EP2222956A1

EP2222956A1 - Power generation assemblies

Info

Publication number: EP2222956A1
Application number: EP08851017A
Authority: EP
Inventors: Shigeyuki Yamamoto; Warren E. Colburn, Jr.
Original assignee: Oceanwind Technology LLC
Current assignee: Oceanwind Technology LLC
Priority date: 2007-11-12
Filing date: 2008-11-11
Publication date: 2010-09-01
Also published as: WO2009064737A1; EP2222956A4; JP2011503422A; CN101965454B; CN101965454A

Abstract

A floating power generation assembly has at least three floating units provided with power generation means and floating in a body of water. At least one of the three floating units is a tension leg platform. The assembly also comprises first anchors secured to a surface beneath the water, and first cables connecting the buoyant body to the first anchors. Second anchors are secured to the underwater surface and connected by second cables to the floating units. The floating units are arranged substantially at the vertices of at least one triangle or quadrilateral.

Description

POWER GENERATION ASSEMBLIES

[Para 1 ] This application is related to International Application WO 2005/040604. [Para 2] This invention relates to power generation assemblies, and apparatus for use therewith. More specifically, this invention relates to (a) waterborne power generation assemblies; and (b) a process for placing a floating unit on water.

[Para 3] The aforementioned WO 2005/040604 discusses the advantages of wind-driven power generation assemblies, and especially the advantages of siting wind farms (i.e., collections of wind-driven power generation assemblies) in deep water without mounting a rigid structure on the sea bed or other underwater solid surface. This application describes a floating power generation assembly having as components at least three floating units floating on a body of water, and at least three anchors secured to a solid surface beneath the body of water, each of the floating units being provided with power generation means, each of the anchors being connected by cables to at least one of the floating units, and each of the floating units being connected by cables to at least two other components, the floating units being arranged substantially at the vertices of at least one equilateral triangle.

[Para 4] The power generation assemblies or wind farms described in the aforementioned applications use vertically-free-floating ("VFF") units, that is to say buoyant units which float freely without any tension leg connecting them to the sea bottom; the cables used in the wind farms are used to provide horizontal tension support from multiple directions, thus making the VFF unit more stable against horizontal forces. The cables also ensure that the VFF units do not drift away from the predetermined locations, and maintain correct position relative to each other. While such VFF units can produce good results, the need to ensure that the center of gravity of each floating unit is a substantial distance below the water surface and that each unit has a substantial metacentric height (the distance between its centers of gravity and metacenter) of several meters, coupled with the need to mount a relatively heavy wind turbine and generator high above the water surface, means that in practice the VFF units must be heavy, typically of the order of several hundred to a couple of thousand tonnes. Such heavy VFF units require large quantities of construction materials and hence are costly to produce, especially in view of the recent substantial increases in the costs of construction materials such as concrete and steel due to increased energy costs.

[Para 5] It is known that wind turbines and other devices which it is desired to use at sea can be mounted on a tension leg platform (TLP). A tension leg platform comprises a buoyant body connected to at least one, and typically three or more, cables or similar connecting devices which are anchored to the sea bed. The cables are kept under substantial tension, and the buoyant body is effectively tethered to the seabed.

[Para 6] TLP's can be made more stable than VFF units of the same height, but, as discussed in more detail below with reference to Figures 13 to 15, they can undergo catastrophic tipping under large horizontal forces than VFF units, and this susceptibility to horizontal forces poses problems with mounting apparatus which requires locating units of substantial weight, such as rotors and generators, on TLP's at substantial distances above the water surface, since the mounting of substantial weight high above the water surface exacerbates the tendency of TLP units to suffer catastrophic tipping.

[Para 7] It has now been found that if some or all of the VFF units in the wind farms described in the aforementioned applications are replaced by TLP's substantial advantages accrue; the TLP's can be lighter and less expensive than similar VFF units, while the interconnections between the floating units provided by the anchors and cables of the wind farm itself (as opposed to anchors and cables associated with any individual TLP) reduce the sensitivity of the TLP's to tipping and horizontal forces and hence render the TLP's a more stable mounting for rotors and generators used for power generation.

[Para 8] This invention also relates to certain modified forms of the wind farms described in the aforementioned WO 2005/040604.

[Para 9] Accordingly, this invention provides a floating power generation assembly having as components at least three floating units provided with power generation means and floating in a body of water, the assembly being characterized in that at least one of the three floating units is a tension leg platform comprising a buoyant body, the assembly also comprising at least one first anchor secured to a solid surface beneath the body of water, and at least one first cable maintained under tension and connecting the buoyant body to the first anchor, the floating power generation assembly further comprising at least three second anchors secured to the solid surface beneath the body of water, each of the second anchors being connected by second cables to at least one of the floating units, and each of the floating units being connected by cables to at least two other components, the floating units being arranged substantially at the vertices of at least one triangle or quadrilateral. [Para 1 0] This aspect of the present invention may hereinafter be referred to as the "TLP assembly" of the invention. Also, for convenience the first anchors and first cables associated with the TLP's may be referred to as the "TLP anchors" and "TLP cables", while the second anchors and second cables which serve to restrain the entire assembly in position may referred to as the "assembly anchors" and "assembly cables". In such a TLP assembly, the floating units may all be TLP's, or some of the floating units may be TLP's and others VFF units. Each power generation means may comprise at least one of a wind turbine and a means for extracting power from waves or marine currents. The three assembly anchors may be arranged substantially at the vertices of a triangle with the floating units arranged within, or along the sides of, this equilateral triangle. In one form of the anchored floating assembly, intended for use where rough weather and/or strong currents may cause problems, each of the floating units is connected by assembly cables to at least three other components of the assembly. The TLP assembly may comprise at least six floating units arranged substantially at the vertices of a hexagon, typically with a seventh floating unit disposed at the center of the hexagon.

