Patent Publication Number: US-2021164437-A1

Title: Replacement insert for repair of a joint connecting a wind turbine rotor blade to a rotor hub

Description:
TECHNICAL FIELD 
     The invention relates generally to wind turbines, and more particularly to a replacement insert for the repair of a joint connecting a wind turbine rotor blade to a rotor hub on a wind turbine. 
     BACKGROUND 
     Wind turbines are used to produce electrical energy using a renewable resource and without combusting a fossil fuel. Generally, a wind turbine converts kinetic energy from the wind into electrical power. A horizontal-axis wind turbine includes a tower, a nacelle located at the apex of the tower, and a rotor having a central hub and a plurality of blades coupled to the hub and extending outwardly therefrom. The rotor is supported on a shaft extending from the nacelle, which shaft is either directly or indirectly operatively coupled with a generator which is housed inside the nacelle. Consequently, as wind forces the blades to rotate, electrical energy is produced by the generator. 
     In recent years, wind power has become a more attractive alternative energy source and the number of wind turbine, wind farms, etc. has significantly increased, both on land and off-shore. Additionally, the size of wind turbines has also significantly increased, with modern wind turbine blades extending between 50 to 80 meters in length, and is expected to further increase in the future. The increased length in the wind turbine blades has introduced a number of interesting design considerations for wind turbine designers and manufacturers. For example, with increasing blade length, the joint between the wind turbine blade to the rotor hub may experience increased stresses that present challenging design considerations in order to ensure that the joint can withstand the loads expected during the operating life of the wind turbine. 
     Conventional joints between wind turbine rotor blades and the rotor hub include threaded stud bolts coupled to and extending from the root end of the wind turbine blade, which are in turn coupled to a pitch bearing associated with the rotor hub. Wind turbine blades are typically made from one or more composite materials formed from fibrous material and resin. Such materials generally do not have the structural integrity to provide a secure fixing mechanism into which the threaded stud bolts may be directly inserted. A hole or bore, for example, may be tapped into the composite material at the root end of the rotor blade to provide a complementing thread upon which the stud bolt may achieve a connection. However, the composite material has insufficient shear strength to transfer the loads between the blades and hub via the stud bolts and deterioration of the composite material at the interface would occur. 
     For this reason, manufacturers attempt to design a joint that more evenly distributes the forces occurring at the connection between the root end of the blade and the rotor hub. For example, in one design axial bores are formed along the circumference of the end face of the wind turbine blade. Internally threaded metal bushings are then positioned within the axial bores and adhesively bonded therein to essentially embed the metal bushings within the composite material of the rotor blade. Alternatively, the metal bushings may be positioned in the blade mould within the various shell laminates at the root end of the blade during the layup process of blade manufacture. The blade may then be infused with resin and cured to form the final blade, with the bushings being integrated into the root end of the blade as part of the original blade manufacture. In any event, the stud bolts are then threadably engaged with the metal bushings. The forces acting between the rotor blade and rotor hub act through the stud bolts, and thus are transferred via the metal bushings, which operate to more uniformly distribute the forces over the interface area with the softer composite material. 
     While current connection joints are generally sufficient to achieve their intended purpose of supporting the loads between the rotor blades and rotor hub, in some instances the adhesive bond or the blade material at a bushing/composite interface could become compromised leading to a loosening of the bushing within the composite blade material. This may occur over a blade&#39;s lifetime, perhaps due to incorrect operation or unusual stresses or unusual levels of fatigue or to a range of unpredictable factors. Should a sufficient number of bushings become compromised, then the joint between the blade and the rotor hub may not be able to easily sustain the loads and stresses that occur during operation of the wind turbine, and a risk of failure may arise. For example, a bushing whose adhesive or attachment is compromised though overload or such like, may tend to work itself loose over time. To avoid such a scenario, periodic inspections of the blade/rotor hub joint are recommended to ensure the integrity of the joint. Should a sufficient number of bushings be determined to be compromised during the inspection, then the entire wind turbine blade may need to be replaced before operation of the wind turbine may be resumed. Of course, such a replacement is very costly. This includes, not only the cost of an additional blade, but further includes costs associated with the large cranes and other equipment needed for a blade replacement, the labor required to achieve the replacement, and the lost operation time of the wind turbine during the replacement process. 
     Accordingly, there is a need in the wind turbine industry for a method of repairing a wind turbine blade, and more particularly repairing the metal bushings at the root end of the wind turbine blade in a more cost-effective manner so as to reestablish the strength and integrity of the joint between the blade and the rotor hub. 
     SUMMARY 
     To these and other ends, aspects of the invention are directed to a replacement insert for the repair of a joint connecting a wind turbine blade to a rotor hub of a wind turbine. The replacement insert includes a bushing having a first end, a second end, an outer surface, and an internal passageway. The replacement insert further includes a cover around at least a portion of the outer surface of the bushing. The replacement insert has a non-circular cross-sectional profile. 
     In another embodiment, a replacement insert includes a bushing have a first end, a second end, an outer surface, and an internal passageway, a cover around at least a portion of the outer surface of the bushing, the cover defining an outer surface, and a plurality of protrusions extending from the outer surface of the cover. The plurality of protrusions is configured to operate as spacers when the replacement insert is positioned in a bushing cavity in the wind turbine blade. The plurality of protrusions defines a gap between the outer surface of the replacement insert and the walls that define the bushing cavity. 
