NZ546825A

NZ546825A - Floating breakwater with half wavelength spacing for energy absorbtion

Info

Publication number: NZ546825A
Application number: NZ546825A
Authority: NZ
Inventors: Christopher Anthony Budd
Original assignee: Wave Ltd C
Priority date: 2003-10-21
Filing date: 2003-10-21
Publication date: 2009-04-30
Also published as: AU2003274336A1; EP1689938A1; JP2008516107A; WO2005045136B1; US20080022915A1; CA2543439A1; WO2005045136A1; NO20062214L

Abstract

A breakwater for absorbing energy from the waves and leaving an area of calm water in the lee of the breakwater includes an energy absorber 104 between vertically orientated substantially parallel structures 100, 102 having neutral buoyancy and pair-wise separated from each other by a distance of (n + ½)l, l being the wavelength of the waves in the location at which the breakwater is to be installed, n being an integer greater than or equal to zero. The resistive force of the energy absorbers is adjusted to resist but permit relative movement of the parallel structures to absorb the energy of the waves and leave an area of calm water in the lee of the breakwater.

Description

<div class="application article clearfix" id="description"> . j ... 546825 FLOATING BREAKWATER Background The present invention relates to a breakwater. By definition a breakwater creates an area of calmed water behind it by 5 reflecting, scattering or absorbing energy from the waves. Existing breakwaters have varied in their design and complexity. One example of a very simple breakwater is a sandbar or embankment on which rocks may be deposited. Such breakwaters may be reinforced using groynes and/or concrete piling. 10 Prior Art An example of another type of breakwater device is described in published UK patent application number GB-A-2370594 (Kepner Plastics Fabricators Inc) and describes an elongate sealed envelope containing a liquid and pressurised air. The breakwater is adapted to float. By 15 varying the amount of internal pressure the breakwater can be arranged to attenuate waves. A disadvantage with the aforementioned breakwater has been that it is relatively complex to manufacture and therefore has proven quite expensive. Also, due to the relatively high internal pressure of fluids and 20 the nature of the flexible material, it has been prone to puncture. This has entailed maintenance on a regular basis. It is also (as with all other known breakwater devices) inertia dependant, that is to say, the mass of the breakwater must be similar to that of the largest waves that it repels. This further adds to the overall cost. 546825 2 The present invention stems from some work aimed at overcoming those disadvantages of existing breakwaters, by providing a simple breakwater device, having relatively few moving parts, which is not inertia dependant, which absorbs rather than reflects or scatters wave energy, which generates minimum mooring loads and which may be fabricated, transported and assembled cheaply and easily. A particular advantage of a breakwater, in accordance with a preferred aspect of the present invention, is that it can be relatively cheap to manufacture and maintain, and relatively light in weight. Summary of the Invention According to one aspect of the present invention, there is provided a breakwater device in which one or more energy absorbers arranged between a plurality of vertically orientated substantially parallel structures having neutral buoyancy are adapted to permanently remove energy from waves incident on the device by resisting the relative motion of the structures caused by opposing forces which are created between those structures by virtue of the fact that the structures are located in different parts of the irrotational oscillating cycle of the water mass which occurs naturally during the passage of waves, wherein the distance between any two of the structures is (n + V2) X where A, is a wavelength of waves in the particular location where the breakwater is to be deployed and n is zero or a positive integer, and the resistive force of the energy absorber/s being so adjusted in use to resist but permit relative movement of the structures so as to calm the area of water to the rear of the device. The invention is therefore not reflecting or scattering the wave energy, as has been the case with existing breakwaters, but rather is absorbing 546825 3 energy by virtue of the relative displacement between displaceable structures. The expression neutral buoyancy is intended to encompass anything that does not sink. Neutral buoyancy can include articles that float, partially 5 float or can be made sufficiently buoyant so that they are submersible, but still float. For example, an object of neutral buoyancy includes one that is lightweight (and naturally floats) and has been weighted with a ballast; or one that naturally sinks and has been made buoyant so that it floats. 10 Ideally the first and second parallel structures are planar when viewed from a perspective of the direction of an incident wave. Advantageously they are positioned in an area of open sea so that the major plane of each of the structures is substantially orthogonal to the incident wave front. Optionally a mechanical interconnection connects the first and second 15 structures; the interconnection preferably comprising means for supporting the energy absorber. The mechanical interconnection may, for example, be a sliding link. The interconnection may be substantially straight or of arched or generally calliper shape. 20 The energy absorber is preferably supported between the structures and is adapted to absorb energy resulting from relative displacement of the structures towards one another as well as apart from one another and the forces which occur thereto. 