Stabilized wind turbines, outrigger structures, and cable arrangements for stabilizing wind turbines

Abstract

A wind turbine (10) includes a tower (12) that defines a tower axis (24). The wind turbine (10) further includes an energy generating unit (14) disposed on the tower (12) and configured to produce electrical energy from wind (40). The tower (12) has a tower radius and a plurality of cables (20). Each cable (20) is anchored to the earth at one end and to an outrigger structure (18) at the opposing end. The outrigger structure (18) is coupled to the tower (12) and includes a plurality of tension / compression bars (60a, 60b, 62a, 62b, 64a, 64b) extending outwardly from the tower (12). At least one of the tension / compression bars (60a, 60b, 62a, 62b, 64a, 64b) has an outrigger apex (60c, 62c, 64c) at a cable interface radius from the tower axis (24). A cable (20) is coupled to the tension / compression bar (60a, 60b, 62a, 62b, 64a, 64b) proximate the outrigger apex (60c, 62c, 64c). A ratio of the cable interface radius to the tower radius is at least 1.2.

Description

[0001] STABILIZED WIND TURBINES, OUTRIGGER STRUCTURES, AND CABLE ARRANGEMENTS FOR STABILIZING WIND TURBINES

[0002] Technical Field

[0003] The invention relates generally to wind turbines, and more particularly to a system for stabilizing a wind turbine.

[0004] Background

[0005] 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 and an energy generating unit positioned atop of the tower. The energy generating unit typically includes a nacelle to house mechanical and electrical components, such as a generator, and a rotor operatively coupled to the components in the nacelle through a main shaft extending from the nacelle. The rotor, in turn, includes a central hub and a plurality of blades extending radially therefrom and configured to interact with the wind to cause rotation of the rotor. The rotor is supported on the main shaft, which is either directly or indirectly operatively coupled with the generator housed inside the nacelle. Consequently, as wind forces the blades to rotate, electrical energy is produced by the generator.

[0006] The tower elevates the energy generating unit above the earth and is typically a tubular steel structure. A base end of the tower is fixed to a concrete foundation, and the energy generating unit is coupled to an opposing end of the tower. The tower must have a bearing capacity capable of supporting its own weight, the weight of the energy generating unit, and dynamic loads during operation of the wind turbine. Transportation and / or production restrictions limit the tower size and thus bearing capacity. Yet, increasing the bearing capacity of the tower is desirable to increase the hub height and increase the electrical generating capacity of the wind turbine.

[0007] To increase the bearing capacity of the tower, the diameter of the tower may be increased. Increasing the diameter of the tower increases the bearing capacity by the power of two. That is, doubling the diameter of the tower quadruples the bearing capacity of the tower. Advantageously, tower stiffness also increases but by a power of three. So, increasing the diameter permits larger, more powerful energy generating units to be utilized.

[0008] While increasing the diameter is effective, there is an upper limit to the diameter of the tower. As identified above, production and / or transportation limits increases in diameter and increases in height of the tower. Rather than an increase in diameter to increase bearing capacity, the shell thickness or wall thickness may be increased. The drawback to increasing the shell thickness is that it is an inefficient way of increasing the bearing capacity and the stiffness as compared to increasing the tower diameter. That is, increasing thickness is not cost effective because bearing capacity and stiffness increase only linearly with the shell thickness. With transportation and / or production being a limiting factor for increasing bearing capacity, other solutions have been developed.

[0009] To address transportation and production limitations, some towers are composed of multiple tower sections coupled end-to-end at joints. A tower may be composed of two or more sections joined end-to-end. Even according to this solution, the diameter of each section is limited by transportation and / or production because of the overall dimensions of the sections or because of the weight of each tower section.

[0010] As an alternative or in addition to changing the dimensions of the tower, bearing capacity of a tower can be increased and the stresses in parts of the tower reduced by stabilizing the tower with cables or wires. The cables extend from anchors or foundations in the earth to points on the tower. The cables are tensioned and so improve stability by reducing tower oscillations produced by the wind. With this so- called guyed tower or cable stayed tower, certain loads are carried by the cables and so are not carried by the tower. Each tower section may therefore be constructed with a relatively smaller diameter, which is more economical to produce and / or transport, while also obtaining a predetermined tower height.

[0011] There are drawbacks to cable stayed towers. One drawback to a stayed tower is that it takes up more land. Land consumption must be accounted for during planning and especially during erection and maintenance of the wind turbine. Also, proper tensioning and attachment of the cables is crucial for the wind tower to withstand the varying and potentially large wind forces.

[0012] The wind turbine industry is searching for solutions to installation and attachment of cables at minimal cost while stabilizing wind turbine towers during energy production.

[0013] Summary

[0014] To these and other ends, embodiments of the invention are directed to stabilized wind turbines and outrigger structures for use with wind turbine towers. According to one embodiment a wind turbine includes a tower fixed at one end to a foundation. The tower defines a tower axis and has a tower radius. The stabilized wind turbine also has a plurality of cables. Each cable is anchored to the earth at one end and to the tower at an opposing end. An outrigger structure is coupled to the tower. The outrigger structure includes a plurality of tension / compression bars extending outwardly from the tower. At least one of the tension / compression bars of the plurality of tension / compression bars has an outrigger apex at a cable interface radius from the tower axis. At least one cable of the plurality of cables is coupled to the at least one tension / compression bars of the plurality of tension / compression bars proximate the outrigger apex. The wind turbine further includes an energy generating unit disposed on the tower and configured to produce electrical energy from wind. A ratio of the cable interface radius to the tower radius is at least 1.1.

[0015] In one embodiment, the at least one of the tension / compression bars of the plurality of tension / compression bars extends downwardly or extends upwardly to define an outrigger height. A ratio of the outrigger height hp to a cable interface radius rp is at least 1 / 3.

[0016] In one embodiment, the plurality of tension / compression bars includes at least one pair of tension / compression bars. The at least one pair of tension / compression bars forms a triangular portion in which individual ones of the tension / compression bars of the at least one pair of tension / compression bars extend outwardly from the tower and intersect to define the outrigger apex. In one embodiment, the plurality of tension / compression bars forms three, four, five, or six triangular portions. In one embodiment, adjacent triangular portions of three triangular portions meet at vertices. In that embodiment, the vertices are spaced circumferentially apart by 120°. In one exemplary embodiment, the vertices are at the tower radius.