     In this embodiment, the plurality of protrusions may be formed from the same material as the cover. Additionally, the plurality of protrusions may be integrally formed with the cover, such as through a machining process of the outer surface of the insert. The plurality of protrusions may have substantially the same height. In this way, the gap between the outer surface of the insert and the walls of the bushing cavity may be configured to be substantially uniform. In one embodiment, the plurality of protrusions may be arranged into a plurality of rows, wherein each row is at a fixed position along a longitudinal axis of the bushing. Each row may also have a plurality of protrusions disposed about the outer surface of the cover. In one embodiment, the rows may be uniformly spaced apart from each other along the longitudinal axis of the bushing. Additionally, the protrusions in each row may be uniformly or non-uniformly spaced apart about the outer surface of the insert. 
     In one embodiment, the cover may be formed from a composite material. More particularly, the cover may be formed from a composite material that is substantially fully cured. In this way, the replacement inserts may be subject to post-processing techniques to provide size and dimension aspects as well as other desirable surface features. A cured cover also facilitates the handling, storage and transport of the replacement inserts. 
     In an exemplary embodiment, the replacement insert includes an oblong cross-sectional profile having a major axis and a minor axis, wherein the major axis is greater than the minor axis. By way of example, the cover may further include a first planar surface, a second planar surface opposed to the first planar surface, a first arcuate surface extending between the first and second planar surfaces, and a second arcuate surface extending between the first and second surfaces and opposed to the first arcuate surface. The first and second arcuate surfaces may have a substantially constant radius of curvature. The substantially constant radius of curvature may be substantially equal to the minor axis. In a further embodiment, the replacement insert may include a chamfer adjacent a tip of the replacement insert. 
     In an embodiment of the invention, the cover includes a plurality of layers of fibrous material. The fibers of adjacent layers may be oriented in different directions. For example, the fibers of each of the plurality of layers may be oriented in different directions. At least one of the plurality of layers of fibrous material may include a fiber roving, wherein the fibers of the roving are oriented in a direction substantially perpendicular to the longitudinal axis of the bushing. At least one of the plurality of layers of fibrous material may include a biaxial fiber sheet, wherein the fibers of the biaxial fiber sheet are oriented at +/− degrees relative to the longitudinal axis of the bushing. At least one of the plurality of layers of fibrous material includes a unidirectional fiber sheet, wherein the fibers of the unidirectional fiber sheet are oriented substantially parallel to the longitudinal axis of the bushing. In one exemplary embodiment, the plurality of layers of fibrous material includes an inner layer, the inner layer including a fiber roving wrapped around the outer surface of the bushing; an intermediate layer, the intermediate layer including a biaxial fiber sheet wrapped around the inner layer; and an outer layer, the outer layer including a unidirectional fiber sheet wrapped around the intermediate layer. 
     In yet another aspect, the replacement insert may include a plug positioned in the second end of the bushing and extending therefrom. The cover may be disposed around at least a portion of an outer surface of the plug. The replacement insert may further include a seal disposed in the internal passageway of the bushing. The seal is configured to prevent a contamination agent introduced into the passageway at the first end from reaching the second end of the bushing, thereby protecting the adhesive bond at the bushing/composite material interface. 
     In accordance with an aspect of the invention, the replacement insert is configured to initially be a separate, stand-alone element, which may be coupled to the wind turbine blade as part of a repair procedure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate one or more embodiments of the invention and, together with a general description of the invention given above, and the detailed description given below, serve to explain the invention. 
         FIG. 1  is a perspective view of a wind turbine in which embodiments of the invention may be used; 
         FIG. 2  is a partial perspective view of a root end of a wind turbine blade; 
         FIG. 3  is a partial view of the root end of the wind turbine blade shown in  FIG. 2 ; 
         FIG. 4  is a flow chart of an exemplary repair process of a bushing of a wind turbine blade; 
         FIG. 5  is a perspective view of a jig used to extract a bushing from the wind turbine blade and to recondition the blade cavity that results from the extraction; 
         FIG. 6A  is a schematic end view of a wind turbine blade illustrating a bushing cavity after extraction of a bushing; 
         FIG. 6B  is a schematic end view of a wind turbine blade illustrating a reconditioned bushing cavity; 
         FIG. 7  is a perspective view of a replacement insert in accordance with an embodiment of the invention; 
         FIG. 8  is a cross-sectional view of the replacement insert shown in  FIG. 7 ; 
         FIG. 9  is another cross-sectional view of the replacement insert shown in  FIG. 7 ; 
         FIGS. 10A-10C  are conceptual schematic illustrations of the reshaping of the bushing cavity in accordance with an embodiment of the invention; 
         FIG. 11  is a flow chart of an exemplary method of making the replacement insert illustrated in  FIG. 7 ; 
         FIGS. 12A and 12B  are cross-sectional views illustrating the insertion of the replacement insert into the reconditioned bushing cavity in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 1 , a wind turbine  10  includes a tower  12 , a nacelle  14  disposed at the apex of the tower  12 , and a rotor  16  operatively coupled to a generator (not shown) housed inside the nacelle  14 . In addition to the generator, the nacelle  14  houses miscellaneous components required for converting wind energy into electrical energy and various components needed to operate, control, and optimize the performance of the wind turbine  10 . The tower  12  supports the load presented by the nacelle  14 , the rotor  16 , and other components of the wind turbine  10  that are housed inside the nacelle  14  and also operates to elevate the nacelle  14  and rotor  16  to a height above ground level or sea level, as may be the case, at which faster moving air currents of lower turbulence are typically found. 
     The rotor  16  of the wind turbine  10 , which is represented as a horizontal-axis wind turbine, serves as the prime mover for the electromechanical system. Wind exceeding a minimum level will activate the rotor  16  and cause rotation in a plane substantially perpendicular to the wind direction. The rotor  16  of wind turbine  10  includes a central hub  18  and at least one rotor blade  20  that projects outwardly from the central hub  18  at locations circumferentially distributed thereabout. In the representative embodiment, the rotor  16  includes three blades  20 , but the number may vary. The blades  20  are configured to interact with the passing air flow to produce lift that causes the central hub  18  to spin about a central longitudinal axis. 
