EP4739908A1

EP4739908A1 - Fastening system for a wind turbine tower assembly and method for using same

Info

Publication number: EP4739908A1
Application number: EP24742820.4A
Authority: EP
Inventors: Peter FYNBO; Christian Elkjær HØEG; Søren Bøgelund MADSEN
Original assignee: Vestas Wind Systems AS
Current assignee: Vestas Wind Systems AS
Priority date: 2023-07-07
Filing date: 2024-07-04
Publication date: 2026-05-13
Also published as: WO2025011718A1; CN121443843A

Abstract

A fastening system (60) and method for connecting a flange (64) of one tower section (62) to a flange (74) of an adjacent tower section (72) is disclosed. The first and second flanges (64, 74) have aligned through holes (66, 76). The fastening system (60) includes a plurality of bolts (82) received in a plurality of aligned through holes (66, 76) in the respective flanges (64, 74) and first and second bolt extenders (100, 104, 198, 202, 210, 214). Each bolt extender (100, 104, 198, 202, 210, 214) has a plurality of extender bores (102, 106), each receiving one of the plurality of bolts (82). For each adjacent pair of extender bores (102, 106) one extender bore is threaded and the other is unthreaded. For each adjacent pair of bolts (82), one bolt (82) has a first orientation and the other bolt (82) has a second orientation opposite to the first orientation.