[0017] In one embodiment, the at least one of the tension / compression bars of the plurality of tension / compression bars defines a plane. The plane intersects the tower axis at an angle from 25° to 60°. In one embodiment, the at least one cable extends to an anchor, and the at least one cable lies in the plane of the at least one tension / compression bar of the plurality of the tension / compression bars. In one embodiment, the at least one cable includes two cables of the plurality of cables. The two cables are coupled proximate the outrigger apex and extend in opposing directions to individual anchors. The two cables of the plurality of cables lie in the plane of the at least one tension / compression bar of the plurality of the tension / compression bars.

[0018] In one embodiment, the tower includes at least two tower sections including an upper section and a lower section. The outrigger structure is positioned between the upper tower section and the lower section of the at least two tower sections.

[0019] In one embodiment, the outrigger structure includes an assembly of three radial segments. Each radial segment includes a base segment. The at least one tension / compression bar of the plurality of tension / compression bars extends from a respective one of the base segments. The base segments collectively form an interface between the upper tower section and the lower section.

[0020] In one embodiment, the outrigger structure is configured to be secured to the tower and comprises a truss-like configuration. In one embodiment, the truss-like configuration includes a frame segment. The frame segment includes two top chords and a bottom chord. The two top chords intersect the bottom chord. The outrigger apex is proximate the intersection of at least one of the top chords and the bottom chord. In one embodiment, the outrigger structure includes an assembly of three frame segments. In one embodiment, the plurality of cables consists of three cables.

[0021] In one embodiment, the plurality of cables consists of six cables.

[0022] According to one aspect of the invention, there is an outrigger structure for use with a wind turbine tower having a tower radius and a plurality of cables. The outrigger structure includes a plurality of tension / compression bars. When the outrigger structure is coupled to the tower, at least one of the tension / compression bars of the plurality of tension / compression bars extends outwardly from the tower. The at least one tension / compression bar of the plurality of tension / compression bars has an outrigger apex at a cable interface radius. A ratio of the cable interface radius to the tower radius is at least 1.1. The outrigger structure is configured to receive at least one cable of the plurality of cables proximate the outrigger apex.

[0023] In one embodiment, when the outrigger structure is coupled to the tower, the at least one of the tension / compression bars of the plurality of tension / compression bars is configured to extend downwardly to define an outrigger height. A ratio of the outrigger height to the tower radius is at least 1 / 3.

[0024] In one embodiment, the plurality of tension / compression bars includes at least one pair of tension / compression bars and the at least one pair of tension / compression bars forms a triangular portion. When the outrigger structure is coupled to the tower, individual ones of the tension / compression bars of the pair of tension / compression bars extend outwardly from the tower and intersect to define the outrigger apex.

[0025] In one embodiment, the plurality of tension / compression bars forms three triangular portions.

[0026] In one embodiment, adjacent triangular portions of the three triangular portions meet at vertices. The vertices are spaced circumferentially apart by 120°.

[0027] In one embodiment, when outrigger structure is coupled to the tower, the vertices are configured to be at the tower radius. In one embodiment, the at least one of the tension / compression bars of the plurality of tension / compression bars defines a plane. When the outrigger structure is coupled to the tower, the plane intersects the tower axis at an angle from 25° to 60°.

[0028] In one embodiment, the tower includes at least one tower section. The outrigger structure is configured to be positioned on an upper end of the at least one tower section.

[0029] In one embodiment, the outrigger structure further includes an assembly of three radial segments. Each radial segment includes a base segment. The at least one tension / compression bar extends from a respective one of the base segments. The base segments collectively form an interface configured to be coupled to the upper end of the at least one tower section.

[0030] In one embodiment, the outrigger structure is configured to the secured to the tower and comprises a truss-like configuration.

[0031] In one embodiment, the truss-like configuration comprises a frame segment including two top chords and a bottom chord. The two top chords intersect the bottom chord. The outrigger apex is proximate an intersection of at least one of the top chords and the bottom chord.

[0032] Brief Description of the Drawings

[0033] 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.

[0034] Fig. 1 is a perspective view of a wind turbine having a cable stayed tower according to one embodiment of the invention;

[0035] Fig. 2 is a schematic perspective view of a tower section of a wind turbine tower having three cables; Fig. 3 is a schematic perspective view of a tower section of a wind turbine tower having six cables;

[0036] Fig. 4 is a static load diagram at a cable interface of the tower section shown in Fig. 2;

[0037] Fig. 5 is a static load diagram at a cable interface of the tower section shown in Fig. 3;

[0038] Fig. 6 is a static load diagram in perspective view of the tower section shown in Fig. 2;

[0039] Fig. 7 is a static load diagram in perspective view of the tower section shown in Fig. 3;

[0040] Fig. 8 is a schematic plan view of an outrigger structure on a tower section with three cables according to one embodiment of the invention;

[0041] Fig. 9 is a schematic plan view of an outrigger structure on a tower section with six cables according to one embodiment of the invention;

[0042] Fig. 10 is a perspective schematic view of the outrigger structure of each of Figs. 8 and 9;

[0043] Fig. 11 is a perspective schematic view of the outrigger structure of Fig. 8 for use with three cables (only one cable is shown) according to one embodiment of the invention;

[0044] Fig. 12 is a perspective schematic view of the outrigger structure of Fig. 9 with six cables (only four cables are shown) according to one embodiment of the invention;

[0045] Fig. 13 is a plan view of an outrigger assembly according to one embodiment of the invention; Fig. 14 is an elevation view of a portion of the outrigger assembly of Fig. 13;

[0046] Fig. 15 is a plan view of an outrigger assembly according to one embodiment of the invention;

[0047] Fig. 16 is an elevation view of the outrigger assembly of Fig. 15; and

[0048] Fig. 17 is a collection of charts of hub height versus required torsional stiffness for four different tower diameters.

[0049] Fig. 18 is a graph of cable interface radius (rF) to tower radius (rT) ratios versus ovalization load ratio of pretension load for 4 cable / outrigger configurations.

