Abstract Various uses including energy transportation can be considered for the flexible marine riser system. We had an opportunity of studying on a new marine riser system for electric power transmission cable, participating in the power generating system experiment program of Japan Marine Science & Technology Center. The experiment was conducted on a double-catenary riser system which was provided with substand-buoys to cope with the movement of the floating power generating system. This paper discusses the results of system behavior analysis and model experiment and the in service performance of the full-size system. Introduction Japan which is surrounded by the sea on all four sides is in an advantageous position to utilize ocean energy, and as a matter of fact its utilization is one of the important current subjects of inquiry. Wave force which has higher energy density than other forms of natural energy such as wind andsunlight is considered a very promising energy source to be tapped. Japan Marine Science & Technology Center has been working on the floating wave power generating system since 1977, carrying out sea trial of power generation and transmission. To obtain the highest efficiency possible of power generation the power generating system, which will subsequently be also referred to as a wave-power plant, is moored in an area of violent wave motion so that the ocean power transmission cables, especially the riser cable hung under water, are susceptible of deformation under the stress of repeated wave motion. Further, the wave-power plant is moored by slack chains to allow it make wild movements at the mercy of rough waves, storms, currents and other elements. It is therefore necessary to adopt a suitable method of cable installation that will enable the cable to withstand such severe environmental conditions. We developed a riser cable system that meets those conditions and assessed its in-service performance. Mooring System for the Wave-power Plant The wave-power plant "Kaimei" is a ship-shaped buoy which is 80 m in length and 12 m in breadth and has.a total weight of about 800 tons. This system is deslgned to carryon board 13 air-turbine generating units. The current prototype is equipped with 5 power generating units whose combined output is 560 kW. The power generating system is moored about 3.5 km off Yura, Yamagata Prefecture, and the power generated is transmitted to land via a submarine cable system consisting of a marine riser cable and a field splice as shown in Fig. 1 and Fig. 2. The configuration of the wave-power plant is indicated in Fig. 3. This wave-power plant is tethered fore with four chains hung in different directions. This mooring system is designed so that the floating plant can freely move in all directions in a relatively wide area, but the turning of the floating plant is restricted within limits of ±50 degrees by auxiliary chains at the stern. The mooring site is located in one of limited sea areas of Japan where billows reach a wave height of 6 to 10 min the winter season.