cfd for offshore applications

CFD for Floating Systems

Bob Gordon

Granherne Americas, Inc.

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Outline

Overview of CFD Present Offshore Industry Use of CFD Applications

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Overview of CFD

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

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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

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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

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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)

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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

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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

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Offshore Industry Use of CFD

Oil CompaniesChevron 9Shell 5Petrobras 4BP 2ExxonMobil 2

Service Co. & ConsultantsTechnip 8Marintek 5Marintek 3Principia 2SBM 1BPP 1Force 1

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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

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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

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Riser VIV

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

SOURCE: C.H.K. Williamson, Cornell U.

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DeepStar/MIT Lake Seneca Tests 2004

QuickTime™ and aYUV420 codec decompressor


SOURCE: K. Vandiver, MIT

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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

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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.


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SOURCE: BP

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VIV Suppression









SOURCE: BP, GlobalSantaFe, Shell

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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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Wave Slamming





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

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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





SOURCE: Diamond Offshore Drilling

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Damage from Hurricane Waves



SOURCE: Dave Wisch, Chevron

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Damage from Hurricane Waves



SOURCE: Dave Wisch, Chevron

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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

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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

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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

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Comparisons with Lab Data






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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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Mesh








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VIM Results






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Wave Impact - Idealized Case



SOURCE:OTRC 11/05A156





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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





SOURCE: OMAE2005- 67097

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Tank SloshingTank Sloshing

Observed and predicted wave profile

QuickTime™ and aCinepak decompressor


SOURCE: CD-adapco

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Tank Sloshing ValidationTank Sloshing Validation

SOURCE: CD-adapco

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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

cfd for offshore applications

Technology

cfd source

riser viv source

viv suppression source

hurricane waves source

finite volume

nasa pegasus5 cfd code

critical issue source

velocity dependent source