WO2018063090A1

WO2018063090A1 - Corrugated liner for mechanically lined pipe installable by the reel-laying method

Info

Publication number: WO2018063090A1
Application number: PCT/SG2016/050487
Authority: WO
Inventors: Paul Anthony Montague
Original assignee: Cladtek Holdings Pte. Ltd.
Priority date: 2016-09-30
Filing date: 2016-09-30
Publication date: 2018-04-05

Abstract

A method of manufacturing a mechanically lined pipe (MLP) comprising at least the steps of: (a) forming the carbon steel outer pipe as a single length; (b) forming the inner liner which is a Corrosion Resistant Alloy (CRA) liner with the same length as the outer pipe; (c) inserting the CRA liner into the outer pipe; (d) forming corrugations inside the CRA liner; (e) expanding each end of the CRA liner to come into contact with the outer pipe and seal-welding each expanded end to the outer pipe; (f) evacuating and sealing by resistance or laser welding the annulus between the outer pipe and the CRA liner; (g) finishing the ends of the MLP in overlay welding, left as welded or machined condition; and (h) externally coating the MLP. The present invention also provides for a method of manufacturing a mechanically lined pipeline (MLPL) by conjoining a plurality of MLPs manufactured in accordance with the method of manufacturing an MLP described above.

Description

CORRUGATED LINER FOR MECHANICALLY LINED PIPE

INSTALLABLE BY THE REEL-LAYING METHOD

TECHNICAL FIELD

[0001 ] This invention relates to the production of piping materials for use in the offshore production of oil and gas, specifically pipes that can be installed by the reel-laying method. In particular, it relates to methods of manufacturing a mechanically lined pipe (MLP) having an outer pipe and an inner corrugated liner that is installable by the reel-laying method.

BACKGROUND OF THE INVENTION

[0002] As it is known in the art, bi-metallic pipes may be used to form corrosion-resistant pipelines for submarine transportation of corrosive fluids, such as gas or crude oil. A bi-metallic pipe is generally composed of two layers. The outer layer, usually made from a carbon steel metal, provides general strength to the design so as to resist external hydrostatic pressure as well as protection against buckling on the reel or seabed. On the other hand, the internal layer, also called a "liner", protects the outer layer from damage caused by the corrosive fluids being transported. A corrosion resistant alloy (CRA) is therefore commonly chosen as the material for the liner. In this way, the bi-metallic pipes are able to combine the strength of the low-cost carbon steel and the corrosion-resistance of the liner.

[0003] One form of bi-metallic pipe as described above is a single "clad" pipe, whereby an internal CRA layer is metallurgically bonded to the carbon steel outer layer.

[0004] A second form of said bi-metallic pipe may be termed a mechanically lined pipe (MLP). According to the definition provided in the Det Norske Veritas (DNV) Offshore Standard DNV-OS-F101 Appendix A (Standard DNV-OS-F101 ), a lined pipe is a "pipe with internal (corrosion resistant) liner where the bond between (linepipe) backing steel and liner material is mechanical". The MLP may be manufactured using mechanical expansion, whereby a loosely fitting liner is plastically expanded from the inside, so as to elastically or plastically deform the outer carbon steel pipe, thereby tightly fitting the inner liner within the pipeline when the expanding internal pressure is removed and an interference stress, or "mechanical bond", between the liner and the host pipe remains. It is this "mechanical bond" that gives MLP its reference name.

[0005] There are two common methods of laying submarine pipelines, namely, the "stove piping" method and the "reel-laying" method. In the "stove piping" method, a pipeline is produced by assembling and welding together individual lengths of pipe, called "pipe stalks", on a pipe-laying vessel.

[0006] In the reel-laying method, the pipeline is assembled onshore and spooled onto a large reel (sometimes called the "drum"), typically around 12 to 24 meters (40 ft to 80 ft) by 6 meters (20 ft) in size and mounted on board a purpose- built vessel. The vessel then goes out to the designated location to lay the pipeline. Once offshore, the pipeline is spooled of the reel, straightened and/or aligned before being laid onto the seabed (James G. Speight, Handbook of Offshore Oil and Gas Operations (Elsevier Inc., 2015) at p.199).

