Patent Publication Number: US-2012045345-A1

Title: Offshore wind turbine and methods of installing same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of U.S. provisional patent application Ser. No. 61/375,551 filed Aug. 20, 2010, and entitled “Offshore Wind Turbine and Methods of Installing Same,” which is hereby incorporated herein by reference in its entirety. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND 
     1. Field of the Invention 
     The invention relates generally to wind turbines. More particularly, the disclosure relates to an offshore wind turbine having a floatable tower and support truss, and associated methods of deployment and installation. 
     2. Background of the Technology 
     Wind turbines are commonly used to convert the kinetic energy of wind into mechanical power. The mechanical power, in turn, may be used to perform a specific task. Alternatively, the mechanical power may be converted into electricity by a generator. The wind turbines may be installed in close proximity, forming a wind farm, and connected to an electricity grid. Electricity produced by the wind farm may then be provided to the electricity grid for widespread distribution and use. 
     The location of a wind turbine, or wind farm, is crucial. The wind farm should be located such that it is exposed to as much wind as possible. Offshore locations offer a number of advantages, such as the availability of large areas over which a wind farm may be installed, higher wind speeds, and less turbulence, such as that caused by buildings, or other obstructions, which subject the wind turbines to fatigue. 
     Offshore locations also have their drawbacks. In particular, the wind turbines must be designed to withstand additional loads imparted by waves and currents in the surrounding water.  FIG. 1  illustrates two conventional offshore wind turbines  10 ,  15 . Each wind turbine  10 ,  15  includes a vertical tower  20 ,  25 , respectively, that supports the weight of a rotor  30 , a nacelle  35 , and the various generating components (e.g., generator, gearbox, drive train, brake assembly, etc.) housed within nancelle  35 . Each tower  20 ,  25  extends from the sea floor  11  and pierces the sea surface  12 , and thus, is configured to withstand loads imparted by wind and the surrounding water (e.g., waves and currents). Consequently, the cross-sectional areas of towers  20 ,  25  increase with increasing water depth. Furthermore, turbine  15  is located in deeper water and is therefore subject to higher forces from the surrounding water. To accommodate the increased loading, tower  25  is greater in size than tower  20  (e.g., tower  25  has a greater diameter than tower  20 ). 
     Given their design configurations, towers  20 ,  25  must have significant mass to prevent collapse under the applied wind, wave, and current loads. This, in turn, affects the manufacturing cost and installation complexity of turbines  10 ,  15 . Accordingly, there remains a need in the art for offshore wind turbines that are more adept at withstanding wind, wave, and current loads. Such offshore turbines would be particularly well-received if they were less expensive to make and easier to install. 
     BRIEF SUMMARY OF THE DISCLOSURE 
     These and other needs in the art are addressed in one embodiment by an offshore wind turbine. In an embodiment, the wind turbine comprises an elongate base having a longitudinal axis, a first end, and a second end opposite the first end. In addition, the wind turbine comprises a tower moveably coupled to the base. The tower has a first end distal the base and a second end disposed within the base, and the tower is configured to telescope axially from the first end of the truss. Further, the wind turbine comprises a nacelle coupled to the first end of the tower. Still further, the wind turbine comprises a rotor including a hub and a plurality of blades coupled to the hub. The hub is coupled to the nacelle. 
     These and other needs in the art are addressed in another embodiment by a method for deploying and installing an offshore wind turbine. In an embodiment, the method comprises (a) transporting a truss-tower assembly to an offshore installation site. The truss-tower assembly includes an elongate truss having a central axis and an tower moveably coupled to the truss. In addition, the method comprises (b) rotating the truss-tower assembly from a horizontal orientation to a vertical orientation at the installation site. Further, the method comprises (c) engaging the sea floor with a lower end of the truss. Still further, the method comprises (d) coupling a nacelle to an upper end of the tower. Moreover, the method comprises (e) coupling a rotor to the nacelle. The method also comprises (f) telescoping the tower axially from the truss. 
     These and other needs in the art are addressed in another embodiment by an offshore wind turbine. In an embodiment, the wind turbine comprises an elongate truss having a longitudinal axis, a first end, and a second end opposite the first end. In addition, the wind turbine comprises an elongate tower extending axially from the truss. Further, the wind turbine comprises a nacelle coupled to the first end of the tower. Still further, the wind turbine comprises a rotor coupled to the nacelle. 
