Abstract
A mooring line (10) comprising a metal chain (24) and an elastomeric element (28) which is attached between two points of attachment (30, 32) thereby defining a bypass section (34) of metal chain (24). The elastomeric element (28) is arranged such that when an initial tensile load is applied, the elastomeric element (28) stretches from an initial length to a longer length wherein the bypass section (34) is not taut and thus the initial tensile load is transmitted from and to further sections (24) of the metal chain through the elastomeric element (28). When a further tensile load is applied, the elastomeric element (28) stretches to a length wherein the bypass section (34) becomes taut, and thus the further tensile load is transmitted directly from and to the further sections (24) of the metal chain through the bypass section (34).
Claims
1. A mooring line comprising a metal chain and at least one elastomeric element having an initial length; wherein the at least one elastomeric element is attached between two points of attachment on the metal chain, thereby defining a bypass section of the metal chain between the two points of attachment with a predetermined length that is greater than the initial length of the at least one elastomeric element; wherein one or more further sections of the metal chain are directly linked to the bypass section by chain links so as to transmit a tensile load directly from one section of the metal chain to the next; and wherein the at least one elastomeric element is arranged such that: when an initial tensile load, below a threshold tensile load, is applied to the mooring line in use, the at least one elastomeric element stretches from the initial length, to a second, longer, length wherein the bypass section of the metal chain between the two points of attachment is not taut and thus the initial tensile load is transmitted from and to the further sections of the metal chain through the at least one elastomeric element; and when a further tensile load equal to or above the threshold tensile load is applied to the mooring line in use, the at least one elastomeric element stretches further from the second length to the predetermined length wherein the bypass section of the metal chain between the two points of attachment becomes taut, and thus the further tensile load applied to the mooring line is transmitted directly from and to the further sections of the metal chain through the bypass section of the metal chain; wherein the at least one elastomeric element is attached to the metal chain at each attachment point by a connector; wherein each connector comprises a first attachment point for the at least one elastomeric element, a second attachment point for the metal chain, and a third attachment point for the bypass section of chain, and the connector is shaped so as to pivot about the second attachment point; and wherein each connector is L-shaped or V-shaped and comprises first and second arms that are substantially straight and that are connected by a bend, wherein the first attachment point is provided at the end of the first arm, the second attachment point is provided at the bend between the first and second arms, and the third attachment point is provided at the end of the second arm, each connector thereby being L-shaped or V-shaped such that the bypass section of chain that is not able to come in contact with the at least one elastomeric element at least when the bypass section of chain is taut.
2. A mooring line according to claim 1, wherein the bypass section and further sections of the metal chain are directly linked by chain links so as to form a continuous metal chain.
3. A mooring line according to claim 1, wherein the bypass section and further sections of the metal chain consist of chain links of the same grade.
4. A mooring line according to claim 1, wherein the predetermined length is at least 1.5 times greater than the initial length of the at least one elastomeric element.
5. A mooring line according to claim 1, wherein the predetermined length is at least double the initial length of the at least one elastomeric element.
6. A mooring line according to claim 1, wherein the initial length of the at least one elastomeric element, or the distance between the two attachment points on the metal chain, is chosen such that the elastomeric element is capable of extensions of at least 80%, 90%, 100%, 110%, 120%, 130%, 140%, 150%, 160%, 170%, 180%, 190% or 200% before reaching the predetermined length.
7. A mooring line according to claim 1, wherein the initial length of the at least one elastomeric element is chosen so that, when the initial tensile load, below the threshold tensile load, is first applied to the mooring line in use, the at least one elastomeric element is taut when attached between the two points on the metal chain and the at least one elastomeric element subsequently stretches from the chosen initial length to a second, longer, length before the bypass section of the metal chain between the two points of attachment becomes taut.
8. A mooring line according to claim 1, wherein an elastic limit of the at least one elastomeric element is no greater than about 50% of the yield strength of the metal chain.
9. A mooring line according to claim 1, wherein the at least one elastomeric element comprises at least one tensile elastomeric element and/or at least one compressive elastomeric element.
10. A mooring line according to claim 9, wherein the compressive elastomeric element has a higher elastic modulus than the tensile elastomeric element.
