The present invention relates to improvements relating to a mechanically lined pipe (MLP), particularly but not exclusively to provide a submarine pipeline; and to methods of manufacture of such an MLP. It relates also to the laying the MLP in reel laying methods, including those described in Standard API 5L/ISO 3183:2007 for seamless and welded steel pipes, and in API 5LD for clad or lined pipes.

Corrosion resistance pipelines for the submarine or otherwise underwater transportation or conveying of corrosive fluids such as gas or crude oil can be 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 is to provide resistance against buckling on the reel or sea bottom and provides general strength to the design so as to resist hydrostatic pressure, whilst the internal layer protects the outer layer from damage due to the chemical composition of the fluid being conveyed. The inner layer is sometimes also termed a "liner". Thus, such pipes are also termed bi-metallic lined pipes. As the main purpose is to protect the outer layer from corrosion, a corrosion resistant alloy [CRA] is commonly chosen as the liner.

The minimum thickness of the liner in the API standard is 2.5mm, and the minimum thickness of the liner in the DNV standard is 3 mm.

There are two common methods of laying underwater or submarine pipelines. The so- called 'stove piping method' involves assembling pipe stalks on a pipe-laying vessel, and then welding each one as the laying progresses. In the so-called 'reel laying method', the pipeline is assembled onshore and spooled onto a large reel, sometimes also termed a storage reel or drum. Once offshore, the pipeline is spooled off from the reel, straightened and/or aligned and finally laid on the seabed. With this method, no welding is required during the offshore operation, saving time for the vessel operation.

The reel laying method is faster and more economical than the stove piping method, such that it is preferred where possible, and pipes or pipelines which can be laid using this method are termed 'reelable' or 'reel-layable'. However, the reeling process obviously generates significant multiple bending strains in the pipeline, which would cause a conventional 2.5-3.0 mm liner in a conventional lined pipe to wrinkle. And it is generally considered that wrinkles are detrimental to an MLP. Thus, it is generally preferred to have a thicker liner which would be less susceptible or likely to wrinkle when undergoing bending, but this would significantly increase the CAPEX, which is undesirable. Thus, it is preferred where possible to use the thinnest possible liner thickness.

For these reasons, there are currently only few commercial applications of the reel lay method for bi-metallic lined pipes.

W097/34101 A discloses the use of temporary bonding in bi-metal lined piping to control the separation between the liner and the outer layer during bending. Its describes a pipe comprising a carbon steel outer pipe and a liner of corrosion resistant metal, in which the liner is secured along its length to the inner surface of the outer pipe by circumferential fixing means having a circumferential shear strength greater than the circumferential shear stress which would be induced by bending the pipe to a predetermined minimum radius of curvature. There are various circumferential fixing means described, one of which is circumferential solid-phase welding. However, such circumferential bonding requires a lot of welds because wrinkling is a periodic phenomenon with a very short wavelength; approximately 600 welds per normal 40 foot (12 m) pipe section length, which is economically difficult to justify for most pipeline manufacture.

WO 2008/072970 Al discloses a method for laying a pipeline having an inner corrosion proof metallic liner that is held inside an outer pipe material by interference stresses. In its method, a section of the pipeline is reeled onto a pipe laying drum, whilst an overpressure is maintained within the section by means of a pressurised fluid. When the pipeline is motionless, the overpressure is relieved, and a further pipeline section is joined to the first section. A new overpressure is then applied within the sections, and the further section is reeled onto the pipe laying drum. Whilst this method may assist to avoid wrinkling when the pipeline sections have "mechanical movement" [defined in WO 2008/072970 Al as meaning reeling the pipeline onto or unwinding the pipeline from, the pipe laying drum), this method requires the overpressuring and pressure-relieving steps each and every time two pipe sections are joined. The pipe laying drum is described in WO 2008/072970 Al as typically having installed "many" pre-fabricated sections, requiring significant repetition of the overpressuring and pressure-relieving steps.

WO2011/048430 Al describes methods of reel-laying an MLP including the steps of spooling and unspooling the MLP in the complete absence of internal pressure based on the clad liner thickness being calculated by a specific formula able to achieve an MLP only having wrinkles <2mmhigh after reeling, especially after hydrotesting the laid MLP.

One object of the present invention is to provide an MLP and a more practical method of manufacturing such an MLP suitable for reel-laying, in particular spooling the MLP onto and off a reel, by better controlling any separation between the liner and the outer layer during bending.

