WO2023072354A1

WO2023072354A1 - Method for performing maintenance on a yaw system of a wind turbine

Info

Publication number: WO2023072354A1
Application number: PCT/DK2022/050220
Authority: WO
Inventors: Nicolaj Biltoft KRISTENSEN
Original assignee: Vestas Wind Systems A/S
Priority date: 2021-11-01
Filing date: 2022-10-20
Publication date: 2023-05-04

Abstract

A method for performing a maintenance action on a wind turbine yaw system. The wind turbine comprises a tower having a top flange, and a base frame mounted to the top flange at a yaw bearing including a yaw ring. The method comprises: providing a jacking device configured to lift the base frame with respect to the top tower flange, mounting the jacking device between the tower and the base frame; lifting the base frame away of the top tower flange using the jacking device; disconnecting the yaw ring from the top tower flange; removing the yaw ring; and assembling a replacement yaw ring on the top tower flange from a plurality of yaw ring segments. A benefit of the invention is that a process is provided for replacing a single-piece yaw ring with a segmented yaw ring in situ which minimises the disassembly work required on the wind turbine. Single piece yaw rings are heavy items which are usually impractical to replace after installation. However, using a jacking device to lift the nacelle and a segmented yaw ring enables the replacement yaw ring to be installed piece-by-piece whilst the nacelle has been lifted using the jacking device. This approach reduces the cost of maintenance.

Description

METHOD FOR PERFORMING MAINTENANCE ON A YAW SYSTEM OF A WIND TURBINE

Technical Field

The invention relates to a method of performing a maintenance action on a yaw system of a wind turbine, such as replacement and/or installation of such a yaw ring. The invention also relates to the configuration of a yaw ring.

Background to the Invention

A common configuration of wind turbine is the so-called horizontal axis wind turbine, or HAWT. In such a wind turbine, a nacelle is supported on top of a tower. The nacelle houses the power generation equipment for the wind turbine and is coupled to a rotatably supported rotor, usually having three blades.

Since wind flow can come from different directions, a wind turbine nacelle is typically connected to its tower by a yaw system. That yaw system comprises a bearing located between the top ofthe tower and a suitable support structure of the nacelle. A yaw system further comprises a drive system which is configured to rotate the nacelle on the yaw bearing with respect to the tower.

Driven by the overall cost of energy, wind turbines are becoming ever larger. It is now known for offshore wind turbines to exceed 10MW generation capacity. This means that towers are getting taller and larger in diameter, and nacelles and rotors are becoming larger and heavier. These factors drive up the size and weight of the yaw system. For example, it is known for yaw bearings to have a diameter in excess of 5m, corresponding to the diameter of the flange at the top of the tower. This means that yaw bearing are very heavy components which makes them challenging to work on after installation. Further, since yaw bearings are sandwiched between the nacelle and the top tower flange, the nacelle must be removed for the bearing to be replaced. Typically, this will require a crane, but this is not desirable for increased tower top heights, and particularly in offshore locations where a floating vessel would also be required. It is an object of the present invention to provide a solution to one or more of the problems mentioned above.

Summary of the Invention

According to a first aspect of the invention, there is provided a method for performing a maintenance action on a wind turbine yaw system. The wind turbine comprises a tower having a top flange, and a base frame mounted to the top flange at a yaw bearing including a yaw ring. The method comprises: providing a jacking device configured to lift the base frame with respect to the top tower flange, mounting the jacking device between the tower and the base frame; lifting the base frame away of the top tower flange using the jacking device; disconnecting the yaw ring from the top tower flange; removing the yaw ring; and assembling a replacement yaw ring on the top tower flange from a plurality of yaw ring segments.

A benefit of the invention is that a process is provided for replacing a single-piece yaw ring with a segmented yaw ring in situ which minimises the disassembly work required on the wind turbine. Single piece yaw rings are heavy items which are usually impractical to replace after installation. However, using a jacking device to lift the nacelle and a segmented yaw ring enables the replacement yaw ring to be installed piece-by-piece whilst the nacelle has been lifted using the jacking device. This approach reduces the cost of maintenance.

