WO2018082756A1

WO2018082756A1 - A foundation for a wind turbine tower

Info

Publication number: WO2018082756A1
Application number: PCT/DK2017/050359
Authority: WO
Inventors: Anders BROHM
Original assignee: Vestas Wind Systems AS
Current assignee: Vestas Wind Systems AS
Priority date: 2016-11-01
Filing date: 2017-10-31
Publication date: 2018-05-11
Anticipated expiration: 2019-05-01

Abstract

The invention relates to a foundation (105) for a guyed wind turbine tower (103) having a vertical tower centreline. The foundation (105) comprises a plinth (201), a base plate (202), and at least one and at most two fins (203,204). The plinth (201) is arranged to have a wind turbine tower (103) mounted thereon. The base plate (202) extends radially outwards from the plinth (201) to a first radius (n), and is arranged circumferentially with respect to the plinth (201). Each fin (203, 204) projects from the base plate (202) and extends at least partly below a soil surface level (106). Said at least one and at most two fins (203, 204) are configured for realising a soil pressure with a horizontal component when the tower (103) is subjected to torsion.

Description

A FOUNDATION FOR A WIND TURBINE TOWER

FIELD OF THE INVENTION

The present invention relates to a foundation for a guyed wind turbine tower. The foundation of the invention is capable of improved handling of torsional loads arising during operation of the wind turbine.

BACKGROUND OF THE INVENTION

Foundations for wind turbine towers normally comprise a part which is arranged under the ground, thereby anchoring the wind turbine tower to the ground. The foundation must be able to withstand various kinds of loads expected during operation of the wind turbine, including gravity loads, bending loads and torsional loads to a limited extend. To this end, the size and weight of the foundation are normally designed in order to be able to handle such loads. The plan shape being circular or near circular by a polygonal shape to realise a foundation while optimizing the work needed to construct the foundation. Such a foundation is not able to counter torsional forces of significance. Sometimes the wind turbine tower is provided with guy-wires which are anchored to the ground via anchor blocks. The guy-wires provide stability to the wind turbine tower and handle part of the expected loads during normal operation. Thereby it is possible to reduce the size and weight of the foundation, while still being able to handle the expected loads during normal operation as especially the bending loads are reduced. However, guy-wires are not able to handle torsional loads of significance on the wind turbine tower, and neither is the foundation having a reduced size and weight.

DESCRIPTION OF THE INVENTION

It is an object of embodiments of the invention to provide a foundation for a guyed wind turbine tower being capable of efficient handling of torsional loads arising during operation of the wind turbine.

The invention provides a foundation for a wind turbine tower having a vertical tower centreline, the foundation comprising :

- a plinth arranged to have a wind turbine tower mounted thereon, - a base plate extending radially outwards from the plinth to a first radius, the base plate being arranged circumferentially with respect to the plinth, and

- at least one and at most two fins, each fin projecting from the base plate and

extending at least partly below a soil surface level, wherein at least one and at most two fins are configured for realising a soil pressure with a horizontal component when the tower is subjected to torsion.

A foundation represents a structure carrying all the weight of a wind turbine, the weight mainly represented in vertical load. Furthermore, due to wind acting on the rotor, the tower and the foundation are subject to bending moments and torsional loads, which the foundation needs to handle.

In case of multirotor wind turbines, torsional loads may even be more severe as there is a risk that one of the rotors stops, while the other(s) keep operating. These torsional loads also need to be appropriately restrained. Therefore, the foundation needs to be carefully designed. The foundation may be built from concrete, steel, or the like. It may be formed in one piece, or it may comprise multiple structures and elements of different materials, all together forming a robust unit. The size of the foundation depends on dimensions of the wind turbine carried by the foundation.

A wind turbine tower represents a structure carrying one or more wind turbines each having a nacelle and a rotor. The tower may be a guyed tubular steel tower, a guyed concrete tower, a guyed lattice tower, or the like. The type of tower may be selected based on various factors, such as size of the wind turbine, cost, installation site, aerodynamic considerations, or similar. The tower has a vertical tower centreline extending from the top of the tower and all the way down to a base of the tower. The centreline typically has a cross-point with the centre of gravity of the entire wind turbine.

