Three type of offshore structure on which an Ocean Thermal Energy Conversion (OTEC) power plant will be constructed are now available; that is, land based (on shore) type, bottom mounted (offshore) type and floating type. Among them, the floating type offshore structure is popular and examined extensively in Japan. As one of the typical and practical applications of the “SUNSHINE PROJECT”, “Study of the Ocean Thermal Energy Conversion (OTEC) Power Generation System” has been commenced. In studying this system, actually using a floating type pilot plant provided with a water intaking device, various experiments are scheduled to conduct. Since the power plant site is often a sea area where might be well visited by a typhoon. In order to protect such offshore structures from the above disasters, it is required to provide the structure with sufficient strength that is bearable against severe sea conditions (such as wave, wind etc.) after analyzing the dynamic response. So, we have studied the calculation method of a fluid force acting on the floating platforms and Cold Water Pipe (CWP), and a formulation of the mathematical models for the coupled platforms and CWP systems, as well as the equivalent linealized treatment of nonlinear fluid force. Then we have carried out the tank test by using 1/50 scale model of the OTEC plant which was designed in the “SUNSHINE PROJECT” for the experiment at sea, and compared the experimental results with the theoretical calculation, there by a reasonable agreement of both has been confirmed. This work was supported by Agency of Industrial Science and Technology under the contract of the “SUNSHINE PROJECT”.