the system according to the present invention facilitates the offshore assembly of prefabricated field joints with internal liners , in a manner compatible with field - proven techniques of pipeline assembly for s - lay or j - lay . furthermore , this method and apparatus for interconnecting lined pipes is designed in such a way that it would not impact the laying rate of s - lay or j - lay spread . it also does not necessitate the establishment of a fabrication base on shore . in addition , using pipe reeling often requires an increase in wall thickness of the pipe to accommodate the anticipated strain . the technique of the present invention , however , allows using thinner pipe , substantially reducing procurement costs . the interconnection process of the present invention to be described is an adaptation of the existing welding process developed for welding pipe field joints in s lay or j lay mode . single , double or multiple pipe sections with the internal liner already fitted are pre - fabricated onshore , or on - deck . the sections may be pre - installed with components used during the assembly process , or the components may be provided separately . the sections and other required components are then shipped out to the offshore site , where they are assembled to form a continuous pipeline . fig1 shows , for the sake of example only , a known pipe laying vessel upon which the novel process may be employed . this vessel corresponds to that described in u . s . pat . no . 5 , 975 , 802 ( willis ), mentioned above . the invention may equally be applied in a j - lay type vessel such as that described in u . s . pat . no . 6 , 213 , 686 ( baugh ). the vessel 5 has a deck 10 , on which is mounted a pipe line assembly arrangement 20 , comprising the lined pipe interconnection system , and various coating and testing stations , for assembling a continuous pipeline from a stock of pipe segments . pipe 30 formed in this way progresses in the direction of the arrow , over first and second radius controllers 40 , 50 . a tiltable ramp 60 is provided for launching the pipe over the stern of vessel 5 . in solid lines , ramp 60 and other equipment are shown in a near - horizontal orientation , appropriate to lower water depths . in chain - dotted lines , the same components are shown in a steeply elevated orientation . the radius controllers 40 to 50 guide the pipe and restrict bending within set limits , according to the angle of the ramp 60 . on ramp 60 there are mounted various pipe handling devices , such as straightener 70 , tensioning and paying - out device 80 and fixed clamp 90 . sections of pipe for joining are stored in the hold of the vessel . continuous lined pipe is formed by joining sections of pipe using the apparatus and method of interconnection . the vessel is provided with swaging apparatus ( not shown in fig1 ) for interconnecting lined pipes on the vessel , by the methods that will now be described in detail , with reference to the accompanying diagrams . the swaging tool will be based at the forward end of the assembly arrangement 20 , from where it can be threaded into the open end of the pipe , and down to the location of a joint between sections . in a vertical tower ( j - lay ) arrangement , such as that of u . s . pat . no . 6 , 213 , 686 , the swaging tool can be housed in an “ attic ” area at the top of the tower , to be lowered into the open end of an erect pipe section . fig2 is a cut - away diagram of a tubular bridging sleeve 102 , used to bridge internally between two sections of lined pipe being interconnected . it comprises a hollow cylindrical tube with a smooth interior surface of substantially equal bore , and a modified exterior surface , modified to enhance the seal between the sleeve and a liner , when fitted . such modifications comprise a number of circumferential channels 104 , grouped at each end . the recesses are used to maximize the efficiency of the seal and to enhance the grip between liner and sleeve . the sleeve therefore has three definable regions , these being a ) at a first end , a first sealing portion 106 having multiple circumferential channels , b ) at the other end , a second sealing portion 108 having multiple circumferential channels and c ) an elongate intermediate portion 109 , interconnecting the two sealing portions . the sealing portions and their channels are described in detail , later in the document . as part of the sleeve will be exposed to the fluid being transported , it needs to be fabricated from a corrosion - resistant material . a typical choice of material might be inconel ™ or stainless steel , however the skilled reader will appreciate that the choice of material is not limited to metals , but to any material providing the required physical attributes . fig3 is a schematic cross - sectional diagram of two prepared lined pipe sections 110 , 120 being brought together for assembly with the bridging sleeve 102 . preparation involves operations that can effectively be performed on - shore , such as lining , end - beveling and finishing sections of pipe . the pipes 110 , 120 are typically 6 ″ (˜ 150 mm ) or greater in diameter , having a grade range of x52 to x65 and a wall thickness of at least ½ (˜ 12 mm ). the liner material is a plastic material , such as polyethylene or polyvinylidene fluoride , of hdpe / pe100 quality , of a thickness of at least 8 mm . the liner 130 exhibits a thermal expansion of approximately 0 . 