Techniques for coating pipes

Abstract

A method and apparatus are for coating a field joint of a pipeline. The method includes positioning a mold tool around the field joint to define a mold cavity, positioning an insert to lie within the mold cavity, and injecting plastics material into the mold cavity to embed the insert in the plastics material. The apparatus has a mold tool positionable around the field joint to define a mold cavity, and an insert positionable within the mold cavity to be embedded in plastics material injected into the mold cavity. Use of an insert in this way reduces the volume of the molten thermoplastics material required to coat the field joint, thereby increasing the interface area of the melt relative to its volume, to the benefit of cooling time.

Claims

1. A method of coating a field joint of a pipeline, comprising: positioning a mold tool around the field joint to define a mold cavity; positioning an insert to lie within the mold cavity; and injecting molten plastics material into the mold cavity to embed the insert in the plastics material; wherein the insert comprises a body and at least one passage extending through the body, the plastics material flows through the passage in the body of the insert as the mold cavity fills, wherein the insert is of a different material to the plastics material injected into the mold cavity, and the material of the insert is relatively insulative in comparison with the plastics material injected into the mold cavity.

2. The method of claim 1, wherein the insert is positioned on the field joint before the mold tool is positioned around the field joint.

3. The method of claim 2, wherein the insert is positioned on a field joint at an upstream station and the mold tool is positioned around a field joint at a downstream station with respect to a direction of movement of the pipeline.

4. The method of claim 1, wherein the insert comprises two or more sections, and wherein the method comprises bringing the two or more sections together around the field joint.

5. The method of claim 1, comprising maintaining a gap between a body of the insert and the pipes joined by the field joint to allow the plastics material to flow around the insert as the mold cavity fills.

6. The method of claim 1, wherein the plastics material flows through a body of the insert as the mold cavity fills.

7. The method of claim 1, wherein the method is performed on a pipelaying vessel comprising a pipeline production facility.

8. The method of claim 1, wherein the method produces a pipeline or field joint for a pipeline.

9. An apparatus for coating a field joint of a pipeline, comprising a mold tool positionable around the field joint to define a mold cavity, an insert positionable within the mold cavity to be embedded in plastics material injected into the mold cavity, the insert comprising a body and at least one passage that extends through the body of the insert through which passage molten plastics material can flow, the insert is of a different material to the plastics material injected into the mold cavity, and the material of the insert is relatively insulative in comparison with the plastics material injected into the mold cavity.

10. The apparatus of claim 9, wherein the insert comprises at least one spacer formation arranged to maintain a gap between a body of the insert and the pipes joined by the field joint.

11. The apparatus of claim 9, wherein the insert comprises at least one passage extending through a body of the insert.

12. The apparatus of claim 9, wherein the insert comprises two or more sections capable of being brought together around the field joint.

13. The apparatus of claim 9, wherein the apparatus is on a pipelaying vessel.

14. The apparatus of claim 9, wherein the apparatus produces a pipeline or field joint for a pipeline.

15. A pipeline production facility comprising: an apparatus for coating a field joint of a pipeline, comprising a mold tool positionable around the field joint to define a mold cavity, and an insert positionable within the mold cavity to be embedded in plastics material injected into the mold cavity, the insert comprising a body and at least one passage that extends through the body of the insert through which passage molten plastics material can flow, the insert is of a different material to the plastics material injected into the mold cavity, and the material of the insert is relatively insulative in comparison with the plastics material injected into the mold cavity; and wherein the insert is positioned on a field joint at an upstream station and the mold tool is positioned around a field joint at a downstream station with respect to a direction of movement of a pipeline being produced by the facility.

16. The pipeline production facility of claim 15, wherein the pipeline production facility is a pipelaying vessel.

17. The pipeline production facility of claim 15, wherein the pipeline facility produces a pipeline or a field joint for a pipeline.

Description

(1) Reference has already been made to FIGS. 1a to 1c of the accompanying drawings to describe the prior art. In order that the invention may be more readily understood, reference will now be made, by way of example, to the remaining drawings in which.