The step of removing the yaw ring may comprise disassembling or even cutting the yaw ring into a plurality of yaw ring pieces. In this way, the yaw ring is easier to remove since each piece is more portable.

The step of assembling the replacement yaw ring on the top tower flange may include connecting a first one of the plurality of yaw ring segments in a predetermined position on the top tower flange, and assembling others of the plurality yaw ring members to the top tower flange in a predetermined sequence. The first one of the yaw ring segments to be connected therefore acts in effect as a datum for the other yaw ring segments, thereby ensuring accurate positioning and connection of the other yaw ring segments.

Brief Description of the Drawings The present invention will now be described, by way of example only, with reference to the attached drawings, in which:

Figure 1 is a schematic view in cross section of a wind turbine tower connected to a nacelle by a yaw system;

Figures 2a-2c show a sequence of steps indicating how a yaw ring may be installed on a wind turbine tower;

Figure 3 is a view from above of a yaw ring of the yaw system in Figure 1 ;

Figure 4 is a circumferential cross section through a portion of the yaw ring shown in Figure 3, together with a plan view of that portion, which shows more clearly how neighbouring yaw segments of the yaw ring interlock together;

Figures 5 to 7 illustrate further aspects of installing a yaw ring assembly onto a wind turbine tower; and

Figures 8a, 8b and 9a, 9b illustrate various views of an example of a jacking device as may be used in a yaw ring installation process in accordance with the invention.

Detailed Description

A specific embodiment of the present invention will now be described in which numerous features will be discussed in detail in order to provide a thorough understanding of the inventive concept as defined in the claims. However, it will be apparent to the skilled person that the invention may be put into effect without the specific details and that in some instances, well known methods, techniques and structures have not been described in detail in order not to obscure the invention unnecessarily.

Yaw rings are heavy components that are complicated to work on. They are sandwiched between the nacelle and the tower and so it is necessary to separate those two components to replace the yaw ring. Conventionally, an external crane is provided to lift the nacelle up to provide access to the yaw ring. However, crane access becomes problematic as tower height increases, and offshore installations provide an added complication. The examples of the invention provide an approach for lifting a nacelle off the tower to provide yaw ring access that does not require a crane. Instead, the nacelle is jacked off the tower using a dedicated jacking device or set of jacking devices which are attached to the tower and connected to the nacelle base frame. Significantly, the jacking point on the base frame is located radially inwards of the tower wall, or top tower flange. This means that the jacking devices can be accessed, installed, and operated from inside the tower which is beneficial particularly in offshore environments where high wind speeds and moist, salty air are present. The nacelle lifting approach of the examples of the invention provides opportunities for various yaw ring maintenance actions to be performed more easily than has been possible with conventional approaches. Further discussion is provided below.

With reference to Figure 1 , a wind turbine 2 includes a tower 4 and a nacelle 6 mounted on top of the tower 4. It should be noted that Figure 1 is schematic in form and that the tower 4 is shown only partially so as not to obscure the invention. Consequently, other features of the tower 4 that may usually be present such as access structures, power cables, lightning protection systems and so on have not been shown for brevity.

The nacelle 6 is mounted to the tower 4 by way of a yaw system 8. The yaw system 8 includes a yaw ring 10 that is situated between the nacelle 6 and the tower 4 and allows the nacelle 6 to rotate with respect to the tower 4.

As shown here, the yaw ring 10 is a single part which sits between the tower 4 and the nacelle 6 and provides a slide bearing formation, as is known in the art. Conventionally slide bearing are used as yaw bearings because they are well-suited to cope with the high loads that are generated from the mass of the nacelle.

