GB2363431A

GB2363431A - Segmented wavepower converter

Info

Publication number: GB2363431A
Application number: GB0022869A
Authority: GB
Inventors: Alan Arthur Wells
Original assignee: Applied Research and Technology Ltd
Current assignee: Applied Research and Technology Ltd
Priority date: 2000-06-14
Filing date: 2000-09-18
Publication date: 2001-12-19
Also published as: GB0022869D0; GB0014597D0; GB2363430B; GB2363430A

Abstract

The water wave converter 101 comprises rigid sections 102 spaced apart by steel laminated HDPE spacers 103 and held in compression by tendon cables 105. Each section has internal 104 fluid passages connected to adjacent section corresponding passages by olives 106. Tendon compression is variable according to wave condition and causes resonant collector buckling and cyclic olive compression/elongation and fluid pumping in the passages to an accumulator. The olives may be: cylindrical with tapered outer surfaces and of rubber/polymer with wound wire reinforcement (Figs 4a,b); replaced by tyre like bellows or; cylindrical pipes sealed to passage ends or in sliding seals (Figs. 8a,b, 9). An alternative (Fig. 13) comprises sections with external passages, each section arranged and weighted to roll relative to adjacent sections. The sections comprise a concrete shell filled with gas concrete or HDPE cylinders. Collector may be arranged horizontally or vertically.

Description

2363431 M&C Folio: GBP82418X A WAVEPOWER COLLECTOR This invention relates

to an apparatus for extracting power from water waves and a method of constructing such an apparatus.

There is a considerable amount of energy available from the waves generated in the sea and in larger lakes. Much effort has been put into attempting to harness this considerable source of energy by converting the oscillatory motion of the body of water into a more readily useable form of mechanical energy and/or electrical energy. However, these efforts have been relatively unsuccessful and consequently this valuable source of renewable power remains substantially untapped.

One of the main problems associated with wavepower collectors is providing a structure which is reliable and which will last in the hostile environment of the sea for a reasonable period of time without being unnecessarily expensive to produce or maintain. These factors lead to the problems faced by previous wavepower collectors, namely that the cost of manufacturing and maintaining collectors has been so high as to make them insufficiently cost effective to provide a viable alternative to more traditional forms of power generation, except in extreme conditions or locations.

Various methods have been proposed for extracting power from waves. Among these are those; which are formed by a long slender floating beam which is sufficiently flexible to follow the profile of the waves as they pass by them. This type of construction is described in the paper entitled "Porpoise, The Buckling Resonant Raft" by Farley, Parkes and Grimshaw (Second International Symposium on Wave and Tidal Energy, September 23-25, 198 1) and "Wave Energy Conversion by Resonant Rafts" by FIM Farley (Royal Military College of Science, Shrivenham, Wilts. 3rd International Conference of _Future Energy Concepts, London, IEE 198 1). These documents relate to 2 a series of connected floating rafts which flex relative to each other. The flexing action causes pistons attached to one raft to pump a hydraulic fluid out of a cylinder in an adjacent raft which can be used to provide useful power.

Resonance is commonly used in wave energy collectors, firstly to cancel inertial forces, allowing the wave force to act essentially on the power extraction mechanism; and secondly to make the motion of the device greater than the wave amplitude, effectively sucking in power from the surrounding sea. The amplified motion allows a relatively small body to radiate sufficiently to cancel the incoming waves over a width which is related more to the length of the collector than its beam.

Shallow rafts of barge form have natural periods of vertical oscillation which are too short to provide resonance because they have too much floating stiffness and too little displacement mass. Whilst the period can be increased by superimposing an end-to-end compression which is sufficient to almost induce longitudinal buckling, there are fundamental problems with having a number of substantially rigid structures hinged together. The principle weakness of hinged rafts relates to the low relative velocities and extremely large hinge forces and moments at power extraction levels comparable with hydro-dynamic performance potentials. Furthermore, hinges and hydraulic rams which are well proportioned in engineering terms are too large to be accommodated.

According to the present invention there is provided a wavepower collector comprising: a plurality of sections, and at least one passage for fluid communication for each section, wherein each passage is aligned with a corresponding passage in an adjacent section, and is connected for fluid communication to its corresponding passage by a connection means, and the sections are retained in a spaced-apart configuration such that the sections are able to pivot relative to one another such that the distance between the mouth of each passage and the mouth of its corresponding passage can be varied.

According to a first embodiment of the invention, the sections of the collector are preferably spaced apart by less than 1 metre, more preferably about 0.25m. The sections 3 themselves are preferably disc shaped i.e. their axial length or thickness is less than their diameter. The sections are preferably around 6.5 metres in diameter and less than 6.5 metres thick, more preferably around 4 metres thick. A collector preferably comprises 20 sections or more. The sections are generally cylindrical. The sections may however have other cross sectional shapes. For example, it may be advantageous to form the sections with a D-shaped cross-section with the curved portion facing dovm towards the sea bed. Similar dimensions to those described above are applicable. Other shapes may also be used to provide varying degrees of buoyancy and centres of buoyancy. Furthermore, a flat or partially flat top may be useful for maintenance purposes. Each section is preferably arranged so that it can pivot relative to an adjacent section without contacting it. In this way, the mouths of the passages in adjacent sections are brought closer together such that the axial length of the pipe formed by the passages and connection means is reduced or increased to provide a pumping action. The sections preferably have generally flat ends facing each adjacent section. The passages may be provided within the body of the section. Alternatively, the passages may be mounted either partially or completely on the outside of the sections.

