Due to the steadier and stronger wind, and the long distance from residential area, the offshore wind farm has been considered as an ideal alternative energy solution. A floating wind turbine system is being considered as a key solution to make this offshore wind farm feasible in economic view point. This research is focusing on an optimal design of the Spar buoy / TLP type floating wind turbines running at a shallow / intermediate water depth. Nacelle acceleration, static and dynamic tensions on catenaries, the maximum tension at the anchors on seafloor are considered as design performances, and a stochastic analysis method has been used to evaluate those quantities based on sea state spectral density functions. The performance at a 100 year hurricane condition is being defined as a limiting case, and a linear wave theory has been the most fundamental theory applied for the present analysis.