Dewatering subsea conduits

Abstract

A method of dewatering a liquid-containing subsea conduit comprises fixing at least one subsea dewatering module to the conduit on a seabed. The dewatering module comprises a self-contained source of a dewatering gas that is injected into a gas pocket disposed within the conduit. Liquid is expelled from the conduit as the gas pocket expands within the conduit. An end portion of the conduit containing the gas pocket is lifted away from the seabed to recover the conduit to the surface, with the dewatering module still attached to the conduit. The dewatering module is landed on the conduit to straddle the conduit like a saddle and is then clamped to the conduit before effecting fluid communication between the source of the dewatering gas and a lumen of the conduit.

Description

[0001] Dewatering subsea conduits

[0002] This invention relates to the challenges of dewatering subsea conduits such as pipelines, flowlines, risers and umbilicals for transporting hydrocarbons or other liquids. The invention relates especially to the challenges of partial dewatering to recover flooded conduits such as pipelines, flexible risers or steel catenary risers from the seabed to the surface.

[0003] References in this specification to dewatering are intended to encompass removal from a conduit not only of water but of fluids in addition to water, or other than water.

[0004] Subsea conduits typically undergo various pre-commissioning tests before being brought into operation. For example, hydrotesting, pressure testing or leak testing may involve filling a conduit with fresh water, seawater or other liquids. A conduit may also be flooded with a liquid to preserve it for later operation or may become flooded with water during installation or decommissioning. Once testing is over or before installation, operation or decommissioning can proceed, it is necessary to inject a gas such as air or nitrogen to purge, expel or displace the liquid from the conduit in a dewatering operation.

[0005] Conventionally, dewatering operations are performed by injecting gas into a subsea conduit from a source at the surface, for example located on a surface vessel, a platform or a floating production storage and offloading (FPSO) unit. The source may, for example, comprise high-pressure compressed gas equipment including pumps, compressors, boosters, gas generators and / or accumulators, known collectively as a dewatering spread. That equipment is connected to the conduit by a downline extending downwardly through the water column, typically comprising a considerable and expensive length of coiled steel tubing.

[0006] A dewatering spread at the surface must be capable of delivering a satisfactory flow of dewatering gas at a pressure exceeding the hydrostatic pressure prevailing at the depth of the conduit. Thus, the deeper and the larger the conduit, the greater the challenge of injecting a volume of gas that will be sufficient to expel liquid from the conduit with reasonable speed or indeed at all.To purge liquid from a large conduit situated at great depth, a dewatering spread at the surface must be very bulky and heavy. The bulk of that equipment may exceed the space available on a vessel or surface installation and its weight can present challenges to the stability of the vessel or installation. Also, the downline that conveys the compressed gas to the conduit must be able to withstand the hydrostatic pressure prevailing at the depth of the conduit, particularly if the downline is ever depressurised when submerged. High-capacity dewatering spreads and lengthy, robust downlines are expensive and may be subject to limited availability.

[0007] US 7815744 attempts to address the problems of a large dewatering spread by instead using a chemical reaction to generate a gas that propels a pig through a pipeline. However, the reaction still takes place aboard a vessel or a surface installation and ends of the pipeline are held at the surface.

[0008] To reduce the need for surface support, US RE42358 discloses a method for commissioning a subsea pipeline by deploying a fill and test package on a submersible vehicle to purge and dewater the pipeline using a compressed gas. The vehicle powers pumps that are located on the seabed or are carried by the vehicle. The pipeline remains stationary on the seabed before and after dewatering. The vehicle therefore couples the fill and test package to the pipeline for dewatering and removes the fill and test package from the pipeline after dewatering, hence moving away from the pipeline to leave the pipeline on the seabed.

[0009] Similarly, US 10823311 discloses a method for emptying a flooded subsea pipeline using a submersible pump in conjunction with injection of pressurised gas. A set of cylinders containing nitrogen at high pressure, placed on the seabed, is connected to an end of the pipeline. A submersible vehicle activates the pump at the other end of the pipeline, which empties the pipeline of water as nitrogen flows into the pipeline from the cylinders. The pump is then stopped and isolated from the system before being recovered to the surface and similarly the cylinders are isolated from the system before being recovered to the surface. Again, therefore, the dewatering equipment is separated from the pipeline, which remains on the seabed. In any subsequent recovery operation, the dewatering equipment has already been removed from the pipeline.

