GB2493023A - Wind turbine foundation with pontoons - Google Patents

Abstract

A wind turbine foundation structure for supporting an offshore wind turbine 52 at sea comprising a tower 54, which forms at least part of a tower suitable for supporting a wind turbine rotor, a plurality of pontoons 18 arranged around the base of the tower member, where each pontoon is adapted to operate in a floating configuration in which the pontoon is buoyant in water in order to allow said structure to be moved and a sunk configuration in which the pontoon is able to sink to the sea floor; an upper support structure which extends between an upper part of said tower member and each pontoon and a lower support structure which extends between a lower part of said tower member and each pontoon; wherein bending moments in said tower member during use are transferred as forces acting on said upper and lower support structures in opposite directions.

Description

Wind turbines and floating foundations

Field of the Invention

The invention relates to wind turbines, and particularly to floating foundations for wind turbines.

Background of the Invention

It is known to position wind turbines offshore (i.e. at sea) in order to generate electricity from wind power. It is also known to tow out complete wind turbines with towers from shore to the final offshore location on floating gravity foundations and finally ballast the tower down on to the seabed. A gravity foundation, or gravity based structure, is one which rests on the sea floor and which relies on gravity to hold the foundation in a stable position on the sea floor. A challenge with using a floating gravity foundation is the large mass of the tower (eg 1,000 tons) and the height of the wind turbine, typically 8Dm or more above the sea level. This calls for large floating foundations.

US2004/0169376 describes a telescopic tower for a wind turbine, which can be towed into position on a floating base, with the telescopic tower in a retracted position. The buoyancy of the base is increased using additional floates 5a, Sb fixed to the top of the base (see paragraph 55). The base is then sunk to form a gravity base. After this the telescopic tower can be raised, for example by pumping sea water into the bottom part of the telescopic tower (see eg paragraph 82).

Summary of the Invention

The invention provides a wind turbine foundation structure and a wind turbine as set out in the accompanying claims.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings.

Brief Description of the Figures

Figure 1 shows the base section of a telescopic tower supported by tour cylindrical pontoons in a dry dock; Figure 2 shows the pontoons in more detail; Figure 3 illustrates the dry dock now filled with water, and after removal of a water barrier; Figure 4 shows the pontoons and tower base section after they have been removed from the dry dock; Figure 5 shows the 4 pontoons and base section of the telescopic tower moored at a quayside, ready for assembly of the upper section of the telescopic tower; Figure 6 shows the process of assembling the upper section of the telescopic tower within the base section; Figure 7 shows a supporting structure being fitted to the top of the upper section of the telescopic tower; Figure 8 shows a rotor in a horizontal position being fitted to the supporting structure; Figure 9 shows the rotor in a vertical position, after being turned on a hinge provided on the supporting structure; Figure 10 shows the wind turbine at sea, after having being towed into position by a tug boat; Figure 11 shows the position of the pontoons on the seabed after the pontoons are filed with water; Figure 12 shows the wind turbine in its final position after the upper section of the telescopic tower is jacked up hydraulically; Figure 13 is a cross-sectional view showing the telescopic tower in a partially raised position; Figure 14 is a cross-sectional view of the telescopic tower in its fully raised position; Figure 15 is a cross-sectional view showing the fixing of the upper section to the lower section when the telescopic tower is fully raised.

Figure 16 is a side view of a wind turbine foundation structure for use in a method such as that illustrated in Figures ito 12; Figure 17 is a cross-sectional view taken along line B-B in Figure 16; Figure 18 is a cross-sectional view taken along line A-A or B-B in Figure 16; Figure 19 is an alternative embodiment of the wind turbine foundation structure of Figures iGto 18; Figure 20 shows in greater detail the portion of Figure 19 at the top of the pontoons, and shows a rubber ring around the tower member; and Figure 21 shows in greater detail the portion of Figure 19 at the bottom of the pontoons, and shows a further rubber ring and rubber support at the bottom of the tower member.

Description of Preferred Embodiments

Figures 1 to 12 are schematic illustrations showing a method for assembling and installing an offshore wind turbine. It is important to note that Figures 1 to 12 illustrate the steps in the method, and do not show all the features of the pontoons and foundation structure shown in the embodiment of Figures 16 to 18, which is described below.

Figure 1 shows how the process is begun in a dry dock 2, which is formed by a quay 4 provided with a recessed area 6 and a water barrier 8 which prevents dockland water from entering the dry dock 2.

