DNV-RP-C205 – what is new?
Analysis of wave-in-deck loads
Konstruksjonsseminar, Petroleumstilsynet
Arne Nestegård, Det Norske Veritas
27.08.2008
DNV Offshore Codes:
3-level document hierarchy
n
Offshore Service Specifications (OSS):
–
n
Principles and procedures for DNV offshore verification,
classification, qualification and asset operation services
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.
Version
02 September 2008
Slide 2
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 the
offshore 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.
Version
02 September 2008
Slide 3
Structure of OS’s and RP’s
u
u
u
u
u
u
u
u
u
Version
A: Quality and Safety Methodology
B: Materials Technology
C: Structures
D: Systems
E: Special Facilities
F: Pipelines and Risers
G: Asset Operation
H: Marine Operation
J: Wind Turbines
02 September 2008
ABC
DE
FGH
J
Slide 4
RP-C205 Environmental Conditions and Environmental Loads
Background:
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).
Version
02 September 2008
Slide 5
Developed in Joint Industry Project 2005-06
Objectives:
n
Establish a Recommended Practice for
assessment of environmental conditions
and environmental loads on marine
structures
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Establish a common basis for DNV’s
offshore standards with respect to
assessment of load effects
Participants:
n
Hydro, Statoil, BP, DNV (funding)
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Aker Kværner, Moss Maritime, PGS, PSA (observers)
Version
02 September 2008
Slide 6
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 and
drag coefficients
Version
02 September 2008
Slide 7
Wind conditions
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Definition of wind parameters
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Wind data and wind speed statistics
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Wind modelling
- Mean wind speed and standard deviation
- Long term probability distributions
- Wind speed profiles (logarithmic, power law, Frøya)
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Wind turbulence
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Wind spectra (offshore / over land) – limitations/recommendation for use
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Wind speed process and wind speed field (coherence spectra)
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Wind profiles and atmospheric stability
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Transient wind conditions (gusts & squalls)
Version
02 September 2008
Slide 8
Wave conditions
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Wave theories and wave kinematics
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Short term wave conditions
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Long term wave statisitics
n
Extreme value predictions
Version
η2
η1
(+ )
∆η 2
∆η 2(− )
∆η 2
02 September 2008
Slide 9
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 and
drag coefficients
Version
02 September 2008
Slide 10
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
-
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Diffraction parameter D/λ
Reynolds number Re=DU/ν
Roughness ∆ = k/D
KC number KC=UMT/D
Current flow velocity ratio α =
Uc
Uc + Uw
Mass and drag coefficients – dependency on
-
Version
Cross sectional shape
Parameters (KC, Re, ..)
Shielding/wake effects
Wall interaction effects and effect of free surface
02 September 2008
Slide 11
Wave loads on large volume structures
n
Frequency domain analysis
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Time domain analysis
n
Forward speed effects
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Numerical methods (panel methods)
n
Hydrostatic and inertia loads
n
Wave frequency loads
-
Version
n Mean and slowly varying loads
Random wave loads
- Difference frequency QTFs
Equivalent linearization
- Mean drift force
Panel mesh requirements
- Viscous effect on drift forces
Irregular frequencies
- Damping of low frequency motions
Multi-body hydrodynamic interactions
- Viscous hull damping
Generalized body modes
Shallow water and restricted areas
n High frequency loads
Moonpool effects
- Sum-frequency wave loads (springing)
Fluid sloshing in tanks
- Higher order wave loads (ringing)
02 September 2008
Slide 12
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 and
drag coefficients
Version
02 September 2008
Slide 13
Wave in deck - background
n
~1972 – designed according to API:
Safety margin: 1.5m airgap for 100 yr
wave
n
~1985 – subsidence detected
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~1993 – Kaplan’s simplified wave-in-deck
formulaes
n
2005 – Renewed attention to wave-indeck loads. Lifetime extension of exisiting
jackets.
