The present invention relates to a bi-metallic mechanically lined pipe, a method 5 making such a pipe, and a mechanically lined pipeline formed from such pipes, particularly but not exclusively to provide a marine pipeline.

Corrosion resistance pipelines for the marine or otherwise underwater transportation or conveying of corrosive fluids such as gas or crude oil can be 0 provided by pipes having an internal metallic liner.

A double-walled or bi-metallic pipe is generally composed of two metallic layers. The outer layer or host pipe or outer pipe as termed hereinafter, is for resisting hydrostatic pressure, and/or internal pressure depending on the water depth,5 whilst the internal layer prevents damage to the outer pipe from the chemical composition of the fluid being conveyed. The inner layer is sometimes also termed a "liner". As one of its main purposes is to protect the outer pipe from corrosion, commonly a corrosion resistant alloy (CRA) is chosen for the liner. 0 One form of bi-metallic pipe is a single "clad" pipe having an internal CRA layer fully

metallurgically bonded all along its length to the outer pipe, which could be formed from a carbon steel base metal.

A second form of bi-metallic pipe can be termed a mechanically lined pipe (MLP). A5 bi-metallic mechanically lined pipe is well known in the Oil & Gas subsea market as a very cost effective product used for the transportation of corrosive hydrocarbon products or water. Such product is manufactured by a range of companies through the world (Butting, Proclad, Cladtek, etc) and a range of such products have been successfully installed, and are in operation.

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Although the manufacturing process for such pipes can slightly vary from one manufacturer to another, the general principle remains the same, i.e. a corrosion resistant (preferably CRA) liner (typically Inconel alloy 316L, 825 or 625) is sleeved inside a carbon steel host pipe. The liner is then expanded inside the host pipe until the outside diameter (OD) of the liner touches the inner diameter (ID) of the carbon steel pipe. The expansion operation is further progressed to a degree that both liner and host pipe outside diameters have grown. Typically, the internal liner will have been plastically deformed, where the host pipe remains within elastic expansion range (this is not strictly true for all processes). Then the expansion operation is released and both pipes try to regain their initial diameter. However, as the internal liner has been plastically expanded, it relaxes to a diameter greater than its initial diameter, and greater than the initial inside diameter of the host pipe. Hence residual hoops stresses in either pipe remain and a mechanical interference stress remains between the two layers of the bi-metallic product.

Such bi-metallic pipes are generally provided as 'stalks', and are generally manufactured in 12m or 24m lengths at a manufacturing location, and then subsequently transported to a base or site next to a river or sea for welding together into conventionally 'standard' 1km lengths. These longer lengths are then spooled onto a vessel reel before being unspooled offshore by known reel laying processes. Or, 12m or 24m stalks can be stored directly on a pipeline laying vessel and joined together offshore to be laid subsea by a known J-lay or S-lay method.

In this way, a mechanically lined pipeline (MLPL), generally comprising a plurality of MLPs brought together, optionally with one or more intermediate apparatus or units, is provided.

Currently, mechanically lined pipes have a length of clad over-lay to cover the extremities of the mechanically lined pipe. The over-lay section provides the ability to cut and re-carry a girth weld between two pipes, without breaching the gap between host and liner layers.

To achieve this, the liner inserted in the host pipe is shorter than the host pipe, and a length of clad is applied using the over-lay process. However, this type of over- laying has various limitations, especially in relation to higher specification pipes, where higher fatigue performance is specified.

For example, clad overlaying is a relatively time consuming process. Also, it is prone to volumetric defects or non-uniform pattern at the carbon steel/over-lay interface. This results in difficulties for existing non destructive testing (NDT) technologies to have a sufficient level of accuracy to fully satisfy that the integrity of the over-lay is met. Another major "weak point" is the interface between the over-lay and the section. Lack of fusion can occur, but it is very difficult to identify this during the NDT inspection phase. Such lack of fusion has often been demonstrated to be the site of fatigue failure in full scale fatigue tests. Figure 1 shows the interface between a liner 6 and a clad over-lay 8 in a mechanically lined pipe 2. The host pipe is indicated by the reference numeral 4 and the lack of fusion location is indicated by the arrow LF.

It is an object of the invention to provide an improved bi-metallic mechanically lined pipe and method of manufacture. According to a first aspect, there is provided a bi-metallic mechanically lined pipe (MLP) having an outer pipe of a first carbon steel based material, and an inner liner of a second corrosion resistant material, characterised in that the inner liner is metallurgically bonded to the outer pipe at the extremities of the outer pipe. Metallurgically bonding the inner liner to the outer pipe at the extremities achieves a more certain, a faster and a more cost effective method than using clad overlay at the terminations of the MLP.

