COMPRESSIVE ELEMENT FOR A MOORING COMPONENT

Abstract

A compressive element for a mooring component includes multiple shells. Each shell has first and second annular portions, and a central section that extends between the first and second annular portions. The first and second annular portions lie in a plane that is perpendicular to a central axis of the compressive element. The first annular portion has a maximum dimension in a direction perpendicular to the central axis that is greater than a corresponding maximum dimension of the second annular portion. The shells are arranged along the central axis such that the first or second annular portion of one shell is joined to the first or second annular portion of an adjacent shell. The compressive element is arranged such that when a compressive stress causes the compressive element to be compressed, the central section of one shell contacts the central section of an adjacent shell.

Claims

1. A compressive element for a mooring component, the compressive element comprising a plurality of shells, wherein each of the plurality of shells comprises a first annular portion, a second annular portion and a central section; wherein the first annular portion and the second annular portion each lie in a plane that is substantially perpendicular to a central axis of the compressive element, wherein the first annular portion has a maximum dimension in a direction substantially perpendicular to the central axis that is greater than a maximum dimension of the second annular portion in a direction substantially perpendicular to the central axis, and wherein the central section connects and extends between the first annular portion and the second annular portion; wherein the plurality of shells are arranged along the central axis such that the first annular portion of one of the plurality of shells is joined to the first annular portion of an adjacent shell of the plurality of shells or such that the second annular portion of one of the plurality of shells is joined to the second annular portion of an adjacent shell of the plurality of shells; wherein the compressive element is arranged such that when a compressive stress is applied to the compressive element substantially in the direction of the central axis, the compressive element is compressed; and wherein the compressive element is arranged such that when the compressive stress applied to the compressive element causes the compressive element to be compressed by a particular fraction of an uncompressed length of the compressive element, the central section of one of the plurality of shells contacts the central section of an adjacent shell of the plurality of shells.

2. The compressive element of claim 1, wherein the compressive element is arranged such that the annular portions of the compressive element have a hoop stress that substantially resists the component of the compressive stress acting in tension on the first annular portion and in compression on the second annular portion.

3. The compressive element of claim 1, wherein a thickness of the first and/or second annular portions is greater than a minimum thickness of the central section.

4. The compressive element of claim 1, wherein a length of the central section is greater than a height of the first and/or second annular portions.

5. The compressive element of claim 1, wherein a thickness of the first and/or second annular portions is greater than a height of the first and/or second annular portions, respectively.

6. The compressive element of claim 1, wherein the maximum dimension of the first annular portion is greater than 40% larger than the maximum dimension of the second annular portion.

7. The compressive element of claim 1, wherein the first annular portion and/or the second annular portion is configured to deform when the compressive force exceeds a threshold that causes the compressive element to be compressed by more than the particular fraction.

8. The compressive element of claim 1, wherein the central section is configured to deform when the compressive stress exceeds a threshold that causes the compressive element to be compressed by more than the particular fraction.

9. The compressive element of claim 1, wherein the central section has a non-uniform thickness along its length.

10. The compressive element of claim 1, wherein the central section has a non-uniform shape, such that a thickness of the central section parallel to the central axis varies along the central axis.

11. The compressive element of claim 1, wherein the compressive element is arranged such that the compressive stress applied to the compressive element up to a first stress value of the compressive stress compresses the compressive element in a first stage of compression by up a first fraction of an uncompressed length of the compressive element, wherein the compressive element is arranged such that the compressive stress applied to the compressive element above a first stress value of the compressive stress and up to a second stress value of the compressive stress further compresses the compressive element in a second stage of compression by greater than the first fraction of the uncompressed length of the compressive element and up to a second fraction of the uncompressed length of the compressive element, and wherein the second fraction of the uncompressed length of the compressive element is less than or approximately equal to the particular fraction of the uncompressed length of the compressive element at which the central sections of adjacent shells are arranged to contact each other.

12. The compressive element of claim 11, wherein the compressive element exhibits a non-plastic response up to the second stress value.

13. The compressive element of claim 1, wherein the central section of each of the plurality of shells comprises a shoulder portion projecting from an inner surface or an outer surface of the central section, wherein shoulder portions of adjacent shells are arranged to contact each other under compression.

14. The compressive element of claim 13, wherein the first shoulder portion projects from the inner surface of the central section.

