Wind turbine tower

Abstract

A wind turbine tower comprising a tubular body with a length along a central axis, and a cross-sectional shape perpendicular to the central axis, and an axially-extending groove 110 that projects inwardly from the outer surface 120 of the wind turbine tower. Preferably the tubular body has a circular cross-section and has multiple axially-extending grooves spaced apart around the tubular body. The tubular body may comprise multiple axially-extending arcuate or multifaceted panels that are connected by an axially-extending connection panel comprising a groove. Preferably an axially-extending rail 130 is provided in the groove and projects outwardly, where the rail has clamping faces on opposite sides that are symmetric about a central plane extending outwardly from the tower body through the middle of the width of the base of the rail at the tower body. A wind turbine tower section for interconnection with one or more further wind turbine tower sections for forming a wind turbine tower is also claimed.

Description

TECHNICAL FIELD The present application relates to wind turbine towers, and more particularly, but not exclusively, to wind turbine towers having a carriage engagement feature extending lengthwise up the tower, and wind turbine towers formed from interconnected stacked sections. BACKGROUND It is known to provide a wind turbine tower with a rail extending up the tower for use by a carriage transporting a load, such as a wind turbine nacelle, for example from WO2018193260A1. There is a requirement for improved wind turbine towers having a carriage engagement feature extending lengthwise up the tower. It is known to construct a wind turbine tower by stacking and interconnecting tower sections, for example from WO2024003543A1. There is a requirement for improved wind turbine towers having an improved connection system for wind turbine towers formed from interconnected stacked sections. SUMMARY OF THE DISCLOSURE According to an aspect, there is provided a wind turbine tower comprising: a tubular body with a length along a central axis (X), and an cross-sectional shape perpendicular to the central axis, and an axially-extending groove that projects inwardly from the outer surface of the wind turbine tower. The tubular body may have a circular cross-section. The outer surface may extend to a maximal radial extent from the central axis (X) in a region adjacent each side of the entrance of the groove. The tubular body may comprise a tube with a circular cross-section and, adjacent each side of the groove, an axially-extending tapered fairing is provided on the exterior of the tube. The tubular body may comprise a tube with a non-circular cross-section having a first radius from the central axis (X) adjacent the groove that is less than a second radius away from the groove, and adjacent each side of the groove, an axially-extending tapered fairing is provided on the exterior of the tube. The fairing may extend to a radius from the central axis (X) that is no more than the second radius. The groove may be provided within a groove assembly that extends to a greater radial extent than the tubular body. The groove may have clamping faces (110S) on opposed sides that are symmetric about a central plane (CP) extending outwardly from the tower body through the middle of the entrance of the groove, and wherein the tubular body comprises a wall (120) aligned with each of the clamping faces (11 OS). The groove may have clamping faces on opposed sides that are symmetric about a central plane (CP) extending outwardly from the tower body through the middle of the entrance of the groove. The groove may have a depth of at least 100 mm. The groove may have a circumferential width of at least 200 mm adjacent the periphery of the tubular body. The tubular body may have a plurality of axially-extending grooves that project inwardly from the periphery of the tubular body and which are spaced apart around the tubular body. The tubular body may comprise a plurality of axially-extending arcuate or multifaceted panels, wherein each pair of adjacent acuate panels are connected by an axially-extending connection panel comprising a groove. An axially extending rail (130) may be provided within the groove (110) and project outwardly. The rail may project outwardly by no more than the depth of the groove. The rail may project outwardly by more than the depth of the groove. The rail may comprise an axially extending tubular rail member. The rail (130) may have clamping faces (130S2) on opposed sides that are symmetric about a central plane (CP) extending outwardly from the tower body through the middle of the width (B) of the base of the rail at the tower body. The rail (130) may comprise a plurality of tubular rail members (130A). The tubular body may have a plurality of axially-extending grooves that project inwardly from the periphery of the tubular body and which are spaced apart around the tubular body, wherein a rail is provided in each groove. The tower may comprise an upper tower portion (100U) and a lower tower portion (100L), wherein the upper tower portion comprises the tubular body, and the tubular body has a circular crosssection, and the lower tower portion does not have a circular cross-section. The upper tower portion (100U) may comprise at least 40% of the height of the wind turbine tower. The lower tower portion (100U) may comprise a lattice tower structure having an exposed framework. The wind turbine tower may comprise a plurality of axially interconnected tower sections. The rail comprises an axially extending tubular rail member comprising a plurality of axially interconnected rail sections, wherein the interconnected rail sections are connected together by pipe connectors. According to a second aspect, there is provided a wind turbine tower section for interconnection with one or more