[Para 1 1 ] When VFF units are present in a TLP assembly, desirably at least one of the VFF units comprises: a mast extending from above to below the water surface; a wind turbine comprising a plurality of blades and rotatably mounted at or adjacent the upper end of the mast such that the blades do not contact the water as they rotate; a buoyancy section provided on the mast adjacent the water surface and arranged to provide buoyancy to the assembly; and a base section provided on the mast below the water surface and having the cables attached thereto, the base section being weighted such that the center of gravity of the floating unit is substantially below the water surface.

[Para 1 2] Desirably, in such an assembly, the center of gravity of the VFF unit is at least about 30 meters below the water surface, and the VFF unit desirably has a metacentric height (the distance between its centers of gravity and metacenter) of at least about 10 meters. Also, the TLP assembly may further comprise at least two auxiliary assembly cables extending from the buoyancy section to the cables connecting the base section to other components of the assembly, or to other components of the assembly (including other floating units). The base section of the mast may be provided with a peripheral hoop arranged to increase the hydrodynamic mass of the floating unit and to lengthen the natural heave period thereof. The mast may have a portion of reduced cross-section at the water surface, and the portion of the mast lying below the water surface may be provided with at least one ballast tank. [Para 1 3] The TLP assemblies of the present invention may be deployed by methods exactly analogous to those described in the aforementioned WO 2005/040604 and in the description of Figures 29A-D below, subject of course to the need to deploy the TLP anchors and cables. [Para 1 4] This invention also provides a floating power generation assembly having as components at least three floating units floating on a body of water, and at least three anchors secured to a solid surface beneath the body of water, each of the floating units being provided with power generation means, each of the anchors being connected by cables to at least one of the floating units, and each of the floating units being connected by cables to at least two other components, the assembly being characterized in that the floating units are arranged substantially at the vertices of at least one quadrilateral.

[Para 1 5] In such a "quadrilateral" power generation assembly, each power generation means may comprise at least one of a wind turbine and a means for extracting power from waves and/or currents. Also, each of the floating units may be connected by cables to at least two other components of the assembly. At least one of the floating units may comprises: a mast extending from above to below the water surface; a wind turbine comprising a plurality of blades and rotatably mounted at or adjacent the upper end of the mast such that the blades do not contact the water as they rotate; a buoyancy section provided on the mast adjacent the water surface and arranged to provide buoyancy to the assembly; and a base section provided on the mast below the water surface and having the cables attached thereto, the base section being weighted such that the center of gravity of the floating unit is substantially below the water surface. In such a mast-containing floating unit, the center of gravity of the floating unit may be at least about 30 meters below the water surface, and the assembly may further comprise at least two auxiliary cables extending from the buoyancy section to the cables connecting the base section to other components of the assembly, or to other components of the assembly. The assembly may have at least four floating units disposed at the vertices of a rectangle or square. At least one base section may be provided with a peripheral hoop arranged to increase the hydrodynamic mass of the floating unit and to lengthen the natural heave period thereof.

[Para 1 6] Figure 1 of the accompanying drawings is a schematic perspective view from above, in front and to one side of a tension leg platform unit which may be used in the TLP assemblies of the present invention.

[Para 1 7] Figure 2 is an enlarged schematic perspective view from above and to one side of the floatation section of the TLP unit shown in Figure 1. [Para 1 8] Figure 3 is a view, similar to that of Figure 2, of a modified floatation section which may be substituted for the corresponding section shown in Figures 1 and 2.

[Para 1 9] Figure 4 illustrates variations of the TLP unit shown in Figures 1 and 2, together with a prior art VFF unit.

[Para 20] Figures 5-8 illustrate details of the manner in which cables are joined to various

TLP units used in the assemblies of the present invention.

[Para 21 ] Figures 9-12 illustrate various cable arrangement by which VFF and TLP units may be interconnected in the assemblies of the present invention.

[Para 22] Figures 13-15 illustrate schematically the forces acting upon a TLP unit and the failure modes thereof.

[Para 23] Figure 16 is a schematic side elevation of a preferred TLP unit for use in the assemblies of the present invention and illustrates the manner in which this unit reduces forces tending to cause failure of the unit.

[Para 24] Figure 17 is a schematic side elevation of a modified TLP unit in which the floatation chamber can move relative to other components of the unit.

[Para 25] Figure 18 is a schematic side elevation of part of a VFF unit in which the floatation chamber can move relative to other components of the unit.

[Para 26] Figure 19 is a side elevation (partly in section) of a cable stabilizer unit which may be attached to one or more of the assembly cables of the assemblies of the present invention.

[Para 27] Figure 20 is an end elevation of the cable stabilizer unit shown in Figure 19.

[Para 28] Figures 21 and 22 show alternative cable arrangements which may be substituted for that shown in Figure 11.

[Para 29] Figure 23 shows a side elevation (partly in section) of a modified form of the cable stabilizer unit shown in Figures 19 and 20.

[Para 30] Figures 24A and 24B show forms of the floating power generation assemblies of the aforementioned WO 2005/040604 modified to take advantage of locations where the wind is known to be predominantly in one direction.

[Para 31 ] Figures 25A to 25C show forms of the floating power generation assemblies of the present invention in which the floating units are arranged in quadrilaterals. [Para 32] Figures 26A and 26B show floating power generation assemblies of the present invention which use power generating floating units in association with non-power generating floating units.

[Para 33] Figures 27 and 28 show two types of non-power generating floating units used in the assemblies of Figures 26A and 26B.