In addition, the MLP of the present invention has a clean and flat interface profile between the liner and the outer pipe, which makes the NDT implementation easier. Furthermore, there is no intricate liner-to-over-lay interface which occurs when an MLP is terminated by the length of clad over-lay. This has the benefit of reducing the criticality of that section towards fatigue performance. The present invention also provides repeated quality and reliability of the MLP over known methods for producing an MLP is that it provides for.

Generally the inner liner is provided from an inner liner former located or sleeved within the outer pipe, and then expanded to form the final shape and form of the inner liner. That is, the inner liner is an expanded inner liner former.

Generally, the bi-metallic mechanically lined pipe is formed wholly or substantially straight or in a straight form or length. The present invention is characterised in that the inner liner is metallurgically bonded to the outer pipe only at the extremities of the outer pipe. That is, the inner liner is not metallurgically bonded to the outer pipe not at the extremities of the outer pipe. The extremities of the outer pipe can be considered as the ends of the outer pipe, without limitation to the extent thereof other than the requirement of achieving the present invention.

In an embodiment, the inner liner is metallurgically bonded directly with the outer pipe at the extremities of the outer pipe. In an alternative embodiment, the inner liner is metallurgically bonded to the outer pipe using an intermediate layer to assist the wetting therebetween.

In another embodiment, the inner liner former is metallurgically bonded to the outer pipe by a resistance welding. Preferably, the resistance welding is a resistance clad welding. In a preferred embodiment, the resistance welding includes pipe circumferential welding.

In another preferred embodiment, the resistance welding uses one or more contact resistance rollers either within the pipe, externally of the pipe, or both. More preferably, the resistance welding includes at least one pair of complementary contact resistance rollers on either side of the pipe.

In an alternative embodiment the inner liner former is metallurgically bonded to the outer pipe by laser welding. In such embodiments, the laser welding may be pin laser welding, laser overlay welding, or laser overlay clad welding.

The mechanically lined pipe (MLP) according to the present invention may be any length, generally 12m or 24m lengths.

Mechanically lined pipes can be formed with any number of layers, liners, coating etc., known in the art, but including at least one Outer layer' or Outer pipe', such as a carbon steel outer pipe, fixed to at least one 'inner layer' or 'liner', being formed from a corrosion resistant alloy (CRA), for example a clad liner such as an alloy 316L, 825, 625 or 904L, without metallurgical bonding.

According to a further embodiment of the present invention, the MLP comprises a clad liner, and the clad liner has a thickness >2.5mm, preferably >3mm. According to a second aspect of the present invention, there is provided a method of manufacturing a bi-metallic mechanically lined pipe (MLP) having an outer pipe of a first carbon steel based material, and an inner liner of a second corrosion resistant material, comprising at least the steps of: forming the outer pipe;

(b) forming an inner liner former;

(c) locating the inner liner former of step (b) within the outer pipe; (d) expanding the inner liner former within the outer pipe to form the MLP with the inner liner; and

(e) metallurgically bonding the inner liner with the outer pipe at the extremities of the outer pipe.

Preferably, the method comprises using resistance welding to metallurgically bond the inner liner to the outer pipe.

The location of the inner liner former within the outer pipe can be carried out as a simple mechanical step of inserting the inner liner former into the host outer pipe length using one or more simple mechanical operations.

The expanding of the inner liner former within the outer pipe to form the final shape and form of the inner liner and MLP can be provided by any one of a number of conventional methods, including one or more mechanical processes and/or one or more hydraulic processes.

Mechanical expansion processes include passing an expander through the pipe length so as to expand the inner liner former to meet the inner surface of the host outer pipe. A hydraulic process can be the use of pressurized water passing through the pipe length, such as carried out during hydrotesting at a pressure in the range 20-50MPa for example, which testing is carried out in accordance with known standard procedures. The method of manufacturing according to the present invention provides a mechanically line pipe (MLP) of any length, optionally but not limited to up to 1km long. The present invention extends to a mechanically lined pipe whenever formed according to a method as described hereinabove. 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 mechanically lined pipes (MLPs) as defined hereinabove. Generally, the conjoining is provided by welding a plurality of consecutive mechanically lined pipes by the use of one or more pipeline-welding processes.

One example of a pipeline-welding process is to provide girth welds between two consecutive MLPs comprising welding using consumables of the same material as the inner liner material. Although inner liner material is usually more expensive than outer pipe material, this process only requires welding every pipe length.

