Injecting fluid into a hydrocarbon production line or processing system

Abstract

An injection fluid is pressurized for injection into a subsea installation or equipment by using a flow of hydrocarbon production fluid to draw the injection fluid from a source and using pressure of the production fluid to pressurize the drawn injection fluid to an elevated pressure. In response to an ejector that is powered by the flow of the production fluid, a pressure booster draws in the injection fluid for pressurization. A portion of the production fluid is diverted to the pressure booster to pressurize and expel the injection fluid and is then exhausted to the ejector as more injection fluid is drawn in.

Claims

1. A method of pressurising an injection fluid for injection into an installation or equipment subsea, the method comprising: using a flow of hydrocarbon production fluid to generate a reduced pressure relative to pressure of the injection fluid in a source so as to draw the injection fluid from the source; and using pressure of the production fluid to pressurise the drawn injection fluid to an elevated pressure above the pressure of the production fluid.

2. The method of claim 1, wherein the injection fluid is under hydrostatic pressure at the source.

3. The method of claim 1, comprising drawing and pressurising the injection fluid in alternating cycles.

4. The method of claim 1, comprising drawing the injection fluid from the source through a non-return valve.

5. The method of claim 1, comprising using the flow of production fluid to drive a pump that generates the reduced pressure.

6. The method of claim 1, comprising: drawing the injection fluid into an injection fluid chamber by reducing pressure in the injection fluid chamber; and pressurising the injection fluid in the injection fluid chamber.

7. The method of claim 6, comprising reducing pressure in the injection fluid chamber to a level below hydrostatic pressure outside the injection fluid chamber.

8. The method of claim 6, comprising diverting a portion of the flow of production fluid to pressurise the injection fluid in the injection fluid chamber.

9. The method of claim 8, comprising: passing production fluid through an open pressurisation control valve when pressurising the injection fluid in the injection fluid chamber; and closing the pressurisation control valve when drawing the injection fluid into the injection fluid chamber.

10. The method of claim 9, comprising controlling the pressurisation control valve to control pressurisation of the injection fluid in the injection fluid chamber.

11. The method of claim 9, comprising introducing the diverted portion of the production fluid into a production fluid chamber to pressurise the injection fluid in the injection fluid chamber.

12. The method of claim 11, comprising: expanding the production fluid chamber under pressure from the introduced portion of the production fluid; and contracting the injection fluid chamber in response to expansion of the production fluid chamber.

13. The method of claim 11, comprising generating a reduced pressure in the production fluid chamber to reduce pressure in the injection fluid chamber.

14. The method of claim 13, comprising: contracting the production fluid chamber; and expanding the injection fluid chamber in response to contraction of the production fluid chamber.

15. The method of claim 13, comprising returning production fluid from the production fluid chamber into the flow of production fluid.

16. The method of claim 15, comprising: passing the returned production fluid through a filling control valve when drawing the injection fluid into the expanding injection fluid chamber; and controlling the filling control valve to control filling of the injection fluid chamber with the injection fluid.

17. The method of claim 11, comprising reciprocating a piston between the production fluid chamber and the injection fluid chamber, that piston having opposed heads that partially define the respective chambers.

18. The method of claim 1, wherein the flow of production fluid is in a subsea production pipeline.

19. The method of claim 18, wherein the pipeline extends from a subsea wellhead.

20. The method of claim 1, further comprising injecting the pressurised injection fluid into the subsea installation or equipment.

21. The method of claim 20, comprising: passing the pressurised injection fluid through an injection control valve before injection into the subsea installation or equipment; and controlling the injection control valve to control injection of the injection fluid into the subsea installation or equipment.

22. The method of claim 20, comprising passing the pressurised injection fluid through a non-return valve before injection into the subsea installation or equipment.

23. The method of claim 1, comprising separating and removing gas from the production fluid before using the production fluid to pressurise the injection fluid.