Description

FASTENING SYSTEM FOR A WIND TURBINE TOWER ASSEMBLY AND METHOD FOR USING SAME Technical Field This application relates generally to wind turbines, and more particularly to a fastening system for connecting wind turbine tower sections and a method for using that fastening system. 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 plurality of blades and supported in the nacelle by means of a shaft. The shaft couples the rotor either directly or indirectly with a generator, which is housed inside the nacelle. Consequently, as wind forces the blades to rotate, electrical energy is produced by the generator. To this end, wind turbines may be located either on a land mass (onshore) or within a body of water (offshore). The tower of a wind turbine may be constructed from a plurality of tower sections stacked one on top of another. A tower section may have a circumferential flange at one or both of its ends where the flange will have a plurality of spaced-apart through holes for receiving respective bolts therethrough. In one configuration, the through holes are spaced equidistant from each other around the entire flange. During assembly, one tower section is stacked on top of another tower section such that the flange of the upper tower section rests upon the flange of the lower tower section. The through holes on both flanges are aligned and bolts are inserted into the through holes of the top flange and extend through the through holes of the lower flange where nuts are threaded onto the ends of the bolts. The nut may be tightened by placing a tool (e.g., a socket) over the nut and using a power tool to turn and thereby tighten the nut onto the bolt. The coupled flanges form a joint between the two adjacent tower sections. The tightened nuts and bolts create a clamping force between the two flanges at the joint. As wind turbines increase in size to produce even greater energy, the wind turbines and the corresponding towers increase in height to accommodate larger (longer) wind turbine blades. As the towers increase in height, they need greater strength at the joints between adjacent tower sections. One approach is to simply increase the diameter of the tower. This approach, however, has a number of drawbacks including increased material costs, transportation costs, and manufacturing limitations. Another approach to increasing the strength is to increase the number of bolts around the flanges of adjacent tower sections. For example, this may be achieved by spacing the bolts closer together around the circumference of the flange. However, the minimum distance the bolts may be spaced next to each other is often dictated by the size of the tool needed to engage the nut to tighten it onto the bolt. If the bolts are spaced too closely, the tool will be unable to positively engage the nut and, therefore, will be unable to tighten the nut onto the bolt. A washer is typically used with the nut. The through hole must be spaced radially inward from the wall of the tower section so that the washer does not contact the radiused fillet located by between the tower wall and the flange. The load transfer may be increased by moving the through holes radially as close as possible to the tower walls. The presence of the radiused fillet, however, limits how close the through holes and thus the bolts may be located next to the tower section walls. In light of the problems for decreasing bolt spacing and moving the bolts closer to the tower section walls, there is a need for a fastening system and method that allows the bolts to be spaced closer together but allow conventional tools to tighten the nuts on the bolts. The fastening system should further allow the bolts to be placed closer to the tower section walls even in the presence of a radiused fillet. Summary According to a first aspect of the invention, a wind turbine tower assembly includes a first tower section including a first flange with a plurality of through holes, where the first flange includes an inner surface and a mating surface and a second tower section including a second flange with a plurality of through holes, where the second flange includes an inner surface and a mating surface. The wind turbine tower assembly further includes a fastening system for connecting the first flange and the second flange at a connection interface such that the respective mating surfaces face toward each other, the respective inner surfaces face away from each other, and the plurality of through holes in the first flange and the plurality of through holes in the second flange respectively align with each other. The fastening system includes a plurality of bolts, each of the plurality of bolts having a head and a shaft extending from the head and received in a respective one of the aligned plurality of through holes in the first and second flanges. As discussed more fully below, a “bolt” may include a standard bolt with a head and a shaft or a stud bolt with a shaft and a nut that effectively operates like the head of a bolt. The fastening system also includes at least one first bolt extender including a plurality of first extender bores contacting the inner surface of the first flange, wherein for each adjacent pair of the plurality of first extender bores, one first extender bore is threaded and the other first extender bore is unthreaded, and each of the plurality of first extender bores receives one of the plurality of bolts. The fastening system further includes at least one second bolt extender including a plurality of second extender bores contacting the inner surface of the second flange, wherein for each adjacent pair of the plurality of second extender bores, one second extender bore is threaded and the other second extender bore is unthreaded, and each of the plurality of second extender bores receives one of the plurality of bolts. For each adjacent pair of the plurality of bolts, one bolt may have a first orientation and the other bolt may have a second orientation opposite to the first orientation. In one embodiment, for each bolt of the plurality of bolts, at least one adjacent bolt may define an overlap region between the head of the bolt and the head of the adjacent bolt. In another embodiment, each bolt of the plurality of the bolts has a first adjacent bolt and a second adjacent bolt each having a centerline. The centerlines of the bolt and the first adjacent bolt are spaced apart with a centerline distance C₁ and the centerlines of the bolt and the second adjacent bolt are spaced apart with a second centerline distance C2, and wherein the second distance C2 may be greater than or equal to the first distance C1. In one embodiment, the shaft of each of the plurality of bolts has a shaft diameter D_sh and the first centerline distance C₁ may be in the range of 1.01 to 1.75 times the shaft diameter Dsh, and preferably may be 1.25 times the shaft diameter Dsh. In one aspect, for each adjacent pair of the plurality of first extender bores, the unthreaded extender bore may be a through bore, and the threaded extender bore may be a through bore or a blind bore. In another embodiment, the at least one first bolt extender may include only two first extender bores. In yet another embodiment, the at least one first bolt extender of the fastening system may include a plurality of first bolt extenders. In another embodiment, the first tower section includes a first tower wall extending from the first flange and the at least one first bolt extender may include a curved edge. The curved edge may be received within a corner defined by an intersection of the first flange and the first tower wall. In one embodiment, the at least one first bolt extender and the at least one second bolt extender may be identical to each other. In one embodiment, each of the at least one first bolt extender and the at least one second bolt extender includes sidewalls that may define a trapezoidal perimeter. In other embodiment, the at least one first and second bolt extenders have sidewalls and each of the sidewalls may include at least one of the plurality of extender bores. The at least one of the plurality of extender bores in each opposing sidewall may extend at least 50% of a complete extender bore circumference. In yet another embodiment, each of the at least one first bolt extender and the at least one second bolt extender have first and second opposing sidewalls. The first sidewall may be concave and the second sidewall may be convex, such that the concave second sidewall of one first bolt extender is configured to nest within the convex first sidewall of an adjacent first bolt extender. The first sidewall may have a first curved portion and the second sidewall may have a second curved portion. The center of curvature of the second curved portion may be defined at a central axis of the one threaded first extender bore. The other unthreaded first extender bore (102) extends at least 70% of a complete extender bore circumference. In one embodiment of the invention, a wind turbine may comprise the wind turbine tower assembly as described above. Another aspect of the invention includes a method of assembling a wind turbine tower assembly. The wind turbine tower assembly includes a first tower section including a first flange with a plurality of through holes, where the first flange includes an inner surface and a mating surface and a second tower section including a second flange with a plurality of through holes, where the second flange including an inner surface and a mating surface. The method includes positioning the first flange relative to the second flange so that the respective mating surfaces face toward each other, the respective inner surfaces face away from each other, and the plurality of through holes in the first flange respectively align with the plurality of through holes in the second flange. The method includes positioning on the inner surface of the first flange at least one first bolt extender including a plurality of first extender bores so that the plurality of first extender bores align with respective ones of the plurality of through holes in the first flange. For each adjacent pair of the plurality of first extender bores, one first extender bore may be threaded and the other first extender bore may be unthreaded. The method also includes positioning on the inner surface of the second flange at least one second bolt extender including a plurality of second extender bores so that the plurality of second extender bores align with respective ones of the plurality of through holes in the second flange. For each adjacent pair of the plurality of second extender bores, one second extender bore may be threaded and the other second extender bore may be unthreaded. Using a plurality of bolts, inserting one of the plurality of bolts into each one of the unthreaded first extender bores of the at least one first bolt extender so as to be received in and engaged with one of the second threaded extender bores of the at least one second bolt extender and inserting one of the plurality of bolts into each one of the unthreaded second extender bores of the at least one second bolt extender so as to be received in and engaged with one of the threaded first extender bores of the at least one first bolt extender. The method includes tensioning each of the plurality of bolts to connect the first flange and the second flange. The plurality of bolts is inserted such that for each adjacent pair of the plurality of bolts, one bolt may have a first orientation and the other bolt may have a second orientation opposite to the first orientation. In one embodiment, the first tower section includes a first tower wall extending from the first flange at an intersection that defines a corner and the at least one first bolt extender may include a curved edge. Positioning on the inner surface of the first flange the at least one first bolt extender may include positioning the at least one first bolt extender such that the curved edge may be received in the corner. Similarly, the second tower section includes a second tower wall extending from the second flange at an intersection that defines a corner and the at least one second bolt extender may include a curved edge. Positioning on the inner surface of the second flange the at least one second bolt extender includes positioning the at least one second bolt extender such that the curved edge may be received in the corner. An aspect of the invention includes a method of retrofitting a fastening system to a wind turbine tower assembly that includes a first tower section including a first flange with a plurality of existing through holes, where the first flange including an inner surface and a mating surface and a second tower section including a second flange with a plurality of existing through holes, where the second flange including