Each section may be formed in a single piece by forming a thin outer concrete wall which is subsequently wound with steel wire under tension in order to provide a compressive hoop stress in the concrete. The interior of each section can be filled with high density polyethylene buoyancy cylinders having closed ends. Preferably, seven cylindrical buoyancy cylinders are used with an arrangement where six cylinders are arranged regularly around a central cylinder. However, the buoyancy elements need not be cylindrical. Any suitable shape and arrangement being applicable appropriately. Spaces are left between the buoyancy cylinders in which to include the passages and tubes through which the tendons may pass. Alternatively the passages may be set in the walls of the section.

Alternatively, the sections may be constructed by forming a thin reinforced concrete outer shell, as before, but by then filling the interior space with a gas concrete. Such a gas concrete is- preferably formed from a mix of sand and Portland cement with a lean 4 mix of aluminium powder included. This composition is mixed with a metered addition of hot water which induces expansion through the generation of hydrogen and steam. As the increased temperature is maintained during curing due to the thermal insulation of the gas concrete, a small amount of thermal contraction will result once the curing has finished and the concrete cools down. This induces a residual tensile strain in the concrete which again puts the outer concrete skin into hoop compression. The end faces of the gas concrete at least and preferably the entire section is protected by applying a sealing coating to the surface. The coating is preferably glass reinforced resin.

As an alternative, the outer layer of the collector may be constructed by forming two spaced apart steel sheets into an annular-section to form a steel composite, e.g. Bi-steel (RTM) which can be filled with a structural ballast material or concrete, preferably low density concrete. Alternatively, the outer layer may be left unfilled, for example if additional ballast is not required due to ballast in the interior of the collector sections. The central zone of the collector sections may be filled with ballast or simply filled with sea water which although not fully entrapped, functions adequately to provide the required ballast.

Preferably, the buoyancy of the collector is such that when it is placed in the water, it floats level, with around 60%-90% of its external volume submerged. Advantageously, the freeboard is less than 3 metres preferably 2m. Beneficially, the axial elastic bending stiffness of the collector is such that the natural period of bending while afloat matches that of the incident wave period, usually around 8-12 seconds for typical principal prevailing waves but this may be varied according to the specific conditions of the installation location.

According to a second embodiment of the present invention, the sections are again preferably generally cylindrical. However, in this embodiment, sections are arranged so that they are side by side with the axis of each section being generally arranged parallel to the axis of each adjacent section and thus the sections are generally perpendicular to the axis of the collector as a whole. Again, the sections may have a non-circular cross- section, In this construction, the passages are preferably provided on the upper and lower parts of each section. In this way, as the sections pivot/roll relative to each other, the mouths of the passages are moved towards each other in a similar manner to that described above. A number of passages may be provided on each section arranged along the axial length of the section. Each section may be arranged to have a centre of mass which is offset from its centre. This may be achieved by providing additional mass at one side of the section or by providing additional buoyancy at the opposite side of the section. The sections are preferably arranged such that the centre of mass is offset to be vertically above the axis of each section when in the water. In this way the natural period of the whole collector is increased. Some or all of the offset may be provided by providing a different number of passages on the top than on the bottom.

The cylinders are preferably provided with generally flat ends which provides the sections with appropriate conditions for linearity and good capture of waves as they pass by the collector. Advantageously, the length of the cylinders is more than the diameter. This provides good transfer stability, strength and an adequate displacement.

The sections of the collector are preferably held together by providing one or more steel (or other suitable material) cables or tendons down the length of the collector. The or each cable may pass through a tube in each section. The cables are preferably tensioned in use in order to provide a continuous longitudinal compressive force along the length of the collector on a plane passing through the neutral axis, A spacer is preferably provided between each section, around each cable in order to space the sections apart while still allowing them to pivot relative to each other. The spacers are preferably made from rubber or high density polyethylene. The spacers may advantageously be formed in a laminated structure with rigid washers (preferably steel) placed at regular intervals within the spacer. This provides improved reaction to bursting of the spacer under high load. Although two tendons are preferably provided, a single tendon may be used in some circumstances. In particular, where wave fluxes arise simultaneously from different azimuth directions, as in the open sea, one central tendon is preferable.

In either embodiment, the connection means are preferably formed as axially compressible resilient members referred to herein as olives. The olives are preferably only compressible in an axial direction. They are preferably inextensible in a radial direction such that the diameter of a bore through the olive does not increase significantly due to axial compression or internal pressure. This resistance to radial expansion may be provided by internal steel wire windings in the inner surface of the bores in the olives. The olives are preferably formed from an elastic material such as natural rubber or reinforced polyethylene.

The connection means (olives) are preferably generally cylindrical in shape but with a diameter which decreases from the centre towards each of the two ends. Thus the two halves of the olive have a generally conical exterior shape. The passages in the collector sections are preferably provided with correspondingly shaped conical mouths at the ends of each of the passages to accommodate the olives. Preferably, the angle (relative to the longitudinal axis) of the taper of the mouths is greater than the angle of taper of the olives to allow for compression of the olives in use.

Alternatively, the connection means may be formed in a generally cylindrical shape. In this construction, the mouths of the passages include a portion with a larger diameter than the rest of the passage such that the end of the connection means can be inserted into the larger diameter portion and abut against the inner end face of the recess. In this construction, the connection means is also preferably compressible only in an axial direction and inextensible in a radial direction. The connection means may also be provided with internal reinforcement to prevent radial expansion in a similar way to that described above. In this construction, the connection means is preferably formed from reinforced natural rubber or reinforced polymer such as polyethylene. The connection means may be provided with a plurality of rigid reinforcing annular discs, preferably made of steel. These discs provide the connection means with a laminated structure to resist collapse or permanent deformation, in use, under high loads. These reinforcing members may be provided as substantially flat linear washers. Alternatively, the 7 washers may have a slightly conical shape similar to a Belleville washer. This arrangement allows the connection means to operate in a manner similar to bellows whereby the internal volume is reduced.

