Nonlinear wave effects on the extreme stress of Semi-submersible FOWT using equivalent design wave approach

The nonlinear wave effects are critical for the extreme stress prediction of floating offshore wind turbine (FOWT) under the harsh environment. Typically, the free-surface induced nonlinear Froude–Krylov and hydrostatic loads, and the viscosity induced drag force are the main contributors to the nonlinear wave effects of FOWT. However, the extreme stress prediction in the time domain involves time-consuming hydro-structure analysis, making it nearly impossible to conduct simulations under a large amount of environmental conditions. Equivalent design wave (EDW) is an efficient approach by using a few regular wave time-domain simulations to predict extreme stress, which is less explored for FOWT. To investigate the mentioned nonlinear wave effects, the extreme stress is predicted based on the OC5 Semi-submersible FOWT using EDW approach, and the hydro-structure interaction of hull structures is analyzed using boundary element and finite element methods. The stress superposition approach is proposed to consider the aerodynamic effects in the linear analysis. The nonlinear wave effects on the global response and local stress are discussed in details. The results show that the nonlinear wave effects significantly affects the motion response and bending moment due to the free-surface corrections. The nonlinear Morison loads have limited influence on the global response while they significantly amplifies the local extreme stress in braces and intersection regions. Moreover, the aerodynamic loads have a pronounced influence on the extreme stress at the tower-hull intersection. The EDW approach provides a practical way to take into account the nonlinear wave effects and improves the efficiency of structural optimization.