Efficient shallow water mooring system was investigated for deployment of a floating offshore wind turbine. Static mooring performance analysis of a mooring system with buoy and/or clamp weight was formulated and incorporated into optimization program. Optimal design parameters were investigated which realize an efficient mooring design equivalent to a mooring system with chain of larger nominal diameter designed for deeper water. In the formulation of static analysis of a mooring system, whole system is divided into several regions which can be assumed uniform within a region. Catenary solution is applied to the regions and whole system is solved by obtaining continuity of displacement and equilibrium of forces at the connecting points between the regions by Newton's iterative method. This static analysis program was incorporated into optimization algorithm and the design parameters of weight of clamp weight, position and size of chain which minimize mooring spring constant under large drifting force typical for FOWT were obtained. It was understood that 1) a mooring system with clamp weight is desirable to restrain the motion of FOWT and minimize mooring spring constant, 2) effect of water depth, weight and position of clamp weight on mooring spring constant under large drift force was investigated and it was understood that there are optimal values for the parameters to minimize the mooring spring constant, 3) It was shown that mooring spring constant of a mooring system with chain of 132mm nominal diameter installed at water depth of 100m can be realized by a mooring line with clamp weight and chain of nominal diameter 68mm at water depth of 50m.%浮体式洋上風車は，洋上風力エネルギー利用実用化に向けて，わが国の海洋再生可能エネルギー利用のなかでも最も開発が進んでおり，事業化に向けてコスト低減が重視されている.特に，コストの関係から，離岸距離の小さい水深50m～100m程度の比較的浅い海域への展開が望まれるが，水深50mは着底式には深すぎ，浮体式には係留の観点からは浅すぎる領域である.
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