The determining of aerodynamic coefficients is one of the essential steps in the design of an offshore structure such as an FPSO (Floating Production Storage and Offloading). It is extremely important as they are one of the dimensioning criteria for the mooring design. Nowadays, these loads are mainly assessed through wind tunnel tests performed at model scale. Estimating realistic wind loads however, remains a big challenge. The complexity and associated simplification level of FPSO topside structures, the scale effects and the establishment of the atmospheric boundary layer imply that many simplifications are to be made. Nowadays with the evolution of CFD (Computational Fluid Dynamics) software, and the increase of the meshing capacity, new scopes open to CFD. Aerodynamic simulations on complex FPSO structures are therefore now possible, but need specific developments and validations that are presented in this paper. The first steps for setting up a procedure for computing these coefficients are thus presented.

The main objective of the work presented here is to investigate the ability of CFD for evaluating wind loads on complex FPSOs topsides. In a first stage, the first elements of the numerical model used as a numerical wind tunnel are presented and studied. Then, some specific effects such as the blockage effects, the atmospheric boundary layer and their numerical modelling are studied. The geometric model used corresponds to the one used in wind tunnel. The same Atmospheric Boundary Layer is simulated and a thorough effort is performed to ensure the mesh convergence. Then, the accuracy of the blockage effect correction is evaluated by performing computations with and without blockage, and results are compared with classical corrections applied in wind tunnel tests.

In the last step, a relevant comparison between wind tunnel tests and simulations on different geometries with grids density up to 50 million cells is presented. The ability for CFD to evaluate aerodynamic coefficients is then discussed.

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