According to a first aspect of the present invention, there is provided a reelable mechanically lined pipe [MLP] having a metallic outer pipe and a metallic inner liner, wherein the inner liner is secured to the outer pipe by one or more welds extending longitudinally along the MLP.

Longitudinal welds are more efficient in pipeline manufacture, and in preventing 'wrinkling' of the inner liner when the MLP undergoes bending, in particular spooling the MLP onto and off a reel. Alternatively, any wrinkles formed are sufficiently minor or small that they can be reduced or removed once the MLP is installed on a seabed and after a hydrotesting step.

Mechanically lined pipes MLP can be formed with any number of layers, liners, coatings etc, known in the art, but at least including:

at least one metallic outer pipe, sometimes also termed in the art as an Outer layer' or 'host pipe', such as a carbon steel outer pipe, and herein generally termed an Outer pipe', fixed to at least one metallic inner liner, sometimes also termed in the art as a 'liner' or 'inner layer', and herein generally termed an 'inner liner'.

And any reference herein to the term "metal" or "metallic" includes alloys, or components formed from an alloy.

According to one embodiment of the present invention, the inner liner is secured to the outer pipe by 4-20 welds extending longitudinally along the MLP, possibly evenly or regularly spaced radially around the MLP. The invention is not limited by the number of longitudinal welds, and the greater the diameters of the outer pipe and inner liner, the greater number of longitudinal welds expected to be desired or necessary. Thus, the inner liner could be secured to the outer pipe using 4, 6, 8, 10, 12, 14, 16, 18 or 20 welds, or any range based on these numbers.

The number of longitudinal welds may also be partly based on the thickness of the inner liner. A thicker inner liner may require fewer welds. For example, a 3 mm inner liner could have the same number of welds as per MLP diameter in feet, i.e. 12" [3.66m) = 12 longitudinal welds, but a 5 mm liner for the same diameter MLP may have less, for example only 6 or 8 longitudinal welds.

The present invention is not limited by the diameters of the outer pipe and inner liner. For example, the outer pipe may have a diameter of any size from less than 12 inch (323.9 mm) OD or outer diameter, to greater than 20 inch (508 mm) OD. MLP pipelines for use in the transportation of gas or crude oil are commonly provided in medium sized diameters generally measured in "inches", such as 12 inch (323.9 mm) OD, 14 inch (355.6 mm) OD and 16 inch (406.4 mm) OD. Generally, the commonly used size is expressed for a 'nominal diameter'; that is, a "12 inch nominal diameter" in fact corresponds to a 12.75 inch outer diameter (323.9 mm OD).

A preferred inner liner is formed from a corrosion resistant alloy (CRA), for example a liner such as an alloy 316L, 825, 625 or 904L. Preferably, the MLP comprises a CRA inner liner having a thickness greater than 2.5mm, preferably greater than 3mm.

According to a second embodiment of the present, the inner liner is secured to the outer pipe by at least two welds formed simultaneously along the MLP.

Simultaneous welding clearly saves time and reduces manufacturing costs. This can be clearly contrasted with conventional method of creating circumferential welds, each one of which is preferably formed individually as the joining of the inner liner and outer pipe is carried out along the length of the MLP. Even if simultaneous circumferential weld could be carried out, such operations are still very time consuming due to the high number of welds required.

Preferably all longitudinal welds are formed simultaneously along the length of the MLP.

Joining two metals together by the act of welding is well known, usually by raising the temperature at or near the intended joint so that the metals can be united by fusing. There are many forms of welding including solid state welding.

The ability to join different metals and alloys by welding increases the flexibility of design and production. However, the joining of dissimilar metals can be a challenging task, usually owing to the sometimes large differences in physical and chemical properties which may be present.

Thus, according to another embodiment of the present invention, the inner liner is preferably secured to the outer pipe by one or more laser welds extending longitudinally along the MLP.

Laser welding is a known bonding technique providing several particular advantages over conventional welding, including precise working, i.e. exact placing of the energy spot, welding of complicated joint geometry, a low heat input and small heat affected zone (HAZ) [thereby providing only minor local distortion), large possible working distances, and a low mixing of dissimilar materials, (for example the low dilution of any CRA with the metal of the outer pipe).

Preferably, the longitudinal laser welds are provided from within the MLP, and more preferably such laser welds are simultaneously provided, for example by a multi-torch laser head. Such a laser head could be provided by a suitable internal pipeline apparatus, device or means, such as a pig.