24. A system for pressurising an injection fluid for injection into an installation or equipment subsea, the system comprising: an injection fluid source containing the injection fluid; a production fluid source containing a hydrocarbon production fluid; an ejector that is powered by a flow of the production fluid; and a pressure booster that is responsive to the ejector to draw injection fluid from the injection fluid source for pressurisation; wherein the pressure booster comprises: an injection fluid inlet connected to the injection fluid source; a production fluid inlet connected to the production fluid source to receive production fluid under pressure to pressurise the injection fluid to above the pressure of the production fluid; a production fluid outlet connected to the ejector to exhaust the production fluid; and an injection fluid outlet for the pressurised injection fluid.

25. The system of claim 24, wherein a subsea production pipeline serves as the production fluid source and channels the flow of the production fluid.

26. The system of claim 24, wherein the injection fluid in the injection fluid source is under hydrostatic pressure.

27. The system of claim 24, further comprising a non-return valve in an injection fluid supply line between the injection fluid source and the injection fluid inlet of the pressure booster.

28. The system of claim 25, wherein the pressure booster comprises: a production fluid chamber communicating with the production fluid inlet for receiving the production fluid from the production fluid source and communicating with the production fluid outlet for exhausting the production fluid, the production fluid chamber being expansible in response to receiving the production fluid; and an injection fluid chamber for pressurising the injection fluid by contracting in response to expansion of the production fluid chamber, the injection fluid chamber communicating with the injection fluid inlet for receiving the injection fluid from the injection fluid source and communicating with the injection fluid outlet for outputting the pressurised injection fluid.

29. The system of claim 28, wherein the pressure booster further comprises a piston between the production fluid chamber and the injection fluid chamber, that piston having opposed piston heads that partially define the respective chambers.

30. The system of claim 29, wherein the piston head of the production fluid chamber has a greater diameter than the piston head of the injection fluid chamber.

31. The system of claim 28, further comprising a production fluid diversion line that connects the pressure booster to the production fluid source.

32. The system of claim 31, further comprising a pressurisation control valve in the production fluid diversion line for controlling expansion of the production fluid chamber and contraction of the injection fluid chamber.

33. The system of claim 31, further comprising a gas separator in the production fluid diversion line.

34. The system of claim 28, further comprising a return line that connects the pressure booster to the ejector, and a filling control valve in the return line for controlling contraction of the production fluid chamber and expansion of the injection fluid chamber.

35. The system of claim 24, further comprising an injection line connected to the injection fluid outlet of the pressure booster, and an injection control valve in the injection line for controlling injection of the injection fluid into the subsea installation or equipment.

36. The system of claim 35, wherein the injection line further comprises a non-return valve between the injection fluid outlet of the pressure booster and an injection point for introducing the injection fluid into the subsea installation or equipment.

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 general layout diagram of a flushing system of the invention, powered by energy from a subsea well;

(3) FIG. 2 corresponds to FIG. 1 but shows the system when it is filling a pressure booster with a flushing medium; and

(4) FIG. 3 corresponds to FIGS. 1 and 2 but shows the system when it is injecting the flushing medium into a subsea processing system.

(5) The drawings exemplify a system 10 of the invention as a flushing system for injecting a fluid flushing medium into an output such as subsea processing unit 12. The system 10 is powered by energy from a subsea well.

(6) The system 10 is beneath the surface 14 and so is submerged in, and surrounded by, seawater 16 that applies hydrostatic pressure to the components of the system 10.

(7) On the seabed 18, the system 10 comprises a subsea wellhead 20, which commonly comprises a Christmas tree, and a production fluid conduit 22 such as a subsea pipeline for conveying hydrocarbon production fluid 24 from the wellhead 20 to the processing unit 12.

(8) An ejector 26 is interposed in the production fluid conduit 22 between the wellhead 20 and the processing unit 12. The ejector 26, which may for example be a jet pump or suction pump, is powered by the flow of production fluid 24 along the production fluid conduit 22.