[Para 34] Figures 29A to 29D, are schematic top plan views illustrating a preferred method for deploying floating power generation assemblies of the present invention. [Para 35] As already indicated, the first aspect of the present invention relates to a modification of the power generation assemblies or wind farms described in the aforementioned WO 2005/040604, with some or all of the floating units having the form of tension leg platforms. Accordingly, the TLP assemblies of the present invention may include any of the optional features of the floating units described in WO 2005/040604. [Para 36] Figure 1 of the accompanying drawings is a schematic perspective view from above, in front and to one side of a tension leg platform unit (generally designated 100) which may be used in the TLP assemblies of the present invention. The unit 100 comprises a rotor 102 comprising a plurality of blades (three are illustrated) and mounted on a hub (or nacelle) 104 for rotation about a horizontal axis sufficiently far above the water that the rotor blades do not contact the water as they rotate; indeed, to ensure that the rotor blades receive the full velocity of the wind unhindered by surface drag, it is desirably that the rotor blades have, at their lowest point, at least 15 meters clearance above water level. The hub 104 houses a generator (or other form of power take off system, not shown) and is supported on a tower or mast 106. Units comprising a rotor and a hub containing a generator are available commercially, and the commercial units can readily be employed in wind farms of the present invention. The commercial units are already provided with means (not shown) to keep the rotor facing the wind, and with a rotation joint (also not shown) located a short distance below the hub to enable the hub and rotor to turn on a fixed mast, thus minimizing the weight which has to rotate as the rotor turns to face the prevailing wind. [Para 37] Thus far, the construction of the unit 100 is conventional. However, instead of being secured to a rigid support, either land or sea bed, the unit 100 is as a tension leg platform for anchoring in deep water. As shown in Figure 1, the unit 100 further comprises a floatation section (generally designated 108), which is described below in more detail with reference to Figure 2. The unit 100 also comprises a cable mounting section having an annular hub HOH from which extend at 120° intervals three legs or spokes HOS; the number of spokes or legs and the angles between adjacent spokes or legs may of course vary widely depending upon the exact form of the TLP assembly. (This cable mounting section 110 may be of alternate structural design suitable to maintain the relative positions of the TLP cable connections to it, for example a set of three beams configured as a equilateral triangle with the TLP cables 112 connected at each corner.) Three first or TLP cables 112 extend vertically downwardly from the floatation section 108, pass through and are secured to the outer ends of the spokes HOS, and thence extend vertically downwardly to TLP anchors (not shown), which anchor the lower ends of the cables 112 to the sea bed in the same way as in a conventional tension leg platform, for example by gravity anchors and/or suction piles. Three lower assembly cables 114 extend outwardly and slightly upwardly from the outer ends of the spokes HOS, while three upper assembly cables 116 extend outwardly and downwardly from the floatation section 108. As discussed in more detail below, the lower and upper assembly cables 114 and 116 are joined to each other at points distant from the unit 100. [Para 38] As shown in Figure 2, which is a schematic perspective view from above and to one side of the floatation section 108 shown in Figure 1, the floatation section 108 comprises a triangular platform 120, which supports the mast 106. A central support member 122 and three outer support members 124 extend vertically downwardly from the center and the vertices respectively of the triangular platform 120, and pass through the water surface. The provision of the vertical support members 122 and 124 reduces the cross-section of the floatation section 108 at the water line and hence reduces the susceptibility of the unit 100 to wave action; cf. U.S. Patent No. 7,293,960, Figures 13A, 13B, 14A and 14B and the paragraph bridging columns 13 and 14 of the description. The lower ends of the vertical support members 122 and 124 are fixedly secured to a hollow floatation chamber 126, which provides the buoyancy for the TLP unit 100. As shown in Figure 2, the floatation chamber 126 has substantially the form of an oblate cylinder surmounted at both ends by frusto- conical sections which taper inwardly away from the central cylindrical section. The central support member 122 is fixedly secured to the flat upper surface of the upper frusto-conical section, while the outer support members 124 are fixedly secured to the outer edges to the upper frusto-conical section adjacent its junction with the central cylindrical section. [Para 39] Three horizontal cable attachment struts 128 extend horizontally outwardly at 120° intervals from the central support member 122 (again, the number of and angles between these struts may vary widely depending upon the exact structure of the TLP assembly in which they are to be used), and are connected to and pass outwardly beyond the outer support members 124. To prevent excessive flexing of the outer portions of cable attachment struts 128 under the substantial loads imposed thereon, braces 130 extend upwardly and outwardly from the junctions of the outer support members 124 and the floatation chamber 126, and are connected to the cable attachment struts 128 adjacent the outer ends of these struts. One TLP cable 112 is connected to the underside of each strut 128 adjacent the outer end of the strut, while one upper assembly cable 116 is connected to an eyelet provided at the outer end of each strut 128.

[Para 40] Figure 3 is a view, similar to that of Figure 2, of a modified floatation section (generally designated 308) which may be substituted for the corresponding section 108 shown in Figures 1 and 2. From Figure 3, it will be seen that in the modified floatation section 308, the mast 106 is again mounted on a triangular platform 120, which has a central support member 122 extending vertically downwardly from its center. However, in the floatation section 308, outer support members 324 extend downwardly and outwardly from the vertices of the triangular platform 120. Also, in the floatation section 308, the single floatation chamber 126 of the floatation section 108 shown in Figure 2 is replaced by three separate floatation chambers 326A, 326B and 326C, each of which has a prolate cylindrical section surmounted by a hemispherical section. The chambers 326A, 326B and 326C are held spaced from one another at the same depth below the water surface and at the vertices of an equilateral triangle by retaining members 328. From the center of each retaining member 328 a horizontal support strut 330 extends inwardly and is connected to the lower end of the central support member 122. The TLP cables 112 are attached to the centers of the circular bases of the chambers 326A, 326B and 326C, while the upper assembly cables are attached adjacent the upper ends of the cylindrical sections of the chambers.

[Para 41 ] Figure 4 illustrates further variations of the unit 100 shown in Figures 1 and 2, together with a prior art VFF unit. To give a general idea of the scale of the units, a vertical scale calibrated in meters is shown on the left-hand side of Figure 4. In Figure 4, the Unit designated "A" is a prior art VFF unit substantially as illustrated in Figure 15 of U.S. Patent No. 7,293,960 and as described therein, except that in Unit A the lowest part of the base portion has been replaced by a framework. Unit F in Figure 4 is a unit as illustrated in Figures 1 and 2, with the a minor modification of the shape of the floatation chamber. [Para 42] Unit B in Figure 4 is a so-called "single leg high floater" unit, which may be regarded as notionally produced by removing the lower part of the base portion of Unit A and replacing it with a single TLP cable and a modified version of the cable mounting section HOH, 11 OS shown in Figure 1. As in the unit 100 shown in Figures 1 and 2, the lower assembly cables are attached to the outer ends of the spokes of the cable mounting section, while the upper assembly cables are attached, as in Unit A, to the upper end of the floatation chamber.

