LAYING OF UNDERSEA PIPES

Abstract: Method and apparatus for connecting first (102) and second (104) sections of double-walled pipe (10) comprising joining the inner pipes of the first and second sections, sliding the outer pipe of the second section relative to the inner pipe of that section such that it meets the outer pipe of the first section, and welding the outer pipes of the first and second sections. The inner insulation layer can also be translated together with the outer pipe, avoiding the need to insert additional insulation sections. It is particularly preferred that bulkheads (112) are fitted between the outer and inner pipes, the bulkheads comprising resilient annular members compressed longitudinally with respect to the pipe. These bulkheads (112) provide a frictional force which holds the outer (104) and inner pipes (102) in place but which can be overcome to slide the outer pipe relative to the inner. In addition, novel double-walled pipe sections suitable for use in this method are disclosed.

Description: LAYING OF UNDERSEA PIPES The present invention relates to the laying of undersea pipes. It is particularly concerned with the manner in which double walled pipes may be laid, and overcomes difficulties encountered in laying at greater than normal depths. Double walled pipes comprise an inner flow pipe encased within an outer protective pipe. The region in between is filled with an insulating material to prevent excessive heat loss from fluid within the inner flow pipe. This allows the safe transport of fluids at elevated temperatures, for example crude oil, without precipitation of lower melting point fractions causing blockage of the pipe. These pipes are usually laid by welding together successive sections 14 on a barge 10, and then submerging the string thus formed behind the barge along the intended route to the pipeline. This is illustrated diagrammatically in Figure 1 . The string 12 exits the barge 10 horizontally and then bends downward before returning to the horizontal at the sea bed. This is commonly referred to as "S"-lay, due to the shape taken up by string 12 during descent. An S-lay arrangement is suitable at shallow depths, but encounters difficulties when the pipe is to be laid in deeper water for the simple reason that the tension in the pipe and in particular the bending forces become excessive simply due to the depth of necessary descent. It is therefore desired to employ a "J"-lay procedure as diagrammatically shown in Figure 2. In this arrangement, the string 12 leaves the barge 10 vertically downward or nearly so and therefore experiences less tension and lower bending forces. However, J-lay requires a tower 1 6 to be built at the rear of the barge to accommodate the sections 14 during welding together to form the string. There are inherent limits on the possible height of this tower 16 imposed by the necessary stability of the barge 10. The relevance of this limitation is that in an S-lay arrangement, several sections 14 would be welded together in advance prior to welding onto the end of the string. Thus, section welding can in effect be carried out in parallel allowing a very high progress rate. In J-lay, the number of sections 14 which can be pre- welded together will be limited by the height of the tower 1 6. This in turn limits the speed in which the pipeline can be laid. Thus, whilst lay speed is critical in both S and J-lay, it is the speed at which the string 1 2 is welded together which is rate-limited in J-lay. In single-walled pipe structures, the constraint is not usually pressing. However, for a double-walled pipe structure it is necessary to weld the inner flow pipe and place insulating packing around the joint. The outer pipe will normally be formed short of the inner pipe to allow access for welding, and therefore a further cover must be put in place around the insulating packing and welded to the two ends of the outer pipe. This is a somewhat lengthy procedure. The result of this is that J-lay has hitherto been generally limited to single-walled pipe structures and that therefore double-walled pipe structures have effectively been limited to relatively shallow depths only. The present invention seeks to provide a manner of connecting sections of double-walled pipe so that they can be formed into a string sufficiently swiftly for an economic J-lay operation. The present invention therefore provides a method of connecting first and second sections of double-walled pipe comprising joining the inner pipes of the first and second sections, sliding the outer pipe of the second section relative to the inner pipe of that section such that it meets the outer pipe of the first section, and welding the outer pipes of the first and second sections. This method is in contrast to previous methods in that by sliding the outer sections in a J-lay operation, the step of adding connecting covers between the outer sections is avoided. There has not previously been a safe and controllable means of doing so. The inner insulation layer can also be translated together with the outer pipe, avoiding the need to insert additional insulation sections. This significantly accelerates the connection procedure. It is particularly preferred that bulkheads are fitted between the outer and inner pipes, the bulkheads comprising resilient annular members compressed longitudinally with respect to the pipe. These bulkheads provide a frictronal force which holds the outer and inner pipes in place but which can be overcome to slide the outer pipe relative to the inner. The present invention also