Annular connecting interface for a wind turbine tower

Abstract

A wind turbine tower arrangement comprising an annular connection interface is provided. The wind turbine tower arrangement comprises an upper tower section, a lower tower section, and the annular connecting interface is disposed between and connects the upper and lower tower sections of a wind turbine tower. The annular connecting interface comprises an opposing pair of radially inward flanges fastened by a plurality of mechanical fasteners of a first type defining a radially inward mechanically fastened joint and an opposing pair of radially outward flanges fastened by a plurality of mechanical fasteners of a second type defining a radially outward mechanically fastened joint. Beneficially, the radially inward mechanically fastened joint is configured as a mechanical fuse such that indicia of mechanical failure are displayed in the radially inward mechanically fastened joint before being displayed in the radially outward mechanically fastened joint.

Description

[0001] ANNULAR CONNECTING INTERFACE FOR A WIND TURBINE TOWER

[0002] Technical Field

[0003] The present disclosure generally relates to the construction of a wind turbine, particularly a T-flange type annular connecting interface coupling together adjacent tower sections of wind turbines.

[0004] Background

[0005] The current trend is for wind turbines to be designed ever larger in an effort to capture more energy from the wind. Wind turbine towers therefore need to be higher and constructed more strongly to accommodate the required operational loads of larger wind turbine generators.

[0006] Typically, wind turbine towers are constructed from steel or concrete and are formed usually from multiple tower sections which are stacked on top of one another and secured together at connecting flanges. Such a construction is well known to the skilled person.

[0007] As wind turbines increase in size, it is more challenging to manufacture and transport tower sections, particularly if overland transport is required. Therefore, it is generally accepted that there is an upper limit to the diameter of tower sections used in wind turbine construction.

[0008] As mentioned above, the use of connecting flanges to secure one tower section to another is well known. Conventionally, L-shaped flanges are used as connecting interfaces between adjacent tower sections. Fasteners of the L-shaped flanges are positioned radially inward so that the connection can be made and monitored from within the tower. For the connection between the tower and the foundation, a T-shaped flange may be used as the outer bolts can be accessed for tightening and inspection at ground level.

[0009] Inspection of T-shaped flanges at higher locations is impractical since it would require rope access or a crane / lift to be accessible. Furthermore, working at height requires favourable weather and will likely require the wind turbine to be shut down during inspection. However, as wind turbines become larger, this challenges the ability of conventional L-flanges to accommodate the increased loading. It is against this background that the present invention resides.

[0010] Summary of the Invention

[0011] According to an aspect of the invention, a wind turbine tower arrangement comprising an annular connection interface is provided. The wind turbine tower arrangement comprises an upper tower section, a lower tower section, and the annular connecting interface is disposed between and connects the upper and lower tower sections of a wind turbine tower. The annular connecting interface comprises an opposing pair of radially inward flanges fastened by a plurality of mechanical fasteners of a first type defining a radially inward mechanically fastened joint and an opposing pair of radially outward flanges fastened by a plurality of mechanical fasteners of a second type defining a radially outward mechanically fastened joint. Beneficially, the radially inward mechanically fastened joint is configured as a mechanical fuse such that indicia of mechanical failure are displayed in the radially inward mechanically fastened joint before being displayed in the radially outward mechanically fastened joint.

[0012] A mechanical fuse is a design feature in a system that is configured with a lower but adequate factor of safety relative to the other design features of the system. Beneficially, this provides predictability on how and where the system will display signs or indicia indicative of wear leading to mechanical failure, if the system is subjected to overload.

[0013] Providing a mechanical fuse in the annular connecting interface between tower sections ensures predictability in the event of overload of the bolted joints of the interface. Furthermore, by locating this mechanical fuse on the radially inward mechanically fastened joint, it allows for indicia of mechanical failure to be detected during routine structural inspection from within the tower. Beneficially, this design avoids the need for the difficult task of inspecting the radially outward mechanically fastened joint; a task that is heavily dependent on favourable weather and involves working at height.

