For the growth of demand for utilization of the vast Japanese Economic Exclusive Zone (EEZ), Ministry of Land, Infrastructure, Transport and Tourism of Japan (MLIT) started the R&D project of offshore platform technologies in 2007. National Maritime Research Institute (NMRI) had developed the integrated design support tool, called “Harmonic design tool” for evaluating economical, safety and environmental aspects of the offshore platforms. The tool conducts conceptual design of offshore platforms for seafloor massive sulphides mining, experimental drilling of methane hydrate, combined application of food and energy production. Using the tool, basic design of semi-submersible rig type platform for experimental drilling of methane hydrate is carried out and optimum hull size, predicted steady environmental forces acting on the platform (current, wave, wind forces), thruster specification for Dynamic Positioning System (DPS) such as required thrust force, construction cost are estimated.

The purpose of this paper is to evaluate the feasibility of proposed basic design of the platform aspects of stability and position keeping ability. From results of experiments and numerical calculation, it is confirmed that the basic design of the platform proposed by the “Harmonic design tool” is appropriate.

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