Patent Publication Number: US-2010111610-A1

Title: connector system for a wave energy converter

Description:
FIELD OF THE INVENTION 
     The present invention relates to wave energy converters and in particular to a wave energy converter comprising at least two interconnected parts. The invention more particularly relates to connector system for connecting the related parts of such a wave energy converter. 
     BACKGROUND 
     Within the context of wave energy converters a number of different approaches are known in the art. One particular type is known a point absorber. Such known point absorbers and one particular type of such point absorbers is described in our earlier patent, EP1295031. In this arrangement a floating body or surface float is connected to a submerged body. Both the floating body and the submerged body are of a large dimension, albeit the submerged body is much greater than that of the surface float. 
     In such an arrangement, the surface float with quite a small displacement supports a very large submerged tank of water (x˜10 times the displacement mass). A slender connection enables the device to function more effectively as a wave energy absorber. 
     Furthermore, in typical ocean waves the distances from crest-to-crest are significantly greater than the diameter of either the float or tank; with any appreciable wave height the float/tank assembly is inclined to pitch (oscillate like a compound pendulum). In the arrangement of EP1295031, the float is inside a concentric floating torus. In big seas and storm conditions the lateral forces arising from different pitch periods and axes of rotation between the torus and float/tank may be destructively great. 
     A problem therefore exists with how to connect such large and massive dimensioned bodies in a manner which provides a narrow connection between the two such that tensile and compressive forces may be transmitted along a major axis between the two devices with little or no loss, but at the same time minimising unwanted lateral bending moments and/or dynamic shocks. 
     SUMMARY 
     These and other problems are addressed in accordance with the teachings of the present invention by the provision of a connection system for use with a wave energy converter of the type having a floating body and a submerged body, the connection system being configured, in use, to provide for a coupling of the floating body to the submerged body and wherein the connection system comprises a plurality of discs threadable on at least one connection cable which is under tension, the connection cable being configured to be coupled at one end to the floating body and at a second end to the submerged body, and wherein the discs are arranged relative to one another to restrict lateral movement of the floating body relative to the submerged body. 
     By providing such an arrangement the invention enables a transmission of tensile and compressive forces along a major axis between the submerged and floating bodies with little or no loss, but at the same time dissipating unwanted lateral bending moments. 
     The discs are desirably arranged in a stack arrangement. 
     By providing the at least one cable in tension, it is possible to maintain the stack arrangement in compression. It is desirable that the degree of tension is sufficient to hold all the discs together when the lower body is accelerating relative to the upper body. 
     Preferably a compressible layer is provided between adjacent discs (these being substantially incompressible) such that the discs may flex relative to one another. Such a layer may be formed from a resilient material such as neoprene or some other suitable material. 
     There are desirably at least two cables provided. The cables are preferably arranged adjacent to a central axis between the two bodies. Such an arrangement allows for bending of the discs relative to one another. The spacing between the adjacent cables is desirably such as to provide a degree of flexing. 
     Effectively the arrangement of the discs and cable provides a pseudo-spine between the two bodies enables a flexing of the two bodies relative to one another yet still maintains the two bodies substantially co-linear with one another. 
     These and other features of the invention will be understood with reference to the following drawings 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described with reference to the accompanying drawings in which: 
         FIG. 1  is a schematic showing an example of a wave energy converter with a connection system in accordance with the teachings of the invention. 
         FIG. 2  is a schematic showing a more detailed view of the connection system components. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     The invention will now be described with reference to  FIGS. 1 and 2 . 
     As shown in  FIG. 1 , a wave energy converter  100  comprises a floating body  105  which is coupled to a submerged body  110  by means of a connection system  115 . The dimensions of the submerged body are typically much greater than that of the floating body, but both may be considered as physically large devices. For example typical top to bottom dimensions of the floating body  105  are about 10 m whereas for same top to bottom dimension of the submerged body is about 35 m. Further information as to the type of construction that may be used for the floating and submerged bodies is described in our earlier European patent EP1295031. It will be noted that the two bodies are arranged about a central axis A-A′. In such an arrangement the bodies are subjected to lateral motion arising from circulation of the water around the bodies. The particle motion within a wave provides a large rotational movement which is proportional to the height from the surface. As such the movement exerted on the lower submerged body is different to that of the floating body, yet it is desirable to maintain the two bodies along the same vertical axis. 
     In order to maintain the two bodies about such a central or main axis, the invention provides a connection system  115 , that is to some degree compliant when subjected to forces other than those along the main axis. Such a system comprises a plurality of discs  120  threadable on at least one connection cable  125 , the connection cable  125  being configured to be coupled at one end to the floating body  105  and at a second end to the submerged body  110 . The connection cable(s) are provided in tension. The arrangement of the discs relative to one another is such to restrict lateral movement of the floating body  105  relative to the submerged body  110 . 
