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Revision of
Norsok Standard N-003
on
Actions and Action Effectswith a focus on
airgap, freeboard and wave actions
in relation to the deck structure
by
Torgeir Moan, NTNU
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Outline• Introduction (relating to airgap, freeboard, green water and wave in deck)
- Framework for regulatory requirements; i.e. NORSOK and other standards
- N-001 vs. N-003 and other standards & guidelines
- Hazardous scenarios to be considered
- Basic principles for standardization
- Consistent ULS requirements – relating to wave in deck
• N-003 standard relating to airgap, freeboard, green water and wave in deck
- Metocean data
- Wave theories
- Airgap criterion
- Wave in deck action
- Air gap and wave in deck analysis for transparent fixed structures
- Local wave impact
- Airgap and wave in deck analysis for floating structures
- Freeboard exceedance and green water relating to ships
- Green water analysis
- Model tests
- Concluding remarks
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Framework for regulatory requirements
NORSOK (from 1997)
N-001 Integrity of offshore structures;
1996 (Rev. 8, September 2012)
N-003 Actions and action effects; (Ed.
2, Sept. 2007, Ed.3 2016)
N-004 Design of steel structures; (Rev.
3, February 2013)
N-005 Condition monitoring of
loadbearing structures
N-006 Assessment of structural
integrity for existing offshore load-
bearing structures
NORSOK M-; R-002; S-001; Z-013
ISO
ISO 2394ISO 19900 General principles
(2013)
ISO 19901-1 Metocean
ISO 19902 Fixed steel
structures
ISO 19903 Fixed concrete
structures
ISO 19904 Floating structures
ISO 19905-1-3 MODU
ISO 19906 Arctic structures
Other standards & guidelines
Classification societies: DNVGL
Eurocodes
Apparent safety regimes:
Prod.platforms –
MODUs - Jack-ups
PSA Regulations (e.g. Framework -; Facilities - )
Introduction
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NORSOK StandardizationThe industry’s strategy with respect to
NORSOK standards
• In general use international standards; but in some
NORSOK standards are necessary
• Withdraw national and industry standards when equivalent
international standards are available
• Contribute to the development of international standards that
serve the needs of the industry (and the PSA) – based on
«home made standards»
• Devote efforts to develop and maintain NORSOK standards
Scope of N-003General principles and guidelines for determination of
characteristic actions and action effects for
design, (re-)assessment and verification of structures
• Standards
should specify
best practice
• If experiences
are limited,
the formulation
will be general
Until about 1997
Regulations, Design standard
and guidelines were managed
by NPD (PSA)
Introduction
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N-003 Committee
Members
• Gerhard Ersdal, Ptil
• Ove Tobias Gudmestad, UiS
• Sverre Haver, Statoil (til 01.05.2014), UIS/NTNU
• Torgeir Moan, NTNU
• Arne Nestegård, DNVGL
• Finn Gunnar Nielsen, Statoil (til 01.07.2016), UiB
• Tone M.G. Vestbøstad, Statoil
• Ole David Økland, Marintek
Supporting members
• Knut Arnesen, DNVGL (seismic actions and - effects)
• Kenneth Eik Johannesen, Statoil, Mosleth DNVGL (ice, icing, snow)
• Jon Kristian Haugland, Øyvind Fjukmoen, DNVGL (marine growth)
• Einar Nygaard, Statoil (metocean data)
Introduction
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Background for the revision of N-003
• Existing version of 02.09.2007
• Ongoing revision – focus areas:
- cold climate (Barents) activities (sea ice, icebergs, level ice; icing; snow)
- accidental actions - ship collision risk
- metocean data - e.g. introduction of important issues in N-002,
hindcasting data basis; e.g. NORA-10 and
- climate change
- design waves, sea state (contour method), longterm analysis
- requirements to numerical, experimental and in-service obs.
