FI ELD OF THE I NVENTION AND PRIOR ART

The present invention refers to a wind power plant designed to convert wind energy at sea to electrical energy, comprising a plurality of wind turbines, a floatable framework including support elements adapted to support the wind turbines, and an arrangement for connecting the framework to a fundament in the sea.

Such a wind power plant is, for example, known from US 201 1037272. The wind power plant includes a plurality of wind turbines and a floating framework includ ing two essentially vertical support elements and an essentially horizontal support element connected to the vertical support elements. The turbines are attached to the vertical support elements. The wind power plant further comprises an arrangement for connecting the framework to a fundament in the form of a post arranged in the sea, which includes two rods and a plurality of wires arranged between the post and the support elements.

Another example of a floating wind power plant is disclosed in EP 2324244. The plant includes a beam structure arranged to form a frame structure and at least one upright for stabilizing the frame structure position in the body of water. This plant is supposed to be attached to a buoy.

A disadvantage with the prior art wind power plants is that the arrangement for connecting the framework to the fundament are bulky and heavy. Another disadvantage is the risk for obtaining resonance problem. A problem with the prior art wind power plants is that they are not suitable to locate in shallow water due to the fact that the power cable between the plant and the bottom of the sea does not have enough space to move and accordingly the cable will be subjected to large strains.

OBJ ECTS AND SUMMARY OF THE I NVENTION

The object of the present invention is to provide an improved wind power plant suitable for converting wind energy to electri- cal energy at sea, which alleviates the above mentioned problems.

According to the invention , this object is achieved by a wind power plant as defined in claim 1 .

Such a wind power plant is characterized in that the arrangement for connecting the framework to the fundament comprises an elongated beam extending through an opening in the frame work and protruding on both sides of the frame work, a connec- tion unit for connecting one end of the beam to the fundament, and a plurality of wire cables arranged between the beam and the frame work in order to mechanically connect the beam to the frame work. Preferably, the wire cables are made of stain less steel.

The wire arrangement includes at least six wire cables wherein at least three of the wire cables are arranged between one end of the beam and the framework, and at least three of the wire cables are arranged between the opposite end of the beam and the framework. Such an arrangement keeps the beam in a fixed position relative the framework in both horizontal and vertical directions.

The arrangement with the support beam and the wire cables at both ends of the beam provides a flexible connection between the fundament and the framework and reduces the dynamic stress on the framework.

Due to the arrangement with the wires cables, the utilization of material is optimized and accord ingly the weight of the arrangement is reduced compared to the prior art arrangements. The reduced weight of the wind power plant also results in a reduced cost of manufacturing the wind power plant in comparison to prior art in comparison prior art wind power plants. Resonance problems are also reduced due to the characteristics of the wire cables.

According to one embodiment of the invention , the connection between the beam and the frame work is established exclusively by means of said plurality of wire cables.

According to one embodiment of the invention , the elongated beam is arranged extending through said opening in the frame work so that elongated beam is spaced apart from the frame work and so that the elongated beam protrudes on both sides of the frame work.

According to an embodiment of the invention, the wire arrangement includes at least eight wire cables, wherein at lest four of the wire cables are arranged between one end of the beam and the framework and at least four of the wire cables are arranged between the opposite end of the beam and the framework. This embodiment provides a robust arrangement connecting the framework to the fundament. Having eight wire cables distrib- utes the forces better so that stresses and material is reduced .

According to an embodiment of the invention, the support elements form a plane and the beam is arranged angular with respect to the plane. Preferably, the angle between the beam and the plane is within the interval of 80° - 30°. This embodiment enables the beam to be connected to the fundament beneath the surface of the water and close to the bottom of the sea while the turbines and the support elements are upright. It is advantageous to have the connection point between the plant and the fundament close to the seabed since the bend ing moment and stress on the fundament, will be dramatically reduced .

According to an embodiment of the invention , the wire cables are arranged so that the lengths of the wire cables are adjustable to enable adjustment of the angle between the beam and the plane formed by the support elements. The angle needed between the beam and the plan formed by the support elements depends on the depth of the sea at the location of the plant. This embodiment makes it possible to adapt the angle between the beam and the plane formed by the su pport elements to dif- ferent water depths, still getting the end of the beam close to level of the sea bottom while the support elements are vertical . Further, this embodiment offers a better possibility to launch the structure at shipyard by gradually change the ang le to erect the structure in water. This embodiment also offers a possibility to eliminate the rear buoyancy element and to lean the structure forward towards the front buoyancy element.

According to an embodiment of the invention , said arrangement comprises connection elements for connecting said wire cables to the frame work and to the beam , and said connection elements are arranged to enable adjustment of the lengths of the wire cables. By utilizing adjustable connections of the wire cables to the frame work is possible to change the angle between the beam and the frame work.

