Field of the invention

The present disclosure relates to a method for repairing damage to a foundation of an onshore wind turbine installation, which may involve partial-replacement of regions of the foundation.

Background of the invention

Onshore wind turbines are supported on a concrete slab that is known as a foundation. Such a foundation typically includes a part that defines a top surface that extends above the surrounding ground level and which is adapted for fixing to a base of a wind turbine tower; this part is typically referred to as a pedestal. In a conventional mounting approach, a base flange of the wind turbine tower is connected to the pedestal of the foundation either directly or by way of an adapter plate, coupled with an arrangement of grout and pre-tensioned anchor bolts.

The concrete foundation is exposed to harsh weather conditions and is subject to high compressive and tensile loads due to the movement of the tower. These factors can lead to cracking and flaking of the concrete structure, particularly around the base of the tower where the foundation is most exposed. Although such damage is understood not to compromise the structural integrity of the foundation initially, it may mean that the foundation is vulnerable to freeze-thawing effects and so repair or replacement is desirable before the structural integrity is affected. Such issues can also occur if the compressive strength of the cured concrete is not at the required level. When inspection identifies unacceptable degradation and cracking of the foundation, a repair strategy must be employed. Currently, remedial work on surface damage may involve injecting mortar or resin into the cracks and subsequent water sealing. However, if the damage is severe and extends below the tower base flange then a full or partial pedestal replacement may be required. In this case, known approaches demand that the wind turbine is dismantled and disconnected from the foundation. With the wind turbine removed, the damaged foundation may be repaired. Alternatively, the wind turbine may be moved to an entirely new foundation. Both solutions are costly and can result in significant wind turbine downtime and further loss in revenue. It is against this background that the invention has been developed.

Summary of the invention

According to a first aspect of the invention there is provided a method of repairing a wind turbine concrete foundation in a wind turbine installation. The wind turbine installation includes a wind turbine tower mounted on the foundation at a tower flange, and the foundation comprises an anchor cage embedded within a central pedestal. The anchor cage comprises a plurality of circumferentially spaced tensioned anchoring rods to which the tower flange is attached. The method comprises: identifying at least one first section of the pedestal which requires replacement; de-tensioning anchoring rods located within the first section; excavating the first section to form a cavity, wherein that cavity extends at least partially under the tower flange; filling the cavity with a filler material; allowing the filler material to cure until a predetermined strength of the filler material is achieved; and re-tensioning the de-tensioned anchoring rods located in the first section.

The method of the invention provides a process by which the pedestal of the foundation can be repaired or at least partially replaced whilst the tower remains situated on the pedestal. This avoids the need for the wind turbine to be disassembled before the pedestal can be repaired therefore providing a saving in both time and cost. Since the pedestal can be repaired more quickly, the wind turbine can be returned to operational readiness more rapidly.

Before the identified section or sections have been evacuated it is required to de-tension the anchor rods to reduce the compressive force exerted by the tower flange on the region of the pedestal that is to be replaced. In addition, it may be beneficial to de tension at least one anchoring rod located adjacent the first section.

In the case where the anchoring rods comprise protection sleeves, the method may include replacing said protection sleeves on respective anchoring rods within the cavity, before filling the cavity with filler material. This ensures that the anchoring rods do not bond with the filler material.

Although the method of the invention may be used to replace a single section of the pedestal, the method may also involve the identification of a plurality of sections of the pedestal that require replacement. In such a case, the method may further include excavating more than one of the identified sections to form two or more cavities in the foundation. The two or more cavities may be spaced from one another.

In one embodiment, the plurality of identified sections extend about the tower flange. For example the plurality of identified sections extend about the tower flange in an annular or part-annular arrangement. Effectively, therefore, the plurality of identified section may provide a contiguous ring-like arrangement of sections that define an underlying annular section of the pedestal that will be replaced underneath the tower flange.

