GB2471874A

GB2471874A - Support for Water Turbine

Info

Publication number: GB2471874A
Application number: GB0912317A
Authority: GB
Inventors: Michael Torr Todman; John Richard Carew Armstrong
Original assignee: Individual
Current assignee: Individual
Priority date: 2009-07-15
Filing date: 2009-07-15
Publication date: 2011-01-19
Anticipated expiration: 2029-07-15
Also published as: GB0912317D0; GB2471874B

Abstract

An underwater turbine device has a frame or support 10 for turbines 11. The frame 10 is formed by two identical buoyant legs 12 having long 21 and short 22 parts. The legs 12 are connected together by limbs 13 having a hydrofoil shape. A tether 14 for an underwater anchorage 15 allows the device to rotate and align itself with current flow. The buoyancy of the legs 12 can be varied so that the device may be in a working position underwater, where the short legs 22 pierce the surface of the water to act as a visual identification, and a maintenance position, where the whole device floats on the water surface (fig 2).

Description

I

Support for Water Turbine This invention relates to turbines for the generation of power from water currents, especially those from coastal regions or river estuaries.

Many systems have been proposed for the mounting of such turbines in the stream flow. A particular problem is how to arrange that turbines can be delivered, deployed in an operating position and then recovered for maintenance or removal.

A feature of some such systems is the use of a mounting frame with variable buoyancy to assist in the deployment and recovery stages. GB 2348249B (Armstrong) for instance discloses the use of variable buoyancy to roll a mounting frame about an inclined longitudinal axis passing through an underwater anchorage, from a deployed operating position to a horizontal floating maintenance position. GB 2434410A (Todman) discloses the use of variable buoyancy combined with controlled flexure of a mounting frame about a rigid tether arm to achieve a similar effect, but without roll rotation. Several other proposals have been made on the basis of flexible tethers such as cables, for instance GB 2256011 (Fraenkel).

Other factors to consider include the stability of pitching of the turbines in operating flows (i.e. rotation about a transverse horizontal axis through the underwater anchorage) Some prior disclosures, such as those referenced above and GB 2409885 (MCT), use surface penetration by one or more parts of the mounting to achieve buoyancy-driven passive pitching stability, so that under varying flow and drag conditions the turbines will rise and fall within controlled limits. Others, wishing to achieve complete submersion of their turbines, use instead of surface-piercing members, active control surfaces to achieve such stability (e.g. WO 00/42318 Dehlsen). Some, for instance GB 2422878B (Mackie), propose surface-piercing members to control pitching and active control surfaces to regulate roll of a flexibly tethered turbine.

Alternative means have been proposed for arranging an underwater anchorage. In some proposals, anchoring of the turbine tether is at the seabed itself. In other proposals, for instance GB 2422878B (Mackie), the turbine is tethered to a buoyant chamber located at mid-depth, which is in turn anchored to the seabed. It may be appreciated that these variations can lead to differences in the stability of the turbine and its mountings under various conditions of flow magnitude and direction, turbulence and wave-induced motion.

The present invention is aimed at combining functional and practical deployment of a tidal stream turbine with enhanced hydrodynamic stability in operation.

According to a first aspect of the invention, there is provided a mounting for an underwater turbine, and comprising: a support adapted to receive thereon one or more turbines, and having a substantially upright member adapted to pierce the surface of the water in use, a rigid tether on said support and extending in use at a pre-determined downward orientation therefrom, one end of said tether being adapted for pivotal connection to an underwater anchorage, and variable buoyancy means to raise and sink said support about a substantially transverse pivot axis of the anchorage, between an underwater operating position in which turbines are in use submerged and a surface maintenance position in which said turbines are above the waterline.

Movement from operating to maintenance positions and vice versa is effected by pitching (rotation about a generally horizontal transverse axis through a seabed hinge), which is both simple and effective.

The arrangement avoids articulation between the support and tether, as is required in the prior art; in use the tether and support move through the same arc in a generally vertical plane, between the operating and maintenance positions. More than one substantially upright member may be provided on the mounting In one embodiment the support is pivoted about a transverse axis extending through said anchorage so as to be movable in pitch and yaw only. In another embodiment the support pivots as a free body in pitch, yaw and roll. In use the support trails in the stream flow, and is self-aligning.