(2) FIG. 2 is a schematic side view of a lay barge configured for S-lay operation, showing a typical context for the coating techniques of the present invention;

(3) FIG. 3 is a schematic cross-sectional view on line III-III of FIG. 4a, showing a mould tool in accordance with the invention positioned around a field joint;

(4) FIGS. 4a to 4c are schematic longitudinal sectional detail views of the mould tool and field joint on line IV-IV of FIG. 3, showing the progression over time of a sequential injection moulding operation in accordance with the invention;

(5) FIGS. 5a and 5b are schematic longitudinal sectional detail views of an alternative mould tool positioned around a field joint, showing the progression over time of another sequential injection moulding operation in accordance with the invention.

(6) FIGS. 6a and 6b are schematic cross-sectional views of another mould tool in accordance with the invention positioned around a field joint, showing the progression over time of a further sequential injection moulding operation in accordance with the invention;

(7) FIG. 7 is a schematic side view of upstream and downstream coating stations in the firing line on a vessel as shown in FIG. 2;

(8) FIGS. 8a and 8b are schematic longitudinal sectional detail views of different mould tools around a field joint in a variant of the invention in which successive moulding operations are performed at the upstream and downstream coating stations shown in FIG. 7;

(9) FIG. 9 is a schematic longitudinal sectional detail view of a mould tool around a field joint in a further variant of the invention in which an insert is embedded in injection-moulded plastics;

(10) FIG. 10 is a side detail view of a stinger arrangement of a lay barge, in which rollers are replaced by continuous track supports;

(11) FIG. 11 comprises enlarged side and end views of a track support shown in FIG. 10;

(12) FIG. 12 is a schematic side view of a field joint passing over a track support, showing measures to protect the field joint coating; and

(13) FIG. 13 is a schematic side view of a field joint passing over a track support, showing another measure to protect the field joint coating.

(14) Referring firstly to the schematic view of FIG. 2 of the drawings, a pipelaying vessel is configured for the S-lay installation method and moves from left to right as illustrated during a pipelaying operation. The vessel 10 carries a supply of pipe joints 12 on its deck 14 that are welded together at one or more welding stations 16 to form a pipe string 18 that moves aft with respect to the vessel 10 along a firing line. The welds are tested at one or more testing stations 20 located downstream (i.e. aft) of the welding stations 16 and are then coated at one or more coating stations 22 located downstream of the testing stations 20. The welding stations 16, testing stations 20 and coating stations 22 thus lie on the firing line along which the pipe string 18 moves as it is assembled, checked and coated before being launched from the vessel 10 into the sea 24.

(15) The pipe string 18 is supported by a tensioner system 26 located downstream of the coating stations 22. The pipe string 18 is launched from the vessel 10 over a stinger 28 extending aft of the vessel 10, located downstream of the tensioner system 26. The stinger 28 comprises rollers 30 that support the overbend of the pipe string 18 as it enters the sea 24. The pipe string 18 hangs from the stinger 28 in a shallow S-shape under tension acting between the tensioner system 26 and a touchdown point on the sea bed (not shown).

(16) It is of course possible for a pipe string to experience a much greater deflection through the overbend than is shown in FIG. 2, especially in so-called Steep S-lay operations in which the departure angle of the pipe string is close to vertical as it leaves the stinger.

(17) The present invention is mainly concerned with coating operations performed at the coating stations 22 on the firing line, which will now be described with reference to FIGS. 3 to 6 of the drawings.

(18) FIGS. 3 and 4a to 4c of the drawings show a mould tool 32 in accordance with the invention, encircling a welded field joint of a pipeline at a coating station 22.