According to a further aspect of the present invention, the connection means is arranged so that it slides axially within the passages into which its ends are respectively inserted. In such an arrangement the connection means is preferably a tubular member which is slidably mounted with each end in two different opposing passages. The tubular member is provided with sealing portions to form a seal between each end and the respective passage. The tubular member connects one passage with an opposing passage for fluid to flow through it from one passage to the other. This again provides for a continuous passage or pump pipe through which pressurised fluid is passed to one or more valves either at one end of the collector or along its length.

The tubular member is preferably flexible along its length to allow for the passages in the collectors no longer being co-axial as they deflect relative to each other. The tubular members are preferably formed from reinforced rubber or reinforced polymer such as polyethylene. The tubular members may also be provided with internal windings to resist radial expansion of the tube. Alternatively, the tubular members may be provided with partially spherical ends to ensure a seal is maintained whilst being able to pivot relative to the collector sections.

In order to tune the collector to the desired natural frequency so as to equate to that of the prevailing waves, the collector is preferably placed under an axial compressive load in use. This may be achieved by using one or more tendons or cables running down the inside of the collector and anchoring the cable to the ends of the collector. The cable may then be put under a tensile strain to apply a force along the longitudinal axis of the collector. Preferably, the tensile strain can be modified in use. This allows accurate tuning of the collector in varying weather conditions and also allows the collector to be detuned during extreme weather conditions to reduce the possibility of the collector becoming damaged.

The passages are preferably connected to a header via one-way valves which allow fluid to pass from the plurality of passages and connection means into the header when the pressure in them exceeds that of the header. The header is preferably maintained at a pressure between 3 0 and 100 bar and more preferably in excess of 5 0 bar. The header is also provided with an outlet to provide high pressure fluid to be used as required, e.g. to feed a turbine for a generator or a desalination plant, or for dosed sea water injection into oil wells terminated at the seabed.

The outlet of the header may be connected to an accumulator. This allows the variable flow rate produced by the pump pipes to be smoothed out to provide a more constant pressure and flow from the collector. The accumulator can be provided by a length of pipe, attached to the outlet of the collector, containing a volume of air. The pipe is preferably a steel pipe with an internal rubber air sack. The pipe may also comprise a concrete jacket to act as ballast to retain the accumulator on the sea bed and also to protect the accumulator pipe.

To provide a highly efficient system, the collector needs buoyancy, a sufficiently large diameter and sufficient displacement in order to convert wavepower into high pressure fluid flow. The length of the collector is preferably more than half a wavelength of the prevailing waves. The length is preferably selected to be less than the wavelength of the prevailing waves. In addition, the collector is preferably arranged to have adequate strength and stiffness to ensure that elastic flexural waves within it travel at the speed of incident waves. The collector is preferably arranged to flex only in a vertical plane.

By selecting the length of the collector accordingly and also the compressive load applied along the length of the collector, the collector can be tuned to the prevailing wave frequency. In this way the collector is coupled to the waves in order to extract the maximum amount of power from the waves passing by the collector.

9 Since the collector requires no fixed reference, it is capable of operating in both offshore and coastal sites. It is particularly convenient to locate the collectors in shallow water close to shore, for example up to a depth of about 50 metres. This allows the high pressure fluid generated by the collector to be carried by a pipeline back to a fixed installation on the shore where the equipment required to utilise the pressurised fluid is more conveniently located. It also permits mobilisation of greater bending strains and pumped fluid displacements arising from the reduced wavelengths and increased wave amplitudes at given wavepower fluxes in shallow water. This clearly allows a greater output of pumped fluid for a given wave flux when compared to locations offshore.

Additionally, the collector can be used in a vertical configuration by collecting energy from the surge action as waves pass over it. This is particularly useful for offshore applications.

The present invention overcomes the problems of previous systems by providing a low cost structure which requires little maintenance but, where required, such maintenance can be easily carried out due to the simplicity of the structure. Furthermore, by mounting the generating or processing equipment away from the collector, either on the shore or on a separate fixed structure, the complexity or the collector structure is minimised. This ensures less chance of malfunction and less maintenance. Coupled with this, the structure of the present invention is capable of efficiently converting the wavepower presented to it. The net result of this is that the collector of the present invention is able to provide power continuously (assuming waves are present) at a cost which compares favourably with traditional types of power generation.

The present invention will now be described by way of a specific example with reference to the accompanying drawings in which:- Figure 1 shows a schematic representation of a collector on the surface of a body of water; Figure 2 shows a schematic view of the present invention; Figure 3 shows a schematic cross-sectional view through the construction shown in Figure 2; Figure 4A shows an enlarged view of an olive in the construction of Figure 2 when the olive is in the uncompressed condition; Figure 4B shows an enlarged view of an olive of the construction of Figure 2 when the faces of the sections are close together and the olive is under compression; Figures 5A and 5B show cross-sectional and side views of the present invention clearly showing the buoyancy cylinders used to provide buoyancy to the collector; Figure 6 shows a modified construction of the present invention; Figures 7A-7C show views of the construction shown in Figure 6; Figure 8A is a view of an alternative arrangement of the present invention; Figure 8B is a partial cross-section along A-A in figure 8A; Figure 9A shows a schematic view of a second aspect of the present invention; Figure 9B shows an enlarged view of the end of one tubular member of the construction -of Figure 9A; Figure 9C shows a further enlarged view of the sealing on a tubular member of the construction of Figure 9A; Figure 10 shows an example of a tyre-shaped member usable in a further modification of the invention; Figure 11 shows an application of the tyre-shaped member of figure 10; Figure 12 shows an alternative application of the tyre-shaped member of figure 10; Figure 13) shows a second embodiment of the present invention; Figure 14 shows a schematic representation of a section according to the second embodiment; and Figure 15 shows an enlarged view of the connection means and externally mounted passages as applied to a construction of the second embodiment.