In a preferred embodiment the variable buoyancy means comprises a chamber of the support, which in use is more or less filled with water so as to determine the operating depth of the support. Thus water is admitted to submerge the support, and voided, for example by pump, to bring the support to the surface. The substantially upright surface piercing member may comprise said chamber.

Preferably the support is disposed in a generally vertical plane and comprises a first end portion which is substantially vertical in the operating position, and a second end portion which is substantially horizontal in the maintenance position. Typically the first and second end portions have an included angle in the range 120° -1500. The second end portion is longer than the first end portion in a preferred embodiment. It will be appreciated that in the operating position, the first end portion pierces the water surface, whereas in the maintenance position the first end portion is substantially clear of the surface. Typically the first end portion will be shaped to minimize hydrodynamic drag.

Typically the turbines will be disposed on the support with a horizontal rotor axis in the operating position of the support, and thus be at an angle to the horizontal when pivoted to the maintenance position.

Preferably the turbines are wholly out of the water in the maintenance position, and are thus substantially offset with respect to the second end portions (so that in the operating position they are downstream with respect to stream flow).

In a preferred embodiment the support includes one or more hydrofoil surfaces effective in the operating position, and typically constituted by substantially horizontal members of the support. Such surfaces aid pitch stability of the support in use, and are preferably fixed relative to the support so as to be immovable in use.

In a preferred embodiment the tether comprises a separate rigid member attached to the support by a lockable hinge so as to be movable from a transport condition to a locked operating condition. The tether is arranged so that in use the first end portion is upstream of the second end portion.

The hinge may be lockable also in the transport condition. Such an arrangement allows the tether to lie in substantially the same plane as part of the support for deployment, for example by floatltow to the operating site, and be locked in the operating condition (in which it is anchored to the seabed) upon arrival at the operating site. In a preferred embodiment the tether lies in the same plane as the second end portion during travel on the water surface.

The means of locking the tether in the deployed condition comprises, in a preferred embodiment, a rigid strut extending between the support' and a point intermediate the ends of the tether so as to triangulate the assembly.

In a preferred embodiment the support comprises two or more symmetrical legs linked by one or more transverse limbs so as to form a generally cruciform structure.

In this preferred embodiment the tether is preferably substantially V' shaped, and comprises upper arms for connection one to each leg of the support, and a lower arm for connection to the anchorage. Disconnectable bracing struts may extend between the upper arms and respective legs of the support.

In an alternative embodiment, said support and tether are integral, the support comprising a first end portion for piercing the water surface, and the tether comprising a second end portion for direct attachment to an underwater anchorage. In use the first end portion lies downstream of the second end portion.

In this alternative embodiment, the support may also comprise two or more symmetrical legs linked by one or more transverse limbs to form a generally cruciform structure, the tether being substantially V' shaped so as to terminate at a single anchorage.

In the case of the first embodiment, the method of deployment is to allow a rigid tether to pivot down to the anchorage whilst the support lies in the maintenance position; the tether is then locked to fix its relationship to the support, and the support is submerged by pivoting about the anchorage. In the case of the alternative embodiment the second end portion(s) of the support are drawn to a seabed anchorage by cable, and guided by a vertically extending restraint.

The supportltether combination requires adaptation for an appropriate underwater anchorage which takes into account roll loads which may occur for example in a transverse swell. Typically a single leg support can be used in conjunction with an anchorage which permits movement in pitch and yaw only. In the case of a multi-leg support movement in roll may be permitted so as to avoid excessive twisting forces at the anchorage.

A key aspect of the invention is the means adopted to regulate roll of the turbines (i.e. rotation about a longitudinal horizontal axis through the seabed anchorage). Where articular freedom is provided in all planes by the seabed pivot, stability especially of roll, is provided by hydrodynamic means combined with buoyancy -especially important in strong tidal flows. In the alternative, roll is prevented by restricting seabed articulation to pitching and yawing only (i.e. rotations about a transverse horizontal axis and a vertical axis respectively, both extending substantially through the seabed anchorage), but in this case a multi-leg support will generally result in unacceptable roll induced loads at the anchorage, and thus the requirement to accommodate roll at the anchorage.

Preferably a plurality of transversely spaced wave piercing elements are provided, and arranged to provide toe-in about a longitudinal axis. By toe-in we mean that the support is constructed and arranged so that transverse forces are generated tending to force laterally disposed members thereof towards a centre line. Thus for example a suitable support having two symmetrically arranged legs will display toe-in tending to force the legs together.