15. (canceled)

16. (canceled)

17. The compressive element of claim 13, wherein the first shoulder portion is shaped so that, when the compressive stress applied to the compressive element causes the compressive element to be compressed by the particular fraction of the uncompressed length of the compressive element, the first shoulder portion projects further towards the adjacent shell than any other part of the central section.

18. The compressive element of claim 1, wherein the first annular portion of one of the plurality of shells is welded or glued to the first annular portion of an adjacent shell of the plurality of shells and/or such the second annular portion of one of the plurality of shells is welded or glued to the second annular portion of an adjacent shell of the plurality of shells.

19. (canceled)

20. The compressive element of claim 1, wherein the compressive element is formed from at least one polymer material.

21. The compressive element of claim 1, wherein the compressive element is formed integrally as a single piece.

22. The compressive element of claim 1, further comprising a first inner plate, connected to one end of the compressive element, a second inner plate connected to the other end of the compressive element, a first outer plate adjacent to the first inner plate for connecting to a first portion of a mooring line, a second outer plate adjacent to the second inner plate for connecting to a second portion of a mooring line, a first connecting member connected to the first inner plate and the second outer plate, and a second connecting member connected to the second inner plate and the first outer plate.

23. (canceled)

24. A mooring system comprising: a mooring line, and the compressive element of claim 1, wherein the compressive element is arranged between a first section of the mooring line and a second section of the mooring line such that tensile stress applied to the mooring line, which compresses the compressive element, causes the overall length of the mooring system to increase.

25. (canceled)

Description

[0116] Certain preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0117] FIG. 1a shows a set of floating offshore wind turbines, moored using a mooring;

[0118] FIG. 1b shows the stress-strain response produced by the mooring system shown in FIG. 1a;

[0119] FIG. 2a shows a set of floating offshore wind turbines, moored using a mooring line according to an embodiment of the present invention;

[0120] FIG. 2b shows the stress-strain response produced by the mooring system shown in FIG. 2a;

[0121] FIG. 3 shows an example of a desirable stress-strain response for a mooring system;

[0122] FIG. 4 shows a cross-sectional view of an elastomeric compressive element according to an embodiment of the present invention;

[0123] FIG. 5a shows the external appearance of the elastomeric compressive element shown in FIG. 4;

[0124] FIG. 5b shows a cutaway perspective view of the elastomeric compressive element shown in FIG. 4;

[0125] FIG. 5c shows a cross-sectional view of the elastomeric compressive element shown in FIG. 4;

[0126] FIG. 5d shows an exploded perspective view of the elastomeric compressive element shown in FIG. 4;

[0127] FIG. 6a shows a perspective view of a single shell of the elastomeric compressive element shown in FIGS. 5a to 5d;

[0128] FIG. 6b shows a cutaway perspective view of the single shell shown in FIG. 6a;

[0129] FIG. 7 shows an exemplary cross-sectional profile of two adjacent shells of an elastomeric compressive element;

[0130] FIG. 8 shows a cross-sectional profile of a series of pairs of shells, as shown in FIG. 7, joined together to form a bellowed spring;

[0131] FIG. 9 shows the spring of FIG. 8 compressed to approximately the start of the second compression stage;

[0132] FIG. 10 shows the spring of FIG. 8 compressed to approximately the end of the second compression stage;

[0133] FIG. 11 shows the spring of FIG. 8 compressed to a point within the third compression stage;

[0134] FIG. 12 is a graph showing the force response of the bellowed spring of FIGS. 8-11;

[0135] FIG. 13 shows another exemplary cross-sectional profile of two adjacent shells of an elastomeric compressive element;

[0136] FIG. 14 is a cross-sectional profile of the shells of FIG. 13, when the adjacent shoulder portions have been brought into contact;

[0137] FIG. 15a shows an example of an elastomeric compressive element in which the central section comprises a number of profile sections; and

[0138] FIG. 15b shows the elastomeric compressive element of FIG. 15a under a compressive force.

[0139] Floating marine structures, such as floating offshore wind turbines, generally require a mooring system connected between the sea bed and the floating marine structure to keep the structure in place. Embodiments of a mooring component, for use in such a mooring system, will now be described.

[0140] FIG. 1a shows a floating offshore wind turbine 1, moored using a conventional catenary mooring system 2. The floating offshore wind turbine 1 is shown in two different positions, representing the position before and after the wind thrust is applied. A catenary mooring system comprises a length of chain, arranged so that one end is laying along the sea floor, whilst the other end is attached to the object that is moored. The horizontal arrow 3 represents the force acting on the mooring system as a result of wind causing action of the turbines 1.

