Time-domain structural integrity assessment of the jacket substructure of a bottom-fixed offshore wind turbine under ULS design load cases

초록

This study presents a comprehensive time-domain structural integrity assessment of the jacket substructure supporting a bottom-fixed offshore wind turbine (BOWT) under ultimate limit state (ULS) design conditions. To achieve fully coupled aero-hydro-servo-structural interaction, the open-source OpenFAST framework was utilized in conjunction with UnityDyn, a custom-developed evaluation module that computes stress ratios according to API RP-2A standards. The IEA 15 MW reference turbine and a representative four-legged jacket model were employed as the case study. The jacket was modeled in Abaqus for stiffness extraction and translated into OpenFAST-compatible SubDyn and HydroDyn input formats. Thirteen stress ratio criteria were evaluated at every time increment using UnityDyn, capturing the effects of combined axial, bending, shear, torsional, and external pressure loads. Comparative simulations with gravity-only, aerodynamic-only, and hydrodynamic-only load cases revealed that aerodynamic loads significantly influence leg member stress ratios, while hydrodynamic forces dominate brace responses. The highest stress ratios occurred in middle jacket legs. Results confirm the critical role of wind-induced loads on leg members and demonstrate the effectiveness of UnityDyn in capturing transient structural responses in a fully coupled simulation environment.

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