Abstract:
The invention relates to a wind turbine blade bearing removal apparatus and method for facilitating the change-over of the blade root bearing assembly ( 58 ) of a wind turbine blade ( 54 ). A plurality of support shoulders or yokes ( 60 ) are located on the hub ( 50 ) of the wind turbine rotor and provide attachment points for respective actuators ( 62 ). Corresponding attachment points ( 66 ) are provided in the wind turbine blade ( 54 ) via a blade support (section  64 ), which is constructed in the blade between the blade root bearing assembly ( 58 ) and the tip. The actuators ( 62 ) are mounted between the hub ( 50 ) and the blade root bearing assembly ( 58 ) and therefore support the blade ( 54 ) in a suspended position when the bearing is to be retained. The actuators ( 62 ) allow the blade to be easily reattached to the hub ( 50 ) when the blade root bearing replacement is completed.

Description:
[0001]    This invention relates to wind turbines, and in particular to a wind turbine blade bearing removal apparatus and method for facilitating the change-over of the blade bearing segment of a wind turbine blade. 
         [0002]      FIG. 1  illustrates a wind turbine  1 , comprising a tower  2  on which a nacelle  3  is mounted. At least one turbine blade  5  is mounted on a hub  6  to form a rotor  4 . The hub  6  is connected to the nacelle  3  through a low speed shaft (not shown) extending from the nacelle front. The wind turbine illustrated in  FIG. 1  may be a small model intended for domestic or light utility usage, or may be a large model, such as those that are suitable for use in large scale electricity generation on a wind farm. In the latter case, the diameter of the rotor may be as large as 150 metres or more. 
         [0003]    A rotor blade is shown in more detail in  FIG. 2 , to which reference should now be made. The blades  5  comprises a reinforced spar  21  that runs along the length of the blade from the blade root  22  to the blade tip  23 . Blade shells  24 , made out of a composite fibre material, are attached to the spar and provide an aerodynamically shaped blade surface, causing the blade to experience a lifting force as the wind passes across the blade surface. The blade root section of the blade  22  is typically circular in cross section and is provided with a circular blade bearing ring  25 , made of steel or other suitable material, for attachment to a corresponding circular bearing ring on the hub  6 . 
         [0004]    This is shown in more detail in  FIGS. 3 and 4  to which reference should now be made. In  FIG. 3 , the hub is viewed in elevation from the front. Three wind turbine blades  5  are shown as attached to the hub via the blade root bearing assembly  25 . In  FIG. 4 , the corresponding circular bearing ring on the hub  6  is shown in a side elevation view. In practice, the blade root bearing assembly  25  comprises one or more blade bearing rings, at least one of which is coupled to the hub circular bearing ring  26  by fasteners such as bolts retained in bolt holes  27 . 
         [0005]    In larger scale wind turbines, the blades can approach 100 m in length, and weigh as much as several tens of tonnes. The weight of the blade is therefore borne entirely by the blade root bearing assembly  25 , the corresponding circular bearing ring  26  on the hub  6 , and the fastening means. Although, these elements are constructed to withstand a great deal of operational loading and strain, it is occasionally necessary to release the connection between the blade  5  and the hub  6  for maintenance and repair. In particular, it can often be necessary to replace the blade root bearing assembly  25 , with a new part. 
         [0006]    Due to the weight of the blades and the location of the blades at the top of tall wind turbine tower situated in windy locations, connecting and disconnecting the blades from the hub can be a cumbersome process. A crane can be used to support the blade, while an engineer working in the hub releases the fasteners between the blade root bearing assembly  25  and circular bearing ring on the hub  6 . However cranes are expensive and time consuming to use. Additionally, there is a problem that, once detached from the wind turbine hub  6 , a blade  5  is susceptible to buffeting from the wind and if in use with a crane can be set in vibration. 
