in agreement with GL from case to case.

C.3.8.2 The test programme shall cover the different joint configurations, tube diameters and wall thicknesses, angles included between the axis of the tubes as well as the different welding positions for each individual groove type. If not otherwise required, the test pieces may be formed as a brace to can connection according to Fig. 4.21.

C.3.8.3 When the diameter “D” (of the can) exceeds 500 mm, this tube may be replaced for the test weld by a plate of same or equivalent steel grade and of relevant size and similar thickness, but not less than 25 mm. When other types of welding (double side, partial penetration groove or fillet welds) are in-tended and approved for fabrication, these types are to be qualified separately. Double side welding may be covered by single side welding qualification with GL’s consent in each individual case.

5"

*

*

!

81

9

Fig. 4.21 Tubular node test piece

C.3.8.4 The joint is to be tested by ultrasonic and magnetic particle method. From each position marked with a circle (A, B, C) in Fig. 4.21 one set of test specimens as required in Table 4.20 is to be prepared and tested. Charpy V-notch impact test specimens are to be taken from positions “A” and “C” only and those from fusion line and heat affected zone shall be placed on the bracing side, if not required otherwise. The acceptance requirements given in Table 4.21 apply.

C.3.8.5 In addition to the limitations and essential variables given in C.3.7 for tubular node connec-tions the following limitation shall apply:

Change of more than 5° from the included angle between the axis of the tubes or girders to smaller an-gles requires re-qualification when the included angle between 45° and 30°. Below 30° the limitation is +20°/-0°, see C.3.7.7.

C.3.9 Qualification of grout beads

C.3.9.1 Type of shear key

The test programme shall cover the different types shear keys used, see Fig. 4.2b.




A welding procedure qualification is required for the specific shear key type used in the project. This WPQR shall be conducted as follows: depositing of a full circumferential bead on a can, with material of the diameter and thickness shown on the drawings, using appropriate production welding equipment.

C.3.9.2 Testing programme

Following tests to be performed: • visual inspection in compliance with the drawing (height, width and toe profile) • VT + MT 100 % • macro-examination 2 sections, minimum distance of 1 m: • external defects quality group B and internal defects group C acc. ISO 5817 • hardness indents shall be made as follows and shall not exceed 325 HV10:

1. weld bead (Fig. 4.2b), key type 1) according to Fig. 4.22 2. square and round bar with fillet beads (Fig. 4.2b), key types 2 and 3 according to Fig. 4.22b.

) )

))

)7

.*

.

:

Fig. 4.22a Hardness indents for shear key type 1

& # #

Fig. 4.22b Hardness indents for both shear key types 2 and 3

C.3.10 Qualification for underwater welding

C.3.10.1 Underwater welding, to be used for construction or repair, is to be qualified by a test, following the scheme given for welding above water, conducted under actual welding conditions at the welding site




or simulated at a test facility. The test is to be carried out at the same water depth or at similar conditions as for the actual welding operation.

C.3.10.2 For underwater welding generally a low hydrogen process shall be used, carried out in a dry chamber under pressure equivalent to the water depth or under one atmosphere pressure (habitat weld-ing).

Underwater wet welding, where the arc is working in the water or in a small gas-filled cavern, is normally restricted to minor or temporary repair work in shallow waters and requires GL’s consent in each individ-ual case.

C.3.10.3 Prior to the qualification test, a detailed preliminary welding procedure specification (pWPS) is to be prepared by the manufacturer and submitted to GL for review. In addition to the particulars men-tioned in C.3.2, the following data are to be specified: • maximum and minimum water depth • chamber (habitat) atmosphere (gas composition and pressure) • maximum humidity in the chamber (habitat) • temperature fluctuations inside the chamber • electrode transport and monitoring system • inspection schedule and methods

C.3.10.4 Based on the preliminary welding procedure specification (pWPS) according to C.3.10.3 a welding procedure qualification test programme is to be established by the manufacturer and submitted to GL for approval. The manufacturer shall also document his previous experience with similar welding works, otherwise an extended testing may be required by GL. Testing and acceptance requirements given in C.3.5 apply analogously.

C.3.10.5 In addition to the limitations and essential variables given in C.3.7 for underwater welding the following variations are considered to be essential: • change from habitat welding to wet welding and vice versa • change in water depth (working pressure) beyond the agreed range • change in habitat atmosphere (gas composition) • change in welding consumables • change in welding current/polarity • change in welding parameters (amperage, voltage, travel speed) of more than 10 % of mean values

Regarding production tests to be performed prior to actual welding, see both C.6.10 and C.6.11.3.

C.3.11 Evaluation of test results, requirements, repeat test specimens, test reports

C.3.11.1 Designation of test results

To ensure that the description and evaluation of welding processes and positions, test results, etc. are as clear and uniform as possible, use shall be made of the terminology and symbols in the relevant stan-dards (e.g. ISO 857, ISO 6947, ISO 6520, ISO 5817, ISO 10042). The position of a defect or fracture shall be indicated and may be designated as follows: • WM : weld metal • FL : fusion line • HAZ : heat-affected zone of base material • BM : base material

C.3.11.2 Requirements, repeat test specimens

C.3.11.2.1 The requirements are specified in Table 4.20.

C.3.11.2.2 Repeat test are according to ISO 15614-1 Chapter 7.6




C.3.11.3 Reports, storage times

C.3.11.3.1 Reports shall be prepared of all trial welds and tests and submitted to GL in duplicate, signed by the tester and the testing supervisor, see GL Rules for General Requirements, Proof of Qualifications, Approvals (II-3-1), Section 4 and Annex D.

C.3.11.3.2 The debris of test pieces, specimens and the test documentation are to be kept until all the tests and inspections are concluded by the confirmation of approval issued by GL. For the storage time of documents relating to the non-destructive testing of welds (e.g. radiographs), see D.1.8.

C.3.12 Range and period of validity

All new welding procedure tests are to be carried out in accordance with this Rule from the date of its issue.

However, this Rule does not invalidate previous welding procedure test made to previous issues of this Rule.

Where additional tests have to be carried out to make the qualification technically equivalent, it is only necessary to do the additional test on a test piece, which should be made in accordance with this Rule.

C.4 Welders’ qualifications

C.4.1 Qualification requirements

The welders shall be certified by an accredited body according to AWS D.1.1, EN 287-1 and /or ISO 9606-4. Welding operators shall be certified according to ISO 14732/EN 1418.

C.4.2 Qualification of underwater welders

C.4.2.1 For underwater welding only those welders may be qualified, who are well trained, experi-enced and qualified for relevant welding above water. Prior to the qualification tests under water, the welders are to be given sufficient training to get familiar with the underwater welding conditions such as pressure, atmosphere, temperatures, etc.

C.4.2.2 The test welds are to be performed using materials comparable in chemical composition (weldability) and strength to those used in the actual welding job, and with the welding process in ques-tion under actual or simulated diving conditions.

C.4.2.3 Type and scope of testing are to be based on the actual welding job analogously to those given above and will be decided from case to case. Limitations given above apply analogously, the limita-tion of applicable (greater) water depth for welding will be decided in each single case.

C.4.3 Validity of qualification Certificates

C.4.3.1 The validity of a welder’s qualification Certificate is generally two years, details see in the men-tioned standards in C.4.1, provided that during this period the welder has been working under the super-vision of GL and the welder’s work is monitored by visual and/or non-destructive testing. GL may demand annual repeat tests if monitoring mainly takes the form of visual inspections or if the results of NDT can-not be correlated to the welder.

C.4.3.2 A repeat test is required where a welder, who has been tested in more than one welding proc-ess, has not used the process in question for a period exceeding six months but has meanwhile used another process. A repeat test is also required if a welder has not performed any welding work as defined in C.4.1 for a period exceeding three months.

GL may demand a repeat test at any time if reasonable doubts should arise as to a welder’s performance.

C.5 Design of weld connections

C.5.1 General requirements

C.5.1.1 The shop drawings, etc. shall provide clear and complete information regarding location, type, size and extent of all welds. The drawings shall also clearly distinguish between shop and field welds. Symbols and signs used to specify welding joints shall be in accordance with recognized codes or stan-




dards (to be mentioned in the drawings, specifications, etc.) or shall be explained in these documents. Special weld (groove) details shall be shown by sketches and/or relevant additional remarks.

C.5.1.2 It is recommended that standard welding details, symbols, etc. including any workmanship or inspection requirements, are to be laid down in standard drawings submitted to GL for general approval. Dimensions of welding should not form a part of those standard drawings, but shall be designated in the contract or shop drawings for each individual structural part or welding respectively.

C.5.1.3 All welds shall be planned and designed in such a way that they are readily accessible during fabrication and can be executed in an adequate welding position and welding sequence, see also C.6.5. Welded joints which are subject to NDT inspection shall be placed and designed to facilitate application of the required inspection procedure, so that tests offering reliable results may be carried out.

C.5.1.4 Welds of special and primary structural members shall have unique weld numbers. Welds of secondary structural members may be group numbered, but only within the same drawing sheet. The shop drawings shall have all information to select the correct WPS.

C.5.2 Weld shapes and dimensions

C.5.2.1 Weld shapes (groove configurations) shall be in accordance with ISO 2553 or ISO 9692 or AWS D 1.1. As far as possible, full penetration butt or T-joints shall be designed as double side welds with the possibility of sufficient root treatment (gouging) instead of single side welds. Fillet welds shall be made on both sides of the abutting plate or web.

C.5.2.2 Weld shapes shall be designed to ensure that the proposed weld type and quality (e.g. full penetration) can be satisfactorily achieved under the given fabrication conditions. Failing this, provisions shall be made for welds which are easier to execute. The (possible lower) load-bearing capacity of these welds shall be taken into consideration in the dimensional design. According to Fig. 4.23, the effective throat thickness of a partial penetration groove weld shall be the depth of bevel less 3 mm, if not other-wise proved in way of welding procedure qualification tests and agreed by GL.

C.5.2.3 Special weld shapes, differing from those laid down in the above Rules and standards (e.g. single side, full penetration groove welds in tubular T-, K- or Y-connections) or weld shapes for special welding processes shall have been proved in connection with the welding procedure qualification tests according to both C.3.3 and C.3.5 and are to be approved by GL. Typical welds of tubular T-, K- and Y-connections with and without back welding are shown in both Figs. 4.24 and 4.25.

Welding details to be observed, especially for Fig. 4.25, see both Tables 4.23 and 4.24

Table 4.23 Minimum weld thicknesses for tubular joints

Weld angle α Weld thickness tw [mm]

Over 130° To continue line of brace/stub, except that tw need not exceed 1.75 · t

60° to 130° 1.25 · t

40° to 60° 1.50 · t

Under 40° 1.75 · t

Remark:

Smooth profile required between brace/weld/chord generally t = wall thickness of brace or stub [mm]




Table 4.24 Root gap requirements for tubular joints

Groove angle β Root opening g [mm]

Over 90° 0 – 5

45° to 90° 2 – 5

Under 45° 3 – 6.5

Remark:

Details of welding process to be considered.

C.5.2.4 The weld contour is to be designed with smooth transitions tangent to the parent material in order to achieve the actual calculated fatigue life. The requirements regarding the weld contour, including additional treatment, such as grinding of weld toes to a smooth profile depending on the actual detail category, see Section 3, I.2.8, are to be laid down in the drawings and/or specifications. A typical im-proved weld contour of a T-joint is shown in Fig. 4.26. The cap layer should be made with single passes (no excessive weave technique), starting at base material and making the last passes in temper bead technique on top of the weld to avoid excessive hardness in the heat affected zone.

C.5.2.5 Dimensions (effective throat thicknesses and lengths) of welds are to be determined in context with the static and fatigue strength calculations and are to be specified in the drawings submitted to GL for approval.

C.5.2.6 The effective throat thickness of a complete penetration groove weld shall be the thickness of the thinner part to be joined. For partial penetration groove welds, the drawings shall specify the groove depth applicable to the effective throat thickness required for the welding procedure to be used, see both C.5.2.2, Fig. 4.23 and Fig 4.27, as well as the calculated effective throat thickness.

The throat thickness of fillet welds is the size “a” in Fig. 4.27. In the drawings, fillet weld dimensions may be specified by the throat thickness “a” or by the leg length “ ”, which is = a 2⋅ .

The throat thickness of fillet welds shall be as follows:

amax : 0.7·t1

amin :3

t+t 21 [mm], but not less than 3 mm

t1 : lesser (e.g. the web) plate thickness [mm]

t2 : greater (e.g. the flange) plate thickness [mm]

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Fig. 4.23 Effective throat thickness “a” of partial penetration groove welds




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Fig. 4.24 Typical tubular weld connections with back welding

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Fig. 4.25 Typical tubular weld connections without back welding




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Fig. 4.26 Typical improved weld contour

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Fig. 4.27 Effective throat thickness “a” and leg length “ ” of fillet welds

C.5.3 Details of welded joints

C.5.3.1 All welded joints on special and/or primary members shall be designed to provide a stress flow as smooth as possible without major internal or external notches, discontinuities in rigidity and obstruc-tions to strains.

The transition between differing component dimensions, e.g. of girders or stiffeners, shall be smooth and gradual. The length of the transition should be at least 4 times the difference in depth, see Fig. 4.28.

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Fig. 4.28 Transitions between components with differing dimensions




C.5.3.2 The requirements given in C.5.3.1 apply in analogous manner to the welding of secondary members onto primary (or special) supporting members. The ends should have smooth transitions to the main structure, see Fig. 4.29.

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Fig. 4.29 Ends of secondary members welded on the main structures

C.5.3.3 Where the plate or wall thickness differs more than 3 mm at joints mainly stressed perpendicu-larly to the (butt) weld direction, the difference shall be smoothed by bevelling and/or buttering according to Fig. 4.30 prior to butt welding. Differences in surface levels up to 3 mm may be equalized within the weld.

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Fig. 4.30 Transition in butt welds of different thickness

C.5.3.4 If a misalignment due to component or plate respective wall thickness tolerances cannot be eliminated by adjusting (e.g. in case of circumferential butt welds in tubes) and the difference “X” is not greater than one quarter of the thinner plate or all thickness, the misalignment may be equalized by chamfer or built-up welding according to Fig. 4.31. Where the difference is greater and/or highly fatigue stressed joints are involved, equalization is to be decided and agreed upon with GL in each individual case.




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Fig. 4.31 Equalization of misalignment in butt welds

C.5.3.5 Where plates (including girder web and flange plates) or tube walls are subjected locally to increased stresses, thicker plates are to be inserted in preference to doubler plates. This applies analo-gously to flanges, stuffing-boxes, hubs, bearing bushes or similar components to be welded into plates. The minimum size of those reinforcement plates shall be:

Dmin : 170 + 3 · (t - 10), but not less than 170 mm

D : the diameter of round inserts or the length/breadth of angular inserts [mm]

t : plate thickness [mm]

The corner radii of angular reinforcing plates shall be at least 50 mm. The welding between those rein-forcement plates or inserts and the plating shall be full penetration butt welds, and the transition shall be in accordance with C.5.3.3.

C.5.3.6 Doubler plates (cover plates) may be used to reinforce girder flanges or similar components in the area of maximum bending moments. Doubler plates shall be limited to one per flange, and their thick-ness “t” shall not exceed 1.5 times the thickness of the flange, their width “w” should not exceed 20 times their thickness. The ends of doubler plates shall be extended beyond the calculated, theoretical start or end conforming to Fig. 4.32. The fillet welds connecting the doubler plate to the flange shall be continu-ous welds. The welds at the end of the doubler plates shall conform to Fig. 4.32.




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Fig. 4.32 Configuration and welding of ends of doubler plates on flanges

C.5.3.7 Local clustering of welds and short distances between welds are to be avoided. Adjacent butt welds should be separated from each other by a distance of at least:

50 mm + 4 t

t : plate thickness [mm]

Fillet welds should be separated from each other and from butt welds by distance of at least:

30 mm + 2 t

The width of sections (strips) of plate to be replaced should, however, be at least 300 mm or 10 t, which-ever is greater.

C.5.3.8 Where tubular joints (node sections) without overlapping requires increased wall thicknesses and/or special steel in the main member (chord, can) and/or in the branch members (bracing stubs), the separation of the welds shall be in accordance with both Figures 4.33a) and 4.33b). The greater meas-ures apply, see also B.15.7.




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Fig. 4.33 Separation at tubular joints and weld seams

C.5.3.9 In overlapping tubular joints (node sections), in which part of the load is transferred directly from one branch member (bracing) to another through their common weld, the overlapping of the welds shall be at least in accordance with both Figures 4.33a) and 4.33b). The greater measure applies. The actual length of the overlapping weld is to be determined by calculation. The heavier brace shall prefera-bly be the through brace with its full circumference welded to the chord.

C.6 Performance of welding, workmanship

C.6.1 General requirements

C.6.1.1 All welding is to be performed observing the applicable paragraphs of this Section as well as the common actual standard of welding technology to achieve the required quality. Detailed recommen-dations are given in relevant parts of EN 1011 – series. Compliance with these Rules and with any other conditions, which may be agreed or stated in the drawings, specifications and other related documents or imposed in connection with approvals as well as with good workmanship, is the responsibility of the manufacturers. Inspections performed by GL do not relieve the manufacturers of this responsibility.




C.6.1.2 The preparation and performance of all welding operations shall be supervised by manufac-turer’s welding supervisors, see C.1.5. Regarding the qualification and the functions of the welding super-vising personnel, see GL Welding Rules. In the event of any deviations from the above stated operating conditions, the welding supervisory personnel shall, in agreement with GL, take steps to ensure the con-stant and adequate quality of the welds. For inspection personnel see F.5.

C.6.2 Overweldable shop primers

C.6.2.1 Overweldable shop primers which are applied to plates, sections, etc. prior to welding and are not removed shall be tested and approved in accordance with GL Rules for General Requirements, Proof of Qualifications, Approvals (II-3-1), Section 6.

C.6.2.2 By suitable checks – on the thickness of the coating especially – on and by sample production tests in the course of normal fabrication welding, shops shall ensure that the welded joints suffer no un-acceptable deterioration, see GL Rules for General Requirements, Proof of Qualifications, Approvals (II-3-1), Section 6, C.

C.6.3 Weld preparation assembly, tack welding

C.6.3.1 Machining, oxygen cutting, air carbon arc gouging, oxygen gouging, chipping or grinding may be used for joint preparation, back gouging or the removal of unacceptable work or metal, except that oxygen gouging shall not be used on normalized or quenched and tempered steels.

C.6.3.2 When preparing and assembling components, care shall be taken to ensure compliance with the weld shapes and root openings (air gaps) specified in the drawings and specifications. With single and double-bevel T-joints, especially, attention shall be paid to an adequate root opening in order to achieve sufficient root penetration.

C.6.3.3 The root opening shall not exceed the specified gap. If the gap exceeds this value locally over a limited area, the gap may be reduced by build-up welding of the groove edges, subject to the prior con-sent of the Surveyor. Details see also both B.10.4 and C.6.8.

With fillet welds, the “a” dimension shall be increased accordingly, or a single or double-bevel weld shall be made, if the air gap is large. Inserts and wires shall not be used as fillers.

C.6.3.4 With the Surveyor’s agreement, large gaps – except in highly stressed areas of special struc-tural members – may be closed by means of a strip of plate with a width of at least 10 times the plate thickness or 300 mm, whichever is the greater.

C.6.3.5 Members to be welded shall be brought into correct alignment, see also B.10, and held in po-sition by bolts, clamps, wedges, guy lines, struts and other suitable devices or by tack welds until welding has been completed. The use of jigs and fixtures is recommended, where practicable. Suitable allow-ances shall be made for warpage, shrinkage and for expansion due to ambient changes, e.g. direct sunlight.

C.6.3.6 Clamping plates, temporary ties, aligning pins, etc. shall be made of structural steel of good weldability and should not be used more than necessary. They are to be carefully removed to prevent damage to the surface of members when the components have been permanently welded, see also B.12.

C.6.3.7 Clamping plates, temporary ties, aligning pins, etc. shall not be welded to components or ar-eas subject to particularly high stresses, nor shall they be welded to the edges of flange plates of girders or similar members. The same applies to the welding of handling lugs and other auxiliary fixtures.

C.6.3.8 Tack welds should be used as sparingly as possible and shall be performed by qualified weld-ers using qualified welding procedures and consumables.

The preheating temperature shall not be less than for welding the joint.

Where their quality does not meet the requirements applicable to the subsequent welded joint, they are to be carefully removed before the permanent weld is made. All tack welds which will form part of the per-manent weld shall be cleaned, ground down to a feather edge at both ends and visually inspected prior to welding of the root pass.

Any cracked tack weld shall be completely removed by grinding and not incorporated into the weld, see also B.10.9.




C.6.3.9 Particularly with mechanized welding processes – and invariably when end craters and de-fects at the start and end of the weld have to be avoided – run-in and run-off plates of adequate section shall be attached to components and cleanly removed on completion of the weld.

C.6.3.10 Surfaces to be welded shall be visually examined, to ensure freedom from laminations, cracks, slag inclusions and cutting notches and additionally clean and dry.

Any scale, rust, cutting slag, grease, paint or dirt is to be carefully removed before welding or dressed out by grinding. If the depth of excavation exceeds 5 mm, a weld repair shall be made, using an approved WPS.

Moisture is to be removed by preheating. Edges (groove and root faces) are to have a smooth and uni-form surface.

Bevels are to be ground after weld preparation by acetylene cutting.

For overweldable shop primers, see C.6.2.

C.6.4 Weather protection, preheating, heat input during welding

C.6.4.1 Weather protection, welding at low temperatures

C.6.4.1.1 The area in which welding work is performed has to be sheltered from wind, damp and cold, particularly outdoor. Where gas-shielded arc welding is carried out, special attention is to be paid to en-sure adequate protection against draughts. When working in the open under unfavourable weather condi-tions it is advisable to dry welding configurations by heating.

C.6.4.1.2 At ambient temperatures below +5 °C, additional measures shall be taken, such as shielding of components, extensive preliminary heating and pre-heating, especially when welding with a relatively low heat input (energy input per unit length of weld), e.g. when laying down thin fillet welds or in the case of rapid heat dissipation, e.g. when welding thick-walled components. Wherever possible, no welding should be performed at ambient temperatures below -10 °C.

C.6.4.2 Preheating for the welding of ferritic steels

C.6.4.2.1 The need for preheating of ferritic steels and the preheating temperature depend on a number of factors. Among these are especially important: • the chemical composition of the base material (carbon equivalent) and the weld metal/consumable • the thickness of the work piece and the type of weld joint (two or three dimensional heat flow, see

combined thickness in EN 1011-2) • the welding process and the welding parameters (energy input per unit length of weld) • the shrinkage and transformation stresses • the temperature dependence of the mechanical properties of the weld metal and the heat affected

zone • the diffusible hydrogen content of the weld metal

C.6.4.2.2 The operating temperature to be maintained (minimum preheating temperature and maximum interpass temperature) for structural steels may be determined in accordance with EN 1011-2.

C.6.4.2.3 Preheating is always necessary for following duties:

tack welds generally • welding of lugs or other auxiliary welds • during erection of major components • welding of large wall thicknesses • welding of castings and forgings • during repair welding should be minimum 50 °C higher than the preheat used for the original weld

A detailed procedure shall be available, where all items mentioned for the project used.

Additional information is required from the steel manufacturer.




C.6.4.2.4 Preheating shall be applied uniformly throughout the thickness of the plate or component over a distance of four times the plate thickness, minimum of 100 mm, on both sides of the weld.

Preheating above 50 °C should be achieved by electric heating elements. Cutting torches are not allowed for preheating.

C.6.4.2.5 To prevent cold cracks in higher-strength and high strength (quenched and tempered) steels, thick-walled components or components of complex design, it is advisable to use measures which give the hydrogen induced into the weld metal during welding sufficient time to escape. The following methods are well established: • maintenance of a specific minimum preheating and interpass temperature throughout the welding

operation • delayed cooling after welding • holding at approx. 230 – 250 °C for about 2 hours prior to cooling (hydrogen-reducing heat treatment

or soaking) • heat treatment immediately after welding (without cooling in between) • further information required from the steel manufacturer

C.6.4.2.6 Where structural steels or fine-grained structural steels have undergone thermo-mechanical processing (TMPC steels), the need for and degree of preheating shall be decided separately on the ba-sis of the carbon equivalent and the results of the approval or welding procedure tests as applicable. Dry-ing of the areas to be welded by heating may be sufficient.

C.6.4.3 Monitoring of interpass temperatures

The guide values contained in EN 1011 or AWS D.1.1 for the interpass temperatures relating to the vari-ous steels shall not be significantly exceeded.

The minimum interpass temperature shall not drop below the minimum required preheat temperature.

C.6.4.4 Welding with controlled heat input per unit length of weld

In addition to controlling the preheating and interpass temperature, the heat input per unit length of weld shall be controlled during welding, especially in the case of weldable, high strength (quenched and tem-pered) fine-grained structural steels. The heat input per unit length of weld shall not fall below or exceed the values indicated by the steel manufacturer or those used in the welding procedure tests and specified in the welding procedure specifications (WPS) by any significant amount.

C.6.4.5 Preheating and heat input during the welding of other steels or metallic materials

C.6.4.5.1 Preheating is not normally required for austenitic materials. Preheating may be necessary for austenitic-ferritic materials. A maximum permitted interpass temperature which is normally between 150 °C and 180 °C shall be complied with in order to prevent hot cracking.

C.6.4.5.2 Ferritic and stainless martensitic steels shall be adequately preheated and welded using con-trolled heat input per unit length of weld. Guide values for the preheating and interpass temperatures are prescribed in EN 1011-3.

C.6.4.5.3 Preheating is not normally required for welding of aluminium alloys, but should not exceed 50 °C. A maximum permitted interpass temperature of 100 °C to 120 °C shall be complied with in order to prevent undesirable phase dispersion. EN 1011-4 contains guide values for both the preheating tempera-ture to be applied and the interpass temperature.

C.6.5 Welding positions, welding sequence

C.6.5.1 Welding should be performed in the optimum welding position, and welding in unfavourable positions is to be limited to the indispensable minimum. For similar and repetitive welding operations it is advisable to use a (rotary) jig enabling all welds, as far as practicable, to be made in the downhand posi-tion. Vertical-downward welding may not be used to join special and primary structural members.

C.6.5.2 The welding sequence shall be chosen to allow shrinkage to take place as freely as possible and to minimize shrinking stresses. In special cases, GL may require to set down the assembly procedure and welding sequence in an appropriate schedule.




C.6.5.3 It is recommended to prepare welding sequence plans for special designs. This shall be at-tached to the relevant WPS used.

C.6.6 Weld quality requirements

C.6.6.1 The requirements to be met by the weld, e.g. seam geometry, surface quality, admissible ex-ternal and internal flaws, depend on the significance of the respective welds for the overall integrity of the structure. The operability of individual components as well as the kind of loads (static/dynamic) and the stress level shall also be considered.

C.6.6.2 The requirements to be met by the welds are to be laid down in the plans for production, in-spection and testing, e.g. drawings, specifications etc., see also F.4.4, and on the basis of relevant codes and standards, see ISO 5817 Table 1, and contractual matters and taking into account the criteria as stated in C.6.6.1. These particulars are to be presented to GL for review and approval. For inspection and testing of welds, see D to F.

C.6.7 Repairs

C.6.7.1 General

For repair welds generally the requirements are identical to those applicable to the original weld, never-theless a repair procedure to be qualified. Welds containing cracks shall not be repaired, until the reason for the cracking has been determined. Prior to carrying out extensive repair welds or the repetition of re-pair welds, e.g. owing to inadequate results of non-destructive inspections, consultation with the GL Sur-veyor shall be sought. The kind and scope of envisaged repair measures have to be agreed with him. For examination of repair welds, see both F.4.4 and F.4.5.

C.6.7.2 Correction of welds containing defects

Repair welding shall be qualified by a separate welding repair qualification test based on WPQRs for the type of weld repair to be applied, see C.6.7.5.

If necessary, the defective part of the weld shall be cut out for further examination. Crater cracks may be repaired by grinding followed by NDT and subsequent repair welding according to an accepted repair welding procedure.

Other defects shall be corrected by grinding, repair welding or re-welding.

When weld defects are removed by grinding only, the final weld surface and the transition to the base material shall be smooth. Removal of defects shall be verified by local visual inspection, aided by appli-cable NDT methods. If applicable, the remaining thickness in the ground area shall be measured. Repair welding is required, if the remaining thickness is less than that specified.

C.6.7.3 Repair by welding

Repair of welded joints shall be carried out by completely removing the unacceptable portion of the weld without substantial removal of the base material.

The excavated area shall have smooth transitions to the metal surface and allow good access for both NDT after excavation and subsequent repair welding. After excavation the defect shall be confirmed by MT.

The excavated groove shall be minimum 50 mm long, measured at defect depth even if the defect itself is smaller. Defects spaced less than 100 mm shall be repaired as one continuous defect.

If in one seam section several flaws requiring repair are located close to each other, the whole seam sec-tion has to be gouged out and welded new.

After repair welding the complete weld, i.e. the repaired area plus at least 100 mm on each side, shall be subjected at least to the same NDT as specified for the original weld.

C.6.7.4 Preheating and interpass temperature

Preheat for repair welding shall normally be 50 °C above minimum specified preheat for production weld-ing.

The interpass temperature shall be maintained until the repair has been completed.




C.6.7.5 Repair welding procedure

Repair and re-repair may be performed in accordance with the WPS of the original weld, or a separately qualified procedure, if the repair process or/and consumable are different.

Qualification of repair welding procedures shall be made by excavating a repair groove in an original weld, welded in accordance with a qualified welding procedure, e.g. partial and through thickness repair. For the mechanical testing the excavated groove shall be of a sufficient length to obtain the required number of test specimens + 50 mm at each end.

Mechanical testing may be limited to HAZ Charpy V-notch testing in the original weld, see both Figs. 4.34 and 4.35.

I), I), I) (A I) I), I),

I), I)

Fig. 4.34 Charpy-V notch impact test specimen positions – full thickness repair

The “FL” specimen shall sample 50 % WM and 50 % HAZ

The “FL+ 5mm” sample is applicable to WPQT only.

FL : fusion line

WM : weld metal

HAZ : heat affected zone

I), I), I) (A I) I), I),

Fig. 4.35 Charpy-V notch impact test specimen positions – partial thickness repair

C.6.7.6 NDT methods

MT, UT or other suitable methods shall be used to verify the complete removal of the defects. PT is only allowed after consultation with the Surveyor.

Repair welding in the same area may be carried out twice. Further repairs shall be evaluated in each individual case.




C.6.7.7 Post-weld heat treatment

In general, major repair welds, at parts which have already undergone post weld heat treatment, necessi-tate another heat treatment of the whole part. GL may agree to local post weld heat treatment or – in the case of minor repair welds – dispense with it completely; this has to be decided in each individual case.

C.6.7.8 Correction of distortion

Improperly fitted parts should be cut apart and re-welded in accordance with the applicable qualified WPS.

Parts distorted by welding, beyond the tolerances, should be straightened. Material properties are to be observed, see also B.13.

C.6.8 Buttering

When the gap of weld joint exceeds the limitations a buttering of the weld groove be qualified by a sepa-rate WPQT and the largest gap in production shall be used.

For special and primary members the maximum build-up shall be 8 mm and the maximum root gap 10 mm. Further details, see both B.10.4 and C.6.3.3.

No weaving is allowed on this welding procedure.

After welding MT shall apply before filling the groove and no surface linear indications are accepted.

C.6.9 Post weld heat treatment

Note

Stress-relief annealing will be dealt with in the following under the heading of “post weld heat treatment”. Should other kinds of post weld heat treatment be intended, agreement shall be sought from GL, who will consider the conditions on a case to case basis.

C.6.9.1 Post weld heat treatment should generally be performed at structural welds of joint thick-nesses of 50 mm and over, unless adequate fracture toughness can be proved in the as welded condi-tion. For restrained joints of complicated design, post weld heat treatment may be required for thick-nesses less than 50 mm, see A.4.5.

