Analyzing the structural integrity of retrofitted jacket substructures for an offshore wind turbine under nonlinear dynamic loading

• Configurations for retrofitting jackets to 15 MW turbines were compared under nonlinear dynamic marine loading. • Configuration 2 reduced peak bracing stresses by over 50%, improving integrity and fatigue resistance. • Translational and rotational displacements dropped by more than 50% in Configuration 2. • Configuration 2 showed lower directional sensitivity and steadier response across 0°–90° phase angles. • Stress triaxiality stayed within a favorable fatigue range, with smoother cycles in Configuration 2. The worldwide impetus for sustainable energy has catalyzed an expansion in offshore wind farm installations; however, the adaptation of 15 MW turbines to existing jacket foundations introduces novel challenges due to the intricate, non-linear marine load conditions. In this research, comprehensive finite-element analyses were performed to assess two retrofitting strategies under the influence of combined wind, wave, and current forces, evaluated across seven distinct wind-wave orientations and three actual geographic locations (Gunsan, Oido, Buan), in addition to one controlled synthetic environment. The findings reveal that Configuration 2, which integrates optimized bracing and tailored member dimensions, markedly decreases peak stress concentrations by over 50 %, mitigates strain hotspots, and reduces lateral and rotational displacements by nearly 50 % when compared to Configuration 1. These enhancements correspond directly to improved fatigue lifespan and decreased sensitivity to variations in load directions, which are essential for ensuring reliable, low-maintenance operation over extended periods in marine environments. By elucidating how targeted design alterations can modernize original jacket systems for next-generation turbines, this study addresses a critical knowledge deficit and provides actionable insights for engineers involved in the retrofitting of foundations to accommodate increasingly larger offshore wind projects.