[0014] An example of how to achieve this mechanical fuse in the annular connecting interface is for a load required to yield mechanical fasteners of the second type to be greater than a load required to yield the mechanical fasteners of the first type. For example, a diameter of at least some of the mechanical fasteners of the second type may be greater than a diameter of the mechanical fasteners of the first type.

[0015] Providing a comparatively larger diameter, for example a thread diameter or a shank diameter, for the mechanical fastener of the second type when compared to the mechanical fastener of the first type ensures that the radially outward mechanically fastened joint is capable of sustaining a higher load than that of the radially inward mechanically fastened joint.

[0016] Additionally, or alternatively, a material grade of the mechanical fastener that determines a yield strength for the mechanical fastener may be greater for the mechanical fastener of the second type when compared to a material grade of the mechanical fastener of the first type.

[0017] Providing a comparatively greater material grade for the mechanical fastener of the second type when compared to the mechanical fastener of the first type ensures that the radially outward mechanically fastened joint is capable of sustaining a higher load than that of the radially inward mechanically fastened joint.

[0018] For example, the aspect of providing the mechanical fasteners of the second type with a greater diameter than the diameter of the mechanical fasteners of the first type may be combined with providing a comparatively greater material grade for the mechanical fastener of the second type when compared to the mechanical fastener of the first type.

[0019] As an alternative, or in addition to the above measures to result in a comparatively ‘weaker’ radially inward mechanically fastened joint, it is also envisaged that the number of fasteners may be varied. For example, a quantity of mechanical fasteners in the radially inward mechanically fastened joint may be equal to a quantity of mechanical fasteners in the radially outward mechanically fastened joint which allows for rotational symmetry of load bearing capability of the annular connecting interface. However, in some examples it may be acceptable to have a different number of fasteners in the radially inward mechanically fastened joint compared to the radially outward mechanically fastened joint.

[0020] The angular offset of the mechanical fasteners of the radially inward mechanically fastened joint relative to the mechanical fasteners of the radially outward mechanically fastened joint may be zero. In other words, the mechanical fasteners in the radially outward joint may be aligned radially with the mechanical fasteners of the radially inward joint.

[0021] Setting the angular offset between the fasteners of the inner and outer mechanical fasteners to zero makes for a simpler load path between the mechanical fasteners of the radially inward and radially outward mechanically fastened joints.

[0022] As an alternative or in addition to embodying the ‘mechanical fuse’ in the design of the annular connecting interface by the configuration of the mechanical fasteners, a ‘mechanical fuse’ may also be achieved by configuring the inward flanges and the outward flanges to have different thicknesses. For example, at least one of the outwardly projecting flanges may be configured to be thicker than at least one of the inwardly projecting flanges.

[0023] A thickness of the upper flanges may be different to a thickness of the lower flanges. provides Beneficially, adding differing thickness between the upper and lower flanges of the annular connecting interface allows the skilled person to control which tower section of the wind turbine will fail, thereby showing signs of wear, before the other in the event of mechanical failure. It is advantageous for the failure to occur in a tower section that is lower cost and / or more easily replaced than the others, for example, a simple tower section with no additional features as opposed to the foundation section or a section with a door.

[0024] A further option is to select the distance of the inner mechanical fasters from a wall of the tower section to be different to the distance of the outer mechanical fasteners to the wall of the tower section. For example, the distance of the central axis of the mechanical fasteners joining the radially inward flange parts from a wall of the wind turbine tower may be less than a distance of a central axis of the mechanical fasteners joining the radially outward flange parts from the wall of the wind turbine tower.

[0025] By positioning the inner mechanical fasteners closer to the wall of the tower section when compared to the outer mechanical fasteners, the inner mechanical fasteners are subjected to a larger component of the force applied to the annular connecting joint in service. This further contributes to the ‘mechanical fuse’ effect.

[0026] Another aspect of the invention contemplates a wind turbine comprising the wind turbine tower arrangement described above. Optional features of the aspects of the invention are set out in the dependent claims. Note that these optional features are combinable with each without limitation, save for the case that a specific limitation is discussed explicitly in the discussion that follows.

