Adapting hydrocarbon pipelines to transport hydrogen

Abstract

A metallic coating is electrodeposited onto an inner surface of a pipeline wall by conveying a pig along the pipeline and driving an electric current through a body of electrolyte fluid that extends from an electrode of the pig to the pipeline wall. To deposit an internal coating of copper, the pig supports a copper anode and the electrolyte fluid is of copper sulphate. The electrodeposition method may be performed on a steel pipeline in situ after the pipeline has conveyed hydrocarbons, and may be followed by conveying hydrogen along the pipeline. The internal coating mitigates migration of hydrogen atoms into the pipe wall and the consequent risk of embrittlement of the steel.

Description

[0001]Adapting hydrocarbon pipelines to transport hydrogen This invention relates to transporting hydrogen gas using pipelines that were used previously to transport hydrocarbons such as oil or natural gas. The invention particularly relates to techniques for adapting, reinforcing or retrofitting previously used or decommissioned steel pipelines to convey hydrogen. Effecting a switchover of existing hydrocarbon pipelines to the bulk transport of hydrogen gas is expected to play a major role in decarbonising the energy industry. However, achieving this is not simply a matter of switching from hydrocarbon fluids to hydrogen. This is because most large hydrocarbon pipelines are made from carbon steel. Hydrogen absorption tends to decrease the ductility and crack resistance of the steel pipeline wall, leaving the pipeline susceptible to failure. Hydrogen embrittlement of steel can arise even under static conditions, although the problem may be exacerbated by dynamic or cyclical loading of a pipeline caused by fluctuating internal pressure or by thermal cycling. Thus, existing hydrocarbon pipelines that are not designed for hydrogen transport must be adapted if they are to be capable of conveying hydrogen at high pressure. Conventionally, used pipelines are retrofitted by lining them with a polymer sleeve. However, permeation of fluids through polymer materials renders this technique unsuitable for some applications, for example for hydrogen transport or for carboncapture involving transport of CO2 for storage in depleted oil reservoirs. In suchapplications, a polymer liner may not sufficiently protect against gas permeation, hence leading to degradation of the surrounding steel pipe due to embrittlement or corrosion. Various studies have suggested that the take-up of hydrogen by pipeline steel could be mitigated by the addition of other fluids such as carbon monoxide, natural gas or water to dilute the hydrogen. However, such measures would add complexity and reduce the efficiency of hydrogen transport. At present, there is no technology known that can line the inside of a long pre-installed steel pipeline in situ to mitigate hydrogen absorption into the pipeline wall, especially if the pipeline is to be used to convey undiluted hydrogen under high pressure. In this respect, the inventor has explored electroplating of pipelines but has found no practical solutions. Copper plating of steel tubes is known in the art. For example, US 3875027 describes electroplating layers of copper and nickel within a ferrous pipe. For this purpose, the pipe is plunged into a ionic solution while current is applied. However, that method cannot be used to retrofit an existing pipeline, which is commonly underwater or buried, because an in situ pipeline cannot be immersed in an ionic solution to perform electrolysis. EP 3710177 describes a pig with built-in anodes to clean and protect a pipeline that serves as a cathode. There is no teaching of metal deposition, which would be precluded by the arrangement of the electric field in any event. Against this background, the invention resides in a method of electrodepositing a metallic coating on an inner surface of a pipeline wall. The method comprises conveying a pig along the pipeline while driving an electric current through a body of electrolyte fluid that extends from an electrode of the pig to the pipeline wall. The electric current is apt to be driven by a voltage source such as an array of battery cells carried by the pig. The method of the invention is apt to be performed on a steel pipeline in situ after the pipeline has been used to convey hydrocarbons, and is further apt to be followed by conveying hydrogen along the pipeline. The body of electrolyte fluid may be part of an electric circuit that comprises an additional connection extending between the pig and the pipeline wall. In that case, the additional connection may be segregated from a chamber of the pig that contains the body of electrolyte fluid and the electrode of the pig. The electrode of the pig may be an anode, for example of copper, in which case the pipeline wall serves as a cathode. Where the coating is of copper, the electrolyte fluid may comprise a copper salt such as copper sulphate. The method may comprise conveying at least one slug of the electrolyte fluid along the pipeline, that slug being bounded by the pig. There could be a succession of such slugs along the pipeline. At least one slug of a washing fluid such as an