(19) As in the prior art arrangement of FIGS. 1a to 1c, the field joint in FIGS. 4a to 4c is created between abutting pipe joints 34 where a circumferential weld 36 attaches the pipe joints 34 to each other. Each pipe joint 34 is coated, for example with a 5LPP coating 38, and that coating 38 terminates short of the end of each pipe joint 34 with a typically chamfered end shape. An annular gap lies between the opposed ends of the coating 38 around the weld 36, where the exposed external surfaces of the pipe joints 34 need to be coated. For this purpose, the mould tool 32 is fixed around the field joint, extending from one coating 38 to the other and overlapping those coatings 38 to define a mould cavity 40 including the annular gap between the coatings 38, into which molten thermoplastics material such as PP is injected as a field joint coating.

(20) The mould tool 32 comprises a tube 42 of generally circular cross-section, divided longitudinally on a diameter of the cross-section into two halves. Opposed end portions 44 of the tube 42 seat against the coatings 38 of the respective pipe joints 34 and so have an internal diameter corresponding to the external diameter of the coated pipe joints 34. A central portion 46 of the tube 42 aligned with the gap between the coatings 38 has an increased internal diameter that exceeds the external diameter of the coated pipe joints 34. This enlarges the mould cavity 40 to allow for shrinkage of the injected plastics material as it cools.

(21) The two halves of the mould tool 32 are assembled together to encircle the field joint. Where they meet, the two halves have flanges 48 that are clamped together by external clamps 50 represented schematically in FIG. 3. The clamps 50 hold together the two halves against internal pressure within the mould tool 32 in use; they also hold the mould tool 32 in sealing engagement with the coatings 38 of the pipe joints 34. Inwardly-facing seals 52 are provided in the end portions 44 of the mould tool for that purpose, as can be seen in FIGS. 4a to 4c. Stiffening rings 54 encircling the end portions 44 of the mould tool 32 also help to maintain structural integrity and sealing.

(22) The tubular wall of the mould tool 32 is penetrated by an array of gates 56 for injection into the mould cavity 40 of molten PP 58 supplied through feed lines 60 under pressure from a supplying reservoir or machine 62. A total of nine gates 56 are shown in the example of FIGS. 3 and 4a to 4c; those gates 56 are disposed in three axially-spaced circumferential groups, each group comprising three gates 56 equi-angularly spaced around the circumference of the tubular wall. The groups of gates 56 are substantially equi-spaced but they are together offset toward one end of the mould tool 32, being the downstream end with respect to the firing line of the vessel 10.

(23) Each gate 56 has a respective valve 64 that controls the injection of molten PP 58 through that gate 56. The valves 64 are controlled by a central control unit 66 shown in FIG. 3 and they may be operated independently of each other. To simplify illustration, poppet valve elements 68 are shown schematically in the valves 64 of FIGS. 4a to 4c; other valve types are, of course, possible.

(24) A vent 67 at an upstream end of the tubular wall of the mould tool 32 allows air to escape as the mould cavity 40 fills with molten PP 58. The mould tool 32 also has an optional cooling system comprising a water jacket created by water pipes 69 disposed in or on the tubular wall of the mould tool 32.

(25) In this example, the cooling system is supplemented by an optional pipe cooling device 71 that is positioned inside the pipe to cool the melt by accelerating conduction of heat through the pipe wall. The pipe cooling device 71 is movable longitudinally along the pipe to apply cooling where it is needed.

(26) The pipe cooling device 71 could be a refrigerated pig but in this example it simply comprises a spray head 73. The spray head 73 sprays water radially outwardly against the inner circumference of the pipe wall to quench the molten PP 58 in the mould cavity on the other side of the pipe wall. Water is thereby projected in a disc that lies in a plane orthogonal to the central longitudinal axis of the pipe. The spray head 73 is supported by a shaft 75 that lies on the central longitudinal axis of the pipe and that supplies the spray head 73 with water under pressure.

(27) The shaft 75 is movable longitudinally with respect to the pipe to move the spray head 73 in a corresponding manner. A wheeled support 77 mounted on the shaft beside the spray head 73 allows for this longitudinal movement, while keeping the spray head 73 centred within the inner circumference of the pipe wall.