[Para 43] Units D and E in Figure 4 are variants of Unit F. Unit D is a so-called "triple leg high floater" and may be regarded as derived from the unit 100 shown in Figures 1 and 2 by shortening the vertical support members 122 and 124, eliminating the braces 130 and attaching the upper assembly cables directly to the points where the vertical support members are joined to the flotation chamber. Since in Unit D the floatation chamber is close to the water surface, this design is most suitable for use in sheltered locations where large waves are not expected. The cable attachment struts in Unit D are mounted directly on the lower end of the floatation chamber, so there is no need for braces corresponding to the braces 130 shown in Figure 2.

[Para 44] Unit E in Figure 4 is a so-called "triple leg low floater" unit, which is essentially a variant of Unit F designed to keep the floatation chamber deeper below the water surface; Unit E may thus be better adapted for locations where waves of substantial height are expected. The lower part of Unit E is essentially identical to that of Unit D. However, in the upper part of Unit E, the vertical support members are lengthened, and render the long outer support members rigid, they are joined to each other adjacent their midpoints by a triangular "collar" joining the three outer support members. This collar may be comprised of struts similar to the cable attachment struts 128 shown in Figure 2 or of retaining members similar to the members 328 in Figure 3 and of struts similar to the struts 330 shown in Figure 3. The outer support members may also be braced by diagonal braces extending between their midpoints and the lower end of the central support member. The upper assembly cables are attached to the outer support members at their junction with the collar.

[Para 45] Finally, Unit C in Figure 4 is a so-called "single leg low floater" unit, which may be regarded as combining the upper part of Unit E with the lower part of Unit B, and hence does not need to be described further.

[Para 46] Figures 5-8 illustrate details of the manner in which the TLP and assembly cables are joined to various units used in the assemblies of the present invention. Figure 5 illustrates the cables attached to the unit 100 shown in Figures 1 and 2. As previously described, the TLP cables 112 extend vertically downwardly from the struts 128 to TLP anchors (not shown), which anchor the lower ends of the cables 112 to the sea bed in the same way as in a conventional tension leg platform. Three lower assembly cables 114 extend outwardly and slightly upwardly from the outer ends of the cable mounting section 110, while three upper assembly cables 116 extend outwardly and downwardly from the struts 128. At points distant from the unit 100, the lower and upper assembly cables 114 and 116 are received in cable blocks 130, and from each cable block 130 a single cable 132 extends to adjacent TLP or VFF units.

[Para 47] Figure 6 is a view similar to that of Figure 5 but showing the manner in which the TLP and assembly cables are joined to the unit shown in Figure 3. As previously described, the TLP cables 112 extend vertically downwardly from the chamber 326A, 326B and 326C (the last is not visible in Figure 6), pass through and are secured to the outer ends of the cable mounting section 110, and thence extend vertically downwardly to TLP anchors (not shown). It should be noted that the cable mounting section 110 used in this unit may be larger than, though of similar design to, that shown in Figure 1 in order to accommodate greater spacings between the mountings of the TLP cables 112 in the unit shown in Figure 3. The arrangements of the cable blocks 130 and cables 132 in Figure 6 are identical to those in Figure 5.

[Para 48] Figures 7 and 8 show views similar to those of Figures 5 and 6 but using the Units E and C respectively shown in Figure 4. It should be noted that the mounting points for the upper assembly cables are arranged to that these mounting points are at substantially the same distance below the water surface in each of Figures 5-8, even though the floatation chambers in Figures 7 and 8 are substantially deeper below the water surface than those in Figures 5 and 6.

[Para 49] The preferred arrangements of the TLP and VFF (if any) units in the assemblies of the present invention are the same as those described in detail in the aforementioned WO 2005/040604 and those described below; see Figures 10A- 1OF and 20A-20J and the related description of WO 2005/040604, and Figures 24A, 24B, 25A-25C, 26A and 26B described below.

[Para 50] Attention will be directed to the arrangements of cables extending between adjacent units in such assemblies. Figures 9-11 each show cable arrangements for interconnecting a VFF Unit A as shown in Figure 4 on the left-hand side of the Figure to a unit 100 as shown in Figures 1 and 2 on the right-hand side of the Figure. In Figures 9-11, it should be understood that both the lower and upper assembly cables are under substantial tension (on the order of many tonnes), and that the drawings greatly exaggerate the curvatures in certain cables due to the weight of the cables themselves. In Figure 9, the lower assembly cable runs essentially directly between the base section of Unit A and the cable attachment section of unit 100, while the upper assembly cables extend from the upper sections of the units.

[Para 51 ] In the modified arrangement shown in Figure 10, the arrangement of the lower assembly cable and the upper assembly cables is similar to that shown in Figure 9, but the length of the upper cables has been adjusted such that the junctions of the lower and upper cables occur at a smaller depth than in Figure 9. Depending upon the length of the cables, the points of attachment of the upper cables to the lower cable may be located about halfway between the depths at which lower and upper cables are attached to the two units. [Para 52] Figure 11 illustrates a different type of upper assembly cable. This type of upper cable is not connected to the lower cable but extends directly between the two adjacent floating units, running essentially parallel to but above the lower cable. This type of interconnection may provide a higher tensile strength connection between units. It provides a more robust connection between the units but does have the disadvantage that it renders the upper cable more susceptible to wave motion, and if the floating assembly is in an area through which vessels may need to pass, the upper cable allows only a limited draft for such vessels.

