TENSION MEMBER

Abstract: A tension member (1) comprises a core of load-bearing elements (2), a protective braided cover (5), and a filter layer (4) which may comprise a polypropylene needlefelt fabric or meltblown material. The filter layer (4) prevents particles of a size greater than e.g. 2 microns from entering the core and abrading the load-bearing elements (2), whilst allowing the ingress of water for cooling purposes. The filter layer (4) may be sandwiched between layers of water impermeable material which have offset openings therein, so that the water passes along a length portion of the filter layer (4) before entering the core. The filter layer (4) may comprise water impermeable material having openings therein across wh ich is filter material, and may be provided on the outside of the braided cover (5).

Description: Tension Member The present invention relates to tension members. Tension members, including rope, are used widely in industry to transfer load from one point to another. They consist of one or more load bearing elements which are connected to a termination at each end, so that an external load can be transmitted via the terminations along the elements. Tension members can be manufactured in many different constructions and materials using many different techniques. Tension members may be made, for example, from synthetic fibres, natural fibres, polymeric materials, metallic materials, including steel wires, and other materials. They may be manufactured in the form of composites using two or more different materials. Tension members may be in the form of cables, pipes, or hoses which contain other materials whose main function is not to transmit load (e.g. oil hoses and umbilicals) . Manufacturing techniques may include twisting, stranding, plaiting, braiding, laying, parallel laying, extruding, pultruding and many others. Several techniques may be used to manufacture one tension member . In the present application, the term "tension member" should be taken to include any of the above members . It is important that tension members retain their strength well and do not degrade too quickly, as otherwise frequent replacement would be required, if functionality and safety are not to be compromised. Accordingly, tension members are often provided with a protective outer layer that protects the member against mechanical abrasion, e.g. from rubbing or chaffing against handling equipment, such as rollers, fairleads and other rough surfaces. The present invention aims to provide a tension member that has improved strength retention over prior art tension members, and relates particularly to tension members that are used at sea. Viewed from one aspect, the present invention provides a tension member having a filter layer which allows liquid into the tension member, whilst preventing particles over a certain size from entering the body of the tension member. The invention also extends to a method of making a tension member, the method including the step of including a filter layer in the tension member . Generally, the tension member will include one or more load-bearing elements, and, if a tension member is in a liquid with a suspension of solid particles (e.g. in sea water or in the mud on the sea bed) , then the solid particles may move into the structure of the tension member, e.g. between the load-bearing elements, and, once inside, may abrade the load-bearing elements as the member moves in the sea and the elements move relative to one another. This abrasion may cause the tension member to lose strength. However, by providing a filter layer about the load-bearing elements, these particles are prevented from entering the tension member, and the member's integrity is maintained. Thus, the present invention provides a filter layer about the body of the tension member that prevents the ingress of particles of a size which might abrade the tension member. Furthermore, by allowing liquid, e.g. water, to enter the tension member, the member may be cooled more rapidly due to the liquid increasing the thermal conductivity of the tension member and/or by the circulation of the liquid therethrough. Thus, although it would be possible to encapsulate the tension member in a non-porous layer to guard against the ingress of solid particles, such an arrangement would also prevent cooling water from entering the member, and so could cause undue temperature rises in the tension member, as it is heated by e.g. hysteresis heating caused by dynamic axial and transverse loads due to wave motion of the sea, currents and the motions of tethered objects. This heating could well degrade the rope. In contrast, however, a filter layer both protects against the ingress of undesired solid particles, whilst also allowing cooling water to circulate. The use of a filter layer as opposed to a non- porous layer also has the advantage that air is not trapped inside the tension member. It is thought that trapped air may also degrade a tension member by e.g. creating local hot spots. Preferably, the filter layer prevents particles of a size over about 20 microns from entering the tension member. More preferably, the filter layer prevents particles of about 2 microns or more from entering the tension member, and, further preferably, prevents particles of sizes greater than about 0.2 or 0.1 microns from entering the body of the tension member. The filter layer may be made of any suitable