CFD For Offshore Applications
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Transcript of 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
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SOURCE: C.H.K. Williamson, Cornell U.
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DeepStar/MIT Lake Seneca Tests 2004
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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.
SOURCE: A. H. Techet, MIT
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SOURCE: BP
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VIV Suppression
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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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SOURCE: A. H. Techet, MIT
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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
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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
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SOURCE: Diamond Offshore Drilling
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Damage from Hurricane Waves
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SOURCE: Dave Wisch, Chevron
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Damage from Hurricane Waves
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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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SOURCE: OMAE2006-92124
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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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SOURCE: OMAE2005-67238
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Mesh
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SOURCE: OMAE2005-67238
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VIM Results
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SOURCE: OMAE2005-67238
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Wave Impact - Idealized Case
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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
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SOURCE: OMAE2005- 67097
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Tank SloshingTank Sloshing
Observed and predicted wave profile
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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