11. A catenary mooring system comprising a floating body and a mooring line comprising a first section of metal chain resting on the floor and a second section of metal chain connecting the first section of metal chain to the floating body, and further comprising at least one elastomeric element having an initial length that is attached between two points of attachment on the second section of metal chain, thereby defining a third section of metal chain between the two points of attachment of the at least one elastomeric element with a predetermined length that is greater than the initial length of the at least one elastomeric element; wherein the third section is directly linked to the second section by chain links so as to transmit a tensile load directly from the second section of the metal chain to the third section; wherein the at least one elastomeric element is arranged such that: when an initial tensile load, below a threshold tensile load, is applied to the mooring line in use, the at least one elastomeric element stretches from the initial length, to a second, longer, length wherein the third section of metal chain between the two points of attachment is not taut and thus the initial tensile load is transmitted from and to the second section of metal chain through the at least one elastomeric element; and when a further tensile load equal to or above the threshold tensile load is applied to the mooring line in use, the at least one elastomeric element stretches further from the second length to the predetermined length wherein the third section of metal chain between the two points of attachment becomes taut, and thus the further tensile load applied to the mooring line is transmitted directly from and to the second section of metal chain through the third section of metal chain; wherein the at least one elastomeric element is attached to the metal chain at each attachment point by a connector; wherein each connector is shaped such that the at least one elastomeric element and the third section of metal chain between the attachment points are not able to come in contact with each other at least when the third section of metal chain is taut; wherein each connector comprises a first attachment point for the at least one elastomeric element, a second attachment point for the second section of metal chain and a third attachment point for the third section of metal chain, and the connector is shaped so as to pivot about the second attachment point; wherein each connector is L-shaped or V-shaped and comprises first and second arms that are substantially straight and that are connected by a bend, wherein the first attachment point is provided at the end of the first arm, the second attachment point is provided at the bend between the first and second arms, and the third attachment point is provided at the end of the second arm.
12. A catenary mooring system according to claim 11, wherein the first section of metal chain is thicker than the second section of metal chain.
13. A catenary mooring system according to claim 11, wherein the metal chain comprises a plurality of shot lengths connected in series and the attachment points of the at least one elastomeric element are located at each end of one shot length.
Description
(1) Some preferred embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings, in which:
(2) FIG. 1 is a schematic representation of a prior art catenary mooring system;
(3) FIG. 2 is a schematic representation of a prior art catenary mooring system in which multiple catenary mooring lines are used;
(4) FIG. 3 is a schematic representation of a mooring line in accordance with an embodiment of the present invention;
(5) FIG. 4 is a detailed view of the mooring line seen in FIG. 3 in which a first tensile load is applied to the mooring line;
(6) FIG. 5 is a detailed view of the mooring line seen in FIG. 3 in which a second, larger, tensile load is applied the mooring line;
(7) FIG. 6 is a detailed view of the mooring line see in FIG. 3 when the tensile load is equal to or greater than the threshold tensile load;
(8) FIG. 7 is a graph showing the reduction of peak loads using different mooring lines;
(9) FIG. 8a-8b are two graphs comparing the peak loads experienced by a traditional mooring line to a mooring line in accordance with embodiments of the present invention;
(10) FIG. 9 is an image of a link in a stud-link chain;
(11) FIGS. 10a-10b are two graphs showing the stress-strain response of a typical elastomeric tensile element and a typical elastomeric compressive element;
(12) FIG. 11 is a graph showing the impact of increasing the elastomeric element length on the peak loads experienced by a moored body;
(13) FIG. 12 is a graph showing the impact of increasing the elastomeric element thickness on the peak loads experienced by a moored body;
(14) FIG. 13 is an image of a typical chain clamp;
(15) FIGS. 14a-14c illustrate the chain clamp seen in FIG. 13 being attached in a mooring line (not shown in full) with the mooring line being under different tensile loads;
(16) FIG. 15 shows an alternative chain clamp;
(17) FIG. 16 illustrates one half of the chain clamp seen in FIG. 15; and
(18) FIGS. 17a-17c illustrates the chain clamp seen in FIG. 15 being attached in a mooring line with the mooring line being under different tensile loads.