(28) It is possible for cooling to be applied locally to the walls of the mould cavity 40 and for the cooling effect to be applied progressively or step-wise along the length of the mould tool 32 and/or the pipe to suit the desired progression and cooling of the melt within the mould cavity 40. In this context, the spray head 73 applies cooling in an advantageously localised manner so that an appropriate region of the melt may be cooled while an adjacent region of the melt remains internally molten due to continued injection of molten PP 58. This allows the molten PP 58 to continue to flow without excessive viscosity and without introducing excessive discontinuities in the cooling melt.

(29) Before the injection moulding operation begins, the bare uncoated external surfaces of the pipe joints 34 are cleaned, primed and heated, as are the chamfered end surfaces of the coatings 38.

(30) In FIG. 4a, the injection moulding operation has begun by opening the valves 64 of the first circumferential group of gates 56 at one end of the mould cavity 40. The result is an annular melt front 70 that has already filled that end of the mould cavity 40 and is now progressing along the mould cavity 40 toward the other end of the mould cavity 40. The outer perimeter of the injected melt has started to freeze to form solid PP 59 but the inner core of the injected melt remains as molten PP 58 while injection continues through the first circumferential group of gates 56. The valves 64 of the second and third circumferential groups of gates 56 remain closed at this stage.

(31) Freezing of the injected melt to form solid PP is helped by the water pipes 69 of the mould tool 32 that cool the radially outer side of the mould cavity 40 and by the spray head 73 of the pipe coding device 71 that cools the radially inner side of the mould cavity 40 via the wall of the pipe. In this respect, it will be noted that the spray head 73 is initially aligned with a downstream end of the mould cavity 40 with respect to the firing line of the vessel 10. It would similarly be possible, but perhaps less effective, to enable or disable the flow of cooling water through certain ones of the water pipes 69 or to vary the relative flow of cooling water through different water pipes 69 to concentrate cooling on certain parts of the mould tool 32.

(32) In FIG. 4b, the melt front 70 has passed the second circumferential group of gates 56. Now, the valves 64 of the first circumferential group of gates 56 close and the valves 64 of the second circumferential group of gates 56 open. This cascade technique further propels the melt front 70 along the mould cavity 40 while maintaining homogeneity of the melt. In this respect, it is advantageous that the valves 64 of the second circumferential group of gates 56 only open when the melt front 70 has passed them; otherwise, there will be two melt fronts that may weld with dissimilar crystalline structures where they come together, which could introduce a weakness in the finished field joint coating.

(33) Meanwhile, as the valves 84 of the first circumferential group of gates 58 have closed, the melt introduced through that first circumferential group no longer receives heat input and so is allowed to start cooling early while injection of melt continues elsewhere in the mould cavity 40. Cooling of the melt introduced through the first circumferential group of gates 56 is promoted by the optional application of local cooling. In this respect, the spray head 73 of the pipe cooling device 71 now applies coding locally to a position slightly downstream of the second circumferential group of gates 56 with respect to the firing line of the vessel 10. It will be noted in FIG. 4b how the core of the melt near the first circumferential group of gates 56 has started to freeze to form solid PP 59.

(34) In FIG. 4c, the injection moulding operation is almost complete. When the melt front 70 passed the third circumferential group of gates 56 near the opposite end of the mould cavity 40, the valves 864 of the second circumferential group of gates 56 closed and the valves 64 of the third circumferential group of gates 56 opened. Molten PP 58 injected through the third circumferential group of gates 56 fills the remainder of the mould cavity 40 while the melt in the region of the second circumferential group of gates 56 is able to start cooling quickly. Meanwhile the melt in the region of the first circumferential group of gates 56 has already cooled significantly as it approaches its ultimate strength.

(35) Again, cooling of the melt introduced through the second circumferential group of gates 56 is promoted by the optional application of local cooling. In this respect, the spray head 73 of the pipe cooling device 71 now applies cooling locally to a position slightly downstream of the third circumferential group of gates 56 with respect to the firing line of the vessel 10. It will be noted in FIG. 4c how the core of the melt near the second circumferential group of gates 56 has started to freeze to form solid PP 59.