(9) The system 10 further comprises a pressure booster 28, which may also be referred to as an amplifier or intensifier, that can receive production fluid 24 from the production fluid conduit 22 via a production fluid deviation line 30. The production fluid conduit 22 therefore serves as a production fluid source for the pressure booster 28. A first control valve 32 controls the flow of production fluid 24 along the production fluid deviation line 30 from the production fluid conduit 22 to the pressure booster 28.

(10) The ejector 26 draws production fluid 24 from the pressure booster 28 and returns that production fluid 24 to the production fluid conduit 22 along a return line 34. A second control valve 36 controls the flow of production fluid 24 along the return line 34 from the pressure booster 28 to the production fluid conduit 22.

(11) The pressure booster 28 also receives an injection fluid 38 such as a flushing medium, as shown in FIG. 2, from a subsea injection fluid source 40 such as a pressure-compensated reservoir or vessel or flexible bladder. The injection fluid 38 in the injection fluid source 40 is therefore at the hydrostatic pressure of the surrounding seawater 16. The injection fluid source 40 is fluidly connected to the pressure booster 28 by an injection fluid supply line 42. A non-return valve 44 ensures that the injection fluid can flow only one way in the injection fluid supply line 42 from the injection fluid source 40 to the pressure booster 28 rather than vice versa.

(12) The pressure booster 28 elevates the pressure of the injection fluid 38 and outputs the thus-pressurised injection fluid 38 to the processing unit 12 via an injection line 46. A third control valve 48 controls the flow of injection fluid along the injection line 46 from the pressure booster 28 to the processing unit 12. A non-return valve 50 ensures that the injection fluid can flow only one way in the injection line 46 from the pressure booster 28 to the processing unit 12 rather than vice versa.

(13) The pressure booster 28 may take various forms. The example shown here comprises an asymmetric piston 52 that reciprocates in a complementary casing 54. The piston 52 is shown in an intermediate position within the casing 54 in FIG. 1.

(14) The piston 52 has opposed heads 56, 58. The heads 56, 58 are of different diameters and hence areas. The casing 54 has complementary bores whose diameters correspond to the respective pistons 56, 58.

(15) The smaller head 56 of the piston 52 cooperates with its corresponding bore of the casing 54 to define an injection fluid chamber 60, as shown in FIG. 1. The injection fluid chamber 60 communicates with the injection fluid source 40 via the injection fluid supply line 42 to receive injection fluid 38, as shown in FIG. 2.

(16) The larger head 58 of the piston 52 cooperates with its corresponding bore of the casing 54 to define a production fluid chamber 62, as shown in FIG. 1. The production fluid chamber 62 communicates with the wellhead 20 via the production fluid deviation line 30 to receive production fluid 24 as shown in FIG. 3.

(17) The state of one or more of the control valves 32, 36, 48 may be controlled by a controller 64 as shown. The controller 64 may be a part of the system 10 or may be remote from the system 10. The controller 64 may receive control signals via wired or wireless connections or may communicate control signals to the control valves 32, 36, 48 via such connections.

(18) Control of the control valves 32, 36, 48 may also be effected by a UUV such as an ROV, which may couple torque tools to the control valves 32, 36, 48 to turn associated control elements. A UUV may be used for primary or auxiliary control of the control valves 32, 36, 48.

(19) The drawings also show an optional gas separator 66, such as a gas harp, in the production fluid deviation line 30 between the production fluid conduit 22 and the pressure booster 28. As oil is a multiphase fluid, separation and removal of gas from the production fluid 24 before the pressure booster 28 ensures consistent pressure. Separation and removal of gas from the production fluid 24 may also ensure effective pumping by the ejector 26.

(20) The operation of the system 10 will now be described with reference to FIGS. 2 and 3. Those drawings observe the convention that the control valves 32, 36, 48 are shown in white when open and in black when closed.

(21) FIG. 2 shows the system 10 when it is filling or charging the injection fluid chamber 60 of the pressure booster 28 with the injection fluid 38 drawn from the injection fluid source 40. To achieve this, the second control valve 36 is opened so that the ejector 26 reduces the pressure in the production fluid chamber 62 of the pressure booster 28.