(19) FIG. 1 depicts a conventional catenary mooring system 2. A floating platform 4 is connected to the seabed 6 by a catenary line 8. The catenary line 8 is typically formed of a plurality of shot lengths of metal chain. It can be seen from FIG. 1 that the circular motion of the platform 4 caused by the waves results in a large horizontal motion envelope for the mooring
(20) line 8 as it is picked up from the ocean floor 6. As the water depth increases due to large waves the catenary chain 8 is lifted off the seabed and the platform 4 moves upwards and to the left. For small waves, the chain 8 is laid along the seabed 6 as the water depth decreases and the platform 4 drifts downwards and to the right. Thus very large amounts of chain and a large space envelope is required to allow horizontal movement as water depths rise and fall. The mooring forces (F.sub.max) are high and transmitted through the entire chain 8, experienced at all points.
(21) FIG. 2 depicts a conventional catenary mooring system for a ship 4 in which several synthetic mooring lines or steel chains 8 are anchored to the seabed. It can be seen that the lines or chains must be long, e.g. up to 2 km, in order to cope with changes in water depth and to provide the required load along the surface of the seabed. The length of the chain 8 must provide sufficient weight to resist horizontal forces when the heavy ship moves, even on relatively small e.g. 5 m waves. As the chain 8 experiences high forces during the changes in water depth, multiple chains 8 are provided to reduce the load on any one chain 8 and also provide a failsafe in the case of failure of one of the chains 8.
(22) FIG. 3 shows an embodiment of the present invention. FIG. 3 shows a mooring line 10 connecting a floating body 12 to an anchor 14 on the seabed. The anchor 14 comprises a line 16 and float 18 for identifying the location of the anchor 14. The mooring line 10 is a catenary mooring line comprising two shot lengths of 38 mm steel chain 20 attached to a shorter, sinker chain 22 which is attached to the anchor 14. Attached to the end of the second shot length of 38 mm chain 20 is a length of 28 mm steel chain 24 which is connected to the floating body 12. Attaching the shot lengths of 38 mm chain 20 to the sinker chain 22 and to the 28 mm chain 24 are 38 mm shackles 26. It can be seen from FIG. 3 that the shot lengths of the 38 mm chain 20 and the sinker chain 22 rest on the seabed.
(23) Attached in parallel with a section of the 28 mm chain 24 is an elastomeric element 28 having an initial length in its unstretched state. The elastomeric element 28 is attached to the 28 mm chain 24 at a first point 30 and a second point 32 such that a bypassed chain section 34 of the 28 mm chain 24 between the attachment points 30, 32 is longer than the initial length of the elastomeric element 28. The bypassed chain section 34 is therefore slack while the elastomeric element 28 is taut. In one example, the elastomeric element 28 has an initial i.e. unstretched length of 4.5 m and the bypassed chain section 34 between the attachment points 30, 32 has a length of about 10.25 m. As the floating body 12 rises and lowers due to changes in the sea depth (e.g. tidal or wave motion), a tensile load is transmitted along the 28 mm chain 24. If there is a sudden change in depth of the water, this load increase is transferred into the elastomeric element 28 causing it to stretch. This allows the floating body 12 to rise with the increased water depth, without transferring shock loads to the steel chains 20, 24. Depending on the elasticity of the elastomeric element 28 and its initial length relative to the bypassed section of chain 34, after a certain extension the chain 34 will become taut and further tensile load will be transmitted along the 28 mm chain 24. Accordingly, as the floating body rises it may also cause the length of 38 mm chain 20, 22 on the seabed to be lifted away from the seabed. This provides a restoring force such that when the sea level lowers, the floating body 12 also lowers and remains securely moored. In the example mentioned above, the bypassed section of chain 34 is only taut once the elastomeric element 28 stretches by 125% or more.
(24) The length and/or elasticity of the elastomeric component may be chosen such that the 38 mm chain 20, 22 on the seabed is only caused to lift when the elastomeric element 28 has been fully extended to just below its elastic limit. Whilst this may be the case in some embodiments, it is preferable that the length and/or elasticity is chosen such that the 38 mm chain 20, 22 lifts before the elastomeric element 28 is fully extended, i.e. well before reaching its elastic limit. This ensures that the elastomeric element 28 is safely operational, and can assist in reducing shock loads, at a range of different sea depths.