[Para 53] Finally, Figure 12 illustrates a form of cable connection which may be used to connect two TLP units, and may be suitable for connecting VFF units. Essentially the cable arrangement shown in Figure 12 is derived from that shown in Figure 11 by eliminating the lower assembly cable, leaving only the upper assembly cable interconnected the two TLP units. The cable arrangement shown in Figure 12 substantially reduces costs by eliminating the lower assembly cable and its mountings. The reasons why the cable arrangement of Figure 12 is effective in stabilizing TLP units is explained below with reference to Figures 13 to 16.

[Para 54] In Figures 9-12, the connecting cables have been shown drooping downwardly under their own weight. The extent of such drooping is greatly exaggerated in these drawings, since the drooping is limited by the substantial tension on the cables. To further reduce drooping, the cables can be designed to be buoyancy neutral; a "buoyancy neutral cable" provides a straight line connection and hence a stiff er formation of units. However, such a stiffer formation may be found (by further research, such as by hydrodynamic simulation) to be more or less appropriate in some cases, because it tends to spread lateral movement and forces of one unit to the surrounding units. Hence, the present invention extends to the use of both regular and buoyancy neutral cables. Buoyancy neutral cables can be formed with a thick protective layer, typically formed of synthetic materials, such as syntactic foam, and therefore buoyant, around a steel core. Buoyancy neutral cables can be used with both VFF and TLP units, and with all the cable configurations described herein, including the cross tensioning cables described below with reference to Figures 21 and 22. [Para 55] The manner in which the present invention stabilizes TLP units will now be explained. Figure 13 shows in a highly schematic manner the forces acting on a single conventional TLP unit which is not connected to any similar unit. Arrow A represents the wind thrust on the rotor, hub and other parts of the unit disposed above the water surface. This thrust varies with the design and size of the blades and hub. If an extreme case is reached where thrust A exceeds the design limits of the TLP unit, the whole TLP unit will rotate clockwise (as illustrated) about Point E, a "tension leg connecting arm" where a TLP cable is connected to the unit. Accordingly, rotational moment of the whole TLP unit under extreme wind thrust can be reduced by positioning Point E and other tension leg connecting arms more shallowly below the water surface.

[Para 56] Arrow B in Figure 13 represents horizontal wave load on the TLP unit. This wave load (for a unit of structure) is at its maximum near the water surface. Accordingly, minimizing the cross sectional area of the TLP unit near the water surface (as by the provision of narrow vertical support members 122 and 124 in Figure 2) can reduce this load. Also, positioning tension leg connecting arms more shallowly below the water surface can reduce the moment of load B around Point E.

[Para 57] Arrow C represents the horizontal wave load and Arrow D the vertical wave load on the submerged portion of the TLP unit. In both cases, the wave load depends on hydrodynamic design of the structure, and positioning the large components of the TLP unit (especially the floatation section) deeper can reduce the load.

[Para 58] The condition which it is most desired to avoid in TLP units is the "fall-backward" condition shown in Figure 14. In this extreme case, the horizontal wave force B and the vertical wave force D combine with the wind thrust A to create a catastrophic moment around Point E, causing the opposed tension legs to become slack, so that the entire unit falls backwards, with possible serious damage to, or even destruction of the blades and generator. It is also necessary to guard against the "pushed backward" condition shown in Figure 15. In this condition, the wind thrust A, the horizontal wave force B and the horizontal wave load C all act in the same direction to cause a substantial horizontal force, resulting in excessive horizontal acceleration of the entire TLP unit. (Small horizontal movements occur constantly and the pushed backward condition only becomes problematic when excessive horizontal acceleration of the TLP unit occurs.) The existence of the two separate failure modes illustrated in Figures 14 and 15 places the designer of a conventional TLP in a dilemma; positioning the large components (especially the floatation section) of the TLP unit more deeply below the water surface can substantially reduce the waves loads C and D, but increases the moment around Point E if this point is positioned as deep as the floatation section.

[Para 59] Prior art attempts to solve the aforementioned problems include lengthening the tension leg connecting arms to increase the moment of the buoyancy forces (provided the floatation section of the TLP unit) around the ends of the arms (such as Point E mentioned above) which serve as rotation points during failure. However, such arm lengthening results in a larger and heavier structure for the submerged portion of the TLP unit, and there are few effective solution for the "pushed-backward" problem other than making the entire unit larger, heavier and more costly.

[Para 6O] As illustrated in Figure 16, preferred TLP units used in the present invention reduce the aforementioned problems by minimizing the cross-sectional area of the unit at the water surface (as by the provision of narrow vertical support members 122 and 124 in Figure 2) to reduce the horizontal wave load B, and by positioning the floatation section deep enough to minimize the wave loads C and D, while keeping the tension leg connecting Point E closer to the water surface. The provision of the substantially horizontal lower and upper assembly cables provides support to attenuate the "pushed-backward" problem, and this is one of the key benefits of the illustrated design. Also, as the wind and wave forces as shown on Figure 15 increase, any push-backward movement of the TLP will reduce the vertical component of force on the right upper assembly cable 116, and will increase the vertical component of force on the left upper assembly cable 116, thus aiding to the stabilization of the TLP and reducing the risk of the "fall -backward" condition shown in Figure 14. [Para 61 ] Figure 17 illustrates a modification of the floatation section of the TLP units previously described which can further reduce wave loads on the unit. Essentially, the modified unit shown in Figure 17 allows controlled swinging motion of the largest hydrodynamic component, the floatation chamber, reduces the dynamic wave load for the rest of the structure. [Para 62] As shown in Figure 17, the modified TLP unit has a triangular platform 120 and vertical support members 122 and 124, all of which are generally similar to the corresponding parts of the unit 100 shown in Figures 1 and 2 (the mast 106 is omitted from Figure 17 for ease of illustration. However, instead of being fixedly connected to a floatation chamber, the lower ends of the support members 122 and 124 are secured to a framework comprising a first horizontal section 720 (which may resemble the struts 128 shown in Figure 2 or the members 328 and 330 shown in Figure 3), three vertical sections 722 (only two of which are visible in Figure 17), and a second horizontal section 724, which may resemble the struts 128 shown in Figure 2 or the members 328 and 330 shown in Figure 3. The TLP cables 112 and the upper assembly cables 116 are attached to the periphery of the first horizontal section 720. A floatation chamber 726 is retained within the framework 720, 722, 724; the lower end of the chamber 726 is mounted on the second horizontal section 724 by means of a gimbaled joint 728, while each of the vertical sections 722 is connected to the upper end of the chamber 726 by an active motion control damper 730.

