Effects of mooring line failure on the dynamic responses of a semisubmersible floating offshore wind turbine including gearbox dynamics analysis

Floating offshore wind turbines (FOWTs) are located in complex ocean environments; therefore, they typically exhibit more significant dynamic responses than land-based wind turbines. Examples from the marine and offshore industries have shown that mooring line systems may fail under mild to severe sea conditions during their life cycle, possibly changing the global responses of the FOWTs and affecting their internal drivetrain dynamics. Therefore, this paper considered an OC4 DeepCwind semisubmersible FOWT as a representative model to investigate the influence of one broken mooring line on the steady-state and transient responses of the system, as well as on the internal drivetrain responses when the turbine operated normally. A fully coupled wind turbine model was constructed to analyze the global response, and then a decoupled method was applied to analyze the internal drivetrain responses. Furthermore, a set of load cases were considered to perform the simulations. The results showed that accidental fracture of the upwind line significantly affected platform motions, turbine structural loads, and tensions in the remaining lines. However, the effects of upwind line failure on the electrical power output and the internal gearbox dynamics were relatively small due to the almost unaffected nacelle yaw errors and the actions of pitch-torque control.