Integrated towhead and fluid processing system

Abstract

A subsea processing system has a lattice frame made up of structural members and at least one fluid-processing device. At least one of the structural members effects fluid communication to or from the or each fluid-processing device.

Claims

1. A towable subsea structure, comprising: a pipeline or a pipeline bundle; and a subsea processing system in fluid communication with the pipeline or pipeline bundle and being disposed at an end of the pipeline or pipeline bundle, the subsea processing system comprising: a lattice frame defining a towhead, the lattice frame comprising an upper frame and a base frame each made up of structural members configured to maintain sufficient structural integrity of the towable subsea structure; and at least one fluid-processing device supported by the lattice frame; wherein structural members of the base frame are in fluid communication with each other and at least one of the structural members of the base frame effects fluid communication to or from the or each fluid-processing device.

2. The structure of claim 1, wherein the or each fluid-processing device is carried on at least one structural member of the frame.

3. The structure of claim 1, wherein the or each fluid-processing device is contained within at least one structural member of the frame.

4. The structure of claim 1, wherein at least one of the structural members defines a tank in fluid communication with the or each fluid-processing device.

5. The structure of claim 4, wherein the or each fluid-processing device is immersed in fluid in the tank.

6. The structure of claim 1, wherein at least one of the structural members effects fluid communication between two or more fluid-processing devices.

7. The structure of claim 1, wherein at least one of the structural members of the base frame is in fluid communication with the pipeline or pipeline bundle.

8. The structure of claim 1, wherein at least one of the structural members of the lattice frame is made of polymer or composite materials.

9. The structure of claim 1, wherein the or each fluid-processing device is an immersion heater in fluid communication with a hot-water heating circuit.

10. The structure of claim 9, wherein the immersion heater is disposed within at least one of the structural members of the lattice frame, which structural member serves as an expansion tank for water heated by the immersion heater.

11. The structure of claim 1, wherein the base frame comprises longitudinally-extending, substantially parallel tubular structural members.

12. The structure of claim 11, wherein the longitudinally-extending, substantially parallel tubular structure members of the base frame are connected in series as part of a fluid circuit.

13. A method of processing fluid subsea, comprising conveying fluid to or from a fluid-processing device through at least one structural member of a base frame a lattice frame that supports the fluid-processing device wherein the at least one structural member of the base frame is in fluid communication with a pipeline or a pipeline bundle and with another structural member of the base frame; and wherein the frame is configured to define a towhead disposed at an end of the pipeline or the pipeline bundle and the structural members are configured to maintain sufficient structural integrity of the towhead.

14. The method of claim 13, preceded by transporting the frame to a subsea location while providing buoyant support to the frame by holding a gas in said structural member of the frame.

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 cross-sectional view of a typical subsea pipeline bundle;

(3) FIG. 2 is a schematic side view of a towhead of the invention attached to an end of the bundle shown in FIG. 1;

(4) FIG. 3 is a cut-away perspective view showing a base frame of the towhead shown in FIG. 2 at an end of the bundle;

(5) FIG. 4 is a schematic outline of the base frame of FIG. 3 in top plan view, showing hot water circulation through the bundle and the base frame in accordance with the invention; and

(6) FIG. 5 is a schematic outline of a base frame of a towhead in top plan view, showing a further embodiment of the invention applied to processing of production fluid in a wellstream.

(7) FIG. 1 shows a pipeline bundle 10 in transverse cross-section. The bundle 10 comprises a flowline 12 and parallel hot water lines 14, all contained within, and extending along, a thermally-insulated inner carrier pipe 16.

(8) In turn, the inner carrier pipe 16 containing the flowline 12 and the hot water lines 14 is contained within, and extends along, an outer carrier pipe 18. Rollers 20 support the inner carrier pipe 16 for longitudinal movement relative to an outer carrier pipe 18, as will occur on assembly of the bundle 10 and under differential thermal expansion in use.

(9) The bundle 10 is an example of many possible bundle arrangements. It shows one way in which production fluid can be kept warm by transfer of heat from one or more hot fluid lines such as the hot water lines 14 to an adjacent flowline 12.

(10) In this example, the hot water lines 14 lie parallel to each other as a pair but are fluidly connected in series to serve as parts or legs of the same hot fluid loop or circuit. Thus, the flow direction in one hot water line 14 is the reverse of the flow direction in the other hot water line 14.

(11) FIG. 2 shows a towhead 22 at an end of the bundle 10. The towhead 22 has a lattice structure that comprises a base frame 24 and an upper frame 26 spaced from the base frame 24. The upper frame 26 stiffens the structure of the towhead 22 and encloses and protects equipment carried by the base frame 24. These parts of the towhead 22 would conventionally be fabricated of steel but one or more structural members of the towhead 22 could be of polymer or composite materials.

(12) The base frame 24 is shown in detail in FIG. 3 and in outline in FIG. 4. Only the main structural members of the base frame 24 are shown in FIGS. 3 and 4, namely parallel longitudinal tubes 28 and tubular cross-members 30 that join the tubes 28. An end portion of the bundle 10 lies centrally between and extends parallel to the tubes 28.