acid wash or a hydrocarbon solvent such as glycol may be propelled along the pipeline ahead of the slug or slugs of electrolyte fluid. The or each slug of washing fluid may be accompanied by at least one brush pig that is adapted to abrade the inner surface of the pipe wall. At least one slug of water may be propelled along the pipeline after the or each slug of washing fluid and / or to flush away the electrolyte fluid. A gas can then be conveyed along the pipeline to flush out the water. The various aforementioned slugs can transit the pipeline in series as part of a continuous pig train, and liquid ones of those slugs can be recovered into respective tanks as they arrive at an end of the pipeline. Residual gas such as nitrogen can be vented from within the pipeline ahead of the pig train. The inventive concept also embraces a pig for electrodepositing a metal coating on an inner surface of a pipeline wall, the pig therefore being adapted to implement the method of the invention. The pig of the invention comprises: a body supporting a voltage source, which may be sealed within and insulated by the body; at least one sealing disc mounted on and extending radially from the body; at least one electrode mounted on the body and connected electrically to a first pole of the voltage source; and at least one electrically-conductive connector member extending from the body to a radial extent substantially corresponding to that of the or each sealing disc, the or each connector member being connected electrically to a second pole of the voltage source. The or each connector member can also extend longitudinally from the body. Where the first pole is a positive terminal of the voltage source, the or each electrode serves as an anode. Such an anode may be made of copper. The or each electrode may be disposed between sealing discs spaced longitudinally along the body. At least one of the sealing discs may be disposed at a longitudinal position between the or each connector member and the or each electrode. Thus, the invention exploits the potential for repurposing decommissioned offshore hydrocarbon pipelines for a new use in transporting hydrogen gas. For example, such pipelines would be apt to convey hydrogen that is produced offshore using electrolysers powered by renewable energy from wind or solar sources. The inventive concept may be exemplified by internal cleaning of the pipeline left in situ followed by lining its radially inner surface with a thin layer of a metal, such as copper, formed by electroplating using a copper anode pig and a copper sulphate slug. In principle, the denser copper lining will form a less absorbent barrier that restricts the degree of hydrogen absorption into the steel pipeline wall. The internal cleaning and copper coating processes of the invention are envisaged to leave an internal copper coating that is approximately 0.01mm thick. This coating forms a radially inner layer to reduce the permeation of hydrogen atoms into and through the steel pipe wall. By reducing the permeation of hydrogen atoms into the pipe wall, the quantity of hydrogen that can be pumped through a former hydrocarbon trunk line can be increased to the extent that offshore production of hydrogen becomes a commercially viable option. Embodiments of the invention implement a method to retrofit a used steel pipeline, the method comprising at least the steps of: acid-washing the pipeline, optionally after preliminarily cleaning and flushing the pipeline; flushing the acid with fresh water; and depositing a layer of a second type of metal such as copper. The method may further comprise depositing another or intermediate layer of a third type of metal, such as tin or nickel in the case of hard steels. Each step of the method may comprise circulating or conveying a volume or slug of an appropriate fluid from a first end of the pipeline to a second end of the pipeline between at least two pigs that are displaced or propelled along the pipeline by a pressure differential. The method may further comprise selectively recovering the respective fluids at the second end of the pipeline for evacuation and disposal. Depositing copper as the second type of metal may involve providing copper anode pigs and circulating those pigs inside the pipeline with a volume of liquid copper sulphate or other copper salt disposed between the pigs. Copper plating is feasible only with some types of steel or other ferrous alloys in view of galvanic corrosion, but the practical limits in this respect are well understood in the art. For example, many older offshore pipelines are made of X50 to X60 steel alloys, which present no issue with copper plating. Embodiments of the invention also provide a copper anode pig for electrodepositing copper on a pipeline inner wall, for example derived from copper sulphate. The pig comprises: a pig structure comprising a sealed and insulated body and outer sealing discs mounted on the body for guiding the pig along the pipeline inner wall; at least one copper anode outside the body; at least one electrical battery inside the body, comprising two electrical poles, one pole of the battery being electrically connected to the copper anode; and at least one electrically conductive