GB2263752A - Protection of underwater pipelines and other structures - Google Patents

Abstract

An underwater object eg. pipe 2, is protected by securing adjacent to the object a jacket 6 formed of a flexible material, the jacket containing an elastomeric material 10 capable of being swollen by a liquid to which the jacket is effectively impermeable and introducing said liquid into the jacket to cause swelling of the elastomeric material either before or after the object is located within the jacket. Spaces between adjacent jackets can be shielded by a cover 11 of similar construction to the jackets. The jacket can be divided by bulkheads which incorporate flow-restricting valves (15, Fig. 3). Thermal insulation 9 can be included and may be separated from the swellable material by an elastomer sleeve (16, Fig 4). Jackets filled simply with liquid are also described. <IMAGE>

Description

PROTECTION OF UNDERWATER PIPELINES AND OTHER STRUCTURES This invention relates broadly to the protection of underwater pipelines and other structures.

Underwater structures are liable to mechanical damage when they suffer impact from fishing gear such as trawl boards and beams, or from anchors or heavy objects dropped overboard from vessels. The structures may also require thermal insulation from the surrounding water.

Protection of objects lying on the sea bed is presently achieved by installing over them a substantially rigid structure that may be made of, for example, steel, concrete or composite glass reinforced plastic. The installation of such protection can often pose greater risk of damage to the object to be protected. Thus, unless the protection structure is secured to the sea bed by piles or other means it may be dragged to contact and thus damage the object. If an attempt is made to mitigate this by using a lightweight composite structure then installation difficulties may be encountered, and the structure itself becomes liable to permanent damage on impact.

My co-pending application number 9016173.8 is directed to a method of protecting an underwater object by locating the object within a surrounding elongate jacket formed of a flexible material, the jacket being filled with liquid to form a compliant structure capable of absorbing externally applied impact energy. That application also extends to an underwater structure comprising an object located within a jacket as aforesaid.

The flexible jacket and the hydraulic reservoir which it envelopes together form an elastic structure which can absorb the energy of impact by other objects.

The present invention extends the earlier work, and has a number of different aspects.

From a first aspect of the invention I provide a method of protecting an underwater object by securing adjacent to the object a jacket formed of a flexible material, the jacket containing an elastomeric material capable of being swollen by a liquid to which the jacket is effectively impermeable and introducing said liquid into the jacket to cause swelling of the elastomeric material either before or after the object is located within the jacket.

According to a second aspect of the invention an underwater structure comprises an object and, adjacent to the object, a jacket formed of a flexible material, the jacket containing an elastomeric material that has been swollen by a liquid introduced into the jacket and to which the jacket is effectively impermeable.

From a third aspect the invention extends to a pipe for conveying fluid, the pipe being surrounded by an elongate jacket formed of a flexible material, the jacket containing an elastomeric material capable of being swollen by a liquid to which the jacket is effectively impermeable.

According to a fourth aspect of the invention a protective structure for fitting to a pipe comprises a tubular inner liner and a tubular outer jacket each extending between and secured to first and second spaced end caps, the space between the liner and the jacket containing an elastomeric material capable of being swollen by a liquid to which both the jacket and the liner are effectively impermeable.

It has been found that the use of swollen elastomeric material within the jacket can provide significantly greater energy absorbtion characteristics than simply filling the jacket with, for example, water and can also improve the thermal protection characteristics of the protective structure.

The swollen elastomeric material may, or may not, completely fill any free space within the jacket. If not, and if voids are left, then those voids may be filled with liquid only, usually the same liquid as used to swell the elastomeric material, or with rigid foam material strong enough to withstand the pressure at the anticipated working depth.

The liquid used to swell the elastomeric material is preferably water or oil. A wide range of elastomeric materials may be used, among which are natural rubbers, styrene-butadiene rubbers (SBR), ethylene-propylene-diene rubbers (EPDM) and polychloroprene rubbers. Other general purpose or speciality compounds may be used when specific applications so require.

EPDM is particularly preferred when protection is required for a pipe destined to convey fluids at temperature above 80 C, and natural rubber or SBR when low cost and easy processing are the most critical factors in elastomer selection. In order to provide the swelling capability the elastomeric material is desirably filled with a partially cross-linked neutralised acrylate resin or related material. Suitable fillers may be selected from partially cross-linked poly sodium acrylate, bentonite, potassium acrylate and a polyvinyl monomer such as disclosed in EP-A-037751, GB-A-1566552, US-A-3929741 or US-A4443019, starch-polyacrylonitrile, hydroxymethylcellulose and hydroxyethylcellulose.

