Preliminary design and dynamic analysis of constant tension mooring system on a 15 MW semi-submersible wind turbine for extreme conditions in shallow water

Floating offshore wind in China faces the challenge of shallow water depths and frequent typhoons, which are very different from the environmental conditions in European waters. The performance of the mooring system directly affects the overall cost of the floating offshore wind turbine and its survivability in extreme conditions. Under extreme conditions in shallow water, line dragging or breaking can frequently occur when the floating platform restrained by a conventional chain catenary mooring system drifts with waves. A constant tension mooring system concept is proposed to solve the design challenges of shallow water mooring systems in harsh conditions for floating wind turbines. The newly proposed mooring system consists of clump weight and winch wires, which release kinetic energy by increasing the platform's moving distance and ensuring that the instantaneous maximum mooring force never exceeds the breaking strength of the chain. Based on the time-domain potential flow theory, the coupling dynamic response of the UMaine VolturnUS-S semi-submersible reference wind turbine moored by a constant tension mooring system is studied. The mooring line tension and the platform motion are analysed. By comparing with the traditional catenary mooring system, it is found that the constant tension mooring system can significantly reduce the length of the mooring line used and the maximum mooring line tension. It completely prevents the mooring chain from being tightened and the turbine from being damaged by excessive mooring loads, effectively improving the survivability of a floating offshore wind turbine in extreme conditions in shallow water areas.