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This invention relates to a cable clamp, in particular for a subsea connector.
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In subsea connectors one of the critical regions is the termination of the subsea cable to the back end of the connector in a cable gland. To ensure a reliable connection to the cable, it is important that the cable is held in place correctly. Furthermore it is important that any external pulling or twisting forces acting on the cable cannot cause the cable to move, or to be pulled out of the gland.
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Difficulties with clamping the cable result from the general construction of the cables. Cables often comprise a metallic core, typically copper, with several layers of rubber, plastic and metal to form the cable insulation, earth screen and protective outer jacket. The rubber and plastic layers mean that these cables expand and contract by a reasonably large percentage of their diameter in response to changes in temperature. A further complication is that the extrusion processes used to form the cables lead to quite large tolerances on the diameters on the cables.
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In accordance with a first aspect of the present invention a cable clamp comprises a plurality of cable clamp sections coupled together by one or more resilient members to form an orifice having an adjustable cross section, the orifice being adapted to receive a cable; wherein the orifice has an inner surface adapted to co-operate with an outer surface of the cable; and wherein the inner surface of the orifice is provided with one or more friction elements adapted to interact with corresponding friction elements on the cable, whereby the clamp and cable are releasably coupled.
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The resilient members joining the cable clamp sections accommodate expansion and contraction of the cable and the friction elements allow the clamp to maintain a fixed location on the cable if a pulling force is applied, but the clamp can still be quickly and easily stripped from the cable, if required.
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Preferably, the at least one of the friction elements comprises at least one of annular protrusions, grooves, teeth, hooks or loops, high friction material, or chemically or mechanically modified cable casing.
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Simply gluing the clamp onto the casing makes the cable difficult to strip in the case of a fault, but carving grooves minimises the damage to the cable casing, whilst holding the clamp better against pull forces or modifying the structure of the cable casing, for example by softening it, enables teeth on the clamp to grip more effectively, than with a conventional casing, yet still be easily and replacably removed, if required.
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In one embodiment, the friction elements comprise surfaces of a double sided insert, each surface comprising a high friction material.
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This has the further advantage that no modification to the surface of the cable or clamp is required, yet the position holding against a pull force is improved.
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Preferably, the inner surface of the orifice is discontinuous and comprises inner surfaces of each cable clamp section.
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When sufficient force is applied to the cable clamp sections by the resilient members to bring each one into contact at its edges, the inner surface of the orifice may be substantially continuous, but typically, the effect of expansion of the cable is to part the sections by a small about making the inner surface of the orifice discontinuous.
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Preferably, the cable is a subsea cable.
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The invention is particularly beneficial for subsea cables which are expensive to repair if a connection is pulled out, because of the difficulty in retrieving and relaying the cable.
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In accordance with a second aspect of the present invention, a cable clamp assembly comprises at least one cable inserted into a cable clamp according to the first aspect.
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Preferably, the cable is electrically connected to a pin and inserted into the cable clamp.
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In accordance with a third aspect of the present invention, a method of releasably coupling a cable clamp to a cable, the cable clamp comprising a plurality of cable clamp sections coupled together by one or more resilient members to form an orifice having an adjustable cross section, the orifice being adapted to receive the cable; the orifice having an inner surface adapted to co-operate with an outer surface of the cable, comprises providing one or more friction elements on the inner surface of the orifice and on the outer surface of the cable, whereby the one or more friction elements on one surface interact with the one or more friction elements on the other surface to maintain the location of the cable clamp relative to the cable.
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In one embodiment, the method comprises providing the friction elements as surfaces of a double sided insert, each surface of the insert comprising a high friction material, and inserting the double sided insert between the cable and the cable clamp before the cable clamp is closed around the cable.
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The friction elements may be embossed on the outer surface of the cable, or on the inner surface of the orifice.
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The friction elements may be engraved in the outer surface of the cable, or in the inner surface of the orifice.
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The method may comprise adhering the friction elements to the outer surface of the cable, or to the inner surface of the orifice by adhesive bonding.
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The method may comprise forming the friction element by chemical or mechanical modification of the outer surface of the cable.
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The subsea cable may be connected before fitting the cable clamp, but typically. the method further comprises electrically connecting a subsea cable to a pin after fitting the cable clamp around the electrical connection.