         [0007]    We have therefore appreciated that there is a need for an improved mechanism for releasing the blade from the blade hub so that maintenance work can be carried out in the blade root bearing assembly  25 , or the hub  6  and connecting bolts. 
       SUMMARY OF THE INVENTION 
       [0008]    The invention is defined in the independent claims to which reference should now be made. Advantageous features are set forth in the dependent claims. 
         [0009]    An embodiment of the invention provides a wind turbine blade comprising: a blade body, having a blade root portion and a blade tip portion; a blade support section connected to the blade root portion, wherein the blade support section comprises: attachment points for mounting a plurality of actuators between the blade and the rotor hub of a wind turbine at which the blade is attached; a support structure for reinforcing the blade root portion in the vicinity of the one or attachment points such that the blade can be suspended by the plurality of actuators and the blade support section, from a wind turbine rotor hub. 
         [0010]    Blade root bearing replacement can therefore be carried out in a more secure, straightforward fashion, as the blade can remain mounted on the hub while the blade root bearing assembly is repaired or replaced. As a result, the process does not require a large crane to carry the blade, leading to reduced costs and easier scheduling of repair operations. The plurality of actuators allows heavy blades to be worked upon, as well as providing stability of the blade when it is detached from the hub. 
         [0011]    The attachment points may be located on the external surface of the blade body, so that the actuators can be easily attached and removed when desired, and so that the spacing of the actuators around the periphery of the blade can be used to create clearance facilitating the removal of the blade root bearing assembly. The attachment points may support the actuators at an angle relative to the blade surface in order to increase the clearance. 
         [0012]    In one embodiment, the support structure is an annular flange connected to the end of the blade body at the blade root portion, and the blade support section has a plurality of connectors for making a secure, releasable connection to a blade root bearing assembly. The flange provides a robust anchor portion for supporting the weight of the blade body. 
         [0013]    In one embodiment, the support structure comprises a frame member comprising a plurality of reinforcing spokes connected within the internal circumference of the blade body in the blade root portion. This provides internal support for the actuator attachment points which may be located on the exterior of the blade surface. The reinforcing spokes may terminate at a reinforced central hub for extra support. 
         [0014]    At least one or more of the reinforcing spokes may connect to the inner surface of the blade body internally of the location where the attachment point brackets are situated. In this way the load placed on the blade by the actuator can be transmitted most effectively to the support structure and damage to the blade can be avoided. With this in mind, the at least one or more of the reinforcing spokes may be connected by a fastener to an attachment point bracket. 
         [0015]    The blade support section may also comprise a reinforcing brace having one or more radial tie arms for providing a cable attachment point. 
         [0016]    In one embodiment, the attachment points are located towards one side of the blade support section such that the other side of the blade support section is substantially free from attachment points. This allows the blade root bearing assembly, once detached from the blade support section, to be slid out to one side of the blade during the repair process past the supporting actuators. 
         [0017]    Advantageously, the attachment points are all located within a 180 degree arc around the circumference of the blade support section, as this leaves one side of the blade support section open for removing the blade support section. 
         [0018]    More advantageously, the attachment points are all located within a 200 degree arc around the circumference of the blade support section, as this leaves one side of the blade support section open for removing the blade support section, while providing a more stable arrangement of actuators. 
         [0019]    The blade support section may comprise three attachment points for attaching three actuators for stability. 
         [0020]    In one embodiment, the blade support section comprises a reinforcing brace, located within the circumference of the annular flange. This strengthens the blade support section to make it more capable of bearing the weight of the wind turbine blade, when the blade is suspended from the hub, as well as any additional loading or strain resulting from the incident wind. 
         [0021]    The reinforcing brace may comprise reinforcing members that span from one attachment point location to another within the inner circumference of the blade support section. In this way, reinforcement is provided at the points on the ring where most of the load is carried. 
