Flowing-water driveable turbine assembly

Abstract

A flowing-water driveable turbine assembly (104) for location in river or sea areas with unidirectional and bidirectional water flows. The turbine assembly comprises a turbine support (106) with positive buoyancy in water. The turbine support (106) is arranged to be anchored by an anchoring system (108) to a water bed. The turbine assembly comprises at least one turbine (110). The positive buoyancy of the turbine assembly in water has an upward force to constrain the turbine support 106 and the at least one turbine (110) to a position of floating equilibrium against a downward force of the anchoring system (108). The turbine assembly may have variable buoyancy, a duct around each turbine for directing water through the turbine to generate power from water flow, and a winch or winches for submerging the turbine assembly or parts thereof.

Claims

1. An anchoring system for anchoring a positively buoyant turbine assembly in water, wherein the turbine assembly comprises a turbine support and has at least one flowing-water driveable turbine for generating power from water flow, wherein the turbine is secured to the turbine support, wherein the anchoring system comprises at least three anchoring cables anchorable to at least three mutually spaced anchoring points on a water bed covering a footprint greater in width and in length than the turbine assembly, wherein each anchoring point on the water bed is attachable to two mutually spaced attachment points on the turbine support to provide directional support to counteract pitch, roll, and yaw of the turbine support, and wherein the anchoring system is arranged to provide a downward force to constrain the turbine assembly to a position of floating equilibrium against the upward force of the positive buoyancy of the turbine assembly.

2. An anchoring system as claimed in claim 1, wherein the attachments points on the turbine support are mutually spaced in at least a direction of upward force of buoyancy, each anchoring cable couples the two attachment points on the turbine support to a single anchor point on the water bed, and each anchoring cable bifurcates into a pair of cable branches for coupling to the pair of mutually spaced attachment points on the turbine support.

3. An anchoring system as claimed in claim 2, wherein each anchoring cable comprises a mooring line to constrain the turbine assembly against the upward force of the positive buoyancy of the turbine assembly, wherein each anchoring cable comprises a tag line to provide directional support to the turbine assembly and wherein the tag line branches from the mooring line at an intermediate point along the length of the mooring line.

4. An anchoring system as claimed in claim 1, wherein the at least three anchoring cables are at least six pairs of anchoring cables and the ends of each pair of anchoring cables are coupled to a respective anchor point on the water bed, wherein the anchoring cables of each pair of anchoring cables diverge from said anchoring point to where opposite ends of the anchoring cables are fixed to a pair of mutually spaced points of the turbine assembly, and wherein at least three pairs of anchoring cables are fixed to each end of the elongate turbine support.

5. An anchoring system as claimed in claim 1, wherein an angle of inclination of the anchoring cables from the water bed and with respect to the horizontal is 30 degrees+/−15 degrees.

6. An anchoring system as claimed in claim 1, wherein the anchoring cables are streamlined and/or equipped with vortex suppressants.

7. A submersible turbine assembly comprising the anchoring system of claim 1, and wherein the turbine assembly is positively buoyant in water.

8. A turbine assembly according to claim 7, wherein the turbine assembly comprises at least one winch each with a respective pull line connectable to the anchoring system, and wherein the or each winch is operable to pull the turbine assembly towards the water bed by tensile forces acting through the pull line and wherein the or each winch is lockable against tensile forces acting through the pull line.

9. A turbine assembly as claimed in claim 8, wherein each pull line is integrally connected to a part of a respective anchoring cable.

10. A method of anchoring a positively buoyant turbine assembly having variable positive buoyancy in water and comprising at least one winch having a respective pull line connectable to an anchoring system to a water bed wherein the turbine assembly comprises at least one flowing-water driveable turbine for generating power from water flow and the anchoring system comprises at least three anchoring cables, wherein the method comprises the steps of: (a) floating the turbine assembly to an installation site; (b) attaching each anchoring cable to two respective mutually spaced attachment points on the turbine assembly; (c) anchoring each anchoring cable to a respective one of at least three mutually spaced anchoring points on a water bed, wherein the at least three mutually spaced anchoring points cover a footprint greater in width and in length than the turbine assembly and wherein each anchoring point on the water bed is arranged to provide a downward force to constrain the turbine assembly to a position of floating equilibrium against the upward force of the positive buoyancy of the turbine assembly; (d) reducing the positive buoyancy of the turbine assembly; (e) operating the or each winch to submerge the turbine assembly to a target location by force of tension in the or each pull line; and (f) locking the or each winch upon arrival at the target location.