546825 intellectual property office of n.z. -1 MAR 2009 RECEIVE The energy absorber may be supported in the body of the liquid or above the surface of the liquid through which the wave is propagating, but is advantageously supported between the structures within the body of the liquid supporting them. 5 The energy that is absorbed may be extracted and used to drive a generator for producing an electric current, pump water or do another form of work, or could be used to drive a propulsion system for use by the device itself. By definition absorbing of the wave energy will create a calmed area of sea which can be used to provide protection on the 10 leeward side of the device from the waves. Such a device may be incorporated in a myriad of situations. Preferably the breakwater device comprises three substantially planar structures, arranged substantially parallel one to another. Ideally the distance between the first and second structures is substantially twice the 15 distance between the second and third structure. Ideally the distance between the first and third structures is approximately A/2, where A represents the maximum wavelength of waves in the particular location where the breakwater device is to be deployed. The distance between the first and third structures should be capable of varying by at least 2x the 20 maximum wave height about the nominal spacing of A/2 of waves of wavelength A. The distance between the first and second structures should be nominally 2/3 the distance between the first and third structures (ie A/3) and should be capable of varying by at least 2x the maximum wave height 25 about the nominal spacing of A/3 of a wave of wavelength 2A/3. Likewise the distance between the second and third structures should nominally be half that of the distance between the first and second 546825 Intellectual property I office of n.z. 5 - 4 NAR 2009 IRECEIVFnj structures and the distance between the second and third structures should be capable of varying by at least 2x the maximum wave height of a wave of wavelength A/3. This particular combination of these relative distances has been found to provide effective energy absorbing qualities 5 and is very well adapted at absorbing a myriad of wave lengths, of principal wavelength A downwards. A further mechanical interconnection can be provided to link the second and third structures, and this interconnection may support a further energy absorber. 10 A plurality of such breakwater devices may be arranged so as to create a breakwater system. Such a breakwater system may be used, for example, to maintain or modify coastal deposition and/or erosion patterns. Other uses of a plurality of breakwater devices, hereinafter referred to as a breakwater system, are explained later. 15 Ideally the plate like structures are substantially parallelepiped in shape and external appearance. However, the structures may be ovaloid or ellipsoid, provided they present a substantially large surface area to an incident wave. The definition of a parallelepiped plate like structure is hereindefined as: the ratio between the area of the plate like structure, 20 which is presented to the direction of a wave, and the square of the thickness of the plate like structure. Ideally this ratio should be in excess of 10, preferably in excess of 20 and ideally in excess of 30. Plate like structures may be formed from a variety of materials or composites. What is important is that the structure formed is able to 25 float, or it may be modified to have neutrally buoyancy, and the structure is strong. Ideally structures are able to withstand compressive and c/eooc [INTELLECTUAL PROPERTY t)4b«^0 OFFICE OF N.Z. - h NAR 2009 RECEIVED bending forces imposed by the action of incident waves, as well as occasional impacts with buoys and sea life. An example of a suitable material is reinforced glass fibre. Other examples are mild steel, flexible concrete or wood. Other materials may 5 be used and it will be apparent to a skilled artisan what types of materials and their respective dimensions, depending upon the particular environment in which the structures are to be deployed and prevailing weather, sea and other conditions. Due to the fact that the majority of energy transmitted via wave action 10 appears relatively close to the surface of water, through which the wave progresses, there is relatively little energy apparent below a depth of about one third to a half of a wavelength (A/3-A/2). Therefore the height of the plate like structure is ideally less than half a wavelength (A/2) of the prevailing wave conditions (and preferably less than A/5) of the sea 15 area where the breakwater device is to be deployed. The length and width of the structure is dependent upon, amongst other things, the strength of the material used to fabricate the structure and the depth of the local sea. The energy absorber acting between adjacent plates may take one of a 20 variety of forms. For example, the energy absorber may be a water choke arranged to squeeze water through a throttle so as to dissipate energy. This is a simple but effective manner to remove energy from waves. What is most desirable is that the energy absorbers should not contain any storing capability, such as a spring, as this could give rise to 25 resonance of the breakwater device, with the result that energy is temporarily removed (stored) and reflected back into the water, rather than being permanently removed from the system and all energy " *1 MAR 2009 I 5J|CElVEjDj removing devices must be capable of removing energy as plates move towards one another as well as when they move apart. In this way the requirements of Bernoulli's theory of irrotation are satisfied with forward and reverse forces within the wave cancelling each other out whilst 5 energy is extracted and only a nominal "second order" external reaction being created. The