Techniques for coating pipes

Abstract

A thermoplastics injection molding process coats a field joint of a pipeline by positioning a mold tool around the field joint to define a mold cavity. Thermoplastics material injected into the mold cavity forms a field joint coating that will set in the mold cavity. As the thermoplastics material shrinks in the mold cavity while the field joint coating sets, compacting pressure is applied radially inwardly within the mold cavity against a radially outer side of the field joint coating. A compacting fluid introduced into the mold cavity between the mold tool and the field joint coating may be used to apply pressure against the field joint coating. This accelerates and controls cooling of the field joint coating while maximizing quality.

Claims

1. A method of coating a field joint of a pipeline, comprising: positioning a mould tool around the field joint, the mould tool having a tubular wall to define an annular mould cavity around the field joint; injecting thermoplastics material into the mould cavity through the tubular wall, to form a field joint coating that will set in the mould cavity; introducing a compacting fluid through the tubular wall into the mould cavity between the tubular wall and the field joint coating to apply compacting pressure within the mould cavity radially inwardly against a radially outer side of the field joint coating, other than by continued injection of the thermoplastics material; and separating the compacting fluid from the field joint coating by an intermediate partition that moves in the mould cavity under pressure from the compacting fluid to exert pressure from the compacting fluid against the field joint coating.

2. The method of claim 1, comprising applying said compacting pressure against the field joint coating as the thermoplastics material shrinks in the mould cavity while the field joint coating sets.

3. The method of claim 1, wherein the compacting fluid is a liquid.

4. The method of claim 1, comprising forcing the compacting fluid along an interface between the mould tool and the field joint coating to separate the field joint coating from the mould tool.

5. The method of claim 1, comprising bringing the compacting fluid into contact with the field joint coating in the mould cavity.

6. The method of claim 1, comprising deflecting the partition to conform to the radially outer side of the field joint coating under pressure from the compacting fluid.

7. The method of claim 1, comprising demoulding the field joint coating while holding the compacting fluid in a chamber partially defined by the partition.

8. The method of claim 1, comprising cooling the field joint coating, while liquid in the mould cavity, by transferring heat from the field joint coating to the compacting fluid that applies pressure against the field joint coating.

9. The method of claim 8, comprising passing a flow of compacting fluid through the mould cavity to remove heat from the field joint coating.

10. The method of claim 8, comprising cooling the compacting fluid.

11. The method of claim 10, comprising transferring heat from the compacting fluid to the mould tool and passing a flow of cooling fluid through the mould tool to reject that heat.

12. The method of claim 10, comprising passing a flow of the compacting fluid outside the mould cavity to reject heat from the compacting fluid.

13. The method of claim 1, wherein said application of compacting pressure is preceded by an initial compacting step comprising continued injection of the thermoplastics material into the mould cavity after the mould cavity has been filled.

14. The method of claim 1, comprising ceasing injection of thermoplastics material into the mould cavity before said application of compacting pressure.

Description

(1) In order that the invention may be more readily understood, reference will now be made, by way of example, to the accompanying drawings in which:

(2) FIG. 1 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. 2 is a schematic cross-sectional view on line II-II of FIG. 3, showing a mould tool in accordance with the invention positioned around a field joint;

(4) FIGS. 3 to 7 are longitudinal sectional detail views of the mould tool and field joint on line III-III of FIG. 2, showing the progression over time of an injection moulding operation in accordance with the invention; and

(5) FIGS. 8 and 9 are longitudinal sectional detail views corresponding to FIGS. 3 to 7 but showing variants of the mould tool in other embodiments of the invention.

(6) Referring firstly to the schematic view of FIG. 1 of the drawings, a pipelaying vessel 10 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.

(7) The pipe string 18 is supported by a tensioner system 26 located downstream of the coating stations 22. The tensioner system 26 typically comprises multiple tensioners but such details are not relevant to the invention and so have been omitted from the drawings.

(8) 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.

(9) In this example, 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). It is possible for a pipe string to experience a much greater deflection through the overbend than is shown in FIG. 1, 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.