11. The method of claim 10 further comprising a step of: (g) increasing the buoyancy of the turbine assembly.

Description

(1) Embodiments of the turbine assembly, the anchoring system, and the anchoring cables of the present invention will now be described with reference to the drawings of which:

(2) FIG. 1 is a perspective view of an embodiment of the modular turbine assembly of the present invention anchored to a sea bed;

(3) FIG. 2 is a perspective view a turbine module docking with a turbine support of the modular turbine assembly of FIG. 1;

(4) FIG. 3 is a plan view of the top of the turbine support;

(5) FIG. 4 is perspective view of the turbine module;

(6) FIG. 5 is a front elevation view of the turbine module;

(7) FIG. 6 is a cross-sectional view of the turbine module with a flared annular section at each end; and

(8) FIG. 7A to 7C show three stages of disassembling the turbine module.

(9) FIG. 8 is a perspective view of an embodiment of a turbine assembly anchored to a sea bed by an anchoring system;

(10) FIG. 9 is a front elevation view of a modification to the turbine assembly of FIG. 8 having two turbines;

(11) FIG. 10 is a front elevation view of a modification to the turbine assembly of FIG. 8 having four turbines;

(12) FIG. 11 is a perspective view with cut-away of a winch for adjusting a tag line of an anchor cable of the anchoring system of FIG. 8;

(13) FIG. 12 is a vertical cross-section through a buoyancy device of the turbine assemblies of FIGS. 8 to 10;

(14) FIG. 13 is a cross-section through an anchoring point of the anchoring system of FIG. 8; and

(15) FIG. 14 is a perspective view of a tag line or a mooring line of an anchor cable of the anchoring system of FIG. 8.

(16) Referring to FIG. 1, there is shown a sea bed 2 in a region of the sea where water flows in two directions due to tidal forces. Submerged in the water is a modular turbine assembly 4 which is for converting the kinetic energy of the flowing water into electrical energy and delivering it to a facility located on shore or offshore. The turbine assembly comprises a turbine support 6 which is positively buoyant in water and which is anchored to the sea bed by an anchoring system 8. The turbine assembly 4 has an array of five turbine modules 10 arranged in a line in the turbine support 6. Each turbine module 10 is positively buoyant in water. Each turbine module 10 is detachably docked to the turbine support 6. The combined positive buoyancy of the turbine support 6 and the five turbine modules 10 has an upward force which constrains them to a position of floating equilibrium against the downward force of an anchoring system 8.

(17) As the turbine assembly of this embodiment is anchored at sea, a double-headed arrow T shows both directions in which the tidal forces cause the water to flow. The modular turbine assembly 4 is orientated with the array of five turbine modules 10 generally in line with arrow T so that as much water as possible flows through the turbine modules in a straight path.

(18) The turbine assembly 4 is described as modular because the turbine modules 10 are interchangeable with each other and are docked to the in the support structure 6 in the same way.

(19) Referring to FIG. 2, there is shown a turbine module 10′ being pulled downward by a winch on the turbine support with two pull lines 12. The turbine module 10′ docks with the turbine support 6. Once docked with the support structure, the turbine module 10′ is anchored to the sea bed by the anchoring system unless, or until, at some time in the future the turbine module 10′ is detached for maintenance, repair or replacement.

(20) If, or when, maintenance, repair or replacement is required, the turbine module 10′ is detached from the turbine support and allowed to float under its own inherent buoyancy in water to the surface. The assent of the turbine module 10′ is controlled by the winch with two pull lines 12.

(21) Alternatively, the winch with two pull lines can be substituted by a remote operated vehicle to perform the task of submerging the turbine module to dock with the turbine support. The remote operated vehicle can perform the task of controlled floatation of the turbine module to the surface too.

(22) Referring to FIG. 3, the turbine support 6 comprises a frame 14. The frame can be made of any material strong enough to support the turbine modules (i.e. steel, aluminium, fibre reinforced concrete, inflated material or composite). The frame has elements that are filled with buoyant material, or that are attached to buoyant material, to provide the positive buoyancy of the turbine support. The positive buoyancy may be adjusted by means of compressed air or buoyant gel or by another medium pumped from the surface or supplied by sub-sea reservoir. The positive buoyancy of the turbine support 6 is enough to be towed to the anchoring site.

(23) The frame 14 is divided into five turbine module docking bays 16. Each docking bay 16 is accessible through the top of the frame to receive a respective turbine module 10. Each docking bay has a pair of flared annular sections 20a, 20b connected to the frame 14 of the turbine support 6. One flared annular section is located at each end of the docking bay. Each flared annular section tapers towards the docking bay.

