Fatigue assessment of wet-towed suction-pile jacket foundations for deep-water offshore wind turbines

A fatigue damage per kilometer index is proposed to quantify transportation-induced fatigue accumulation and to directly assess fatigue-controlled transportability. This study addresses the largely overlooked problem of transportation-stage fatigue in wet-towed suction-pile jacket foundations for deep-water offshore wind turbines (OWTs). Although wet-towing can reduce dependence on heavy-lift vessels and complex offshore installation, it exposes the semi-submerged foundation–structure system to long-duration cyclic hydrodynamic loading before installation. A comprehensive fatigue assessment framework is developed by integrating hydrodynamic load modeling, time-domain finite element analysis, rainflow cycle counting, and S–N fatigue evaluation. A total of six OWT models, comprising 15-MW and 22-MW turbines corresponding to 50-, 65-, and 80-m site depths, are analyzed under different towing speeds, wind–wave directions, and environmental conditions. The results show that hydrodynamic loading dominates the towing response, with maximum fatigue damage consistently occurring at the brace–chord joint near the towing connection. The result clearly reveals the strong influence of wind–wave direction and the nonlinear effect of tugboat speed on fatigue accumulation. Under typical towing conditions, the maximum cumulative fatigue damage over 200 km is only 0.79%. These results strongly demonstrate that wet-towing is structurally feasible for deep-water jacket-supported OWTs when transportation-stage fatigue is considered.