C.6.9.2 Any heat treatment is to be performed in accordance with a procedure specification, to be prepared by the manufacturers and submitted to GL for approval. This specification shall detail heating facilities, insulation, control devices and recording equipment, as well as heating and cooling rates, tem-perature gradients, holding temperature ranges and times.

C.6.9.3 Wherever possible, post weld heat treatment is to be carried out in an enclosing furnace. Where this is impractical, local heat treatment may be performed with the consent of GL. When local heat treatment is performed, an area extending over at least 3 times the material thickness on both sides of the weld is to be kept at the specified temperature.

C.6.9.4 Any heat treatment cycle is to be recorded using thermocouples equally spaced externally and – whenever possible – internally throughout the heated area. The heat treatment records are to be sub-mitted to the GL Surveyor. Regarding inspections and tests after heat treatment see F.4.

C.6.10 Production tests

C.6.10.1 Production tests, i.e. tests with test pieces welded simultaneously at specified intervals during fabrication may be called for where the base material, the welding process or the loading conditions re-quire that proof be provided of the sufficient mechanical characteristics of the welded joints produced under fabrication conditions.

C.6.10.2 Production test pieces shall be welded and tested in a manner analogously to that prescribed in C.3.5. The scope of the tests and the requirements to be met shall be determined on a case to case basis.

C.6.10.3 If a production test fails, the reason for the failure shall be determined and remedial action implemented.




Welding should be suspended until the cause of cracks and defects has been identified and measures to be taken to prevent their occurrence. Cracks or other persistent weld defects may lead to a revision and/or a re-qualification of the WPS.

C.6.11 Underwater welding

C.6.11.1 Underwater welding is generally to be carried out in a large chamber from which the water has been evacuated (habitat welding). Underwater wet welding characterized by the arc working in the water or in a small gas-filled confinement, is normally restricted to minor repair work at secondary structures only in shallow water and requires the special consent of GL from case to case.

C.6.11.2 Underwater welding is to be performed in accordance with a detailed procedure specification to be prepared by the manufacturers and submitted to GL for approval. It is to be carried out using espe-cially qualified welding procedures (see C.3.10) and by special qualified welders (see C.4.2). The transfer and handling of welding consumables as well as baking and storage in the welding habitat shall be con-sidered in the welding procedure specification.

C.6.11.3 Production test welds are to be carried out prior to commencing the underwater welding at the site in a manner which, as far as possible, reproduces the actual welding conditions, thus checking that all systems are properly functioning, and results in sound welds. The production test welds are to be visually inspected and non-destructively tested prior to production welding. Subsequently they are to be subjected to mechanical and technological tests. Type and number of test pieces and tests are to be specified and agreed by GL from case to case.

C.6.11.4 Visual inspections and non-destructive testing are to be carried out on the actual production welds by qualified operators, applying qualified and approved procedures, see also F. The methods and extent of inspections and testing are to be specified and agreed by GL from case to case. The finished welds shall fulfil the requirements specified for the individual structural parts or welds respectively.

C.6.11.5 All underwater welding work, including qualifications, production tests, actual welding data and inspection and test results shall be recorded. The records are to be submitted to GL for review and shall be kept for 5 years, see D.1.8.

D Survey, Inspection and Testing

D.1 General requirements

D.1.1 Inspection and testing shall be performed by the manufacturers in accordance with an ap-proved inspection and test plan (ITP) by the Owner/Purchaser, to confirm that all project requirements meet the demands of these Rules, the approved specifications and drawings or any other requirements stated or agreed upon.

ITPs to be submitted to GL for acceptance.

D.1.2 Inspection and testing are to be carried out during fabrication, construction and erection phases and shall cover following items: • correct identification and documentation and use of materials • qualification and acceptance procedures and qualified/certified personnel • inspection of preparatory work (assembly, fit-up form work, reinforcements, etc.) • welding inspection • inspection of fabrication work for compliance with specifications and procedures • witnessing of NDT, control and testing • inspection of repairs • inspection of corrosion protection systems

D.1.3 For the quality control required during fabrication and construction, see B.4.




D.1.4 Manufacturers will be responsible for the due performance of the inspections and testings as required, as well as for compliance with the requirements laid down in the documents. Inspections and testings performed by GL do not relieve manufacturers of their responsibility.

D.1.5 The GL Surveyor shall have the opportunity of witnessing and observing the inspections and tests conducted, or of conducting inspections and tests himself. To this effect the Surveyor shall have permission to enter production areas and building sites and be assisted, wherever possible.

D.1.6 Following inspection and testing by the manufacturers the structures or parts thereof are to be presented to the GL Surveyor for final checking. Parts are to be presented in suitable sections enabling proper access for inspection generally before painting. The Surveyor may reject components that have not been adequately inspected and tested by the manufacturers, and may demand their resubmission upon successful completion of such inspections and corrections by the manufacturers.

D.1.7 Assessments of defects (e.g. weld defects), which are identified by their nature, location, size and distribution, shall take due account of the requirements applicable to the welded joints, see also F.4. Inspection or testing results respectively shall be evaluate by the testing department and/or the welding supervisory staff and submitted to GL for review. The final assessment, together with the right of decision as to the acceptance or repair of defects in the material or weld, shall rest with the GL Surveyor.

D.1.8 Reports shall be prepared on all (initial and repeat) inspections and tests, and these shall be submitted to the GL Surveyor together with other documentation (e.g. radiographs etc.) for review. The reports shall contain all the necessary details relating to the particular test method used, see F.7.2, G.7, H.11, I.13, J.15, K.9, and the position at which the test was performed and the results obtained. The re-ports shall at any time, during fabrication and construction, as well as during erection and installation, permit the irrevocable tracing of tested areas and test results. Test reports and documentation shall be kept for minimum 5 years.

D.1.9 Welds shall be subject to NDT in progress with fabrication. The results of these activities shall be consecutively reported to the Owner/Purchaser.

D.2 Codes and standards

D.2.1 The codes, standards and other relevant Rules and Regulations mentioned in the following constitute an integral part of these Rules and require no special consent. The version in force on the date of publication of these Rules shall be applied. New editions of the codes, standards, etc. may be used in agreement with GL.

D.2.2 The application of other equivalent standards, codes etc. (e.g. AWS D 1.1 “Structural Welding Code-Steel“ and API RP 2X “ Recommended Practice for Ultrasonic and Magnetic Examination of Off-shore Structural Fabrication and Guidelines for Qualification of Technicians”) may be agreed by GL. They are to be listed in the construction particulars, (e.g. in the inspection and testing plans, specifications or other contractual Rules and Regulations) and presented to GL on request.

D.2.3 Where the codes, standards, etc. are contradictory to these Rules, the latter shall automati-cally take precedence. Any deviations from these Rules require approval by GL on a case to case basis.

D.2.4 Generally the latest edition of the codes and standards shall apply due to the contractual date for the whole fabrication and construction phase.

D.2.5 Mandatory principles and/or recommendations are subjected to the responsible Authority in use.

D.3 Terms and definitions

The terms and definitions as contained in recognized codes, standards, etc. are to be employed. The code or standard used is to be indicated in the inspection and testing documentation and presented to GL on request. Any other deviating terms and definitions are to be explained separately in the inspection and testing specifications and/or procedures.




D.4 Testing equipment

D.4.1 The testing facilities and equipment employed have to meet the latest technical standards and have to be in good condition.

The manufacturer has to ensure functionally of examination or testing equipment and of recording and/or measuring devices vital for correct functioning of equipment and machinery used in fabrication.

If manufacturers do not possess equipment of their own, they may use test equipment of third parties, e.g. testing institutes.

D.4.2 The equipment has to be calibrated and/or gauged and the respective Certificates have to be on hand.

D.4.3 GL may check the equipment or insist on having the equipment checked in the presence of the Surveyor. Ultrasonic testing equipment and accessories may be checked within the scope of examin-ing the testing personnel.

D.5 Vendor inspection

Such components have to comply with the relevant specifications. The grade of supervision depends on the type of component. A type approval of special components to be performed by GL HO or another authorized agency. Statutory Rules and Regulations have to comply with.

E Survey and Inspection Scheme

E.1 General

E.1.1 The survey and inspection plan has the task to all areas of concern: • Granting of human live and avoiding of death • Protection of the environment • Saving of properties, operating interests and other economic aspects

E.1.2 This survey and inspection scheme gives an overview of periodical and non-periodical surveys and inspections. The periodical surveys to be performed on an annual basis, except the underwater sur-vey to be carried out 3 months later after installation of the jacket. The next underwater survey to be per-formed after 9 months and is subject to the regular annual survey. Periodical inspections of Offshore Wind Turbines and Transformer substations is specified in BSH Standard”Design of Offshore Wind Tur-bines”.

E.2 Periodical surveys

E.2.1 Above water

General components: • stairs, ladders and other access facilities • boat landing • walkways

Structural components: • steel members for both jacket and topside (dents, deformations, fatigue cracks, bolt pre-tension) • corrosion protection (painting) • foundation structures

E.2.2 Splash zone • denting of the structure




• missing and deformed structural members • pittings • marine growth

E.2.3 Submerged zone

Structural components: • general damages • jacket structure incl. boat landing, mud plates, pile guide supports, incl. grouting • J-tubes incl. cable entrances • cathodic protection system

Other requirements:

marine growth • scour protection and scouring of the sea bed • build-up of cuttings or sediments • movements in bottom sediments • biological activities

E.2.4 Initial condition survey

During the first year of operation of the installation, an initial condition survey should be carried out in order to enable an overall assessment of the ability of load bearing structures to meet the redefined ac-ceptance criteria. The condition survey will be able to disclose damage or defects if any at an early stage such that corrective measures can be initiated as needed.

This activity may be included as part of the first periodic survey program.

E.3 Non-periodical survey

General such surveys are required after extreme/accidental events or as a result of operational changes. A non-periodical inspection, subject to evaluation, be advanced or delayed to coincide with a scheduled inspection program, if reasonable. Following surveys are subject to non-periodical matters: • Damage and repair of structural components

List of possible damages: • structural • excessive deformations • fabrication or installation • due to overloading • due to man-made hazards • corrosion, etc.

F Non-Destructive Testing

F.1 General

F.1.1 Prior to commencement of fabrication the manufacturer shall submit a plan for NDT and NDT procedures.

F.1.2 Welds shall be subject to NDT in progress with the fabrication. The results of these activities shall be consecutively reported to Owner/Purchaser.




F.1.3 Methods of NDT shall be chosen with due regard to the sensitivity of the method and the method’s ability to detect defects likely to occur as a consequence of the welding process.

F.1.4 Final inspection and NDT of structural steel welds shall not be carried out before 48 hours after completion, except where PWHT is required. The time delay may be reduced to 24 hours for steel grades with SMYS of 355MPa or lower, and for steel grades with SMYS of 420 MPa or lower for plate thickness below 40 mm, provided delayed have not been observed for the materials and/or welding con-sumables in question.

F.1.5 When heat treatment is performed, the final NDT shall be carried out when all heat treatments have been completed.

F.1.6 All NDT shall be properly documented. The reports shall identify all defects exceeding the acceptance criteria unless more stringent reporting requirements have been agreed. All weld repair shall be documented.

F.1.7 The reporting system shall ensure that there is no doubt what is examined and where it is examined. The system shall give a clear and exact description of reportable defect location.

F.1.8 Nevertheless, where defects requiring repair are located by the discretionary inspection a de-tailed report shall be submitted to the Owner/Purchaser.

F.2 NDT procedures

NDT procedures shall be performed in accordance with agreed written procedures, that as a minimum are in accordance with GL Rules and give detailed information on the following aspects: • applicable codes and/or standards • materials, dimensions and temperature of tested material • periodically verification of equipment requirements • joint configuration and dimensions • technique (sketches / figures) to be referenced in the NDT report • equipment and consumables • both calibration techniques and references • testing parameters and variables • assessment of imperfections and the surfaces from which the examination has been performed • reporting and documentation of results • personal qualifications • acceptance criteria

F.3 Selection of NDT - Methods for different materials

For different types of materials following selection of NDT methods shown in Table 4.25.

Table 4.25 Minimum scope of non-destructive inspections and testing [%]

Welds Clad NDT

method Material Butt T-

joint T-joint partial Fillet

Plate Cast-ings

For-gings weld plate

VT All x x x x x x x x x

MT Ferromag-netic steels/ Duplex 1

x x x x x x x - -




PT Aluminium/Cu alloys/ SS/Duplex 2

x x x x x x x x -

RT All x4 - - - - 3 3 - -

UT 5 All excl. Cu x x 3 - x x X x x

ET 3 All x x x x x 3 3 x -

1 Methods are applicable with limits for Duplex, shall be approved case by case 2 May be used for other materials also after special approval in each case 3 May be approved after special approval in each case 4 Recommended for t ≤ 40 mm 5 Only applicable for welds with t ≥ 8 mm

F.4 Extent of NDT

F.4.1 The required minimum extent of testing is given in Table 4.26. For the categorization of struc-tural members, see A.2; the relevant weld connections are to be subdivided accordingly.

The extent shall be based on type and level of design stresses and on the importance of the connection in question. The welds shall be assigned inspection categories equal to be highest structural category of the two components.

All welds which will become inaccessible or very difficult to inspect in-service to be non-destructive tested over their full length, according to the scope of non-destructive inspections and testing, see Table 4.26.

The specified percentage to be tested in Table 4.26 refers to the total length of welds for each structural member category.

Table 4.26 Minimum scope of non-destructive inspections and testing [%]

Category of structural members/ Type of inspection

Special structural members

Primary structural members

Secondary structural members Type of

connection

VT RT UT 4 MT or PT 5 VT RT UT 4

MT orPT 5

VT RT UT 4 MT or PT 5

Butt welds 100 10 1 100 100 100 10 1 100 100 100 - - spot 3

T-joints (full pene-tration) 100 - 100 100 100 - 100 100 100 - - spot 3

T-joints (partial penetration) 100 - 2 100 100 - 2 100 100 - - spot 3

Fillet welds 100 - - 100 100 - - 100 100 - - spot 3

1 The radiographic testing may be replaced by ultrasonic testing, if the weld thickness is ≥ 40 mm 2 Where partial penetration T-joints are admissible in highly stressed areas, an ultrasonic inspection to determine the size of

incompleteness and the soundness of the welds may be required 3 Approximately 2 to 5 % 4 Ultrasonic examination shall be replaced by RT for plate thickness < 8 mm 5 Selection according to Table 4.25

F.4.2 The respective inspection and testing plans as well as relevant specifications shall be com-piled by the manufacturers for each structure, taking into consideration their individual design and stress




conditions. These plans and specifications shall contain the scope and methods of inspection and testing as well as the acceptance limits for surface and internal weld defects. These limits are intended to be used for the assessment of the welds in accordance with common standards, e.g. ISO 5817, details see C.6.6.

Particular attention is to be paid to materials and processes (possibilities for faults) besides the stresses.

When partial testing is defined for welds in an area, the testing shall be done in areas of welds most sus-ceptible to weld defects, like on the transition of different thickness or crossing of longitudinal and/or circumferential welds.

Specifications and plans shall be submitted to GL for approval.

F.4.3 All non-destructive testings of complicated and heavy structures (e.g. tubular node joints and other, thick-walled T-, K- and Y-connections) or of higher-strength and especially high-strength structural steels shall be performed not earlier than specified in F.1.4 after weld completion in order to be able to detect delayed cracking. Where components are subjected to post-weld heat treatment, the non-destructive testings shall be performed after this heat treatment.

F.4.4 Should the inspections and tests reveal defects of any considerable extent, the scope of the tests shall be increased. Unless otherwise agreed, tests shall then be performed on two further sections of weld of the same length for every weld section tested and found to be in need of repair. Where it is not certain that a defect is confined to the section of weld under test, the adjoining weld sections shall be additionally tested. GL may stipulate further inspections and tests, especially in the event of doubts as to the professionally competent and satisfactory execution of the welds.

F.4.5 Repaired welds shall be re-inspected. Where welds have been completely remade, retesting at least equal in scope to the initial inspections shall be performed in accordance with the Surveyor’s in-structions. Re-inspections are to be identified as such in test reports and on radiographs, e.g. by a letter “R“(= repair) placed next to the reference of the radiograph.

F.4.6 The repair rate shall be submitted regularly on a weekly basis to the Owner/Purchaser.

F.4.7 Frequent repairs shall result in increased extent of NDT. The extent of NDT shall be increased in a manner such that all relevant defects are discovered in the areas of concern and representative sam-pling is carried out on all welds.

A reduction of examination can be agreed with the Owner/Purchaser after the quality level has been re-stored.

F.4.8 If severe defects (i.e. cracks, other planar defects or excessive slag lines) occur repeatedly, all welds made with the same welding procedure during the period in question, shall be examined full length.

F.4.9 NDT shall cover start and stop points of automatically welded seams, except in the case if run-on, run-off tabs are used.

F.5 Inspection personnel, supervisors

F.5.1 Inspection personnel (inspectors)

F.5.1.1 The non-destructive weld tests shall be performed by persons trained in the use of the test method concerned, possessing adequate practical experience and qualification according to the type of testing/inspection applied, see both EN 473 and ISO 9712. GL shall be supplied with appropriate docu-mentary proof of such training and experience, e.g. conforming to both EN 473 and ISO 9712.

F.5.1.2 Inspection of welds by NDT shall only be performed by inspectors holding a Level II of EN 473, ISO 9712 or ASNT SNT-TC-1A. Moreover personnel who inspect tubular nodes shall be especially qualified in accordance with API RP 2X.

F.5.2 Inspection supervisors

F.5.2.1 An appropriately qualified works inspection supervisor shall be available for scheduling and monitoring the performance of the non-destructive weld tests and evaluating the results. Personnel re-




sponsible for all NDT activities shall be qualified to EN 473, ISO 9712 or ASNT SNT-TC-1A, Level III. Additional qualification in accordance to API RP 2X is required.

F.5.2.2 The inspection supervisor is responsible for ensuring that the non-destructive weld tests are competently and conscientiously carried out and recorded by suitable inspectors in accordance with these Rules, the relevant codes, standards and the approved inspection schedule.

F.5.2.3 When using the services of outside inspection bodies, the works shall ensure that the above conditions are satisfied and shall inform GL accordingly.

F.6 Acceptance criteria for NDT

F.6.1 All welds shall show evidence of good workmanship. For primary and special members the visual inspection and NDT acceptance criteria level shall comply with ISO 5817 level B for surface imper-fections (5817, Table 1, item no. 1 and 3) and level C for internal imperfections (ISO 5817, Table 1, item no. 2). Welds of category secondary shall comply with ISO 5817 level C, unless otherwise stated in speci-fications, drawings, etc. For aluminium and aluminium alloys ISO 10042 shall apply, see also ISO 17635: NDT of welds.

F.6.2 Acceptance of defects exceeding the given limits may be granted based on fracture mechan-ics testing and appropriate calculations. If this approach is considered, the inherent inaccuracy of the NDT method shall be considered when the critical defect size is determined.

F.6.3 The soundness of welds shall comply with the acceptance criteria for each of the NDT method used. Defects exceeding the limit shall be repaired and after repair welding has been performed, the com-plete weld, (i.e. the repaired area plus at least 100 mm on each side) shall be subjected to at least the same NDT methods as specified for the original weld.

F.7 Final report

F.7.1 General

All NDT shall be properly documented in such a way that the performed testing can be easily retraced at a later stage. The report shall identify the unacceptable defects present in the tested area, and a conclu-sive statement as to whether the weld satisfies the acceptance criteria or not.

The report shall include a reference to the applicable standard, NDT procedure and acceptance criteria.

F.7.2 Minimum requirements shall be stated: • object and drawing references • place and date of examination • material type and dimensions • post-weld heat treatment, if required • location of examined areas, type of joint • welding process used • name of the company and operator carrying out the testing including certification level of the opera-

tor • surface conditions • temperature of the object • number of repairs, if specific area repaired twice or more • contact requirements, e.g. order no., specifications, special agreements, etc. • sketch showing location and information regarded detected defects • extent of testing • testing equipment used • description of the parameter used for each method • description and location of all recordable indications




• examination results with reference to the acceptance level/criteria • number of repairs to be specified in the report, such as once or twice repairs in the same area

G Visual Inspection

G.1 General

The testing shall be performed during fit-up (see APIRP2X, Chapter 7.6) and in the as-welded condition in accordance to ISO 17637, in case of post-weld heat treatment after the treatment. The results of the vis-ual inspection during the fit-up shall be reported to ascertain areas likely to result in poor weld quality or areas where the root location has been modified.

G.2 Testing conditions

Details see ISO 17637, Chapter 2

G.3 Testing volume

Details see Table 4.26.

G.4 Evaluation of indications

The weld shall be examined to determine whether it meets the requirements of the acceptance criteria in accordance to F.6.1.

Details see ISO 17637, Chapter 4.4.1 to 4.4.5.

Additional requirements: • any attachments temporarily welded to the object shall be removed. The area where the attachment

was fixed shall be checked to ensure freedom of unacceptable imperfections • all sharp corners adjacent to the weld are to be rounded. Preparation of edges/structural shapes to

be prepared to an acceptable surface finish.

Non-ferreous materials:

Weld zones in stainless steels, Nickel and Titanium alloys shall be visually inspected and fulfil the criteria for oxidation levels (surface colour).

G.5 Visual testing of repaired welds

When welds fail to comply wholly or in part with the acceptance criteria and repair is necessary, the fol-lowing actions shall be made: • If removal of metal is required and the remaining thickness is less than that specified repair welding

shall be done according to an approved procedure • If the weld is partly removed it shall be checked that the excavation is sufficiently deep and long to

remove all imperfections. It shall also be ensured, that there is a gradual taper from the base of the cut to the surface of the weld metal at the ends and sides of the cut. The width and profile of the cut must be prepared such that there is adequate access for re-welding, see also C.6.7.3.

• It shall be checked that, when a cut has been made through a faulty weld and there has been no se-rious loss of material, or when a section of materials containing a faulty weld has been removed and a new section is to be inserted, the shape and dimensions of the weld preparation meet the re-quirements, see also C.6.3.4.

• In case where part of a weld is gouged out the excavated area shall be ground and magnetic particle testing should be carried out prior to re-welding in order to ensure that the imperfection is removed, see also C.6.7.6.

G.6 Acceptance criteria

The quality shall comply with the requirements of F.6.1.




G.7 Reporting The information specified in F.7.2 shall be included in the report.

H Magnetic Particle Inspection

H.1 Scope

Magnetic particle testing (MT) techniques apply for the detection of surface imperfections in ferromagnetic steels, forgings, castings and welds including the heat affecting zones using continuous wet or dry method.

MT shall be carried out in accordance with ISO 17638.

H.2 Information

Required information prior to testing, see F.3.

H.3 Equipment

Unless otherwise agreed the following types of alternating current-magnetizing equipment shall be used: • electromagnetic yoke • current flow equipment with prods • adjacent or threading conductors of coil techniques

This equipment shall be in accordance to ISO 9934-3.

H.3.1 Alternating current

The use of alternating current gives the best sensitivity for detecting surface imperfections and an AC electromagnetic yoke shall be used. Each AC electromagnetic yoke shall have a lifting force of at least 45 N (10 lbs.) at the maximum pole space that will be used.

H.3.2 Direct current

Current flow equipment with prods and adjacent or threading conductors or coil techniques shall be spe-cially approved by the Society in each case. Each DC electromagnetic yoke shall have a lifting force of at least 174 N (40 lbs.)

Unless otherwise agreed, use of permanent magnets shall be avoided due to limitations of the different equipment and the difficulty to obtain sufficient magnetic field/strength for several configurations.

Where prods are used, precautions shall be taken to minimize overheating, burning or arcing at the con-tact tips. Removal of arc burns shall be carried out where necessary. The affected area shall be tested by a suitable method to ensure the integrity of the surface. The prod tips should be lead, steel or aluminium to avoid copper deposit on the part being tested. Prods should not be used on machined surfaces.

When black light is used the black light must be capable of developing the required wavelengths of 330 to 390 nm with an intensity at the examination surface of not less than 1000 µW/cm² when measured with a suitable calibrated black light meter.

H.4 Overall performance test

Before testing begins, a test is recommended to check the overall performance of the testing. The test shall be designed to ensure a proper functioning of the entire chain of parameters including equipment, the magnetic field strength and direction, surface characteristics, detecting media and illumination.

H.5 Surface preparation

The testing surfaces shall be free from loose scale, rust, weld spatter and other impurities (i.e. oil, grease, dirt, heavy and loose paint). Notches, grooves, scratches, edges and other surface irregularities which can produce false indications, are to be removed by grinding or machining prior to the test. The surface shall be in accordance to acceptance level 1 of ISO 23278, see H.10.




When testing of welds is required, the surface and all adjacent areas within 20 mm has to be prepared as described above.

There shall be a good visual contrast between the indications and the surface under test. For non-fluorescent technique, it may be necessary to apply a uniform thin, adherent layer of contrast paint. The total thickness of any paint layer shall normally not exceed 50 µm.

H.6 Testing

Testing shall be done in accordance to ISO 17638

H.7 Detecting media

H.7.1 General

The detecting media shall be in accordance with ISO 9934-2.

H.7.2 Dry powder

The colour of the dry particles shall provide adequate contrast with the surface being examined and it may be of fluorescent or non-fluorescent type. Dry particles shall only be used if the surface temperature of the test object in the range from 57 °C to 300 °C.

H.7.3 Wet particles

The colour of the wet particles shall provide adequate contrast with the surface being examined and they are available in both fluorescent or non-fluorescent type. These particles are suspended in a suitable liquid medium such as water or petroleum distillates. The temperature range of wet particle suspension and the test object surface shall be in the range between 0 °C and 57 °C. For temperature below 0 °C or above 57 °C, procedures approved in accordance with recognized standard for this purpose shall be used. For temperatures exceeding 57 °C dry powder shall be used.

H.7.4 Fluorescent particles

With fluorescent particles the testing is performed using an ultraviolet light, called black light. The testing shall be performed in darkened area where the visible light is limited to a maximum at 20 lx.

The test surface shall be viewed under a UV-A radiation source. The UV-A irridiance at the surface in-spected shall not less than 10 W/m2 (1000 μW/cm2).

H.7.5 Visible light intensity

The test surface for colour contrast method shall be inspected under daylight or under artificial white lu-minance of not less than 500 lx on the surface of their tested object. The viewing conditions shall be such that glare and reflections are avoided.

H.8 Application of detecting media

After the object has been prepared for testing, magnetic particle detecting medium shall be applied by spraying, flooding or dusting immediately prior to and during the magnetization. Following this, time shall be allowed for indications to form before removal of the magnetic field.

When magnetic suspension are used, the magnetic field shall be maintained within the object until the majority of the suspension carrier liquid has drained away from the testing surface. This will prevent any indications being washed away.

H.9 Evaluation of imperfections

Certain indications may arise from spurious effects of scratches and change of section, which create a boundary between regions of different magnetic properties. These are defined as false indications and not as real imperfections. The operator shall carry out any necessary testing and observations to identify such false indications.




H.10 Acceptance criteria

The quality for welds shall comply with the requirements of F.6.1, i.e. linear indications in accordance to ISO 23278 are not acceptable. Any linear indications shall be ground and re-examined.

H.11 Reporting

Additionally to the items listed in F.7.2, following details have to be included in the testing report: • type of magnetization equipment • type of current • detection media • viewing conditions • demagnetization, if required • lifting force • other means of magnetic field strength verification.

I Penetrant Testing

I.1 Scope

This part describes penetrant testing (PT) used to detect imperfections which are open to the surface of the tested material. PT in accordance with ISO 3452-1 should be used for non-magnetic materials.

I.2 Equipment / testing material

Details see ISO 3452-1, Chapter 5.4 and 6.1, Table 1.

I.3 Compatibility of testing materials with the parts to be tested

Details see ISO 3452-1, Chapter 7.1.

I.4 Preparation, pre-cleaning and testing


I.5 Application of penetrant

Details see ISO 3452-1, Chapter 8.3.1 to 8.3.3

I.6 Excess penetrant removal

The application of the remover medium shall be done such that no penetrant is removed from the defects/ discontinuities.

The excess penetrant shall be removed using a suitable rinsing technique. Examples: spray rinsing or wiping with a damp cloth. The temperature of the water shall not exceed 50 °C.

General, the excess penetrant shall be removed first by using a clean lint-free cloth. Subsequent cleaning with a clean lint-free cloth lightly moistened with solvent shall then be carried out.

Any other removal technique shall be approved by the contracting parties particularly when solvent re-mover is sprayed directly on to the object to be tested.

First the excess water washable penetrant shall be removed with water. Subsequent cleaning with a clean lint-free cloth, lightly moistened, with solvent shall be then carried out.

I.7 Drying

Details see ISO 3452-1, Chapter 8.4.7.




I.8 Application of developer


I.9 Development time

Details see ISO 3452-1 Chapter 8.5.7

I.10 Viewing conditions


I.11 Acceptance criteria

The quality shall comply with the requirements of F.6.1,

i.e. cracks, laps and lack of fusion , which open to the surface, are not acceptable.

These linear indications shall be ground and re-examined.

I.12 Post cleaning and protection

After final inspection, post cleaning of the object is necessary only in those cases where the penetrant testing products could interfere with subsequent processing or service requirements.

If required a suitable corrosion protection shall be applied.

I.13 Reporting

Additionally to the general terms in F.7.2, following items to be included: • penetrant system used, e.g. coloured or fluorescent • penetrant product • application methods • penetration and development time • viewing conditions • test temperature

J Radiographic Testing

J.1 Scope

This part applies to the radiographic testing (RT) of fusion welded joints in metallic materials. RT shall be carried out in accordance with ISO 17636, Class A.

J.2 Radiation sources

Details see ISO 17636, both Chapters 7.2.1 and 7.2.2

J.3 Films and intensifying screens

Details see ISO 17636, Chapter 7.3.

For different radiation sources, the minimum film system classes and type and thickness given in both Tables 3 and 4 of ISO 17636 shall be used.

J.4 Radiographic parameters

J.4.1 Surface preparation





J.4.2 Film identification

Details see ISO 17636, both Chapters 6.4 and 6.5.

J.4.3 Overlap of films


J.5 Types and position of Image Quality Indicator (IQI)

Details see both ISO 19232-1 or 19232-2, Chapter 6.7

J.6 Test arrangement

The radiographic techniques in accordance with ISO 17636-1, Chapter 7.1.1 to 7.1.9 shall be used.

J.7 Choice of tube voltage and radiation source

J.7.1 X-ray devices up to 500 KV

Details see J.2.

J.7.2 Other radiation sources

Details see both, J.2 and ISO 17636-1, Chapter 7.2.2.

J.8 Film systems and screens

Details see both, J.3 and ISO 17636-1, Table 2.

J.9 Alignment of beam


J.10 Reduction of scattered radiation

J.10.1 Filters and collimators


J.10.2 Interception of back-scattered radiation


J.10.3 Source to object distance

Details see ISO 17636-1, Chapter 7.6

J.11 Density of radiographs

Details see ISO 17636-1, both Chapter 7.8 and Table 5.

J.12 Film processing

Details see ISO 17636-1, Chapter 7.9

J.13 Viewing conditions


J.14 Acceptance criteria

Details see F.6.1




J.15 Reporting

Additionally to the general terms in F.7.2, following items to be included: • radiographic technique and class • type and position of image quality indicator • IQI sensitivity • test arrangement in accordance with ISO 17636-1, Chapter 7.1 • system of marking used • film position plan • radiation source • etc.

Further details see ISO 17636-1, Chapter 8.

K Ultrasonic Testing

K.1 Scope

This part specifies techniques for the manual ultrasonic testing (UT) of fusion-welded joints in metallic materials of thickness greater than or equal to 8 mm. It is primarily intended for use on full penetration welded joints where both the welded and parent material are ferritic.

