CFD Application in Offshore Structures Design at PETROBRAS

Transcription

1 CFD Application in Offshore Structures Design at PETROBRAS Marcus Reis ESSS CFD Director

2 Mooring System Design of Floating Production Systems; Current and Wind Loads; Wave Induced Drag Coefficients.

3 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.

4 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.

5 The model test was performed at Danish Maritime Institute; Scale: 1:200; Tunnel Speed: 15m/s; Re = 4.28 x 10 5 (Current). The Model Test

6 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;

7 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).

8 Parametric Study

9 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;

10 About the Flow State... Steady State run => 2 hours Transient run => 2 weeks 180,000 nodes mesh running in a Pentium 4 PC

11 About the Flow State...

12 About the Steady State Convergence... Converged Run (90 iterations)

13 About the Steady State Convergence... Mean values are representative Partially Converged Run (300 iterations)

14 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;

15 About the SST Turbulence Model... The Shear Stress Transport is a two-equation RANS turbulence model; It blends the κ ε and the κ ω models were they work better; It is a good choice for drag calculations; Full κ ω in RED

16 About the Yplus... For drag calculations, Y + values below 1 would be excellent, but expensive for this proposal; Mean value 10

17 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;

18 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)

19 Superficial Mesh (On the Hull)

20 Superficial Mesh (Farfield and Sea Surface)

21 Tetrahedral Mesh

22 Mesh report: Element types : TETRA_4 : PENTA_6 : (prism) Total elements : Total nodes :

23 Mesh Checking Recommended: >10 Recommended: > 0.2 Recommended: <10 Recommended: <100

24 Boundary Conditions Hull: Wall No Slip; Far Field: Opening with Prescribed velocity; Sea Surface: Wall Free Slip;

25 Post Processing...

26 Current Simulation (Heading=140 Degrees). Pressure Streamlines plotting.

27 Current Simulation (Heading=140 Degrees). Velocity vector plotting (domain). Velocity vector plotting (plane Y=0)

28 Numerical X Experimental Results

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32 Conclusions: The results obtained from the CFD analysis can be used to predict the drag coefficients; The discrepancies can be associated with the greater model complexity used in the tests compared to the CFD model.

33 Other Studies

34 Other Studies: Roll Motion in FPSOs

35 Wind Drag Coefficients Calculation Pressure Field Velocity vector plotting (plane Y=0) Streamlines plotting (plane Y=0)

36 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