GB2480112A

GB2480112A - Recovery of oil for a spilling subsea well

Info

Publication number: GB2480112A
Application number: GB1008805A
Authority: GB
Inventors: Jean-Francois Saint-Marcoux
Original assignee: Acergy France SAS
Current assignee: Acergy France SAS
Priority date: 2010-05-06
Filing date: 2010-05-26
Publication date: 2011-11-09
Also published as: GB201007594D0; GB2480093A; GB201008805D0

Abstract

A method of recovering well fluid from a spilling undersea well involves positioning a container 12 over the well, the container having an open bottom and a closed top 22 and initially containing water; allowing well fluid to enter the container 12 through the open bottom to be trapped by the closed top 22 while displacing water from the container; and recovering well fluid from the container. The container 12 may rise to the surface under buoyancy imparted by the well fluid, during which gas can be vented. Also claimed is a system for recovering spilling well fluid that includes a container 12 positionable over the well, the container having an open bottom to admit well fluid and a closed top 22 to trap well fluid; and a container support system operable to move the container 12 from the well for periodic recovery (see fig 6) of well fluid from the container 12. The container 12 when filled with water and submerged has a centre of gravity substantially below its centre of buoyancy.

Description

Recovery of oil from a spilling subsea well This invention relates to the recovery of oil leaking at a high rate from a subsea well.

In recent weeks following the explosion and loss of the Deepwater Horizon drilling rig, shorelines of the Gulf of Mexico have been threatened by oil rising uncontrollably in large volumes from the wellhead to the surface. For unknown reasons, subsea blowout preventers failed to stem the flow and subsequent efforts to activate them with ROVs also failed.

The wellhead lies in deep water (circa 1500m), beyond diver depth. It is likely that the spill will only be ended by the last-resort measure of drilling a relief well. However in water of that depth, a relief well may take one or two months to complete. Meanwhile it is estimated that at least 5000 barrels per day (BPD) of oil is flowing into the sea and adding to the growing slick at the surface.

An unsuccessful attempt has been made to enclose the wellhead with a specially-fabricated inverted box-like containment structure lying on the seabed surrounding the wellhead. The pointed top of the box was surmounted by a vertical pipe to channel oil to the surface for collection by a recovery vessel stationed directly above the wellhead.

Similar containment structures have been used before in the Gulf of Mexico -for example, after Hurricane Katrina to channel oil to the surface where platforms had been damaged -but that was in shallow water.

Sadly, the box containment did not work on this fast-flowing spill in deep water. The box and the pipe were rapidly clogged by the formation of a methane hydrate slurry due to the gas content of the well fluid and the low-temperature and high-pressure environment of the wellhead.

Even if the box containment had worked, there was a long delay in fabricating the structure while oil continued to escape. The containment structure could also have acted as a separator, in that gas phase would easily escape from the oil during its passage through the structure and the pipe. Such separation could lead to unstable vertical flow of the mixture of oil and gas in the pipe. Also, use of the containment structure involved stationing a recovery vessel vertically above the well, which is not a safe location due to flammable gas bubbling up from the well.

There is a precedent for the recovery of oil leaking from a deep water location, namely the operation in 2004 to recover fuel oil from the wreck of the tanker Prestige, which sank in 2002 off Spain to a depth of nearly 4000m. This was a very different challenge to that now faced in the Gulf of Mexico, mainly because the oil carried by the Prestige was extremely viscous and was largely static. In that case, the oil was not flowing at high speed under pressure in a turbulent jet from an uncontrollably leaking well. Nor was there a problem posed by gas phase entrained with the oil in a flow of well fluid.

Instead, once the vessel sank, oil leaked slowly from the wreck and there was time to plug many of the leaks using ROVs while designing and implementing a bespoke solution over a period of more than a year.

