Computational fluid dynamics (CFD) simulations were carried out on the OC4-DeepCwind semisubmersible to obtain a better understanding of how to set hydrodynamic coefficients for the structure when using an engineering tool such as FAST to model the system. This study focussed on the drag behavior and the effects of the free surface, free ends and multimember arrangement of the semisubmersible structure. These effects are investigated through code-to-code comparisons and flow visualizations. The implications on mean load predictions from engineering tools are addressed. This study suggests that a variety of geometric factors should be considered when selecting drag coefficients. Furthermore, CFD simulations demonstrate large time-varying loads caused by vortex shedding that FAST’s hydrodynamic module, HydroDyn, does not model. The implications of these oscillatory loads on the fatigue life needs to be addressed.

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