GB2268564A - Installation of submarine pipelines - Google Patents

Abstract

The invention provides a means of installing by towing and a novel installed submarine pipeline system, whilst also providing mechanical protection. Where required the invention may also provide thermal insulation. The ability to tow is provided by containing a pipeline 1, 2 or group of pipelines within a single or group of elongate outer jackets 4, 5 the annulii around the pipe or pipes and within the jacket or jackets being packed with hollow spherical material 10 having a specific gravity less than that of water, The assemblage of jacket and microspheres forms a compliant structure possessing mechanical strength, impact resistance and resistance to collapse when subjected to hydrostatic pressure. When installed on the seabed, the annulii formed by the jacket and pipes and between the void space between and around the spherical material is flooded with seawater or other suitable liquid or filling material to form an impact absorbing structure. <IMAGE>

Description

INSTALLATION AND PROTECTION OF SUBMARINE PIPELINES The invention relates to the installation and protection of submarine pipelines.

With the growth of an offshore oil and gas industry, it has been necessary to install pipelines from offshore oil and/or gas fields to onshore processing facilities, and between different offshore locations, (for processing or for loading into shuttle tankers from offshore loading buoys).

To meet the need to install pipelines with a high degree of reliability, it is possible to use barges and vessels with several aligned welding, inspection and coating stations, that can fabricate a pipeline from a large number of short lengths of coated pipe (typically lOm long) and then inspect and coat welded joints of the pipeline, and then lower the pipeline to the seabed down a long curved stinger or ramp.

As an alternative (suitable for pipelines of smaller diameter) the pipeline may be supplied in long lengths which are wound on a large reel housed on a vessel. The pipeline is unreeled from the vessel, straightened to remove the curvature created by the reeling procedure, and then lowered directly into the sea, and thus down to the sea bed.

A further alternative is to fabricate at a suitable shore site, a bundled assembly formed of a pipeline or group of pipelines housed within a larger diameter outer steel pipeline, known as a carrier pipe. The void space within the carrier pipe and around the pipelines housed within it is sealed to prevent the ingress of water when towed. Thus the dry cavity or void space within the carrier and around the pipelines housed within it, provides buoyancy when for installation of the fabricated bundle assembly is towed from its shore site. The bundle is assembled such that the buoyancy provided, results in the bundled assembly having just sufficient submerged unit weight (negatively buoyant) for the purpose of towing.

Submarine pipelines 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 pipelines may also require thermal insulation from the surrounding water.

Protection of pipelines lying on the sea bed is presently achieved by applying a substantial layer of concrete or elastomeric material. The carrier pipe featured in the towed bundle, also provides protection to the individual pipelines housed within it.

My co-pending applications, numbers 9016173.8 and 2246413A is directed in so far as pipelines, to a method of protection by locating the pipeline within a surrounding elongated jacket formed of a flexible material, the jacket being filled with liquid or liquid and elastomer, to form a compliant structure capable of absorbing externally applied impact energy. That application also extends to an underwater structure (including a pipeline) 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 installing and protecting submarine pipelines by locating a single pipeline or group of pipelines within an elongate jacket formed of a flexible material which is effectively impermeable to the surrounding water, the jacket containing tightly packed ceramic microspheres or other suitable material, herein after referred to as microspheres. To facilitate installation, the void space within the jacket and hence between the microspheres will be sealed to prevent ingress of water by the jacket.

By preventing the ingress of water the microspheres, having specific gravity of typically 0.4 and a bulk specific gravity when packed of typically 0.7, derive buoyancy. The volume of packed microspheres will be selected to give a resultant submerged unit weight of the pipeline assembly appropriate for towing. When installed the void space within the jacket and around the microspheres may be filled with water or other suitable liquid. Other filling materials are possible; with water combined with a thickener or viscous modifying agent, or with a liquid adhesive, or bonding polymer, or other suitable substance which could later set and bond the microspheres to form a consolidated or visco-elastic compliant composite structure capable of absorbing externally applied impact energy and forces and providing thermal insulation.