A guyed tower is a tower comprising guy-wires attached to the tower with their one end and anchored to the ground at anchor blocks by another end at a distance from the tower base. The guy-wires provide support and stability to the free-standing tower. The tension in the diagonally positioned guy-wire allows the tower to withstand bending moments that may be created by wind, or may appear during operation of multirotor wind turbines. Having a tower supported with guy-wires allows a smaller foundation to be used compared to free-standing wind turbine towers. However, the smaller foundation limits the magnitude of torsional loads that can be handled by the foundation if of conventional type, and therefore the invention is particularly useful for guyed wind turbine towers. Additionally, the tower itself can be smaller in diameter when supported by guy-wires. According to the invention, the foundation comprises a plinth, base plate and at least one and at most two fins.

The plinth is arranged to have a wind turbine tower mounted thereon. It is to be interpreted as a heavy base supporting a wind turbine tower base. It is in a direct contact with the tower base. The plinth may be made of steel, concrete, mix of the two, etc. It may comprise a number of bolts and cords for connecting the tower thereto. The plinth may partly be below and partly above the soil surface level. Having a part of the plinth above the soil surface level may ease mounting of the wind turbine tower.

The base plate extends radially outwards from the plinth to a first radius. The base plate may be arranged with a circular footprint, but may alternatively have any other suitable footprint shape. The first radius may not be constant along the entire height of the base plate. The base plate is arranged circumferentially with respect to the plinth so that at least a part of the plinth is embedded in the base plate. The plinth and the base may be provided with reinforcements, for example a rebar cage structure known to the skilled person. The connection may also be ensured by additional reinforcement or rebars between the plinth and base plate.

The base plate may have a conical frustum or cylindrical shape, or a combination of the two.

The plinth and base plate are designed to handle bending moments experienced during normal operation of the wind turbine. The fin(s) of the foundation projects from the base plate. It is to be understood that the fin(s) may project from the sides of the base plate as well as from the bottom part of the base plate. The fin(s) extends at least partly below a soil surface level so that it becomes part of the load path from the wind turbine tower to the soil. Each fin is configured for realising a soil pressure with a horizontal component when the tower is subjected to torsion. This is the main role of the fins. The fin is an appendage to the base plate and has its own width and height. Efficiency of the fin(s), i.e. the torsional load that can be handled by the fin(s), may depend on the size of the fin(s), its length and height.

Having one or two fins of a sufficient size ensure that there is a large interface between the earth and the fins creating sufficient pressure from the earth to the fins to withstand extreme torsional loads. This makes the entire foundation capable of handling torsional loads.

The soil pressure, due to torsional loads, spreads out in a wedge shape in a horizontal plane due to friction. Surprisingly, the inventor has discovered that more than two fins could have the consequence that the fins cause a 'shadow effect' for each other, thereby reducing the efficiency of each fin. As the plinth and base plate, the fins may be made of steel, concrete, or a mix of the two. Connection between the base plate and the fins may be established in similar manner as the connection between the plinth and the base plate, as it is described above. The fins may take rectangular, trapezoidal, squared, triangular, ellipsoidal, or similar shape.

The plinth, base plate, and fins may be formed from one concrete unit.

Each fin may extend from the base plate along a radial direction to a second radius. The second radius represents a distance between the tower centreline and a free end of the fin. The second radius is larger than the first radius of the base plate. A difference between the second and the first radius may define an active length of the fin. The active length of the fin defines the length that is in a direct contact with the ground. By having a fin extending away from the tower centreline and the base plane, the foundation is enabled to handle significant torsional loads. Furthermore, the fins which extend radially outwards from the base plate provide a larger contract area between the fins and the earth.

The foundation may comprise two fins. As mention above, the fins may extend from the base plate along two different radial directions. These two directions may define an angle there between of at least 90 degrees, preferably between 90 and 180 degrees, more preferably approximately 180 degrees. As in the embodiment described above, two torsional fins with 90 - 180 degrees radial separation will counteract torsional loads induced during the wind turbine operation in an effective way.

