Vortex-Induced Motion of a Deep-Draft Semi-Submersible In Current And Waves

A deep-draft semi-submersible (DDS) has better vertical motion characteristics than a conventional semi-submersible due to the smaller wave exciting forces on the pontoon structure, but elongated submerged column and enlarged projected area to current may induce larger in-line and transverse horizontal motions in current due to the increase of drag force and vortex-induced lift force. A seakeeping model test of a newly developed DDS is carried out to assess the global motion performance in wind, wave and current environments. Model test results showed that a DDS may experience significant vortexinduced motion (VIM) in the direction normal to the current, and its amplitude is strongly dependent not only on the current speed but also on the wave excited particle velocity.