In a first aspect, the present invention relates to a vessel for transporting a wind turbine, and in a second and a third aspect a method of moving a wind turbine, the method of the second aspect relating to moving the turbine from land to an offshore wind farm at its erection, and the method of the third aspect relating to removing a worn-out wind turbine from the offshore wind farm. In a fourth aspect the invention relates to a wind turbine.
The vessel according to the first aspect of the invention can load at least one wind turbine mounted on a base, the wind turbine, after the loading, being placed on the vessel on a loading space in an upright position corresponding to the upright operating position of the wind turbine.
Such a vessel is known from EP 1 101 935 A2, according to which the base, tower, nacelle and rotor blades are assembled on shore into an entire wind turbine to avoid the substantially higher costs involved in assembling the main components of the wind turbine at the offshore wind farm. Since a wind turbine for offshore use has a very considerable size, such as a height of more than 100 m and a mass of e.g. 1000 t in its assembled state, it is obviously no simple task to sail an entire wind turbine out to an offshore position and place it on the seabed. In the said publication, a crane on the vessel is used to lift the entire wind turbine from land onto the vessel, which then sails with this one turbine to the offshore position and lifts the turbine into place on the seabed. A vessel with a crane for lifts of about 1000 t is expensive to operate. One drawback is that this vessel is used to sail each individual wind turbine from land to the offshore farm, particularly if the farm is far from land.
WO 99/43956 describes how to build two wind turbines on a barge and tow them out to an offshore wind farm. On the farm, each turbine is lifted off the barge by means of a crane on another vessel and lowered into place on the farm. Experience shows that it is risky to lift a large structure from a floating vessel by means of a crane on another floating vessel, and it requires really calm weather and small wave heights to carry out a safe lift without damage to the structure. This means that only a limited number of days are available to erect a wind farm. Since crew and vessels are only available in coherent periods, this results in high costs for installing the farm.
Lifting the assembled turbine with base also implies the drawback that the turbine tower must be dimensioned in terms of strength to withstand the lift. Since the loads of a lift act differently from the dynamic loads in operation, the turbine tower has to be of sturdier dimensions to withstand the lift than would otherwise have been required. The heavier dimensions entail more weight and thus larger dynamic loads in operation and higher costs for manufacturing the wind turbine.
WO 02/088475 describes how one or two wind turbines with a base designed as so-called suction piles with an open bottom for being pressed down into the seabed material to create anchoring is/are suspended by wires in a vessel and sailed out to an offshore wind farm where the wind turbine is lowered into place on the seabed. Before sailing, the wind turbine is pulled up against the bottom or side of the vessel. However, the wind turbine is still a self-floating structure with an extremely elevated centre of gravity in relation to the centre of gravity of the vessel. This inevitably causes mutual movement of the structures during sailing, and it is only possible to sail out to the farm when the weather is good and the wave heights small, which limits the number of days available, as mentioned above, and entails high costs.
WO 01/34977 describes a wind turbine which is lifted into the water by a crane, whereupon a vessel grips around the tower, and ballast is removed from the base, which contains a single chamber communicating with the inside of the tower. When sufficient ballast has been removed, the wind turbine floats up to abut the vessel, which can then sail the wind turbine out to a farm where the base is filled with ballast so that the wind turbine sinks to the seabed. Quite apart from the problems of the crane lift of the turbine and sailing with two floating structures abutting each other, the grip around the turbine tower involves a risk of damage to the tower caused by the grip. Even minor scratches are of importance to the durability and strength of the turbine tower.