Each fin may define a height which increases as a function of the radial distance from a centre of the base plate. Namely, the height of the fins may be smaller at the end attached to the base plate than at the opposite end further away from the base plate. A fin of this geometrical shape provides sufficient torsional load handling while having a reduced size, as outer portions of the fins are mainly responsible for torsional load damping. Reduced size of the fins, i.e. a foundation, provides reduction in material used, as well as a cost of the foundation. In an embodiment with two fins, the fins are preferably identical, even though small differences are allowed. The fins need to provide the same pressure to the ground, such that extreme torsional loads are counteracted by the soil pressure. Furthermore, having nearly identical fins, the torsional loads travel symmetrically through the plinth and base plate.

In one embodiment of the invention, the fin(s) or at least part of the fin(s) may project downwards from underneath the base plate without extending beyond the radius of the base plate. This embodiment is advantageous in that the excavation pit can be maintained within the horizontal extend of the base plate. The fin(s) in the embodiment above may extend beyond the radius of the base plate. Thus, the level of torsional loads that the foundation can counteract is increased. In further embodiments of the invention, the fin(s) extend below the vertical level of the base plate outside the horizontal extend of the base plate without having a part projecting downwards from underneath the base plate.

Each fin may comprise at least two opposite directed ground anchors. Part of the forces from the wind turbine tower may be transferred into the ground via these ground anchors. The ground anchors may be connected to the fins in an anchor point with a horizontal distance from the tower centreline. The anchor point may be positioned at the end of the fin that is the furthest from the tower centreline. The ground anchors may be connected to the fins via a hook, bolt, screw, or the like. Each ground anchor may be configured for realising a force with a component that is tangential to a radius between the anchor point and the tower centreline. This increases the ability to handle torsional loads, as the ground anchors increase the resistance of the fins towards the soil pressure caused by the torsional loads. The ground anchor may comprise a wire at one end and an anchor body at the opposite end. The wire may be attached to the anchor point via a hook, bolt, screw or the like. The anchor body may be attached to the wire by the same means. The length of the wire may be selected based on the size of the wind turbine, or based on the ground properties at a particular installation site. The anchor body may have a spiral shape, so it can penetrate the ground if driven by a power tool. The anchor body may be installed after the fins are installed, and there may be no need for separate digging of the ground. The size of the anchor body together with the length of the wire may play a key role in realising a force with a component that is tangential to a radius between the anchor point and the tower centreline.

The ground anchor may be oriented such that the anchor body is at the same vertical level as the anchor point. Moreover, the ground anchor may extend from the anchor point in a direction tangentially to a radius from the tower centreline to the anchor point.

The torsional load that the ground anchor can counteract increases with depth as the soil pressure increases with depth due to the weight of the column of soil above the anchor body. Therefore, the ground anchor may be oriented such that the anchor body is at a deeper level than the anchor point. However, the tangential component decreases with depth as a vertical component is added. The depth where the anchor body is most efficient depends on soil characteristics and geometrical dimensions of the foundation. Furthermore, erosion of the ground, if there is any, may be less influential at deeper ground levels than closer to the ground surface. Alternatively or additionally, the ground anchor may be oriented such that the anchor body is located radially further away from the tower centreline than the anchor point. The wires may extend diagonally compared to vertical direction and the direction of the tower centreline. This could be necessitated by soil conditions. However, the effectiveness of the ground anchor may decrease due to a reduction of the tangential force component and increase of a component parallel to the radius of the base plate.

The plinth and/or the base plate and/or the fin(s) may be in-situ cast as one integrated element.

The plinth and/or the base plate may be pre-cast part(s). According to this embodiment, the plinth and/or the base plate may be cast into their final form elsewhere, and then may be transported to a site of wind turbine installation. It may be that the plinth/base plate cast elsewhere is of a better quality than one which may be cast at the site, because they may be cast under controlled conditions and without the impact of wind and weather prevailing at the site of the wind turbine. Additionally, pre-cast plinth/base plate may reduce the amount of material needed compared to a case when these are cast at the site. The site where a wind turbine is to be erected may not be easily accessible and this may be a reason for pre-casting parts of the foundation.

