CFD For Offshore Applications


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CFD For Offshore Applications

  1. 1. CFD for Floating Systems <ul><li>Bob Gordon </li></ul><ul><li>Granherne Americas, Inc. </li></ul>
  2. 2. Outline <ul><li>Overview of CFD </li></ul><ul><li>Present Offshore Industry Use of CFD </li></ul><ul><li>Applications </li></ul>
  3. 3. Overview of CFD <ul><li>What is CFD? </li></ul><ul><li>Brief History </li></ul><ul><li>Overview of CFD Methods </li></ul><ul><li>Validation & Verification </li></ul>
  4. 4. Fluid Dynamics <ul><li>Theoretical </li></ul><ul><ul><li>Analytical Solutions (Heyday in 19 th & early 20 th Century) </li></ul></ul><ul><ul><ul><li>Potential Flow </li></ul></ul></ul><ul><ul><ul><ul><li>Many analytical solutions, including nonlinear equations (Airy, Stokes, Kelvin, Lamb, Korteweg and de Vries, Stoker, …) </li></ul></ul></ul></ul><ul><ul><ul><li>Viscous Flow </li></ul></ul></ul><ul><ul><ul><ul><li>Very few analytical solutions (Stokes, Poiseuille, Blasius, Ekman, …) </li></ul></ul></ul></ul><ul><ul><li>Theory of Turbulence (Reynolds, 1889 ->) </li></ul></ul><ul><li>Experimental </li></ul><ul><ul><li>Many advances in laboratory and field instrumentation continue to appear (e.g., Particle Image Velocimetry, Acoustic Doppler Current Meters) </li></ul></ul><ul><li>Computational </li></ul><ul><ul><li>Many advances continue in physical models, algorithms, software (parallelization) and computing hardware </li></ul></ul><ul><ul><li>Advances in CFD depend on good experimental data for verification </li></ul></ul>
  5. 5. Why CFD? <ul><li>Real world flows are too complex to be addressed solely by theory or experimentation </li></ul><ul><ul><li>Nonlinear </li></ul></ul><ul><ul><li>Complicated Geometry </li></ul></ul><ul><ul><li>Coupled (Heat & Mass Transfer, Chemical Reaction, Fluid-Structure Interaction) </li></ul></ul><ul><ul><li>Turbulent </li></ul></ul>
  6. 6. Some Historical Milestones <ul><li>1922 - L. F. Richardson developed first numerical weather prediction system using finite differences calculated by hand ( Humans ~10 -9 GFlop ) </li></ul><ul><li>1946 - J. von Neumann develops program for ENIAC to calculate hydrogen bomb explosion ( ENIAC ~10 -6 GFlop ) </li></ul><ul><li>1965 - Harlow & Welch develop the MAC method at LANL; first successful technique for incompressible flows ( CDC 6600 ~10 -3 GFlop ) </li></ul><ul><li>1981 - Spalding (ICL & CHAM) develops the first commercial CFD code - PHOENICS ( CRAY X-MP ~10 0 GFlop ) </li></ul><ul><li>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 ~10 5 GFlop ) </li></ul>
  7. 7. Components of a Numerical Solution Method <ul><li>Mathematical Model </li></ul><ul><ul><li>Incompressible vs. Compressible, Laminar vs. Turbulent, 2D vs. 3D, etc </li></ul></ul><ul><li>Discretization Method </li></ul><ul><ul><li>Finite Difference, Finite Volume, Finite Element </li></ul></ul><ul><li>Coordinate System </li></ul><ul><ul><li>Cartesian, Orthogonal and Non-orthogonal Curvilinear, etc </li></ul></ul><ul><li>Numerical Grid </li></ul><ul><ul><li>Structured, Block-structured, Unstructured </li></ul></ul><ul><li>Finite Approximations </li></ul><ul><ul><li>Accuracy vs. speed </li></ul></ul><ul><li>Solution Method </li></ul><ul><ul><li>Time stepping for transient; Iteration schemes for steady state </li></ul></ul><ul><li>Convergence Criteria </li></ul>
  8. 8. Validation & Verification <ul><li>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 </li></ul><ul><li>Validation ~ Solving the right equations </li></ul><ul><ul><li>Compare against measured data </li></ul></ul><ul><ul><li>Compare against benchmark analytical and/or numerical solutions </li></ul></ul><ul><li>Verification ~ Solving the equations right </li></ul><ul><ul><li>Check convergence with mesh and time step refinement </li></ul></ul><ul><ul><li>Make sure that numerical errors are sufficiently small </li></ul></ul>
  9. 9. Offshore Industry Use of CFD
  10. 10. Enabling Technology <ul><li>Physical Models </li></ul><ul><ul><li>Turbulence Models (DNS, LES, RANS) </li></ul></ul><ul><ul><li>Heat & Mass Transfer, Multi-Phase Flows, Combustion </li></ul></ul><ul><li>Algorithms </li></ul><ul><ul><li>Finite Element & Volume Methods </li></ul></ul><ul><ul><li>Grids </li></ul></ul><ul><ul><ul><li>Moving Grids </li></ul></ul></ul><ul><ul><ul><ul><li>Arbitrary Lagrangian-Eulerian Methods (ALE) </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Level Set Methods </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Sliding Grids </li></ul></ul></ul></ul><ul><ul><ul><li>Chimera Grids </li></ul></ul></ul><ul><li>Software </li></ul><ul><ul><li>Parallelization </li></ul></ul><ul><li>Hardware </li></ul><ul><ul><li>Low Cost, High Performance Parallel Computing Architectures </li></ul></ul><ul><ul><ul><li>Clusters </li></ul></ul></ul><ul><ul><ul><li>Grids </li></ul></ul></ul>
  11. 11. Some Offshore Problem Areas of Interest for CFD <ul><li>Fluid-Structure Interaction </li></ul><ul><ul><li>Vortex-induced vibrations of risers </li></ul></ul><ul><ul><li>Vortex-induced motions of floating platforms </li></ul></ul><ul><li>Flow Around Vessel Hulls and Superstructure </li></ul><ul><ul><li>Wind and current forces </li></ul></ul><ul><li>Slam and water impact loading </li></ul><ul><li>Sloshing in Tanks </li></ul>
  12. 12. Riser VIV SOURCE: C.H.K. Williamson, Cornell U.