[0007] The "reel-laying" method has several inherent advantages. Firstly, it is faster and more economical than the "stove piping" method as no welding is required during the offshore operation. Secondly, as the facilities to assemble the pipeline are located onshore, they are not affected by the weather or the sea state and are less expensive than seaborne operations. Thirdly, the process of pipeline supply can be coordinated: as one pipeline is being laid offshore, another one can be spooled onshore. Lastly, a single reel can have enough capacity for a full length flow line (Speight at p. 199).

[0008] That said, the "reel-laying" method is not without problems. Firstly, it can only handle lower diameter pipelines up to approximately 500 mm (20 in). Secondly, the kind of steel used for making the pipes must be capable of undergoing the required amount of plastic deformation as the pipes are bent to the proper curvature when reeled around the drum, and straightened back (by a straightener) during the layout operations at the installation site. Further, when winding the pipeline onto a drum or during reeling out from the drum, the inner liner may become deformed by buckling or wrinkling. Such deformation causes the inner liner to loosen from the carbon steel outer pipe's interior surface. Further, the deformation may cause other problems as well. For example, it may hinder the flow of the liquids, reduce the pipe's resistance to corrosion, obstruct the inspection and cleaning of the pipe, and may result in failure of the MLP pipeline altogether. This wrinkling effect is currently regarded as detrimental to an MLP. Accordingly, all methods for reeling an MLP that have been developed to date have been aimed at avoiding the wrinkling effect experienced during the reeling and/or unreeling process, at all costs. Nevertheless, such flaws are typically inevitable and as such, the goal is to reduce such flaws accordingly, and these are measured in the industry. In this regard, flaw acceptance criteria can be derived directly from the workmanship-based acceptance standard or alternative weld acceptance standards based on engineering critical acceptance which can be developed in accordance with the procedures and requirements also included in pipeline codes and standards, such as Standard DNV- OS-F101 Appendix A (R. Winston Revie, Oil and Gas Pipelines: Integrity and Safety Handbook (John Wiley & Sons, Inc., 2015) at p.237).

[0009] The current state of the art requires that a pressure be maintained inside the pipeline during each step of the reeling and/or unreeling process to avoid the formation of wrinkles. For example, US Patent No. 8,226,327 discloses a method for laying a pipeline having an inner corrosion proof metallic cladding closely fitted with metallic contact to an outer pipe material. According to this method, a section of the pipeline is reeled onto a pipe laying drum while an overpressure is maintained within the section by a pressurised fluid inside the section. When the pipeline is motionless, a further section of the pipeline is joined to the section already reeled, and the overpressure is relieved. A new overpressure is then applied within the sections, and the further section is then reeled onto the pipe laying drum. These steps are repeated until the predetermined pipeline length is achieved.

[0010] While this method may help to avoid the formation of wrinkles and thus ensure that the inner liner remains in close metallic contact with the outer pipe as the pipeline is reeled onto the pipe laying drum, it requires the overpressure to be applied and relieved each and every time two pipe sections are joined. This creates significant safety concerns especially where many pipe sections are joined and reeled onto the pipe laying drum, as envisaged by the patent itself.

[001 1 ] US Patent No. 8,876,433 provides another method of reeling and unreeling an internally clad metal pipeline that involves the use of pressure to minimise or avoid wrinkling. According to this method, an initial length of pipeline is assembled by welding together a succession of pipe stalks. The pipeline is then internally pressurised by deploying a single sealing pig within the pipeline near to the trailing end of the pipeline and filling the pipeline with fluid to a required absolute pressure, before being advanced up the ramp over the chute and onto the reel. After winding onto the storage reel, the pipeline is depressurised, the sealing pig unlocked, another length joined to the trailing end, pressurised with fluid, and further advanced onto the reel. This process is repeated until the reel is full and the vessel can then be transferred to the worksite. At this point, the pipe is re-pressurised and then laid onto the seabed in a conventional manner.

