WO2020048573A1

WO2020048573A1 - Wind turbine foundation

Info

Publication number: WO2020048573A1
Application number: PCT/DK2019/050260
Authority: WO
Inventors: Gerner Larsen; Gunnar K. Storgaard Pedersen; Ivar J.B.K. JENSEN; Christian Skov FREDERIKSEN
Original assignee: Vestas Wind Systems AS
Current assignee: Vestas Wind Systems AS
Priority date: 2018-09-06
Filing date: 2019-09-02
Publication date: 2020-03-12
Anticipated expiration: 2021-03-06

Abstract

A tension stay foundation element for a wind turbine. The tension stay foundation element has a container with an internal cavity configured to receive ballast when disposed at site. The tension stay foundation element also has an anchor coupled to the container and configured to secure a tension stay for a wind turbine tower. The tension stay foundation element is disposable at site separate from a wind turbine tower foundation. The internal cavity can then be filled with ballast to provide a tension stay foundation assembly for a tension stay of a wind turbine.

Description

WIND TURBINE FOUNDATION

FIELD OF THE INVENTION

The present invention relates to a tension stay foundation element for a wind turbine, a tension stay foundation assembly and a method of installing the same.

BACKGROUND OF THE INVENTION

In order to meet global energy demands, renewable energy sources, such as wind turbines, are being placed under increasing pressure to increase power outputs, which is driving technology in the field. Due to space constraints and also due to the high costs associated with installing individual winds turbines, increasing power output by simply building and installing an increasing number of wind turbines is often not an economically viable solution to meet global demands.

Therefore, wind turbines are now being designed and manufactured to yield increased “per unit” power outputs. This is principally achieved in two ways, firstly, by increasing the size of the turbine blades and rotor to increase turbine capacity, and secondly, by placing the turbine blades higher in the atmosphere so that they are able to access a more steady supply of wind.

In order for these features to be implemented, modern wind turbines require increasingly tall support. In addition, towers can be made solely of steel, concrete or a combination of the two materials in what is known as a hybrid tower with a concrete lower portion and a steel upper portion. Towers may have a height ranging from 75m to 160m or more. This height can lead to issues with tower stiffness.

The concrete lower portion of hybrid wind turbine towers are typically stiffened by internal tension stays which ensures that the concrete is in compression at all times as concrete does not provide much structural integrity when under tension.

Moreover, tall towers may benefit from external tension stays at a stay cable angle of about 45° to improve the bending characteristics of the tower.

A compromise between the near vertical internal tension stay and the 45° external tension stay is disclosed in WO 2016/1 16645 A1 where external tension stays are provided at a shallow angle and replacing the internal tension stays. This construction provides both compression of the concrete lower portion and stability of the tower. The tension stays are secured to a base point on the main foundation. However, such structures are typically large and very heavy which makes transportation of these structures difficult. They are therefore typically manufactured on site which increases wind turbine installation time, leading to increased costs. Furthermore, such foundations are typically concrete casted structures, and are therefore expensive and also require time for curing which further adds to the overall installation time, and associated costs, for installing such foundation structures.

Therefore, the aim of the present invention is to provide a way of alleviating at least some of the aforementioned issues.

SUMMARY OF THE INVENTION

A first aspect of the invention provides a tension stay foundation element for a wind turbine comprising a containerwith an internal cavity configured to receive ballast when disposed at site, and an anchor coupled to the container and configured to secure a tension stay for a wind turbine tower, wherein the tension stay foundation element is disposable at site separate from a wind turbine tower foundation.

The term“tension stay” is used herein to refer to an anchoring tether, or tendon, having a high axial stiffness so as to induce a compressive load on the wind turbine tower structure to which it is attached. The term“tension stay” is well known in the art of wind turbine systems.

The tension stay foundation element may be a permanent foundation element, permanently sited. Alternatively, due to the tension stay foundation element having a loadable container, the foundation element may be a temporary foundation element, temporarily sited. By extracting the ballast from the internal cavity of the container, the foundation element may be easily removed and optionally re-sited at another location. Prior to loading with ballast, the container is advantageously lightweight, easy to transport and arrange in place at a desired site.

The container may comprise a base and at least one sidewall defining the internal cavity. The container may be a trapezoidal prism shape or a rectangular prism shape. In a further alternative, the container may have a circular cross-section or any other suitable shape.