From a second aspect the invention also extends to method of installing and protecting a submarine pipeline by locating a single or group of pipelines within an elongate jacket formed of a more rigid material, typically steel, the jacket containing dry microspheres, the void space which is filled as aforesaid. With the presence of the microspheres a thinner thickness jacket may be employed than otherwise would be required to obviate collapse or buckling due to hydrostatic pressure from the surrounding water when installed.

From a third aspect the invention also extends to an underwater structure comprising a pipeline or group of pipelines located within a jacket as aforesaid.

According to a fifth 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 tightly packed dry ceramic microspheres the void space within which is filled after installation with liquid or substances to which the jacket and the liner are effectively impermeable as aforesaid.

Maximum advantage may be achieved when the pipeline or group of pipelines are actually located in the jacket. The pipelines are then protected from all sides by the jacket and material within it. The jacket then being an elongate jacket extending from adjacent one end to adjacent the other end of the pipelines. In an alternative arrangement for protecting a pipe a pre-formed jacket may be spirally wrapped around the pipe.

From another aspect of the invention I provide a method of installing and protecting an underwater pipeline or group of pipelines each 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 dry microspheres for the purpose of installation and subsequently filled as aforesaid to form a compliant structure capable of absorbing externally applied impact energy and forces. 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 fabrication 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 part-cylindrical 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.

Alternatively, the jacket may comprise a plurality of short length jacket sections, typically of length equal to the pipe joint from which the pipeline or group of pipelines is formed. The short length sections of jacket may be abutted to each other and secured at their end-to-end caps or bulkheads which are in turn secured to the pipeline or group of pipelines.

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.

For some applications it is desirable that a layer of additional thermal insulating material surround the pipe, and the microspheres should then be present between the additional thermal 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 A and B show parts of a pipeline, partly shown in cross section; Figure 2 is a section on the line Il-Il of figure 1; Figures 3 & 4 show alternative embodiments of pipes according to the invention, each in cross section; Figure 5 is a section through a further protected pipeline according to the invention; Figures 6 & 7 are schematic radial sections through further embodiments of the invention.

Referring to figure 1A 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.

Alternatively, jacket 4 may be abutted to jacket 5 following welding of pipes 1 and 2 and application of corrosion protective wraps and thermal insulation, all as illustrated in figure 1 B. 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 jacket, either by adhesive or, more desirably, by co-curing the jacket and the end caps to form an integral assembly. Within the jacket the pipe may be spirally wound with a layer of thermally insulating material 9. The space between the pipe 2 or the insulating material 9 and the cover 6 contains tightly packed ceramic microspheres or other suitable material and remain dry during the installation of the pipeline. The ingress of the surrounding water is prevented by the impermeable material from which the jacket and end caps are made.Liquid adhesive or bonding polymer material may be pumped into the jacket and void space within the microspheres, or the jacket may be allowed to self-fill when installed.

A wide range of materials are 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 and the surrounding water.

The construction and thickness of the jacket should be such as to avoid tearing, and the jacket may incorporate one or more layers of re-inforcing material, or a wound wire or textile cord. Re-inforcing 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 plastic material such as SBR, EPDM, polyzinylchloride, polyolefines or glassreinforced resins. When the pipe will be at high temperature it is preferred to use EPDM or other heatresistant 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. Heatresistant material should be used until the temperature gradient from the inside to the outside of the end cap drops to below about 70 degree C; the outer part may then be of SBR.

The microspheres or other suitable hereafter referred to as microspheres material will have a bulk specific gravity less than that of water when tightly packed. Microspheres having a bulk specific gravity of 0.7. In this was the volume of the microspheres or other suitable material can be adjusted to result in the pipeline having a unit weight, weight per metre, appropriate for installation of the pipeline by the "Controlled Depth Towing" method. When tightly packed the microspheres will prevent the collapse of the jacket 6 due to the hydrostatic pressure from the surrounding water. The microspheres themselves having collapse pressure beyond that to which they would be subjected by the surrounding water during installation.

Once the pipeline has been installed at its site location, the void space within the jacket and between the microspheres will either be filled with water, with water combined with a thickener or viscous modifying agent such as hydroxymethylcellulose, hydroxyethylcellulose, partially cross linked sodium poly-acrylate, carboxymethylcellulose or hetropolysacharide Welan gum; or with any other high viscosity oil known as aromatic extract or residue; or a liquid adhesive or bonding polymer, or other suitable substance which will later set and bond the microspheres to form a consolidated or visco-elastic compliant composite structure capable of absorbing externally applied energy and forces. The microspheres and void infill material provide thermal insulation.