     The wind turbine  10  may be included among a collection of similar wind turbines belonging to a wind farm or wind park that serves as a power generating plant connected by transmission lines with a power grid, such as a three-phase alternating current (AC) power grid. The power grid generally consists of a network of power stations, transmission circuits, and substations coupled by a network of transmission lines that transmit the power to loads in the form of end users and other customers of electrical utilities. Under normal circumstances, the electrical power is supplied from the generator to the power grid as known to a person having ordinary skill in the art. 
     As is well known in the industry, for certain wind turbine designs, the rotor blades  20  are coupled to the rotor hub  18  in a manner that allows the blades  20  to rotate or pitch about a longitudinal axis of the blades  20 . This is achieved by coupling the root end  22  of a blade  20  to a pitch bearing (not shown) operatively coupled to the rotor hub  18 . The pitch bearing generally includes a ring rotatable relative to the hub  18  to which the root end  22  of the blade  20  is coupled. Pitch bearings are generally well known in the art and thus will not be described in further detail herein. 
     As illustrated in  FIGS. 2 and 3 , a connection joint  24  between a rotor blade  20  of the wind turbine  10  and the rotor hub  18  includes a plurality of bushings  26  coupled to the rotor blade  20  at the root end  22  thereof, and a plurality of stud bolts  28  configured to be coupled to the bushings  26  in the rotor blade  20  ( FIG. 3 ) and further configured to be coupled to the rotor hub  18  (FIG.  2 ), such as through the pitch bearing. As illustrated in  FIG. 3 , the bushings  26  may be circumferentially spaced about an end face  30  at the root end  22  of the blade  20  and embedded within the composite material of the blade  20 . The number of bushings  26  along the circumference of the root end  22  of the blade  20  depends on the size of the blade, among potential other factors, but may be anywhere from 70 to 180 inserts for blades between 50 m-80 m in length. It should be realized, however, that more or less inserts may be used depending on the specific application. 
     As noted above, in some instances, one or more bushings  26  may become compromised such that the bond strength at the bushing/composite material interface may be significantly reduced. In this case, the bushings  26  may be described as being generally loose within the composite blade material that surrounds the bushing  26 . Aspects of the present invention are directed to repairing a loose or otherwise compromised bushing  26  at the root end  22  of the wind turbine blade  20 . In this regard,  FIG. 4  broadly illustrates steps of a bushing repair process in accordance with aspects of the invention. To this end, the process includes extracting a compromised bushing  26  from the root end  22  of the blade  20 , as illustrated by step  32 . Next, the bushing cavity  34  which results from the extraction of the bushing  26  may be reconditioned in order to receive a replacement insert  36  which includes a new bushing. This is illustrated as step  38  in  FIG. 4 . A replacement insert  36  may be prepared for positioning within the reconditioned bushing cavity  34 , as represented by step  40 . As will be explained in more detail below, in one advantageous aspect the replacement insert  36  may be formed in a process that is separate from the repair process such that one or more replacement inserts  36  are readily available during the repair process. Lastly, the replacement insert  36  may be positioned in the reconditioned bushing cavity and adhesively bonded to the root end  22  of the blade  20 . This is illustrated as step  42  in  FIG. 4 . 
     In one embodiment, the repair process described above may occur at a manufacturing, repair, or other facility having the requisite machinery to effectuate the repair. Accordingly, the repair process may further include removing the impacted blade  20  from the wind turbine  10 ; transporting the blade  20  to the manufacturing/repair facility; repairing one or more compromised bushings  26  as described above and described in more detail before; transporting the repaired blade  20  back to the wind turbine site; and reattaching the repaired blade  20  to the wind turbine  10 . As the details of removing/attaching a wind turbine blade  20  to the wind turbine  10  and transporting a blade between a facility and wind turbine site is generally well known, these aspects will not be described in further detail herein. In an alternative embodiment, the repair process may be implemented at the wind turbine site, thus forgoing the transport of the wind turbine blade  20  back and forth from a manufacturing/repair facility. 
     In an exemplary embodiment and as illustrated in  FIG. 5 , the extraction of an existing bushing  26  may be achieved using a jig  44  configured to be operatively coupled to the root end  22  of the blade  20 . In one embodiment, the jig  44  may include a support frame  45  configured to be in engagement with the end face  30  of the blade  20 . The jig  44  includes a central hub or chuck  46  that defines a central axis  48 . The jig  44  is coupled to the root end  22  of the blade  20  in an adjustable manner such that the central axis  48  of the chuck  46  may be substantially coaxially aligned with the center axis  50  of the blade  20  at its root end  22 . By way of example and without limitation, the central chuck  46  may include a plurality of extendable pins  52  which may engage with an inner surface of the blade  20  at the root end  22  or with a template  54  disposed over the end face  30  of the blade  20 . The extendable pins  50  may be adjusted accordingly such that the axes  48 ,  50  are substantially coaxially aligned. 
     The jig  44  further includes an arm  56  rotatably coupled to the central chuck  46  and rotatable about the central axis  48 . The arm  56  extends radially outward from the central axis  48  to a tool head  58  configured to be carried adjacent a terminating end of the arm  56 . The length of the arm  56  may be adjustable such that the tool head  58  may be positioned immediately above a bushing  26  that is to be extracted. Various tool heads  58  may be selectively operatively coupled to the arm  56  depending on the particular task being performed. By way of example and without limitation, to effectuate the extraction of a bushing  26 , the tool head  58  may include an extraction tool that essentially pulls the bushing  26  out of the root end  22  of the blade  20 . In one exemplary embodiment, the extraction tool may include a hydraulic jack or actuator. In this case, a rod or stud  28  bolt may be threadably engaged with a selected bushing  26  and operatively coupled to the hydraulic actuator. The hydraulic actuator may then be activated so as to apply enough force (e.g., up to 10 tons, for example) to the selected bushing  26  to break any remaining adhesive bonds between the selected bushing  26  and the composite blade material, and thereby pull the bushing  26  from the blade  20 . 