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à Computational Fluid Dynamics for
wave-in-deck calculations
Version
02 September 2008
Slide 14
Wave-in-deck and jacket loads
Wave-in-deck load
10000 y
1000 y
100 y
22o
N
SWL
Jacket wave load
Version
02 September 2008
Slide 15
Present jacket load analysis methodology
Loads 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 = Crest max
§ No velocity reduction, VRF = 1.0.
§ Long-crested waves d(η,u)/dy = 0
§ CFD (VOF) wave-in-deck analysis with inflow
Stokes wave
Version
02 September 2008
Slide 16
5th order Stokes wave
Wave period, T
120
100
Wave height, H
80
60
Crest
Water
depth
40
20
0
0
Version
50
100
150
200
250
02 September 2008
300
350
400
450
Slide 17
Computational Fluid Dynamics – ComFLOW
Deck structure
Inflow boundary,
Stokes 5th wave
Fluid
domain
Courtesy of
Jørn Birknes, DNV
Benedicte Brodtkorb, DNV
Version
02 September 2008
Slide 18
Modelling of deck geometry
NP wave
NWP wave
Version
02 September 2008
Slide 19
Fluid domain – 3D view
Incoming
wave
Version
02 September 2008
Slide 20
Wave in deck – Fluid grid
Wave from NP
Detailed fluid grid
close to structure,
~0.5 x 0.5 x 0.5m
Wave from NWP
Version
02 September 2008
Slide 21
Global wave-in-deck loads (1)
Direction:
110
100
Fx-Deck
90
Fz-Deck
Deck
force
[MN]
.
80
Fz
70
60
50
40
30
20
Fx
10
0
-102.0
3.0
4.0
5.0
6.0
7.0
8.0
-20
-30
-40
-50
Time (sec)
Version
02 September 2008
Slide 22
Global wave-in-deck loads (2)
max F z
deck
min F z
deck
max F x
deck
Direction:
110
225, PL NW 1000yr DNV ( H = 29.31m)
100
Fx-Deck
90
Fz-Deck
Deck
force
[MN]
.
80
Fz
70
60
50
40
30
20
Fx
10
0
-102.0
3.0
4.0
5.0
6.0
7.0
8.0
-20
-30
-40
-50
Time (sec)
Version
02 September 2008
Slide 23
Deck vs jacket loads
n
Wetted deck area varies with time
n
Time correlation with jacket load
Direction:
110
Fx-Deck
100
Fz-Deck
90
Jacket Horisontal loading
.
70
Wave load (MN)
80
60
Jacket vertical loading
50
40
30
20
10
0
-10 2
3
4
5
6
7
8
-20
-30
-40
-50
Version
02 September 2008
Time (sec)
Slide 24
1) Simplified API method (solid decks)
1
Fh = ρ Cd A v 2
2
u

Cd = 

Version
2.5
for end -on and broadside waves
1.9
for diagonal (θ w = 45 o ) waves
02 September 2008
Slide 25
2) Fh – head-on waves
n
Deck structures
- Box-shaped, 30 m x 50 m
- 6 other configurations
15 to 20 analyses
n
Normalized horizontal force
curves versus the API method
3.5
deck girders
3
2
x-max
- 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 m
Vertical
~0.2 m to ~0.5 m
Fx / 0.5
ρ A v force
[-]
Normalized
(-)
n
Cd API = 2.5
2.5
2
Boxshaped
deck
1.5
1
0.5
0
0
0.5
1
1.5
2
2.5
3
3.5
time [s]
Version
02 September 2008
Time (s)
Slide 26
2) Fh – head-on waves – selected deck
6
4m 7.6m/s fx 101_11b_non_dim
Multiple
under-deck
2.3m 7.8m/s fx 2101_8p5m_1_non_dim
API Head-On
girders
Fx / 0.5 rho A vx_max2 [-]
.