(24) Referring to FIGS. 4 and 5, the turbine module 10 has a major body shell 22 and a minor body shell 24 joined to form a duct 26 which defines a hollow generally cylindrical bore. An external surface 27 of the joined minor and major body shells has a generally elliptical profile transverse the cylindrical bore of the duct. The body shells 24, 26 are filled with buoyant material (i.e. a fluid, solid or a combination of both), or are attached to buoyant material, to provide the positive buoyancy of the turbine module. The positive buoyancy of each turbine module 10 is enough to be towed to an anchoring site.

(25) The turbine module 10 has a water-driveable horizontal axis turbine 28 mounted upon a bracket 30 inside the duct. The turbine has a rotor 32 co-axial with the duct. The rotor is matched to the diameter of the duct. The duct shields the turbine from turbulence caused by adjacent turbines so that the array of five turbine modules may be closely spaced. The turbine is driveable by water flowing in either direction through the duct and generates electrical power.

(26) Returning to FIG. 2 in more detail, each turbine module 10 is docked with a respective docking bay 16 in a complementary locating arrangement which automatically orientates a major axis 18 of the elliptical profile of the external surface 27 in a generally upright position in the turbine support 6 where the turbine module is locked in place by a locking mechanism. This reduces the horizontal cross-sectional area of the turbine module. As a result, the turbine module is streamlined to reduce interaction with upward or downward wave motion in the surrounding water.

(27) Electrical connections between the turbine modules 10 and the turbine support 6 are made before or after docking. The electrical power generated by the turbines varies with water flow rate. Each turbine module has electrical power equipment (not shown) for conditioning the electrical power generated by the turbines. The turbine support has electrical power management equipment (not shown) for combining the conditioned electrical power from the five turbine modules. The turbine support's electrical power management equipment includes a step-up transformer (not shown) for transmission of the generated electrical power to a shore, or offshore, facility via a power cable 40. A communication cable 42 from the turbine assembly accompanies the power cable.

(28) Referring to FIG. 6, there is shown the duct 26 in fluid communication with the pair of flared annular sections 20a, 20b when one of the turbine modules 10 is docked in one of the docking bays 16 of the turbine support 6. The annular sections are suited for bi-directional water flow. Single headed arrow T indicates a direction of water flow which results in the up-flow annular section 20a performing the role of concentrator to scoop water into the duct and the down-flow annular section 20b performing the role of diffuser to emit water from the duct. This situation will be reversed when the tide changes and water flows through the duct in the opposite direction and arrow T is reversed (i.e. annular section 20b becomes the concentrator and annular section 20a becomes the diffuser).

(29) The geometry of the annular sections 20a, 20b is matched to the water flow requirements of the turbine. The annular sections may be made of steel, aluminium, fibre reinforced concrete, inflated material or composite. The annular sections are connected may contribute the positive buoyancy of the turbine support.

(30) The boundaries between the duct 26 and the flared annular sections 20a, 20b each have an annular gap 34a, 34b. The gaps enable water flowing outside the turbine module to enter the diffuser (i.e. the down-flow annular section 20b in this example) by venturi effect. This promotes water flow augmentation which reduces water eddies by re-establishing a boundary layer connection between water flow and the diffuser. A reduction in water eddies is beneficial because it reduces parasitic energy losses and drag.

(31) The ends of the flared annular sections 20a, 20b facing away from the duct 26 are each equipped with an array of transverse vanes 36a, 36b. The vanes help prevent ingress of debris into the duct and help straighten the water flowing into the turbine 28. The vanes induce a rotational flow into the water flow to increase the energy extraction of the turbine.

(32) Referring to FIG. 7A, the positive buoyancy of the turbine module 10 is localised about the major axis 18 of the elliptical external surface 27. As a result, the turbine module tends to float on the water surface with an increased horizontal cross-sectional area. This improves stability, and reduces the draft, of the turbine module when it is being towed at sea.

(33) Referring to FIG. 7B, the major body shell 22 has positive buoyancy to enable removal of the minor body shell 24 while the major body shell and the turbine 28 remain afloat. Removal of the minor body shell allows complete access to the turbine, and even removal of the turbine by floating crane, for the purpose of repair or maintenance to the turbine module, as is shown by FIG. 7C.

(34) Returning in more detail to FIG. 1, the anchoring system 8 comprises eight pairs of anchoring cables 44a, 46a-44h, 46h. Each pair of anchoring cables includes an upper anchoring cable 44 and a lower anchoring cable 46. Two pairs of anchoring cables are fixed to each corner edge of the frame 14 of the turbine support 6 (i.e. the upper anchoring cable of each pair is fixed to the corner edge above where the lower anchoring cable of each pair is fixed to the corner edge). The other ends of each pair of anchoring cables are permanently fixed to a respective anchor point 48a-48h on the sea bed.