In the illustrated embodiment the nacelle 6 includes a base frame 18 which is slidingly coupled to the yaw ring 10. Yaw claws 20 may be provided for this purpose which, as is known, constrain axial movement of the base frame 18 but allow the base frame 18 to rotate angularly about the tower axis A. The base frame 18 can be any structure that is suitable to transfer the load of the nacelle 6 to the tower 4 via the yaw ring 10. Typically, the base frame 18 is connected directly or indirectly to the main bearings of the nacelle 6, as well as the supporting structure for the outer walls of the nacelle 6. The yaw ring 10 is fixed to an upper flange of the tower 4, which will hereinafter be referred to as a top tower flange 22. The fixing may be achieved by any suitable means, which is typically a circular array of bolts 23.

The yaw ring 10 defines a gear surface 24 which faces radially outwards in the illustrated embodiment, with respect to the tower axis A. Note that in some arrangements this configuration may be reversed such that the gear surface faces radially inwards.

The yaw system 8 further comprises a yaw drive 26. The yaw drive 26 includes a yaw gear 28 that is engaged with the gear surface 24 of the yaw ring 10. As shown, the yaw drive 26 is associated with the nacelle 6. Therefore, operation of the yaw drive 26 turns the nacelle 6 with respect to the tower 4.

As has been discussed above, it is conventional for an external crane to lift the nacelle 6 off the tower 4 in the event that a maintenance action needs to be performed on the yaw ring 10. However, the examples of the invention propose a different approach, as discussed above, and as will now be explained further with respect to Figures 2a to 2c. It should be noted that although Figures 2a-2c show a wind turbine 2 similar to that shown in Figure 1 , the wind turbine has been simplified for the purposes of this discussion.

In Figure 2, the wind turbine 2 includes a jacking device 30 for jacking up the nacelle 6 with respect to the tower 4. As can be seen, the jacking device 30 is mounted between the nacelle 6 and the tower and, more specifically, between the base frame 18 of the nacelle 6 and the top tower flange 22 of the tower 4.

The jacking device 30 includes a jack body 32 that supports a movable jacking arm 34. As shown, the jacking arm 34 is oriented in this example to move along a jacking axis J that is parallel to the tower axis A. It should be noted that the axis J along which the jacking arm 34 acts is radially inwards of the tower wall. Also, here, the jacking axis J is radially inwards of the top tower flange 22.

The jack body 32 is shown here as mounted to the underside of the top tower flange 22. The mounting of the jack body 32 may be achieved by a suitable array of bolts 40. The bolts 40 may be inserted in selected ones of a circumferentially arranged set of bolt holes that are used to connect the yaw ring 10 to the top tower flange 22. Beneficially, this uses existing bolt holes so that purpose-drilled bolt holes do not need to be provided. Alternatively, the jack body 32 may be mounted to the top tower flange 22 by a specially drilled set of bolts holes. The skilled person would appreciate that the number of bolts required, and the size/length of the bolts would be designed to be appropriate for the load that the jack body 32 must withstand. This would be within the capabilities of the skilled person.

In Figure 2a, a single jacking device 30 is shown for clarity. However, it should be appreciated that more than one such jacking device may be used. Where multiple jacking devices are used, these may be distributed circumferentially around the top tower flange 22 to ensure an even lifting load is applied to the base frame 18.

The jacking arm 34 engages with the base frame 18 at a suitable jacking point 42 provided on the base frame 18. The jacking point 42 can be any suitable location that is strong enough to support the weight of the nacelle 6 as the base frame 18 is lifted from the top tower flange 22. In one example, the jacking point 42 may be a portion of a circumferential bolt array provided on the base frame 18 and to which yaw claws of the yaw system usually attach. Thus, in order to attach the jacking arm 34 to the jacking point 42, a selected one or more yaw claws may be removed thereby providing empty bolt holes to which the jacking arm 34 may be connected. The jacking point 42 is engaged by a jacking rest 44 of the jacking arm 34.

In terms of the form that the jacking device 30 may take, it is envisaged that the jacking device 30 may most appropriately be a hydraulic device, although this is not essential and screw-operated jacking devices are also anticipated. However, since in most applications the nacelle will be very heavy, a hydraulic jacking device 30 is considered most appropriate. One option is to use a manually operated hydraulic device in which a manually operated pump generates hydraulic pressure. This may be appropriate for smaller nacelles, but for larger nacelles it is envisaged that an electrically powered hydraulic system would be more effective. Indeed, the jacking device 30 may be connectable to a hydraulic system of the nacelle to provide the necessary hydraulic power.