device therefore being arranged to generate small mooring loads. One way of achieving energy absorption is with an electromagnetic arrangement, sealed inside suitable waterproof containers, configured to 10 present a resistive force against relative displacement of the plate like structures resulting in generation of an electromotive force. A rack and pinion arrangement is another way in which energy can be removed. The rack and pinion may be fitted with suitable gears to transmit incident energy to a rotating resistive force, so that the energy 15 of a wave can be extracted. A yet further example of an energy absorber is a piston in cylinder arrangement acting as a dashpot. Alternatively the energy absorber may comprise a piston in cylinder having a fluid with variable rheological properties. Another type of energy absorber is a bi-directional hydraulic pump, which is adapted to remove energy during relative displacement of adjacent plates towards one another and away from one another. In a preferred embodiment, using this type of energy absorber, two nonreturn valves are arranged at each end of a cylinder containing a bidirectional piston which itself is connected to adjacent plates. Relative motion between the adjacent plates moves the piston in either direction which in turn pumps fluid at high pressure out of the relevant non-return 546825 20 25 546825 8 valve at one end of the cylinder and draws fluid in at the other. In this way energy in the form of fluid at high pressure is continuously extracted from the wave system and delivered to an external storage or use system regardless of the direction of motion of the adjacent plates. All that is 5 required is relative motion to occur. Again however, it is important that no compressible fluid is used as this could act as a pneumatic spring reinjecting energy back into the wave system. Advantageously a plurality of these devices can be interconnected, in the form of a loosely coupled barrier, so as to provide a breakwater system. 10 Due to the nature of the energy absorbers little relative displacement or external reaction is experienced between adjacent devices. This means that only modest tethering or anchoring of such a breakwater system is required. Brief Description of the Drawings 15 Preferred embodiments of the invention will now be described, by way of example only, and with reference to the accompanying drawings in which: 20 Figures 1 and 2 illustrate diagrammatically how energy, in the form of waves passing through a body of water affects and moves submerged bodies; Figures 3a to 3d illustrate diagrammatically how breakwater device walls, according to a first aspect of the invention move relative to one another during the passage of waves of wavelength twice the distance between them; 546825 Figures 4a to 4 d illustrate diagrammatically how wave energy is transmitted and can be absorbed by the device as illustrated in Figures 3 a to 3d when an energy absorbing system is placed between the walls. 5 Figures 5a to 5d illustrates diagrammatically how by adding a third wall and associated energy absorbing device to the system illustrated in Figures 4a to 4d wave energy can be absorbed by a myriad of different wavelengths at the same time constituting a complex sea state. 10 Figure 6 is a diagrammatical representation of an embodiment of the breakwater device using a simple water choke to absorb the wave energy. Figure 7 shows operation of a plurality of breakwater devices arranged as a breakwater system for protecting coastal regions 15 and/or managing coastal erosion and deposition. Figures 8 and 9 illustrate diagrammatically irrotational oscillation of a body of water and how this motion is affected by water depth in relation to wave length; Explanation of Theory on which the Invention is based 20 Referring to Figures 1,2,8 and 9 there is depicted a diagrammatical series of images, which explain how energy progresses through a body of liquid. This has been included as it provides a useful explanation to assist the reader with the theory on which the invention relies. OF \ ^ 4 MAR 2009 546825 intellectual property] office of n.z - \ MAR 2009 IRECEIveh Physics demonstrates that energy is transmitted through a body of water by means of a submerged oscillatory motion of the water mass about a relatively fixed datum. The fixed datum moves only gradually in the wave direction. This motion is known as an irrotational oscillation but 5 can be referred to as "wobbling". The wobbling motion is both up and down, as well as back and forth, and creates a coherent circular or elliptical oscillating, "wobbling" pattern about a point. The point is substantially stationary relative to the seabed. A phase shift between the vertical and horizontal oscillations determines: the direction of "rotation" 10 of the oscillating pattern; the direction of travel of the progressive waves on the surface; and the transmission of energy in that direction. The presence or absence of this "wobbling" motion is the only difference between still water and that which has waves passing across it. The coherent oscillatory motion of the water mass extends downwards 15 from the surface, reducing exponentially in amplitude to about 5% of its size at the surface at a depth of 1/2 wavelength (A/2). The oscillatory motion in the water is phase dependant. That is to say, when it is oscillating in the wave direction, it creates a crest and when it is oscillating against the wave direction it creates a trough. The 20 momentum, force applied and distance travelled by the coherent mass of fluid in the wave is substantially the same in all directions, with fluid particles returning to almost the same position, relative to the datum, at the end of each cycle. The wave profile and it's motion across the water, therefore, only represents