Other filler material may also be incorporated into the elastomeric compound. One suitable addition is tyre crumb, which is very inexpensive and provides additional thermal protection. Thermal protection would also be enhanced by the incorporation of ceramic microspheres.

Although the invention may be put into practice by placing one or more jackets adjacent to or over the object to be protected, maximum advantage may be achieved when the object is actually located in the jacket. The object is then protected from all sides by the hydraulic reservoir of swollen elastomeric material. In this form the invention is particularly suitable for the protection of a pipe, the jacket then being an elongate jacket extending from adjacent one end to adjacent the other end of the pipe. In an alternative arrangement for protecting a pipe a pre-formed jacket may be spirally wrapped around the pipe; swelling would then normally be carried out subsequent to the wrapping operation.

From another aspect of the invention I provide a method of protecting an underwater pipeline comprising a plurality of pipes that are joined together end-to end, in which each pipe is located within a surrounding elongate jacket formed of a flexible material, each jacket terminating short of each end of the respective pipe and each end of each jacket being secured to an end cap which circumferentially surrounds and is secured to the respective pipe, each jacket being filled with liquid to form a compliant structure capable of absorbing externally applied impact energy. The invention further extends to an underwater pipeline comprising a plurality of pipes that are joined together end-to-end, each pipe being protected by a jacket as aforesaid.

Terminating the jacket short of each end of the respective pipe significantly facilitates the laying of continuous pipeline by leaving unprotected end sections where adjacent pipes may be secured together by welding or other suitable technique. These relatively small areas may be left unprotected or, more preferably, each exposed pipeline region between two adjacent jackets may be shielded by a protective cover fitted after the joint is made.

Such a cover may be formed by two or more partcylindrical sections which are held together around the respective exposed pipeline region. Each section may itself be in the form of a jacket according to the invention, although other constructions are possible.

For some applications it may be advantageous to divide the space within the jacket into compartments or chambers.

Thus, a jacket may include at least one internal flexible or rigid diaphragm member that divides the jacket into a plurality of separate compartments, desirably spaced along the length of the jacket. Any of the diaphragm members may have flow restrictor means to allow controlled flow of liquid between adjacent compartments. Such means may simply be one or more holes in a diaphragm member, or may include a valve fitted to a diaphragm member.

Additionally or alternatively one or more sleeves may divide the space within the jacket or within a compartment of the jacket into a plurality of sealed annular chambers. When swollen elastomeric material is used within the jacket it may then be present in less than all of the chambers.

For some applications it is desirable that a layer of additional thermally insulating material surround the pipe, and the swollen elastomeric material should then be present between the additional thermally insulating material and the jacket.

The additional insulating material may be applied to the pipe in any suitable manner, for example by spiral or other wrapping, extrusion, spraying or casting. The pipe itself, particularly when of steel, may have its outer surface coated with an anti-corrosion material prior to application of additional insulating material and/or the jacket.

In order that the invention may be better understood specific embodiments thereof will now be described in more detail, by way of example only, with reference to the accompanying drawings in which : Figure 1 shows part of a pipeline, partly shown in crosssection; Figure 2 is a section on the line Il-Il of figure 1; Figures 3 and 4 show alternative embodiments of pipes according to the invention, each in cross-section; Figure 5 is a section through a further protected structure according to the invention; Figures 7 and 8 are schematic radial sections through further embodiments of the invention; and Figures 8 and 9 schematically illustrate protection of a submarine well head.

Referring to figure 1 this shows two pipes 1, 2 of an underwater pipeline, the pipes being joined end-to-end by a welded connection 3. Each pipe is located within a surrounding elongate jacket 4, 5 respectively, the jackets terminating short of each end of the respective pipe. The jackets are of similar construction and only that associated with the pipe 2 will be described in detail.