[0010] US 9188246 discloses a method for recovering a section of pipeline from a seabed including the steps of: installing termination heads on first and second ends of thepipeline section; coupling a source of compressed gas to the first end; using a submersible vehicle to couple a high pressure pump to the second end; and using the submersible vehicle to power the pump to pull water out of the pipeline and displace the water with a relatively modest volume of gas from the compressed gas source, thereby dewatering the pipeline; and recovering the dewatered section of pipeline from the seabed.

[0011] Partial dewatering of a flooded conduit can be sufficient in some circumstances, particularly when recovering an end of the conduit from the seabed to the surface. In that case, partial dewatering expels at least some of the liquid from an elevated end of the conduit that has been lifted from the seabed and so is suspended in the water column. The apparent weight of the conduit experienced by a crane or winch of a surface vessel is thereby reduced and there is a correspondingly lower risk of overstressing or buckling the conduit.

[0012] By way of example, US 3777499 discloses a method for recovering a subsea pipeline by introducing gas into one end of the pipeline and pumping out liquid from the other end of the pipeline, until a section of the pipeline long enough to reach the surface has been substantially voided of liquid. Thereafter, the end of the pipeline is lifted to the surface. Similarly, in US 3751932 and US 3788084, an end of a subsea pipeline is recovered from the seabed by inserting a pigging element into the end of the pipeline, injecting a gas into the end of the pipeline behind the pigging element until a section of the line sufficiently long to reach to the surface of the body of water has been voided of liquids, and thereafter lifting the end of the line to the water's surface.

[0013] In practice, however, the difficulty of even partially dewatering a conduit in deep water using topside-supplied gas means that a high-capacity crane or winch and a correspondingly large vessel may still be necessary to recover the conduit. Such vessels are also expensive and their availability may be limited.

[0014] The invention contemplates partial dewatering to allow recovery of a partially flooded pipeline, flexible riser or steel catenary riser without requiring mobilisation of a large dewatering spread, an expensive downline or coiled tubing, or a vessel with a large crane or winch.US 8240952 describes a universal pump platform for deepwater pipelines. The platform contains an electric motor that drives a hydraulic pump for producing high pressure hydraulic fluid. The pump is selected for a desired commissioning method to be carried out, such as filling, chemical treating, pigging, hydrostatic testing or dewatering a pipeline.

[0015] US 2010 / 0089126 relates to apparatus and methods for subsea pipeline servicing, and notes the use of a dewatering pig that may be inserted in one end of the pipeline.

[0016] US 2003 / 0010094 is directed to a pumping skid carried by a subsea vehicle, which may be used in the commissioning of a pipeline.

[0017] US 8770892 relates to apparatus and methods for subsea pipeline servicing, including dewatering and chemical containment.

[0018] Against this background, the invention resides in a method of dewatering a liquidcontaining subsea conduit, the method comprising: fixing at least one subsea dewatering module to the conduit on a seabed, the or each dewatering module comprising a self-contained source of a dewatering gas; injecting the dewatering gas from the or each dewatering module into a gas pocket disposed within the conduit; and progressively removing the liquid from the conduit as the gas pocket expands within the conduit.

[0019] The gas pocket may be disposed in an end portion of the conduit, in which case the liquid can be removed as the gas pocket expands along the conduit. Thus lightened, the end portion of the conduit can be lifted away from the seabed to recover the conduit toward the surface, for example with the end portion containing the gas pocket atop a column of the liquid. Advantageously, the end portion can be lifted with the or each dewatering module remaining attached to the end portion, whereby injection of the dewatering gas can conveniently continue while lifting the end portion.

[0020] Conveniently, the dewatering gas can be injected into a first end of the conduit corresponding to the end portion, for example through a pulling head disposed at the first end of the conduit. Similarly, liquid can be removed from a second end of the conduit opposed to the end portion, for example through a non-return valve. Liquidcould be removed from the conduit simply by expulsion due to overpressure of the dewatering gas.

[0021] The or each dewatering module can be landed onto the conduit, for example by embracing the conduit in a downwardly-open recess of the or each dewatering module. The or each dewatering module can then be clamped around the conduit. The source of the dewatering gas can extend to mutually opposed sides of the conduit and / or over the conduit.