(10) The invention is concerned with coating operations performed at the coating stations 22 on the firing line, which will now be described with reference to FIGS. 2 to 7 of the drawings.

(11) FIGS. 2 to 7 show a mould tool 32 in accordance with the invention, encircling a welded field joint of a pipeline at a coating station 22. The field joint is created between abutting pipe joints 34 where a circumferential butt weld 36 attaches the pipe joints 34 to each other.

(12) Each pipe joint 34 is coated with a parent coating, for example 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. The mould cavity 40 includes the annular gap between the coatings 38, into which molten thermoplastics material is injected as a field joint coating.

(13) 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.

(14) A central portion 46 of the tube 42 encompassing the gap between the coatings 38 has an increased internal diameter that exceeds the external diameter of the coated pipe joints 34. This increases the depth of the mould cavity 40 to allow for shrinkage of the injected thermoplastics material as it cools. The enlarged central portion 46 extends beyond the chamfered ends of the coatings 38 to define extensions 48 of the mould cavity 40.

(15) The two halves of the mould tool 32 are assembled together to encircle the field joint. Where they meet, the two halves have flanges 50 that are clamped together by external clamps 52 represented schematically in FIG. 2. The clamps 52 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 54 are suitably provided in the end portions 44 of the mould tool for that purpose, as can be seen in FIGS. 3 to 7.

(16) The tubular wall of the mould tool 32 is penetrated by an array of injection nozzles or gates 56 for injection into the mould cavity 40 of molten PP 58 supplied through respective feed lines 60 under pressure from a supplying reservoir or machine 62. A circumferential array of three gates 56 are shown in this example, equi-angularly spaced around the circumference of the tubular wall at a central longitudinal position.

(17) 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. 2 and may be operated together or independently of each other. To simplify illustration, poppet valve elements 68 are shown schematically in the valves 64; other valve types are of course possible.

(18) Vents 70 at both ends of the tubular wall of the mould tool 32 allow 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 an array of parallel pipes 72 embedded in or disposed on the tubular wall of the mould tool 32. Other cooling fluids such as oil or a gas could be pumped through the pipes 72 instead of water. It is also possible for a warm fluid to be pumped through the pipes 72 so as to warm up the mould tool 32 before use.

(19) In accordance with the invention, the tubular wall of the mould tool 32 is further penetrated by one or more fluid inlets 74 connected to a fluid supply system 76. The fluid supply system 76 is entirely separate from the system that supplies molten PP 58 to the gates 56. In this example, there are two fluid inlets 74, located inboard from the ends of the mould tool 32 to align approximately with the chamfered ends of the coatings 38 of the pipe joints 34, just inboard of the extensions 48 of the mould cavity 40.

(20) The fluid supply system 76 communicating with the fluid inlets 74 comprises a fluid reservoir 78, a high-pressure pump 80 and a valve 82, all of which are interconnected by fluid lines 84 that lead to the fluid inlets 74. The pump 80 and the valve 82 are both controlled by a controller 86 to admit a compacting fluid 88 from the reservoir 78 into the mould cavity 40. The compacting fluid 88 may be a gas or, preferably, a liquid such as oil.

(21) A vent may be provided in the mould tool 32 to allow air to escape as compacting fluid 88 enters the mould cavity 40, but such a vent has been omitted from the drawings for simplicity.

(22) 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.

(23) In FIG. 3, the injection moulding operation has begun by opening the valves 64 associated with the gates 56 (only one of which is shown in this view) to admit a melt of pressurised molten PP 58. As injection continues during mould filling as shown, the injected melt has two melt fronts 90 that advance in opposite longitudinal directions from the gates 56 toward respective ends of the mould cavity 40.

(24) FIG. 4 shows the mould cavity 40 now full of PP 58 when the melt fronts 90 reach and fill the extensions 48 at respective ends of the mould cavity 40. The interior of the melt of PP 58 remains molten at this stage but the melt starts to freeze as the PP 58 cools. The melt solidifies from the outside in by virtue of heat transfer via its exterior.