The solution that was eventually adopted for the Prestige involved tapping ROy-operable extraction valves through the hull into the oil tanks of the vessel. Water was injected into the tanks as oil was allowed to escape through an extraction valve into one of a series of shuttle tanks positioned above the extraction valve. For this purpose, the bottom of each shuttle tank was provided with an ROV-operable door allowing oil oozing from the extraction valve to float up from the extraction valve into the shuttle tank. During the extraction process, the flow of oil was controlled by ROV operation of the extraction valve, with failsafe systems to avoid overfilling. Once a shuttle tank was full, the extraction valve and the door were closed and the tank was recovered to near the surface for unloading the oil. Meanwhile a further empty shuttle tank was positioned above the extraction valve for filling in the same manner.

The shuttle tanks were fabricated specially for the Prestige project. Each shuttle tank comprised a hollow aluminium body with a diameter of approximately 5.3m and an overall length of approximately 23m, suitable to carry up to 350m3 of oil. Buoyancy units installed externally to the shuttle tank maintained buoyancy throughout and allowed controlled transportation through the water column using dedicated shuttle handling equipment and ballast chains. A top interface comprised a mechanical interface to a riser termination unit for unloading the oil to a FPSO.

Shuttle tanks such as those used in the recovery operation on the Prestige would not be suitable to capture oil flowing at a high rate from a subsea wellhead. The fast-moving jet of well fluid would threaten the stability of the shuttle tank when it is lowered and as it passes over the well jet. There would be further challenges in controlling the position of the shuttle tank during filling, controlling filling of the shuttle tank, releasing the expanding gas phase during recovery of the shuttle tank through the water column and thereafter recovering the oil from the shuttle tank to a tanker. Nor was there any problem with gas hydrates.

In acute emergency circumstances such as are now faced in the Gulf of Mexico, there is an urgent need for a temporary solution that is effective to prevent substantial uncontrolled leakage of oil into the sea until a more permanent solution can be effected. It is essential that the temporary solution can be implemented quickly using readily-available resources and assets, for example by using apparatus that can be fabricated easily if needs be and that can be handled by existing service vessels.

There will also be benefit in having a solution available in the event of a similar problem arising in the future.

It is against this background that the present invention has been devised. In accordance with the invention, it is proposed to use an inverted bucket (i.e. a hollow closed-topped, open-bottomed catching structure) to be positioned over the well to catch the well fluid rising from it. Advantageously, the inverted bucket is similar in size and construction to a subsea suction pile. The inverted bucket is held in place by service vessels (typically tugs, ROV support vessels or similar service vessels) preferably operating in pairs opposed about the bucket and the wellhead.

The bucket may be sized for the expected daily (or half-daily) production of the well to suit continuous recovery to the surface in a shuttle process. Several such buckets should be used to ensure continuous recovery in a noria' arrangement in which buckets are brought into position one after the other over the well by respective sets of two support vessels.

The invention thus resides in a method of recovering well fluid from a spilling undersea well, the method comprising: positioning a container over the well, the container having an open bottom and a closed top and initially containing water; allowing well fluid to enter the container through the open bottom to be trapped by the closed top while displacing water from the container; and recovering well fluid from the container.

The method of the invention suitably comprises raising the container to a depth compatible with the volume of the container and the Gas-Oil Ratio (GOR) before recovering well fluid from the container. From this location, a conventional (not deepwater and therefore readily available) flexible hose can be used to recover the fluid.

The container may be allowed to rise under buoyancy imparted to the container by the well fluid. In that case, the buoyant rise of the container is preferably restrained by one or more anchors. For example, the container may be suspended from a surface vessel on a chain, wire or the like from which at least two dead-man anchors are suspended at spaced locations. This enables the container to be held at an intermediate depth to allow gas hydrates in the container to decompose in a controlled manner.

The container is preferably positioned over a jet of well fluid emanating upwardly from the well and more preferably is positioned such that the jet enters the open bottom of the container. Elegantly, the container may be aligned with the jet by impingement of the jet against the container.

Gas in the well fluid may be allowed to expand within the container as the container is raised toward the surface. In an emergency to avoid escape of oil, the gas may be vented from the container to control the volume of gas in the container as the container is raised toward the surface. However venting of gas is not desirable in normal situations as the gas will rise to the surface and may threaten the safety of vessels located where it rises.