relates to novel double-walled pipe sections suitable for use in this method. Thus, it provides a double-walled pipe section comprising an inner and an outer pipe with an insulating material therebetween, the inner and outer pipes being of corresponding lengths but mutually offset such that at least one end of inner pipe is accessible. It is particularly intended that the inner and outer pipes will be of the same length but staggered so that the protruding inner pipe can be welded in place and the outer pipe translated, leaving the previously hidden end of inner pipe now protruding for subsequent connection. However, the invention is still applicable with different but corresponding lengths of inner and outer pipe, although this may require other less desirable consequences such as the use of alternating designs of pipe section. It is again particularly preferred that bulkheads are fitted between the outer and inner pipes, the bulkheads comprising resilient annular members compressed longitudinally with respect to the pipe. These bulkheads provide a frictional force which holds the outer and inner pipes in place but which can be overcome to slide the outer pipe relative to the inner. The invention also relates to apparatus which is useful in handling such pipes to achieve the above method. Thus, it further provides an apparatus for handling double-walled pipe structures, comprising a means for supporting a first sectional length in an upright state, a means for supporting a second sectional length in a generally aligned state, thereby to allow joining of the inner pipes, and means for displacing the outer pipe of the second sectional length relative to the inner pipe thereof, thereby to allow jointing of the outer pipes. The means for displacing can comprise an expandable gripping member for applying relative tension to the interior of the inner pipe. Likewise, the means for displacing the outer pipe can comprise suitable abutments for applying a relative longitudinal compression to the outer pipe. The apparatus can include a suitable drive means for imparting relative motion, such as a hydraulic jack. An embodiment of the present invention will now be described, by way of example, with reference to the accompanying Figures, in which: Figure 1 , already described, shows the S-lay procedure; Figure 2, also already described, shows the J-lay procedure; Figure 3, shows a pipe section according to the present invention; Figure 4 shows the manner of connecting three such pipe sections; Figure 5 shows the connection of two sets of three sub-sections to form a 72 m section; Figure 6 shows how such 72 m sections can be attached to the end of a string; and Figure 7 shows apparatus as used in Figure 6. Referring to Figure 3, this shows a pipe section 100 comprising an inner flow pipe 102 and an outer pipe 104. An insulating material 106 is placed in the annular space therebetween. The inner flow pipe 102 and the outer pipe 104 are of identical length, but are somewhat staggered so that at a first end 108, the inner pipe 102 protrudes beyond the end of the outer pipe 104, whilst at the opposed face 1 10, the inner flow pipe is effectively recessed within the outer pipe 104. The pipes are held in this configuration by bulkheads 1 1 2 at either end. These bulkheads are preferably elastomeric and longitudinally compressible so as to grip the respective walls of the pipes. A degree of compression can easily be found for such bulkheads which prevents free sliding but which allows controlled sliding at a few tonnes of force. This allows safe and controllable sliding in the content of J-lay. Suitable bulkheads are disclosed in our International Application published as WO 96/36831 . Figure 4 illustrates the connection of a pair of sub-sections 100. They are placed in end-to-end abutting relationship such that the ends 108 meet. The bulkheads 1 1 2 at the facing ends are removed, and the inner flow lines 102 of the sub-sections 100 brought into contact. The outer pipes 104 will not contact, due to the relative staggering illustrated in Figure 3. The inner flow lines 102 are then welded and additional insulation material placed in the annulus base immediately around the weld. This is shown in Figure 4(a). The bulkhead 1 1 2' from the free end 1 10 of one of the sub-sections 1 00' is then removed, releasing the outer pipe 1 04' of that sub-section. This is then moved along the section until it abuts the outer pipe of the other section 100. The two are then welded, as shown in Figure 4(b). The process is then repeated at the free end of the sub-section 100 prime with a further sub-section 1 00". This is shown in Figures 4(c) and 4(d). In the same way, two groups of three sub-sections can be brought together as shown in Figures 5(a) and 5(b) and joined in the same way to form a single pipe section 1 14. The original sub-sections 100 are normally 1 2 m in length, meaning that the section 1 14 will be a total of 72 m in length. This is a typical length for working on the vertical tower of a J-lay barge such as shown in Figure 2. Figure 6 shows how the individual sections 1 14 can be assembled into a pipeline. In Figure 6(a) the free end 1 1 6 of a partially submerged pipeline is held by a gripping means 1 1 8 in a vertical position. In Figure 6(b) a section length 1 14 is held vertically above the free end 1 1 6 and with its end 108 lowermost such that the protruding inner pipes 102 are able to contact. They are then welded in this position, as shown in Figure 6(c). Figure 6(d) shows a manipulator 1 20 attached to the upper end of the section 