18 mm / m /° c . and maximum operational temperature of approximately 80 ° c . service conditions for the lined pipe are a maximum external pressure of approximately 200 bars and maximum internal pressure of approximately 345 bars . a typical fluid conveyed by the pipe would be deoxygenated seawater o 2 & lt ; 5 ppb plus biocides batch injections . a section 110 of pipe is mid - way through the process of being connected to a continuous pipeline 120 , formed out of previously assembled sections of the same . as can be seen , the outer , steel pipe 122 is of continuous diameter , although the skilled person will appreciate that this is not critical to the operation or efficacy of the present invention , applying equally to pipe including a widened end region , where the bore of the pipe is enlarged ( not shown ). both ends of the pipe section are shaped in the same way , unless items other than pipe sections are being fitted , such as pipe ends or flanges . the pipe section 110 , 120 has been pre - lined with a plastic liner 130 . the lining is dimensioned when fitted to contact the inner surfaces of the pipe , with the exception of an unlined region 144 , approximately 120 mm from the ends to be welded , which distances the liner from the region where high weld temperatures would permanently and detrimentally modify the characteristics of the liner . as a result , with a sleeve in place an annular void region 146 behind the weld is formed by the combination of gaps either side of the weld . this void region ensures that there is no material behind the weld region whilst the hot phase of the welding is being performed , thus providing the aforementioned advantages . the unlined distance 144 can be reduced where a quicker welding time is used , as less heat is put into the weld region . the converse applies . the two sections of pipe 110 , 120 are interconnected by insertion of the single additional bridging sleeve 102 , bridging between the internal surfaces of the liners of both pipe sections . prior to fitting , the profiles of the inside and outside diameters of the sleeve are substantially constant and cylindrical . the ends 162 , 164 of the bridging sleeve 102 and liner 130 are beveled to ease insertion of the bridging sleeve into the bore of the liner , although the skilled person will appreciate that only one of either needs to be beveled , to achieve substantially the same effect . the sleeve and liners are dimensioned such that the sealing portions 106 , 108 are adjacent the internal surfaces of both liners , when fitted . in order to keep the fluid being transported from contacting any of the susceptible steel pipe there is required a fluid - tight seal between the contacting surfaces of the plastic liner 130 and the sleeve 102 . this is achieved by swaging the sealing portions 106 , 108 of the sleeve onto the liner , adjacent to a region 166 where both liner and sleeve are co - located , and which upon expansion of the sleeve by deformation forces the surfaces of the sleeve and plastic pipe together , thus forming a fluid - tight seal and ensuring that the liner is locked in its intended location . upon application of pressure the grooves gradually bite into the liner , providing a gradual increase in resistance as force is applied . the grooves allow expansion to proceed in a gradual , controlled manner using force feedback and closed - loop control . doing so is more effective at providing a good seal between liner and sleeve . the liner is unable to move with respect to the steel pipe because it is gripped at both ends . when the liner is compressed by the swaging process the plastic deforms some way into circumferential channels 104 . doing so hinders any longitudinal movement of the pipe liner with respect to the sleeve , firmly capturing the liner in its intended location . the detailed form of the grooves may be the same , for example , as that described in gb 2298256 , mentioned in the introduction . the skilled person will appreciate that different methods may be equally effective for adhering the contacting surfaces to each other , such as adhesives . it is