CFD Simulation of Vortex-Induced Motions of a Deep Draft Semi-Submersible Platform

The vortex-induced motion (VIM) of semi-submersible platforms becomes an important issue with the recent development of deep draft semi-submersible platforms. As a result of the increased draft, the semi-submersibles are susceptible to coherent vortex shedding, and the platform VIM increases significantly. The VIM of semi-submersibles is more complex than those of spars and mono-column hulls due to the wake interaction of vortices shed from multiple columns. In general, the vortex-induced motion of deep draft semi-submersible platform is characterized mainly by three degree-of-freedom motions with surge (in-line), sway (transverse), and yaw motions. In the present study, numerical simulations are performed for a semi-submersible with four square columns subjected to a current at a 45 degree incidence angle. Calculations were performed using the Finite-Analytic Navier-Stokes (FANS) code in conjunction with a moving overset grid approach to accommodate the relative motions between the semi-submersible hull, wake, and background grid blocks. Simulations are performed both for the full scale and the 1:70 model platforms to check the validity of the Froude scaling law. Various current speeds corresponding to different reduced velocities are simulated. Motion responses and the flow fields for both the model and full scale platforms are studied. Comparisons are made with experimental data to demonstrate the capability of the present CFD approach.