[0141] This force 3 pushes the turbine 1 away from the direction of the wind. The initial tension in the mooring line is not sufficient to resist this motion and so the platform moves. As it moves more catenary chain 2 is lifted from the seabed increasing the tension in the mooring line, until an equilibrium position is reached where the horizontal component of the tension 5 in the mooring line balances the additional thrust due to the wind, shown by the dotted line 4.

[0142] FIG. 1b shows the stress strain response curve 11 produced by the catenary mooring system 2 of FIG. 1a. The x-axis 6 represents the distance X, in arbitrary units, that the turbine has been displaced from its neutral position. This is the position in which the catenary mooring system 2 holds the turbine 1 when there is no thrust acting on the system due to wind, i.e. the position of the wind turbine 1 when the catenary chain is in position 2. The y-axis 8 represents the tension, in arbitrary units, present in the chain of the catenary mooring system. The additional wind thrust moves the platform to a new position 10, where the stiffness response of the mooring system (the additional tension required to move the platform) is much higher than in the original neutral position.

[0143] It can be seen from this graph that, at large displacements from the neutral position, a small amount of wave induced motion 7 results in a very large change 9 in tension experienced by the mooring system. This increases the size that the mooring components need to be so as not to break under the maximum tension. At these large displacements, for example at a point 10, the system is said to have high stiffness. This stress-strain response is undesirable as in high sea states the waves can induce large changes in the displacement X of the turbines 1. This can cause huge tension peaks to occur in the mooring system, which in turn induces fatigue in the system and raises the likelihood of failure of the mooring line.

[0144] FIG. 2a shows a mooring system containing a mooring component having multiple elastomeric compressive elements, in accordance with an embodiment of the present invention.

[0145] In this example the elastic compressive elements are connected along the same length of mooring line, i.e. in series. The turbine 1 is once again subject to the wind force 3, moving the platform away from the direction of the wind until the horizontal component of the tension 5 matches the wind thrust. FIG. 2b is a stress-strain response graph of the mooring component.

[0146] The graph shows the response curve 20 provided by the mooring system shown in FIG. 2a containing the mooring component in comparison to the response curve 11 of the conventional catenary mooring system (as shown in FIG. 1a).

[0147] Certain features of the compressive elements are designed to give a stress-strain response as described herein, and the thrusts at which each stage of the stress-strain response begins are selected by adjusting these features, so as to be suitable for a particular mooring environment.

[0148] In this case, the compressive elements are designed such that the thrust load, which moves the turbine to the position shown on the right of FIG. 2a, in which the thrust load is balanced by the horizontal component of the tension in the mooring line 5, compresses each compressive element to within the second stage of compression. As seen in the graph of FIG. 2b, at this position 10, which is within stage 2 of compression (the operational range of the component), the stress-strain response curve flattens out such that a change of the platform displacement 7, for example due to a wave, only results in a small change in the tension 9.

[0149] As shown in FIG. 3, an example of a stress-strain response (as also shown in FIG. 2b), which is more desirable than the conventional response shown in FIG. 1b, can be broken down approximately into three separate stages. The x-axis 36 in FIG. 3 shows stress, in arbitrary units, whilst the y-axis 38 represents strain, again in arbitrary units. This stress-strain response may be scaled to a particular value depending on the system in which it is intended to be incorporated.

[0150] In the first stage 30, up to a first value 35 of the stress, a mooring system having the response of FIGS. 2b and 3 exhibits a high stiffness. This high stiffness causes a small extension of the mooring system to result in a large increase in thrust. In some examples, the mooring system will not operate within this range, in use, as pre-tension and thrust load acting on the mooring system when in use will pre-compress the compressive element such that the system operates in the second stage of compression 32. Should for some reason the thrust load not be present and the pre-tension not be high enough so that the experienced tensions are within this second stage, the high stiffness of the component in the first stage results in the overall mooring system behaving as a traditional mooring system within this range.

[0151] In the second stage 32, above the first value 35 of the stress and up to a second value 37 of the stress, the mooring system has a gently sloping response curve, thus having a lower stiffness than in the first stage 30. This is the operational range of the component and the first value is chosen based on the turbine thrust and pre-tension as described above, while the second value 37 is chosen based on the ultimate limit state. In this second stage 32, a change in platform position away from the anchor (e.g. due to a wave) will result in a small but appreciable increase in tension on the mooring line, and vice versa. If the response in the second stage 32 of the stress-strain response curve is too flat, then a small increase in the wind thrust applied to the platform will result in a large increase in the extension of the mooring line, leaving very little extension available for managing wave motions.

