GB1585764A

GB1585764A - Supporting a pipe line or string

Info

Publication number: GB1585764A
Application number: GB35782/76A
Authority: GB
Original assignee: Viking Jersey Equipment Ltd
Current assignee: Viking Jersey Equipment Ltd
Priority date: 1976-08-27
Filing date: 1976-08-27
Publication date: 1981-03-11
Also published as: SE7709562L; BR7705678A; JPS5333944A; NO772929L; ES461896A1; DK376777A; AU2814577A; AU512849B2; FR2362699A1; CA1078228A; DE2738618A1; NL7709414A; ZA774921B

Description

(54) IMPROVEMENTS IN OR RELATING TO SUPPORTING A PIPE LINE OR STRING (71) We, VIKING JERSEY EQUIPMENT LIMITED, a British Company, of RO. Box 72, St. Helier, Jersey, Channel Islands and FILBERT MARCEL CHABRIER, a Citizen of France, of 7 Rue Bastienne, Montmorency 95160, France, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement The present invention relates to supporting a pipe line or string, where a pipe line or string is formed by successively welding fresh sections of pipe to the end of the string.

In the laying of subsea pipe lines, sections of pipe are commonly welded to the end of the pipe string, during the laying process, on a pipe laying barge. As further lengths of pipe are added to the string, the pipe is progressively lowered frolp the stern of the barge to the sea bed as the barge is moved forward. Typically, during the lowering, or launching; operation, the pipe string on the barge is moved rearwardly stepwise in increments corresponding to the length of each fresh pipe section welded to the end of the string.

It will be understood that, employing known welding techniques, the welding together of two sections of pipe takes a significant length of time, requiring several welding passes over the joint or seam. It will also be understood that the speed with which a pipe laying barge can lay pipe is greatly dependent on the speed with which the pipe sections can be welded together on the barge. In order to increase the welding speed, a production line system is commonly used, whereby joints between successive sections are welded at a plurality of welding stations. In a typical arrangement three such welding stations are provided spaced apart along the portion of the pipe string on the barge by distances approximately equal to the length of each fresh section of pipe. Then, considering any one joint between a new section of pipe and the end of the string, the fresh pipe section is initially aligned with the end of the pipe string at a first station and an initial welding pass is performed on the joint to secure the fresh section .to the string. The assembled string is then moved rearwardly along the barge by a distance substantially corresponding to the length of each fresh section of pipe, until the joint under consideration is adjacent a second welding station. The pipe is then held substantially stationary relative to the barge while further welding passes are performed, building up the joint. During this time a further pipe section is being aligned and secured to the end of the string at the first welding station.

Then the pipe string is moved rearwardly once again by a similar distance until the joint under consideration is adjacent to a third welding station, where further welding is performed to complete the welding of the joint. Still further stations may be provided, further "downstream" of the pipe string on the barge for, for example, X-raying the welded joints, repairing faulty joints and preparing "field joints", i.e.

encapsulating the finished weld. The line of welding stations is referred to in the art as "the firing line Clearly the pipe string along the firing line has to be supported at spaced intervals along its length in such a way as to permit the pipe to move along the firing line when required. Hitherto, the pipe has been supported along the firing line on a plurality of roller boxes mounted fixedly on the deck of the barge. The roller boxes each comprise a plurality of rollers arranged in a V formation to support the pipe in the horizontal transverse direction as well as vertically. It will be appreciated that when performing a welding pass at a joint between two abutting ends of pipe, it is essential to have sufficient access circumferentially right round the pipe along the joint or seam.

Automated welding machines are typically used which straddle the pipe around the joint and have one or more carriages carrying welding rods which circulate around the pipe. Such welding devices rest on the pipe when operating on a particular joint and can, therefore, follow any relatively small "to and fro" movement of the pipe relative to the barge. However, the pipe string along the firing line must be supported, typically by the roller boxes, at regular intervals along its length, for example, at one position for every pipe section. Clearly, any movement of a pipe relative to the barge during welding must be sufficiently small so that the joint at which welding is taking place does not traverse, or approach too near to one of the fixed pipe supports.