A second pipeline-welding process involves welding consecutive outer pipes together using consumables of the same outer pipe material, such as carbon steel, followed by internal pipeline welding of consecutive inner liners. To assist avoiding any mixing of welding materials to form non-desired alloys, the inner liners of consecutive pipes could be cut back to allow clean outer pipe welding, and the internal welding can then be carried out cleanly between the two inner liners to avoid the already formed outer pipe welds.

The MLPL is preferably formed/manufactured/assembled at a spoolbase, generally where the MLPL is spooled onto a reel. Generally this is onshore or at an onshore location, optionally where the outer pipe and/or inner liner is formed, preferably to form an MLPL which is >500m long, and is ready for laying, preferably as part of a longer (generally many kilometres) pipeline.

It would be understood that any excess inner liner former will be removed from the MLP after the inner liner and outer pipe have been metallurgically bonded together so that the inner liner and the outer pipe are of substantially the same length prior conjoining the MLP to another MLP.

According to a further aspect of the present invention, there is provided a mechanically lined pipeline (MLPL) whenever formed according to one or more of the methods of manufacturing a mechanically lined pipeline as described above. The methods of manufacturing described herein above are not limited to the size, shape, design, physical and/or chemical properties of the outer pipe and/or inner liner. There are two common methods of laying underwater or marine pipelines. The so- called 'stove piping method' involves assembling pipes on a marine pipe-laying vessel, and then welding each one as the laying progresses. In the so-called 'reeled laying method', the pipeline is assembled onshore from a number of pipe lengths, and then directly spooled onto a large reel, sometimes also termed a storage reel or drum. Once offshore, the pipeline is unwound from the reel as a single entity and is directly available for laying because no welding is required during the offshore operation.

The present invention encompasses all combinations of various embodiments or aspects of the invention described herein. It is understood that 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. Furthermore, any elements of an embodiment may be combined with any and all other elements from any of the embodiments to describe additional embodiments.

The invention will now be described by way of non-limiting example, with reference being made to the accompanying drawings, in which:

Figure 1 shows the interface between a liner and clad overlay in a mechanically lined pipe;

Figure 2 is a diagrammatic view of one step of a method of forming a mechanically lined pipe (MLP);

Figure 3 is a diagrammatic view of a second step of a method of forming a mechanically lined pipe (MLP)

Figure 4 is a schematic cross-sectional view of an MLP formed according to the method shown in Figures 2 and 3; Figures 5a and 5b are schematic cross-sectional view showing the metallurgical bonding of the liner and outer pipe according to an embodiment of the present invention; and

Figure 6 is a schematic cross-sectional view of a method of conjoining two consecutive MLPs in accordance with the invention.

Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example "comprising" and "comprises", means "including but not limited to", and is not intended to (and does not) exclude other components, integers or steps.

Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Referring to the Figures 2 to 6, Figure 4 shows a schematic cross-sectional view of parts of a mechanically lined pipe (MLP) 12. The MLP 12 generally comprises a number of layers, only two of which are shown in Figure 4 for clarity, comprising an outer pipe 14 of a first carbon steel based material, which can be a carbon steel pipe, and an inner layer or liner 16 a second corrosion resistant material, preferably being formed from a corrosion resistant alloy, such as alloy 316L. The relative dimensions shown in Figure 4 are not to scale, and are provided for clarity of representation.

It is emphasised that the present invention relates to the forming and use of specific bi-metallic pipes, having a metallic liner rather than any form of plastic liner. The present invention still allows the forming of an MLP having an extensive length, such as a length of >500m.

Figure 2 shows the locating of an inner liner former 6a within the outer pipe 14 following the direction of arrow A. Figure 3 shows the application of a suitable pressure, such as water pressure from hydrotesting, in the direction of arrow B into the inner liner former 16a so as to expand the inner liner former 16a circumferentially, and as generally shown by arrows C, to form interference contact stress between the two formed layers 14, 16 and hence their bonding together to form a mechanically lined pipe 12.

Figures 5a and 5b show the metallurgical bonding process of the liner 16 and outer pipe 14 at the extremities of the outer pipe. The liner 16 is metallurgically bonded to the outer pipe 14 to form a bond 17 by a resistance welding process or laser welding process. For example, the liner 16 may be metallurgically bonded to the outer pipe 14 by a resistance welding process, a pin laser welding process, a laser overlay welding process, or a laser overlay clad welding process. Laser welding in its various forms is generally well known, and uses one or more lasers to direct sufficient heat where required.