In this context, it is the jacking arm 34 that is mobile with respect to the jack body 32 and which bears against the jacking point 42 on the base frame 18. In other embodiments, however, it is envisaged that the jacking device 30 could be reconfigured such that the jacking arm 34 is connected to the top tower flange 22 and the jack body 32 is connected to the jacking point 42. In either configuration, the jacking device 30 is configured to lift the base frame 18 with respect to the top tower flange 22.

Figure 2b shows the jacking device 30 being operated. As such, the base frame 18 is being lifted away from the top tower flange 22. It will be appreciated at this point that any mechanical fastenings, such as the yaw claws 20 shown in Figure 1 , between the base frame 18 and the top tower flange 22 must be removed in order for the base frame 18 to be lifted. Here, it will be appreciated that the yaw claws 20 are not shown in Figs 2a-2c.

The purpose of lifting the base frame 18 off the top tower flange 22 is to enable the yaw ring 10 to have a maintenance action performed on it or for the yaw ring 10 to be removed entirely to allow replacement. The base frame 18 must therefore be able to be lifted a suitable distance for this to be permitted. Currently it is envisaged that a lift distance of between 10cm and 50cm would be appropriate, and more particularly between 10cm and 25cm.

Figure 2c illustrates that once the base frame 18 has been lifted from the top tower flange 22, the yaw ring 10 may be removed.

The discussion will now focus on various maintenance actions that may be performed on the yaw ring 10 once the base frame 18 has been lifted from the tower 4.

Figures 3, 4, 5 and 6 show a process for replacement of a single-piece yaw ring with a segmented yaw ring. In this context it will be appreciated that a yaw ring is typically a heavy component and so is problematic to replace entirely. However, yaw rings can wear to unacceptable levels in use, in which case replacement of that yaw ring becomes necessary.

A segmented yaw ring 10 is shown in Figures 3 and 4 and is constituted by a yaw ring assembly 50 comprising a plurality of yaw ring segments or members 52. At least some of the yaw ring members 52 are configured to interlock with mutually adjacent ones of the yaw ring members 52. The yaw ring assembly 50 may be comprised of any number of yaw ring members 52. In general, however, the number of yaw ring members 52 may be influenced by the portability required from them. For example, smaller yaw ring members 52 may be carried by hand, although including too many yaw ring members in a yaw ring assembly may prove more complicated to manufacture and assemble. Each yaw ring member 52 may be of equal size, but this is not essential. Notably, each yaw ring member 52 would have substantially identical cross section along their length. However, the circumferential span of the yaw ring members may differ. For instance the circumferential span of one or more of the yaw ring segments may be between 20° and 90°, such as between 25° and 60° or between 30° and 45°.

Figure 3 demonstrates an example where there the yaw ring members 52 are different sizes. Here, it will be noted that there is one large yaw ring member, shown as 52a, which spans approximately 180 degrees, and a series of smaller yaw ring members. It should be noted however that this is only exemplary and should not be considered limited. Moreover, other arrangements are acceptable. In a preferred arrangement, the yaw ring members are the same size. It is envisaged that in practice at least four yaw ring members would be a practical number to ensure portability, each of which spanning approximately 90 degrees. For lifting by hand, a limit of around 50kg to 70kg is to be expected so this would mean a larger number of yaw ring members would be required for larger and heavier yaw ring assemblies.

Each of the yaw ring members 52 fit together in an interlocking manner to form the completed yaw ring 10, which is shown here as defining the radially outward facing gear surface 54.

Interlocking of the yaw ring members 52 with their adjacent neighbours provides various advantages. One benefit of the interlocking configuration is that the yaw ring members 52 are made to fit together without the use of external components such as fixing plates. A further benefit is that the interlocking formations between adjacent yaw ring members 52 can be configured to ensure that tension is applied to the assembly which improves its rigidity.