[0022] Two or more of the dewatering modules can be fixed to the conduit in longitudinal series to supply the dewatering gas from each of their respective sources for injection into the gas pocket. For example, the sources of the dewatering modules may be in daisy-chained serial fluid communication to convey the dewatering gas for injection into the gas pocket.

[0023] Correspondingly, the inventive concept embraces a liquid-containing subsea conduit having at least one subsea dewatering module attached in fixed relation thereto, the or each dewatering module comprising a self-contained source of a dewatering gas in fluid communication with a lumen of the conduit, for example through a pulling head disposed at an end of the conduit.

[0024] The inventive concept also embraces a subsea dewatering module that is configured to be fixed to a cylindrical subsea conduit and that comprises a self-contained source of a dewatering gas, for example an array of longitudinally-extending gas cylinders in substantially parallel relation.

[0025] The source of the dewatering gas may be supported by a frame that defines a recess accessible through an opening for receiving the conduit. The frame may, for example, be arched. The source can extend to mutually opposed sides of the recess and / or across the recess.

[0026] The module may further comprise longitudinally-spaced clamps that are configured to embrace the conduit. In that case, the source could be disposed longitudinally between the clamps.Embodiments of the invention implement a method for dewatering a subsea flowline that is to be recovered from the seabed, the method comprising: affixing a subsea dewatering system to the flowline; introducing gas into one end of the flowline; extracting liquids from the opposite end of the flowline until a section of the flowline, sufficiently long to reach the surface of the water, is partially or substantially devoid of liquids; and elevating the end of the flowline to the water's surface.

[0027] Embodiments of the invention also provide a dewatering apparatus comprising a plurality of interconnected gas cylinders that are configured to be clamped around a flowline, in addition to clamping methods and clamping arrangements for such an apparatus.

[0028] In summary, the invention involves dewatering a liquid-containing subsea conduit by mounting at least one subsea dewatering unit or module to or onto the conduit on the seabed. The dewatering module comprises a self-contained source of a dewatering gas that is injected into a gas pocket disposed within the conduit. Liquid is expelled from the conduit as the gas pocket expands within the conduit.

[0029] The dewatering module can be landed on the conduit to straddle the conduit like a saddle and can then be clamped to the conduit before effecting fluid communication between the source of the dewatering gas and a lumen of the conduit.

[0030] An end portion of the conduit containing the gas pocket can be lifted away from the seabed to recover the conduit to the surface. The dewatering module can remain mounted on the conduit, thereby potentially still injecting dewatering gas into the gas pocket during recovery of the conduit.

[0031] 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:

[0032] Figure 1 comprises schematic side and end views of a dewatering module of the invention in conjunction with a subsea conduit;

[0033] Figure 2 corresponds to Figure 1 but shows the dewatering module being lowered toward the conduit laid on the seabed;Figure 3 corresponds to Figure 2 but shows the dewatering module now lowered onto the conduit;

[0034] Figure 4 corresponds to Figure 3 but shows the dewatering module latched onto and being fluidly connected to the lumen of the conduit;

[0035] Figure 5 corresponds to Figure 4 but shows an additional dewatering module also latched onto the conduit and fluidly connected to the lumen of the conduit via an intermediate dewatering module also latched onto the conduit;

[0036] Figure 6 is a schematic side view of a flooded subsea conduit laid on the seabed;

[0037] Figure 7 corresponds to Figure 6 but shows a dewatering module of the invention lowered onto the conduit by a surface vessel, to be latched onto and fluidly connected to the conduit as shown in Figure 4;

[0038] Figure 8 corresponds to Figure 7 but shows the vessel beginning to recover an end of the conduit toward the surface, with the dewatering module remaining latched to the conduit while beginning to dewater the conduit;

[0039] Figure 9 corresponds to Figure 8 but shows the vessel continuing to recover the end of the conduit toward the surface as the latched dewatering module continues to dewater the conduit to a partially dewatered state; and

[0040] Figure 10 corresponds to Figure 9 but shows a pulling head at the end of the partially dewatered conduit recovered to and secured to the vessel.

