CFD for Floating Systems

1. CFD for Floating Systems
Bob Gordon
Granherne Americas, Inc.

2. 2
Outline
Overview of CFD
Present Offshore Industry Use of CFD
Applications

3. 3
Overview of CFD
 What is CFD?
 Brief History
 Overview of CFD Methods
 Validation & Verification

4. 4
Fluid Dynamics
 Theoretical
• Analytical Solutions (Heyday in 19th
& early 20th
Century)
• Potential Flow
• Many analytical solutions, including nonlinear equations (Airy, Stokes, Kelvin,
Lamb, Korteweg and de Vries, Stoker, …)
• Viscous Flow
• Very few analytical solutions (Stokes, Poiseuille, Blasius, Ekman, …)
• Theory of Turbulence (Reynolds, 1889 ->)
 Experimental
• Many advances in laboratory and field instrumentation continue to
appear (e.g., Particle Image Velocimetry, Acoustic Doppler Current
Meters)
 Computational
• Many advances continue in physical models, algorithms, software
(parallelization) and computing hardware
• Advances in CFD depend on good experimental data for verification

5. 5
Why CFD?
 Real world flows are too complex to be
addressed solely by theory or experimentation
• Nonlinear
• Complicated Geometry
• Coupled (Heat & Mass Transfer, Chemical Reaction,
Fluid-Structure Interaction)
• Turbulent

6. 6
Some Historical Milestones
 1922 - L. F. Richardson developed first
numerical weather prediction system
using finite differences calculated by
hand (Humans ~10-9
GFlop)
 1946 - J. von Neumann develops
program for ENIAC to calculate
hydrogen bomb explosion (ENIAC ~10-6
GFlop)
 1965 - Harlow & Welch develop the
MAC method at LANL; first successful
technique for incompressible flows
(CDC 6600 ~10-3
GFlop)
 1981 - Spalding (ICL & CHAM)
develops the first commercial CFD code
- PHOENICS (CRAY X-MP ~100
GFlop)
 2002 - NASA Pegasus5 CFD code is
used by Boeing to design the Sonic
Cruiser aircraft with much reduced
reliance on wind tunnel tests (IBM
BlueGene ~105
GFlop)

7. 7
Components of a Numerical
Solution Method
 Mathematical Model
• Incompressible vs. Compressible, Laminar vs. Turbulent, 2D vs. 3D, etc
 Discretization Method
• Finite Difference, Finite Volume, Finite Element
 Coordinate System
• Cartesian, Orthogonal and Non-orthogonal Curvilinear, etc
 Numerical Grid
• Structured, Block-structured, Unstructured
 Finite Approximations
• Accuracy vs. speed
 Solution Method
• Time stepping for transient; Iteration schemes for steady state
 Convergence Criteria

8. 8
Validation & Verification
 As with all Engineering Analysis codes, it is essential
that the model (i.e., code, conceptual modeling
assumptions, and input data) be verified and the
predicted results be validated
 Validation ~ Solving the right equations
• Compare against measured data
• Compare against benchmark analytical and/or numerical
solutions
 Verification ~ Solving the equations right
• Check convergence with mesh and time step refinement
• Make sure that numerical errors are sufficiently small

9. 9
Offshore Industry Use of CFD
Oil Companies
Chevron 9
Shell 5
Petrobras 4
BP 2
ExxonMobil 2
Service Co. & Consultants
Technip 8
Marintek 5
Marintek 3
Principia 2
SBM 1
BPP 1
Force 1

10. 10
Enabling Technology
 Physical Models
• Turbulence Models (DNS, LES, RANS)
• Heat & Mass Transfer, Multi-Phase Flows, Combustion
 Algorithms
• Finite Element & Volume Methods
• Grids
• Moving Grids
• Arbitrary Lagrangian-Eulerian Methods (ALE)
• Level Set Methods
• Sliding Grids
• Chimera Grids
 Software
• Parallelization
 Hardware
• Low Cost, High Performance Parallel Computing Architectures
• Clusters
• Grids

11. 11
Some Offshore Problem Areas
of Interest for CFD
 Fluid-Structure Interaction
• Vortex-induced vibrations of risers
• Vortex-induced motions of floating platforms
 Flow Around Vessel Hulls and Superstructure
• Wind and current forces
 Slam and water impact loading
 Sloshing in Tanks

12. 12
Riser VIV
QuickTimeª and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
SOURCE: C.H.K. Williamson, Cornell U.