     The use of a hydraulic actuator as the extraction tool is merely exemplary, and the extraction tool may take other forms sufficient to pull the selected bushing  26  from the root end  22  of the blade  20 . For example, if the selected bushing  26  cannot be readily pulled from the blade  20  in a first instance, then part of the bushing  26  may have to be drilled out (such as with a core bit), and then any remaining portion of the bushing  26  may be pulled from blade  20 . In this instance, the extraction tool may include or further include a drill bit for achieving removal of at least a portion of the bushing  26 . It should be recognized that if there is more than one compromised bushing  26  that needs to be extracted from the blade  20 , then the arm  56  may be rotated about the central axis  48  of the chuck  46  (as illustrated by arrow A) so as to be above and generally aligned with another compromised bushing  26 . The steps described above may then be repeated so as to extract the selected bushing  26  from the blade  20 . 
     As discussed above and illustrated in  FIG. 6A , upon removal of a bushing  26  from the blade  20 , a bushing cavity  34  remains in the blade  20 . While some of the compromised or otherwise damaged composite blade material may be pulled away upon extraction of the compromised bushing  26 , some of the composite blade material that is left behind and thereby forms the bushing cavity  34 , may be damaged and thus unable to form a strong adhesive bond with a new bushing of element positioned back in the bushing cavity. Accordingly, in a preferred embodiment, some of the composite blade material around the bushing cavity  34  may be excised or removed so as to expose uncompromised or undamaged composite blade material to the new element replacing the original bushing  26 . Accordingly, the bushing cavity  34  may be reconditioned by removing a relatively small amount of material along at least a portion of the bushing cavity  34  to define a new and oversized (relative to blade cavity  34 ) reconditioned bushing cavity  60  which is configured to receive a replacement insert  36  therein. For example, and without limitation, between about 70%-95% of the area of the bushing cavity  34  may have material removed to define the new bushing cavity  60 . The reconditioned bushing cavity  60  is configured to expose new and undamaged composite blade material, and thereby form a more secure bond between the composite blade material and the replacement insert  36 , as will be explained in more detail below. The particular shape and dimensions of the reconditioned bushing cavity  60  may be generally dictated by the shape and dimensions of the replacement insert  36 . More particularly, the shape and dimensions of the bushing cavity  60  may be substantially similar to the shape and dimensions of the replacement insert  36 . By way of example, the reconditioned bushing cavity  60  may be just slightly oversized relative to the replacement insert  36  so as to define a substantially constant gap (e.g., about 0.5 mm) between the outer surface of the replacement insert  36  and the inner wall of the bushing cavity  60 , as will be explained in more detail below. 
     In accordance with one embodiment, the jig  44  may be used to form the reconditioned bushing cavity  60 . In this regard, the tool head  58  may be configured as a reshaping tool, such as a mill bit, drill bit or other suitable boring or reaming tool for removing material from about the bushing cavity  34 . In this regard, once the shape and dimensions of the reconditioned bushing cavity  60  are determined (e.g., such as by the shape and dimensions of the replacement insert  36 ), the jig  44  may be used to reshape the bushing cavity  34 . To this end, the jig  44  may include a controller  62  operatively coupled to the tool head  58  so as to control the position and operation of the tool head  58 , and thereby engage with the composite material so as to arrive at the desired shape and dimensions for the reconditioned bushing cavity  60 . An exemplary reconditioned bushing cavity  60  is illustrated in  FIG. 6B . 
       FIGS. 7-9  illustrate a replacement insert  36  in accordance with an exemplary embodiment of the invention. In one aspect of the invention, the original bushing  26  is replaced by an insert  36 , which includes not only a new bushing  70 , but also an outer wrapping or cover  72  of composite material and one or more plugs  74 . In one particular aspect of the invention, the replacement insert  36  is a stand-alone element that is formed in a separate process from the repair process described above in reference to  FIG. 4 . More particularly, the composite material cover  72  is substantially fully cured prior to the insert  36  being positioned in the reconditioned bushing cavity  60  in the root end  22  of the blade  20 . This may provide a number of advantages. One advantage, for example, is that when the replacement insert  36  is adhesively bonded in the reconditioned bushing cavity  60 , that bond takes place at an interface between two cured composite materials. This provides for a strong bond of the replacement insert  36  (and the new bushing  70 ) with the composite blade material that forms the walls of the reconditioned bushing cavity  60 . Second, forming the replacement insert  36  separately from the repair process allows manufacturers to take more time and care in ensuring that the adhesive bond at the metal/composite interface between the new bushing  70  and the cover  72 , which can sometimes be problematic, is of high quality. In other words, the formation of the potentially more problematic bonding interface is performed on a smaller, more controllable scale, and therefore results in generally higher quality adhesive bonds at the metal/composite interface. Third, since the composite material cover  72  of the replacement insert  36  is cured, the outer surface of the cover  72  may be subject to various machining processes in order to provide a desired shape or dimension, and to provide additional features (such as spacers, described below) to the replacement insert  36  that are useful in the repair process. The replacement inserts  36  may also be more easily handled during use, storage, and transport. 