     Embodiments described herein comprise a combination of features and advantages intended to address various shortcomings associated with certain prior devices, systems, and methods. The various characteristics described above, as well as other features, will be readily apparent to those skilled in the art upon reading the following detailed description, and by referring to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a detailed description of the preferred embodiments of the invention, reference will now be made to the accompanying drawings in which: 
         FIG. 1  is a front schematic view of two conventional offshore wind turbines; 
         FIG. 2  is front view of an embodiment of an offshore wind turbine in accordance with the principles disclosed herein; 
         FIG. 3  is a front view of an embodiment of a pinned connection for connecting the truss of  FIG. 2  to the sea floor; 
         FIG. 4  is an enlarged cross-sectional view of the truss of  FIG. 2  taken along section  4 - 4 ; 
         FIG. 5  is a top view of the support truss of  FIG. 2 ; 
         FIG. 6  is a top view of an alternative embodiment of the truss; 
         FIG. 7  is an enlarged view of  FIG. 5 , illustrating the guide tubes and rails. 
         FIGS. 8-19  are sequential schematic views illustrating an embodiment of a method for the offshore transport and installation of the wind turbine of  FIG. 2 ; 
         FIG. 20  is front view of an embodiment of an offshore wind turbine in accordance with the principles disclosed herein; and 
         FIGS. 21-33  are sequential schematic views illustrating an embodiment of a method for the offshore transport and installation of the wind turbine of  FIG. 19 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following discussion is directed to various exemplary embodiments. However, one skilled in the art will understand that the examples disclosed herein have broad application, and that the discussion of any embodiment is meant only to be exemplary of that embodiment, and not intended to suggest that the scope of the disclosure, including the claims, is limited to that embodiment. 
     Certain terms are used throughout the following description and claims to refer to particular features or components. As one skilled in the art will appreciate, different persons may refer to the same feature or component by different names. This document does not intend to distinguish between components or features that differ in name but not function. The drawing figures are not necessarily to scale. Certain features and components herein may be shown exaggerated in scale or in somewhat schematic form and some details of conventional elements may not be shown in interest of clarity and conciseness. 
     In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . ” Also, the term “couple” or “couples” is intended to mean either an indirect or direct connection. Thus, if a first device couples to a second device, that connection may be through a direct connection, or through an indirect connection via other devices, components, and connections. In addition, as used herein, the terms “axial” and “axially” generally mean along or parallel to a central axis (e.g., central axis of a body or a port), while the terms “radial” and “radially” generally mean perpendicular to the central axis. For instance, an axial distance refers to a distance measured along or parallel to the central axis, and a radial distance means a distance measured perpendicular to the central axis. 
     Referring now to  FIG. 2 , an embodiment of an offshore wind turbine  100  in accordance with the principles disclosed herein is shown installed in a body of water  105 . In general, turbine  100  is employed to harness wind energy to generate power (e.g., electrical and/or mechanical). In this embodiment, wind turbine  100  includes an elongate base  110  extending vertically upward from the sea floor  101 , a tower  120  moveably coupled to truss  110 , a nacelle  130  mounted to the upper end of tower  120  above the sea surface  102 , a rotor  140  coupled to nacelle  130 , and a plurality of guide wires  150  supporting base  110 . The various power generating components such as the generator, gearbox, drive train, and brake assembly or turbine  100  are housed within nacelle  130 . 
     In this embodiment, base  110  is an elongate truss having a central or longitudinal axis  115 , a first or upper end  110   a , and a second or lower end  110   b . As will be described in more detail below, truss  110  receives tower  120  through upper end  110   a . Lower end  110   b  is coupled to the sea floor  101  and upper end  110   a  extends above the sea surface  102 . In general, lower end  110   b  may be coupled to the sea floor  101  by any suitable means including, without limitation, a pinned connection, a rigid connection, etc. In  FIG. 2 , lower end  110   b  comprises a ballasted tank  116  that functions as a gravity anchor to rigidly secure lower end  110   b  to the sea floor  101 . In  FIG. 3 , lower end  110   b  comprises a pinned coupling  118  disposed axially below ballasted tank  116  and the sea floor  101 . The ballast in tank  116  provides the weight to urge coupling  118  into engagement with the sea floor  101 . Pinned coupling  118  between lower end  110   b  and the sea floor  101  may employed to allow turbine  100  to pivot about lower end  110   b  relative to the sea floor  101  in response to changing wind, wave, and current loads. Movement of turbine  100  in this manner offers the potential to alleviate stress to truss  110  and reduces fatigue damage to truss  110  that may otherwise occur. 
