Time Domain Ultimate Buckling Limit State Assessment of a 15MW FOWT Substructure Using FLOWStress

Abstract Some codes such as OpenFAST, OrcaFlex, and BLADED that have been used popularly for integrated load analyses (ILAs) of floating offshore wind turbines (FOWTs) solve the equation of motions at the center of mass, hence substructure should be assumed as a rigid body and structural stresses could not be captured from the ILAs. To address this issue, this study utilized a software FLOWStress that has been developed by present authors and capable of producing quasi-dynamic stress history at each shell element by recycling the ILA results with stress amplitude operators (RAOs). The objective of this study is to perform a buckling strength assessment in time domain for a 15 MW capacity FOWT using FLOWStress. For buckling evaluation, we developed the FLOWStress buckling module, which enables the creation of panels in a finite element model and the visualization of buckling evaluation results for each panel. The finite element analysis (FEA) model for the substructure was generated with shell elements. Because the ILAs were conducted for the design load case 6.2 (DLC 6.2) which corresponds to the ultimate limit state case, FEAs were to obtain stress RAOs. With combination of the ILA results and stress RAOs, FLOWStress generated stress history at each element. The buckling strength assessment was in accordance with DNV-RP-C201. Biaxial and shear stress components were used to assess panel buckling strength that were subjected to biaxial in-plane loads and shear loads. The buckling strengths and buckling utilization factor the time domain for the substructure of a 15MW FOWT are included.