A CFD Based Analysis of the Vortex Induced Motion of Deep-Draft Semisubmersibles

A platform that has shown promise for larger payload and ultradeepwater applications is the Deep Draft Column Stabilized Floater (DDCSF) concept. An area of concern for this concept is its susceptibility to Vortex Induced Motions (VIM) due to its deeper draft that results in higher column slenderness ratios than conventional semisubmersibles. The VIM characteristics of the platform have traditionally been assessed through experimental measurements in model scale. An important issue with experiment based VIM predictions is that of Reynolds number scaling. The model test Reynolds numbers are typically in the subcritical regime. Limitations in experimental setup make it difficult to test VIM of semisubmersibles at supercritical Reynolds numbers or higher that are needed to preserve the flow similitude between model and prototype scales. A Computational Fluid Dynamics based approach is presented here that addresses the Reynolds number scale effects related to the VIM prediction of semisubmersibles. The effect of external damping due to the risers and mooring system on the VIM characteristics is also presented.