This paper addresses numerical simulations of the lifting operation of an offshore wind turbine monopile foundation considering both shielding effects from the vessel and the spreading of the waves. A numerical model of the coupled monopile-vessel system 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 lifting operation are accounted for in this study by interpolating fluid kinematics between pre-defined wave points near the vessel using SIMO software and an external Dynamic Link Library (DLL). The effects of short-crested waves on the wave field and on responses of the system are investigated by implementing the directional spreading function in the wave spectrum. Based on the time-domain simulations, the critical responses of the lifting system in various conditions are studied. The results indicate that the effects of the wave spreading are considerable in both incident and disturbed waves. The shielding effects are less significant in short-crested waves than in long-crested waves.
Monopile (MP) substructures are the most commonly used foundations for offshore wind farms in water depths up to 40 meters. It has been estimated that more than 75% of all installations are founded on monopiles by the end of 2013 (EWEA, 2014). Monopiles can be transported to site by the installation vessel or a feeder vessel, they can be barged to the site or can be capped and wet towed (Kaiser and Snyder, 2013). An offshore crane is often employed to upend the monopile to a vertical position and lower it down through the wave zone to the seabed. During the lifting operation, the monopile and the installation vessel are coupled through the lift wire and a gripper device which limits the horizontal motions of the monopile during the lowering. The monopile is lowered at a position which is very close to the hull of the crane vessel, so the wave forces on the monopile are affected by the presence of the vessel. Furthermore, since the lifting operation is commonly performed at a relative low sea states, the waves may spread in different directions and affect the motions of the vessel as well as the wave forces on the monopile. Therefore, it is of great interest to evaluate the effects of the wave spreading as well as the shielding effects from the vessel on the behavior of the lifting system.