(25) FIG. 4 shows a close-up view of the attachment points 30, 32. It can be seen that the length of 28 mm chain 24 is attached to each end of the elastomeric element 28 by a connector 36. The connector 36 has an L-shape with one arm of the L shorter than the other. On each connector 36 are three attachment holes 38, 40, 42. The elastomeric element 28 is attached at both ends using a shackle 29a to connect to the first attachment hole 38. Each shackle 29a is connected to a metal end connector 29b that is inserted into the elastomeric element 28 with the elastomeric material moulded around the metal insert of the connector 29b. The moulded design of the end connector 29b allows for a much higher level of elongation and much higher loads than could typically be held by a crimped, clamped, or knotted end connection for the elastomeric element 28.
(26) The 28 mm chain 24 is then attached to the second attachment hole 40 positioned at the bend between the two arms of the L-shaped connector 36. A slack portion 44 of 24 mm chain 24 is then kept loose and the 28 mm chain 24 is attached again at the third attachment point 42. The purpose of keeping the slack portion 44 is to allow the connector 36 to freely pivot during operation without putting excess forces on the 28 mm chain 24. The other end of the elastomeric element 28 is attached to the 28 mm chain 24 in a similar manner. The distance between the third attachment points 42 of the two connectors 36 is chosen such so as to bypass a slack chain section 34 of the 28 mm chain 24 which has a predetermined length greater than the initial e.g. unstretched length of the elastomeric element 28. This bypassed chain section 34 will only become taut and transmit tensile load once the elastomeric element 28 has reached an extension corresponding to its predetermined length. Of course the 28 mm chain 24 may be attached to the elastomeric element 28 via different connectors, some examples of which will be discussed in more detail below.
(27) Operation of the mooring line 10 will now be described with reference to FIGS. 4 to 6. FIG. 4 shows the elastomeric element 28 attached to bypass a section of 28 mm chain 24 and in an unstretched state where it is taut at its initial length. FIG. 5 illustrates the case where an increased tensile load has been applied to the mooring line 10. A tensile load applied to the mooring line 10 is transmitted directly along the 28 mm chain 24, which is incapable of elastic deformation, and therefore passes through the connector 36 and thus into the elastomeric element 28. In FIG. 5 it can be seen that the tensile load has caused the elastomeric element 28 to stretch from an initial unstretched length to a second, longer, length. It can also be seen that the bypassed chain section 34 is no longer as slack as it was in the first case seen in FIG. 4 however it is still not taut. As the tensile load applied to the mooring line is below the threshold tensile load, the entire tensile load is transferred along the elastomeric element 28 to the rest of the 24 mm chain 24. It can be seen that the connectors 36 are orientated such that the applied tensile load is transferred linearly along the elastomeric element 28 from the connected sections of 24 mm chain 24.
(28) FIG. 6 illustrates the scenario in which a larger tensile load, equal to or greater than the threshold load, is applied to the mooring line 10. Here it can be seen that the tensile load is sufficiently large to stretch the elastomeric element 28 to the point where its length is the same as the predetermined length of the bypassed chain section 34. Once the bypassed chain section 34 becomes taut, the connectors 36 rotate about their second attachment point 40 such that the elastomeric element 28 is no longer directly in line with the connected sections of 24 mm chain 24. The tensile load is now shared between the elastomeric element 28 and the bypassed chain section 34, which run in parallel. It can be seen that the bypassed chain section 34 is now taut and has a predetermined length equal to the maximum stretched length of the elastomeric element 28. It will be appreciated that the elastomeric element 28 may be able to stretch to larger lengths than that shown in FIG. 6, however the predetermined length of bypassed chain section 34 is chosen specifically such that the elastomeric element 28 does not reach its elastic limit i.e. a length less than that at which it may fail. This prevents the elastomeric element 28 from over-extending and thus ensures longevity of the mooring line 10. The relevance of the small slack portions 44 can be seen in FIG. 6. When the bypassed chain section 34 becomes taut and the L-shaped connector 36 is pivoted, the slack in the small slack portions 44 is taken up and it can become taut. Without these slack sections 44 the L-shaped connectors 36 would not be able to rotate about the second attachment point 40. Of course, it will be appreciated that the connectors 36 do not need to have an L-shape. They could, for example, be V-shaped instead e.g. with arms of substantially the same length. Such connectors 36 would still act to prevent the bypassed chain section 34 from coming into contact with the elastomeric element 28 when they are both pulled taut side-by-side. The ability of an L-shaped or V-shaped connector to pivot, and change the direction along which tensile loads are applied, helps to bring the elastomeric element 28 into alignment with the connected sections of 24 mm chain 24 during its elongation phase. However this is not essential. In yet other embodiments the connector may have an alternative shape without substantially straight arms but still functioning to prevent the bypassed chain section 34 from coming into contact with the elastomeric element 28 when they are both pulled taut side-by-side.
