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DNV-RP-C205 – what is new?Analysis of wave-in-deck loads
Konstruksjonsseminar, Petroleumstilsynet
Arne Nestegård, Det Norske Veritas27.08.2008
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Version Slide 202 September 2008
DNV Offshore Codes:3-level document hierarchy
n Offshore Service Specifications (OSS):
– Principles and procedures for DNV offshore verification, classification, qualification and asset operation services
n Offshore Standards (OS):- Technical provisions and acceptance criteria for general use by the
offshore industry as well as the technical basis for DNV offshore services.
n Recommended Practices (RP):- Proven technology and sound engineering practice as well as
guidance for the higher level Offshore Service Specifications and Offshore Standards.
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Version Slide 302 September 2008
Offshore Standard vs. Recommended Practice
n Offshore Standard (OS)- A DNV offshore standard is a document which presents the principles and
technical requirements for design of offshore structures. The standard is offered as DNV’s interpretation of engineering practice for general use by theoffshore industry for achieving safe structures.
n Recommended Practice (RP)- The recommended practice publications cover proven technology and
solutions, which have been found by DNV to represent good practice, and which represent one alternative for satisfying the requirements stipulated in the DNV offshore standards or other codes and standards cited by DNV.
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Version Slide 402 September 2008
Structure of OS’s and RP’s
u A: Quality and Safety Methodologyu B: Materials Technologyu C: Structuresu D: Systemsu E: Special Facilitiesu F: Pipelines and Risersu G: Asset Operationu H: Marine Operationu J: Wind Turbines
AA BB CC DD EE FF GG HH JJ
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Version Slide 502 September 2008
RP-C205 Environmental Conditions and Environmental Loads
n RP-C205 is an updated and enhanced version of DNV Classification Notes 30.5 Environmental conditions and Environmental loads.
n CN 30.5 provides key information on main issues related to environmental loads on ships and offshore structures.
- description on wave, wind and current conditions- methods for load prediction on various types of structures
n CN30.5 has been widely used in the industry for design of offshore structures
n The document has also been widely used by DNV in verification and advisory services and it serves as a basic reference for several other DNV rules, standards and recommended practices (RP).
Background:
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Version Slide 602 September 2008
Developed in Joint Industry Project 2005-06
n Establish a Recommended Practice for assessment of environmental conditions and environmental loads on marine structures
n Establish a common basis for DNV’s offshore standards with respect to assessment of load effects
Objectives:
Participants:
n Hydro, Statoil, BP, DNV (funding)
n Aker Kværner, Moss Maritime, PGS, PSA (observers)
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Version Slide 702 September 2008
Contents of DNV RP-C205
1. Introduction
2. Wind conditions
3. Wave conditions
4. Current and tide conditions
5. Wind loads
6. Wave and current induced loads on slender structures
7. Wave and current induced loads on large volume structures
8. Airgap, wave-in-deck loads and wave slamming
9. Vortex induced oscillations
10. Hydrodynamic model testing
Appendices: Scatter diagrams, added mass anddrag coefficients
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Version Slide 802 September 2008
Wind conditions
n Definition of wind parameters
n Wind data and wind speed statistics
n Wind modelling- Mean wind speed and standard deviation- Long term probability distributions- Wind speed profiles (logarithmic, power law, Frøya)
n Wind turbulence
n Wind spectra (offshore / over land) – limitations/recommendation for use
n Wind speed process and wind speed field (coherence spectra)
n Wind profiles and atmospheric stability
n Transient wind conditions (gusts & squalls)
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Version Slide 902 September 2008
Wave conditions
n Wave theories and wave kinematics
n Short term wave conditions
n Long term wave statisitics
n Extreme value predictions
2
)(2
)(2
1
2
ηη
η
ηη
∆∆
∆−
+
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Version Slide 1002 September 2008
Contents of DNV RP-C205
1. Introduction
2. Wind conditions