the transmission of energy through the water 25 and not the motion of the water mass itself. It can be shown that wave energy is transferred only by the difference in potential energy (height) of the coherent water mass when oscillating with the wave direction at the crest to that of the same water mass when 546825 oscillating against the wave direction in the trough. The fluid motion described is in accordance with the Bernoulli steady state integrated equation of motion and assumes irrotational flow and invariant fluid density throughout the bulk of the fluid. This theory therefore underpins 5 the primary mechanism of energy transfer through water in the form of waves and is the theory on which this patent is based. Figure 1 represents the oscillating motion of a "discrete" block of water 3 (shown hatched for clarity), during the passage of a wave 2. For the purpose of explanation only, impermeable» infinitely thin and flexible 10 diaphragms 4 and 5 can be imagined to be positioned at the front and rear boundaries of this discrete block of water, so that its VOLUME, MASS AND IDENTITY remain the same throughout the process. During wave transit this mass oscillates back and forth, yet remains approximately in the same position relative to a fixed seabed datum 16. The diaphragms 4 15 and 5 bend backwards and forwards not in phase with each other, but respectively in phase with that part of the wave profile that is passing across that part of the surface of the water. As well as swaying backwards and forwards relative to the datum, this discrete block of water becomes taller and narrower (as shown in Figure lc) and shorter 20 and wider (as shown in Figure la) in a sequential and oscillating "wobbling" manner as each wave cycle passes. During this process a floating vertical plate like structure 7 will move backwards and forwards relative to the datum 16, by a total distance (measured at the water surface of approximately the wave height. The 25 plate itself, however, has minimal effect upon the passage of the wave, and is virtually transparent to the passage of the energy. [intellectual property OFFICE OF N.Z -4MAR20D9 RECEIVED 546825 - 4 MAR 2009 Received 12 A buoy 1, floating on the surface of the water transcribes a circle about the datum 16 of diameter approximately equal to the wave height. However, the buoy 1 does not itself rotate. This type of fluid motion is called an irrotational oscillation. 5 From Figure 1 it can be seen that the oscillation of the block represents cyclic motion of a large volume and therefore large mass of water a total distance at the surface of approximately the wave height every wave cycle in the horizontal direction. The kinetic energy and momentum of this block of water is also large, being a measure of the total quantity of 10 energy, which is contained within the wave. If the horizontal motion of plate 7 is resisted, the whole oscillating mass of the water reacts on it and generates large forces in the process. Since this is an oscillating process, the direction of action of the forces reverses twice during the passage of each wave cycle. For this reason vertical plates positioned one 15 wavelength apart are always acted upon by forces and displacements in the same direction. However, plates, located half a wavelength apart, will always be acted upon by equal forces and displacements in opposite directions. This phenomenon is shown by the direction of arrow 8 in Figures la and lc. 20 As mentioned above, the oscillating process occurring in the body of water is not restricted to the horizontal direction. Oscillation occurs in the vertical axis during the same time interval. This results in a circular or even elliptical complex oscillating motion. Figure 2 shows how numerous plate like structures 10, located in different parts of a water 25 mass, which is oscillating and causing the passage of waves 2 overhead, all experience different parts of the oscillating cycle at any instant in time. The part of the cycle experienced by a plate depends upon its position relative to the part of the wave passing overhead. Also, as the 546825 13 depth at which plates are located increases, the size of the oscillation excursion reduces until below a certain depth it tends to disappear. Therefore each of the plate like structures move relative to other structures located in different parts of the water mass as the waves pass 5 overhead and their distances apart are continuously changing. The one exception to this is if plates are positioned exactly one wavelength apart in the horizontal direction as detailed above. The orientation of these structures however will not substantially change during the oscillation process. That is to say end B of structure 10 continues to point to the 10 right throughout its circular, orbital path. Figures 8 & 9 show how these motions apply to vertical plates 56 suspended in deep water where depth > A/2 Figure 8 and shallow water where depth < A/20 Figure 9. In Figure 8 the motion of the top edge of the plate (in this case suspended by a buoyancy device 60) is 15 approximately circular, of excursion approximately equal to wave height about seabed datum 54 and clockwise with respect to waves 52 approaching from the left. The horizontal motion of the bottom edge of the plate is reduced in amplitude as explained earlier but the vertical excursion (being controlled by wave height) is the same as the top edge 20 and this results in a vertical elliptical motion of the bottom edge. Figure 9 demonstrates how the plate motion changes in shallow water. Here the motion of the top edge of the plate is elliptical with a vertical excursion axis of approximately wave height and a rotation clockwise in relation to waves 52 approaching from the left. The motion of the 25 bottom edge of the plate is again the same