possible to have a lined section of pipe pre - fitted with a sleeve of the first embodiment . in this instance each section of pipe is provided pre - fitted with the liner 130 and bonded sleeve 102 , thereby minimizing the number of operations per section performed offshore . in principle , two bridging members could be fitted into both ends of each pipe in half of the pipe sections , while the remainder of the pipe sections are left with two open ends . it will generally be preferable to treat each section identically , however , rather than treat different sets of sections differently . this preferred approach is adopted for the following illustration . all of the components necessary for joining two pipe sections together have now been described . pipe interconnection is consequently achieved in accordance with the following procedure and with reference to fig3 to 5 , on the basis that the first sections of pipe have already been laid , forming a continuous section 120 extending into the sea from the pipe laying vessel , and using the pipe sections pre - fitted with sleeves as described in the previous paragraph : 1 ) lower continuous section 120 of pipe ( further ) into the sea to allow the next pipe section 110 to be fitted ; 2 ) clean the mating surfaces of the protruding sleeve using , for example , compressed air ; 3 ) lower a swaging tool 170 into the sleeve until adjacent the lowermost sealing portion 106 of the sleeve , corresponding to lower overlap region 166 between liner and sleeve , then radially expand the sleeve onto the liner . ( fig3 , step 1 ); 4 ) lower the next pipe section 110 onto the protruding sleeve until the two ends of the steel pipes abut ; 5 ) perform a preliminary weld 180 (“ root pass ” and “ hot ”) of the two pipes together ( fig4 , step 2 ); 6 ) during performing of the cooler phase of the welding (“ weld fill ” and “ cap ”), swage the intermediate portion 109 of the sleeve , in a manner that forces any fluid trapped in the region behind the weld past the as yet unswaged region ( s ) ( fig4 , step 3 ); 7 ) swage the uppermost sealing portion 108 of the sleeve ( fig5 , step 4 ), or if not performed earlier , swage both overlap regions of the sleeve , to form in each case a seal between sleeve and liner ; 8 ) perform any validation steps , such as non - destructive testing ; 9 ) to continue laying further pipe , repeat the process from step 1 . note that the weld surfaces are maintained in alignment with respect to each other during this process by the field proven interconnection equipment , rather than by using internal clamps . furthermore , the welding process used is typical for s - lay and j - lay pipe laying methods currently in use , allowing plastic lined pipes to be used commercially for subsea pipe laying . in j - lay systems , where the pipe is suspended almost vertically during the jointing process , the pre - fitting of the bridging sleeve 102 ensures that the sleeve will not slide down the bore of the pipeline . the skilled reader will appreciate that the alternative means can be employed , if pre - fitting is not convenient . for example , a removable plastic collar could be provided around the intermediate portion of each bridging member , holding it in the mouth of the suspended pipe , until it has been sealed to the liner . fig6 a and 6 b show an alternative joining process , suitable for use with the interconnection method and apparatus . instead of a swaging tool being lowered into the sleeve 102 after it has been installed in the pipeline , as described previously , here the sleeve and swaging tool are lowered into the pipeline as a single entity . also , instead of a single swaging tool being radially expanded and moved longitudinally between lower and upper sealing portions 106 , 108 and intermediate portion 109 portions , there are two separate swaging tools 200 , 210 physically interconnected ( not shown ), each positioned adjacent the upper and lower sealing portions . these can be activated with a restricted pressure to grip the sleeve , without expanding it . to effect swaging , all that is required is radial expansion of the swaging tools , eliminating the requirement for drawing the swaging tool along each sleeve sealing portion . a further difference to the previous process is that while the swaging tools are expanded , a seal is formed between them . this is used to facilitate expansion of the intermediate 109 portion , by injecting fluid such as water via pipe 220 through a port 230 in the upper swaging tool 200 into the region between the tools , thus generating an expansion pressure . the gripping is also used as a means of transporting the bridging member to the location of the joint . ( fluid or other power supply to the swaging tools 200 , 210 is not shown in this