1 14, which acts to grip the inner and outer pipes 102, 104. As shown in Figure 6(e) the manipulator 1 20 pushes the outer pipe 104 downwardly relative to the inner flow pipe 102, bringing it into contact with the outer pipe of the free end of the string 1 1 6. The outer pipes are then welded in place, amalgamating the previous section 1 14 into the string 1 1 6. This can then be lowered further. In practice, subsections 100 can be assemble into 72 m sections 1 14 on board the barge and then placed in the vertical position. The final link can then be made as shown in Figure 6. Through the present invention, that final link can be made swiftly allowing pipe laying to proceed in a swift and economic fashion. Figure 7 shows the manipulator 1 20 of Figure 6 in more detail. A central shaft 1 22 carries an outer gripper 1 24 and an inner gripper 1 26. The inner gripper 1 26 comprises a tapered plug 1 28 on the end of the shaft 1 22, the widest point being at the free end, together with a tapered annulus 130 around the tapered plug 1 28. The annulus 1 30 is tapered on its internal face, such that its narrowest point (ie widest internal bore) is adjacent the free end of the plug 1 28. The outer diameter of the annulus 1 30 is approximately the inner diameter of the inner flow line 102. Thus, the plug 1 28 and annulus 130 can be inserted within the inner pipe 102, but if the shaft 1 22 is sought to be removed, the plug 1 28 will expand the slightly resilient annulus 1 30 and grip the inner faces of the inner pipe 102. The inner gripper 126 also has a small hydraulic jack 132 mounted on the shaft 1 22 and onto a plate 1 34 adapted to overlie the inner pipe 102. This serves to hold the shaft 122 in the correct position with the plug 1 28 within the end of the inner pipe 102, and provide a slight upward force so as to provide the necessary gripping action with the inner pipe 102. The upper grip 1 24 simple comprises an end plate 1 36 below which are mounted hydraulic jacks 1 38. These end in abutments 140 which are adapted to contact the end of the outer pipe 104. Thus, the shaft 1 2 is inserted into the end of the inner pipe 102 and the jacks 1 32 extended so as to cause a gripping action 1 28 and the pipe 102. The jacks 1 38 are then extended until the abutments 140 at the ends thereof contact the end face of the outer pipe 104 and impart a downward force thereto. This causes the outer pipe 104 to slide over the inner pipe 102, from the position shown in Figure 6(d) to that shown in Figure 6(e). The jacks 1 32 and 1 38 can then be released, in enabling the shaft 1 22 to be withdrawn and the manipulator 1 20 to be removed. It will of course be understood that many variations are possible within the scope of the present invention.

Claims: CLAIMS 1 . A method of connecting first and second sections of double-walled pipe, comprising joining the inner pipes of the first and second sections, sliding the outer pipe of the second section relative to the inner pipe of that section such that it meets the outer pipe of the first . section, and welding the outer pipes of the first and second sections. 2. A method according to claim 1 in which bulkheads are fitted between the outer and inner pipes, the bulkheads comprising resilient annular members compressed longitudinally with respect to the pipe. 3. A double-walled pipe sections comprising an inner and an outer pipe with an insulating material therebetween, the inner and outer pipes being of corresponding lengths but mutually offset such that at least one end of inner pipe is accessible. 4. A pipe section according to claim 3 in which the inner and outer pipes are of the same length but staggered. 5. A pipe section according to claim 3 or claim 4 in which bulkheads are fitted between the outer and inner pipes, the bulkheads comprising resilient annular members compressed longitudinally with respect to the pipe. 6. Apparatus for handling double-walled pipe structures, comprising a means for supporting a first sectional length in an upright state, a means for supporting a second sectional length in a generally aligned state, thereby to allow joining of the inner pipes, and means for displacing the outer pipe of the second sectional length relative to the inner pipe thereof, thereby to allow jointing of the outer pipes. 7. Apparatus according to claim 6 in which the means for displacing comprises an expandable gripping member for applying relative tension to the interior of the inner pipe. 8. Apparatus according to claim 6 or claim 7 in which the means for displacing the outer pipe comprises suitable abutments for applying a relative longitudinal compression to the outer pipe. 9. Apparatus according to any one of claims 6 to 8 including a drive means for imparting relative motion to the inner and outer walls. 10 Apparatus according to claim 9 in which the drive means is a hydraulic jack. 1 1 . A method of laying double walled pipe structures substantially as herein described with reference to and/or as illustrated in the accompanying drawings. 1 2. A double walled pipe structure substantially as herein described with reference to and/or as illustrated in the accompanying drawings. 13. Apparatus for handling double walled pipe structures substantially as herein described with reference to and/or as illustrated in the accompanying drawings.

Inventors: SUMMERFIELD PAUL, WELCH STUART, SUMMERFIELD, PAUL, WELCH, STUART
Applicants: CORUS UK LTD, SUMMERFIELD PAUL, WELCH STUART, CORUS UK LIMITED, SUMMERFIELD, PAUL, WELCH, STUART
Application year: 1999
Main classification: F16L39
IPC codes: F16L1