In a particular example the specific material 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 or 1.3 parts NR tp l 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 microspheres a glass hard, inert, hollow silicate sphere, whose mechanical properties are directly attributable to the spherical nature of the material.The microspheres have a typical density range 0.65 to 0.75 g/cc and a corresponding bulk density of 0.38 to 0.42 g/cc with a packing factor of 60 - 65 %. The microspheres have a thermal conductivity of 0.1 to 0.11 WM-1 K-l. Particle size of the microspheres range from 5-300 microns, but a particle size distribution can be selected for utilisation in the invention.

For the end caps having an inner diameter of 17.5 cm and an outer diameter of 40.6 cm 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.

Void space filler may be water, or water combined with a thickener or viscous modifying agent, such as hydroxymethylcellulose, hydroxyethylcellulose, partially iron-linked sodium poly-acrylate, carboxymethylcellulose or heteropolysaccharide Welan gum; or with any other high viscosity oil known as aromatic extract or residue, or liquid adhesive, or bonding polymer which sets and/or bonds the microspheres.

The assembly formed by applying the two end caps in uncured form to the pipe. Thermal insulation wrapped around the pipe, followed by tightly packed microspheres. The cover built and cured on a suitable mandrel, removed from the mandrel and slid over the end caps. The end cap regions submitted to an additional cure. cycle either before or following packing of the jacket with microspheres.

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 column of void filled microspheres longitudnally and circumferentially around and towards the back of the jacket, 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.

Where the jackets 6 are not abutted, the exposed pipeline region that lies adjacent to the welded joint, may be left exposed, but more desirably this is shielded by a protective shrink wrapping material 11 and cover 12 as indicated by broken lines in figure 1, and shown in figure 2A. This illustrates the use of two semi-cylindrical segments 13 fitted around the pipe and held in place by any suitable means, such as adhesive or circumferential straps. The segments typically comprising an elastomeric envelope containing microspheres and suitable liquid, adhesive or bonding polymer.

When the cover 11 is positioned it is possible to wrap the whole of the joint area with a suitable shrink wrapping material.

Where the jackets 4 and 5 are abutted the exposed pipeline region that lies adjacent to the welds, may be left exposed, but more desirably this is shielded by a protective shrink wrapping material 11 and thermal insulation 9(i) as indicated in figure 1B, and shown in figure 2B. This illustrates the use of two semi-cylindrical thermal insulation segments 9(i) fitted around the pipe and held in place by any suitable means, such as adhesive or circumferential straps. Multiple cylindrical segments may be used.

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 on bulkhead 14 is shown in the figure. The bulkheads divide the jacket into a number of compartments, each compartment being filled with tightly packed microspheres and with liquid or adhesive following installation.

The bulkheads may be of flexible or more rigid 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 enery-absorbing mechanism to the construction.

This embodiment may equally be used where the jackets 4 and 5 are abutted as illustrated in figure 1B.

Figure 4 shows a further embodiment of pipe, again similar to that of figure 1A 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 microspheres and liquid, or adhesive subsequent to installation lOb are contained in the outer chambers and are 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 the void space within the jacket and between the microspheres may be filled with liquid after installation. 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 puncturing.

This embodiment may equally be used where the jackets 4 and 5 are abutted as illustrated in figure 1B.

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 shaped end caps 22, 23. The space between the liner and the jacket is filled with tightly packed microspheres and subsequently with liquid or adhesive following installation 24. A pipe 25 as indicated by the broken lines may be inserted through sealing rings 26, 27 bonded to the end caps.

This embodiment may equally be used where the jackets 4 and 5 are abutted, as illustrated in figure 1B.

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 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 tightly packed microspheres and additionally following installation by a liquid or adhesive.

This embodiment may equally be used where the jackets 4 and 5 are abutted as illustrated in figure 1B.