UT of welds in plate and tubular butt welds and double side welded joints shall be performed in accor-dance with ISO 17640 or API RP 2X and in tubular nodes (see Figs. 4.24 and 4.25) in accordance with API RP 2X. This part specifies especially node joint testing not specified in ISO 17640. Testing in accor-dance to ISO 17640 shall be performed on examination level C.

Alternative detection and sizing methods, e.g., Time-of- Flight Diffraction (TOFD), Automated UT, AUT Imaging, Computerized UT, or Computerized Imaging may be used to supplement the recommended manual UT methods. However, TOFD shall not be used as the sole method of sizing. Where these meth-ods have not been reduced to routine practice, they shall have demonstrated an acceptable track record of verification, as well as having been represented in successful practical tests for personnel qualification.

K.2 Applicability of UT to offshore structures

Offshore structures require extensive use of T, K and Y tubular member intersections and ring and dia-phragm-stiffened tubulars. Considering the limitations of alternate methods, ultrasonic examination is recommended for the detection of internal discontinuities and MT for surface imperfections. Therefore in accordance to F.4, Table 4.26 butt welds and T-Joints (full penetration) in special and primary structural members shall be 100 % UT and 100 % MT inspected and only 10 % RT in case of butt welds.

K.2.1 Advantages and limitations of UT of welds

UT has a heavy dependence on the skill and training of personnel, therefore UT inspection personnel who inspect tubular nodes shall be especially qualified in accordance with API RP 2X. This section de-fines the major attributes and limitations of the ultrasonic technique.

K.2.1.1 Detection

Radiography is most sensitive to three-dimensional discontinuities, such as lack-of-penetration, slag in-clusions, and porosity. Other discontinuities, such as cracks and lack-of-fusion, are less reliably detected, especially when oriented askew of the radiation beam. In order to be readily discernible on the film, the thickness of the discontinuity parallel to the radiation beam must be on the order of two percent of the weld thickness. As the thickness of the weld increases, the quality of the discontinuity image decreases due to radiation scattering within the weld. Therefore RT may be completely replaced by UT, if the weld thickness is > 40 mm (see note 1 of Table 4.26).

In contrast to RT, the UT is highly sensitive to two-dimensional discontinuities and less sensitive to three-dimensional ones. Tight cracks and lack-of-fusion discontinuities are also difficult to detect with UT; how-ever, the threshold limit for UT is considerably smaller than for RT.




After limitation in the use of shear wave UT is the failure to detect large two-dimensional (planar) discon-tinuities as a result of the inherent direction of the reflected beam. Large flat discontinuities reflect the acoustical energy away from the receiving transducer and therefore often go undetected in single-angle or single-transducer examination. Conversely, small discontinuities produce a scattering of reflected en-ergy over a broad angular envelope, increasing the chance of detection.

The direction of the ultrasonic beam also causes difficulty in detection of certain types of discontinuities. For example, single pores or randomly dispersed porosity is particularly difficult, if not impossible, to de-tect at sensitivities recommended for the detection of planar discontinuities significant to service perform-ance. Spherical discontinuities (and to a lesser extent those of cylindrical shape) have only a fraction of their area perpendicular to the acoustic beam and therefore do not return acoustical energy proportional to their physical size.

In most cases, this limitation should not be cause for great alarm since the influence of such internal im-perfections is limited as stress raiser.

Based on the high influence of surface imperfections to the fatigue strength reference value the required quality levels for such imperfections are higher than for internal imperfections. Therefore the quality level B of ISO 5817 is required for surface imperfections detected by VT, MT and UT in some cases, like root imperfections in tubular node joints without back welding and level C for internal imperfections detected by UT and in case of butt welds 10 % RT (see Section 3, I, B.15.1.2 and F.6.1)

K.2.1.2 Evaluation

Evaluation implies the identification and sizing of detected discontinuities since some discontinuities, such as porosity and isolated slag, are of little significance. It is worthwhile to identify the character and source of each anomaly, not only to effect removal of those of rejectable size but also to permit preventive cor-rective action.

The information yielded by UT is only a portion of that required for identification. To reach a reasonable conclusion as to indication type, qualified UT personnel must be thoroughly familiar with the welding proc-ess and the degree of perfection in fit-up, and have accurately located the reflector within the weld profile. Combining this information with that obtained by secondary manipulations of probe angle and beam, qualified UT personnel are able to reliably identify only three geometric discontinuity configurations with spherical, cylindrical, and planar. The technique is inherently incapable of differentiation between such discontinuities as lack-of-fusion at the root and incomplete penetration or, more importantly, the coinci-dental presence of cracks radiating from these fusion defects.

For UT discontinuity size measurement are two techniques commonly practiced, each with its own advan-tages.

The first, called amplitude measurement, is rather simple and particularly suited to measuring discontinui-ties of small size that can be completely contained within the cross section of ultrasonic beam. Accuracy of the technique deteriorates rapidly as discontinuities increase in size and approaches a limit of applica-bility when the discontinuity exceeds 6 mm. The major disadvantage of the technique lies in the fact that the total reflecting area is responsible for the echo height, rendering it impossible to differentiate between length and width of the reflector.

The second, called beam boundary intercept technique, offers greater advantage in determining both the length and width of most discontinuities of significance. Theoretically the technique is accurate when used properly. However, the unavoidable variations in probe characteristics plus the requirements for pre-calibration of equipment and the need for exceptionally skilled personnel somewhat diminish the accuracy of results (seldom better than 1.6 mm).

K.2.1.3 Influence of material properties, thickness, surface finish and curvature

Some steel manufacturing processes, such as control rolled and thermo-mechanical controlled process (TMCP= TM) steels, can cause minor variations in acoustic properties. This effect on velocity may cause changes to the transducer beam angle in comparison to the reference standard.

The physical condition of the weld and base metal significantly influence the detection and evaluation of weld flaws. The acoustical characteristics of the base metal will determine the amount of sound lost through scattering and absorption to interception with the weld discontinuity. Inclusions and lamination in the base metal will misdirect shear wave beams away from intended areas of examination and produce reflections from unknown und unidentified areas.




Therefore where ultrasonic testing is to be performed on steel produced by control rolled or TMCP cali-bration (reference) blocks shall be produced both perpendicular to, and parallel to, the direction of rolling. The rolling direction shall be clearly identified.

The reference block shall be either: 1. a block of the same acoustic properties, and specified surface finish, and having a thickness within

+/- 10 % of the test object; or

2. a block of the same acoustic properties, surface finish, shape and curvature as the test object.

In the case of Type 1), correction for any differences in attenuation, curvature and coupling losses is re-quired before equipment performance checks and Distance Amplitude Curve (DAC) can directly applied.

K.3 Procedure Qualification and Approval

The following applies to procedure qualification and approval: • Written UT-Procedure shall be prepared by the UT supervisor. • The following list of essential variables shall be detailed in the written procedure and employed dur-

ing trial and subsequent weld examinations. Significant variations from the proven procedure shall be cause for requalification.

• Type of weld configuration and surface temperature range • Acceptance Standard/Criteria • Type of UT instrumentation • Use of electronic gates, suppression, alarms, and the like • Equipment calibration and frequency (time intervals in acc. to API RP 2X, Table 1) • Equipment performance checks and frequency (min. requirements see API RP 2X, Table 2 und

K.4.1) • Length of coaxial cable • Transducer frequency, size and shape, beam angle and type of wedge an angle beam probe • Surface preparation • Couplant (shall be in accordance with EN 583-1 or equivalent) • Base metal examination • Transfer correction (shall be in accordance with API RP 2X, Chapter 7.7.4, EN 583-2 or equivalent) • Scanning sensitivity • Scanning pattern • Triangulation methods for determining effective beam angle, indexing of root area, and flaw location. • Method of discontinuity length determination • Method of discontinuity width determination • Computer hardware and software used for locating and sizing reflectors. • Reporting and retention

K.4 UT Equipment

The equipment used for ultrasonic testing has to: • be applicable for the pulse echo technique and for the double probe technique • cover the frequency range from 2 to 6 MHz • allow echoes with amplitudes of 5 % of full screen amplitude to be clearly detectable under test con-

ditions • include straight beam transducers, and angle beam transducers of 45°, 60° and 70° • see also API RP 2X, Chapter 7.5.1 and 7.5.2




K.4.1 Equipment performance checks

Since determination of the reflector location and size is the primary intent of an ultrasonic examination, it is essential for the instrumentation to yield accuracy commensurate with the examination requirements. Before any new instrument or component is employed for weld examination, it shall be examined to de-termine its performance characteristics with respect to the requirements of these rules.

A standard operating procedure and quality control manual shall detail the method adapted to assure traceability based on the following requirements: • Equipment Performance Checks (Standardization) shall be performed with the Standards (e.g. cali-

bration blocks) of API RP 2X, table 2 considering the following requirements • The IIW and IOW calibration blocks shall be fabricated from material to be examined (see K.2.1.3) • Transducer beam angle:

Beam angle determination using the large hole (for angles of < 70°) in the IIW block may contain a significant error, therefore, transducers employed for evaluating discontinuity position and size be further calibrated using the IOW block (see Fig. 4.36).

• Beam Profile: The IOW block should be employed to determine the characteristics and shape of each beam pro-file. The intensity of any echo signal rise from a secondary axis should not exceed 10 % of the major axis intensity.

• Determining the Beam Spread: The beam boundary is defined as the surface of a cone where the echo intensity from a small reflec-tor intersecting the beam will be some predetermined percent of the maximum echo obtained from the same reflector on the beam axis. Commonly defined boundaries are those where the intensity is 6 dB and 20 dB below the maximum signal. The IOW block is designed to facilitate the measurement of the cone angle at varying distances from the index point for plotting the profile in both the vertical and horizontal plan.

• Resolution: The IOW block is employed to assess the resolution capabilities of each instrument-transducer com-bination. The ability to independently resolve two closely spaced reflectors on the sound path is a mandatory requirement for accurate weld-quality assessment. For example, the differentiation of a root discontinuity from the root protrusion of an acceptable weld is required when examination must be conducted from one side only (see Fig. 4.25). Using the five 1.5 mm resolution holes as reflectors, one should be able to clearly separate the indi-vidual holes on 4 mm spacings with a 45° transducer oscillating at 2.25 MHz.

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Fig. 4.36 Institute of Welding (IOW) block




K.5 Preparation for examination

Prior to attempting evaluation of a weld in the structure, UT personnel should become thoroughly familiar with the design by reference to the specification and construction drawings. The nominal thickness of specific connections should be noted and compared to values obtained by measurement during the weld examination.

The results of the visual inspection during the fit-up should be reviewed to ascertain areas likely to result in poor weld quality or areas where the root location has been modified.

A final visual inspection of the weld should be performed for detection of undercut, incomplete fill or ex-cess roughness which would interfere with a meaningful ultrasonic examination.

K.5.1 Index of weld root

Subsequent to the weld bevel preparation, including any field trimming during fit-up, and prior to welding of the root pass, the member from which scanning is to be performed should be scribed or punch-marked at a specific distance from the root face to ensure an exact index of the root face location after completion of welding (see Fig. 4.37).

This marking is particularly important for T, K, and Y connections where measurement from other index points becomes difficult or impossible after completion of welding. These marks provide an exact location of the root face during the ultrasonic examination and aid in differentiation of root defects from acceptable root protrusions.




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Fig. 4.37 Weld root index

K.5.2 Surface preparation

The surface from which scanning is to be achieved should be cleaned to remove all scale or coating (for example by grit-blasting or power brushing) to assure continuous coupling during the examination, par-ticularly if flaw sizing is based on amplitude technique. Regardless of the quality of the surface finish, it is recommended that transfer corrections be utilized in all cases (see EN 583-2, Chapter 8 and Annex E).

K.5.3 Thickness measurement

The thickness of each member from which scanning is to be achieved should be determined and re-corded for use in the flaw location determination. Thickness should be ascertained at four points around the circumference on tubular members and every 2 m along welds in flat plate connections.




K.5.4 Base metal examination

The entire area from which scanning is to be achieved should be examined by the longitudinal wave tech-nique to assure freedom from lamination or other laminar-type flaws which could interfere with sound wave propagation. If defects or considerable variation in attenuation are found, it is important that their influence on the weld examination be taken into account and the scanning technique adjusted to ensure complete examination of the weld.

K.6 Scanning techniques

Probe manipulations employed to detect discontinuities in the weld area are termed scanning und the success of the entire examination depends on the selected probe, instrument sensitivity, and transducer movements employed during this phase of the examination.

K.6.1 Probe selection

The selection of a scanning probe is generally a compromise between sensitivity, coverage, resolution, and mechanical coupling stability. Large high-frequency probes produce a narrow ultrasonic beam of high-resolution capabilities and a reduced sensitivity to detection of discontinuities obliquely oriented to the sound beam.

Consequently, examination of welds in thick sections with miniature and small dimension probes gener-ally will require two or more scans at different sensitivity levels and base line adjustments to ensure that all discontinuities of interest will reflect sufficient energy to be visible on the cathode ray tube.

For angle beam scanning of weld discontinuities of the size of interest in offshore structural fabrication, a probe with a transducer element of 12 mm round or square, operating between 2 MHz and 2.25 MHz is suggested as a good compromise of all desirable attributes. The beam produced from this probe will permit examination of thick sections with moderate beam decay, resolve most discontinuities of interest without indicating the presence of fine inclusions within the steel, and exhibit a beam profile included an-gle sufficiently broad to ensure that discontinuities at all orientations between the nominal 45°, 60°, and 70° angles will yield a significant echo signal. Other transducer frequencies and sizes may be desirable for specific applications.

K.6.2 Sensitivity

Scanning must ensure the detection of all discontinuities of interest; therefore, a sensitivity greater than that required to produce a full-scale echo signal from the reference reflector is always employed. An in-crease in sensitivity 6 dB above that required to produce a full-DAC (Distance Amplitude Correction: A method of correcting for the changes in the ultrasonic wave caused by attenuation or divergence) echo signal from the reference reflector will ensure full- DAC echo signals for discontinuities oriented 7° to 10° from perpendicular when scanning with the recommended probe. An increase of 12 dB above the refer-ence will aid in detection of transient reflectors at high scanning speeds.

K.6.3 Extent of coverage

K.6.3.1 The complex geometry of welded connections on offshore structure particularly T, K, and Y connections, typically requires the use of nominal angle transducers (i.e. 45°, 60°, and 70°) in multiple scans of the entire circumference and almost always from the brace side of the weld only. Thick wall di-agonal braces with full- throated weld connections may also require the use of an 80° transducer to en-sure that the sound beam intercepts the root area.

In all scans the transducer cant angle is continually adjusted to maintain the beam perpendicular to the length of the weld. See Fig. 4.38 for definitions of nomenclature associated with tubular member intersec-tions. The recommended effective probe angle for examination of the root area should be that which pro-duces an incident angle nearest to perpendicular to the anticipated weld discontinuity.




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Fig. 4.38 Parameters associated with geometry of pipe intersection

K.6.3.2 Examination of the weld root areas separately from the remainder of the weld is recom-mended. If the probe is moved parallel to the toe of the weld at the intercept distance (see K.5.1 –index of weld root) to the root protrusion, any change in the root geometry can be ascertained by a lateral move-ment on the horizontal scale of the Cathode Ray Tube (CRT). The generally continuous echo from the root protrusion indicates a sound weld-whereas, an interruption or shift in echo signal position indicates a change in root geometry and the presence of a discontinuity. Detection of the metal path distance shift is enhanced by expanding the horizontal scale to include only the region of interest (see Fig. 4.39)

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Fig. 4.39 Weld root examination

K.6.3.3 After examination of the root area, the remainder of the weld is scanned using a back-and-forth motion accompanied by a slight rotational movement (see Fig. 4.40). The length of the transverse movement must be sufficient to ensure that the center of the beam crosses the weld profile in two direc-




tions, a full-vee path, or a surface distance equivalent to approximately one and one-fort times the skip distance (see Fig. 4.41)

A

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Fig. 4.40 Scanning patterns

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Fig. 4.41 Weld scanning

K.6.3.4 Additional, axial weld scanning is recommended for welded connections in material with 355 MPa SMYS and above and thickness of >= 25 mm for the detection of transverse planar discontinuities (see Fig. 4.40). Scanning coverage should be parallel to the weld axis in two directions and include trans-verse scanning atop the weld reinforcement surface, where practical. If scanning atop the reinforcement is not practical, all efforts for axial weld scanning should be performed from the adjacent base material, each side of the weld. A 45° probe angle is recommended as the primary scan for the detection of trans-verse planar discontinuities.

K.6.3.5 As a minimum, the welds shall be examined through the entire volume of the weld and the heat-affected zone.

K.7 Discontinuity evaluation

Evaluation consists of the various steps required to assess the unknown character, orientation, and size of discontinuities whose presence has been established during scanning. Characterization and orientation are reasonable reliable determinations of a well-executed ultrasonic examination. The methods of evalua-tion are described in the following section.

K.7.1 Characterization and orientation

Weld imperfection geometries corresponding to the cylindrical shape are hollow beads, some slag lines, and un-fused slugs. Single pores and widely spaced porosity produce reflections similar to the ideal spherical reflector.

The group of weld imperfections identified by planar configurations are lack of fusion, un-fused root faces, fusion line slag, undercut, and cracks.

To differentiate between the spherical or cylindrical reflectors from the planar, it is necessary to employ several different angles (see API RP 2X, Chapter 7.9).




K.7.2 Size evaluation

Three methods which produce useful information on sizing are recommended for use on offshore struc-tures: • The amplitude comparison technique • The beam boundary intercept technique • The maximum amplitude technique

In some instances, it will be necessary to employ all three using best judgment for the final estimation of size. However, if the reflector is large with respect to the sound beam, the methods of sizing will generally be restricted to the beam boundary intercept and maximum amplitude techniques.

K.7.2.1 Amplitude comparison technique

Size determination by the amplitude comparison technique is achieved by comparing the discontinuity reflection to the reflection obtained from an artificial reflector in reference blocks. In order to improve the accuracy of sizing of weld discontinuities, which generally are elongated parallel to the weld length, a multitude of artificial reflectors of various widths and lengths should be available for comparison with the discontinuities detected during scanning. Selection of the proper length artificial reflector requires prior determination of the length of the unknown by one of the alternate methods of sizing.

Since the amplitude of a reflector diminishes with increased distances from the probe, it is necessary to construct a DAC curve, e.g. in accordance to EN 583-2, Chapter 6.3 for reflector comparison when the unknown reflector fails to fall at the same distance on the display-screen horizontal scale as the reference reflector.

The inaccuracy is considered too great to qualify a procedure meeting the minimum standards recom-mended herein if the comparison technique is to be the sole method employed (for more details see API RP 2X, Chapter 7.9.3.1).

K.7.2.2 Beam boundary intercept technique

Determination of the transverse dimension of a reflector by the beam boundary intercept technique is accomplished by manipulating the probe to obtain the maximum signal from the reflector and adjusting the echo signal to some convenient screen height, for example, 75 % of full scale. The sensitivity is then increased 20 dB using the calibrated gain control and the probe moved forward until the echo signal re-turns to the original maximized value, that is, 75 % of full scale. At this point, the bottom edge of the 20 dB beam boundary determined by the procedures outlined in K.4.1, will be intercepting the top edge of the reflector (see Fig. 4.42).

The bottom of the reflection is located by moving the probe backward through the maximum position until the echo signal again returns to 75 percent of full screen indicating that the top of the 20 decibel beam profile is intercepting the bottom edge (see Fig. 4.42).

The beam boundary intercept technique is best suited to members approximately 20 mm and thicker and evaluation on the first traverse (half skip) of the thickness.

Multiple determinations should be obtained from the edges of the reflector in order to minimize errors due to coupling variations and contact pressure. At least three attempts should be made from each edge, and the results for all three should fall within 1.6 mm of the same location.

The method yields uncertain results when applied to measuring the depth of corner reflectors, such as lack of penetration in single-sided welds. Alternate methods are suggested for that type of reflector. More about Beam Boundary Intercept Technique see API RP 2X, Chapter 7.9.3.2.




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Fig. 4.42 Beam boundary plotting

K.7.2.3 Maximum amplitude technique

The maximum amplitude technique utilizes the fact that most of the energy in the ultrasonic beam is con-centrated near the center of the beam in a very narrow band. This characteristic is employed to find the top of reflectors by maximizing the reflection as in the beam boundary technique and moving the probe forward until the echo signal just begins to trop from the peak value. The bottom edge is determined by moving the probe backward until the echo signal just begins to drop. Measurements of the sound beam path lengths, A1 and B2, and surface distances for each probe position D1 and D2 see Fig. 4.43.

Best results from the maximum amplitude technique occur when the instrument pulse length is very short and the probes have high-resolution capabilities. Probes of a high diameter-to-wave-length ratio produce a sharply defined central beam and are preferred for this technique. These conditions dictate the use of frequencies above that for normal scanning and the largest practical diameter for the surface conditions of the examination. Probes with a frequency of 4 MHz and diameters of 12.5 mm or larger are recom-mended.

This technique can also be used to determine the diameter of cylindrical shaped defects. More about Maximum Amplitude Technique, see API RP 2X, Chapter 7.9.3.3.

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Fig. 4.43 Maximum amplitude technique

K.7.2.4 Inaccuracies in size elevation

All three of the described techniques are capable of yielding accurate results on flat plate weld examina-tions within the limitations described, but all three suffer significant loss of accuracy when examining tubu-lar members. In the absence of a large number of curved-reference standards of the same geometry as the members to be examined, the accuracy of all techniques diminish due to a spreading of the beam at the coupling interface and the opposite curved wall of tubulars. The inherent broadening of the beam results in reduction of the centerline intensity and alteration of intensity to the beam edge. The magnitude of the error increases with the difference in diameters between reference and examined member.

When scanning in the circumferential direction or at a cant angle to the pipe axis (see Fig. 4.38), the sound beam also changes dimensions depending on the area of contact and the thickness-diameter ratio of the member. Fig. 4.44 illustrates the increase in divergence after the beam has struck the back wall of a tubular during a circumferential scan. The effect is to alter the angular position of the 6 dB, 20dB, or any other constant intensity ray of the beam. This invalidates the beam intercept technique and diminishes the accuracy of the amplitude comparison technique. When possible, the size measurements should be made in the first half-skip distance of the sound beam to minimize the effects of beam spreading at the intercept with the back wall.




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Fig. 4.44 Circumferential beam profile

K.7.2.5 Recommended techniques

In practice the three sizing techniques should be performed in an organized sequence from simplest to most difficult usually in the following order. First, the amplitude comparison technique should be per-formed during initial characterization and orientation assessment with the discontinuity compared to known size implants. The maximum amplitude technique can usually be performed concurrently while the discontinuity remains at the maximized amplitude. The beam boundary technique should be performed if there is uncertainty in the previous sizing estimates or if characterization and orientation suggest using this technique.

The amplitude comparison technique is recommended for evaluating corner reflectors if the anticipated width is not much greater than 3 mm. Length should be approximated by the maximum amplitude tech-nique. For all other circumstances, all possible means should be employed. From the results, a best esti-mate should be offered with considerations to the effects of curvature and configuration.

K.8 Acceptance criteria

The appropriate level of examination is an integral part of the overall fracture control plan, which main-tains a quality level consistent with the design performance. For example, Figs. 4.45 and 4.46 show weld surface profile qualities and fatigue S-N curves which are consistent with each other. For offshore plat-forms designed to fatigue performance Level C (curve X), with as-welded surface profiles, Level C pro-vides a compatible internal examination criteria.

Level C – Experience-based fitness-for-purpose quality – is based upon generalized consideration of fatigue, brittle fracture, and tensile instability modes of failure, and intended for use in applications where the design stress, fatigue analysis, and minimum toughness meet the requirements of API RP 2A-WSD. This also requires consideration of the fracture toughness of welds and heat-affected zones.

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K.8.1 Level C acceptance criteria

The following applies concerning Level acceptance criteria:

K.8.1.1 Discontinuities are evaluated using a combination of beam boundary techniques and ampli-tude calibration utilizing the transfer and distance amplitude corrections addressed in K.2.1.3 and K.7.2.4. The reference level calibration for root reflectors are the 1.6 mm notches shown in the reference block for Level C examination (Fig. 4.47). The reference level for internal reflectors is the side hole with a diameter of 1.6 mm.

Evaluation of a discontinuity is to be based on the ultrasonic operator’s best determination of actual flaw dimensions, bearing in mind that interior planar flaws which are poorly oriented with respect to the ultra-sonic beam may return a smaller echo than a surface (root) notch of the same size.

Flaw size is to be given in terms of width and length as illustrated in Fig. 4.48. In addition, the reflector shall be classified as spherical, cylindrical, or planar and its position within the weld cross section be ac-curately determined. Identification as to type (such as, crack versus incomplete fusion) is not required.




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K.8.1.2 All reflectors 6 dB smaller than the reference level (that is 50 % DAC) may be disregarded (DRL = disregard level). Reflectors above the DRL shall be further evaluated as described in the following sections. The operator may elect to decrease the DRL based on the criticality of a particular component.

K.8.1.3 Isolated, random, spherical reflectors are acceptable, regardless of signal amplitude.

K.8.1.4 Clustered multiple reflectors with indications above 50% DAC curve and two times the branch thickness in length should be evaluated in terms of their encompassing with and length using the limits of Figs. 4.49 and 4.50.

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Fig. 4.50 T, K and Y root defects

K.8.1.5 Cylindrical or planar reflectors whose dimensions exceed the limits of Figs. 4.49 and 4.50 (de-pending on location) are rejectable.

K.8.1.6 Internal reflectors near the fusion line of a weld shall be evaluated with various probe angles while making an effort to obtain a beam path perpendicular to the fusion line.

K.8.1.7 Reflectors within the base metal of the main member (joint can) at welds shall be evaluated as described in the following. In addition, any such reflector that exceeds the disregard level shall be re-ported to the operator:

1. Individual reflectors exceeding the limits of Fig. 4.49 are rejectable. 2. Accumulated reflectors exceeding 8 % of the area under the weld in any 150 mm or D/2 (D = Out-

side diameter of the main member) length (whichever is less) are rejectable. 3. Rejectable base metal reflectors shall be reviewed by the operator prior to any excavation or at-

tempted repair. Consideration should be given to the risk of causing further problems (such as la-mellar tearing) with the additional thermal strain cycles such excavation and repair would entail.

K.9 Reporting

Additional to the items listed in F.7.2, following details have to be included in the testing report: • The details of the examination shall be documented in sufficient detail to permit repetition of the ex-

amination at a later date. • Details of acceptable discontinuities shall be documented to acknowledge their presence (e.g. incor-

porated tack welds, porosity material imperfections) • Full-scale sketches and drawings are desired to augment descriptions of the weld or material and

locations of all rejectable discontinuities. • The form shown in API RP 2X, Appendix D provides the required details of documentation




Section 5 Corrosion Protection A General ....................................................................................................................... 5-1 B Material Selection ....................................................................................................... 5-6 C Coating Selection...................................................................................................... 5-11 D Cathodic Protection................................................................................................... 5-14

A General

A.1 Scope

A.1.1 This Section covers corrosion protection of fixed or mobile steel and concrete structures. De-sign, calculation methods, material selection, fabrication, installation and commissioning of the corrosion protection system are subject to approval by GL in connection with the overall Certification procedure.

A.1.2 The DIN 81249 and ISO 15156 series standards shall apply to the selection of materials and the design of offshore structures and equipment components in case the technical designer has to con-sider the corrosion behavior of an unprotected metallic material in sea water or sea atmosphere and if this metallic material is listed in one of these standards.

A.1.3 The corrosion protection of fixed offshore steel structures, including platforms (jackets), ten-sion leg platforms (TLPs) and subsea templates shall be conform to EN 12495 or NACE SP0176 supple-mented with the requirements listed below.

A.1.4 The corrosion protection of mobile floating structures which are static during their usual opera-tion conditions include barges, jack-ups, semi-submersible platforms, storage tankers, buoys and appur-tenances, such as chains shall be conform to EN 13173 or NACE SP0176 supplemented with require-ments below. It does not cover the cathodic protection of ships.

A.1.5 The corrosion protection of steel in concrete shall be conform to NORSOK M-503.

A.2 Terms, definitions

A.2.1 General

The basic terms and definitions for corrosion of metals and alloys in ISO 8044 and ISO 15156, the gen-eral principles of cathodic protection in EN 12473 and paint systems in NACE SP0108, ISO 12944 and NORSOK M-501 shall be applied. For the different special structure types the terms and definitions in according with EN 12495, EN 13173 and NORSOK M-503 shall be used.

A.2.2 Anode

Anode is an Electrode at which anodic reaction predominates. (ISO 8044)

A.2.3 Anodic reaction

Anodic reaction is an electrode reaction equivalent to a transfer of positive charge from the electronic conductor to the electrolyte. An anodic reaction is an oxidation process. An example common in corrosion is: Me → Men+ + ne- (ISO 8044)

A.2.4 Atmospheric zone

Zone located above the splash zone, i.e. above the level reached by the normal swell, whether the struc-ture is moving or not. (EN 12495)


Section 5 Corrosion Protection


A.2.5 Back e.m.f.

Back Electro Motive Force = Voltage produced in a conductor that tends to neutralize the present volt-age. The back e.m.f. is also the naturally occurring open circuit potential difference between the anode and the cathode in sea water.

A.2.6 Boot topping

Boot topping is the section of the hull between light and fully loaded conditions, which may be intermit-tently immersed. (EN 13173)

A.2.7 Bracelet anode

Anode shaped as half-rings to be positioned on tubular items. Two half-ring anodes will have to fit to-gether to become a bracelet anode. (EN 12496)

A.2.8 Buried zone

Zone located under the mud line. (EN 12495)

A.2.9 Calcareous deposits

Calcareous deposits are minerals precipitated on the steel cathode because of the increased alkalinity caused by cathodic polarization. Well formed calcareous deposits reduce the rate of diffusion of dissolved oxygen in the sea water to the steel surfaces and thereby reduce the current density necessary to main-tain cathodic polarization. (EN 12473)

A.2.10 Cathode

Cathode is an electrode at which cathodic reaction predominates. (ISO 8044)

A.2.11 Cathodic disbonding

Failure of adhesion between a coating and a metallic surface that is directly attributable to the application of cathodic protection. (EN 12473)

A.2.12 Cathodic protection

Cathodic protection is an electrochemical protection by decreasing the corrosion potential. (ISO 8044)

A.2.13 Cathodic reaction

Cathodic reaction is an electrode reaction equivalent to a transfer of negative charge from the electronic conductor to the electrolyte. A cathodic reaction is a reduction process, e.g.: Ox + ne- → Red. (ISO 8044)

A.2.14 Coating breakdown factor

The coating breakdown factor is the anticipated reduction in cathodic current density due to the applica-tion of an electrically insulating coating when compared to that of bare steel. (EN 12473)

A.2.15 Conductor pipes

Conductor pipes form the first installed casing of an offshore well. (EN 12495)

A.2.16 Corrosion protection

Corrosion protection means modification of a corrosion system so that corrosion damage is reduced. (ISO 8044)

A.2.17 Corrosion – resistant alloy (CRA)

Alloy intended to be resistant to general and localized corrosion of oilfield environments that are corrosive to carbon steels. (ISO 15156)

A.2.18 Crevice corrosion

Locally more intensified corrosion in crevices. It results from corrosion cells caused by differing concentra-tions within the corrosive medium, especially by differences of the oxygen concentration between the




crevice and the environment. As a result of the corrosion cell formation pH value reductions as well as increased chloride concentrations occur within the crevices (DIN 81249-1).

A.2.19 Critical crevice temperature (CCT)

The CCT is that temperature at which, in a 6 % FeCl3 solution, crevice corrosion first occurs (see ASTM G 48).

A.2.20 Critical pitting temperature (CPT)

CPT is that temperature at which, in a 6 % FeCl3 solution, pitting corrosion first occurs (see ASTM G 48).