A crane 12 is used to position a steel tube 14 which forms the base section 16 of a telescopic tower. The base section 16 is held in a vertical position by 4 cylindrical pontoons 18 which surround the base of the steel tube 14. The pontoons 18 are formed from a suitable material, such as concrete, and fixed to the steel tube 14 in any

suitable manner.

Figure 2 is a more detailed view of the four pontoons 18 and base section 16 within the dry dock 2. In Figure 2 the pontoons 18 are illustrated partially transparent to show a layer of sand 20 in the bottom of each pontoon 18 to act as ballast when the pontoons 18 are floated. As shown in Figure 2, each pontoon 18 is a hollow cylindrical structure containing a layer of sand 20.

Figure 3 shows the next stage in the process. Water is slowly introduced into the dry dock 2 so that the pontoons 18 and base section 16 are raised off the floor 22 of the dry dock 2 and float within the dry dock 2. The water level within dry dock 2 is illustrated by water level line 24. The water barrier 8 is removed so that the whole assembly of the pontoons 18 and base section 16 can be moved out of the dry dock 2, for example using a tug boat.

Figure 4 shows the next stage. Once the pontoons 18 have been removed from the dry dock 2 more sand 20 is introduced into each pontoon 18 so that the pontoons 18 sink down further below the water level 24, but part of each pontoon 18 remains above the water level 24. The additional sand 20 can be added, for example, via an opening 26 provided at the top of each pontoon 18. By referring to Figures 2 to 4, it can be seen that only a small water depth is required within the dry dock 2 in order to float the pontoons 18 and base section 16 out of the dry dock 2, and the pontoons 18 are then sunk to a deeper level once they are outside the dry dock 2.

Figure 5 shows the next stage in the process. In Figure 5 the pontoons 18 and base section 16 are moored at a quayside 28, whilst still floating in dockland water 10 as described above. The upper section 30 of the telescopic tower is positioned in a number of parts on the adjacent quay 32. In this embodiment the upper section 30 is formed from 3 separate parts 34, 36 and 38, each formed from steel tubing. A crane is used to lift each part 34, 36, 38 into the base section 16 in turn. Part 34 is the first part to be lifted into the base section 16, and the first part 34 is provided with a frustroconical base portion 42, which forms a closed base of the upper section 30 of the telescopic tower. Parts 36 and 38 are next lifted into the base section 16, and all 3 parts 34, 36 and 38 are secured together in any suitable manner to form a single upper section 30 within base section 16, as shown in Figure 6.

Figure 7 shows how, once the upper section 30 has been assembled, a supporting structure 44 is fixed to the top of the upper section 30 using crane 40. The crane 40 may of course be the same crane as crane 12 shown in Figure 1, if appropriate.

As shown in Figure 8, next the crane 40 is used to lift a rotor 48, provided with for example with three air blades 50, to the top of the tower for fixing on the supporting structure 44. The rotor 48 is lifted in a horizontal position, in which the blades 50 lie in a substantially horizontal plane. The rotor is then fixed to the supporting structure 44 by means of a hinge (not shown in Figure 8), which allows the rotor 48 to turn to the vertical position shown in Figure 9, in which the blades 50 lie in a substantially vertical plane. The weight of the rotor is typically in the range of 50 to 400 tonnes (ie. 50,000 to 400,000 Kg).

The completed wind turbine 52 is shown moored at the quayside 28 in Figure 9. The wind turbine 52 includes the rotor 48, a telescopic tower 54, comprising upper and lower sections 30 and 16, and the pontoons 18. At this position alongside a quay the complete wind turbine system can be pre-commissioned and tested prior to the offshore installation.

Figure 10 shows the wind turbine 52 after it has been towed out to an offshore position by a tug boat 56. It can be seen in Figure 10 that the pontoons 18 continue to float partially above the water line 24.

As shown in Figure 11, the pontoons 18 are then filled with water so that they sink to the seabed 58. As a result of the ballast added to the pontoons the centre of gravity of the entire structure is positioned below the centre of buoyancy of the system during the lowering to the seabed, hence the system is stable. This is possible due to the low center of gravity provided by the telescopic tower bringing the masses of the upper tower section and the support structure and rotor down to only approximately half its final height. The concrete pontoons 18! together with the sand 20 they contain, are heavy enough to form a gravity foundation for the wind turbine 52, and it is not necessary to fix the pontoons 18 to the seabed 58 by separate fixing means. Suitable dimensions for each pontoon 18 are diameter 15 to 25 meters and height 15 to 20 metres for carrying a 5-10MW wind turbine, although other ranges are possible. A typical pontoon 18 may therefore have diameter 2Dm and height 17m making it possible to assemble and tow out the wind turbine from a harbour and tow route with a depth of only 13-15m.