(25) Solidification of the PP 58 results in shrinkage that is compensated initially by an optional brief packing step as shown in FIG. 4. It will be noted in this respect that the valves 64 associated with the gates 56 remain open at this stage so that additional PP 58 is forced into the mould cavity for a short period to keep the mould cavity full and so to compensate for shrinkage.

(26) With reference now to FIGS. 5 and 6 of the drawings, the packing step ceases before PP 58 in the gates starts to freeze. The valves 64 associated with the gates 56 are then closed so that no further PP 58 is admitted to the mould cavity 40. Consequently, the melt no longer receives heat input and so cools more quickly than if the packing step was prolonged. Cooling can be accelerated by passing cooling fluid through the pipes 72 of the cooling system of the mould tool 32.

(27) From the end of the packing step, in the prior art, continued cooling of PP 58 already in the mould cavity 40 would result in the drawbacks of uncompensated shrinkage. In contrast, the invention compensates for shrinkage by a compaction step that, unlike packing, does not involve continued injection of PP 58. Instead, in this example, the compaction step uses the compacting fluid 88 to apply radially inward pressure against the PP 58 in the mould cavity 40.

(28) The prior art does not teach in-mould compaction of a field joint coating by injecting a fluid additional to the molten thermoplastics material that forms the field joint coating.

(29) Indeed, no fluid other than a viscous coating material, a curing product or a chemical additive has been known to be used inside a mould cavity between a pipe and a mould tool.

(30) When the controller 86 of the fluid supply system 76 activates the pump 80 and opens the valve 82, the compacting fluid 88 is drawn through the lines 84 from the reservoir 78 to be injected under pressure into the mould cavity 40 through the fluid inlets 74. The fluid inlets 74 inject the compacting fluid 88 as a thin layer between the tubular wall of the mould tool 32 and the PP 58 that is also in the mould cavity 40. In this example, there is direct contact between the compacting fluid 88 and the PP 58 in the mould cavity 40.

(31) Advantageously, as shown in FIG. 5, the compacting fluid 88 is forced under pressure to propagate along the interface between the mould tool 32 and the PP 58 in the mould cavity 40 to separate the PP 58 from the mould tool 32 with a peeling action. This prevents adhesion of the PP 58 to the mould tool 32 or detaches the PP 58 from the mould tool 32 if such adhesion has already occurred.

(32) Eventually the compacting fluid 88 extends nearly the full length of the mould cavity 40 to separate the PP 58 from the mould tool 32 as shown in FIG. 6. This eases eventual demoulding and is one way in which the compacting fluid 88 helps to reduce stress in the solidifying PP 58. Pressure is maintained in the compacting fluid 88 as the solidifying PP 58 shrinks; more compacting fluid 88 is introduced into the mould cavity 40 as may be necessary to account for continued shrinkage of the PP 58.

(33) As the compacting fluid 88 will generally be injected at a lower temperature than the PP 58 at that stage, the compacting fluid 88 acts as a heat sink to draw heat from the PP 58 and so to accelerate cooling of the PP 58. Intimate thermal contact between the compacting fluid 88 and the PP 58 across a large surface area is advantageous in this respect. The cooling system of the mould tool 32 may remain active to draw heat from both the compacting fluid 88 and the PP 58.

(34) When the operation parameters have been checked and the PP 58 in the mould cavity 40 has cooled and solidified to an appropriately self-supporting extent, the compacting fluid 88 is depressurised and drained, for example by reversing the pump 80 to return the compacting fluid 88 to the reservoir 78. The clamps 50 shown in FIG. 2 are then released to separate and remove the two halves of the mould tool 32 from the field joint in a demoulding operation. The PP 58 is then air-cooled to ambient temperature by exposure to ambient air as shown in FIG. 7.

(35) If faster cooling is required, air or other cooling gases may be blown over the exposed PP 58 to cool the PP 58 by conduction and convection. Alternatively, or additionally, a cooling liquid such as water may be sprayed or poured over the exposed PP 58, to cool the PP 58 by conduction and evaporation. The temperature and/or the flow rate of cooling liquids or gases may be modified to control the rate of cooling; for example, such liquids or gases may be refrigerated to below ambient temperature.