According to the present invention, apparatus for supporting a pipe string for movement along a firing line where fresh sections of pipe are welded to the end of the string comprises a plurality of supports for supporting the pipe string at spaced locations along its length, said supports being movable with the pipe string, whereby welding access is provided completely around the pipe string at joints between sections thereof continuously even during movement of the pipe string along the line by a distance greater than the support spacing, and means for engaging the supports with the pipe string to support the line and disengaging them from the pipe string after moving with the pipe string a predetermined distance along the line. This arrangement obviates the problem of fixed pipe supports, such as roller boxes, interfering with welding access at the pipe joints. The movable supports can be engaged with the pipe string at locations between joints. The supports then travel with the pipe string as it moves along the firing line, so that welding access at the joints between pipe sections is continuously provided.

The means for engaging and disengaging may comprise an endless belt conveyor extending along the firing line anr carrying the supports, the supports being movable with the pipe by circulation of the belt, and peing engaged and disengaged from the pipe at respective ends of the conveyor.

Alternatively, the supports may comprise trolleys running on a length of track parallel with the firing line. The trolleys may run on a closed circuit track having a return portion, for returning trolleys disengaged from the pipe back to the beginning of the firing line for re-engagement. The trolleys may be interconnected as a chain.

With the trolleys, the means for engaging and disengaging may comprise upwardly and downwardly sloping portions respectively of track at the ends of said length.

Thus the trolleys engage the pipe on the upwardly sloping portions, and disengage on the downwardly sloping portions. However, instead the means for engaging and disengaging may comprise respective track sections at the ends of said length which are raisable and lowerable.

The trolleys may run on a track comprising a return portion parallel to said length and transfer portions interlinking the adjacent ends of said length and the return portion, the transfer portions comprising means for conveying trolleys transversely between said length and the return portion.

Where said length and the return portion of the track comprise respective pairs of rails on which the trolleys can run, each transfer portion may comprise an endless belt conveyor having at least one pair of rail portions mounted transversely on the belt for carrying a trolley. Then, trolleys are conveyed transversely, for example from the end of said length of the track to the beginning of said return portion, by aligning the pair of rail portions on the endless belt with the end of said length of track and running the trolley onto the belt, and then disengaging the trolley from the pipe and circulting the belt to transfer the trolley sideways until aligned with the return portion.

It is essential for obtaining the advantages of the present invention that said movable supports are arranged to carry the pipe string so that there is sufficient radial space around the joints to provide access for welding the joints. The amount of space needed depends on the welding process employed. However, for typical welding processes, said supports desirably carry the pipe string so that there is a radial space of at least two feet completely round the string over the region between adjacent supports.

Very conveniently, the apparatus of the present invention is employed in combination with welding apparatus which is engageable with the pipe string at joints to be welded and is arranged to be movable along the firing line with the pipe, there being provided means for disengaging the welding apparatus from the pipe after movement therewith and for returning the welding apparatus back along the pipe string for re-engagement at a new joint.

It can be seen then that welding at a joint can be continued even while the pipe is being moved along the firing line. Thus, where the present invention is employed in a pipe laying barge, pipe welding can be continued even while a length of pipe is being launched from the barge.

According to a further aspect of the present invention, a method of welding fresh sections of pipe to the end of a pipe string in a firing line comprises the steps of (a) supporting the pipe string along the firing line by the supporting apparatus described above, (b) positioning a fresh pipe section suit ably for welding to the end of the string, (c) engaging the joint so formed with a welding device, (d). performing a first welding pass to fasten the fresh section to the string, (e) performing further welding passes using the same welding device to complete the joint whilst simultaneously moving the pipe string along the firing line and successively repeating steps (b) to (d) with respective further welding devices, and (f) disengaging each welding device when the joint is complete and transferring the disengaged welding device back along the firing line for re-engagement at a new joint.

Examples of the present invention will now be described making reference to the accompanying drawings in which: Figure 1 is a schematic elevational representation of one embodiment of the invention, Figure 2 is a plan schematic view of a second embodiment, Figure 3 is an elevational view of a third embodiment, Figure 4 is a plan view of the embodiment of Figure 3, and Figure 5 is a detailed view of part of the embodiment of Figures 3 and 4 taken along line I-I of Figure 3.

Referring to Figure 1, there is shown a pipe string 10 comprising a plurality of pipe sections 11 joined together at joints 12.