cathode trailing arm outside the body, electrically connected to the other pole of the battery, a tip of the cathode trailing arm being in contact with the inner wall of the pipeline. In summary, a metallic coating is electrodeposited onto an inner surface of a pipeline wall by conveying a pig along the pipeline and driving an electric current through a body of electrolyte fluid that extends from an electrode of the pig to the pipeline wall. To deposit an internal coating of copper, the pig may support a copper anode and the electrolyte fluid may be of copper sulphate. The electrodeposition method is apt to be performed on a steel pipeline in situ after the pipeline has conveyed hydrocarbons, and may then be followed by conveying hydrogen along the pipeline. The internal coating mitigates migration of hydrogen atoms into the pipe wall and the consequent risk of embrittlement of the steel. 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: Figure 1 is a schematic plan view of an offshore oil / gas field comprising platforms connected directly or indirectly to a terminal onshore; Figure 2 is a piping and instrumentation diagram showing a typical piping system on an offshore oil / gas platform that conveys production fluid into a subsea export pipeline; Figure 3 is a piping and instrumentation diagram showing a typical piping system of an onshore terminal that receives production fluid from a subsea export pipeline such as that shown in Figure 2; Figure 4 is a schematic side view of a pig of the invention; Figure 5 is a schematic end view of the pig shown in Figure 4; Figure 6 is a schematic cross-sectional view on line VI-VI of Figure 4; Figures 7, 8 and 9 are schematic side views of pigs that can be used in methods of the invention; Figure 10 is a flow diagram that outlines method steps of the invention; Figures 11a and 11b are schematic side views of the piping systems of Figures 2 and 3 connected by a subsea export pipeline, showing pre-existing hydrocarbons being purged from the pipeline in a first step of the method set out in Figure 10; Figures 12a to 12g are schematic side views of a subsea export pipeline undergoing further steps of the method set out in Figure 10; Figure 13 is a schematic side view of a pig train of the invention in transit along a subsea export pipeline; and Figure 14 is a cross-sectional view of a pipeline with an internal coating of copper applied in accordance with the invention. Figure 1 exemplifies how an offshore oil / gas field 10 typically comprises a cluster of individual platforms 12. In this example, the hydrocarbon production fluid is collected by infield pipelines 14 extending from satellite platforms 12 to a single gathering platform 12, from which a single export pipeline 16 or trunk line extends across the seabed and crosses the shoreline 18 to reach a terminal 20 situated on shore. Figure 2 shows an example of a topsides piping system on an offshore platform 12 for the production of hydrocarbons. Hydrocarbon production fluid flows along an export line 22 through production export valves 24 and a boarding valve 26 before descending below sea level 28 through a riser 30 and into a subsea export pipeline 16 that leads to an onshore terminal 20 such as that shown in Figures 1 and 3. A pig system comprises an offshore pig trap 32 that can be loaded with pigs through a trap door 34 at one end. The pigs are launched into the export line 22 through a launch pipe 36 at the other end of the pig trap 32. Trap isolation valves 38 are disposed in the launch pipe 36 downstream of the pig trap 32, upstream of the confluence between the launch pipe 36 and the export line 22. Where the pig trap 32 is used to launch pigs, the pig trap 32 is supplied with production fluid by a branch from the export line 22 via bypass valves 40 and pig trap valves 42. The pig trap 32 can also be supplied with other fluids, such as glycol for cleaning or water for flushing, via a flanged inlet 44 controlled by double block valves 46. Correspondingly, Figure 3 shows an example of a topsides piping system of an onshore terminal 20. The subsea export pipeline 16 emerges above sea level 28 as it crosses the shoreline 18 and extends through an in-line dunes valve 48 into a production import pipeline 50 that extends to onshore processing facilities via production import valves 52. In a mirror arrangement of the pig system shown in Figure 2, an onshore pig trap 54 can receive pigs through a receiving pipe 56 branched from the production import pipeline 50. Trap isolation valves 58 are disposed in the receiving pipe 56 upstream of the pig trap 54, downstream of the branch between the production import pipeline 50 and the receiving pipe 56. Again, where the onshore pig trap 54 is used to launch pigs back along the subsea export pipeline 16 via the production import pipeline 50, the pig trap 54 is supplied with production fluid by a branch from the production import pipeline 50 via bypass valves 40 and pig trap valves 42. The pig trap 54 can also be supplied with other fluids via a flanged inlet 44 controlled by double block valves 46. Figures 4 to 9 illustrate various pigs that may be used to implement the invention. Like numerals are used for like features. Figures 4, 5 and 