     The bushing  70  may be similar to the bushing  36 , and may include a generally cylindrical elongate body  76  having a first end  78 , an opposing second end  80 , and a passageway  82  extending between the first and second ends  78 ,  80 . The elongate body  76  may be made of a suitable metal, such as steel, having sufficient strength for accommodating the expected loads during operation. The first end  78  may include a flange  84  and is configured to be positioned adjacent to the end face  30  of the blade  20  such that a stud bolt  28  may access the passageway  82 . The second end  80  is configured to be positioned away from the end face  30  and embedded within the blade material. The passageway  82  includes a first tapered portion  86  (e.g., diverging in a direction toward the first end  78 ) adjacent the first end  78 , a first generally straight portion  88 , a threaded portion  90 , a second generally straight portion  92 , and a second tapered portion  94 . The first tapered portion  86  may be configured to guide the insertion of stud bolts  28  into the bushing  70 , the threaded portion  90  may be configured to engage with the threads of the stud bolts  28 , and the second tapered portion  94  (e.g., diverging in a direction toward the second end  80 ) may be configured to engage with a plug, such as plug  74   a . The arrangement of passageway  82  described above is exemplary and other arrangements may also be possible, and aspects of the invention should not be limited to that shown in the figures. 
     The outer surface  96  of the bushing  70  may have a smooth portion  98  along the flange  84  and an undulating portion  100  (e.g., a series of crests and troughs) along a remaining portion of the bushing  70 . The undulating profile is configured to increase the surface area and provide an improved interface with the composite material of the cover  72 . In an exemplary embodiment, the diameter of the flange  84  may be greater than the diameter of the bushing  70  along the undulating portion  100 . By way of example, the undulating portion  100  may have a diameter that is reduced between about 10% and about 20% of the diameter of the flange  84 . In an exemplary embodiment, for example, the length of the bushing  70  may be between about 30 cm and about 40 cm, and preferably about 36 cm, the outer diameter along the flange  84  may be between about 5 cm and about 6 cm, and preferably about 5.8 cm, and the outer diameter along the undulating portion  100  may be between about 4.5 cm and about 5.5 cm, and preferably about 5 cm. Of course, the bushing  70  may have other dimensions and remain within the scope of the present invention. 
     The outer cover  72  of composite material may include a plurality of layers of composite material disposed about the outer surface  96  of the bushing  70 . For example, the outer cover  72  may include three layers of composite material, more or fewer layers may be possible, however. In this regard and in an exemplary embodiment, the cover  72  may include a first inner layer  102 , an intermediate layer  104 , and an outer layer  106  of composite material. The configurations of the fibers in the composite material may be different for each of the layers  102 ,  104 ,  106  of the outer cover  72 . By way of example and without limitation, the inner layer  102  may have fibers (e.g., carbon or glass fibers) extending in the transverse or perpendicular direction (e.g., 90 degrees) relative to the longitudinal axis of the bushing  70 . The intermediate layer  104  may have fibers extending in a crisscross fashion at an angle relative to the longitudinal axis of the bushing  70 . For example, the fibers of the intermediate layer  104  may extend at +/−45 degrees relative to the longitudinal axis. Lastly, the fibers of the outer layer  106  may be unidirectional fibers extending substantially parallel (e.g., 0 degrees) to the longitudinal axis of the bushing  70 . In general, the orientation of the fibers in the outer layer  106  should generally correspond to the orientation of the fibers in the composite material that forms the walls of the reconditioned bushing cavity  60 . However, aspects of the invention are not limited to this particular arrangement of the fibers in the composite material that forms the layers  102 ,  104 ,  106  of the cover  72 , and those of ordinary skill in the art should recognize that other fiber arrangements may be possible. 
     As illustrated in the figures, the replacement insert  36  includes a first plug  74   a  coupled to the second end  80  of the bushing  70  and extending therefrom. The first plug  74   a  includes a generally cylindrical body  108  having a first end  110  and a second end  112 . In one embodiment, the first plug  74   a  may be generally solid. Alternatively, however, the first plug  74   a  may be hollow. In a preferred embodiment, the first plug  74   a  may be formed of a fibrous material, such as wood, for example. However, other materials for the first plug  74   a  may also be possible. The first plug  74   a  may include a first tapered portion  114  adjacent the first end  110  and converging in a direction toward the first end  110 , a generally straight portion  116 , and a second tapered portion  118  adjacent the second end  112  and converging in a direction toward the second end  112 . The second end  112  of the plug  74   a  terminates in a raised ridge  120 . The outer surface  122  of the plug  74   a  may be generally smooth in an exemplary embodiment. 
     The first tapered portion  114  of the plug  74   a  is sized and dimensioned so as to be received in the second tapered portion  94  of the passageway  82  of the bushing  70 . More particularly, the taper angle of the first tapered portion  114  of the plug  74   a  may be substantially the same as the taper angle of the second tapered portion  94  of the bushing  70  such that the plug  74   a  is seated therein. To this end, the plug  74   a  may include a shoulder  124  that engages with an end face of the bushing  70  at the second end  80 . The straight portion  116  and the second tapered portion  118  extend beyond the second end of the bushing  70 . In an exemplary embodiment, the length of the first plug  74   a  may be between about 10 cm and about 15 cm, and preferably about 12 cm. Additionally, the diameter of the plug  74   a  along the straight portion  116  may be about the same as the outer diameter of the undulating portion  100  of the bushing  70 , e.g., between about 4.5 cm and about 5.5 cm, and preferably about 5 cm. As demonstrated in the figures, one or more of the layers  102 ,  104 ,  106  of the cover  72  may surround most of the plug  74   a  that extends beyond the second end  80  of the bushing  70 . For example, in one embodiment only the second tapered portion  118  may not be encased within the cover  72 . Additionally, in one embodiment each of the three layers  102 ,  104 ,  106  are disposed about the plug  74   a.    