(29) FIG. 7 shows a graph of the peak loads experienced by four different mooring lines: (1) a traditional catenary mooring line (e.g. steel chain alone), (2) a mooring line comprising a compressive spring, (3) a mooring line comprising a bypass elastomeric element as described above, and (4) a mooring line comprising a dynamic tether (e.g. an elastomeric element and a compressive spring in combination). The load examples are taken from a Scottish feed barge in an exposed environment experiencing a 50 year storm. It can be seen that the traditional catenary mooring line (1) (8 lines, each 200 m long comprised of 36 mm metal chain) experiences the largest peak loads at around 600 kN. A mooring line with an in-line spring (2) (500 kN and 5 m long) experiences reduced loads at around 430 kN. The bypass elastomer line (3) (15 kN and 10 m long) reduces the peak loads to around 220 kN. This is approximately one third of the peak loads experienced in a traditional catenary mooring line and highlights the benefits of the present invention. It can be seen that the mooring line comprising a dynamic tether (4) (100 kN) provides the best reduction in peak loads (>80%). However, such a dynamic tether is costly and does not provide the fail-safe features of a bypass elastomeric element as described above.
(30) Thus a mooring line comprising an elastomeric element rated at 15 kN and having an initial length of 10 m is shown to provide >50% peak load reduction, as well as shock protection, as compared to traditional catenary mooring lines.
(31) FIG. 8a shows the effective tension against time on a typical catenary mooring line and FIG. 8b shows the same catenary mooring line with a bypass elastomeric element added. It can be seen that the peak effective tension 46 experienced by the typical catenary mooring line is considerably higher than the peak effective tension 48 experienced by the modified mooring line comprising a bypass elastomer. It can also be seen that the bypass elastomer helps to generally reduce the effective tension on the mooring line as FIG. 8b shows lower, and smoother, peaks of the effective tension compared to that of the typical catenary mooring line seen in FIG. 8a. It can also be seen that FIG. 8b has far smoother changes to loads (even at low loads) than FIG. 8a, which should lead to much lower wear and tear.
(32) A mooring line in accordance with embodiments of the present invention can be provided to assist in reducing peak loads in a wide variety of mooring situations. For example, many structures moored at sea have multiple different types of mooring lines. For example, aquaculture cages are large structures that are moored at sea. They typically comprise a large cage formed from polymer rope. Such aquaculture cages are typically moored by multiple bridle lines attached to each corner of the aquaculture cage. These bridle lines are subsequently attached to chain mooring lines. A bypass elastomeric element as described above may be added to the bridle lines and/or the mooring lines.
(33) FIG. 9 is an image of a single chain link in a stud-link chain showing the shape of the metal. For a new chain the diameter of the chain will be circular. Over time, as wear occurs from the grinding of one chain link against another, the shape of the diameter will shift to ellipsoidal and the link needs to be replaced when the chain is worn too thin. One measurement often used to determine when to replace the chain link is a ratio of 0.88. When d2<0.88.Math.d1 then the link needs replacement.
(34) FIG. 10a shows a graph of the stress/strain response curve of a 4.5 m long, 160 mm diameter elastomeric element. The Young's Modulus at any point is defined as the applied force divided by the ratio of extension times the cross-sectional area. The Young's Modulus changes as the elastomeric element stretches, lowering as the elastomeric element elongates. FIG. 10b shows a graph of the stress/strain response curve of a 1.2 m long, 270 mm diameter thermoplastic compressive spring element. The Young's modulus of this element increases smoothly as it is compressed.