     Referring now to  FIGS. 2 and 4 , truss  110  includes a plurality of laterally spaced vertically-extending legs  111  interconnected by a plurality of stiffening members  112  to form a truss frame. Together, legs  111  and members  112  define the radially outer perimeter of truss  110 . As best shown in  FIG. 4 , interior to the frame, truss  110  further includes a plurality of internally positioned guide members  113 . Guide members  113  enhance the structural integrity of truss  110  and define a central through passage  114  extending through truss  110  from upper end  110   a  and sized to coaxially receive tower  120 . In this embodiment, legs  111  and members  112 ,  113  are tubulars interconnected by welded joints or other means known in the art. In particular, legs  111  and members  112 ,  113  are sealable tubulars that may be ballasted with water or de-ballasted with air, and thus, provide a means for adjusting buoyancy of truss  110  and turbine  100  as needed. 
     A plurality of uniformly circumferentially-spaced vertical rails  117  extending axially from upper end  110   a  and are coupled to guide members  113 . Tower  120  is coaxially inserted into truss  110  at upper end  110   a  and engages rails  117 . As will be described in more detail below, tower  120  telescopes from truss  110 . Tower  120  is axially moveable relative truss  110  along rails  117 , and thus, may telescope from truss  110  between a fully refracted position as shown in  FIG. 18  and a fully extended position shown in  FIG. 2 . Tower  120  is disposed in the fully refracted position during deployment, and transitioned to the fully extended position during installation after rotor  140  and nacelle  130  have been coupled thereto. 
     In general, tower  120  may be transitioned between the fully refracted and fully extended positions by any suitable means. For example, tower  120  may be transitioned between the fully refracted position and the fully extended position by de-ballasting and ballasting tower  120  (i.e., buoyancy forces are used to raise tower  120  relative to truss  110 ). As another example, tower  120  may have an inherent positive net buoyancy such that tower  120  moves upward through truss  110  upon release of a coupling mechanism that maintains tower  120  in the fully refracted position. As yet another example, tower  120  may be transitioned between the fully refracted position and the fully extended position with a lifting device such as a crane. As still yet another example, a motor and drive mechanism such as the jacking mechanism employed to move the legs on a jackup platform may be used to transition tower  120  between the fully retracted and fully extended positions. In embodiments where tower  120  is transitioned between the fully retracted position and the fully extended positions via buoyancy forces or with a crane, rails  117  preferably comprise guides that slidingly engage the outer surface of tower  120 . However, in embodiments wherein tower  120  is transitioned between the fully refracted position and the fully extended positions via a jacking mechanism, rails  117  may function as guides that slidingly engage tower during jacking operations, or alternatively, may comprise toothed racks or the like that positively engage a pinion or stepping jack coupled to tower  120 . 
     Once tower  120  has been fully extended, it is releasably locked relative to truss  110  such that tower  120  is restricted and/or prevented from moving relative to truss  110  along rails  117  during operation of turbine  100 . Tower  120  may be locked to truss  110  by any suitable releasable mechanism, coupling or device such as removable bolts. When desired, for instance during disassembly or maintenance of turbine  100 , the coupling mechanism may be released or removed to again enable tower  120  to move axially downward relative to truss  110  along rails  117 . 
     Referring again to  FIG. 2 , truss  110  also includes tank  116  at lower end  110   b . Tank  116  includes at least one closeable port or valve  117  that allows tank  116  to be closed from and opened to the surrounding environment. As will be described in more detail below, during deployment of truss  110 , valve  117  remains closed and tank  116  is filled with a gas such as air to give truss  110  to provide buoyancy. At the installation site, valve  117  is opened to flood tank  116  and sink and anchor truss  110  to the sea floor  101 . Thus, tank  116  may be ballasted and de-ballasted. Tank  116  preferably comprises a plurality of sub-compartments that are independently ballasted and deballasted, each sub-compartment having its own closeable port or valve. 