Toe-in forces may be generated by a suitable geometrical arrangement or shape of laterally disposed members, or by hydrodynamic surfaces designed to generate toe-in.

Toe-in may for example be generated by two oppositely curved and laterally disposed side members.

According to a second aspect of the invention there is provided an underwater turbine mounting adapted to be tethered by a rigid arm downstream of an underwater anchorage, and including variable buoyancy means for pivoting said mounting from an underwater operating position in which a turbine is in use submerged, and a surface maintenance position in which a turbine is above the waterline, said mounting including laterally disposed frame members adapted to generate toe-in forces with respect to the stream flow direction.

Toe-in is an aid to stability. Preferably the frame members are aligned substantially with the direction of stream flow, and in the preferred embodiment the toe-in forces generated by each member are balanced. As is well-known toe-in forces tend to act transversely to the direction of relative movement, in this case the direction of stream flow, and towards a centre line aligned with that direction.

BRIEF DESCRIPTION OF DRAWINGS

Other features of the invention will be apparent from the following description of preferred embodiments shown by way of example only in the accompanying drawings in which:-Figure 1 is an isometric view of the turbine mounting support according to a first embodiment of the invention in an operating position.

Figure 2 is the turbine mounting support of Fig. 1 in a maintenance position.

Figure 3 is the turbine mounting support of Fig. I in an operating position, in plan.

Fig. 4 is a side view of second embodiment of the invention in the operating position.

Fig. 5 corresponds to Fig. 4 and is a side view of the second embodiment in maintenance position.

Fig. 6 corresponds to Figs. 4 and 5 and shows an installation procedure.

DESCRIPTION OF PREFERED EMBODIMENTS

The following text and figures describes an embodiment of the invention suitable for installation of multiple turbines, to thereby achieve a high power output.

With reference to Fig. 1, a frame assembly 10 consists of two identical buoyant chambers 12 each consisting of a long leg 21 and a short leg 22. The legs 2 1,22 may for example comprise hollow welded vessels, and are approximately in the form of a hockey stick. As illustrated, the included angle between the legs is about 1350.

The chambers 12 are connected symmetrically by one or more generally transverse limbs 13, which lie generally horizontal in use. Each limb has a hydrofoil section to generate lift. Each limb may further have a streamlined shape to reduce water drag when cutting through a tidal stream. The number of limbs 13 and the shape of the hydrofoil section is selected to give a pre-determined lift in use, by reference to the mass of the frame assembly and the velocity of stream flow.

A rigid V' shaped tether 14 connects the chambers 12 to a seabed pivot 15 so as to permit angular movement about a generally vertical axis. In this way the frame is self-aligning with the direction of the stream flow. Rigid struts 17 brace the tether 14 at the upper end, so that the tether has four frame attachment points 16, respectively in the vicinity of the junctions between the struts 13 and chambers 12.

Attached to arms 18 of the frame assembly 10 are respective turbines 11 for generation of electrical power in a stream flow. The precise mounting arrangements of the turbines can be selected to suit flow characteristic across the frame assembly, and the number and size of turbines is also selectable. The arms 18 extend to rear of the plane of the long legs 21 for reasons which will become apparent. The arms may have a hydrofoil section to generate additional lift, and may further compromise a single arm/hydrofoil as shown in Fig. 3.

The dimensions of the frame and tether are selected to suit the depth of the seabed pivot 15, so that in use the short legs 12 pierce the surface 19 of the sea so as to give improved stability in addition to acting as a warning to ships and the like. These surface piercing components also provide a means of access and connection between the support and a service vessel when in the operating position.

In order to determine the floating depth of the frame assembly, the chambers 12 are ballasted with water, and suitable onboard pumps and valves may be provided.

Fig. 2 illustrates a surface configuration whereby the ballast is pumped out of the chambers to cause the long legs 21 of the frame assembly to lie on the surface 19.

The tether pivots upwardly, as illustrated, but retains the frame assembly to the seabed in a trailing condition with respect to stream flow direction, The anchorage permits up and down movement as the tide rises and falls.

The orientation of the arms 18 is selected to lift the turbines clear of the surface 19, as illustrated, for the purposes of repair and maintenance. It will be appreciated that raising the frame assembly requires only a pump, which may conveniently be provided on the frame or on a maintenance vessel or the like, and connected to a suitable coupling on the short leg 12. Sinking the frame assembly back to the operating position of Fig. I requires a suitable valve and controls, which may be depth or pressure sensitive, to admit sea water as ballast.

