Vortex-induced motion (VIM) phenomenon is a great challenge for design and operation of offshore structures subjected to ocean flow. Semi-submersibles with three columns are often applied to the field of wind turbine, suffering VIM motions probably. In recent years, it is showed that many factors have more or less influence on VIM of platforms. A comparison of circular columns with square columns on VIM characteristics of three-column semisubmersibles is carried out using the detached eddy simulation (DES) method via Star-ccm+ software. This paper analyzes motions in the transverse direction and yaw, as well as fluid forces including drag force and fluctuating lift force. The results show that transverse amplitudes of semisubmersible with three square columns are much lower than those of semisubmersible with three circular columns at all incidences. Besides, the authors conclude that the semisubmersible with three square columns do not experience obvious resonance behavior, which is different from the semisubmersible with three circular columns. Besides, galloping phenomenon occurred at large reduced velocities at 0°-incidence for the semisubmersible with three square columns, which is likely not induced by lift force directly. Meanwhile, in these cases, yaw amplitudes are also larger than the others. This may be due to the galloping behavior, which is just a conjecture.

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