[0027] The skilled person will be able to combine the various aspects mentioned in the preceding using for example finite element analysis and thereby optimizing the radially inward and radially outward mechanically fastened joints with due consideration of the manufacturing process and the tools available to tensioning the fasteners of the joints.

[0028] Brief Description of the Drawings

[0029] So that it may be more fully understood, the invention will now be described, by way of example only, with reference to the following drawings, in which like features are assigned like reference numerals, and in which:

[0030] Figure 1 is a front view of a wind turbine tower and includes an inset panel illustrating a flange connection between adjacent sections of the tower;

[0031] Figure 2 is a partial cut-away view illustrating the flange connection in Figure 1 from a different perspective;

[0032] Figure 3 is a cross section through part of the flange connection shown in the inset panel of Figure 1 ;

[0033] Figure 4 is similar to Figure 3 but shows an alternative flange arrangement.

[0034] Detailed Description

[0035] In general terms, embodiments of the invention provide a wind turbine tower arrangement comprising at least one annular connecting interface that itself comprises radially inward and radially outward pairs of mating flanges. The radially inward pair of mating flanges acts as a mechanical fuse for the annular connecting interface such that indicia of mechanical failure will display on the radially inward pair of mating flanges before being displayed on the radially outward pair of mating flanges. Expressed another way, the radially outward pair of mating flanges and specifically the mechanical joining thereof, are configured to be structurally stronger than the adequately specified radially inward pair of mating flange parts, such that signs of wear appear on the inner pair of flanges sooner than the radially outward pair of flange parts. In this way, inspection work can pick up signs of wear earlier so that appropriate action can be taken, whilst at the same time mitigating against excessive weight and cost at over-engineering both the radially outward and radially inward mating flange parts simultaneously.

[0036] To provide context for the invention, Figure 1 shows a typical horizontal axis wind turbine 2, that includes a nacelle 4, mounted on top of a tower 6, which supports a front facing rotor 8 comprising a plurality of coplanar blades 10. Although not shown in Figure 1 , the rotor 8 is connected to a powertrain housed within the nacelle 4. The drivetrain comprises components required to convert rotation of the rotor 8 into electricity, including a generator, a gear system and a controller, although these components are not shown in Figure 1 or described in detail for brevity. Although a horizontal-axis wind turbine is shown in Figure 1 , which is a common configuration of wind turbine, it should be noted that the invention may be applicable to other wind turbine configurations.

[0037] Typically, the tower 6 may be made from steel or concrete, the choice of which depends on many factors such as expected loading conditions, hub height, and location, to name a few examples. Hybrid towers of concrete and steel are also known.

[0038] The tower 6 is constructed from annular or tubular tower sections 20, as can be seen in Figure 1 . Due to the slight tapering of the tower 6, each of the annular tower sections 20 are slightly frustoconical, in this example. In the illustrated example, the annular tower sections 20 can be considered to be entirely, or mostly, made of steel which is a common material of construction for wind turbine towers.

[0039] In the illustrated wind turbine 2, there are two tower sections 20, comprising a lower tower section 22 and an upper tower section 24. Note that the tower 6 may comprise more than two tower sections 20, for example between three and ten tower sections 20.

[0040] Each of the tower sections 20 are connected together by way of an annular connecting interface 26 or flanged connection. A portion of the annular connecting interface 26 between the lower tower section 22 and the upper tower section 24 can be seen in the inset panel in Figure 1 and later in Figures 3 and 4. As can be seen, the upper tower section 24 comprises an annular wall 28 which terminates at its lower end at a set of flanges 30, 32. Likewise, the lower tower section 22 comprises an annular wall 34 which terminates at its upper end in a respective set of flanges 36, 38. Such sets of flanges may also be referred to as T-flanges due largely to their cross- sectional shape, and are generally known in the art. Generally speaking, T-shaped flanges comprise two distinct flange parts; one projecting radially inwards, from now on known as a radially inward flange 30, 36, from its respective wall portion and another projecting radially outwards, from now on known as a radially outward flange 32, 38, from the respective wall portion. As illustrated, the flanges 30, 32, 36, 38 are integral with the annular walls 28, 34 although in principle the flanges 30, 32, 36, 38 may be separate to the wall portions 28, 34 and welded thereto to form unitary components.