[0041] Figures 1 to 4 show a dewatering module 10 of the invention that comprises an elongate arched frame 12 defining a downwardly-open tunnel-like recess 14 accessible through a slot-like bottom opening. The frame 12 is wide enough to straddle a subsea conduit 16, such as a pipeline, flowline, riser or umbilical, that is pre-laid on the seabed 18 defining a mudline. In this way, the pre-laid conduit 16 can be received in and accommodated by the recess 14 within the frame 12 when the module 10 is lowered onto the conduit 16 like a saddle, as shown in Figures 2 and 3.The frame 12 is surmounted by lifting points 20 to which a lifting line 22 is attached as shown in Figures 2 and 3. The lifting line 22 extends upwardly through the water column to a crane or winch of a surface vessel, which is not shown in those drawings.

[0042] The conduit 16 includes a recovery system that is exemplified here by a pulling head 24 mounted to one end of the conduit 16. The module 10 is landed on the conduit 16 slightly inboard of that end of the conduit 16, hence being offset longitudinally along the conduit 16 toward the pulling head 24.

[0043] Once the module 10 has been lowered onto the conduit 16 as shown in Figure 3, clamping jaws 26 at opposed ends of the frame 12 are closed around the conduit 16 to secure or latch the module 10 to the conduit 16 as shown in Figure 4. The jaws 26 may, for example, by actuated by an ROV 28 or otherwise remotely. Clamping movement of the jaws 26 relative to the frame 12 can be driven by a power source provided on the module 10 or onboard the ROV 28.

[0044] In this example, the jaws 26 are semi-circular and are hinged to the frame 12 at the central apex of the arch, hence closing together to encircle the conduit 16 fully. The jaws 26 are lined internally with resilient concave liners 30 to engage the exterior of the conduit 16, hence preventing the module 10 slipping along the conduit 16.

[0045] The frame 12 also supports onboard gas storage that is exemplified here by a group of gas storage cylinders 32. In this example, the cylinders 32 are elongate in the longitudinal direction of the frame 12 and are disposed in parallel in an array that extends over and around the arch of the frame 12. The cylinders 32 hold a dewatering gas such as air, nitrogen or an inert gas under high pressure that suitably exceeds the hydrostatic pressure experienced by the conduit 16 at the depth of the seabed 18.

[0046] The gas storage cylinders 32 are in fluid communication with each other and with a piping header and connection system 34 that is also supported by the frame 12, in this example surmounting the frame 12 at the apex of the arch. The piping header and connection system 34 enables the cylinders 32 to be coupled to the conduit 16 for fluid communication with the interior of the conduit 16. For this purpose, Figure 4 shows the ROV 28 having connected a gas injection hose 36 between the piping header and connection system 34 and the adjacent end of the conduit 16, for example through the pulling head 24. As will be explained with reference to Figures 6 to 10, dewatering gasintroduced into the conduit 16 under high pressure via the gas injection hose 36 purges water from the conduit 16 to effect dewatering.

[0047] Figure 5 shows a variant of the arrangement shown in Figure 4, in which a second dewatering module 10 is similarly latched onto the conduit 16 inboard of the first dewatering module 10. An interconnecting hose 38 extending between their piping header and connection systems 34 effects fluid communication between the first and second modules 10, thus substantially increasing the volume of dewatering gas that is available to purge water from the conduit 16. The first module 10 remains in fluid communication with the interior of the conduit 16 through the gas injection hose 36, hence serving as an intermediate module 10 between the second module 10 and the conduit 16. More than two modules 10 could be daisy-chained in series in this way.

[0048] Turning now to Figures 6 to 10, these drawings show how dewatering modules 10 of the invention fixed to or mounted on a subsea conduit 16 can be used to effect partial dewatering of the conduit 16 while an end of the conduit 16 is being recovered toward the surface 40.

[0049] Figure 6 shows a flooded conduit 16 lying on the seabed 18 with the pulling head 24 fitted at one end whereas Figure 7 shows a vessel 42 at the surface 40 having lowered a module 10 onto the conduit 16 near to the end to which the pulling head 24 is fitted. The module 10 is lowered toward the conduit 16 on a lifting line 22 and is then clamped and fluidly coupled to the conduit 16 as shown in Figure 4.

[0050] Figure 7 also shows a non-return valve 44 fitted to an outlet at the other end of the conduit 16. The non-return valve 44 could be installed with the conduit 16 or after the conduit 16 has been installed, before or after the module 10 is clamped to the conduit 16.