A.2.21 Current density

Current density is the current per unit area of the electrode. (ISO 8044)

A.2.22 Current drain

Current drains are components which are not considered to require cathodic protection but will drain cur-rent from the system. (NORSOK M-503)

A.2.23 Dielectric shield

Alkali resistant organic coating applied to the structure being protected in the immediate vicinity of an impressed current anode to enhance the spread of cathodic protection and minimize the risk of hydrogen damage to the protected structure in the vicinity of the anode. (EN 12473)

A.2.24 Driving potential

The driving potential is the difference between the structure/electrolyte potential and the anode/electrolyte potential. (EN 12473)

A.2.25 Duplex stainless steel

Duplex stainless steel is a stainless steel whose microstructure at room temperature consists primarily of a mixture of austenite (Face-centred cubic crystalline phase of iron-based alloys) and ferrite (Body-centred cubic crystalline phase of iron-based alloys). (ISO 15156).

A.2.26 Extended tidal zone

Extended tidal zone is a zone including the tidal zone, the splash and the transition zone. (EN 12495)

A.2.27 Ferritic steel

Ferritic steel is steel whose microstructure at room temperature consists predominantly of ferrite (body-centred cubic crystalline phase of iron-based alloys). (ISO 15156)

A.2.28 Flush mounted anode

Flush mounted anodes are designed to limit hydrodynamic effects when fitted to the structure. (prEN 12496)

A.2.29 Galvanically – induced hydrogen stress – cracking (GHSC)

Cracking that results due to the presence of hydrogen in a metal, induced in the cathode of a galvanic couple, and tensile stress (residual and/or applied). (ISO 15156)

A.2.30 Galvanic protection

Galvanic protection is an electrochemical protection in which the protecting current is obtained from a corrosion cell formed by connecting an auxiliary electrode to the metal to be protected. (ISO 8044)

A.2.31 H.A.T.

Level of the highest astronomical tide. (EN 12495)




A.2.32 Hydrogen (induced) stress cracking (HSC, also known as HISC)

Cracking that results from the presence of hydrogen in a metal and tensile stress (residual and/or ap-plied).

HSC describes cracking in metals that are not sensitive to SSC (Stress Corrosion Cracking) but which may be embrittled by hydrogen when galvanically coupled, as the cathode, to another metal that is cor-roding actively as an anode. The term galvanically induced HSC has been used for this mechanism of cracking. (ISO 15156)

A.2.33 Holiday

As holiday a coating discontinuity is understood that exhibits electrical conductivity when exposed to a specific voltage. (ISO 21809-1)

A.2.34 Immersed zone

Zone located below the extended tidal zone and above the mud line. In case of floating structures, zone located below the waterline at draught corresponding to normal working conditions. (EN 12495)

A.2.35 Impressed current protection

Impressed current protection is an electrochemical protection in which the protection current is supplied by an external source of electric energy. (ISO 8044)

A.2.36 Insert

Insert is the form over which the anode is cast and which is used to connect the anode to the structure requiring protection (prEN 12496).

A.2.37 J-tube

Curved tubular conduit designed and installed on a structure to support and guide one or more pipeline risers or cables. (EN 12495)

A.2.38 L.A.T.

L.A.T. is the level of the lowest astronomical tide. (EN 12495)

A.2.39 Marine sediments

Top layer of the sea bed composed of water saturated solid materials of various densities. (EN 12495)

A.2.40 M.T.L.

Mean tide level (also known as M.S.L or M.W.L.). (EN 12495)

A.2.41 Pitting corrosion

Type of corrosion with the anodic formed on the material surface being considerably smaller than the cathodic regions. Pitting corrosion frequently occurs if the protective layer of a metal is locally damaged or if the passive layer of a stainless steel is penetrated e.g. by chloride ions (DIN 81249-1).

A.2.42 Pitting resistance equivalent number (PREN)

PREN is a number, developed to reflect and predict the pitting resistance of a CRA, based upon the pro-portions of Cr, Mo, W and N in the chemical composition of the alloy.

PREN = % Cr + 3.3 (%Mo + %W) + 16x %N (ISO 15156-3)

A.2.43 Protection current density

Current protection density is the density that is required to maintain the corrosion potential in a protection potential range. (ISO 8044)




A.2.44 Reference electrode

Reference electrode is an electrode, having a stable and reproducible potential that is used as a refer-ence in the measurement of electrode potentials. (ISO 8044)

A.2.45 Resistivity of an electrolyte

Resistivity is the resistance of an electrolyte of unit cross section and unit length. It is expressed in ohm · metres (Ω · m). The resistivity depends, amongst other things, upon the amount of dissolved salts in the electrolyte. (EN 12473)

A.2.46 Riser

Vertical or near vertical portion of an offshore pipeline between the platform piping and the pipeline at or below the seabed, including a length of pipe of at least five pipe diameters beyond the bottom elbow, bend or fitting. (EN 12495)

A.2.47 Salinity

Amount of inorganic salts dissolved in the sea water. The standardized measurement is based on the determination of the electrical conductivity of the sea water. Salinity is expressed in grammes per kilo-gramme or in ppt. (EN 12495)

A.2.48 Splash zone

Height of the structure which is intermittently wet and dry due to the wave action just above the H.A.T. (EN 12495)

A.2.49 Submerged zone

The submerged zone is a zone including the buried zone, the immersed zone and the transition zone. In case of floating structures, the zone includes the immersed and the buried zones. (EN 12495)

A.2.50 Stand off anode

Stand off anodes are designed to be off set a certain distance from the object on which they are posi-tioned. The distance is typically 0.3 metre. (prEN 12496)

A.2.51 Tidal zone

Zone located between the L.A.T. and the H.A.T. (EN 12495)

A.2.52 Transition zone

Zone located below the L.A.T. and including the possible level inaccuracy of the platform installation and a depth with a usually higher oxygen content due to the normal swell. (EN 12495)

A.2.53 Underwater hull

Underwater hull is the part of the hull vital for its stability and buoyancy of a floating structure, i.e. below the light water line. (EN 13173)

A.3 Structural design

A.3.1 Design parameters

Some factors that may influence the design of a corrosion protection system shall be outlined. More de-tailed information is given in each of the referred Standards in A.1 and A.2.1. When designing a corrosion protection system, it is important to ensure that the whole structure is adequately protected and that e.g. areas are not substantially over-protected, uniform distribution of current and restriction are placed on the sitting of the anodes (e.g. in way of nodes) and it may be advantageous to use coatings. The detailed design of offshore structures shall include corrosion protection. This shall involve: • type of production installation • material selection • specification of coating




• cathodic protection (sacrificial or impressed current) • selection of anode material and type • reliability • maintenance • monitoring • The major parameters affecting corrosion protection are: • dissolved oxygen content • wind • earthquake • vibrations • waves • currents • sand-, ice-loads, formation and accretion • temperature • marine growth • salinity

B Material Selection

B.1 General

The intensity of corrosion of an unprotected steel structure in seawater varies markedly with position rela-tive to the sea levels as shown in Fig. 5.1. The splash zone above the mean tide level (M.T.L) is the most severely attacked region due to continuous contact with highly aerated sea water and the erosive effects of spray, waves and tidal actions. Corrosion rates as high as 0.9 mm/y at Cook Inlet, Alaska, and 1.4 mm/y in the Gulf of Mexico have been reported. Cathodic protection in this area is ineffective because of lack of continuous contact with the seawater, the electrolyte, and thus no current flows for much of the time. Corrosion rates of bare steel are often also very high at a position just below M.T.L in a region that is very anodic relative to the tidal zone, due to powerful differential aeration cells which form in the well aerated tidal region.

Protection of a steel structure can be achieved by various means; each corrosion zone being separately considered: • atmospheric zone • extended tidal zone • immersed zone • buried zone

Three generally accepted methods are • cathodic protection • painting or coating • and sheathing

Corrosion in the atmospheric zone is typically controlled by the application of a protective coating system.

For the extended tidal zone, with the highest damage/corrosion rates NACE RP0176-2003 provide a list of Corrosion Control Measures, e.g. additional Corrosion Allowance, Steel Wear Plates, Alloy-clad and diverse coatings.




!"

#"

Fig. 5.1 Schematic representation of levels, zones and thickness loss in sea water environment

This clause outlines some of the more important metals which may be protected by cathodic protection in sea water and gives guidance for • metal sheathing of extended tidal zone with 65 % Ni-Cu alloy 400 and 90-10 Cu-Ni • Critical Pitting and Crevice Temperature (CPT and CCT) at which the CRAs are susceptible to chlo-

rides in seawater • potentials recommended for protection of metals

However, in view of the wide range of alloys and applications it is essential that these values are used for general guidance only and for more specific recommendations the standards of A.1, A.2.1 and literature review may be necessary.

B.2 Metal sheathing

Metal sheathing with 65 % Ni-Cu alloy 400 and 90-10 Cu-Ni has proved to be a very successful approach when applied to the legs and risers in the extended tidal zone.

B.2.1 65Ni-Cu (Alloy 400) is prone to pitting under stagnant conditions (0-0.6 m/sec) but becomes passive in flowing seawater and high resistance even at 40 m/sec.

In early trials of 65Ni-Cu (Alloy 400) welded directly to the steel, it was assumed that corrosion of the an-odic steel below the tidal zone would be accelerated because it is in direct contact with the more noble sheathing alloy. On the contrary, steel below the tidal zone was found to be cathodic relative to the noble alloy sheathing material, since the sheathing alloy became polarized to the potential of the adjacent steel below. Hence the submerged steel below the sheathed pilling corroded at a lower rate than the sub-merged steel on unsheathed bare steel because the resulting galvanic current between the sheathed tidal zone and the submerged steel below it is lower.

B.2.2 90-10 Cu-Ni is an established alloy for seawater systems and recognized for its unique combi-nation of high resistance to corrosion and macrofouling.

Maximum resistance to biofouling relies on 90-10 Cu-Ni being freely exposed and not galvanically or ca-thodically protected by less noble materials. It is thought that this allows the availability of free copper ions in the surface film to inhibit the growth of macrofouling although some microfouling will colonize. Attach-ment of the sheathing material to the steel structure by welding or mechanical fasteners will result in ca-thodic polarization of the sheath material and a reduction in the antifouling capability of the 90-10 Cu-Ni- alloy. Therefore it is necessary to electrically insulate the sheath from the steel jacket members to




gain the full advantage of the biofouling resistance properties of the alloy. Electrical insulation can be achieved by pumping cement or an epoxy into the annular space between the component and the sheath or, more simply, by use of an elastomer or rubber-base insulator.

B.2.3 Long term data has shown complete protection of the steel behind the sheathing. The corro-sion rate of the 65Ni-Cu (Alloy 400) and 90-10 Cu-Ni was very low and uniform; after 10 years no meas-urable loss of thickness of the sheathing itself in the case of the directly welded and insulated steel. Coat-ing should be applied and maintained on the upper steel/65Ni-Cu or steel/90-10 Cu-Ni interface and any steel area above in the atmospheric zone. The accumulation of biofouling on insulated Cu-Ni was 1-4 % of the bare steel. In the case of directly welded sheathing, there were over 50 and 60% reduction in the biofouling mass after 5 and 10 years exposure respectively representing a significant reduction in struc-ture weight and wave loading (see also ISO 12473 Chapter 8.3.2.3 Cooper alloys).

B.3 High strength steel and corrosion resistant alloys (CRAs)

B.3.1 For deep water developments, high pressure, high temperature, sour gas and condensate fields there is a need for high strength and corrosion resistant alloys (CRAs). Although CRAs are consid-ered to be immune from corrosion, they are really only resistant to general corrosion in oil and gas pro-duction fluids compared to carbon and low alloy steels. Internal corrosion problems in steels are essen-tially related to corrosion by CO2 and H2S. ISO 15156 Part 3 lists the CRAs that are acceptable and the conditions for which they may be used in terms of pH, H2S partial pressure and chloride levels. External corrosion problems increases at the anode with • an increasing free corrosion potential difference • an increasing cathodic/anodic area ration • an increasing specific electrical conductivity and an increasing temperature of the sea water

The types of corrosion attack depend upon the material used, the component design, and the operating conditions. Uniform corrosion and shallow pit formation usually occur on unalloyed steels and low- alloy steels. Pitting and crevice corrosion generally occurs on materials that are passivatable and tend to form a surface layer (high-alloy steels, aluminum and aluminum alloys as well as copper and copper alloys).

B.3.2 The pitting resistance equivalent number (PREN) is a measure of the pitting corrosion resis-tance of stainless steels as well as chrome and molybdenum nickel-base alloys. The larger the value of the PREN, the better the resistance to pitting and crevice corrosion in seawater. Using the following for-mula, various materials can be ranked based upon their chemistry.

PREN = % Cr + 3.3 (%Mo + %W) + 16x %N (ISO 15156-3)

Crevice corrosion is very similar to pitting corrosion. However, since the tighter crevice allows higher con-centration of corrosion products, it is more insidious than pitting. This drives the pH value lower. The end result is that crevice corrosion can happen at temperatures 30 - 50 °C lower than pitting in the same envi-ronment.

The Table 5.1 to 5.3 present some of the more important metals used for subsea collecting systems.




Table 5.1 Reference materials

UNS, M-No. Standard Grade, class Material-types

S32205 ASTM A 240 2205 Duplex 22Cr

1.4462 EN 10028-7 X2CrNiMoN 17-12-2 Duplex 22Cr

S32750 ASTM A 240 2207 Super-Duplex 25Cr

1.4410 EN 10028-7 X2CrNiMoN 25-7-4 Super-Duplex 25Cr

S30400 ASTM A 240 304 Austenic SS

1.4301 EN 10028-7 X5CrNi 18-10 Austenitic SS

S31603 ASTM A 240 316 L Austenitic SS

1.4404 EN 10028-7 X2CrNiMo 17-12-2 Austenitic SS

N06625 625 Nickel-base- Alloy

2.4856 DIN 17744 NiCr22Mo9Nb Nickel-base- Alloy

N08825 825 Nickel-base- Alloy

2.4858 DIN 17744 NiCr21Mo Nickel-base- Alloy

Table 5.2 Chemical composition of the reference materials

Weight, %, max. UNS M-No. C Mn Si P S Cr Ni Mo Fe others

S32205 0.03 2.0 1.0 0.03 0.02 22.0-23.0 4.5-6.5 3.0-3.5 Rest N=0.1

1.4462 0.03 2.0 1.0 0.035 0.015 21.0-23.0 4.5-6.5 2.5-3.5 Rest N=0.1

S32750 0.03 1.2 0.8 0.035 0.02 24.0-26.0 6.0-8.0 3.0-5.0 Rest N=0.2

1.4410 0.03 2.0 1.0 0.035 0.015 24.0-26.0 6.0-8.0 3.0-4.5 Rest N=0.2

S30400 0.08 2.0 0.7 0.045 0.03 18.0-20.0 8.0-10.5 - Rest N=0.1

1.4301 0.07 2.0 1.0 0.045 0.015 17.0-19.5 8.0-10.5 - Rest N=0.1

S31603 0.03 2.0 0.7 0.045 0.03 16.0-18.0 10-14 2.0-3.0 Rest N=0.1

1.4404 0.03 2.0 1.0 0.045 0.015 16.5-18.5 10-13 2.0-2.5 Rest N=0.1

N06625 0.1 0.5 0.5 0.015 0.015 20.0-23.0 Rest 8.0-10.0 5,0 Nb4.1

2.4856 0.1 0.5 0.5 0.02 0.015 20.0-23.0 58 8.0-10.0 5,0 Nb,Co

N08825 0.05 1.0 0.5 - 0.03 19.5-23.5 38-46 2.5-3.5 Rest Ti=1.2

2.4858 0.02 1.0 0.5 0.02 0.015 19.5-23.5 38-46 2.5-3.5 Rest Co,Cu

Table 5.3 Mechanical properties, PREN, CPT and CCT

UNS M-No.

R0,2 MPa min.

Rm MPa

A5 %

PREN CPT °C 1, 2

CCT °C 1, 2

S32205 450 640 25 31-37 ~30 ~20 1.4462 460 640 25 31-38 ~30 ~20 S32750 550 800 20 38-44 ~50-60 ~25-35 1.4410 530 730 20 38-46 ~50-60 ~25-35 S30400 210 515-690 45 20-22 < 0 < 0 1.4301 210 520-720 45 19-21 < 0 < 0




S31603 220 515-690 45 18-20 < 0 < 0 1.4404 220 530-680 40 18-20 < 0 < 0 N06625 410 800-1000 30 46-56 ~60-90 ~40-60 2.4856 410 800 30 46-56 ~60-90 ~40-60 N08825 240 590-750 30 28-35 ~20-40 < 0-15 2.4858 235 550 30 28-30 ~20-25 < 0

Notes 1 DIN 81249 2 Klöwer, J., Schlerkmann, H., Pöpperling, R.: H2S Resistant Materials for Oil & Gas Production. Paper No. 01004-2001 by

NACE International

B.3.3 The potential susceptibility of some alloys used for jumpers, manifolds, valves, etc. to HISC due to polarization from cathodic protection of carbon steel subsea flowlines or subsea structures shall be considered. Within the potential range of cathodic protection by e.g. Al-anodes (i.e. –0.80 to –1.10V Ag/AgCl/ seawater) the production of hydrogen increases exponentially towards the negative potential limits. Forgings, casts and welds are more prone to HISC than wrought materials due to the course mi-crostructure allowing HISC to propagate.

The potential susceptibility of HISC of ferritic and ferritic- pearlitic structural steels with specified minimum yield strength above 700 MPa or 300/350 HV, ferritic-austenitic Duplex stainless steels and certain pre-cipitation hardening nickel based alloys with hardness above 300/350 HV due to polarization from ca-thodic protection shall be considered. Whilst this is the subject of current research, guidance will be pro-vided in Table 5.4 (see also EN 12473, Chapter 8.3, Other metallic materials).

Table 5.4 Summary of potential versus Ag/AgCl/ sea water reference electrode recommended for the cathodic protection of various metals in sea water

Material Min. negative po-tential [Volt]

Max. negative po-tential [Volt]

High strength steel: ReH ≤ 700 MPa and ≤350 HV: Aerobic environment Anaerobic environment

-0.80 -0.90

-1.10 -1.10

High strength steel: ReH > 700 MPa or >350 HV: -0.80 -0.95 1

Stainless steel: Austenitic steel: PREN ≥ 40 Austenitic steel: PREN < 40 Ferritic-austenitic Duplex:

-0.30

-0.60 2 -0.60 2

No limit No limit -0.90 3, 1

Al Mg and Al Mg Si alloys -0.80 -1.0 Copper alloys without Al Copper alloys with Al

-0.45 to -0.60 -0.45 to -0.60

No limit -1.10

Nickel base alloys –0.20 4

Notes 1 For materials susceptible to HISC the simultaneous presence of at least two conditions is needed:

• more negative than -0,83 V • high level of strain in the material (> 80 % of ReH for global and local stress; 67 % of ReH for regions influenced by re-

sidual stresses) 2 For most applications these potentials are adequate for the protection of crevices although higher potentials may be consid-

ered. 3 Forgings, casts and welds are more prone to HISC than wrought materials due to the course microstructure allowing HISC to

propagate preferentially in the ferritic phase. 4 High strength nickel copper and nickel chromium iron alloys can be susceptible to HISC and potentials that result in signifi-

cant hydrogen evolution should be avoided. Nickel based alloys in the solution annealed condition are generally considered immune to HISC. For certain nickel based alloys (i.e. austenitic alloys including e.g. UNS N05500 and UNS N07750) precipi-tation hardening may induce sensitivity to HISC.

ReH = minimum specified yield strength




C Coating Selection

C.1 General

C.1.1 The most suitable form of coating depends on the type of structure and its environment. In the selection of a coating, the aim should be to achieve overall economy in the combined cost of the pro-tected structure and of the initial and running costs of the protection schemes. Due regard should be paid to the design life of the structure and the economics of repairing the coating should this become neces-sary. More information is given in the respective Standards, see NACE SP0108, ISO 12944, ISO 20340 and NOSOK M-501.

C.1.2 While organic coatings are not entirely impermeable to oxygen and water they do restrict cor-rosion when applied to the surface of a metal. The bulk of the corrosion on a painted surface does not occur beneath the intact coating but at the base of holidays and pin holes. If cathodic protection is applied to a painted surface, the coating acts as a substantial resistive barrier to current flow and where the cur-rent flows, it is to the holidays or pin holes. In terms of cathodic protection the presence of a coating im-proves current distribution and reduces current demand and interference effects.

C.1.3 Coating on offshore structures can be divided into four major areas: • atmospheric zone • extended tidal zone • immersed zone • buried zone

Fig. 5.1 presents the levels, zones and profile of the thickness loss resulting from corrosion of an unpro-tected steel structure in seawater.

Corrosion in the atmospheric zone is typically controlled by the application of a protective coating system (see NACE SP0108-Table 3A to Table 5 and ISO 12944-Corrosion Category C5-M). Table 5.5 illustrates typical coating systems used in the atmospheric zone for a design life of more than 15 years.

For the extended tidal zone, with the highest damage/corrosion rates (until 100 times higher than in the atmospheric zone and three times higher than in the immersed zone) NACE SP0108–Table 6A provide diverse coatings like Vulcanized Chloroprene Rubber (thickness of 6 to 13 mm), two layer glass flake epoxy with 1 mm DFT and Thermal-sprayed Aluminum with epoxy sealer. Polyurethane topcoat does not have good water resistance and shall not used in the splash zone.

In the past, conventional protective coatings were seldom applied to structure in the immersed and buried zone. However, increased current requirement and anode weight restriction can affect the decision to coat complex structures to be installed in deeper waters with higher current density requirements, in shielded areas as large conductor bundles and/or on structures with extended design lives.

Table 5.6 illustrates typical coating systems used in the extended tidal and immersed and buried zone.

C.1.4 Organic coatings promote the formation of a dense calcareous deposit at coating holidays and bare areas because the initial current density may be relatively high at such locations. However, the solu-bility of potential film- forming calcareous deposits normally increases with decreasing temperature such that colder waters might not allow the formation of a protective calcareous deposit or could require higher initial current density to achieve polarization. This includes deep water applications for which the forma-tion of calcareous deposits may be slow.

C.1.5 The application of coatings may not be suitable for parts of submerged structures requiring frequent inspection for fatigue cracks, e.g. critical welded nodes of jacket structures.

For components of materials sensitive to HISC by CP (see B.3), an organic coating should always be considered as a barrier to hydrogen adsorption.

C.2 Coatings and coating breakdown factors

C.2.1 Coating breakdown factor is the anticipated reduction in cathodic current density due to the application of an electrically insulating coating when com-pared to that of bare steel. If the Coating break-




down factor is zero, the coating is 100 % electrically insulating, thus decreasing the cathodic current den-sity to zero. Coating breakdown factor 1 means that the coating has no current reducing properties.

An initial coating breakdown factor (fci) related mainly to mechanical damage occurring during the instal-lation of the structure should be considered and a coating deterioration rate shall be thereafter evaluated in order to take into account the coating aging and possible small mechanical damage occurring to the coating during the structure life.

C.2.2 Due to possible interactions between the cathodic protection and the coating, all coatings to be used in combination with cathodic protection shall be tested beforehand to establish that they have ade-quate resistance to cathodic disbondment. Alkali is generated on the cathodically protected surface and this may result in the cathodic disbondment of the coating in way of any coating defects.

C.2.3 The conventional coatings of the oleo-resinous or alkyd types are attacked by alkali, i.e. they are subject to saponification and are not recommended for use in association with cathodic protection. The use of polyvinyl butyral shop primers has also caused loss of adhesion when combined with cathodic protection.

The Coating Categories I to VII of Table 5.6 und the recommended constants k1 and k2 of Table 5.7 are based on recommended values of EN 12495, EN 13173, ISO 15589 and NORSOK M-503 as well the acceptance criteria of the coating standards ISO 12944, inclusive ISO 20340 and ISO 21809-1.

Table 5.5 Typical coating systems used in the atmospheric zone (design life >15 years)

Primer coat (s) Subsequent coat(s) Paint system

System No. 1, 2 Binder type Type of

primer No. of coats

NDFT [ym]

Binder type

No. of coats

NDFT [ym]

A5M.02 EP, PUR Misc. 1 80 EP, PUR 3 3-4 320

A5M.04 EP, PUR Misc. 1 250 EP, PUR 3 2 500

A5M.06 EP,PUR,ESI Zn (R ) 1 60 EP, PUR 3 4-5 320

A5M.07 EP,PUR,ESI Zn (R ) 1 60 EPC 3-4 400 Notes

1 ISO 12944-5, corrosivity category C5-M

2 The systems shall be pre-qualified in accordance to ISO 20340 (Ageing resistance)

3 Polysiloxane provides better UV restistance than polyurethane ( see NACE SP0108)

EP : Epoxy

PUR : Polyurethane

ESI : Ethyl silicate

EPC : Epoxy combination

Zn (R ) : Zinc-rich

Misc. : Primers with miscellaneous types of anticorrosive pigments.

NDFT : Nominal dry film thickness




Table 5.6 Typical coating systems used in the extended tidal, immersed and buried zone

Primer coat (s) Subsequent coat (s) Paint system

System No. Binder

type Type

of primer

No. of

coats NDFT [ym]

No. of

coats NDFT [ym]

fcf

Coat. Cat. (see

Table 5.7)

EP Misc. 1 20 1,0 3 I

A6.05 Mod 1,3,5

EP Misc. 1 80 EP 2 250 0,8 3 II

A6.05 Mod. 1,3,5

EP Misc. 1 80 EP 2 350 0,26 3 III

A6.05 Mod. 1,2,4

EP Misc. 1 80 EP 2 600 0,26 2 IV

A6.08 1,2,4 EP Misc. 1 80 EPGF 3 800 0,26 2 IV

3LPE- Cl.A/B 6

EP 7 or 8 Misc. 1 25 7 or 125 8

Adhesive 150 ym +PE 10 3 1300-

4700 11 0,16 2 V

3LPP- Cl.C 6

EP 7 or 8 Misc. 1 25 7 or 125 8

Adhesive 9 150 ym +PP 10

3 1300- 3800 11

0,10 2 VI

Rubber 2, 12 EP Misc. 1 25 Adhesive 9 25 ym +PC

3 6000- 13000 0,07 2 VII

Notes 1 ISO 12944-5, corrosivity category Im1, Im2 and Im3 (for water and soil)

2 for extended tidal zone recommended

3 for immersed zone recommended

4 The system shall be pre-qualified in accordance to ISO 20234 (Ageing resistance, Cathodic disbonding, Sea water immer-sion)

5 The system shall be pre-qualified in accordance to ISO 20234 (Cathodic disbonding, Sea water immersion)

6 ISO 21809-1

7 Liquid

8 Powder (FBE= Fusion Bond Epoxy)

9 Powder sprayed or extruded

10 Applied by extrusion

11 Thickness is function of pipe weight

12 NACE SP0108, Polychloroprene (neoprene) rubber

EP : Epoxy EPGF : Epoxy glass flake PE : Polyethylene PP : Polypropylene PC : Polychloroprene Fcf : Final coating breakdown factor for design life of 30 years




Table 5.7 Recommended constants k1 and k2 for calculation of coating breakdown factors

Coating categories (Table 5.6) Zones (Fig.1)

Con-stants I II III IV V VI VII

Extended k1 0.10 0.05 0.02 0.015 0.01 0.01 0.005

Tidal k2 0.10 0.025 0.012 0.008 0.005 0.003 0.002

k1 0.10 0.05 0.02 0.015 0.01 0.01 0.005 Immersed

k2 0.05 0.015 0.008 0.005 0.003 0.002 0.0015

C.2.4 Calculation of coating breakdown factors:

fci = k1 (1)

fcm = k1 + k2 x t/2 (2)

fcf = k1 + k2 x t (3)

If fcm or fcf exceeds 1, fcm = 1 or fcf = 1 shall be applied in the design. (Coating breakdown factor 1 means that the coating has no current reducing properties.)

k1 : constant for the initial condition of coating depending mainly on mechanical damage occur-ring during the installation of the structure mechanical damage

k2 : constant for coating damage depending mainly on marine growth (incl. cleaning operations to remove such growth) and the erosion effects of waves and currents

t : design life (years)

fci : initial coating breakdown factor

fcm : mean (or maintenance or average) coating breakdown factor

fcf : final (or re-polarization) coating breakdown factor

D Cathodic Protection

D.1 General

In order to achieve the cathodic protection criteria on the whole structure it is necessary to consider the electrical current demand on each part of the structure.

The electrical current demand of each part of the structure is the product of its steel surface area multi-plied by the electrical current density required.

The current density required is not the same for all parts of the structure as the environmental conditions are variable. Therefore, the following areas and parts should be considered, referring to zones as defined in Fig. 5.1. • areas located in the extended tidal zone (usually coated or cladded) • areas located in the immersed zone • areas located in the buried zone • wells to be drilled; a current allowance per well shall be considered, depending on projected size,

depth and cementing of the wells • Neighboring structures and pipelines in electrical contact with the fixed steel offshore structure to be

protected.




D.2 Design current densities

The selection of design current densities may be based on experiences from similar structures in the same environment or from specific tests and measurements.

The electrical current density required for cathodic protection depends upon the kinetics of the electro-chemical reactions and varies with parameters such as listed in A.3.1.

For each particular set of environmental condition (see Table 5.8) and surface condition of the structure (see Table 5.6 and 5.7), the following electrical current densities shall be evaluated: • initial electrical current density required to achieve the initial polarization of the structure, i.e. to

achieve the lowering of the steel potential down to value within the range recommended in Table 5.4 • mean (or maintenance or average) electrical current density required to maintain this polarization

level on the structure • final (or re-polarization) electrical current density required for a possible re-polarization (i.e. for re-

establishing the potential to the initial polarization level) of the structure after severe storms or clean-ing operations (i.e. marine growth)

Table 5.8 presents a general guide to the design of CP systems in major offshore petroleum producing areas and offshore wind farms for carbon steel, stainless steel, aluminum and other metallic materials. These data may be used as a starting point for investigation prior to selection of final design parameters for a specific application. For special cases and deep water Table 5.9 and 5.10 shall be used.

Table 5.8 Typical design current density values of bare metal surface in immersed zone until 100 m water depth

Design current density (mA/m2)

Cur-rent Den-sity

levels

Code 1

Geographic area

Water Resistivity (ohm-m)

Water Temp. (C°) 2

Wave Action 3

Lateral Water Flow Initial Mean Final

Very high NACE Cook Inlet 0.50 2 Low High 430 380 380

EN, NACE

North Sea > 62°N 4

0.26-0.33 0-12 High Mod. 220 100 130

NACE Gulf of Mex-

ico Deep water

0.29 4-12 Low Varies 194 75 86

EN, NACE

North Sea 55 – 62°N 4

0.26-0.33 0-12 High Mod. 180 90 120

High

NACE Brazil 0.20 15-20 Mod. High 180 65 86

EN North Sea < 55°N 4

0.26-0.33 0-12 High Mod. 150 80 100

NACE U.S. West Coast 0.24 15 Mod. Mod. 150 86 100 Me-

dium

GL Baltic Sea

< 60°N 0.85-1.15 5 0-12 Low Low 150 65 90

NACE Arabian Gulf 0.15 30 Mod. Low 130 65 86

NACE Australia Southeast 0.23-0.30 12-18 High Mod. 130 86 86

NACE West Africa 0.20-0.30 5-21 Low Low 130 65 86 Low

EN India 130 70 90




NACE Australia

Northwest 120 65 65

NACE Gulf of Mexico 0.20 22 Mod. Mod. 110 54 75

NACE Indonesia 0,19 24 Mod. Mod. 110 54 75

NACE South China Sea 0,18 30 Low Low 100 32 32

Very Low

EN Mediterran.