[0041] As can be seen in Figure 2, the upper and lower tower sections 24, 22 define a central tower axis A. Moreover, the respective wall portions 28, 34 extend about and are generally aligned parallel to the tower axis A.

[0042] Two circular arrays of mechanical fasteners 40, 42 in the form of, and hereinafter referred to as, bolts extend circumferentially about the annular connecting interface 26 and serve to mechanically join together the upper tower section flanges 30, 32 and the lower tower section flanges 36, 38. A first circular array of bolts 40 joins together the radially inward projecting flanges 30, 36, of the upper and lower tower sections 24, 22, hereinafter referred to as the radially inward joint 44. Likewise, a second array of mechanical bolts 42 joins together the radially outward projecting flanges 32, 36 of the upper and lower towers sections 24, 22, hereinafter referred to as the radially outward joint 46.

[0043] As can be seen in Figure 3, the bolts 48, 50 are headed bolts that are T-shaped in cross section thereby defining an upper bolt head that is integral to shank. Respective nuts 52, 54 are secured on a threaded portion of the bolts 48, 50 and tightened suitably to join the annular connecting interface 26. In principle the diameter of the tower 6 may be any size, but the invention is envisaged to be most suited to large-diameter towers 6 in which the ‘bolt circle diameter’ or BCD of the bolts is in excess of 3m, for example between 3m and 10m, and more preferably between 3.5m and 8m.

[0044] It is also customary to turn the bolts 48, 50 upside down, so the bolt heads are below the lower flanges 36, 38 and the nuts 52, 54 are above the upper flanges 30, 32. Also, the bolts 48, 50 may instead be in the form of stud bolts having no fixed heads but first and second removable nuts 52, 54, as would be well understood by the skilled person. Alternatively, the mechanical fasteners may take the form of rivets.

[0045] Moving on and referring specifically to the upper tower section 24, it will be noted that the flanges 30, 32 extend in a direction that is generally perpendicular to the annular wall 28. Thus, the annular wall 28 and the respective flanges 30, 32 define a T-shape in a vertical cross section, as shown. In the illustrated example the upper and lower tower section flanges 30, 32, 36, 38 join with each other at respective contact faces.

[0046] Similarly, with respect to the lower tower section 22, it will be noted that the flanges 36, 38 extend in a direction that is generally perpendicular to the annular wall 34. Thus, the annular wall 34 and the respective flanges 36, 38 define a T-shape in a vertical cross section, as shown.

[0047] The annular connecting interface 26 between the tower sections 22, 24 is configured to define a ‘mechanical fuse’ in terms of the functionality of the radially inward joint 44. In effect, the configuration of the radially outward joint 46 is structurally stronger or ‘over engineered’ compared to the radially inward joint 44. This means that the radially outward joint 46 can be considered to be maintenance free by design and provides the advantage that inspection / maintenance need only be implemented routinely on the radially inward joint 44.

[0048] Any wear indications will appear on the radially inward joint 44 sooner than the radially outward joint 46 which means that only the radially inward joint 44 needs to be inspected to assess the integrity of the annular connecting interface 26 as a whole. In other words, implementation of a ‘mechanical fuse’ in the radially inward joint 44 of the annular connecting interface 26 ensures that indicia of mechanical failure will appear first on the radially inward joint 44. Indicia of mechanical failure may be, for example, loose bolts, signs of corrosion on the bolts, paint chipping or fretting at the interface of the joint indicating relative movement between the upper and lower flange parts of the radially inward joint, and water ingress through the joint.

