The present invention relates to a polymer lined pipe (PLP) assembly, a connector for use in such a PLP assembly, a method of connecting to PLP pipe sections two form a PLP assembly and a PLP pipeline using at least one said PLP assembly.

One type of pipeline in the subsea and offshore field generally comprises an outer metal pipe made of for example, steel, in particular carbon steel, and a protective inner liner or sleeve. Such pipelines are well known in the art, and are generally termed 'polymer lined pipelines' often abbreviated to "PLP", which can be formed from a series of joined polymer lined pipes, or pipe sections or pipe stalks. The inner liner is generally formed of a plastics or polymer materials such as high density polyethylene, for the protection of the outer metal pipe against hydrocarbons and other corrosive liquids being transported by the pipeline, although they may also be used to transport non-corrosive fluids such as water.

One possibility is to provide a continuous liner along the full length of the pipeline, but this can be difficult to form. More commonly, the pipeline is formed of PLP pipe sections or stalks, with each outer metal pipe being welded to its neighbour, most conventionally using butt or girth welding. Meanwhile, it is required to achieve an internal continuation of the lining across each join of the PLP pipe sections.

The use of liner junction sleeves for this purpose is well known, and US7722085 shows a junction sleeve having terminal portions with notched surfaces enabling it to be inserted in and anchored against the inside surface of the terminal portions of inner liners of two pipe elements.

W095/22713A1 shows a lining sleeve intended to achieve the same purpose of continuation of the lining material between two PLP pipes, which uses tapered screw- threaded portions to create a seal at the thread interface.

However, the use of any type of 'notches' or 'grooves' or 'threads' etc. to form the securement between such sleeves, also creates corners, angles and edges, which lead to stress concentrations either during securement, or after securement, or both, and stress concentrations are weak spots. Any weak spot will be exploited by the usually corrosive and pressurised fluid being transported by the pipeline in use, leading to cracks, leaks and potential catastrophic failure. It is an object of the present invention to provide a PLP assembly that avoids the provision of any such stress concentrations both during the connection of the PLP assembly and during use of the PLP assembly in a pipeline.

Thus, according to one aspect of the present invention there is provided a polymer lined pipe (PLP) assembly comprising:

two PLP pipe sections, each PLP pipe section comprising an outer metal pipe and a fitted inner polymer liner, each inner polymer liner terminating before one end of each outer metal pipe, and the final portion of each such end of each liner having a smooth female inner bore with an original internal diameter D2, and

a connector having a cylindrical central section between opposing cylindrical ends, each connector end having a smooth male outer shaft with an original external diameter of Dl, wherein Dl > D2 prior to mating, and adapted to mate by interference fit with the female inner bores of the inner polymer liners of the PLP pipe sections to form a continuation of the liner along the PLP assembly.

In this way, the smooth mating of the female inner bores and the male outer shafts can be carried out without inducing any stress concentrations or weak points thereinbetween, leading to a more trustworthy seal and connection, and trustworthy pipeline in use.

In the present application, the term 'cylindrical' as used here relates to a tubular or substantially tubular cross section with a hollow inner bore, optionally being similarly cylindrical. The wall thickness of the cylindrical central section may or may not be uniform.

In the present application, the term 'cylindrical end' as used here relates to a tubular end or substantially tubular end, with a hollow inner bore, and the outer shaft of each end being substantially cylindrical, i.e. almost fully cylindrical except in one or more limited areas, such as comprising chamfers or circumferential grooved areas.

According to a preferred embodiment of the present invention, the inner bore of cylindrical central section and the inners bore of both cylindrical ends are coaxial and have the same diameter.

Each PLP pipe section comprises an outer metal pipe and a fitted inner polymer liner. Each PLP pipe section maybe any length, including typical stalk lengths of either 12m or 24m, but possibly up to 1km or longer. The present invention is not limited by the length of each PLP pipe section, and the person skilled in the art is aware how to form PLP pipe sections in general. The outer metal pipe may be any suitable metal, and may have any suitable thickness or dimension, typically intended to achieve a certain degree of flexibility in the formed pipeline, especially reelability to assist with reel-laying. Such outer metal pipes are typically formed of steel, in particular carbon steel, and include without limitation carbon resistant alloys.

The inner polymer liner may be formed of any suitable material, generally being a plastic material, able to be fitted within the outer metal pipe so as to be wholly or substantially pressed against the inside face of the outer metal pipe and fitted thereto, generally by compression of interference fit.

Each inner polymer liner terminates before one ends of each outer metal pipe, optionally before both ends of the outer metal pipe where it is intended to extend the pipeline using further PLP pipe sections. Optionally, the inner polymer liner is provided as a longer length relative to the outer metal pipe, and then 'cut back' to terminate before the end of the outer metal pipe.