an inner surface and a mating surface. The first flange is connected to the second flange via a plurality of existing bolts in the plurality of existing through holes of the first and second flanges such that the respective mating surfaces face toward each other and the respective inner surfaces face away from each other. The method of retrofitting includes removing a subset of the plurality of existing bolts from the plurality of existing through holes in the first and second flanges and forming a plurality of intermediate through holes in the first and second flanges, where each of the plurality of intermediate through holes may be positioned between adjacent pairs of existing through holes in the first and second flanges. The method includes positioning on the inner surface of the first flange at least one first bolt extender including a plurality of first extender bores so that at least one of the plurality of the first extender bores aligns with one of the plurality of existing through holes in the first flange and an adjacent one of the plurality of first extender bores aligns with the adjacent one of the plurality of intermediate through holes in the first flange. For each adjacent pair of the plurality of first extender bores, one first extender bore may be threaded and the other first extender bore may be unthreaded. The method includes positioning on the inner surface of the second flange at least one second bolt extender including a plurality of second extender bores so that at least one of the plurality of the second extender bores aligns with one of the plurality of existing through holes in the second flange and an adjacent one of the plurality of second extender bores aligns with an adjacent one of the plurality of the intermediate through holes in the second flange. For each adjacent pair of the plurality of second extender bores, one second extender bore may be threaded and the other second extender bore may be unthreaded. The method includes using a plurality of replacement bolts and inserting one of the plurality of replacement bolts into each one of the unthreaded first extender bores of the at least one first bolt extender so as to be received in and engaged with one of the second threaded extender bores of the at least one second bolt extender and inserting one of the plurality of replacement bolts into each one of the unthreaded second extender bores of the at least one second bolt extender so as to be received in and engaged with one of the first threaded extender bores of the at least one first bolt extender. Finally, tensioning each of the plurality of replacement bolts to connect the first flange and the second flanges. The plurality of replacement bolts is inserted such that for each adjacent pair of the plurality of replacement bolts, one replacement bolt may have a first orientation and the other replacement bolt may have a second orientation opposite to the first orientation. In one embodiment, each of the plurality of the existing through holes in the first and second flanges may be positioned at a first radius R₁ from a center of the first and second tower sections and each of the plurality of the intermediate through holes may be positioned at a second radius R₂ less than or equal to the first radius R₁. In another embodiment, before removal, each of the plurality of existing bolts may be in the first orientation. 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 diagrammatic front view of an exemplary wind turbine with multiple wind turbine tower sections. Fig.2A is an elevational view of section of adjacent tower sections coupled together with bolts and nuts in a known manner. Fig.2B is a plan view of the bolt spacing shown in Fig.2A. Fig.2C is a cross-sectional view of the tower sections shown in Fig.2A. Fig.3 is an elevational view of a section of adjacent tower sections coupled together according to one known configuration. Fig.4 is an elevational view of a section of adjacent tower sections coupled together according to a second known configuration. Fig.5A is a perspective view of adjacent tower sections coupled together using a fastening system according to an embodiment of the invention. Fig.5B is a plan view of the tower sections shown in Fig.5A. Fig.6 is a cross-sectional view of the tower sections of Fig.5A. Fig.7 is a cross-sectional view of the tower sections of Fig.5A. Fig.8 is perspective view of adjacent tower sections coupled together according to another embodiment of the invention. Fig.9 is a cross-sectional view of the tower sections of Fig.8. Fig.10 is a cross-sectional view of the tower sections of Fig.8. Fig.11 is a perspective view of adjacent tower sections coupled together according to another embodiment of the invention. Fig.12 is a cross-sectional view of the tower sections of Fig.11. Fig.13 is a cross-sectional view of the tower sections of Fig.11. Fig.14 is a perspective view of adjacent tower sections coupled together according to another embodiment of the invention. Fig.15 is a cross-sectional view of the tower sections of Fig.14. Fig.16 is a cross-sectional view of the tower sections of Fig.14. Fig.17A is a perspective view of a bolt extender according to one embodiment of the invention. Fig.17B is a plan view of the bolt extender of Fig.17A. Fig.17C is a plan view of the bolt extender of Fig.17A with overlapping heads of adjacent bolts. Fig.17D is a plan view of the bolt extender of Fig.17A with a nut overlapping adjacent through holes. Fig.18 is perspective view of adjacent tower sections coupled together according to another embodiment of the invention. Fig.19 is a cross-sectional view of the tower sections of Fig.18. Fig.20 is a cross-sectional view of the tower sections of Fig.18. Fig.21 is a plan view showing intermediate through holes positioned between existing through holes in a flange. Fig.22 is a plan view of a plurality of bolt extenders according to one embodiment of the invention. Fig.23 is a plan view of a plurality of bolt extenders according to another embodiment of the invention. Fig.24 is a plan view of a plurality of bolt extenders according to another embodiment of the invention. Fig.25 is a plan view of a plurality of bolt extenders according to another embodiment of the invention. Detailed Description Figure 1 depicts an exemplary wind turbine 10 including a wind turbine tower assembly (“tower assembly”) 12 and an energy generating unit 14 disposed at the apex of the tower assembly 12. The following description provides relevant background context on the environment and primary components of an exemplary wind turbine. The tower assembly 12 is coupled to a foundation 16 at a lower end thereof, shown specifically along a ground surface. The foundation 16 may be a relatively large mass formed from concrete, steel, etc., that transfers forces acting on the wind turbine 10 into the ground surface. In an alternative embodiment, the foundation 16 may include a pile or other structure at an offshore location to which the wind turbine 10 is attached. The tower assembly 12 is configured to support the weight of the energy generating unit 14 and elevate the energy generating unit 14 to a height above ground level or sea level at which faster moving air currents of lower turbulence are typically found. The tower may be formed from a plurality of tower sections 18 coupled together vertically at a connection interface 18a. The energy generating unit 14 includes a nacelle 20, a rotor 22, and a generator (not shown). The rotor 22 includes a central hub 24 and a plurality of wind turbine blades 26 (e.g., three blades) that are operatively coupled to the hub 24. Each blade 26 may extend from a root end to a tip end, with the root end being coupled to the hub 24 through a pitching system or the like. The pitching system selectively rotates (or “pitches”) each blade 26 about a pitch axis thereof to adjust their angle of attack with respect to the wind. The energy generating unit 14 is typically coupled to the tower assembly 12 by a yaw system (not shown) that rotates the energy generating unit 14 relative to the tower assembly 12 so that the rotor 22 is pointed in an optimum direction with respect to the incoming wind. The generator is operatively coupled to the hub 24, e.g., by a drive train including a gear arrangement that interconnects the rotor 22 and the generator. The rotational speed of the generator is typically a fixed multiple of the rotational speed of the rotor 22. Thus, the speed of the generator may be determined from the speed of the rotor, and vice-versa. The generator and a substantial portion of the drive train may be positioned inside the nacelle 20. The generator converts mechanical energy received from the rotor 22 into electrical energy. The blades 26 are configured to interact with the wind in a manner that generates lift. This lift causes the rotor 22 to rotate to generally define a sweep area of the blades 26. The energy generating unit 14 thus generates power from the wind that passes through the sweep area of the rotor 22. Fig.2 illustrates a partial section of two tower sections 18 forming the connection interface 18a. Each tower section 18 includes first and second tower walls 30a, 30b and a flange 32a, 32b with a plurality of through holes 34. The tower sections 18 are connected with a plurality of bolts 36, with each bolt 36 extending through the flange 32a and out of flange 32b. A washer 38 and nut 40 are placed onto the end of the bolt 36. A tool 42 is placed over the nut 40 so as to tighten the nut onto the bolt 36. The tool 42 has a diameter D_T. The tool 42 may be any suitable tool such as a standard impact socket for torquing the nut 40 onto the bolt 36. Alternatively, the tool 42 may be a combination tool that applies tension to the bolt 36 to stretch the bolt 36. The combination tool may then turn the nut 40 onto the threads of the stretched bolt 36. The combination tool may then release the tension on the bolt 36 to create a clamping force. Other tools may be used to tension the bolt 36 to create a clamping force. For example, a torque/angle tool may be used. In another example, a yield controlling tool may be used. In any event, the tensioning of the bolt 36 is not limited to any particular tool for tensioning the bolts 36. The bolts 36 must be spaced sufficiently apart to allow the tool 42 to extend over the nut 40 and not be obstructed by the adjacent nuts 40. Washers 38 may also be used between the heads and the flange 32a. In an embodiment, the washer 38 may be integrated with the nut 40 such that the nut 40 and washer 38 are one piece. Fig.2B illustrates a bottom view of the three bolts 36 in Fig.2A. The centerline of adjacent bolts 36 are equally spaced a centerline distance C around both flanges 32a, 32b. The bolts 36 are spaced as closely as possible while still allowing the tool 42 to fit between the adjacent bolts 36. The bolts 36 cannot move any closer to each other because that would prevent full engagement of the tool 42. Fig.2C shows a cross-sectional view of the tower sections 18 in Fig.2A. A corner 44a is defined by an intersection of the first flange 32a and the first tower wall 30a. A corner 44b is defined by an intersection of the second flange 32b and the second tower wall 30b. The corners 44a, 44b may be radiused fillets that have a curved profile. As illustrated in Fig.2C, the bolt 36 is spaced radially inwardly from the radiused fillets 44a, 44b between the first and second tower walls 30a, 30b and the respective flanges 32a, 32b. Advantageously, the bolt 36 should be located as close to the first and second tower walls 30a, 30b as possible to increase the effectiveness of the bolts 36 and to minimize any gap that may form between the flanges 32a, 32b when the tower assembly 12 and tower sections 18 move because of wind or other external forces. Having said that, the bolt 36 must be spaced away from the radiused fillets 44a, 44b to allow the tool 42 to fully engage the nut 40 without interference from the tower wall 30a. The bolt 36 should also be sufficiently spaced away from the first and second tower walls 30a, 30b so that the washers 38 contact only the flat surface of the flanges 32a, 32b and do not contact the radiused fillets 44a, 44b. Fig.3 illustrates one know approach for locating the bolts 36 closer together compared to the traditional approach illustrated in Fig.2A. In this approach, not every bolt 36 extends through the top flange 32a and extends out of the bottom flange 32b. Instead, studs 46 are threaded into threaded bores in the flanges 32a, 32b. Every other stud 46 is threaded into the bottom flange 32b and the intervening studs 46 are threaded into the upper flange 32a. A nut 40 is then threaded onto the exposed end of the stud to generate the clamping force. This approach allows the bolts 36 to spaced more closely together and still