[0012] While this method may help to miniminise or avoid the wrinkling effect, the repetition of the pumping, locking, pressurising and winding operations as pipeline sections are joined to the trailing end until the desired pipe length is achieved, means that a significant volume of pressurised fluids are applied and maintained, which increases the risk of serious injury in case of pipeline failure during the operations.

[0013] An alternative method is to make the liner thicker to make it sufficiently resistant to wrinkling and separating from the outer pipe. For example, US Patent No. 8,864,417 discloses a method of reel-laying a MLP comprising the steps of spooling the MLP onto a reel in the complete or substantial absence of internal pressure above ambient pressure in the MLP, where the MLP has a defined liner thickness, to thereby provide an unspooled MLP having wrinkles with maximum height of 4mm, which can then be removed during the subsequent straightening procedure and/or pre-commissioning. In particular, the maximum height of 4mm of the wrinkles is achieved by increasing the wall thickness of the inner liner of the MLP over a minimum liner wall thickness. However, increasing the wall thickness of the liner will result in a more expensive pipe.

SUMMARY OF THE INVENTION

[0014] The current invention therefore seeks to provide methods of manufacturing a mechanically lined pipe (MLP) that is installable by the reel-laying method without requiring the MLP to have a thicker liner or internal pressure to be applied, such that the laid MLP has residual wrinkles that are within an acceptable range of height resulting from the installation process in accordance with Standard DNV-OS-F101 , whereby raised indications are up to 0.5mm. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 is a cross-sectional view of the mechanically lined pipe (MLP) before and after installation of the MLP by the conventional reel-laying method. It shows an MLP having an outer pipe and a corrugated CRA liner.

[0016] Figure 2 is a cross-sectional view of the MLP after the field hydrostatic test is carried out and the liner corrugations are removed.

DETAILED DESCRIPTION OF THE INVENTION

[0017] References will now be made in detail to the description of the present invention, an example of which is shown in figures. The example is provided to explain the invention and not a limitation. Accordingly, changes and modifications obvious to one skilled in the art to which the invention pertains are deemed to be within the spirit, scope and contemplation of the invention.

[0018] According to one aspect of the present invention, there is provided a method of manufacturing a MLP comprising at least the steps of: (a) forming the carbon steel outer pipe as a single length; (b) forming an inner liner having the same length as the outer pipe; (c) inserting the liner into the outer pipe; (d) forming corrugations inside the liner; (e) expanding the ends of the liner to come into contact with the outer pipe and seal welding each expanded end to the outer pipe; and (f) evacuating and sealing by resistance or laser welding the annulus between the outer pipe and the CRA liner.

[0019] According to this aspect of the invention, the MLP comprises a carbon steel host outer pipe and a Corrosion Resistant Alloy (CRA) liner. The outer pipe for the MLP can be formed as a single length using one of the processes known in the art, including, but not limited to, continuous forming as a single piece or by welding a plurality of outer pipe stalks together. Similarly, the forming of a CRA liner being of the same length as the outer pipe could be provided by a number of processes known in the art, including, but not limited to, continuous forming.

[0020] The methods of manufacturing MLPs described herein are not limited to the size, shape, design, physical and/or chamical properties of the outer pipe and/or the inner liner. However, according to one embodiment of the present invention, the CRA liner may be made from alloy 316L, 825, 625 or other alloys as defined in the American Petroleum Institute Specification for CRA Clad or Lined Steel Pipe Fourth Edition (March 2015) (API Specification 5LD).

[0021 ] As it is known in the art, the API Specification 5LD covers seamless and welded clad steel line pipe and lined steel line pipe with enhanced corrosion- resistant properties suitable for use in pipeline transportation systems in the petroleum and natural gas industries. The clad and lined steel line pipe specified in the API Specification 5LD comprises a carbon steel backing or base material outside, and a CRA liner inside of the pipe. The API Specification 5LD thus provides guidance on industry standard for the manufacture of lined steel pipe, and is accordingly referred to for the purposes of the present invention.