The final portion of each such end of each liner has a smooth female inner bore with an original internal diameter D2. This can be formed by machining the inside of the final portion of the liner, using suitable cutting machinery known in the art such as a counter boring tool. Optionally, one or more of the circumferential corners of the inner polymer liner where its internal diameter changes can be bevelled or chamfered or rounded.

The PLP assembly includes a connector having a cylindrical central section between opposing cylindrical ends, each connector end having a smooth male outer shaft with an original external diameter of Dl. The connector may be formed from any suitable material, including any suitable plastic or metal or combination of same, and including composite materials. One possible connector material is the same material used to form the inner polymer liners of the PLP pipe sections. Another possible connector material is a metal, such as a corrosion resistant alloy, such as alloy 625. The appropriate material for the connector can be selected to take account of the method of pipeline laying, and the nature of the fluid being transported by the pipeline. For example, and especially for a smaller diameter or thicker pipeline which do not tend to 'ovalise' during laying, metallic connectors could be used. In contrast, cheaper plastic or composite connectors may be more appropriate for S-laying and J-laying the pipeline, due to the high number of connectors required. Suitable machinery is known for providing each connector end with a smooth male outer shaft having an original external diameter of Dl. The present invention requires Dl > D2 before mating.

In the present invention, the male outer shafts of the connector are adapted to mate by interference fit with the female inner bores of the inner polymer liners of PLP pipe sections, to form a continuation of the liner along the PLP assembly, i.e. along the bore of the PLP assembly, and so-formed PLP pipeline where extended further. Mating by interference fit avoids forming or developing any stress concentrations, and therefore any conventional weak spots or fatigue during or after mating, thus increasing the integrity of the PLP assembly in use. As a result of the mating, one of Dl and D2, or both Dl and D2, change, such that D1=D2 within a reasonable level of accuracy. Which of Dl and D2 change depend on the nature and material of the male outer shafts and the female inner bores. The interference fit can be achieved by bringing together, usually with a degree of lateral or longitudinal or axial force, the smooth female inner bores and the smooth outer male shafts together, and extending their overlap until a sufficient seal is achieved thereinbetween. One form of such mating is by the use of press-interference fit, wherein at least one of the PLP pipe sections and the connector are pressed using a sufficient force, such as in the range 4-40 Te, preferably in the range 10-30 Te, said force being generally oriented along the longitudinal centre line of the PLP pipe section, and optionally whilst holding one or other of the PLP pipe section or the connector in a suitable clamp or press or holder.

According to one embodiment the present invention, the calculation (Dl-D2)/2 is in the range 0.2mm - 1mm prior to mating. Preferably, the calculation (Dl-D2)/2 is in the range 0.3mm to 0.7mm prior to mating, preferably within the range 0.45mm to 0.55mm. According to another embodiment of the present invention, the length L of the interference fit between the male outer shaft of the connector and the female inner bore of the inner polymer liner is >30mm, and preferably >50mm. Preferably, the length L of the interference fit is in the range 70mm to 200mm, and preferably in the range 100mm and 150mm.

Examples of the present invention formed by the applicants within the above definitions and ranges have been found to achieve a waterproof connection which is easy to install, and which is also very reliable as large stress concentrations in the polymer liner are avoided. In the present invention, the surfaces which are compressed against each other to achieve the water tightness are both smooth and cylindrical, which leads to an even and progressive stress distribution in the polymer liner, thus significantly reducing the risk of crack initiation. This is clearly more reliable than using 'notches' or 'grooves' or 'threads'. The present invention also relies upon a suitable combination between the contact pressure and the contact surface which is obtained when the above interference fit conditions are fulfilled.

According to a preferred embodiment of the present invention, the contact pressure between the smooth male outer shaft of the connector and the smooth female inner bore of the inner polymer liner is higher than 75 bar, preferably comprised between 100 bar and 400 bar, more preferably comprised between 150 bar and 300 bar.

Optionally, the present invention further comprises adhesive between the male outer shaft of the connector and the female inner bore of the inner polymer liner. The adhesive may be applied either on the male outer shaft, or the female inner bore or both, by any method known in the art, including coating, spraying or painting, etc., to provide additional securement and/or sealing between the mating surfaces. Optionally, the ends of the male outer shafts of the connector are chamfered to assist their introduction to the female inner bores.

