Ocean energy, as a renewable resource that covers a large area of the earth, is increasingly used for POWER generation in forms of wave, tidal and thermal. Ocean thermal energy conversion (OTEC) SYSTEMs utilize from the difference between the temperatures of surface and deep water and drive a thermodynamic Rankine cycle to generate electric POWER. The generated POWER of these plants depends on the temperature of the surface water as warm source. Due to the variation in the temperature of ocean surface, the output POWER of the OTEC SYSTEM frequently changes in the year. This uncertainty nature results in the variation in the generated POWER and so, integration of large-scale OTEC generation units to the POWER SYSTEM is a challenging problem. Thus, new techniques must be developed to study the effects of these variable resources on the POWER SYSTEM. In operation studies of POWER SYSTEMs, the balance between generation and consumption is important and from reliability point of view, some spinning reserve must be scheduled to prevent load curtailment in the events such as forced outages of generation units, transmission lines and so on. In a POWER SYSTEM containing large-scale OTEC POWER plants, the uncertainty nature of these plants must be considered in the reserve scheduling. Thus, for this purpose, a reliability model considering both failure of composed components and variation in the output POWER, is developed for OTEC units. To determine a suitable multi-state model for OTEC units, fuzzy c-means clustering technique and XB index are utilized. Then, the proposed multi-state model is used to determine spinning reserve value of a POWER SYSTEM containing OTEC plants using of the modified PJM method. Numerical results associated to RBTS and IEEE-RTS present the effectiveness of the proposed technique for operation studies of POWER SYSTEMs containing OTEC SYSTEMs.