     In addition to the first plug  74   a , the replacement insert  36  may include a sealing plug  74   b  disposed adjacent the second end  80  of the bushing  70 . More particularly, the sealing plug  74   b  may be formed from a suitable material that forms a seal with the inner wall of the passageway  82 . For example, the sealing plug  74   b  may be formed from a synthetic rubber, such as a fluoro rubber (fluoroelastomers). Other materials may also be possible. In one embodiment, the sealing plug  74   b  may be positioned in the second straight portion  92  to seal the passageway  82 . In this way, any hydraulic oil or other contaminating elements that enter the passageway  82  of the bushing  70  are not able to penetrate into the metal/composite interface or the wood/composite interface associated with the replacement insert  36 . This is intended to resolve what is believed to be a primary cause of the contamination of the bushings and surrounding composite material at least along a path from the interior of the insert  36 . 
     In one aspect of the invention, the outer profile  130  of the replacement insert  36  is configured to improve the adhesive bond between the replacement insert  36  and the reconditioned bushing cavity  60 . In this regard, the concept is to provide the replacement insert  36  with a non-circular cross-sectional profile. In an exemplary embodiment, for example, the replacement insert  36  may have an elliptical or oblong cross-sectional profile, which may be generally characterized by a major axis  132  and a minor axis  134 , wherein the major axis  132  is greater than the minor axis  134 . The change from a generally circular cross-sectional profile (e.g., from the cross-sectional profile of the original bushing  34 ) to an elliptical or oblong cross-sectional profile of the replacement insert  36  is intended to increase the contact surface area between the insert  36  and the composite blade material of the reconditioned bushing cavity  60 , and to provide an interface of increased uncontaminated composite blade material. 
       FIG. 9  illustrates an exemplary cross-section of the replacement insert  36  having a major axis  132  and a minor axis  134 , wherein the major axis is greater than the minor axis  134  to provide an oblong cross-sectional profile. By way of example, the major axis  132  may be between about 10% and about 25% greater than the minor axis  134 . More particularly, in the exemplary embodiment illustrated in  FIG. 8 , the outer profile  130  includes a first generally planar surface  136 , a second generally planar surface  138  opposed to the first planar surface  136 , a first generally arcuate side surface  140  extending between the first and second planar surfaces  136 ,  138 , and a second generally arcuate side surface  142  also extending between the first and second planar surfaces  136 , 138  and being opposed to the first arcuate surface  140 . In one embodiment, the first and second arcuate surfaces  140 ,  142  may be formed as portions of a circle, i.e., they have a generally constant radius of curvature. The circular portions may be semicircular such that the center of the circular portions lie on a line that connects the edges of the planar surfaces  138 ,  140 . 
     The particular cross-sectional profile described above may be a consequence of starting with a generally circular cross-sectional profile (e.g., from the original cylindrical bushing  34 ) with the desire to produce an oblong cross-sectional profile (to improve adhesive bonding at the new interface) but without expanding the bushing cavity in the radial direction, and all with attention to the ability to produce such an outer profile  130  on the replacement insert  36  and the ability to produce a reconditioned bushing cavity  60  having a similar profile. In one embodiment, and as schematically illustrated in  FIGS. 10A-10C , these may be conceptually achieved by separating a circular profile  146  have a generally constant radius of curvature R at its centerline  148 , moving the two semicircular profiles  146   a ,  146   b  apart along a linear axis  150  perpendicular to the centerline  148 , and filling the gap with planar surfaces  152 . Such a cross-sectional profile is believed relatively straight forward to form (in the blade  20 ) starting from a circular cross-sectional profile, ending with an oblong cross-sectional profile, and using conventional machining tools, such as drill bits, mill bits, reaming tools, etc. 
     A tip  154  of the replacement insert  36  may also have a particular configuration. More particularly, the tip  154  of the replacement insert  36  may have a taper or chamfer  156  wherein the cross-sectional area decreases in a direction toward the raised ridge  120 . The degree to which the outer profile  130  converges inwardly may be characterized by a taper angle a 1 . In an exemplary embodiment, the taper angle a 1  may be between about 2 degrees and about 10 degrees, and preferably between about 4 degrees and 7 degrees. The oblong profile of the upper portion of the replacement insert  36  may be maintained in the tapered tip  154 . For example, the major/minor axis ratio may be maintained in the tapered tip  154  to produce a uniform taper in the tip  154 . Other configurations of the tip are possible however. The taper  156  occurs primarily in the outer layer  106  of the cover  72 , but may extend into the intermediate and inner layers  104 ,  102  as illustrated in  FIG. 7 , for example. As described above, the second tapered portion  118  of the plug  74   a  may be exposed and not surrounded by the cover  72  ( FIG. 7 ). The taper angle a 2  in this region of the plug  74   a  may be between about 50 degrees and about 70 degrees, and preferably between about 58 degrees and 62 degrees. The tapered configuration in the tip  154  of the replacement insert  36  may be configured to improve the flow of adhesive and air removal during the insertion and bonding of the replacement insert  36  in the reconditioned bushing cavity  60 , as will be explained in more detail below. 
     An exemplary method of making replacement insert  36  will now be described in reference to  FIG. 11 . With a metal bushing  70  provided, a plug  74   a  may be coupled to the second end  80  thereof by inserting the first tapered portion  114  of the plug  76   a  into the second tapered portion  94  of the passageway  82  at the second end  80  of the bushing  70 . This is illustrated in step  160 . In a next step  162 , a glass or carbon spun fiber roving may be wrapped about the outer surface  96  of the bushing  70  along the undulating portion  100  and along the straight portion  116  of the plug  76   a . Preferably, the roving is wrapped about the bushing  36  and plug  76   a  in a direction transverse to the longitudinal axis of the bushing  36 . In one embodiment, this roving does not overlie the flange  84 , but starts just down from the flange  84  in the reduced diameter region. This roving constitutes the inner layer  102  of the cover  72  and may have a thickness such that the outer diameter of the inner layer  102  is between about 5% and 10% greater than the outer diameter of the bushing along the undulating portion  100 . For example, the thickness of the inner layer  102  may be about 2 mm in an exemplary embodiment. Other thicknesses are also possible. 