Optionally, the female inner bores of the inner polymer liners are wholly cylindrical. Optionally, the outer diameter of the central section of the connector is less than the inner diameter of the outer metal pipes, to form an annulus between the central section and the outer metal pipes. Any such annulus may have a depth of a few millimetres, such as in the range between 1.5mm and 3mm. Optionally, the annulus includes a thermal insulation such as a fire blanket, intended to be located below the ends of the PLP outer metal pipes where they are to be welded together, such that during welding, the heat does not affect the nature of the connectors.

The outer diameter of the cylindrical central section of the connector is optionally larger than the outer diameter Dl of the opposing cylindrical ends prior to mating. Optionally, the outer diameter of the cylindrical central section of the connector can be equal to the outer diameter Dl of both opposing cylindrical ends prior to mating.

Where the connector is formed from a corrosion resistant alloy, it is also preferable to avoid contact between the connector and the outer metal pipe to avoid corrosion of the carbon steel pipes in case of water ingress.

Optionally, the PLP assembly of the present invention claims further comprises a lock ring within the inner polymer liner of each PLP pipe section, prior to the end of the final portion of the inner polymer liner. A lock ring can be an integrity check of the PLP assembly, and may also assist prevention of any sliding of an inner polymer liner during deep water installation or SCR service. A lock ring may also ensure a central position of the connector with respect to the welding of the outer metal pipes together. This assists in ensuring the correct location of any thermal insulation in an annulus between the central section and the outer metal pipes as discussed hereinabove.

According to another embodiment of the present invention, the PLP assembly of the present invention further comprises one or more sealing rings or elastic seals such as 0- rings, D-rings or T-rings and the like, optionally internally reinforced, between the male outer shafts of the connector and the female inner bores of the inner polymer liner. Whilst the use of sealing rings are well known in the art in forming a contact between two surfaces in many situations, the use of sealing rings in the present invention is intended to further or otherwise enhance the interference fit already formed as part of the present invention.

In one embodiment of the present invention, the PLP assembly comprises at least one circumferential grove in the male outer shafts of the connector, optionally at least two circumferential grooves in each male outer shaft of the connector. Optionally, each such circumferential groove wholly or substantially houses a sealing ring, and the sealing ring can be compressed into the height of the groove to achieve the same height dimension as the remainder of the male outer shaft. Such sealing rings can be compressed by the surface of the female inner bores to achieve a compression ratio of <20%, optionally <30%.

In another embodiment of the present invention, the PLP assembly comprises at least one sealing ring or elastic seal between each male outer shaft and the central section of the connector, able to abut the end of each female inner bore of the inner polymer liners after mating. The abutment of the ends of each female inner bore may be against an edge or a shoulder of the central section, such that the elastic seal is located in, at or against a portion of the connector between the central section and the male outer shafts. This may include locating an elastic seal in a radial groove in a portion, such as a shoulder or edge, between the central section and each male outer shaft of the connector.

Optionally, the central section of the connector of the present invention includes one or more lateral pressure relief valves extending between the outer and inner surfaces of the connector. Optionally, the connector comprises 2 to 8 pressure relief valves in a circumferential pattern around the central section. Where the PLP assembly is in a pipeline used for hydrocarbon transport, the pressure relief valves can relieve any pressure built up behind the inner polymer liner due to the diffusion of gas therethrough as discussed hereinafter, from the outside surface of the connector to the inner surface, i.e. the bore of the connector. The valves can be designed to open at a differential pressure not higher than needed to collapse the inner polymer liner.

According to a second aspect of the present invention, there is provided a connector for a polymer lined pipe (PLP) assembly comprising two PLP pipe sections, each PLP pipe section comprising an outer metal pipe and a fitted inner polymer liner, each inner polymer liner terminating before one end of each outer metal pipe, and the final portion of each such end of each liner having a smooth female inner bore with an internal diameter D2,

wherein the connector has a cylindrical central section between opposing cylindrical ends, each connector end having a smooth male outer shaft with an external diameter of Dl wherein Dl > D2, and adapted to mate by interference fit with the female inner bores of the inner polymer liners of the PLP pipe sections to form a continuation of the liner along the PLP assembly.

The connector may include one or more of the embodiments described hereinabove in relation to the PLP assembly, including shape, size, dimensions, and optional further features including elastic seals and pressure check valves. According to a further aspect of the present invention, there is provided a method of connecting two plastic lined pipe (PLP) sections to form a PLP assembly, comprising at least the steps of:

providing two PLP pipe sections, each PLP pipe section comprising an outer metal pipe and a fitted inner polymer liner, each inner polymer liner terminating before one end of each outer metal pipe, and the final portion of each such end of each liner having a smooth female inner bore with an internal diameter D2;

providing a connector having a cylindrical central section between opposing cylindrical ends, each connector end having a smooth male outer shaft with an external diameter of Dl, wherein Dl > D2;

mating by interference fit the female inner bores of the inner polymer liners of the PLP pipe sections with the males outer shafts to form a continuation of the liner along the PLP assembly; and

welding the outer metal pipes of the PLP pipe sections together to form the PLP assembly.