6 show a copper anode pig 60 comprising a cylindrical body 62 from which longitudinally-spaced pairs of sealing discs 64 extend radially. The sealing discs 64 lie in parallel planes that are orthogonal to the central longitudinal axis 66 of the body. The sealing discs 64 bear against the inner surface of the surrounding wall 68 of the pipeline 16 as shown in Figure 4 to allow the pig 60 to be propelled by differential fluid pressure within the pipeline 16 ahead of and behind the pig 60. The longitudinal spacing between the pairs of sealing discs 64 defines an annular chamber 70 between the body 62 and the pipe wall 68. The body 62 is sealed and electrically insulated and contains an onboard power source that is exemplified here by batteries 72 embedded within the body 62. Positive terminals of the batteries 72 are connected to an array of longitudinally-extending copper anodes 74 that are angularly spaced around the body 62. The copper anodes 74 are disposed in the space between the pairs of sealing discs 64, hence being exposed to electrolyte fluid in the annular chamber 70. In this example, an aqueous solution of copper sulphate (CuSO4) is envisaged as the electrolyte fluid. The negative terminals of the batteries 72 are connected to cathode connections that are exemplified here by conductive trailing arms 76 extending outwardly from one end of the body 62. The arms 76 extend longitudinally and radially outwardly in respective radial planes that are spaced angularly around the central longitudinal axis 66. The arms 76 are resiliently biased to bear elastically against the inner surface of the pipe wall 68 at locations longitudinally outboard of the annular chamber 70. By sliding contact with the pipe wall 68, the arms 76 thereby complete a circuit between the pipe wall 68 serving as a cathode and the copper anodes 74 that are opposed to the pipe wall 68 across the annular chamber 70, that circuit extending through the electrolyte fluid in the annular chamber 70. The arms 76 are segregated from the annular chamber 70 by the intervening sealing discs 64. In this example, the batteries 72 each have a cylindrical format and extend longitudinally on parallel axes disposed in concentric arrays around the central longitudinal axis 66. The batteries 72 are connected in parallel by ring connectors 78 that connect their like terminals to each other and to the copper anodes 74 and the trailing arms 76 respectively. Other onboard power sources or other battery formats or arrangements are possible. Figure 7 shows a bidirectional brush pig 80. Again, the pig 80 comprises a cylindrical body 62 from which longitudinally-spaced pairs of sealing discs 64 extend radially. The sealing discs 64 lie in parallel planes that are orthogonal to the central longitudinal axis 66 of the body 62 to bear against the inner surface of a surrounding pipe wall 68, allowing the pig 80 to be propelled by differential fluid pressure as is conventional. In the longitudinal gap between the pairs of sealing discs 64, the body 62 supports angularly-spaced arrays of wire brushes 82 that bear against and abrade the inner surface of the surrounding pipe wall 68 as the pig 80 advances along the pipeline 16, hence removing deposits and exposing bare metal. Magnets 84 supported by the body 62 of the pig 80 capture ferrous particles that are dislodged from the pipe wall 68 by the scouring action of the brushes 82. Figure 8 shows a bidirectional flushing pig 86 that is similar to the brush pig 80 of Figure 7 but omits the brushes 82. Optionally, as in this example, the body 62 of the pig 86 supports circumferential arrays of magnets 84 to capture ferrous particles that may be dislodged from the pipe wall 68. Figure 9 shows a foam pig 88 that comprises a resilient, generally cylindrical foam body 62 whose circular cross section is a tight sliding fit within the surrounding pipe wall 68. Longitudinally-spaced circumferential ribs 90 around the body 62 improve sealing between the body 62 and the pipe wall 68. An exemplary method of the invention is summarised in Figure 10 and will be described in detail in Figures 11a to 13. As Figure 10 shows, the pipeline 16 is first purged of hydrocarbons at 92 and then cleaned internally at 94, for example with a hydrocarbon solvent such as a glycol. After a flush with fresh water at 96, the pipeline 16 is washed internally with a dilute acid at 98, flushed again with fresh water at 100, and electroplated internally at 102 using a copper sulphate solution to form a copper coating on the inner surface of the pipe wall 68. Finally, after a further flush with fresh water at 104, the contents of the pipeline 16 are purged with nitrogen at 106. Figures 11a and 11b illustrate the initial purging step 92 applied to a pipeline 16 that was previously used to convey natural gas. A bidirectional pig 86 such as that shown in Figure 8 is launched from the offshore pig trap 32 as shown in Figure 11a and along the pipeline 16 in the onshore direction, propelled by compressed nitrogen. The piston effect of the transiting pig 86 drives residual natural gas toward the onshore end of the pipeline 16, where the gas is flared off. When the pig 86 arrives ashore, it is diverted into the onshore pig trap 54 