Subsea Pipelines Equipped with Direct Electrical Heating Systems

Abstract

A heated subsea pipeline includes a direct electrical heating (DEH) system that heats a central major portion of the pipeline. Supplementary heating systems extend along respective end portions of the pipeline, longitudinally outboard of the central portion heated by the DEH system. A flow of heating fluid is circulated along the end portions and may be circulated through an underwater vehicle that pumps and heats the flow.

Claims

1-27. (canceled)

28. A heated subsea pipeline, comprising: a direct electrical heating (DEH) system that is arranged to heat a central elongate portion of the pipeline; at least one supplementary heating system that extends along an end portion of the pipeline, longitudinally outboard of the central portion heated by the DEH system; and external connectors for the supply of energy from an underwater vehicle to the supplementary heating system, wherein the connectors are supported by a bulkhead, connector plate or end structure of the pipeline.

29. The pipeline of claim 28, wherein the central portion extends along the pipeline between current transfer zones that are mutually spaced along the pipeline.

30. The pipeline of claim 29, wherein the end portions include longitudinally outboard portions of the current transfer zones.

31. The pipeline of claim 28, wherein the central portion extends along the pipeline between current injection points that are mutually spaced along the pipeline.

32. The pipeline of claim 31, wherein the end portions are longitudinally outboard of the current injection points.

33. The pipeline of claim 28, wherein the supplementary heating system comprises at least one heating element in thermal communication with an inner flowline of the pipeline.

34. The pipeline of claim 33, wherein the heating element is in contact with the flowline.

35. The pipeline of claim 33, wherein the heating element is surrounded by thermal insulation that encircles the flowline.

36. The pipeline of claim 35, wherein the heating element is disposed within an annulus between the inner flowline and the thermal insulation.

37. The pipeline of claim 35, wherein the heating element is embedded in the thermal insulation.

38. The pipeline of claim 33, wherein the heating element is a hot fluid conduit.

39. The pipeline of claim 38, wherein the conduit has a diameter of less than two inches (5.1 cm).

40. The pipeline of claim 33, in combination with a feeder cable that is connected electrically to the flowline.

41. The pipeline of claim 33, in combination with a return cable that is connected electrically to the flowline.

42. The pipeline of claim 28, wherein the connectors are hot stab couplings.

43. In combination, the pipeline of claim 28 with an underwater vehicle that is configured to supply energy to the supplementary heating system.

44. The combination of claim 43, wherein the underwater vehicle carries connectors that are cooperable with the connectors of the pipeline to complete a heating circuit.

45. The combination of claim 44, wherein the underwater vehicle carries a pump that is arranged to drive a flow of heating fluid around the heating circuit.

46. The combination of claim 45, wherein the underwater vehicle carries a heater that is arranged to heat the flow of heating fluid.

47. A method of heating a subsea pipeline that is heated primarily by a direct electrical heating (DEH) system, the method comprising: connecting an underwater vehicle to external connectors of at least one supplementary heating system that extends along an end portion of the pipeline, longitudinally outboard of a central portion of the pipeline heated by the DEH system, the external connectors being supported by a bulkhead, connector plate or end structure of the pipeline; and activating the supplementary heating system by circulating a flow of heating fluid between the underwater vehicle and the supplementary heating system, along the end portion of the pipeline.

48. The method of claim 47, comprising pumping the flow of the heating fluid from the underwater vehicle into the supplementary heating system.

49. The method of claim 47, comprising heating the flow of the heating fluid aboard the underwater vehicle.

50. The method of claim 47, comprising activating the supplementary heating system when the pipeline is shut down.

Description

[0058] In order that the invention may be more readily understood, reference will now be made, by way of example, to the remainder of the accompanying drawings in which:

[0059] FIG. 2 is a schematic side view of a subsea pipeline fitted with a DEH system and supplementary heating systems in accordance with the invention;

[0060] FIG. 3 is a cross-sectional view taken through an end portion of the pipeline of FIG. 2 heated by one of the supplementary heating systems;

[0061] FIG. 4 is a cross-sectional view showing a variant of the arrangement shown in FIG. 3;

[0062] FIG. 5 is a schematic side view of an end portion of the pipeline of FIG. 2 showing a supplementary heating system arrangement;

[0063] FIG. 6 is a schematic side view showing a variant of the arrangement shown in FIG. 5;

[0064] FIG. 7 is a schematic system diagram of a supplementary heating system of the invention, including parts of the system that may be implemented by an ROV;

[0065] FIG. 8 is a schematic side view of a pipe joint adapted to implement a supplementary heating system of the pipeline of FIG. 2; and

[0066] FIGS. 9 to 11 are schematic side views of various arrangements for field joints made between conjoined pipe joints of FIG. 8.