Thus, the jacket 5 comprises a cover 6 of flexible material, extending between end caps 7 circumferentially surrounding the pipe 2. If desired, the end caps may be secured to the pipe by a suitable adhesive. The end caps will also be bonded to the cover, either by adhesive or, more desirably, by co-curing the cover and the end caps to form an integral assembly. Within the jacket the pipe is spirally wound with a layer of thermally insulating material 9. The space between the insulating material 9 and the cover 6 contains an elastomeric material 10 capable of being swollen by a liquid to which the cover material 6 is impermeable. A suitable swelling liquid is introduced into the space after the jacket has been located around the pipe, in order to cause swelling of the elastomeric material 10.The liquid may be pumped into the space, or the jacket may be allowed to selffill on submersion by providing suitable holes in the end caps and/or in the jacket itself.

A wide range of materials is suitable for the construction shown in figure 1. The cover material is preferably fabricated of a suitable elastomeric material such as SBR or EPDM. It should be effectively impermeable to the liquid with which the jacket is filled. The construction and thickness of the jacket should be such as to avoid tearing, and the jacket may incorporate one or more layers of reinforcing material such as a woven or non-woven fabric of nylon or other textile material, or a wound wire or textile cord. Reinforcing ribs or slats may be incorporated to stiffen the cover. It is desirable, however, that the cover has a low modulus in order that the jacket is elastic.

The end caps 7 are of a suitable rubber or plastics material such as SBR, EPDM, polyvinylchloride, polyolefines or glass-reinforced resins. When the pipe will be at high temperature it is preferred to use EPDM or other heat-resistant material for at least the radially inner part of each end cap; the radially outer part could be of a cheaper material such as SBR. The dimensions of the parts will be chosen to suit the anticipated pipe temperature. Heat-resistant material should be used until the temperature gradient from the inside to the outside of the end cap drops to below about 70 C; the outer part may then be of SBR.

The swellable elastic material will generally be selected from natural rubbers, SBR, EPDM and polychloroprene rubbers suitably compounded with a filler that may be selected from partially cross-linked poly sodium acrylate, bentonite, potassium acrylate and a polyvinyl monomer, starch polyacrylonitrile, hydroxymethylcellulose and hydroxyethylcellulose. The elastomer may be swollen by the introduction of water (either fresh water or sea water) or of an oil appropriate to the elastomer used. Examples of suitable oils are naphthenic process oil, ester (such as dioctyl phthalate) and aromatic oils. The thermal insulation may be of rubber filled with ceramic microspheres.

In a particular example the specific materials used were as follows:for the 1.5 cm thick cover an elastomeric compound of SMR 20 natural rubber and of SBR 1606 supplied by Enichem Elastomers Ltd (in a ratio of 1.3 parts NR to 1 part SBR) compounded with carbon black filler, a suitable curing system, antioxidant and antiozonant with a single-ply reinforcing breaker of an RFL dipped nylon Leno fabric: of 250 - 280 g/m2; for a 4 cm wrap of swellable elastomer a compound made up of WRT Neoprene 100 parts, partially cross-linked polysodium acrylate under the trade name AquaKeep lOSH 200 parts, zinc oxide 5 parts, magnesium oxide 4 parts, N550 carbon black 5 parts, octamine 1 part and dioctyl phthalate 10 parts; for the end caps having an inner diameter of 17.5 cm and an outer diameter of 40.6 cm an inner portion having a radial dimension of 5 cm of an elastomeric compound of Nordel (Trade Mark) 1320 and 1040 EPDM from DuPont compounded with carbon filler, a suitable curing system, antioxidant and antiozonant with the remaining 6.55 cm the same SBR compound as used for the cover.

The assembly was formed by applying the two end caps in uncured form to the pipe. Thermal insulation was then wrapped around the pipe, followed by a wrap of the pre-cured swellable elastomer. The cover was built and cured on a suitable mandrel, removed from the mandrel and slid over the wrapped elastomer and the end caps. The end cap regions were then submitted to an additional cure cycle.

It will be seen that the arrangement creates an energy absorbing jacket around the pipe that is to be protected. Upon impact on the jacket the energy will be absorbed by displacement of the enclosed volume of swollen rubber longitudinally and circumferentially around and towards the back of the jacket, by the elasticity and plastic deformation of the jacket material and by the shear effect of the volume of sea water that is displaced by the jacket in comparison to the unprotected pipe.

The exposed pipeline region that lies between the adjacent jackets 4 and 5 may be left exposed, but more desirably this is shielded by a protective cover 11 as indicated by broken lines in figure 1, and shown in figure 2.