     As best shown in  FIGS. 2 and 5 , truss  110  is held in position with guide wires  150 . Each guide wire  150  has a first or upper end  150   a  secured to truss  120  between ends  120   a, b  and a second or lower end  150   b  secured to the sea floor  101 . In general, lower ends  150   b  of wires  150  may be secured to the sea floor  101  by any suitable means including, without limitation, a gravity anchor, a Pyle, or combinations thereof. Guide wires  150  preferably comprise resilient elastic material(s) that allow wires  150  to stretch and retract as truss  110  moves under changing loads from the surrounding water  105 . Examples of such elastic materials include, without limitation, wire, composite or polyester ropes. Such elastic guide wires  150  offer the potential to minimize the effects fatigue to guide wires  150 , thereby prolonging their service life. Further, such elastic guide wires  150  may help dampen vibrations induced in truss  110  from the rotation of blades  142  and other forces acting on turbine  100  and prevent resonant vibration of turbine  100 . 
     In the embodiment shown in  FIGS. 2 ,  4 , and  5 , truss  110  includes four legs  111  and has a rectangular or square shape in top view. However, in other embodiments, the truss (e.g., truss  110 ) may include a different number of legs (e.g., legs  111 ) and/or have another suitable geometry in top view. For example, in  FIGS. 6 and 7 , an embodiment of a truss  110 ′ including three legs  111  arranged in a triangle in top view is shown. Otherwise, truss  110 ′ is the same as truss  110 . 
     Referring again to  FIGS. 2 and 4 , tower  120  has a central axis  125 , a first or upper end  125   a , and a second or lower end  125   b . Lower end  125   b  is coaxially inserted into truss  110  with the radially outer surface of tower  120  engaging rails  117  previously described. Upper end  120   a  is coupled to nacelle  130  with a pair of axially-extending parallel support members  121 . Support members  121  are laterally spaced apart a sufficient distance to enable nacelle  130  to be slidingly disposed therebetween and coupled thereto. In this embodiment, nacelle  130  is pivotally coupled to tower  120  such that nacelle  130  may be rotated about an axis  121   a  oriented perpendicular to axis  125  in front view. In particular, nacelle  130  may be rotated about axis  121   a  relative to tower  120  between a first position, in which the longitudinal axis of nacelle  130  is oriented generally parallel to axis  125  as shown in  FIG. 17 , and a second position, in which the longitudinal axis of nacelle  130  is oriented generally perpendicular to axis  125  as shown in  FIG. 1 . The ability to pivot nacelle  130  relative to tower  120  enables nacelle  130  to be positioned vertically upright to generally position rotor  140  away from the sea surface  102  and water  105  during installation and maintenance of rotor  140 , nacelle  130 , and/or the generating components within nacelle  130 . In this embodiment, an elevator  122  is moveably disposed within tower  120  and enables relatively easy access to rotor  140 , nacelle  130  and the components therein for repair and/or maintenance. 
     Referring still to  FIG. 2 , rotor  140  includes a hub  141  and a plurality of blades  142  extending radially outward therefrom. Hub  141  is coupled to the generator components housed within nacelle  130 . In particular, hub  141  is coupled to a generator via a rotatable shaft and a gear box. As wind loads act on blades  142 , blades  142  rotate hub  141 . This, in turn, rotates the shaft. The generator, coupled to the shaft, converts the rotational mechanical energy of the shaft into electricity that may then transmitted to a remote location, such as an onshore electricity grid via electrical wiring or cables. 
     Referring now to  FIGS. 8-19 , an embodiment of a method for the offshore transport and installation of wind turbine  100  is shown. In this embodiment, a truss-tower assembly  160  comprising tower  120  coaxially disposed in truss  110  is transported to the installation site along the sea surface  102  as shown in  FIGS. 8-11 . At the installation site, truss-tower assembly  160  is transitioned to an upright position and secured to the sea floor  101  as shown in  FIGS. 12-14 . Next, the nacelle  130  (including the generating components housed therein) and rotor  140  are mounted to upper end  120   a  of tower  120  to complete the assembly of wind turbine  100  as shown in  FIGS. 15-18 . With wind turbine  100  fully assembled, tower  120  is transitioned to the extended position and locked in positioned relative to truss  110  for subsequent power generation operations as shown in  FIGS. 18 and 19 . 
     Referring first to  FIGS. 8 and 9 , truss-tower assembly  160  is moved from a land into the water  105  via a barge  171 . In particular, assembly  160  is staged at a loading dock  170 , and barge  171  is positioned adjacent dock  170 . Next, truss-tower assembly  160  is disposed on barge  171 . In general, assembly  160  may loaded onto barge  171  with a crane, rolled onto barge  160  with rollers, or other suitable means known in the art. Once truss-tower assembly  160  is loaded onto barge  171 , barge  171  is moved offshore a suitable distance from dock  170 . 