allow the tool 42 to engage the nut 40 because the adjacent stud 46 does not extend out of the respective flange 32a, 32b. Fig.4 illustrates another known approach for locating the bolts 36 closer together compared to the traditional approach illustrated in Fig.2A. As illustrated, not every bolt 36 extends through the top flange 32a and extends out of the bottom flange 32b. Instead, every other bolt 36 extends upwardly through the bottom flange 32b and extends through the top flange 32a. An individual spacer 50 is then located between the washer 38 and the flange 32a, 32b. The spacer 50 positions the washer 38 and the nut 40 away from the surface of the flange 32a, 32b. As such, the bolts 36 may be spaced more closely together and still allow the tool 42 to engage the nut 40 and not contact the adjacent bolts 36. Figs.5A-7 illustrate a fastening system 60 according to one embodiment of the invention. Figs.5A, 6, and 7 illustrate a first tower section 62 including a first flange 64 and a plurality of through holes 66. The first flange 64 includes an inner surface 68 and a mating surface 70. Figs.5A, 6, and 7 also illustrate a second tower section 72 including a second flange 74 and a plurality of through holes 76. The second flange 74 includes an inner surface 78 and a mating surface 80. The fastening system 60 is used to connect the first flange 64 to the second flange 74 such that the mating surfaces 70, 80 face toward each other and the inner surfaces 68, 78 face away from each other. In addition, the plurality of holes 66 in the first flange 64 are aligned with the plurality of holes 76 in the second flange 74. The fastening system 60 includes a plurality of bolts 82 that connect the first flange 64 and the second flange 74. The term “bolt” as used herein may include a standard bolt with a head connected to a shaft or it may further include a stud bolt where a shaft and nut(s) are used. With the stud bolt, the shaft may include two threaded ends where one threaded end may be threaded into a threaded bore and a nut is threaded onto the opposing threaded end. In another stud bolt arrangement, a nut is threaded onto one threaded end and another nut is threaded onto the opposing threaded end. When using the stud bolt, one nut effectively operates like the head 84 of bolt 82. Each bolt 82 has a head 84 and a shaft 86 extending from the head 84. As illustrated in Figs.5A-7, the shaft 86 has a head end 88 and a tip end 90. The head 84 is connected to shaft 86 at or adjacent to the head end 88. At least a portion of the shaft 86 adjacent the tip end 90 is threaded. Each of the plurality of bolts 82 is received in a respective one of the aligned through holes 66, 76 in the first and second flanges 64, 74. The fastening system 60 further includes at least one first bolt extender 100 including at least one extender bore 102 receiving one of the plurality of bolts 82. As used herein, the term “extender bore” means a bore with a sidewall extending between about 50% to about 100% of a complete extender bore circumference. For example, the sidewall of the extender bore 102 in Fig.5B extends 100% of a complete extender bore circumference. In contrast, some of the extender bores 102 (see, e.g., Fig.23) extend approximately 50% of a complete extender bore circumference. In an embodiment, the at least one first bolt extender 100 may include a plurality of first bolt extenders 100. The first bolt extender 100 contacts the inner surface 68 of the first flange 64. The fastening system 60 also includes at least one second bolt extender 104 including at least one extender bore 106 receiving one of the plurality of bolts 82. The second bolt extender 104 contacts the inner surface 78 of the second flange 74. In the embodiment in Figs.5A-7, each of the first and second bolt extenders 100, 104 have two extender bores 102, 106, where one of the two extender bores 102, 106 is threaded and the other of the two extender bores 102, 106 is unthreaded. Moreover, because the first and second bolt extenders 100, 104 have two extender bores 102, 106, there must be an even number of bolts 82 used and a corresponding even number of through holes 66, 76 in the first and second flanges 64, 74 when connecting the first and second tower sections 62, 72. In an embodiment, the extender bores 102, 106 extend all the way through the respective first and second bolt extenders 100, 104. To that end, the bolt 82 may be sized so that the tip end 90 does not extend completely through the extender bores 102, 106. In other words, the tip end 90 of bolt 82 does not penetrate the inner surfaces of the first and second bolt extender 100, 104. In an embodiment, the first and second bolt extenders 100, 104 may be identical to each other. In an embodiment, the fastening system 60 may include a washer 108 positioned between the head 84 and the first and second bolt extenders 100, 104. For the fastening system 60, for each adjacent pair of bolts 82, one bolt 82 has a first orientation and the other bolt 82 has a second orientation opposite the first orientation. For example, as illustrated in Figs.5A and 6, one bolt 82 has a head-up orientation where the head 84 is positioned above the first and second flanges 64, 74. The adjacent bolt 82, however, has a head-down orientation where the head 84 is positioned below the first and second flanges 64, 74. The through holes 66, 76 in the first and second flanges 64, 74 may not be equally spaced. With reference to Fig.5B, the centerlines of adjacent bolts 82 of one first bolt extender 100 are spaced apart a first centerline distance C1. The centerline of the bolt 82 of one first bolt extender 100 is spaced from the centerline of the adjacent bolt 82 of the adjacent first bolt extender 100 a second centerline distance C₂. In an embodiment, the centerline distance C2 may be greater than the centerline distance C₁. Advantageously, the effective distance C_eff which equals the average of the centerline distance C₁ and the centerline distance C₂ is less than the centerline distance C (Fig.2B). In one scenario, the bolts 36 (Fig.2A) and the bolts 82 may be M72 bolts. As such, the centerline distance C would be around 133 mm whereas C_eff would be 105 mm. In an embodiment, the shaft 86 of each bolt 82 has a shaft diameter (Dsh) and the first centerline distance C₁ is in the range of 1.01 to 1.75 times the shaft diameter (D_sh), and preferably 1.25 times the shaft diameter (D_sh). The centerline distance C₁ may also have other values outside this range. With reference to Figs.5B and 6, for each bolt 82 of the plurality of bolts 82, at least one adjacent bolt 82 defines an overlap region 110 between the head 84 of the bolt 82 and the head 84 of the adjacent bolt 82. With reference to Fig.7, the first and second tower sections 62, 72 include respective first and second tower walls 112, 114 which extend from respective first and second flanges 64, 74. The intersection of the first and second tower walls 112, 114 with the first and second flanges 64, 74 define corners 116, 118 which may have a curved, rounded, radiused, or chamfered profile (“curved profile”). In an embodiment, the first and second bolt extenders 100, 104 may have a curved, rounded, radiused, or chamfered edge 120, 122 (“curved edge 120, 122”), which approximates the curved profile of the corners 116, 118. As such, the curved edges 120, 122 may be received within the curved profile of the corners 116, 118. As a result, the bolts 82 may be moved closer to the first and second tower walls 112, 114 as compared to the configuration illustrated in Fig.2. Consequently, the bolts 82 may more effectively provide load transfer between the first and second tower sections 62, 72 during operation of the wind turbine 10. Figs.8-10 illustrate another fastening system 60 according to an embodiment of the invention which is similar to the fastening system described above. In that regard, like reference numerals represent like features where applicable compared to the embodiment described above with respect to Figs.5A-7. In fastening system 60, the extender bores 102, 106 are blind bores and, therefore, extend only partially through the respective first and second bolt extenders 100, 104. Because the extender bores 102, 106 are blind bores, the length of the bolts 82 must be appropriately chosen so that the tip end 90 of the bolts 82 fully fit within the blind bore of the extender bores 102, 106. Figs.11-13 illustrate another fastening system 60 which is similar to the fastening system described above. In that regard, like reference numerals represent like features where applicable compared to the embodiment described above with respect to Figs.5A-7. In the embodiment illustrated in Figs.5A-7, each of the first and second bolt extenders 100, 104 have two extender bores 102, 106, and one of those two extender bores 102, 106 have threads therein. In an embodiment, the bolt extenders 100, 104 may include four or more extender bores 102, 106 as long as the number of extender bores is even. In that regard, the bolt extenders 100, 104 may include a single, closed ring (not shown) with an even number of extender bores 102, 106. Alternatively, the first and second bolt extenders 100, 104 may include a smaller number of ring segments (not shown) with an even number of extender bores 102, 106. In the embodiment illustrated in Figs.11-13, first and second bolt extenders 100, 104 have six extender bores 102, 106, of which three of those six extender bores 102, 106 having threads therein and the other three are unthreaded. In the embodiment shown in Figs.11-13, the threaded extender bores 102, 106 extend all the way through the first and second bolt extender 100, 104. In another embodiment, the threaded extender bores 102, 106 may be blind bores like the type illustrated in Figs.8-10. In an embodiment of fastening system 60, the bolts 82 may be equally spaced from each other as well as spaced further apart such there is no overlap region 110 as is present in fastening systems 60 described above. Figs.14-17C illustrate another fastening system 142 which is similar to the fastening system 60. In that regard, like reference numerals represent like features where applicable compared to the embodiment described above with respect to Figs.5A-7. The fastening system 142 includes at least one first bolt extender 144 including at least one extender bore 146 (Fig.17A) receiving one of the plurality of bolts 82. In an embodiment, the at least one first bolt extender 144 may include a plurality of first bolt extenders 144. The first bolt extender 144 contacts the inner surface 68 of the first flange 64. The fastening system 142 also includes at least one second bolt extender 148 including at least one extender bore 150 receiving one of the plurality of bolts 82. The second bolt extender 148 contacts the inner surface 78 of the second flange 74. In an embodiment, the first and second bolt extenders 144, 148 may be identical to each other. Figs.17A-17C illustrate the first bolt extender 144. The description of the first bolt extender 144 is equally applicable to the second bolt extender 148. The first bolt extender 144 includes two cutouts 152a, 152b on opposite sides of the extender bore 146. The cutouts 152a, 152b are configured to receive the head 84 of each bolt 82 adjacent to the extender bore 146. The fastening system 142 includes a plurality of nuts 154 that threadingly engage the threaded end of the bolts 82. When threaded on, the nuts 154 secure the first and second bolt extenders 144, 148 to the respective inner surfaces 68, 78 of the first and second flanges 64, 74 so as to connect the first flange 64 to the second flange 74. In an embodiment, a washer 156 may be placed between the nut 154 and the respective first and second bolt extenders 144, 148. In an embodiment, the through holes 66, 76 are equally spaced around the first and second flanges 64, 74. The use of the first and second bolt extenders 144, 148 allows the bolts 82 to be more closely spaced than the through holes 34 in Figs.2A and 2B. The close spacing of the bolts 82 in fastening system 142 means that for each bolt 82 of the plurality of bolts 82, at lest one adjacent bolt 82 defines an overlap region 160 between the head 84 of the bolt 82 and the head 84 of the adjacent bolt 82 as illustrated in Fig.17C (see also Fig.15). In one scenario, the bolts 36 (Fig.2A) and the bolts 82 used in fastening system 142 may be M48 bolts. As such, the centerline distance C for bolts 36 would be around 102 mm and a centerline distance C_blt for bolts 82 (Fig.17C) would be around 62 mm or an approximately 40% reduction in the bolt spacing. In an embodiment, the first and second bolt extenders 144, 148 may have a curved, rounded, radiused, or chamfered edge 162, 164 (“curved edge 162, 164”), which approximates the curved profile of the corners 