- wave load effects on jackets etc ( Kinematics; Drag and inertia coefficients)
- airgap (water level, wave kinematics (crest)…)
- wave impacts, green water; ringing/springing/whipping
- CFD methods in wave load calculation
- marine growth
- action combinations (ULS, ALS – also for damaged condition)
- editorial – direct it to users involved in fixed versus floating facilities
reorganized to separate the treatment of action effects on fixed
and floating platforms
Introduction
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Relevant sections for airgap; freeboard and
green water and wave-in-deck
6.1 General
- data, climate
- Sect. 6.1.3 Determination of char.
actions
6.2 Modelling of metocean conditions
-including wave kinematics
6.3 Hydrodynamic actions
- nonlinear actions (use of CFD)
- slamming, breaking wave impacts, run-
up11.5 Metocean AE applicable to all types of
structures- wave slamming and impact by breakingwaves
- air gap and wave in deck analysis- Green water
Commentary
Introduction
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Brief historical notes- on experiences in the GoM and North Sea
• GoM Camille (1969) – the strongest hurricane since 1935
in the GoM; Andrew (1992); Lilli (1996); Katrina (2005) –
the next largest
• New metocean guideline API (2007) 1000 years
airgap/robustness check
• NPD (1977) 1.5 m (but with no clear spec. of ref. crest)
- sometimes a larger airgap was specified
• NPD (1984) PLS (ALS) req. and «the 10-4 probability»
• In the mid-1980s the Ekofisk field was found to be suffering
from an unexpected degree of subsidence
Introduction
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Relevant regulatory requirements• The PSA Facilities Regulations state that accidental loads/actions and
environmental loads/actions corresponding to 10 000 year return-period
shall not result in loss of a main safety function. Furthermore, in
Guidelines regarding the facilities regulation it is referred to the
NORSOK-standards N-001, N-003 and N-004.
• According to NORSOK N-001 a facility shall be checked for different
safety limit states (ULS, ALS, FLS)
• According to NORSOK N-001 Section 6.4.1 Facilities with insufficient
deck clearance shall be designed for actions caused by waves and
current and impact actions should be verified by properly defined model
tests.
• Design criteria depend on whether the facility is
- manned or not,
- operating or shut-down
Introduction
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N-001 vs. N-003
• N-001 (and N-006 for existing structures shall specifyDesign Criteria in view of acceptable safety level- characteristic values ( second step in ALS)
- (possible) explicit req. to air gap and green water versus
direct design check
- safety factors or intentionally conservative methods
(e.g. reflecting increased uncertainty in
hydrodynamic actions on platform decks; use of
nonlinear load effect analysis of complex structures
(vs. Limit states)
N-003 vs. Other standards and guidelinesrelating to airgap, freeboard, green water and wave in deck- ISO 19902 (for jackets)
- DNV(GL) RP-C205
- DNVGL OTG-13
- DNVGL OTG-14
The challenge is the balance between
safety and costs - cfr. ALARP
Introduction
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Hazardous scanarios to be consideredrelating to airgap, freeboard, green water and wave in deck
• Overall failure modes:
- global structural failure
(possible imposed forced
deformations on risers and conductors
- excessive heeling or capsizing of floaters
• Water impact on the topside
• Water on deck (even lower deck for topsides
with truss girders and light cladding cover
- implication on strength and stability
(of floaters) - in case of progressive flooding
• Water impact on life boats and other safety
critical equipment
• Direct effect on personnel
SEMI Thunder Horse
Introduction
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Basic principles for standardization
- Standards should represent best practice
(air gap, wave in deck; green water are complex problems
and with limited support from field experiences and
research)
- Requirements should be such that compliance can be
demonstrated
- The standard should be consistent; i.e. with respect to
safety level.
- Various methods for prediction of actions are envisaged.
Moreover, they are described by a «procedure» with
opportunities of interpretation – as opposed to strength
formulations expressed by formulae
Introduction
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Consistent ULS requirementsULS requirements to the structure are ensured by
- the definition of characteristic action effects and resistances
and load and resistance factors
- different methods are applied to determine actions and action
effects, with different uncertainty; but the action factor is
normally not differentiated.
Conclusion: Then adequate safety needs to be achieved by
using a properly conservative approach
c R S cR /γ γ S S S C S
R R C R R
B S ,B 1;
B R ,B 1; V 0.1
The design format:
Reliability basis (design value format):
hence expd S S S S cS V S
22
SR
S
S
VV
V
exp /S S S S cV S
Action model uncertainties estimated by comparison with model tests used
in «Ekofisk subsidence» reliability assessment (OTC13187, 2001)
CoV: 0.18 (wave height); 0.25 (jacket hydrodynamic load)
0.35 (Kaplan’s method for wave action);
Introduction
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Metocean data
• Regarding wind and waves high quality and verified
hindcast data base (e.g. NORA10).