as shown in Figure 11b. The entire pipeline 16 is then purged with a flow of nitrogen, which may be released into the atmosphere at the onshore end. Figure 12a shows the internal surface cleaning step 94, which involves injection of glycol into the pipeline via the offshore pig trap 32 and using the offshore pig trap 32 to launch a succession of brush pigs 80 like that shown in Figure 7. The objective of this step 94 is to remove residual hydrocarbon deposits from the inner surface of the pipe wall 68 and to begin to bring that surface back to bare metal. For this purpose, it is envisaged that at least three brush pigs 80 may be fired from the offshore pig trap 32 at intervals such that the pigs 80 of the resulting pig train are separated from each other by slugs of glycol 108. Those slugs 108 may, for example, be about 100m long. Figure 12b shows the first flushing step 96 involving injection of a slug of fresh water 110 into the pipeline 16 via the offshore pig trap 32. The slug of water 110 clears residual glycol 108, continues to clean the inner surface of the pipe wall 68 toward a bare metal state and acts as a neutral barrier for the following acid wash. The slug of water 110 is bounded by brush pigs 80 that may also be about 100m apart. The forward one of those pigs 80 is the trailing pig 80 of the pig train that transports the glycol slugs 108 injected in the preceding step 94. The rearward one of those pigs 80 is launched from the offshore pig trap 32 to be added to the pig train, which therefore now also embraces the water slug 110. Figure 12c shows the acid washing step 98 involving injection of a slug of diluted acid 112 into the pipeline 16 via the offshore pig trap 32. The acid slug 112 continues to clean the inner surface of the pipe wall 68 toward a bare metal state and is bounded by brush pigs 80 that may also be about 100m apart. The forward one of those pigs 80 is the trailing pig 80 of the pig train that transports the glycol and water slugs 108, 112 injected in the preceding steps 94, 96. The rearward one of those pigs 80 is launched from the offshore pig trap 32 to be added to the pig train, which therefore now also embraces the acid slug 112. The injection of diluted acid propels the pig train along the pipeline 16 in the onshore direction. Figure 12d shows the second flushing step 100 involving injection of a second slug of fresh water 114 into the pipeline 16 via the offshore pig trap 32. The second water slug 114 clears residual acid 112, continues to clean the inner surface of the pipe wall 68 toward a bare metal state and acts as a neutral barrier for the copper sulphate slugs that will follow. Again, this second water slug 114 is bounded by brush pigs 80 that may, for example, be spaced about 100m apart. The forward one of those pigs 80 is the trailing pig 80 of the pig train that transports the glycol, water and acid slugs 108, 110, 112 injected in the preceding steps 94, 96, 98. The rearward one of those pigs 80 is launched from the offshore pig trap 32 to be added to the pig train, which therefore now also embraces the second water slug 114. The injection of water propels the pig train further along the pipeline 16 in the onshore direction. Optionally, the rearmost pig of the enlarged pig train, at the trailing end of the second water slug 114, could be a copper anode pig 60 like that shown in Figures 4 to 6. This possibility is shown in Figure 12e, which illustrates the electroplating step 102 where slugs of copper sulphate solution 116 are injected into the pipeline 16 via the offshore pig trap 32. The offshore pig trap 32 is used to launch a succession of copper anode pigs 60 like that shown in Figures 4 to 6. The copper sulphate solution fills the annular chamber 70 of each copper anode pig 60 bounded by the pipe wall 68, the body 62 and the longitudinally-spaced pairs of sealing discs 64, hence bathing the copper anodes 74 in that electrolyte fluid to complete the electrical circuit. The objective of the electroplating step 102 is to deposit a thin copper lining on the bare metal inner surface of the pipe wall 68. Thus, elemental copper derived from the copper sulphate in the electrolyte fluid is deposited on the pipe wall 68 that serves as the cathode. In other words, during electrolysis, copper atoms derived from the copper sulphate form a coating on the pipe wall 68, leaving residual sulphur in the electrolytic solution. It is envisaged that at least three copper anode pigs 60 may be fired from the offshore pig trap 32 at intervals such that the pigs 60 of the resulting pig train are separated from each other by slugs of copper sulphate solution 116. Those slugs 116 may, for example, be about 200m long. As noted above, the foremost one of the copper anode pigs 60 is the rearmost pig 60 of the pig train that transports the glycol, water, acid and second water slugs 108, 110, 112, 114 that were injected in the preceding steps 94, 96, 98, 100. The other copper anode pigs 60 launched from the offshore pig trap 32 are added to the pig train, which therefore now also embraces the copper sulphate slugs 116. The continued injection of copper sulphate solution propels the pig train further along the pipeline 16 in the onshore direction. Figure 