     Moving now to  FIGS. 10 and 11 , once offshore, truss-tower assembly  160  is offloaded from barge  171  into water  105  and floated out to the offshore installation site. Truss-tower assembly  160  may be offloaded from barge  171  by loading assembly  160  onto rails disposed on the deck of barge  171 , de-ballasting one end of barge  171 , and then sliding assembly  160  along the rails off the deballasted end of barge  171  as shown in  FIG. 10 . Alternatively, the entire barge  171  may be deballasted below the sea surface  102  and assembly  160  simply floated off barge  171 . 
     For float out offshore transport, assembly  160  is configured to have a positive net buoyancy for offshore transport. As previously described, legs  111  and members  112 ,  113  may be de-ballasted to provide buoyancy. In addition, for offshore transport, tank  116  is filled with air and valve  117  is closed. Together, legs  111 , members  112 ,  113 , and tank  116  provides sufficient buoyancy to enable assembly  160  to float and be towed along the sea surface  105  to the offshore installation site. Tower  120  may also be configured to provide buoyancy. For example, tower  120  may include sealed or ballast adjustable compartments. 
     Referring now to  FIGS. 11-14 , upon arrival at the installation site, truss-tower assembly  160  is transitioned from the horizontal float out position to a vertical upright position, and lower end  110   b  is secured to the sea floor  101 . First, as shown in  FIGS. 11-13 , valve  117  of buoyancy tank  116  is opened, and tank  116  is flooded (i.e., ballasted), causing tank  116  and lower end  110   b  of truss  110  (and hence assembly  160 ) to sink towards the sea floor  101 . Due to the buoyant nature of legs  111  and members  112 ,  113 , and with tank  116  is ballasted, tank  116  will land on the sea floor  101  and assembly  160  will transition to the upright configuration shown in  FIG. 13 . To complete installation of truss-tower assembly  160 , lower end  110   b  is moved into engagement with the sea floor  101  and guide wires  150  are installed as shown in  FIG. 14 . 
     Moving now to  FIGS. 15-18 , with assembly  160  secured in position and tower  120  in the refracted position, a nacelle-rotor assembly  180  comprising nacelle  130  (and the generating components housed therein) and rotor  140  is mounted to upper end  120   a  of tower  120 . Thus, nacelle  130 , the generating components housed within nacelle  130 , and rotor  140  are pre-assembled before being attached to tower  120 . First, nacelle-rotor assembly  180  is lifted and positioned generally above and coaxially aligned with tower  120  as shown in  FIGS. 15 and 16 . Assembly  180  is oriented such that rotor  140  is positioned above nacelle  130 . In this embodiment, a crane  172  mounted to a floating vessel  173  is used to lift, position, and orient assembly  180 . Next, as shown in  FIG. 17 , nacelle-rotor assembly  180  is lowered to position nacelle  130  between support members  121  of tower  120 , and nacelle  130  is pivotally coupled to support members  121 . Rotor-nacelle assembly  180  is subsequently pivoted about axis  121   a  to move rotor  140  downward and into position to face oncoming wind. In particular, rotor  140  is positioned such that blades  142  extend generally parallel to a plane normal to the sea surface  102  as shown in  FIG. 18 . Crane  172  is preferably used to controllably rotate nacelle-rotor assembly  180 . 
     With nacelle-rotor assembly  180  securely attached to support members  121 , tower  120  is transitioned from the fully refracted position to the fully extended position as shown in  FIGS. 18 and 19 . As previously described, tower  120  may be transitioned to the fully extended position by a variety of suitable means. For example, if tower  120  has a positive net buoyancy, a coupling mechanisms that maintains tower  120  in the fully retracted position maybe released, thereby allowing tower  120  naturally rises upward to the fully extended position. Alternatively, tower  120  may be lifted, for example, using crane  172  or with a jacking mechanism. Regardless of the means to extend tower  120 , once in the fully extended position, tower  120  is secured in position and prevented from moving relative to truss  110 . At this point, installation of offshore wind turbine  100  is complete. Unless crane  172  is used to lift tower  120  into the fully extended position, crane  172  may be detached from nacelle-rotor assembly  180  once it is secured to support members  121  and controllably pivoted downward. 