3. Wave conditions
4. Current and tide conditions
5. Wind loads
6. Wave and current induced loads on slender structures
7. Wave and current induced loads on large volume structures
8. Airgap, wave-in-deck loads and wave slamming
9. Vortex induced oscillations
10. Hydrodynamic model testing
Appendices: Scatter diagrams, added mass anddrag coefficients
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Version Slide 1102 September 2008
Wave and current induced loads on slender structures
n Morison’s equation- Combined current and waves- Fixed and moving structures- Normal and axial forces
n Governing parameters- Diffraction parameter D/λ- Reynolds number Re=DU/ν- Roughness ∆ = k/D- KC number KC=UMT/D- Current flow velocity ratio
wc
c
UUU+
=α
n Mass and drag coefficients – dependency on- Cross sectional shape- Parameters (KC, Re, ..)- Shielding/wake effects- Wall interaction effects and effect of free surface
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Version Slide 1202 September 2008
Wave loads on large volume structuresn Frequency domain analysis
n Time domain analysis
n Forward speed effects
n Numerical methods (panel methods)
n Hydrostatic and inertia loads
n Wave frequency loads- Random wave loads- Equivalent linearization- Panel mesh requirements- Irregular frequencies- Multi-body hydrodynamic interactions- Generalized body modes- Shallow water and restricted areas- Moonpool effects- Fluid sloshing in tanks
n Mean and slowly varying loads- Difference frequency QTFs- Mean drift force- Viscous effect on drift forces- Damping of low frequency motions- Viscous hull damping
n High frequency loads- Sum-frequency wave loads (springing)- Higher order wave loads (ringing)
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Version Slide 1302 September 2008
Contents of DNV RP-C205
1. Introduction
2. Wind conditions
3. Wave conditions
4. Current and tide conditions
5. Wind loads
6. Wave and current induced loads on slender structures
7. Wave and current induced loads on large volume structures
8. Airgap, wave-in-deck loads and wave slamming
9. Vortex induced oscillations
10. Hydrodynamic model testing
Appendices: Scatter diagrams, added mass anddrag coefficients
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Version Slide 1402 September 2008
Wave in deck - background
n ~1972 – designed according to API: Safety margin: 1.5m airgap for 100 yr wave
n ~1985 – subsidence detected
n ~1993 – Kaplan’s simplified wave-in-deckformulaes
n 2005 – Renewed attention to wave-in-deck loads. Lifetime extension of exisitingjackets.
n à Computational Fluid Dynamics for wave-in-deck calculations
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Version Slide 1502 September 2008
22o
N
Wave-in-deck load
Jacket wave load
SWL
Wave-in-deck and jacket loads
10000 y1000 y
100 y
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Version Slide 1602 September 2008
Present jacket load analysis methodologyLoads on jacket:
§ According to Norsok (API/ISO)
§ Stokes 5th order (Hmax, THmax)
§ VRF = 0.95 for North Sea conditions
§ Morison’s equation with CD = 0.65 (smooth), 1.05 (rough) (+ marine growth)
§ Loads from disturbed kinematics beneath the deck (jet effect)
Loads on deck:
§ Stokes 5th order (Hmax, THmax)
§ u(z) distribution shifted upwards (adjust water depth) so that Creststokes = Crestmax
§ No velocity reduction, VRF = 1.0.
§ Long-crested waves d(η,u)/dy = 0
§ CFD (VOF) wave-in-deck analysis with inflow Stokes wave
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Version Slide 1702 September 2008
0
20
40
60
80
100
120
0 50 100 150 200 250 300 350 400 450
5th order Stokes wave
Wave period, T
Wave height, H Crest
Water depth
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Version Slide 1802 September 2008
Computational Fluid Dynamics – ComFLOW
Inflow boundary,Stokes 5th wave
Deck structure
Fluid domain Courtesy of
Jørn Birknes, DNV
Benedicte Brodtkorb, DNV
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Version Slide 1902 September 2008
Modelling of deck geometry
NWP wave
NP wave
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Version Slide 2002 September 2008
Fluid domain – 3D view
Incomingwave
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Version Slide 2102 September 2008
Wave in deck – Fluid grid
Wave from NWP
Wave from NP
Detailed fluid grid close to structure,
~0.5 x 0.5 x 0.5m
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Version Slide 2202 September 2008
Global wave-in-deck loads (1)
Fz
Fx
-50-40
-30
-20-10
010
20
3040
50
6070
8090
100
110
2.0 3.0 4.0 5.0 6.0 7.0 8.0
Time (sec)
Dec
k f
orc
e [
MN
]
.