Figure 4 illustrates an example of the interlocking configuration in more detail. In Figure 4, the lower view shows a view from above of the yaw ring assembly 50 of Figure 2, focussing on two neighbouring yaw ring members, which are labelled here as 52a and 52b, whereas the upper view shows a cross section through the line A-A. As can be seen the two yaw ring members 52a, 52b interlock at an interlock region 56. In this region, a first interlocking portion 60 of the first yaw ring member 52a overlaps and interlocks with a second interlocking portion 62 of the second yaw ring member 52b. As can be seen, the interlocking portions 60,62 extend in the circumferential direction. It will be appreciated that each yaw ring member 52a, 52b will include a first interlocking portion 60 and a second interlocking portion 62 disposed at either circumferential end thereof.

The first interlocking portion 60 of the first yaw ring member 52a includes interlocking elements which, in the illustrated embodiment, comprise a recess 64 and a projection, wedge, lug or lobe 66. Similarly, the second interlocking portion 62 of the second yaw ring member 52b includes interlocking elements, which in the illustrated example comprise a recess 68 and a projection, wedge, lug or lobe 70.

The first interlocking portion 60 is shaped in a complementary way to mate with the second interlocking portion 62. As such, the lug 66 of the first interlocking portion 60 fits into the recess 68 of the second interlocking portion 62. Correspondingly, the lug 70 of the second interlocking portion 62 fits into the recess 64 of the first interlocking portion 60.

In this example, it should be appreciated that the cross section shown in the upper view of Figure 4 is uniform throughout the yaw ring members 52a, b. Therefore, the recesses 64,68 and the lugs 66,70 extend across the full radial width of the yaw ring members 52a, b. This is an advantage in terms of assembly because the yaw ring members 52 can be slid against one another in the radial direction, allowing easy placement of a yaw ring member against other members.

Although the interlocking portions 60,62 provide a useful means of coupling neighbouring yaw ring members 52a, 52b together, an optional yet preferred feature is to provide a supplementary means to fix the yaw ring members 52a, 52b to each other. This may be achieved by suitable mechanical fasteners such as bolts 72. As shown in Figure 4, the bolts 72 extend through the lug 66 of the first yaw ring member 52a and the recess 68 of the second yaw ring member 52b. Other positions would be acceptable, however, within the principle of mechanically fixing the first yaw ring member 52a to the second yaw ring member 52b. A possible adaptation is that the recesses 64,68 and lugs 66,70 may be configured to define inclined interlocking surfaces. For example, the recesses and lugs may define trapezoidal cross sections, such as being in the form of pyramids or cones. Such a configuration of interlocking elements provides the effect of applying tension between yaw ring members and so provide a more rigid final assembly.

An approach for manufacturing the yaw ring members may be as follows. Each yaw ring member may be cut from a piece of sheet material, for example a large steel sheet. The steel sheet may be constituted from high strength steel suitable for a high load application, as would be understood by a skilled person. Preferably a suitable type of steel is of a grade appropriate to be induction hardened, for example medium-carbon steel (e.g. 0.3 to 0.5% carbon) with or without alloying components. It is envisaged that once each yaw ring member is cut from the sheet, the yaw ring members would be processed appropriately with interlocking elements as discussed above for interlocking with adjacent yaw ring members, and also have formed therein suitable bores for receiving coupling bolts. This can be achieved using suitable milling and drilling/boring processes.

Following the formation of the individual yaw ring members, they are assembled together to form a complete yaw ring assembly. At this point, appropriate surface finishing such as grinding and polishing processes can be performed in order to create a smooth bearing surface, as required. At this point the yaw ring assembly can also be processed/cut appropriately to form its gear surface. In principle, the gear surface could be applied by an additive manufacturing process, but it is envisaged most likely that a subtractive process such as milling would be most appropriate to maximise strength.