13. 13
DeepStar/MIT
Lake Seneca
Tests 2004
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YUV420 codec decompressor
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SOURCE: K. Vandiver, MIT

14. 14
Classic VIV Catastrophe
If ignored, these vibrations can prove catastrophic to
structures, as they did in the case of the Tacoma
Narrows Bridge in 1940.
SOURCE: A. H. Techet, MIT

15. 15
VIV in the Ocean
 Non-uniform currents effect
the spanwise vortex shedding
on a cable or riser.
 The frequency of shedding
can be different along length.
 This leads to “cells” of vortex
shedding with some length, lc.
SOURCE: A. H. Techet, MIT

16. 16
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SOURCE: BP

17. 17
VIV Suppression
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SOURCE: BP, GlobalSantaFe, Shell

18. 18
Platform Vortex-Induced Motions
 Same phenomenon as Riser
VIV
 Vortex-induced motion
amplitudes (A) for a Spar can
up to 1.5 times the Platform
Diameter (D), if no VIV
suppression is used
 Motion is typically in a Figure
8 pattern
 Magnitude of A/D is velocity
dependent
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SOURCE: A. H. Techet, MIT

19. 19
Wave Slamming
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 Basic Physics
• Drag forces: caused by
viscosity resulting in flow
separation
• Inertia forces: related to the
acceleration of the incident
flow and the modification of
the incident wave pattern by
the member.
• Slam forces: occur when a
wave engulfs a member
causing a volume of water to
be decelerated (conservation
of fluid momentum)
 Progress has bee made in
predicting loads using CFD
SOURCE: MARINTEK

20. 20
Surface Blow-Out Preventer
(SBOP)
 Uses high pressure
casing riser
 Allows wells to be drilled
quickly
 Has been used in areas
with relatively calm
weather
 Industry is looking to
extend to harsher
climates
 Wave impact is a critical
issue
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SOURCE: Diamond Offshore Drilling

21. 21
Damage from Hurricane Waves
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SOURCE: Dave Wisch, Chevron

22. 22
Damage from Hurricane Waves
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SOURCE: Dave Wisch, Chevron

23. 23
Wind Forces
 Typical industry practice
for offshore platform
design is to determine
wind loads from scaled
wind tunnel tests
 Changes during design
or after installation may
require revision to wind
loads
 CFD is being used to
determining effects of
changes
SOURCE: Force Technology

24. 24
Example Applications
 Vortex-Induced Vibration of a Long Riser
 Vortex-Induced Motion of a Spar
 Wave Slamming
 Tank Sloshing
 Drag on a Riser Fairing
 Wind Loads

25. 25
VIV of a Long Riser
 Work performed by
Chevron
 Comparisons made
against high quality lab
data from Norwegian
Deepwater Program
 Fully 3D simulations for a
riser with L/D=1400. This
is a world record!
 Procedure was to find the
coarsest mesh that yields
the required accuracy
SOURCE: OMAE2006-92124
Riser Configuration
Elevation View of Mesh

26. 26
Comparisons with Lab Data
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SOURCE: OMAE2006-92124

27. 27
VIM of a Spar
 Work performed by
Chevron
 Tow tests made of 1:46
scale model of Genesis
spar
 Care was taken to
include appurtenances in
both physical & numerical
models
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SOURCE: OMAE2005-67238

28. 28
Mesh
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SOURCE: OMAE2005-67238

29. 29
VIM Results
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SOURCE: OMAE2005-67238

30. 30
Wave Impact - Idealized Case
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SOURCE:OTRC 11/05A156
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31. 31
Wave Slamming on GBS Deck
 Work performed by
Marintek
 Wave basin model of
Statfjord GBS at 1:54
scale
 Deck instrumented to
record wave impact loads
 Excellent agreement with
CFD calculation
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SOURCE: OMAE2005- 67097

32. 32
Tank Sloshing
Tank Sloshing
Observed and predicted wave profile
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SOURCE: CD-adapco

33. 33
Tank Sloshing Validation
Tank Sloshing Validation
SOURCE: CD-adapco

34. 34
Summary
 CFD has become a “mainstream” engineering tool for
many industrial applications
• Appropriate for initial studies
• Appropriate to interpolate and extrapolate measurements
 Adoption in the Offshore Oil & Gas industry is growing
rapidly