schematic representation ) fig6 b shows the liner after fluid expansion of the intermediate portion has been performed and the upper and lower swaging tools have expanded their corresponding regions . once all swaging has been performed the internal fluid pressure is reduced and the swaging tools are retracted and withdrawn from the pipe , leaving behind a sleeve similarly shaped to that in fig5 , swaged by the alternative process . pipe interconnection in the embodiment of fig6 a and 6 b is consequently achieved in accordance with the following procedure , on the basis that the first sections of pipe have already been joined , forming a continuous section 120 extending into the sea from the pipe laying vessel : 1 ) lower continuous section 120 of pipe ( further ) into the sea to allow the next pipe section 110 to be fitted ; 2 ) clean the mating surfaces of the liners and pipes using , for example , compressed air ; 3 ) position the next pipe section 110 such that the two ends of the steel pipes align and abut ; 4 ) using restricted pressure in the swaging tooling 200 to 240 , grip and lower the sleeve 102 and tooling into the lined pipe section 110 until the upper and lower sealing portions locate adjacent the ends of the liners ( as shown in fig6 a ); 5 ) perform a preliminary weld 180 (“ root pass ” and “ hot ”) of the two pipes together ( fig6 a , step a ); 6 ) during performing of the cooler phase 182 of the welding (“ weld fill ” and “ cap ”) ( or subsequently , if the timing for this stage is not critical ), swage by fluid injection the intermediate portion 109 of the sleeve , in a manner that forces any fluid trapped in the region behind the weld past the as yet unswaged sealing portions ( fig6 b , step b ); 7 ) using full pressure in the swaging tools 200 , 210 , swage the lower and upper sealing portions 106 , 108 of the sleeve to form in each case a seal between sleeve and liner ( fig6 b , steps c and d ); 8 ) retract swaging tools and remove tooling ; 9 ) perform any validation steps , such as non - destructive testing ; 10 ) to continue laying further pipe , repeat the process from step 1 . as in the first embodiment , the weld surfaces are maintained in alignment with respect to each other during this process by the field proven interconnection equipment , rather than by using internal clamps . furthermore , the welding process used is typical for s - lay and j - lay pipe laying methods currently in use , allowing plastic lined pipes to be used commercially for subsea pipe laying . in j - lay systems , where the pipe is suspended almost vertically during the jointing process , the swaging tool ( which also carries the bridging member in the pipe ) can be suspended from the top of a pipe lay tower or ramp , and advanced and retracted by unreeling and winching in the tether 240 . the fluid pressure for expanding the intermediate portion 209 can be contained within an expanding bladder , rather than contained entirely by the seals formed between the swaging tools 200 , 210 and the inside of the bridging sleeve . in a variation of the second embodiment , the welding can begin even before the bridging sleeve is located behind the joint . this may be desirable for reasons of speed , for example , but otherwise brings no particular advantage . it must be ensured in this case that the plastic liner does not deform in the heat of welding , so as to prevent insertion of the bridging member . in both embodiments , the skilled person will appreciate that the individual steps of swaging can be deferred until the most appropriate stage in the process , and as such are not rigidly bound to the order provided above . in all embodiments , however , the form of the bridging sleeve , and the sequence of operation , combine to ensure that there is space , not bridging sleeve material , at the back of the weld during the initial passes . in general , it will be seen that the bridging sleeve proposed herein allows a much greater degree of choice in the sequence of assembly , compared with the sleeves proposed in u . s . pat . no . 6 , 226 , 855 . finally , note that the internal diameter of the join between pipes is not significantly reduced by the method , allowing pigs , for example , to travel the pipeline relatively unhindered . the internal diameter of at least the conduit may be increased at its end , so as to substantially maintain the bore of the liner at the joints , after expansion of the bridging member . the skilled reader will appreciate that numerous variations are possible within the principles of the apparatus described above . accordingly it will be understood that the embodiments illustrated herein are presented as examples to aid understanding , and are not intended to be limiting on the spirit or scope of the invention claimed .