The pipe string 10 moves in the direction of arrow 13 as further sections 14 of pipe are supplied and joined to the end 15 of the string. The fresh pipe sections 14 are supported on roller boxes 16 which, though fixed in the longitudinal direction of the pipe, are adjustable vertically and in the horizontal transverse direction so that the fresh section 14 can be properly aligned with the pipe string 10 for an initial weld to be made to fasten the section 14 to the end 15.

The arrangement illustrated in Figure 1 is as employed on the deck of a barge for laying subsea pipe lines. The pipe is launched from the stern of the barge (which is in the direction of arrow 17) and, as is well known in the offshore pipe laying art, the suspended portion of the pipe between the barge and the seabed is kept under tension to prevent ovalization of the pipe and possible buckling or fracture. To maintain the pipe under a desired tension, tensioners 18 are provided which grip the pipe and automatically let the pipe out, to the left in Figure 1, when tension is too high, and pull pipe in when tension is too low.

Betwen the line up station, where fresh sections 14 of pipe are initially aligned with the pipe string 10, and the tensioners 18, the pipe string 10 is supported by a conveyor support assembly 19. This assembly 19 comprises a plurality of supports 20 mounted on an endless belt 21. fhe supports 20 are spaced apart to support the pipe string 10 at intervals along its length, with one support 20 for each section 11 of pipe. When the pipe string 10 moves in the direction of arrow 13, the supports 20 move with the pipe on the circulating belt 21. The belt 21 may be positively driven appropriately to correspond with the movement of pipe string 10. However, the belt 21 may circulate by being dragged along by the pipe string 10, in which case there must be sufficient friction between the supports 20 and the pipe.

It can be seen that the conveyor support assembly 19 permits welding access to the joints 12 of the pipe string continuously during movement of the pipe up to the tensioners 18. Thus, welding of the various joints 12 along the pipe string can be performed on the region of pipe before the tensioners 18 even when the pipe is moving in the direction of arrow 13.

A further conveyor support assembly 22 is provided for supporting the pipe in a region after the tensioners 18. Thus, access is again provided to the joints in the pipe string over this region for further welding, X-ray checking and final encapsulation of the joints. After the joints are completed, the pipe is supported on fixed roller boxes 23 to the stern of the barge for launching.

It can be seen that the conveyor support assemblies 19 and 22 cause the pipe supports 20 to disengage from the pipe string at a point 24 where the belt 21 is deflected around the. roller at the end of the run of the conveyor. The supports 20 are then returned back along the firing line on the return run of the belt 21 and reengaged with the pipe string at a point 25.

So that the supports 20 -provide a firm support for the pipe string 10, means (not shown) are provided to prevent deflection of the belt 21 under the load of the pipe.

Considering now Figure 2, there is shown in plan view an arrangement for supporting a pipe along the.firing line, which corresponds to the arrangement of Figure 1, except that instead of conveyor support assemblies 19 and 22 there are provided series of trolleys 30 running on closed circuit tracks 31. Each track 31 has a length 32 extending parallel and beneath the line of the pipe string 10. Trolleys 30 running on the length of track 32 support the pipe 10 and move with the pipe. The supporting trolleys 30 are disengaged from the pipe near the tensioners 18 and return on the closed circuit path 31 along a return portion 33 for re-engagement with the pipe 10 near where fresh sections of pipe are welded on.

For disengagement from the pipe, the length 32 of the track 33 conveniently has a downwardly sloping portion at its end near the tensioners, so that the trolleys 30 drop away from the pipe 10 sufficiently to allow them to be deflected laterally from the pipe string around the track. Similarly, there is a correspondingly upward sloping portion at the other end of the length 32 of the track, so that the trolleys 30 come up into supporting engagement with the pipe. In the arrangement of Figure 2 the trolleys 30 may be linked together as an endless chain.

It will be understood that it is important that supports 20 in the embodiment of Figure 1 and trolleys 30 in the embodiment of Figure 2 are brought into engagement with the pipe string 10 at positions between adjacent joints 12 in the pipe string 10 so that welding access is provided at the joints. The spacing between adjacent pairs of supports 20, and between adjacent pairs of trolleys 30 is made to correspond with the length of the pipe sections 11.