     In step  164 , one or more glass biaxial fabric sheets may be wrapped about the inner layer  102  and constitute the intermediate layer  104 . Preferably, and as noted above, the fiber of the biaxial fabric sheets may be oriented at +/−45 degrees relative to the longitudinal axis of the bushing  60 . In one embodiment, the biaxial fabric sheets also do not overlie the flange  84 , but start just down from the flange  84 . In one embodiment, the thickness of the intermediate layer  104  is such that the outer diameter of the intermediate layer  104  may be substantially the same as the outer diameter of the flange  84 . More broadly perhaps, the thickness of the intermediate layer  104  may be such that the outer diameter of the intermediate layer  104  may be between about 5% and about 10% greater than the outer diameter of the inner layer  102 . For example, the thickness of the intermediate layer may be about 2 mm in an exemplary embodiment. Other thicknesses are also possible, however. 
     In a further step  166 , one or more glass unidirectional fiber sheets may be wrapped about the intermediate layer  104  and constitute the outer layer  106  of the cover  72 . Preferably, the fibers of the unidirectional fabric sheets may be oriented at 0 degrees so as to be generally parallel to the longitudinal axis of the bushing  70 . In one embodiment, the unidirectional fabric sheets do overlie or surround the flange  84  so that an edge of the sheets are generally flush with an end face of the bushing  70  at the first end  78 . The thickness of the outer layer  106  may be such that the outer diameter of the outer layer  106  is between about 20% and about 30% greater than the outer diameter of the intermediate layer  104 . For example, the thickness of the outer layer  106  may be between about 6 mm and about 9 mm, and preferably about 7.5 mm in an exemplary embodiment. Other thicknesses are possible. 
     After the fiber layers  102 ,  104 ,  106  of the cover  72  have been positioned relative to the bushing  70 , the assembly may be infused with resin and cured, as demonstrated by step  168 . By way of example, an RTM process or vacuum infusion process may be used in this regard. These processes are well understood and will not be described in further detail herein. As noted above, the formation of the replacement insert  36  as described above occurs separate from the repair process described in reference to  FIG. 4  above. Accordingly, the infusion and curing processes may be more carefully controlled to ensure a high quality composite structure with an excellent adhesive bond at the bushing/composite interface at the outer surface  96  of the bushing  70 . After this process, the replacement insert  36  may be slightly oversized and thus additional processing steps may be implemented in order to provide the replacement insert  36  with the desired shape, dimensions and features. 
     Thus, in a further step  170  in accordance with the method, material may be removed from the outer surface  158  of the replacement insert  36  so as to arrive at the desired shape, dimensions and to form other additional features. In accordance with one aspect of the invention, since the composite material cover  72  of the insert  36  is substantially fully cured, then well-known processes may be used to modify the cross-sectional profile of the replacement insert  36 . For example, various machining processes may be used to remove material from the outer surface of the oversized replacement insert  36  and thereby arrive at the desired shape and dimensions of the insert. In this regard, the outer surface of the oversized insert may be machined to provide the oblong cross-sectional profile as described above and illustrated in  FIG. 9 . Again, since the formation of the insert is done in a separate process from the repair process, the machining step (or other material removing steps) may be more carefully controlled in order to produce a high-quality replacement insert. 
     As noted above, during removal of material from the oversized replacement insert, additional features may be attributed to or incorporated into the insert  36 . In this regard,  FIGS. 7-9  illustrate a number of discrete spacers  172  projecting from the outer surface  158  of the replacement insert  36 . While in one embodiment, these spacers  172  may be separate elements which are coupled to the outer surface  158  of the insert  36 , in an exemplary embodiment, the spacers  172  are integrally formed in the outer surface  158 . In this regard, as material is being removed from the oversized insert, such as by a controllable milling machine of the like, selected material may be spared or left behind in order to form the spacers  172 . In the illustrated embodiment, the spacers  172  may be illustrated as being generally square or rectangular. For example, the spacers may have a length and width between about 3 mm and about 10 mm, and preferably are about 5 mm. It should be realized, however, that the spacers  172  may take on other sizes and shapes and remain within the scope of the invention. The spacers  172  may be used to provide a gap between the outer surface of the replacement insert  36  and the inner wall of the reconditioned bushing cavity  60 . In that regard, the height of the spacers  172  may be selected to provide the desired gap. By way of example, the height of the spacers  172  may be between about 0.2 mm and about 1 mm, and preferably about 0.5 mm ( FIGS. 11A and 11B ). This will result in a relatively uniform gap between the outer surface  158  of the insert  36  and the walls of the reconditioned bushing cavity  36 . 
     The arrangement of the spacers  172  on the outer surface  156  of the replacement insert  36  may vary. The arrangement is configured to provide a stable and consistent gap between the outer surface  158  of the insert  36  and the inner wall of the bushing cavity  60 . In an exemplary embodiment, the arrangement may include a plurality of rows  174  along the length of the insert  36 , wherein each row includes a plurality of spacers  172  distributed about the periphery of the insert  36  (e.g., in a direction transverse to the longitudinal axis of the insert). For example, the arrangement may include four rows  174   a ,  174   b ,  174   c ,  174   d , uniformly spaced along the length of the insert  36 . Each row  174  may be separated from an adjacent row by about 10 cm. This distance is exemplary and different separation distances may be possible. Additionally, in an alternative embodiment the rows  174  may be non-uniformly spaced along the length of the insert  36 . 
