Yielding strength assessment of a 15 MW offshore floating wind turbine substructure with a fully coupled approach

With the development of large-scale floating offshore wind turbines (FOWTs), the structural integrity of the floating support structures has become increasingly critical for ensuring operational reliability. The objective of the present study is to carry out a comprehensive dynamic response analysis of a 15 MW FOWT substructure under extreme loads specified by the International Electrotechnical Commission (IEC), with particular focus on yield strength evaluation at critical locations. A framework was proposed to simulate the stress time history of the floating support structure under combined wave, wind, and current actions using a fully coupled approach. The proposed method was validated through a representative case study involving the UMaine VolturnUS-S platform substructure. The potential damage locations were identified by screening regions exhibiting the highest Von Mises stress concentrations at the incident wave frequency corresponding to the peak surge acceleration Response Amplitude Operator (RAO). The proposed method was subsequently employed to systematically investigate wave-wind-current interactions and their effects on local stress characteristics in the time domain. Statistical analysis was conducted to quantify the influence of varying wave heights and wind speeds on stress distributions at identified critical locations. Finally, the yielding strength assessment was performed to assess the possibility of structural collapse at the potential damage locations. The findings showed that the total stress response is predominantly governed by Rotor Nacelle Assembly (RNA) excitation and wave actions. As wind speeds increase, the mean value of the total stress response presents explicit offsets from the zero averages. Conversely, while wave height variations minimally affect mean stress values, they substantially amplify stress fluctuation amplitudes.