Wind turbines for converting wind energy to electrical power have been known and applied for many years but have found a dramatically increased application as an alternative energy source during the last couple of decades. It has become common to place wind turbines together in large groups of turbines often counting hundreds of wind turbines within a restricted area. Such large collections of wind turbines can provide an environmentally less desirable solution both from an aesthetic point of view and also due to the inevitable noise problems they cause. Furthermore, the positioning of wind turbines on land may not always be an optimal placement, as it is preferable that the blades of the wind turbine be located in a laminate flow of air which is not always obtained on land due to for instance the presence of hills, woods, buildings, etc. It has hence become popular to locate groups of wind turbines offshore, not too far from the coast at locations where water depths allow the wind turbines to be fixedly attached to a foundation provided at the bottom of the sea. Over water, the flow of air is not disturbed by the presence of various obstacles as mentioned above and furthermore, such placements may be advantageous from an environmental point of view.
Due to the large dimensions of present-day wind turbines—dimensions which furthermore tend to increase due to the relation between the diameter of the rotor and the maximum electrical power which the wind turbine can provide—it is vitally important that the wind turbine be provided with a stable foundation. This is not in principle a problem for wind turbines located on the ground but becomes a problem in connection with offshore wind turbines. One prior art solution is to mount the tower of the wind turbine on a suitable construction of pillows and grids, this construction being fixed to a firm foundation on the bottom of the sea, but this solution is expensive and primarily applicable at relatively shallow waters, i.e. up to depths of around 25 meters.
For the application in connection with offshore wind turbines positioned at larger water depths, floating foundations have been described in various prior art documents. Thus, for instance DE 100 34 847 A1 discloses a floating foundation for instance for offshore wind turbines designed for water depths of up to 100 meters. This floating foundation comprises a pyramidal buoyancy body (or rather structure) with a generally quadratic base, the buoyancy structure being formed by a number of cylindrical bodies or tubes. The tower of the wind turbine is placed centrally on this buoyancy structure. The buoyancy structure is maintained on site by means of four tensioned tethers attached to each of the four corners of the quadratic base. The tower itself penetrates the surface of the sea and extends to the base of the buoyancy structure.
A floating offshore wind power installation is disclosed in WO 01/73292 A1. The installation comprises a raft consisting of two cylindrical carrying pontoons acting as buoyancy bodies placed in parallel relationship to each other for carrying a tower provided with a wind turbine at its upper end. The tower extends normally to the plane of the cylindrical pontoons and in order to attain sufficient stability of the connection between the tower and the pontoons, a stiffening wire extends between one end of the pontoons and a point in the vicinity of the upper end of the tower. However, the wires between the buoyancy bodies and the top portion of the tower can not be applied in systems where the nacelle must be relatable relative to the tower, which is necessary, when the buoyancy body or -bodies are fixed to the bottom of the sea in a manner that prevents rotation of the buoyancy bodies about the longitudinal axis through the tower. Hence, the solution disclosed in this document is not applicable for a wind turbine according to the present invention. Such wire stiffeners would at any rate not provide a solution of satisfactory stability in systems with high towers and hence long wires.