Patent ID: 8701579

Claim:
An offshore wind turbine installation vessel for installation of an offshore wind turbine, wherein the wind turbine comprising: a vertical mast to be fitted with its lower end onto a foundation, and a nacelle with a hub and blades supported on top of said mast, the vessel comprising: a floating hull; and a crane structure extending upward from said hull, wherein the crane structure is provided with a wind turbine hoisting device having one or more wind turbine suspension elements and a wind turbine engagement device supported by said one or more suspension elements, wherein said wind turbine engagement device is adapted to engage with the mast of said wind turbine, said wind turbine hoisting device being adapted to support and to raise and lower in controllable manner at least the mast of the wind turbine while in vertical orientation; wherein the vessel includes an active horizontal motion device adapted to actively compensate for sea-state induced horizontal displacement of said wind turbine engagement device in at least one horizontal direction, while the vertically oriented mast of the wind turbine is engaged by said wind turbine engagement device, wherein the active horizontal motion device comprises an actuator attached to the vessel.