[0152] In the third stage 34 of the stress-strain response curve, above the second value 37 of the stress, the extension of the mooring line is large. In the third stage 34, the mooring exhibits a high stiffness once again, such that a small extension of the mooring system results in a large increase in thrust. This is designed to ensure that the platform is kept within a target surge (distance from the neutral position) and to ensure that the component can manage unexpected loads.

[0153] The Applicant has designed a polymer mooring component, in accordance with at least preferred embodiments of the present invention, with particular design features that aim to implement each of the stages 30, 32, and 34 of the stress-strain curve. These various features will be described in greater detail below.

[0154] The stress-strain curve, as achieved by the polymer mooring component, in accordance with at least preferred embodiments of the present invention, provides a number of benefits to a mooring system. The risk of failures during shock loading is reduced, which reduces repair and insurance costs; smaller components can be used to deliver the same capability of a much larger mooring chain, thereby reducing the component cost and the deployment cost; and also reducing the operational costs since fewer repairs to the infrastructure are required.

[0155] FIG. 4 shows a cross-sectional view of an elastomeric compressive element 40 according to an embodiment of the present invention. FIG. 5a shows the external appearance of the elastomeric compressive element 40 shown in FIG. 4. FIG. 5b shows a cutaway perspective view of the elastomeric compressive element 40 shown in FIG. 4. This cutaway view shows a section of the outer surface cut away to show the inner structure. FIG. 5c shows a cross-sectional view of the elastomeric compressive element 40 shown in FIG. 4. This cross-sectional view shows the front half of the element cutaway. FIG. 5d shows an exploded perspective view of the elastomeric compressive element shown in FIG. 4, where the polymer spring is assembled from eight identical individual polymer shells.

[0156] The elastomeric compressive element 40 in the embodiment shown in FIGS. 4, 5a, 5b, 5c and 5d comprises four bellows, or convolutes, consisting of eight shells in total, arranged end-to-end along a single axis. Through the centre of the bellows there are arranged four steel rods, parallel to the central axis along which the bellows are arranged. As one of the steel rods is in front of the other from the perspective of FIG. 4, only three steel rods 44a, 44b, 44c can be seen. Two of the steel rods 44a, 44b are each attached at a first end to a first outer plate 46a and are attached at a second end to a second inner plate 48b, which is attached to the end of the row of bellows. The other two steel rods 44c are similarly attached between a second outer plate 46b and a first inner plate 48a. The steel rods 44a, 44b, 44c each comprise an I-beam.

[0157] The elastomeric compressive element 40 can be incorporated into a mooring line by attaching the outside of each of the outer plates 46a, 46b to sections of the mooring line. The end of the mooring line sections can then be in contact with a sea bed, e.g. via an anchor, whilst the end of the other section of the mooring line can be connected to a floating body which is to be moored, for example a floating offshore wind turbine.

[0158] Owing to the arrangement of the inner and outer plates 46a, 46b, 48a, 48b, as the tension in the line increases, each of the first and second outer plates 46a, 46b is acted on by a tensile force, in the direction along the axis of the bellows and away from the bellows. These tensile forces are shown by the arrows 41, 41. As a result of these tensile forces, the inner end plates 48a, 48b each apply an inwards compressive force onto the bellows, as shown by the force arrows 43, 43.

[0159] Each of the bellows comprises two halves, also known as shells 42a, 42a, 42b, 42b, 42c, 42c, 42d, 42d. Each of these shells is approximately identical. The shells can be joined together by a number of possible methods, including welding. Alternatively, the elastomeric compressive element, including the bellowed shape, can be formed as a single piece. FIG. 5d shows an exploded perspective view of the elastomeric compressive element 40 as shown in FIG. 4. The elastomeric compressive element 40 is shown in FIG. 5d in a blown-up format, so that each shell is shown separated and the steel rods can be seen through the gaps between adjacent shells.

[0160] FIG. 6a shows a perspective view of a single shell 42a of the elastomeric compressive element 40 shown in FIGS. 5a to 5d, during the second stage of compression (i.e. compressed to a stress value higher than 35 in FIG. 3). FIG. 6b shows a cutaway perspective view of the single shell 42a shown in FIG. 6a. The cutaway perspective view shown in FIG. 6b shows the profile of the thickness of the shell material.