However, in practice pipe sections 11 vary in length by an appreciable amount, so that an accumulated error can arise causing supports 20 or trolleys 30 to be engaged too close to a joint 12. To prevent this occurring, the supports 20 or the trolleys 30 may be provided with rollers for engaging the pipe string 10 so that a relative movement between the supports 20 or trolleys 30 can be obtained when desired to move supports 20 or trolleys 30 back away from joints 12. Sufficient friction may still be provided between supports 20 or trolleys 30 and the pipe string 10 for movement of the pipe to drag the supports 20 or trolleys 30 along in normal operation. However, it may be desirable to positively drive the belt 21, or the chain of trolleys 30, at a speed suitable to keep the support 20 or trolleys 30 well away from joints 12.

Referring now to Figures 3, 4 and 5, there is shown a further embodiment of the present invention as applied for supporting a pipe string 39 along a firing line between a pipe tensioner 42 and an alignment station where fresh sections 41 of pipe are brought into alignment with the pipe string. As before, the fresh sections 41 are supported on roller mountings 40 which are fixed to the deck 43. However, the portion 39 of the pipe string in the firing line is supported on trolleys 44 which run on rails 46 provided parallel with and beneath the line of the pipe. The trolleys 44 are not connected together and are dragged along the rails 46, when the pipe string 39 moves to the left in Figure 3 by friction.

Pipe is typically made in sections which are approximately 40 feet long. However, in order to speed production of the pipe string for laying in a pipe laying operation, it is desirable that each fresh section of pipe welded to the end of the string is as long as possible, so that maximum new length of pipe is nodded to the string for each welded joint. In the arrangement illustrated in Fgures 3 to 5 each fresh section 41 of pipe comprises two individual pipe sections welded together at a joint 45. Thus, using the usual 40 ft. pipe lengths, each fresh section 41 is approximately 80 ft. However, as shown in Figure 3 one supporting trolley 44 is provided for each 40 ft. length of pipe, i.e. two trolleys are provided to support each fresh section 41 of pipe added to the string 39.

The rails 46 are mounted above the level of the deck 43 by means of pillars 47. As can best be seen from Figure 4, trolleys disengaged from the pipe string 39 at the downstream end of the firing line near the tensioner 42 are returned to the upstream end of the firing line near the roller mounting 40 by a return rail 48. The trolleys are transferred laterally between the ends of the rail 46 and the rail 48 by respective transfer devices 49 and 50.

Figure 5 is a view on a larger scale, taken along line I-I in Figure 3, which shows the transfer device 49 in greater detail. The device 49 comprises a transfer trolley 51 running on rails 52 mounted substantially at right angles to the rails 46 and 48. The trolley 51 comprises a base 53 on which are mounted wheels 54 which run on the rails 52. A platform 55 is carried above base 53 on a plurality of jacks 56, which may typically be quick-acting hydraulic jacks. A pair of rail portions 57 at a gauge corresponding to the gauge of rails 46 and 48 are provided on the top surface of the platform 55.

In operation, the trolley 51 is moved to the left-hand position shown in Figure 5, and with the platform 55 raised on the jacks 56, so that the rail portions 57 align with the rails 46. A buffer 58 may be pro- vided to stop the trolley 51 at the correct position in alignment. The trolley 51 may be driven, for example, by an electric motor, in which case the buffer 58 may include a trip for disconnecting supply to the motor when the trolley is in alignment.

A trolley 44 supporting the pipe string 39 is moved along the rail 46 and onto the aligned rail portions 57 of the trolley 51 by move ment of the string 39. When the trolley 44 is completely on the rail portions 57, as shown in Figure 3, the jacks 56 are operated quickly to lower the platform 55, disengaging the bearing sruface 62 of the trolley 44 from the bottom of the pipe.

