Wear of Shallow Water Moorings for a Floating Offshore Wind Turbine

When a floating offshore wind turbine is commercialized and many floating offshore wind turbines are operated as wind farms, basically, the mooring system is required to function without trouble during the assumed operating life to reduce the maintenance and repair costs. In this study, attention is paid to wear. A wear index is formulated to evaluate the degree of wear relatively. The index is incorporated into a quasi-static analysis method of mooring which uses analytical solution and the degree of wear of simple catenary mooring, buoy-sinker mooring and synthetic fiber rope mooring are evaluated.The wear index when the horizontal load at the mooring point was changed by unit load was evaluated. The amount of wear at touchdown point is constant and relatively large. The wear index is small at the general part of the mooring chain. It is large at the mooring point. As for the connection point with sinker, it is large until the sinker lifts from the seabed, and when the sinker lifts from the sea floor. The index gets bigger at the connection point with the buoy. When the buoy sinker lifts from the seabed, the wear index becomes smaller, and the mooring point becomes the largest when viewing the entire mooring system. With regard to synthetic fiber rope mooring, the angle of rotation at the connection point with the buoy and sinker due to the change in the horizontal force is large, but the value of the wear index is small. The value at the mooring point became large. The wear index at the touchdown point becomes larger than t the value at mooring point when the horizontal force is smaller. The value at touchdown point becomes larger. This is consistent with observation that wear progress fast sometimes in a mooring line with a small tension in the leeward side.