This illustrates the use of two semi-cylindrical jackets fitted around the pipe and held in place by any suitable means, such as adhesive or circumferential straps. Each jacket comprises an inner wall 12 of EPDM and an outer cover 13 desirably of EPDM reinforced with RFL dipped nylon Leno fabric as aforesaid, the ends being closed by end caps similar in construction to those described for jacket 5. Space within the jacket may be filled with a liquid, and may incorporate swellable elastomeric material in a manner similar to the jacket 5. When the cover 11 is positioned it is possible to wrap the whole of the joint area with a suitable shrink wrapping material.

Figure 3 shows a pipe similar to that shown in figure 1, and corresponding parts are shown by the same reference numerals used in figure 1 with the suffix a. The structure differs from that shown in figure 1 in that the jacket includes a number of axially spaced radially extending bulkheads, of which one bulkhead 14 is shown in the figure. The bulkheads divide the jacket into a number of compartments, each compartment being filled with swollen elastomeric material.

The bulkheads may be of flexible or rigid material, desirably a suitable elastomeric material. They may incorporate one or more two-way flow restricting valves 15 which have the effect of restricting flow of displaced liquid from an individual compartment that is influenced by the impact load. The displacement effect through the restrictors adds a further energy-absorbing mechanism to the construction.

Figure 4 shows a further embodiment of pipe, again similar to that of figure 1 and the same parts are indicated by the reference numerals of figure 1 with the suffix b. In this embodiment the jacket incorporates an elastomeric sleeve 16 that divides the space within the jacket into two separate, sealed annular chambers. The thermal insulation material 9b is contained in one of these chambers and the swellable elastomeric material 10b is contained in the other chamber and is thus kept out of contact with the thermal insulation.

The jacket constructions already described will generally be built onto the pipe before this is laid, and they may be filled with liquid before or after laying. To allow filling, the construction may include a valve or connection system that will allow a liquid supply pipe to be connected, or the cover and/or end caps may be formed with holes that will allow ingress of water after submersion.

Alternatively, a pipe may be fitted into a prefabricated jacket such as that shown in figure 5. This comprises a tubular inner liner 20 and a tubular outer jacket 21, each of suitable elastomeric material and each extending between and secured to first and second spaced end caps 22, 23. The space between the liner and the jacket is filled with swellable elastomeric material 24. A pipe 25 as indicated by the broken lines may be inserted through sealing rings 26, 27 bonded to the end caps. Either before or after insertion of the pipe liquid is introduced into the structure in order to swell the elastomeric material 24.

In the foregoing embodiments the jacket is filled with swelled elastomeric material. Other filling materials are possible and the space may simply be filled with water (usually sea water); with water combined with a thickener or viscous modifying agent such as hydroxymethylcellulose, hydroxyethylcellulose, partially cross-linked sodium polyacrylate, carboxymethylcellulose, or heteropolysaccharide Welan gum; or with any other high viscosity liquid such as an inexpensive high viscosity oil known as aromatic extract or residue.

Figure 6 illustrates schematically a further jacket arrangement, wherein a pipe 30 is located within a structure having an inner liner 31 and a tubular outer jacket 32. The space between the liner and the jacket is divided by a number of flexible longitudinally extending bulkheads 33, which may be present in any required number and disposition. Any one or more of the bulkheads may include one or more two-way flow restricting valves. If required, the structure may also be further divided by radial bulkheads, which may also incorporate flow restricting valves. The protective structure may again terminate short of each end of the pipe that it protects, and the free space within the jacket is filled with any suitable liquid, and preferably with elastomeric material swollen by the addition of a suitable liquid.

Figure 7 illustrates that a plurality of pipes 40 may be grouped into a bundle, and the whole of the bundle enclosed within a protective structure comprising a tubular inner liner 41 and a tubular outer jacket 42. Again, the space within the jacket may be divided by bulkheads as required, flow restricter valves may be incorporated in any one or more of the bulkheads and the space may be filled with a suitable liquid or, preferably, with swollen elastomeric material. Both the jacket and the liner should be effectively impermeable to the liquid with which the jacket is filled.

Figure 7 also illustrates the provision of a longitudinally extending sealed membrane 43 within one of the compartments within the jacket. This membrane is empty on installation, and it allows the jacket to be made buoyant by inflating the membrane with air or other -gas in order to facilitate recovery of the installation from the sea bed. It will be appreciated that a similar floatation arrangement may be incorporated in any of the previously described embodiments.