Figure 12 shows the final stage of the installation process. The telescopic tower 54 is jacked up into the extended and raised position shown in Figure 12, for example by pumping water into the base section 16, as will be described below. Once the telescopic tower 54 reaches its fully extended position, the upper and lower sections 30 and 16 are secured together, for example using bolts, as will be described below. It will be appreciated that when the telescopic tower 54 is in its lowest position, shown in Figure 11 the centre of gravity of the wind turbine 52 is significantly lowered. As a result, the whole floating structure shown in Figure 11 is much more stable. This greater stability allows a smaller floating foundation (in the form of pontoons 18) to be used.

Figure 13 shows the telescopic tower 54 in more detail. The base 60 of upper section 30 is sealed, thus forming a chamber 62 into which water can be pumped by a pump 64 in order to raise the telescopic tower 54. The walls of the tower itself thereby have the dual function as both being the structural supporting tower for the installed wind turbine as well as acting as a gigantic hydraulic cylinder during the installation, resulting in an auto-installable tower. The chamber 62 is sealed by a rubber lip seal 66 fixed to the base 60 of upper section 30. In a preferred embodiment the area of the base 60 of upper section 30 is 38 square metres. This allows for example 1000 tons to be lifted using a water pressure within water chamber 62 of only 2.6 bar.

The upper section 30 is provided with a Teflon (registered trade mark) coated flange 68 (or other sliding material), shown in greater detail in Figure 15. When the telescopic tower 54 is fully extended the flange 68 abuts a 1-flange 70 at the top of the lower section 16. Bolts 72 are then used to fix the T-flange 70 to the Teflon coated flange 68 in order to fix the telescopic tower 54 in its extended position. The fact that flange 68 is Teflon coated allows it to slide more freely within the lower section 16. Alternatively, this sliding surface can be replaced by a sliding material mounted on the inside of the T-flange 70 sliding against the outside of the upper section 30 wall. Optionally, a roller 76 shown in Figure 15, may be fixed above T-flange 70 in order to guide the upper section 30. The upper section 30 is also provided with a Teflon coated spacer 74 which slides within the lower section 16.

Figure 13 shows an upper section 30 formed from two separate parts bolted together at internal flanges 78. As mentioned above! the upper section 30 may be formed from any number of parts.

Figure 14 shows the telescopic tower 54 in its fully extended position. In Figure 14 the upper section 30 is shown as a frustroconical tube which has a diameter of about 7 metres at its base and about 5 metres at its top. A suitable place for the upper section to house electrical equipment is illustrated by an electrical housing 80 in Figure 14.

Although the telescopic tower 54 has been described as hydraulic, it will be appreciated that it could be jacked up using air pressure, a different fluid or any other suitable means, including mechanical means. In addition, although the telescopic tower 54 of the preferred embodiment has two sections (i.e. upper and lower sections and 16) other embodiments are possible in which the telescopic tower has more than two telescoping sections.

Although not shown in the drawings, suitable rotating means (guides) is preferably provided for rotating the upper section 30 relative to the lower section 16, thus allowing the bolts 72 to pass through suitable apertures in the 1-flange 70 and flange 68 described above.

Figures 16 to 18 show an embodiment of a wind turbine foundation structure 81 which is suitable for use in a method such as that illustrated in Figures 1 to 12. Components in Figures 16 to 18 corresponding to those in Figures 1 to 12 are given like reference numerals, although it should be noted that the embodiment of Figures l6to 18 includes additional features not shown in Figures ito 12.

Figure 16 is a side view of a wind turbine foundation structure comprising four pontoons 18 arranged around a central tower member 14, in the form of a steel tube which forms the base section 16 of a telescopic tower, as described above.