12f shows the third flushing step 104 involving injection of at least a third slug of fresh water 118 into the pipeline 16 via the offshore pig trap 32. In this example, there are third and fourth water slugs 118 disposed in succession to clear residual copper sulphate solution from the pipeline. Each of those slugs 118 may, for example, be about 100m long. The third water slug 118 is bounded at its leading end by the trailing copper anode pig 60 of the preceding pig train and at its trailing end by a flushing pig 86 like that shown in Figure 8, hence also disposed at the leading end of the fourth water slug 118. The fourth water slug 118 is further bounded at its trailing end by a second flushing pig 86. The flushing pigs 86 are launched from the offshore pig trap 32 to be added to the pig train, which therefore now also embraces the third and fourth water slugs 118. The injection of water propels the pig train further along the pipeline 16 in the onshore direction. Figure 12g shows first and second slugs of nitrogen 120 injected into the pipeline 16 via the offshore pig trap 32. In this example, the first and second slugs of nitrogen 120 are disposed in succession to clear the various previously-injected liquids 108 to 116 of the pig train from the pipeline 16. Again, each of those slugs 120 may, for example, be about 100m long. The first nitrogen slug 120 is bounded at its leading end by the trailing flushing pig 86 of the preceding pig train and at its trailing end by a foam pig 88 like that shown in Figure 9, hence also disposed at the leading end of the second nitrogen slug 120. The second nitrogen slug 120 is further bounded at its trailing end by a second foam pig 88. The foam pigs 88 are launched from the offshore pig trap 32 to be added to the pig train, which therefore now also embraces the first and second nitrogen slugs 120. The injection of nitrogen propels the pig train further along the pipeline 16 in the onshore direction. Where slugs of the same fluid are in immediate succession within the pipeline, the pigs that separate those slugs could instead be considered as subdividing one longer slug of that fluid. Figure 13 is a schematic representation of the various slugs of fluid of the pig train travelling along the pipeline 16 in the onshore direction. The pig train may, for example, transit at a speed of about 0.1m / s. In order from the leading end to the trailing end of the pig train, the slugs are of glycol 108, water 110, dilute acid 112, water 114, copper sulphate solution 116, water 118 and nitrogen 120. Eventually, the pig train is pushed to shore, with each pig of the pig train being received in sequence by the onshore pig trap 54. In each of the steps shown in Figures 12a to 12g, and as also shown in Figure 13, residual nitrogen 122 left from the initial purging step and disposed ahead of the pig train is expelled into the atmosphere from the onshore end of the pipeline 16 as the pig train extends and advances along the pipeline 16. As each liquid slug reaches the terminal 20 at the onshore end of the pipeline 16, the slugs are captured selectively by the onshore pig trap 54 and drained into respective tanks provided at the terminal 20, namely a glycol tank 124, a water tank 126, an acid tank 128 and a copper sulphate tank 130. The nitrogen 120, 122 at the leading and trailing ends of the pig train can simply be vented to atmosphere, whereupon the pipeline 16 is completely purged with nitrogen. Turning finally to Figure 14, this cross-sectional view of an internally coated steel pipeline 16 of the invention shows a copper lining 132 with a thickness of, for example, 0.01mm. The steel pipe wall 68 may, for example, have a thickness of 13mm and an outer diameter of 24 inches (610mm). In this example, the pipeline 16 has an external anti-corrosion coating 134 of fusion-bonded epoxy, typically with a thickness of 0.6mm, surrounded by a weight coating 136 of concrete that may be 50mm thick. Other corrosion-resistant or thermally insulating coatings are possible. Many variations are possible within the inventive concept. For example, anodes and cathodes of various metals can be used to deposit copper from an electrolyte fluid that comprises a copper salt. Also, in principle, copper cathode elements on a pig could be used to transfer copper to the pipe wall serving as an anode, if the resultant erosion of the copper cathode would be manageable. It would be possible to deposit a layer of a different metal, such as tin or nickel, using similar techniques with corresponding adjustments to electrolytes and electrodes. Such a layer could, for example, be deposited onto or beneath a layer of copper. The electrical connection between the pig and the pipeline that is made through the trailing arms could instead be made through one or more of the sealing discs. Where the invention is applied to an offshore pipeline, it would be possible in principle to convey the pig train and the associated slugs in the opposite direction to that described above, hence from the onshore end toward the offshore end of the pipeline. Also, whilst the invention has been described in the context of an offshore pipeline, the inventive concept can also be applied to an onshore pipeline.