According to another embodiment of the present invention, the MLP is a multi-section pipe. For example, the MLP could be pre-assembled from a number of smaller pipe sections. Such pipe sections may extend from several metres long up to approximately 1km long, or be greater than 1km long. Methods and apparatus for joining two pipe sections are well known in the art and are not described herein in detail. Generally the joining comprises one or more welds, such as tie-in welds. The two pipe sections form a combined pipe section. Typically, a reeled pipeline for subsequent laying via the reel lay method can be formed from many pipe sections, and so be many kilometres long.

Thus, according to another embodiment of the present invention, there is provided a reelable mechanically lined pipe (MLP], wherein the MLP comprises a series of welded pipe sections, and the inner liner is secured to the outer pipe by one or more welds extending longitudinally and continuously along the MLP between the ends of the welded pipe sections.

The MLP of the present invention may include one or more further means of permanent and/or temporary bonding of the outer pipe to the inner liner using one or more other securing methods known in the art to reduce, hopefully avoid, the formation of wrinkles in an MLP in any bending.

It is also known from a previous patent application filed by the applicant that the extent of wrinkles formed in reeling an MLP depends on one or more of the following group: the interference contact stresses in the liner, the liner thickness, the radial insertion gap, liner yield strength, liner strain hardening; the tensile response of the liner material, the applied bending strain, and the number of reverse bending cycles. There are two reverse bending cycles during a typical reeling operation, and there may be as many as five bending cycles during a contingency re-reeling operation.

The present invention is not limited by the thickness of the inner liner. As mentioned above, the API or DNV standards recite 2.5 or 3 mm as a minimum thickness. Thus, the minimum thickness is generally a thickness of >2.5mm or >3mm such as 4mm or 5mm.

In W02011/048430 Al, a minimum inner liner thickness can be calculated by a formula involving the factors of maximum reeling strain and the radial insertion gap. The maximum bending strain (nominal bending strain) during reeling depends on the pipe outer diameter and the reel or aligner radius, whichever is smaller. The smallest bending radius of the reel means the radius of the reel on which the pipe is to be spooled or the radius of the aligner of the laying vessel or any other reel on which the pipe is to be bent before it is straightened and laid, whichever is the smallest.

The bending radius of a reel can be as low as 1.5m, and go up to 10m or more. The radius of the reel and spooled pipe obviously increases as the pipe is reeled onto the reel. One typical example of a reel for laying a marine pipeline has a smallest reel bending radius of 8.23m.

However, using the present invention the liner thickness can now be less than the minimum liner thickness previously calculated. This is due to the local longitudinal welds which are benefit to the wrinkling sensitivity. We have found that longitudinal welds generally improve the bending capacity of an MLP without the risk of liner wrinkling.

Thus, according to another particular embodiment of the present invention, the thickness of the inner liner is less than "t" calculated by the formula:

a _1Q (.sD ) ⁱ g° + _αι1 .εΏ°· ⁷ ψ g ¹ + a^ * ⁷⁵ ) ¹ g ² + 0.16 where αοο, αοι, ... are constants given in Table 1 below,

ε is th um reeling strain or nominal strain which can be calculated as follows where ^d H is the outer diameter of the host pipe and ^ is the smallest bending radius of the reel;

3 is the radial insertion gap (i.e. the nominal gap between the outer diameter of the inner liner and the inner diameter of the outer pipe) in mm; and

D = DH - 2t _H is the liner outer diameter, where ^D n and ^t n are the outer diameter and wall thickness of the host pipe in mm respectively.

Using index notation, the formula can also be written as: t = di _j ieD*- ⁷⁵ )* g ^j + 0.16 where z ^' =0,l,2 and y ^' =0,l.

All dimensions in the above equations are in millimetres.

Table 1: Constants in Formula for Minimum Liner Thickness

Whilst this formula can calculate a range of liner thicknesses, and in particular minimum liner wall thicknesses, one or more other requirements or desires may still result in the use in practice of a liner wall thickness greater than the possible minimum. However, it is clearly beneficial that the present invention can achieve a thinner liner thickness than previously recognised by the WO2011/048430 Al patent, and thus reduce CAPEX.

As local longitudinal welds benefit the wrinkling sensitivity, the skilled man can see that the number of longitudinal welds can be balanced with liner thickness. That is, the more longitudinal welds used in the present invention, the less or thinner the possible liner thickness, and vice versa. Thus, it is possible to compute the number of longitudinal welds required to use a 3mm liner thickness without wrinkling the liner, or the possibility of using a slightly thicker liner such as 4mm, with less longitudinal welds. Other parameters affect this computation, including the materials used and the pipe OD etc, but the skilled man can consider the balance between using a liner thickness that is thinner than that previously considered as a minimum thickness, and the OPEX required to achieve the number of welds to weld the liner to the outer pipe of the MLP during manufacture.