Figure 17 is a cross-sectional view taken along line B-B in Figure 16, and Figure 18 is a cross-sectional view taken along line A-A or C-C in Figure 16. The four pontoons 18 are formed of concrete by a slip forming construction method. This is a known method for forming large concrete structures, in which a device known as a slip is slowly moved upwards to form concrete walls. As concrete sets at the bottom of the slip, new concrete is poured into the top of the slip. In the present case the slip allows the walls of the four pontoons to be formed at the same time, and to be integrally formed with each other, as shown in Figures 17 and 18. As shown in Figures 17 the walls of the pontoons 18 are joined to each other tangentially at four connecting portions 82. Each pontoon 18 has a cylindrical portion 84 provided with a domed roof 86 and a domed base 88. Each connecting portion 82 extends along the whole height of the cylindrical portion 84 of each pontoon 18. Each pontoon 18 has a cylindrical cavity 90 which provides buoyancy when the foundation structure is floated.

As shown in Figures 16 to 18, the foundation structure 81 is provided with upper and lower support structures 92 and 94 at the top and bottom of the cylindrical portions 84 respectively. These support structures 92 and 94 are in the form of horizontal flat concrete slabs which are integrally formed with all four of the pontoons 18, and which fill the gaps between the cylindrical portions 84 of the pontoons 18 and the steel central tower member 14. In this embodiment the concrete slabs forming the support structures have substantially the same thickness as the walls of the cylindrical portions 84 of the pontoons 18.

During use there may be significant bending moments on the steel tower member 14 as a result of horizontal forces on a wind turbine rotor at the top of the tower, and the support structures 92 and 94 act to hold the tower member 14 in position against these bending moments, and act to transfer and spread the load from these bending moments to the pontoons 18 in an efficient manner.

In use there may also be significant vertical forces on the tower member! as a result of the weight of the tower and the wind turbine rotor. Because the tower member 14 is "cast in" during the construction of the pontoons 18, that is to say it is positioned between the pontoons 18 as the pontoons are created by the slip forming method, the tower member 14 is to some extent embedded in the walls of each pontoon 18, or at least has a large area of contact with the walls of the pontoons 18, so that a good degree of friction is formed between the tower member 14 and the pontoons 18. This friction supports the tower member 14 against the vertical forces mentioned above. In addition, protrusions, such as welded protrusions, (not shown) may be formed on the outside of the tower member 14, and these protrusions embedded in the walls of the cylindrical portions 84 of the pontoons, to provide extra engagement between the tower member 14 and the pontoons 18.

Figure 19 shows a foundation structure 96 which is an alternative embodiment of the foundation structure 81 of Figures 16 to 18, in which corresponding elements are labelled with the same reference numerals. Figure 20 shows in greater detail the portion of Figure 19 at the top of the pontoons 18, and shows a rubber ring 102 around the tower member 14, as will be described below. Figure 21 shows in greater detail the portion of Figure 19 at the bottom of the pontoons 18, and shows a further rubber ring and rubber support at the bottom of the tower member 14, as will be described below.

In use, when a large bending moment is applied to the tower for example during high winds, the steel tower member 14 may bend or flex slightly. In Figure 19 this bending is shown greatly exaggerated for the purposes of illustration, and the flexed tower member is labelled 14a.

In the embodiment of Figure 19, the circular base 97 of the tower member 14 rests on a circular rubber support 98, which is held within a circular recess 100 formed in the upper surface of the lower support structure 94. In addition an upper rubber ring 102 is fitted around the inside of the circular opening in the upper support member 92, so that the tower member 14 rests against the rubber ring 102 and not directly against the concrete upper support structure 92. A lower rubber ring 108 is fitted around the base of the tower member 14, as shown in Figure 21 to protect the circular edge wall of the recess 100 against movement of the base of the tower member 14.

The flexed tower member 14a shown in Figures 19. 20 and 21 illustrates that when the tower member 14 bends the circular base 97 of the tower member 14 effectively rotates about a horizontal axis through a small angle. The rubber support 98 and the lower rubber ring 108 are able to compress and expand to accommodate this rotation, and thus avoid cracking of the concrete lower support structure 94. For a tower member 14 having a diameter of for example 8 meters the rotation of the circular base 97 may cause the edges of the circular base 97 to move vertically up and down by a small distance of for example 2mm, but in the absence of the rubber support 98 this small movement may nevertheless be sufficient to damage the concrete lower support structure 94.