[0050] Detailed Description

[0051] Exemplary embodiments of the invention are directed to improved cable stayed wind turbines, particularly with respect to structures by which cables are attached to a wind turbine tower and the arrangement of anchors for tethering the cables to the earth. To those and other ends, with reference to Fig. 1 , an exemplary cable stayed wind turbine 10 includes a tower 12 and an energy generating unit 14 disposed at the apex of the tower 12. The tower 12 may be coupled to a foundation 16 at a lower end thereof. The exemplary tower 12 shown is modular and includes three sections 12a, 12b, and 12c. The cable stayed wind turbine 10 also includes an outrigger structure 18 (described in detail below) between section 12a and section 12b. The outrigger structure 18 serves as a point of attachment for cables 20 to the tower 12. The cables 20 are secured to anchors 22 in the earth. The tower 12 may be referred to as a cable stayed tower. The turbine 10 including the outrigger structure 18 in combination with cables 20 and anchors 22 are an effective stabilization system while also providing additional advantages not before known (described below). The three sections 12a, 12b, 12c, and outrigger structure 18 (when installed between sections 12a, 12b, 12c) collectively define a generally vertical tower axis 24 about which the energy generating unit 14 may rotate via a yaw mechanism (not shown). Each tower section 12a, 12b, 12c may have a tubular configuration in which a sidewall having a tower radius r? (see, e.g., Figs. 8, 9, and 10) generally defines a right circular cylinder of each tower section 12a, 12b, 12c. The foundation 16 may be a relatively large mass (e.g., concrete, anchor cage, etc.) embedded in the ground and through which forces on the wind turbine 10 may be ultimately transferred. Although not shown, in an alternative embodiment, the foundation 16 may include an offshore platform or the like used in offshore wind turbine applications. The anchors 22 may be a portion of the foundation 16 or be separate anchor structures from the foundation 16. These anchors 22 may be in a predetermine arrangement (described below) around the tower 12 to enhance the torsional strength of the wind turbine 10. The tower 12 supports the weight of the energy generating unit 14 and operates to 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.

[0052] In that regard, the energy generating unit 14 transforms the energy of the wind into electrical energy. The energy generating unit 14 typically includes a housing or nacelle 26, a rotor 30 having a central hub 32 and a plurality of blades 34 (e.g., three blades) mounted to the central hub 32 and extending radially therefrom, and a generator (not shown) for converting mechanical energy into electrical energy. The energy generating unit 14 may further include a drive train (not shown), including a gear arrangement, interconnecting the rotor 30 and the generator. The generator and a substantial portion of the drive train may be positioned inside of the nacelle 26 of the wind turbine 10. In addition to the generator, the nacelle 26 typically houses miscellaneous components required for converting wind energy into electrical energy and various components needed to operate, control, and optimize the performance of the wind turbine 10. The wind turbine blades 34 are configured to interact with the wind. As is shown in Fig. 1 , wind 40 (shown as arrows) produces lift and causes the rotor 30 to spin or rotate generally within a plane defined by the wind turbine blades 34. The energy generating unit 14 generates power from the airflow 40 that passes through the swept area of the rotor 30. In addition to generating power, the airflow 40 produces dynamic loads on the wind turbine 10. The dynamic loads and the static loads from the dead weight of the energy generating unit 14 must be borne by the tower 12 as supported by the cables 20. With reference to Fig. 1 , the outrigger structure 18 is secured between the tower section 12a and the tower section 12b. Cables 20 are coupled to the outrigger structure 18 so as to transmit static and dynamic loads on the wind turbine 10 to the cables 20. These loads are then distributed to the earth via the anchors 22. For instance, the outrigger structure 18 distributes at least a portion of the dynamic loads from movement of the tower 12 due to the wind 40 and rotation of the rotor 30 to the cables 20. The offloading of those loads from the tower 12 to the cables 20 and to the earth via anchors 22 is advantageous. As an example, the tower 12 may be redesigned with the diameter of the tower being reduced as compared to a tower without cables while maintaining a necessary stiffness for operation. Other improvements are also possible and may include increasing the height of the tower and / or placing a larger rotor / more powerful energy generating unit on the tower than would otherwise be permissible.

[0053] More specifically, once anchored and properly tensioned, the cables 20 permit shared loading of bending moments in the tower 12. From an attachment point of the cable 20 on the outrigger structure 18 and downward, bending moments are significantly reduced. From a different perspective, the torsional stiffness of the tower 12 is reinforced by connection of the cables 20 and anchors 22 to the tower 12. That is, a system of the tower 12, cables 20, and anchors 22 has a greater torsional stiffness than the tower 12 alone. Embodiments of the invention are not limited to the location of the outrigger structure 18 and cables 20 / anchors 22 shown in the figures. For example, while the outrigger structure 18 is shown in Fig. 1 between tower sections 12a and 12b, the outrigger structure 18 may be inserted between any two sections of a tower. The outrigger structure 18 may be inserted between each adjacent pair of tower sections 12a and 12b and 12b and 12c. As a further example, the outrigger structure 18 may be positioned at an interface between the tower 12 and an energy generating unit 14 (as long as the cables 20 are arranged to avoid interference with the blades 34). As yet a further example, the outrigger structure 18 may be coupled at any elevation along the height of the tower 12. Thus, the outrigger structure 18 is not limited to being inserted between tower sections 12a, 12b, 12c. Although six cables 20 are shown in Fig. 1 , embodiments of the invention are not limited to six cables. Specifically, embodiments of the invention include more than six cables, for example, eight, ten, or twelve cables, or less than six cables, for example, three, four, or five cables. However, using a larger number of cables, means that each cable may be relatively smaller, which advantageously eases transportation and handling of the cables themselves while reducing the cost of transportation and handling.

[0054] The wind turbine 10 according to the exemplary embodiments are advantageous relative to prior art cable stayed towers in at least two aspects. The improvements include a reduction in, or elimination of, ovalization loads during construction and an improvement in torsional stiffness of the tower 12. Specifically, Applicant identified problems with cable stayed towers not before known and then identified exemplary solutions to those problems, including the outrigger structure 18 and the wind turbine 10 having a cable arrangement with the outrigger structure 18 shown, for example, in Fig. 1 and further described below.

[0055] The problems identified are described with respect to Figs. 2-7. Generally, ovalization loads according to prior cable stayed tower designs cause tower construction issues, specifically during assembly of towers having multiple tower sections. Coupling of the cables to the tower sections is necessary to stabilize the tower during construction. Typically, once one or more tower sections is installed, cables are attached to an interface at the upper-most end of the erected tower portion. The cables are then pretensioned. Coupling and pretensioning the cables stabilizes the partly constructed tower, which aids subsequent construction of the wind turbine. For example, during construction, once two tower sections are assembled end-to-end, an interface is attached to an upper most end of the highest tower section. Cables are attached to the interface and pretensioned. The pretensioned cables stabilize the two tower sections against movement and permit one or more additional sections to be assembled on the interface to complete construction of the wind turbine tower. However, pretensioning the cables coupled to the interface may elastically deform the interface. In other words, the force on the interface from the cables may cause the interface to be bent out of round. For example, whereas prior to pretension of the cables, the interface is circular, after tensioning the cables, the interface is oval or another non-circular shape. This distortion at the interface is referred to as ovalization. If the interface at which the next tower is to be placed is deformed from design, coupling the next tower section, which is undistorted, to the distorted interface is problematic. For example, distortion at the interface causes misalignment of bolt holes of the adjacent sections by which the sections are bolted together. As such, insertion of bolts through the bolt holes is made difficult and installation of the wind turbine is more time-consuming and costly.