In this way, the two factors of the number of longitudinal welds and the possible thickness of the liner can be, but are not limited to being, inversely proportional.

By way of nominal example only, 12 longitudinal welds could be used to secure a 3mm thick CRA liner to a 12 inch OD outer pipe, or 8 longitudinal welds could be used to secure a 5mm thick CRA liner to the same outer pipe.

The present invention further allows faster spooling and spooling off to be effected to provide faster, safer and so more economical reel-laying of an MLP. In particular, such an MLP can be designed and manufactured to be directly loaded onto a reel in a conventional manner without requiring any internal pressure either fully or in sections.

In another aspect of the present invention, there is provided a method of manufacturing a mechanically lined pipe [MLP] having a metallic outer pipe and a metallic inner liner for reel-laying comprising the steps of: providing the metallic inner liner providing the metallic outer pipe; and

locally welding the inner liner longitudinally into the outer pipe to form the MLP.

Preferably, the method comprises laser welding longitudinally the inner liner into outer pipe to form the MLP.

Optionally, the method provides 4-20 longitudinal welds to weld the inner liner into the outer pipe to form the MLP.

Optionally, the method provides a multi-torch head laser-welder along the inner liner to weld the inner liner into the outer pipe to form the MLP.

Optionally, the MLP comprises an outer pipe and/or inner liner as defined in one or more of the embodiments described hereinabove.

According to another aspect of the present invention, there is provided a method of spooling, reel-laying and hydrotesting an MLP as defined herein comprising at least the steps of:

(a) spooling the MLP onto a reel in the complete or substantial absence of internal pressure above ambient pressure in the MLP;

(b) spooling off the MLP from the reel in the complete or substantial absence of internal pressure above ambient pressure in the MLP;

(c) straightening the spooled off MLP of step (b);

(d) laying the MLP of step (c); and

(e) hydrotesting the laid MLP, to provide a laid MLP wholly or substantially having wrinkles < 2 mm high.

After hydrotesting, any residual wrinkles in the MLP will be less than 2mm high and preferably less than 1 mm high, unless in the instance of mismatched pipe where local wrinkles may be increased due to mismatch. Preferably, step (e) comprises hydrotesting the MLP at a maximum hydrotest pressure in accordance with DNV-OS-F101 [offshore standard, Det Norske Veritas, DNV-OS-F101, submarine pipeline systems, October 2007] where the internal pressure applied for the hydrotest depends on the pipe diameter and wall thickness.

The term "internal pressure" as used herein relates to the pressure within the MLP during the method of spooling or spooling off such an MLP onto or off a reel, during a whole or complete process thereof, as opposed to the spooling and/or spooling off processes involving one or more stopping instances or time periods requiring internal pressure or internal pressure changes for the bending or unbending of the MLP. The spooling of step (a) is preferably wholly or substantially continuous or otherwise on-going or uninterrupted, compared to previous stop-start spooling processes.

The term "straightening" as used herein includes one or more processes or steps of making the MLP straight after it has been spooled and spooled off from the reel, and before it is delivered to its intended laying position or location. This can include one or more bending cycles, alignments and/or straightening steps, usually before the MLP enters the marine environment. The reeled lay method generally involves at least the steps of aligning and straightening the MLP.

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.

Thus, various aspects of the present invention can be practiced alone or in combination with one or more of the other aspects, as will be appreciated by those skilled in the relevant arts. The various aspects of the invention can optionally be provided in combination with one or more of the optional features of the other aspects of the invention. Also, optional features described in relation to one embodiment can typically be combined alone or together with other features in different embodiments of the invention.

Various embodiments and aspects of the invention will now be described in detail with reference to the accompanying drawings. Still other aspects, features, and advantages of the present invention are readily apparent from the entire description thereof, including the figures, which illustrates a number of exemplary embodiments and aspects and implementations. The invention is also capable of other and different embodiments and aspects, and its several details can be modified in various respects, all without departing from the spirit and scope of the present invention. Accordingly, the drawings and descriptions are to be regarded as illustrative in nature, and not as restrictive. Furthermore, the terminology and phraseology used herein is solely used for descriptive purposes and should not be construed as limiting in scope. Language such as "including," "comprising," "having," "containing," or "involving," and variations thereof, is intended to be broad and encompass the subject matter listed thereafter, equivalents, and additional subject matter not recited, and is not intended to exclude other additives, components, integers or steps. Likewise, the term "comprising" is considered synonymous with the terms "including" or "containing" for applicable legal purposes.