[0049] The integrity of a mechanically fastened joint is largely dependent on maintaining pretension in the bolts of the joint. The cumulative bolt pretension correlates directly to a clamping force holding the joint together. Bolt pretension reduction occurs in several ways, for example, a bolt may plastically deform or stretch, a fastened nut could slip on the thread of the associated bolt, or a bolt could mechanically fail, snap or break.

[0050] If bolt pretension is lost in one or more bolts of the joint, the clamp load in the joint diminishes, and the sum of any applied loads on the joint are shared by the remaining intact bolts, however, with a reduced margin to yield. This additional load could cause more bolts in the joint to fail, potentially leading to complete mechanical failure of the joint. It is for this reason that the routine inspection of the annular connecting interface is important.

[0051] A design pretension force or clamping force of any bolt is typically expressed as a portion or percentage of the load required to yield said bolt. The load required to yield the bolt is generally considered to be the tensile force limit of the bolt, although there are some exceptions to this which are outside of the scope of this discussion.

[0052] It is normal for a new bolt to exhibit a level of strain or stretch when it is fastened for the first time. This is usually referred to as bolt ‘relaxation’ or ‘torque relaxation’. In many bolted joint applications, this relaxation does not pose an issue or can be accounted for in the design of the bolted joint. However, in the application of the annular connecting interface 26 of a wind turbine tower 6 arrangement, maintenance of bolt pretension is considered critical.

[0053] An initial pretension force or clamping force of any bolt in the annular connecting interface 26 is usually limited to 70% of the yield strength of said bolt. Routine initial relaxation of the bolt can reduce this pretension force, for example, by 30%. Performing maintenance or retightening the bolts can limit this reduction of pretension force, for example, to 10%. Therefore, the bolts 48 on the accessible radially inward joint 44 could have a resultant pretension force of 63% (10% reduction of the original pretension of 70%) of the force required to yield the bolt 48. Correspondingly, the bolts 50 on the inaccessible radially outward joint 46 could have a resultant pretension force of 49% (30% reduction of the original pretension of 70%) of the force required to yield the bolt 50. Retightening of the external bolts 50 is difficult, if not practically impossible, to implement. To include a factor of safety to ensure that the bolts 50 of the radially outward joint 46 are truly maintenance free, the radially outward joint 46 may be designed to have a pretension force of 40% of the force required to yield the bolt 50. Principally, the ‘mechanical fuse’ aspect of the annular connecting interface 26 operates by designing a T-flange wherein the allocation of percentage pretension force of the force required to yield the bolt between the inner and outer bolts 48, 50 is, for example, 63% / 40%. These values are provided by way of example only and are not intended to be limiting.

[0054] There are a number of design methodologies that can be implemented to achieve the parameters of the mechanically fastened joints 44, 46 described above. Some examples are discussed below.

[0055] In one embodiment, different types of bolts 48, 50 are used to fasten the radially inward and radially outward joints 44, 46. For clarity, the first type of bolts 48 used to fasten the radially inward joint 44 will be referred to as the inner bolts 48 and the second type of bolts 50 used to fasten the radially outward joint 46 will be referred to as the outer bolts 50. Since the radially outward joint 46 is required to be stronger than the radially inward joint 44 to provide the ‘mechanical fuse’ functionality, then one option is that the outer bolts 50 are configured to require a higher tensile force to yield the bolt than the inner bolts 48. To achieve this, the outer bolts 50 may have a larger diameter, for example a thread diameter or a shank diameter, than that of the inner bolts 48. In a wind turbine tower 6 application, the outer bolts 50 may be M72, whereas the inner bolts 48 may be M64, for example. Consequently, increasing the diameter of the bolt thereby increases the tensile stress area of the bolt which is directly proportional to the tensile force required to yield the bolt.

[0056] Alternatively, the inner and outer bolts 48, 50 may differ in material composition, classification, or grade to achieve the ‘mechanical fuse’ effect. Bolt classification standards define the mechanical and physical properties for bolts based on their material properties, for example, metric bolt grades are defined by ISO 898-1 :2013. The inner bolts 48 may be a class 10.9 according to ISO 898-1 :2013 and the outer bolts 50 may be a higher class 12.9 according to ISO 898-1 :2013. It is known from ISO 898-1 :2013 that a class 12.9 bolt has a higher yield strength than the same size class 10.9 bolt.