Once the yaw ring assembly has been fully formed, it can be appropriately marked so that the order of assembly can be followed later. Since the gear surface has been formed on the assembly yaw ring, it is important that the separate yaw ring members are assembled in the correct order. Therefore, the yaw ring members may be marked appropriately with identifying marks that indicate the assembly sequence. At this stage, in the event that a spare yaw ring is required in the future, a suitable three — dimensional scan of the fully assembled yaw ring assembly can be performed so that an identical yaw ring may be manufactured at a later date when needed. The yaw ring assembly can then be disassembled and stored ready for use. Returning to Figure 2c, it has been described above that the yaw ring 10 may need to be removed because it is damaged. However, removal and installation of a single-piece yaw ring may be problematic in the situation shown here, where the nacelle is jacked up from a position inside the outer walls of the tower. Therefore, a beneficial approach is to remove the existing single-piece yaw ring piece-by-piece and then replace that yaw ring with a segmented yaw ring assembly 50 as described above.

Figure 5 illustrates an example of the removal of a single piece yaw ring 10. Here, the yaw ring 10 is depicted as being cut into a plurality of pieces using a suitable cutting device 71. In this way each piece is of a suitable size that it may be lifted manually by maintenance personnel. Cutting the yaw ring 10 into several pieces also means that the positioning of the jacking devices 30 does not obstruct removal of the yaw ring.

Although Figure 5 illustrates the situation where the yaw ring 10 is comprised of a singlepart, this approach also applies to a yaw ring assembly comprising multiple segments that are installed on the top tower flange. Such a yaw ring assembly could thus be disassembled from the tower top piece-by-piece ready for the installation of a replacement yaw ring assembly.

Once the yaw ring has been removed from the top tower flange 22, a replacement yaw ring can be brought into position and fixed to the top tower flange 22. This is depicted in Figure 6 in which the individual yaw ring members 52 are shown as being assembled onto the top of the top tower flange 22 of a wind turbine tower 4. The yaw ring assembly 50 is shown in its assembled position in dashed lines.

Various approaches may be taken to assemble the yaw ring assembly 50 on top of the tower 4. In one example, all of the yaw ring members 52 can be placed into their predetermined locations and interlocked with one another. Once the yaw ring assembly 50 is in place on the tower, then appropriate fasteners such as bolts can be applied to secure the yaw ring assembly 50 to the top tower flange 22.

In another example, a starting segment can be applied to the top tower flange 22 as an initial step and connected to the tower using e.g. bolts, as is common in the art. The starting segment therefore acts as a datum piece from which the rest of the yaw ring assembly 50 can be assembled. Each segment can therefore be added to the assembly in a piece-by- piece basis until the entire yaw ring assembly 50 has been completed. The individual segments can be bolted down at an appropriate time, which may be when the yaw ring assembly is completed. Alternatively, each segment can be bolted down one it has been placed in position.

The discussion will now turn to a further aspect of the invention as is depicted by Figure 7 which shows another maintenance action being performed. In Figure 7, a yaw ring 10 is lifted from the top tower flange 22 to address a worn section of the yaw ring 10. As can be seen in the figure, the yaw ring 10 has a worn gear region, illustrated here as ‘80’. During use of a wind turbine, wind tends to flow from a prevailing direction and so the nacelle spends most of its operational life oscillating around a main wind heading. This means that mechanical wear of the gear surface is accelerated in a particular circumferential region or regions of the yaw ring 10 due to consistent operation of the yaw drive in that region. In this case, the wear may take the form of eroded, chipped or cracked gear teeth, which can interrupt the proper functioning of the yaw drive and can result in inaccurate detection of yaw position since a gear tooth position sensor may miss movement of the yaw ring due to the worn gear teeth.

In the process of Figure 7, at step 100 the yaw ring 10 is removed from the top tower flange 22. Before the yaw ring 10 can be removed, the nacelle 6 must be lifted from the top tower flange 22. This may be achieved by a nacelle lifting process as described above with reference to Figures 2a to 2c.

