Integrated fatigue assessment framework for jacket-supported bottom-fixed offshore wind turbines using updated OpenFAST SubDyn module and UnityDyn-FLS

초록

Direct joint-level fatigue assessment from fully-coupled OpenFAST simulations remains limited for jacketsupported bottom-fixed offshore wind turbines. This study develops an integrated framework for fatigue limit state (FLS) assessment by combining OpenFAST updates with the newly developed UnityDyn-FLS module. The original SubDyn module was updated to enable member-dependent discretization based on nominal element length and to expand the member output capacity to 299 members. The mathematical compatibility of the update with finite element assembly and Craig-Bampton reduction was clarified. UnityDyn-FLS was developed to automatically construct coplanar joint groups, extract real-time member forces, evaluate joint classification ratios and stress concentration factors (SCFs), and calculate hotspot stresses, cumulative fatigue damage, and fatigue life. The framework was applied to a 15 MW jacket-supported bottom-fixed offshore wind turbine (BOWT) under DLC1.2 conditions. Verification studies confirmed accurate reproduction of prescribed joint classification ratios, SCFs, and hotspot stresses. Convergence analyses showed that the updated discretization strategy improved structural response accuracy with acceptable computational cost, while producing noticeable differences in hotspot stresses and fatigue damage predictions compared with the original SubDyn implementation. The proposed framework enables direct joint-level FLS assessment from fully-coupled OpenFAST simulations and provides a practical tool for fatigue-driven design of BOWT jacket substructures.

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