In a similar way, the upper rubber ring 102 accommodates movement of the tower member 14 within the upper support structure 92. For the purposes of illustration, in Figures 19 and 20 a horizontal line 104 is drawn across the tower member 14 at the position of the upper support structure 92, and for the flexed tower member 14a the same horizontal line is illustrated and labelled 106. It will be seen that the two lines 104 and 106 are rotated relative to each other through a small angle. This illustrates the fact that when the tower member 14 bends the portion of the tower member 14 within the opening of the upper support structure 92 rotates about a horizontal axis by a small amount, so that the walls of the tower member 14 move up and down vertically. The upper rubber ring 102 accommodates this vertical movement, and avoid potential cracking of the concrete upper support structure 92.

In Figure 19 the entire weight, or vertical load, of the tower member 14 is supported by the lower support member 94. In addition a space (not shown) is provided between the tower member 14 and the vertical side walls of the pontoons 18, to accommodate flexing of the tower member 14 within the pontoons 18. The walls of the pontoons 18 are cast without being in contact with the tower member 14. However alternative embodiments are possible which use both resilient members and casting in of the tower member 14. For example, the tower member 14 may be cast in at the lower support structure 94 and provided with a resilient ring 102 at the upper support structure 92, or vice versa.

Although the circular support 98 and the rings 102 and 108 are described above as being formed of rubber, they may be formed from any suitable material which allows some movement of the tower member 14 during flexing. Resilient and/or flexible materials for example are suitable. A hard rubber may be used. The material of the rings 102 and/or 108 may also use a low friction material.

In this specification the term "concrete" is defined broadly to include any type of aggregate, such as sand and/or small rocks, held together by any type of binder, such as Portland cement or another cement.

Although embodiments have been described with four pontoons, any number of pontoons may be used.

Claims (1)