116, 118. As such, the curved edges 162, 164 may be received within the curved profile of the corners 116, 118. As discussed above, the bolts 82 may be moved closer to the first and second walls 112, 114 as compared to the configuration illustrated in Fig.2. Consequently, the bolts 82 may more effectively provide load transfer between the first and second tower sections 62, 72 during operation of the wind turbine 10. Fig.17D illustrates a nut 154 partially extending over both adjacent through holes 66, 76 in the first and second flanges 64, 74 thereby defining overlap regions 166a, 166b. Figs.18-20 illustrate another fastening system 142 which is similar to the fastening system in Figs.14-16. In that regard, like reference numerals represent like features where applicable compared to the embodiment described above with respect to Figs. 14-16. For fastening system 142, the first and second flanges 64, 74 include respective grooves 172, 174. The fastening system 142 further includes first and second bolt extenders 144, 148 that are essentially the same as first and second bolt extenders described above, except that first and second bolt extenders 144, 148 do not have the cutouts 152a, 152b. The first and second bolt extenders 144, 148 contact the inner surfaces 68, 78 of the first and second flanges 64, 74 so as to span the respective grooves 172, 174. The grooves 172, 174 are configured so that heads 84 of the bolts 82 may be positioned within the grooves 172, 174. When a stud bolt is used, the nut is secured to the shaft of the stud bolt and the nut is positioned within the grooves 172, 174, effectively operating like the head 84 of bolt 82. Because the first and second bolt extenders 144, 148 are essentially the same dimensionally as the first and second bolt extenders described above, the spacing of the bolts 82 in the fastening system 142 is essentially the same as the spacing of the bolts 82 in the fastening system described above. In an embodiment, the first and second bolt extenders 144, 148 may be identical to each other. The invention further contemplates a method of assembling a wind turbine tower of the type described above with multiple tower sections 18. The method described below applies to at least fastening systems 60. The method includes positioning the first flange 64 relative to the second flange 74 so that the respective mating surfaces 70, 80 face toward each other, the respective inner surfaces 68, 78 face away from each other, and the plurality of through holes 66 in the first flange 64 respectively align with the plurality of through holes 76 in the second flange 74. Then, the at least one first bolt extender 100 including the plurality of first extender bores 102 may be positioned on the inner surface 68 of the first flange 64 so that the plurality of first extender bores 102 align with respective ones of the plurality of through holes 66 in the first flange 64. For each adjacent pair of the plurality of first extender bores 102, one first extender bore 102 is threaded and the other first extender bore 102 is unthreaded. Next, the at least one second bolt extender 104 including the plurality of second extender bores 106 may be positioned on the inner surface 78 of the second flange 74 so that the plurality of extender bores 102 aligns with respective ones one of the plurality of through holes 76 in the second flange 74. For each adjacent pair of the plurality of second extender bores 106, one second extender bore 106 is threaded and the other second extender bore 106 is unthreaded. Using a plurality of bolts 82, the method may include inserting one of the plurality of bolts 82 into each one of the unthreaded first extender bores 102 of the at least one first bolt extender 100 so as to be received in and engaged with one of the second threaded extender bores 106 of the at least one second bolt extenders 104 and also inserting one of the plurality of bolts 82 into each one of the unthreaded second extender bores 106 of the at least one second bolt extender 104 so as to be received in and engaged with one of the threaded first extender bores 102 of the at least one first bolt extender 100. Finally, the method may include tensioning each of the plurality of bolts 82 to connect the first flange 64 and the second flange 74. As discussed above, the bolt 82 may be tensioned by torquing the head 84 of the bolt 82 using an impact socket for example. The bolt 82 may also be tensioned using the combination tool (as described above). When using a stud bolt, the shaft may be threaded into the threaded first extender bore 102 and then a nut may be threaded onto the opposing threaded end of the shaft. A socket may be used to torque the nut or the combination tool (as described above) may be used to tension the stud bolt. The plurality of bolts 82 is inserted such that for each adjacent pair of the plurality of bolts 82, one bolt 82 has a first orientation (e.g., head-up) and the other bolt 82 has a second orientation (e.g., head-down) opposite to the first orientation. When a stud bolt is used, a head-up orientation means the nut is secured to the shaft of the stud bolt and is positioned atop and contacting the first bolt extender 100 and a head- down orientation means the nut is secured to the shaft of the stud bold and is positioned below and contacting the second bolt extender 104. The method may further include positioning the at least one first bolt extender 100 such that the curved edge 120 is received in the corner 116 and positioning the at least one second bolt extender 104 such that the curved edge is received in the corner 118. As note above, this positions the bolts 82 as close as possible to the tower walls 112, 114. In an embodiment and as illustrated in Figs.18-20, the first flange 64 includes a groove 172 and the second flange includes a groove 174 and the method further includes positioning the at least one first bolt extender 144 to span the groove 172 and positioning the at least one second bolt extender 148 to span the groove 174. In an embodiment, the head 84 of the bolt 82 may be positioned within the grooves 172, 174. When a stud bolt is used, the nut is secured to the shaft of the stud bolt and the nut is positioned within the grooves 172, 174, effectively operating like the head 84 of bolt 82. Each tower section 62, 72 has essentially a circular cross section and the through holes 66, 76 in the first and second flanges 64, 74 define a bolt circle with a bolt circle diameter. The first and second bolt extenders 100, 104 are generally rectangular when viewed from above, i.e., in plan view. That rectangular shape may lead to excessive gaps between adjacent ones of the first and second bolt extenders 100, 104 when they are installed along the bolt circle diameter. To minimize the gap between adjacent first and second bolt extenders 100, 104, a bolt extender 198 has sidewalls 200 that define a trapezoidal perimeter when viewed from above as shown in Fig.22. The bolt extender 200 may include all the features of first and second bolt extenders 100, 104, including the corner 116 with the curved edge 120. The bolt extender 198 may also include a plurality of extender bores 102 to receive bolts 82. For each adjacent pair of extender bores 102 in the bolt extender 198, one extender bores 102 may be threaded and the other extender bore may be unthreaded. The bolt extender 198 may be used on both the first and second flanges 64, 74. To further reduce the spacing between the threaded bore and the unthreaded bore of adjacent first and second bolt extenders 100, 104, opposite sidewalls of the bolt extenders 100, 104 may include unthreaded bores. As shown in Fig.23, a bolt extender 202 includes opposing sidewalls 204 and opposing sidewalls 206. The sidewalls 204 each including the extender bores 102. The bolt extender 202 is generally rectangular when viewed from above, i.e., in plan view. The extender bores 102 in the sidewalls 204 may be unthreaded. In this embodiment, the extender bores 102 extend approximately 50% of a complete extender bore circumference. As shown in Fig.23, one bolt extender 202 may be positioned adjacent another bolt extender 202 such that the sidewall 204 of the bolt extender 202 is adjacent the sidewall 204 of the other bolt extender 202. In this arrangement, opposing extender bores 102 are positioned close to one another so as to form a nearly complete circumference of a circle. As such, the opposing extender bores 102 are configured to receive bolts 82. The threaded end of each of the bolts 82 may be threaded into the threaded extender bore 106 of a corresponding bolt extender (not shown) mounted to the second flange 74 of the second tower section 72. The bolt extender 202 may be used on both the first and second flanges 64, 74. The bolt extender 202 with the three extender bores 102 and rectangular shape is configured to be used with a specific bolt circle diameter. To use the bolt extender 202 with other bolt circle diameters, large gaps between adjacent bolt extenders 202 or oversized extender bores 102 may be required. To minimize the gap between adjacent bolt extenders, a bolt extender 210 similar to bolt extender 202 but with a modified geometry may be employed. The bolt extender 210 may include all the features of the bolt extender 202, including the sidewalls 204, 206, where each of sidewalls 204 include extender bores 102. The sidewalls 204, 206 of the bolt extender 210 may define a trapezoidal perimeter when viewed from above, i.e., in plan view, as shown in Fig.24. The bolt extender 210 may be used on both the first and second flanges 64, 74. Fig.25 shows another bolt extender 214 with extender bores 102. One of the extender bores 102 is threaded and the other extender bore 102 is unthreaded and extends at least 70% of a circle diameter with an opening 216 in the unthreaded extender bore 102 disposed furthest away from the threaded extender bore 102. The extender bores 102 are configured to receive bolts 82. The bolt extender 214 includes opposite sidewalls 218, 220. As shown in Fig.25, the sidewall 218 includes a curved portion 222 that curves inwardly (i.e., concave) towards the unthreaded extender bore 102 and the sidewall 220 includes a curved portion 224 that curves outwardly (i.e., convex) away from the thread extender bore 102. The curved portion 224 of the sidewall 220 is complimentary of the curved portion 222 of the sidewall 218. As such, the curved portion 224 of the sidewall 220 of one bolt extender 214 may nest within the curved portion 222 of the sidewall 218 of an adjacent bolt extender 214. The center of curvature of the curved portion 224 of the sidewall 220 is the center of the threaded extender bore 102. As such, the bolt extender 214 may rotate about a central axis extending through the threaded extender bore 102 so that the gap between adjacent bolt extenders 214 remains constant had one bolt extender 214 rotates relative to the other (adjacent) bolt extender 214. As a result of the curved portions 222, 224 of sidewalls 218, 220, the bolt extender 214 may be used with different bolt circle diameters without creating excessive gaps between adjacent bolt extenders 214. The bolt extender 214 may be used on both the first and second flanges 64, 74. A method for assembling a wind turbine tower assembly 12 using the fastening system 142 will now be described. This method is similar to the method described above using fastening systems 60. The method includes positioning the first flange 64 relative to the second flange 74 so that the respective mating surfaces 70, 80 face toward each other, the respective inner surfaces 68, 78 face away from each other, and the plurality of through holes 66 in the first flange 64 respectively align with the plurality of through holes 76 in the second flange 74. Then, the method includes at least one first bolt extender 100 including at least one first extender bore 102 positioned on the inner surface 68 of the first flange 64 so that the at least one first extender bore 102 aligns with at least one of the plurality of through holes 66 in the first flange 64. Next, the method includes positioning at least one second bolt extender 104 including at least one second extender bore 106 on the inner surface 78 of the second flange 74 so the at least one second extender bore 106 aligns with at least one of the plurality of through holes 76 in the second flange 74. Using a plurality of bolts, the method includes inserting one of the plurality of bolts 82 into the at least one first extender bore 102 of the at least one first bolt extender 100 and also inserting one of the plurality of bolts 82 into the at least one second extender bore 106 of the at least one second bolt extender 104. Next, engaging each of the plurality of bolts 82 with a nut 154. Finally, the method includes tensioning each of