When access to the yaw ring 10 has been achieved, it can be disconnected from the top tower flange 22 and removed. This may be achieved using an internal crane system of the nacelle which is typically used to hoist components up through the tower. Once the yaw ring 10 has been removed from the top tower flange 22, it is turned angularly through a predetermined angle, as is shown at step 102. It is to be noted that the yaw ring 10 is turned angularly about its axis which is coincident with the tower axis A.

In principle any predetermined angular position is acceptable, but it is envisaged that the position change should be sufficient so that the worn gear region 80 is no longer subject to wear by the gear drive. The precise repositioning that is required may be determined through analysis of how the nacelle yaw system operates over an extended period of time. An example of the angular positioning is shown in Figure 7, in which the worn gear region 80 is rotated angularly through approximately 90 degrees to a new position illustrated as ‘90’. Following the rotation of the yaw ring 10, it can be reconnected to the top tower flange 22 at a different angular position. By ‘different angular position’ it is meant that the yaw ring has been moved angularly about the central tower axis, and therefore the rotational axis of the yaw ring, by a predetermined angle and then reconnected to the tower. It may be moved as a single piece. So, comparing the orientation of the yaw ring before and after it has been disconnected from the tower and reconnected to the tower, a point on the circumference of the yaw ring will have moved in the circumferential direction for a distance that corresponds to the angular movement of the yaw ring.

Note that the angular displacement of the yaw ring 10 may be achieved in different ways. If the yaw ring 10 is a single component, then the entire yaw ring may be lifted and then rotated. Alternatively, if the yaw ring is a segmented yaw ring comprising a plurality of individual segments, then the yaw ring may be disassembled and re-assembled on the top tower flange 22 in a different angular position.

Through the use of this approach, the useful life of the yaw ring can be extended which defers the cost of replacement.

The skilled person would appreciate that various modifications may be made to the examples of the invention shown and described here without departing from the inventive concept as defined by the claims.

As discussed above, the jacking devices 30 may take any suitable form and so may be hydraulically operated or be motorised screw-type jacking devices, by way of example. Although the jacking devices are shown in a simplified schematic form, the configuration of a jacking device is considered to be within the capacity of a skilled person. However, an example of a jacking device is also shown in Figures 8a, 8b, 9a, 9b, and is labelled generally as 100. Parts in common with other drawings will be denoted with the same reference numbers.

The jacking device 100 of this example comprises a jacking cradle 102 which is generally L-shaped in form, when considered in the orientation shown in the drawings, in which the jacking device 100 is mounted to the tower in an upright configuration. The jacking cradle 102 therefore is generally L-shaped in cross section when considered in a plane taken radially through the tower centre . As such, the jacking cradle 102 includes a cradle base 104 and an upright cradle stanchion 106. The cradle base 104 and the cradle stanchion 106 extend transversely to one another, and are approximately perpendicular in this example.

The cradle stanchion 106 defines a free end 108, being the upper end as illustrated, which is configured to engage the underside of the nacelle base frame 18. As shown in the Figure, the free end 108 of the stanchion 106 is engaged with the base frame 18 and is bolted to it by way of a set of bolts 110.

The cradle base 104 supports an actuation system 112 for operating the jacking device 100. The actuation system 112 may be hydraulically operated such as a hydraulic ram, an electrically driven linear actuator, or another suitable actuator. As illustrated, the actuator system 112 includes a central support arm 113 that includes a support bolt 114 (see Figures 9a and 9b) which is installed through a bolt hole 115 in the top tower flange 22. A pair of linear actuators 116 flank the support arm 113, one on either side. In this example, each of the linear actuators 116 include a hydraulic supply input 116a. Although a single actuator 116 would be acceptable, a pair of actuators provides greater force and a balanced application of that force. The actuators 116 and the support arm 113 are connected at a yoke 117.

The cradle 102 is movable between jacked and unjacked positions. In Figures 8a and 9a, it is shown in an unjacked position. In this position, the cradle base 104 is shown in a position that is spaced from the underside of the top tower flange 22.