[0056] The forces tending to cause ovalization are illustrated with reference to the schematic diagrams of Figs. 2 and 3, which depict a cable stayed tower section 44 having a height h and with a cable arrangement 46 having three cables 42 attached and a cable arrangement 50 having six cables 42 attached, respectively. Each cable 42 is shown directly coupled to an interface (not shown) at the upper end of the tower section 44. For the purposes of calculations below with reference to Figs. 2-7, a radius of the interface r is assumed to be equal to a tower radius iy In Fig. 2, the cable 42 is connected in a straight line between a tower axis 52 and an anchor 54. The cables 42 are coupled to the interface at three locations 56, which are spaced 120° apart around the circumference of the interface. Each cable 42 is coupled to the tower 44 at one location 56 and to an anchor 54 coupled to the earth. The cable 42 extends radially outwardly and downwardly at a cable angle 0Gmeasured from the attachment location 56. The cable 42 lies in a plane coincident with an anchor radius and the tower axis 52. Fig. 3 differs slightly in that pairs of cables 42 are shared by three anchors 54. And, different pairs of cables 42 attach to the tower 44 at three locations 56. The attachment locations 56 at the tower 44 are rotated 60° around the circumference of the tower 44 from a plane coincident with the anchor radius TA and the tower axis 52. The cables 42 therefore do not reside in the plane of the anchor 54 and the tower axis 52.

[0057] Referring now to Figs. 4 and 5, basic torsional stiffness of the system kewith cable arrangements 46 and 50 is shown in Figs. 2 and 3, respectively. With reference to Figs. 2 and 4, torsional stiffness kewith the cable arrangement 46 is shown to be proportional to an interface radius r (which is assumed to be approximately the same as the tower radius iy of Figs. 2 and 3), a force Fpproduced by the cables 42 on the tower 44, which is approximately equal to a pretension force FPR in the cables 42, and a number n of cables 42 as follows: keoc nFPr (1 ) Per (1 ), the stiffness k of the cables 42 does not contribute to system torsional stiffness kein Figs. 2 and 4. That is, the length of the cables 42 is not changed by rotating the tower 44 over small angles that are anticipated during tower construction.

[0058] By contrast, with reference to Figs. 3 and 5, torsional stiffness of the system kewith the cable arrangement 50 is basically proportional to the stiffness k of the cables 42, the interface radius r squared for the condition of a 120° angle between the cable 42 and radius r, and the number n of cables 42 as follows: k(joc nkr2(2)

[0059] In (2), the force Fp on the tower 44, which may only be a very small component of FPR, is ignored to focus on variables that have greatest influence on system torsional stiffness ke. More specifically, by way of example and with reference to Fig. 5, when the cables 42 are attached at locations 56 and extend to the anchors 54 at a 30° angle with respect to a plane tangent to the tower 44 at location 56, the force Fp from the two cables 42 is 2 FPRsin 30° = FP, where FPR is the pretension force in the cables 42. Only a component of the pretension force FPR in the cable 42 may be perpendicular to a wall of the tower 44. Only that component of the pretension force FPR in the cable 42 contributes to the radial force Fpon the tower 44. As a result, the force Fp in the radial direction in Figs. 3 and 5 is less (e.g., one-half in the arrangement 50) than the sum of the pretension force FPR in the cables 42. The orientation of cables 42 relative to the tower 44 can change the stiffness k contribution of the cables 42 to the torsional stiffness of the system ke. In one instance, the orientation of the cables 42 reduces the ovalization force on the tower 44. For example, when the cables 42 are oriented at 90° with respect to r, ovalization force from the cable 42 is zero. As the angle of the cable 42 relative to the tower 44 at location 56 increases from 90° (e.g., Fig. 5), ovalization forces increase and the stiffness contribution k of the cable 42 is reduced with the square of sine of the angle tangent to the tower 44. With respect to the orientation of cables 42 relative to the tower 44, the cable arrangements 46 and 50 are further shown schematically in perspective view in Figs. 6 and 7, in which, additional detail is added to the above relationships. For one, in Figs. 6 and 7, because the cable 42 is coupled to the tower 44 at an elevated location at one end and to the anchor 54 in the earth at the other, the cable 42 forms the cable angle 0Gwith the tower 44. Figs. 6 and 7 show cable angle 0Gin the system ke. As such, for the cable arrangement 46 in Fig. 6, the torsional stiffness of the system is: ke= nFPr sin 0G(3)

[0060] Again, n is the number of cables 42, Fpis the pretension force in the cables 42, and r is the interface radius. In Fig. 7, in addition to the cable angle 0G, the torsional stiffness of the system keis calculated with a rotation angle 0Fbetween the location 56 at which the cable 42 is coupled to the tower 44 and a plane intersecting the tower axis 52 and the anchor 54. With that, (2) is further defined as: ke« n[FPr cos 0Fsin 0G+ kr2sin20Fsin20G] (4)

[0061] With the basic proposition that (3) describes Fig. 6 and (4) describes Fig. 7, it was identified that increasing the interface radius r in (3) and in (4) will increase the torsional stiffness keof each arrangement 46 and 50. As a distinction between interface radius r and tower radius iy, it is possible to increase the torsional stiffness by increasing the tower radius iy (see Figs. 2 and 3). However, there is a physical limit imposed on the tower dimension (i.e., diameter) by transportation of the tower sections of larger diameter and increasing the tower diameter is also limited by cost.