[0022] According to a preferred embodiment of the present invention, the outer pipe has an outside diameter (101 ) of 1 14mm to 610mm and a wall thickness (102) of 7mm to 60mm. On the other hand, the wall thickness (103) of the liner is preferably from 2mm to 6mm, and its average outside (104) diameter is to be determined.

[0023] The CRA liner is then inserted into the carbon steel outer pipe using one or more simple mechanical operations known in the art. Corrugations (105) are then formed inside the liner by one of the processes known in the art, for example by using a hydroforming tool or by rolling, to form a corrugated liner.

[0024] A "corrugated liner" refers to a liner having the ridges or grooves of a corrugated surface. Commonly, the corrugations (105) will go alternately from peak (108) to valley (109), and so on, and this term is known in the art. The corrugations (105) shall be formed circumferentially inside the liner. Further, it is essential that the corrugations are of uniform size, as this will ensure that the corrugations are completely flattened and the liner becomes smooth after straightening and pressurising of the MLP. An optimal ratio of the side corrugation radius (1 12) to the center corrugation radius (1 1 1 ) is about 2:1 , and reducing the respective radiuses (while maintaining the radius ratio) will result in better performance. The pitch (106), amplitude (107) and frequency of the corrugations will be determined by the strain to be resisted, the final expansion parameters and the properties of the CRA liner material used, using the finite element analysis. [0025] As it is known in the art, the finite element analysis (FEA) is a computer-based method of stimulating and/or analysing the behaviour of engineering structures and components under a variety of conditions. The FEA helps to predict how a product reacts to real world forces, vibration, heat, fluid flow, as well as other physical effects, thus showing whether the product will break, wear out, or work the way it was designed. Using the strain to be resisted, the final expansion parameters and the properties of the CRA liners as loads, boundary conditions and material properties respectively for the FEA, the optimal pitch (106), amplitude (107) and frequency of the corrugations may be determined.

[0026] Care should be taken in ensuring that the gap (1 13) between two consecutive corrugations is minimal. Larger gaps may be beneficial, as it may help to reduce liner deformation during the expansion process, given that the corrugated surface before expansion is smaller. However, smaller gaps allow for earlier contact of the liner and the host pipe, which in turn increases the buckling force and reduces risks of liner collapse. In a preferred embodiment of the present invention, the gap (1 13) between two consecutive corrugations is set at 1 mm.

[0027] In another embodiment of the present invention, the corrugations may be formed inside the CRA liner first, before the CRA liner is inserted into the carbon steel outer pipe using one or more simple mechanical operations known in the art.

[0028] Each end of the corrugated CRA liner will then be expanded sufficiently to be in contact with the carbon steel outer pipe and within the range of 20 to 100mm at each end. Each end may be expanded using one of the several methods currently known in the art, for example by mechanical expansion or hydraulic expansion.

[0029] Each end of the CRA liner is then seal-welded to the outer pipe for the same length of expansion. The annulus between the liner pipe and the outer pipe would be evacuated and sealed by resistance or laser welding. These are the two commonly used welding processes. In resistance welding, the welding energy is provided by two welding electrodes which in turn provide welding force and also provide a conduit for welding current. The heat that is produced by resistance to flow of current through the part stack produces welding heat. On the other hand, laser welding involves the supply of energy to the weld location in the form of a laser beam that interacts with the materials and produces heat. [0030] Regardless of which of the above welding processes is used, the evacuating and seal-welding of the annulus between the inner liner and the outer pipe is vital, as it is the vacuum between the corrugations and the carbon steel outer pipe that allows the corrugations to be pushed up and flattened during the pressurising step, thus smoothening the CRA liner.