1. <claim-text>CLAIMS: 1. A wind turbine foundation structure for supporting an offshore wind turbine at sea, said foundation structure comprising: a tower member, which forms at least pad of a tower suitable for supporting a wind turbine rotor; a plurality of pontoons arranged around the base of the tower member, each pontoon being adapted to operate in a floating configuration in which the pontoon is buoyant in water in order to allow said structure to be moved, and a sunk configuration in which the pontoon is able to sink to the sea floor; an upper support structure which extends between an upper part of said tower member and each pontoon; and a lower support structure which extends between a lower part of said tower member and each pontoon; wherein bending moments in said tower member during use are transferred as forces acting on said upper and lower support structures in opposite directions.</claim-text> <claim-text>2. A foundation structure as claimed in claim 1, wherein said upper and lower support structures are formed from concrete.</claim-text> <claim-text>3. A foundation structure as claimed in claim 2, wherein each of said upper and lower support structures is integrally formed from a single piece of concrete.</claim-text> <claim-text>4. A foundation structure as claimed in any preceding claim, wherein said pontoons are formed of concrete.</claim-text> <claim-text>5. A foundation structure as claimed in claim 4, wherein said tower member is positioned between said pontoons during the formation of said pontoons, so that said tower member is cast in position between said pontoons.</claim-text> <claim-text>6. A foundation structure as claimed in claim 5, wherein said tower member is provided with protrusions which, when embedded in said concrete, prevent the tower member from moving relative to the concrete when vertical loads are applied to the tower member during use.</claim-text> <claim-text>7. A foundation structure as claimed in claim 4, 5 or 6, wherein each pontoon is slip formed by a slip forming construction method.</claim-text> <claim-text>8. A foundation structure as claimed in any preceding claim, wherein said tower member is formed of steel.</claim-text> <claim-text>9. A foundation structure as claimed in any preceding claim, wherein said plurality of pontoons are integrally formed with each other.</claim-text> <claim-text>10. A foundation structure as claimed in any preceding claim, wherein said upper and lower support structures are integrally formed with said pontoons.</claim-text> <claim-text>11. A foundation structure as claimed in any preceding claim, wherein each pontoon has an internal cavity for providing all or most of the pontoon's buoyancy, and also has a cylindrical portion which encloses all or most of said cavity.</claim-text> <claim-text>12. A foundation structure as claimed in claim 11, wherein said pontoons are positioned all the way around said tower member, and each pontoon is integrally formed with, or at least touches, at least two adjacent pontoons.</claim-text> <claim-text>13. A foundation structure as claimed in claim 12, wherein said cylindrical portions of the pontoons touch or meet each other tangentially.</claim-text> <claim-text>14. A foundation structure as claimed in any preceding claim, wherein said tower member is cylindrical, said pontoons are positioned all the way around said tower member, and each pontoon touches said tower member.</claim-text> <claim-text>15. A foundation structure as claimed in claim 14, wherein said cylindrical portions of the pontoons touch said tower member tangentially.</claim-text> <claim-text>16. A foundation structure as claimed in any preceding claim, wherein said plurality of pontoons includes at least three pontoons.</claim-text> <claim-text>17. A foundation structure as claimed in any preceding claim, wherein said plurality of pontoons includes at least four pontoons.</claim-text> <claim-text>18. A foundation structure as claimed in any preceding claim, wherein at least one of said pontoons is provided with an aperture for placing a ballast material and/or water inside the pontoon.</claim-text> <claim-text>19. A foundation structure as claimed in any preceding claim, wherein said upper support structure allows vertical movement of the outside edges of said tower member relative to said upper support structure.</claim-text> <claim-text>20. A foundation structure as claimed in claim 19, wherein said allowed vertical movement is at least 2mm or at least 5mm.</claim-text> <claim-text>21. A foundation structure as claimed in claim 19 or 20, wherein an upper resilient and/or flexible member is positioned between said tower member and said upper support structure.</claim-text> <claim-text>22. A foundation structure as claimed in any preceding claim, wherein said lower support structure allows vertical movement of the outside edges of said tower member relative to said lower support structure.</claim-text> <claim-text>23. A foundation structure as claimed in claim 22, wherein said allowed vertical movement is at least 2mm or at least 5mm.</claim-text> <claim-text>24. A foundation structure as claimed in claim 22 or 23, wherein a lower resilient and/or flexible member is positioned between said tower member and said lower support structure.</claim-text> <claim-text>25. A foundation structure as claimed in claim 24, wherein said lower resilient and/or flexible member comprises a tower support member which is positioned below said tower member and which supports substantially all vertical loads produced by the tower member.</claim-text> <claim-text>26. A foundation structure as claimed in claim 24 or 25, wherein said lower resilient and/or flexible member comprises a ring member which is positioned around the base of said tower member.</claim-text> <claim-text>27. A foundation structure as claimed in any one of claims 24 to 26, wherein said lower resilient and/or flexible member is positioned within a recess formed in said lower support structure.</claim-text> <claim-text>28. A wind turbine comprising: a foundation structure as claimed in any preceding claim; a rotor for converting wind power into stored energy; an extendable tower supporting said rotor, said tower being movable between lower and raised positions, and said tower comprising the tower member of said foundation structure; and pressure means arranged to raise said tower from said lower to said raised position by means of increased liquid or gas pressure.</claim-text> <claim-text>29. A wind turbine as claimed in claim 28, wherein said tower is telescopically extendable in at least two parts.</claim-text> <claim-text>30. A wind turbine as claimed in claim 29, wherein said pressure means is arranged to increase the pressure within at least the lowest of said parts in order to raise said tower from said lower to said raised position.</claim-text> <claim-text>31. A wind turbine as claimed in claim 30, wherein the base of an upper one of said parts is sealed, thus forming a chamber below said upper one of said parts into which hydraulic or pneumatic pressure can be applied in order to raise the telescopic tower.