(36) It will be noted from FIGS. 6 and 7 that the oversized central portion 46 of the mould tool 32 and the compensated, controlled shrinkage of the PP 58 under pressure from the compacting fluid 88 determines and controls the external shape and dimensions of the finished field joint coating. The end result is predictable, uniform and consistent from one field joint coating to the next, both in external shape and internal structure.

(37) The external diameter of the finished field joint coating broadly corresponds to the external diameter of the coated pipe joints 34 to either side of the field joint. By virtue of the extensions 48 at the ends of the mould cavity 40, the ends of the field joint coating overlap the pipe coatings 38 slightly. Those overlaps beneficially lengthen and hence increase the area of the interfaces between the pipe coatings 38 and the field joint coating.

(38) FIGS. 8 and 9 show variants of the invention in which a flexible, resiliently-stretchable membrane 92 is interposed between the compacting fluid 88 and the PP 58. The membrane 92 is sealed around its periphery to the interior of the mould tool 32, outboard of the fluid inlets 74, to define an expandable chamber that encloses the compacting fluid 88 that is admitted through the fluid inlets 74.

(39) As the chamber between the mould tool 32 and the membrane 92 expands under increasing pressure from the compacting fluid 88 within it, the membrane 92 exerts fluid pressure on the PP 58. The membrane 92 is thin enough not to interfere excessively with heat transfer from the PP 58 to the compacting fluid 88. However, direct contact between the compacting fluid 88 and the PP 58 is avoided and the compacting fluid 88 remains contained by the membrane 92 for ease of handling, especially when depressurising and draining the compacting fluid 88 for demoulding.

(40) FIG. 9 differs from FIG. 8 by an optional provision for the compacting fluid 88 to flow into, through and out of the chamber defined by the membrane 92, while continuing to apply fluid pressure to the PP 58 through the membrane 92. This allows the compacting fluid 88 to convey heat from the PP 58 out of the mould cavity 40 and to reject that heat outside the mould cavity 40. A similar provision for flowing compacting fluid 88 through the mould cavity 40 could be made in a variant of the first embodiment shown in FIGS. 2 to 7.

(41) Specifically, FIG. 9 shows a heat-exchange circuit comprising a circulation pump 94. The pump 94 recirculates the compacting fluid 88 from a fluid outlet 96 through a heat exchanger 98 outside the mould cavity 40 and returns the compacting fluid 88 through a fluid inlet 74 to the mould cavity 40 at a lower temperature.

(42) In some cases, compacting fluid 88 need not be recirculated: for example, if the compacting fluid 88 is water and so is inexpensive and has no environmental impact, it could be discarded after passing though the mould cavity 40.

(43) Whilst FIG. 9 shows a cooling system for the mould tool 32 comprising pipes 72 as in the preceding embodiments, it is possible that cooling via the compacting fluid 88 could remove the need for a separate cooling system for the mould tool 32.

(44) The present invention is particularly apt to be used in S-lay operations but its use is not limited to S-lay. The invention can also be used in J-lay operations and when fabricating and spooling reel-lay pipelines at a spoolbase. For example, like S-lay, the pipeline is horizontal and is guided on a system of rollers at a spoolbase, where weld testing and field joint coating takes place between two ends of the pipeline. Welding takes place at one end to assemble the pipeline; once tested and coated, the pipeline is spooled onto a reel at the other end.

(45) Many other variations are possible within the inventive concept. For example, the mould tool may have more or fewer gates as appropriate and they may be distributed differently over the mould tool. Also, gates of the mould tool may open in longitudinal or circumferential succession to enable cascade moulding as proposed in WO 2012/004665.

(46) Optionally, a mould tool cooling system may be supplemented by a pipe cooling device positioned inside abutting pipe joints to cool the melt by accelerating conduction of heat through the pipe wall. Such a pipe cooling device may, for example, be a refrigerated pig or a spray head that is movable longitudinally along the pipe to apply cooling where it is needed. An example of such a spray head is also disclosed in WO 2012/004665.

(47) 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 EPDM (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.