Methods of manufacturing and connecting two PLP pipe sections are discussed in more detail hereinafter. According to another aspect of the present invention, there is provided a polymer lined pipeline comprising of a plurality of PLP pipe sections comprising an outer metal pipe and a fitted inner polymer liner, such as those defined herein, connected together by at least one PLP assembly as also defined herein. According to another aspect of the present invention, there is provided a polymer lined pipe (PLP) assembly comprising:

a PLP pipe section comprising an outer metal pipe having an end, a fitted inner polymer liner terminating a first distance before the said end of the outer metal pipe, the final portion of the end of the liner having a smooth female inner bore with an original internal diameter D2, and a clad ring, fitted at the end of the outer metal pipe and extending along the outer metal pipe to a second distance less than the first distance, and

a metallic cylindrical connector extending between the first distance and the second distance, and having at one end a smooth male outer cylindrical shaft with an original external diameter of Dl, wherein Dl > D2 prior to mating, and adapted to mate by interference fit with the female inner bore of the inner polymer liner of the PLP pipe section to form a continuation of the liner along the PLP assembly, and whose other end is able to form a seal with the clad ring. The other end of the metallic cylindrical connector could form a seal with the clad ring by welding.

Optionally the metallic cylindrical connector and the clad ring are made of the same material, such a corrosion resistant alloy (CRA).

According to another aspect of the present invention, there is provided a metallic cylindrical connector for a PLP pipe section comprising an outer metal pipe having an end, a fitted inner polymer liner terminating a first distance before the said end of the outer metal pipe, the final portion of the end of the liner having a smooth female inner bore with an original internal diameter D2, and a clad ring fitted at the end of the outer metal pipe and extending along the outer metal pipe to a second distance less than the first distance,

wherein the metallic cylindrical connector is adapted to extend between the first distance and the second distance, and has at one end a smooth male outer cylindrical shaft with an original external diameter of Dl, wherein Dl > D2 prior to mating, and is further adapted to mate by interference fit with the female inner bore of the inner polymer liner of the PLP pipe section to form a continuation of the liner along the PLP assembly, and whose other end is able to form a seal with the clad ring.

Embodiments of the present invention will now be described by way of example only, and with reference to the accompanying drawings in which:

Figure 1 is a perspective view of a longitudinal cross sectional view of a PLP pipe section and a connector for use in the present invention:

Figure 2 is a longitudinal cross sectional view of the connector shown in Figure 1;

Figure 3 is a longitudinal cross section of the PLP pipe section shown in Figure 1;

Figure 4 is a longitudinal cross section of the PLP pipe section and connector shown in Figures 2 and 3;

Figure 5 is a longitudinal cross section of a PLP assembly according to one embodiment of the present invention, based on the addition of further PLP pipe section shown in Figure 3 to Figure 4;

Figure 6 is a longitudinal cross sectional view of a variation of the connector shown in Figure 2; Figure 7 is a longitudinal cross sectional view of the connector in Figure 6 with the PLP section shown in Figure 3;

Figure 8 is a variation of Figure 7;

Figure 8a is an enlarged portion of Figure 8;

Figure 9 is an enlarged view of a portion of a variation of Figure 7;

Figure 10 is a perspective half view of another connector for use with the present invention;

Figures 11-0 to 11-6 are schematic steps of a method of forming a PLP assembly according to another embodiment of the present invention;

Figure 12 is a perspective view of an insertion tool useable with the present invention; Figure 13 is a variation of Figure 9;

Figure 14 is a longitudinal cross sectional view of a variation of the connectors shown in Figures 2 and 6;

Figure 15 is a perspective view of a transverse cross sectional view of a variation of a seal ring useable for the connector shown in Figures 6 and 7;

Figure 16 is a perspective view of a transverse cross sectional view of another variation of a seal ring useable for the connector shown in Figures 6 and 7; and

Figure 17 is a longitudinal cross sectional view of an end termination of a PLP pipe section.