The container may be pre-formed and transportable to site. The container may be configured to sit on the ground when disposed at site, substantially at ground level. Alternatively, the container may be configured to be at least partially submerged in the ground, below ground level.

The anchor may be at least partially located within the container.

The anchor may be located substantially at a centre-point of the container.

The term“centre- point” is used herein to refer to a location corresponding to the centre of mass of the container.

The anchor may comprise a tension stay connection, or any other suitable connectors or fasteners to engage the tension stay.

The anchor may comprise an upstanding arm with the tension stay connection to engage the tension stay.

The tension stay connection may protrude above the container.

The upstanding arm may have a position to secure the tension stay at a desired angle to the wind turbine tower. The upstanding arm may be movable to a position according to the desired angle of the tension stay.

The upstanding arm may comprise one or more upstanding arm portions.

The anchor may further comprise a cross-bar extending between opposing sidewalls of the container and a joint coupling the upstanding arm and the cross-bar.

The joint may be a pivotal joint to allow for rotation between the upstanding arm and the cross-bar.

A second aspect of the invention provides a tension stay foundation assembly comprising a plurality of tension stay foundation elements according to the first aspect of the invention and ballast received within each container of the plurality of tension stay foundation elements.

The plurality of tension stay foundation elements may be disposed radially about the wind turbine tower foundation.

The plurality of tension stay foundation elements may be disposed substantially encircling the wind turbine tower foundation.

The plurality of tension stay foundation elements may be spaced apart from the wind turbine tower foundation. The ballast is provided to achieve a weighting effect. By using ballast, the manufacturing and installing of the tension stay foundation element is cheaper and faster. The ballast may comprise ground materials. The ballast may comprise gravel. The ballast may comprise rock. The ballast may comprise low-cost waste materials. The ballast may be local to site.

The tension stay foundation assembly may further comprise an excavated pit. At least part of each container of the plurality of tension stay foundation elements may be submerged within the excavated pit.

A third aspect of the invention provides a wind turbine system comprising a wind turbine tower, a plurality of tension stays and a tension stay foundation assembly according to the second aspect of the invention, wherein a first end of each tension stay is connected to the wind turbine tower and a second end of each tension stay is connected to the anchor of a respective tension stay foundation element.

The wind turbine tower may comprise concrete. The wind turbine tower may comprise steel. The wind turbine tower may comprise concrete substitute material.

In a preferred embodiment of the invention a lower part of the wind turbine tower comprises concrete and an upper part of the tower comprises steel.

The plurality of tension stays provides compression of the concrete in the lower part and stability against bending forces.

The plurality of tension stays may be made from steel. Alternatively, the plurality of tension stays may comprise a composite material exhibiting a high tensile strength. The composite material may be a carbon fibre reinforced composite. Alternatively, the composite material may be a glass fibre reinforced composite and/or an aramid fibre reinforced composite. In a further alternative, the plurality of tension stays may be made from a high tensile strength polymer material, such as Nylon.

The wind turbine may be an on-shore wind turbine.

The term“on-shore” is used herein to refer to a location situated on or near to the shore. This may include areas such as rivers or shorelines but will be considered to exclude any deep, open water areas such as locations in the middle of oceans or other comparable open water areas, such as large lakes.

A fourth aspect of the invention provides a method of installing a tension stay foundation assembly for a wind turbine comprising the steps of providing a plurality of tension stay foundation elements, each element comprising a container with an internal cavity and an anchor, disposing the tension stay foundation elements separate from a wind turbine tower foundation and filling the internal cavity of each container with ballast.

The method may further comprise the steps of manufacturing the plurality of tension stay foundation elements off site and transporting the plurality of tension stay foundation elements to site.

The method may further comprise the steps of excavating a section of ground to form an excavated pit and installing the plurality of tension stay foundation elements within the excavated pit such that at least part of each container is submerged within the excavated pit.

The method may comprise filling the internal cavity with ground materials. The method may comprise filling the internal cavity with gravel. The method may comprise filling the internal cavity with rock. The method may comprise filling the internal cavity with low cost waste materials. The method may comprise filling the internal cavity with ballast local to site.

The method may comprise disposing the tension stay foundation elements radially about the wind turbine tower foundation.

The method may comprise disposing the tension stay foundation elements substantially encircling the wind turbine tower foundation.

The method may comprise disposing the tension stay foundation elements spaced apart from the wind turbine tower foundation.