further wind turbine tower sections for forming a wind turbine tower in accordance with the first aspect. DESCRIPTION OF THE DRAWINGS Examples are further described hereinafter with reference to the accompanying drawings, in which: • Figure 1A shows a first wind turbine tower having grooves and rails extending lengthwise up the tower, Figure 1B shows a cross-sectional view through the tower of Figure 1A; • Figures 1C and 1D show an enlarged cross-sectional views through a rail of a wind turbine tower; • Figure 2A shows a second wind turbine tower having grooves and rails extending lengthwise up the tower, Figures 2B and 2C show enlarged views of the top and bottom of the tower, and Figure 2D shows a cross-sectional view through the tower of Figure 2A; • Figure 2E shows a carriage-rotor-nacelle assembly mounted onto a rail of the tower of Figure 2A in each of two different positions at different stages during the installation of the rotor-nacelle assembly onto the tower, during the assembly of a wind turbine assembly; • Figure 2F shows a cross-sectional view through a third tower; • Figure 3A shows a fourth tower and Figure 3B shows a cross sectional view of the fourth tower; • Figure 3C shows a fifth tower and Figure 3D shows a cross sectional view of the fifth tower; • Figure 4A shows a sixth tower having grooves extending lengthwise up the tower, Figure 4B shows a cross sectional view of the tower of Figure 4A, and Figure 4C shows an enlarged view of the cross-section of the groove; • Figure 5A shows a cross-sectional view through a seventh tower having grooves extending lengthwise up the tower; • Figure 5B shows a cross-sectional view through a eighth tower having grooves extending lengthwise up the tower; • Figure 6A shows a ninth tower having grooves extending lengthwise up the tower, and Figure 6B shows a cross sectional view of the tower of Figure 6A; • Figure 7A shows a tenth tower having grooves extending lengthwise up the tower between fairings, and Figure 7B shows a cross sectional view of the tower of Figure 7A; • Figure 7C shows an eleventh tower having grooves and rails extending lengthwise up the tower between fairings; • Figure 8A shows a twelfth tower having grooves and rails extending lengthwise up the tower, Figure 8B shows a plan view of the tower of Figure 8A, and Figure 8C shows an enlarged view of the top of the tower of Figure 8A; • Figure 9A shows a thirteenth tower having rails extending lengthwise up the tower, with a lower lattice tower construction, and Figures 9B and 9C show plan views of the tower of Figure 9A respectively with and without a tower top section; and • Figure 10A shows an enlarged view of part of the tower of Figure 8A or Figure 9A, Figure 10B shows a partially disassembled view of the part of the tower of Figure 10A, and Figure 10C shows a further enlarged view of part of the tower in Figure 10A with rail connectors, and Figure 10D shows a cut-away view of a rail connector of Figure 10C. DETAILED DESCRIPTION The present application describes a wind turbine tower having a carriage engagement feature extending lengthwise up the tower for transporting loads up and down the tower, such as when transporting a wind turbine nacelle up the tower. The present application discloses such a wind turbine tower that has reduced aerodynamic drag and lift from wind exposure, compared with a corresponding tower with rails installed on the outside of a cylindrical tower, enabling a taller tower to be constructed whilst reducing bending of the tower caused by aerodynamic drag and lift arising from wind exposure. The present application describes a wind turbine tower formed by vertically stacking and interconnecting tower sections that has reduced manufacturing complexity, reduced assembly complexity, and enhance strength to reduce bending caused by wind exposure. The interconnection of the tower section may enable manufacturing by less specialist manufacturers and methods, and facilitate transportation in sections whose dimensions can be tailored to match route restrictions (e.g. road width and bridge heights) along the route to the installation site. In the described examples, like features have been identified with like numerals, albeit in some cases having one or more of: increments of integer multiples of 100; suffix letters; and typographical marks (e.g. primes, asterisks and daggers). For example, in different figures 100 and 100’ have been used to indicate a wind turbine tower. Figure 1A illustrates a wind turbine tower 100, having grooves 110 extending up the outside of the tower between exterior panels 120. The grooves 110 are each a long narrow space in the outside of the tower 100. A rail 130 extends up the tower 100 within each groove 110, for one or more clamps or guides of the carriage to mechanically engage with when the carriage transports loads up or down the tower. The tower 100 has a central axis X (e.g. vertical axis), perpendicular to which (e.g. horizontally, e.g. along the line H-H) the tower has a generally circular cross-sectional shape, formed by the exterior panels 120, which provide the tower with a smooth outer surface between the grooves 110. The grooves 110 project inwardly from the smooth outer surface of the exterior panels 120. The exterior panels 120 fit a circular cross-sectional envelope E, as indicated in Figure 1B. The rails 130 do notextend outside of the circular envelope E of the exterior panels 120. The generally circular cross-sectional shape of the tower 100, with the rails 130 that are recessed within respective grooves and remain entirely within the circular envelope E of the exterior panels 120 provides the tower 100 with a lower aerodynamic drag and lift when exposed to incident wind, with marginal additional load on the tower compared with a cylindrical tubular tower without one or both of grooves and rails extending up the tower. Further, the