In one embodiment, the step of identifying a first section of the pedestal that requires replacement may includes grouping the identified sections into at least first and second groups, wherein the sections in each group are associated with a set of respective identifiers, such that at least some of the sections in the first group are associated with identifiers that are in common with at least some of the sections in the second group. In such a case, the method may include selecting a first section in the first group and selecting a first section in the second group, those first sections having a first identifier in common. Those identified first sections having a common identifier may be replaced first before moving on to replacement of other sections identified with other common identifiers during a separate working procedure.

Therefore, the may include allowing the filler material within each of the respective cavities associated with the first sections to cure before starting a further working procedure. To provide a controllable technique of excavating the cavities, preferably the excavation of a section includes water-jet excavation.

It will be appreciated that preferred and/or optional features of the first aspect of the invention may be combined with the other aspects of the invention. The invention in its various aspects is defined in the independent claims below and advantageous features are defined in the dependent claims below.

Brief description of the drawings

For a better understanding of the invention, some embodiments of the invention will now be described with reference to the following drawings, in which:

Figure 1 is a front view of an onshore wind turbine installation to which the invention is applicable, including a wind turbine mounted to a subterranean foundation;

Figure 2 is a detailed view of a part of the foundation show in Figure 1 ;

Figure 3 is a perspective view of the foundation that illustrates possible wear- related degradation that may occur;

Figure 4 is a plan view of the foundation which depicts an arrangement of identified sections that extend about the pedestal of the foundation;

Figure 5 is a flowchart of the method of the invention; and

Figures 6 to 9 show a series of steps that correspond to the flowchart of Figure 5.

Detailed description of embodiments of the invention

Figure 1 shows a wind turbine installation comprising a wind turbine 2 which in this example is a horizontal-axis wind turbine (HAWT) that includes a nacelle 6 mounted on top of a tower 8. The nacelle 6 supports a rotor 10 including a hub 12 and three blades 14. The tower 8 of the wind turbine 2 is mounted to a foundation 16 which is made of reinforced concrete and is embedded in the ground 18 in the usual way. Although a HAWT is shown here, it should be noted hat the embodiments of the invention as will be described below are also applicable to other forms of wind turbines, for example vertical axis or‘Darrieus’ type wind turbines.

In its most basic form, the base of the tower 8 is supported by and is affixed to the foundation 16 by a suitable fastening system. As the skilled person will appreciate there are various approaches to achieving this fixing, but one example will be described here for context. However, this specific example should not be taken to be limiting to the scope of the invention as defined by the claims.

Figure 2 shows a more detailed view of the mechanical interface between the tower 8 and the foundation 16. As can be seen, the tower 8 includes a tower base flange 20 that is secured to an upper surface 21 of an upstanding pedestal 22 of the foundation 16. Embedded within the concrete body of the foundation 16 is an annular strengthening structure that is referred to as an anchor cage 24. Such a structure is well-known to the skilled person as a system that is conceived to transfer the loads of the wind turbine to the foundation in a more effective way. It is known for such anchor cages to be embedded within their foundation so that they are fully‘submerged’ but also where parts of the anchor cages extend beyond the concrete mass. The term‘embedded’ therefore should be interpreted accordingly.

The anchor cage 24 includes a lower base flange or‘anchor plate’ 30, a load distribution flange 32 (which is optional) and a plurality of circumferentially spaced anchor rods 34 that extend between the lower base flange 30 and the load distribution flange 32. The anchor rods 34 are arranged in a pair of parallel rows. During assembly of the foundation 16, and before pouring of the concrete, the anchor cage 24 is assembled and placed within a lattice rebar structure of the foundation 8. Concrete is then poured into the rebar structure which is surrounded by suitable formwork to form the completed foundation. For simplicity, it should be appreciated that the rebar structure of the foundation has been omitted from the drawings, but the skilled person would understand the form that such a rebar structure would take.

In the position shown in Figures 1 and 2, the load distribution flange 32 is situated at the upper surface 21 of the pedestal therefore forming an interface to which the tower base flange 20 of the wind turbine tower 8 can be connected. The tower base flange 20 is provided with a plurality of thru-holes (not shown in the Figures, but implied) which mate to the arrangement of the anchor rods 34. Respective nuts 40 are tightened over the anchor rods and tensioned appropriately to secure the tower base flange 20 in position and provide a secure anchoring point for the wind turbine.