in the vertical direction but much magnified in the horizontal direction thus resulting in the elongated horizontal ellipse as shown. miiCSTSrJwinfTi OFRCF OF "4 MAR 2009 RECEIVED 546825 14 Detailed Description of Preferred Embodiments Figure 3 shows an example of an embodiment of the invention that could extract energy from a wave. This device 12 comprises first 13 and second 14 floating vertical structures arranged substantially parallel one 5 to another. Structures 13 and 14 are spaced nominally half a wavelength apart. The device 12 is oriented in use, so that the planes of structures 13 and 14 substantially orthogonal to the general direction of waves. Structures 13 and 14 are coupled together by a device 15. The device 15 may be an energy absorbing double acting hydraulic pump, but 10 in this example it is allowed to move freely in and out without extracting any energy. In this configuration an embodiment of the invention is envisaged under conditions whereby tilt, stroke and distance measuring devices are incorporated into the device 15 to accurately measure wave height, 15 wavelength and wave period. Further to this, delicate equipment (or personnel) located and supported approximately midway between plates 13 and 14 are subjected to only a minimum degree of lateral or vertical motion relative to the seabed. As explained in Figure 1, a floating vertical plate like structure can be 20 shown to oscillate backwards and forwards about a datum a total distance of approximately one wave height during the passage of each wave cycle. Figure 3a shows how structure 13 is behind datum 16, as a wave trough passes and a wave crest approaches whereas structure 14 is in front of datum 17, as wave crest passes and wave trough approaches. The two 25 structures 13 and 14 are therefore further apart than their nominal spacing, by about one wave height at the instant shown in Figure 3a. Likewise, as the progressive wave profile passes, structures 13 and 14 [ OFFICE OF n3 ERTYI -4'MA*20B nj 546825 15 move two wave heights closer together. This is shown in Figure 3c. Since the datums 16 and 17 are fixed, both relative to each other and the seabed, the structures 13 and 14 move relative to each other a distance of approximately two wave heights. This occurs each wave cycle. 5 However, it will be noted that the assembly remains substantially stationary relative to the seabed. Motion is symmetrical about datums 16 and 17 when no energy is being extracted by device 15. In this condition plate like structures 13 and 14 are free to move backwards and forwards solely under the influence of 10 oscillating water mass and the waves proceed virtually unaffected as explained above with reference to Figure 1. Figures 4a to 4d show how wave motion changes when energy is being extracted by pump 15. In this example progressive waves 2 are considered to approach from the left. Extraction of energy by the 15 pump 15 means that relative motion must occur between plates 13 and 14 against a force f. It also follows that the external forces transmitted by plate 14 into the water (on its right hand side) and its motion (relative to datum 17) must always be zero or wave energy would be transmitted and lost. Figure 4a shows how the extraction of energy by pump 15 through the application of a force -f causes a reduction in trough depth 18 across plate 13 (In this description forces and motions to the left ie against the direction of travel of the waves are considered as negative and forces and motions to the right that is to say with the direction of the waves are considered to be positive although the exact opposite notation would work just as well). During this process plate 13 moves a distance -d to the left that is to say with the oscillating mass of water and the force times the intelA^tual property office OF HZ MAR 2009 RECEiven 20 25 546825 OFFICE OF N.Z. - 'i MAR 2009 -RECElVFO 16 distance means that a positive amount of energy +W will have been extracted from the wave. In Figure 4a this motion is to the left because the direction of oscillation in a wave trough is in the opposite direction to that of the wave itself. 5 Because plate 13 is moving, it transmits a progressive "in phase" wave into the space between plates 13 and 14 and this has the same wavelength as the incident wave. However, its trough and crest amplitude are reduced in direct relation to the quantity of energy remaining from that extracted by the pump 15. The reduction in wave energy means that the 10 oscillating motion within the wave is also reduced. This is shown as a reduction in deflection of "imaginary" diaphragms for example from position 19 shown dashed to the lesser deflected position 20 shown solid. As mentioned earlier, plate 13 must apply a force to the pump 15 as well as move relative to it so as to enable energy to be extracted. The equal 15 and opposite reaction to this force however appears on plate 14 and this would cause it to be moved to the left and generate a wave trough to its right, if it were not resisted by an equal and opposite force to the right. From the previous description of the oscillating motion within the waves, it is apparent that the direction of motion of the forces, within the wave, 20 are reversed every half cycle. Therefore if plates 13 and 14 are positioned nominally half a wavelength apart, plate 14 will be acted upon by a force to the right which counteracts the force generated by pump 15. In closer examination it is apparent that the level (energy change) across plate 13 must always be equal and opposite to the level (energy change) 25 across plate 14 at all times, as action and reaction across the pump 15 must always be equal. The "level" change across plate 13 is thus replicated in reverse by the "level" change across plate 14, whereas the 546825 17 degree of