[0062] In view of the above, exemplary embodiments shown in Figs. 1 , 8, 9, and 10 increase the interface radius from a value of r, essentially the tower radius, to some larger value designated rp in Figs. 8, 9, and 10. Increasing the interface radius rp is achieved with embodiments of the outrigger structure 18. The outrigger structure 18 permits an increase in the radius at which the cables 20 are coupled independent of the radius r? of the tower 12. The interface radius rp is therefore larger than the tower radius r?. The cables 20 are coupled to the outrigger structure 18 at a location proximate the interface radius rp, and the cables 20 are arranged relative to the outrigger structure 18. According to embodiments of the invention, stiffness of the system 10 is increased while ovalization problems during construction are reduced. As shown in the exemplary embodiment of Fig. 1 , the outrigger structure 18 is between tower section 12a and tower section 12b. However, the outrigger structure 18 is not limited to being between sections 12a and 12b. Rather, the outrigger structure 18 may be secured directly to the tower 12 at any location, such as by bolting or welding the outrigger structure 18 to the tower 12. See, for example, the exemplary outrigger structure 18 shown in Figs. 15 and 16 and described below.

[0063] Generally, with reference to Figs. 8, 9, and 10, the outrigger structure 18 has an overall triangular configuration with three triangular portions 60, 62, 64. In an exemplary embodiment, the triangular portions 60, 62, 64 each include corresponding pairs of tension / compression bars 60a, 60b and 62a, 62b and 64a, 64b, respectively. Individual tension / compression bars 60a, 60b and 62a, 62b and 64a, 64b extend outwardly and downwardly (Fig. 10) from vertices 66 toward the earth. Although not shown, the tension / compression bars 60a, 60b, 62a, 62b, 64a, 64b may extend outwardly and upwardly in accordance with exemplary embodiments of the invention. As shown, the vertices 66 are adjacent the tower 12 at approximately the tower radius r?. In the exemplary embodiment, the outrigger structure 18 meets the tower 12 at three vertices 66, which are spaced circumferentially apart at 120° from one another around the circumference of the tower 12. While three triangular portions 60, 62, 64 are shown, embodiments of the invention are not limited three. Other arrangements are contemplated. By way of example only, and not limitation, the outrigger structure 18 may include four triangular portions. Other exemplary arrangements include five and six triangular portions with two cables per triangular portion. Thus, for four triangular portions there may be eight cables coupled to four anchors, for five triangular portions there may be 10 cables coupled to five anchors, and for six triangular portions there may be 12 cables coupled to six anchors.

[0064] Pairs of tension / compression bars 60a and 60b, 62a and 62b, 64a and 64b extend outwardly from respective vertices 66 to intersect at outrigger apex 60c, 62c, 64c, respectively. In the exemplary embodiment, the outrigger apices 60c, 62c, 64c each define a cable interface radius r / = from the vertical tower axis 24 to the corresponding apex 60c, 62c, 64c. As shown in Figs. 8 and 9, cables 20 are coupled to the outrigger structure 18 proximate the outrigger apices 60c, 62c, 64c.

[0065] In Fig. 8, in one exemplary embodiment, a cable arrangement 70 includes three cables 20 attached to the outrigger structure 18 with one cable 20 attached to each of the apices 60c, 62c, and 64c. Each cable 20 extends radially outwardly and downwardly in a plane coincident with the cable interface radius rp. In Fig. 9, in one exemplary embodiment, a cable arrangement 72 includes six cables 20 attached to the outrigger structure 18 with two cables 20 attached to each of the apices 60c, 62c, and 64c. Unlike the cables 20 in the cable arrangement 70 of Fig. 8, the cables 20 in the arrangement 72 do not extend outwardly and downwardly in the plane coincident with the cable interface radius rp.

[0066] Applicant identified a relationship between the configuration of the outrigger structure 18 and the forces on the tower 12. Without being bound by theory, an angle 6B, which represents an angle between each tension / compression bar 60a, 60b, 62a, 62b, 64a, 64b and the tower radius iy at the vertices 66, is determined by the cable interface radius rp relative to the tower radius i As these dimensions change relative to one another, the angle 0Bchanges. In turn, the cable interface radius rp is determined by the dimensions of the triangular portions 60, 62, 64, particularly the tension / compression bars 60a, 60b, 62a, 62b, 64a, 64b. Noting this relationship, in one exemplary embodiment, the outrigger structure 18 has a cable interface radius rp at each apex 60c, 62c, and 64c that is twice the tower radius iy (not shown). With this relationship, the angle 0Bequals 90° and the angle 0Fequals 60°. Advantageously, with the cable interface radius rp being twice the tower radius iy, loads from the cable 20 in either arrangement 70 and 72 on the outrigger structure 18 cannot form a radial load on the tower 12. With all loads from the cable 20 being transferred to the tower 12 in a predominately tangential direction, ovalization loads are reduced or eliminated. In accordance with these relationships, the outrigger structure 18 is then dimensioned in accordance with the dimensions (e.g., radius) of the tower 12 to reduce or to eliminate ovalization during construction. To that end, embodiments of the invention are not limited to the cable interface radius rp being twice the tower radius r-r. As an example, a ratio of / p / ry and ovalization loads is shown in Fig. 18 for a plurality of cable arrangements. In the example, a ratio of rp / rr of 2 for an “oval ratio - 3 / 6 cables” eliminates ovalization loads on the tower 12. That is, ovalization load ratio of pretension load is zero. Other rp / rr ratios result in an ovalization load ratio of pretension load of zero for different cable arrangements. As shown in Fig. 18, adding cables to the system permits a reduction in the rp / rr ratio needed to eliminate the ovalization load ratio of pretension load. For example, for an oval ratio of 4 / 8 cables, in which there are four or eight cables attached to four apices, the rp / rr ratio that produces no ovalization load is about 1 .4. For an oval ratio of 5 / 10 cables, the rp / rr ratio that produces no ovalization load is about 1 .25. And, for an oval ratio of 6 / 12 cables, the rp / rr ratio that produces no ovalization load is about 1.2.