tower 100 may have substantially lower drag and lift than a corresponding tower having ‘surface-mounted’ rails that are installed entirely on the outside of a body having a circular cross-section (e.g. cylindrical). The grooves 110 may have a depth of at least 100 mm (e.g. depth of the deepest part of the groove, with respect to the circular cross-sectional envelope E). The grooves 110 may have a width adjacent the surface of the tower of at least 200 mm. The rails 130 may be welded into the groove 110 along their length (e.g. with a continuous or intermittent welding pattern). The formation of the tower 100 from rails 130 and a plurality of exterior panels 120 (e.g. to form a circular cross-sectional envelope E) may enable the rails and exterior panels to be transported to a wind turbine installation site in a partially dissembled state, and then to be assembled onsite, for example being bolted together or welded together to form the tower. The tower may be transported as a series of tower sections for interconnecting in a vertical stack. Tower sections having a greater width (e.g. diameter) may be transported in two or more segments of the circumference of the tower section, which are connected together at the installation site to form the tower sections. Transportation in a disassembled state may enable a larger width tower to be constructed, by avoiding the limitations of tower width arising from bridge height restrictions. In the tower illustrated in Figures 1A to 1D, the rails 130 are composite, each comprising a pair of tubular rail members 130A extending along a respective groove 110 (e.g. extending parallel). Forming the rail 130 from one or more tubular elements may facilitate a high strength construction, and may facilitate an inexpensive construction, for example by enabling use of standard sizes of tubes. The tubular rail members 130A may be in contact along their length, as shown in Figures 1A and 1B, for example being welded together (e.g. with a continuous or intermittent welding pattern). Alternatively, the tubular rail members 130A may be spaced apart, for example being spaced apart by one or more spacing plates 132, as shown in Figure 1C. The one or more spacing plate(s) 132 may be welded along the length of the length of the tubular rail members 130A (e.g. with a continuous or intermittent welding pattern). For example, Figures 1C and 1D respectively illustrate tubular rail members 130A that are spaced apart by one and two spacing plates 132. The one or more spacing plate 132 may be coplanar with the circumference of the tubular rail members 130A, or may be set back from being coplanar with the circumference of the tubular rail members 130A (e.g. a dumbbell shape). The spacing plate 132 may be provided further from the central axis X of the tower than the centres of the tubular rail members 130A. Being spaced apart may enhance the transfer of load from a carriage engaged on the rail 130 into the structure of the tower 100. Being spaced apart may enable the tubular rails to support greater torque, for example when an ascending or descending elevator carriage carrying a load (e.g. a wind turbine nacelle) is exposed to wind, including providing improved load transfer from the rail into the rest of the tower. In particular, the use of a spacing plate 132 increases the stiffness of the rails 130 in the circumferential direction of the tower, which may enable the use of rails having a smaller radial projection (with respect to the central axis X of the tower 100), enabling the use of a shallower groove 110 into which to recess the rail 130. Further, providing a spacing plate 132 extending between the tubular rail members 130A may reduce the aerodynamic drag and lift of the rails from wind exposure. In Figures 1A to 1C, the groove 110 has a maximum depth of D1, between the circular cross-sectional envelope E of the exterior panels 120 and the back of the rail 130 (e.g. defined by a plane parallel to the central axis X and perpendicular to the radius from the central axis X), and the rails 130 are entirely within the circular envelope E. Alternatively, the rails 130 may project out beyond the circular cross-sectional envelope E of the exterior panels 120, as shown in Figure 1D, e.g. projecting (radially with respect to the central axis X of the tower 100) beyond the circular envelope E by no more than the maximum depth D2 between the circular cross-sectional envelope E of the exterior panels 120 and the back of the rail 130, within which the rail 130 is provided (e.g. projecting beyond the circular envelope E by no more than half of the maximum depth D2). The groove 110 in Figure 1D is shallower than the groove in Figure 1C. The centrelines CL of the tubular rail members 130A are shown in Figure 1D. In the tower illustrated in Figures 1A to 1D, the rails 130 are composite, each comprising a pair of tubular rail members 130A. Alternatively, each rail 130 may be a single tubular member 130A, e.g. each groove 110 has a rail 130 comprising only one tubular member 130A, or comprising more than two tubular rail members. In the illustrated tower 100 of Figures 1A and 1B, the tower is provided with three grooves 110, which are approximately equally spaced apart around the central axis X. Alternatively, the tower may have only one groove, two grooves, or more than three grooves, e.g. with a respective rail each provided within the or each groove. For example, in the case of only one groove, a carriage may be mounted to a primary groove (or rail within the groove) and have one or more stabilisation arms that bias against one or more secondary grooves (or corresponding rails). For example, a carriage may be mounted to a groove (or a rail within the groove) and have one or more stabilisation arms that bias against the exterior panels 120, for example in the case that the tower 100 has only one groove (or rail). Providing only one groove or a small number of grooves may reduce the aerodynamic drag and lift of the tower, compared with a larger number of grooves. Providing a larger number of identical grooves may facilitate selection of a preferred position around the tower’s central axis X to attach a carriage, e.g. enabling selection of the position that results in reduced loads on the tower (with respect to the foundation) due to wind exposure of the carriage and any load. Additionally, one or both of the grooves and rails (where rails are present within the grooves) may provide substantial strengthening of the tower, and providing a larger number of grooves (and rails) may strengthen the tower against bending due to loads from wind exposure (and asymmetric loads, such as a carriage). As shown in Figure 1A, the tower 100 may be provided with a capping flange 122 to which a tower top section 106 may be secured (the tower top section 106, which may comprise a yaw bearing, may be pre-assembled to the nacelle 194). In the illustrated towers with a plurality of grooves spaced apart around the tower, the grooves are all the same. Alternatively, the grooves may have different sizes, for example with one or more secondary grooves being smaller in cross-section than a primary groove. Similarly, in the illustrated towers with a plurality of grooves spaced apart around the tower with respective rails, the rails are all the same. Alternatively, the rails may have different sizes, for example with one or more secondary rails being smaller in cross-section than a primary rail. The grooves 110 and corresponding rails 130 in Figure 1A extend up substantially the full height of the tower. However, alternatively, the grooves 110 (and any corresponding rails 130) may start above the bottom of the tower, may stop before the top of the tower, or both. The grooves 110 (and any corresponding rails 130) may extend along at least 50% of the height of the tower 100, e.g. extending along at least 75% of the height of the tower. Figure 2A shows a further wind turbine tower 100, having rails 130 within grooves 110 that extend up the tower between exterior panels 120 that form a generally circular (horizontal) cross-section. Figures 2B and 2C respectively show enlarged views of the top and bottom of the tower, and Figure 2D shows an exemplary horizontal cross-sectional view, along the line H-H. In Figure 2A, each groove 110 is provided with a single rail 130, which are entirely within the circular cross-sectional envelope E of the exterior panels 120. The rail 130 has an outer portion (e.g. adjacent an outer face 130S1) that is wider than a waist formed behind the outer portion (e.g. between opposed clamping faces 130S2), to facilitate securely connecting a carriage to the rail. Opposed sides 132S2 (clamping surfaces) of the rail 130 may have generally flat surfaces (e.g. surfaces that are parallel to a central plane CP that is coplanar with the central axis X and extending through the centre of the groove 110, or symmetrically inclined about the central plane CP, e.g. having a relative inclination of less than 30 degrees to the central plane CP) for clamps or guides of an elevator carriage to bias against. Figure 2E shows a wind turbine assembly 190 in two different stages of assembly, as a rotornacelle assembly (wind turbine rotor 192 having a rotor hub and rotor blades; wind turbine nacelle 194) is transported up a wind turbine tower 100 (e.g. the tower of Figure 2A), by a carriage 196 that is mechanically engaged with one of the tower rails 130. The lower position shows the carriage-rotor-nacelle assembly 196, 192, 194 where it has been installed onto the rail 130 near the bottom of the tower 100, and the upper position shows the carriage-rotor-nacelle assembly when the rotor-nacelle assembly is being pivoted onto the top of the tower (the tower top section 106, which is connected to the nacelle, is lowered onto the top of the tower) for use, and prior to the carriage returning back down the rail 130. The other towers disclosed here may also be used in the same manner, with a complementary carriage engaged upon with one or more rails or grooves as the carriage travels up or down the tower. As is shown more clearly in Figure 2C, the bottom of each rail 130 may have a portion 130Z in which the rail is narrower, to facilitate connection of a carriage to the rail. Similarly, a lower portion of the tower in which the rails are narrower may be provided in other tower designs described here. As shown in Figure 2F, the groove 110 may extend between the adjacent exterior panels 120 with a part-circular cross-sectional shape (e.g. part-cylindrical, or having a multifaceted shape), having a constant radius of curvature. Having a constant radius of curvature may simplify manufacturing of the groove 110, compared with a more complex shape, by being more easily formed by a rolling process or cut from a cylindrical shape. In Figure 3A, as is also shown in the corresponding cross-sectional view in Figure 3B, each rail 130 (e.g. one or more rails and respective grooves) may be supported on a bridging panel 112 extending between adjacent exterior panels 120. The bridging panel 112 connects (e.g. by welding) to portions of the exterior panels that are spaced apart from the respective groove 110. The bridging panel 112, rail 130 and groove 110 may be pre-fabricated into a bridging assembly, enabling simplified construction of the tower, requiring only connection of the bridging assemblies 112, 130, 110 and exterior panels 120, e.g. by welding. The bridging panel 112 may be flat (planar) as shown in Figure 3A. Alternatively, as shown in Figures 3C and 3D, the bridging panel 112 may be arcuate, having a convex shape towards the central axis X of the tower 100. The use of an arcuate bridging panel 112 that has a convex shape towards the central axis X