level change is determined by the quantity of energy extracted by the pump 15. Different amounts of energy extracted produce different effects from these level changes. For example, if only a small percentage of the available energy is extracted, the level change 18a would be small 5 in relation to the trough depth 24. Because this is replicated in reverse on plate 14, the level change 21a would also be small in relation to (intermediate) wave crest height 26. The reverse reaction force (generated on plate 14 by pump 15) is therefore not large enough to resist all of the force generated by wave crest 26. Any unresisted surplus 26a 10 moves plate 14 to the right, transmitting some of the wave energy through to the right of plate 14 which is therefore transmitted through the system and lost. If however the backpressure on pump 15 is increased the level (energy change) 18 across plate 13 correspondingly increases. The effect is that 15 the transmitted wave trough depth 27 is reduced since the impinging trough depth 24 does not change. However, the trough depth 27, also determines the crest height 21, since they are both functions of the same reduced amplitude oscillating process. Thus, as the level difference 18 is increased, with increased backpressure from the pump 15, both the 20 remaining trough depth 27 and the remaining crest height 21 reduce correspondingly. At a certain level of energy extraction (back pressure) the "level" difference 21a across plate 14 will match the still water level 22. Under these conditions there are no residual forces remaining on plate 14 to create a wave on its right hand side and therefore no 25 horizontal motion occurs. The wave has therefore theoretically disappeared because the pump 15 has extracted all the oscillating energy entrained in it. 546825 18 To assist in explanation the phrase "level" (energy change) has been used to define the different states occurring across the plates. This is because the energy in a wave is not directly proportional to its height but to the square of its height. Therefore directly measured height differences 5 across the plates have to be mathematically computed using a square law to compare them with changes of energy. Secondly the shape of "real" waves is approximately Stokian and not sinusoidal. That is to say the steepness of curve is greater over wave crest 28 than it is through wave trough 29, as can be seen in Figure 4b. 10 Figure 4b shows how wave trough 31 is longer than wave crest 30 as measured along the mean "still" water line and demonstrates how, as the progressive waves pass, the natural motion of plate 13 as it moves back and forth will follow the point where the still water level intersects the wave surface profile. 15 Figures 4a to 4d show the motions of a wave 2 and plates 13 and 14 throughout a full progressive wave cycle. From this series of "snapshots" it can be seen how balancing and cancelling of wave forces continues throughout the process. For example in Figure 4b, as wave crest 2 approaches from the left, both plates 13 and 14 coincide with a 20 wave "node" point; and therefore level differences (i.e. forces) across both will disappear. Between states Figure 4a and 4b trough depth 27 progressively decreases at the same rate as the crest 21 reduces thus maintaining a state of equilibrium and force balance. Between Figure 4b and 4c the wave crest approaches plate 13, generating a wave trough 25 against plate 14, which provides a reaction force to counteract the crest force acting on plate 13. Figure 4d shows the situation has returned to that of Figure 4b but in mirror image. As the progressive wave '^^CTUATpROPERrv/ f OFFICE OF rvZ RTVf - 4 MAR 2009 -5ECEJVED 546825 19 continues the configuration of Figure 4a returns and the process repeats continuously in a cyclic manner. Two fundamental properties of the device can now be defined from the forgoing analysis. Firstly, plates placed one wavelength apart oscillate in 5 a circular or elliptical pattern relative to the seabed, but in unison and without measurable differential motion between them. Secondly, plates placed half a wavelength apart oscillate in the same pattern, but diametrically opposed to each other, and create a differential motion approximately equivalent to two wave heights each wave cycle at the sea 10 surface. When the plates are positioned one wavelength apart, and fixed together, wave energy passes right through the device virtually unaffected; whereas when the plates are positioned approximately half a wavelength apart, or (n+l/2)A wavelengths apart where n is a positive whole number including 15 zero, theoretically energy can be extracted up to a quantity equal to the total amount available in the wave, by adjusting the resistive force (back pressure) of an energy absorber adapted to extract energy from the relative displacement occurring between the plates. A further embodiment of the invention is now described with reference to 20 Figure 5 wherein vertically orientated floating plates 32, 33 and 34 are positioned orthogonal to the general direction of the waves and coupled together by two double acting hydraulic pumps 30 and 31. Plate 33 is nominally located 2/3 and 1/3 from outer plates 32 and 34 respectively. This embodiment has been found to be capable of extracting wave energy 25 from a wide range of wavelengths. Figure 5b shows how plates 32 and 33 move in a similar manner to plates 13 and 14 of Figure 4, when acted upon by waves of wavelength of intellectual property office of n.z - RECEIVED 546825 20 the order of twice the distance between these plates. As explained above, with reference to Figure 4, theoretically substantially all the energy contained within the wave can be absorbed. Plate 34 is then effectively redundant. If the device is now acted upon by waves of shorter 