the plurality of bolts 82 to connect the first flange 64 and the second flange 74. Tensioning of the bolts 82 may be accomplished in the same manner as described above for assembling a wind turbine tower assembly using fastening system 60. The plurality of bolts 82 is inserted such that for each adjacent pair of the plurality of bolts 82, one bolt 82 has a first orientation and the other bolt 82 has a second orientation opposite to the first orientation. Further, the shaft 86 of each of the plurality of bolts 82 has a shaft diameter (D_sh), wherein the centerlines of two adjacent bolts (82) are spaced apart with a centerline distance (C_blt), and wherein the centerline distance (Cblt) may be in the range of 1.01 to 1.75 times the shaft diameter (D_sh), and may preferably be 1.25 times the shaft diameter (D_sh). As described above, each of the first and second bolt extenders 144, 148 used in the fastening system 142 have cutouts 152a, 152b that each receive at least part of the head 84 of the bolt 82. As such, the fastening system 142 may require a particular sequence for installing the bolts 82 and the first and second bolt extenders 144, 148 so that the heads 84 of the bolts 82 are captured by the first and second bolt extenders 144, 148. This sequence assumes the placement of the bolts 82 is occurring in a counterclockwise direction around the first and second flanges 64, 74. The following sequence will use the term “down bolt” where the bolt 82 is placed tip end 90 down through the first and second flanges 64, 74 and the head 84 contacts the inner surface 68 of the first flange 64. Similarly, the term “up bolt” means the bolt 82 is placed tip end 90 up through second and first flanges 74, 64 and the head contacts the inner surface 78 of the second flange 74. When a stud bolt is used, a down bolt orientation means the nut is secured to the shaft of the stud bolt and is positioned at least partially within the cutouts 32a, 32b of the first bolt extender 144 and an up bolt orientation means the nut is secured to the shaft of the stud bolt and is positioned at least partially within the cutouts 32a, 32b of the second bolt extender 148. The term “top bolt extender” refers to the bolt extender 144 contacting the inner surface 68 of the first (top) flange 64. The term “bottom bolt extender” refers to the bolt extender 148 contacting the inner surface of the second (bottom) flange 74. One sequence may include placing first and second down bolts 82 into through holes 66, 68 with a set of through holes 66, 68 therebetween in the first and second flanges 64, 74, where the second down bolt 82 is left of the first down bolt 82. A first top bolt extender 144 is placed over the heads 84 of the first and second down bolts 82 and contacts the inner surface 68 of the first flange 64. A first up bolt 82 is placed in the through holes 66, 76 between the first and second down bolts 82 and through the first top bolt extender 144 and a nut 154 is placed on the tip end 90 of first up bolt 82. A third down bolt 82 is placed into through holes 66, 68 two through holes to the left of the second down bolt 82. A second top bolt extender 144 is placed of the heads 84 of the second and third down bolts 82. A second up bolt 82 is placed in the through holes 66, 68 between the second and third down bolts 82 and through the second top bolt extender 144 and a nut 154 is placed on the tip end 90 of the second up bolt 82. A first bottom bolt extender 148 is placed over the heads 84 of the second and third up bolts 82 and the second down bolt 82 is placed through the first bottom bolt extender 148 and a nut 154 is placed on the tip end 90 of the second down bolt 82. The sequence then repeats by placing a fourth down bolt 82 in through holes 66, 68 two to the left of the third down bolt 82. This sequence repeats counterclockwise until only the through holes 66, 68 to the right of and adjacent to the first down bolt 82 remain unfilled. The final top bolt extender 144 is placed over the heads 84 of the first down bolt 82 and the last-placed down bolt 82. The final up bolt 82 is placed into the unfilled through holes 66, 68 and through the final top bolt extender and a nut 154 is placed onto the tip end 90 of the final up bolt 82. The second to last bottom bolt extender 148 is placed over the heads 84 of the last two down bolts 82 and a nut 154 is placed on the tip end 90 of the final up bolt 82. The last bottom bolt extender 148 is then placed of the heads 84 of the last up bolt 82 and the first up bolt 82 and a nut 154 is placed on the tip end 90 of the first down bolt 82. In an embodiment, the above sequence may also be completed by starting with first and second up bolts 82, instead of starting with the first and second down bolts 82 as described above. In an embodiment, the sequence may be completed by moving in a clockwise direction around the first and second flanges 64, 74. Other sequences are contemplated for assembly the wind turbine tower using the fastening system 142. For example, all the down bolts 82 may be placed in every other through hole 66, 76 around the first and second flanges 64, 74. A unitary, continuous top bolt extender (e.g., a closed ring) with cutouts like cutouts 152a, 152b may be placed over the heads 84 and each down bolt 82. A first up bolt 82 may be place through an open through hole 66, 76 and through the unitary top bolt extender and a nut 154 placed on the tip end 90 of the first up bolt 82. In a like manner, the remainder of the up bolts 82 may be placed into the open through holes 66, 76 until all the up bolts are connected via a nut 154 to the unitary top bolt extender. A unitary bottom bolt extender (e.g., a closed ring) may be placed over the heads 84 of the up bolts 82 and nuts 154 placed on the tip ends 90 of the down bolts 82. The unitary top bolt extender and the unitary bottom bolt extender may be divided into smaller sections, like a quarter of a circle sections, for example, to make handling and installing them more reasonable. The fastening systems and methods described above are in the context of constructing a new wind turbine tower assembly. In another embodiment, however, the fastening systems and methods described above may be used to retrofit an existing wind turbine tower assembly. An exemplary method of retrofitting the fastening systems 60. An existing wind turbine tower assembly 12 may include a first tower section 18 including a first flange 32a with a plurality of existing through holes 34, where the first flange 32a includes an inner surface 68 and a mating surface 70. A second tower section 18 may include a second flange 32b with a plurality of existing through holes 34, where the second flange 32b includes an inner surface 78 and a mating surface 80. The first flange 64 is connected to the second flange 74 via a plurality of existing bolts 36 in the plurality of existing through holes 34 of the first and second flanges 32a, 32b and the respective mating surfaces 70, 80 face toward each other and the respective inner surfaces 68, 78 face away from each other. The method includes removing a subset of the plurality of existing bolts 36 from the plurality of existing through holes 34 in the first and second flanges 32a, 32b. Forming, such as by drilling, for example, a plurality of intermediate through holes 190 (Fig.21) in the first and second flanges 32a, 32b, each of the plurality of intermediate through holes 190 being positioned between adjacent pairs of existing through holes 34 in the first and second flanges 32a, 32b. Positioning on the inner surface 68 of the first flange 64, at least one first bolt extender 100 including a plurality of first extender bores 102 so that at least one of the plurality of the first extender bores 102 aligns with one of the plurality of existing through holes 34 in the first flange 64 and an adjacent one of the plurality of first extender bores 32a aligns with the adjacent one of the plurality of intermediate through holes 190 in the first flange 64. For each adjacent pair of the plurality of first extender bores 102, one first extender bore 102 is threaded and the other first extender bore 102 is unthreaded. Next, positioning on the inner surface 78 of the second flange 32b at least one second bolt extender 104 including a plurality of second extender bores 106 so that at least one of the plurality of the second extender bores 106 aligns with one of the plurality of existing through holes 34 in the second flange 32a and an adjacent one of the plurality of second extender bores 106 aligns with an adjacent one of the plurality of the intermediate through holes 190 in the second flange 32b, wherein for each adjacent pair of the plurality of second extender bores 106, one second extender bore 106 is threaded and the other second extender bore 106 is unthreaded. Then, using a plurality of replacement bolts 82, inserting one of the plurality of replacement bolts 82 into each one of the unthreaded first extender bores 102 of the at least one first bolt extender 100 so as to be received in and engaged with one of the second threaded extender bores 106 of the at least one second bolt extender 104 and also inserting one of the plurality of replacement bolts 82 into each one of the unthreaded second extender bores 106 of the at least one second bolt extender 104 so as to be received in and engaged with one of the first threaded extender bores 102 of the at least one first bolt extender 100. Finally, tensioning each of the plurality of replacement bolts 82 to connect the first flange and the second flanges 32a, 32b. The plurality of replacement bolts 82 is inserted such that for each adjacent pair of the plurality of replacement bolts 82, one replacement bolt 82 has a first orientation and the other replacement bolt 82 has a second orientation opposite to the first orientation. In an embodiment, before removal, all of the plurality of existing bolts 36 are in the first orientation. In an embodiment, at least one or more of the replacement bolts 82 may be new or one or more of the existing bolts 36. In an embodiment, each of the plurality of the existing through holes (34) in the first and second flanges (32a, 32b) may be positioned at a first radius (R1) from a center of the first and second tower sections (18) and each of the plurality of the intermediate through holes (190) may be positioned at a second radius (R₂) that is less than or equal to the first radius (R1). A method of retrofitting a wind turbine tower assembly using fastening system 142 will now be described. This retrofitting method is similar to the retrofitting method discussed above using fastening systems 60. The method includes removing a subset of the plurality of existing bolts 36 from the plurality of existing through holes 34 in the first and second flanges 64, 74. Then, forming a plurality of intermediate through holes 190 in the first and second flanges 64, 74, each of the plurality of intermediate through holes 190 being positioned between adjacent pairs of existing through holes 34 in the first and second flanges 64, 74. Then, positioning on the inner surface 68 of the first flange 64 at least one first bolt extender 100 each including at least one first extender bore 102 so the at least one first extender bore 102 aligns with one of the plurality of existing through holes 34 in the first flange 64 or with one of the plurality of intermediate through holes 190 in the first flange 64. Then, positioning on the inner surface 78 of the second flange 74 at least one second bolt extender 104 each including at least one second extender bore 106 so the at least one second extender bore 106 aligns with one of the plurality of existing through holes 34 in the second flange 74 or with one of the plurality of intermediate through holes 190 in the second flange 74. Using a plurality of replacement bolts 82, inserting one of the plurality of replacement bolts 82 into the at least one first extender bore 102 of the at least one first bolt extender 100 and inserting one of the plurality of replacement bolts 82 into the at least one second extender bore 106 of the at least one second bolt extender 100. Next, engaging each of the plurality of replacement bolts 82 with a nut 154. Finally, tensioning each of the plurality of replacement bolts 82 to connect the first flange 64 and the second flanges 74. The plurality of replacement bolts 82 is inserted such that for each adjacent pair of the plurality of replacement bolts 82, one replacement bolt 82 has a first orientation and the other replacement bolt 82 has a second orientation opposite to the first orientation. For this retrofit method, the centerline distance (Cblt) is in the range of 1.01 to 1.75 times the shaft diameter (Dsh), and preferably 1.25 times the shaft diameter (D_sh). 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.