[0161] The Applicant has appreciated that various features of the profile of the shell contribute to the three stages 30, 32 and 34 as shown in FIG. 3, as will be described below. The desired stress-strain response can be achieved by adjusting a large variety of parameters of the shells or compressive element, and the examples given below are intended to be exemplary but not limiting.

[0162] FIG. 7 shows a cross-sectional profile of two adjacent shells 42b, 42b of an elastomeric compressive element, according to an embodiment of the present invention, in an uncompressed state. In order to assist in understanding, the dashed line 78 shows the separation between the upper shell 42b and the lower shell 42b, as shown in FIG. 7. This distinction may be merely conceptual, since a series of such shells i.e. a compressive element, may be integrally formed.

[0163] Each shell 42b, 42b comprises a first, outer, annular portion 74, 74 and a second, inner, annular portion 72, 72, with a central section 76, 76 extending between them. The shells 42b, 42b are formed by rotating the shell profile, as shown in FIG. 7, through 360 degrees around a central axis 70, giving a two-sided symmetric profile shape as shown in FIG. 7.

[0164] One or both of the first, outer, annular portion 74, 74 and the second, inner, annular portion 72, 72 may be strengthened. For example, these annular portions 72, 72, 74, 74 may be thicker than the central section 76, 76 of the shell and/or they could be made of a higher grade or stiffer polymer material than the central section 76 of the shell.

[0165] FIG. 8 shows a series of such shells 42b, 42b, joined together to form the bellows of the elastomeric element of the mooring component. The bellows 80 (also referred to as convolutes) are in an uncompressed state in FIG. 8 i.e. 0% compression, when no load is applied to the bellows 80. The stages of compression of the bellows 80 will be described below with reference to FIGS. 9, 10 and 11, and the response curve of FIG. 12.

[0166] FIG. 12 is a graph representing the force response of bellows 80 to compression. The x-axis shows displacement in units of mm, and the y-axis shows the resistance force produced by the bellows 80, in units of kilo-Newtons (kN). FIG. 8 represents the compression at point 90, i.e. 0% compression of the bellows 80.

[0167] FIG. 9 shows the cross-sectional profile of the two adjacent shells 42b, 42b, in a partially compressed state, i.e. at point 92 on the graph of FIG. 12. The partially compressed state of the shells 42b, 42b corresponds approximately to the first stress value 35 shown in FIG. 3, i.e. the start of the second phase of compression. In these exemplary bellows 80, this occurs at a compression of approximately 10% (of the uncompressed length shown in FIG. 8). It is clear from comparison of FIGS. 8 and 9 that the shells 42b, 42b of the bellows 80 substantially maintain their shape during the first phase of compression (i.e. compression from the arrangement of FIG. 8 to the arrangement of FIG. 9), and thus upon initial compression, the compressive element 80 comprising shells 42, 42 with a cross-sectional profile as shown in FIG. 7 will deform very little. This may for example be achieved through selection of a suitable material stiffness.

[0168] When the shells 42b, 42b are joined together to form the bellows of the elastomeric compressive element of the mooring component, the relative distance of the annular portions 72, 72, 74, 74 (where the shells 42b, 42b join) from the central axis 70 defines a load pathway 77. It is along this pathway 77 (for a particular shell) that the compressive force 79 applied to the elastomeric compressive element is transmitted. This occurs because a load pathway 77 as shown in FIG. 7 forms a relatively small angle with the central axis 70, causing the compressive element to have a relatively high stiffness in the configuration shown in FIG. 7.

[0169] This stiff response can be seen in the steep gradient of the stress-strain response curve shown in FIG. 3 (and FIG. 12).