As will be appreciated in Figure 2, much of the foundation 8, including the pedestal 22, is subterranean although a small margin encircling the base of the tower is generally exposed at above ground level. Although concrete is a tough material that has favourable properties for the job the foundation has to perform, it is sometimes the case that the cyclical loads imposed by the wind turbine and severe weather conditions can combine to cause cracks 42 in the concrete material of the foundation, particularly where the pedestal 22 of the foundation 16 is exposed. Such a scenario is depicted in Figure 3.

Although some degree of surface degradation is to be expected and accommodated, in some circumstances it may be desirable to replace a part of the foundation, and particularly the mass of concrete that forms the pedestal 22 on which the tower 8 is supported. As discussed above, replacement of the pedestal 22 is a significant undertaking which is time consuming and expensive because existing approaches involve the de-installation of the wind turbine only after which the affected parts of the foundation can be repaired. In some instances, an approach involving replacement of the entire foundation is preferred.

Against this context, the embodiments of the invention propose a solution to the problems described above in which the pedestal 22 of the foundation 16 can be repaired and, indeed at least partially replaced, whilst the wind turbine tower 8 is still anchored to it. From a broad perspective, the inventive concept involves identification of a damaged section or multiple sections of the foundation and excavating the section(s) so as to form a respective cavity that extends below the tower base flange 20, whereinafter that cavity is filled with a suitable filler material such as grout. A comparatively non-destructive excavation technique is preferred such as high-pressure water jetting, as this is capable of removing concrete from the identified section whilst not affecting either reinforcing bars in the section nor the anchor rods that extend vertically through the section.

An embodiment of the invention will now be described in more detail with reference to Figure 4-9. At this point it should be noted that the method of the invention is directed to replacing substantially the entire pedestal 22 of the wind turbine foundation 16 on a section-by- section basis as it is envisaged that this approach will provide the most reliable and predicable repair of the foundation. However, it is possible that only a smaller number of sections may be replaced, or even just a single section, if that is considered as suitable for providing a complete repair to the pedestal 22.

Figure 4 illustrates a diagrammatic plan view of the foundation and is centred on the pedestal 22, thus depicting the load distribution flange 32. It should be noted that the following repair and replacement procedure is undertaken with the tower situated on the load distribution flange 32. For clarity, however, the tower is not shown in Figure 4, although its presence is implied.

Also with reference with Figure 5, the method of the invention is started by determining the scope of repair, as shown at step 100. Although only a limited repair may be needed, the illustrated embodiment assumes that a significant repair is required such that a partial replacement of the pedestal 22 is needed.

Once the damage and the extent of necessary repair have been determined, the pedestal 22 is divided into a section arrangement 50 comprising a plurality of sections 52 (Fig 5 - step 102). Only three of the individual sections are labelled in Figure 4. In this embodiment, the arrangement of sections 52 extend about or encircle the pedestal 22 in an annular arrangement. The sections 52 are contiguous in the sense that one section is located so as to be adjacent to a neighbouring section and shares a common boundary with it. It will be appreciated that this type of arrangement assumes that the significant proportion of the pedestal will be replaced. However, a different arrangement would be suitable if only a part of the pedestal 22 requires remedial work. For example, the section arrangement 50 could extend around a restricted arc of the pedestal 22.

As will be appreciated from Figure 4, each of the sections 52 extends radially and circumferentially. In this embodiment, the radial‘depth’ and the circumferential‘width’ of each section 52 is equal, although it is envisaged that this need not be the case in other arrangements as the geometry of each of the sections 52 could be determined individually. In the illustrated arrangement, the section arrangement 50 and so extends from a first or ‘inner’ radius position R1 , when measured from the central axis 54 of the pedestal 22, to a second or‘outer’ radius position R2. The precise dimensions of the inner and outer radius positions R1 and R2 for each section may differ or they may be the same. In general, however, the inner and outer radius positions R1 and R2 may be determined based on understanding of how extensive the damage to the pedestal may be. Alternatively the dimensions R1 and R2 may be specified as extending a sufficient horizontal distance beyond radially inner and outer edges (32a, 32b) of the load distribution flange 32.