The present invention provides a floating flexible beam structure formed from a plurality of sections each comprising one or more passages. The collector structure formed in this way is floated on the surface of the sea such that as waves pass by the collector, the various sections move relative to each other, varying the spacing between the passages provided thereon. By providing appropriate connection means between each of the sections this variation in the spacing between the sections can be used to vary the overall length of a plurality of passages in respective sections causing an increase or decrease in pressure of a fluid contained within. As waves repeatedly pass by the collector structure, the structure flexes in one direction and then another causing this repeated change in length to provide a pumping action. This pumping action is used to provide a source of pressurised fluid which can then be utilised, for example for power generation. This may be by feeding the high pressure fluid into a turbine to provide mechanical power which may in turn be used to drive a generator to provide electrical power. Alternatively, the pressurised fluid may be used for other purposes depending upon the application. For example pressurised sea water may be used in a permeable membrane desalination process to provide fresh water. Also, the pressurised 12 sea water may be used in oil extraction for pumping into oil wells to replace and displace oil in oil bearing rocks.

Two embodiments of the present invention will now be described. Although the embodiments operate in a similar manner, the arrangement and construction of the sections is different.

A first embodiment of the present invention comprises a collector formed by a number of large diameter sections arranged end to end to form an elongated compressed spine shaped body. In this embodiment the sections are circular in cross-section but they may be other shapes such as Dshaped. The cylindrical body is arranged to float at the surface of the sea or lake and is tethered to hold it in position. The collector is arranged such that its axis lies generally perpendicular to the wave front such that the body oscillates with the wave, causing the body to flex. This flexing of the body causes the length of the plurality of pump pipes to increase or decrease repeatedly. This causes the axial length of each of the pump pipes to be cyclically varied. The pipes are filled with a hydraulic fluid, usually sea water, and the variation in length and hence volume of the pipes acts as a pump for producing high pressure fluid.

Figure I shows a representation of a collector I floating on the surface of the sea 2. The bed 3 of the sea is shown for reference. Figure 2 shows a more detailed view of the collector 1. The collector sections are fonned as hollow structures which are at least partially ballasted, preferably so that 60% to 90% of the cross-sectional area is submerged. The interior of the collector sections are non-nally sealed although they may be flooded with sea water.

In order to obtain the maximum possible power from a wave, the collector needs to be tuned so that its natural resonant bending frequency corresponds to that of the incident waves. Clearly the frequency and amplitude of the waves on the water surface will vary according to prevailing conditions and it will be necessary to select a resonant frequency for the collector which lies in the range of wave frequencies which occur in the location 13 where the collector is to be deployed. In order to "tune" the collector to the appropriate frequency, the entire body of the collector is put under a compressive load. This compressive load is provided by one or more cables, in this case two, connecting each of the ends of the collector together. The cables are tensioned so as to compress the body of the collector in the axial direction. By appropriately selecting the tension in the cables, the compression force in the collector body can be chosen and the collector can thus be tuned. The compression provided by the cables tends to induce buckling in the beam which helps to maximise the bending of the collector and hence the strain as each wave passes.

Figure 2 shows part of a collector 10 1 which is formed from a plurality of rigid sections 102 which are spaced apart by flexible spacers 103 and olives 106. As shown in Figure 2, each section is formed of a substantially cylindrical body provided with passages 104 in the interior of the body. The cylindrical sections are preferably arranged so that their length is less than their diameter so as to have a disk like form.

The sections are preferably formed from a thin outer concrete wall which is formed from centrifugally cast concrete. The concrete outer walls are relatively thin and are preferably subsequently wound with steel wire under tension. This maintains a continuous compressive force on the concrete sections. In the interior of each section there are provided one or more polymer, e.g. high density polyethylene (HDPE), buoyancy cylinders having closed ends. The remaining space in the interior of the sections allows the passages to be inserted. The passages are then embedded in more concrete in order to provide a substantially unitary structure. In an alternative construction, the sections may be formed with an annular outer wall, with passages embedded therein. The tubes for the tendon cables may either be also embedded therein or pass through the central part of the section.

As shown in Figure 2, the sections are preferably provided with four passages arranged around the collector sections 102. The passages 104 are preferably located close to the outer periphery of the sections in order to provide the maximum displacement between 14 the sections as the wave passes by the collector. In operation, the passages will be operating at relatively high pressures and it is important that radial expansion of the cylinders is restricted in order to maximise the hydro-dynamic efficiency of the collector. The passages are therefore preferably formed from steel. This coupled with them being embedded in concrete helps to provide good resistance to radial expansion of the cylinders.

As indicated. above, the collector is formed from a plurality of sections which are held together longitudinally with a pair of cables or tendons 105. These tendons 105 are passed through each of the sections and posttensioned in order to hold the sections in close contact and also to tune the collector according to the wavelength of the prevailing waves. The tendons are placed syrnmetrically on a plane through the neutral axis of the collector but spaced apart towards the periphery of the sections 102.One or more large steel laminated H13PE spacers 103 are provided around the tendons between each of the sections in order to space the sections apart. For instance, in the example given above, the sections are preferably spaced apart by up to 1 metre. The laminated structure of the spacers helps to maintain their integrity under the extreme operating loads of the collector. When the collector is floated on the water, it is preferably arranged so that the two tendons are in a horizontal plane. This allows the collector to bend significantly along its longitudinal axis, i.e. in a vertical plane, whilst still being restrained from bending in a direction perpendicular to the longitudinal axis.

The collector, is Provided with valves (not shown) for collecting the pressurised fluid, produced by the pump pipes, in a header. As the collector flexes, some of the pump pipes are compressed and suffer an increase in pressure in the fluid within them. As the pressure rises above the pressure in the header, the valves open and fluid is discharged into the header. As the collector bends in the opposite direction, the pressure in those pump pipes drops and an inlet valve opens drawing fluid back into the pump pipes to replace that expelled during the pumping cycle. This process continues repeatedly in each of the pump pipes as the waves pass by the collector.