According to an embodiment of the invention , the two su pport elements are arranged at a distance from each other, and the framework includes a lattice structure connected to the support elements in order to guy the support elements. For example, the two su pport elements are diagonally braced . By utilization of a lattice structure and wire cables, the flexural stress in the sup- port elements are significantly reduced compared to traditional land-based wind power plants. The support elements are for example tu be shaped , which makes them floatable. According to an embodiment of the invention , the elongated beam extends through the open ing so that it is spaced apart from the two su pport elements and from the pontoon unit. Accordingly, there is no direct attachment between the elongated beam and the support elements or the pontoon unit.

According to an embodiment of the invention , the elongated beam is connected to the framework exclusively by means cables. The connection by means cables between the elongated beam and the framework has the advantage of reducing the weight of the wind power plant.

BRI EF DESCRI PTI ON OF TH E DRAWI NGS

The invention will now be explained more closely by the descri ption of different embodiments of the invention and with reference to the appended figures.

Fig . 1 shows a perspective view of a wind power plant according to a first embodiment of the invention .

Fig . 2 shows a side view of the wind power plant shown in figure 1 , when the wind power plant is in a transportation position . Fig . 3 shows a side view of the wind power plant shown in figure 1 , when the wind power plant is connected to a fundament in the sea and ready for operation .

4 shows a rear view of the wind power plant shown in figure 1 . Fig . 5 shows a rear view of a wind power plant according to a second embodiment of the invention .

Fig . 6 shows an example of a connection element for connect- ing wire cables to a frame work and a beam of the wind power plant.

DETAI LED DESCRI PTI ON OF PREFERRED EM BOD I MENTS OF THE I NVENTION

Figures 1 - 4 shows a wind power plant adapted for converting wind energy to electrical energy at sea according to a first embodiment of the invention. The wind power plant includes a plurality of wind turbines 2. In this embodiment the wind power plant includes two wind turbines 2. However, in another embodiment of the invention , the wind power plant may include three or more wind turbines. The wind power plant further includes a floating framework including at least two support elements 3a, 3b adapted to su pport the wind turbines 2. The sup- port elements are elongated . I n this embodiment, the su pport elements are two hollow tubes arranged in parallel and at a distance from each other so that an opening 7 is formed between the su pport elements. The wind turbines 2 are mechanically connected to upper ends of the support elements. The wind power plant is designed so that the support elements 3-b are vertically arranged when the plant is in operation .

The framework further includes an elongated pontoon unit 4 arranged between the support elements 3a-b. The pontoon unit 4 is also tube shaped . The pontoon unit 4 is mechanically connected to lower ends of the su pport elements. I n another embodiment it is possible to have three or more support elements to support three or more wind turbines. For example, the su pport elements 3a-b and the pontoon 4 are made by tu bes. The tubes contribute to a certain floating power and stabilizing in roll direction . The framework further includes a lattice structure, e.g . a diagonal brace 5 connected to the support elements 3a-b in order to guy the support elements. It is advantageous to use a diagonal brace to stabilize the support elements and to reduce the flex- ural stress in the framework. Of course, it is possible to use other type of arrangement to guy the support elements.

The wind power plant further includes an arrangement for connecting the framework to a fundament arranged in the sea in order to keep the wind power plat at a fixed position in the sea. The fundament is, for example, a buoy, a pole/post or submarine tower. I n this example, the fundament is a pole 8 The pole 8 is anchored in the bottom of the sea, as shown in figure 3. This arrangement comprises an elongated beam 9, preferably made of stain less steel or aluminium. For example, the beam can be solid , tube-shaped or a lattice structure. The elongated beam 9, in the following named support beam, is arranged so that it extends through the open ing 7 between the support elements 3a-b and protrudes on both sides of the framework as shown in figure 2 and 3. The su pport elements 3a-b and the pontoon 4 form a plane. The beam 9 is arranged angu lar with respect to the plane. The support beam is neither perpendicular to the plane nor parallel with the plane. Preferably, the beam is arranged with an ang le a relative the plane which is within the interval of 30° - 80°, depending on water depth.

One end of the beam 9 is provided with a connection unit 10 for connecting the beam 9 to the pole 8. The connection unit 10 is designed so that it is possible to move the connection unit in a vertical direction relative the pole, but not in a horizontal direction . Each end of the beam 9 is provided with a buoyancy element 12a, 12b, which also can be used as weight elements. A first end 1 7 of the beam 9 is provided with a front buoyancy ele- ment 12a and a second end 1 8 of the beam 9 is provided with a rear buoyancy element 12b. At least the front buoyancy element 12a is hollow and includes a space with an inlet adapted to receive air and water, and the plant includes means for controlling the level of water in the space to allow a user to change the relation between air and water in the space in order to change the floating power of the buoyancy element. In a preferred embodiment of the invention , the rear buoyancy element 12a also includes a space including an inlet adapted to receive air and water, and the plant includes means, for instance a valve, for controlling the level of water in the space to allow a user to change the relation between air and water in the space in order to change the floating power of the buoyancy element. It is advantageous to fill the buoyancy elements with water after the plant has been installed at site. This means that mass is supplied to the plant which reduces movements of the water. It is more effi- cient to add mass at a position farther away from the connection point to the fundament, than at a position closer to the connection point.