(35) The anchoring cables 44a, 46a-44h, 46h of each pair of anchoring cables converge from the frame 14 of the turbine support 6 to their respective anchoring points 48a-48h. The mean angle of inclination of the anchoring cables of each pair of anchoring cables with respect to the horizontal is approximately 30 degrees.

(36) The anchor points 48a-48h are arranged about the turbine support 6 to suit the sea bed topography and to maintain the turbine support in a generally horizontal position. The anchor points cover a footprint greater in width and in length than the turbine support.

(37) The upward force of the combined positive buoyancy of the turbine support 6 and the five turbine modules 10 cause tensile forces along the full length of the anchoring cables 44a, 46a-44h, 46h.

(38) The anchoring cables may be (preferably high performance) synthetic rope, steel/wire rope, chain, solid metallic rod or solid composite rod.

(39) The anchoring cables are equipped with vortex suppressants to reduce their hydrodynamic drag and reduce any vibration caused by water flow. For example, a vortex suppression system may be fibre or tape strands incorporated or attached to the anchoring cables. The fibre or tape strands stream with the water flow to form a fairing, or a hydrofoil. Rotating faired sections which fit over the anchoring cables and align with the water flow, spiral sections either fitted to or incorporated into the structure of the anchoring cable, or other proprietary vortex suppression systems are also suitable.

(40) Returning to FIG. 2, the power cable 40 and the communication cable 42 from the turbine assembly are incorporated within the upper anchoring cable 44a.

(41) The modular turbine assembly is assembled at sea by towing the turbine support to an anchorage site, submerging it, and anchoring it to the sea bed with the anchoring system where it remains permanently. The five turbine modules are towed to the anchorage and submerged, each one in turn, to dock with the turbine support.

(42) To recap, the following are important features of at least some preferred embodiments of the present invention, and each can be provided independently or in different embodiments.

(43) A tethered sub-sea installation base which, when populated with devices, in itself comprises a small array of horizontal axis Tidal Energy Convertors (TECs). The base is for use at deep water sites (over 40 msw) and enables the TECs to be positioned at the optimum depth dictated by the compromise between power output (strongest current found close to the surface) and adverse structural and flow influences from wave interaction. Alternatively, the base may be used at shallower sites where it is submerged very close to, or even slightly protruding above (provided the TECs are submerged), the water surface.

(44) As an integral part of the design a method is disclosed of installing and retrieving the TECs using buoyant modules into which individual TECs are loaded. The loaded modules are then towed to site and connected to the sub-sea base electrically and via a pull in line. The module is pulled sub-sea by the pull in line and interfaces with and locks into the sub-sea base.

(45) An alternative to the above method, the buoyant modules may be driven to and retrieved from the PMSS by means of a Remote Operated Vehicle (ROV) specifically designed for the purpose and having the required thrust capability. This may include the use of variable buoyancy within the buoyant module to reduce the quantity of thrust required to drive the module subsea.

(46) The turbine support may be a permanently installed buoyant subsea structure PMSS comprising: Structural space frame which may be of steel, aluminium or composite construction—the elements of which may be sealed to form pressure vessels, or may be filled with, surrounded by, or have attached buoyant material (including air or other gas) providing all or part of the buoyancy required to support the structure. ‘Conical’ diffuser and concentrator sections—the precise geometry of the concentrator and diffuser can be matched to the flow requirements of the TEC. The diffuser and concentrator sections can be suited to bi-directional flow The diffuser and concentrator sections can incorporate ‘slots’ to enable flow augmentation to re-establish boundary layer connection within the diffuser. The diffuser and concentrator sections may be constructed from steel, aluminium, fibre reinforced concrete, inflated material (i.e. ‘hyperlon’ or similar), composite (i.e. glass or other fibre reinforced plastic). The diffuser and concentrator sections may contribute to the buoyancy of the PMSS Buoyancy of the PMSS may be adjustable by means of compressed air or buoyant gel or other medium pumped from the surface or supplied from a subsea reservoir. Step up transformer for transmission of generated electrical power to shore or offshore processing facility via power cable. Power conditioning and switching equipment as required to combine and transmit the output of one or more tidal energy convertors as electrical power.