(36) When the mould cavity 40 has been filled and all of the melt within has solidified to a self-supporting extent, the clamps 50 are released to separate and remove the two halves of the mould tool 32 from the field joint. As it sets, the injection-moulded material will shrink but the oversized central portion 46 of the mould tool 32 allows for this shrinkage so that the external diameter of the finished field joint coating approximates to the external diameter of the coated pipe joints 34 to either side of the field joint.

(37) The sequential injection moulding operation described above has several advantages in the context of pipeline field joint coating. It enables quick cooling of thermoplastics materials, reducing cycle time to a level compatible with S-lay installation methods although it is emphasised that the process is also suitable for J-lay installation methods and for pipeline fabrication including reeling/spooling operations. Yet, the process of the invention produces a high-quality field joint coating. Also, it enables lower moulding pressure and hence lower clamping force as the viscous melt need only travel the short distance between the gates 56 rather than traversing the mould cavity 40 as a whole.

(38) Other arrangements of gates 56 and valves 64 are possible, both in the number of gates 56 and their relative disposition around the tubular wall of the mould tool 32: for example, the gates 56 of neighbouring circumferential groups may be angularly offset with respect to each other. Variations are also possible in the sequence of operation of the valves 64: for example, the valves 64 of a circumferential group need not open simultaneously but their opening could be staggered, for example by delaying opening of the valve 64 of one gate 56 until the melt front 70 from another, previously opened gate 56 has passed it.

(39) The valves 64 may open and close on a predefined timescale based on the assumption that the melt front 70 will move a certain distance over a certain time. It is also possible to open and close the valves 64 in response to detecting the position of the melt front 70, for example using temperature sensors or pressure sensors (not shown) on the mould tool 32.

(40) It may be beneficial to advance the melt front 70 through the mould cavity 40 in the direction of motion of the vessel 10 during pipelaying, or in an upstream direction with respect to the firing line of the vessel 10. This ensures that the coolest and hence strongest part of the field joint coating is the first to meet the rollers 30 of the stinger 28, which gives more time for hotter parts of the field joint coating to cool and solidify before they too meet the rollers 30.

(41) FIGS. 5a and 5b show an alternative sequential injection moulding arrangement using a mould tool 33 in which a first circumferential group of gates 56 is disposed between a pair of second circumferential groups of gates 56. Like numerals are used for like parts. Here, the injected melt has two melt fronts 70 that advance in opposite longitudinal directions from the first circumferential group of gates 56 toward and beyond respective ones of the second circumferential groups of gates 56.

(42) At and soon after the start of the injection moulding operation as shown in FIG. 5a, the valves 64 of the first circumferential group of gates 56 are open and the valves 64 of the second circumferential groups of gates 56 are closed. Conversely FIG. 5b shows the injection moulding operation almost complete. When the melt fronts 70 passed the second circumferential groups of gates 56 near opposite ends of the mould cavity 40, the valves 64 of the first circumferential group of gates 56 closed and the valves 64 of the second circumferential groups of gates 56 opened. In FIG. 5b, molten PP 58 injected through the second circumferential groups of gates 56 has filled the remainder of the mould cavity 40 while the melt in the central region of the mould cavity 40 near the first circumferential group of gates 56 was able to start cooling quickly as soon as the valves 64 of that group of gates 56 closed. Consequently, the melt in the central region of the mould cavity 40 has started to freeze to form solid PP 59.

(43) Whilst the pipe cooling device 71 of FIGS. 4a, 4b and 4c has been omitted from the embodiment shown in FIGS. 5a and 5b, it will be apparent that localised cooling may be applied through the pipe wall by similar means if required.

(44) FIGS. 6a and 6b show a further sequential injection moulding arrangement using a mould tool 35 whose tubular wall is penetrated by an array of gates 56 that are angularly spaced from each other in a circumferential arrangement. A total of five gates 56 are shown in the example of FIGS. 6a and 6b in an arrangement that is symmetrical about a central longitudinal vertical plane of the mould tool 35. Those gates comprise a first gate 56 disposed centrally at the bottom of the mould tool 35, a pair of second gates 56 above the level of the first gate and a pair of third gates 56 above the level of the second gates 56.