A fifth aspect of the invention provides a method of installing a wind turbine system comprising the steps of providing a wind turbine tower and a plurality of tension stays, installing a tension stay foundation assembly for a wind turbine according to the fourth aspect of the invention, attaching respective first ends of each tension stay to the wind turbine tower and attaching respective second ends of each tension stay to the anchor of a respective tension stay foundation element.

The step of attaching respective first ends of each tension stay to the wind turbine tower and attaching respective second ends of each tension stay to the anchor of a respective tension stay foundation element may be done to induce a compressive load on the wind turbine tower. A sixth aspect of the invention provides a method of de-constructing a tension stay foundation assembly according to the second aspect of the invention comprising extracting ballast from the internal cavity of each container, and removing the plurality of tension stay foundation elements from site.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to the accompanying drawings, in which:

Figure 1 illustrates a perspective view of a wind turbine system having the tension stay foundation assembly according to one example;

Figure 2 illustrates a perspective view of the tension stay foundation assembly according to one example;

Figure 3 illustrates a perspective view of a tension stay foundation element according to one example;

Figures 4 to 8 illustrate a method of installing a tension stay foundation assembly according to the illustrated examples; and

Figures 9 to 12 illustrate a method of installing a wind turbine system onto the tension stay foundation assembly according to the illustrated examples.

DETAILED DESCRIPTION OF EMBODIMENT(S)

Figure 1 shows a wind turbine system 1 utilising an example of a tension stay foundation assembly according to the present invention.

The wind turbine system 1 comprises a wind turbine 2 including a nacelle 3 supported on a tower 10 which is mounted on a wind turbine tower foundation 12. The nacelle 3 supports a rotor 4 comprising a hub 5 to which three blades 6, 7, 8 are attached. It will be noted that the wind turbine 2 is the common type of horizontal axis wind turbine (HAWT) such that the rotor 4 is mounted at the nacelle 3 to rotate about a substantially horizontal axis defined at the centre at the hub 5. However, it shall be appreciated that in an alternative example, the wind turbine 2 may be a vertical axis wind turbine (VAWT). As is known, the blades 6, 7, 8 are acted on by the wind which causes the rotor 4 to rotate about its axis thereby operating generating equipment through a gearbox (not shown) that is housed in the nacelle 3. The generating equipment is not shown in Figure 1 since it is not central to the examples of the invention. The tower 10 comprises an upper member 10a, a lower member 10b and a joint member 10c located therebetween, coupling the upper 10a and lower 10b members. Typically, the upper member 10a and the joint member 10c are made from steel, or other similar materials, due to its beneficial tensile properties whereas the lower member 10b is made from concrete, or concrete substitute materials, due to its beneficial properties in compression. However, it shall be appreciated that the tower 10 may be made entirely from steel, entirely from concrete, or may be made from any other suitable material, or combination of suitable materials. Furthermore, whilst the tower 10 illustrated in Figure 1 is formed from multiple member sections, it shall be appreciated that the tower 10 may alternatively be formed as a unitary structure.

The wind turbine system 1 further comprises a plurality of tension stays 20a-e, configured so as to induce a compression at the lower member 10b of the tower 10. In Figure 1 , the plurality of tension stays 20a-e are attached at respective first ends 21a- e to the joint member 10c of the tower 10 and are attached at respective second ends 22a-e to a plurality of respective tension stay foundation elements 30a-e which form part of the tension stay foundation assembly 30. However, it shall be appreciated that the first ends 21 a-e of the plurality of tension stays 20a-e may alternatively be attached to the lower member 10b of the tower 10, or may be attached at any other suitable point.