[Para 63] The gimbaled joint 728 allows limited swinging motion of the chamber 726 within the framework, while the active motion control dampers 730 control this swinging motion and limit the maximum movement of the chamber 726 relative to the framework. The dampers 730 can be of pneumatic, hydraulic or spring types, and may be arranged to generate additional electricity.

[Para 64] The use of a movable floatation chamber to reduce wave loading is not confined to TLP units but can extend to VFF units. For example, Figure 18 shows a movable floatation chamber 726 being used in a modified form of VFF Unit A shown in Figure 4.

[Para 65] Figures 19 and 20 are respectively a side elevation (partly in section) and an end elevation (looking from the left in Figure 19) of a cable stabilizer unit ("CSU" - generally designated 800) which may be attached to one or more of the assembly cables of the assemblies described above in order to increase the resistance of the cable against horizontal movement of the VFF or TLP units. As shown in Figures 19 and 20, the cable stabilizer unit 800 comprises a float 802 which rests below the water surface and a short distanced above the remaining components of the CSU, and controls the position and buoyancy of the cable stabilizer unit and the cable to which it is attached, as described below. The float 802 is connected via a cable 804 to the main section of the CSU, which comprises a disc 806 attached at its periphery to a hollow cylinder 808. An assembly cable (illustrated as the cable 132 from any one of Figures 5-8) passes through the center of the disk 806, and six CSU cables 810, three on each side of the disk 806) extend from spaced points on the cylinder 808 to points on the cable 132 spaced from the CSU 800, thus holding the disk 806 perpendicular to the cable 132.

[Para 66] The CSU 800 acts to provide additional resistance to horizontal movement of the cable 132 and hence additional resistance to horizontal movement to the VFF or TLP units attached to the cable 132. The CSU 800 should be located so that it is not affected by wave action (which might cause unwanted horizontal movement of the cable 132) and hence is desirably located at the deepest part of the cable 132, typically midway between the VFF or TLP units attached to this cable. The CSU 800 may be placed along the cable between VFF and /or TLP units to take advantage of the expected phase of the wave forces on the CSU relative to the phase of the wave forces on the VFF and/or TLP.

[Para 67] By way of example, the disk 806 may have a diameter of 8 meters and the cylinder 808 an axial length of 4 meters. The hydrodynamic mass (i.e., the weight of captured water) of such a CSU would be about 200 tonnes and the displacement of the float 802 would be about 20 tonnes. The floater 802 could be formed of fiber-reinforced plastic or a similar material, and the cable 804 of synthetic rope, steel cable or steel chain. The disk 806 and cylinder 808 could be formed of fiber-reinforced plastic or a similar composite material, steel or steel reinforced concrete.

[Para 68] Figures 21 and 22 show alternate cable arrangements which may be substituted for that shown in Figure 11. In Figures 21 and 22, cross tensioning cables 830 are provided extending between the attachment point of an upper cable on one VFF or TLP unit and the attachment point of a lower cable on an adjacent unit, so that the cross tensioning cables 830 extend "diagonally" between the two units. Where two cross tensioning cables 830 pass each other, they can be attached to each other, or left free to move relative to each other at this point. Figure 21 illustrates cross tensioning cables being used to connect two VFF units, while Figure 22 illustrates cross tensioning cables being used to connect one VFF and one TLP unit; cross tensioning cables can of course also be used to connect two TLP units. The use of cross tensioning cables may be used to reduce wave induced motions of the VFF or TLP platform and may enable the individual cables to be made smaller and lighter since the total tension required can be distributed among four cables rather than two. Alternatively, the upper and lower cable shown in Figures 21 and 22 can be eliminated so that only the cross tensioning cables are present. [Para 69] Figure 23 shows a modified form (generally designated 900) of the cable stabilizer unit shown in Figures 19 and 20. The CSU 900 shown in Figure 23 differs from the CSU 800 shown in Figures 19 and 20 in that the section of the main cable 132 passing through the center of the disk 806 is eliminated. This requires that the CSU cables 810 be sufficiently strong that the three (or other number provided) CSU cables on each side of the disk 806 can carry the total tension in the main cable 132. Eliminating the section of the main cable 132 passing through the center of the disk 806 in the manner shown in Figure 23 may simplify attachment of the CSU to the main cable since there is no need to pass the main cable though the disk of the CSU.

[Para 70] Attention will now be given to the second main aspect of the present invention, namely quadrilateral and similar power generation assemblies, which may be used regardless of whether the units are of the VFF or TLP types, or a mixture of both. In the various floating power generation assemblies shown in Figures 10 and 20 of the aforementioned WO 2005/040604, the VFF units are arranged in equilateral triangles. This is, however, not an essential feature of the power generation assemblies; arranging the floating units in equilateral triangles maximizes the number of floating units per area of water surface, but non-equilateral triangles or other polygons, especially quadrilaterals may be desirable to allow for particular circumstances. In particular, when a floating power generation assembly is in a location where the winds are predominantly from one direction, it has been found advantageous to distort equilateral triangle based arrangements such as those shown in Figures 10 and 20 by extending the arrangements along the prevailing wind direction. In theory, the floating units should be arranged so that they minimize shading of one another ("shading" refers to the tendency of an upwind floating unit to reduce the power output of a downwind floating unit) when the wind is in the prevailing direction. In practice, this is essentially impossible to realize, since even in areas where the winds are unusually constant in direction, for example areas where trade winds blow all year, the "prevailing wind direction" will actually be a range of (say) 45 degrees. Accordingly, in practice it is desirable to arrange the floating units so that the distances between units are larger along directions close to the prevailing wind direction. Examples of assemblies based on distorted equilateral triangles are shown in Figures 24 A and 24B.