material, and in any suitable structure and dimensions. The filter layer may also be designed to perform the additional function of increasing the coefficient of friction between the load-bearing elements and e.g. a protective outer cover layer. This may help to prevent damage to the tension member when placed in a clamping device, as relative movement between the load-bearing elements and the cover is reduced. The filter layer may be made from polypropylene, polyester, natural fibre or any other suitable material, including non-woven materials, such as meltblown fabrics. It may be in the form of a flat fabric, yarn, amorphous bundle or any other suitable form. The filter material may preferably comprise a polypropylene needlefelt fabric, or some other material of matted construction. The filter material may be applied by wrapping, braiding, manually laying or by any other suitable method. The filter material is preferably applied in such a manner that it will remain surrounding the load-bearing elements whilst the elements stretch and contract in use. The filter layer may be wrapped around e.g. the one or more load-bearing elements in e.g. a helical fashion. Preferably, the filter layer is wrapped about the tension member so as to have an overlap. The overlap helps to accommodate any stretching of the tension member. Thus, a particularly preferred form, the filter layer comprises a polypropylene needlefelt fabric wound helically around the load-bearing elements, the fabric having a width of e.g. about 30 cm and an overlap of e.g. about 6 cm . The filter material may preferably have a thickness in the range of 0.1 mm to 10 mm, e.g. about 2 mm. The filter material may be held in place through mechanical friction, bonding or any other suitable method. Further layers of other materials may also be used with the filter layer for other purposes. For example, an outer layer may be provided which holds the filter layer and the load-bearing elements together. Preferably, an outer layer is provided about the tension member to provide protection against mechanical abrasion of the tension member. This layer may take any suitable form, but should be porous to allow the ingress of liquid, e.g. water, into the tension member for cooling purposes. The protective layer may take any suitable form, and may comprise e.g. a braided outer layer of for example a polyester sheath, e.g. a 32 plait braided polyester sheath. The filter layer may comprise filter material sandwiched between a pair of layers of water impermeable material having openings therein to allow the water therethrough, and the openings of one of the water impermeable layers may be offset from those of the other, so that water will follow a tortuous route to the core, along a length portion of filter material. This increases the performance of the filter material, as the water must pass through a greater amount of filter material than if passing directly across the material. The filter layer need not be completely manufactured from filter material, and instead may comprise a water impermeable layer in which are provided openings that are covered by filter material . This then allows the use of less filter material, and so facilitates the use of more expensive and finer gauged filter materials, such as meltblown f bric. These filter materials may be in the form of rings, patches or sheet material wrapped about the core of the tension member. Thus, a filter layer may be produced by wrapping a water impermeable material and a filter material preferably simultaneously about the core of the tension member, the water impermeable material filling the gaps between the windings of the filter material. In an alternative embodiment, rings of filter material are manually or mechanically placed over the core of the tension member, and sheets of water impermeable material, e.g. of polyethylene material, are placed between the rings. The rings and polyethylene may then be bonded together, e.g. by glue. The rings may be made by a single preformed ring of filter material slipped onto the core of the tension member, or may comprise a strip of material that is wrapped about the core one or more times over itself or helically. The water impermeable portions may be similarly formed. In a further alternative embodiment, the filter layer may comprise a layer of water impermeable material formed from a sheet or sheets of the material which have openings in them, which are laid or wrapped about the core and then have patches of filter material applied across the openings, e.g. by adhesive. The filter layer need not be the layer adjacent the core, nor must it be provided under an outer cover, and it may itself surround an inner cover which keeps the load-bearing elements of the core together and may comprise the outer cover or have a further outer cover thereabout . When acting as the outer cover, the water impermeable material of the layer may need to provide a protective function, and so may comprise polyurethane . Thus, in one embodiment, the tension member may comprise a core of load-bearing elements held together by a covering layer, e.g. of braided material, and a number of filter patches may be placed onto the covering layer by e.g. glue about their edges. Further covering patches may then be placed over the filter patches so that tabs or other projections on the covering patches extend upwards. The water impermeable material may then be applied over the whole of the surface of the covering layer, e.g. by being sprayed or poured over the core, and the tabs or other projections may be then pulled on, before the water impermeable layer cures, so as to remove the covering patches and expose the filter elements. This layer may then be the final outer layer, or a further protective layer, e.g. a braided cover may be provided. The covering patches may be made of any suitable material, such as polyethylene or adhesive-backed card, and may be smaller in size than the filter patches so that the impermeable material may engage the edges of the filter patches and hold them in place. Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 shows a typical tension member under pure tension; Fig. 2 shows the movement of and forces acting on the tension member of Fig. 1 when in use in the sea; Figs. 3a to 3g show cross-sections through the line 3-3 of the tension member 1 of Fig. 1 for various possible configurations of the load-bearing elements; Fig. 4 shows a tension member made in accordance with one embodiment of the present invention; Fig. 5 shows schematically a tension member made in accordance with a second embodiment of the present invention; Fig. 6 is a longitudinal cross-section through the tension member of Fig. 5; Fig. 7 is a longitudinal cross-section through a tension member according to a third embodiment of the present invention; Fig. 8 is a longitudinal cross-section through a tension member according to a third embodiment of the present invention; Fig. 9 is a schematic view of a tension member according to a fourth embodiment of the present invention in which a portion of the outer cover has been removed; Fig. 10 is a longitudinal cross-section through the tension member of Fig. 9 ; Fig. 11 is a schematic view of a tension member according to a fifth embodiment of the present invention in which a portion of the outer cover has been removed; Fig. 12 is a longitudinal cross-section through the tension member of Fig. 11; Referring to Fig. 1, a tension member 1 comprises a plurality of parallel load-bearing elements 2 having a termination 3 at each end. The tension member 1 is used to transfer load between two points connected to the terminations 3, and is put under tension in the direction of the arrows T. The load-bearing elements 2 may be arranged in a number of different configurations, as shown in Figs. 3a-3g. These show cross-sections of tension members made up from load-bearing elements with various cross- sectioned shapes. For example, (a) shows elements of circular cross-section surrounded by other such elements to fill in the shape, (d) shows the same as (a) with a surrounding ring-shaped element, and (g) shows several ring-shaped elements surrounding each other. Fig. 1 shows a tension member 1 having the arrangement of load- bearing elements shown in Fig. 3b. Such a tension member is often used in the sea to e.g. tether oil and drilling rigs and buoys, and may be partially buried in mud at the bottom of the sea. Whilst in the sea, wave motion, currents and wind may cause the tension member 1 to be subjected to transverse loads as shown by the arrows in Fig. 2 and to dynamic axial loads. The motion caused by the waves, current and wind tends to heat the tension member 1, and this may degrade the tension member. Also, if particles in the sea/mud are able to penetrate the tension member, the movement of the load-bearing elements 2 relative to one another may cause the particles to abrade the load-bearing elements . Fig. 4 shows the tension member of Fig. 1 made in accordance with one embodiment of the present invention. The tension member of Fig. 4 comprises a central portion of about 150 mm diameter made up from a number of load-bearing elements 2 laid parallel to one another, a filter layer 4, and an outer cover 5. The load-bearing elements may be e.g. 3 or 4 strand polyester ropes, braided polyester rope, or any other suitable elements such as aramid or high modulus polyethylene . The filter layer 4 comprises a 30 cm wide polypropylene needlefelt fabric wrapped helically around the load-bearing elements 2 with an overlap of about 6 cm. It is about 2 mm in thickness. The filter layer is typically able to prevent particles of a size greater than about 2 microns from entering the central portion of the tension member. This therefore prevents the tension member from losing strength by abrasion of the load-bearing elements 2 by such particles, whilst allowing water to circulate through the tension member and cool it. The outer protective cover 5 is about 8 mm in thickness and is made from 32 plait braided polyester. This also allows the water through, whilst protecting the tension member from mechanical abrasions due to chaffing or rubbing of the tension member against handling equipment. Such tension members are particularly useful in the mooring of oil rigs and drilling rigs and buoys, and in similar such situations. Figs . 