[0031 ] The ends (1 10) of the MLP can then be finished in overlay welding left as welded or machined condition. Weld overlay, also known as cladding, weld cladding or weld overlay cladding, refers to the process of applying a corrosive resistant layer onto the parent material, thus providing a surface protection which allows the MLP to provide strength requirements to meet the desired standards and operate in a cost-effective manner. The commonly used methods for weld overlay include shielded metal arc welding, C0₂ welding, Metal Inert Gas welding/Tungsten Inert Gas welding, Submerged arc welding, and Plasma Transferred Arc welding. The length of the overlay weld can be from 3mm to 250mm. The liner may be resistance-welded to the outer pipe at each end for up to 100mm.

[0032] The so-formed MLP may be left in the bare condition, ready for spooling. Alternatively, according to one embodiment of the present invention, the method further comprises the step of externally coating the MLP with one or more coatings known in the art.

[0033] The method of manufacturing according to the present invention provides an MLP of a substantial length, from the lengths of 4 to 25m long, such that the present invention extends to an MLP whenever formed according to a method as described hereinabove. In the preferred embodiment, the MLP is produced to be in the length of 12.2m.

[0034] The MLP is preferably manufactured at a spoolbase, generally where the MLP is spooled onto a reel. Generally this is onshore or at an onshore location, preferably to form an MLP which is more than 500m long, and is ready for laying, preferably as part of a longer (which may be up to several kilometers) pipeline.

[0035] According to another aspect of the present invention, there is provided a method of manufacturing a mechanically lined pipeline (MLPL) comprising conjoining a plurality of consecutive mechanically lined pipes (MLPs), the method comprising at least the steps of: (a) forming the carbon steel outer pipe of an MLP as a single length; (b) forming an inner liner of an MLP having the same length as the outer pipe; (c) inserting the liner into the outer pipe; (d) forming corrugations inside the inner liner; (e) expanding the ends of the liner to come into contact with the outer pipe and seal welding each expanded end to the outer pipe, and evacuating and sealing by resistance or laser welding the annulus between the outer pipe and the CRA liner; (f) finishing the ends of the MLP in overlay welding left as welded or machined condition; (g) externally coating the MLP of step (f); and (h) conjoining a plurality of MLPs of step (g) to form an MLPL.

[0036] Similar to the method of manufacturing an MLP, the corrugations may be formed insider the CRA liner first, before the CRA liner is inserted into the carbon steel outer pipe.

[0037] Methods and apparatus for conjoining the MLPs together are well- known in the art and thus will not be described here in detail. It suffices to say that generally the conjoining comprises one or more welds, such as tie-in welds.

[0038] According to a further aspect of the present invention, there is provided a MLPL whenever formed according to the method of manufacturing a MLPL as described above.

[0039] The pipeline would then be spooled onto a reel of a reel ship in the absence of internal pressure above ambient pressure in the pipeline. The vessel then goes out to the designated location to install the pipe in the field.

[0040] Once the vessel is offshore, the pipeline is spooled off, straightened and/or aligned, in the absence of internal pressure above ambient pressure in the pipeline, before being laid onto the seabed. The terms "straighten" or "straightened" as used herein includes one or more steps of unbending the spooled pipeline after it leaves the reel, and generally when it is delivered to the location where it is intended to be laid. This can include one or more bending cycles, alignments and/or straightening steps, and usually take place before the pipeline enters the marine environment.

[0041 ] The present invention thus provides for a method of manufacturing an

MLP which may be reel-laid in a safer and faster manner without stopping for any internal pressure to be applied during the reeling and/or unreeling process to minimise or avoid deformation of the liner in any way. Rather, the corrugations (105) will absorb the strains of reeling and/or unreeling without causing any permanent deformation of the liner.

[0042] Further, it is also not necessary to make the liner thicker to withstand the bending strains, as the corrugations will absorb the strains of reeling and/or unreeling. In other words, the MLP manufactured from the method presented in the present invention may be used in the conventional reel-laying method without any modifications or extra steps required.