</claim-text> <claim-text>32. A wind turbine as claimed in claim 31, wherein the tower has a dual function of being both a hydraulic cylinder at least during installation of the wind turbine and also being a supporting tower for supporting said rotor during power production after installation.</claim-text> <claim-text>33. A wind turbine as claimed in any one of claims 28 to 32, wherein said pressure means is a pump.</claim-text> <claim-text>34. A wind turbine as claimed in any one of claims 28 to 33, wherein said liquid or gas is water.Amendments to the claims have been filed as follows CLAIMS: 1. A wind turbine foundation structure for supporting an offshore wind turbine at sea, said foundation structure comprising: a tower member, which fornis at least part of a tower suitable for supporting a wind turbine rotor: at least three pontoons arranged around the base of the tower member, each pontoon being adapted to operate in a floating configuration in which the pontoon is buoyant in water in qrder to allow said structure to be moved, and a sunk configuration in which the pontoon is able to sink to the sea floor; an upper support structure which extends between an upper part of said tower member and each pontoon; and a lower support structure which extends between a lower part of said tower member and each pontoon; wherein bending moments in said tower member during use are transferred as forces acting on said upper and lower support structures in opposite directions; and 0*** * wherein said pontoons are integrally formed with each other and with said upper and lower support structures. * 252. .A foundation structure as claimed in claim 1, wherein said upper and lower !* support structures are formed from concrete.* 3. A foundation structure as claimed in claim 2, wherein each of said upper and lower support structures is integrally formed from a single piece of condrete.4. A foundation structure as claimed in any preceding claim, wherein said pontoons are formed of concrete.5. A foundation structure as claimed in claim 4, wherein said tower member is positioned between said pontoons during the formation of said pontoons, so that said tower member is cast in position between said pontoons.5. A foundation structure as claimed in claim 5, wherein said tower member is provided with protrusions which, when embedded in said concrete prevent the tower member from moving relative to the concrete when vertical loads are applied to the tower member during use.1. A foundation structure as claimed in claim 4, 5 or 6, wherein each pontoon is slip formed by a slip forming construction method.8. A foundation structure as claimed in any preceding claim, wherein said tower member is formed of steel.9. A foundation structure as claimed in any preceding claim, wherein each pontoon has an internal cavity for providing aU or most of the pontoon's buoyancy, and also has a cylindrical portion which encloses all or most of said cavity.10. A foundation structure as claimed in claim 9, wherein said pontoons are positioned all the way around said tower member, and each pontoon is integrally formed with, or at least touches, at least two adjacent pontoons.11. A foundation structure as claimed in claim 10, wherein said cylindrical portions :... 25 of the pontoons touch or meet each other tangentially. -S..12. A foundation structure as claimed in any preceding claim, wherein said tower member is cylindrical, said pontoons are positioned all the way around said tower member, and each pontoon touches said tower member.13. A foundation structure as claimed in claim 12, wherein said cylindrical portions of the pontoons touch said tower member tangentially.14. A foundation structure as claimed in any preceding claim, wherein said pontoons include at least four pontoons 19 -- 15. A foundation structure as claimed in any preceding claim, wherein at least one of said pontoons is provided with an aperture for placing a ballast material and/or water inside the pontoon.16. A foundation structure as claimed in any preceding claim, wherein said uppei support structure allows vertical movement of the outside edges of said tower member relative to said upper support structure, 17. A foundation structure as claimed in claim 16, wherein said allowed vertical movement is at least 2mm or at least 5mm.18. A foundation struct&e as claimed in claim 16 or 17, wherein an upper resilient and/or flexible member is positioned between said tower member and said upper support structure.19. A foundation structure as claimed in any preceding claim, wherein said lower support structure allows vertical movement of the outside edges of said tower member relative to said lower support structure.20. A foundation structure as claimed in daim 19. wherein said allowed vertical movement is at least 2mm orat least 5mm.21. A foundation structure as claimed in claim 19 or 20, wherein a lower resilient ... 25 and/or flexible member is positioned between said tower member and said lower support structura 22. A foundation structure as claimed in* claim 21, wherein said lower resilient and/or flexible member comprises a tower support member which is positioned below said tower member and which supports substantially all vertical loads produced by the tower member.23. A foundation structure as claimed in claim 21 or 22, wherein said lower resilient and/or flexible member comprises a ring member which is positioned around the base * 35 of said tower member. 20.24. A foundation structure as claimed in any one of claims 21 to 23, wherein said lower resilient and/or flexible member is positioned within a recess formed in said lower support structure.25. A wind turbine comprising: a foundation structure as claimed in any preceding claim; a rotor for converting wind power into stored energy; an extendable tower supporting said rotor, said tower being movable between lower and raised positions, and said tower comprising the tower member of said foundation structure; and pressure means arranged to raise said tower from said lower to said raised position by means of increased liquid or gas pressure.26. A wind turbine as claimed in claim 25, wherein said tower is telescopically extendable in at least two parts. - 27. A wind turbine as claimed in claim 26, wherein said pressure means is arranged to increase the pressure within at least the lowest of said parts in order to raise said tower from said lower to said raised position.28 A wind turbine as claimed in claim 27, wherein the base of an upper one of said parts is sealed, thus forming a chamber below said upper one of said parts into which *... 25 hydraulic or pneumatic pressure can be applied in order to raise the telescopic tower.29. A wind turbine as claimed in claim 28, wherein the tower has a dual function of being both a hydraulic cylinder at least during installation of the wind turbine and also being a supporting tower for supporting said *rotor during power production after .. installation.A wind turbine as claimed in any one of claims 25 to 29, wherein said pressure means is a pump.31. A wind turbine as claimed in any one of claims 25 to 30, wherein said liquidor gas is water.</claim-text>