In order to float the frame assembly to the installation site, the tether 14 may be attached to the chamber 12 by a horizontal axis hinge, and the struts 17 may be similarly pivoted at one end, and have a disconnectable coupling (not illustrated) at the other end. Thus by disconnecting the struts 17 at one end, both the struts and the tether can adopt a substantially horizontal configuration, and lie approximately parallel to the long legs 21 of Fig. 2. If preferred the struts 17 may remain rigidly attached to the tether 14 so as to project upwardly in the transit condition.

In the embodiment of Figs. 1-3, yaw stability about the seabed attachment is provided by fluid drag on the turbines and frame combined with the lift forces from the hydrofoil surfaces provided. Yaw stability may be provided adequately by selecting suitable downstream dimensions for the frame assembly.

Pitch stability is provided by the balance of moments of buoyancy and lift forces on the one hand, and gravity and drag on the other. The determination of stability is within the scope of a suitably skilled designer, and is influenced for example by changing the cross-sectional area of the short legs 22 which pierce the wave surface in use.

Roll stability (rotation about a longitudinal horizontal axis through the seabed hinge in the stream flow direction) is difficult to ensure, especially at high stream flow rates.

At low flows, it is provided by the differential buoyancy between the laterally opposed buoyancy chambers 12 piercing the water surface -as one side sinks down it is opposed by the greater buoyancy that this movement generates.

However, a second destabilizing force may operate as the frame and turbines roll. If rolling to the left (looking downstream), a yaw moment is generated by the offset drag of the structure tending to bring it back to the right into alignment with the stream.

This movement creates an angle of attack on the lift surfaces of the buoyant chambers which, because there is typically more lift surface above the centre of gravity than below, creates a net moment tending to increase the roll angle of the frame and turbines. As the flow rate increases, this destabilizing moment increases in magnitude, and may exceed the fixed roll stabilizing moment provided by chamber buoyancy.

With a single vertical chamber, the only way to counter this tendency and provide adequate roll stability is to arrange for sufficient lift surface below the centre of gravity. This is difficult in practice because of the structural implications of large surfaces attracting substantial lift forces but having low mass.

Even with two vertical chambers, destabilizing roll effects may dominate at high flows unless attention is given to the hydrodynamic design of the chambers, such that with two vertical chambers (in use), a further stabilizing mechanism becomes possible, and constitutes part of the invention described herein. Again considering a frame such as that illustrated above rolling to the left (looking downstream), the port chamber becomes immersed more than the right chamber. If the axes of two chambers are now directed towards each other a shown at 24 in Figure 3 (described as "toe-in"), then there will be a net force to the right in the figure because of the extra depth of immersion of the left hand (port) chamber. This force will have the effect of rolling the frame and turbines to the right, thereby stabilizing them in roll. It is worth noting that if the chambers had been inclined in the opposite sense to each other ("toe-out"), then the effect would have been reversed and stability reduced. With neither toe-in nor toe-out there would still be the roll destabilizing effect described above, resulting from the yaw angle rotation produced by the moment of drag on the structure.

Roll stability may be further enhanced by the reduction of structural drag, in accordance with the drag-related stability mechanism described above. The following features can be incorporated to reduce drag: 1. Adoption of well-profiled hydrofoil sections for all submerged structures.

2. Adoption of "ships bow" shapes for sections piercing the water surface, especially on the leading edge of the short legs 22.

In order to enhance roll stability in the maintenance position, it may also be beneficial, for the same reasons pertaining to vertical chamber "toe-in" described above, to apply toe-in to long legs 21, which in Fig. 3 have a respective axes in common with the axes 24, Referring now to Figure 2, showing the maintenance position, as the frame and turbines roll to the left, looking downstream, a greater part of the left hand side member becomes immersed, and thereby because of its inward inclination relative to the flow, generates a lift force to the right tending to opposite the roll, and thereby stabilizing this movement.

Referring now to Figure 4, two horizontal axis turbines 111 are laterally disposed about a mounting frame 112 and attached to it by transverse crossarms 113. The frame 112 is pivotally attached to a seabed anchorage 114 by pivots allowing pitch and yaw articulations relative to the anchorage. In further embodiments of this arrangement, it is possible to fit two or more rows of turbines to the frame in a similar manner.

