The present invention concerns a foundation for an offshore wind turbine generator and a method of constructing and installing the foundation for a such wind turbine generator which is located preferably at sea depth above 45 meter.

It is previously known to locate wind turbine generators or wind mills offshore at water depth up to 45 meter. Frame work structures of steel such as "jackets" have been utilized as foundation structure. Such foundation structures will normally be constructed onshore, towed out to the location site, submerged and anchored to the bottom, for example by piling. Wind mill including tower structure, generator and rotor blades will then be mounted on the foundation structure at the installation site offshore.

Such jackets will, as mentioned above, be transported to the installation site and located there after which the wind mill tower etc. is mounted to jacket offshore. This method will require comprehensive work offshore, which is a cost increasing factor since work offshore is substantially more costly than work onshore or at shore. Another alternative is that the jacket and the wind mill structure are put together onshore, barge etc. and transported to the field by means of special installation vessels. However, this will also be a very costly operation.

When establishing a foundation of wind mills at larger sea depth, preferably above 45 m, considerations must be taken to a combination of a large number of loads which will act on the structure, for example wind, waves, current and earth quake. The natural periods of foundation and wind mills and the fatigue etc. also create large challenges when calculating and designing such structures.

An object of the present invention therefore is to provide a foundation for a wind turbine generator which solves the problem with regard to environmental loads, as stated above, and hence achieves a foundation having high stiffness and favourable dynamic characteristic which gives high effective life of the foundation and reduces fatigue in the mill tower itself. In this connection it should be mentioned that wind mills are sensitive to dynamic and require low structural natural periods which differ from the frequency of the rotor blades itself in order to avoid fatigue of the steel tower in addition to the generator and the rotor blades itself. The foundation should "survive" three-four mills in its effective lifetime and therefore having an additional value in addition to the mill itself since wind is a resource which will live in "all future". Therefore, the foundation shall be used again for several mills.

Another object is that the foundation shall be built and assembled with wind mill tower, generator and rotor in protected waters close to shore and shall be towed in a stable way to the installation site completely assembled. Ballast of solid material shall also be supplied the structure preferably prior to tow out to the installation site, where only water ballast is required for submerging the structure at the installation site. Offshore works are substantially more costly that onshore works or close to shore, such that costly work offshore shall be reduced to a minimum.

A third object is that the foundation shall be adapted to the soil conditions at the installation site.

A forth object is that the foundation shall have a relatively low weight, shall be simple to construct, could be adapted to the draft at the construction site and be suited for mass production of several wind mills for wind mill parks offshore and be cost efficient in relation to existing solutions.

A fifth object is that the foundation with or with out wind tower could be removed and towed to shore by deballasting the foundation.

The objects of the present invention are achieved by a foundation for an offshore wind turbine generator, preferably for sea depth above 45 metre, characterized in that the foundation in the construction, transport and installation phases is a self floating and stabile construction of reinforced concrete comprising:

- a foundation structure,

- a hollow, substantially conical tower structure arranged on the foundation structure and having a lower external diameter less than the smallest transversal dimension of the foundation structure, since the diameter of the tower structure gradually decreases upwards toward the upper portion of the tower structure.

Preferred embodiments of the foundation are further specified in the claims 2 to 13.

Further, the object of the invention is achieved by a method of constructing and installing a foundation for a wind turbine generator offshore, substantially for sea depth above 45 meter, characterized in that:

- a foundation structure of reinforced concrete in addition to a lower part of a substantially conical hollow tower structure of reinforced concrete, having a lower external diameter less than the smallest transversal dimension of the foundation structure is cast in a dry dock or on a barge, possibly onshore and is skidded over to a barge or out on a skid track running out into the sea,

- the foundation structure including a lower part of the tower structure is made floating by filling water in the dry dock, by lowering the barge or by skidding out into sea,

- the substantially conical tower structure including a substantially cylindrical upper portion, is slip casted in floating condition by ballasting with floating ballast and/or ballast of solid material,

- an interconnecting device for wind turbine generator is arranged on the upper portion of the tower structure,

- the wind turbine generator is mounted on the interconnecting device,

- the structure including the foundation and the tower structure is ballasted, towed to the installation site, lowered down to the sea bottom by supply of ballast and connected to the sea bottom.

The invention is further described with reference to the attached drawings wherein: figure 1 illustrates a foundation for wind mills including a foundation structure and a tower structure according to a first embodiment of the invention, figure 2 illustrates the foundation of figure 1 , but now with a partly cut off foundation structure figure 3 illustrates a section of the foundation structure having a mounted mill tower, figure 4 illustrates a second embodiment of a foundation according to the invention, where the foundation structure itself now has another form than in the embodiment described in figure 1 , 2 and 3, and figure 5 illustrates the foundation of figure 4 , but now in a partly cut off embodiment.