(47) The anchoring system is a tension spread mooring system (TSM) Sea-bed fixing points which may be drag anchors; gravity anchors; suction piles; pinned template structures; attachment to sub-sea geographical features; Tension members which may be high performance synthetic rope such as UHMwPE (i.e. dyneema); steel/wire rope; chain; solid metallic rod (i.e. nitronic 50; 17-4 pH; 316 stainless steel etc.) Vortex Induced Vibration (VIV) suppression system which may be fibre or tape strands incorporated or attached to the tension member which streams with the flow to form a fairing (‘hairy’ or ‘ribbon’ fairing); Rotating faired sections which fit over the tension member and align with the flow; Spiral sections either fitted to or incorporated into the structure of the tension member, or other proprietary vortex suppression system. Power transmission cable incorporating power conductors and communications (i.e. fibre optic or conventional signal pair conductors) The power transmission and communication cables may be incorporated into one or more of the tension members (i.e. the structural cable casing may act as tension member(s)).

(48) The turbine module is a buoyant module (BM) Parallel annular duct matched to TEC to reduce the effects of off axis flow and wave interaction by straightening and aligning current flow with the TEC axis. Once installed the BM Integrates with the ‘conical’ diffuser and concentrator sections (which form part of the PMSS) to enhance performance over that achievable in open ocean conditions. BM's installed into the subsea structure by sub-sea pull in lines, buoyancy control or a combination of the two. The BM has an elliptical (or otherwise non-circular) distribution of volume to reduce the horizontal area presented to wave motions, and to give stability when on the surface. The BM can contain power conditioning equipment as required for each individual TEC to enable the power produced to be fed to the centralised step up transformer for onward transmission. The BM can be split to allow installation of the TEC by means of overhead crane. This minimises the crane capacity required.

(49) Power for Sub-Sea Operations:

(50) Power to drive the subsea winches may be provided by equipment permanently or temporarily fitted to the sub-sea structure, or may be provided by means of an umbilical connection from a surface ship, or by specially equipped Remote Operated Vehicle (ROV).

(51) Protection against object ingress. Vanes on the concentrator and diffuser may provide some or all of the following functions: Prevent the ingress of marine fauna, flora and flotsam/debris Guide objects clear of the duct and sub-sea structure Further straighten the flow into the turbine blades Induce counter rotational flow into the water stream to increase the energy extraction potential.

(52) The vanes are not a fundamental part of the design but may have significant efficiency benefits if considered as part of the turbine design as it may allow significantly higher rotor speeds and therefore lighter, lower cost generators. The design of the vanes may simply look similar to two traditional ‘cow catchers’, mirrored and joined on the centre-line to form an inlet guard—one such unit at each end to catch and guide any objects clear of the inlet to the turbine duct.

(53) Referring to FIG. 8, there is shown a sea bed 102 in a region of the sea where water flows in two directions due to tidal forces. Submerged in the water is a turbine assembly 104 which is for converting the kinetic energy of the flowing water into electrical energy and delivering it to a facility located on shore or offshore. The turbine assembly comprises a turbine support 106 which is anchored to the sea bed by an anchoring system 108. The turbine assembly 104 has a single turbine 110 secured to the turbine support 106. The turbine support 106 comprises a frame 106a and a pair of buoyancy devices 107 located at opposite ends of the frame 106a for stability.

(54) The turbine 110 is secured to the frame 106a between the buoyancy devices 107. The turbine is located approximately midway between the buoyancy devices to minimise instability about the X axis of the turbine assembly which passes through the central axis of the turbine. Optionally, the turbine 110 may be detachable and interchangeable with other turbines.

(55) The turbine support 106 has positive buoyancy in water which is variable by virtue of the buoyancy devices 107, as is explained in more detail below. Optionally, the turbine 110 may have variable positive buoyancy in water. The combined positive buoyancy of the turbine assembly 104 in water has an upward force which constrains it to a position of floating equilibrium against the downward force of the anchoring system 108.

(56) As the turbine assembly of this embodiment is anchored at sea, a double-headed arrow T shows both directions in which the tidal forces cause the water to flow. The turbine assembly 104 is orientated with axis X through the turbine 110 generally in line with arrow T so that as much water as possible flows through the turbine in a straight path. The buoyancy devices 107 are streamlined and elongate in the direction of axis X of the turbine assembly to minimise hydrodynamic resistance to water current flowing in line with arrow T.

(57) Referring to FIG. 9, there is shown a first alternative turbine assembly 204 which comprises a turbine support 206 similar to the turbine support 106 mentioned above albeit having two turbines 110 secured to the turbine support 206. The turbine support 206 comprises a frame 206a and three buoyancy devices 107, two located at opposite ends of the frame 206a for stability and a third buoyancy device 107 in the middle of the frame 206a for additional buoyancy. The turbines 110 are secured to the frame, one turbine in each gap between the buoyancy tanks. The turbines are located approximately midway between the buoyancy devices to minimise instability about the X axis.