In the foregoing description the object to be protected has been one or more pipes, and the object has been located within the jacket. However, protection may be achieved by locating one or more jackets adjacent to the structure to be protected. For example, two half-moon shaped jackets may be wrapped together around a pipe to give continuous protection even though the pipe is not located within either of those jackets. Alternatively, jackets may simply be placed adjacent to both sides of a pipe to be protected, although the protection achieved in this way will be limited. If the object to be protected is other than a pipe then appropriately shaped jackets may be located adjacent to the object and secured either to the object or to the seabed in any suitable manner.

Figures 8 and 9 illustrate how this may be achieved for a typical seabed wellhead 50 or other structure. Protection jackets 51, 52 are strategically placed around the object and are secured in position. When so positioned, each jacket is inflated with water or other liquid to the required pressure so that it assumes the desired shape for protection; inflation may be effected through a hose from a pump on the surface or on a remote operated underwater vehicle. Obviously the jacket can be configured to suit the installation being protected, and if necessary the arrangement may include tubular members which can be pre-inflated to create the required shape, prior to the main body of the protection system being inflated. In these arrangements the jackets will all incorporate elastomeric material which is capable of being swollen by the liquid used for inflation.

Claims (38)

1. A method of protecting an underwater object by securing adjacent to the object a jacket formed of a flexible material, the jacket containing an elastomeric material capable of being swollen by a liquid to which the jacket is effectively impermeable and introducing said liquid into the jacket to cause swelling of the elastomeric material either before or after the object is located within the jacket.

2. A method according to claim 1 in which said liquid is water or oil.

3. A method according to claim 1 or claim 2 in which said elastomeric material is an elastomeric compound containing as filler a partially cross-linked neutralised acrylate resin.

4. A method according to claim 3 in which the elastomer is selected from natural rubbers, styrene-butadiene. rubbers, ethylene-propylene-diene rubbers and polychloroprene rubber, and said filler is selected from partially cross-linked poly sodium acrylate, bentonite, potassium acrylate and a polyvinyl monomer, starch-polyacrylonitrile, hydroxymethylcellulose and hydroxyethylcellulose.

5. A method according to any one of the preceding claims in which said object is located within the jacket.

6. A method according to any one of the preceding claims in which said object is a pipe, and said jacket is an elongate jacket extending from adjacent one end to adjacent the other end of the pipe.

7. A method of protecting an underwater pipeline comprising a plurality of pipes that are joined together endto-end, in which each pipe is located within a surrounding elongate jacket formed of a flexible material, each jacket terminating short of each end of the respective pipe and each end of each jacket being secured to an endcap which circumferentially surrounds the respective pipe, each jacket being filled with liquid to form a compliant structure capable of absorbing externally applied impact energy.

8. A method according to claim 7 in which each exposed pipeline region between two adjacent jackets is shielded by a protective cover.

9. A method according to any one of claims 6 to 8 in which the or each jacket includes at least one internal diaphragm member dividing the jacket into a plurality of separate compartments.

10. A method according to claim 9 in which at least one of said diaphragm members has flow restrictor means to allow controlled flow of liquid between adjacent compartments.

11. A method according to any one of claims 7 to 10 in which the jacket contains an elastomeric material that is swollen by the liquid with which the jacket is filled.

12. A method according to any one of the preceding claims in which one or more sleeves divide the space within the jacket into a plurality of sealed annular chambers, and liquid is present in any one or more of said chambers.

13. A method according to any one of claims 5 to 12 in which a layer of thermally insulating material surrounds said pipe and said liquid is present between said thermally insulating material and said jacket.

14. An underwater structure comprising an object and, adjacent to the object, a jacket formed of a flexible material, the jacket containing an elastomeric material that has been swollen by a liquid introduced into the jacket and to which the jacket is effectively impermeable.

15. An underwater structure according to claim 14 in which said liquid is water or oil.

16. An underwater structure according to claim 14 or claim 15 in which said elastomeric material is an elastomeric compound containing as filler a partially cross-linked neutralised acrylate resin.