With reference to the figures, particularly figure 1 , 2 and 3, a foundation 1 for an offshore wind turbine generator consists of a foundation structure 5 and a hollow, substantially conical tower structure 15. The diameter of the tower structure 15 decreases gradually upwards toward the wave zone of the foundation 1 and the tower structure 15 is across this area extended with a substantially cylindrical upward portion 20. The tower structure 15 is in its upper portion 22 provided with interconnecting devices for a mill tower 25 including generator and rotor blades. The foundation structure 5 may have a circular, square or any other shape. The foundation structure 5 further comprises a bottom plate 6 having a centrically arranged cylindrical tower structure foundation 8 with exterior radial walls 9 extending from the tower structure foundation and to the outer edge of the bottom plate. In the shown embodiment, see particularly figure 2, the bottom plate has an angular shape with inner diameter corresponding to the tower structure foundation 8 and with an outer diameter corresponding the outer diameter of the foundation structure. The cylindrical tower structure foundation 8 is further provided with an internal downwardly directed dome structure 12 extending from the upper area of the tower structure foundation. The bottom plate 6 is along its outer edge provided with vertical upwardly directed walls 10 with substantially the same height as the tower structure foundation 8 and the exterior radial walls 9, whereby a number of upward open box structures 11 are formed. The box structures 11 are formed such that the room for ballast in the form of sand or alike is provided. The form of the box structure is adapted such that solid ballast can be filled in a simple way from above into the open chambers. The foundation structure 5 is further formed such that it can float in very shallow water which results in that it is possible to construct the foundation structure in a shallow dry dock or on a submergible barge. Ballast of solid material is preferably filled prior to towing to the field in order to avoid heavy expenses related to offshore work.

The foundation structure 5 is adapted to the soil conditions at the site. When the soil conditions are solid the foundation structure 5 may consist of a flat bottom plate, possibly with ribs in order to direct the forces to the strongest areas. When the soil conditions are soft so-called skirts 7 will be utilized which penetrate into the earth to earth masses which are solid enough to withstand influence of waves, current and wind.

The foundation further consists of, as mentioned above, a conical tower structure 15 having a cylindrical upper portion 20. The tower structure is as slender as possible in order to minimize wave loads, but the tower structure shall nevertheless provide sufficient strength and stiffness during the operation in addition to buoyancy and stability during towing to the field. The foundation 1 is cast in reinforced concrete in order to achieve good stiffness qualities, good fatigue qualities and long life time. In the upper portion 22 of the tower structure connecting devices are provided for a mill tower 25 which will be in steel. The interconnecting devices can include possibilities for adjusting the verticality of the mill tower 25.

Figures 4 and 5 show a second embodiment of the foundation 1 according to the invention. This embodiment differs substantially from the first in that it is provided with a complete bottom plate 6 under the whole structure and not an annular bottom plate 6 according to the first embodiment. The previously discussed dome structure 12 extending from the upper portion of the tower structure foundation is here deleted.

The tower structure foundation 8 is here reinforced with the exterior and interior radial diagonal struts 13 extending from the tower structure foundation and down towards the bottom plate. This second embodiment of the foundation 1 will thus not be provided with the open box structures 11 as in the first embodiment. The second embodiment of the foundation will thus not be provided with ballast external in the open box structures 11 as in the first embodiment of the foundation. Ballast of solid material will in this embodiment of the foundation 1 be filled internally in the tower structure foundation and possibly in the lower part of the conical tower structure 15.

Constructing the foundation 1 will be carried out two main phases. Firstly, the foundation structure 5 of reinforced concrete in addition to the lower part of the substantially conical shaped hollow tower structure 15 of reinforced concrete are casted in a dry dock or on a barge, possibly onshore and skidded onto a barge or on to a skid track running out in the sea after which the foundation structure 5 including lower part of the tower structure 15 is made floating by filling water into the dry dock, by submerging the barge or by skidding out into the sea after which the structure is anchored at deeper water. In the second main phase the substantially conical shaped tower structure 15 including a substantially cylindrical upper portion 20 is slip casted in floating condition by ballasting with lliquid ballast and/or ballast of solid material. Further an interconnecting device is provided for the mill tower 25 on the upper portion 22 of the tower structure after which the mill tower 25 is mounted on the interconnecting device and one or several wind mills is (are) mounted to the mill tower. The structure including the foundation 1 and the tower structure 15 in addition to the wind turbine generator is ballasted to required draught, towed to the installation site, lowered to the sea bottom by supplying ballast (sea water) and connected to the sea bottom.