(58) Referring to FIG. 10, there is shown a second alternative turbine assembly 304 which comprises a turbine support 306 similar to the turbine supports 106, 206 mentioned above albeit having four turbines 110 secured to the turbine support 306. The turbine support 306 comprises a frame 306a and three buoyancy devices 107, two located at opposite ends of the frame 306a for stability and a third buoyancy device 107 in the middle of the frame 306a for additional buoyancy. The turbines 110 are secured to the frame, two turbines in each gap between the buoyancy tanks. The centre of gravity of each pair of turbines is located approximately midway between the buoyancy devices to minimise instability about the X axis.

(59) Referring to FIGS. 8 to 10, the frame 106a, 206a, 306a of the turbine support can be made of any material strong enough to support the turbine or turbines 110 (i.e. steel, aluminium, fibre reinforced concrete, inflated material or composite).

(60) Referring to FIG. 11, the buoyancy devices 107 are secured to the frame 1006a, 206a, 306a. Each buoyancy device comprises a fixed buoyant material 112, such as foam, at opposite ends of the buoyancy device and a ballast tank 114 located in between the fixed buoyant material. The buoyancy of the turbine support may be adjusted by filling the ballast tank with water to reduce positive buoyancy. Reduced positive buoyancy may be desirable to reduce the force required to submerge the turbine assembly during its installation at a target location. The buoyancy of the turbine support may also be adjusted by emptying the water from the ballast tank and replacing it with air to increase positive buoyancy. Increased positive buoyancy may be desirable to make it easier to tow the turbine assembly to the anchoring site. The air may be compressed air stored on the turbine support. Alternatively, buoyant gel may be used or by another buoyancy medium pumped from the surface or supplied by sub-sea reservoir. The ballast tank 114 is located midway between equal amounts of fixed buoyant material 112 to minimise instability about the Y axis of the turbine assembly when the buoyancy of the ballast tank is being varied.

(61) FIG. 11 shows a cross-section through one buoyancy device 107 secured at an end of the turbine support 106, 206, 306 and it shows parts of the frame 106a, 206a, 306a equipped with a tag line winch 116 and a mooring line winch 118. There is shown a tag line winch at each nose end 120 of the buoyancy device. Each nose end 120 is partially formed of fixed buoyant material 112 and has a fairing which covers the tag line winch. There is also shown a mooring line winch underneath the water ballast tank. The purpose of the winches is explained in more detail below.

(62) Returning to FIG. 8, the turbine 110 has an elongate generally cylindrical body shell 122 coaxial with axis X to minimise hydrodynamic drag. As mentioned above, the turbine may have positive buoyancy and, if so, the body shell 122 is filled with buoyant material (i.e. a fluid, solid or a combination of both), or is attached to buoyant material.

(63) The turbine 110 is a water-driveable horizontal axis turbine with a rotor 132 having two rotor blades. The rotor may have three of more rotor blades. The rotor diameter is typically 16 m for a 1 MW turbine although the rotor diameter may range from 2 meters for a 50 kW turbine, 6 meters for a 200 kW turbine and 20 meters for a 2 MW turbine. The turbine shown in FIG. 1 is not ducted, although as duct may be fitted around the rotor to shield the turbine from turbulence caused by any adjacent turbines or wave action and to increase efficiency of energy extraction from the water current. For example, the rotor 132 of the turbine 110 may be located in a duct in fluid communication with a concentrator at one end of the duct and a diffuser at the other end of the duct, like the duct 26 and the pair of flared annular sections 20a, 20b shown in FIG. 6. The turbine 110 is driveable by water flowing in either direction through the rotor 132 and generates electrical power.

(64) Electrical connections between the turbine 110 and the turbine support 106, 206, 306 are made when the turbine is secured to the turbine support. The electrical power generated by the turbines varies with water flow rate. Each turbine has electrical power equipment (not shown) for conditioning the electrical power generated by the turbines. The turbine support has electrical power management equipment (not shown) for combining the conditioned electrical power from the turbine or turbines. The turbine support's electrical power management equipment includes a step-up transformer (not shown) for transmission of the generated electrical power to a shore, or offshore, facility via a power cable 140. A communication cable 142 from the turbine assembly accompanies the power cable.