Sea, Adriatic Sea

110 60 80

Notes

1 NACE SP0176, EN 12495, EN 13173 and GL (Germanischer Lloyd) 2 Surface water temperature (sea level) 3 Turbulence Factor 4 Conditions in the North Sea can vary greatly from the northern to the southern area, from winter to summer, from the coast

near to the coast far, and during storm periods. 5 Salinity 7 to 10 ppt

D.3 Cathodic protection systems

Cathodic protection can be achieved using: • the galvanic (or sacrificial) anodes system • the impressed current system • a combination of both cathodic protection systems (Hybrid Systems)

For permanently installed offshore structures, galvanic anodes are usually preferred.

However, due to weight and drag forces caused by galvanic anodes, impressed current cathodic protec-tion systems are sometimes chosen for permanently installed floating structures.

When using hybrid systems the galvanic anodes should provide cathodic protection during float out, initial installation and subsequently during the impressed current systems shutdowns.

Galvanic anodes should also be installed in areas where it may be difficult to achieve adequate level of polarization by the impressed current system due to shielding effects.

As the electrical current demand is not constant with time, the cathodic protection system shall be able to deliver the re- polarization current density required for short periods throughout the life of the structure.

D.4 Galvanic anodes system

D.4.1 Design considerations

The three design electrical current densities as defined in D.2 shall be considered: • The initial electrical current density shall be used to verify that the output current capacity of new an-

odes, i.e. their initial dimensions, is adequate to obtain a complete initial polarization of the structure within a few weeks.

• The mean (or maintenance or average) electrical current density shall be used to determine the weight of the anodes. This current density is required to maintain an adequate polarization level of the structure during its design life.

• The final (or re-polarization) electrical current density shall be used to verify that the output current capacity of the anodes when they are consumed to an extent commensurate with their utilization fac-tor, i.e. their final usable dimensions, is adequate to re-polarize the structure after severe storms or after marine growth cleaning operations.




Table 5.9 Typical design current density values for special cases

Design Current Density [mA/m2] Case No. Code 4 Description

Initial Mean Final

1 EN Bare metal in extended tidal zone 1.2 × Table 5.8

1.2 × Table 5.8

1.2 × Table 5.8

2 NACE Deep-water structures depth > 100 m Table 5.10 Table 5.10 Table 5.103 EN Bare metal in marine sediment 25 20 20 4 NACE Freely flooded compartments Table 5.8 Table 5.8 Table 5.8 5 NACE Closed compartments with free access to air Table 5.8 Table 5.8 Table 5.8 6 NACE Closed and sealed flooded compartments 0 0 0 7 NORSOK Current Drain by mud mats, skirt and piles 1 20 20 20 8 NORSOK Open pile ends 2 Table 5.8 Table 5.8 Table 5.8 9 EN Current Drain per platform wells 5000 5000 5000 10 NORSOK Current Drain per subsea wells 8000 8000 8000

11 NORSOK Current Drain to anchor chain Table 5.8 and 5.9 3

Table 5.8 and 5.9 3

Table 5.8and 5.9 3

12 NORSOK For components heated by an internal fluid for each °C exceed 25 °C, additional to base values

1 1 1

13 NORSOK For embedded steel in concrete structures exposed on one side in immersed zone 2 2 2

14 NORSOK For embedded steel in concrete structures exposed on both sides in immersed zone 1 1 1

15 NORSOK For embedded steel in concrete structures exposed on one side in extended tidal zone 3 3 3

16 NORSOK For embedded steel in concrete structures exposed on both sides in extended tidal zone 1.5 1.5 1.5

17 NORSOK For Al- and Zn- components or coated with Al or Zn 10 10 10

18 NORSOK For Al- and Zn- components or coated with Al or Zn and heated by an internal fluid for each °C exceed 25 °C, additional to case 17

0.2 0.2 0.2

Notes 1 based on the outer (sediment exposed) external surface area 2 The top internal surface shall be included for a distance of 5 times the internal diameter 3 For anchor systems with mooring topside only, 30 m of each chain shall be accounted for in the CP design. For anchor

system with mooring point below sea level, the seawater exposed chain section from sea level to mooring point and 30 m from the mooring point shall be accounted for in the CP design for each chain.

4 NACE SP0176, EN 12495, EN 13173 and NORSOK M-503




Table 5.10 Typical design current density values of bare metal surface in immersed zone > 100 m water depth (NACE SP0176) 1

Design Current Density [mA/m2] Temperature °C

Initial Mean Final 4 °C 350 140 300

5 °C 300 85 250

6 °C 250 75 200

7 °C 210 65 160

8 °C 170 60 120

9 °C 140 55 90

10 °C 120 52 70

11 °C 115 48 65

12 °C to 17 °C 110 45 to 40 60

18 °C to 23 °C 100 40 to 35 50

24 °C to 30 °C 100 to 90 35 to 30 50 to 40 Note 1 For deep-water structures different design current density values should be used for different temperature zones. To opti-

mize the design the structure should be split up into separate zones over which the temperature does not vary by more than 1 °C at temperatures ≤ 11 °C and 5 °C at temperatures > 11 °C. The depth average temperature of each interval should be used to assess the required current densities.

A large variety of shapes and sizes can be used to deliver protective electrical current to the structure in order to optimize the electrical current distribution (see D.4.3).

The performance of galvanic anodes in sea water depends critically upon the alloy composition, particu-larly when aluminum or zinc alloys are used (see EN 12496, NACE SP0387 and NACE SP0492)

The electrochemical properties of the anodic material shall be documented or determined by appropriate tests.

The information required includes: • the driving voltage to polarized steel, i.e. the difference between closed circuit anode potential and

the positive limit of the protection potential criterion • the practical electrical current capacity (Ampere hour per kilogram = Ah/kg) or consumption rate

(kilogram per Ampere per year = kg/Ay) • the susceptibility to passivation • the susceptibility to intergranular corrosion

D.4.2 Galvanic anode materials

D.4.2.1 Chemical composition and physical quality requirements

The requirements specified in EN 12496 shall be applied. Further information for inspection of anodes is given in NACE SP0387 and NACE SP0492. For the Aluminum and Zinc anodes the chemical composi-tions of Table 5.11 are required.

D.4.2.2 Electrochemical properties

For design purposes, the data given in Table 5.12 shall be used unless otherwise documented. The use of practical current capacity greater than the values of Table 5.12 shall be justified by long term free run-ning test of minimum 12 month in accordance to DNV RP-B401, Annex C. Electro chemical quality control testing shall be in compliance with the requirements in DNV RP-B401. The test procedure shall be in ac-cordance to DNV RP-B401, Annex B (short time test of 96 h).




Table 5.11 Chemical composition for Al- and Zn anodes gives as mass fraction in %

Elements Al alloy A2 1,3,4 Al alloy A3 2,3,4 Zn alloy Z1 5

Aluminum (Al) Remainder Remainder 0.1 – 0.5

Zinc (Zn) 3.0 – 5.5 4.75 – 5.75 99.314 min.

Indium (In) 0.016 – 0.040 0.016 – 0.020 -

Iron (Fe) 0.09 max. 0.06 max. 0.005 max.

Silicon (Si) 0.10 max. 0.08 – 0.12 -

Copper (Cu) 0.005 max. 0.003 max. 0.005 max.

Cadmium (Cd) 0.002 max. 0.002 max. 0.025 – 0.07

Lead (Pb) - - 0.006 max.

Other Impurities (each) 0.02 max. 0.02 max. -

Other Impurities (total) 0.1 max. 0.05 max. 0.10 max. Notes 1 Al alloy A2 is normally used for offshore applications.

2 Al alloy A3 is normally used for deepwater and cold water applications.

3 Al alloy A2 and A3 have proved to be applicable in marine sediments, i.e. covered with mud

4 Al alloys A2 and A3 are only applicable for environments with salinity of greater than 5 ppt, i.e. typically in seawater / brackish water environments with resistivity of less than 2 ohm-m. Higher resistivity may occur at the mouths of major riv-ers in the Baltic Sea (see Table 5.8)

5 Zn alloy Z1 should not be used at temperatures above 50 °C in sediments

Table 5.12 Design values for galvanic anodes

Seawater Sediments

Anode type Closed circuit

potential Ag/AgCl/seawater

(mV) Ea

Current capacity (Ah/kg)

Closed circuit potential

Ag/AgCl/seawater (mV) Ea

Current capacity (Ah/kg)

Temperature Limits °C

Al alloy A2,A3 1,3

-1050 2000 -1000 1500 Max. 30

Zn alloy Z1 2 -1030 780 -980 750

580 Max. 30 30 to 50

Notes 1 Al-Density 2725 kg/m3

2 Zn-Density 7130 kg/m3

3 Anode capacity for all Al-based alloys will be significantly lower at higher operating temperature. For example anode capacity for alloy A2 will drop essentially linearly from 2500 Ah/kg at 25 °C (short time test) to 500 Ah/kg at 80 °C. In saline mud, at elevated temperatures, the capacity values will be lower than those indicated for seawater.

D.4.3 Anode shapes and utilization factors

D.4.3.1 Stand-off anodes

Stand-off anodes shall be used as far as possible with a minimum distance to the steel surface of 300 mm (see Fig. 5.2). The insert should be preferably cylindrical in order to avoid gas entrapment during the alloy casting. The physical tolerances, anode weight, anode dimensions and straightness, steel inserts and metallurgical requirements shall be in accordance to EN 12496 or NACE SP0387.




The dimensions of the insert and the support are to be designed taking into account the weight of the anodicalloy and with the various environmental loading conditions as detailed in EN 12495 Chapter 5.5.4 –Structural considerations.

D.4.3.2 Flush-mounted anodes

Flush-mounted anodes are designed to limit hydrodynamic effects when fitted to the structure (see Fig. 5.2). The physical tolerances, anode weight, anode dimensions and straightness, steel inserts and metal-lurgical requirements shall be in accordance to EN 12496 or NACE SP0387.

Zn anodes located underneath mud mats shall be flush-mounted.

Flush-mounted anodes may be recommended for the protection of mobile floating units.

D.4.3.3 Bracelet anodes

Bracelet anodes are designed to reduce the wave loads on steel jackets. Bracelet anodes are shaped as half-rings to be positioned on tubular items. Two half-ring anodes will have to fit together to become a bracelet anode (see Fig. 5.2). The physical tolerances, anode weight, anode dimensions and straight-ness, steel inserts and metallurgical requirements shall be in accordance to EN 12496 or NACE SP0492.

$%%

&

' (

)

* $

(+++ Fig. 5.2 Typical anode shapes

D.4.3.4 Utilization factor

The utilization factor is determined by the proportion of anodic material that may be consumed before the anode ceases to provide the required current output. The shape of the anode will affect the utilization factor.




Table 5.13 Recommended anode utilization factors

Anode type Anode utilization factors u

Long slender stand-off, if L ≥ 4 ⋅ r 0.90

Short slender stand-off, if L < 4 ⋅ r 0.85

Long flush-mounted, if

L ≥ 4 width

L ≥ 4 thickness

0.85

Short flush-mounted, bracelet 0.8

D.4.4 Calculation of anode resistance

The anode resistance, Ra shall be in accordance with the following formulas:

Stand-off anodes, if L ≥ 4 ⋅ r:

4 LRa ln 12 L rρ ⋅⎡ ⎤= ⋅ −⎢ ⎥⋅ π ⋅ ⎣ ⎦

(4)

Stand-off anodes, if L < 4 ⋅ r:

2

2

2 L rln 1 1r 2 L

Ra2 L

r r12 L 2 L

⎡ ⎤⎧ ⎫⎛ ⎞⋅⎪ ⎪⎛ ⎞⎢ ⎥⎜ ⎟+ +⎨ ⎬⎜ ⎟⎢ ⎥⎜ ⎟⋅⎝ ⎠⎪ ⎪ρ ⎝ ⎠⎢ ⎥⎩ ⎭= ⋅ ⎢ ⎥⋅ π ⋅ ⎢ ⎥⎛ ⎞⎢ ⎥+ − + ⎜ ⎟⎢ ⎥⋅ ⋅⎝ ⎠⎣ ⎦

(5)

Flush-mounted anodes:

Ra2 Sρ

=⋅

(6)

Bracelet anodes:

0,315RaA⋅ρ

= (7)

Ra : anode resistance [Ohm]

ρ : sea water/sediment resistivity [ohm m] (see Table 5.14)

L : length of the anode [m]

r : radius of the anode [m] (for other shapes than cylindrical, r = C/2 ⋅ π, where C [m] = cross section periphery)

S : arithmetic mean of anode length and width [m]

A : exposed surface area of anode [m2]




Table 5.14 Sea water resistivity as a function of temperature, salinity and density

Temperature [°C] Salinity[ppt]

Density [kg/m3] Resistivity [ohm m]

-5 30 1024 0.47

-5 35 1028 0.40

-5 40 1032 0.34

0 30 1024 0.41

0 35 1028 0.35

0 40 1032 0.29

5 30 1024 0.35

5 35 1028 0.30

5 40 1032 0.24

10 30 1023 0.30

10 35 1027 0.26

10 40 1031 0.21

15 30 1022 0.27

15 35 1026 0.23

15 40 1030 0.18

20 30 1021 0.24

20 35 1025 0.21

20 40 1029 0.17

25 30 1020 0.22

25 35 1023 0.19

25 40 1027 0.16

30 30 1018 0.20

30 35 1022 0.18

30 40 1026 0.15

D.4.5 Sea water resistivity and temperature

D.4.5.1 Temperature has a significant influence on sea water resistivity, dissolved oxygen and cal-careous deposit formation.

As the temperature along with salinity of sea water has a significant effect on the resistivity of sea water and as this is directly related to the effective resistance of galvanic anodes, it should be taken into ac-count in the design of cathodic protection systems.

In the open sea the salinity does not vary significantly and temperature is the main factor. The relation between resistivity and temperature at a salinity of 30 to 40 ppt (parts per thousand) is given in Table 5.14. Fig. B.1 of ISO 12495 shall be used for sea water densities between 1000 kg/m3 and the values of Table 5.14.

D.4.5.2 Compared to sea water, the resistivity of marine sediments is higher by a factor ranging from about 2 for very soft clays to 5 for sand. Unless sediment data for the location are available, the highest factor shall be assumed for calculation of the resistance of any buried anodes.




D.4.6 Cathodic protection calculation and design procedure

The purpose of this paragraph is primarily the clarification of the Cathodic Protection (CP) Calculation Procedure. The procedure will be demonstrated by using two different examples:

Example 1: Tripod

A tripod with mudmats, built from carbon steel, located in West Africa, designed for a life time of 30 years

Example 2: PLET (Pipeline end termination unit)

Carbon steel subsea structure (PLET =Pipeline end termination unit) connected to carbon steel flow line and jumper with Super-Duplex 25Cr flow meter, located in the East Mediterranean Sea in 500 m water depth, sea bed temperature 13 °C, internal fluid temperature 50°C, designed for a life time of 25 years

D.4.6.1 Subdivision of the object in sections

The object has to be subdivided into sections using the zone definitions of Fig. 5.1.

Example 1: Tripod

The four following zones shall be separately considered: • atmospheric zone • extended tidal zone • immersed zone • buried zone

The following text, Figures and Tables shall be considered:

Fig. 5.1: Zones and profile of the thickness loss of an unprotected steel structure in sea water

B.1 and B.2: Methods of corrosion protection in the four zones

C.1 and C.2: Coating selection for the four zones (Table 5.5, 5.6, and 5.7)

D.1 and D.2: The subdivision shall consider all sections which influence the cathodic protection current demand and the different design current densities (Table 5.8, 5.9, and 5.10).

Possible solution:

Atmospheric Zone will not drain current from the CP system. This zone shall not include in the CP system and shall be protected minimum with a coating system of Table 5.5.

Extended Tidal Zone may be protected with 90-10 Cu-Ni on rubber-base insulator and will not drain cur-rent from the CP system.

Section 1: Immersed Zone may be protected with coating systems of coating category IV of Table 5.6.

Section 2: Mudmat underside may be none coated

Example 2: PLET

The following zones shall be considered: • immersed zone

The following text, Figures and Tables shall be considered:

B.1 and B.2: Material Selection for deep water applications (Table 5.1, 5.2, 5.3, 5.4)

C.1 andC.2: Coating Selection for deep water structures and piping components (Table 5.6, 5.7)

D.1 and D.2: The Subdivision shall consider all sections which influence the cathodic protection current demand and the different design current densities (Table 5.8, 5.9, 5.10).

Possible solution:

Section 1: Subsea structure (PLET) may be protected with a coating system of coating category IV

Section 2: Flow line may be protected with a coating system of coating category V




Section 3: Jumper with Super-Duplex 25Cr flow meter may be protected with a coating system of coating category IV, because the CPT and CCT of Super-Duplex 25Cr are equal or lower than the fluid tempera-ture, so this material shall cathodic protected.It shall be considered that Super-Duplex 25Cr is potential susceptible to HISC due to over polarization. Within the potential range of cathodic protection by Al-anodes (i.e. -0.80 to -1.10V Ag/AgCl/seawater) the production of hydrogen increases exponentially to-wards the negative potential limit. Forgings, casts, like flow meters and welds are more prone to HISC than wrought materials due to the course microstructure allowing HISC to propagate preferentially in the ferrit phase. I.e. the anodes shall be located as far as possible away from the Super-Duplex 25Cr compo-nent.

For Example 2 only the cathodic protection of Section 1 will be shown. In case of Section 2 the require-ments of ISO 15589-2 shall be considered.

D.4.6.2 Surface area of sections

For each section, with or without coating the surface areas shall be calculated separately.

Example 1: Tripod

Solution:

Section 1: Ac1 = 659 m2


Example 2: PLET

Solution:


D.4.6.3 Calculation of current demand

For each section the following required current demands shall be calculated:

Ici1 = Ac1 ⋅ ici1 ⋅ fci1 (8)

Icm1 = Ac1 ⋅ icm1 ⋅ fcm1 (9)

Icf1 = Ac1 ⋅ icf1 ⋅ fcf1 (10)

Ici1 : required initial current demand of section 1 [mA]

Icm1 : required mean current demand of section 1 [mA]

Icf1 : required final current demand of section 1 [mA]

Ac1 : surface area of section 1 [m2]

ici1 : required initial design current density of section 1 [mA/m2]

icm1 : required mean design current density of section 1 [mA/m2]

icf1 : required final design current density of section 1 [mA/m2]

fci1 : initial coating breakdown factor, if section 1 is coated; fci1 = 1, if bare metal surface

fcm1 : mean coating breakdown factor, if section 1 is coated; fci1 = 1, if bare metal surface or fcm1 >1.

fcf1 : final coating breakdown factor, if section 1 is coated; fci1 = 1, if bare metal surface or fcf1 >1

Example 1: Tripod

The following clauses, figures and tables shall be considered:

C.2: Calculation of coating breakdown factors (Table 5.7)

D.1 and D.2: Required design current densities of geographic areas and in specific areas of the structure (Table 5.8, and 5.9)




Solution:

Section 1: Coating category IV

Selection of k1 and k2 (Table 5.7: Immersed zone):

k1 = 0.015, k2 = 0.005

Selection of design current densities (Table 5.8: West Africa):

ici1 = 130 mA/m2, icm1= 65 mA/m2, icf1= 86 mA/m2

Calculation of coating breakdown factors:

fci1 = k1 = 0.015

fcm1 = k1 + k2 ⋅ t/2 = 0.015 + 0.005 ⋅ 30/2 = 0.09

fcf1 = k1 + k2 ⋅ t = 0.015 + 0.005 ⋅ 30 = 0.165

Calculation of required current demands: Ici1, Icm1 and Icf1 for Ac1 = 659 m2

Ici1 = Ac1 ⋅ ici1 ⋅ fci1 = 1285 mA

Icm1 = Ac1⋅ icm1⋅ fcm1= 3855 mA

Icf1 = Ac1⋅ icf1⋅ fcf1= 9351 mA

Section 2: Mudmat underside with bare metal surface

Selection of design current densities (Table 5.9: Case No. 3: Bare metal in marine sediment):

ici2 = 25 mA/m2, icm2 = 20 mA/m2, icf2 = 20 mA/m2

Calculation of required current demands Ici2, Icm2 and Icf2 for Ac2 = 238 m2:

Ici2 = Ac2 · ici2 = 5950 mA

Icm2 = Ac2 · icm2 = 4760 mA

Icm3 = Ac2 · icf2 = 4760 mA

Example 2: PLET



D.1 and D.2: Required design current densities in deep water (Table 5.10).

Solution:

Section 1: Coating category IV.


k1 = 0.015, k2 = 0.005

Selection of design current densities (Table 5.10):

ici1 = 110 mA/m2, icm1 = 44 mA/m2, icf1 = 60 mA/m2


fci1 = k1 = 0.015

fcm1 = k1 + k2 · t/2 = 0.015 + 0.005 · 25/2 = 0.078

fcf1 = k1 + k2 · t = 0.015 +0.005 · 25 = 0.14

Calculation of required current demands: Ici1, Icm1 and Icf1 for Ac1 = 288 m2

Ici1 = Ac1 · ici1 · fci1 = 475 mA

Icm1 = Ac1 · icm1 · fcm1 = 988 mA

Icf1 = Ac1 · icf1 · fcf1 = 2419 mA




D.4.6.4 Selection of anode materials and shapes

D.4.1, D.4.2, und D.4.3 shall be considered.

Example 1: Tripod

Solution:

Stand-off aluminum alloy anodes for immersed zone (see Fig. 5.2)

Flash-mounted zinc alloy anodes on mud mats facing downwards (see Fig. 5.2).

Example 2: PLET

Solution:

Stand-off aluminum alloy anodes placed on subsea structure

D.4.6.5 Calculation of anode mass

Calculation of the required net anode mass (Ma) for the required mean current demand Icm (see D.4.6.3) for each section.

The Table 5.8 (Geographic area, Water temperature), D.4.1, D.4.2 (Table 5.12), und D.4.3 (Table 5.13) shall be considered.

Ma = Icm ⋅ t ⋅ 8760 / 1000 ⋅ u1 ⋅ ε1 (11)

Ma : net anode mass [kg]

Icm : required mean current demand [mA] (D.4.6.3)

t : design life (years)

8760 : refers to hours per year

u : anode utilization factor (Table 5.13)

ε : anode current capacity [Ah/kg] (Table 5.12)

Example 1: Tripod

Solution:

Section 1:

Selection of Anode current capacity ε1 (1 = section 1) in seawater, considering anode material (Table 5.12: Alloy A2) and sea water temperature (Table 5.8: West Africa water temperature 5 -21): ε1 = 2000 [Ah/kg]

Selection of utilization factor u1 (1 = section 1) considering Anode Type Stand-off with L ≥ 4 ⋅ r (Table 5.13): u1 = 0.9

Calculation of net anode mass Ma1 of section 1:

Ma1 = Icm1⋅ t ⋅ 8760 / 1000 ⋅ u1⋅ ε1 = 3855 ⋅ 30 ⋅ 8760/ 1000 ⋅ 0.9 ⋅ 2000 = 562 kg

where: Icm1 = 3855 mA, t = 30 years

Section 2:

Selection of Anode current capacity ε2 (2 = section 2) in sediment, considering anode material (Table 5.12: Alloy Z1) and sea water temperature (Table 5.8: West Africa water temperature 5-21): ε2 = 750 (Ah/kg)

Selection of utilization factor u2 (2 = section 2) considering Anode Type Flush-mounted (Table 5.13): u2 = 0.85


Ma2 = Icm2 ⋅ t ⋅ 8760 / 1000 ⋅ u2 ⋅ ε2 = 4760 ⋅ 30 ⋅ 8760 / 1000 ⋅ 0.85 ⋅ 750 = 1962 kg





Example 2: PLET

Solution:

Section 1:

Selection of Anode current capacity ε1 (1 = section 1) in seawater, considering anode material (Table 5.12: Alloy A3) and subsea water temperature of 13 °C: ε1 = 2000 (Ah/kg)

Selection of utilization factor u1 (1 = section 1) considering Anode Type Stand-off with L ≥ 4 ⋅ r (Table 5.13): u1 = 0.9


Ma1 = Icm1 ⋅ t ⋅ 8760 / 1000 ⋅ u1 ⋅ ε1 = 988 ⋅ 25 ⋅ 8760 / 1000 ⋅ 0.9 ⋅ 2000 = 120 kg


D.4.6.6 Calculation of anode number

D.4.6.6.1 Acceptance criteria

The calculation shall be iterative, starting with a commercially available product sizing, e.g. example Tri-pod, section 1 with Ma1 = 562 kg, N = 4 (stand-off anode with anode net weight of ma = 150 kg). The result of the iteration shall be:

.1 The number of initial (new) anodes (Ni) of one section multiplied with the initial anode current out-put (Iaio) of one anode shall be equal or larger than the required initial current demand (Ici) of the section.

Ici ≤ Ni ⋅ Iaio (12)

.2 The Number of final (consumed) anodes (Nf) of one section multiplied with the final anode current output (Iafo) of one anode shall be equal or larger than the required final current demand (Icf) of the sec-tion.

Icf ≤ Nf ⋅ Iafo (13)

.3 The Number of anodes (N) based on mean current output of one section multiplied with the mean current capacity (Cao) of one anode shall be equal or larger than the required mean current output of the section.

Ca = N ⋅ Cao ≥ Icm ⋅ t ⋅ 8760 / 1000 (14)

The following clauses, Figures and Tables shall be considered:

B.3, Table 5.4: Protective Potentials for various metals in sea water

D.4.2, Table 5.12: Closed circuit anode potential Ea

D.4.4, D.4.5, and Table 5.14: Calculation of Anode Resistance

In the case of stand-off anodes the dimensions of the insert and the support are to be designed taking into account the weight of the anodic alloy and with the various environmental loading conditions as de-tailed in ISO 12495 Chapter 5.5.4 – Structural considerations, see D.4.3.

The design protective potential Ec for all metals of Table 5.4 in seawater and anaerobic environment (sediment, mud) shall be –800 mV.

Iteration with N

o(Ec Ea)Iai

Rai−

= (15)

o(Ec Ea)Iaf

Raf−

= (16)

oCa mam u= ⋅ε ⋅ (17)

Iaio : initial current output of one anode [mA]




Iafo : final current output of one anode [mA]

Ea : closed circuit anode potential [mV] (see Table 5.12)

Ec : Design protective potential [–800 mV]

Rai : initial anode resistance of the new anode [ohm] (D.4.4)

Raf : final anode resistance consumed to its utilization factor u [Ohm]

Cao : Anode mean current capacity of one anode [A⋅h]

mam : mean anode net mass of one anode [kg]

u : Anode utilization factor (Table 5.13)

ε : Anode current capacity [Ah/kg] (Table 5.12)

D.4.6.6.2 Calculation of initial and final anode resistance

The final net anode mass maf and final volume Vaf is given by:

maf = mai ⋅ (1 – u) (18)

mafVaf =ζ

(19)

mai : initial anode net mass of the new anode [kg]

maf : final anode net mass consumed to its utilization factor u [kg]

ζ : Density of the anode material [kg/m3] (Table 5.12)

Vaf : final volume of the final anode [m3]

The final dimensions and final anode resistance Raf of the different anode shapes can be calculated as follows:

Stand-off anodes:

2 2maf (daf dac )Vaf Laf4− ⋅π

= = ⋅ζ

(20)

daf : final diameter of the anode (daf = 2 · raf) [m]

dac : core diameter [m]

raf : final radius of the anode [m]

Laf : final length of the anode shall be: Laf = Lai ⋅ 0.9 [m]

Lai : initial length of the anode [m]

2maf dac Lai 0.9 4daf

4 Lai 0.9⎛ ⎞⋅ π ⋅ ⋅

= + ⋅⎜ ⎟⎜ ⎟ζ π⋅ ⋅⎝ ⎠ (21)

Raf calculation with the equations (4) and (5) of D.4.4:

Stand-off anodes, if L ≥ 4 ⋅ r:

4 LafRaf ln 12 Laf raf

ρ ⋅⎡ ⎤= −⎢ ⎥⋅π ⋅ ⎣ ⎦ (22)




Stand-off anodes, if L < 4 ⋅ r:

2

2

2 Laf rafln 1 1raf 2 Laf

Raf2 Laf

raf raf12 Laf 2 Laf

⎡ ⎤⎧ ⎫⎛ ⎞⋅⎪ ⎪⎛ ⎞⎢ ⎥⎜ ⎟+ +⎨ ⎬⎜ ⎟⎢ ⎥⎜ ⎟⋅⎝ ⎠⎪ ⎪ρ ⎝ ⎠⎢ ⎥⎩ ⎭= ⎢ ⎥⋅ π ⋅ ⎢ ⎥⎛ ⎞⎢ ⎥+ − + ⎜ ⎟⎢ ⎥⋅ ⋅⎝ ⎠⎣ ⎦

(23)

Raf : final anode resistance [ohm]

ρ : sea water resistivity [ohm ⋅ m] (see D.4.5, Table 5.8 and Table 5.14)

Laf : Lai ⋅ 0.9 = final length of the anode [m]

raf : daf/2 = final radius of the anode (for other shapes than cylindrical, assuming that the final anode shape is cylindrical) [m]

Flush-mounted anodes:

The shape of flush-mounted anodes are almost similar to the non cylindrical stand-off anodes (see Fig. 5.2). The final shape shall be assumed to be semi-cylindrical and the final length (Laf), final radius (raf) shall be calculated with the equations (20) and (21).

2 2maf (daf dac )Vaf Laf4− ⋅π

= = ⋅ζ

(20)

daf : final diameter of the anode (daf = 2 ⋅ raf) [m]

dac : core diameter (for other shapes than cylindrical, dac = C/π, where C [m] = cross section pe-riphery) [m]




2maf dac Lai 0.9 4daf4 Lai 0.9

⎛ ⎞⋅ π ⋅ ⋅= + ⋅⎜ ⎟⎜ ⎟ζ π⋅ ⋅⎝ ⎠

(21)

Raf calculation with the equation (6) of D.4.4:

Raf2 Safρ

=⋅

Raf : final anode resistance [Ohm]

ρ : sea water resistivity [Ohm ⋅ m] (see D.4.5, Table 5.8 and Table 5.14)

Laf : Lai ⋅ 0.9 = final length of the anode [m]

daf : final diameter of the anode (for other shapes than cylindrical, assuming that the final anode shape is cylindrical) [m]

Saf : (Laf + daf) / 2 = arithmetic final of anode length and diameter [m]

Bracelet anodes:

For bracelet anodes the final exposed area (Aaf) shall be assumed to be equivalent to the initial exposed area (Aai).

Raf calculation with the equation (7) of D.4.4:

0,315RafAaf

⋅ρ=

Raf : final anode resistance [Ohm]




ρ : sea water resistivity [Ohm ⋅ m] (see D.4.5, Table 5.8 and Table 5.14)

Aaf : exposed final surface area of anode [m2]

D.4.6.6.3 Calculation of initial and final current output and mean current capacity of the anodes

Example 1: Tripod

Solution:

Section 1:

Selection of stand-off anode size (see Fig.5.2, not cylindrical), closed circuit anode potential Ea =-1050 mV (Table 5.12), design protective potential Ec = -800 mV, anode current capacity ε =2000 Ah/kg (Table 5.12) and Anode utilization factor u = 0.9 (Table 5.13).