         [0022]    In one embodiment, the reinforcing brace comprises a cable attachment point. This allows the blade to be lifted into position for reattachment to the hub by means of a cable attached to the brace. The other end of the cable may be attached to a crane or a winch. The reinforcing brace is preferably positioned within the blade support section such that when a cable is attached to the cable attachment point, the blade adopts a tilted angle with respect to the tower, facilitating the connection of the blade root bearing assembly to the wind turbine hub. 
         [0023]    In a further embodiment, the invention provides a wind turbine blade-hub support apparatus, comprising: the wind turbine blade discussed above; one or more attachments points provided on the wind turbine rotor hub, wherein each respective attachment point corresponds to a respective attachment point on the blade support section; and one or more actuators mounted between one of the one or more attachment points on the hub and the corresponding attachment point on the wind turbine blade, the one or more actuators having an actuated position in which they supports the wind turbine blade in a separated position from the wind turbine rotor hub. 
         [0024]    The actuator may be a hydraulic or pneumatic ram. 
         [0025]    The invention also provides a method of replacing a blade root bearing assembly in a wind turbine, in which at least one wind turbine blade is connected to a wind turbine rotor hub via a blade root bearing assembly, the wind turbine blade having a blade support section located at the blade root portion of the blade comprising attachment points for mounting a plurality of actuators between the blade and the rotor hub of the wind turbine at which the blade is attached, and wherein one side of the blade support section is left substantially free of attachment points; the method comprising: attaching a plurality of actuators between the respective attachment points on the wind turbine rotor hub, and on the blade support section; releasing the connection between the wind turbine rotor hub and the wind turbine blade root bearing; activating the one or more actuators to separate the wind turbine blade root bearing from the wind turbine hub; removing the blade root bearing assembly while the blade is suspended from the hub on the one or more actuators, through the one side of the blade support section that is left substantially free of attachment points. 
         [0026]    The method may further comprise activating the actuators to draw the wind turbine blade into contact with the wind turbine rotor hub such that the blade can be connected to the wind turbine rotor hub, or drawing the wind turbine blade into contact with the wind turbine rotor hub such that the blade can be connected to the wind turbine rotor hub using a cable attached to a brace section provided on the wind turbine blade. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    Embodiments of the invention will now be described, by way of example only, and with reference to the accompanying drawings, in which: 
           [0028]      FIG. 1  is a front view of a horizontal axis wind turbine; 
           [0029]      FIG. 2  is an elevation view of a wind turbine blade; 
           [0030]      FIG. 3  is a front elevation view of the wind turbine hub showing the attachment of the blade root and blade root bearing assembly; 
           [0031]      FIG. 4  is a side elevation view of a wind turbine hub showing the connection point for a wind turbine blade root bearing assembly; 
           [0032]      FIG. 5  illustrates an example of the invention permitting a blade to be detached from the hub for blade bearing replacement; 
           [0033]      FIG. 6  illustrates the blade support section according to a first example of the invention; 
           [0034]      FIG. 7  is a close-up view of the connection mechanism between the hub, blade root ring bearing assembly and blade support section; 
           [0035]      FIG. 8  illustrates a blade support section according to a second example of the invention in a top elevation view; 
           [0036]      FIG. 9  illustrates a blade support section according to a second example of the invention in cross section; and 
           [0037]      FIG. 10  illustrates a front view of an actuator attachment point according to a second example of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0038]    The invention relates to a blade-hub support apparatus that permits blade root bearing replacement for maintenance or repair.  FIG. 5  illustrates the apparatus in a side view. 
         [0039]    The hub  50  of the wind turbine rotor is attached to the nacelle of the wind turbine (not shown) by hub bearing ring  52 . Rotor blades  54  are attached to respective hub blade bearing rings  56  by means of their respective blade root bearing assemblies  58 . In this view, a rotor blade  54 , a blade root bearing assembly  58  and the hub  50  are shown as detached from one another, as they would be if the blade root bearing assembly were being replaced. 
