Abstract: Optimization study toward semi-submersible platform in deep water is investigated on the basis of computational fluid dynamics (CFD) numerical method. A new type of semi-submersible platform which can suppress vortex-induced motion (VIM) is presented. Model test is also carried out to testify the validity of the numerical method. The characteristics of VIM toward new and conventional platforms are studied respectively. Some key factors such as longitudinal offset, sway response, sway distribution tendency and lock-in interval are compared. The characteristics of platform motion trajectory coupled from each degree of freedom (DOF) are analyzed. Mechanism of vortex shedding suppression in new type of platform is investigated in detail. The effects of column cross section geometry and column arrangement on VIM performance of platform is studied from the uniformity, stability and amplitude of VIM response and the vortex shedding form. It indicates that this new configuration of platform where cross section geometry of each column is trapezoidal and the columns are arranged according to splayfooted type, plays a significant part in suppressing VIM, and contribute to decreasing and postponing the destructive effects toward mooring and riser system due to strong non-linear VIM.

Key words: vortex-induced motion (VIM), sway response, lock-in phenomena, mechanism of vortex shedding suppression, cross section geometry