There is described a method for anchoring of a floating wind turbine, more particularly where the method comprises the steps :

a coupling unit provided with multiple tension moorings attached to a seabed is arranged submerged in a mass of water at a prescribed first depth under a water surface;

a floating turbine unit provided with a buoyancy element having a draught equal to a second depth, wherein the first depth is greater than the second depth, is displaced in over the coupling unit;

the wind turbine unit being ballasted and displaced vertically until the first depth is equal to the second depth and the wind turbine unit is supported by the coupling unit.

Also described is an anchoring system for use in applying the method.

A wind turbine positioned offshore is placed on a foundation arranged on the sea bed, or, at least for wind turbines hav- ing a horizontal axis, having a tower arranged floating in the mass of water, for example by being partly submerged in the mass of water and having an end portion towering above the mass of water, or that the wind turbine is an integrated, extending part of a floating device such as a floating plat- form or the like. The floating wind turbine is anchored directly or indirectly in the seabed.

There is a long series of ways to anchor floating wind turbines . The anchoring provides for the wind turbine maintain- ing its position, i.e. is prevented from drifting off, and it as a rule provides for an increase in stability, i.e. heave and heel movements are dampened. Anchoring a floating installation is time-consuming, costly and complicated, and in ocean areas where wind turbines are installed, there is natu- rally often a lot of wind and waves, leading to installation operations taking a long time and are often difficult to accomplish due to lack of sufficiently long periods of suitable working conditions.

US 7476074 B2 describes a floating foundation submerged under a sea surface and anchored to the seabed by cables or chains. On the foundation being below the wave movement depth is raised a structure, particularly a wind turbine tower.

US 7293960 B2 describes a floating power generator assembly comprising at least three floating units anchored to the sea- bed below the mass of water.

US 2009/0072544 Al describes a wind turbine having a vertical axis mounted on a platform. The platform may be made up of assembled module elements. The platform is semi-submerged with the turbine tower extending up over a water surface and with a counterweight extending downward from the platform.

Alternatively the platform may float on the water surface and have several arms extending outward from the platform to increase the extent of the platform. For anchoring the turbine an anchor system being connected with the seabed may be used allowing the wind turbine to be adjusted actively or pas- sively to adapt to changes in water level due to tide or swell .

EP 1666722 Al describes an attachment system for floating wind turbines, wherein to the footing of a shank is arranged a submerged structure being arranged to be able to provide buoyancy to the wind turbine, as said submerged structure is designed to function as a guide element for a cable connecting the footing with at least a central anchor element. Several buoys are arranged in the vicinity of the wind turbine, and each buoy is connected to independent anchor elements via chains and to the footing by means of cables, and the buoys are arranged about the shank a certain distance away.

GB 2454585 A describes a method for transport of a building structure, such as a wind turbine, in a mass of water. At least one float is connected with the structure in such a way that the structure is floating stably in the water as the float encircles the structure, and a bottom portion of the structure extends downwards from the lower edge of the float. Two barges connected to each other to hold on to the struc- ture may form the float.

The object of the invention is to remedy or reduce at least one of the prior art drawbacks .

The object is achieved by the features stated in the below description and in the following claims. The invention provides a foundation for a floatable wind turbine unit, as the foundation is provided as a coupling unit anchored to a seabed by several tension moorings and arranged submerged in a water surface. The wind turbine unit is provided with buoyancy elements and may be displaced in the mass of water having a draught less than the distance of the coupling unit from the water surface. Ballasting may lower the wind turbine to supporting abutment against a contact surface on the coupling unit. The wind turbine unit and the coupling unit are preferably locked to each other by means of coupling elements detachably meshing with each other. The buoyancy of the connected unit is balanced to create tension in the moorings during all operating conditions and to give desired stability to the unit.

By the connection between the wind turbine unit and the coupling unit being reversible, the wind turbine may be brought to a port or other waters for maintenance.

In a first aspect the invention relates more specifically to a method for anchoring of a floating wind turbine, characterised in that the method comprises the steps:

a coupling unit provided with multiple tension moorings attached to a seabed is arranged submerged in a mass of water at a prescribed first depth under a water surface;

a floating wind turbine unit provided with a buoyancy body having a draught equal to a second depth, wherein the first depth is larger than the second depth, is displaced in over the coupling unit;

the wind turbine unit being ballasted and displaced vertically until the first depth is equal to the second depth and the wind turbine unit is supported by the coupling unit.

The coupling unit may be brought to the prescribed first depth by ballasting and/or by being pulled by means of one or more anchor winches connected to the tension moorings.