(35) FIG. 11 shows a graph of the peak load experienced in the same scenario as FIG. 7 when different length 3T elastomeric elements are used. As the length of the elastomeric element is extended, the energy stored by the time the elastomeric element reaches the threshold load is greater and the peak load experienced is lower. The benefit of increasing the length of the elastomeric element reduces as the length is increased and an optimum length is normally chosen based on the cost benefits of load reduction versus the cost of the elastomeric element.
(36) FIG. 12 shows a graph of the peak load experienced in the same scenario as FIG. 7 when different diameter 9 m elastomeric elements are used. As the diameter of the elastomeric element is extended (increased Elastomer Design load capacity), the energy stored by the time the elastomeric element reaches the threshold load is greater and the peak load experienced is lower. Increasing the diameter also increases the stiffness of the elastomeric element making it less responsive to lower loads. The benefit of increasing the diameter of the elastomeric element reduces as the diameter is increased and an optimum diameter is normally chosen based on the cost benefits of load reduction versus the cost of the elastomeric element. This is normally considered along with FIG. 11 to choose the optimum elastomeric element for a given deployment.
(37) FIG. 13 shows an example of a typical chain clamp 50 which may be used to connect a chain 24 to the connector 36 (seen in earlier Figures). The chain clamp 50 may be used when connecting a stud-link chain 24 to the connector 36. Typically, each link 52 in the stud-link chain 24 comprises a stud 54 which connects two sides of the link 52 and divides the internal opening into two separate openings 56. When another link 52 is passed through one of the openings to form a length of chain, the remaining space in the opening 56 is typically too small to enable a shackle, or equivalent attachment means, to be attached to the stud-link chain 24. Attachment to this type of chain 24 may be achieved through the use of a chain clamp 50. The chain clamp 50 essentially comprises two plates 58, 60 which are placed around a link 52 and fixedly held together, e.g. through the use of multiple nuts and bolts. The clamp 50 further comprises an attachment portion 62 extending from the clamp plates 58, 60 and comprising an eyelet 64 which can be used to attach the chain clamp 50, and hence the chain 24, to another component, e.g. the connector 36. The attachment portion 62 may optionally be able to pivot relative to the clamp plates 58, 60.
(38) FIG. 14a shows the chain clamp 50 being used to attach the 28 mm stud-link chain 24 to the connector 36. A chain clamp 50 is attached at both attachment points 40, 42 instead of shackles as previously described. The chain clamp 50 may be modified from the typical chain clamp 50 seen in FIG. 13, for example the eyelet 64 may be formed in a thinner attachment portion 62 e.g. flat plate enabling it to mate more closely with the connector 36. Alternatively, the attachment portion 62 may comprise a forked plate that passes over either side of the connector 36. The eyelet 63 may be attached to the attachment points 40, 42 by any suitable means, for example using a nut and bolt fastener. Although not shown in FIGS. 14a-14c, it will be understood that an elastomeric element 28 can be connected to the fastener 36 at the other attachment point 38 in the same way as previously described. An arrow depicts the force applied by the elastomeric element 28 at the attachment point 38.
(39) FIG. 14a illustrates the scenario, similar to FIG. 4 discussed above, whereby no tensile load is applied to the chain 24. In this scenario a bypassed chain section (not shown) of the chain 24 which is positioned between the connectors 36 at each end of the elastomeric element (not shown) will hang down under gravity.
(40) FIG. 14b illustrates the scenario whereby a tensile load is applied to the chain 24 such that the elastomeric element 28 (not shown) which is attached to the connector 36 at the point 38 is stretched, but not to full extension, i.e. the tensile load is below the threshold, and so the elastomeric element 28 is not stretched to a length which is equivalent to the length of the bypassed chain section between the connectors 36. This scenario is equivalent FIG. 5 discussed above. Arrows depict the tensile loads being applied.
(41) FIG. 14c illustrates a third scenario whereby a sufficient tensile load is applied to the chain 24, i.e. equal to or above the threshold load, which causes the elastomeric element (not shown) to stretch to a length equal to the length of the bypassed chain section (not shown) attached between the two connectors 36. This scenario is similar to FIG. 6 discussed above.
(42) In each of the scenarios discussed above, the chain clamp 50 functions to attach the chain 24 to the connector 36 and also allows the nearby sections of the chain 24 to pivot and rotate as the chain 24 is brought into tension under a tensile load. It can be seen from FIGS. 14a-14c that the connector 36 rotates as the loads increase, keeping the chain 24 and elastomeric element (not shown) apart.