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 schematic cross-sectional view of an MLP;

Figure 2 is a diagrammatic view of a method of spooling an MLP onto a reel;

Figure 3 is a diagrammatic view of the reel lay method, which illustrates the spooling off, aligning, straightening and laying an MLP from a reel;

Figure 4 is a diagrammatic cross-sectional longitudinal view of a prior art MLP before welding, and then after welding an inner liner circumferentially to an outer pipe;

Figures 5a and 5b are diagrammatic cross-sectional longitudinal and transverse views respectively of an MLP according to one embodiment of the present invention after welding an inner liner longitudinally to an outer pipe;

Figure 6 is a cross-sectional view of a longitudinal weld in Figure 5a/b; and Figure 7 is a diagrammatic cross-sectional longitudinal view of a method of manufacturing an MLP according to another embodiment of the present invention.

The present invention provides an MLP suitable for spooling, spooling off, aligning, straightening and pre-commissioning as a submarine pipeline.

Figure 1 shows a schematic cross-sectional view of parts of a mechanically lined pipe (MLP) 2. The MLP 2 generally comprises a number of layers (including coating), only two of which are shown in Figure 1 for clarity, comprising a metallic outer layer 4 which can be a carbon steel pipe, and a metallic inner liner 6 being formed from a corrosion resistant alloy (CRA), such as alloy 316L. The relative dimensions shown in Figure 1 are not to scale, and are provided for clarity of representation.

Figure 2 shows a diagrammatic reel 10 having a smallest bending radius "R", and a mechanically lined pipe (MLP) 12 having an outer pipe diameter "D". The MLP may be formed in sections joined together to form a single pipe. Sections are normally 1km long, but can be longer or shorter as required. Figure 2 shows spooling of the MLP 12 onto the reel 10.

By way of example only, the reel 10 could have a bending radius R of 8.23m, and the MLP 12 could have a diameter D of 12.75 inches (323.9 mm) and a wall thickness of 15.9 mm.

Figure 3 shows a method of spooling off the spooled MLP 12 of Figure 2 from the reel 10 now located on a suitable vessel 14, from a sea surface 16, to a seabed 18; generally known as the reel lay method. The spooled off MLP provides a laid marine pipeline 12a after it has been aligned by passing over an aligner 20 and straightened by passing through a straightener comprising multiple tracks 22, prior to passing below the vessel 14 to be laid on the seabed 18.

Pre-commissioning of the laid MLP 12a (not shown) usually involves pressurising the laid MLP 12a in a manner known as 'hydrotesting'. Such pressurising can generally be provided by passing a pressurised fluid such as water along the laid pipelinel2a, generally at a pressure depending on the pipe size and the wall thickness such as 30-40MPa.

In-particular, a combination of Figures 2 and 3 shows a method of laying a mechanically lined pipe (MLP) 12 in a marine environment comprising;

(i) spooling the MLP 12 onto a reel 10;

(ii) spooling off the MLP 12 of step (i) from the reel 10 and directing the MLP 12 to the aligner 20 of a lay tower 24 [during which the MLP 12 undergoes a reverse plastic bending such that the MLP 12 is or may be almost fully straightened in the span between the reel 10 and the aligner 20), and then to the straightener track 22 of the lay tower 24 in order to lay the MLP 12 in a marine environment as a marine pipeline 12a; and

(iii) hydrotesting (not shown) the laid MLP 12a of step (ii) to remove any wrinkles in the MLP 12a once on the seabed 18.

It could be useful to note that hydrotesting during pre commissioning of the pipeline is made to verify its integrity after installation, it is of course also useful for wrinkles removal.

However, bending of a pipeline formed from the two layers shown in Figure 1, such as spooling on or off the reel, can create wrinkles which are detrimental to the laid pipeline. Thus, increasing the bonding, even temporary bonding, to reduce or prevent wrinkling of the inner liner compared with the outer layer during bending, would clearly be beneficial.

Figure 4 shows an MLP 30 having only two layers for clarity like Figure 1. The MLP 30 comprises an outer layer 32 and an inner liner 34 in longitudinal cross-section. The lower part of Figure 4 shows a known method of applying a series of circumferential welds 36 to help secure the inner liner 34 to the outer pipe 32 to avoid their separation during bending of the MLP.