[0067] With continued reference to Fig. 18, embodiments of the invention are not limited to elimination of ovalization loads. Rather, there is a range of ovalization loads that are acceptable as not causing undue construction difficulties. These ovalization loads may not produce measurable deformation of the tower or the deformation observed is insufficient to prevent alignment and assembly of tower sections. The range of permissible ovalization loads may be selected based on the tower, which may be dependent on the structural features of the tower itself, any construction limitations, and the final pretension magnitude needed to at least prevent the cables from not going slack in any single operational and / or idle condition of the wind turbine. For example, with reference to Fig. 18, with an oval ratio - 3 / 6 cables arrangement, a rp / rr of at least 1.1 may sufficiently reduce ovalization loads. As shown, this relationship produces less than 0.5 ovalization load ratio of pretension load on the tower 44. As an additional example, a rp / rr of greater than 1.1 reduces ovalization loads. In one example, a 50% relative reduction in the ovalization load ratio of pretension load is achieve at about rp / rr of 1.4 and greater. That is, at rp / rr of 1.4, the ovalization load ratio of pretension load is about 0.25. As yet another example for an oval ratio - 3 / 6 cables arrangement, a range in the ratio rp / rr of from 1.6 to 2.7 may sufficiently eliminate ovalization loads on the tower. With reference to Fig. 10, in the exemplary embodiment, each triangular portion 60, 62, 64 points toward the ground from the respective vertices 66. This relationship creates an elevation difference between the vertices 66 and the apices 60c, 62c, and 64c that is determined by a length LB of the tension / compression bars 60a, 60b, 62a, 62b, 64a, 64b and an angle 0GA. The elevation difference is referred to as an outrigger height hp and represents a height dimension of the outrigger structure 18. By way of example only and not limitation, in one embodiment, a ratio of outrigger height hp to interface radius rp is from 1 / 3 to 3. By way of further example, the outrigger height hp is at least equal to the tower radius t .

[0068] The elevation difference between vertices 66 and the apices 60c, 62c, and 64c (i.e., the outrigger height hp) requires an orientation of each of the triangular portions 60, 62, 64 in a plane at an angle 0GArelative to the tower axis 24. By way of example only and not limitation, the angle 0GAis from 25° to 60°. By way of further example, the angle 0GAis from 30° to 60°. In addition to the height of the tower 12, the number and arrangement of the cables 20 coupled to the outrigger structure 18 changes each of the outrigger height hp, the length LB of the tension / compression bars 60a, 60b, 62a, 62b, 64a, 64b, and the angle 0GA.

[0069] More specifically, referring to Figs. 11 and 12, in the exemplary embodiments, for each of the cable arrangements 70 and 72, the cables 20 lie in a plane of the tension / compression bars 60a, 60b, 62a, 62b, 64a, 64b. That is, the cables 20 lie in a plane at angle 0GArelative to the tower axis 24. To maintain this relationship in each the cable arrangements 70 and 72, the outrigger structure 18 is different in Fig. 11 relative to the outrigger structure 18 in Fig. 12 in that the angle 0GAof the outrigger structure 18 changes according to the cable arrangement. Stated another way, the angle 0GAfor the cable arrangement 70 is different from the angle 0GAof cable arrangement 72. For example, the cable arrangement 70 (i.e., three cables, Fig. 11 ), angle 0GAis greater than the angle 0GAof the cable arrangement 72 (i.e., six cables, Fig. 12). As a consequence of the difference in angle 0GA, the interface height hp of the outrigger structure 18 of the cable arrangement 72 is greater than the interface height hp of the outrigger structure of the cable arrangement 70. The interface height hp may be determined by:

[0070] In (5), rp is the cable interface radius (i.e., distance from the tower axis 24 to the apices 60c, 62c, and 64c), r? is the tower radius, 0Fis the angle between the tower radius iy at an intersection of the tower radius iy with a vertex 66 and the cable interface radius rp, and 0GAis the angle of a plane defined by respective apices 60c, 62c, and 64c and vertices 66 with the tower axis 24. From a relative dimension of the outrigger structure 18 perspective, as applied to the cable arrangements 70 and 72, the outrigger structure 18 of the cable arrangement 72 requires longer tension / compression bars 60a, 60b, 62a, 62b, 64a, 64b than the tension / compression bars 60a, 60b, 62a, 62b, 64a, 64b of the outrigger structure 18 of the cable arrangement 70. The outrigger structure 18 and the cable arrangement 72 results in relatively higher compression loads than the structure 18 and cable arrangement 70. In other words, the outrigger height hp introduces a moment at the installed location of the outrigger structure 18 on the tower 12. The moment is proportional to a horizontal force provided by the cables 20 from a tower displacement. The greater the outrigger height hp, the greater the moment and that moment is transferred through push and pull forces in the tension / compression bars 60a, 60b, 62a, 62b, 64a, 64b.

[0071] With further refinement of (4) in view of Fig. 12, the torsional stiffness of the system keshown is: ke= n [cos(0F+ dA)FPsin 0Gr + kr2sin2(0F+ dA) sin20G(6)

[0072] In (6), 0Ais an angle between a first line and a second line defined in the system 72 and shown in Fig. 12. The first line is labeled rpA in Fig. 12 and is defined by the cable 20 from a respective one of the apices 60c, 62c, and 64c to a corresponding anchor 22 as the cable 20 is projected onto a plane perpendicular to the tower axis 24. The second line is labeled in Figs. 11 and 12 and is also in the plane perpendicular to the tower axis 24 and extends from the tower axis 24 to the same anchor 22. Equation (6) applies to each of the cable arrangements 70 and 72. As an example, the torsional stiffness of the cable arrangement 70 (three cables shown in Fig. 11) is calculated in (6) when 0Fequals zero, 0Aequals zero, and n equals three.

[0073] According to the above expressions, exemplary embodiments of the outrigger structure 18 are shown in Figs. 13 and 14 and in Figs. 15 and 16. In Figs. 13 and 14, an exemplary embodiment of the outrigger structure 18 is configured to form an interface between two tower sections, such as between the tower section 12a and the tower section 12b of Fig. 1. With reference to Fig. 13, the exemplary outrigger structure 18 is an assembly 80 of individual, separate radial segments 82, 84, 86. When the radial segments 82, 84, 86 are attached at an end of the tower 12, they collectively form the outrigger structure 18.

[0074] Each radial segment 82, 84, 86 defines an individual one of the triangular portions 60, 62, and 64 with respective tension / compression bars 60a, 60b, 62a, 62b, 64a, 64b. The tension / compression bars 60a, 60b, 62a, 62b, 64a, 64b extend from a respective base segment 90, 92, 94 to a respective apices 60c, 62c, and 64c. Each base segments 90, 92, and 94 is configured to be bolted to the tower 12, such as to the tower section 12b (Fig. 1 ), at an upper end. In that regard, each base segment 90, 92, and 94 lies in a plane of the tower radius r? at the upper end of the tower section 12b. The base segments 90, 92, 94 collectively form an interface 88 of the outrigger structure 18, which receives an additional tower section 12a. The interface 88 is therefore captured between the tower section 12a and the tower section 12b and so forms part of the tower 12.