     Referring now to  FIG. 20 , an embodiment of an offshore wind turbine  200  in accordance with the principles disclosed herein is shown installed in body of water  105 . In general, turbine  200  is employed to harness wind energy to generate power (e.g., electrical and/or mechanical). In this embodiment, wind turbine  200  includes a support truss  210  extending vertically upward from the sea floor  101 , a tower  220  moveably coupled to truss  210 , a nacelle  230  mounted to the upper end of tower  220  above the sea surface  102 , a rotor  240  coupled to nacelle  230 , and a plurality of guide wires  150  as previously described supporting truss  210 . The various power generating components such as the generator, gearbox, drive train, and brake assembly or turbine  200  are housed within nacelle  230 . 
     Truss  210  is substantially the same as truss  110  previously described. Namely, truss  210  is an elongate frame having a central or longitudinal axis  215 , a first or upper end  210   a , and a second or lower end  210   b . As will be described in more detail below, tower  220  is retractable and extendable through upper end  210   a . Lower end  210   b  is coupled to the sea floor  101  and upper end  210   a  extends above the sea surface  102 . In general, lower end  210   b  may be coupled to the sea floor  101  by any suitable means including, without limitation, a pinned connection or a rigid connection as previously described. In addition, truss  210  is formed by a plurality of legs  111 , stiffening members  112 , and guide members  113 , each as previously described. However, unlike truss  110  previously described, in this embodiment, legs  111  and members  112 ,  113  may or may not be filled with air. A plurality of uniformly circumferentially-spaced vertical rails  117  as previously described are coupled to guide members  113 . Tower  220  is coaxially inserted into truss  210  at upper end  210   a  and engages rails  117 . 
     Tower  220  is axially moveable relative truss  210  along rails  117 , and thus, may telescope from truss  210  between a fully refracted position as shown in  FIG. 22  and a fully extended position shown in  FIG. 20 . As will be described in more detail below, tower  220  is disposed in the fully refracted position during deployment, and transitioned to the fully extended position after installation of nacelle  230  and rotor  240 . In this embodiment, tower  220  is transitioned between the fully refracted and fully extended positions with a jacking mechanism such as those used to move the legs on a jackup platform. However, in other embodiments, tower  220  may be transitioned between the fully refracted and fully extended positions via buoyancy. Once tower  220  has been fully extended, it is releasably locked relative to truss  210  such that tower  220  is restricted and/or prevented from moving relative to truss  210  during operation of turbine  200 . Tower  220  may be locked to truss  210  by any suitable releasable mechanism, coupling or device such as removable bolts. When desired, for instance during disassembly or maintenance of turbine  200 , the coupling mechanism may be released or removed to allow the jacking mechanism to lower tower  220  axially downward relative to truss  210 . 
     Referring again to  FIG. 20 , truss  210  also includes a ballast tank  216  at lower end  210   b . Tank  216  may be filled with removable ballast, fixed ballast, water ballast, solid ballast, or combinations thereof. A closeable port  217  is included in this embodiment to allow ballast to be added and removed from tank  216 . In this embodiment, tank  216  is not relied upon for buoyancy, and thus, tank  216  may be filled with ballast at any time prior to installation. As will be described in more detail below, during installation of turbine  200 , the ballast in tank  216  enables truss  210  to be sunk into engagement with the sea floor  101 . Truss  210  is held in position with guide wires  150  as previously described. 
     Referring still to  FIG. 20 , tower  220  is substantially the same as tower  120  previously described. Namely, tower  220  has a central axis  225 , a first or upper end  220   a , and a second or lower end  220   b . Lower end  220   b  is coaxially inserted into truss  210  with the radially outer surface of tower  220  engaging rails  117 . However, in this embodiment, upper end  220   a  does not comprise a pair of parallel support members  121 . Rather, in this embodiment, upper end  220   a  comprises a base  221  to which nacelle  230  is mounted. In this embodiment, nacelle  230  sits atop base  221  and is attached thereto. 
     Referring still to  FIG. 20 , rotor  240  is the same as rotor  140  previously described. Specifically, rotor  240  includes a hub  141  and a plurality of blades  142  extending radially outward therefrom. Hub  141  is coupled to the generator components housed within nacelle  230 . In particular, hub  141  is coupled to a generator via a rotatable shaft and a gear box. As wind loads act on blades  142 , blades  142  rotate hub  141 , which, in turn, rotates the shaft. The generator, coupled to the shaft, converts the rotational mechanical energy of the shaft into electricity that may then transmitted to a remote location, such as an onshore electricity grid via electrical wiring or cables. 