The tension stay foundation assembly 30 typically comprises sixteen tension stay foundation elements coupled to respective tension stays. However, for clarity and conciseness, only five of the tension stay foundation elements 30a-e and associated tension stays 20a-e shall be referenced and described in this application. It shall also be noted that in other examples more than sixteen, or fewer than sixteen, tension stays and tension stay foundation elements may be used, although typically the number of tension stays and associated tension stay foundation elements will be no fewer than three. In embodiments the number of tension stays are a multiple of three, for example three, six, nine, twelve, fifteen, eighteen, twenty-one or twenty-four. In other embodiments the number of tension stays are an uneven number from three and upwards, for example three, five, seven, nine, eleven, thirteen, fifteen, seventeen, nineteen, twenty-one or twenty-three. Each tension stay 20a-e is typically made from steel. However, it shall be appreciated that the plurality of tension stays may alternatively comprise a composite material exhibiting a high tensile strength such as carbon fibre reinforced composite or a different fibre reinforcement, such as glass fibre or aramid fibre or high tensile strength polymer materials, such as Nylon. The tension stay foundation assembly 30 is shown in greater detail in Figure 2. As has been mentioned previously, the tension stay foundation assembly 30 comprises a plurality of tension stay foundation elements 30a-e, which are disposed separately to the wind turbine tower foundation 12. The tension stay foundation elements 30a-e are arranged radially about, and spaced from, the wind turbine tower foundation 12. In Figure 2, the plurality of tension stay foundation elements 30a-e are disposed in a ring substantially encircling the wind turbine tower foundation 12, with each tension stay foundation element 30a-e is spaced from the wind turbine tower foundation 12. The tension stay foundation elements 30a-e may be spaced from the wind turbine tower foundation 12 by one or more spacer bars so as to maintain a predetermined distance. However, it shall be appreciated that the tension stay foundation elements may alternatively be configured to abut the wind turbine tower foundation, or in a further alternative, may be disposed in any other suitable orientation, separate from the wind turbine tower foundation 12.

The tension stay foundation elements 30a-e are configured such that the tension stays extend from the tower 10 and are secured to the respective tension stay foundation elements 30a-e at a desired tension stay angle with respect to the tower 10. The tension stay angle is selected to achieve an appropriate compressive and stabilising effect on the tower 10. Typically, the tension stay angle may range between 10 and 45 degrees.

An individual tension stay foundation element 30a shall now be described with reference to Figure 3. Like reference numerals denote equivalent parts in subsequent tension stay foundation elements 30b-e.

The tension stay foundation element 30a comprises a container 31a and an anchor 40a coupled to the container 31 a for securing a respective tension stay 20a of the wind turbine system 1. In the example illustrated in Figure 3, the container 31a is made up of a base 32a and a plurality of sidewalls 33a-36a which define an internal cavity 37a within the container 31 a, configured to receive ballast. The container 31a is typically a trapezoidal or rectangular prism shape no larger than 3m wide, 7.2m long and 3m in height. These dimensions correspond to standard dimensions for haulage on a flatbed trailer. Since the tension stay foundation elements 30a-e are typically pre-formed as individual modular units off-site to be later assembled on-site to form the tension stay foundation assembly 30, their smaller size and lower weight enables the individual tension stay foundation elements 30a-e to be easily transported to site, and around site, using standard road haulage means. This has the advantage of further reducing on-site installation times when installing the wind turbine system 1.

However, it shall be appreciated that the container 31 a may be substantially circular comprising a single sidewall, or may alternatively be any suitable shape.

Typically, the container 31a is made from concrete and/or steel. However, it shall be appreciated that the container 31a may be made from any other suitable materials.

In an embodiment of the invention the container 31 a is an intermodal container which is a standardized shipping container built for intermodal freight transport. Such containers are available worldwide and of a standardized size which enables local sourcing and foundation standardization. The container is modified to be suitable for the present application by for example removing the top of the container and attaching means for attachment of the tension stay 20a. The container may also be an open top container in which case removal of the top is unnecessary. The most common size of these containers are twenty, forty or fifty-three feet standard length, but other formats are available and can be used dependent on the installation requirements.

In the exampled depicted in the Figures, the anchor 40a comprises means for attachment of the tension stay 20a in the form of a cross-bar 42a located within the internal cavity 37a of the container 31 a. The cross-bar 42a extends between a pair of opposing sidewalls 34a, 36a and is secured to each sidewall 34a, 36a respectively via welding, or any other suitable securing means.

The anchor 40a further comprises an upstanding arm 44a coupled to the cross-bar 42a. The upstanding arm comprises a first end 46a proximal to the cross-bar 42a and a second end 48a distal from the cross-bar 42a. The upstanding arm 44a is oriented substantially upright to the cross-bar 42a. The upstanding arm 44a is positioned to secure the tension stay 20a at a desired tension stay angle relative to the wind turbine tower 10, and a joint 49a for coupling the upstanding arm 44a with the cross-bar 42a. Typically, the upstanding arm 44a and the cross-bar 42a are made from a steel material. However, it shall be appreciated that other suitable materials may be used. In the example illustrated in Figure 3, the joint 49a is a pivotal joint comprising a through-hole which extends through a first end 46a of the upstanding arm 44a, proximal to the cross-bar 42a, and is configured to receive the cross-bar 42a so as to allow rotation between the upstanding arm 44a and the cross-bar 42a. However, it shall be appreciated that the joint 49a may be any other suitable pivotal, or non-pivotal joint.