Figure 1 is a perspective longitudinal cross sectional view of a polymer lined pipe (PLP) section 2 discussed hereinafter in more detail, a connector 4 discussed hereinafter in more detail, and a lock ring 6 discussed in more detail hereinafter. Figure 2 shows the connector 4 having a cylindrical central section 10 between opposing cylindrical ends 12, each connector end 12 having a smooth male outer shaft 14 with an external diameter of Dl. The ends of the male outer shafts 14 are chamfered 16. Between the male outer shafts 14 and the central section 10 are shoulders 18. Figure 3 shows one end of a PLP pipe section 2, having an outer metal pipe 20, optionally formed of carbon steel in a manner known in the art, and optionally having a length starting from several metres, such as 12m or 24m, up to 1km or longer. Within the outer metal pipe 20 is a fitted inner liner polymer 22. Inner polymer liners for use in PLP pipe sections are known in the art, and are generally formed from a polymer or plastic material intended to be fitted within the outer metal pipe 20 and be wholly or substantially the same length.

In the present invention, the inner polymer liner 22 terminates before the end 24 of the outer metal pipe 20. This can easily be achieved by inserting a preformer polymer liner into the outer metal pipe 20, and cutting the end of the preformer using a suitable tool within the outer metal pipe 20 to a desired 'cut back' length having an end 25.

A final portion 26 of the end 25 of inner polymer liner 22 is formed with a smooth female inner bore 28 with an internal diameter D2, providing a shoulder 29 with the remainder of the inner polymer liner 22. Figure 4 shows the combination of the connector 4 shown in Figure 2 and the PLP pipe section 20 shown in Figure 3, wherein the smooth male outer shaft 14 mates by interference fit with the female inner bore 28 to form a continuation of the liner therein between. Figure 4 also shows a length L of the interference fit between the male outer shaft 14 and the female inner bore 28, L being greater than 50mm.

Figure 4 also shows the end 25 of the inner polymer sleeve 22 abutting the shoulder 18 of the connector 4 between the central section 10 and the cylindrical ends 12.

Because Dl is greater than D2 before mating, force is required to mate the male outer shafts 14 and the female inner bore 28 and create the interference fit thereinbetween. Where the force is provided directly against either the connector or the PLP pipe section, this may be press-interference fit. The force required clearly must be sufficient to achieve the interference fit, without being too great to be detrimental to the nature of the connector, or the inner polymer liner, or both. As such, the applicants have conducted a number of trials to establish that the best mode of invention is an interference fit length L being in the range 70mm to 200mm and preferably in the range of 100mm and 150mm. Similarly, such trials have established that the best mode of the invention is achieved by the calculation of (Dl - D2) /2 before mating being in the range of 0.2mm - 1mm, optionally in the range 0.3mm to 0.7mm, and preferably being in the range of 0.45mm to 0.55mm, such as 0.5mm. According to a preferred embodiment of the present invention the temperature of the end portions of the inner polymer liner and/or of the connector is controlled and maintained in a suitable range before and during mating. The range is above a low temperature, as the polymer material of the inner polymer liner could become brittle and damaged during mating and below a high temperature where the mechanical properties of the polymer material of the inner polymer liner could be reduced, which would then reduce the contact pressure between the connector and the liner. Before and during mating, the temperature of the final portion of the end of the liner and the temperature of the connector are preferably maintained in the range 5° Celsius to 35° Celsius, more preferably in the range 10° Celsius to 30° Celsius, more preferably in the range 15° Celsius to 25° Celsius. Well known heating and/or cooling devices are used to achieve this temperature control.

The arrangement shown in Figure 4 has the advantages of being easy to implement with a simple press, and reducing the risk of damaging the inner polymer liner, when compared to conventional arrangements using notched or toothed surfaces instead of smooth surfaces.

Figure 4 also shows the lock ring 6 within the inner polymer liner 22 of the PLP pipe section 2 prior to the final portion 26. The lock ring 6 constrains the inner polymer liner 22 axially, and this can provide an integrity check of the connector-liner connection, helps prevents the inner polymer liner 22 sliding during deep water installation or SCR service, and ensures a central position of the connector 4 with respect to the outer metal pipe 20 when being welded, as discussed hereinafter. Figure 5 shows an embodiment of the present invention, being a polymer lined pipe (PLP) assembly comprising two PLP pipe sections 2, each PLP pipe section 2 comprising an outer metal pipe 20 and a fitted inner polymer liner 22, each inner polymer liner 22 terminating before the end 24 of each outer metal pipe 20, and the final portion 26 of each end of each inner polymer liner 28 having a smooth female inner bore 28 with an original internal diameter D2; and a connector 4 having a cylindrical central section 10 between opposing cylindrical ends 12, each cylindrical end 12 having a smooth male outer shaft 14 with an original external diameter Dl and adapted to mate by interference fit with the female inner bores 28 of the inner polymer liners 22 of the PLP pipe sections 2 to form a continuation of the liner along the PLP assembly. The interference fit has a length L, and the outer metal pipes 20 of each PLP pipe section 2 are joined by a girth weld 34 to form the complete PLP assembly.