5 wavelengths, for example of wavelength equal to the distance between plates 32 and 33, as shown in Figure 5c, then plates 32 and 33 move backwards and forwards in unison allowing the waves to pass "through" unimpeded. Plates 33 and 34, however, which are positioned half the distance apart of plates 32 and 33, are now at the correct spacing of A/2 10 to absorb all the wave energy via pump 31. In fact wave energy can be extracted with maximum efficiency by the device from any wavelengths A where the plate spacing is A(n + 1/2) between any two plates and n is a positive full number including zero. Thus for example spacings of A(0 + 1/2) = 0.5A, A(1 + 1/2) = 1.5A, 15 A(2 + 1/2) = 2.5A etc between any two plates provide maximum energy absorption. In between these specific wavelengths the function of energy extraction is divided between different pairs of plates; an example of which is shown in Figure 5a. In this situation the exact half wavelength exists between plates 32 and 34. However, because plate 33 is located 20 1/3 wavelength from plate 34, a small proportion of the wave energy is extracted by pump 31 with the remainder being extracted by pump 30. This works because each plate only "sees" the horizontal differential motion occurring between it and the other two plates and extracts energy from this motion to an amount equal to (displacement) x (the resisting 25 force). In this way wavelengths, where the half wavelength does not exactly equate to any of the plate pair spacing, still achieve a high energy extraction efficiency. For example a wavelength equal to the distance between plates 32 and 34 cannot extract any energy from this pair, but (i^TELLECTUAL^TOPEiry] OFRCE OF M.z HrYl ~ * MAR 2009 RECEJVED _ OFWCE OF N.Z B25 -1, MAK 2009 RECEIVED 21 only from intermediate plate 33, which is now a maximum of 1/6 of a wavelength offset from the nominal 1/2 wavelength position. This creates a small, but not significant, drop in energy extraction efficiency at this wavelength. Energy can also be extracted from 5 wavelengths, which are shorter than the distance between plates 33 and 34 and Figure 5d shows how using equation A(n + 1/2) enables a maximum energy absorption to be achieved with much shorter wavelengths. Further to the above, real seas invariably comprise combinations of 10 wavelengths creating a complex surface shape and pattern and this is the most common form usually encountered. As previously explained the shape, (which in this case can be more accurately described as the velocity and elevation of a particle at any instantaneous point on the surface), defines the single motion occurring at that point under the 15 surface which has been created by the sums and differences of all the waves of different lengths passing through that point. The proposed embodiment of this invention employs this resultant differential motion, effectively extracting energy from all of the entrained different wavelengths, as if they were individually isolated one from the other. 20 Figure 6 shows an embodiment of a breakwater device having two rectangular plates 100 and 102 and an energy absorber 104 pivotally mounted to each plate by pivots 108 and 110. The energy absorber is submerged and comprises a loose fitting piston or flow restricting device 111 located in a cavity 112 which has a loose fitting aperture 113 25 and choke passage 114. In operation differential motion between the plates during the passage of waves causes motion of the flow restricting device 111 and water to be forced in and out of cavities 115 and 116 and intellectual property 546825 office of n.z. - h um 2009 RECEIVED also past the flow restricting device creating a resistive force both when the plates are moving apart and together thereby extracting energy from the waves. Figure 7 shows an alternative embodiment of the invention, in which a 5 plurality (in this case eight) floating devices 205 and 206 (as described in more detail in Figures 4 and 5) are linked together in the form of a chain so as to provide a breakwater system to protect the shoreline 207. Waves are present in the open sea 200, whereas the water surface 210 in the lee of the breakwater system is calm as a result of energy having been 10 absorbed by the breakwater system. In this embodiment two or three plate arrays could be grouped to deliberately adjust the wave climate to manage coastal erosion or deposition patterns on the shoreline 207. Wave energy absorption, or compensating means have been described. Plates or plate like structures provide the effect. The structures may, or 15 may not, allow the passage of liquid therethrough. Valves may be incorporated in the structures so as to allow or facilitate the passage of liquid in one direction. The structures are submerged in different parts of, or below, a body of liquid, which is subject to the oscillating pattern caused by the passage of waves. Ideally wave energy absorption, or 20 compensation is achieved through the control of the interaction between two or more of the aforesaid structures or between two or more structures interacting against the inertial mass of the body of liquid. This may be enhanced by exploiting, in a controlled manner, the flow of liquid through the structures in one direction only. 25 Switching on or off a breakwater device can be achieved by manually resetting the distance between its plates. For example moving the plates from half a wavelength to one wavelength apart will switch off the 546825 23 device. Switching off can also be achieved by removing resisting forces from interconnecting means. The invention has been described by way of exemplary embodiments. It will be appreciated that variation to the embodiments described may be 5 made without departing from the scope of the invention. I'MTEu £ctual MAR zoos mreu 546825 24 </div>