Fx-Deck Fz-Deck
Direction:
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Version Slide 2302 September 2008
max Fzdeck
max Fxdeck
min Fzdeck
Global wave-in-deck loads (2)
-50-40-30-20-10
010
20304050
60708090
100110
2.0 3.0 4.0 5.0 6.0 7.0 8.0
Time (sec)
Dec
k f
orc
e [
MN
]
.
Fx-Deck Fz-Deck
Direction: 225, PL NW 1000yr DNV ( H = 29.31m)
Fz
Fx
-50-40-30-20-10
010
20304050
60708090
100110
2.0 3.0 4.0 5.0 6.0 7.0 8.0
Time (sec)
Dec
k f
orc
e [
MN
]
.
Fx-Deck Fz-Deck
Direction: 225, PL NW 1000yr DNV ( H = 29.31m)
Fz
Fx
Fz
Fx
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Version Slide 2402 September 2008
Deck vs jacket loads
n Wetted deck area varies with time
n Time correlation with jacket load
-50-40-30-20-10
010
2030405060708090
100110
2 3 4 5 6 7 8
Time (sec)
Wav
e lo
ad (
MN
)
.
Fx-Deck Fz-DeckJacket Horisontal loadingJacket vertical loading
Direction:
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Version Slide 2502 September 2008
1) Simplified API method (solid decks)
u
2
21 vACF dh ρ=
==
waves)45( diagonalfor 9.1 wavesbroadside andon -endfor 5.2
ow
dCθ
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Version Slide 2602 September 2008
n Deck structures- Box-shaped, 30 m x 50 m- 6 other configurations
n 15 to 20 analyses- Varying wave inundation- Rp: 100 year to 10 000 year- Horizontal top of crest velocity:
7 m/s to 12 m/s- Fluid mesh:
Horizontal ~0.3 m to ~0.5 mVertical ~0.2 m to ~0.5 m
n Normalized horizontal force curves versus the API method
0 0.5 1 1.5 2 2.5 3 3.50
0.5
1
1.5
2
2.5
3
3.5
time [s]
Fx
/ 0
.5 ρ A
v x-m
ax2
[-]
Cd API = 2.5
Box-shaped
deck
Time (s)
Nor
mal
ized
forc
e (-)
2) Fh – head-on waves
deck girders
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Version Slide 2702 September 2008
2) Fh – head-on waves – selected deck
0
1
2
3
4
5
6
0.0 0.5 1.0 1.5 2.0 2.5time (s)
Fx /
0.5
rho
A v x
_max
2 [-]
.
4m 7.6m/s fx 101_11b_non_dim
2.3m 7.8m/s fx 2101_8p5m_1_non_dim
API Head-On
30 m by 50 m smooth deck
Multiple under-deck girders
Cd API = 2.5
Time (s)
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Version Slide 2802 September 2008
2) Fh – oblique waves
n Normalized horizontal force curves versus the API method, 45° oblique waves
0 0.5 1 1.5 2 2.5 3 3.5 40
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
time [s]
Fx
/ 0.5
ρ A
v x-m
ax2
[-]
Cd API = 1.9
Time (s)
Nor
mal
ized
forc
e (-)
Box-shaped
deck
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Version Slide 2902 September 2008
Deck structure, elevation view
Undisturbed surface elevation •
Wave propagation
vz bos
3) Simplified vertical force – DNV-RP-C205 (1)
2
21
boszvv vACF ρ=
= waves.oblique 45for 10
wavesbroadside andon -headfor 5ovC
Wetted deck area at the time of maximum impact force
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Version Slide 3002 September 2008
3) Simplified vertical force – DNV-RP-C205 (2)
n Definition of wetted length for maximum vertical impact force
Undisturbed surface elevation •
Deck – elevation view
Lp
vz bos
Wave crest
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Version Slide 3102 September 2008
3) Simplified vertical force – DNV-RP-C205 (3)
0° head-on wave
Wetted length = Lp
Dec
k br
eadt
h =
B
Lp
Deck structure, elevation view
vz bos
Top view
45° oblique wave
Wetted
leng
th = L
p
Projected deck breadth = Bp
Top view
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Version Slide 3202 September 2008
3) Fv – head-on waves
n Normalized vertical force curves versus DNV-RP-C205, 0° head-on waves
0 0.5 1 1.5 2 2.50
1
2
3
4
5
6
time [s]
Fz
/ 0
.5 ρ A
v z-bo
s2
[-]
CV DNV= 5
Box-shaped
deck
Time (s)
Nor
mal
ized
forc
e (-)
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Version Slide 3302 September 2008
3) Fv – head-on waves – selected deck
0
1
2
3
4
5
6
7
8
0.0 0.5 1.0 1.5 2.0 2.5time (s)
Fz /
0.5
rho
A v z
bos
2 [-]
.