Figure 5 also shows the formation of an annulus 32 between the top surface of the central section 10 of the connector 4 and the inner surface of the outer metal pipes 20. This annulus 32 is also formed by the ends 25 of the final portions 26 of the inner polymer liners 22. Optionally, thermal insulation (not shown) is located in the annulus 32 to protect the connector 4 from excessive heat generated during the forming of the weld 34 between the outer metal pipes 20.

Figure 6 shows a variant of the connector shown in Figure 2, wherein two circumferential grooves 36 are located along each end 12 of the connector 4a in the surface of the male outer shafts 14. Within the grooves 36 are located sealing rings 38, optionally O-rings, i.e. i.e. an elastomeric toric joint of substantially circular transverse cross section, such that a portion of the sealing rings 38 extends beyond the surface of the male outer shafts 14.

O-rings can be made of any suitable elastic material, such as ethylene propylene or AFLAS, or fluoroelastomer, or copolymer nitrile or indeed other elastomers with similar thermo-mechanical characteristics, including high resistance to heat, tear, abrasion and high tensile strength. Suitable O-rings may have a diameter between 1-lOmm, with a nominal compression range between 4% and 40%. Preferably, the majority of the O- ring is accommodated within the grooves 36 to avoid displacing O-rings during the mating step, and in order to achieve a compression ratio of approximately 7-20%.

Figures 15 and 16 show in enlarged cross-sectional form two possible variants of toric sealing rings that can be used instead of O-rings. Figure 15 shows a first variant sealing ring 102 comprising an elastomeric body 100, preferably made of a high modulus elastomer, reinforced by one anti-extrusion Inconel spring 104 which is embedded in the elastomeric body 100. The transverse cross section of this sealing ring 102 is similar to a letter 'D'. Such sealing rings 102 are called D-rings and are commercialized by PPE (Precision Polymer Engineering). Figure 16 shows a second variant sealing ring 106 comprising an elastomeric body 108, preferably made of a high modulus elastomer, reinforced by two anti-extrusion Inconel springs 110 which are embedded in the elastomeric body 108. The transverse cross section of this sealing ring 106 is similar to an inverted letter ', both springs 110 being located symmetrically in both branches of the ', the summit of the inverted 'T' having further a smooth rounded shape. Such sealing rings 106 are called spring seals and are also commercialized by PPE (Precision Polymer Engineering).

Figure 7 shows the interference fit of the connector 4a shown in Figure 6 with a PLP pipe section 2 shown in Figure 3, mated in a similar manner to that shown in Figure 4, and further comprising the compression of the sealing rings 38 during the mating of the male outer shaft 14 with the female inner bore 28 of the PLP pipe section 2.

The interference fit between the connector 4a and PLP pipe section 2 shown in Figure 7 is still substantially achieved by the interference fit between the male outer shaft 14 and female inner bore 28, with the O-ring compression being in addition to the strength of the interference fit, and assisting resistance to water penetration along the seal between the male outer shaft 14 and the female inner bore 28 as a further or additional safety measure, especially in a failure of the interference fit between the male outer shaft 14 and female inner bore 28.

Figure 8 shows a further variation of the arrangement shown in Figure 4 between the connector 4 and a PLP pipe section 2. Figure 8, and an enlarged portion thereof shown in Figure 8a, shows the location of an elastic seal such as an O-ring 40 located in a corner of the connector 4 between the shoulder 18 and the beginning of the male outer shaft 14 on each connector end 12 of the connector 4. Figures 8 and 8a also show the end of the final portion 26 of the inner polymer liner 22 being chamfered in a complementary position 42 when PLP pipe section 2 is mated with connector 4. As shown in Figure 8a the mating of the PLP pipe section 2 and connector 4 presses the O- ring 40 between the corner and the chamfered edge 42 to assist sealing thereinbetween, and to assist any fluid leakage or ingress through the seal between PLP pipe section 2 and the connector 4.

Figure 9 shows a further variant of mating between a PLP pipe section 2 and a connector 4b, wherein the connector 4b has a circular groove 44 located in the shoulder 18 between the central section 10 and the connector ends 12 of the connector 4b, and an elastic seal such as an O-ring 46 housed within the groove 44. Figure 9 shows the compression of O-ring 46 between the end of the final section 26 of the inner polymer liner 22 and the shoulder 18 of the connector 4b. In this way, the sealing integrity formed by the mating of the PLP pipe section 2 and the connector 4b can be maintained during reeling of a PLP pipeline formed with the PLP assembly, as the O-ring 46 is only compressed in an axial direction.

