Numerical Simulation of a Semi-Taut Moored Floating Offshore Wind Turbine Under a Mooring Line Failure in Shallow Waters

Abstract The potential sites for floating offshore wind turbines (FOWTs) in Taiwan are mostly located in shallow water depths ranging from 50 m to 100 m. Recent studies reveal that semi-taut mooring appears to be a feasible solution for FOWTs in shallow waters. However, the potential area is also one of the busiest navigation channels in the world, indicating that the issue of mooring line failure should be carefully considered. Thus, this study aims to investigate the feasibility of a semi-taut mooring configuration applied to FOWTs in shallow waters and the impact of mooring line failure on it. For this study, the 15 MW semi-submersible FOWT reported by the International Energy Agency (IEA) is adopted, equipped with three mooring lines. The study area is situated at a water depth of 70 meters off northeast Taiwan, and the extreme sea state is a combination of a 50-year return period of irregular waves, wind, and current. Comparison results among the catenary chain and two types of semi-taut mooring (Polyester and nylon) demonstrate that both semi-taut mooring configurations notably decrease the probability of touchdown points nearing anchor locations and substantially reduce the maximum load. Additionally, they reduce surge and pitch responses but increase heave responses, with little differences in other degrees of freedom. The comparison results of upstream mooring line failure on the three mooring configurations indicate that after the failure, the semi-taut mooring reaches balance states faster, but the drift distance of the catenary chain is shorter. In conclusion, the two semi-taut moorings could be considered as viable and superior solutions for FOWTs in the shallow water areas of Taiwan.