Structural Weak-Point Identification and Strength Assessment of a 15 MW Semi-Submersible Floating Offshore Wind Turbine Under Representative Stages of Typhoon Evolution

Abstract

Conventional typhoon-resistant assessments of floating offshore wind turbines commonly adopt a single stationary extreme environmental condition, although wind and wave characteristics vary substantially among different stages of typhoon evolution. This study investigates the structural weak points and strength performance of the IEA 15 MW reference wind turbine supported by the VolturnUS-S semi-submersible platform under five representative stages of Typhoon Lekima: the front outer vortex, front eyewall, eye center, back eyewall, and back outer vortex regions. Stage-dependent wind profiles, turbulence spectra, and wave spectra were constructed and applied to a fully coupled aero-hydro-servo-elastic model. The resulting global motions and environmental loads were transferred to a shell finite-element model for time-domain load reconstruction and local stress assessment. The overall stress distribution pattern remained broadly similar among the analyzed conditions, whereas the stress levels and stress ranges varied considerably. Stress concentrations consistently occurred at the column–pontoon connections. Stress Concentration Zones 1 and 3 satisfied the adopted allowable stress of 511 MPa under all load cases, with Zone 1 reaching 493.2 MPa under the back eyewall condition. In contrast, Zone 2 exceeded the allowable stress under the front outer vortex, eye center, back eyewall, and back outer vortex conditions. Its maximum von Mises stress reached 591.0 MPa under the front outer vortex condition, corresponding to an exceedance of 15.7%. The eye center condition generally produced relatively low and spatially uniform stresses at the selected monitoring points, whereas the back eyewall condition generated substantially increased stress ranges at several locations, indicating more severe cyclic loading. These results demonstrate that the governing structural strength condition does not necessarily coincide with the maximum wind-speed stage and highlight the need for stage-dependent typhoon assessment and targeted reinforcement of side column–pontoon connections.