The provision of a pivotal joint 49a between the cross-bar 42a and the upstanding arm 44a allows the position of the upstanding arm 44a to be varied by rotating the upstanding arm 44a freely or between set points dependant on the tension stay angle required by the wind turbine system 1. Therefore, the pivotal joint 44a enables the tension stay foundation element 30a to be used with a variety of different wind turbine systems having different tension stay angle requirements without the need for modifying the manufacturing process and without requiring substantial alteration of the tension stay foundation element 30a to be performed on-site. This provides the tension stay foundation element 30a with greater adaptability for use on different wind turbine system designs and also helps to reduce on-site installation times, since adjustment time is also reduced.

The second end 48a of the upstanding arm 44a comprises a tension stay connection 50a configured to engage with a respective tension stay 20a of the wind turbine system 1. In the example illustrated in Figure 3, the tension stay connection 50a comprises a through-hole sized to receive the tension stay. However, the tension stay connection 50a can comprise any suitable connector or fastening means. The upstanding arm 44a is sized such that the tension stay connection 50a protrudes above the top edge of the container 31 a. However, in an alternative example, the tension stay connection 50a may be located within the container 31a, below the top edge of the container 31a. In one such example, the upstanding arm 44a may be omitted with the tension stay connection 50a being located on the cross-bar 42a. In another example, the anchor 40a may comprise an upstanding arm 44a with the cross bar 42a being omitted. In this example, the upstanding arm 44a may be secured to a base 32a or to one of the plurality of sidewalls 33a-36a of the container 31 a. Furthermore, the upstanding arm 44a may comprise a single upstanding arm portion or multiple upstanding arm portions. The multiple upstanding arm portions may each be coupled to respective sidewalls 33a-36a of the container 31 a. In one example, the upstanding arm 44a may comprise a pair of upstanding arm portions disposed on opposing sidewalls of the container 31a. In yet another alternative, both the cross-bar 42a and the upstanding arm 44a may be omitted and the anchor 40a may comprise a tension stay connection 50a located on the base 32a within the internal cavity, or one of the sidewalls 33a-36a of the container 31 a external or internal to the internal cavity.

The anchor 40a is preferably located substantially at a centre-point of the container 31 a. This provides the advantage of reducing moments acting on the tension stay foundation element 30a when securing the respective tension stay 20a, and therefore enables the tension stay foundation element 30a to better secure the respective tension stay 20a.

Furthermore, by placing the anchor 40a at the centre point of the container, the weighting effect of the foundation element 30a on the tension stay 20a is optimised. This feature enables the tension stay 20a to be secured using less, or lighter, ballast than would be required for an anchor 40a positioned away from the centre-point of the container 31 a. This provides the further advantage of reducing material cost during installation since each tension stay foundation element 30a-e can be sufficiently ballasted with less or lighter ballast, such as soil.

A method of installing a tension stay foundation assembly 30 shall now be described with reference to Figures 4 to 12. In a first step of installing a tension stay foundation assembly 30, a section of ground 52 is excavated using an excavator 53, or any other suitable means, to form an excavated pit 54. The excavated pit 54 typically has a diameter in the region of 40m, corresponding to the desired diameter of the completed tension stay foundation assembly 30, and a depth of 3.2m, thereby ensuring that each container 31 a-e of the tension stay foundation assembly 30 is completely submerged within the excavated pit 54 upon installation.

However, in an alternative example, the excavated pit 54 may be shallower such that each container 31 a-e of the tension stay foundation assembly 30 is only partially submerged within the excavated pit 54. In yet another alternative, the excavated pit 54 may be omitted entirely and each container 31a-e may be mounted directly on the ground.

Once the excavated pit 54 has been dug, the wind turbine tower foundation 12 is installed at a central location within the excavated pit 54 via a crane 55, or any other suitable lifting means, as illustrated in Figure 5. The wind turbine tower foundation 12 typically comprises two modular foundation sections 12a, b which are each typically no larger than 3.6m wide, 7.2m long and 3.5m in height. Similarly to dimensions of the tension stay foundation elements 30a-e, the dimensions of the foundation sections 12a, b correspond to standard dimensions for haulage on a flatbed trailer. The foundation sections 12a, b are typically manufactured off site as individual elements before being transported to site prior to installation, thereby helping to further reduce on site installation times. However, it shall be appreciated that the wind turbine tower foundation 12 may be any other suitable foundation type and may be manufactured on-site.

