Analysis of lifting operation of a monopile for an offshore wind turbine considering vessel shielding effects

• The lowering operation of a monopile was studied in the time-domain.
• A method to consider shielding effects in the time-domain analysis was proposed.
• A great reduction of extreme responses was obtained considering shielding effects.
• The responses depended greatly on the wave direction and the wave length.
• The responses using a floating vessel and a jack-up vessel were compared.

This study addresses numerical simulations of the lifting operation of a monopile for an offshore wind turbine with a focus on the lowering process. A numerical model of the coupled system of the monopile and vessel is established. The disturbed wave field near the vessel is investigated and observed to be affected by the diffraction and radiation of the vessel. The shielding effects of the vessel during the continuous lowering operation are accounted for in this study by developing an external Dynamic Link Library (DLL) that interacts with SIMO program in the time-domain simulations. The DLL is implemented by interpolating fluid kinematics between pre-defined wave points near the vessel. Based on the time-domain simulations, the critical responses, such as the motions of the monopile, the tensions in the lift wire and the contact forces in the gripper device in the disturbed wave fields, are compared with those in incident wave conditions. The results indicate that a great reduction in these extreme responses can be achieved when the shielding effects are considered. The sensitivity study of the responses in different wave directions is performed. The results indicate different behaviours with different wave directions and with short or long waves. A comparison of the responses when using a floating vessel and a jack-up vessel is also studied and can be used to support the choice of installation vessel type.