At least part of the foundation may be cast on the site. Any of the elements of the foundation may be cast on the site. For instance, the base plate may be cast from concrete mixed on the site. Then a precast plinth may be arranged above the base plate, and connected to the base plate with a plurality of bolts, bores, cords, or the like. Casting the foundation or at least a part of it at the site may save cost of transporting massive concrete blocks, and other huge and heavy parts comprising the foundation. Whether the elements will be precast or cast at the site may depend on their dimensions and weight, as too large and heavy pieces may be difficult for transportation.

In one embodiment, the fins may be realised by a metallic beam extending through the plinth and baseplate. Examples of metallic beams are an I-beam, a U-beam, an X-beam or any other suitable profile that may be selected from a range of standard commercially available profiles. To increase the efficiency of the fins when in the form of metallic beams that has a comparably small area to realise ground pressure, such an embodiment may advantageously be combined with ground anchors. This embodiment is a particularly easy to realise and is very cost-effective, as standard components may be used. The wind turbine may be a multirotor wind turbine. Multirotor wind turbines comprise more than one nacelle, and thereby more than one rotor, supported by a single tower. Multirotor wind turbines produce significantly larger amount of electrical energy compared to traditional single rotor wind turbines, while saving ground area by installing only one foundation and one tower. These are, however, particularly prone to torsional loads. An example of an extreme load case for torsional loads may appear in a failure condition for a multirotor wind turbine if one turbine or the multirotor wind turbine stops operating. Therefore, a special type of foundation such as one described by this invention may be required.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in further detail with reference to the accompanying drawings in which

Fig. 1 illustrates a multirotor wind turbine with a foundation according to an embodiment of the invention,

Fig. 2 illustrates side views of foundations according to eight different embodiments of the invention,

Fig. 3 illustrates top views of foundations according to two different embodiments of the invention, and

Fig. 4 illustrates a top view and a side view of a foundation according to an embodiment of the invention, with ground anchors. DETAILED DESCRIPTION OF THE DRAWINGS

Fig. 1 illustrates a multirotor wind turbine 100 having two rotors 101 and 102. The rotors 101 and 102 of the wind turbine 101 are mounted on a wind turbine tower 103. The wind turbine tower 103 is provided with two guy-wires 104a and 104b which provide stability to the wind turbine 100. The wind turbine 100 further comprises a foundation 105 according to an embodiment of the invention to which the wind turbine tower 103 is attached and which is positioned partly below a soil surface level 106. A tower supported with guy-wires 104a, 104b allows a smaller foundation 105 to be used, compared to free-standing towers.

However, the smaller foundation 105 is not capable of handling torsional loads of any significant magnitude, and therefore a foundation according to the invention is particularly suitable for guyed wind turbine towers and/or multirotor wind turbines where asymmetrical operation can occur. Thus, the foundation 105 of the wind turbine 100 of Fig. 1 is provided with two fins 203, 204, extending from a base plate 202. The foundation 105 could be any of the foundations 105 illustrated in Figs. 2-4, and described below. The details of the foundation 105 will also be described below.

Fig. 2 illustrates side views of foundations 105 according to eight different embodiments of the invention. Fig. 2a illustrates a foundation 105 comprising a plinth 201 to which a wind turbine tower 103 will be mounted. The foundation 105 further comprises a base plate 202 extending radially outwards from the plinth 201 to a first radius n. The base plate 202 is arranged circumferentially with respect to the plinth 201. Two fins, 203 and 204, are projecting from the base plate 202. At least part of the fins 203 and 204 is extending below a soil surface level (not shown). The fins 203 and 204 extend from the base plate 202 along a radial direction to a second radius r₂. Different radii of the base plate 202 and the fins 203 and 204 can also be seen in other embodiments shown in Figs. 2d-2f. Typically, the entire foundation 105 may be cast at the site. Alternatively, the plinth 201 and base plate 202 are cast into their form before being transported to a site of wind turbine installation and the fins 203 and 204 are cast on the site. However, it may be the case that the entire foundation 105 is cast elsewhere and brought to the site. By having fins 203, 204 extending away from the tower centreline and the base plate 202, torsional loads can be counteracted by the soil pressure acting on the side of the fins 203, 204. Furthermore, the fins 203, 204 which extend radially outwards from the base plate 202 provide a larger contract area between the fins 203, 204 and the soil. All the features described above apply to the embodiments illustrated in Figs. 2b-2g.