facilitate fully recessing a rail 130 with a greater (radial) projection or recessing a larger proportion of the rail, within the circular cross-sectional envelope E. An arcuate bridging panel 112 may also be used with other towers 100, including where a rail 130 is not used, for example as shown in Figures 4A and 4B. Figures 3C and 3D show a tower 100 with a T-shaped rail 130. T-shaped rails 130 may enable low cost manufacturing. In Figures 1A to 3D, the wind turbine towers 100 have been provided with rails 130 extending along their grooves 110, for use with carriages to engage with a rail (or more than one rail) whilst transporting loads up and down the tower. Alternatively, the towers 100’ may be provided without rails, and having grooves 100, for use with carriages to engage with a groove (or more than one groove) whilst transporting loads up and down the tower. One or more clamps or guides of the carriage may mechanically engage within one or more grooves 110. Direct mechanical engagement between the clamps or guides of the carriage and the groove may enable forces to be more directly transmitted into the exterior panels 120 of the tower 100. In Figure 4A, the tower 100’, which is shown in corresponding cross-sectional view in Figure 4B, has grooves 110 extending along the tower, without rails. Figure 4C shows an the enlarged view of the groove 110, and clamps or guides of the carriage may bias B against opposed sides 110S of the groove 110. In cross-section (perpendicular to its length), the groove 110 has a partcircular shape (e.g. the groove has a part-cylindrical shape), which may reduce manufacturing complexity and cost, for example by enabling formation of the groove 110 from cylindrical tube. In Figure 5A, the tower 100’, which is shown in cross-sectional view, again has grooves 110 extending along the tower, without rails. Opposed sides 110S (clamping surfaces) of the groove 110 may have generally flat surfaces (e.g. surfaces that are parallel to a central plane CP that is coplanar with the central axis X and extending through the centre of the groove 110, or symmetrically inclined about the central plane CP, e.g. having a relative inclination of less than 30 degrees to the central plane CP) for clamps or guides of an elevator carriage to bias against. For example the groove 110 may have a rectangular shape (e.g. having three perpendicular walls). Flat opposed sides 110S may reduce manufacturing complexity and cost, for example by enabling formation of the groove 110 from flat sheet material. The innermost surface of the groove 110 may be part of a bridging panel 112 extending between adjacent exterior panels 120 and strengthening the opposed sides 110S. The or each groove 110 may be formed from an arrangement of tubular elements 114A, 114B, as shown in the tower 100” in Figure 5B. Two tubular elements 114B at the entrance of the groove 110 are connected to a further tubular element 114A (e.g. welded together, along their length or intermittently) and may be connected directly to the exterior panels 120, and the tubular elements 114B are spaced apart to provide a groove 110 between opposed convex sides 110S. The further tubular element 114A may have a larger diameter than the tubular elements 114B at the entrance of the groove 110. Forming the groove 110 from tubular elements may facilitate a high strength construction, and may facilitate inexpensive construction, for example by enabling forming using standard sizes of tubes. The sides 11 OS of the groove 110 may project out beyond the adjacent exterior side panels 120. For example, the groove 110 may project beyond the circular cross-sectional envelope E of the exterior panels 120, as shown in Figure 6A and the corresponding cross-sectional view in Figure 6B, e.g. having opposed sides 110S projecting (radially with respect to the central axis X) beyond the circular envelope E of the exterior panels 120. Projection of the sides 110S of the groove 110 beyond the adjacent exterior side panels 120 may facilitate closer alignment between biasing forces B of a clamp or guide of an elevator that is travelling along the groove and the adjacent exterior panels 120, e.g. enabling the bias B to be substantially aligned with the adjacent panel 120. The groove 110 may be provided between fairings 122, as shown in the tower 100” of Figure 7A and the corresponding cross-sectional view of Figure 7B. The fairings 122 extend to a maximum distance (radius) from the central axis X of the tower 100 adjacent or proximate the groove 110, and are smoothly faired to the exterior panels 120. The fairings 122 may be connected onto a substantially cylindrical underlying tube, with the exterior panels 120 being provided the exposed portions of the underlying tube. As shown in Figure 7B, the fairings 122 extend project beyond the circular cross-sectional envelope E of the exterior panels 120, e.g. having opposed sides 11 OS of the groove 110 also projecting (radially with respect to the central axis X) beyond the circular envelope E of the exterior panels 120. Forming the groove(s) 110 between fairings 122 may enable the groove(s) 110 to be provided with a low aerodynamic drag and lift for wind exposure, whilst reducing manufacturing complexity and cost by enabling the use of an underlying tube having a fully (substantially) circular cross-sectional shape (e.g. a cylindrical tube). Recessing the rails 130 or grooves 110 within a generally circular cross-sectional envelope E, or shrouding the rails or grooves with fairings 122 with a cross-sectional shape that is close to circular reduces drag and lift, compared with surface-mounted rails or grooves. The tower 100” of Figures 7A and 7B have grooves 110 between fairings 122 and without rails. Alternatively, the tower 100” may be provided rails 130 between respective fairings 122. Similarly to the grooves 110 in Figures 1C and 