Offshore wind turbine installation

An offshore wind turbine installation vessel for installation of an offshore wind turbine, the wind turbine comprising:

• a vertical mast to be fitted with its lower end onto a foundation, and with 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 comprises:
• a wind turbine hoisting device having one or more wind turbine suspension elements attached to and extending downwardly from the crane structure and
• a wind turbine engagement device supported by said one or more suspension elements and adapted to engage with said wind turbine, said hoisting device being adapted to support and
• to raise and lower at least the mast of the wind turbine while in vertical orientation;
• wherein the vessel has single rigid crane structure, the wind turbine hoisting device being arranged to hold the wind turbine, suspended along a first vertical side of the crane structure, a foundation hoisting device also being provided that allows to support as well as raise and lower in a controllable manner the foundation suspended there from along an opposed second vertical side of the crane structure.

An offshore wind turbine installation tool for installation of an offshore wind turbine on a foundation that is installed on the seabed prior to the installation of the wind turbine on the foundation,

• the wind turbine comprising a vertical mast to be fitted with its lower end onto the foundation,
• wherein the tool is adapted to be fastened to the foundation prior to mating the mast of the wind turbine with the foundation,
• wherein the mast is provided with a circumferential flange on its exterior, said flange being adapted to be engaged by the wind turbine installation tool, the tool providing a temporary fastening of the mast to the foundation allowing to effect permanent fastening of the mast to the foundation,
• wherein the tool is adapted to allow for rotation of the mast about its longitudinal axis while the mast is temporary fastened to the foundation with the tool, and
• wherein the tool comprises a set of lower rollers engaging the flange of the mast from below and a set of upper rollers engaging said the flange from above.