Claims

Claims 1. A wind turbine tower assembly (12), comprising: a first tower section (62) including a first flange (64) with a plurality of through holes (66), the first flange (64) including an inner surface (68) and a mating surface (70); a second tower section (72) including a second flange (74) with a plurality of through holes (76), the second flange (74) including an inner surface (78) and a mating surface (80); and a fastening system (60) for connecting the first flange (64) and the second flange (74) at a connection interface (18a) such that the respective mating surfaces (70, 80) face toward each other, the respective inner surfaces (68, 78) face away from each other, and the plurality of through holes (66) in the first flange (64) and the plurality of through holes (76) in the second flange (74) respectively align with each other, the fastening system (60) comprising: a plurality of bolts (82), each of the plurality of bolts (82) having a head (84) and a shaft (86) extending from the head (84) and received in a respective one of the aligned plurality of through holes (66, 76) in the first and second flanges (64, 74); at least one first bolt extender (100, 198, 202, 210, 214) including a plurality of first extender bores (102) contacting the inner surface (68) of the first flange (64), wherein: for each adjacent pair of the plurality of first extender bores (102), one first extender bore (102) is threaded and the other first extender bore (102) is unthreaded, and each of the plurality of first extender bores (102) receives one of the plurality of bolts (82); and at least one second bolt extender (104, 198, 202, 210, 214) including a plurality of second extender bores (106) contacting the inner surface (78) of the second flange (74), wherein: for each adjacent pair of the plurality of second extender bores (106), one second extender bore (106) is threaded and the other second extender bore (106) is unthreaded, and each of the plurality of second extender bores (106) receives one of the plurality of bolts (82), and wherein for each adjacent pair of the plurality of bolts (82), one bolt (82) has a first orientation and the other bolt (82) has a second orientation opposite to the first orientation.

2. The wind turbine tower assembly (12) of claim 1, wherein for each bolt (82) of the plurality of bolts (82), at least one adjacent bolt (82) defines an overlap region (110) between the head (84) of the bolt (82) and the head (84) of the adjacent bolt (82).

3. The wind turbine tower assembly (12) of claim 1 or 2, wherein each bolt (82) of the plurality of the bolts (82) has a first adjacent bolt (82) and a second adjacent bolt (82) each having a centerline, wherein the centerlines of the bolt (82) and the first adjacent bolt (82) are spaced apart with a centerline distance (C₁), wherein the centerlines of the bolt (82) and the second adjacent bolt (82) are spaced apart with a second centerline distance (C2), and wherein the second distance (C2) is greater than or equal to the first distance (C₁).

4. The wind turbine assembly (12) of claim 4, wherein the shaft of each of the plurality of bolts (82) has a shaft diameter (D_sh) and the first centerline distance (C₁) is in the range of 1.01 to 1.75 times the shaft diameter (D_sh), and preferably 1.25 times the shaft diameter (Dsh).

5. The wind turbine tower assembly (12) of any of the preceding claims, wherein for each adjacent pair of the plurality of first extender bores (102), the unthreaded extender bore (102) is a through bore, and the threaded extender bore (102) is a through bore or a blind bore.

6. The wind turbine tower assembly (12) of any of the preceding claims, wherein the at least one first bolt extender (100, 198, 202, 210, 214) includes only two first extender bores (102).

7. The wind turbine tower assembly (12) of any of the preceding claims, wherein the at least one first bolt extender (100, 198, 202, 210, 214) of the fastening system (60) includes a plurality of first bolt extenders (100, 198, 202, 210, 214).