     In one embodiment, there may be four spacers  172  in each of the rows  174 . The spacers  172  may be uniformly distributed about the periphery of the insert  36  (and from a circumferential angle perspective). Thus, for example and in reference to rows  174   b  and  174   d , the spacers may be positioned at 0, 90, 180, and 270 degrees about the periphery of the replacement insert  36 . This is merely exemplary and other uniformly arranged spacers  172  may be possible. Alternatively, the spacers  172  may have a non-uniform distribution about the periphery of the insert  36 . By way of example, the spacers  172  may be at a specified angle on either side of the first and second planar surfaces  136 ,  138 . For example, spacers  172  may be positioned at about 30 degrees to either side of each of the planar surfaces  136 ,  138 . Such an arrangement is shown in  FIG. 7  for rows  174   a  and  174   c , for example. Other non-uniform distributions of spacers  172  are also possible. It should be realized that the raised ridge  120  at the terminating end of the plug  74   a  may also operate as a spacer between the insert  36  and the wall of the reconditioned bushing cavity  60 . Thus, in one embodiment, the height of the raised ridge  120  should be substantially equal to the height of the spacers  172 . 
     As noted above, the reconditioned bushing cavity  60  is configured to have a shape and dimensions similar to the shape and dimensions of the replacement insert  36 . Accordingly, the original bushing cavity  34  that remains after the extraction of the selected compromised bushing  26  may be reshaped to match the outer profile  130  of the replacement insert  36 . In an exemplary embodiment, this would include forming the bushing cavity  60  to have an oblong cross-sectional profile as described above. In regard to the end face  30  of the blade  20 , the amount of blade material radially inboard and outboard of the original bushing cavity  34  is not significant, and typically is less than the amount of blade material between circumferentially adjacent bushing cavities. (See  FIGS. 6A and 6B ). Accordingly, in an exemplary embodiment, the major axis  132  of the cross-sectional profile of the replacement insert  36  may be configured to extend generally in the circumferential direction of the root end  22  of the blade  20 . With the shape, dimensions and orientation now established, the jig  44  may be used, along with the appropriate tool heads  58 , to reshape the bushing cavity  34  to have the desired profile. 
       FIGS. 12A and 12B  schematically illustrate the details of an exemplary method of coupling the replacement insert  36  to the blade  20 , and more particularly to the reconditioned bushing cavity  60 . In this regard, a certain amount of adhesive  180  may be positioned in the bottom of the bushing cavity  60 . In one aspect of the invention, a precise, pre-determined amount of adhesive may be determined based on the dimensions of the bushing cavity  60 , the replacement insert  36 , and the size of the gap or cavity  182  that is desired for the adhesive to occupy. That amount may then be deposited in the bottom of the bushing cavity  60 . The replacement insert  36  may then be slowly inserted into the bushing cavity  60 , with the tapered tip  154  being inserted into the bushing cavity  60  first. Of course, the orientation of the replacement insert  36  must match the orientation of the bushing cavity  60 . In other words, the major axis  132  and the minor axis  134  of the replacement insert  36  must align with the major axis and minor axis of the bushing cavity  60 . With the spacers  172  at the appropriate height, the insert  36  is essentially self-centering relative to the bushing cavity  60 . As the tip  154  of the replacement insert  36  is moved in proximity to the bottom of the cavity  60 , the insert  36  contacts the adhesive  180  and starts driving the adhesive  180  upwardly through the gap  182  and toward the end face  30  of the root end  22  of the blade  20 . When the replacement insert  36  is fully seated, the raised ridge  120  engages with the bottom of the cavity  60 , the spacers  172  substantially engage with the sidewalls of the cavity  60 , and the adhesive  180  substantially fills the gap  182  such that the adhesive  180  is substantially flush with the end face  30  of the blade  20 . The filling process described above may be particularly beneficial to prevent air pockets in the adhesive, and thereby provide an improved adhesive bond at the interface between the outer surface  158  of the insert  36  and the inner wall of the bushing cavity  60 . 
     With the replacement insert  36  fully seated in the bushing cavity  60 , the adhesive  180  may be cured, such as by heating. In this regard, a heating blanket or other type of heater may be used to facilitate curing of the adhesive  180 . In an additional step, a filet of sealant  184  may be disposed around the flange  84  at the first end  78  of the bushing  70 . The sealant  184  may be configured to prevent hydraulic oil or other contaminating agents from infiltrating the interface between the metal bushing  70  and the inner surface of the cover  72 , and optionally between the interface between the outer surface of the cover  72  and the inner wall of the bushing cavity  60 . With the seal  184  protecting the external accessibility of the sensitive bond interfaces, and the plug  74   b  protecting the internal accessibility of the sensitive bond interfaces, the impact of fluids (e.g., hydraulic fluids, etc.) and other contaminating agents from damaging bushings  70  may be significantly reduced. 
     As discussed above, once all of the contaminated bushings  26  have been repaired in the manner described above, the repaired blade  20  may be returned back to the wind turbine site and reattached to the wind turbine  10 . Again, these steps are well known in the industry and will not be described further herein. It is believed that the various elements and methods outlined above represent a more cost-effective way of addressing the issue of loose and damaged bushings that hold a wind turbine blade onto a rotor hub. The cost-effectiveness of the repair method may be further increased when the repair process is implemented at the wind turbine site. In this way, the transportation costs associated with moving the blade to and from a manufacturing/repair facility may be avoided. 
     While the present invention has been illustrated by a description of various preferred embodiments and while these embodiments have been described in some detail, it is not the intention of the Applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. Thus, the various features of the invention may be used alone or in any combination depending on the needs and preferences of the user.