The ends of the collector are typically provided with a support member such as a spider to provide a connection for tensioning cables, mooring ropes and an outlet pipe.

As the flow generated by each of the tube pumps will not be constant, there will also be a fluctuation in the amount of flow generated by the collector. This variation in the amount of hydraulic fluid generated throughout the cycle will cause a variation in the pressure in the header and consequently the pressure generated by the collector. Where the pressurised fluid is being used to drive an electrical generator or other equipment requiring a constant flow and pressure, then any variation in the pressure is disadvantageous. Although electronic systems can be used to filter variations in the voltage generated by a generator in order to overcome the variation in supply pressure, they cannot store energy and are usually expensive to buy and maintain.

The outlet from the high pressure header can be connected to an accumulator (not shown) for smoothing out any variations in the pressure generated in the header. The accumulator is effectively an air filled volume which, by virtue of air being a compressible material, acts as a reservoir to receive pressurised fluid when it is being supplied by the pump pipes and maintains the flow to the generator equipment, or the like, when the pump pipes are not providing pressurised fluid. This ensures that the outlet pressure from the collector is maintained at a substantially constant level. The accumulator is conveniently provided by a length of steel pipe connected to the main high pressure outlet from the header. The pipe is selected to have sufficient length and bore to provide a volume according to the fluid flow generated by the collector.

In this embodiment, the design preferably has twenty or more individual sections which are 5-8 metres in diameter and preferably around 6.5 metres in diameter. The length is preferably less than the diameter of the sections and usually between 3-6 metres is length. Ideally, the sections are 4 metres long.

When the pumps are filled with water, the balance of the concrete mass and the internal buoyancy is such that the collector as a whole floats in the water such that 60%-90% of 16 the external volume is submerged. The axial elastic bending stiffness is also arranged to be relatively small so that the natural period of bending oscillation is in the region of 712 seconds, matching the period of incident waves thus avoiding the need for additional longitudinal strut action. Of course this can be modified according to different incident wave periods when the collector is located in different locations with different prevailing conditions. The natural period of the collector when afloat is governed, not only by its buoyancy distribution, but also by aggregate elastic bending stiffness in the axial direction and mass, including entrained water mass. Thus reducing the bending stiffness increases the natural period. This can be achieved by applying a load through the longitudinal tendons attached between the ends of the collector The arrangement of the present invention is advantageous in that when adjacent sections reach the limit of bending compression displacement, for example during the high waves of a storm, the olives progressively provide increased resistance to further movement by virtue of their volumetric compression and thereby prevent shock contact between the sections. This avoids the need for additional protection to prevent contact between the sections which clearly avoids the potential for damage to the sections caused by impact. In addition the olives act to provide a spring function which counter balances the compressive strain applied by the cable and stabilises the overall structure. This also helps with the tuning of the collector.

In an alternative to this embodiment, rather than using HDPE buoyancy cylinders or cavities, the individual sections can be formed homogeneously by completely filling the interior of the section, other than the passages using a gas concrete (or other low density material) of a low specific gravity of about 0.47. Gas concrete of this type would, for instance, use sand and Portland cement with a lean mix addition of aluminium powder. This mixture, with controlled addition of hot water, would induce expansion in the concrete due to the generation of a large number of evenly distributed small closed pores caused by the reaction between the cement and aluminium to generate both hydrogen and steam. In addition, the heat of reaction would supplement the temperature in the mixture and enhance cured strength. This increased temperature would also be 17 maintained during curing by the reduced thermal conductivity of the aerated concrete allowing the temperature to be maintained until the concrete has cured.

This process would result in some thermal contraction relative to the dense concrete skin during final cooling and would induce tensile residual stress at the interior. This stress would be insufficient to cause cracking of the gas concrete because it has a low elastic modulus but would be sufficient to put the outer concrete skin into hoop compression. This compression would allow the tensioned outer steel wire winding to be dispensed with, providing a considerable saving in the overall cost of the collector. The end faces of the sections are preferably protected by a coating to prevent ingress of sea water whilst afloat and also to protect them structurally. This coating can be formed of reinforced resin which would also have the advantage of protecting the outer. skin of dense concrete against local cracking and damage.

During construction, the gas concrete core is preferably added to the inside of the dense concrete skin using a centrifugal moulding process. The outer skin of dense concrete would have end plates put on to define the interior space. The passages and hoops through which the tendon cables pass would be precisely held in position and then concrete would be inserted around them. The whole arrangement would be rotated to ensure distribution of the concrete around the circumference of the section. After an initial curing period, the end plates could be removed from the mould, usually within a few hours and after further curing the outer cylindrical part of the mould can be removed.

A further alternative for the construction of the collector is provided by forming the outer wall using a composite structure, such as Bi-steel. Such a structure would comprise a pair of steel plates retained in a spaced apart configuration by an array of transverse bars to form hollow panels. These panels are then attached to each other to form the annular outer wall of the section. Of course each section may be formed from a single panel fonned using to cylindrical steel plates. These panels are then put into position and may be filled with concrete, preferably low density concrete. In this way 18 the steel portions of the construction provide resistance to tensile strains whilst the inner concrete filling provides resistance to compressive loads. Figure 6 shows a construction formed in this way. The outer wall of the collector is formed of a sandwich of two steel plates 301 and 302 separated by a plurality of bars with a filling of concrete 303 therebetween. This arrangement provides a very reliable yet strong and easily constructed structure. Alternatively, the panels may be filled with ballast instead of concrete.