Once the wind turbine tower foundation 12 has been installed, the plurality of foundation elements 30a-e are disposed individually and separately from the wind turbine tower foundation 12. In the example illustrated in Figure 6, the plurality of foundation elements 30a-e are individually disposed radially about and spaced from the wind turbine tower foundation 12 in a ring substantially encircling the wind turbine tower foundation 12. The foundation elements 30a-e may be spaced from the wind turbine tower foundation using one or more spacer bar so as to maintain a predetermined distance. However, it shall be appreciated that in other examples, the plurality of foundation elements 30a-e may abut the wind turbine tower foundation or be disposed in any other suitable orientation. The foundation elements 30a-e The container 31 a-e of each of the respective tension stay foundation elements 30a-e is typically manufactured from steel off site and then transported to the site prior to installation. However, in an alternative example, the container 31 a-e of each tension stay foundation element 30a-e may be manufactured using concrete, with each container 31a-e being pre-cast off-site prior to being transported to site for installation. Manufacturing the containers 31 a-e off-site provides the advantage of significantly reducing on-site installation times. A further advantage of using loadable and individual containers for each foundation element 30a-e is that they are comparatively smaller and lighter weight when compared to the conventional unitary foundation.

The anchor 40a may be coupled to the container before or after transporting the container to site, or before or after disposing the container in the desired position on site. For example, as shown in Figure 6, each foundation element 30a-e is typically manufactured off site comprising the container 31 a-e and cross-bar 42a-e. The upstanding arm 44a-e is then attached to each respective foundation element 30a-e after the plurality of foundation elements 30a-e have been installed within the excavated pit 54, as shown in Figure 7. Since the respective tension stay connections 50a-e of the upstanding arms 44a-e typically protrude above the respective containers 31 a-e, attaching the upstanding arms 44a-e following installation provides the advantage of reducing the overall height of the tension stay foundation element 30a- 30e during storage and transport, thereby enabling easier transportation and allowing for more efficient storage of the tension stay foundation elements 30a-e prior to installation. However, in an alternative example, the foundation element 30a-e may be pre-formed off-site with the upstanding arm 44a-e. Optionally, the upstanding arm may be moveable between a storage position and a securing position. In the securing position, the upstanding arm is able to secure the tension stay.

Once each of the respective upstanding arms 44a-e have been attached to the respective cross-bars 42a-e of the plurality of foundation elements 30a-e, they are pivoted to a securing position. The securing position is typically determined by the desired tension stay angle of the tension stay relative to the wind turbine tower 10. Once each of the upstanding arms 44a-e have been secured at the desired position, the internal cavities 37a-e of each container 31 a-e of the plurality of foundation elements 30a-e are filled with ballast 60a-e using a tipper truck 59, or other suitable means, as illustrated in Figure 8. Each tension stay foundation element 30a-e is filled with enough ballast 60a-e to provide a sufficient weighting effect. When filled with ballast, the tension stay foundation element is preferably able to counteract a tension stay force of approximately 160 Tonnes during use.

The ballast 60a-e in the illustrated example comprises ground materials located local to the site. In one example, the ballast 60a-e comprises ground material excavated from the excavated pit 54. Alternatively, the ballast 60a-e may instead be in the form of gravel, rock, low cost waste materials or any combination thereof. This provides the advantage of reducing installation times since such materials do not require time to cure and are located proximal to the installation site, thereby reducing transport time. Such materials are also cheap and readily available, which also helps to reduce the cost of installation when compared with other known methods.

The completed tension stay foundation assembly 30 comprising the plurality of tension stay foundation elements 30a-e and ballast 60a-e received with the respective internal cavities 37a-e of each container 31a-e is illustrated in Figure 9. Once installed, the containers of the foundation elements 30a-e are submerged within the excavated pit 54 and covered by ground materials and/or other ballast materials. The tension stay connections 50a-e of the upstanding arms 44a-e protrude above the ground materials and/or other ballast materials to enable access to the tension stay connections 50a-e for connection of the tension stays 20a-e.