5 and 6 show a tension member 1 in accordance with a second embodiment of the present invention. In this embodiment, the filter layer 4 is formed on the outside of the braided cover 5. Thus, in this embodiment, the main use of the braided cover 5 is to hold together the load-bearing elements (not shown individually) in the core 6 of the tension member 1. The filter layer 4 in this case comprises a plurality of patches 7 of filter material and a coating of a water impermeable material 8, such as polyurethane, which serves both to keep water out (except where it can pass through the filter material) and to protect the tension member from damage. This embodiment facilitates the use of more expensive filter materials, such as meltblown filter fabrics, as only a portion of the filter layer is made from the filter material, and the rest of the layer is inexpensive polyurethane. Meltblown filter fabrics are available which are able to filter out materials above 0.1 microns . In order to manuf cture the tension member of Figs . 5 and 6, the outer cover 5 may be braided onto the core 6 of the load-bearing elements in the usual manner, and then filter patches 7 may be affixed to the braided cover 5 by e.g. glue about their edges. A further patch of suitable covering material (e.g. polyethylene or adhesive-backed card) with a tab or other projection thereon may then be placed over each filter patch 7 so that the tab sticks up. Polyurethane may then be sprayed or poured over the whole of the surface of the outer layer 5 and over the patches, and before the layer has fully cured, the covering patches may be removed by pulling on the tabs so as to leave the filter patches 7 exposed. The covering patches may be smaller in size than the filter patches 7 so that the polyurethane can coat the edges of the filter patches 7 to hold the patches 7 in place. The filter material is indented from the surface of the polyurethane layer, and so will be protected from abrasion damage, etc. The water may flow into the core 6 through each of the filter patches 7 as shown by the arrows. To provide a good supply of water to the core 6, the filter material should preferably cover e.g. between about 1% and about 20% of the surface area of the filter layer 4, e.g. about 5%. Fig. 7 shows a third embodiment of the present invention, in which the filter layer 4 is between the core 6 and braided outer cover 5 and comprises a plurality of spaced rings 11 of filter material, between which is provided water impermeable material 12. In this embodiment, the water impermeable layer could also be polyurethane, but, as the water impermeable layer is not used for protection, it may also comprise, e.g., polyethylene. During manufacture, the rings 11 may be placed mechanically or by hand over the core 6, and for example could either be preformed and slipped over the core 6 or could comprise strips wound once or more about the core. Polyethylene sheeting may then be wrapped about the core 6 between the rings 11 again either mechanically or manually, and bonded to the rings 11, e.g. by adhesive. Fig. 8 shows a fourth embodiment of the present invention in which the filter layer comprises a layer of filter material 13 sandwiched between two water impermeable layers 14 which have openings 15 therein for allowing the ingress of water. These openings 15 are offset from one another, so that the water enters the core 6 through a tortuous route along a length portion of the filter material 13, as shown by the arrows in the Fig. 8. Thus, in this embodiment, the water flows through a greater part of the filter material 13, so that it is more effective at filtering than merely when the water passes straight across the width of the material . The filter material may be polypropylene needlefabric, whilst the water impermeable layers may be polyethylene. The tension member may be made by manually wrapping polyethylene sheeting about the core, then filter material and then further polyethylene sheeting. The openings may be made by leaving gaps between the wraps of the sheets or using sheets with openings therein. A further embodiment is shown in Figs. 9 and 10, in which the filter layer 4 is again provided between the core 6 and an outer braided cover 5. In this case, the method of forming the filter layer 4 from both filter and water impermeable materials 16,17 is to wind the materials preferably simultaneously onto the core 6, e.g. in a helical fashion. This may be achieved using known wrapping machines, the core 6 being passed through the wrapping machines before reaching the braiding machine . Again, this embodiment facilitates the use of more expensive filter materials, as less in used then if the filter layer 4 is formed completely of filter material 16. Figs. 11 and 12 show a still further embodiment of the present invention, in which the filter layer 4 is between the core 6 and braided outer cover 5 , and in which the filter layer 4 comprises a layer of water impermeable material 18, having openings therein, across which are provided filter patches 19. Thus, this embodiment is similar to that of Figs. 5 and 6, but with the filter layer 4 on the inside of the tension member. This embodiment may be made by hand-laying or wrapping polyethylene sheeting over the core 6, and then e.g. adhesively affixing the patches 19 over openings which are formed in the sheeting before laying. The above are only particular embodiments of the present invention, and various modifications and alternative arrangements and combinations of the above features are also possible. For example, the filter materials and impermeable materials could be made from any other suitable materials and constructions, and could be arranged to prevent particles of different sizes from entering the tension member, e.g. particles over 0.2 or over 20 microns in size. The tension member itself may also be made in a number of different forms.