     Referring now to  FIGS. 21-33 , an embodiment of a method for the offshore transport and installation of wind turbine  200  is shown. In this embodiment, the various components of wind turbine  200  are transported to the installation site on a barge  271  as shown in  FIGS. 21 and 22 . At the installation site, a truss-tower assembly  260  is offloaded from barge  271 , transitioned to an upright position, and secured to the sea floor  101  as shown in  FIGS. 23-26 . Next, a nacelle-hub assembly  280  (including nacelle  230 , the generating components housed therein, and hub  141  coupled thereto) is mounted to upper end  220   a  of tower  220  as shown in  FIGS. 27 and 28 , followed by attachment of blades  142  as shown in  FIGS. 29-31 . With wind turbine  200  assembled, tower  220  is transitioned to the extended position and locked in positioned relative to truss  210  for subsequent power generation operations as shown in  FIGS. 32 and 33 . 
     Referring first to  FIGS. 21 and 22 , truss-tower assembly  260 , nacelle-hub assembly  280 , and blades  142  are loaded onto a barge  271 . These components may be staged on a loading dock (e.g., dock  170 ) and loaded onto barge  271  from the loading dock (e.g., dock  170 ) in any of the manners previously described. In this embodiment, a pair of cranes  172  are also disposed on barge  271 . 
     As best shown in  FIG. 22 , in this embodiment, barge  271  includes a pair of parallel laterally spaced pontoons  273  and a cross-member  274  extending perpendicularly therebetween. Cross-member  274  extends between ends of pontoons  273 , thereby defining an opening  275  in barge  271  between pontoons  273  and extending from cross-member  274  to the opposite ends of pontoons  273 . Decks  273   a ,  274   a  are disposed on the top of pontoons  273  and cross-member  274 , respectively. Opening  275  provides direct access to the sea surface  102  through barge  271 . A rotatably, cylindrical pin  276  extends between pontoons  273  across opening  275 . In this embodiment, nacelle-hub assembly  280  is loaded onto deck  274   a , and blades  142  and cranes  172  are disposed on pontoon decks  173   a . Truss-tower assembly  260  is loaded onto barge  271  such that it is cantilevered over opening  275  and supported by deck  274   a  and pin  276 . In particular, upper ends  210   a ,  220   a  are supported on deck  274   a , and pin  276  is disposed between ends  210   a ,  220   a  and ends  210   b ,  220   b , respectively. With the components of wind turbine  200  loaded onto barge  271  as shown in  FIGS. 21 and 22 , barge  271  is moved offshore to the installation site. Thus, in this embodiment, the components of wind turbine  200  are transported to the installation site on barge  271 . 
     Moving now to  FIGS. 23-26 , at the offshore installation site, truss-tower assembly  260  is offloaded from barge  271  into water  105 , transitioned a generally horizontal position to a vertical, upright position, and secured to the sea floor  101 . First, tank  216  is ballasted if it has not already been ballasted, and then truss-tower assembly  260  is pushed axially off deck  274   a  and across pin  276 . In this embodiment, pin  276  is allowed to roll along tracks disposed on the inside of pontoons  273 . As upper ends  210   a ,  220   a  move off of deck  274   a  and truss-tower assembly  260  rolls across pin  276 , lower end  210   b  moves axially away from pin  276  and upper end  210   a  moves axially towards pin  276 . When the lower end  210   b  is sufficiently spaced from pin  276 , truss-tower assembly  260  will pivot or rotate about pin  276  with end  210   b  swinging downward and end  210   a  swinging upward as shown in  FIGS. 24 and 25 . Lower end  210   b  swings downward and into engagement with the sea floor  101 . To complete installation of truss-tower assembly  260 , lower end  210   b  is moved into engagement with the sea floor  101  and guide wires  150  are installed as shown in  FIG. 26 . Barge  271  may be used to urge truss-tower assembly  260  to the full upright position after lower end  210   b  engages the sea floor  101  and/or wires  150  may be used to move truss-tower assembly  260  to the full upright position. 