Figure 6 also shows alternative constructions of the inner parts of the collector of the present invention in which the passages 104 are held in place by plates 304. The passages 104 are provided with a diverging end 305 into which the olives 106 are inserted. Tubes 305 are also provided for the steel tendons 105 (not shown). Figure 7C shows an end view of the construction shown in Figure 6. Figures 7A and 7B show cross-sections through this construction also.

The second embodiment of the present invention is similarly constructed from a number of large diameter sections arranged next to each other to form an elongated collector body. As with the first embodiment, the collector is arranged to float on the surface of the sea or a lake such that it is partially submerged. The sections are preferably 50% submerged.

Figure 13 shows a representation of a collector 401 according to the second embodiment of the present invention floating on the surface of the sea 2. The collector is formed from a plurality of sections 402 which are generally cylindrical. The sections 402 are arranged with their axis generally parallel to the axes of the adjacent sections. Each section is pivotally attached to the adjacent section such that they can pivot or roll relative to each other. In this way the passages 404 mounted on each section are caused to move towards or away from the corresponding passage on adjacent sections (see figure 15). As the mouths of the passages come towards each other, the connection means 406 provided between the mouths of the corresponding passages are axially compressed or_ slide into the passages. In this way, the overall length of the passages 19 and connecting means is reduced. The connecting means are arranged such that the increase in internal pressure does not cause radial expansion of the connecting means. In this way, the overall volume in the passages and connecting means is reduced causing an increase in pressure in the fluid contained within. This pressurised fluid can then be extracted to be put to useful work.

Although Figure 13 only shows a single passageway provided on each section., it is envisaged that two or more passages may be provided on either or both of the top and bottom of each of the sections.

One important aspect of the present invention is arranging the collector to have the frequency tuned to that of the incident wave frequency. In the first embodiment this is principally achieved by modifying the tension applied to the collector using the tendon cables. However, with the second embodiment, tuning in pitch rather than heave allows the structure to dispense with the need for ancillary longitudinal tension to be applied. The collector can still be tuned by arranging the centre of gravity of each section to be offset from the centre of the section. This is preferably achieved by providing a mass 460 (see figure 14) offset from the centre of the section which is otherwise generally uniform. This mass may be provided in any number of ways. For example additional offset mass could be achieved by providing a different number of passages on the top than on the bottom of the collector sections. Alternatively, ballast may be provided in the upper part of each section. It will be appreciated that the provision of this top mass has a destabilising effect on the sections which has the effect of increasing the natural period of the whole collector.

When afloat, preferably at half depth, the hydro-dynamic conditions for linearity and good capture can be met by providing relatively flat ends to the cylinders. Furthermore, by ensuring that the sections are longer than their diameter, the sections have good transverse stability, strength and appropriate displacement.

The sections can be held together in a number of ways, although it is preferred that a pair of light longitudinal cables passing down the length of the collector retains the assembly in one piece. In this embodiment, the cables are not required to tune the structure and so the tension in them is considerably less than in the first embodiment. This allows the cables to be smaller and lighter and also reduces complexity, cost and maintenance. However, whilst the cables are not required to tune the collector, it is still possible to provide some tuning of the collector using the cables nonetheless. Even so, this would still require considerably lower tensions in the cables than in the construction of the first embodiment.

With the construction of this embodiment, the collector can flex in a horizontal plane as well as a vertical plane and this can provide additional power.

The sections of the second embodiment can be formed relatively cheaply as concrete structures. The cylinders can be spun cast to form the main circumferential wall of the cylinder. Then, when the main structure has been formed, the two ends can be cast on as flat members without any need for supporting framework. In order to provide strength to the sections, post stressing cables may be applied to the outside of the structure in a helical arrangement. The flat ends of the sections would be intrinsically post-compressed without extra cables. The external diameter is preferably around 5 metres and the sections are preferably around 7 metres long. The thickness of the circumferential walls is preferably around 0.3 metres and the thickness of the end sections is preferably around 0.5 metres.

The arrangement of the passages and connecting means will now be described in relation to both the embodiments of the present invention.

In a first aspect, the passages in each of the sections are arranged coaxially with those in the adjacent sections such that a continuous pump pipe is formed down the length of the collector through the passages and connecting olives. At each end of the passages there is provided a conical section such that the diameter of the passage increases 21 towards its end to define a mouth 107, The mouth of the passages of facing sections define a recess into which an olive 106, 406 is inserted as shown in Figure 3.

The olives 106, 406 are formed from reinforced rubber or reinforced polymer, e.g. polyethylene. They are generally cylindrical but their diameter reduces from a central portion towards the ends. They are provided with a substantially cylindrical bore through the centre. The olives contain an internal steel wire winding 110 as shown in Figure 4A. These windings I 10 are arranged around the internal surface I I I of the central cylindrical bore through the olives. These windings provide an arrangement which permits considerable axial elastic compression but greatly restricts any radial expansion of the bores. As indicated above, radial expansion of the passages and the intervening olives leads to hydro-dynamic inefficiencies.

In Figure 4A, the angle of taper A of the outer surface relative to the longitudinal axis of the olive is less than the angle B of the diverging conical portions at the mouths of the passages in the collector sections. Thus when the olives are initially inserted in the recess formed between the mouths of cylinders of opposing sections, a gap 112 remains between the inner surface of the mouth of the passages and the outer surface of the olive. However, as the collector sections are moved closer together as shown in figure 4B due to wave action, the olives are compressed axially so that they deform, as shown in Figure 4B. The gap 112 provided between the mouths of the passages and the olives provide a region into which the outer part of the olives can radially expand as they are compressed, However, as indicated above, the diameter of the internal bore is maintained substantially constant due the windings I 10.