         [0040]    The hub  50  bears one or more hub support shoulders or yokes  60  spaced around the circumference of the hub blade bearing rings  56  where the blades  54  are attached. Each hub support shoulder  60  provides a point of attachment for one end of a releasably securable actuator  62 , such as a hydraulic or pneumatic ram, or an electrical or mechanical actuator for example. 
         [0041]    The blade  54  comprises a blade body section  55  and a blade support section  64 , attached between the blade body  55  and the blade root bearing assembly  58 . The end of each actuator  62  that is not attached to a hub support shoulder  60  is attached to the blade support section  64  at a corresponding blade support section support shoulder  66 . 
         [0042]    The actuators  62  can be coupled to the support shoulders on the hub  50  and the blade support section  64  by a suitable fastener, such as a retaining bolt or screw. In this example, each support shoulder  60  or  66  comprises a pair of metal buttress plates  67  having a hole defining an axis for receiving a fastener. A retaining bolt can be threaded through the holes in the buttress plates  67 , and though a corresponding hole in the end of the actuator  62  to secure it in place. 
         [0043]    The blade support section is shown in  FIG. 6  to which reference should now be made. The blade support section  64  is a reinforced steel or metal ring around the periphery of which the blade support section support shoulders  66  are formed. As noted above, each blade support section support shoulder  66  comprises a pair of buttress plates  67  defining an intermediate space in which one end of the actuator  62  can be positioned and secured. In  FIG. 6 , three blade support section support shoulders  66  are shown, separated from one another by an angular displacement of just over 90 degrees. The position of the actuators  62  is illustrated in  FIG. 6  by the dotted lines shown around each support shoulder  66 . 
         [0044]    Due to the separation of the shoulders  66 , one side  69  of the blade support section  64  is left without a support shoulder or actuator  62 . As will be appreciated from  FIG. 5 , this means that in use the side  69  of the blade support section is open, and that is there is a space between the actuators  62  at positions  62   a  and  62   b,  through which the blade root bearing assembly  58  can be inserted or removed during maintenance. 
         [0045]    The circumference of the blade root bearing assembly  58  is essentially the same as that of the blade support section  64 , excluding the additional circumference due to the buttress plates  67 . This means that attachment of the actuators  62  to the buttress plates at a position  62   a,    62   b  or  62   c,  slightly outside of the outer circumference of blade support section  64  provides clearance between the outer circumference of the blade root bearing assembly  58  and the circumference defined by the position of the actuators  62  around the perimeter of the blade support section  64 . With this in mind, it is worth noting an angular separation of 90 degrees between the attachment positions  62   a,    62   b  and  62   c  would allow for optimal clearance between the blade root bearing assembly  58  and the actuators as the blade root bearing assembly  58  is inserted into or removed from position. However, as the optimal positioning for stability of three actuators  62  would be a triangular configuration, that is an equal separation between all three actuators  62 , it is preferred if the separation between at least two of the actuators, in this cases  62   a  and  62   c,  and  62   b  and  62   c  is more than 90 degrees (thus, the angular separation between positions  62   a  and  62   b  is slightly less than 180 degrees). In other words, in this example, the attachment points for the actuators are all located within a 180 degree arc, or more advantageously say a 200 degree arc, around the circumference of the blade support section. 
         [0046]    The extent by which the separation can be more than 90 degrees, but with which clearance can still be maintained between the bearing root bearing assembly and the actuators, is a function of the particular dimensions used in the apparatus and will not be discussed further here. 
         [0047]    In embodiments where the point of attachment of the actuators to the blade support section is at a location distant from the blade root bearing assembly, such that clearance is not a problem, the attachment points could be located around the blade over a 200 degree arc or greater. 
         [0048]    The purpose of the reinforced blade support section  64  is to provide a strong anchor point for the actuators  62 , as well as to support the weight of the blade, by means of the actuators, when the blade is detached from the hub  50  and the blade root bearing assembly  58  is removed. This is essential as when the blade is removed from the hub it will be in a less stable configuration and will be more susceptible to buffeting or induced motion from the incident wind forces. 