Resistance welding, sometimes also termed electric resistance welding, refers to a group of welding processes that produce coalescence of meeting or faying surfaces where the heat to form the weld is generated by the electrical resistance of the relevant materials, combined with time and force used to hold the materials together during welding. Some factors influencing the process are the size, thickness etc. of the work pieces, any metal coatings, the nature of the electrode materials, the electrode geometry, the forces used, the usual welding parameters such as electrical current, welding time, etc. Generally, molten metal is formed at the point of most electrical resistance (the meeting or "faying" surfaces) as an electrical current (such as in the range 100-100,000 A) is passed through the material(s). Some resistance welding forms the weld progressively, whilst relying on two electrodes to apply pressure and current. The electrodes can be in the form of contact resistance rollers to contact the material as it passes between the rollers, and typically to move the material, such as to rotate the material, as it passes between the rollers. In this way, the electrodes stay in constant contact with the material to form continuous welds. Optionally, the electrodes may also create the movement of the material.

A particular difficulty in relation to the present invention is the different nature of the first carbon steel based material of the outer pipe, and the second corrosion resistant material of the inner liner. Carbon steel often has a higher melting point or solidus compared with corrosion resistant materials, an example being 1495°C compared with typically a range of 1270 °C to 1370°C for corrosion resistant alloys commonly used in oil and gas pipelines. Thus, there can be a temperature difference greater than 100°C or even greater than 200°C between the two materials to consider when joining. It is known that bridging this temperature difference can be assisted using one or more intermediate layers or materials, such as a brazing interlayer known in the art, often having a melting temperature lower than at least the solidus of the two materials to be welded, such that it can flow readily below the solidus temperature. Preferably, the resistance welding includes pipe circumferential welding. That is, welding carried out in a circumferential manner in, along or otherwise about, the circumference of the MLP at its extremities. This is generally being between the outer pipe and inner liner at the extremities or at each end of the MLP, and for the required depth into the MLP from the actual ends of the MLP and extending inwardly into the MLP.

Contact resistance rollers can act as the electrodes required to provide the current flow into the materials, as well as applying the force to the materials during the welding. The resistance heating of the materials created by the current flow, combined with the force, results in the formation of a localised bond or weld. By using contact rollers, which can roll around either the inner circumference, or the outer circumference, or preferably simultaneously both circumferences, of the MLP extremities, the resistance welding can provide continuous welding around the extremities. Preferably, at least one contact resistance roller is located within the pipe, and at least one complementary contact resistance roller is located oppositely on the outer circumference of the MLP extremity, and the two rollers act as the required electrodes to provide the required current flow thereinbetween through the outer pipe and inner liner to achieve a smooth and continuous bond or weld thereinbetween.

The use of resistance welding is particularly suitable for welding dissimilar and/or dissimilar thickness materials. As mentioned above, the melting or solidus temperatures of typical outer pipes and corrosion resistant inner liners for MLPs are dissimilar. In particular, the relative thicknesses of the outer pipe and the inner liner are different, especially where it is preferred to seek a reduction in inner liner thickness (due to its comparatively higher cost).

Welding apparatus and welding parameters to achieve electric resistance welding are known in the art. Typically the electrodes are formed wholly or substantially of copper. The actual parameters required for the resistance welding process to achieve the best heat balance will depend, as generally discussed above, on many factors. These include the thickness of the outer pipe, which can easily be greater than 10mm thick, such as 12.5mm thick, and the thickness of the corrosion resistant inner liner, which can be in the range l-4mm, typically 2mm, 2.5mm or 3mm thick. Other factors are the operating current, the conductivity of the materials, the speed of any rollers, and the force applied by the rollers, etc.

The present invention uses a resistive current based methodology to create metallurgical bonds or bonding at the point of contact, typically including the use of force to push together the outer pipe and inner liner to come into greater or more contact during the process. As such, the bonding between the outer pipe and the inner liner at the extremities of the MLP is not only smoother because of the use of force, but easier to inspect internally, such that the n bonds or bonding are more reliable and more open to NDT inspection. Figure 6 shows a method of manufacturing a mechanically lined pipeline (MLPL) 100 comprising conjoining by welding a plurality of mechanically lined pipes (MLPs). Figure 6 shows two consecutive MLPs 112a, 112b such as those formed as described above in relation to Figures 2 to 5b, and involving an outer host pipe 114 and an inner liner 116.

The method comprises using consumables of the same material as the inner liner material to provide a butt weld 118, for example a girth weld, between the two consecutive MLPs 112a, 112b, so that at least the material of the inner liner is continuous along the consecutive MLPs 112a, 112b. This process can provide an MLPL 100 ready for the reeled-lay method of laying a marine pipeline.