[0043] After the MLP is spooled off and straightened, it is pressurised to remove the liner corrugations. This involves hydrotesting the MLP, in accordance with the Standard DNV-OS-F101 . During this testing operation, the liner corrugations (105) will be removed and the liner to end cladding interface remains in elastic compression during operation. The result is a laid MLP having residual wrinkles that are within an acceptable range of height and in accordance with Standard DNV-OS- F101 resulting from the installation process. The hydrostatic test fluid would then be removed and the pipe prepared for operation.

[0044] It may be noted that during this elastic compression mentioned above, the liner will undergo a certain degree of shrinkage. At the same time, as the corrugated liner is straightened, the liner will also extend. This extension of the liner will partially make up for the shrinkage of the liner and help to alleviate the residual strain resulting from the reeling and/or unreeling.

[0045] The present invention encompasses all combinations of various embodiments or aspects of the invention described herein. Any and all embodiments of the present invention may be taken in conjunction with any other embodiment to describe additional embodiments of the present invention. Similarly, any elements of an embodiment may be combined with any elements from any embodiments to describe additional embodiments.

[0046] Further, various modifications and variations to the described embodiment of the invention will be apparent to those skilled in the art without departing from the scope of the invention as defined in the appended claims. Although the invention has been described in connection with a specific preferred embodiment, it should be understood that the invention as claimed should not be unduly limited to such specific embodiment.

Claims

1 . A method of manufacturing a mechanically lined pipe (MLP) having an outer pipe and an inner liner comprising at least the steps of:-

(a) forming the carbon steel outer pipe as a single length;

(b) forming the inner liner which is a Corrosion Resistant Alloy (CRA) liner made from alloy 316L, 825, 625 or other alloys as defined in API Specification 5LD and with a wall thickness of at least 2mm and at most 6mm, having the same length as the outer pipe;

(c) inserting the CRA liner into the outer pipe;

(d) forming corrugations inside the CRA liner;

(e) expanding each end of the CRA liner to come into contact with the outer pipe and seal welding each expanded end to the outer pipe; and

(f) evacuating and sealing by resistance or laser welding the annulus between the outer pipe and the CRA liner, wherein a vacuum is created between the outer pipe and the CRA liner.

2. A method as claimed in claim 1 wherein the corrugations are formed inside the CRA liner, before the CRA liner is inserted into the carbon steel outer pipe.

3. A method as claimed in claim 1 wherein the outer pipe is a carbon steel outer pipe having an outside diameter of 1 14mm to 610mm and a wall thickness of 7mm to 60mm.

4. A method as claimed in claim 1 wherein the corrugations are circumferential about the pipe.

5. A method as claimed in claim 4 wherein the gap between two consecutive corrugations is 1 mm.

6. A method as claimed in claim 4 wherein the corrugations are of a uniform size.

7. A method as claimed in claim 6 wherein the amplitude and the frequency of the corrugations are determined by the strain to be resisted, the final expansion parameters and the properties of the material used.

8. A method as claimed in claim 7 wherein the ratio of the side corrugation radius to the center corrugation radius is approximately 2:1 .

9. A method as claimed in claim 1 wherein the ends of the CRA liner are expanded to contact the outer pipe for at least 20mm and at most 100mm from each end and seal-welded to the outer pipe to form the MLP.

10. A method as claimed in claim 1 further comprising, after step (f), the step of

(g) finishing the ends of the MLP in overlay welding, left as welded or machined condition, wherein the length of the overlay weld is at least 3mm and at most 250mm, and wherein the liner may be resistance welded to the outer pipe at each end for up to 100mm.

1 1 . A method as claimed in claim 1 further comprising, after step (g), the step of

(h) externally coating the MLP.

12. A manufactured MLP which is made up of :-

(a) a carbon steel outer pipe; and

(b) a corrugated CRA liner.

13. An MLP as claimed in claim 12 wherein the carbon steel outer pipe has an outside diameter of 1 14mm to 610mm and a wall thickness of 7mm to 60mm.