[0067] The steel pipeline 40 shown on the seabed 18 in FIG. 2 shares many similarities with the pipeline 16 shown in FIG. 1. Like numerals are therefore used for like features.

[0068] In this example, a coaxial umbilical riser 14 connects to the DEH system 10 of the pipeline 40 via a subsea junction box 20. Again, subsea structures in fluid communication with the pipeline 40, such as PLETs, have been omitted from FIG. 2. A surface installation such as an FPSO has also been omitted from FIG. 2.

[0069] As in FIG. 1, a first conductor of the umbilical riser 14 is connected electrically to the pipeline 40 via a feeder cable 28 coupled to a first connection plate 24 situated close to one end of an isolated section of the pipeline 40. A second conductor of the umbilical riser 14 is connected electrically to the pipeline 40 via a piggybacked DEH cable 30 coupled to a second connection plate 26 situated close to the opposite end of the isolated section. The connection plates 24, 26 are electrically connected to the wall of the pipeline 40 to serve as current injection points.

[0070] The DEH system 10 is electrically connected to the surrounding seawater by arrays of additional sacrificial anodes 34 mounted on the pipeline 40. Those arrays of anodes 34 define respective CTZs 36 that, as before, may extend up to about fifty metres to either side of each connection plate 24, 26.

[0071] End sections 38 of the pipeline 40 positioned longitudinally outboard of the connection plates 24, 26 extend at least to the outboard ends of the respective CTZs 36 and so each have a length of up to about fifty metres. The end sections 38 are bounded by the connection plates 24, 26 and are contiguous with the central DEH-heated portion of the pipeline 40.

[0072] In accordance with the invention, each end section 38 of the pipeline 40 is heated by a respective supplementary heating system 42 that is independent of the DEH system 10. Each supplementary heating system 42 comprises a heating element in the form of a looped heating conduit 44 for conveying a flow of hot fluid such as water along most or substantially all of the length of each end section 38. This warms the end sections 38 by thermal transmission between the conduit and the steel wall of the pipe 40 to avoid, or to remediate, plugging of the pipe 40 with solids at those locations.

[0073] In this example, each heating conduit 44 has two generally parallel limbs conjoined at one end by a U-section. At the opposite end of the heating conduit 44, free ends of the limbs communicate with respective hot-stab connectors 46 that enable an external pumping and heating system to be coupled temporarily to the conduit 44 when mitigation or remediation is required. As will be explained below with reference to FIG. 7, such a system is apt to be implemented by an ROV that docks with the conduit 44 via complementary connectors to complete a heating loop or circuit in which a hot fluid can be recirculated.

[0074] The cross-sectional view of FIG. 3 shows the two tubular limbs of the heating conduit 44 in contact with the exterior of a steel inner flowline 48 of the pipeline 40 to maximise thermal conduction between them. In this example, the conduit 44 is accommodated in an annulus 50 that is defined by a radial gap between the flowline 48 and a tubular layer of wet thermal insulation 52 encircling the flowline 48. Heat can propagate circumferentially and axially around and along the flowline 48 by convection through a volume of gas, such as air, trapped in the annulus 50.

[0075] As is well known in the art, the annulus 50 may be maintained by spacers extending radially between the flowline 48 and the insulation 52. Such spacers have been omitted from the drawings for ease of illustration but may be spaced longitudinally along and/or angularly around the flowline 48.

[0076] FIG. 4 corresponds to FIG. 3 but shows a variant in which multiple heating conduits 44 are spaced angularly around the flowline 48 to distribute heat more evenly. Alternatively those tubular features may represent multiple limbs of the same conduit 44 in fluid communication with each other. It will be apparent that similar arrangements may also be adopted within the annulus 50 of the variant shown in FIG. 3.

[0077] FIG. 4 also shows that the conduits 44 may be embedded in the insulation 52, which in this example is in direct contact with the flowline 48 to leave no annulus between them.

[0078] The insulation 52 shown in FIGS. 3 and 4 typically comprises a polymer material such as polypropylene or polyurethane. Such a polymer material may be provided as a solid layer or may instead be a matrix of a syntactic foam or of a composite material.

[0079] Turning next to FIGS. 5 and 6, these drawings show possible arrangements for supporting the hot stab connectors 46 shown in FIG. 2. In each case, part of the CTZ of the pipeline 40 is shown, comprising sacrificial anodes 34 spaced longitudinally along the pipeline 40. In these examples, a bulkhead 54 performs the current injection function of the connector plate 26 shown in FIGS. 1 and 2 and so is connected electrically to the DEH cable 30. A connector hub 56 is provided at the adjacent end of the pipeline 40.