Part or all of said frame 114 consists of a chamber which may be filled or drained of ballast water. In the operating position of the turbines 111, shown in Figure 4, the chambers are filled with sufficient ballast water to keep frame 1114 at a level in the water which keeps the rotors of turbine 114 well below the surface of the water at low flow rates, but well clear of the seabed at the highest operating flow rates.

When it is required to access the turbines, for maintenance or removal, ballast water is pumped out of the frame chambers, causing the frame to rise in the water, as shown in Figure 5. A workboat can now be brought alongside for access to the parts or systems needing work.

The procedure for installation is shown in Figure 6.

The frame 112 is floated out to site with its chambers slightly ballasted so that the turbines Ill and a crossarms 113 can provide sufficient buoyancy to provide the frame with sufficient roll stability until it is connected to its pre-installed seabed anchorage 114, after which roll movements are constrained. The buoyancy of frame 112 is arranged so that its lower end, carrying the pitch and yaw hinge unit 115, and its location spigot 16, is just positively buoyant on the water surface. The electrical power cable 117 may be attached to the hinge unit 115 so that it may also be drawn down in position.

The hinge pin 116 is connected to a pull-down line 118 which then draws it down into the anchorage 114, by means of a surface winch or other such means commonly used in offshore operations. The spigot may be locked in place by apparatus such as that commonly used in offshore operations.

This operation may be reversed if it is necessary to bring to the surface the seabed hinge unit, or the electrical cable, for repair or replacement.

Figs. 4-6 illustrate a single boom arrangement with turbines on either side. Roll forces induced by a transverse swell may be low enough to permit an anchorage which allows relative movement in yaw and pitch only. However the arrangement of Figs. 4-6 is also suitable for a multi-boom assembly having several tether portions which meet at a common anchorage connection, for example in a V' shaped configuration.

Multi-boom arrangements tend to generate larger roll forces, and for this purpose the anchorage may be modified to accommodate movements in roll, pitch and yaw.

Claims

Claims 1. An underwater turbine mounting comprising: a support adapted to receive thereon one or more turbines and having a substantially upright member adapted to pierce the surface of the water in use, a rigid tether on said support and extending in use at a pre-determined downward orientation therefrom, one end of said tether being adapted for pivotal connection to an underwater anchorage, and variable buoyancy means to raise and sink said support about a substantially transverse pivot axis of the anchorage for movement between an underwater operating position and a surface maintenance position.
2. A mounting according to claim 1 wherein said support comprises buoyancy chambers.
3. A mounting according to claim I or claim 2 wherein said support comprises a plurality of substantially parallel legs linked by cross members to form a cruciform structure.
4. A mounting according to claim 3 wherein said cross members comprise turbine attachments.
5. A mounting according to any of claims 3 or 4 wherein said cross members comprise hydrofoils adapted to generate lift in a stream flow.
6. A mounting according to claim 5 wherein hydrodynamic lift surfaces are provided on said legs.
7. A mounting according to any of claims 3-6 wherein said legs comprise respective buoyancy means.
8. A mounting according to any of claims 3-7, wherein said legs toe-in toward said tether.
9. A mounting according to any of claims 3-8 wherein each of said legs includes a substantially upright member.
10. A mounting according to claim 9 wherein the upright members generate respective toe-in forces.
11. A mounting according to claim 9 or claim 10 wherein each upright member comprises a wave piercing bow.
12. A mounting according to claim 11 wherein each upright member is symmetrical about a substantially longitudinal axis.
13. A mounting according to any preceding claim, and further including an articulation for attachment of said tether to an underwater anchorage, said articulation permitting movement in roll, pitch and yaw.
14. A mounting according to any preceding claim wherein said tether is attached to said support by a lockable hinge, and is movable between a substantially horizontal transport condition to a downwardly directed operating condition.
15. A mounting according to claim 14, and further including a strut extending between said support and tether in the operating condition.
16. A mounting according to any preceding claim and further including a turbine mounted on said support.
17. A mounting according to claim 16 and further including a plurality of turbines symmetrically arranged about said support.
18. A mounting according to claim 16 or claim 17 wherein said turbines are arranged to have a substantially horizontal rotational axis in the operating position.
19. A mounting according to claim 18 wherein said turbines are arranged to be substantially clear of the waterline in the maintenance position.
20. A mounting substantially as described herein with reference to the accompanying drawings.