Similarly, each of the sections 52 has a predetermined height H, which is shown in the inset panel of Figure 4. In the same way as the radial dimensions R1 and R2 are predetermined, the height dimension H of each section may be specified for example based on the depth of the damaged concrete. In the event that the pedestal is known to comprise a different grade of concrete, than the depth of the pedestal may be influential on the determination of the height H of the section.

The plurality of sections 52 in the section arrangement 50 are divided into at least first and second groups. In the illustrated embodiment, however, the section arrangement 50 comprises four groups (A-E) each containing five sections, such that the section arrangement 50 comprises twenty individual sections 52 in total.

The sections in each group are associated with a set of respective identifiers, such that at least some of the sections in the first group are associated with identifiers that are in common with at least some of the sections in the second group. In the illustrated embodiment, the identifiers are simply integer numbers (e.g. 1 , 2, 3 and 4) although in principle any appropriate identifier scheme could be used.

Once the scope of the damage has been determined, and the repair area has been worked up into an appropriate number of sections and section groups, the repair process can begin by identifying a first one of the sections 52 to excavate (Fig 5 - step 104). It will be appreciated from Figure 4 that the plurality of sections 52 extend at least partially under the load distribution flange 32. Therefore, before the concrete within a section 52 can be excavated, the process includes de-tensioning the anchor rods within that section (Fig 5-step 106). It will be appreciated that the anchor rods 34 are placed in tension by the action of the associated nuts 40 that are affixed to the respective anchor rods 34. Removing the concrete under the load distribution flange 32 could therefore compromise its structure, due to the force exerted on the flange 32 by the nuts 40. Therefore, step of de-tensioning the anchor rods 34 that are within the identified first section avoid any risk of the load distribution flange 32 from buckling. Although it is envisaged that it would be necessary only to de-tension the anchor rods located in the identified section 52, it is also possible to de-tension one or more of the anchor rods in the neighbouring section to the identified section that is to be excavated.

Figure 6 shows this step diagrammatically as the nuts 40 of the relevant anchor rods 34 are have been loosened, thereby relieving the tension in the anchor rods 34 that pass through the identified first section 52 to be excavated. It should be noted that the nuts 40 need to be loosened only slightly, although the position of the nuts 40 in Figure 6 have been exaggerated for clarity.

Once the appropriate anchor rods 34 have been de-tensioned, the first identified section 52 can be excavated (Fig 5 - step 108). A first identified section having been excavated to form a cavity is shown in Figure 6 as‘60’.

Although power tools such as pneumatic drills could be used for the excavation of the identified sections, it is envisaged that a less aggressive approach would be more optimal such as the use of a high pressure water jet device. Such water jet devices are commercially available for blast-removing concrete, but they have the advantage that they do not damage metal objects within the concrete. So, in this case a high pressure water jet device would enable maintenance personnel to excavate the identified section to create a cavity that extends underneath the load distribution flange 32 without damaging the anchor rods 34 that pass through the section or any reinforcing bars that are exposed as the concrete is blasted away.

The process of excavating the identified section to form a cavity 60 may damage any protective sleeves (Figure 2 - 62) that are installed on the anchor rods 34. As the skilled person will appreciate, protective sleeves are often installed on anchor rods in order to prevent the poured concrete from bonding to the anchor rods and interfering with the tensioning process. Such protective sleeves 62 are typically simple plastic tubes or tape and so are easily damaged by the high pressure blasting that will be used to excavate the concrete within the section. Any protective sleeves that are damaged should therefore be replaced before the excavated section 52 is filled (Figure 5 - step 110).

During the excavation work, debris removal may be achieved by the use of a suitable technique such as the use of a slurry vacuum machine.