[Para 71 ] Figure 24A shows a variant of the 6 anchor, 19 floating unit assembly shown in Figure 2OG of WO 2005/040604 modified to allow for a prevailing wind which, for purposes of illustration, is assumed to blow a few degrees above or below the horizontal in the drawing (allowance being made of course for the landscape orientation of the drawing as presented). To allow for the more prevailing wind, the assembly is stretched approximately 50 per cent along a direction which is a few degrees away from the prevailing wind direction, so that the adjacent floating units are separated by six times the blade diameter in the "stretching" direction but by only four times the blade diameter perpendicular to this direction. This increased spacing between floating units in the stretching direction reduces the "shading" effect of one floating unit on an adjacent downstream floating unit.

[Para 72] Assemblies such as those shown in Figure 24A, in which the spacing between adjacent floating units is increased in one direction, may have open angles greater than 120 degrees. Such large open angles tend to make the assembly more vulnerable to disruption by weather. Depending upon the expected weather conditions at the location of the assembly, it may be desirable to provide additional anchors to increase assembly stability and to reallocate some cables to eliminate large open angles. For example, Figure 24B shows a modified form of the assembly of Figure 24A provided with four additional anchors 2802, and with several cables attached to these anchors 2802 reallocated to remove large open angles. [Para 73] As already mentioned, the floating units used in the floating power generation assemblies of the present invention need not be arranged at the vertices of triangles but may be arranged at the vertices of a quadrilateral, preferably a rectangle or square. Figures 25A-25C illustrate assemblies of this type. Figure 25 A illustrates an assembly comprising eight anchors and 21 floating units, the floating units essentially being arranged in a modified 5 x 5 array with the corner units omitted, so that the floating units are arranged at the vertices often squares and four right angled triangles. Figure 25B illustrates a modified version of the assembly of Figure 25 A which has been distorted in the same manner as the assembly shown in Figure 24B, i.e., the assembly of Figure 25B is produced by elongating the assembly of Figure 25 A in the horizontal direction, which is assumed to be close to the direction of the prevailing wind. The assembly of Figure 25C is also produced by elongating the assembly of Figure 25 A, but this time in a diagonal direction so that the floating units in Figure 25 C are actually arranged at the vertices of parallelograms rather than squares (as in Figure 25 A) or rectangles (as in Figure 25B).

[Para 74] Figures 26A and 26B illustrate floating assemblies which combine power generating and non-generating floating units and are intended primarily for research and surveillance purposes. The assembly shown in Figure 26A comprises four anchors, a power generating floating unit 900 and a helicopter deck unit 3102. The assembly shown in Figure 26B comprises six anchors, a power generating floating unit 900, a helicopter deck unit 3102, and a radar unit 3104, with the three units 900, 3102 and 3104 being arranged in an equilateral triangle and each connected to two anchors.

[Para 75] Figures 27 and 28 are enlarged side elevations of the units 3102 and 3104 respectively shown in Figures 26 A and 26B. The submerged part of each of the units 3102 and 3104 closely resembles that of the floating unit 1100 shown in Figure 11 of WO 2005/040604 and is labeled accordingly. The submerged part of the helicopter deck unit 3102 is provided with a submersible docking station 3220. Above the water line, both units 3102 and 3104 are provided with a small vessel dock 3222 and with laboratory or work space 3224. The flat upper surface of unit 3102 forms a helicopter deck 3226. [Para 76] The upper part of unit 3104 comprises a radar or communications dome 3228. In addition, unit 3104 is provided with a sensor unit which comprises floats 3230 tethered by a cable 3232 to an auxiliary cable 1114. A rod or cable 3234 hangs down from the floats 3230 and carries one or more sensor units 3236 (only one such unit is shown in Figure 33); these sensor units 3236 may measure wave motion (as indicated by the double arrow in Figure 33), water temperature and salinity, currents and any other desired parameters. Additional sensors or sensor units may be provided on the main and auxiliary cables if desired, and all sensors can feed back to instruments in the laboratory space 3224.

[Para 77] It will be appreciated that the provision of the various auxiliary structures such as the submersible docking station, vessel dock and sensor units on the units 3102 and 3104 is highly variable and that any of the auxiliary structures can be provided on either unit. Indeed, by enlarging the deck 3226, it might be possible to provide both a helicopter deck and a radar or communications dome, thus essentially combining the functions of the units 3102 and 3104.

[Para 78] Figures 29A to 29D illustrate, in a highly schematic manner, the manner in which the anchors and cables needed to form a floating power generation assembly are assembled and the VFF and/or TLP units attached to the anchors and cables. For simplicity, Figures 29 A to 29D illustrate the assembly of the floating power generation assembly shown in Figure 2OD of WO 2005/040604, which has seven floating units and six anchors, since it is believed that the necessary modifications of the process needed to assemble more complex layouts will readily be apparent to those skilled in the art of deploying anchored floating units. [Para 79] As illustrated in Figure 29A, the process begins by placing anchors 2002 at the position which they occupy in the final floating power generation assembly. The anchors 2002 are connected by cable portions to cable adjustment devices 2004 which serve to lengthen or shorten their associated cables, temporary connectors 2006 and temporary buoys 2008. (Note that one of the anchors 2002, the left-hand one as illustrated in Figure 29A, carries two temporary connectors 2006, while the other anchors carry only one each.) In the next step of the process, as illustrated in Figure 29B, the lengths of the cables are adjusted by the cable adjustment devices 2004 and the various cables are interconnected so that the temporary connectors 2006 occupy substantially the positions which the floating units 900 will occupy in the final assembly, and the cable network is topologically the same as the final network, though not occupying exactly the same positions. The temporary buoys 2008 are omitted from Figure 29B for the sake of clarity, but occupy positions closely adjacent their associated temporary connectors 2006 and serve to keep the temporary connectors 2006 and the adjacent portions of the cables close to the water surface.