Comparing embodiments 2a-2c, small modifications can be noticed. The plinth 201 and the base plate 202 are identical in all three cases. The fins 203 and 204 are, however, different. In Fig. 2b, portions of the fins 203 and 204 are also projecting downwards from underneath the base plate 202, forming one unit, as these portions are connected underneath the base plate 202. This is not the case in the embodiment of Fig. 2a. It should be noted, however, that the fins 203, 204 illustrated in Fig. 2b could, alternatively, be separate parts, each attached to the base plate 202. The fins 203 and 204, illustrated in Figs. 2a and 2b, are parallel to the soil surface (not shown), as well as to the lower side of the base plate 202, i.e., perpendicular to the wind turbine tower 103 unlike the fins 203 and 204 illustrated in Fig. 2c. The fins 203, 204 illustrated in Fig. 2c have angled sides as they extend outwards from the plinth 201.

The embodiment illustrated in Fig. 2d represents a foundation 105 having a plinth 201 which is longer than the one shown in 2a-2c. Furthermore, the base plate has a first radius n which is smaller than the first radius n of the embodiments in 2a-2c. However, radius r₂, is equal in embodiments 2a-2c. The embodiment in 2d enables installation of a base plate 202 smaller in its first radius n, without compromising stability of the wind turbine 100. This is because the base plate 202 is able to bear the weight of the tower, while the vertical area of the fins 203, 204 is large enough to counteract torsional loads to which the wind turbine tower 103 is subjected. This embodiment could be, e.g., suitable for a guyed tower, where a smaller foundation can be used, as the guy-wires handle part of the bending moments. Figs. 2e, 2f, and 2g illustrate three other embodiments of a foundation 105 in which each of the fins 203 and 204 define a height, which increases as a function of the radial distance from a centre of the base plate 202. Namely, the height of the fins 203, 204 is smaller at the end attached to the base plate 202 than at the other end further away from the base plate 202. Additionally, a part of each fin 203, 204 projects below the vertical level of the base plate 202, as indicated with a dashed line 205 and without having a part projecting downwards from underneath the base plate 202. This can also be seen in Fig. 2g. In contrast to Fig. 2b, the fins 203 and 204 do not merge under the base plate 202.

Fig. 2h illustrates an embodiment having only one fin 203 entirely projecting downwards from underneath the base plate 202 without extending beyond the radius of the base plate 202. Apart from contributing to improved stability of the foundation 105, the fin 203 counteracts torsional load. This embodiment enables the extend of the excavation for the foundation to stay within the boundary of the base plate.

Fig. 3 illustrates top views of foundations 105 according to two different embodiments of the invention. From this top view it is very easy to distinguish the plinth 201, the base plate 202 extending radially outwards and circumferentially with respect to the plinth 201, and the fins 203 and 204 projecting from the base plate 202. Fig. 3a may represent the top view of any of the embodiments from Figs. 2a-2g. The fins 203 and 204 extend from the base plate 202 along radial directions defining an angle of 180° between them. In another embodiment, this angle may take any other value smaller than 180° and larger than 90° . Fig. 3b illustrates a foundation with only one fin 203. The fins 203, 204 each provides a surface which interacts with the soil in such a manner that rotation of the tower and the foundation 105 is prevented in response to torsional loads. In particular, since there are no additional fins which could provide 'shadow' for the fins 203, 204, the interaction between the fins 203, 204 and the soil is at a maximum level, thereby allowing torsional loads to be efficiently handled. Fig. 4 illustrates a top view and a side view of a foundation 105 according to an embodiment of the invention with ground anchors. From Fig. 4a, which illustrates a foundation 105 with one fin 203, it can be seen that the fin 203 comprises two opposite directed ground anchors 401 and 402. The ground anchors 401 and 402 are connected to the fin 203 in an anchor point 403. The ground anchors 401, 402 comprise a wire 401a, i.e. 402a, connected to the anchor point 403 and an anchor body 401b, i.e. 402b connected to another end of the wire 401a, i.e. 402b. The anchor bodies 401b and 402b are arranged with a horizontal distance further away from the tower centreline, coinciding with the centre of the plinth 201. This horizontal distance is indicated in Fig. 4a as di distance. In the embodiment, the distance di is larger than a distance between the tower centreline and the anchor point 403. Each ground anchor 401 and 402 is configured for realising a force with a component that is tangential to a radius between the anchor point 403 and the tower centreline.