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An example of a cable clamp for a subsea connector in accordance with the present invention will now be described with reference to the accompanying drawings in which:
- Figures 1a to 1d illustrate an example of a conventional tapered collet style cable clamp;
- Figure 2 illustrates an example of a multiple segment spring-loaded compliant cable clamp;
- Figures 3a and 3b illustrate more detail of the example of Fig.2;
- Figure 4 illustrates part of an embodiment of the present invention showing surface modification of the cable;
- Figure 5 illustrates a multiple segment compliant cable clamp with surface modification to engage with the cable of Fig.4;
- Figure 6 illustrate alternative surface modifications for a cable and cable clamp according to the invention;
- Figure 7 illustrate further alternative surface modifications for a cable and cable clamp according to the present invention; and,
- Figure 8 is a flow diagram illustrating a method of coupling a cable and cable clamp according to the present invention.
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As discussed above, it is necessary to ensure a reliable connection between the cable and the connector in the cable gland, so a cable clamp has to be designed to be able to withstand the forces applied, as well as accommodating size variations due to manufacturing tolerances, and/or thermal expansion in use. This applies particularly for subsea operation, where access is difficult.
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Conventionally, these problems have been addressed by using a cable clamp to protect the connection to the cable, which might otherwise be susceptible to damage if one or other cable suffered rough handling. Cables are typically terminated to a solid pin (the back end of a plug or receptacle front end or a module penetration), but the issue also applies for joining two cables together. Pulling forces, in particular, may cause damage. Environmental factors such as temperature, or pressure and consequent changes to any of the components in the system may alter the geometry of clamping components, or the cable itself. These changes to the geometry may cause fluctuations in the pressure exerted by a clamp, which can result in a reduction in clamping force reducing the effectiveness of the connector and potentially damaging the cable or other connected equipment. One example of a cable clamp is a collet type clamp, as illustrated in Figs.1a to 1d. Fig.1d shows a section A - A of Fig.1c. A typical cable 1 comprises a conductive core 2, a braided or woven conductive sheath 3 separated from the core by an insulator and an outer protective casing 4, or sheath, which is generally non-conducting. For example a typical coax cable has a copper core, an ethylene propylene rubber (EPR) or cross linked polyethylene (XPLE) insulator, a copper conductive sheath and a PVC protective sheath. A flexible split collet 5 fits around the cable sheath 4 within a tapered housing 6. The tapered housing may be formed from more than one piece, although it acts as a single piece to hold split fingers of the collet 5 in place against the sheath 4 of the cable 1. The collet 5 is driven into the fixed housing 6 by a spring around the circumference. The action of driving the two tapers against each other compresses the collet.
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This type of clamp performs very well on initial pull tests, but is uncompliant. When the temperature of the cable increases, the cable expands diametrically, but the section of cable under the clamp is unable to expand because of the uncompliant nature of the clamp. This section of cable 1 under the clamp is constrained by the clamp and unable to expand diametrically in the way that the rest of the cable can. As a result, the material of the cable under the clamp ends up extruding along the length of the cable, creeping away from the clamped region. When the temperature of the cable 1 reduces again, there is less material of the cable under the clamp and so the clamping force applied by the clamp is reduced. Over time, the effect of exposure to increased temperatures and thermal cycling may cause a clamp that has been set up at room temperature, or a relatively low temperature compared to its normal operating temperature, to lose most of its clamping effect.
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One approach to address this is to design the clamp so that it is formed from multiple segments which are individually pressed onto the sheath of the cable by way of elements which are capable of undergoing elastic deformation, such as springs, rubber or elastomeric components. This is illustrated in the example of Fig.2 which shows an example of a multipart clamping device 10, or cable clamp and a cable 1, as before comprising a core 2, insulator 8, metal braid 3 and casing, or jacket 4. As discussed above, as well as protecting the connection, it is desirable that the cable clamp is able to adapt to thermal expansion or contraction of the cable 1, or related parts. This aspect is addressed by using a multi-part cable clamp 10, of the type illustrated in Fig.2, made up of two or more separate sections 11 spaced apart by a slot 12 and coupled together around the cable 1 by resilient members 13, 14. These resilient members may for example be spring loaded bolts, using rubber, elastomer, or metal springs. Examples of springs include coil springs, wave springs, or disc springs. The resilient members apply the required pressure to the outer casing 4 of the cable 1, but allow some movement, so as to reduce or remove cable deformation due to the pressure exerted by the clamp on the cable if the cable expands. Similarly, as the cable contracts, the spring force of the resilient members 13, 14 maintains the clamping effect.