[Para 80] Next, as illustrated in Figure 29C, the temporary connectors 2006 are replaced by the floating units 900, without substantially changing the geometry of the assembly; the temporary buoys 2008 are also removed at this stage since the buoyancy of the floating units 900 renders the temporary buoys 2008 unnecessary. Finally, as illustrated in Figure 29D, the cable adjustment devices 2004 are used to adjust the length of the cables so as to provide the necessary tension in the cables and produce the final floating power generation assembly. [Para 81 ] The types of floating units described herein are highly scaleable without substantial modification of the design, and accordingly the preferred dimensions, weights and power outputs mentioned above can vary widely depending upon the particular power output desired.

Claims

1. A floating power generation assembly having as components at least three floating units provided with power generation means and floating in a body of water, the assembly being characterized in that at least one of the three floating units is a tension leg platform comprising a buoyant body, the assembly also comprising at least one first anchor secured to a solid surface beneath the body of water, and at least one first cable maintained under tension and connecting the buoyant body to the first anchor, the floating power generation assembly further comprising at least three second anchors secured to the solid surface beneath the body of water, each of the second anchors being connected by second cables to at least one of the floating units, and each of the floating units being connected by cables to at least two other components, the floating units being arranged substantially at the vertices of at least one triangle or quadrilateral.

2. An assembly according to claim 1 characterized in that each power generation means comprises at least one of a wind turbine and a means for extracting power from waves or marine currents.

3. An assembly according to claim 1 characterized in that each of the floating units is connected by assembly cables to at least three other components of the assembly.

4. An assembly according to claim 1 wherein at least one of the floating units comprises: a mast extending from above to below the water surface; a wind turbine comprising a plurality of blades and rotatably mounted at or adjacent the upper end of the mast such that the blades do not contact the water as they rotate; a buoyancy section provided on the mast adjacent the water surface and arranged to provide buoyancy to the assembly; and a base section provided on the mast below the water surface and having the cables attached thereto, the base section being weighted such that the center of gravity of the floating unit is substantially below the water surface.

5. An assembly according to claim 1 wherein at least one of the floating units comprises: a wind turbine comprising a plurality of blades and rotatably mounted above the water surface such that the blades do not contact the water as they rotate; a mast supporting the wind turbine and extending downwardly therefrom; a platform disposed at the lower end of the mast; a plurality of support members extending downwardly from the platform; a least one floatation chamber attached to the lower ends of the support members.

6. An assembly according to claim 5 further comprising cable attachment struts fixed to one of the support members and having means for attaching cables thereto.

7. An assembly according to claim 5 having a plurality of floatation chambers attached to the support members and provided with means for attaching cables thereto.

8. An assembly according to claim 5, wherein at least on floatation chamber comprises a framework secured to at least one support member, and a floatation member movably secured to the framework so as to reduce the horizontal acceleration of the assembly caused by wave load on the floatation member.

9. An assembly according to claim 1 further comprising at least one cable stabilizer unit attached to a cable, the cable stabilizer unit comprising a float, and a disk connected to the float via a connector member, the disk being attached to the cable and serving to increase resistance of the cable to movement through the water.

10. A floating power generation assembly having as components at least three floating units floating on a body of water, and at least three anchors secured to a solid surface beneath the body of water, each of the floating units being provided with power generation means, each of the anchors being connected by cables to at least one of the floating units, and each of the floating units being connected by cables to at least two other components, the floating units being arranged substantially at the vertices of at least one quadrilateral.

11. A floating power generation assembly according to claim 10 wherein each power generation means comprises at least one of a wind turbine and a means for extracting power from waves and/or currents.

12. A floating power generation assembly according to claim 10 wherein each of the floating units is connected by cables to at least three other components of the assembly.

13. A floating power generation assembly according to claim 10 wherein at least one of the floating units comprises: a mast extending from above to below the water surface; a wind turbine comprising a plurality of blades and rotatably mounted at or adjacent the upper end of the mast such that the blades do not contact the water as they rotate; a buoyancy section provided on the mast adjacent the water surface and arranged to provide buoyancy to the assembly; and a base section provided on the mast below the water surface and having the cables attached thereto, the base section being weighted such that the center of gravity of the floating unit is substantially below the water surface.

14. A floating power generation assembly according to claim 13 wherein the center of gravity of the floating unit is at least about 30 meters below the water surface.

15. A floating power generation assembly according to claim 13 further comprising at least two auxiliary cables extending from the buoyancy section to the cables connecting the base section to other components of the assembly, or to other components of the assembly.

16. A floating power generation assembly according to claim 13 having at least three floating units disposed at the vertices of an equilateral triangle, or a triangle formed by elongating an equilateral triangle along one axis while leaving it unchanged along the other axis.

17. A floating power generation assembly according to claim 10 having at least four floating units disposed at the vertices of a rectangle or square.

18. A floating power generation assembly according to claim 13 wherein the base section is provided with a peripheral hoop arranged to increase the hydrodynamic mass of the floating unit and to lengthen the natural heave period thereof.

19. A process for assembling a floating power generation assembly having as components at least three floating units floating on a body of water, and at least three anchors secured to a solid surface beneath the body of water, each of the floating units being provided with power generation means, each of the anchors being connected by cables to at least one of the floating units, and each of the floating units being connected by cables to at least two other components, the process comprising: placing the anchors in the desired positions, with each anchor having attached thereto a cable, means for varying the length of the cable, at least one temporary connector capable of interconnecting at least two cables, and floatation means capable of keeping the end of the cable remote from the anchor at the water surface; interconnecting the cables by means of the temporary connectors to provide the connections between cables required in the final assembly; replacing the temporary connectors with the floating units; and reducing the length of at least one cable to produce the final assembly.