[0170] As the compressive force on the compressive element increases, the shells 42b, 42b flex (about the first, outer, annular portion 74, 74 and the second, inner, annular portion 72, 72). As the compression of the compressive element increases, the angle of the load pathway 77 with the central axis 70 increases. Approaching and through the change from the first to second stages 30, 32 of stress-strain response curve (as shown in FIG. 3), the gradient of the stress-strain response curve lessens as the compressive element becomes less stiff. This is also seen in the particular response curve graph of FIG. 12. This lower gradient of the stress-strain response curve continues through the second stage 32 until the second stress value 37 is reached, as described below with reference to FIG. 10. FIG. 10 shows the cross-sectional profile of the two adjacent shells 42b, 42b, in a compressed state, corresponding approximately to point 94 in FIG. 12. Thus FIG. 10 shows the state of compression of the bellows 80 approximately at the end of the second phase 32 of compression. It can be seen in FIG. 10 that some of the adjacent shells 42b, 42b are just starting to come into contact at the adjacent outer sides of their respective central sections 76, 76. This results in a sharp increase in the stiffness of the bellows 80, and therefore produces the large increase in gradient seen after the point 94 on the graph of FIG. 12. Specifically, the contacting of the outer surfaces of the central sections 76, 76 of adjacent shells 42b, 42b changes the load pathway 77 to reduce the angle between the load pathway 77 and the central axis 70. This produces the increased stiffness seen in the graph of FIG. 12.

[0171] Further compression beyond this compression value further increases contact between adjacent shells 42b, 42b, as shown in FIG. 11. FIG. 11 shows the compression of the bellows 80 at approximately the value 98 shown on the graph of FIG. 12. It can be seen that this contact gives a load pathway 77 approximately parallel to the central axis 70 resulting in the high stiffness. This high stiffness at large compression ensures that the bellows are able to withstand thrust values which occur in the Ultimate Limit State (ULS) of the mooring system.

[0172] Additionally, or alternatively, some or all of the features of the response curve achieved herein may be achieved by including one or more shoulder portions on the shell. A shoulder portion is essentially a more pronounced thickening of a portion of the shell, extending in a direction away from the shell, as described above.

[0173] One example of such a shell is shown in FIG. 13.

[0174] FIG. 13 shows a cross-sectional profile of two adjacent shells 142b, 142b of an elastomeric compressive element, according to another embodiment of the present invention. In order to assist in understanding, the dashed line 178 shows the separation between the upper shell 142b and the lower shell 142b, as shown in FIG. 13. This distinction may be merely conceptual, since a series of such shells i.e. a compressive element, may be integrally formed.

[0175] Each shell 142b, 142b comprises a first, outer, annular portion 174, 174 and a second, inner, annular portion 172, 172, with a central section 176, 176 extending between them. The shells 142b, 142b are formed by rotating the shell profile, as shown in FIG. 13, through 360 degrees around a central axis 170, giving a two-sided symmetric profile shape as shown in FIG. 13.

[0176] Each central section 176, 176 comprises a respective inner shoulder portion 102, 102 that projects from the inner surface of the central section 176, 176 towards the first, outer, annular portion 74, 74. Contact between adjacent shoulder portions 102, 102 may give rise to the third phase of the response curve, in a similar manner to that described above. Contact of the adjacent shoulders 102, 102 is illustrated in FIG. 14. Alternatively, the shoulder portions 102, 102 may be arranged to contact at any desired point during the stress-strain response curve, to assist in providing the desired response curve. Multiple such shoulders may be provided on both the inner and/or the outer surfaces of the shells 42b, 42b, both in the annular portions and in the central section.

[0177] As described, the shells 42b, 42b, 142b, 142b are formed by rotating the shell profile, as shown through 360 degrees around the central axis 70, 170, thus forming a solid of revolution.

[0178] Alternatively, a shell may comprise a plurality of profile sections, each consisting of rotations of the profile shown about the axis 70 through only certain limited angles, of less than 180 degrees. In this latter case, multiple profile sections are then joined to the first, outer, annular portion 74, 74, 174, 174 and the second, inner, annular portion 72, 72, 172, 172. Each of the annular portions 72, 72, 74, 74 extend (and are thus continuous) through 360 degrees. One such example is shown in FIGS. 15a and 15b.

[0179] FIG. 15a shows an example of an elastomeric compressive element in which the central section comprises a number of profile sections, which are joined at the top and bottom circumferences. The Applicant has appreciated that these portions must be at least partial revolutions of the shell profile described already through at least a minimum angle, in order to give the desired non-linear response curve, otherwise the response profile of the shell will be similar to that of a beam, and not show the desired non-linear response. FIG. 15b shows the deformation of the elastomeric compressive element of FIG. 8a when acted on by a compressive force.

[0180] It will be appreciated by those skilled in the art that the invention has been illustrated by describing one or more specific embodiments thereof, but is not limited to these embodiments; many variations and modifications are possible, within the scope of the accompanying claims.