The wind power plant further comprises a plurality of wire cables 14a-h, preferably made of steel or polyester arranged between the support beam 9 and the framework in order to mechan ically connect the beam to the framework. The beam 9 and the support elements are provided with a plurality of cable attachments 15 for facilitating the attachment of the wire cables. The cable attachment is, for example, a loop or a ring . The ends of the cable wire are also provided with corresponding attachments, for example hooks. The arrangement with wire cables achieves an optimal utilization of material and accordingly reduces the weight of the power plant. The attachment of wires to the frame- work can also be adjustable, e.g . wire 14a and wire 14e can be one continuous wire and 14b and 14f can be one continuous wire, as shown in figure 4. The connection to the framework can be a clamping device inside the su pport elements 3a-b, keeping the relative position of wire connection point and the tube 3 in a wanted position . In the embodiment shown in figure 1 - 4, eight wire cables are arranged between the beam and the framework. Four of the wire cables 14a-d are arranged between the framework and the end of the beam, which includes the connection unit 10, and four of the wire cables 14e-h are arranged between the framework and the opposite end of the beam. I n this embod iment, cable wires 14a and 14c are connected between a first end 1 7 of the beam and the support element 3a, cable wires 14b and 14d are connected between the first end 1 7 of the beam and the support el- ement 3b. Cable wires 14e and 14g are connected between a second end 18 of the beam and the support element 3a, cable wires 14f and 14h are connected between the second end 18 of the beam and the su pport element 3b. As seen from figures 1 and 4, the support beam 9 is free from the framework, i.e. is not in contact with the framework, and is only connected to the framework through the wire cables 14a-h. Since the support beam 9 has no connection to the framework except through the wire cables 14a-h no forces are introduced between the beam 9 and the framework.

Figure 2 shows the wind plant in a transportation position, in which it is possible to take the plant in tow. The wind power plant is transported to the pole 8 by boat. During transportation of the wind power plant, the buoyancy elements 12a-b are filled with air to keep the wind power plant floating during transportation . By changing the relation between water and air in the front buoyancy element 1 2a, it is possible to rise and lower the front part of the wind power plant. When the wind power plant has reached the pole 8, the connection unit 10 is connected to the upper part of the pole 8. When the power plant has been connected to the pole 8, the front buoyancy element 1 2a is filled with water so that the wind power plant is tilted forward , as shown in figure 3. The front end 1 7 of the beam 9 including the connection unit 10 sin ks until they reach a stop position close to the bottom of the pole 8. I n this position the buoyancy element 12a-b are not effected by unfavourable movements of the framework due to wave movements in the water. The framework 3a-b, 4 and the beam 9 are designed so that the support elements 3a-b are essentially vertical and the plane of the turbines are parallel with the vertical plane when the plant is anchored . Due to the fact that the anchoring point is close to the bottom of the sea, bending stresses on the pole 8 is significantly reduced , and therefore the pole only has to be dimensioned main ly for sheer forces with small bend ing movement.

A power cable 1 6 is positioned on the bottom of the sea and extends through the pole 8. The power cable 1 6 is connected to the wind power plant above the surface of the water after the connection to the pole. An advantage with this is that the power cable 16 does not move in the water due to movements of the framework, as compared to the solution when the beam 9 is connected to a buoy. Figure 5 shows a rear view of a wind power plant accord ing to a second embodiment of the invention . This embodiment differs from the first embodiment in that the wire arrangement on ly has six wire cables, instead of eight as in the first embodiment. The wire cables 14c and 14d have been exchanged by a single wire cable 14i. In the same way, the wire cables 14g and 14h have been exchanged by a single wire cable (not shown) .

Figure 6 shows an example of a connection element 16 for connecting the wire cables 14a-i to the frame work and to the beam, which connection element makes it possible to adjust the lengths of the wire cables on both sides of the framework to enable adjustment of the angle a between the beam 9 and the plane formed by the support elements. The connection element 6 includes a bobbin rotatably arranged relative the support ele- ment. The wires 14e and 14a are parts of one wire which are winded on the bobbin 16. The lengths of the parts 14e and 14a will change upon rotation of the bobbin such that when wire part 14a is lengthen , wire part 14e is shorten and vice versa. For example, the rotation of the bobbin is drive by a motor. I n this example, a motor 14 is arranged inside the hollow support element 3a. If such connection elements 16 is used in the embodiment shown in figure 1 , the framework is provided with four bobbins, a first for wires 14a and 14e, a second for wires 14b and 14f, a third for wires 14c and 14g , and a fourth for wires 14d and 14h .

This wind power plant is large, for example, the length of the beam is about 150m. The part of the beam extend ing in a direction towards the pole 8 is, for example, about 100m and the part of the beam extending in the opposite direction is, for example, about 50m.

An advantage with the power plant according to the present invention is that it is possible to position in shallow water. Another advantage compared to the prior art power plants is that the wind turbines on ly has one su pport element to the centre of a rotor of the wind turbine. This arrangement reduces disturbances and problems with dynamic dimensioning of the framework.

The present invention is not limited to the embodiments dis- closed but may be varied and mod ified within the scope of the following claims. For example, the plant may include more than two support elements and more than two wind turbines.