An Ocean Thermal Energy Conversion (OTEC) floating plant which is converted from a pre-owned ship may be able to reduce the cost, and thus such a concept has been developed targeting for 100MW-NET power plant. The distinction of the plantship is the attachment of the Cold Water Pipe (CWP) which has 800m length and a diameter of 12m. For discussion of the position keeping system and the CWP, coupled behavior between the plantship and the CWP is analyzed in this study. An analysis model of the plantship is designed from KVLCC2M and the CWP is assumed as made of FRP. The environmental conditions for Indonesia seas are assumed for the extreme analysis. A spread mooring system is considered preferable as a position keeping system. Preliminary designs by several combinations of flexible joint, clump weight, taut mooring system and catenary mooring system are compared on their dynamic behavior by using OrcaFlex. Two kinds of models which are calculated by direct coupled system and only CWP under the forced oscillation obtained by the moored ship without CWP are compared in order to examine those interactions. In addition, a simplified model is proposed, in such a way that comprehend the character of the coupled behavior. As a result of the comparison of these models, the simplified model is generally consistent with the numerical simulation. Also it is found that the interaction is significant and thus should not be ignored around the resonant frequency of the CWP and the slowly-varying motion of the plantship.