(13) In this example, the cross-members 30 extend orthogonally to the tubes 28 so that the base frame 24 has a ladder configuration. The base frame 24 could instead have a triangulated configuration in which cross-members 30 are at acute angles to the tubes 28.

(14) The upper frame 26 comprises upper tubes 32 that extend parallel to the tubes 28 of the base frame 24 and may also be of ladder or triangulated configuration.

(15) The bundle 10 is connected to the towhead 22 mechanically and also to establish fluid communication between the bundle 10 and the towhead 22. In this example, the connections between the bundle 10 and the towhead 22 are made via the base frame 24.

(16) Specifically, the base frame 24 is arranged and equipped to receive production fluid from the flowline 12 or to convey production fluid to the flowline 12. The base frame 24 is also arranged and equipped to receive water from the hot water lines 14, to heat that water, to output the heated water back into the hot water lines 14 and to drive circulation of the water along the hot water lines 14.

(17) The end portion of the bundle 10 is attached to and supported by one of the cross-members 30 near one end of the base frame 24. A typically welded interface between the outer carrier pipe 18 of the bundle 10 and the cross-member 30 effects mechanical coupling between the bundle 10 and the base frame 24.

(18) The other cross-member 30 near the other end of the base frame 24 supports a tie-in porch 34. The flowline 12 emerges from a closed end of the outer carrier pipe 18 of the bundle 10 and extends along the base frame 24 to the tie-in porch 34 via a production check valve 36.

(19) As noted above, the hot water lines 14 are parts of the same hot fluid loop or circuit. In accordance with the invention, that circuit includes structural members of the base frame 24, in this example the tubes 28, which serve as flow conduits, reservoirs and/or expansion tanks for hot water. Thus, the hot water lines 14 emerge from the end of the bundle 10 and extend from the bundle 10 in respective opposite lateral directions to connect to respective ones of the tubes 28 for fluid communication with the tubes 28.

(20) Each hot water line 14 has a respective hot water check valve 38 disposed between the bundle 10 and the associated tube 28.

(21) The hot fluid loop or circuit is completed by a cross-pipe 40 that extends between and couples the tubes 28 for fluid communication between them. A hot water pump 42 in the cross-pipe 40 drives the flow of hot water through the hot fluid circuit.

(22) Each tube 28 is equipped with electrically-powered immersion heaters 44 for heating water as it resides within and flows along the tubes 28. Power units 46 of the immersion heaters 44 conveniently protrude from ends of the respective tubes 28. As shown in FIG. 4 of the drawings, heating elements 48 of the immersion heaters 44 extend from the power units 46 along the interior of the tubes 28.

(23) FIG. 4 also shows the path of hot water circulation around the system. Water returning from the bundle 10 along one of the hot water pipes 14 emerges from the bundle 10 and enters one of the tubes 28, where the water is heated by the associated heating element 48. The heated water flows along that tube 28 and then along the cross-pipe 34 and into the other tube 28, where the water is heated further by the heating element 48 associated with that other tube 28. The further-heated water flows along that other tube 28 and then back into the bundle 10 along the other hot water pipe 14.

(24) Turning finally to FIG. 5, this exemplifies how the inventive concept may be applied to processing of production fluid in a wellstream. As in FIG. 4, arrows show the various fluid flow paths. Like numerals are used for like features.

(25) Here, a base frame 50 of a towhead or other subsea structure again comprises parallel longitudinal tubes 28 and tubular cross-members 30 that join the tubes 28. In this example, the base frame 50 supports a gas separator 52 and a water separator 54 that are connected in series to remove gas and then water from a wellstream. For this purpose, the gas separator 52 and the water separator 54 communicate with a flowline 12 that carries the incoming wellstream.

(26) Gas removed from the wellstream by the gas separator 52 is held in, and conveyed along, one of the tubes 28 of the base frame 50 to a pump 56 that pressurises the gas, for example for re-injection into a subsea well.

(27) Oily water removed from the wellstream by the water separator 54 is held in, and conveyed along, the other tube 28 of the base frame 50 to a treatment unit 58. The treatment unit 58 cleans the water by removing residual oil, allowing the water to be re-injected into a well or to be expelled into the surrounding sea.

(28) Typically, control valves will be interposed between the flowline 12 and the tubes 28 and/or between the tubes 28 and the pump 56 and the treatment unit 58. Such valves have been omitted from FIG. 5 for simplicity.

(29) FIG. 5 shows that different structural members of a subsea structure may be used to hold or to convey different fluids. More generally, using structural members of the subsea structure to hold and/or to convey fluids simplifies and lightens the assembly by reducing pipework and by obviating holding tanks. Also, by providing more options to position functional modules space-efficiently, the assembly can be made more compact.

(30) Many variations are possible within the inventive concept. For example, hot gases such as steam or fluids or liquids other than water could be circulated in the system.

(31) Fluid communication between the tubes 28 could be effected via one or more structural cross-members 30 rather than via separate piping such as the cross-pipe 40. The hot water pump 42 could be positioned elsewhere in the hot fluid circuit.

(32) Only one tube 28 or other structural member could be used to heat or to convey fluids; conversely, more than two tubes 28 or other structural members could be used to heat or to convey fluids.