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Figs.3a and 3b show more detail of the example of Fig.2. The clamping device 10 may be circumferentially surrounded by a housing (not shown), for example a metal structure and fixed to the structure in an axially defined position in the housing by bolts 21 to axially restrict movement of the clamping device 10. In this example, the clamp 10 comprises three clamp segments 11, the inner surfaces 9 of which are in contact with the outer surface of the cable, in use. A plurality of elastically deformable elements 13, 14 provide the necessary preloading for the clamp 10 to grip the cable 1 and also to provide thermal compliance. Thus, when a change causes the cable to expand, for example a change in environmental conditions, the spring elements 14 compress, allowing the clamp segments 11 to move with the cable 1. This prevents thermal damage or deformation and so the clamping force is retained. In this example, the clamping member 10 comprises three sections 11 adjacent one another and each separated from the next by slots 12 extending in a radial direction and in an axial direction. Each section 11 and its adjacent slot 12 spans about 120° of a circumference of the clamping member 10, when the clamping member is embodied as a cylindrical tube, or as a clamp sleeve.
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As stated above, the clamping device 10 is able to compensate for changes of geometry of the cable 1. In this example, the adjustment of the clamp is by means of preloadable elements, such as bolts 13 and springs 14, or a coil spring, resulting in a clamping force with little variance in performance despite changes in the cable geometry with change in environmental conditions. The springs 14 may be mounted on the bolt 13 in recesses in the sections 11 and hold adjacent sections together. As can be seen from the figures, a plurality of these preloadable elements 13, 14 may be arranged in parallel to one another. The spring loading force required depends upon the cable and the spring characteristics. Thus, a combination of clamp area, spring rate, and number of springs may be used to determine the design for a particular cable. The pressure generated by the specific clamping device 10 needs to apply enough force through the cable layers to maintain the required load, without causing significant extrusion of the cable from the clamped region.
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Another problem with cable clamps is that if a smooth cable clamp inner surface 9 is used, then the pull force is transmitted by friction and so the force achievable is quite variable. This is because the pull force depends on surface cleanliness, finish, material and other such factors. This variability can be reduced by introducing small teeth to the clamping surface to bite into the cable jacket 4. The teeth may cause a small amount of localized damage, but greatly increase the repeatability and achievable force. However the teeth need a high enough force to allow them the actually bite into the cable jacket 4, which can be a very tough material and so the number of teeth allowable is driven directly by the availability of springs with sufficient stiffness and strength.
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The present invention addresses the issue of effective clamping, in particular, maintaining the location of the clamp on the cable, by providing a resilient, high friction coupling between the cable 1 and the clamp 10. The coupling may be formed on the inner surface of the clamp and the outer surface of the cable, or provided as an intermediate element for insertion between the inner surface of the clamp and the outer surface of the cable. For example, in the case of forming a high friction coupling on the sheath of the cable and the inner surface 9 of the clamp 10, by means of ridges and grooves, it is easier to manufacture the high friction coupling by forming the grooves in the surface of the cable and the corresponding ridges in the inner surface 9 of the clamp 10, than to form the coupling in the opposite way with ridges on the cable sheath and grooves in the inner surface of the clamp. This is because cutting sufficient material from the cable casing to form ridges would weaken the cable more than just forming grooves in it. However, the invention does not exclude the possibility of forming the high friction coupling in this alternative way, with ridges on the cable sheath and grooves in the inner surface of the clamp.
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Fig. 4 illustrates a cable surface modification comprising axial grooves 24 and circumferential grooves 25 formed in the cable sheath or casing 4 using a tool, for example by cutting. The circumferential grooves 25 and axial grooves 24 are designed to cooperate with circumferential ridges 22 and nodules 23 formed in the inner surface of the clamp sections, for example by extrusion, machining, or bonding, as shown in Fig.5. The clamp is preferably a multiple segment clamp 10 with resilient members 13, 14 so that the segments 11 can move with the cable 1 as it expands, reducing extrusion of cable sheath material under pressure in case of elevated temperatures. The resilient members may comprise a rubber, elastomer, or spring. Examples of springs include coil springs, wave springs, or disc springs. The cable clamp may be fixed to a cable housing by longitudinal bolts in the clamp. Three clamp sections 11 is effective for accommodating expansion and contraction of the cable whilst maintaining the shape of the cable, However, the number of segments 11 is not limited to the three segment variant shown in Fig.5. Four segments, as illustrated in Fig.6, or two segments, as illustrated in Fig.7, may also be used. However, more than four sections increases the complexity and cost.
