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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 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 the present study, numerical simulations are performed for a semi-submersible with four square columns subject to a current at a 45° incidence angle and allowed surge (in-line), sway (transverse), and yaw motions. Calculations were performed using the Finite-Analytic Navier–Stokes (FANS) code in conjunction with a moving overset grid approach. Computations are conducted over a wide range of reduced velocities, from pre-lock-in to post-lock-in conditions. Both the full scale and the 1:70 model platforms are studied and detailed results compared to check the scale effect. In addition, three corner geometries are simulated, and the semi VIM is found to be sensitive to the corner rounding. Comparisons are made with experimental data to demonstrate the capability of the present CFD approach. [ABSTRACT FROM AUTHOR]