17. An underwater structure according to claim 16 in which the elastomer is selected from natural rubbers, styrenebutadiene rubbers, ethylene-propylene-diene rubbers and polychloroprene rubber, and said filler is selected from partially cross-linked poly sodium acrylate, bentonite, potassium acrylate and a polyvinyl monomer, starch polyacrylonitrile, hydroxymethylcellulose and hydroxyethylcellulose.

18. An underwater structure according to any one of claims 14 to 17 in which said object is a pipe and said pipe is located within the jacket.

19. An underwater pipeline comprising a plurality of pipes that are joined together end-to-end, in which each pipe is located within a surrounding elongate jacket formed of a flexible material, each jacket terminating short of each end of the respective pipe and each end of each jacket being secured to an endcap which circumferentially surrounds the respective pipe, each jacket being filled with a liquid to form a compliant structure capable of absorbing externally applied impact energy.

20. An underwater pipeline according to claim 19 in which each exposed pipeline region between two adjacent jackets is shielded by a protective cover.

21. An underwater pipeline according to claim 19 or claim 20 in which the jacket contains an elastomeric material that is swollen by the liquid with which the jacket is filled.

22. An underwater pipeline according to any one of claims 18 to 21 in which the or each jacket includes at least one internal diaphragm member dividing the jacket into a plurality of separate compartments.

23. An underwater pipeline according to claim 22 in which at least one of said diaphragm members has flow restrictor means to allow controlled flow of liquid between adjacent compartments.

24. An underwater structure according to any one of claims 14 to 22 in which one or more sleeves divide the space within the jacket into a plurality of sealed annular chambers, and liquid is present in any one or more of said chambers.

25. An underwater structure according to any one of claims 18 to 24 in which a layer of thermally insulating material surrounds said pipe and said liquid is present between said thermally insulating material and said jacket.

26. A pipe for conveying fluid, the pipe being surrounded by an elongate jacket formed of a flexible material,the jacket containing an elastomeric material capable of being swollen by a liquid to which the jacket is effectively impermeable.

27. A pipe according to claim 26 in which said elastomeric material is selected from natural rubbers, styrenebutadiene rubbers, ethylene-propylene-diene rubbers and polychloroprene rubber, and said filler is selected from partially cross-linked poly sodium acrylate, bentonite, potassium acrylate and a polyvinyl monomer, starchpolyacrylonitrile, hydroxymethylcellulose and hydroxyethylcellulose.

28. A pipe according to claim 26 or claim 27 in which the or each jacket includes at least one internal diaphragm member dividing the jacket into a plurality of separate compartments.

29. A pipe according to any one of claims 26 to 28 in which at least one of said diaphragm members has flow restrictor means to allow controlled flow of liquid between adjacent compartments.

30. A pipe according to any one of claims 26 to 29 in which one or more sleeves divide the space within the jacket into a plurality of sealed annular chambers and swellable elastomeric material is present in less than all of said chambers.

31. A pipe according to any one of claims 26 to 30 in which a layer of thermally insulating material surrounds said pipe and said swellable elastomeric material is present between said thermally insulating material and said jacket.

32. A pipe according to any one of claims 26 to 31 in which the swellable material is swollen with water or oil.

33. A protective structure for fitting to a pipe, the structure comprising a tubular inner liner and a tubular outer jacket each extending between and secured to first and second spaced endcaps, the space between the liner and the jacket containing an elastomeric material capable of being swollen by a liquid to which the jacket and the liner are effectively impermeable.

34. A protective structure according to claim 33 in which said elastomeric material is selected from natural rubbers, styrene-butadiene rubbers, ethylene-propylene-diene rubbers and polychloroprene rubber, and said filler is selected from partially cross-linked poly sodium acrylate, bentonite, potassium acrylate and a polyvinyl monomer, starch polyacrylonitrile, hydroxymethylcellulose and hydroxyethylcellulose.

35. A protective structure according to claim 33 or claim 34 and including at least one internal diaphragm member dividing the space between the liner and the jacket into a plurality of separate compartments.

36. A protective structure according to claim 35 in which at least one of said diaphragm members has flow restrictor means to allow controlled flow of liquid between adj acent compartments.

37. A protective structure according to any one of claims 33 to 36 in which one or more sleeves divide the space between the liner and the jacket into a plurality of sealed annular chambers.

38. A protective structure according to any one of claims 33 to 37 in which the liner is immediately surrounded by a layer of thermally insulating material and said swellable elastomeric material is present between said thermally insulating material and said jacket.