In operation, the collector is placed in the water and partially submerged. As a wave passes by the collector, the collector tends to bend along its longitudinal axis causing the gap between sections to vary. As the collector bends in one direction, the gap between collector sections narrows on one side and widens on the other side. Then, as the collector bends in the opposite direction, the sections deflect in the opposite direction relative to each other such that the compressed olives are relaxed and the previously 1) 1) relaxed olives are compressed. This rhythmic compression and relaxation of the olives causes a cyclic reduction and increase in the axial length of the olives. As shown in Figure 413, as the opposing faces of the sections moves together, the olive is deformed and reduced in length. However, as indicated above, the internal steel wire windings prevent radial expansion of the bore of the olive thus the volume of the internal bore of the olive is reduced. This reduction in volume of the bore causes a reduction in the overall volume of the pump pipe defined by the passages and olives and pressurises fluid contained therein. This pressurised fluid can then be bled off and utilised.

The olives are axially pre-compressed such that when the sections move apart the olives maintain a good seal with the mouths of the passages. The olives may be bonded (408) in position. Whilst a specific example of the shape of the olives has been described above, this is not the only form which could be used. For example, the invention could equally be applied to olives which had a cylindrical outer shape with a suitable cylindrical slot into which they could be inserted into the sections.

In an alternative construction 80 (see figures 8A and 813), the olives are replaced with connection members 82 which are substantially cylindrical pipes. The mouths 81 on the sections at the end of each passage 104 are formed as a larger diameter section at the end of the passage. The cylindrical connection member is inserted into the larger diameter portion until its end abuts against the lip 83 where the diameter of the passage reduces. In this way the cylinder is retained in intimate contact with the sections. As the sections move toward each other, the connection member is axially compressed in much the same way as the olives described above. The connection members are constructed so that their internal diameter does not expand under pressure such that as their axial length increases, their internal volume is reduced and the fluid contained within is compressed.

The cylindrical pipes 82 are arranged with a plurality of annular disc shaped steel plates 84 embedded in a reinforced rubber or polymer body 85. The discs help to reinforce the 23 pipe against deformation or rupturing in use. Whilst the plates are shown as flat in figure 813, then may be slightly conical in shape.

In a further modification to the present invention, the olives or connecting members are replaced by one or more tyre-like structures 100 as shown in figures 1 OA to 1 OC. In this arrangement, the mouths of the passages through the sections 102 would be connected by a tyre or a plurality of tyres arranged side by side. Figures 1 OA to 1 OC show a typical high pressure tyre such as an aircraft tyre which can withstand the high pressures exerted by the collector. Figure 11 shows an example of a system utilising a tyre. In this construction, the tyre is squeezed between the sections as they move towards each other. This causes the volume of the tyre to be reduced and pressurised fluid to be pumped into the passages. Alternatively, the edges of the tyre may be attached directly to the side of the sections around each passage to form a seal therebetween. Similarly, where more than one tyre is used, the edges of adjacent tyres are attached to its neighbour to form a seal, see figure 12. In this way the plurality of tyres act like a type of bellows such that as the sections move together, the fluid is forced out of the body of the tyres under pressure. Although in the above description, tyres have been referred to as a convenient and readily available high pressure structure, it will be appreciated that other similar structures may be used which are not tyres.

As a further alternative, the olives may be replaced by sliding tubular members 20 1, as shown in figures 9A to 9C. In this construction, the passages have a substantially constant cross section throughout their length and have a tubular member 201 inserted into each end. The tubular member bridges the gap between the mouths on opposing passages 203 effectively connecting them. Each tubular member is provided with a sealing section 202 at each end to form a seal to prevent fluid passing form the interior of the passages and the outside. The sealing section is provided with heavy duty hydraulic seals 204 as shown in figure 9C. The construction of the collector is otherwise similar to that of the first aspect of the present invention. This includes the provision a plurality of passages arranged circumferentially around each section and having tensioning tendons.

In this construction, as the collector bends along its longitudinal axis, the sections deflect so that the gap between adjacent sections reduces at one side of the section and increases at the other side. As opposing sections move closer together, the ends of the tubular members 201 move axially into the collector sections, axially reducing the effective volume of the passages and compressing the pumping fluid in the passages. As described previously, this causes an increase in the pressure of the fluid which can be released through valves at the ends of the collectors.

The hollow interior of each tubular member allows pressurised fluid to flow from one passage to the next.

When the collector sections move relative to each other, the axes of their passages deviate from the corresponding passage in the adjacent section. In order to allow for this, the tubular members may be flexible or their seals 203 may be arranged to work at varying angles.

All the arrangements of connection members described above are useable on both the embodiments.

The present invention has been described for use on the surface of the water. However, it is also suitable for use in a vertical mode. In this arrangement, the collector is provided with a vertical mooring. The lower part may be tethered to the seabed or simply weighted down whilst the other end is retained close to the surface of the sea. This is generally achieved by tethering the upper part to a float. Rather than obtaining energy from bending due to the oscillation of the surface of the water, the collector in this arrangement is caused to bend by the surge of water as a wave passes overhead. At the surface of the sea, the water is moving up and down. However, below the surface for a significant depth, the water is moving backwards and forwards in the direction of propagation of the waves. This forward and backward motion diminishes with depth. Thus, a collector held in the position described above is caused to deflect significantly at the top whilst the bottom part moves relatively little. This difference in the amount of movement causes the deflector to flex significantly as each wave passes over the top of it. This flexing of the collector provides a similar action to that when the collector is arranged horizontally and thus can be used to provide a source of high pressure fluid.

This is particularly advantageous in offshore applications since the collector only needs to be tethered at the bottom to a vertical tether. For example in offshore oil extraction facilities, such wavepower devices can be used to generate power and also as a source of high pressure water which can be used as part of the extraction process whereby high pressure water is pumped into oil bearing rock to displace oil.