Figure 7 and 8 illustrate that a plurality of pipes 40 may be grouped into a bundle, and the whole of the bundle enclosed within a protective structure which may comprise of 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 restrictor valves may be incorporated in any one of more of the bulkheads and the space within the jacket filled with tightly packed mircospheres and subsequent to installation by liquid or adhesive.

Both the jacket and the liner should be effectively impermeable to the liquid with which the jacket is filled.

Figure 7 and 8 also illustrate the provision of a longitudinally extending sealed membrane 43 within one of the compartments or generally within the jacket. A plurality gr such membranes may be housed within the jacket also filled with microspheres for installation, and it allows the jacket to be made buoyant by removing the microspheres and inflating the membrane with air or other gas in order to facilitate recovery of the installation from the sea bed. Alternatively, the membrane may be filled with material having a specific gravity greater than water, to provide additional weight to pipeline assembly. It will be appreciated that a similar flotation or additional weight 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 pipelines and the pipeline or group of pipelines have been located within the jacket. However, protection may be achieved by locating one or more jackets adjacent to the pipelines to be protected. For example, two half-moon shaped or other appropriately 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 be simply placed adjacent to both sides of a pipe to be protected, although the protection achieved in this way will be limited.

Claims (34)

1. A method of installing and-protecting an underwater pipeline or group of pipelines by locating them within a surround elongate jacket formed of a flexible material, the jacket being tightly packed with microspheres or other suitable buoyant material to result in the pipeline assembly having a submerged weight per metre length to facilitate installation by towing. The void space within the jacket and between the microspheres being filled with liquid after installation to form a compliant structure capable of absorbing externally applied impact energy.

2. A method according to claim 1 in which the jacket is of more rigid material or construction, typically steel, which is prevented from collapse due to hydrostatic pressure, by the presence of the microspheres or other suitable material.

3. Amethod according to claims 1 and 2 in which the jacket includes at least one internal diaphragm member dividing the jacket into a plurality or separate compartments.

4. A method according to claims 1 and 2 in which at least one of the said diaphragm members has flow restrictors means to allow controlled flow of liquid between adjacent compartments.

5. A method according to claims 1 and 2 in which the liquid is an adhesive or which will solidify or set to derive the mechanical properties of an elastomeric material.

6. A method according to claims 1 and 2 in which the jacket includes a membrane inflated with gas to increase the buoyancy of the structure.

7. A method according to claims 1 and 2 in which a layer of thermally insulating material surrounds said pipelines and said microspheres and subsequent liquid or adhesive is present between said thermally insulating material and said jacket.

8. A method according to claims 1 to 7 in which said jacket is an elongate jacket extending from adjacent one end to adjacent the other end of the pipeline or pipeline assembly, where the pipelines comprise a plurality of pipes joined end to end.

9. A method according to claims 1 to 7 in which said jacket comprises a plurality of elongate jackets which are abutted to each other and secured at there ends to the end caps or bulkheads.

10. A method according to claims 1 to 7 in which the said jacket comprises a plurality of elongate steel jackets which are joined to each other by welding.

11. A method according to claims 1 to 10 in which a layer of thermally insulating material surrounds said pipe and said microspheres ad subsequent liquid or adhesive is present between said thermally insulating material and said jacket.

12. A method according to claims 1 to 10 in which the exposed region adjacent to the pipe welded joint is shielded by a protective cover comprising a shrinkable warp material and layer of thermal insulation material.

13. A method of protecting an underwater pipeline or plurality of pipelines where each pipeline comprises 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 tightly packed ceramic microspheres or other suitable buoyant material to result in the pipeline assembly having a submerged weight per metre to facilitate installation by towing.

The void space within the jacket and between the microspheres being filled with liquid after installation to form a compliant structure capable of absorbing externally applied impact energy and forces.

14. A method according to claim 13 in which each exposed pipeline region between two adjacent jackets is shielded by a protective cover.

15. A method according to claim 13 in which each jacket includes at least on internal diaphragm member dividing the jacket into a plurality of separate compartments.

