Directional Response and Strength Assessment of Truss-Type Legs for a Wind Turbine Installation Vessel Under Crane Angles and Wave Loads

With the rapid development of offshore wind energy toward deep water, the structural safety of truss-type legs for jack-up wind turbine installation vessels (WTIVs) under complex operational and environmental loads has become a key concern. This study focuses on the directional coupling effect between crane slewing angles and wave directions, which has rarely been systematically investigated in previous research. A finite element model of a 1500 t-class WTIV truss leg is established using SESAM, and its reliability is verified by mesh convergence analysis and literature comparison. The influences of crane slewing angle, wave direction, and their coupling on structural displacement and stress are analyzed quantitatively, and strength evaluation is carried out under typical working conditions in accordance with classification society rules. The results show that the structural response presents significant directional dependence and stiffness anisotropy. The peak displacement and stress occur at a crane slewing angle of 270°, with the maximum displacement approximately 33% higher than the minimum value. Obvious response amplification is observed when the crane slewing angle and wave direction are aligned within 225–270°, which constitutes the most unfavorable loading combination. The strength assessment demonstrates that all conditions meet the specification requirements, and the survival condition is the most critical, with a maximum stress of 289.66 MPa and a maximum displacement of 338.6 mm. This study reveals the coupling mechanism between operational loads and environmental loads and identifies the critical dangerous angle sector. The research findings can provide reasonable references for offshore lifting operation management and operational planning of marine truss leg structures.