(22) Reducing the pressure in the production fluid chamber 62 moves the piston 52 to contract the production fluid chamber 62 and to expand, and hence reduce the pressure in, the injection fluid chamber 60 on the opposite side of the piston 52. The resulting overpressure in the injection fluid 38 held at hydrostatic pressure in the injection fluid source 40 forces the injection fluid 38 along the injection fluid supply line 42 and into the injection fluid chamber 60.

(23) In the filling stage shown in FIG. 2, the first control valve 32 is closed to prevent production fluid 24 reaching the pressure booster 28 along the production fluid deviation line 30. The third control valve 48 is also closed to prevent injection fluid 38 passing along the injection line 46 into the processing unit 12. Conversely, it will be apparent that the open second control valve 32 may serve as a filling or charging control valve for controlling how the injection fluid 38 is drawn into the injection fluid chamber 60.

(24) FIG. 3 shows the system 10 in an injection stage when it is injecting the injection fluid 38 into the processing unit 12. Now, the second control valve 36 is closed and the first control valve 32 is open. In consequence, production fluid 24 flows along the production fluid deviation line 30 from the production fluid conduit 22 into the production fluid chamber 62 of the pressure booster 28. This increases the pressure in, and hence expands, the production fluid chamber 62 and moves the piston 52 to contract, and hence increase the pressure in, the injection fluid chamber 60 on the opposite side of the piston 52.

(25) It will be apparent that the first control valve 32 may serve as a pressurisation control valve for controlling how the injection fluid 38 is pressurised in the injection fluid chamber 60.

(26) By virtue of the asymmetry of the piston 52, the pressure in the injection fluid chamber 60 is boosted to exceed the pressure of the production fluid 24 in the production fluid chamber 62. Thus, the pressure of the injection fluid 38 in the injection fluid chamber 60 is increased to above the pressure of the production fluid 24 in the production fluid conduit 22 and hence in the processing unit 12.

(27) When it is desired to inject the thus-pressurised injection fluid 38 into the processing unit 12, the third control valve 48 is opened to allow the injection fluid 38 to flow from the pressure booster 28 and into the processing unit 12 along the injection line 46. The third control valve 48 may be a choke valve that serves as an injection control valve, being adjustable to adjust the flow and pressure of the injection fluid 38 flowing along the injection line 46.

(28) When the injection fluid chamber 60 has emptied, a filling or charging cycle can begin again by closing the first and third control valves 32, 48 and opening the second control valve 36. This allows the ejector 26 to draw the production fluid 24 from the production fluid chamber 62 of the pressure booster 28 and to exhaust that production fluid 24 back into the production fluid conduit 22.

(29) Many variations are possible within the inventive concept. For example, it should be understood that other fluids such as remediation or preservation liquids, or other chemicals, may be injected in accordance with the invention.

(30) The invention also contemplates that fluids may be injected into, or otherwise outputted to, subsea equipment or installations other than a processing unit, such as a subsea pipeline or into interim storage equipment. For example, once the pressure of a fluid has been elevated in accordance with the invention, fluids may be injected at that elevated pressure into an accumulator or other storage equipment and held at that pressure for subsequent injection into a subsea installation or other subsea equipment.

(31) Alternatives to a piston-based pressure booster are possible, such as a hydraulically-powered pump. For example, a pump may comprise a turbine disposed in the flow of production fluid, a compressor driven by the turbine, conveniently on the same shaft, and a gear system for boosting.

(32) If the system of the invention is used with sour production fluids containing a high level of hydrogen sulphide (H2S), there is a risk of corrosion of components such as piston heads and seals. Consequently, for ease of maintenance and refilling, parts of the system such as the pressure booster and/or the injection fluid source may be mounted on an ROV or a recoverable skid. Parts of the system may also be implemented in a retrievable module, which may comprise a standardised transport and installation frame like those proposed for some subsea processing systems.