Claims

<div class="application article clearfix printTableText" id="claims"> CLAIMS

1. A breakwater device in which one or more energy absorbers arranged between a plurality of vertically orientated substantially parallel structures having neutral buoyancy are adapted to permanently remove 5 energy from waves incident on the device by resisting the relative motion of the structures caused by opposing forces which are created between those structures by virtue of the fact that the structures are located in different parts of the irrotational oscillating cycle of the water mass which occurs naturally during the passage of waves, wherein the distance 10 between any two of the structures is (n + V2)X where X is a wavelength of waves in the particular location where the breakwater is to be deployed and n is zero or a positive integer, and the resistive force of the energy absorber/s being so adjusted in use to resist but permit relative movement of the structures so as to calm the area of water to the rear of the device . 15

2. A breakwater device according to claim 1 wherein the breakwater device includes a third structure, which in use is arranged substantially parallel to the other two structures.

3. A breakwater device according to claim 2 wherein the distance between the first and second structures is substantially twice the distance 20 between the second and third structures.

4. A breakwater device according to claim 2 or claim 3 wherein the distance between first and second structures is XI2 and X is the maximum wavelength of waves in that particular location where the breakwater is to be deployed. 25 4 NASI 200S 546825 25

5. A breakwater device according to any one of claims 2 to 4 comprising a mechanical interconnection from the first to the second structure, and from the second to the third structure, the interconnections supporting the energy absorbers. 5

6. A breakwater device according to any one of claims 1 to 5 wherein the structures are substantially parallelepiped structures.

7. A breakwater device according to claim 6 wherein the structures are plate-like and plate-like is defined as the ratio between the area of the structure, which is presented to the direction of a wave, and the square of 10 the thickness of the structure, said ratio being greater than 10.

8. A breakwater device as claimed in claim 7 in which said ratio is greater than 20.

9. A breakwater device as claimed in claim 8 in which said ratio is greater than 30. 15

10. A breakwater device according to claim 7 wherein the height of the plate like structures is less than a half the wavelength (A./2) of waves in that particular location where the breakwater is to be deployed.

11. A breakwater device as claimed in claim 10 in which said height is less than (X/5) of the waves in that particular location. 20

12. A breakwater device according to any preceding claim in which the or each energy absorber comprises water chokes arranged to squeeze water through a throttle so as to dissipate energy upon relative displacement of the structures. 546825 26

13 A breakwater device according to any one of claims 1 to 11 in which the or each energy absorber comprises an electromagnetic arrangement, sealed inside a suitable waterproof container, configured to generate an electromotive force upon relative displacement of the 5 structures.

14. A breakwater device according to any one of claims 1 to 11 in which the or each energy absorber includes rack and pinion arrangements fitted with suitable gears to convert linear to rotating motion.

15. A breakwater device according to any of claims 1 to 11 in which the 10 or each energy absorber comprises a piston and cylinder arrangement so arranged as to act as a dashpot.

16 A breakwater device according to any of claims 1 to 11 in which the or each energy absorber includes a bi-directional piston and cylinder, with a fluid arranged to pass through energy absorbers so as to absorb 15 wave energy when the structures move towards one another as well as away from one another.

17. A breakwater device according to any one of the preceding claims wherein the breakwater device, in use, is so positioned in a body of water, such as an area of open sea, that the lengthwise axes of the 20 structures extend substantially parallel to an incident wave front.

18. A breakwater system comprising a plurality of breakwater devices according to any one of the preceding claims, said system being capable of maintaining or modifying coastal deposition and/or erosion patterns

19. A method of controlling coastal erosion using the breakwater 25 devices of any of claims 1 to 17 or the systerr 546825 27

20. A breakwater device according to claim 1 and substantially as herein described with reference to any embodiment shown in the accompanying drawings. INTEnlSI^^OPERTY] office OF isi.z I - h MAR 2009 RECEIVED </div>