The wind turbine unit and the coupling unit may be joined together by means of coupling elements.

The buoyancy of the wind turbine unit and the coupling unit may subsequent to connection be adjusted to provide a prescribed tension in the tension moorings. In a second aspect the invention relates more specifically to an anchoring system for a floating wind turbine, characterised in that

a coupling unit is provided with several tension moor- ings arranged to be able to be made fast to a seabed, as the coupling unit is provided with means arranged to be able to arrange the coupling unit submerged in a mass of water in a prescribed first depth below a water surface;

a floating wind turbine unit is provided with one or more buoyancy elements and has a draught equal to a second depth, where the first depth is greater than the second depth;

the wind turbine unit is arranged to be able to be ballasted and be displaced vertically until the first depth is equal to the second depth for support of the wind turbine unit on the coupling unit.

The wind turbine unit and the coupling unit may be provided with means arranged for detachably being able to make the wind turbine unit fast to the coupling unit. The wind turbine unit may be provided with means arranged for ballasting the wind turbine unit.

In a third aspect the invention relates more specifically to use of a coupling unit anchored to a seabed by means of tension moorings and arranged submerged in a mass of water at a prescribed first depth below a water surface, as a foundation for a floatable wind turbine unit.

In the following is described an example of a preferred embodiment illustrated in the accompanying drawings, wherein:

Fig. 1 shows a side view a principle sketch of a device according to the invention, as a floating wind tur- bine is arranged near a submerged, floating foundation anchored by tension moorings to a seabed; and

Fig. 2 shows a side view of the floating wind turbine submerged and attached to the submerged, floating foundation.

In the figures the reference numeral 1 indicates a water mass delimited by a seabed 11 and a water surface 12.

A foundation 2 for a wind turbine is arranged floating in the mass of water 1 as a coupling unit 21 is arranged submerged in the mass of water 1 and anchored to the seabed 11 by means of several tension moorings 22 each connected to an anchor 23 of a per se known type. Each tension mooring 22 is tied to an anchor winch 221 in the coupling unit 21.

The coupling unit 21 is provided with a first ballasting de- vice 213, shown schematically here, typically in the form of a ballast tank having one or more connected pumps and lines (not shown) connecting the ballast tank to the surrounding mass of water 1.

The coupling unit 21 is provided with an upward facing con- tact surface 211 and also several first coupling elements 212.

A wind turbine unit 3 is provided with a buoyancy element 31 comprising a downward facing contact surface 311 arranged to be able to abut the contact surface 211 of the coupling unit 21, several second coupling elements 312 arranged to be able to grip the corresponding first coupling elements 212 of the coupling unit 21, and also a second ballasting device 313, shown schematically here, typically in the form of a ballast tank having one or more connected pumps and conduits (not shown) connecting the ballast tank to the surrounding mass of water 1.

From the buoyancy element 31 extends upwards a tower 32 carrying a per se known wind turbine 33. The wind turbine 33 comprises an engine housing 331, a rotor shaft 332 and a multiple bladed rotor 333. The engine housing 331 houses a generator (not shown) , which via leads may be connected to a grid for electric power (not shown) .

The foundation 2 is preinstalled in the mass of water 1 in a for the purpose suitable location with the coupling unit 21 at a desired depth D ₁ . The installation may comprise connecting up to the electric power grid (not shown) , and the coupling unit 21 and the wind turbine unit 3 may be provided with for the purpose suitable connectors (not shown) to con- nect the engine housing 331 with the grid.

The wind turbine unit 3 is displaced floating in the mass of water 1, for example by means of one or two tugs (not shown), until the wind turbine unit 3 is positioned directly above the coupling unit 21. The wind turbine unit 3 is then lowered by ballasting to supportive abutment against the coupling unit 21. The wind turbine unit 3 is fastened to the coupling unit 21 by means of the coupling elements 212, 312. The buoyancy of the wind turbine unit 3 and the coupling unit 21 is subsequently adjusted by means of the ballasting devices 213, 313 to achieve the desired tension in the tension moorings 22 and the desired weight distribution and stability.

If the wind turbine unit 3 requires maintenance, which may not be carried out offshore, it may be disconnected from the coupling unit 21 by reversing the above described process and be towed to a protected port or protected waters, where the maintenance may be carried out more easily. It is self-evident that even though what is described here is a wind turbine unit 3 having a horizontal rotor shaft 332, the principle of the invention still applies for wind turbines having a vertical rotary axis.