DNV-RP-C205 – what is new? Analysis of wave-in-deck loads · Version 02 September 2008 Slide 2...

DNV-RP-C205 – what is new?Analysis of wave-in-deck loads

Konstruksjonsseminar, Petroleumstilsynet

Arne Nestegård, Det Norske Veritas27.08.2008

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.


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.


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


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:


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)


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


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)


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

∆∆

∆−

+



1. Introduction

2. Wind conditions

3. Wave conditions


5. Wind loads








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


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)



1. Introduction

2. Wind conditions

3. Wave conditions


5. Wind loads








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


22o

N

Wave-in-deck load

Jacket wave load

SWL

Wave-in-deck and jacket loads

10000 y1000 y

100 y


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


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


Computational Fluid Dynamics – ComFLOW

Inflow boundary,Stokes 5th wave

Deck structure

Fluid domain Courtesy of

Jørn Birknes, DNV

Benedicte Brodtkorb, DNV


Modelling of deck geometry

NWP wave

NP wave


Fluid domain – 3D view

Incomingwave


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


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:


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


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:


1) Simplified API method (solid decks)

u

2

21 vACF dh ρ=

==

waves)45( diagonalfor 9.1 wavesbroadside andon -endfor 5.2

ow

dCθ


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


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)


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


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



n Definition of wetted length for maximum vertical impact force

Undisturbed surface elevation •

Deck – elevation view

Lp

vz bos

Wave crest



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


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 (-)


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


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 (-)


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.


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


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


Wave in box without girders

Red line – COMFLOW, blue line – COMET

Horizontal load Vertical load


Red line – COMFLOW, blue line – COMET

Wave in box with girders

Horizontal load Vertical load



1. Introduction

2. Wind conditions

3. Wave conditions


5. Wind loads








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


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


Thank you for your attention!

DNV-RP-C205 – what is new? Analysis of wave-in-deck loads · Version 02 September 2008 Slide 2...

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