(45) As before, each gate 56 has a respective valve 64 (shown for ease of illustration as a poppet valve element) that controls the injection of molten PP 58 through that gate 56. The valves 64 are controlled by a central control unit such as that shown in FIG. 3.

(46) A vent 67 at the top of the tubular wall of the mould tool 32 diametrically opposed to the first gate 56 allows air to escape as the mould cavity 40 fills with molten PP 58. The mould tool 32 also has an optional cooling system comprising a water jacket created by water pipes 69 disposed in or on the tubular wall of the mould tool 32.

(47) In FIG. 6a, the injection moulding operation has begun by opening the valve 64 of the first gate 56 at the bottom of the mould cavity 40. This has created melt fronts 70 that are shown here progressing circumferentially in opposed angular directions around the pipe joints 34 in the mould cavity 40, clockwise and anticlockwise. The outer perimeter of the injected melt has started to freeze to form solid PP 59 but the inner core of the injected melt remains as molten PP 58 while injection continues through the first gate 56. The valves 64 of the second and third gates 56 remain closed at this stage.

(48) Freezing of the injected melt to form solid PP is helped by the water pipes 69 of the mould tool 32 that cool the radially outer side of the mould cavity 40.

(49) In FIG. 6b, the melt fronts 70 have passed the second gates 56. Now, the valve 64 of the first gate 56 closes and the valves 64 of the second gates 56 open. This cascade technique further propels the melt fronts 70 around the mould cavity 40 while maintaining homogeneity of the melt. As before, it is advantageous for homogeneity of the melt that the valves 64 of the second circumferential group of gates 56 only open when the melt front 70 has passed them.

(50) Meanwhile, as the valve 64 of the first gate 56 has closed, the melt introduced through that first gate 56 no longer receives heat input and so is allowed to start cooling early while injection of melt continues elsewhere in the mould cavity 40.

(51) Cooling of the melt introduced through the first gate 56 is promoted by the optional application of local cooling. In this respect, FIG. 6b shows how the cooling system may be supplemented by an optional pipe cooling device 79 that is positioned inside the pipe to cool the melt by accelerating conduction of heat through the pipe wall. The pipe cooling device 79 comprises a spray head that is pivotable about the central longitudinal axis of the pipe to apply cooling where it is needed. The spray head may, for example, oscillate to spray an arc that increases as the melt fronts 70 progress around the mould cavity 40. The spray head sprays water radially outwardly against the inner circumference of the pipe wall to quench the molten PP 58 in the mould cavity 40 on the other side of the pipe wall. This applies cooling in an advantageously localised manner so that an appropriate region of the melt may be cooled while an adjacent region of the melt remains internally molten due to continued iniection of molten PP 58.

(52) As before, it would also be possible to enable or disable the flow of cooling water through certain ones of the water pipes 69 or to vary the relative flow of cooling water through different water pipes 69 to concentrate cooling on certain parts of the mould tool 32.

(53) FIGS. 7, 8a, 8b and 9 show other variants of the invention. They are shown in conjunction with gates 56 and valves 64 arranged for sequential injection moulding as in FIGS. 3 and 4a to 4c.

(54) The variant shown in FIGS. 7, 8a and 8b is specific to S-lay and contemplates successive coating stations 22, 22 in the firing line working simultaneously on successive pipe joints 34, as shown in FIG. 7. An upstream coating station 22 applies a thin inner injection-moulded coating that cools and hardens quickly, particularly as it has a low volume in relation to its surface area. A downstream coating station 22 applies a further thin outer injection-moulded coating atop the inner coating, which similarly cools and hardens quickly. Together, the thicknesses of the inner and outer coatings, after shrinkage, substantially equate to the thickness of the pipe joint coatings.