(65) The anchoring system 108 comprises four anchoring cables 144a-144d. Each anchoring cables comprises a mooring line 145a-145d and a tag line 146a-146d branching from the mooring line at an intermediate point along the mooring line. One end of the mooring line of each anchoring cable is connected to its nearest lower corner of the turbine frame 106a, 206a, 306a. The mooring lines are connected below the centre of buoyancy of the turbine assembly to maintain stable pitch and roll attitude. The tag line of each anchoring cable is connected to the turbine frame at the nearest nose end 120 of its nearest buoyancy device 106a, 206a, 306a (i.e. the tag line of each anchoring cable is connected to a corner of the frame end approximately above the corner of the frame where the mooring line is connected in the direction of upward force of the positive buoyancy). As such, an end of the tag and mooring lines of an anchoring cable are connected to each corner of the frame. The other opposite end of each mooring line is permanently attached to a respective anchor point 148a-148d on the sea bed. The power cable 140 and the communication cable 142 from the turbine assembly are incorporated within the mooring line 145b.

(66) The anchoring cables 144a-144d diverge outwardly from turbine support to the water bed. The mooring lines 145a-145d under tension form the edges of a substantially pyramidal shape on the sea bed. The mean angle of inclination α of the mooring line of each anchoring cable with respect to the horizontal is approximately 25 degrees. The mean angle of inclination β of the tag line of each anchoring cable with respect to the mooring line is approximately 15 degrees.

(67) The anchor points 148a-148d are arranged about the turbine support 106, 206, 306 to suit the sea bed topography and to maintain the turbine support in a generally horizontal position. The footprint of the anchoring system upon the sea bed, as defined by where the mooring lines of the anchoring cables are attached to the anchoring points 148a-148d, is greater in width and in length than the turbine support. The enlarged footprint improves the stability of the turbine assembly.

(68) The upward force of the positive buoyancy of the turbine assembly 104, 204, 304 causes tensile forces along the full length of the anchoring cables 144a-144d. The turbine assembly 104, 204, 304 anchored by the anchoring system 108 typically has an operational depth in the top third of water column where power extraction from water current is optimal. This is unlike traditional anchoring systems, such as gravity anchors or columns driven into sea bed, which have an operation depth in the bottom third of water column where power extraction from water current is sub-optimal because water currents are slower down there.

(69) The fixed buoyant material 112 of the buoyancy devices 107 has sufficient positive buoyancy in water to result in an equilibrium state at the operational depth with zero water current and wave loading when the turbine assembly 104, 204, 304 is anchored to the sea bed by the anchoring system 108. A variable component of positive buoyancy is additionally required to provide sufficient upward force to counteract the drag moment around the anchor points 148a-148d created by longitudinal drag caused by current flow and longitudinal and horizontal wave particle velocities. Water current speeds can vary between 0 m/s (calm) and 8 m/s (storm conditions) and the optimal water current speed for peak power output from the turbines 110 is about 2.5 m/s. The mean water current speed at any particular site depends on factors such as depth of water column, location of turbine assembly and the bathymetry of the sea bed. The variable component of positive buoyancy provided by the ballast tanks 114 can either be varied (i.e. by emptying of ballast tanks of water and filling them with air) upon installation of the turbine assembly at site and then be constant for its operational life or it can be varied during its operational life if required. Additionally or alternatively, the variable positive buoyancy can be supplemented throughout the tidal cycle by hydrodynamic upward force. Hydrodynamic upward force may be provided by hydrofoils, or fins, secured to the turbine support 106a, 206a, 306a. Referring to FIGS. 8 to 10, there is shown hydrofoils 150 protruding outwardly from the ballast tanks 114. The positive buoyancy and hydrodynamicupward force required to counteract the drag moment around the anchoring points is proportional to the maximum water current speed experienced by the turbine assembly. The hydrofoils 150 are shaped to increase upward force with increasing water current and, in doing so, provide increasing counteraction to the drag moment around the anchor points caused by increasing longitudinal drag and maintain the turbine assembly at the desired elevation above the sea bed.

(70) In normal operating conditions, excursion of the turbine assembly may be about +/−2 meters in both the horizontal and vertical planes. In storm conditions, excursion of the turbine assembly may be about +/−10 meters in both the horizontal and vertical planes.

(71) In practice, we have found that the proportion of the variable upward force divided by the total upward force (fixed and variable) of the turbine assembly should be 10% to 20% of the figure (expressed as meters/second) of the maximum current flow speed in line with the turbine assembly. Likewise, we have found that the proportion of the variable upward force divided by the total weight of the turbine assembly should be 20% to 30% of the figure (expressed as meters/second) maximum current flow speed in line with the turbine assembly.

(72) The tag lines 146a-146d and the mooring lines 145a-145d of the anchoring cables may be ropes made of nylon, polypropylene and/or high performance polyethylene materials or the anchoring cables may be steel/wire rope, chain, solid metallic rod or solid composite rod. The tag lines are made of different material to the mooring lines and the tag lines have a greater elasticity than the mooring lines. Thus, the mooring lines are to constrain the turbine assembly against the upward force of the positive buoyancy of the turbine assembly. The tag lines are to provide directional support to counteract pitch, roll or yaw movement of the turbine assembly 104, 204, 304 about the X, Y and Z axes. The tag lines have integral resistance to shock in the event of sudden movement of the turbine assembly. Additional resistance to shock may be provided by dampers connected in series or in parallel with one or more of the tag lines.