Following excavation of the section 52 to create the cavity 60, the cavity is filled with a suitable filler material (Figure 5 - step 112). Figure 7 depicts the cavity 60 being filled with filler material 64 through a suitable filler nozzle 66. In order to shape the filled section, suitable formwork such as plastic, wood or metal panels, as would be known to the skilled person, should be applied around the cavity before it is filled. This will ensure that the exposed exterior form of the filled section matches the form of the sections around it.

The filler material may take a variety of forms but in order for it to provide a suitable base to support the tower base flange 20 it is preferred that the filler material has a compressive strength that is equal to or exceeds the compressive strength of the concrete from which the foundation 16 is or should be formed. It is envisaged that a suitable filler material will be a high performance grout material that is commonly used in wind turbine foundations and which would be well-known to the skilled person. One example of which is Conbextra BB92 by Fosroc UK.

Once the section has been filled, a suitable time period is provided to allow the filler material to cure, and therefore harden to result in a section with the required compressive strength as specified (Figure 5 - step 112). Typically a suitable time period would be between 12 to 36 hours to provide a sufficient time for the grout to cure to a maturity level such that the anchor bolts can be re-tensioned.

Once the filler material within the cavity 60 has cured sufficiently, the anchoring rods 34 within that section are re-tensioned as required (Figure 5 - step 114).

At this point in the discussion, it will be appreciated that in the illustrated embodiment there are twenty identified sections 52 within the section arrangement 50 and four groups, which means that five sections 52 are identified with the same identifier. It will also be noted that due to the arrangement of the groups, all of the sections with the same identifier are spaced from the other like sections. Due to this arrangement it is possible to excavate more than one of the first identified sections during the same working procedure. For example, up to five of the sections identified with the identifier ‘#1’ could be excavated simultaneously, or at least within a sequence during which other sections with the same identifier are being allowed to cure. Therefore, the excavation, filling and curing process of two or more of the identified sections is taking place in parallel, that is to say within the same working procedure. In this context, Figure 8 illustrates an example where two sections 52 have been excavated therefore creating a pair of cavities 60 that are spaced from each other circumferentially, whereas Figure 9 illustrates that pair of cavities 60 having been filled with a suitable filler material 64. It will be appreciated that the scenario shown in Figure 8 and 9, and particularly the relative spacing is illustrative only and is not meant to indicate a practical situation. In practice, it may be preferred to excavate sections at the same time that are not directly opposite one another about the pedestal. As shown in Figure 4, there are five groups of sections, each of which contains four sections. This results in five sections (one in each group) that can be excavated simultaneously. Preferably simultaneous excavation does not occur in diametrically opposed positions because that can adversely affect the base tower flange.

In the illustrated embodiment, all of the first identified sections, that is to say the sections that are identified with the same identifier #1 , are excavated, filled and allowed to cure within the same working procedure. Once all of those sections have been completed, then the process repeats by identifying the next set of sections to be excavated, filled and allowed to cure (Figure 5 - steps 116, 118 and 120).

In order to maximise the curing time for the sections that have just been filled, it is preferable that the next sections to be selected for work are spaced from the previously- filled sections. For example, in the illustrated embodiment, the sections identified with the identifier #3 may be excavated, filled and cured in the next work procedure. Then the sections marked as #2 may be excavated, filled and then cured. Finally, the sections identified as #4 may be excavated, filled and cured. By this approach, all of the twenty sections in the illustrated section arrangement 50 will be replaced, thereby effecting a substantial replacement of the pedestal 22.

The skilled person will appreciate that the embodiment described above provides a process in which the entire ring of sections 52 about the pedestal 22 are replaced. However, the process would also apply where individual sections are replaced one by one, although the process may take longer to complete to the cycle time required for the curing process. However, in such a case, it is preferable to select a section for excavation that is non-contiguous with a previously completed section.

Various modifications and alternatives to the illustrated embodiments have already been discussed above. However, the skilled person would appreciate that other variations may be made to the illustrated embodiments without departing form the inventive concept as defined by the claims.