1D, when the rail 130 is provided between fairings 122, the rail 130 may project (radially from the central axis X) no more than the fairings 122, as shown in Figure 7C, or the rails 130 may project out beyond the radial extent of the adjacent fairings (not shown). Forming the rail(s) 130 between fairings 122 may enable the rail(s) 130 to be provided with a low aerodynamic drag and lift for wind exposure, whilst reducing manufacturing complexity and cost by enabling the use of an underlying tube having a fully (substantially) circular cross-sectional shape (e.g. a cylindrical tube). Up substantially the full height of the wind turbine towers 100 illustrated in the preceding drawings, the towers have a generally smooth circular cross-sectional shape, with the exterior panels 120 providing a circular cross-sectional envelope E. Alternatively, only an upper wind turbine tower portion 100U may have a generally smooth circular cross-sectional shape (provided with groove(s) 110), with the upper exterior panels 120U in the upper portion 100U providing a circular cross-sectional envelope E (as indicated in several preceding drawings), and a lower tower portion 120L may have a non-circular shape, in which the exterior panels 120L in the lower portion 100L do not provide a circular cross-sectional envelope E, for example as shown in Figure 8A and the corresponding, vertically-downward view of Figure 8B. The upper tower portion 100U may comprise at least 40% of the height of the tower (or at least 50 m of the height of the tower). Providing the generally smooth circular shape to the upper tower portion 100U reduces aerodynamic drag and lift due to wind exposure, and correspondingly reduces being bending torque on the tower, in the portion of the tower that is exposed to higher wind speeds (being higher above the ground) and which is furthest from the foundation 102 of the tower (e.g. the foundations are typically in the ground, for on-shore wind turbines, in the seabed for off-shore wind turbines, or may be a floating foundation). The lower tower portion 100L may have lower exterior panels 120L extending approximately straight between adjacent grooves 110 or adjacent rails 130, as shown in Figure 8A. Providing a non-circular shape in the lower tower portion 100L may reduce the manufacturing complexity and cost of the lower tower portion 110L, for example by avoiding the manufacturing of large lower panels that curve around the central axis X of the tower 100. Additionally, lower panels 120L having a relatively flat shape may be directly connected to underlying bracing elements 102 extending between adjacent grooves 110 or adjacent rails 130 (e.g. avoiding the requirements for a more complex underlying structure to support the lower panels). As shown in Figure 8C, the tower 100 may be provided with a tower top section 106 (e.g. a cylindrical turret), which may be provided with a yaw bearing for the rotation of a wind turbine nacelle 194 (the tower top section 106 may be connected to the nacelle 194, before the nacelle is connected to the top of the tower). As is also shown in Figure 8C, the rails 130 may each be provided with a rail termination 132. The rail termination 132 may protect the rail 130 from the ingress of rain water. In a further alternative, only an upper wind turbine tower portion 100U may have a generally smooth circular cross-sectional shape, with the upper exterior panels 120U in the upper portion 100U providing a circular cross-sectional envelope E (as indicated in several preceding drawings), and exterior panels may be absent from some or all of a lower tower portion 120L’, instead having an exposed framework forming a lattice tower structure, for example as shown in Figure 9A and the corresponding, vertically-downward view of Figure 9B. As discussed in relation to Figure 8A, providing the generally smooth circular shape to the upper tower portion 100U reduces aerodynamic drag and lift due to wind exposure. Providing a lower tower portion 100L’ without exterior panels may reduce the manufacturing complexity of the lower tower portion. Having an exposed framework forming a lattice tower structure tower portion may provide a cost effective construction to withstand shear forces, allows wind to blow through the lower tower portion 100L’ and prevents the risk of large exterior panels buckling in the lower tower portion 100L. For purposes of illustration, Figure 9C shows the tower 100 of Figure 9B in the absence of the tower top section 106, enabling the ends of the tubular rail members 130A and spacing plate 132 to be seen. The exterior panels 120 and one or both of the grooves 110 and the rails 130 (where rails are present) may jointly provide the strength of the tower 100, 100’ to resist bending forces. The exterior panels 120 may predominantly provide the strength of the tower (e.g. for the faired structures shown in Figures 7A to 7C). Alternatively, one or both of the grooves 110 and rails 130 (where rails are present) may predominantly or fully provide the strength of the tower, and the exterior panels 120 may provide reduced aerodynamic drag and lift without providing a substantial contribution to the strength of the tower 100 to resist bending forces. However, the exterior panels 120 may provide strength to the tower 100 to resist sheer forces, by providing a sheer resistant structure between the grooves 110 or rails 130, e.g. as an alternative to a lattice bracing arrangement. Where the strength of the tower is mostly provided by one or both of the grooves and rails, the tower has three or more grooves and rails (where rails are present). Providing a tower in which the strength is mostly provided by one or both of the grooves and rails (where rails are present) may enable light-weight exterior panels 120 to be used, reducing the manufacturing cost of the tower. A tower in which the strength is mostly