  13. 13. DeepStar/MIT Lake Seneca Tests 2004 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 <ul><li>Non-uniform currents effect the spanwise vortex shedding on a cable or riser. </li></ul><ul><li>The frequency of shedding can be different along length. </li></ul><ul><li>This leads to “cells” of vortex shedding with some length, l c . </li></ul>SOURCE: A. H. Techet, MIT
  16. 16. SOURCE: BP
  17. 17. VIV Suppression SOURCE: BP, GlobalSantaFe, Shell
  18. 18. Platform Vortex-Induced Motions <ul><li>Same phenomenon as Riser VIV </li></ul><ul><li>Vortex-induced motion amplitudes (A) for a Spar can up to 1.5 times the Platform Diameter (D), if no VIV suppression is used </li></ul><ul><li>Motion is typically in a Figure 8 pattern </li></ul><ul><li>Magnitude of A/D is velocity dependent </li></ul>SOURCE: A. H. Techet, MIT
  19. 19. Wave Slamming <ul><li>Basic Physics </li></ul><ul><ul><li>Drag forces: caused by viscosity resulting in flow separation </li></ul></ul><ul><ul><li>Inertia forces: related to the acceleration of the incident flow and the modification of the incident wave pattern by the member. </li></ul></ul><ul><ul><li>Slam forces: occur when a wave engulfs a member causing a volume of water to be decelerated (conservation of fluid momentum) </li></ul></ul><ul><li>Progress has bee made in predicting loads using CFD </li></ul>SOURCE: MARINTEK
  20. 20. Surface Blow-Out Preventer (SBOP) <ul><li>Uses high pressure casing riser </li></ul><ul><li>Allows wells to be drilled quickly </li></ul><ul><li>Has been used in areas with relatively calm weather </li></ul><ul><li>Industry is looking to extend to harsher climates </li></ul><ul><li>Wave impact is a critical issue </li></ul>SOURCE: Diamond Offshore Drilling
  21. 21. Damage from Hurricane Waves SOURCE: Dave Wisch, Chevron
  22. 22. Damage from Hurricane Waves SOURCE: Dave Wisch, Chevron
  23. 23. Wind Forces <ul><li>Typical industry practice for offshore platform design is to determine wind loads from scaled wind tunnel tests </li></ul><ul><li>Changes during design or after installation may require revision to wind loads </li></ul><ul><li>CFD is being used to determining effects of changes </li></ul>SOURCE: Force Technology
  24. 24. Example Applications <ul><li>Vortex-Induced Vibration of a Long Riser </li></ul><ul><li>Vortex-Induced Motion of a Spar </li></ul><ul><li>Wave Slamming </li></ul><ul><li>Tank Sloshing </li></ul><ul><li>Drag on a Riser Fairing </li></ul><ul><li>Wind Loads </li></ul>
  25. 25. VIV of a Long Riser <ul><li>Work performed by Chevron </li></ul><ul><li>Comparisons made against high quality lab data from Norwegian Deepwater Program </li></ul><ul><li>Fully 3D simulations for a riser with L/D=1400. This is a world record! </li></ul><ul><li>Procedure was to find the coarsest mesh that yields the required accuracy </li></ul>SOURCE: OMAE2006-92124 Riser Configuration Elevation View of Mesh
  26. 26. Comparisons with Lab Data SOURCE: OMAE2006-92124
  27. 27. VIM of a Spar <ul><li>Work performed by Chevron </li></ul><ul><li>Tow tests made of 1:46 scale model of Genesis spar </li></ul><ul><li>Care was taken to include appurtenances in both physical & numerical models </li></ul>SOURCE: OMAE2005-67238
  28. 28. Mesh SOURCE: OMAE2005-67238
  29. 29. VIM Results SOURCE: OMAE2005-67238
  30. 30. Wave Impact - Idealized Case SOURCE:OTRC 11/05A156
  31. 31. Wave Slamming on GBS Deck <ul><li>Work performed by Marintek </li></ul><ul><li>Wave basin model of Statfjord GBS at 1:54 scale </li></ul><ul><li>Deck instrumented to record wave impact loads </li></ul><ul><li>Excellent agreement with CFD calculation </li></ul>SOURCE: OMAE2005- 67097
  32. 32. Tank Sloshing Observed and predicted wave profile SOURCE: CD-adapco
  33. 33. Tank Sloshing Validation SOURCE: CD-adapco
  34. 34. Summary <ul><li>CFD has become a “mainstream” engineering tool for many industrial applications </li></ul><ul><ul><li>Appropriate for initial studies </li></ul></ul><ul><ul><li>Appropriate to interpolate and extrapolate measurements </li></ul></ul><ul><li>Adoption in the Offshore Oil & Gas industry is growing rapidly </li></ul>