         [0049]    For this reason, an optional support or lift brace  70  can be provided in the inner circumference of the blade support section  64  for reinforcement. In this example, the support brace  70  is attached to connection points  63  on the blade support section  64  corresponding in position to that of the support shoulders  66 , but on the inside circumference of the blade support section  64  rather than the outside. This is advantageous as it will be appreciated that when the blade  54  is detached from hub and supported on the actuators  62 , much of the loading on the blade support section  64  will occur though the shoulders  66 . Different shapes and configurations of brace  70  are of course also possible. The brace  70  can also have a lifting function as will be explained later. 
         [0050]      FIG. 7  to which reference should now be made illustrates in more detail how the blade root bearing assembly  58  is attached to the blade support section  64  and to the hub blade bearing ring  56  for the purposes of this example. As is known in the art, the blade root bearing assembly  58  can comprises an inner  72  and an outer bearing ring  74 , and a centre bearing ring  76 . The centre bearing ring  76  has a plurality of bolt holes  76   a  corresponding in position to opposing holes  56   a  on the hub blade bearing ring  56 . By securing bolts or other suitable fasteners between bolt holes  76   a  and  56   a,  the centre bearing ring  76  can be secured in a fixed relationship to the hub blade bearing ring  56  and therefore the hub  50 . The centre bearing ring  76  is carried in an intermediate position between inner and outer bearing rings  72  and  74  of the blade root bearing assembly  58  by a plurality of bearings  78 . Thus, in normal operation of the wind turbine the centre bearing ring  76  can turn in relation to the inner and outer bearing rings  72  and  74 , allowing the angular blade position or pitch relative to the hub  50  to be adjusted. The blade root bearing assembly  58  can therefore be referred to as the pitch bearing. 
         [0051]    In alternative examples, the blade  55  and the blade support ring  64  could be attached to the centre bearing ring  76 , with the hub attached to the inner  72  and outer  74  bearing rings, in what is known as an inverted three ring bearing assembly. Other bearing ring assemblies and connections to the hub, such as two ring bearing assembly for example, could also be used, as will be apparent to the skilled person. 
         [0052]    The inner and outer bearing rings  72  and  74  also have a plurality of bolt holes  72   a  and  74   a  that correspond in position to opposing bolt holes  64   a  and  64   b.  As before, using securing bolts or other suitable fasteners between bolt holes  72   a  and  64   a,  and  74   a  and  64   b,  the inner  72  and outer bearing ring  74  can be secured in a fixed relationship to the blade support section  64 . The blade support section  64  is then connected to the blade body  55  by means of further bolts or fasteners connected to a further plurality of bolt holes  64   c , and opposing holes  55   a  in a reinforced section of the blade body laminate. 
         [0053]    The operation of the blade-hub support apparatus will now be described in more detail. 
         [0054]    In normal operation of the wind turbine, the blade  55  is attached to the hub  50  via the blade support section  64 , the blade root bearing assembly  58 , and the hub blade bearing ring  56 , without the actuators  62  attached. In this arrangement, the pitch of the blades  55  can be adjusted using pitch actuators known in the art, as the inner and outer bearing rings  72  and  74  attached to the blade are free to turn around the centre bearing ring  76  attached to the hub  56 . 
         [0055]    At times when the blade root bearing assembly  58  is to be replaced, an engineer working from within the hub or from the outside by means of a crane or platform, attaches a actuator to each of the support shoulders  60  and  66  on the hub and the blade support section  58 . It will be appreciated that for a three blade wind turbine, a total of nine actuators will be advantageous, and in this example there are consequently nine actuator support shoulders  60  on the hub. However, in practice, only one blade is likely to be worked on at a time, meaning that only three actuators will be needed for a single blade root bearing assembly replacement operation. 