A trip may be provided operated by the trolley 44 to initiate the lowering operation of jacks 56. Clearly, if the pipe 39 is still moving when the trolley 44 is completely on the rail portions 57, the operation of jacks 56 must be Stifficiently fast to disengage the trolley 44 from the pipe before the trolley is pulled by the pipe off the side of the. transfer trolley 51. However, it may be arranged that movement of the pipe string 39 is terminated when each trolley 44 is loaded on the transfer trolley 51, in which case this problem is obviated. Alternatively, the end of the rail 46 near the transfer device 49 may slope downwards so that trolleys 44 are disengaged from the pipe as or just before they move onto the transfer trolley 51. The height of rail portions 57 of the transfer trolley 51 is then, of course, adjusted to align with the lower end of rail 46. Further, means may be provided if necessary to pull each trolley 44 completely onto trolley 51 after disengagement from the pipe. The trolley 51 may still have jacks 56 to lower each trolley 44 to provide the extra clearance from the pipe required to move the trolley 44 laterally from beneath the pipe.

The jacks 56 lower the platform 55 sufficiently so that the supporting surfaces 62 of the trolley 44 are completely clear of the pipe 39. Then the transfer trolley 51 is moved along the rails 52 until the rails 57 on the platform 55 align with the return rails 48. A further buffer and trip device 59 may be provided to halt the transfer trolley 51 accurately in alignment.

The rails 48 may be mounted above the deck 43 at a educed heighit compared with the rails 46 corresponding with the height of the rails 57 on the platform 55 in the lowered position. When the rails 57 are aligned with rails 48, the trolley 44 carried by the transfer trolley Sf is unloaded onto the rails 48 for the journey back to the beginning of the firing line. The trolleys 44 may be driven along the rail 48 by means of an endless toothed belt 60. A tooth projection 61 may be provided at the bottom of each trolley 44 for engaging in the teeth of belt 60. When the trolley 44 is unloaded, the trolley 51 is returned to the left hand position in Figure 5 and the platform 51 raised again ready to receive the next trolley 44.

The transfer device 50 at the other end of the return rails 48 operates in the same manner as described above, but in the reverse sequence. Thus, a trolley 44 arriving at transfer device 50 on rails 48 is driven onto the rail portions 57 on the transfer trolley 51. The transfer trolley 51 is then driven under the line of the fresh pipe section 41, and the jacks 56 are operated to raise the platform 55 so that the trolley 44 engages the fresh section 41, when desired.

An example of a welding sequence will now be described which makes use of the embodiment of the invention described with reference to Figures 3 to 5. In this example, three separate welding devices 70, 71 and 72 are employed. With the pipe string 39 substantially stationary relative to the deck 43, a fresh pipe section 41 is brought into near abutting relationship with the end of the pipe string. Roller mountings 40 supporting the fresh pipe section 41 are adjusted to align the fresh section 41 with the string in a known manner, and the fresh section 41 is then clamped in position relative to the end of the pipe string 39.

The welding device 70 is then lowered, by means not shown in Figure 3, so that it rests on the pipe over the joint between the fresh section 41 and the rest of the string 39. Before the pipe string 39 is moved along the firing line, a first welding pass is performed by the welding device 70 On this joint to secure the fresh section 41 to the end of the pipe string. Thereafter, one or more further welding passes may be performed by welding device 70. During these further welding passes, the pipe string 39 may be moved rearwardly over the deck 43 in the direction of arrow 73. During this movement the welding device 70 moves along with the pipe as it is totally supported by resting on the pipe. The lifting tackle for the device 70 may run along a rail 74 suspended from the roof 75.

When the designated number of passes by welding device 70 have been completed, typically when the pipe has moved in the direction of arrow 73 a distance substantially corresponding to the length of each fresh section 41, the welding device 70 is lifted from the pipe and transferred back along the firing line for re-engagement with the pipe at the joint between the next fresh pipe section 41 and the end of the string.

At the same time the welding device 71 is transferred from further along the line back to the position of the joint just vacated by the welding device 70, i.e. the position of welding device 71 in Figure 3. Then, whilst welding device 70 is operating on the joint with the new section 41, welding device 71 makes further welding passes on the original joint, and may continue operating during a further move in the direction of arrow 73 of the pipe string 39.

Once again, when the designated number of welding passes has been performed by welding device 71, at a time when the pipe has completed its second move, the welding device 71 is raised from the pipe and transferred back to the position shown in Figure 3, while the third welding device 72 is lowered to engage with the joint, which is now in the region of welding device 72 as shown in Figure 3. The welding device 72 performs at least one further welding pass on the joint to complete the welding.

