Mechanism of mooring line breakage and shutdown opportunity analysis of a semi-submersible offshore wind turbine in extreme operating gust

One of the most dangerous conditions for wind turbines is extreme operating gust (EOG), which is characterized by transient increases in wind speed. As for a floating offshore wind turbine (FOWT), however, it is unclear the potential risks and fracture mechanisms associated with the mooring system under the coupling conditions of EOG, waves, and currents. There is no published rule regarding FOWT design for reference at this time. In this study, a full aero-hydro-servo-elastic numerical model was developed based on blade element momentum (BEM) theory with correction factors, potential flow theory, Morison formulation, and finite element analysis model for mooring lines. The model incorporates the dynamic interactions caused by extreme gusts, waves, currents, control, and shutdown on FOWT responses. This is beneficial for revealing the fully coupled risk mechanism of mooring line breakage. The numerical simulation is conducted on a semi-submersible OC4-DeepCwind wind turbine with the designed mooring system in a 50 m-depth ocean area. First, the hazardous EOG parameters on mooring line safety are identified, including rise time and wind speed amplitude. And the reason of largely increased mooring tension is explained by analyzing the aerodynamic and hydrodynamic response mechanisms of the FOWT system. Additionally, the response quantities, including platform motions, tower loads, and mooring tensions, are examined under EOG-current-wave conditions with the identified rise time, wind speed amplitude, and various start times. In the study, we demonstrate that only specific coupling effects between wind and wave loads can cause transiently increased over-limit mooring tension, and we explain the essential risk mechanism of mooring line breakage. EOG has been found to induce continuous mooring line breakage, long-distance platform drift, power grid damage, and even platform capsizing. There is about 55 s between the starting time of EOG and the occurrence of the mooring line breakage incident, providing an excellent oppor-tunity for an emergency shutdown strategy. Afterwards, the optimal response period for a shutdown is identified and proven to effectively reduce the likelihood and severity of the accident. Results of this study may serve as references for the design of floating offshore wind turbines, the optimization of emergency strategies, and the formulation of design rules.