This study is focused on the long-term reliability analysis of loads and motions for a utility-scale floating offshore wind turbine supported by a semi-submersible platform situated in the North Sea. The floating wind turbine, which consists of a 5-MW 3-bladed rotor, is assumed to be mounted on the OC4 semi-submersible floating platform. The platform consists of a main column, three offset columns, a cross frame, and pontoons, deployed at a site with a water depth of approximately 200 meters. A mooring system consists of three catenary lines together with the turbine and platform completes the integrated system. Based on the metocean conditions characterizing the selected turbine site, a number of sea states are identified for which coupled-dynamics simulations are carried out using the aeroelastic computer-aided engineering (CAE) tool for horizontal-axis wind turbines, FAST. Short-term turbine load and platform motion statistics are established for individual sea states that are analyzed. The long-term global performance analysis yields estimates of 50-year loads and platform motions that takes into consideration response statistics from the simulations as well as the metocean (wind-wave) data and distributions. This study seeks to assess long-term loads and motions associated with large utility-scale offshore wind turbines that might be deployed on floating platforms for planned offshore wind power development in the U.S.

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