Structural analysis of a 15 MW floating offshore wind turbine platform based on a novel fully-coupled framework

Abstract

The structural integrity of platform is a critical factor in ensuring the safety of floating offshore wind turbines (FOWTs). In order to address the limitations of existing research in considering the nonlinear coupling effects between wind, wave and current loadings, this study has proposed a novel fully-coupled framework (FAM) for structural analysis of FOWT platforms by integrating OpenFAST with ANSYS. The aero-hydro-servo-elastic analysis of the FOWT is first performed using OpenF2A that incorporates OpenFAST into ANSYS-AQWA. The loads acting on the tower-base, fairleads, and platform wet-surface are obtained and then imported to ANSYS-Mechanical for carrying out the structural analysis. The 15 MW VolturnUS-S platform is re-designed to detailing the compartment and internal structure for the case study. The structural dynamics of the platform under extreme conditions are examined to investigate the influence of mooring breakage and wind-wave misalignment. It is found that mooring breakage causes a 57.05 % increase in maximum stress near the fairlead. The maximum stress over the platform is significantly increased, which is consistent with the trend of platform motion. The misalignment between wind and wave loadings will enhance the stress in platform hotspots, including the connections between the offset-column and pontoon, heave plates and internal ribs. Compared to the results of aligned condition, the maximum stress over the platform is increased by 25.39 % and 29.84 %, respectively, for 90° and 180° wind-wave misalignment scenarios. The buckling analysis of the platform indicates that the proposed structure design has sufficiently high buckling factors to maintain the platform integrity. The relevant analysis has demonstrated that the FAM developed in this study can be used for structural analysis of steel-made platforms of FOWT.