The following parameters may be selected, if Ma1= 562 kg:

N : 4 (Number of anodes)

mai : 150 (initial anode weight) [kg]

ζ : 2725 (density of the anode material – Table 5.12) [kg/m3]

Lai : 1.77 (initial anode length) [m]

W : 0.184 (initial anode width) [m]

D : 0.184 (initial anode depth) [m]

dac : 0.06 (core diameter) [m]

u : 0.9

ρ : 0.24 (average water temperature of West Africa acc. to Table 5.8: 13 °C); salinity may be 35 ppt; resistivity 0.24 [Ohm·m] acc. to Table 5.14) [Ohm · m]

Calculation of Rai according to formula (4):

=⋅Crai

2 π = 0.117 [m] = ⋅ + ⋅C 2 W 2 D = 0.736 [m]

⋅⎡ ⎤= −⎢ ⎥⋅ ⋅ ⎣ ⎦

4 LaiRai ln 12 Lai rai

ρπ

= 0.067 [Ohm]

Calculation of Raf according to formular (22):

maf = mai ⋅ (1 – u) = 15 [kg] (see (18))

=mafVafζ

= 0.0055 [m3] (see (19))

maf : final anode net mass consumed to its utilization factor u [kg]

Vaf : final volume of the final anode [m3]

⎛ ⎞⋅ ⋅ ⋅= + ⋅⎜ ⎟⎜ ⎟ ⋅ ⋅⎝ ⎠

2maf dac Lai 0.9 4daf4 Lai 0.9

πζ π

= 0.089 [m] (see (21))

daf : final diameter of the anode (daf = 2 ⋅ raf) [m]




⎡ ⎤⋅= −⎢ ⎥⋅ ⋅ ⎣ ⎦

4 LafRaf ln 12 Laf raf

ρπ

= 0.095 [Ohm] (see (22))




D.4.6.6.4 Examination of fulfillment of acceptance criteria:

Example 1: Tripod

Solution:

Section 1:

Ec : – 800 mV

Ea : –1050 mV

Iaio : initial current output of one anode [mA]

Iafo : final current output of one anode [mA]

Raf : final anode resistance consumed to its utilization factor u [Ohm]

Cao : Anode mean current capacity of one anode [A⋅h]

mam : mean anode net mass of one anode [kg]

ε : 2000 Ah/kg (anode current capacity)

−=o

Ec EaIaiRai

( ) = 3730 mA (see (15))

−=o

Ec EaIafRaf

( ) = 2632 mA (see (16))

= ⋅ ⋅oCa mam uε = 270 000 A·h (see (17))

Acceptance Criteria:

.1 Ici1 ≤ N ⋅ Iaio = 4 ⋅ 3730 = 14920 mA; Ici1 = 1285 mA; (see (12))

.2 Icf1 ≤ N ⋅ Iafo = 4 ⋅ 2632 = 10528 mA; Icf1 = 9351 mA; (see (13))

.3 Ca = N ⋅ Cao ≥ Icm1⋅ t ⋅ 8760/1000 = 1013094 A⋅h; Ca = 1080000 A⋅h; (see (14))

If the equations 13, 14 and 15 cannot be fulfilled for the initially selected dimensions and/or numbers, another anode size and/or numbers shall be selected and the calculation repeated until the criteria are fulfilled.

D.4.7 Location of anodes

Based on the calculated number of anodes required for different zones of the structure, for the distribution of anodes EN 12495 and EN 13173 and the following aspects shall be considered: • anodes shall be uniformly distributed • anodes for the splash zone shall be located on the upper level of immersed zone • anodes shall be located nearer complex and critical points such as node areas, but not closer than

600 mm to nodes • anodes protecting conductor pipes shall be uniformly distributed at the different levels in their imme-

diate proximity on the conductor guide frames • anodes corresponding to the current drain of wells shall be uniformly distributed over all the im-

mersed part of the structure • on legs, the anodes shall face the centre of the structure • on diagonals, the anodes shall be alternately placed on the upper and lower surface if more than

one anode is required • on horizontal bracings at different levels, the anodes shall be installed alternately facing up and

down with the exception of the uppermost level where they shall be mounted facing downwards




• The adequacy of the current distribution can be improved using a greater number of anodes, which have a lower individual electrical current output.

The location of all individual anodes shall be shown on drawings. With the exception of stand-off anodes, coating with minimum a coating category II (Table 5.6) shall be applied to the anode surface towards the protection object.

D.4.8 Anodes' inserts and attachments design

The method of attachment of the anodes to the structure shall be in accordance with EN 12495 and EN 13173.

D.4.9 Documentation of Cathodic Protection Design Report

The Cathodic Protection Design Report shall contain the following parts: • design criteria including the design life, the environment characteristics (see Table 5.8 and 5.14), the

protection criteria, the electrical current density requirements (see Table 5.8, 5.9, and 5.10), the as-sumed values of the anode current output at these various periods, and the anode theoretical cur-rent capacity and closed circuit anode potential (see Table 5.12) incl. all relevant project specifica-tions, codes, standards, literatures;

• subdivision of the object in sections incl. to all design data (see D.4.6.1) • surface area calculation of all sections (see D.4.6.2) • current demand calculation (see D.4.6.3) • calculation of minimum required net anode mass (see D.4.6.5) • calculation of minimum anode number (see D.4.6.6) • calculation of initial and final anode resistance (see D.4.6.6.2) • calculation of initial and final current output and mean current capacity of the anodes (see D.4.6.6.3) • examination on fulfillment of acceptance criteria (see D.4.6.6.4) • the number of galvanic anodes, their dimensions (see Fig. 5.2), weight, specification, chemical com-

position (see Table 5.11), practical consumption rate or current capacity (as measured during labo-ratory tests, see D.4.2.2), utilization factor (see Table 5.13), as well as the manufacturer/supplier’s references and documentation

• anode location drawings (see D.4.7) • anode attachment drawings (see D.4.8)

D.4.10 Design of monitoring system

Permanently installed monitoring systems may be used with galvanic anodes system.

The monitoring system of a cathodic protection system measures and may be used to control the operat-ing parameters and the effectiveness of the cathodic protection system. EN 12495 and EN 13173 shall be used as a guideline.

D.4.11 Commissioning and surveying

The objectives of the commissioning and the survey of the cathodic protection system are: • to ensure that the cathodic protection system functions in accordance with the intentions of the de-

sign at structure installation • to establish if the cathodic protection system is continuing to perform in accordance with the design

and that the structure remains satisfactorily protected from corrosion • a cathodic protection survey of the entire structure shall be performed within three month for bare

steel structure or one year on coated structures from the installation of the structure • the survey should include the measurement of the potential at selected locations • the actual location and status of the anodes should be verified and recorded

The documentation of commissioning and surveying shall include:




• the location of each and every anodes as checked during construction or after positioning, all dis-crepancies with the design location being highlighted in the as-built drawings, the method of attach-ment, the date of installation; these data should be updated during the life of the structure

• the description, specification and position of any electrical current or potential control or monitoring device, including type of reference electrode, measuring equipment, and connecting cables

• the commissioning results including the potential survey data

D.4.12 Safety and cathodic protection

The cathodic protection system shall comply with all safety standards and regulations related to electrical equipment that may apply to fixed steel offshore installations.

Herewith the safety hazards due to cathodic protection systems and related to diving personnel during their underwater operations are defined. The galvanic anodes system will be considered, in association with physical obstruction and evolution of dangerous gases.

The major risk from anodes is the entanglement of the diver’s umbilical or life line around the anodes or the anode supports, and the mechanical damage to the equipment due to chafing. Thus the following galvanic anode and anode assembly characteristics are recommended (see Fig. 5.2): • stand-off anodes with J-shaped cylindrical stand off supports, protruding from anode extremities • flush- mounted anodes • bracelet anodes

Similarly, anode cable conduits, junction boxes, etc. shall be designed to exclude any sharp edges or corners or protruding extremities.

Polarization of the structure to potential more negative than –800 mV can result in the evolution of hydro-gen gas at the steel surface. If the gas is allowed to collect in confined air spaces, such as cofferdams which are only part full of sea water, it may present a risk of explosion. All designs shall include an ade-quate venting to prevent the build up of hydrogen.

D.5 Impressed current system

D.5.1 Design considerations

The impressed current system for fixed steel offshore structures and floating structures usually includes a variable transformer rectifier and one or several anodes and normally a number of reference electrodes.

Manually adjustable rectifiers are generally selected because the current demand does not vary signifi-cantly with time.

Rectifiers with automatic potential control may be used in particular cases where the environment condi-tion and the structure configuration induce large and frequent variations of the current demand.

Specific areas presenting particular situations may even require the consideration of a multi-zone control system in order to adapt and optimize the current distribution to the protection demands, the control of the protection of each zone being performed by a separate specific impressed current system.

The number and location of the anodes shall be determined in order to achieve, as far as is practical an adequate protective potential level.

Individual impressed current anodes usually deliver higher electrical currents than galvanic anodes, there-fore with impressed current anodes either dielectric shields or larger anode to structure separation should be used to prevent localized over-polarization and improve the electrical current distribution to the cath-ode.

D.5.2 Direct current power source

The transformer rectifier or controllable DC generator shall be able to deliver the total current demand to the zone it is intended to protect.

The output voltage shall take into account the resistance of the electric circuit (cables, anodes) and the back e.m.f. (back electromotive force) between the anode and the cathode. The back e.m.f. is the natu-rally occurring open circuit potential difference between the anode and the cathode in sea water.




The transformer rectifier shall be able to deliver sufficient current to maintain the steel / sea water / reference electrode potential within the design range.

DC power source with automatic potential control shall deliver an electrical current when one of the refer-ence electrodes used for the control of the DC power source leads to a potential reading less negative than the set minimum negative potential or more negative than the set maximum negative potential (see Table 5.4).

There shall be devices to limit the output current to each anode to a pre-set value.

A DC power source without current limitation circuits should have an effective shutdown in the event of an external short circuit.

D.5.3 Anode materials

Anodes used for impressed current systems can be of two main types: semi-consumable or inert anodes.

Platinum coated titanium and niobium anodes were claimed to be the most commonly used electrodes for cathodic protection of large offshore structures. Lead alloy anodes were listed as being popularly used to protect semi-submersible drilling rigs.

Impressed current anodes, unlike galvanic anodes, can be driven over a wide range of anode current densities, depending on the designer’s preferences and the demands of a particular application. Because anode current loadings have a definite bearing on consumption rates, any meaningful listing of consump-tion rates for various anode materials shall necessarily include the approximate anode current density at which such rates have been established.

Table 5.15 lists consumption rates, associated anode current densities of various impressed current an-ode materials used for CP systems on offshore structures.

Platinum is the ideal permanent impressed current anode material. It is one of the most noble metals and in practically all environments forms a thin invisible film which is electrically very conductive. In addition, the exchange current densities of most anodic reactions on the Pt surface are greater than on other an-ode materials. Due to its high cost, platinum is applied as a thin coating (1–5 μm) on metallic substrates such as titanium, niobium and tantalum.

To avoid the dissolution of titanium at unplatinized locations on the surface, the operating voltage of the anode is limited by anodic breakdown potential of titanium which is in the range of 9 to 9.5 V in the pres-ence of chlorides. Hence the maximum recommended operating voltage of platinized titanium anodes is 8 V. The corresponding maximum current density output is approximately 1 kA/m2.

For cathodic protection systems where operating voltages are relatively high, niobium and tantalum based anodes are generally selected. This is because these two substrates have anodic breakdown potentials greater than 100 V in chloride containing electrolytes.

Table 5.15 Typical electrochemical characteristics for impressed current anodes

Anode materials Consumption rate [g/A⋅y]

Max. current density [A/m2] Max. Voltage [V]

Platinized Titanium 0.0012 to 0.004 500 to 3000 8

Platinized Niob 0.0012 to 0.004 500 to 3000 50

Platinized Tantalum 0.0012 to 0.004 500 to 3000 100

Mixed Metal Oxide 0.0006 to 0.006 400 to 1000 8

Lead Silver 25 to 100 250 to 300 24

Chromium silicon iron 250 to 500 10 to 30 50

Note EN 12945 and EN 13173

The rate of platinum consumption has been found to accelerate in the presence of AC current ripple. Most consumption was observed to occur with AC frequencies of less than 50 Hz. Ripples frequencies less




than 100 Hz should be avoided. The repeated oxidation/ reduction processes result in the formation of a brownish layer of platinum oxide. To avoid the occurrence of this phenomenon, a single or a three phase full-wave rectification is recommended. The consumption rate of platinized anodes is also adversely af-fected by the presence of organic impurities such as sugar and diesel fuel.

D.5.4 Anode shapes and installation

Impressed current systems are more critical with respect to mechanical damage because relatively few anodes, each discharging a substantial amount of protective current, are involved. The loss of an anode can seriously reduce system performance.

The electrical connection between the anode lead cable and the anode body shall be watertight and me-chanically sound.

Cable and connection insulating materials should be resistant to chlorine, hydrocarbons, and other delete-rious chemicals.

Care shall be taken to provide suitable mechanical protection for both the anode and its connecting cable. On suspended systems, the individual anodes or anode strings may be equipped with winches or other retrieval means as a damage- preventing measure during severe storms or for routine inspection and maintenance. The loss of protection during these periods should be considered.

Acceptable methods of installing fixed-type impressed current anodes include, but are not limited to, the following:

D.5.4.1 Tubular, rod and wire anodes

Tubular, rod, and wire anodes can be installed at the lower ends of protective vertical steel pipe casings or conduits. Casings should be attached to above-water structure members and supported at repeating members below water. The anodes should be lowered through the casings (which protect the anode lead wires) and should be allowed to extend below a termination fitting at the bottom of each casing. This method provides a means of anode retrieval or replacement using the anode cable, without diver assis-tance. Marine growth or corrosion scale may make anode retrieval difficult.

D.5.4.2 Flush mounted anodes

Flush- mounted anodes, insulating-type holders can be attached directly to submerged structure mem-bers or to auxiliary structural members, such as vertical pipes, which can be removed for anode replace-ment. Properly designed systems of the latter type permit anode retrieval without diver assistance.

D.5.4.3 Stand-off anodes

Stand-off anodes can be installed on submerged structure members. Diver assistance is required for this type of anode replacement.

D.5.4.4 Bottom-installed anodes

Bottom-installed anodes are typically mounted on specially designed concrete sleds for stability; this minimizes the possibility of their becoming covered with mud or silt.

D.5.4.5 Location of anodes

All precautions shall be taken to avoid any leakage of water through the hull penetration. It is usually a requirement to fit a cofferdam. Typical anode mountings with cofferdam arrangement are given in EN 13173 annex D.

Impressed current anodes should be located as far as practical from any structure member (usually a minimum distance of 1.5 m, but proportional to current magnitude (see Table 5.15). If a spacing of 1.5 m is not feasible a dielectric shield should be used to minimize wastage of protective current by localized overprotection.

Anode holder should be designed to avoid such wastage and to minimize the possibility of a short circuit between the anode and the structure.

There may be a delay of several months to a year or more between the time a structure is set until per-manent electrical power becomes available. Plans should be made for either temporary power and early energizing of impressed current systems or a short-term galvanic anode system (see also D.3). Other-




wise, serious corrosion of structure members, as well of as the underwater components of the impressed current system, can occur.

D.5.5 Dielectric shields

The objective of dielectric shields, and coatings used as dielectric shields, is to prevent extremely high current densities and current wastage in the vicinity of the anodes. This serves to promote more uniform protective current distribution.

Materials selected shall be suitable for the intended service.

Liquid applied coatings, fiberglass reinforced plastic, prefabricated plastic or elastomeric sheets can be used on the structure adjacent to the anodes, or incorporated into the anode assembly.

The electrochemical reactions occurring at the anode and cathode surfaces produce corrosive products and gases which may deteriorate the dielectric shield or include its disbanding.

The design of the dielectric shield should take into account the possible deterioration and aging character-istics of the materials.

Liquid applied coatings, prefabricated plastic or elastomeric sheets can be used on the structure adjacent to the anodes, or incorporated into the anode assembly.

D.5.6 Reference electrodes

Reference electrodes are used to measure the steel to electrolyte potential and may be used to control the output current of the cathodic protection system. They are either zinc or silver/silver chloride/sea wa-ter electrodes (see ISO 12473). Zinc electrodes are more robust whereas silver/silver chloride/sea water electrodes are more accurate.

When reference electrodes are used to control the impressed current system, they should be installed at locations determined by calculations or experience to ensure the potential of the structure is maintained within the design set limits (see Table 5.4).

Precautions should be taken in order to avoid any direct electrical contact between the electrodes and the steel structure.

D.5.7 Cathodic protection calculation and design procedure

The required current demand for each section of the object shall be calculated as described in D.4.6.1 to D.4.6.3.

For current demand determination of floating structures, the underwater surface area should always in-clude the boot topping, but never the atmospheric zone.

To compensate for a less efficient current distribution (small number of anodes), the impressed current coating protection system shall be designed to be able to provide 1.25 to 1.5 times the calculated total current demand, which is the total design current demand.

Example 1: Tripod



D.1 and D.2:Required design current densities of geographic areas and in specific areas of the structure (Table 5.8, and 5.9).

Solution:

Section 1: Coating category IV.


k1 = 0.015, k2=0.005

Selection of design current densities (Table 5.8: West Africa):

ici1= 130 mA/m2, icm1= 65 mA/m2, icf1= 86 mA/m2





fci1 = k1 = 0.015

fcm1 = k1 + k2 ⋅ t/2 = 0.015 + 0.005 ⋅ 30/2 = 0.09

fcf1 = k1 + k2 ⋅ t = 0.015 +0.005 ⋅ 30 = 0.165

Calculation of required current demands: Ici1, Icm1 and Icf1 for Ac1=659m2

Ici1 = Ac1 ⋅ ici1 ⋅ fci1 = 1285 mA

Icm1 = Ac1 ⋅ icm1 ⋅ fcm1 = 3855 mA

Icf1 = Ac1 ⋅ icf1 ⋅ fcf1 = 9351 mA

Multiplied with a factor of 1.5 the minimum required total current demand shall be:

Icf1tot = 1.5 ⋅ Icf1 = 14027 mA

For any cathodic protection system, the size of the anodes shall be determined using Ohm's law. In the case of inert anodes, like platinized or mixed metal oxide anodes the anode resistance Ra will be stable during the life time, because no change of the dimensions will occur. In case of semi- consumable an-odes, like lead silver and chromium silicon iron the initial and final anode resistance shall be considered (see D.4.6.6).

( )Ec EaIa

Ra−

= (see 15)

Ia : current output of anodes [mA]

Ea : Closed circuit potential (see Table 5.15) [mV]

Ec : Design protective potential (see Table 5.4) [mV]

Ra : anode resistance (D.4.4)

The anodic resistance is a function of the resistivity of the anodic environment and of the geometry (form and size) of the anode. The equations (4) and (5) for Stand-off anodes, (6) for Flush-mounted anodes and (7) for Bracelet anodes shall be used.

The acceptance criteria:

The maximum value of Ici, Icm, and Icf multiplied with a factor of 1.5 shall be smaller than the current output of the anodes.

D.5.8 Design of monitoring system

Permanently installed monitoring system shall be used with impressed current systems.

The monitoring system of a cathodic protection system measures and may be used to control the operat-ing parameters and the effectiveness of the cathodic protection system.

The cathodic protection is an active system, i.e. it is effective only when the impressed current systems are operating and provide adequate polarization of the steel to achieve the protection criteria stated.

Therefore, the steel potential should be measured during the life of the structure in order to verify the adequacy of the protection.

The monitoring system may include the following: • Permanent reference electrodes. The reference electrode should be installed and secured on each

immersed level of the structure, close to the steel surface, and at locations as mentioned in D.5.4. These reference electrodes shall be electrically insulated from the structure and connected to the data transmission system.

• Conduit systems, cables, junction boxes, data transmission systems shall be in accordance to EN 12495

D.5.9 Potential measurements

The potential measurements shall be conducted in accordance to EN 12495 and EN 13173.




D.5.10 Commissioning and surveying

For the commissioning of the impressed current cathodic protection system the following operations shall be carried out: • check wiring to ensure correct polarity (negative terminal to structure) • switching-on the transformer-rectifier(s) to supply direct current to the structure • monitoring of the structure’s potentials with the monitoring system until the required potential values

are reached

A cathodic protection potential survey of the structure should be performed within one month of the com-missioning of the cathodic protection system.

This survey should include the recording of the structure potential at selected locations. The actual loca-tion and status of anodes, cables, conduits, monitoring system should be verified and recorded.

D.5.11 Documentation

The following data should be kept for reference and permanently updated, if applicable: • design criteria including the design life, the environment characteristics (see Table 5.8 and 5.14), the

protection criteria, the electrical current density requirements (see Table 5.8, 5.9, and 5.10) the as-sumed values of the anode current output (Table 5.15) incl. all relevant project specifications, codes, standards, literatures

• subdivision of the object in sections incl. to all design data (see D.4.6.1) • surface area calculation of all sections (see D.4.6.2); • current demand calculation (see D.4.6.3) • calculation of anode resistance (seeD.4.4) • calculation of the required current output of the anodes (see D.5.7) • examination on fulfillment of acceptance criteria (see D.5.7) • the number of anodes, their dimensions, specification (see D.5.7 and Table 5.15), description of an-

odic equipment and connection, effective output current densities, associated consumption rates and allowable voltage (see Table 5.15), as well as the manufacturer/supplier’s references and documen-tation

• the description and means of attachment of anodes, the composition and location of any dielectric shield, as well as the specification, characteristics and attachment method and through wall or through hull arrangements of the connecting cables (see D.5.4 and D.5.5)

• the location of each and every anodes as checked during construction or after positioning, all dis-crepancies with the design location being highlighted in the as-built drawings, the method of attach-ment, the date of installation; these data should be updated during the life of the structure (see D.5.4)

• the location, detailed specification, drawings, and output characteristics of each DC power source with their factory test reports (seeD.5.2)

• the location, description and specification of any protection, potential control or monitoring device, including reference electrode, measuring equipment and connecting cables (see D.5.6, D.5.8, and D.5.9)

• the commissioning results including the potential survey data; current and voltage output values of each DC power source and any adjustment made for non-automatic devices (see D.5.10)

• the results of data recorded during periodic maintenance inspection including protection potential values, DC output values, maintenance data on DC power sources and downtime periods in order to follow the changes of the protection potential level status of the structure.

D.5.12 Safety and cathodic protection

The cathodic protection system shall comply with all safety standards and regulations related to electrical equipment that may apply to fixed or floating steel offshore installations.

This clause deals with safety hazards due to cathodic protection systems and related to diving personnel during their underwater operations. The galvanic anodes system will be considered, in association with physical obstruction, electric shock and evolution of dangerous gases.




The major risk from anodes is the entanglement of the divers umbilical or life line around the anodes or the anode supports, and the mechanical damage to the equipment due to chafing.

Similarly, anode cable conduits, junction boxes, etc. should be designed to exclude any sharp edges or corners or protruding extremities.

In the event that the impressed current cathodic protection system equipment is defective, or the diver inadvertently makes direct contact with the active anode element, he may suffer an electric shock.

During diving operations not directly related to cathodic protection system, and any diving inspection car-ried out close to impressed current anodes, the DC supply of the anodes shall be switched off. However, diving cathodic protection inspection may be performed, with the impressed current system in operation, providing all relevant safety regulations and precautions are applied (see EN 12495 annex D).

Polarization of the structure to potential more negative than –800 mV can result in the evolution of hydro-gen gas at the steel surface. If the gas is allowed to collect in confined air spaces, such as cofferdams which are only part full of sea water, it may present a risk of explosion.

To avoid this hazard, the following measures should be taken: • all designs to include an adequate venting to prevent the build up of hydrogen • the structure to electrolyte potential to be kept at values less negative than the threshold value at

which hydrogen evolution is not significant. This can be achieved by ensuring that a minimum dis-tance should be kept between the structure and impressed current anodes.

Electrochemical reactions at the surface of impressed current anodes in sea water invariably result in the evolution of chlorine gas which is highly toxic and corrosive. If this is allowed to collect in confined air spaces above the water line, it may present a hazard to personnel and materials.

To avoid this hazard, all design shall prevent the build up of gas.




Section 6 Foundations A General ....................................................................................................................... 6-1 B Geotechnical Investigations........................................................................................ 6-1 C General Design Considerations.................................................................................. 6-2 D Steel Pile Foundations ................................................................................................ 6-4 E Shallow Foundations................................................................................................... 6-8

A General

A.1 Scope

This Section applies to the foundations of fixed offshore structures and covers piled as well as gravity type designs. Requirements regarding self-elevating platforms (mobile units) see GL Rules for Mobile Offshore Units (IV-6-2), Section 2.

A.2 Foundation design

The foundation design of fixed offshore structures shall consider all types of loads which may occur dur-ing installation and operation of the structure. The design can be carried out according to the following standards:

API Recommended Practice 2A-WSD (RP 2A-WSD) for pile and shallow foundations

or

ISO 19902:2007 (same as DIN EN ISO 19902:2008-07) for pile foundations

ISO 19901-04:2003 (same as DIN EN ISO 19901-4:2005-07) for general requirements and shallow foun-dations

A.3 Soil

The soil behaviour and its interaction with the foundation shall be investigated with special regard to static, dynamic and transient loading.

A.4 Evolution of foundations

The design practise for foundations has proved to be very innovative and this evolution is expected to continue. Therefore circumstances may arise when the procedures described herein are insufficient on their own to ensure that a safe and economical foundation design is achieved. In such a case new re-quirements have to be discussed and agreed by GL.

B Geotechnical Investigations

B.1 Field investigation

The field investigation shall be carried out at the actual site, defined as the area within which the structure may be installed, including accepted tolerance limits from theoretical centre. The field investigation pro-gramme shall comprise but not necessarily be limited to the following: • site geology survey in order to establish geological conditions in general • geophysical explorations for the purpose of extending the localized information from the borings and

in situ testing • bottom topography including registration of rocks or hard objects on the bottom


Section 6 Foundations


• soil sampling to ensure a sufficient number of good quality samples, that will allow the properties of all important layers to be determined

• in situ cone penetration tests (or other equivalent in situ test) performed, if possible, to a depth that will cover the critical shear zone

The actual extent of a geotechnical field investigation has to be set up individually for each site.

B.2 Boring vessel

B.2.1 Positioning system

The boring vessels shall be equipped with a positioning system which ensures an adequate accuracy of borehole positioning.

B.2.2 Laboratory and personnel

The boring vessel shall contain a high quality soil laboratory for classification and testing of soil speci-mens. The opening of samples on board, the visual inspection and the classification of the soil shall be performed by qualified geotechnical personnel.

B.3 Samples

The handling, storage and transport of soil samples shall be such as to retain a satisfactory sample qual-ity.

C General Design Considerations

C.1 Influences to be considered

The design of foundations shall take account of all effects which may be of influence on deformations, bearing capacity and installation of the foundation system.

C.2 Characteristics of soil layers

For each soil layer the physical properties shall be thoroughly evaluated with the help of in-situ and labo-ratory testing.

The main parameters for the foundation design are:

For sand: • effective friction angle Φ' [degrees] • submerged unit weight γ' [kN/m3] • Youngs modulus E [Mpa] • initial soil modulus K [MN/m3]

For clay: • undrained shear strength cu [kN/m2]

• submerged unit weight γ' [kN/m3] • Youngs modulus E [Mpa] • Strain at 50 % of failure e50 [-]

C.3 Cyclic loads

C.3.1 The effect of cyclic loading, which may cause a reduction of shear strength and bearing ca-pacity of the soil, shall be investigated.




C.3.2 Especially for structures installed in seismically active areas the proneness of the soil to partial liquefaction and possibly resulting reduction in soil strength and stiffness have to be evaluated with great care.

C.4 Scour

C.4.1 The possibility of scour or undermining around the foundation shall always be investigated. Normally cohesive soils are not prone to scour but for a cohesionless soil condition global and local scour may occur.

In case that scouring is possible, • the foundation has to be protected by suitable means • alternatively, the foundation has to be considered to be partly unsupported

C.4.2 If no other data are available for the specific site conditions, the scour depth at pile founda-tions may be estimated as 1.5 · d (d = pile diameter) for design purposes including an overburden reduc-tion depths of 6 · d, please refer to API or ISO.

Experience shows that 1.5 · d is not conservative for the German sector of the North Sea. We recom-mend applying 2.5 · d for these locations

C.4.3 The design criteria shall be verified by regular surveys, see GL Rules for Certification and Surveys (IV-7-1), Section 4. Countermeasures shall be taken in case of exceeding the limits established in the design.

C.5 Hydraulic stability

The hydraulic stability shall be investigated for those types of foundations which may exhibit significant hydraulic gradients within the supporting soils.

C.6 Soil stability

C.6.1 If necessary, due to existing slope or due to installation effects, the risk of slope failure or the possibility of a deep slip shall be investigated.

C.6.2 Total or effective stress analysis shall be used for soil stability investigations.

C.6.3 Especially for soft, normally consolidated clays or loose sand deposits consideration of sea-floor instability is required.

C.6.4 Seabed movements due to action of waves, earthquake or operational effects, e.g. drilling, dredging, may cause reduced resistance or increased loading and shall be investigated.

C.7 Settlements and displacements

Long term settlements and displacements of the structure as well as the surrounding soil shall be moni-tored.

C.8 Load deformation behaviour

The load deformation behaviour of a piled foundation is to be analysed with special regard to possible structural interaction.




D Steel Pile Foundations

D.1 General

D.1.1 Among several existing types of pile foundations the following are most frequently used off-shore: • open ended and driven piles • driven and underreamed piles • drilled and grouted piles

Many other designs are used to suit the needs of the individual soil conditions.

D.1.2 For each type of pile foundation the soil-pile interaction and pile capacity have to be evaluated with due regard to site specific soil conditions.

D.1.3 The following paragraphs are not intended for monopole foundations.

D.2 Pile design, design methods

D.2.1 The design of piled foundations has to fulfil requirements for axial and lateral bearing capacity, load-deflection behaviour (non-linear springs) and displacements.

D.2.2 Among others the following factors will affect the design: diameter and wall thickness, penetra-tion, mudline restraint, material, pile footing, pile group spacing (if applicable) and installation method.

D.2.3 The design method has to incorporate the pile geometry, properties and arrangement. It should be capable of simulating the non-linear properties of the soil and be compatible with the load dis-placement behaviour of the structure and pile foundation system.

D.2.4 Displacements and rotations shall be checked for individual piles and pile groups.

The design shall consider bending moments as well as axial and lateral loads.

D.2.5 In D.3 axially loaded piles are treated according to API ASD method whereas D.4 describes the method according to ISO LRFD. D.5 and following subparagraphs treat the other pile related require-ments which are identical in both standards (API and ISO).

D.2.6 In D.11 additional requirements of the German authority BSH (Bundesamt für Seeschiffahrt und Hydrographie) are summarized.

D.3 Axially loaded piles acc. to ASD-Method

The allowable stress design (ASD) method has to be carried out according to API RP 2A-WSD latest revision.

D.3.1 Design loads

The design loads are defined in Section 3, C.2 The resulting axial load (from the design load combina-tions) in the pile is FA.

D.3.2 Design format for axial capacity

The required design axial pile capacity may be expressed as follows:

dA

g

QF

V≤

FA : Axial pile load from design load combination

Qd : Ultimate bearing capacity




gg : Global safety factor for pile capacity, factors in Table 6.1 are valid for “main-text”- method of API

Qd has to be calculated according to API RP 2A-WSD. There are 2 principal methods how to calculate Qd. One is described in the main text and the other methods (CPT-based) are explained in detail in the com-mentary part of the API.

If one of the CPT-based methods from the commentary part of the API is applied the global safety factors have to be agreed upon with GL. The applied additional safety factor has to be justified by rational analy-sis and soil expert statement.

Table 6.1 Safety factors γg for different loading conditions

Loading conditions

No. Designation Safety factor γg

2 Operating loads 2.0

3 Extreme environmental loads with specified max. vertical loads 1.5

3 Extreme environmental loads with minimum weights (for pullout) 1.5

D.3.3 Pile stresses

Pile stresses have to be checked according to corresponding interaction formulas (refer to chapter 3 of API) for the design loading conditions. The pile is mostly loaded by axial loads and bending moments. The calculation of stresses should consider the horizontal and vertical support by the soil as well as the deformed shape of the pile by the design loads.

D.4 Axially loaded piles acc. to LRFD-Method

The load resistance factor design (LRFD) method has to be carried out according to ISO 19902 latest revision.

D.4.1 Design loads

The design loads are defined in Section 3, C.3. The resulting axial load (from the design load combina-tions) in the pile is Pd.