8. The wind turbine tower assembly (12) of any of the preceding claims, wherein the first tower section (62) includes a first tower wall (30a) extending from the first flange (64), wherein the at least one first bolt extender (100, 198, 202, 210, 214) includes a curved edge (122), and wherein the curved edge (122) is received within a corner (118) defined by an intersection of the first flange (64) and the first tower wall (30a).

9. The wind turbine tower assembly (12) of any of the preceding claims, wherein the at least one first bolt extender (100, 198, 202, 210, 214) and the at least one second bolt extender (104, 198, 202, 210, 214) are identical to each other.

10. The wind turbine tower assembly (12) of any of the preceding claims, wherein each of the at least one first bolt extender (198, 210) and the at least one second bolt extender (198, 210) includes sidewalls (200, 204, 206) defining a trapezoidal perimeter.

11. The wind turbine assembly (12) of any of the preceding claims, wherein the at least one first and second bolt extenders (210) have sidewalls (204, 206), wherein each of the sidewalls (204) includes at least one of the plurality of extender bores (102), wherein the at least one of the plurality of extender bores (102) in each opposing sidewall (204) extends at least 50% of a complete extender bore circumference.

12. The wind turbine assembly (12) of any of claims 1-9, wherein each of the at least one first bolt extender (214) and the at least one second bolt extender (214) have first and second opposing sidewalls (218, 220), the first sidewall (218) being concave and the second sidewall (218) being convex, wherein the concave second sidewall (220) of the at least one first bolt extender (214) is configured to nest within the convex first sidewall (218) of an adjacent first bolt extender (214).

13. The wind turbine assembly (12) of claim 12, wherein the first sidewall (218) having a first curved portion (222) and the second sidewall (220) having a second curved portion (224), wherein the center of curvature of the second curved portion (224) is defined at a central axis of the one threaded first extender bore (102).

14. The wind turbine assembly (12) of claim 12, wherein the other unthreaded first extender bore (102) extends at least 70% of a complete extender bore circumference.

15. A wind turbine (10) comprising the wind turbine tower assembly (12) according to any of claims 1-9.

16. A method of assembling a wind turbine tower assembly (12), the wind turbine tower assembly (12) comprising: a first tower section (62) including a first flange (64) with a plurality of through holes (66), the first flange (64) including an inner surface (68) and a mating surface (70); a second tower section (72) including a second flange (74) with a plurality of through holes (76), the second flange (74) including an inner surface (78) and a mating surface (70), the method comprising: positioning the first flange (64) relative to the second flange (74) so that the respective mating surfaces (70, 80) face toward each other, the respective inner surfaces (68, 78) face away from each other, and the plurality of through holes (66) in the first flange (64) respectively align with the plurality of through holes (76) in the second flange (74); positioning on the inner surface (68) of the first flange (64) at least one first bolt extender (100) including a plurality of first extender bores (102) so that the plurality of first extender bores (102) align with respective ones of the plurality of through holes (66) in the first flange (64), wherein for each adjacent pair of the plurality of first extender bores (102), one first extender bore (102) is threaded and the other first extender bore (102) is unthreaded; positioning on the inner surface (78) of the second flange (74) at least one second bolt extender (104) including a plurality of second extender bores (106) so that the plurality of second extender bores (106) align with respective ones of the plurality of through holes (76) in the second flange (74), wherein for each adjacent pair of the plurality of second extender bores (106), one second extender bore (106) is threaded and the other second extender bore (106) is unthreaded; using a plurality of bolts (82): inserting one of the plurality of bolts (82) into each one of the unthreaded first extender bores (102) of the at least one first bolt extender (100) so as to be received in and engaged with one of the second threaded extender bores of the at least one second bolt extender (104); and inserting one of the plurality of bolts (82) into each one of the unthreaded second extender bores (106) of the at least one second bolt extender (104) so as to be received in and engaged with one of the threaded first extender bores (102) of the at least one first bolt extender (100); and tensioning each of the plurality of bolts (82) to connect the first flange (64) and the second flange (74), wherein the plurality of bolts (82) is inserted such that for each adjacent pair of the plurality of bolts (82), one bolt (82) has a first orientation and the other bolt (82) has a second orientation opposite to the first orientation.

17. The method of claim 15, wherein: the first tower section (62) includes a first tower wall (30a) extending from the first flange (64) at an intersection that defines a corner (116), the at least one first bolt extender (100) includes a curved edge (120), and positioning on the inner surface (68) of the first flange (64) the at least one first bolt extender (100) includes positioning the at least one first bolt extender (100) such that the curved edge (120) is received in the corner (116); and the second tower section (72) includes a second tower wall (30b) extending from the second flange (74) at an intersection that defines a corner (118), the at least one second bolt extender (104) includes a curved edge (122), and positioning on the inner surface (78) of the second flange (74) the at least one second bolt extender (104) includes positioning the at least one second bolt extender (104) such that the curved edge (122) is received in the corner (118).

18. A method of retrofitting a fastening system (60) to a wind turbine tower assembly (12), the wind turbine tower assembly (12) comprising: a first tower section (18) including a first flange (32a) with a plurality of existing through holes (34), the first flange (32a) including an inner surface (68) and a mating surface (70); a second tower section (18) including a second flange (32b) with a plurality of existing through holes (34), the second flange (32b) including an inner surface (78) and a mating surface (80), wherein the first flange (32a) is connected to the second flange (32b) via a plurality of existing bolts (36) in the plurality of existing through holes (34) of the first and second flanges (32a, 32b) such that the respective mating surfaces (70, 80) face toward each other and the respective inner surfaces (68, 78) face away from each other, the method comprising: removing a subset of the plurality of existing bolts (36) from the plurality of existing through holes (34) in the first and second flanges (64, 74); forming a plurality of intermediate through holes (190) in the first and second flanges (64, 74), each of the plurality of intermediate through holes (190) being positioned between adjacent pairs of existing through holes (34) in the first and second flanges (64,74); positioning on the inner surface (68) of the first flange (64) at least one first bolt extender (100) including a plurality of first extender bores (102) so that at least one of the plurality of the first extender bores (102) aligns with one of the plurality of existing through holes (34) in the first flange (64) and an adjacent one of the plurality of first extender bores (100) aligns with the adjacent one of the plurality of intermediate through holes (190) in the first flange (64), wherein for each adjacent pair of the plurality of first extender bores (102), one first extender bore (102) is threaded and the other first extender bore (102) is unthreaded; positioning on the inner surface (78) of the second flange (74) at least one second bolt extender (104) including a plurality of second extender bores (106) so that at least one of the plurality of the second extender bores (106) aligns with one of the plurality of existing through holes (34) in the second flange (74) and an adjacent one of the plurality of second extender bores (104) aligns with an adjacent one of the plurality of the intermediate through holes (190) in the second flange (74), wherein for each adjacent pair of the plurality of second extender bores (106), one second extender bore (106) is threaded and the other second extender bore (106) is unthreaded; using a plurality of replacement bolts (82): inserting one of the plurality of replacement bolts (82) into each one of the unthreaded first extender bores (102) of the at least one first bolt extender (100) so as to be received in and engaged with one of the second threaded extender bores (106) of the at least one second bolt extender (140), and inserting one of the plurality of replacement bolts (82) into each one of the unthreaded second extender bores (106) of the at least one second bolt extender (104) so as to be received in and engaged with one of the first threaded extender bores (102) of the at least one first bolt extender (100); and tensioning each of the plurality of replacement bolts (82) to connect the first flange (64) and the second flanges (74), wherein the plurality of replacement bolts (82) is inserted such that for each adjacent pair of the plurality of replacement bolts (82), one replacement bolt (82) has a first orientation and the other replacement bolt (82) has a second orientation opposite to the first orientation.

19. The method of claim 18, wherein each of the plurality of the existing through holes (34) in the first and second flanges (32a, 32b) is positioned at a first radius (R1) from a center of the first and second tower sections (18), and wherein each of the plurality of the intermediate through holes (190) is positioned at a second radius (R₂) less than or equal to the first radius (R1).

20. The method of claim 18 or 19, wherein before removal, each of the plurality of existing bolts (36) is in the first orientation.