         [0056]    In order to attach the actuators, an inspection or working window in the surface of the hub  50  can be provided so that, where possible, the engineer has access to the shoulders  60  from inside the hub  50 . 
         [0057]    Once the actuators are securely attached between the hub  50  and the blade support section  64 , the engineer working from within the hub ensures that the actuators are activated and take up the strain between the blade  54  and the hub  50 . The engineer then undoes the bolts or fasteners that connect the centre bearing ring  76  to the hub blade bearing ring  56 , and the bolts that connect the inner and outer bearing rings  72  and  74  to the blade support section  64 . 
         [0058]    The blade root bearing assembly  58  is now fully disconnected from both the hub  50  and from the blade  54 , and the actuators are fully loaded with the weight of the blade  54 . However, the blade root bearing assembly  58  will still be pinned in place between the blade  55  and hub  50  since the activated actuators  62  prevent the movement of the blade  55  and hub  50 . The engineer therefore activates the actuators so that they extend and separate the blade  54  from the hub  50 , providing clearance between them. Before doing this, the engineer preferably takes measures to secure the blade root bearing assembly  58  to the hub  50 , by cables for example, so that the blade root bearing assembly  58  does not fall or slip when the blade  54  is moved. 
         [0059]    Once the blade is supported on the hub by the actuators the blade root bearing assembly  58  can be removed and replaced as appropriate. A crane or pulley arrangement in the hub or nacelle for example can be used to provide the necessary lifting or supporting force. Once a replacement blade root bearing assembly  58  has been manoeuvred into position, the blade  54  can be reattached to the hub  50  by following the reverse of the process described above. In the reverse process, the actuators can advantageously act as a guide for ensuring that the bolt holes  76   a  in the new blade root bearing assembly  58 , when it is attached to the support ring  64 , match up with the corresponding holes  56   a  in the hub blade bearing rings  56 . The process can then be repeated for the other blades  54  on the hub  50  as desired, moving the actuators  62  to the appropriate shoulders  60  and  66  for working on the next blade. 
         [0060]    Lifting or manoeuvring the blades  54  back into position against the hub can be achieved using the actuators  62 , or alternatively by using a cable attached to the lifting brace  70 . In this respect, the asymmetrical shape of the lifting brace  70  shown in  FIG. 6  is advantageous as when used as a lifting brace for a cable winch, means that a starting tilt can be give to the blade allowing it to be more easily manoeuvred into its final position against the hub. 
         [0061]    In the above examples of the invention, the actuators  62  are connected to the blade  55  via the blade support section  64  connected between the blade body  55  and the blade root bearing assembly  58 . With reference to  FIGS. 8 ,  9  and  10 , an alternative example of the blade support section and attachment points for the actuators  62  will now be described. 
         [0062]    In  FIG. 8 , a blade support section  80  is located internally of the blade outer shell  82 . The blade support section comprises a reinforcing inner ring  84  of steel or other suitable material, and a plurality of reinforcing spokes  86  connected between the inner ring  84  and the blade outer shell  82 . In the example shown, eight reinforcing spokes  86  are provided at radial positions, angularly spaced equally around the inner circumference of the blade, and each reinforcing spoke  86  comprises a pair of reinforcing tubular members  88 . As shown in more detail in  FIG. 9 , the reinforcing tubular members  88 , making up each reinforcing spoke  86 , are connected between the inner ring  84  and the blade outer shell  82  by fasteners, such as retaining bolts. 
         [0063]    A number of blade support shoulders  66  are mounted on the blade outer shell  82  to provide attachment points for the actuators  62 . The blade support shoulders  66  are located at positions corresponding to the point at which the reinforcing tubular members  88  are coupled to the blade outer shell  82 . In the example shown in  FIG. 8 , three blade support shoulders for supporting three actuators are shown. 