[0051] In Figure 8, a recovery line 46 suspended from a crane or winch of the vessel 42 has been attached to the pulling head 24. By applying tension to the recovery line 46, the vessel 42 starts to lift that first end of the conduit 16 away from the seabed 18. The recovery process continues in Figure 9 and is shown completed in Figure 10, where the pulling head 24 has been secured to the vessel 42.As the first end of the conduit 16 lifts away from the seabed 18 toward the surface 40, the module 10 injects gas into the conduit 16 to start or to continue dewatering. The high-pressure dewatering gas purges water 48 from within the conduit 16 to exit the conduit 16 through the non-return valve 44 at the other, second end of the conduit 16. As water 48 is expelled from the conduit 16 with continued injection of the dewatering gas as the first end of the conduit 16 lifts further, a growing gas pocket 50 forms within the raised first end of the conduit 16.

[0052] Thus, the gas pocket 50 extends progressively along the conduit 16 as more dewatering gas is injected and more water 48 is thereby expelled from the conduit 16. This reduces the negative buoyancy of the first end portion of the conduit 16 suspended above the seabed 18, hence reducing the apparent weight of the conduit 16 that would otherwise be experienced by the vessel 42.

[0053] As shown in the drawings, the first end portion of the conduit 16 suspended above the seabed 18 is partially dewatered, hence containing a pocket 50 of gas disposed above a column of water 48. If desired, however, that suspended portion of the conduit 16 could be substantially fully dewatered.

[0054] It will be noted that the dewatering module 10 remains attached to the conduit 16 rather than being left on the seabed 18 or removed from the conduit 16 before the conduit 16 is recovered to the surface 40, as happens in the prior art. Thus, if required, the module 10 can continue to provide dewatering gas to the conduit 16, and to control the flow of that gas, as the suspended first end portion of the conduit 16 is being lifted through the water column toward the surface 40. Recovery of the conduit 16 can therefore begin before all of the dewatering gas has been injected and additional dewatering gas can be injected into the gas pocket 50 during recovery of the conduit 16.

[0055] Many other variations are possible within the inventive concept. For example, the clamping jaws of a dewatering module could have other part-circular or non-circular shapes. The jaws need not encircle the conduit fully and could move in other ways relative to each other and to the frame.

[0056] The liners of the jaws could be inflatable to expand radially inwardly, further to improve the frictional grip of the module onto the conduit. Conveniently, in that case, the liners could be inflated by using some of the dewatering gas.Where two or more dewatering modules are employed, the modules could be fluidly coupled to the conduit in parallel instead of being daisy-chained in series as shown in Figure 5.

[0057] Whilst it is convenient for the same vessel to install a dewatering module and then to recover the conduit with assistance from that module, it would be possible to perform those operations using different vessels instead.

[0058] Elegantly, water can be allowed simply to drain from the conduit through the non-return valve at the outlet end of the conduit in response to injection of dewatering gas into the conduit from the dewatering module. However, if desired, drainage of water can be promoted by a suction pump at the outlet end of the conduit.

[0059] Dewatering gas can be injected into the conduit simply by virtue of overpressure in the gas storage cylinders of the dewatering module, relative to the hydrostatic pressure prevailing at the depth of the conduit. However, if desired, the pressure of the dewatering gas can be increased by a booster pump on the dewatering module.

[0060] Dewatering gas could be generated underwater, for example by a chemical reaction aboard the dewatering module.

[0061] If the conduit contains clean water, the water can be expelled from the conduit into the surrounding sea but if the conduit contains a fluid other than water, the expelled fluid can be captured to prevent mixing with and contamination of seawater. For example, a fluid other than water can be captured in a containment skid that is fluidly coupled to the second end of the conduit.

[0062] The pocket of dewatering gas could be divided from the column of liquid in the conduit by one or more pigs that can be driven along the conduit by overpressure of the dewatering gas.

[0063] Whilst it is convenient to effect fluid communication with the lumen of the conduit through an end of the conduit, it would be possible instead to inject dewatering gas and / or to remove other fluid from the conduit through a wall of the conduit.By attaching a recovery line to the dewatering module, the module could be used instead of, or in conjunction with, the pulling head to lift the conduit from the seabed.