When operated, the actuator system 112 functions to move the cradle base 104 towards the top tower flange 22. As shown, the actuators 116 work to push against the yoke 117 thereby pushing the cradle stanchion 106 upwards, in the orientation of the figures. In doing so, the cradle stanchion 106 lifts the base frame 18 clear of the yaw ring 10. This is the position shown in Figures 8b and 9b.

The cradle base 104 extends for a length in the circumferential direction and provides a support block 120 for supporting a plurality of bolts 122. Since the cradle base 104 follows the circumference ofthe tower wall, it is shaped similarly to form an arc. The bolts 122 are engageable with corresponding holes 124 in the tower top flange 22, as can be seen in Figures 8a and 8b.

When the cradle 102 is in the jacked position, the support block 120 abuts the underside of the tower top flange 22. The bolts 122 can then be adjusted such that the nuts 123 of the bolts 123 are tightened up against the support block 120. This ensures that the bolts 122 bear the weight of the nacelle base frame 18 in addition to the actuation system 112. This provides a safety feature in the event of malfunction of the actuation system 112 since the bolts 122 effectively lock the cradle 102 to the tower top flange 22.

In this way, the actuators 116 serve to push the cradle 102 into its engaged position with the tower top flange 22. However, once the jacking device 100 is in the jacked position, it forms a strong bolted connection which can remain in place indefinitely until required service work has been performed. In some embodiments, it is envisaged that the support arm 113 and the actuator units 116 may be removed until such time that the jacking device 100 needs to be moved back to the unjacked position.

Claims

1 . A method for performing a maintenance action on a wind turbine yaw system (8), wherein the wind turbine comprises a tower (4) having a top tower flange (22), and a base frame (18) mounted to the top tower flange at a yaw ring (10), wherein the method comprises: providing a jacking device (30) configured to lift the base frame (18) with respect to the top tower flange (22), mounting the jacking device between the tower and the base frame; lifting the base frame away of the top tower flange using the jacking device; disconnecting the yaw ring from the top tower flange; removing the yaw ring; and assembling a replacement yaw ring (50) on the top tower flange from a plurality of yaw ring segments (52).

2. The method of Claim 1 , wherein the step of removing the yaw ring comprises: disassembling the yaw ring (50) into a plurality of yaw ring pieces.

3. The method of Claim 1 , wherein the step of removing the yaw ring comprises: cutting the yaw ring into a plurality of yaw ring pieces.

4. The method of any of Claims 1 to 3, wherein the step of assembling a replacement yaw ring on the top tower flange includes interlocking at least some of the yaw ring segments (52) with other ones of the yaw ring segments (52).

5. The method of Claim 4, wherein the yaw ring comprises a plurality of identical interlocking yaw ring segments (52).

6. The method of any one of the preceding claims, wherein the step of assembling the replacement yaw ring on the top tower flange includes: connecting a first one of the plurality of yaw ring segments (52) in a predetermined position on the top tower flange; assembling others of the plurality yaw ring segments (52) to the top tower flange in a predetermined sequence.

7. The method of Claim 6, wherein the others of the plurality of yaw ring segments (52) are assembled on the top tower flange to form a complete yaw ring before connecting said yaw ring segments (52) to the top tower flange.

8. The method of any one of the preceding claims, wherein each of the yaw ring segments (52) comprises a radially outward facing gear surface (54).

9. The method of any one of the preceding claims, wherein the yaw ring segments (52) are positioned next to each other along the circumference to form the completed replacement yaw ring (50).

10. The method of any one of the preceding claims, wherein said replacement yaw ring (50) consists of a plurality of yaw ring segments (52) with substantially identical crosssections.

11. The method of any one of the preceding claims, wherein the circumferential span of at least one of the yaw ring segments (52) is between 20° and 90°, such as between 25° and 60°.

12. The method of any one of the preceding claims, wherein the circumferential span of each of the yaw ring segments (52) is between 20° and 90°, such as between 25° and 60°.

13. The method of any one of the preceding claims, wherein the radial span of each of the yaw ring segments (52) is along the full radial width of replacement yaw ring (50).