Claims: Claims 1. A tension member having a filter layer which allows liquid into the tension member, whilst preventing particles over a certain size from entering the body of the tension member. 2. A tension member including one or more load- bearing elements in a central portion of the member about which a filter layer is provided which allows water to pass therethrough but not particles over a set size . 3. The tension member of claim 1 or 2 , wherein the filter layer prevents particles of a size over about 20 microns from passing therethrough. 4. The tension member of claim 1 or 2, wherein the filter layer prevents particles of about 2 microns or more from passing therethrough. 5. The tension member of claim 1 or 2, wherein the filter layer prevents particles of sizes greater than about 0.2 microns from passing therethrough. 6. The tension member of any preceding claim, wherein the filter layer increases the coefficient of friction between the load-bearing elements and a protective outer cover layer. 7. The tension member of any preceding claim, wherein the filter layer is made from polypropylene. 8. The tension member of any of claim 1 to 6 , wherein the filter material of the filter layer comprises a polypropylene needlefelt fabric. 9. The tension member of any preceding claim, wherein the filter material of the filter layer is of matted construction. 10. The tension member of any of claims 1 to 6 , wherein the filter material of the filter layer comprises a meltblown material . 11. The tension member of any preceding claim, wherein the filter material of the filter layer has a water impermeable layer on either side thereof, each of the water impermeable layers having openings defined therein for the ingress of water. 12. The tension member of claim 11, wherein the openings of the impermeable layers are offset from one another . 13. The tension member of any preceding claim, including an outer layer provided about the filter layer . 14. The tension member of claim 13 , wherein the outer layer is a braided outer layer. 15. The tension member of any of claims 1 to 14, wherein the tension member comprises a core of load- bearing elements, a covering layer about the core, and a filter layer about the covering layer. 16. The tension member of any preceding claim, wherein the filter layer comprises portions of water impermeable material and portions of filter material. 17. The tension member of claim 16, wherein the filter layer comprises a plurality of spaced rings of filter material between which is provided water impermeable material . 18. The tension member of claim 16, wherein the filter layer comprises a layer of impermeable material having openings therein across which is filter material. 19. The tension member of claim 16, 17 or 18, wherein the water impermeable material comprises polyurethane . 20. The tension member of claim 16, 17 or 18, wherein the water impermeable material comprises polyethylene . 21. A method of making a tension member, the method including the step of including a filter layer in the tension member. 22. The method of claim 21, wherein the filter layer is wound onto a central core of load-bearing elements . 23. The method of claim 21 or 22, including the step of applying an outer cover over the filter layer. 24. The method of claim 21, 22 or 23, including the step of providing a layer of water impermeable material on either side of the filter material of the filter layer, the water impermeable layers being provided with openings therein. 25. The method of any of claims 21 to 24, wherein the filter layer is comprised of a layer of water impermeable material having openings therein across which is provided filter material. 26. The method of claim 25, including the steps of providing a covering layer over a core of load-bearing elements, applying filter material and filter covering material to the covering layer, applying a water impermeable material over the surface of the covering layer and filter and filter covering materials, and removing the filter covering material to expose the filter material . 27. The method of claim 25, wherein rings of filter material are provided over a core of load-bearing elements, and water impermeable material is provided between the rings of filter material. 28. The method of claim 25, wherein filter material and water impermeable material are wound onto the tension member so as to provide a layer having portions of water impermeable material and portions of only filter material . 29. The method of any of claims 21 to 28, wherein the filter layer is comprised of a matted fabric. 30. The method of any of claims 21 to 29, wherein the filter layer is comprised of a polypropylene needlefelt fabric. 31. The method of any of claims 21 to 28, wherein the filter material comprises a meltblown material. 32. The method of any of claims 25 to 31, wherein the water impermeable material is polyurethane. 33. The method of any of claims 25 to 32, wherein the water impermeable material is polyethylene. 34. A tension member including a filter layer that prevents the ingress of particles of a size which might abrade the tension member. 35. A tension member having a filter layer including a polypropylene needlefelt material as filter material . 36. A tension member having a filter layer including a meltblown material as filter material . 37. A tension member having a core, a core cover and a further cover, the further cover comprising a water impermeable protective layer in which are provided openings to allow water therethrough, the openings having filter elements therein to prevent the ingress of particulate material above a set size. 38. A tension member having a core, a protective outer cover and a filter layer therebetween, the filter layer comprising portions of a water impermeable material and portions of a filter material, water entering the core through the filter material. 39. The tension member of claim 38, wherein the filter layer comprises a plurality of filter rings between which is provided cylindrical sections of the water impermeable material . 40. A tension member including a filter layer provided between layers of water impermeable material which having openings therein. 41. The tension member of claim 42, wherein the openings of one of the water impermeable layers are offset from the openings of the other water impermeable layer.

Inventors: HOOKER JOHN GILBERT, HOOKER, JOHN, GILBERT
Applicants: MARLOW ROPES LTD, MARLOW ROPES LIMITED
Application year: 1998
Main classification: D07B1
IPC codes: D07B1, E21B33