4m 3.9m/s fz 101_11b_non_dim2.3m 3.4m/s fz 2101_8p5m_1_non_dimUnder-deck girders of
varying size
Cv DNV = 5
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Version Slide 3402 September 2008
3) Fv – oblique waves
n Normalized vertical force curves versus DNV-RP-C205, 45° oblique waves
0 0.5 1 1.5 2 2.5 30
2
4
6
8
10
12
time [s]
Fz
/ 0.5
ρ
A v z-
bos
2 [-]
Cv DNV = 10 Box-shaped
deck
Time (s)
Nor
mal
ized
forc
e (-)
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Version Slide 3502 September 2008
Increased jacket substructure loads due to disturbedwave kinematics
Free kinematics DECK
Disturbed kinematics
In Marintek’s Wave Impact JIP PIV measurements of fluid velocities will be performed.
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Version Slide 3602 September 2008
Wave kinematics models
Stokes 5th VRF = 1
VRF= 0.94
Stokes 5th VRF = 1Stokes 5th VRF = 1Stokes 5th VRF = 1
VRF= 0.94
Stokes 5th VRF = 1
VRF= 0.94
Stokes 5th VRF = 1Stokes 5th VRF = 1Stokes 5th VRF = 1
VRF= 0.94
Stokes 5th VRF = 1
VRF= 0.94
Stokes 5th VRF = 1Stokes 5th VRF = 1
VRF= 0.94
Stokes 5th VRF = 1
VRF= 0.94
Stokes 5th VRF = 1Stokes 5th VRF = 1
VRF= 0.94
100 y
10000 y
Horizontal velocity of design wave
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Version Slide 3702 September 2008
Comparison of CFD models
Comflow
By University of Groningen
Comet
by CD-Adapco
Wave-in-deck loads
on regular box with
and without girders.
Courtesy of Oleg Gaidai, DNV
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Version Slide 3802 September 2008
Wave in box without girders
Red line – COMFLOW, blue line – COMET
Horizontal load Vertical load
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Version Slide 3902 September 2008
Red line – COMFLOW, blue line – COMET
Wave in box with girders
Horizontal load Vertical load
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Version Slide 4002 September 2008
Contents of DNV RP-C205
1. Introduction
2. Wind conditions
3. Wave conditions
4. Current and tide conditions
5. Wind loads
6. Wave and current induced loads on slender structures
7. Wave and current induced loads on large volume structures
8. Airgap, wave-in-deck loads and wave slamming
9. Vortex induced oscillations
10. Hydrodynamic model testing
Appendices: Scatter diagrams, added mass anddrag coefficients
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Version Slide 4102 September 2008
Vortex induced oscillations
n Introduction to Vortex induced oscillations- Vortex shedding frequency, reduced
velocity, lock-in, damping, etc. - Cross Flow and In-Line response
n Implications of VIV
n Principles for prediction of VIV- Force models, response models, flow
models (CFD), model tests- Assumptions and limitations
n Vortex induced hull motions
n Wind induced vortex shedding
n Current induced vortex shedding
n Vortex induced oscillations in waves
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Version Slide 4202 September 2008
Hydrodynamic model testingn When is model testing recommended
- Hydrodynamic load characteristics- Global system concept and design verification- Individual structure component testing- Marine operations, demonstration of functionality- Validation of nonlinear numerical models- Extreme loads and response- Unknown or unexpected phenomena
n Test methods and procedures- Modelling and calibration of environment (waves, wind and
current)- Restrictions and simplifications in physical model- Calibration of physical model set-up- Measurement of physical parameters and phenomena- Nonlinear extreme loads and response- Data acquisition, analysis and interpretation- Flow measurements- Accuracy level; repeatability- Photo and video
n Scaling effects- Froude scaling- Reynolds number scaling- Choice of scale- Scaling of slamming load
measurements
Courtesy of Marintek
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Version Slide 4302 September 2008
Thank you for your attention!
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Version Slide 4402 September 2008