[0075] With reference to Fig. 14, the tension / compression bars 60a, 60b, 62a, 62b, 64a, 64b do not lie in the plane of the base segment 90, 92, and 94. Rather, the bars 60a, 60b, 62a, 62b, 64a, 64b extend outwardly and downwardly at the angle 0GA, which as described above, and vary according to the height and diameter of the tower 12 and the number and arrangement of cables 20. At the respective apices 60c, 62c, and 64c, the triangular portions 60, 62, 64 are configured to receive end portions of the cables 20, such as via through bores 96. By way of example only, in the exemplary outrigger structure 18 shown in Figs. 13 and 14, radial segments 82, 84, 86 are each monolithic steel segments, which may be manufactured by machining a steel plate of desired thickness and then bending the tension / compression bars 60a, 60b, 62a, 62b, 64a, 64b relative to the corresponding base segments 90, 92, 94 to the angle 0GA. Advantageously, because the radial segments 82, 84, 86 form the interface 88 a pinned connection is avoided. Loads alternating between tension and compression on the outrigger structure 18 are efficiently transferred to the cables 20 and so the connection between the outrigger structure 18 and the tower 12 is less likely to fail.

[0076] Referring now to Figs. 15 and 16, in an exemplary embodiment, the outrigger structure 18 is configured to be bolted to the tower 12. The outrigger structure 18 may therefore be placed at any elevation along the tower 12, because it does not form an interface between any two of the tower sections 12a, 12b, 12c. To that end, the exemplary outrigger structure 18 is an assembly 100 of three frame segments 102, 104, 106. When the frame segments 102, 104, 106 are attached to the tower 12, such as by bolting, pinning, or welding, they collectively form the outrigger structure 18. As such, the frame segments 102, 104, 106 define the triangular portions 60, 62, 64 when the assembly 100 is assembled. In the exemplary embodiment, the frame segments 102, 104, 106 each define one half of each triangular portion 60, 62, 64. For example, the frame segment 102 defines one half of each of the triangular portions 60 and 62. Similarly, the frame segment 104 defines one half of each of triangular portions 62 and 64. Lastly, the frame segment 106 defines one half of each of triangular portions 60 and 64. With the frame segments 102, 104, 106 defining a corresponding one half of the triangular portions 60, 62, 64, each frame segment 102, 104, 106 includes corresponding ones of tension / compression bars 60a, 60b, 62a, 62b, 64a, 64b.

[0077] With reference to Fig. 16, each frame segment 102, 104, 106 has a truss-like configuration. For the frame segment 104, shown in Fig. 16, the tension / compression bars 62b and 64a each form a top chord 110 and 112, respectively. The top chords 110 and 112 intersect one another at vertex 66. In addition, a bottom chord 114 extends from one apex 60c, 62c, and 64c to an opposing apex 60c, 62c, and 64c. For example, in Fig. 16, the bottom chord 114 extends from apex 62c to apex 64c. At each apex 60c, 62c, and 64c, a flange 120 extends outwardly and provides a location, such as a through bore 96, to which a cable 20 is coupled. In the exemplary embodiment, a web 116 is shown extending from the vertex 66 to the bottom chord 114. While the remaining frame segments 102 and 106 are not specifically described, each has the same configuration as the frame segment 104 described with reference to Fig. 16.

[0078] With this configuration, the individual frame segments 102, 104, 106 form triangular structures in both the vertical direction and in the horizontal direction when assembled. The frame segments 102, 104, 106 may be constructed from steel plates which may be bent along a direction of the web 116 to define vertices 66. The segments 102, 104, 106 are then secured to the tower 12 at the vertex 66 and / or at the bottom chord 114. Advantageously, the assembly 100 transfers bending loads on the tower 12 to the cables 20. In view of the truss-like configuration, the tension / compression bars 60a, 60b, 62a, 62b, 64a, 64b may not be angled according to the angle 0GA. As an advantage, the cables 20 may be assembled to the outrigger structure 18 prior to attaching the outrigger structure 18 to the tower 12. In that regard, the cables 20 may be attached while the structure 18 is at ground level. The assembly of cables and the structure 18 may then be hoisted into position at the desired elevation on the tower 12 and then secured to the tower 12.

[0079] With reference to Fig. 17, it is possible, based on the above, to determine a ratio rp / rr for an outrigger structure 18 needed to achieve a selected torsional stiffness for a system. In Fig. 17, rp / rr ratio curves for selected tower dimensions are shown. For example, a ratio rp / rr for a tower diameter D of 4.0 m and shell thickness t of 20 mm is shown in the upper left graph. For D = 4.0 m and t = 20 mm, required torsional stiffness may be determined for a hub height and outrigger structure 18 having a set rp. For example, at a hub height of 250 m, a required torsional stiffness of 450 MNm is achieved at a rp / rr of 3. In this example then, the interface radius rp of the outrigger structure 18 would need to be at least 6 m (i.e., rr = 2 m therefore 3 (rp / rr) x 2 (rr)= 6) to provide the requisite torsional stiffness. Other tower diameters and shell thickness combinations shown in Fig. 17 include D = 4.3 m, t = 20 mm; D = 4.6 m, t = 20 mm; and D = 4.6 m, t = 20 mm. Ratios rp / r of 1 , 2, 3, and 4 are shown. Also shown is “0” indicating a tower without an interface and cables. Fig. 17 does not represent calculations for determining pretension forces during construction and so does not address ovalization. However, ovalization is addressable according to the identified solutions described herein. As such, the outrigger structure 18 and cable arrangements 70 and 72 advantageously address both a targeted stiffness for the system while simultaneously addressing ovalization.

[0080] 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

CLAIMS1 . A wind turbine (10), comprising: a tower (12) fixed at one end to a foundation (16) and defining a tower axis (24) and having a tower radius r , a plurality of cables (20), each cable (20) of the plurality of cables (20) being anchored to the earth; an outrigger structure (18) coupled to the tower (12), the outrigger structure (18) comprising a plurality of tension / compression bars (60a, 60b, 62a, 62b, 64a, 64b) extending outwardly from the tower (12), at least one of the tension / compression bars (60a, 60b, 62a, 62b, 64a, 64b) of the plurality of tension / compression bars (60a, 60b, 62a, 62b, 64a, 64b) having an outrigger apex (60c, 62c, 64c) at a cable interface radius rp from the tower axis (24), at least one cable (20) of the plurality of cables (20) being coupled to the at least one tension / compression bars (60a, 60b, 62a, 62b, 64a, 64b) of the plurality of tension / compression bars (60a, 60b, 62a, 62b, 64a, 64b) proximate the outrigger apex (60c, 62c, 64c); and an energy generating unit (14) being disposed on the tower (12) and configured to produce electrical energy from wind (40), wherein a ratio of the cable interface radius rp to the tower radius r? is at least 1.1.