However, the operation of welding device 72 should be completed before the next move of the pipe 39, since in the embodiment of Figure 3 there is insuffiicent space before the tenisoners 42 for the welding device 72 to travel along with the pipe during a complete move. Thus, the designated amount of welding to be performed by welding device 72 should be less than that to be performed by device 71.

As explained previously, the application of the movable supports of the present invention in the firing line provides a significant time saving. Employing three welding stations as described above, the welding of joints in a 36 inch diameter pipe typically requires about 6 minutes of welding time at the first station (welding device 70), 8 minutes at the second station (welding device 71) and 6 minutes at the third station (welding device 72). A further 2 minutes is required at station 1 for lining up each fresh pipe section 41 and clamping it in position. In an offshore pipe laying operation, the movement of the pipe string 39 towards the stern of the pipe laying barge is accompanied by a forward movement of the barge by a corresponding distance. A forward move of 80 feet, corresponding to 80 ft. pipe sections, may take a typical minimum time of 2 minutes. Thus, if welding canont be continued during movement of the barge, the total time required for each complete moving cycle is 8 minutes plus 2 minutes equals 10 minutes. However, employing the present invention, welding devices 70 and 71 may operate on the pipe during the 2 minutes of barge movement.

Since the welding operation of device 72 is completed in 6 minutes, i.e. before barge movement begins, the overall cycle time between barge movements is reduced to 8 minutes, i.e. a saving of 20. If a cycle time saving of this order can be reflected in the overall time taken for a pipe-laying job, the saving in terms of operating cost can be very significant.

Instead of transferring each welding device after a designated number of welding passes back to the preceding joint to repeat the welding programme on the new joint, a continuous welding method may be employed, in which each welding device performs a complete welding operation on a respective joint. Thus, referring again to Figure 3 welding device 70 can be used to perform the full number of welding passes on its respective joint, with the welding device moving with the pipe 39 right down the firing line substantially to the position of welding device 72. When welding is completed at the particular joint, welding device 70 is lifted clear of the pipe by means not shown in the Figure and is then moved laterally of the pipe and returned back to the beginning of the firing line (also by means not shown) to begin welding a fresh joint. With three welding devices, there will then be two devices operating on respective joints of the pipe string 39 at any time with the third welding device in transit from the end to the beginning of the firing line.

Clearly, modifications can be envisaged in which more than three welding devices are employed, enabling a faster production rate.

WHAT WE CLAIM IS:- 1. Apparatus for supporting a pipe string for movement along a firing line where fresh sections of pipe are welded to the end of the string, the apparatus comprising a plurality of supports for supporting the pipe string at spaced locations along its length, said supports being movable with the pipe string, whereby welding access is provided completely around the pipe string at joints between sections thereof continuously even during movement of the pipe string along the line by a distance greater than the support spacing, and means for engaging the supports with the pipe string to support the li

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

72 should be completed before the next move of the pipe 39, since in the embodiment of Figure 3 there is insuffiicent space before the tenisoners 42 for the welding device 72 to travel along with the pipe during a complete move. Thus, the designated amount of welding to be performed by welding device 72 should be less than that to be performed by device 71.

As explained previously, the application of the movable supports of the present invention in the firing line provides a significant time saving. Employing three welding stations as described above, the welding of joints in a 36 inch diameter pipe typically requires about 6 minutes of welding time at the first station (welding device 70), 8 minutes at the second station (welding device 71) and 6 minutes at the third station (welding device 72). A further 2 minutes is required at station 1 for lining up each fresh pipe section 41 and clamping it in position. In an offshore pipe laying operation, the movement of the pipe string 39 towards the stern of the pipe laying barge is accompanied by a forward movement of the barge by a corresponding distance. A forward move of 80 feet, corresponding to 80 ft. pipe sections, may take a typical minimum time of 2 minutes. Thus, if welding canont be continued during movement of the barge, the total time required for each complete moving cycle is 8 minutes plus 2 minutes equals 10 minutes. However, employing the present invention, welding devices 70 and 71 may operate on the pipe during the 2 minutes of barge movement.

Since the welding operation of device 72 is completed in 6 minutes, i.e. before barge movement begins, the overall cycle time between barge movements is reduced to 8 minutes, i.e. a saving of 20. If a cycle time saving of this order can be reflected in the overall time taken for a pipe-laying job, the saving in terms of operating cost can be very significant.