Since the protruding tension stay connections 50a-e are accessible, the tension stays can be connected to each respective foundation element 30a-e after the loading of ballast 60a-e to the internal cavity 37a-e. This feature further improves installation process and times.

Once the tension stay foundation assembly 30 has been completely installed, a wind turbine 2 can be installed to complete the installation of the wind turbine system 1. In a first step illustrated in Figure 10, the lower member 10b of the wind turbine tower 10 is installed onto the wind turbine tower foundation 12. The lower member 10b of the tower is typically installed as a series of modular blocks which are stacked on top of each other, via the crane 55. Once the lower member 10b has been installed, the joint member 10c is installed onto the lower member 10c using fasteners or any other suitable means.

The lower member 10b of the tower typically comprises fourteen modular concrete sections and therefore, due to its height, typically requires additional support before the upper member 10a can be fully installed. Therefore, once the joint member 10c has been installed, tension stays 70b-e are attached to the tower 10 to provide a provisional stabilising effect during installation. The tension stays sufficiently stabilise the lower member 10b of the tower 10 to allow the upper member 10a to be installed, as shown in Figure 1 1.

The tension stays 70b-e each have respective first ends 71 b-e which are attached to corresponding tension stay connectors (not shown) located at the joint member 10c, and respective second ends 72b-e which are attached to the respective tension stay connections 50b-e of the tension stay foundation assembly 30.

Once the tension stays 70b-e have been installed, the upper member 10a of the tower 10 can be installed via crane 55. The upper member 10a of the tower is also typically installed as a series of modular blocks which are stacked on top of each other, via the crane 55. However, it shall be appreciated that the upper 10a and lower 10b portions may be installed as single, unitary structures, or in a further alternative, a unitary wind turbine tower 10 may be provide comprising the upper 10a and lower 10b members.

Once the upper member 10a has been installed, additional or alternative tension stays 20a-e can be installed onto the corresponding tension stay connectors of the joint member 10c to induce a compressive load on the lower member 10b wind turbine tower 10 and provide stability during the operation of the wind turbine 2. Typically, the number of tension stays 20a-e used during the operation of the wind turbine 2 is greater than the number of tension stays 70b-e used during the installation of the wind turbine 2. In the example shown in Figures 1 1 and 12, the tension stays 70b-e used during the installation of the wind turbine 2 are subsequently detached and replaced by alternative tension stays 20a-e to provide the desired tension and stability during the operation of the wind turbine 2. However, in an alternative example, the tension stays 70b-e may be permanent tension stays which are left attached to the tower 10 during operation. Once the tower 10 and its associated tension stays 20a-e have been installed, the nacelle 3, rotor 4 and other components of the wind turbine system 1 can be installed onto the upper member 10a of the tower 10 to complete the installation process.

In an alternative embodiment, the containers are initially only partly filled with ballast such that the upstanding arms may rotate freely. Then the procedure is performed until the tension stays are installed and a preliminary tension is applied. Then the containers are filled with ballast and the tension stays are fully tension before the tower is completed. This embodiment has the advantage that the angle of the upstanding arms is self-regulating and forces will be in the longitudinal direction of the arm.

Upon decommissioning, the wind turbine system 1 can be de-constructed via removing the wind turbine components 2 and then removing the wind turbine foundation assembly 30 by extracting ballast 60 from the internal cavity 37a-e of each container 31 a-e. The plurality of tension stay foundation elements 30a-e can then be removed from site. In the illustrated example, the ballast comprises ground materials, gravel and/or rock. Therefore, the ballast 60 can be deposited at site following extraction from the respective containers 31a-e without causing detriment to the local environment.

The provision of using ballast comprising ground materials, gravel and/or rock therefore has the benefit of improved ease of removal for the wind turbine foundation assembly during decommissioning, as well as improving the sustainability of the wind turbine foundation assembly since such materials can be deposited back into the local environment, or can be recycled for use in further wind turbine foundation assemblies (as well as other uses), without any additional processing of the ballast being required.

Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.

Claims

1. A tension stay foundation element for a wind turbine comprising:

a container with an internal cavity configured to receive ballast when disposed at site; and

an anchor coupled to the container and configured to secure a tension stay for a wind turbine tower,

wherein the tension stay foundation element is disposable at site separate from a wind turbine tower foundation.

2. The tension stay foundation element according to claim 1 , wherein the container comprises a base and at least one sidewall defining the internal cavity, and preferably wherein the container is a trapezoidal prism shape or a rectangular prism shape.