Figure 13 shows a further variant of the connector 4b shown in Figure 9, wherein the connector 4c instead has an elastic V-ring 47 located against the shoulder 18 between the central section 10 and the connector ends 12 of the connector. This V-ring 47 seats on a circumferential seal ring support 48 upstanding from the connector end 12 close to the shoulder 18. In this way, the sealing integrity formed by the mating of the PLP pipe section (not shown in Fig 13) and the connector 4c can be maintained during reeling of a PLP pipeline formed with the PLP assembly, as the V-ring 47 is again only compressed in an axial direction. Figure 13 also shows two circumferential grooves 36 as shown in Figures 6 and 7 along each end 12 of the connector 4c in the surface of the male outer shafts 14. The present invention allows for the use of any one of, or any combination of, the arrangements for sealing rings or elastic seals as shown by the examples in the accompanying drawings, and as described herein. The skilled man can see that combinations of the arrangements shown in Figures 6-9, 13-16 can further assist sealing integrity between the PLP pipe sections and the connector, and the skilled man will select the most appropriate arrangement to suit the location and use of the PLP assembly and PLP pipeline.

Figure 10 shows a perspective cross sectional view of another connector 50 for use with the present invention, the connector having a cylindrical central section 52 between opposing cylindrical ends 54. The connector 50 can be used in the present invention to connect PLP pipe sections in the same manner as the connectors 4, 4a, and 4b described and shown herein. The connector 50 includes a series of pressure relief valves 60 arranged in an equidistant circumferential pattern around the middle of the central section 52. It is known that some materials used for inner polymer liners are not completely impermeable to some gases carried by fluids being transported by the PLP pipeline, for example carbon dioxide, methane and hydrogen sulphide carried in hydrocarbon fluids. As such there can be defusing of gases from the hydrocarbon fluid through the liner over time. Such defused gases tend to collect in suitable spaces or open locations, one being any annulus 32 created in the connection of PLP pipe sections.

The pressure relief valves 60 shown in Figure 10 are designed to be able to open at a differential pressure lower than the pressure that could collapse the inner polymer liners, thereby avoiding catastrophic failure of the PLP pipeline. Such differential pressure could be approximately 5 bar, and be for example the difference between gas pressure in the annulus 32 shown in Figure 5, and the bore of the pipeline (reference 33 and Figure 5). In this way, gases that have slowly defused from the bore 33 to the annulus 32 through the inner polymer liner 22 can be released when the pressure relief valves 60 are open, to depressurise the annulus 32. The number of pressure relief valves 60 on each connector 50 may comprise between 2 to 8, and have a diameter up to 10 mm. Figures 11-0 to 11-6 are a series schematic cross sectional drawings showing a method of connecting two polymer lined (PLP) sections to form a PLP assembly according to an embodiment of the present invention.

Figure 11-0 shows providing two PLP pipe sections 62, each PLP pipe section 62 comprising an outer metal pipe 64. As known in the art, it is possible using swagelining to insert an inner polymer liner within an outer metal pipe 64 by first pulling it through a die in order for it to be inserted within the outer metal pipe 64. When the 'pull-in' is completed, the traction used to stretch the inner polymer liner is released, so that the inner polymer liner 'swells' and comes into direct contact with the inner surface of the outer metal pipe 64 in order to form a fitted inner polymer liner 66.

In Figure 11-1 the ends of the inner polymer liner 66 are cut back such that they terminate before the ends of each outer metal pipe 62. A final portion 68 of the polymer inners 66 is then machined to achieve a smooth female bore 70 having an internal diameter D2, which is greater than the internal diameter of the remainder of the inner polymer liner 66. Machining the final portion 68 of the inner polymer liner 66 can be carried out using apparatus or machinery known in the art. One example includes means to attach itself to the PLP pipe section 62, followed by centering means that enables the longitudinal axis of the machine to be adjusted so as to brought into coincidence with the longitudinal axis of the PLP pipe section 62, thus making it possible to machine the inside of the inner polymer liner 66 at its ends in a manner that is accurately concentric with the PLP pipe section 62. Such machining makes it possible to achieve a smooth female inner bore 70 having the required cylindrical geometry for the present invention, with an accurate internal diameter D2 so that the intended interference fit can be achieved. An example of such machinery is shown in US7722085.

Figure 11-2 shows the location of a lock ring 72 within the inner polymer liner 66 of each PLP pipe section 62, prior to the final portion 68 of each polymer liner 66. The lock rings 72 can be push fitted into place. The lock rings 72 may alternatively be located within the inner polymer liner 66 prior to machining of the final portions 68 as shown in Figure 11-1. Machinery suitable for locating the lock rings 72 is also well known in the art, such as described in US5992897. An example of such an insertion tool is shown in Figure 12, and includes an outer metal pipe clamp 74, a guide locator 76, and a plunger 78 moveable by a hydraulic jack 80 to press items located within the firing line of the insertion tool and along as longitudinal axis of the PLP pipe section 72.

