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CFD Application in Offshore Structures Design at PETROBRAS

CFD Application in Offshore Structures Design at PETROBRASCFD Application in Offshore Structures Design at PETROBRAS

Marcus ReisESSSCFD Director



• Mooring System Design of Floating Production Systems;

• Current and Wind Loads;• Wave Induced Drag Coefficients.



Case Study: Platform Hydrodynamic Drag

• Current Practice:– Use the drag coefficients from similar projects or an assembly of

simple geometric shapes at early stages, and adapt them according to the project evolution;

– The final coefficients are obtained experimentally at a more advanced design phase.

• Objective:– A good estimate of hydrodynamic drag coefficients at early

design stages.



Drag Coefficients x Headings Diagram

• The drag coefficients are calculated for various headings (current direction) in order to generate a diagram;

• The diagram represents the environmental loading which is used to design the mooring system.



The Model Test• The model test was

performed at Danish Maritime Institute;

• Scale: 1:200;• Tunnel Speed: 15m/s;• Re = 4.28 x 105

(Current).



About the Geometry Configuration ...• Flow around “BLUFF

BODIES” is necessarily unsteady and transient;

• It is very difficult to achieve a steady state solution of this kind of problem;

• The spatial (mesh) and temporal (time step) variables coarsening can help to filter these instabilities in steady state runs;

• A “Hull” geometry is composed by several “BLUFF” geometries;



The Hull Geometry

• The geometry was modeled in ICEM CFD;

• In order to generate high quality meshes, several geometrical details were suppressed;

• Curves were kept only at strong surfaces tangency discontinuities;

• As well, points were kept only at knuckle (curves tangency discontinuities).



Parametric Study



Motivation• Necessity of reducing the processing time (various heading angles);• Uncertainties concerning the mesh conception, domain definition and

boundary conditions;

Main Parameters• Farfield size;• Mesh refinement (global and superficial);• Prismatic elements layer;• Y plus values;• Mesh quality;• Near hull mesh refinement;



About the Flow State ...

Steady State run => 2 hours Transient run => 2 weeks

180,000 nodes mesh running in a Pentium 4 PC



About the Flow State ...



About the Steady State Convergence ...

Converged Run (90 iterations)



About the Steady State Convergence ...

Partially Converged Run (300 iterations)

Mean values are Mean values are representativerepresentative



About the Steady State Convergence ...• The convergence

difficulties (maximum residual) are located at the vortex street;

• It is not related to the mesh or boundary conditions;

• These residuum's are related to the unsteady characteristics of flows around Bluff Bodies;



About the SST Turbulence Model ...

• The Shear Stress Transport is a two-equation RANS turbulence model;

• It blends the κ−ε and the κ−ωmodels were they work better;

• It is a good choice for drag calculations;

Full Full κκ−−ωω in RED in RED



About the Yplus ...

Mean value Mean value ≈≈ 10 10

• For drag calculations, Y+ values below 1 would be excellent, but expensive for this proposal;



Parametric Study (Conclusions...)

• The Farfield distance from the Hull is very important;

• Surface Mesh and Prism Mesh refinement turns the convergence more difficult in the steady state;

• The results of coarse meshes in steady state are sufficient;

• The mesh refinement away from the Hull (Vortex Street and Farfield) is not significant;

• The mesh quality impact directly the convergence;

• Boundary Layer refinement for a Y+ below 1 is not necessary;



Parametric Study (Final Set Up...)

• Circular Far Field: Radius = 900 m (10L)• Global element size = 64 m• Surface element size = 2 m• Near hull refinement: Size = 16 m• Prism layer: Total height = 4 m

Number of Layers = 6

OBS: L = 90 m (Hull characteristic length)



Superficial Mesh (On the Hull)


CFD Application in Offshore Structures Design at PETROBRASSuperficial Mesh (Farfield and Sea Surface)



Tetrahedral Mesh



Mesh report:

Element types :

TETRA_4 : 336811

PENTA_6 : 224886 (prism)

Total elements : 613148Total nodes : 181137



Mesh Checking

Recommended: >10 Recommended: > 0.2

Recommended: <10 Recommended: <100



• Hull: Wall No Slip;

• Far Field: Opening with Prescribed velocity;

• Sea Surface: Wall Free Slip;

Boundary Conditions



Post – Processing ...



Current Simulation (Heading=140 Degrees).

Streamlines plotting.

Pressure



Current Simulation (Heading=140 Degrees).

Velocity vector plotting (plane Y=0)

Velocity vector plotting (domain).



Numerical X

Experimental Results



Conclusions:

•• The results obtained from the CFD analysis The results obtained from the CFD analysis can be used to predict the drag coefficients;can be used to predict the drag coefficients;

•• The discrepancies can be associated with the The discrepancies can be associated with the greater model complexity used in the tests greater model complexity used in the tests compared to the CFD model.compared to the CFD model.



Other Studies



Other Studies: Roll Motion in FPSOs



Wind Drag Coefficients Calculation

Velocity vector plotting (plane Y=0)

Streamlines plotting (plane Y=0)

Pressure Field



Co-operation:

• Mauro Oliveira - PETROBRAS• Fábio Menezes - PETROBRAS• Fernando Torres - PETROBRAS• Marcos Donato - PETROBRAS• Allan Carre - PETROBRAS• Márcio Maia – PETROBRAS• João Pessoa - PETROBRAS• Ricardo Damian - ESSS• Nicolas Spogis - ESSS• Rodrigo Ferraz - ESSS• Celso Takemori - ESSS

cfd application in offshore structures design at petrobras · cfd application in offshore...

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