[0080] The end section 38 of the pipeline 40 extends from the bulkhead 54 to the connector hub 56. The end section 38 accommodates a supplementary heating system 42 that comprises a heating conduit 44 whose limbs terminate in respective hot stab connectors 46. The hot stab connectors 46 are shown supported by the connector hub 56 in FIG. 5 but are instead shown supported by the bulkhead 54 in FIG. 6.

[0081] FIG. 7 shows an external pumping and heating system 58 that can be coupled temporarily to the heating conduit 44 of a supplementary heating system 42 when mitigation or remediation is required. In this example, the pumping and heating system 58 is implemented on board an ROV 60 or on a skid carried by an ROV 60.

[0082] The ROV 60 docks with the hot stab connectors 46 of the heating conduit 44 via complementary connectors 62 to complete a heating loop or circuit for recirculation of hot water. In addition to the connectors 62, the pumping and heating system 58 comprises a pump 64 in series with an electrical heating unit 66.

[0083] FIG. 7 also shows further details of valve arrangements of the supplementary heating system 42. The flow of hot fluid through the heating conduit 44 is controlled by an inlet valve 68 and one-way flow in the heating conduit 44 is assured by opposed non-return valves 70.

[0084] FIG. 8 shows a pipe joint 72 that can be incorporated into the pipeline 40 to implement at least part of the heating conduit 44. In this example, the pipe joint 72 is arranged to implement a central portion of the conduit 44 and so comprises parallel pipes 74 that extend the full length of the pipe joint 72. Each pipe 74 corresponds to a respective limb of the conduit 44. The pipes 74 are buried under, or encapsulated within, an outer layer of wet insulation 52 that surrounds an inner flowline 48.

[0085] Those skilled in the art will understand that a pipe joint is a length of pipe of a standard length of nominally twelve metres. Pipe joints may also be provided in lengths of multiples of twelve metres. Pipe joints are welded together end-to-end to fabricate a pipe string that is lowered to the seabed when installing a subsea pipeline.

[0086] Two or more pipe joints 72 may need to be joined end-to-end to complete the full length of the heating conduit 44. Other pipe joints 72 could therefore be provided to implement an end portion of the conduit 44, for example where the pipes 74 are conjoined by a U-section at which the flow of hot fluid within the conduit 44 reverses in direction. Alternatively, substantially all of the conduit 44 could be implemented on one pipe joint 72.

[0087] Turning finally to FIGS. 9 to 11, these drawings show field joints at which successive pipe joints 72 are welded together to form a pipeline 40. Here, connections must also be made between the pipes 74 of the successive pipe joints 72 so that those parts of the heating conduit 44 are brought into fluid communication with each other.

[0088] As is conventional, each field joint shown in FIGS. 9 to 11 comprises a circumferential butt weld 76 between successive lengths of the steel inner flowline 48 of the pipe joints 72. To facilitate welding, the thermal insulation 52 around those lengths of the inner flowline 48 is cut away from the end regions that face each other around the interface to be welded.

[0089] After welding together the successive lengths of the flowline 48, the pipes 74 are connected to their counterparts as shown in FIGS. 9 to 11. Thermal insulation is subsequently restored by forming a field joint coating 78 around the field joint, as shown in dashed lines in FIGS. 9 to 11, for example by moulding. Conveniently, the field joint coating 78 embeds or surrounds the pipes 74 and their connections and therefore retains heat applied to the flowline 48 by the heating conduit 44 in use.

[0090] FIGS. 9 to 11 show various connection arrangements between free ends of the pipes 74 that face each other across the field joint. In FIG. 9, the pipes 74 are joined together end-to-end by welds 80. Conversely, the pipes 74 are coupled end-to-end by complementary connectors 82 in FIG. 10. In FIG. 11, an intermediate tube 84 is inserted between each pair of opposed pipes 74. That intermediate tube 84 may comprise a flexible pipe as shown here, terminated by connectors 82. Alternatively, the intermediate tube 84 could be rigid, in which case welds 80 like those of FIG. 9 could be used instead of connectors 82.

[0091] Many variations are possible within the inventive concept. For example, it would be possible for a pumping and heating system to be connected permanently to the heating conduit of a supplementary heating system. This would enable the supplementary heating system to be activated whenever required, without requiring intervention by an ROV.

[0092] Limbs of the heating conduit of a supplementary heating system could have a different shape to that illustrated, for example with sinusoidal, undulating or helical curvature along the underlying pipeline. Also, the conduit could have more than two limbs, for example in a serpentine shape comprising multiple inflections.

[0093] Wet insulation surrounding the heating conduit could be replaced or supplemented by the outer pipe of a PiP assembly.