5
4
3
Cd API = 2.5
2
30 m by 50 m
smooth deck
1
0
0.0
0.5
1.0
1.5
2.0
2.5
time (s)
Time
(s)
Version
02 September 2008
Slide 27
2) Fh – oblique waves
n
Normalized horizontal force curves
versus the API method,
2
45° oblique waves
1.8
Cd API = 1.9
Boxshaped
deck
1.4
1.2
2
Fx / 0.5
[-]
ρA v
x-max
Normalized
force
(-)
1.6
1
0.8
0.6
0.4
0.2
0
0
0.5
1
1.5
2
2.5
3
3.5
4
time [s]
Time
(s)
Version
02 September 2008
Slide 28
3) Simplified vertical force – DNV-RP-C205 (1)
Deck structure,
elevation view
Wave propagation
vz bos
Undisturbed surface
elevation •
1
2
Fv = ρ Cv A v z bos
2
Wetted deck area at the time
of maximum impact force
 5
Cv = 
 10
Version
for head-on and broadside waves
for 45o oblique waves.
02 September 2008
Slide 29
3) Simplified vertical force – DNV-RP-C205 (2)
n
Definition of wetted length for maximum vertical impact force
Wave crest
Deck – elevation view
vz bos
Lp
Version
Undisturbed surface
elevation •
02 September 2008
Slide 30
3) Simplified vertical force – DNV-RP-C205 (3)
Deck breadth = B
0° head-on
wave
Wetted length
= Lp
Deck structure,
elevation view
vz bos
Pr
oj
ec
te
Top view
d
W
et
te
d
le
ng
th
=
L
p
Lp
Top view
de
ck
br
ea
dt
h
=
B
p
45° oblique
wave
Version
02 September 2008
Slide 31
3) Fv – head-on waves
n
Normalized vertical force curves
versus DNV-RP-C205,
0° head-on waves
6
CV DNV= 5
Boxshaped
deck
4
2
z-bos
Fz / 0.5ρ A force
v
[-]
Normalized
(-)
5
3
2
1
0
0
0.5
1
1.5
2
2.5
Time (s)
time [s]
Version
02 September 2008
Slide 32
3) Fv – head-on waves – selected deck
8
4m 3.9m/s fz 101_11b_non_dim
2.3m 3.4m/s fz 2101_8p5m_1_non_dim
Under-deck
girders of
varying size
Fz / 0.5 rho A vz bos2 [-]
.
7
6
Cv DNV = 5
5
4
3
2
1
0
0.0
0.5
1.0
1.5
2.0
2.5
time (s)
Version
02 September 2008
Slide 33
3) Fv – oblique waves
n
Normalized vertical force curves
versus DNV-RP-C205,
45° oblique waves 12
10
2
Fz / 0.5
ρA v
[-]
Normalized
force
(-)
z-bos
Cv DNV = 10
Boxshaped
deck
8
6
4
2
0
0
0.5
1
1.5
2
2.5
3
time [s]
Time
(s)
Version
02 September 2008
Slide 34
Increased jacket substructure loads due to disturbed
wave kinematics
Free kinematics
DECK
Disturbed kinematics
In Marintek’s Wave Impact JIP PIV measurements of fluid velocities
will be performed.
Version
02 September 2008
Slide 35
Wave kinematics models
Horizontal velocity of design wave
100 y
10000 y
Stokes 5th VRF = 1
VRF= 0.94
Stokes 5th VRF = 1
VRF= 0.94
Version
02 September 2008
Slide 36
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
Version
02 September 2008
Slide 37
Wave in box without girders
Horizontal load
Vertical load
Red line – COMFLOW, blue line – COMET
Version
02 September 2008
Slide 38
Wave in box with girders
Horizontal load
Vertical load
Red line – COMFLOW, blue line – COMET
Version
02 September 2008
Slide 39
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 and
drag coefficients
Version
02 September 2008
Slide 40
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
Version
02 September 2008
Slide 41
Hydrodynamic model testing
n
When is model testing recommended
-
n
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
Courtesy of Marintek
Test methods and procedures
- Modelling and calibration of environment (waves, wind and
current)
- Restrictions and simplifications in physical model
n
- 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
Version
02 September 2008
Scaling effects
-
Froude scaling
Reynolds number scaling
Choice of scale
Scaling of slamming load
measurements
Slide 42
Thank you for your attention!
Version
02 September 2008
Slide 43
Version
02 September 2008
Slide 44