Claims

Claims1. A method of dewatering a liquid-containing subsea conduit, the method comprising:fixing at least one subsea dewatering module to the conduit on a seabed, the or each dewatering module comprising a self-contained source of a dewatering gas;injecting the dewatering gas from the or each dewatering module into a gas pocket disposed within the conduit; andprogressively removing the liquid from the conduit as the gas pocket expands within the conduit.

2. The method of Claim 1 , wherein the gas pocket is disposed in an end portion of the conduit and the liquid is removed as the gas pocket expands along the conduit.

3. The method of Claim 2, further comprising lifting the end portion of the conduit away from the seabed to recover the conduit to a surface location.

4. The method of Claim 3, comprising lifting the end portion containing the gas pocket atop a column of the liquid.

5. The method of Claim 3 or Claim 4, comprising lifting the end portion with the or each dewatering module attached to the end portion.

6. The method of Claim 5, comprising continuing injection of the dewatering gas while lifting the end portion.

7. The method of any of Claims 2 to 6, comprising injecting the dewatering gas into a first end of the conduit corresponding to the end portion.

8. The method of Claim 7, comprising injecting the dewatering gas through a pulling head disposed at the first end of the conduit.

9. The method of any of Claims 2 to 8, comprising removing the liquid from a second end of the conduit opposed to the end portion.

10. The method of any of Claims 2 to 9, comprising landing the or each dewatering module onto the end portion.

11. The method of Claim 10, comprising embracing the conduit in a downwardly-open recess of the or each dewatering module.

12. The method of any preceding claim, comprising clamping the or each dewatering module around the conduit.

13. The method of any preceding claim, comprising removing the liquid from the conduit through a non-return valve.

14. The method of any preceding claim, comprising removing the liquid from the conduit by expulsion due to overpressure of the dewatering gas.

15. The method of any preceding claim, wherein the source of the dewatering gas extends to mutually opposed sides of the conduit.

16. The method of any preceding claim, wherein the source of the dewatering gas extends over the conduit.

17. The method of any preceding claim, comprising fixing two or more of the dewatering modules to the conduit in longitudinal series and supplying the dewatering gas from each of their respective sources for injection into the gas pocket.

18. The method of Claim 17, wherein the sources of the dewatering modules are in daisy-chained serial fluid communication to convey the dewatering gas for injection into the gas pocket.

19. A liquid-containing subsea conduit having at least one subsea dewatering module attached in fixed relation thereto, the or each dewatering module comprising a self-contained source of a dewatering gas in fluid communication with a lumen of the conduit.

20. The conduit of Claim 19, wherein the source is in fluid communication with the lumen through a pulling head disposed at an end of the conduit.

21. The conduit of Claim 19 or Claim 20, further comprising a non-return valve in a liquid outlet of the conduit.

22. The conduit of any of Claims 19 to 21 , wherein the conduit is received in a downward ly-open recess of the or each dewatering module.

23. The conduit of any of Claims 19 to 22, wherein the or each dewatering module is clamped around the conduit.

24. The conduit of any of Claims 19 to 23, wherein the source of the dewatering gas extends to mutually opposed sides of the conduit.

25. The conduit of any of Claims 19 to 24, wherein the source of the dewatering gas extends over the conduit.

26. The conduit of any of Claims 19 to 25, comprising two or more of the dewatering modules fixed to the conduit in longitudinal series and with their respective sources in fluid communication with the lumen.

27. The conduit of Claim 26, wherein the sources of the dewatering modules are in daisy-chained serial fluid communication.

28. A subsea dewatering module that is configured to be fixed to a cylindrical subsea conduit and that comprises a self-contained source of a dewatering gas.

29. The module of Claim 28, wherein the source is supported by a frame that defines a recess accessible through an opening for receiving the conduit.

30. The module of Claim 29, wherein the frame is arched.

31. The module of Claim 29 or Claim 30, wherein the source extends to mutually opposed sides of the recess.

32. The module of any of Claims 29 to 31 , wherein the source extends across the recess.

33. The module of any of Claims 28 to 32, further comprising longitudinally-spaced clamps that are configured to embrace the conduit.

34. The module of Claim 33, wherein the source is disposed longitudinally between the clamps.

35. The module of any of Claims 28 to 34, wherein the source comprises an array of longitudinally-extending gas cylinders in substantially parallel relation.

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