In Figure 11-3, a connector 82 is provided, having a cylindrical central section 84 and opposing cylindrical ends 86, each such end having a smooth male outer shaft with an external diameter of Dl, wherein D1>D2 prior to mating with the PLP pipe sections 62. The connector 82 may be pressed into the first PLP pipe section 62 by means of the insertion tool shown in Figure 12, to achieve mating by interference fit of the female inner bores 70 of the inner polymer liners 66 with the smooth male outer shafts of the connector 82, such that there is an equalising of Dl and D2 either by reduction of Dl or expansion of D2, or both.

Figure 11-4 shows the pressing of the other PLP pipe section 62 against the other end 86 of the connector 82 to form a continuation of the liner along the now formed PLP assembly 88. It is noted that the continuation of the liner between the PLP pipe sections 62 shown in Figure 11-4 may not be with a continuous same diameter.

Figure 11-5 shows the conduction of a leak test, by the location of a sealing clamp 90 placed over the junction between the outer metal pipes 64, and the application of a suitable fluid such as air from an air pump (not shown) against the outer metal pipe 64 junction, to determine if there is any drop in pressure (indicating that the sealing created between connector 82 and the PLP pipe sections 62 is leaking).

Assuming the leak test is successful in Figure 11-5, Figure 11-6 shows the application of a girth weld 92 between the outer metal pipes 64 in a manner known in the art, in order to complete the formation of the PLP assembly ready for use. The girth weld 92 can be applied either prior to or during laying of a PLP pipeline using the PLP assembly 88 shown in Figure 11-4.

As mentioned hereinbefore, the lengths of each of the PLP pipe section 62 shown in Figures 11-0 to 11-6 are not limited by the present invention, so that the present invention is able to form a PLP pipeline comprising of plurality of PLP pipe sections 62 connected together by at least one PLP assembly 88 to any suitable length. Optionally, all the PLP pipe sections of such a PLP pipeline are connected together by PLP assemblies as shown and described herein. Figure 14 shows a variant of the connector shown in Figure 2, wherein the central cylindrical section 10 of the connector 4d comprising a wide cylindrical groove 56 on its outside surface. The groove 56 can be located at equal distance to both ends of the connector 4d. The width of the groove 56 is preferably larger than 50mm, more preferably comprised between 100mm and 180mm. Preferably, a thermal protection 57 such as a fire blanket or a thermal insulation layer is located inside the groove 57, so that during welding of the outer metal pipe, the heat does not damage the connector 4d.

Figure 17 shows an end termination of another possible PLP section 112. The PLP section 112 comprises an outer metal pipe 120 and a fitted inner polymer liner 122 terminating a first distance before the end 125 of the outer metal pipe 120. The end of the inner polymer liner 122 has a smooth female inner bore 128 with an original internal diameter D2. The end of the outer metal pipe 120 has been internally cladded with a Corrosion Resistant Alloy (CRA) cladding 132, along a length of around 100mm to 400mm, usually being shorter than the first distance. The termination comprises a tubular end connector 130 having a first cylindrical end 112 and a second opposite thick tubular end 124. The end connector 130 is metallic and made with a Corrosion Resistant Alloy. The cylindrical end 112 has a smooth male outer shaft 114 with an original external diameter Dl, and is adapted to mate by interference fit with the female inner bore 128 of the inner polymer liner 122 of the PLP pipe section 112 to form a seal proof connection between the inner polymer liner 122 and the end connector 130. The opposite end of the end connector 130, i.e. the opposite end of the thick tubular end 124 is welded to the CRA cladding 132 on the inner surface of the outer metal pipe 120. This weld 134 forms a seal proof connection between the end connector 130 and the outer metal pipe 120. The present invention provides a method of forming a PLP assembly which is easy to implement using a series of simple press operations, thereby reducing the risk of damaging the inner polymer liners when compared to conventional arrangements using notched surfaces between liners to form a connection and continuation of the liner there across. This avoids creating easy locations for large stress concentrations to occur where liner material has been deliberately machined to form sharp turns or edges such as teeth or notches, significantly reducing the risk of generating cracks in the seals of the connection, leading to failure and leaks in such liner arrangements. The present invention provides an arrangement based on smooth cylindrical surfaces being made and brought together by an interference fit which is sufficiently calculated to still achieve the required sealing integrity, whilst avoiding excessive forces during press fitting that could affect the integrity of the inner polymer liners.