• For permanent facilities with a planned service life of more
than 50 years, climate change shall be accounted for.
Commentary
- In lack of more detailed documentation the following increase in
metocean values 50 years ahead shall be used:
- extreme significant wave height & wind speed: 4% increase.
- sea level: 0.25 m
• Measurement of current and joint wave, wind and current
conditions
Note: The revised N-003
is not finalized
N-003 standard
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Characteristic action (effects)
N-003 standard
«Modified
contour
approach»?
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16 Background: Wave theories
0 0.2 0.4 0.6 0.8 1 1.2 1.4-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
z (
m)
Horizontal Velocity (m/s)
- second order methods :
Johannessen (blue line);
Stansberg (green line)
Wheeler
stretching
Wave particle kinematics beneath two large wave
events recorded in a 10-4 sea state;
(a) spilling and (b) over-turning (Swan, 2016)
Comparison of calculated
crest kinematics (CresT JIP, Marin)
Second order
formulation
by Sharma and
Dean, 1981)
BEM –a fully
nonlinear
solution
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Wave theories
• Irregular waves
- problems insensitive to crest elevation and kinematics
in the crest: linear wave theory; Gaussian waves
(FD analysis)
- problems sensitive to crest elevation and kinematics
in the crest: second order wave elevation process &
consistent kinematics (see e.g. DNV RP C205)
(linear theory with Wheeler stretching in exceptional cases)
- If short crested sea is introduced in connection with estimating extremes,
the exponent, n, in the wave directional function 𝑐𝑜𝑠𝑛θ should not be
taken to be lower than 10 without a more detailed documentation.
- Special consideration should be made for crest kinematics
in sea states with near breaking waves or breaking waves
N-003 standard
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Airgap criterion (commentary)
• Basic principle: avoid wave in deck or design for wave in
deck
• Relevant to consider 10-2 and 10-4 events
since such characteristic values are used
in ULS and ALS design checks (for facilities unmanned in
storms NORSOK N-001 specify ALS check with annual
probability of exceedance of 10-3 )
• Due to the complexity and uncertainty associated with
determining wave-in-deck actions; designing for a
positive airgap above a wave crest with an annual
probability of exceedance of 10-4 is recommended
(implying 30% increase in 10-2 crest).
N-003 standard
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Wave-in-deck actions
• Inertia and drag, slamming, and buoyancy actions
• Consider water entry and exit; horizontal and vertical actions
• Express the time variation of the transient action
• Methods
- semi-empirical ( Kaplan (1992, 1995), API, DHI, DNV,
in-house methods in various oil companies
- component models: wave loading on each deck component is
determined separately. Validate by CFD and/or experiments
-silhouette method; Horizontal drag force
where where ρ is sea water density 𝑢𝑤 is the water particle
velocity, A = sd · b is the exposed area, 𝑠𝑑 is the inundation
(height) and b the width of the inundated area. Validation!
- CFD
- laboratory experiments
Unless the air gap is sufficient to avoid wave in deck impact,
relevant deck impact analysis shall be performed.
N-003 standard
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Wave in deck actions
Methods
- Simplified method by Kaplan et al. 1995
- CFD methods
- laboratory tests
The approach applied should be validated
by high quality model tests with
due consideration of
- the large inherent stochastic variability and
- uncertainty in predicting deck impact actions.
N-003 standard
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21 Crest height distribution (Swan et al., OSRC 2016)
Crest height distributions recorded in a laboratory, Tp=16.0s
Crest height
observations
in the field (Hs≥12m)
Lab.
Hs=12.5m
Lab.
Hs=15.0m
Lab.
Hs=17.5m
breaking
(by both spilling
and over-turning)
Norsok N-003: Forristall crest height distribution. in agreement with a
second-order surface process. (see e.g. DNV RP C205)
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22 N-003 standard
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Wave in deck actions• A wave event can either be an extreme crest event in an irregular design
seastate or a regular design wave. A regular design wave approach must
be shown to be conservative.
•The wave event approach should include the following:
- Determine design crest height Cq (corresponding to annual probability q)
from Forristall (long) crest height distribution and long term statistics.
- Increase Cq by a factor 1.1.