However, as previously described the ground anchors would counteract the torsional loads most efficiently if placed tangentially to a radius between the anchor point 403 and the tower centreline.

From the side view, illustrated in Fig. 4b it can be seen that the anchor bodies 401b and 402b are at a deeper level than the anchor point 403 with respect to the soil surface level 106.

While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of examples in the drawings and have been described in detail herein. The drawings are not to scale and dimensions of the drawings should not be compared. It should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

Claims

1. A foundation ( 105) for a guyed wind turbine tower ( 103) having a vertical tower centreline, the foundation ( 105) comprising :

- a plinth (201) arranged to have a wind turbine tower ( 103) mounted thereon, - a base plate (202) extending radially outwards from the plinth (201) to a first radius

(ri), the base plate (202) being arranged circumferentially with respect to the plinth

(201) , and

- at least one and at most two fins (203, 204), each fin projecting from the base plate

(202) and extending at least partly below a soil surface level ( 106), wherein said at least one and a most two fins (203, 204) are configured for realising a soil pressure with a horizontal component when the tower ( 103) is subjected to torsion.

2. A foundation ( 105) according to claim 1, wherein each fin (203, 204) extends from the base plate (202) along a radial direction to a second radius (r₂), the second radius (r₂) being larger than the first radius (n) . 3. A foundation ( 105) according to claim 2, the foundation ( 105) comprising two fins, wherein the fins (203,204) extend from the base plate (202) along radial directions which define an angle there between of at least 90 degrees, preferably between 90 and 180 degrees, more preferably approximately 180 degrees.

4. A foundation ( 105) according to any of the preceding claims, wherein each fin (203, 204) defines a height which increases as a function of the radial distance from a centre of the base plate (202) .

5. A foundation ( 105) according to any of the preceding claims, wherein each fin (203, 204) or at least part of each fin (203, 204) projects from under the base plate (202) .

6. A foundation ( 105) according to any of the preceding claims, wherein each fin (203,204) comprises at least two opposite directed ground anchors (401, 402), wherein said ground anchors (401, 402) are connected to said fins (203,204) in an anchor point (403) with a horizontal distance to the tower centreline, wherein each ground anchor (401, 402) is configured for realising a force with a component that is tangential to a radius between the anchor point (403) and the tower centreline.

7. A foundation (105) according to claim 6, wherein the ground anchor (401, 402) comprises a wire (401a, 402a) at one end and an anchor body (401b, 402b) at the opposite end, wherein the wire (401a, 402a) is attached to the anchor point (403).

8. A foundation (105) according to claim 7, wherein the ground anchor (401, 402) is oriented such that the anchor body (401b, 402b) is at a deeper level than the anchor point (403).

9. A foundation (105) according to claim 7 or 8, wherein the ground anchor (401, 402) is oriented such that the anchor body (401b, 402b) is located radially further away from the tower centreline than the anchor point (403).

10. A foundation (105) according to any of the preceding claims, wherein the plinth (201) and/or the base plate (202) is/are pre-cast part(s).

11. A foundation (105) according to any of the preceding claims, wherein at least part of the foundation (105) has been cast on the site.

12. A foundation (105) according to any of the preceding claims, wherein the wind turbine (100) is a multirotor wind turbine.