It will be understood that the present invention provides a collector which is provided with a number of pump pipes passing along its length. As the collector flexes, the axial length of the pump pipe varies and connection means provided between the sections allow for the reduction in the distance between the sections by axially compressing or sliding into the sections without allowing any increase in diameter. This ensures that the fluid in the pump pipe is compressed to a high level which can then be extracted and used in a number of various applications. It will be appreciated that a number of modifications can be made to the invention without departing from the scope of the appended claims.

Claims

CLAIMS:

1. A wavepower collector comprising:

a plurality of sections, and at least one passage for fluid communication for each section, wherein each passage is aligned with a corresponding passage in an adjacent section. and is connected for fluid communication to its corresponding passage by a connection means, and the sections are retained in a spaced-apart configuration such that the sections are able to pivot relative to one another such that the distance between the mouth of each passage and the mouth of its corresponding passage can be varied.

2. A wavepower collector according to claim I wherein the passages in a plurality of sections are connected to each other by said connections means to form a continuous pump pipe.

3. A wavepower collector according to claim I or 2 wherein the connection means are axially compressible.

4. A wavepower collector according to claims 1,2 or 3 wherein the internal volume of the connection means reduces as the distance between the mouth of the passage and the mouth of its corresponding passage is reduced and vice versa.

5. A wavepower collector according to any one of the preceding claims wherein the connection means is formed from an elastic material.

6. A wavepower collector according to any one of the preceding claims wherein the connection means is made of natural rubber or reinforced polyethylene.

7. A wavepower collector according to any one of the preceding claims wherein the connection me-ans is generally cylindrical.

8. A wavepower collector according to any one of the preceding claims wherein the connection means is a generally cylindrical body in which the diameter decreases axially between the middle and the ends.

9. A wavepower collector according to any one of claims 1-7 wherein the connection means is formed from one or more ring like structures each having an annular outer wall and two walls projecting generally radially inward.

10. A wavepower collector according to claim I wherein the connection means is arranged to slide axially within the passage and its corresponding passage.

11. A wavepower collector according to claim 10 wherein the connection means comprises a sealing portion at or near each end for preventing fluid communication between the interior of the passages and the outside.

12. A wavepower collector according to claim I I wherein the sealing means is provided by heavy duty hydraulic seals.

13. A wavepower collector according to any one of claims 10 to 12 wherein the connection means is substantially axially incompressible.

14. A wavepower collector according to any one of claims 10-13 wherein the connection means is flexible along its longitudinal axis.

15. A wavepower collector according to any one of the preceding claims wherein the connection means is internally reinforced to resist radial expansion of the internal volume of the connection means.

16. A wavepower collector according to claim 15 wherein the reinforcement is provided by internal wire windings.

17. A wavepower collector according to claim 16 wherein the wire windings are arranged helically around the inner surface of the connection means.

18. A wavepower collector according to any one of the preceding claims wherein the connection means includes a plurality of rigid washers, 19. A wavepower collector according to claim 18 wherein the washers are arranged co-axially with the body of the connection means spaced along the length of the connection means.

20. A wavepower collector according to claim 19 or 20 wherein the washers are flat.

21. A wavepower collector according to claim 18 or 19 wherein the washers are shaped to be slightly conical.

22. A wavepower collector according to any one of the preceding claims wherein the length of the collector is arranged to have a length between 5 0% and 100% of the wavelength of the prevailing waves.

23. A wavepower collector according to any one of the preceding claims wherein the collector is restrained such that it flexes only in a vertical plane along its length.

24. A wavepower collector according to any one of the preceding claims wherein the collector includes a cable connected to the ends of the collector such that in use the cable may be tensioned to place the body of the collector under compressive stress.

2 5'. A wavepower collector according to claim 24 wherein annular spacers are provided on the cable between each section.

29 26. A wavepower collector according to claim 25 wherein the spacers include a plurality of rigid washers to form a Ixninated structure.

27. A wavepower collector according to any one of the preceding clairns further comprising at least one header vessel provided on the collector and connected to one or more of the pump pipes via a one way valve and further comprising an outlet to provide, in use, pressurised fluid.

28. A wavepower collector according to claim 27 wherein the outlet of the header vessel is connected to an accumulator.

29. A wavepower collector according to claim 28 wherein the accumulator is a pipe.

30. A wavepower collector according to any of the preceding claims wherein one end of the collector is connectable to a vertical tether such that, in use, the collector can be used vertically with one end anchored to hold it below the opposite end.

31. A wavepower collector according to any one of the preceding claims, wherein at least a part of the passage is mounted on the outside of the section.

32. A wavepower collector according to any one of the preceding claims, wherein the sections are substantially cylindrical.

33. A wavepower collector according to any one of the preceding claims, wherein each section has a substantially flat force which is generally perpendicular to the longitudinal axis of the collector and facing an adjacent section.

34. A wavepower collector according to claim 32, wherein the sections are arranged side by side with their longitudinal axes substantially perpendicular to the longitudinal axis of the collector.

35. A connection means for use with a wavepower collector according to any one of the preceding claims, the connection means comprising a generally cylindrical body in which the diameter decreases axially between the middle and the ends.

36. A connection means for use with a wavepower collector according to any one of claims 1-34, the connection means comprising a substantially cylindrical body.

37. A connection means according to claim 35 or 36 formed from an elastic material 38. A connection means according to any one of claims 35-37 formed from a flexible material.

39. A connection means according to any one of claims 35-38 having internal reinforcing to resist radial expansion.

40. A connection means according to any one of claims 35-39 formed from reinforced rubber or reinforced polymer.

41. A wavepower collector substantially as hereinbefore described and as shown in the attached drawings.

42. A method of manufacturing a wavepower collector substantially as hereiribefore described with reference to the attached drawings.

43. A connection member substantially as hereinbefore described and as shown in the attached drawings.