14. An MLP as claimed in claim 12 wherein the CRA liner has a wall thickness of at least 2mm and at most 6mm.

15. An MLP as claimed in claim 12 wherein said corrugations are circumferential about the pipe.

16. An MLP as claimed in claim 15 wherein the gap between two consecutive corrugations is 1 mm.

17. An MLP as claimed in claim 15 wherein said corrugations are of a uniform size.

18. An MLP as claimed in claim 17 wherein the amplitude and the frequency of said corrugations are determined by the strain to be resisted, the final expansion parameters and the properties of the material used.

19. An MLP as claimed in claim 18 wherein the ratio of the side corrugation radius to the center corrugation radius is approximately 2:1 .

20. A method of manufacturing a mechanically lined pipeline (MLPL) comprising at least the steps of :-

(a) forming the carbon steel outer pipe of an MLP as a single length;

(b) forming corresponding inner liners which are CRA liners made from alloy 316L, 825, 625 or other alloys as defined in API Specification 5LD and with a wall thickness of at least 2mm and at most 6mm, having the same length as the outer pipes;

(c) inserting each CRA liner into each of the outer pipes;

(d) forming corrugations inside each of the CRA liners;

(e) expanding each end of the liner to come into contact with the outer pipe and seal welding each expanded end to the outer pipe, and evacuating and sealing by resistance or laser welding the annulus between the outer pipe and the CRA liner, wherein a vacuum is created between the outer pipe and the CRA liner;

(f) finishing the ends of the MLP in overlay welding left as welded or machined condition;

(g) externally coating the MLP of step (f); and (h) conjoining a plurality of MLPs of step (g) to form an MLPL.

21 . A method as claimed in claim 20 wherein the corrugations are formed inside the CRA liner, before the CRA liner is inserted into the carbon steel outer pipe.

22. A method as claimed in claim 20 wherein the carbon steel outer pipe has an outside diameter of 1 14mm to 610mm and a wall thickness of 7mm to 60mm.

23. A method as claimed in claim 20 wherein said corrugations are circumferential about the pipe.

24. A method as claimed in claim 23 wherein the gap between two consecutive corrugations is 1 mm.

25. A method as claimed in claim 23 wherein said corrugations are of a uniform size.

26. A method as claimed in claim 25 wherein the amplitude and the frequency of said corrugations are determined by the strain to be resisted, the final expansion parameters and the properties of the material used.

27. A method as claimed in claim 26 wherein the ratio of the side corrugation radius to the center corrugation radius is approximately 2:1 .

28. A method as claimed in claim 20 wherein the ends of the CRA liner are expanded to contact the outer pipe for at least 20mm and at most 100mm from each end and seal-welded to the outer pipe to form the MLP.

29. A method as claimed in claim 20 wherein the length of the overlay weld is at least 3mm and at most 250mm, and wherein the liner may be resistance welded to the outer pipe at each end for up to 100mm.

30. A method as claimed in claim 20 wherein the MLPs are conjoined together by one or more welds.

31 . A manufactured MLPL which is made up of :- (a) a carbon steel outer pipe; and

(b) a corrugated CRA liner.

32. An MLPL as claimed in claim 31 wherein the carbon steel outer pipe has an outside diameter of 1 14mm to 610mm and a wall thickness of 7mm to 60mm.

33. An MLPL as claimed in claim 31 wherein the CRA liner has a wall thickness of at least 2mm and at most 6mm.

34. An MLPL as claimed in claim 31 wherein said corrugations are circumferential about the pipe.

35. An MLPL as claimed in claim 34 wherein the gap between two consecutive corrugations is 1 mm.

36. An MLPL as claimed in claim 34 wherein said corrugations are of a uniform size.

37. An MLPL as claimed in claim 36 wherein the amplitude and the frequency of said corrugations are determined by the strain to be resisted, the final expansion parameters and the properties of the material used.

38. An MLPL as claimed in claim 37 wherein the ratio of the side corrugation radius to the center corrugation radius is approximately 2:1 .