D.4.2 Design format for axial capacity

The required design axial pile capacity may be expressed as follows:

rd d

R

QP Q

V≤ =

Pd : Design axial load on the pile

Qd : Design axial pile capacity

Qr : Representative value of axial pile capacity

gR : Partial resistance factor for pile capacity, factors in Table 6.2 are valid for “main-text”-method of ISO

Qr has to be calculated according to ISO 19902. There are 2 principal methods how to calculate Qr. One is described in the main text and the other methods (CPT-based) are explained in detail in the Annex A.

If one of the CPT-based methods from the Annex A of the ISO is applied the partial resistance factors have to be agreed upon with GL. The applied additional factor on the resistance has to be justified by rational analysis and soil expert statement.




Table 6.2 Partial resistance factors γR for pile capacity and different loading conditions

Loading conditions Partial Resistance Factor γr

Operating loading condition or loading condition with perma-nent and variable actions 1.5

Extreme environmental loading conditions 1.25

D.4.3 Pile stresses

Pile stresses have to be checked according to corresponding interaction formulas (refer to chapter 13 of ISO) for the design loading conditions. The pile is mostly loaded by axial loads and bending moments. The calculation of stresses should consider the horizontal and vertical support by the soil as well as the deformed shape of the pile by the design loads.

D.5 Axial pile performance

D.5.1 The axial pile performance has to fulfil specified service requirements by the owner.

D.5.2 Pile axial static deflections should be within acceptable serviceability limits and shall be com-patible with internal forces and moments.

D.5.2.1 Cyclic effects on axial performance have to be considered. Repetitive actions can cause tem-porary or permanent decrease in pile resistance and/or accumulation of deformation. Rapidly applied actions can cause an increase in resistance and/or pile stiffness. The design pile penetration shall be sufficient to develop the required design pile capacity.

As an approach cyclic effects shall be considered if the cyclic load amplitude is greater than 10 % or the cyclic load range is greater than 20 % of the ultimate static pile resistance to be considered for the ex-treme loading condition.

D.6 Soil reactions for piles under axial loads

D.6.1 The axial resistance of the soil for pile axial loads is provided by a combination of soil-pile adhesion and associated shear transfer along the skin of the pile. For axial compression loads on the pile the end bearing resistance at the pile tip is activated.

D.6.2 The axial shear transfer is based on t-z curves. The end bearing for compression loads is de-fined by a Q-z curve. Both curves are provided in API and ISO for clay and sand soil types.

D.7 Soil reactions for piles under lateral loads

D.7.1 The pile foundation shall be designed to resist static and dynamic lateral loads. The lateral resistance of the soil near the surface has a significant influence on the pile design. In this respect scour has to be considered.

D.7.2 The non-linear lateral support of the soil is represented by the so called p-y curves. The p-y curves from API or ISO can be used for different soil types (soft and stiff clay as well as sand).

D.8 Pile group behaviour

D.8.1 For pile groups with pile spacing of less than eight times pile diameter, pile group effects have to be evaluated.

D.8.2 For axial pile loads the group capacity in clay may be less than the sum of the single pile ca-pacities, whereas the group capacity in sand may be higher than the sum of the individual pile capacities.




D.8.3 For lateral loads the pile group will normally undergo larger deflection than a single pile under corresponding average loading.

D.8.4 The analysis of a pile group may be carried out according to acknowledged methods 1 as agreed with GL.

D.8.5 All piles of a pile group should end in the same soil layer.

D.9 Pile structure design

D.9.1 General

The pile structure design shall consider all types of loads which may occur during installation and opera-tion phases.

D.9.2 Stresses in piles during installation

D.9.2.1 During pile installation the max. combined stresses shall not exceed the yield stress of the pile material. In general, it can be assumed that buckling of the pile during hammering will not occur as a re-sult of dynamic portion of the driving stresses.

D.9.2.2 Consideration shall be given to axial loads and bending moments due to pile weight and full hammer weight. Bending moments due to pile batter eccentricities are to be accounted for. For vertical piles 2 % of pile and hammer weight shall be assumed as if acting laterally to account for possible im-pacts and eccentricities during hammer placement.

D.9.2.3 Dynamic stresses shall be calculated based on wave propagation theory. Dynamic stresses shall not exceed 80 to 90 % of yield stress depending on specific circumstances, see API or ISO.

D.9.2.4 Pile wall thickness

D.9.2.4.1 The ratio of pile diameter and wall thickness shall be small enough so that local buckling dur-ing installation is avoided. General guidelines are given in API RP 2A and ISO 19902.

D.9.2.4.2 The pile wall thickness may vary along its length according to the stress level in the different sections. Thickened portions should be reasonably extended in order to allow for underdrive or overdrive.

D.9.2.4.3 For piles to be driven in hard soils a driving shoe should be provided.

D.9.3 Stresses in piles during platform operation

D.9.3.1 All loads resulting from the design load conditions of the platform are to be considered for the pile structure design.

Normally these loads will control the pile design in the mudline area.

D.9.3.2 The axial load transfer may be calculated according to the "axial soil resistance - pile deflec-tion" characteristics or alternatively according to the skin friction capacity of the pile, see D.6.

D.9.3.3 The lateral soil stiffness, see D.7, has to be accounted for when calculating the bending mo-ment distribution. Due regard shall be given to the effect of scour and lack of soil adhesion due to large pile deflections.

D.9.3.4 The pile strength shall be designed according to D.3 or D.4.

For piles having large horizontal deflections a stress increase due to second order effects shall be con-sidered.

D.9.3.5 For pile sections embedded in the soil normally column buckling needs not to be investigated.

––––––––––––––

1 See e.g. Focht and Koch, OTC 1896, 1973. This procedure combines the subgrade reaction (p-y) analyses and elastic half

space procedures. It results in modified p-y curves for an isolated pile to represent the single pile behaviour in a pile group.




D.9.4 Pile Fatigue

D.9.4.1 In principal pile fatigue has to be considered due to pile driving and in-service loads.

D.10 Grouted pile to structure connection

D.10.1 Platform loads may be transferred to leg and sleeve piles by grouting the annulus.

The application of shear keys is recommended in order to improve the capacity of the steel grout bond.

D.10.2 Grouted pile to structure connections may be designed according to API or ISO. The design loads have to be taken from the design loading conditions according to the applied standard, refer to ei-ther D.3 or D.4. The design checks have to be carried out according to the chosen standard.

D.10.3 For fatigue check of the grout connection NORSOK N-004, Rev. 3, Appendix K can be ap-plied.

D.10.4 For geometries not covered by the existing formulae the strength of the grouted connection has to be proven by suitable calculation methods or by test.

D.10.5 Grout quality

D.10.5.1 Prior to the installation, the suitability of the grout mix has to be proven. The compressive strength has to be confirmed by laboratory tests on grout samples which were mixed and cured under field conditions.

D.10.5.2 The design grout strength has to be verified by a representative number of specimens taken during grouting operations. It has to be ensured that the specimens will be cured simulating in-situ condi-tions.

D.11 Additional BSH requirements for steel pile foundations

For projects in the German exclusive economic zone BSH (Bundesamt für Seeschiffahrt und Hydro-graphie) is the approving authority. In this case the following additional requirements for pile foundations generally apply. These requirements however are subject to project specific confirmation by BSH. • In principal API RP 2A-WSD and ISO 19902 can be applied for design purposes of steel pile founda-

tions in the German sector. • The “main text method” of both standards has to be applied in general. Any exemption needs to be

explicitly agreed with BSH. • In case that ISO standard is applied the partial resistance factor for extreme condition is 1.40 for

compression and 1.50 for tension (according to the DIN 1054: 2010-12) instead of 1.25.

• The max. value of unit skin friction is limited to 115 kN/m2. • While capacities of the piles and soil have to be calculated with characteristic soil parameters, lower

and upper bound values may have to be used for stiffness related analysis as well as for pile driv-ability analysis.

• The cyclic behaviour of piles has to be considered and designed e.g. according to the methods de-scribed in reference EA Pfähle, 2nd edition, 2012.

• A concept of a dynamic axial load test (“dynamische Pfahlprobebelastung”) after installation of the piles has to be presented prior to installation, please refer to EC 7-1:2009-09 and DIN1054:2010-12 in correspondence with to EA-Pfähle, 2nd edition 2012.

E Shallow Foundations E.1.1 Scope of Application

E.1.2 The shallow foundations considered herein are typically used for temporary support of unpiled structures, especially mudmats during installation phase.




E.1.3 Characterization

Shallow type foundations are characterized as foundations with relatively small penetration into the soil, compared to the width of the foundation bases, and relying predominantly on compressive contact with the supporting soil.

E.1.4 Scope of design

The design of the foundation shall include the following considerations: • On-bottom stability including failure due to overturning, bearing, sliding or combinations thereof. • static deformations • dynamic behaviour, i.e. performance of the foundation under cyclic loads • hydraulic instability, such as scour or underwashing due to wave pressures • flat-bottom components or structures shall be designed with due consideration given to non-uniform

soil reactions which can cause hard points acting on the structure

E.1.5 Design loads

The design loads have to be taken from the design loading conditions according to the applied standard, refer to either D.3 or D.4.

E.1.6 Code Checks

The code checks have to be carried out according to API RP 2A-WSD or ISO 19901-4 for shallow foun-dations according to the required scope defined in E.1.3.




Section 7 Crane and Crane Support Structures A Cranes......................................................................................................................... 7-1

A Cranes

A.1 General

A.1.1 Scope

This Section provides requirements for Certification of lifting appliances, especially cranes, on offshore installations/units for different purposes, e.g. lifting duties like loading and discharging to and from the platform, assistance for platform works, personnel transfers, etc.

A.1.2 Standards and Guidelines

Within the scope of Certification the following standards are accepted for GL: • GL Rules for Loading Gear on Seagoing Ships and Offshore Installations (VI-2-2) • GL Guideline for Personnel Transfers by Means of Lifting Appliances (IV-6-9) • EN 13001 – 1 Cranes - General design - Part 1: General Principles and requirements • EN 13001 – 2 Cranes – General design – Part 2: Load actions • EN 13001 – 3 – 1 Cranes - General design

Part 3 – 1:Limit states and proof competence of steel structures • EN 13001 – 3 – 2 Cranes - General design

Part 3 – 2:Limit states and proof competence of wire ropes in reeving systems • EN 13852 – 1 Cranes – Offshore cranes – Part 1: General-purpose offshore cranes • EC Guideline no. 2006/42/EC: Mechanical engineering Directive on Machinery • ILO – C152 – Occupational Safety and Health (Dock Work) Convention

A.1.3 Other Regulations

Certification may be requested by the manufacturer or Owner including specified items. In case of other regulations, which are not covered by the standard Certification system, this has to be clarified, which Rules and/or combinations shall apply. The covering of additional requirements may be limited. If the additional requirements not comply with the general Certification work, this shall be reported to the Owner in writing.

A.1.4 Extension and reduction of work

Upon request of the Owner, the scope of work may be extended or reduced beyond the subjects and aspects covered by the Certification. Both extent and reduction have to be agreed in writing.

In case of lowering of the safety standards due to leading hazards or other discrepancies this will not agreed by the Certifier.

A.1.5 Cranes and lay-down areas

Offshore substations shall be fitted with a lifting device for handling of cargo and personnel transfer, if requested. The location of crane, working area and lay-down area shall be free from hazard sources. In case of unavoidable positioning adequate impact protection to be installed. Lay-down areas are not to be placed in the vicinity of helicopter decks and not on the same level.


Section 7 Crane and Crane Support Structures


A.1.6 Repairs and maintenance

When important repairs or renewals are required to be made to each lifting device, the repairs are to be carried out under the attendance and to the satisfaction of the GL Surveyor. Tests and examinations of the particular lifting devices, as may be deemed necessary, are to be carried out in accordance to the existing Rules.

Maintenance to be performed according to manufacturer's crane manual and a programme shall be pro-vided for all lifting equipment and shall include all important maintenance tasks highlighted.

A.2 Coverage of Certification Procedure

A.2.1 Activities

Following activities are covered by the Certification: • design examination • survey during fabrication and installations • witness of testing and marking

A.2.1.1 Design examination

The design approval is subject with respect to strength and suitability for its purpose and covers all load-carrying and other important components of the lifting appliance including the relevant substructures.

Environmental conditions to be applied.

If the stiffeners and crane loads may have a significant effect on the load distribution and local stressing, crane pedestals shall be included in the analytic model of the primary structure of the installation/unit.

The design approval may also be obtained either on a case-by-case basis or as a general approval like a Type Approval. In this case certain conditions to be observed and period of validity will be limited.

A.2.1.2 Survey during fabrication and installation

Generally a survey during manufacturing of each separate lifting appliance shall be performed by the GL Surveyor, in order to ascertain compliance with the approved drawings and other requirements of the Certification standard as well as good workmanship.

After completion of fabrication and testing (FAT) at manufacturer's site a provisional Certification to be issued. Attached shall also the manufacturer's documentation, e.g. material Certificates, details of weld-ing, NDT reports, etc.

After installation of the lifting appliance in the final location, and before testing, a final survey shall be per-formed by the Surveyor.

A.2.1.3 Testing and Marking

The testing shall be carried out according to an approved test programme.

After successful testing of both load requirements and function, components and the lifting appliance shall be marked.

A.2.2 Parts, components and systems

Following parts, components and systems to be covered by the Certification procedure: • all load-carrying structural members and components of the lifting appliance • all loose gear, e.g. cargo hooks, chains, rings, blocks, sheaves, shackles, lifting beams, swivels and

ropes • rope drums • slewing bearing including fasteners • power systems for hoisting, luffing, slewing, and travelling • brakes and brake systems • seating and fastening for prime movers, winches and for bearing of power transmitting components




• electrical installation • control and monitoring systems • safety equipment • protection against fire

A.3 Crane manufacturing and construction

A.3.1 Major related items

The manufacturer shall cover all aspects of a quality control system with defined responsibilities and com-petent personnel.

Following items to be observed, but not limited to: • approved drawings and specifications • material designation and handling including surface requirements • dimensional tolerances • calculations for fatigue, buckling, accelerations, etc. • check of design and strength of components, e.g. buckling stability of jibs • personnel transfer applicability • approved welding procedure specifications • weld preparations according to recognized standards • weld performance including preheating • approved repair welding procedure specification • qualified welders • approved welding consumables • specification for forming materials • heat treatment, if required • production welds • performance of works • inspection and testing of welds • weld acceptance criteria • NDT methods, including approved specifications and qualified personnel • minimum NDT requirements to be mentioned • defects and deficiencies to be documented and corrected before painting or coating • material protection against corrosion • machinery components, e.g. winches, drums, gear transmissions, ropes, with fittings and anchor-

age, sheaves, ventilation, etc. • slewing bearing of jib crane • lifting gear including loose gear • electrical equipment and power systems, e.g. prime movers • hydraulic, pneumatic, instrumentation, automation, communication • control and monitoring system • operator's cabin including equipment and arrangement • safety and safety equipment • platform, access possibilities, parking area • specific requirements – cranes with safety functions with reducing the risk connected to crane opera-

tions • monitoring of safety functions




• list of hazards including detailed descriptions • risk analysis, if required • protection and precaution against fire • performance procedure of load tests, e.g. by weights or dynamometer, depending on SWL • specification and examination after testing including sling and lifting tackles • check of electrical installation, insulation resistance • marking and signs • verification procedure including final shut down • etc.

A.3.2 Documentation on board

Crane manual for each crane with following information: • design standard, operation, erection, dismantling and transportation • all limitations during normal and emergency operations with respect to SWL, safe working moment,

maximum wind, design temperature and braking systems • all safety devices • diagrams for electrical, hydraulic and pneumatic systems and equipment • materials used in construction, welding procedures and extent of non-destructive testing • guidance on maintenance and periodic inspections • Certification of components, e.g. hydraulic cylinders • Certification of additional equipment, e.g. baskets, skids, pad eyes, etc. • Certification of wire ropes

A.4 Certification and Classification

A.4.1 Certification

Certification of offshore cranes includes the issuance of a GL Register Book by GL Surveyor as described in the GL Rules for Loading Gear on Seagoing Ships and Offshore Installations (VI-2-2). In addition the GL Surveyor will add the test Certificate(s) for crane(s) and other test Certificate(s), whatever is in use.

A.4.2 Classification

In case of offshore crane Classification in addition to the crane Certification, the acting GL Surveyor will make a relevant note in the Register Book and the Operator will get a crane Class Certificate from the GL Head Office.




Section 8 Helicopter Deck Structures A General ....................................................................................................................... 8-1 B Structure of the Helicopter Deck................................................................................. 8-2

A General

A.1 Scope

A.1.1 This Section summarizes main design considerations relating to helicopter landing facilities. Aspects which are mostly aeronautically determined, like size and marking of the helicopter deck, clear-ances around the platform, sectors for approach and take-off have to be treated according to the relevant international and national regulations or codes, compare the GL Rules for General Safety (IV-7-3), Sec-tion 5, D.

A.1.2 In this Section it is assumed that the structure of the helicopter deck is made of steel. If a structure made of aluminium alloys shall be provided, the design should follow the GL Rules for Hull Structures (I-1-1) or recognized standards, like standards of the American Petroleum Institute (API).

A.1.3 For electrical installations on helicopter facilities, see GL Rules for General Safety (IV-7-3), Section 5, D.

A.2 Standards and regulations

Depending on the location of the offshore installation or the flag state of the offshore unit relevant national and international standards and regulations have to be fulfilled besides of these GL Rules. The following examples can be defined: • ISO/CD 19901-3 Standard: Petroleum and Gas Industries – Specific Requirements for Offshore

Structures – Topside Structure, 8.5 • IMO: Code for the Construction and Equipment of Mobile Offshore Drilling Units (MODU Code),

Chapters 9 and 13 • IMO Res. A.855(20): Standards for on board Helicopter Facilities • ICS (International Chamber of Shipping): Guide to Helicopter/Ship Operations • CAP 437: Offshore Helicopter Landing Areas - Guidance on Standards, Civil Aviation Authority,

Gatwick Airport South, West Sussex, RH6 0YR, UK

A.3 Helicopter data

For providing relevant helicopter facilities the Owner/Operator has to deliver the following information: • types of helicopters to be operated • geometrical main dimensions, especially length of fuselage, number and diameters of rotors, etc. • total overall length of the helicopter when the rotors are turning (D-value) • weight, weight distribution and wheel or skid configuration • highest vertical rate of descent on the helicopter deck, e.g. because of a single engine failure, etc. • data for winching operations, if applicable • lashing systems to be provided • possibility of an unserviceable helicopter stowed on the side of the deck while a relief helicopter is

required to land, if applicable • fuel used and type and capacity of refuelling equipment to be provided


Section 8 Helicopter Deck Structures


• starting equipment, if applicable

A.4 Documentation to be submitted

A.4.1 Plans showing the arrangement, scantlings and details of the helicopter deck are to be submit-ted. The arrangement plan is to show the overall size of the helicopter deck and the designated landing area. If the arrangement provides for the securing of a helicopter or helicopters to the deck, the predeter-mined position(s) selected to accommodate the secured helicopter, in addition to location of deck fittings for securing the helicopter is to be shown.

A.4.2 The helicopter for which the deck is designed is to be specified and calculations for the rele-vant loading conditions are to be submitted.

B Structure of the Helicopter Deck

B.1 General

B.1.1 The helicopter deck shall be dimensioned for the largest helicopter type expected to use the deck.

B.1.2 For scantling purposes, other loads (cargo, snow/ice, etc.) are to be considered simultane-ously or separately, depending on the conditions of operation to be expected. Where these loads are not known, the data contained in 2. may be used as a basis.

B.1.3 The following provisions shall in principle apply to landing areas on special, pillar-supported landing decks or on the upper deck, superstructure deck or deckhouse of a fixed or mobile installa-tion/unit.

B.2 Design loads

The following design load cases (LC) which are described in B.2.1 – B.2.4 are to be considered:

B.2.1 Load case LC 1

Helicopter lashed on deck of mobile units, where helicopter deck is used in floating condition, with the following vertical forces acting simultaneously:

B.2.1.1 Wheel and/or skid force P acting vertically at the points resulting from the lashing position and distribution of the wheels and/or supports according to helicopter construction, see Fig. 8.1.

P = 0.5 ⋅ G (1 + av) [kN]

Fig. 8.1 Skid/wheel loads of a helicopter

G : maximum permissible take-off weight [kN]

P : evenly distributed force over the contact area f = 30 ⋅ 30 cm for single wheel or according to data supplied by helicopter manufacturers; for dual wheels or skids to be determined indi-vidually in accordance with given dimensions

e : wheel or skid distance according to helicopter types to be expected

av : acceleration addition to be defined primarily by a motion analysis

av : acceleration addition for non self-propelled units where the towing speed v ≤ L0.5 [kn] to be estimated as follows:

av : 0.11 ⋅ m




m : 1.61 – 3.05 ⋅ x / L 0 ≤ x / L ≤ 0.2

: 1.0 0.2 < x / L ≤ 0.7

: 1 + 8.7 (x / L – 0.7) 0.7 < x / L ≤ 1.0

L : length of the unit [m] as defined in GL Rules for Mobile Offshore Units (IV-6-2), Section 1, B.4

B.2.1.2 Evenly distributed vertical load over the entire helicopter deck, taking into account snow, car-go, personnel, etc.

p = 2.0 kN/m2

B.2.1.3 Vertical force on supports of the deck due to weight of helicopter deck Me:

Me (1 + av) [kN]


Helicopter lashed on deck with the following vertical and horizontal forces acting simultaneously:

B.2.2.1 Wheel and/or skid force P acting vertically at the points resulting from the lashing position and distribution of the wheels and/or supports according to helicopter construction, see Fig. 8.1.

P = 0.5 ⋅ G [kN]

B.2.2.2 Vertical force on supports of the deck due to weight of helicopter deck:

Me [kN]

B.2.2.3 Evenly distributed vertical load over the entire helicopter deck, taking into account snow, car-go, personnel, etc.

p : 2.0 kN/m2 for fixed installations and units

: 0.0 kN/m2 for floating units

B.2.2.4 Horizontal forces on the lashing points of the helicopter:

H = ah ⋅ G + WHe [kN]

ah : acceleration on the helicopter in horizontal direction

: to be defined primarily by a motion analysis if no details are known, the following values may be used:

: 0 for fixed installations and units

: 0.6 for floating units

WHe : wind load on the helicopter at the lashing points

B.2.2.5 Horizontal force on supports of the deck due to weight and structure of helicopter deck:

H = ah ⋅ Me + WSt [kN]

WSt : wind load on the structure of the helicopter deck, compare B.2.3.4


Normal landing impact on fixed offshore installations and floating offshore units, with the following forces acting simultaneously:

B.2.3.1 Wheel and/or skid load P at two points simultaneously, at an arbitrary (most unfavourable) point of the helicopter deck (landing zone + safety zone), see Fig. 8.1.

P = 0.75 ⋅ G [kN]

P to be increased by 15 % if the helicopter deck is part of a deckhouse with accommodations below.




B.2.3.2 Evenly distributed load over the entire helicopter deck, taking into account snow or other envi-ronmental loads:

p = 0.5 kN/m2

B.2.3.3 Vertical force on supports of the deck due to deadweight of helicopter deck:

Me [kN]

B.2.3.4 Horizontal force on supports of the deck due to structure of helicopter deck:

H = WSt [kN]

WSt : wind load on the structure of the helicopter deck for the wind velocity admitted for helicopter operation vW'

vW' : wind velocity, to be taken according to local weather conditions

: 25 m/s if no other relevant data available


Emergency/crash landing impact on fixed and floating offshore installations/units, with the following vertical force:

B.2.4.1 Wheel and/or skid load P at two points simultaneously, at an arbitrary (most unfavourable) point of the helicopter deck (landing zone + safety zone), see Fig. 8.1.

P = 1.25 ⋅ G [kN]

B.2.4.2 Forces according to B.2.3.2 to B.2.3.4

B.3 Scantlings of structural members

B.3.1 Structural analysis

Structural analysis of the supporting structure shall be effected in accordance with Section 3 by direct calculations. Regarding construction and materials employed, see Section 4.

Proof of sufficient buckling strength is to be carried out in accordance with Section 3, H for structures subjected to compressive stresses.

B.3.2 Permissible stresses

Permissible stresses for stiffeners, girders and substructure:

σzul = 235 / (k ⋅ γ ) [N/mm2]

k : material factor

: 295 / (ReH + 60)

γ : safety factor according to Table 8.1

Table 8.1 Safety factors for different load cases of helicopter decks

γ Structural element

LC1 + LC2 B.3.2.1.1 LC3 B.3.2.1.2 LC4

Stiffeners (deck beams) 1.25 1.10 1.00

Main girders (deck girders) 1.45 1.45 1.10

Load bearing substructure (pillar system) 1.7 2.0 1.20




B.3.3 Plating

The thickness of the plating is not to be less than the greater of the two following values::

ktkPcnt +⋅⋅⋅= [mm]

or

kt 1.1 a p k t= ⋅ ⋅ ⋅ + [mm]

n : 1 in general

: 0.9 for crash landing of helicopters (LC 4)

P : total load in [kN] of one wheel or group of wheels with print area f on a plate panel F = a ⋅ b, see Fig. 8.2

: values for different load cases LC 1 - 4 according to B.2

a : width of smaller side of plate panel (in general beam spacing)

b : width of larger side of plate panel

F need not be taken greater than 2.5 a2

Fig. 8.2 Area of plate panel influenced by a wheel print

In case of narrowly spaced wheels these may be grouped together to one wheel print area.

tK : corrosion allowance depending on material, protection, service and maintenance conditions, if any [mm]

c : factor to be determined as follows: • for the aspect ratio b/a = 1 and for the range 0 < f / F ≤ 0.3:

c = 0.95 1.87 (f/F) (3.4 4.4 f / F)− ⋅ − ⋅

• for the aspect ratio b/a = 1 and for the range 0.3 < f / F ≤ 1.0:

c = 0.95 (1.20 – 0.40 ⋅ f / F) • for the aspect ratio b/a ≥ 2.5 and for the range 0 < f / F ≤ 0,3:

c = 0.95 2.0 (f/F) (5.2 7.2 f / F)− ⋅ − ⋅

• for the aspect ratio b/a ≥ 2.5 and for the range 0.3 < f / F ≤ 1.0:

c = 0.95 (1.20 – 0.517 ⋅ f / F)

For intermediate values of b/a the factor c is to be obtained by direct interpolation.

p : evenly distributed pressure load [kN/m2]

: values for different load cases LC 1 - 4 according to B.2




Section 9 Marine Operations A General ....................................................................................................................... 9-1 B Standards and Guidelines........................................................................................... 9-1 C Lifting Operations........................................................................................................ 9-2

A General

A.1 Marine operations, i.e. operations associated with moving or transporting an offshore structure or parts thereof during the construction, installation or abandonment process, may have decisive influ-ence on the overall design and on the dimensioning of scantlings, see also GL Rules for Certification and Surveys (IV-7-1). Marine operations concerning structural integrity are therefore normally included in the design review by GL.

A.2 Where such operations, i.e. loadout, transportation, lifting/launching, upending, levelling, piling, float-over/lifting, mating and lowering /embedding, impose important loads or critical conditions on the structure, they have to be taken account of in the design.

The review and survey by GL will cover, as far as applicable: • Location Survey and environmental conditions • Loadout Analyses • Transportation Analyses including motion response • Lifting/Launching/Upending Analyses including on-bottom stability • Lifting/float-over, Lowering and Touchdown Analyses including dynamic influences

Additional to above investigations Marine Warranty Survey (MWS) document review and on-site surveil-lance will cover as far as applicable:

• Condition Survey of vessels and equipment involved • Loadout Procedure including quay condition, mooring and retrieval system • Transportation Arrangement including seafastening, stability, towing • Lifting/Launching/Upending Procedure (rigging, mooring, clearances) • Installation Procedures including anchor handling, positioning, piling, grouting, mating

A.3 A complete Certification of the design and construction of an offshore installation will be based on surveys and attendance to all important transport and installation operations. The necessary extent of such attendance will be agreed upon in each individual case, and certified accordingly. However, the operations shall be conducted by the responsible personnel of the service company in charge, which is assumed to be competent and experienced for the respective tasks.

A.4 Construction phases of a (e.g. concrete) platform in a floating, moored condition are not un-derstood to be within the scope of this Section.

B Standards and Guidelines For procedures in connection with marine operations not specifically covered by these Rules GL Noble Denton Guidelines may be applied where appropriate and agreed upon:

• 0001/ND General Guidelines for Marine Projects • 0009/ND Self elevating platforms, guidelines for elevated operations


Section 9 Marine Operations


• 0013/ND Guidelines for Loadouts • 0015/ND Concrete offshore gravity structures, guidelines for approval of construction, towage and

installation • 0016/ND Seabed and sub-seabed data required for approvals of mobile offshore units (MOU) • 0021/ND Guidelines for the approval o towing vessels • 0027/ND Guidelines for marine lifting operations • 0028/ND Guidelines for the transportation and installation of steel jackets • 0030/ND Guidelines for marine transportations • 0031/ND Guidelines for float-over installations • 0032/ND Guidelines for moorings

Within the scope of Certification and Marine Warranty Survey the further the following standards are ac-cepted by GL:

• NORSOK STANDARD: Applicable Chapters regarding Marine Operations • ISO 19901-6 Marine Operations • GL Rules VI – Additional Rules and Guidelines, Part 11 – Other Operations, Chapter 1 – Guidelines

for Ocean Towage

C Lifting Operations

C.1 General

C.1.1 Design considerations

Overall dimensions, weight and centre of gravity of a module to be lifted, number of lifting points, ar-rangement and characteristics of lifting equipment, etc. should be considered at an early design stage in view of the means which will be available, and of the static and dynamic loads associated with the antici-pated procedure.

C.1.2 Calculations

C.1.2.1 Parameters

The analysis of the lifting and lowering procedures shall take into account, where adequate, the elastic properties of both, the structure itself and the lifting equipment, the dimensional tolerances of the lifting slings, dimensional and weight tolerances of the lifted object, and the dynamic conditions which depend on the environment and the type of lifting gear employed. The weight of the rigging/lifting equipment (e.g. spreaders) shall be accounted for where relevant.

C.1.2.2 Additional external influences

Additional external influences and loads such as hydrodynamic and hydrostatic forces acting on sub-merged structures, wind, tug forces and foreseeable impact forces shall be considered where they are deemed to be important.

C.2 Load assumptions

C.2.1 Weight contingency factors

For lifted objects a weight contingency factor fCONT [-] shall be considered as follows:

fCONT : 1.03 for weighed weights

: 1.10 for calculated weights

For rigging equipment a rigging weight contingency factor fRIG shall be considered as follows:

fRIG : 1.03




C.2.2 Dynamic amplification fDAF

C.2.2.1 A dynamic amplification factor fDAF [-] depending on the mass M [t] of the lifted object accord-ing to Table 9.1 has to be considered for any controlled lifting operation using one hook.

C.2.2.2 In any case the dynamic amplification factor can be evaluated in a detailed motion analysis.

C.2.2.3 For lifting and lowering procedures involving floating objects and/or two or more crane barges, a special investigation of the motion characteristics of the coupled system will usually be necessary, unless it can be shown that the maximum forces likely to occur are sufficiently smaller than the lifting ca-pacity provided.