(73) Returning to FIG. 11, each tag line 146a-146d is connected to a respective tag line winch 116 and each mooring line 145a-145d is connected to a respective mooring line winch 118. The winches 116, 118 are operable to vary the length of the tag lines and the mooring lines. The winches are used to submerge the turbine assembly 104, 204, 304 from the water surface to its target operational depth as is explained in more detail below. Additionally, the winches may be used to adjust the orientation of the turbine assembly about the X, Y and Z axes and, in doing so, orientate it in the optimal direction when anchored to the sea bed. The winches are externally activated, typically by a remote operated vehicle, although the winches may be activated by their own electric motor. Each winch is ratcheted to lock it against unintentional release of the tag lines and mooring lines under tension. The frame 106a, 206a, 306a of the turbine assembly is equipped with the tag line and mooring line winches, but the skilled person will understand that the winches may be installed on the anchoring system, such as near or at the anchoring points 148a-148d.

(74) Referring to FIG. 14, there is shown the mooring line 145a of anchoring cable 144a where it is connected to the anchoring point 148a. The anchoring point protrudes from in a hole drilled 15 meters into the sea bed at an angle α of approximately 25 degrees to the horizontal. The drilling operation may be performed by a remote operate vehicle deployed upon the sea bed. Approximately the bottom ten meters of the hole are in bed rock 150 and approximately the top five meters of the hole (including the open mouth of the hole) are in weathered rock 152. The diameter of the hole is approximately 0.25 m into which a single or multiple tendon anchor 154a is installed and grouted. The anchoring point 148a is on the exposed end of the anchor 154a and the mooring line is connected thereto. Once the grout has hardened tension may be applied to the mooring line.

(75) The mooring lines 145a-145d and the tag lines 146a-146d of the anchoring cables are equipped with vortex suppressants to reduce their hydrodynamic drag and reduce any vibration caused by water flowing past them. The vortex suppressant comprises a helical protrusion 156 arranged about the circumference of each line and woven into or bonded to the strands of the rope material used to make the line. The helical protrusion has a pitch 158 of approximately twelve times the diameter 160 of the line, although a pitch 158 falling within the range of four to sixteen times the diameter 160 of the line can be used. The helical protrusion has an outer diameter 162 of approximately 150% of the diameter of the line, although an outer diameter 162 falling within the range of 110% to 200% times the diameter 160 of the line can be used.

(76) The helical protrusion 156 is applied to the rope of the mooring line 145a-145d or tag line 146a-146d by one or more of the following methods: a) Arranging the weave of the material used to make the rope so that a helix is generated which is more pronounced than the other windings, and displays the characteristics of the pitch ratio described above; b) Additional materials may be added during the production of the rope to bulk out the rope to form the helix. The bulking material may the same material as the rope or a rigid section of thermoplastic material pre-formed as a helix and bonded to the rope; and/or c) An outside cover that is either wrapped or whipped around the rope with parts woven in for continuity at various points. Materials could be the same as the core rope or the others listed above.

(77) Other possible vortex suppressants include fibre or tape strands incorporated or attached to the anchoring cables. The fibre or tape strands stream with the water flow to form a fairing, or a hydrofoil. Rotating faired sections which fit over the anchoring cables and align with the water flow, spiral sections either fitted to or incorporated into the structure of the anchoring cable, or other proprietary vortex suppression systems are also suitable.

(78) The floating turbine assembly 104, 204, 304 is initially assembled in harbour whence it is towed to an anchorage site where four anchoring points 148a-148d have been fixed to the sea bed. The mooring lines 145a-145d and the tag lines 146a-146d of the anchoring cables 144a-144d are unwound from their respective winches 116, 118 and are submerged towards the sea bed. The free ends of the mooring lines are fixed to respective anchoring points. The variable positive buoyancy is reduced by filling the ballast tanks 114 with water. The winches are turned slowly to wind up the tag lines and mooring lines. The winches are operated by remote operated vehicle. The turbine assembly is steadily submerged to its operational depth. The ballast tanks are re-filled with air upon arrival at the operational depth. This increases positive buoyancy so that the turbine assembly is anchored to the sea bed in a state of equilibrium by the anchoring system 108 with tensile forces in the mooring lines and the tag lines.

(79) As noted above, each feature may be provided independently and applied to other embodiments or aspects.