Design of a Mooring System for FOWT with Sufficient Yaw Stiffness

Abstract

To further make green energy a mainstay to combat the emissions from the consumption of fossil fuels, alternative solutions for energy generation have been of interest. Offshore wind generation has recently become one of the most promising sustainable energy sources. About 80% of the wind resources are available in offshore regions with water depths of more than 60 m, and using the fixed foundation concept is unfeasible from an economic point of view. The floating wind turbine leverages this abundant wind resource in deep water. One of the critical components of the floating wind turbine is the mooring lines, which keep the floater in place (station keeping). The use of steel chains, polyester, or nylons for the mooring lines is possible. Because of the ease of construction and the unconditional stability offered by the spar-type concept, given that the centre of gravity is lower than the centre of buoyancy, it is used in this study. This report analyses the suitability of taut configuration polyester mooring lines in providing sufficient yaw stiffness for a spar-type floater. The floater is subjected to irregular waves with a water depth of 250 m. The significant wave height and peak periods range from 5.25 m to 6.6 m and 9.03 s to 12.67 s, respectively. In the initial proposed solution, about 95% of the length of each mooring line is made of polyester, while the remaining part is made of stud-less chains with a buoy placed at the connection point; the chain connects to the anchor. A fully coupled analysis is carried out in OpenFAST, and the hydrodynamic input file is prepared with the boundary element method (BEM) solver NEMOH and BEMRosetta. The baseline IEA 15 MW semi-submersible controller and tower properties are used. The MoorDyn program is used to compute the mooring dynamics of the semi-taut mooring configuration. The crowfoot configuration, which is patented, is first modelled, and then the simple catenary system is also modelled. The best configuration is then selected as the one that provides yaw stiffness similar to the crowfoot configuration. The selected configuration is a 3-line semi-taut mooring with three anchor points and six fairlead points. This configuration provides yaw stiffness higher than the initial proposal and utilises 50% less chain and 30% less polyester.