(43) Of course the chain clamp 50 may be used to connect the chain 24 directly to the elastomeric element without a connector 36. FIG. 15 illustrates another example of a chain clamp 64 for attaching a stud-link chain 24 to an elastomeric element 28. The chain clamp 64 is particularly well suited for attaching the chain 24 directly to the elastomeric element 28 without the need for an intermediate connector, e.g. connector 36. This will be discussed in more detail below in relation to FIGS. 17a-c. Such a chain clamp may be particularly advantageous as it may reduce the total number of components for the mooring line and thus potentially reduce its cost. The chain clamp 64 comprises a main body 66 which extends over a central portion of a link of the chain 24. The chain clamp 64 further comprises an eyelet 68 and a shoulder portion 70. Whilst not shown in this Figure, the chain clamp 64 comprises two separate plates which are placed around a link and secured together.
(44) FIG. 16 shows one plate 72 of the chain clamp 64. The plate 72 comprises a recessed portion 74 arranged to accommodate a link of the stud-link chain 24. The recessed portion 74 is sufficiently deep such that when two plates 72 are brought together the combined depth of the two recesses is sufficient to house a link of the stud-link chain 24. The plate 72 also comprises an extending portion 76 through which the eyelet 68 is provided. The plate 72 further comprises a shoulder portion 70 which extends outwards of the main body of the plate 72. The purpose of the shoulder portion 70 will be discussed below in relation to FIGS. 17a-c. The plate 72 further comprises a plurality of holes 78. When two mirror-image plates 72 are brought together, securing means, e.g. nuts and bolts, can be used with the holes 78 to secure the plates 72 together. Alternatively, the plates 72 may not comprise the holes 78 and instead the plates 72 may be placed around a link and welded together. A second plate (not shown) can be attached to the plate 72 to form the chain clamp 64. Preferably the second plate is a mirror image of the plate 72, but with opposite handedness. However this may not be necessary and, for instance, provided that the recess on the plate 72 is sufficient to house a link of the chain 24 the second plate may not comprise a recessed portion. This may result in the second plate being cheaper and/or easier to manufacture. Although not shown, a polymer sheath may be added to avoid wear between the clamp 64 and a chain held therein. A polymer sheath or spacer can also allow for a wider dimensional alignment.
(45) FIGS. 17a-17c show the chain clamp 64 directly connected to the elastomeric element 28 via the metal end connector 29b. FIG. 17a shows the scenario wherein no tensile load is applied to the stud-link chain 24. Here the chain 24 is loose in the chain clamp 64 and the bypassed section of chain (not shown) between the two chain clamps 64 is not taut, hanging down freely under gravity. Typically the length of the bypassed chain section between the chain clamps 64 is 225% to 250% of the length of the elastomeric element 28. For example, an elastomeric element 28 of length 4.75 m is clamped at points 10-11 m apart along the chain 24.
(46) FIG. 17b illustrates the scenario whereby a tensile load is applied, such that the elastomeric element 28 is caused to stretch but to a length which is less than the length of the bypassed chain section between the two chain clamps 64. Arrows depict the forces being applied. The angle of the chain 24 changes as the bypassed section is now looping over a wider gap and therefore at a shallower angle than is seen in FIG. 17a. However the bypassed chain section is not yet taut.
(47) FIG. 17c shows a scenario whereby a sufficiently large tensile load, i.e. equal to or above the threshold load, has been applied causing the elastomeric element 28 to stretch to the point at which its length is the same as the bypassed chain section between the two chain clamps 64. At this point both the elastomeric element 28 and the chain 24 are taut. As the chain 24 becomes taut the chain clamps 64 swing around and the shoulder 70 is positioned between the chain 24 and the elastomeric element 28. Both the elastomeric element 28 and the chain 24 are now linear. The shoulder 70 helps to prevent contact between the chain 24 and the elastomeric element 28 and thus helps to prevent scraping and chaffing which may otherwise damage the elastomeric element 28 and reduce its lifespan.
(48) Of course the chain clamps 50, 64 discussed above are not limited to use with stud-link chain and may be in fact be used to attach various different types of chain, including those without studs, to the elastomeric element 28 or connector 36.