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Producing a high friction coupling may be achieved by mechanical or chemical means, or different surface treatments may be used to increase the frictional force between the clamp 10 and sheath 4. Whatever embodiment is chosen, it is desirable that the coupling is easily releasable so that if a fault occurs within the cable connector which needs to be repaired, the clamp can be removed and the cable, or connectors, stripped down quickly and efficiently, then reconnected again after repair.
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In one example, the coupling comprises engagement elements on the inner surface of the clamp designed to cooperate with corresponding engagement elements on the surface of the cable. The material of the high friction coupling may comprise hook and loop fasteners, for example as illustrated in Fig.6, or the cable jacket may be modified by cutting grooves, forming ridges, or using sanding, filing or sand-blasting to roughen the surface of the sheath 4 to increase the clamping force and repeatability. In the example illustrated in Fig.6 with hook and loop fasteners, loops 31 are applied on one of the cable sheath 4 or the inner surface 9 of the clamp 10, typically by bonding a backing strip on which the loops are formed to the surface, for example using adhesive. On the other of the cable sheath 4 or inner surface 9 of the clamp 10, hooks 30 are applied, again typically by bonding, or gluing to the outside of the cable sheath or the inside of the clamp segments, for example bonding a backing strip, on which the hooks or loops are formed, to the appropriate surface. The example of Fig.6 illustrates loops on the cable sheath and hooks on the inner surface 9 of the clamp 10. In the embodiment illustrated in Fig. 6, using hook and loop fasteners as the high friction material in the interface between the cable sheath and the clamp segments, the bond between the hook and loop fasteners and the component that the fasteners are glued to is made strong in shear by choosing a suitable adhesive or bonding agent. The hook and loop fasteners then allow a very high shear resistance between the cable and clamp for a very low spring force, which may be as little as half that required without the high friction coupling. Another option, not shown, is to modify the surface of the sheath and the inner surface of the clamp, for example by applying a material as the coupling such as an adhesive to one surface, or changing the chemical structure of the sheath in specific locations on the cable to make it sticky, or tacky, so that it bonds more effectively, yet still releasably, to the clamp surface. Alternatively, as illustrated in Fig.7, the sheath 4 may be softened at specific locations 33 to allow teeth 32 on the inner surface 9 of the clamp to cut their own grooves more easily. Another option is that the coupling comprises a double sided high friction material inserted between the inner surface of the clamp and the outer surface of the cable.
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An alternative to the examples described above is to use an ultra high friction material, such as that described in
US9182075 . The high friction material may have a bonding surface allowing it to be bonded to the cable clamp, or the high friction material may be double sided, in which case it may be simply placed at an interface between the clamp and cable components. This allows a much lower spring force, up to half that otherwise required, to be used in the clamp to achieve the required resistance to pull forces. The high shear resistant, or ultra-high friction materials greatly increase the shear force transmission from the cable sheath to the clamp and thereby greatly increase the pull force of the clamp for a given spring force. The advantages include the fact that the cable does not suffer unquantifiable damage through the clamping process, even through thermal cycling. The clamping force does not reduce after heat has been applied because the clamp is compliant and able to move with the cable, unlike the conventional collet type clamp. The invention allows for the clamping force to be greatly increased for a given spring force. This enables smaller, lighter clamps with weaker springs to achieve the same or greater clamping forces than current clamping methods. By their very nature the high friction materials resist both axial and twisting forces without the need for complex machining. The invention may be embodied as a two part high shear, or high friction material such as hook and loop fasteners, or via a single ultra high friction material layer.