[0057] Although in some examples, the ‘mechanical fuse’ functionality of the annular connecting interface 26 may be achieved by configuring the relative geometry of the inner bolts 48 and the outer bolts 50 to be different, the mechanical fuse functionality may also be achieved by selecting the material classification of the bolts in addition to or as an alternative to the relative faster geometry. The skilled person will be capable of selecting bolts of an appropriate size or class for their specific application.

[0058] Other possibilities exist other than the relative fastener geometry and / or material classification for achieving the mechanical fuse functionality of the annular connecting interface 26. For example, it is conventional in an annular connecting interface 26 of a wind turbine tower 6 for there to be equal numbers of bolts 48 in the radially inward joint 44 as compared to the radially outward joint 46. This is because it is generally preferable to match the load bearing capacity of the radially inward and radially outward joint 44, 46. However, it is envisaged that the mechanical fuse functionality could also be achieved by varying the quantities or numbers of bolts 48, 50. For example, the radially inward joint 44 may have a different quantity of bolts 48 to the radially outward joint 46. For an applied service load on the annular connecting interface 26, reducing the quantity of bolts 48, 50 in either of the bolted joints 44, 46 will increase the load of each of the bolts in that joint. For the radially inward joint 44 to act as a mechanical fuse, the quantity of bolts 48 in that joint 44 may be reduced, thereby increasing the load applied to each of the inner bolts 48 such that the inner bolts 48 are subjected to a greater load than the outer bolts 50.

[0059] In general terms, reducing the number of bolts 48 in the radially inward joint 44, compared to the radially outward joint 46, serves to provide a difference in the ability for each joint 44, 46 to transfer load as required to implement a mechanical fuse in the annular connecting interface 26. However, the practicality of implementing this design approach has the potential to introduce unnecessary complexity into the annular connecting interface 26. For example, mismatched quantities of bolts 48, 50 between the radially inward and radially outward joints 44, 46 can cause variability in the ability of the annular connection interface 26, as a whole, to transfer load depending on direction of loading. In other words, the is a potential for bias in directional loading of the wind turbine tower 6. As a result, it is currently considered preferable to for the quantity of inner bolts 48 to match the quantity of outer bolts 50 to simplify the design of the annular connecting interface 26.

[0060] Angular alignment of the inner and outer bolts 48, 50 can also affect the annular connecting interface’s 26 ability to transfer load from one tower section 24 to another 22. As above, for the sake of design simplicity, arranging the inner and outer bolts 48, 50 so they are radially aligned is beneficial to maintain a rotationally symmetric ability to transfer load throughout the annular connecting interface 26. Rotational symmetry can be maintained with differing quantities of bolts 48, 50 between the radially inward and radially outward joints 44, 46 wherein the number of outer bolts 50 is divisible by the number of inner bolts 48. That is to say that if the number of outer bolts 50 is divided by the number of inner bolts 48, the result is a whole number.

[0061] Moving on, another method of adjusting the relative capability of transferring load between the radially inward and radially outward joint 44, 46 is by modifying the distance of the central axes 56, 58 of the bolts 48, 50 to the respective walls 28, 34 of the wind turbine sections 22, 24 for each joint 44, 46. The closer a central axis 56, 58 of a bolt 48, 50 is placed to a wall 28, 34 of the tower 6, the greater the component force of the load applied to that bolt 48, 50 will be. Adjusting the radial distance of the inner and outer bolts 48, 50 to the wall 28, 34 of the tower 6 allows the skilled person to fine tune the component force of the load that is applied to the bolts 48, 50.