Instead of transferring each welding device after a designated number of welding passes back to the preceding joint to repeat the welding programme on the new joint, a continuous welding method may be employed, in which each welding device performs a complete welding operation on a respective joint. Thus, referring again to Figure 3 welding device 70 can be used to perform the full number of welding passes on its respective joint, with the welding device moving with the pipe 39 right down the firing line substantially to the position of welding device 72. When welding is completed at the particular joint, welding device 70 is lifted clear of the pipe by means not shown in the Figure and is then moved laterally of the pipe and returned back to the beginning of the firing line (also by means not shown) to begin welding a fresh joint. With three welding devices, there will then be two devices operating on respective joints of the pipe string 39 at any time with the third welding device in transit from the end to the beginning of the firing line.

Clearly, modifications can be envisaged in which more than three welding devices are employed, enabling a faster production rate.

WHAT WE CLAIM IS:- 1. Apparatus for supporting a pipe string for movement along a firing line where fresh sections of pipe are welded to the end of the string, the apparatus comprising a plurality of supports for supporting the pipe string at spaced locations along its length, said supports being movable with the pipe string, whereby welding access is provided completely around the pipe string at joints between sections thereof continuously even during movement of the pipe string along the line by a distance greater than the support spacing, and means for engaging the supports with the pipe string to support the line and disengaging them from the pipe string after moving with the pipe string a predetermined distance along the line.
2. Apparatus as claimed in claim 1 wherein the means for engaging and disengaging comprise an endless belt conveyor extending along the firing line and carrying the supports, the supports being movable with the pipe string by circulation of the belt, and being engaged and disengaged from the pipe string at respective ends of the conveyor.
3. Apparatus as claimed in claim 1 wherein the supports comprise trolleys running on a length of track parallel with the firing line.
4. Apparatus as claimed in claim 3 wherein the trolleys run on a closed circuit track having a return portion, for returning trolleys disengaged from the pipe string back to the beginning of the firing line for re-engagement.
5. Apparatus as claimed in claim 4 wherein the trolleys are interconnected as a chain.
6. Apparatus as claimed in any of claims 3 to 5 wherein the means for engaging and disengaging comprises upwardly and downwardly sloping portions respectively, of track at the ends of said length.
7. Apparatus as claimed in claim 3 wherein the means for engaging and disengaging comprises respective track sections at the ends of said length which are raisable and lowerable.
8. Apparatus as claimed in claim 7 wherein the trolleys run on a track including a return portion parallel to said length and

transfer portions interlinking the adjacent ends of said length and the return portion, the transfer portions comprising means for conveying trolleys transversely between said length and the return portion.
9. Apparatus as claimed in claim 8 wherein said length and the return portion of the track comprise respective pairs of rails on which the trolleys can run and each transfer portion comprises an endless belt conveyor having at least one pair of rail portions mounted transversely on the belt for carrying a trolley.
10. Apparatus as claimed in any preceding claim and in combination with welding apparatus which is engageable with the pipe string at joints to be welded and is arranged to be movable along the firing line with the pipe string, there being provided means for disengaging the welding apparatus from the pipe string after movement therewith and for returning the welding apparatus back along the pipe string for re-engagement at a new joint.
11. A method of welding fresh sections of pipe to the end of a pipe string in a firing line, comprising the steps of (a) supporting the pipe string along the firing line by the supporting apparatus claimed in any preceding claim, (b) positioning a fresh pipe section suitably for welding to the end of the string, (c) engaging the joint so formed with a welding device, (d) performing a first welding pass to fasten the fresh section to the string, (e) performing further welding passes using the same welding device to complete the joint whilst simultaneously moving the pipe string along the firing line and successively repeating steps (b) to (d) with respective further welding devices, and (f) disengaging each welding device when the joint is complete and transferring the disengaged welding device back along the firing line for re-engagment at a new joint.
12. Apparatus for suporting a pipe string for movement along a firing line substantially as hereinbefore described with reference to and as illustrated in Figures 1 or 2, or Figures 3 to 5.
13. A method of welding fresh sections of pipe to the end of a pipe string, as claimed in claim 11 and substantially as hereinbefore described.