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The flow diagram of Fig.8 illustrates an example of a method of coupling a cable and cable clamp according to the present invention. A cable clamp, for example of the multi-segment type described hereinbefore, comprises a plurality of cable clamp sections. The cable 1, is terminated 40 in a connector in a cable gland, for example to a pin. Depending on the type of high friction coupling chosen the cable may undergo treatment before being terminated at the cable gland, or afterwards. For example, embossing, ridges or grooves in the cable, or inner surface of the clamp, are conveniently formed as part of the manufacturing process, but grooves in the cable may also be cut by an operator with a suitable tool at the point of forming the connection when the required cable length is known.
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Bonding of a high friction material to the cable or clamp surface, or softening of the casing is more typically done at a later stage, for example as part of the installation process and only in a specific region of interest. Thus, before applying the clamp to the cable, necessary surface modifications are carried out 41 and to the extent that the high friction coupling is applied separately, rather than by chemical or mechanical modification of the surface, then the high friction coupling is applied 42 to the cable and the clamp. For bonding with adhesive, the adhesive is chosen to be strong in shear, so that the cable and clamp can be taken apart easily if the cable has to be stripped to repair a fault. In the example, referred to above, of a double sided ultra high friction material, this may be simply a matter of placing the high friction coupling at an interface between the clamp and cable components before the cable clamp is closed around the cable.
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The electrical connection of the subsea cable may be made before or after fitting the clamps. If the cable is being fitted with a clamp before being connected, then the segments of the clamp are coupled together 43 around the cable close to the connector by one or more resilient members. The cable is received in the orifice formed by the clamp segments and the resilient members allow the orifice to have an adjustable cross section. By tightening the resilient members, a desired initial pressure is applied to the cable by the clamp sections for the ambient conditions, then once in use the clamp allows sufficient movement during thermal cycling by compression or expansion of the springs of the resilient members to prevent unacceptable amounts of cable extrusion, or loss of clamping effect. The one or more friction elements provided on the inner surface of the orifice and on the outer surface of the cable interact with the one or more friction elements on the other surface to maintain the location of the cable clamp relative to the cable and so give additional protection to the cable connectors. The clamped and connected cable may then be deployed to its operational location 45. If a fault occurs and the cable has to be retrieved and stripped down, the high friction coupling is designed to be easily removed without further damage to either cable or clamp and in many cases allows the clamp to be refitted after the cable or connector repair, without the need for further material, or processing.
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In summary, the present invention adapts a cable clamp comprising a plurality of cable clamp sections coupled together by one or more resilient members to form an orifice having an adjustable cross section to improve the effectiveness of the clamp at protecting the connection. The orifice is adapted to receive a cable and the inner surface of orifice formed by the clamp sections is adapted to co-operate with the outer surface of the cable, so that one or more friction elements on the inner surface of the orifice interact with corresponding friction elements on the cable, so that the clamp and cable are releasably coupled. Spring-loaded compliant segments of the clamp may have with ridges or teeth, to engage with pre-cut grooves in the cable jacket, or with chemically or mechanically modified cable casing. The invention has the advantages that it allows for a greatly increased clamping force and damage to the jacket can be controlled, pre-designed and repeatable. No further damage should occur through thermal cycling or pulling/twisting forces.
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The surface modification of the cable sheath in combination with a compliant clamp has the advantages the cable is not damaged through thermal cycling and so the clamping force does not reduce as a consequence of heat having been applied. This is due to the clamp being compliant and able to move with the cable unlike the collet type clamp of Figs.1a and 1b. The invention allows the clamping force to be increased and the required clamping force to be achieved more repeatably, by using a surface treatment, or releasable high friction coupling, to improve the engagement between the clamp and the cable sheath. This reduces the need for high spring forces and reduces the likelihood of damage or degradation to the cable.
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Although there may be some damage to the outer sheath of the cable, for example by cutting grooves, sand blasting, or chemical softening, or by engaging the teeth, or ridges in the grooves or modified cable surface, this is damage is repeatable, qualifiable and pre-defmed damage which does not worsen with thermal cycling.
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It should be noted that the term "comprising" does not exclude other elements or steps and "a" or "an" does not exclude a plurality. Also elements described in association with different embodiments may be combined. It should also be noted that reference signs in the claims should not be construed as limiting the scope of the claims.
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Although the invention is illustrated and described in detail by the preferred embodiments, the invention is not limited by the examples disclosed, and other variations can be derived therefrom by a person skilled in the art without departing from the scope of the invention.