(55) For this purpose, different mould tools are used at the different coating stations 22, 22. The upstream coating station 22 has a first mould tool 72 shown in FIG. 8a having a thick wall 74 in its central portion and a correspondingly small internal diameter that is less than the external diameter of the coated pipe joints 34. This produces a thin inner coating in a first sequential injection-moulding operation. In contrast, the second mould tool 78 shown in FIG. 8b used at the downstream coating station 22 is much the same as the mould tool 32 shown in FIGS. 3 and 4a to 4c, with a thinner wall 80 in its central portion and a correspondingly large internal diameter that is greater than the external diameter of the coated pipe joints 34. Here, the second mould tool 78 is shown before starting a second sequential injection-moulding operation to overlay a further thin outer coating over the inner coating 76 already formed at the upstream coating station 22.

(56) The inner and outer coatings may be of the same material, such as PP, or may be of dissimilar materials to optimise properties such as insulation. For example, the inner coating may be GSPP for insulation and the outer coating may be solid PP for protection and increased heat capacity.

(57) Referring next to FIG. 9 of the drawings, FIG. 9 shows a variant of the invention in which an insert 82 is positioned in the mould cavity 40 to be embedded in molten plastics during a sequential injection-moulding operation. The insert 82 is a thermoplastics tube (of, for example, solid PP or GSPP) in two or more sections that are brought together around the prepared and heated field joint; the insert 82 may itself be heated and primed to promote adhesion of the melt to the insert 82.

(58) Like the arrangement shown in FIG. 7, the insert 82 may be attached to the field joint at an upstream station before a mould is positioned around the field joint and the insert at a downstream station.

(59) The insert 82 comprises tapered spacer formations 84 that space the tubular body 86 of the insert 82 from the bare exterior of the pipe joints 34, and annular recesses 88 around the tubular body 86 whereby the sections of the insert 82 may be strapped around the field joint. The tubular body 86 of the insert is also penetrated by holes 90 through which molten plastics may flow during moulding, aiding filling of the mould cavity 40 and ensuring a strong mechanical connection between the hardened melt and the insert 82.

(60) The insert 82 reduces the volume of the melt and increases the ratio of the surface or interface area of the melt relative to its volume, to the benefit of cooling time. The insert 82 serves as a heat sink that promotes cooling of the melt; the insert also reinforces the field joint coating to help it survive the forces of tensioning and launching over the stinger 28. Also, if made of an insulating material like GSPP, the insert 82 may confer desired insulating properties on the field joint coating.

(61) FIGS. 10 to 13 show how the rollers 30 of a stinger 28 may be replaced by continuous track supports 92, which have the benefit of spreading the load of the pipe string 18 across a large contact area. In this way, the field joint coating may be allowed to reach the supports 92 before all or part of the newly-coated region has reached full hardness. A similar effect may be achieved by enlarging the radius of the rollers 30, or with belts running over paired rollers of the stinger 28. The continuous track supports 92 shown in FIGS. 10 to 13 could therefore be replaced by a roller 30 or by a series of rollers, with or without belts running over them.

(62) The overall firing line cycle time may be reduced by spreading the cooling time over more than one cycle. In an S-lay operation, this involves allowing cool-down to take place over one, two or three rollers 30 or other supports 92 after the coating station 22. To achieve this the pipe string 18 must be supported over the length of the field joint coating or at least over the length of the still-soft part of the field joint coating. Thus, the radius of a roller 30, the length of a belt section bridging rollers or the length of a continuous track support 92 must be such as to bear on an already-cooled part of the field joint coating and/or on the adjacent pipe coating 38, effectively to bridge any still-soft part of the field joint coating.

(63) FIG. 10 shows a series of continuous track supports 92 spaced along and aligned with the longitudinal axis of the pipe string 18, whereas FIG. 11 shows one of the continuous track supports 92 of FIG. 9 in more detail. FIG. 11 shows that a T-shaped frame 94 supports a pair of main wheels 96, one at each end of the frame 94. A track 98 comprising resilient articulated links 100 turns around the main wheels 96 in a continuous loop aligned with the longitudinal axis of the pipe string 18.