provided by one or both of the grooves and rails (where rails are present) may be constructed as a lattice tower framework, to which exterior panels are connected, and which provide a reduced aerodynamic drag compared with the underlying lattice tower framework, if it had been left exposed to the wind. A wind turbine tower having three or more legs, where one or each of the legs is provided with one or both of a groove and rail, provides both a strong tower and enables a carriage to transport loads up and down the tower. Figure 10A shows part of the tower 100 of Figure 8A, which has exterior panels 120 (which may be structural, to resist sheer forces experienced by the tower, or may be only aerodynamic). Figure 10B shows a corresponding part of the tower 100, with the exterior panels 120 removed from one of the tower sections 100S, to expose the underlying framework 100F forming a lattice tower structure, having bracing arms 100B between adjacent rails 130, e.g. between the adjacent tubular rail members 130A. Similarly to the rails 130 of Figures 1A to 1C, the rails 130 shown in Figures 8A to 9C are composite, each comprising a pair of tubular rail members 130A, and may be connected with a spacing plate 132. As is shown in Figures 10A to 10D, the tower 100 may be formed from tower sections 100S that are connected in a vertical stack. Each tower section 100S has three or more sections of rail 130, which are connected to respective sections of rail on an adjacent (stacked) tower section 100S by respective by rail connectors (pipe connectors) 140. Correspondingly, tower sections having three or more sections of groove 110 (e.g. tower sections corresponding to the cross-section of Figure 5B) may correspondingly be interconnected with rail connectors 140. The rail connectors 140 may form a connection between tubular rail members 130A having a substantially uniform outer diameter (the rail connector may have a narrow annular gripping rebate on either side of connection). Where the rails 130 each comprise one or more tubular rail members 130A, the ends of each pair of tubular rail members 130A are connected with a respective tubular rail connector 140, which may also be known as pipe connectors. The tubular rail connectors 140 may, for example, be Pipeline Risers manufactured by GMC® Limited, or another suitable pipe connector from the subsea oil and gas industry. Figure 10D shows a cut-away view through an exemplary tubular rail connector 140 and part of the corresponding tubular rail elements 130A. The tubular rail connector 140 comprises a male part 140M and a female part 140F, having respective patterns of engageable teeth 140T (e.g. annular teeth) provided on mating generally conical surfaces. The generally conical surfaces provided with the teeth 140T may each be provided between generally cylindrical portions that form a snug fit when male and female parts 140M, 140F are connected. The tubular rail connectors 140 form joints that may be stronger than the tubes 130A to which they connect, facilitating the use of tubular rail connectors 140 in constructing a strong wind turbine tower 100. The male and female parts 140M, 140F of the tubular rail connector 140 are each connected to the end of a respective tubular rail element 130A, for example being welded together at lap joints 140J, as shown in Figure 10D. Alternatively, the male and female parts 140M, 140F may be directly machined into the ends of the respective tubular rail elements 130A. Assembling the tower 100 from tower sections 100S, with three or more rails 130 (spaced apart around the central axis X), enables rapid, lightweight and low complexity assembly (or disassembly) of the tower by connecting rail connectors between the respective sections of rail on successive tower sections, compared with connecting together flanges between stacked tower body sections using large numbers of bolts, in which the flanges and bolts are both heavy and slow to assemble (or disassemble). In towers 100 in which the rails 130 or grooves 110 predominantly provide the strength of the tower, and where the adjacent tower sections 100S have exterior panels 120, the exterior panels of adjacent tower sections may be sized to provide a panel gap 120G between the exterior panels of adjacent tower sections, as shown in Figure 10C. A resiliency deformable weather-resistant filler material (e.g. a rubber-type seal) may be provided in the panel gap 120G between the exterior panels 120 of adjacent tower sections 100S to seal the panel gap. In Figure 100, a spacing plate gap 132G is shown between the spacing plates 132 on adjacent tower sections 100S, aligned with the rail connectors 140. A weather-resistant cover 132W may be provided to seal the spacing plate gap 132G, as indicated by the dashed line in Figure 10C. The specific embodiments described above may be used in combination or interchangeably depending on the context. It is specifically intended in this disclosure that individual features of the wind turbine towers of the invention which are disclosed in relation to any of the above figures may be considered more generally to represent features of the invention as a whole, within the scope of the appended claims. Thus, for example, particular features of the composite rail 130 described in Figures 1A to 1D may be combined with the use of tubular rail connectors, as described in Figure 10D. The figures provided herein are schematic and not to scale. Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of them mean “including but not limited to”, and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise. Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and / or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and / or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed. The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are 5 incorporated herein by reference.