- Adjust height of top of crest to include storm surge and tidal level
- Construct a regular wave with the relevant crest
• Determine the flow and actions in the design event by using a CFD with
- due consideration of spatial and temporal convergence of the model.
- due account of possible effect of compressed air
- due consideration of the geometric modelling of the deck
- the effect of possible breaking waves shall be assessed.
- model the action as time variant (due to possible dynamic action effects)
N-003 standard
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Wave in deck loads for fixed structures- The enhanced action effect on the substructures below deck due to
disturbed kinematics from wave impact on deck shall be considered.
- For large volume fixed structures wave diffraction effects shall be
included in the airgap analysis. For such structures higher order
diffraction e.g. due to caisson effects may be important.
- High quality model tests are recommended to validate the airgap
analysis
Uncertainties - airgap prediction
- wave action in decksea-states; real irregular vs. regular waves; hydrodynamic models
(semi- emprical vs. CFD vs. experimental methods)
N-003 standard
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Local wave impactLocal wave impact above maximum crest due to effects not accounted
for in the above requirement to positive airgap, may be permitted to
occur on any part of a deck structure provided
- they can be designed for
- the water does not threaten personnel's life, or damage pipes and
other equipment which may lead to environmental damage in ULS
and ALS . Commentary: - guideance on pressure levels /
pressure area is given
- For a fixed platform the affected
area can reach a maximum
height of 1.4 times the ULS
wave crest height for the relevant sector.
- the deck structure adjacent to platform columns shall be designed to
resist the possible pressure actions due to run-up along columns.
N-003 standard
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Airgap and wave in deck analysis for
floating structures- The platform motions contribute relative velocity and acceleration
and the deck (impact) contact with the waves influence to some
extent the motions
- The deck height varies in time and space
- reserve buoyancy for stability wold normally imply that the deck
needs a certain strength due to the hydrostatic and - dynamic
pressure- In wave in deck analysis of large volume
floating structures, higher order wave
diffraction effects may be accounted for by
- using a factor 1.2 on the first order
wave response based on Gaussian sea
states, or
- a higher order wave theory which is
documented to yield reliable predictions.
The analysis of wave/structure interaction
effects should be made. The analysis results
should be validated by model testing.
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Freeboard exceedance and green water
relating to ships
- Green water occurs when the wave elevation exceeds the
ship freeboard
- Areas occupied by personnel, or where safety-related
equipment is located, shall not be exposed to waves with an
annual probability greater than 10-2.
- The freeboard exceedance and actions shall be assessed by
validated calculations or model tests.
- Green water typically induces pressure actions and local
slamming actions on the exposed structures.
- The influence of green water on the stability or global motions
of a floating facility shall be evaluated.
N-003 standard
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Green water analysis
- For ships, relative wave elevation exceeding the freeboard
can be determined using potential theory by computing the
relative motion
- The flow of water on deck and resulting action effects on
deck structures should be determined by using advanced
nonlinear methods and/or model tests.
- The effect of wave slamming, run-up or green water actions
should be appropriately combined with the other wave action
effects.
N-003 standard
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Model testsHydrodynamic model tests should be carried out to:
• confirm that no important hydrodynamic action has been overlooked (for
new types of facilities, metocean conditions, adjacent structure),
• support theoretical calculations (due their large uncertainties),
• verify theoretical methods on a general basis.
• direct determination of action/action effects for complex problems where
numerical methods are insufficient or not available.
Implement experimental test results by carefully considering
• scaling effects,
• model simplifications (e.g. related to damping),
• limitations in testing facilities (e.g. finite dimensions; quality of waves, .)
• uncertainties regarding the data recording and processing,
• uncertainties with regard to long-term variability,
• statistical uncertainties with respect to limited samples (suggesting
e.g. that extreme values should be determined by extrapolation
• The model test shall be executed and documented in such a manner
that they are repeatable.
• Interpretation of experimental results should be supported by numerical
analyses both in model and full scale.
N-003 standard
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Concluding remarks
• Experience shows that airgap, freeboard, wave in
deck or water on deck are important considerations
in design
• These problems, are however, complex - especially
to deal with by theoretical analysis and limited
experiences are available. Current design standards
are in their first stage.
• Unless a proven conservative approach can be used,
experimental evidence for the relevant problem is
crucial.