In 2012, a mooring line of the semi-submersible" Deepsea Atlantic "that had been operating at Gullfaks South Field in the North Sea was broken. A JIP team investigated causes of the accident in Norway and reported that the accident occurred due to the combination of viscous wave drift force and anomalous wave. Until now, potential wave drift force has been used in the mooring design for semi-submersible but viscous wave drift force has not been taken into consideration. Generally, in the

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... cy band where the wave length is long, the ratio of potential wave drift force is small among wave drift forces acting on the floating body, while viscous wave drift force becomes dominant. In a structure of which splash zone has a short diameter, such as a column of semi-submersible, the wave length is long compared to the column, and the viscous wave drift force acts remarkably.