Table 9.1 Dynamic amplification factor fDAF

Mass Unprotected areas offshore 1Sheltered areas

(inshore) 1 and onshore

M ≤ 1000 t DAFMf 1.15 0.15 1

1000⎛ ⎞= + ⋅ −⎜ ⎟⎝ ⎠

DAFMf 1.05 0.10 1

1000⎛ ⎞= + ⋅ −⎜ ⎟⎝ ⎠

M > 1000 t 1.15 1.05

1 it is assumed that operations are carried out under defined, controlled weather conditions

C.2.3 Lifting weight WL

The lifting weight is defined as follows:

WL = M ⋅ g ⋅ fCONT ⋅ fDAF [kN]

M : mass [t], for the lifted object

g : gravity acceleration (9,81 m/s2)

fCONT : weight contingency factor according to 2.1

fDAF : dynamic amplification factor according to 2.2

C.2.4 Rigging weight WR

The rigging weight is defined as follows:

WR = MR ⋅ g ⋅ fRIG ⋅ fDAF * [kN]

MR : mass of all rigging equipment (slings, shackles, etc.)

g : gravity acceleration (9,81 m/s2)

fRIG : rigging weight contingency factor acc. to 2.1

fDAF : dynamic amplification factor according to 2.2

* same fDAF as for WL to be applied

C.2.5 Hook load H

The hook load H may be determined as follows:

H = WL + WR [kN]

C.2.6 Inaccuracy of centre of gravity fCOG

Depending on the type of structure and possible accuracy of calculation (design stage), a tolerance factor fCOG [-] shall be chosen to take into account errors regarding the position of centre of gravity (COG).

Standard procedure should be the consideration of the worst position of the COG inside an envelope of 1 m × 1 m.




Possible tilting effects have to be considered.

Where this procedure can not be applied a minimum factor of

fCOG : 1.10

has to be considered.

C.2.7 Skew load factor fSKL

The skew load factor fSKL [-] is applied to account for dimensional tolerances of the slings and the influ-ence of elasticity of slings length or properties.

For indeterminate (4-sling) lifts a skew load factor of:

fSKL : 1.25

shall be applied to each diagonally opposite pair of lift points in turn.

For determinate (2- and 3-point) lifts the skew load factor is:

fSKL : 1.00

C.2.8 Lift point resolved lift weight PRLW

The lift point resolved lift weight PRLW [kN] is the vertical load on each lift point (padeye / trunnion) taking into account the lift weight WL and the geometric arrangement of the lifting points in relation to the COG only.

C.2.9 Resolved lift point load (sling load) PS

The sling load PS [kN] is the total load acting on the padeye in direction of the sling:

( )α⋅⋅

=sin

ffPP SKLCOGRLWS

α : angle between the sling and the horizontal plane.

C.2.10 Additional influences

Additional influences e.g. from horizontal forces, see C.1.2.2, shall be especially considered. At the points of load application (padeyes), a force of at least 5 % of PS is to be accounted for, acting perpendicular to the padeye plane.

C.2.11 Spreader frames

When using spreader frames, the sling forces may be calculated according to C.2.9 and C.2.10, α being the angle between sling and (horizontal) spreader plane.

C.3 Safety factors

C.3.1 Safety factors for lift points and lifted objects

C.3.1.1 Consequence factors fcons

The following consequence factors fcons [-] shall be applied to the lift points and supporting structure.

fcons : 1.35 for lift points, attachments and supporting members, including those on spreader-frames

fcons : 1.00 for other structural members




C.3.1.2 Global safety factor γg

The global safety factor γg is:

γg : 1.45 for axial and bending stresses

γg : 2.16 for shear stresses

γg : 1.25 for equivalent stresses

C.3.2 Safety factors for lifting equipment

For the safety factor of lifting equipment see C.5

C.4 Design of lifted object and lift points

C.4.1 General

The structure to be lifted, including the elements introducing the loads into the structure shall be designed according to the principles laid down in Section 3 and Section 4.

C.4.2 Shock loading

Elements which may be subjected to shock loading during the lifting and lowering procedure shall be especially considered and duly strengthened. Protection by guides and in special cases fendering may be necessary.

C.4.3 Arrangement of padeyes

Padeyes shall be so arranged that the sling direction lies in the plane of the padeye main plate. A bending force according to C.2.10 in the direction of the eye axis is to be considered. The padeye main plate shall be introduced into the structure, avoiding sling forces to be transmitted across girder or deck plates.

Where through thickness loading of plates cannot be avoided, the material used shall be documented to be free of laminations, with a recognized through-thickness designation.

Pinholes should be bored/reamed, and shall be designed to suit the shackle proposed. Adequate spacer plates shall be provided to centralise shackles.

C.4.4 Design of standard padeyes

Padeyes of usual configuration shall comply with the following indications, Fig. 9.1.

The following formulae are valid for a relation between the pin diameter of the proposed shackle (dp) and the diameter of the pinhole (d) of:

pd0.95

d≥

C.4.4.1 Section A-A:

Shear stress:

s eH

consS1 g

P 1000 Rf2 A

⋅τ = ≤

⋅ γ [MPa]

AS1 : shear area of Section A-A, related to force Ps ( for D / 2R ≥ 0.8, AS1 = A1) [mm2]

A1 : area of Section 1-1 [mm2]

ReH : minimum nominal upper yield strength [MPa]

γg : global safety factor for shear stresses according to C.3.1

: 2.16 fcons : consequence factor for lift points according to C.3.1

: 1.35




Fig. 9.1 Standard padeye

: base length [mm]

h : pin hole height above base [mm]

R : main plate radius [mm]

d : pin hole diameter [mm]

D : Cheek plate diameter [mm]

e1 : neutral axis of Section B-B related to pin hole edge [mm]

e2 : pin hole eccentricity related to the neutral axis y-y of Section C-C [mm]

t : main plate thickness [mm]

t1 : cheek plate thickness [mm]

a : throat thickness of cheek plate weld [mm]

β : sling angle against x-axis

Ps : resolved padeye load (according to C.2.9) [kN]

Fx : PS ⋅ 1000 ⋅ cos (β) [N]

Fy : minimal 0.05 ⋅ PS ⋅ 1000 (see C.2.10) [N]

Fz : PS ⋅ 1000 ⋅ sin (β) [N]




C.4.4.2 Section B-B:

Equivalent stress:

2 22 1 x

eq cons z2 2y S2

eH

g

d 2 e F1f F 32 A 15 Z 2 A

R[MPa]

⎛ ⎞ ⎛ ⎞+ ⋅σ = ⋅ ⋅ + + ⋅⎜ ⎟ ⎜ ⎟⎜ ⎟⋅ ⋅ ⋅⎝ ⎠⎝ ⎠

≤γ

A2 : area of Section B-B (similar A-A) [mm2]

AS2 : shear area of Section B-B, related to force Fx ( for D / 2R ≥ 0.8, AS2 = A2) [mm2]

Z2y : section modulus of Section B-B related to axis y-y [mm3]


γg : global safety factor for equivalent stresses according to C.3.1

: 1.25

fcons : consequence factor for lift points according to C.3.1

: 1.35

C.4.4.3 Section C-C:

Combined axial and bending stress:

y eHz z 2 xa / b cons

3 3y 3x g

F h RF F e F hf [MPa]A Z Z

⎛ ⎞⋅⋅ − ⋅σ = ⋅ ± ± ≤⎜ ⎟⎜ ⎟ γ⎝ ⎠

Location with highest stresses is relevant

A3 : area of Section C-C [mm2]

Z3x(y) : section modulus of Section C-C for location under consideration related to axis x-x or y-y, respectively [mm3]

γg : safety factor for axial and bending stresses according to C.3.1

: 1.45


: 1.35

Shear stress:

eHx(x) cons

S3(x) g

RFf [MPa]

Aτ = ⋅ ≤

γ

y eH(y) cons

S3(y) g

F Rf [MPa]

Aτ = ⋅ ≤

γ

A3 : area of Section C-C [mm²]

: shear area of Section C-C, related to force Fx or Fy [mm2], respectively (for rectangular cross section t ⋅ , without stiffening brackets, A3 = AS3(x) = AS3(y) = t ⋅ )






: 2.16


: 1.35

Equivalent stress:

( ) ( )( )22 eHeq a /b x y

g

R3 [MPa]σ = σ + ⋅ τ + τ ≤γ


γg : global safety factor for equivalent stresses according to C.3.1

: 1.25


: 1.35

C.4.4.4 Weld connection between cheek plate and main plate:

Shear stress:

s eH1cons

1 g

P 1000 Rtf [MPa]D a t 2 t

⋅τ ≅ ⋅ ⋅ ≤

⋅ π ⋅ + ⋅ γ



: 2.16


: 1.35

Minimum weld thickness:

1min

t ta 1,5 [mm]

3+

= ⋅

but not less than 3 mm

C.4.5 Arrangement of trunnions

The diameter of the trunnion has to be chosen with respect to the minimum allowable bending radius of the proposed lifting sling, see Fig. 9.2. In general the diameter of the trunnion shall not be less than:

Dt : 9 ⋅ Ds

Ds : nominal sling diameter [mm]




Fig. 9.2 Typical trunnion arrangement

Dt : trunnion diameter [mm]

b : clearance for sling between keeper plates [mm]

c : stub length of trunnion [mm]

β : sling angle against x-axis

δ : sling angle against y-axis

Ps : resolved trunnion load (according to C.2.9) [kN]

Where practical aspects do not allow keeping this minimum diameter, a reduction of the allowable sling load has to be considered.

Adequate clearance shall be included between trunnion keeper plates, to allow for ovalisation of the sling section under load. In general, the width available for the sling shall be not less than:

b = 1.25 · Ds + 25 [mm]

However, the practical aspects of the rigging and de-rigging operations may demand a greater clearance than this.

Through-thickness loading of the trunnions and their attachments to the structure shall be avoided if pos-sible. If such loading cannot be avoided, the material used shall be documented to be free of laminations, with a recognized through-thickness designation.

C.4.6 Design of trunnions

Trunnions are subject to a strength investigation based on the following loads.

Safety factors are to be chosen according to C.3.1

Fx : PS ⋅ 1000 ⋅ cos(β) [N]

Fy : PS ⋅ 1000 ⋅ cos(δ), but not less than 0.05 ⋅ 1000 ⋅ PS [N] (see C.2.10)




Fz : ( )2 2 2S x yP 1000 F F [N]⋅ − −

where z is the vertical direction:

Fz : PRLW ⋅ 1000 ⋅ fCOG ⋅ fSKL

PRLW : Lift point resolved lift weight (according to C.2.8) [kN]

C.4.6.1 Loadings on trunnion

The following loadings have to be considered for the design of the trunnion and the weld connection. The loads are acting simultaneously.

Bending moment:

2 2x z

bM F F c [Nmm]2

⎛ ⎞= + ⋅ −⎜ ⎟⎝ ⎠

Axial load:

FA : Fy [N]

Shear force:

FS : 2 2x zF F [N]+

For abbreviations see Fig. 9.2.

C.4.6.2 Weld connection of trunnion stub to shell

The trunnion stub shall generally be welded to the shell by means of a full penetration weld.

C.4.6.3 Shell and substructure

The shell and the substructure are also subject to strength investigation with the local loads as defined in C.4.6.1, especially when the shell is unstiffened.




Section 10 Containers A Scope ........................................................................................................................ 10-1 B Certification for Offshore Service Containers ........................................................... 10-2 C Structural Parts ......................................................................................................... 10-6 D Materials and Production .......................................................................................... 10-7 E Electrical Requirements ............................................................................................ 10-8 F Heating, Ventilation and Air Conditioning ............................................................... 10-10 G Public Address and Main Alarm Systems............................................................... 10-10 H Portable Offshore Unit ............................................................................................ 10-11 I Certification Procedure ........................................................................................... 10-12

A Scope

A.1 General These Rules shall cover the different types of offshore containers with respect to design, manufacturing, testing, certification, marking and periodic inspection.

Additional other requirements shall cover the container structure and any permanent equipment for han-dling, filling, emptying, refrigeration, heating and safety purposes.

Regarding handling and operation of offshore containers several items have to be observed for the in-tended use.

Following types shall be covered by this Certification: • Offshore service containers • Offshore portable units

A.2 Offshore service containers The Offshore Service container is designed and equipped on board of a fixed installation or floating unit and equipped for a special service task, e.g. workshop, stores, power plants, control units, etc. Location and installation are subject to the applying regulations.

A.3 Offshore portable units These portable units are not containers as defined in A.2. The specific purpose for these units must be considered when assessing the fitness for each design type and where necessary for each transport event.

Operational restrictions have to be issued in the Certificate.

A.4 Codes and Standards

Following codes and standards shall apply:

EN 12079 – Offshore containers and associated lifting sets:

Part 1: Offshore container – Design, manufacture and marking

Part 2:Lifting sets – Design, manufacture and marking

Part 3: Periodic inspection, examination and testing

IMO MSC/Circ. 860 Guideline on Certification of Offshore Containers

IMO FTP Code - International Code for Application of Fire Test Procedures


Section 10 Containers


IMO FSS Code - International Code for Fire Safety Systems

CSC Convention – International Convention for Safe Containers, 1972

GL Guidelines for the Construction, Repair and Testing of Freight Containers (VI-1-1)

Statutory Rules and regulations, if applicable

B Certification for Offshore Service Containers

B.1 General For the certification of offshore containers the following steps apply: • design assessment • prototype test, if applicable • production inspections • production testing • inspection of completed installation • function testing • issuance of Certificates • plating and marking

B.2 Design assessment The documentation package shall contain all relevant information for performing the design assessment to verify that the design complies with the requirements.

An individual design approval will be performed on “case-by-case”.

If series production of containers is intended, Type Approval of the design is to be carried out.

B.3 Required documentation Following documents are to be submitted for evaluation: • strength of structure, lifting set and other permanent equipment • material specifications • welding and other joining methods • system's evaluation for the Certification • arrangement of systems • suitability of components including cables, materials, etc. • Ex-Certification and type of materials and components, if required • fire protection • plating and marking • testing

B.3.1 Types of documentation

B.3.1.1 General information

General information is required for: • instructions for hook-up/installation on the offshore installation/unit • operating instructions where relevant for equipment of importance to safety




B.3.1.2 Structural matters

Following details are required: • general arrangement plan with dimensions, scantlings of strength members, pad eyes and other de-

sign with specified details of material to be used • particulars of joining methods, e.g. welding, bolting and/or other types of connections • details of corrosion protection and painting, e.g. type application, colour, dry film thickness, etc. • design calculation • any information about details in case of special purposes and/or special equipment • etc.

B.3.2 Electrical matters

Following electrical documents are required: • Arrangement drawings of the container showing the location of electrical equipment and cable

routes. • Explanation how equipment is fixed and protected to resist shocks at transportation. • One line diagrams for power distribution circuits. • Logic wiring diagrams for power distribution and controls. • Electrical material lists giving main data for components. The material lists shall have identification

keys enabling the components to be traced to the single line diagrams, wiring diagrams or the ar-rangement/assembly drawings.

• Manufacturers' data sheets for components of safety importance or for other components with influ-ence on the certification are to be added, where necessary to show their rating and identification and to give the precise data for the components needed for evaluation of their suitability.

• Cable lists showing main data for cables. Alternatively the same information and identification keys for cables may be given in the single line diagrams and wiring diagrams.

• Construction data and test references for cables including certificates for, if available. • Wiring diagrams for intrinsically safe circuits. • Listing of all Ex-certified components. The list shall include identification systems for each compo-

nent towards the drawings, and shall show type of Ex protection, Certificate no. and indicate, whether the Certificate contain special conditions to be taken into account.

• For intrinsically safe circuits the list shall show that all relevant data for compatibility between the barrier and the field components have been complied with i.e. max. voltages, current, capacitance, inductance, etc.

• Ex-Certificates arranged in an indexing system. • Drawings showing the hook-up of electrical, control and signal system, in particular the hook-up to

the installation/vessel control room. • When programmable controllers are used for safety functions, e.g. over pressure monitoring, fire de-

tection, etc., than these types shall be suitable for the environmental conditions on site, e.g. offshore installations/units. Documentation shall be required.

B.3.2.1 Public addresses and alarm systems

Following items are required: • relevant types of documentation listed under electrical installation • operational description of the systems

B.3.2.2 Fire protection

Following documents are required: • relevant types of documentation listed under electrical installation • operational description for fire protection systems




• general arrangement of the fire alarm system, together with type and location of detectors with de-tails of how the signals are transmitted

• description of wall constructions and materials used for passive fire protection • Certificates of non-combustibility and fire resistance of materials used • Certificates/information on doors, windows, bulkheads, ceilings and fire dampers used in the con-

struction • details of penetrations, location of fire dampers and general arrangement of the ventilation system • type and number of portable extinguishers with certificates • description of any fixed fire extinguishing system • GL Certificates or Certificates from other recognized bodies for important components in the fire

alarm and extinguishing system

B.3.2.3 HVAC

Following documents are required: • relevant types of documentation listed under electrical installation • arrangement drawings for the ventilation system • operational description of over-pressure ventilation system, if relevant • calculation of over-pressure ventilation system purge time • calculation of ventilation, heating and cooling capacity

B.3.2.4 Other systems

In case of other requirements additional documentation to be approved, e.g. gas detection system, etc.

B.4 Type approval Type approval has to be related to an on-going production. Testing may be performed with an agreed test programme and is a verification of the ability to meet specific requirements, e.g. type testing.

The renewal of Type Approval is due after 4 years.

B.5 Inspection and testing Surveys for inspection and testing are to be agreed between GL and the client as a part of the certifica-tion agreement.

The steel structure is to be built and tested under supervision of an Inspector according to GL. The re-quirements for manufacturing and survey of the structural parts, i.e. pad eyes, framework, panels (exclud-ing fire protection), etc., details see also GL Guidelines for the Construction, Repair and Testing of Freight Containers (VI-1-1).

The structure should be inspected and structural tests carried out before insulation and/or equipment is installed.

Manufacturing and surveys for structural parts are to be according to the agreed referenced standard for these structures.

Surveys of electrical systems should normally include: • Checking that all documentation represents the as-built status of the unit, and that all equipment is

sufficiently documented. Any deviations are to be dealt with as deemed appropriate by the attending Inspector.

• Final approval of electrical components including checking of marking and main data. Checking of Ex Certificates.

• Installation details such as earthing, cables and other items as deemed appropriate by the attending Inspector.

• Function tests of electrical-, alarm-, HVAC systems etc.




When all surveys have been completed and documentation is approved, the Offshore Service Container Certificate will be issued. Certification marking should be according to GL Guidelines for the Construction, Repair and Testing of Freight Containers (VI-1-1).Copies of reports and of the certificate should be sub-mitted to GL Head Office.

B.6 Design requirements

For containers with special features, additional design requirements may be applicable.

Container shall be designed as structural frames, with non-load bearing cladding where necessary. All connections between frame members and between pad eyes and frame members shall be designed to give good continuity.

Protruding parts on the outside of the container frame that may catch or damage other containers or structures shall be avoided. Doors, handles, hatch cleats, etc. shall be so placed or protected that they do not catch the lifting set. Pad eyes may protrude above the top level of the container frame.

The design temperature TD shall not be taken higher than the statistically lowest daily mean temperature.

The required strength of a container is found by calculations and verified by prototype tests. Minimum material thickness shall apply according to the calculations, but for primary structure t ≥ 4 mm, corner parts and bottom rails t ≥ 6 mm and secondary structure t = 2 mm.

In case of fire resistance, the FTP code shall apply, i.e. minimum thickness for “A 0” class standard is t = 4 mm.

B.7 Design details

B.7.1 Pad eyes

In order to prevent lateral bending moments on pad eyes they should be aligned with the sling to the cen-tre of lift, with a maximum manufacturing tolerance of ± 2.5°.

If the lifting force is transferred through the thickness of a plate, i.e. the z-direction, plates with specified through thickness properties must be used, see also Section 4, A.4.4.3.

B.7.2 Doors and hatches

Doors and hatches including hinges and locking devices shall be designed for at least the same horizon-tal forces as the primary structure.

If weather tightness is required the doors shall be equipped with gaskets.

B.7.3 Equipment

Equipment installed in offshore containers shall be designed and installed to withstand the dynamic load-ing and other environmental forces to which it may be exposed.

B.7.4 Coating and corrosion protection

Offshore containers shall be suitable for the offshore environment by means of construction, use of suit-able material and/or corrosion protection and coating.

All offshore container roofs, including those constructed from checker plate, shall be coated with a per-manent non-slip medium.

B.7.5 Both tank and bulk containers

Offshore containers for liquid or solid bulk cargoes are subject to international regulations or standards according to this subsection and may also be subject to other codes and requirements.

In addition to the design requirements the frame shall be designed to also protect the tank and the fittings. On tank containers for dangerous goods, all parts of the tank and fittings shall be suitably protected from impact damage.

International and/or national Rules shall apply in case of storage of hazardous and/or dangerous goods.




B.7.6 Prototype testing

For all offshore container types prototype tests are required and shall be considered as a design require-ment. The testing has to be witnessed by GL.

The prototype testing may be a partial substitute for strength calculations, but may not replace the design review.

A test programme to be approved by GL. The testing to be performed with verified equipment and cali-brated gauges.

Vertical impact test shall not be performed with containers permanently located on the installation/unit.

C Structural Parts

C.1 Requirements related to offshore transport and handling The general structure of offshore containers including their attached lifting set is part of primary and sec-ondary structures.

Note

Supports for tanks or heavy machinery are primary structures and internal walls are secondary structures. Fittings and sub-frames, e.g. for insulation panels are not part of a container structure.

Structural loads on containers occurring during use on offshore installations are not covered. Only re-quirements related to lifting operation to and from the supply vessel in open sea as well as handling by fork lift trucks.

In the particular specification for such a service container, it is advised, that the rating is chosen higher than the estimated fitted out load, e.g. to specify a certain pay-load even, if the container is not intended to carry cargo.

In case of permanent installed containers the offshore location has to be prepared including the approved substructures on the installation/unit.

C.2 Additional requirements If additional pad eyes or other lifting fittings for moving and/or other offshore handlings are required, this has to be mentioned in the Certificate. Generally these addition lifting fittings are not intended for use of loading and/or discharging of the whole unit.

C.3 Additional loads

Additional loads, statically or dynamically, to be subject in case of external environmental loads and/or internal loads, e.g. machinery or winches, etc.. The matters shall be considered on the basis of client's specification as well as national requirements in the container structure.

C.4 Structural components

C.4.1 Primary structure

Following parts are required: • main load carrying and supporting frame • load carrying panels, e.g.floor • fork lift pockets • pad eyes • substructure for tanks • substructure for heavy equipment • corner brackets/corner posts




C.4.2 Secondary structure

Following items are included: • doors, walls, roof panels • panel stiffeners and corrugations • structural elements for protection purposes • internal securing points

C.4.3 Type of lifting set

Following items are relevant: • single or multi leg system • master links, shackles, chains, etc.

D Materials and Production

D.1 Steel

D.1.1 General

Material selection to be mentioned in the material specification.. Chemical composition, mechanical prop-erties, heat treatment and weldability shall be suitable for the purpose. Steels shall comply with material requirements of the recognized standard. High strength steels, with specified yield strength above 500 MPa, shall not been used.

Steel forgings shall be carbon or carbon-manganese steels. Such forgings shall be made from killed and fine-grain treated non-ageing steel.

Alloy steel shall be delivered in quenched and tempered condition.

ISO-corner fittings made from cast steel shall fulfil the requirements of ISO 3755.

Bolt assembly considered as an essential for structural and operational safety shall conform to ISO 898 or other recognized standard.

Non-ferrous materials shall be suitable for the purpose.

D.1.2 Certificates

Materials for construction of offshore containers shall be according to following types:

type 3.2 (EN 10204) for ISO-corner fittings, pad eyes

type 3.1 (EN 10204) for primary structural members

type 2.2 (EN 10204) for secondary structural members

D.2 Welding

All main welds between the primary frame structure and pad eyes shall always be full penetration welds.

If additional supporting welds are required on pad eyes and/or supporting structures fillet welds may be acceptable after special consideration.

Essential and non-redundant primary structural members shall be welded with full penetration welds.

Fork lift pockets shall be connected to the bottom rails with full penetration welds. In case of other primary members fillet welds may be permitted after special agreement with the Certifying Body.

For secondary structural members fillet welds or intermittent fillet welds are acceptable. Corrosion protec-tion requirements have to be observed.

Welds between primary and secondary structural members are considered to be fillet welds.

Welding consumables shall be according to recognized standards.




Welding procedures shall be approved, depending on the type of material and a certified welding proce-dure qualification record (WPQR), for primary steel. Material test requirements are to be selected accord-ing to Section 4, A.4 for an exposure level L3.

Approved welders according to recognized standards are required.

D.3 Production

D.3.1 General

Production shall be performed according to approved drawings, specifications and procedures.

The manufacturer shall present a quality plan for acceptance before production starts. Relevant produc-tion documents shall be presented for acceptance before start of production, details see also (IV-7-4).

D.3.2 Inspection and NDE

The inspections shall be performed during manufacturing, witnessed testing and review of the production documentation, details see Section 4.

Non-destructive testing shall be carried out according to both approved specifications and qualified opera-tors, details see Section 4.

D.3.3 Marking and plating

Safety markings due to the type of container and manufacturing details are subject of the general mark-ing.

The plate has to be marked according to EN 12079-1

D.4 Lifting sets

The lifting set shall be specially designed for use on offshore containers.

Lifting sets shall be approved and certified by Certifying Body. Testing of the lifting set with its compo-nents shall be performed to recognized standard. Traceabel material certificates to be used, type of Cer-tificates shall be 3.1 according to EN 10204.

Periodic examination shall be performed at intervals not exceeding 12 months.

E Electrical Requirements

E.1 Safety requirements Following safety requirements shall apply: • installation shall be performed by qualified personnel • risk is to minimize for injury of personnel, damage of the environment in case of short circuits, earth

fault, fires, explosions and electrical shocks • protected devices to prevent overload, excessive heating and other damages • fail to safety principles due to missing electrical components

E.2 Components and equipment All items shall be constructed according to an IEC or other recognized standard. Components and cables to be rated based on IEC publications or other recognized standards.

Electrical equipment and systems are to be suitable for the environmental conditions as well as to with-stand of a certain vibration level range. Protection against corrosion is required.

Insulation materials are to be flame retardant.

If the location of electrical components are outside of the container, the ingress protection classification shall be at least IP56 and inside of the container at least IP20. For fire extinguishing systems are pro-




vided, electrical equipment shall be at least IP56 for the safety functions. The environmental requirements to be observed for an installation of electrical equipment, e.g. range of temperatures, humidity and sun radiation, etc.

The indoor spaces shall be acceptable for performing maintenance and repair works in a safe way.

E.3 Arrangements Following requirements of arrangements to be observed: • before final location outside equipment to be sheltered and/or protected against transportation dam-

ages • the heating shall be performed permanently for both onshore and offshore • portable equipment is to be securely fixed

E.4 Earthing system The container earthing arrangement shall be selected to suit the requirements of the installation/unit on which the container may be located.

All insulated electrical and control systems within the container are to have continuous insulation monitor-ing. Earth leakage circuit breakers are to be provided in the final sub circuits or at a suitable location up-stream.

E.5 Requirements for electrical equipment exposed to hazardous atmospheres. Following mandatory requirements shall apply: • Certification of a recognized institution for the equipment used in this area • type of relevant category for the intended area • temperature classification to be mentioned

E.6 Inspections of installation, repair and maintenance of Ex-equipment Both the methods of installation and condition of Certification shall be verified.

Following items shall be prepared: • check lists for parts used and installed • functional testing of single components under normal and under service conditions as well as in

case of emergency requirements • intrinsically safe circuits shall be installed • no repairs shall be performed on Ex-equipment

E.7 Earthing of structure Following earthing of the container structure required: • earth lug plate to be welded to the main steel structure on the outside of the container in the lower

part • AC systems above 30 V and DC systems above 50 V and all electrical power systems for containers

for hazardous areas • earthing system to be described • national requirements, if applicable, to be observed

E.8 Cables Cables shall be securely fastened and terminated in a durable manner. Special attention shall be paid to the gland arrangements and bending of cables near the entries and the bulkhead penetrations.




E.9 Electrical Systems Following systems shall apply: • lighting • gas detection arrangement, if required • emergency shut down • systems required in emergencies, e.g. fire protection circuits, emergency lighting, public address

and general alarm circuits • etc.

All these systems are part of the whole electrical equipment and outfitting system and shall comply with the relevant regulations according to IEC and/or other mandatory Rules, codes and standards.

F Heating, Ventilation and Air Conditioning

F.1 General General requirements for these systems on offshore containers are: • adequate ventilation and comfortable working temperature • ventilation may be used to keep the atmosphere free from explosive gases and vapours • internal over-pressure with respect to the external environment in case of a hazard source outside • calculations shall be performed and be based on a worst case flammable substance release sce-

nario • offshore service containers shall be designed to meet the requirements

F.2 Ventilation arrangement Mechanical ventilation or fan powered inlet ventilation should be applied for containers as working place. Natural ventilation may be applied for storage purposes. Requirements for heating and/or cooling are subject for special containers as a working space or equipment.

F.3 Over-pressure ventilation system In case of such a system all other systems, e.g. electrical equipment, fire protection system, emergency lighting, communication systems, etc. are subject to suitability. Also doors and other openings should be arranged for this purpose and an air lock can support practically the flow out.

G Public Address and Main Alarm Systems

G.1 General According to the Rules such systems are required. Safety alarms, e.g. fire alarm, gas alarm and muster alarms, have to cover all areas.

The final decision to what equipment is necessary will be up to the discretion of the persons responsible on the installation/unit, when the final location of the container is decided.

G.2 Fire protection The actual requirements for fire protection depend on both the fire risk inside the container and on the actual location on the installation/unit.

The fire protection philosophy for offshore containers is subject of 4 main points: • passive fire protection




• fire detection • active fire protection • means of escape

G.2.1 Passive fire protection The purpose of passive fire protection may be both preventing a fire from spreading from the inside of the container to the outside, and protecting equipment and personnel inside the container from flames or heat radiation from the outside.

Ratings of passive fire protection are defined in IMO Res. A.754 (18) or equivalent standards. Materials shall be tested to IMO FTP Code.

G.2.2 Fire detection and alarm system Location and installation as well as type shall be according to the actual fire risk. All fire alarm compo-nents shall be type approved by GL or other recognised Certifying Bodies.

For containers permanently located on the platform the systems shall be part of an integral fire alarm system.

If non-permanent location for offshore containers is required, the installation and all requirements have to be fulfilled. Under such circumstances the fire detection system has been activated, e.g. stop of ventila-tion, activating of the extinguishing system, etc.

G.2.3 Active fire protection Portable fire extinguishers have to be placed near each exit depending on the fire load and the type.

Fixed fire extinguishing system will be subject to evaluation in each separate case based on the fire risk of the equipment mounted inside the container. Release facilities should be located outside of the con-tainer at an easily accessible position and shall be of manual type. In case of emergency the relevant alarms and required shut-offs to be activated.

G.2.4 Escape routes Generally two separate escape routes from each compartment are required, where personnel normally are employed. On both sides these exits to be marked in both English and local language.

G.3 Application Generally for an installation/unit the statutory regulations shall apply. Results from risk analysis and evaluation of dimensional accidental events should be included in the consideration of the required pro-tection and equipment.

G.4 Noise and vibration Any noise and vibration sources situated in the container shall be addressed.

H Portable Offshore Unit

H.1 General The portable offshore unit is a package or unit intended for repeated or single offshore transportation and installation/lifting. Here we have equipment or any other kind of installation intended for a service function offshore.

H.2 Type of Structures

H.2.1 Primary structure The primary structure includes:




• all main structural components • pad eyes

Other elements are lashing points, fork lift pockets, load distribution floor, deck beams, panels supporting, substructures for tanks and/or for heavy equipment.

H.2.2 Secondary structure Following structural parts are non-load carrying: • doors, walls and roof panels • panel stiffeners and corrugations • structural components used for protection only

H.3 Required design documentation Following documents shall be submitted: • general arrangement, dimensions, any protruding parts • maximum gross weight and payload • design drawings of main structure including joints • design drawings of pad eyes • relevant design calculations with details of weight and centre of gravity (CoG) • design loads for main structures and design details • sling set details with maximum and minimum sling angles • material selection including material specification • joining methods • kind of operation, e.g. single event or multiple transport, handling procedures, etc. • corrosion protection, if required • list of applicable codes and standards

I Certification Procedure Following main steps of this procedure shall apply: • application • design verification including all details • production follow-up including manufacturing, inspection and testing • documentation




chapter 2 - structural design

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