Estimation of free vibration in the jack-up leg system of a wind turbine installation vessel in marine environmental conditions

This study investigates the free vibration response of the leg system of a jack-up wind turbine installation vessel (WTIV) under representative marine environmental conditions. The structure is analyzed in the raised operational position, where the hull is elevated above the sea surface and supported by the legs during offshore installation. A SACS-based finite element model is employed to assess the dynamic characteristics under three critical scenarios: preload, regular operation, and survival. The results show that the natural periods of the first three vibration modes are longer than 3.0 s in all analyzed cases, satisfying the minimum values recommended by classification societies and reducing the risk of resonance with dominant wave-induced excitations. Yaw is identified as the most sensitive mode to crane orientation and load configuration. The findings highlight the importance of evaluating vibration behaviour, particularly at structurally sensitive regions such as the leg–spudcan joint.