[0062] Another approach to causing a difference between the load applied to the inner and outer bolts 48, 50 is to modify the geometry of the flanges 30, 32, 36, 38, an example of which is shown in Figure 4. Specifically, flange thickness in the axial direction of the wind turbine tower 6 is proportional to flange bending stiffness. Increasing the thickness of the flange, and therefore the flange stiffness, transmits a greater portion of the load applied to the wind turbine tower to the bolts 48, 50 of the joint 44, 46. This allows control, or fine tuning, of the load applied to the inner and outer bolts 48, 50 independently of one another. In Figure 4, the thickness of the radially inward flange parts 30, 36 is greater than that of the radially outward flange parts 32, 38 thereby exerting a larger portion of the load applied to the tower to the inner bolts 48.

[0063] Similarly, the upper and lower flanges 30, 32, 36, 38 of an annular connecting interface of the wind turbine tower may have different thicknesses. Beneficially, this allows the skilled person to control which tower section of the wind turbine will fail before the other in the event of mechanical failure. It would be advantageous for the failure to occur in a tower section that is lower cost and / or more easily replaced than the others, for example, a simple tower section with no additional features as opposed to the foundation section or a section with a door.

[0064] The inventive concept further contemplates a wind turbine 2 fitted with the wind turbine tower arrangement of the invention. In the above discussion, various alternative examples to the illustrated embodiments have been mentioned. Other variants and examples would be apparent to the skilled person.

Claims

Claims1. A wind turbine tower arrangement (6) comprising an upper tower section (24) a lower tower section (22) and an annular connecting interface (26) disposed between and connecting the upper and lower tower sections (24, 22) of a wind turbine tower (6), the annular connecting interface (26) comprising: an opposing pair of radially inward flanges (30, 36) fastened by a plurality of mechanical fasteners of a first type (48) defining an radially inward mechanically fastened joint (44); an opposing pair of radially outward flanges (32, 38) fastened by a plurality of mechanical fasteners of a second type (50) defining an radially outward mechanically fastened joint (46); wherein the radially inward mechanically fastened joint (44) is configured as a mechanical fuse such that indicia of mechanical failure are displayed in the radially inward mechanically fastened joint (44) before being displayed in the radially outward mechanically fastened joint (46).

2. The wind turbine tower arrangement (6) of Claim 1 , wherein a diameter of at least some of the mechanical fasteners of the second type (50) is greater than a diameter of the mechanical fasteners of the first type (48).

3. The wind turbine tower arrangement (6) of any preceding claim, wherein a material grade of the mechanical fastener determines a yield strength for the mechanical fastener and wherein the material grade for the mechanical fastener of the second type (50) is greater than a material grade of the mechanical fasteners of the first type (48).

4. The wind turbine tower arrangement (6) of any preceding claim, wherein a quantity of mechanical fasteners (48) in the radially inward mechanically fastened joint (44) is equal to a quantity of mechanical fasteners (50) in the radially outward mechanically fastened joint (46).

5. The wind turbine tower arrangement (6) of Claim 4, wherein the angular offset of the mechanical fasteners (48) of the radially inward mechanically fastened joint (44)relative to the mechanical fasteners (50) of the radially outward mechanically fastened joint (46) is zero.

6. The wind turbine tower arrangement (6) of any preceding claim, wherein each flange (30, 32, 36, 38) has a thickness in an axial direction relative to the tower sections (22, 24) and wherein the thickness of at least one inwardly projecting flange (30, 36) is less than a thickness of at least one outwardly projecting flange (32, 38).

7. The wind turbine tower arrangement (6) of Claim 6, wherein a thickness of the upper flanges (30, 32) is different to a thickness of the lower flanges (36, 38).

8. The wind turbine tower arrangement (6) of any preceding claim, wherein each mechanical fastener (48, 50) comprises a central axis (56, 58), wherein the distance of the central axis (56) of the mechanical fasteners (48) joining the radially inward mechanically fastened joint (44) from a wall (28, 34) of the wind turbine tower (6) is less than a distance of a central axis (58) of the mechanical fasteners (50) joining the radially outward mechanically fastened joint (46) from the wall (28, 34) of the wind turbine tower (6).

9. A wind turbine (2) comprising the wind turbine tower arrangement (6) of any preceding claim.

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