Claims

Claims 1. A method of electrodepositing a metallic coating on an inner surface of a pipeline wall, the method comprising conveying a pig along the pipeline and driving an electric current through a body of electrolyte fluid that extends from an electrode of the pig to the pipeline wall.

2. The method of Claim 1, wherein the body of electrolyte fluid is part of an electric circuit that comprises an additional connection extending between the pig and the pipeline wall.

3. The method of Claim 2, comprising segregating the additional connection from a chamber of the pig that contains the body of electrolyte fluid and the electrode of the pig.

4. The method of any preceding claim, wherein the electric current is driven by a voltage source carried by the pig.

5. The method of any preceding claim, wherein the electrode of the pig is an anode and the pipeline wall serves as a cathode.

6. The method of Claim 5, wherein the anode is of copper.

7. The method of any preceding claim, wherein the coating is of copper.

8. The method of any preceding claim, wherein the electrolyte fluid comprises a copper salt.

9. The method of Claim 8, wherein the copper salt is copper sulphate.

10. The method of any preceding claim, comprising conveying a slug of the electrolyte fluid along the pipeline, that slug being bounded by the pig.

11. The method of Claim 10, comprising conveying a succession of such slugs along the pipeline.

12. The method of Claim 10 or Claim 11, comprising conveying at least one slug of a washing fluid along the pipeline ahead of the slug of the electrolyte fluid.

13. The method of Claim 12, comprising accompanying the or each slug of washing fluid with at least one brush pig that is adapted to abrade the inner surface of the pipe wall.

14. The method of Claim 12 or Claim 13, comprising conveying at least one slug of water along the pipeline following the or each slug of washing fluid.

15. The method of any of Claims 12 to 14, wherein the or each slug of washing fluid comprises an acid wash.

16. The method of any of Claims 12 to 15, wherein the or each slug of washing fluid comprises a hydrocarbon solvent.

17. The method of Claim 16, wherein the hydrocarbon solvent is a glycol.

18. The method of any preceding claim, followed by conveying at least one slug of water along the pipeline to flush the electrolyte fluid.

19. The method of Claim 18, followed by conveying a gas along the pipeline to flush the water.

20. The method of any of Claims 11 to 19, wherein the slugs transit the pipeline in series as part of a continuous pig train.

21. The method of Claim 20, comprising recovering liquid slugs into respective tanks as the slugs of the pig train arrive at an end of the pipeline.

22. The method of Claim 21, comprising venting gas from within the pipeline ahead of the pig train.

23. The method of any preceding claim, performed on a steel pipeline in situ after the pipeline has conveyed hydrocarbons, and followed by conveying hydrogen along the pipeline.

24. A pig for electrodepositing a metal coating on an inner surface of a pipeline wall, the pig comprising: a body supporting a voltage source; at least one sealing disc mounted on and extending radially from the body; at least one electrode mounted on the body and connected electrically to a first pole of the voltage source; and at least one electrically-conductive connector member extending from the body to a radial extent substantially corresponding to that of the or each sealing disc, the or each connector member being connected electrically to a second pole of the voltage source.

25. The pig of Claim 24, wherein the first pole is a positive terminal of the voltage source and the or each electrode is an anode.

26. The pig of Claim 25, wherein the or each anode is of copper.

27. The pig of any of Claims 24 to 26, wherein the or each electrode is disposed between sealing discs spaced longitudinally along the body.

28. The pig of any of Claims 24 to 27, wherein at least one of the sealing discs is disposed at a longitudinal position between the or each connector member and the or each electrode.

29. The pig of any of Claims 24 to 28, wherein the or each connector member also extends longitudinally from the body.

30. The pig of any of Claims 24 to 29, wherein the voltage source is sealed within and insulated by the body.

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