         [0064]    The presence of the support shoulders on the blade outer shell  82 , and the actuators when these are in use, place loads on the blade constructions. As a result of the placement of the shoulders  66  at positions corresponding to the reinforcing spokes  86 , these loads are borne by the spokes and transmitted to the inner ring  84 . As shown in  FIGS. 9 and 10 , the blade support bracket  66  can be attached to the blade outer shell by fasteners or bolts  90  that are arranged coaxially with those supporting the tubular members  88 . In some embodiments, the same bolts  90  may fix in place both the support shoulders  66  and the tubular members  88 . In  FIGS. 9 and 10 , reinforced bolt holes  91  for attaching the blade outer shell  82  to a blade root flange are also shown for clarity, though these are not part of the design of the blade support section. 
         [0065]    Additionally, one or more tie bars  92  can be mounted between the inner ring  84  and the blade outer shell  82  in order to provide cable attachment points. In this way, the blade support section can also act as the lifting brace  70  mentioned above. The tie bars  92  can be provided in between the spokes  84  or displaced either above or below for ease of access. 
         [0066]    In alternative embodiments, an additional support ring can be provided immediately inside the blade outer shell and arranged contiguously with it. This outer support ring provides additional support and can simplify the connection of the blade support section  80  to the blade outer shell  82 . In this case, the blade support shoulders  66  need not be located at positions that correspond directly to the position of the reinforcing spokes  86 , but can be located anywhere around the circumference of the blade. 
         [0067]    Other embodiments of the blade support section are also possible and are contemplated within the design. One example is a plurality of actuator support brackets located around the circumference of the blade at suitable angular positions, such as those indicated in the examples above. If required, reinforcement of the blade at these positions could then be provided by internal beams that connect one support position to another. If three actuator support shoulders  66  are provided at substantially 180 degree angular separations from one another, then the beams may be arranged in a triangular shape. If the actuator support shoulders  66  are positioned as shown in  FIG. 6 , then an A-shaped reinforcement frame can be provided. 
         [0068]    By means of the apparatus and methods described above, the process of swapping a blade root bearing assembly becomes easier. However, it will be appreciated that the actuators  62  need to be capable of holding the blades  54  extremely firmly as the blade is jacked down. Without a firm hold on the blade, it will be impossible to stably hold the blade against even small wind gusts. For a 30 or more tonne blade, the actuators used should therefore perhaps provide as much as 150 tonnes of lift, though it will be appreciated that the actual figure required will vary depending on practice and the size and weight of the components used, as well as the safety factors of the replacement operation. 
         [0069]    Although, three actuators are described in the example above, it will be appreciated that in alternative examples only one or two actuators may be used per blade, or even more than three actuators. In each case, what is most important is the stability of the support for the blade when the blade is detached from the hub, and the clearance for the blade root bearing assembly when it is removed. In the case where only a single actuator is used, stability can be improved using guide cables or tag lines connected between the blade and the tower or hub to hold it in place. 
         [0070]    Although the actuators have been described as releasably securable actuators, it will be appreciated that in alternative examples they could be permanently attached between the blade and the hub. 
         [0071]    Although the invention has been described with reference to carrying out repair or maintenance of a blade root bearing assembly, the invention is not limited to this as it will be appreciated that the arrangement of actuators between the hub and the blade support section allows other aspects of the blade hub interface to be worked upon. 
         [0072]    Although the attachment points have been described as forming a blade support section, it will be appreciated that this is intended to refer only to the section of the blade where the attachment points are located, and could encompass the case where only three attachment point brackets are provided in the blade surface, either spaced equally around the circumference in a triangular arrangement, or the case in which the attachment points are located towards one side of the blade circumference only such that the ring can be considered as subscribing only a partial segment of the circumference of a circle. 
         [0073]    Furthermore, although the examples of the invention that are described above show a three blade horizontal axis wind turbine, the invention is not limited to such wind turbines and could also be used with wind turbines having any number of blades, and also with vertical axis wind turbines.