2. The wind turbine (10) of claim 1 , wherein the at least one of the tension / compression bars (60a, 60b, 62a, 62b, 64a, 64b) of the plurality of tension / compression bars (60a, 60b, 62a, 62b, 64a, 64b) extends downwardly or extends upwardly to define an outrigger height hp, and wherein a ratio of the outrigger height hp to the cable interface radius rp is at least 1 / 3.

3. The wind turbine (10) of any preceding claim, wherein the plurality of tension / compression bars (60a, 60b, 62a, 62b, 64a, 64b) includes at least one pair of tension / compression bars (60a and 60b, 62a and 62b, 64a and 64b) and the at least one pair of tension / compression bars (60a and 60b, 62a and 62b, 64a and 64b) forms a triangular portion (60, 62, 64) in which individual ones of thetension / compression bars (60a, 60b, 62a, 62b, 64a, 64b) of the at least one pair of tension / compression bars (60a, 60b, 62a, 62b, 64a, 64b) extend outwardly from the tower (12) and intersect to define the outrigger apex (60c, 62c, 64c).

4. The wind turbine (10) of any preceding claim, wherein the plurality of tension / compression bars (60a, 60b, 62a, 62b, 64a, 64b) forms three, four, five, or six triangular portions (60, 62, 64).

5. The wind turbine (10) of claim 4, wherein adjacent triangular portions (60, 62, 64) of three triangular portions (60, 62, 64) meet at vertices (66), and wherein the vertices (66) are spaced circumferentially apart by 120°.

6. The wind turbine (10) of claim 5, wherein the vertices (66) are at the tower radius r-r.

7. The wind turbine (10) of any preceding claim, wherein the at least one of the tension / compression bars (60a, 60b, 62a, 62b, 64a, 64b) of the plurality of tension / compression bars (60a, 60b, 62a, 62b, 64a, 64b) defines a plane, and wherein the plane intersects the tower axis (24) at an angle from 25° to 60°.

8. The wind turbine (10) of claim 7, wherein the at least one cable (20) extends to an anchor (22), and wherein the at least one cable (20) lies in the plane of the at least one tension / compression bar (60a, 60b, 62a, 62b, 64a, 64b) of the plurality of the tension / compression bars (60a, 60b, 62a, 62b, 64a, 64b).

9. The wind turbine (10) of claim 8, wherein the at least one cable (20) includes two cables (20) of the plurality of cables (20), wherein the two cables (20) are coupled proximate the outrigger apex (60c, 62c, 64c) and extend in opposing directions to individual anchors (22), and wherein the two cables (20) of the plurality of cables (20) lie in the plane of the at least one tension / compression bar (60a, 60b, 62a, 62b, 64a, 64b) of the plurality of the tension / compression bars (60a, 60b, 62a, 62b, 64a, 64b).

10. The wind turbine (10) of any preceding claim, wherein the tower (12) includes at least two tower sections (12a, 12b, 12c) including an upper section (12a) and a lower section (12b), and wherein the outrigger structure (18) is positioned between the upper tower section (12a) and the lower section (12b) of the at least two tower sections (12a, 12b, 12c).

11. The wind turbine (10) of claim 10, wherein the outrigger structure (18) comprises an assembly (80) of three radial segments (82, 84, 86), each radial segment (82, 84, 86) including a base segment (90, 92, 94), the at least one tension / compression bar (60a, 60b, 62a, 62b, 64a, 64b) of the plurality of tension / compression bars (60a, 60b, 62a, 62b, 64a, 64b) extending from a respective one of the base segments (90, 92, 94), wherein the base segments (90, 92, 94) collectively form an interface (88) between the upper tower section (12a) and the lower section (12b).

12. The wind turbine (10) of any of claims 1 -9, wherein the outrigger structure (18) is configured to be secured to the tower (12) and comprises a truss-like configuration.

13. The wind turbine (10) of claim 12, wherein the truss-like configuration comprises a frame segment (102, 104, 106) including two top chords (110, 112) and a bottom chord (114), the two top chords (110) intersecting the bottom chord (114), and wherein the outrigger apex (60c, 62c, 64c) is proximate the intersection of at least one of the top chords and the bottom chord (114).

14. The wind turbine (10) of claim 13, wherein the outrigger structure (18) comprises an assembly (100) of three frame segments (102, 104, 106).

15. The wind turbine (10) of any preceding claim, wherein the plurality of cables (20) consists of three cables (20) or the plurality of cables (20) consists of six cables16. An outrigger structure (18) for use with a wind turbine tower (12) having a tower (12) radius i and a plurality of cables (20), the outrigger structure (18) comprising: a plurality of tension / compression bars (60a, 60b, 62a, 62b, 64a, 64b), wherein when the outrigger structure (18) is coupled to the tower (12), at least one of the tension / compression bars (60a, 60b, 62a, 62b, 64a, 64b) of the plurality of tension / compression bars (60a, 60b, 62a, 62b, 64a, 64b) extends outwardly from the tower (12), wherein the at least one tension / compression bar (60a, 60b, 62a, 62b, 64a, 64b) of the plurality of tension / compression bars (60a, 60b, 62a, 62b, 64a, 64b) has an outrigger apex (60c, 62c, 64c) at a cable interface radius rp, wherein a ratio of the cable interface radius rp to the tower radius r? is at least 1.1 , and wherein the outrigger structure (18) is configured to receive at least one cable (20) of the plurality of cables (20) proximate the outrigger apex (60c, 62c, 64c).

17. The outrigger structure (18) of claim 16, wherein when the outrigger structure (18) is coupled to the tower (12), the at least one of the tension / compression bars (60a, 60b, 62a, 62b, 64a, 64b) of the plurality of tension / compression bars (60a, 60b, 62a, 62b, 64a, 64b) is configured to extend downwardly to define an outrigger height hp, and wherein a ratio of the outrigger height hp to the cable interface radius rp is at least 1 / 3.

18. The outrigger structure (18) of claim 16 or claim 17, wherein the plurality of tension / compression bars (60a, 60b, 62a, 62b, 64a, 64b) forms three, four, five, or six triangular portions (60, 62, 64).

19. The outrigger structure (18) of claim 18, wherein adjacent triangular portions (60, 62, 64) of three triangular portions (60, 62, 64) meet at vertices (66), and wherein the vertices (66) are spaced circumferentially apart by 120°.

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