16. A method according to claim 13 in which at least one of said diaphragm members has flow restrictor means to allow controlled flow of liquid between adjacent compartments

17. A method according to claims to in which one or a plurality of tubular members are housed within the said jacket, the said tubular members being tightly packed with ceramic microspheres or other suitable buoyant material. The microspheres or other material being displaced following installation of the pipeline assembly, being displaced with water or other liquid or filler having material having specific gravity greater than water, or alternatively with gas or other suitable material having specific gravity to make the pipeline assembly buoyant to facilitate its recovery.

18. An underwater pipeline or group of pipelines by locating them within a surround elongate jacket formed of a flexible material, the jacket being tightly packed with ceramic microspheres or other suitable buoyant material to result in the pipeline assembly having a submerged weight per metre length to facilitate installation by towing. The void space within the jacket and between the microspheres being filled with liquid after installation to form a compliant structure capable of absorbing externally applied impact energy and forces.

19. An underwater pipeline or group of pipelines according to claim 1 in which the jacket is of more rigid material or construction, typically steel, which is prevented from collapse due to hydrostatic pressure, by the presence of the microspheres or other suitable material.

20. An underwater pipeline or group of pipelines according to claims 1 and 2 in which the jacket includes at least one internal diaphragm member dividing the jacket into a plurality or separate compartments.

21. An underwater pipeline or group of pipelines according to claims 1 and 2 in which at least one of the said diaphragm members has flow restrictors means to allow controlled flow of liquid between adjacent compartments.

22. An underwater pipeline or group of pipelines according to claims 1 and 2 in which the liquid is an adhesive or which will solidify or set to derive the mechanical properties of an elastomeric material.

23. An underwater pipeline or group of pipelines according to claims 1 and 2 in which the jacket includes a membrane inflated with gas to increase the buoyancy of the structure.

24. An underwater pipeline or group of pipelines according to claims 1 and 2 in which a layer of thermally insulating material surrounds said pipelines and said microspheres and subsequent liquid or adhesive is present between said thermally insulating material and said jacket.

25. An underwater pipeline or group of pipelines according to claims 1 to 7 in which said jacket is an elongate jacket extending from adjacent one end to adjacent the other end of the pipeline or pipeline assembly, where the pipelines comprise a plurality of pipes joined end to end.

26. An underwater pipeline or group of pipelines according to claims 1 to 7 in which said jacket comprises a plurality of elongate jackets which are abutted to each other and secured at there ends to the end caps or bulkheads.

27. An underwater pipeline or group of pipelines according to claims 1 to 7 in which the said jacket comprises a plurality of elongate steel jackets which are joined to each other by welding.

28. An underwater pipeline or group of pipelines according to claims 1 to 10 in which a layer of thermally insulating material surrounds said pipe and said microspheres ad subsequent liquid or adhesive is present between said thermally insulating material and said jacket.

29. An underwater pipeline or group of pipelines according to claims 1 to 10 in which the exposed region adjacent to the pipe welded joint is shielded by a protective cover comprising a shrinkable warp material and layer of thermal insulation material.

30. An underwater pipeline or group of pipelines protecting an underwater pipeline or plurality of pipelines where each pipeline comprises 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 tightly packed ceramic microspheres or other suitable buoyant material to result in the pipeline assembly having a submerged weight per metre to facilitate installation by towing.

The void space within the jacket and between the microspheres being filled with liquid after installation to form a compliant structure capable of absorbing externally applied impact energy and forces.

31. An underwater pipeline or group of pipelines according to claim 13 in which each exposed pipeline region between two adjacent jackets is shielded by a protective cover.

32. An underwater pipeline or group of pipelines according to claim 13 in which each jacket includes at least on internal diaphragm member dividing the jacket into a plurality of separate compartments.

33. An underwater pipeline or group of pipelines according to claim 13 in which at least one of said diaphragm members has flow restrictor means to allow controlled flow of liquid between adjacend compartments

34. An underwater pipeline or group of pipelines according to claims to in which one or a plurality of tubular members are housed within the said jacket, the said tubular members being tightly packed with ceramic microspheres or other suitable buoyant material. The microspheres or other material being displaced following installation of the pipeline assembly, being displaced with water or other liquid or filler having material having specific gravity greater than water, or alternatively with gas or other suitable material having specific gravity to make the pipeline assembly buoyant to facilitate its recovery.