3. The tension stay foundation element according to claim 1 or claim 2, wherein the container is pre-formed and transportable to site.

4. The tension stay foundation element according to any preceding claim, wherein the anchor is at least partially located within the container, and preferably wherein the anchor is located substantially at a centre-point of the container.

5. The tension stay foundation element according to any preceding claim, wherein the anchor comprises an upstanding arm with a tension stay connection to engage the tension stay.

6. The tension stay foundation element according to any preceding claim, wherein the tension stay connection protrudes above the container.

7. The tension stay foundation element according to claims 5 or 6, wherein the upstanding arm has a position to secure the tension stay at a desired angle to the wind turbine tower.

8. The tension stay foundation element according to claims 5, 6 or 7, wherein the upstanding arm comprises one or more upstanding arm portions.

9. The tension stay foundation element according to claims 5 to 8, wherein the anchor further comprises a cross-bar extending between opposing sidewalls of the container and a joint coupling the upstanding arm and the cross-bar.

10. The tension stay foundation element according to claim 9, wherein the joint is a pivotal joint to allow for rotation between the upstanding arm and the cross-bar.

11. A tension stay foundation assembly comprising:

a plurality of tension stay foundation elements according to any preceding claim; and

ballast received within each container of the plurality of tension stay foundation elements.

12. The tension stay foundation assembly according to claim 11 , wherein the plurality of tension stay foundation elements are disposed radially about the wind turbine tower foundation, and preferably wherein the plurality of tension stay foundation elements are disposed substantially encircling the wind turbine tower foundation.

13. The tension stay foundation assembly according to claim 1 1 or 12, wherein the plurality of tension stay foundation elements are disposed spaced apart from the wind turbine tower foundation.

14. The tension stay foundation assembly according to claims 1 1 , 12 or 13, wherein the ballast comprises ground materials, gravel and/or rock, and preferably wherein the ballast is local to site.

15. The tension stay foundation assembly according to any of claims 1 1 to 14, further comprising an excavated pit, and wherein at least part of each container of the plurality of tension stay foundation elements is submerged within the excavated pit.

16. A wind turbine system comprising:

a wind turbine tower;

a plurality of tension stays; and

a tension stay foundation assembly according to any of claims 1 1 to 15, wherein a first end of each tension stay is connected to the wind turbine tower and a second end of each tension stay is connected to the anchor of a respective tension stay foundation element.

17. A method of installing a tension stay foundation assembly for a wind turbine comprising the steps of:

providing a plurality of tension stay foundation elements, each element comprising a container with an internal cavity and an anchor;

disposing the tension stay foundation elements separate from a wind turbine tower foundation; and filling the internal cavity of each container with ballast.

18. The method of installing a tension stay foundation assembly according to claim 17, the method further comprising the steps of:

manufacturing the plurality of tension stay foundation elements off site; and transporting the plurality of tension stay foundation elements to site.

19. The method of installing a tension stay foundation assembly according to claim 17 or 18, further comprising the steps of:

excavating a section of ground to form an excavated pit; and

installing the plurality of tension stay foundation elements within the excavated pit such that at least part of each container is submerged within the excavated pit.

20. The method of installing the tension stay foundation assembly according to any of claims 17 to 19, wherein the step of filling the internal cavity with ballast comprises filling the internal cavity with ground materials, gravel and/or rock, and preferably wherein the ballast is local to site.

21. The method of installing the tension stay foundation assembly according to any of claims 17 to 20, wherein the plurality of tension stay foundation elements are disposed radially about the wind turbine tower foundation, and preferably wherein the plurality of tension stay foundation elements are disposed substantially encircling the wind turbine tower foundation.

22. The method of installing the tension stay foundation assembly according to claim 20 or 21 , wherein the plurality of tension stay foundation elements are disposed spaced apart from the wind turbine tower foundation.

23. A method of installing a wind turbine system comprising the steps of:

providing a wind turbine tower and a plurality of tension stays;

installing a tension stay foundation assembly for a wind turbine according to any of claims 17 to 20;

attaching respective first ends of each tension stay to the wind turbine tower; and

attaching respective second ends of each tension stay to the anchor of a respective tension stay foundation element.

24. A method of de-constructing a tension stay foundation assembly according to claim 11 , the method comprising:

extracting ballast from the internal cavity of each container; and

removing the plurality of tension stay foundation elements from site.