     Moving now to  FIGS. 27 and 28 , with assembly  260  secured in position and tower  120  in the refracted position, nacelle-hub assembly  280  is mounted to upper end  220   a  of tower  220 . Thus, nacelle  230 , the generating components housed within nacelle  230 , and hub  141  are pre-assembled before being attached to tower  220 . First, nacelle-hub assembly  280  is lifted and position above tower  220  as shown in  FIG. 27 . In addition, assembly  280  is oriented such that it may be lowered onto surface  222  and secured to  221 . For example, if bolts are used to secure assembly  280  to base  221 , then the holes in assembly  280  and base  221  through which the bolts are disposed must be aligned. Next, as shown in  FIG. 28 , nacelle-hub assembly  280  is lowered onto base  221  and secured thereto. In this embodiment, a crane  172  is used to lift, position, orient, and lower assembly  280 . 
     With nacelle-hub assembly  280  securely attached to upper end  220   a , blades  142  are attached to hub  141  as shown in  FIGS. 29-31 . In this embodiment, cranes  172  are used to lift and position blades  142  so that they may be secured to hub  141 . As best shown in  FIG. 31 , tower  220  may need to be at least partially raised during installation of blades  142  to provide sufficient clearance between the blades  142  already attached to hub  141  and barge  271 . After all the blades  142  are attached to hub  141 , assembly of wind turbine  200  is complete. Next, tower  220  is transitioned to the fully extended position using the jacking system as shown in  FIGS. 32 and 33 , and barge  172  is moved away from the installation site. 
     In the manner described, embodiments described herein provide systems and methods for transporting, deploying and installing offshore turbines. As best shown in  FIGS. 2 and 20 , lower ends  120   b ,  220   b  of towers  120 ,  220 , respectively, are positioned proximal the sea surface  102  and upper ends  110   a ,  210   a  of trusses  110 ,  210 , respectively, when towers  120 ,  220  are in their fully extended positions. Thus, once turbine  100 ,  200  is installed and tower  120 ,  220 , respectively, is transitioned to the fully extended position, truss  110 ,  210  is generally open and transparent to currents and waves in the surrounding water  105 . In particular, the gaps between legs  111  and members  112 , and the open space within truss  110 ,  210  allows water  105  to flow freely through truss  110 ,  210  with minimal impedance. This is in contrast to conventional towers  20 ,  25  previously described and shown in  FIG. 1 . Water cannot pass freely through towers  20 ,  25 , and must flow around towers  20 ,  25 . Consequently, without being limited by this or any particular theory, forces exerted on towers  20 ,  25  by the surrounding water are significantly higher than those exerted on similarly sized trusses  110 ,  210 , thereby necessitating the mass of towers  20 ,  25  be significantly higher as well. In other words, embodiments of trusses  110 ,  210  described herein need not have a mass comparable to that of similarly sized towers  20 ,  25  because trusses  110 ,  210  will experience less loads from the surrounding water than towers  20 ,  25 . As a result, trusses  110 ,  210 , which are formed by a plurality of tubular elements joined via welding, offer the potential for a less expensive structure than towers  20 ,  25 . In addition, the ability to telescope towers  120 ,  220  from trusses  110 ,  210 , respectively, offers the potential to simplify deployment, installation, and maintenance of turbines  100 ,  200 , respectively. In addition, the ability to telescope towers  120 ,  220  from trusses  110 ,  210 , respectively, enables nacelle  130 ,  230 , and rotor  140 ,  240  to be installed and accessed proximal the sea surface  102 . This offers the potential to reduce installation and maintenance costs since specialized heavy lift vessels and equipment may not be needed, and further, enhances safety since installation and maintenance operations do not need to be performed at high elevations. Furthermore, at least with regard to turbine  100 , the tubular nature and buoyancy of truss  110  enables truss-tower assembly  160  to be floated to an installation site. This offers the potential for a simpler and lower cost deployment and installation as compared to many similarly sized conventional wind turbines such as turbines  10 ,  15  previously described. 
     While preferred embodiments have been shown and described, modifications thereof can be made by one skilled in the art without departing from the scope or teachings herein. The embodiments described herein are exemplary only and are not limiting. Many variations and modifications of the systems, apparatus, and processes described herein are possible and are within the scope of the invention. For example, the relative dimensions of various parts, the materials from which the various parts are made, and other parameters can be varied. Accordingly, the scope of protection is not limited to the embodiments described herein, but is only limited by the claims that follow, the scope of which shall include all equivalents of the subject matter of the claims. Unless expressly stated otherwise, the steps in a method claim may be performed in any order. The recitation of identifiers such as (a), (b), (c) or (1), (2), (3) before steps in a method claim are not intended to and do not specify a particular order to the steps, but rather are used to simply subsequent reference to such steps.