However, it can be seen that the number of circumferential welds 36 per MLP length needed to avoid wrinkling, which is a periodic phenomenon, can be substantial. Such a high number leads to significant time and effort required during the manufacturing process. The number of welds required clearly depends upon many factors including pipe diameter, etc. but a typical MLP pipeline requires a weld every 20-30mm, resulting in the need to make approximately 600 circumferential welds for a typical 40 foot/12m length of pipe section.

Moreover, as wrinkles are overwhelmingly formed radially or circumferentially, making circumferential welds between the outer pipe and inner liner can create locations along the MLP 30 which can help 'initiate' wrinkling, particularly where any welding or welding material induces or creates a line of circumferential weakness in the inner liner 34.

Circumferential welding may generate local liner lift off due to thermal distortion causing the liner to be more susceptible to wrinkling when the MLP is subject to bending during installation.

Figures 5a and 5b show an MLP according to one embodiment of the present invention. Figures 5a and 5b show a reelable mechanically lined pipe (MLP) 40 having a metallic outer pipe 42 and metallic inner liner 44, wherein the inner liner 44 is secured to the outer pipe 42 by a number of welds 46 extending longitudinally along the MLP 40.

As shown in Figures 5a and 5b, the inner liner 44 is secured to the outer pipe 42 by 8 longitudinal welds 46 which extended continuously along the MLP 40, and preferably between the ends of welded pipe sections as described hereinafter in relation to Figure 7. The longitudinal welds 46 are regularly spaced radially around the MLP 40, and their size in Figure 5b is for clarity and location purposes only, and does not represent their extension from the inner liner 44 in practice.

Figure 6 more typically shows a magnified portion of a weld in Figure 5b, and the extent of penetration of the longitudinal weld 46 from the inner liner 44 into the outer pipe 42. Figure 6 also shows a graduated scale 48 to show the relative penetration of the longitudinal weld 46 in relation to the thickness or height of the inner liner 44.

Figure 7 shows a method of manufacturing a mechanically lined pipe (MLP) comprising a series of welded pipe sections. In Figure 7, there is shown a first pipe section 40 such as that shown in Figure 5a, and second pipe section 41 to be joined to the first pipe section 40 in a manner known in the art, such as by girth welding, and not described in further detail herein. The inner liner 44 of each section 40, 41 is or will be secured to the outer pipe 42 of each section by one or more welds extending longitudinally 46 and continuously along the MLP 40 between the ends 48 of each welded pipe section 40, 41.

The longitudinal welding is provided by a pig 50. Pigs able to travel internally along lengths of pipe or pipe section are well known in the art, and generally comprise a central body 51, and one or more, usually a regular series, of wheels 56 or rollers able to travel along the inside surface of the pipe or pipe section and support the pig body 51.

The pig 50 in Figure 7 supports a multi-torch head 52 having a series of laser welders 54 arranged circumferentially therearound, and able to provide simultaneous forming of the longitudinal welds 46 as the pig 50 travels along the MLP 40 in the direction of arrow A shown in Figure 7. In this way, all the longitudinal welds 46 are formed simultaneously by the single passage of the pig 50, which is also then ready to traverse into the next pipe section 41 once the pipe sections 40, 41 are joined.

Either the MLP 40, or an MLP formed by a series of pipe sections such as 40 and 41, is then useable in a method of reel-laying and hydrotesting an MLP as described hereinabove in relation to Figures 2 and 3.

The speed and power of the laser welding is dependant upon the nature and size of the outer pipe and inner liner. A typical laser suitable for the method is a ytterbium fibre laser YLR-15,000 IPG, with laser power varied between 3 and 6 kW. The speed of welding may be as low as lm/min or even lower, and up to 6, 7, 8 or greater m/min.

Typically, the longitudinal weld need only extend into the outer pipe [i.e the weld depth] by one or two millimetres or less.

The skilled man can use these parameters, and knowing the nature of the outer pipe and inner liner, calculate usable power and usable welding speeds. The present invention provides a simple method of manufacturing an MLP that achieves good at least temporary bonding between the inner liner and outer pipe for bending, in particular for spooling on and off a reel, optionally with a thinner liner thickness than previously considered possible. The method of manufacture is relatively fast and simple, especially when being able to provide all welds simultaneously without affecting other steps in the manufacturing process.

In particular, longitudinal welds avoid the problem of circumferential welds providing a place of initiation of wrinkles.