A commercial-scale Ocean Thermal Energy Conversion (OTEC) floating platform will require a large diameter Cold Water Pipe (CWP) to be attached. Several studies have analyzed the dynamic behavior of the coupled system between the floating platform and the CWP. However, the characteristic of the coupled behavior has not yet been fully understood. This study aims to formulate the coupled system of an OTEC floating plant and simplify the formula to clarify the characteristic of the coupled behavior. The formula is suitable for validation of the numerical simulation results and the preliminary design of an OTEC plant. In the first section of this paper, we derive the equation of motion and equilibrium of the direct moored floating body and an elastic pipe hanged off from the floating body. In the second section, we verify the formula for a 100MW OTEC plantship with 800m length and 12m diameter CWP. The Response Amplitude Operator (RAO) is calculated by solving the equation of motion and statistics responses in 3 hours are compared with a numerical simulation by OrcaFlex. As the result of the comparison, we observed that the present formula is applicable in the early stage of the practical design loop.