     A typical arrangement for such a connection system is shown in  FIG. 2 . As will be seen from this schematic, the connection system  115  includes a stack  200  of (typically) high density concrete discs  120  with compressible (e.g. neoprene) discs  205  between each layer and holding the stack together with pre-stressed (stainless steel) cables. It will be appreciated that it is desirable for these to be cables, not reinforcing bars, and as such not bonded to the discs  120 ,  205 ,—the latter are as beads on a string. By providing a cable as opposed to a rigid bar it is less prone to fatigue stresses. 
     The spacing of the cables is important,—close to the central axis means greater scope for bending or flexing, less so if they are wide apart, towards edges  210  of the stack arrangement  200 . By providing the cable(s) in tension it is possible to maintain the compression forces on the stack. 
     The neoprene or other compressible material is of a nature that it will yield when the spine is forced towards bending sideways, it being ‘nipped’ at the edge, but vertical compressive forces will cause little compression as they tend to be spread across the full area of the neoprene discs. The pre-tension on the cables is desirably of a level to be sufficient to keep all elements in the stack always in contact. It is also desirable that the thickness of the compressible layer is small relative to the thickness of the discs on either side of it. The area of the individual discs is desirably such that in tension there is negligible vertical interaction between adjacent discs. 
     Such an arrangement has been subjected to theoretical analysis, numerical modelling and laboratory tests. Finite element analysis indicates that a small and slender solution is achievable, which solves the problem of connecting the two bodies in a manner that allows the transmission of forces between the two devices and yet restricted lateral movement. In such analysis typical dimensions for the geometrical arrangement for the stack arrangement is one having a length L of about 4 m, a diameter d of about 2 m, and a thickness t of about 0.025 m. These dimensions are, it will be appreciated, exemplary of the type of dimensions that may be expected in an device according to the teaching of the invention but that the specific dimensions will depend on the exact location and environment where the device is being deployed. 
     A ¼ scale model of the geometrical arrangement illustrated in  FIG. 2  was produced in ANSYS (a well known computer-aided engineering technology and engineering design analysis software product) in order to observe the affects of scaling on the free vibration properties of the structure. A reduction in scaling was found to have a significant effect on the natural frequencies of the system. The fundamental frequency was calculated at 1.11 Hz, with the second natural frequency found to be 14.7 Hz. Both frequencies are well outside a typical wave energy spectral energy distribution and as such the system would operate well in the intended environment. 
     It will be understood that the relationship between the specific components of the system will depend on the exact dimensions. However for a geometrical arrangement of L=4 m, d=2 m and t=0.025 m, the number of blocks used to create the spine is preferably of the order of six, which may be fabricated in concrete or some other suitable material. It will be appreciated however that this number may depend on an ability to either prefabricate the blocks with certain dimension or pour the blocks on-site with a type specific shuttering. The height of each individual block (h) is thus recommended at 0.645 m, with five intermediate layers of neoprene each of 0.025 m thickness. The diameter of each block and neoprene pad is recommended at being 2 m. 
     Concrete represents a good material for the blocks, as it is relatively cheap, easily transportable on-site and shape malleable. As the connector system is to be used in a harsh marine environment, grade 30 concrete, with a compressive stress capacity of 30 Nmm-2 and neoprene pads as intermediate material with elastic modulus of 2.28×106 Nmm-2 are typical and should be adequate for all geometrical arrangements and loading configurations. It will be appreciated that these dimensions and values and materials are provided as illustrative of the type of arrangement that may be employed and that different dimensions, values and materials may be found equally suitable in application. 
     The magnitude of the pre-stressing force required was observed to be both a function of the length of the spine, and the diameter of the spine. As the lowest possible pre-stressing appears most favourable, decreasing the length of the spine and increasing the diameter of the spine will yield the lowest required pre-stressing force, i.e. for all the geometrical arrangements reported in this document, the most favourable arrangement would be a spine length of 4 m, a spine diameter of 2.0 m, and a neoprene thickness layer of 0.025 m. This assembly would require a pre-stressing force of 9000 kN. 
     It will be appreciated that what has been described herein is a connector system for a wave energy converter comprising two interconnected bodies. The system enables a transmission of forces between the two bodies yet restricts lateral movement between them. By providing a plurality of discs in a stack arrangement and allowing the discs to flex relative to one another, alignment of the interconnected bodies about a central axis may be maintained. Although the invention has been described with reference to preferred embodiment, it will be appreciated that modifications may be made without departing from the scope of the invention which, it is intended, is to be limited by the appended claims only. 
     The words comprises/comprising when used in this specification are to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers , steps, components or groups thereof.