(64) An upper side of the loop of the track 98 defines a support surface 102 for the pipe string 18. The support surface 102 is supported along its length by auxiliary wheels 104 within the loop. As best seen in the end view of FIG. 11, the links 100 of the track 98 are concave-dished in transverse cross-section so that the support surface 102 is longitudinally-grooved to locate the pipe string 18 laterally. Other than that, the support surface 102 is approximately planar allowing for shallow ramp portions at each end. The straight length of the support surface 102 is of the order of the field joint length: for example, about 750 mm.

(65) The main wheels 96 may turn passively with the track 98, whose movement is driven by movement of the pipe string 18 in its launch direction, driven in turn by relative movement of the vessel 10 and controlled by the tensioner system 26. Alternatively, at least one of the main wheels 96 may drive movement of the track 98 to match movement of the pipe string 18 in its launch direction, if a main wheel 96 is powered by a suitable electric or hydraulic motor (not shown).

(66) Moving on now to FIG. 12, this shows schematically how a field joint coating 106 may be protected as a pipe string 18 traverses a continuous track support 92. Here, the field joint coating 106 is kept clear of the support surface 102 of the track 98 by resilient pads 108 disposed under the pipe string 18. The pads 108 may, for example, be of layered neoprene rubber. One pad 108 is disposed on each side of the field joint coating 106 and each pad 108 is suitably strapped to the pipe string 18 by a respective bungee strap 110.

(67) The spacing between the pads 108 is such that the support surface 102 of the track 98 bridges the gap between them and the portion of track 98 between the pads 108 does not come into contact with the field joint coating 106. Once the field joint coating 106 has cleared the support 92, the bungee straps 110 are undone to remove the pads 108 for re-use in protecting the next field joint coating to pass over the support 92.

(68) Turning finally to FIG. 13, this shows schematically how a mould tool 112 may travel with the pipe string 18 across a support 92, thus giving more time for moulding and cooling operations to take place. Indeed, this allows the field joint coating 106 within the mould tool 112 to traverse the support 92 before that coating 106 is cool enough to resist damage or to be self-supporting, or even before the coating 106 is complete.

(69) It will be noted that the mould tool 112 shown in FIG. 13 has adaptations to assist its passage over the support 92. These adaptations comprise longitudinal extensions 114 at each end of the mould tool 112 on its lower side, which extensions overlay the pipe coatings 38 adjacent the field joint coating 106. Each extension 114 comprises a frusto-conical ramp surface 116 tapering upwardly and longitudinally from the central wall 118 of the mould tool 112 to the outer surface of the pipe coatings 38 under each extension 114.

(70) Once the mould tool 112 and the field joint coating 106 have cleared the support 92 and the field joint coating 106 is solid enough to survive further tensioning or overboarding steps, the mould tool 112 is disassembled and removed from the pipe string 18. Further overboarding steps may involve the field joint traversing further rollers or other supports and may therefore use any of the abovementioned solutions to protect the newly-formed field joint coating 106. The mould tool 112 may then be reassembled and re-used to form a subsequent field joint coating on the pipe string 18 upstream of the support 92. Whilst an additional mould tool may be needed in this system, the available cooling time is advantageously increased by the duration of one pipelay cycle and possibly more, without adversely affecting the critical path.

(71) The thermoplastics material used for injection moulding may be PP, polystyrene or other suitable thermoplastics material that is compatible with the coating applied to the pipe joints. Additives or modifiers may be employed, such as an elastomeric modifier like EDPM (ethylene propylene diene monomer rubber) to provide appropriate flexibility and impact resistance, or fibres of glass, aramid or carbon to increase strength and elastic modulus. Additives such as fibres may also reduce shrinkage and speed cooling.

(72) By virtue of the invention, it is envisaged that the cooling time after injection may be reduced to three or four minutes. This enables the use of advantageous injection moulding for field joint coating of compatible thermoplastics in time-critical applications such as S-lay or J-lay pipeline installation operations, and in other pipeline fabrication operations, without the disadvantages of incompatibility suffered by coating materials such as PU.