Claims

1. A wind turbine tower comprising:a tubular body with a length along a central axis (X), and an cross-sectional shape perpendicular to the central axis, andan axially-extending groove that projects inwardly from the outer surface of the wind turbine tower.

2. The wind turbine tower according to claim 1, wherein the tubular body has a circular crosssection.

3. The wind turbine tower according to claim 1, wherein the outer surface extends to a maximal radial extent from the central axis (X) in a region adjacent each side of the entrance of the groove.

4. The wind turbine tower according to claim 3, wherein the tubular body comprises a tube with a circular cross-section and, adjacent each side of the groove, an axially-extending tapered fairing is provided on the exterior of the tube.

5. The wind turbine tower according to claim 1 or claim 2, wherein the tubular body comprises a tube with a non-circular cross-section having a first radius from the central axis (X) adjacent the groove that is less than a second radius away from the groove, andadjacent each side of the groove, an axially-extending tapered fairing is provided on the exterior of the tube.

6. The wind turbine tower according to claim 5, wherein the fairing extends to a radius from the central axis (X) that is no more than the second radius.

7. The wind turbine tower according to any preceding claim, wherein the groove is provided within a groove assembly that extends to a greater radial extent than the tubular body.

8. The wind turbine tower according to claim 7, wherein the groove has clamping faces (110S) on opposed sides that are symmetric about a central plane (CP) extending outwardly from the tower body through the middle of the entrance of the groove, andwherein the tubular body comprises a wall (120) aligned with each of the clamping faces(110S).

9. The wind turbine tower according to any one of claims 1 to 7, wherein the groove has clamping faces on opposed sides that are symmetric about a central plane (CP) extending outwardly from the tower body through the middle of the entrance of the groove.

10. The wind turbine tower according to any preceding claim, wherein the groove has a depthof at least 100 mm.

11. The wind turbine tower according to any preceding claim, wherein the groove has a circumferential width of at least 200 mm adjacent the periphery of the tubular body.

12. The wind turbine tower according to any preceding claim, wherein the tubular body has a plurality of axially-extending grooves that project inwardly from the periphery of the tubular body and which are spaced apart around the tubular body.

13. The wind turbine tower according to any preceding claim,wherein the tubular body comprises a plurality of axially-extending arcuate or multifaceted panels,wherein each pair of adjacent acuate panels are connected by an axially-extending connection panel comprising a groove.

14. The wind turbine tower according to any preceding claim, wherein an axially extending rail (130) is provided within the groove (110) and projects outwardly.

15. The wind turbine tower according to claim 14, wherein the rail projects outwardly by no more than the depth of the groove.

16. The wind turbine tower according to claim 14, wherein the rail projects outwardly by more than the depth of the groove.

17. The wind turbine tower according to any one of claims 14 to 16, wherein the rail comprises an axially extending tubular rail member.

18. The wind turbine tower according to any one of claims 14 to 16, wherein the rail (130) has clamping faces (130S2) on opposed sides that are symmetric about a central plane (CP) extending outwardly from the tower body through the middle of the width (B) of the base of the rail at the tower body.

19. The wind turbine tower according to any one of claims 14 to 18, wherein the rail (130) comprises a plurality of tubular rail members (130A).

20. The wind turbine tower according to any preceding claim, wherein the tubular body has a plurality of axially-extending grooves that project inwardly from the periphery of the tubular body and which are spaced apart around the tubular body,wherein a rail is provided in each groove.

21. The wind turbine tower according to any preceding claim, wherein the tower comprises an upper tower portion (100U) and a lower tower portion (100L),wherein the upper tower portion comprises the tubular body, and the tubular body has a circular cross-section, andthe lower tower portion does not have a circular cross-section.

22. The wind turbine tower according to claim 21, wherein the upper tower portion (100U) comprises at least 40% of the height of the wind turbine tower.

23. The wind turbine tower according to claim 21 or claim 22, wherein the lower tower portion (100L’) comprises a lattice tower structure having an exposed framework.

24. The wind turbine tower according to any preceding claim, wherein the tower comprises a plurality of axially interconnected tower sections.

25. The wind turbine tower according to claim 24, wherein the rail comprises an axially extending tubular rail member comprising a plurality of axially interconnected rail sections, wherein the interconnected rail sections are connected together by pipe connectors.

26. A wind turbine tower section for interconnection with one or more further wind turbine tower sections for forming a wind turbine tower in accordance with claim 24 or claim 25.

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