The authors previously proposed a semi-submersible floating structure for offshore wind power generation system, which is mainly composed of three vertical base floaters and horizontal connecting beams. The present study attempts a structural optimization of the draft depth of the connecting beams by a cumulative fatigue damage analysis. Although the wave induced motion was varied according to the draft depth, it does not interfere the operation of wind turbines, however, fatigue damage was much more influenced by the draft depth. The optimum depth of 30 m has been obtained from the viewpoints of structural safety.