Means such as a float valve is advantageously provided to block oil from exiting the container with the vented gas. Such a valve suitably comprises a float for floating on the oil, arranged to close the valve if the oil level approaches the vent. In a preferred embodiment, the float is constrained for movement in a cage, through which gas may flow to the vent when the valve is open.

Preferred arrangements of the invention involve positioning a hydrate inhibitor in the container in contact with gas hydrates in the container. For optimum effectiveness, the hydrate inhibitor has a density selected to lie between a gas hydrate layer and sea water in the bottom of the container. Such a density may be achieved if the hydrate inhibitor is a mixture of methanol and/or ethanol with monoethylene glycol, diethylene glycol and/or triethylene glycol. As the gas hydrate layer decomposes into gas that rises through the oil, the density of the hydrate inhibitor is such that it preferably lies between the oil and the sea water in the container.

The container is suitably suspended between a plurality of surface vessels that move the container with respect to the well. Advantageously, the surface vessels may be widely spaced in this arrangement to avoid the unsafe area of the surface directly abovethewell.

The method preferably comprises successively positioning a plurality of containers over the well, each successive container taking over from the preceding container when the preceding container is removed from the well for emptying the recovered well fluid.

The inventive concept extends to a system for recovering well fluid from a spilling undersea well, the system comprising: a container positionable over the well, the container having a chamber with an open bottom to admit well fluid into the chamber and a closed top to trap well fluid in the chamber; and a container support operable to move the container from the well for periodic recovery of well fluid from the container; wherein when filled with water and submerged, the container has a centre of gravity substantially below its centre of buoyancy. This is important for stability as the container could encounter a high-speed jet of well fluid emanating from the well.

The chamber is preferably defined by an upright elongate hollow cylinder like a suction pile in size, shape, material and method of construction, having added buoyancy at an upper end and added weight at a lower end.

Where the container has attachment points for chains, wires or the like of the container support, the attachment points are preferably below the centre of buoyancy and above the centre of gravity to keep the closed top of the container uppermost.

Buoyancy means such as floats may be positioned at or near to an upper end of the container. Buoyancy may be directly attached to the container or held by chains or wires. Conversely the container may be weighted at or near to a lower end. For example, the container may have an annular stabilising ring at its lower end. That stabilising ring may be supported by spars extending downwardly from the chamber of the container and is preferably of substantially greater diameter than the chamber of the container.

To maximise the potential for catching well fluid and to make the container self-aligning with respect to a jet of well fluid, the container preferably comprises a flared skirt around its open bottom.

The container support system suitably comprises chains, wires or the like suspending the container between a plurality of surface vessels that together determine the depth and position of the container in the water. The container may be equi-spaced between the support vessels.

To maximise efficiency, the system of the invention preferably comprises a plurality of containers that are positionable in turn over the well.

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: Figures 1(a) and 1(b) are, respectively, schematic plan and side views showing the deployment of apparatus in accordance with the invention; Figure 2 is a sectional side view of a bucket in accordance with the invention, forming part of the apparatus shown in Figure 1; Figures 3(a), 3(b), 3(c) and 3(d) show, respectively, a turbulent jet of well fluid emanating from a subsea wellhead; the bucket of Figure 2 positioned over the jet to catch the well fluid and hence to fill the bucket with oil and gas phase; and the effect of decreasing depth upon the gas phase volume within the filled bucket; Figure 4 is a schematic side view showing oil and gas being emptied from the bucket at a shuttle tanker or an FPSO; Figure 5 is a schematic side view showing the use of dead-man anchors to control the bucket; Figure 6 is a schematic side view showing a variant of the arrangement of Figure 5 in which two dead-man anchors are used to stabilise the bucket at intermediate depth; Figure 7 is a part-sectional schematic side view of a gas release system for use with the invention; and Figure 8 is a series of schematic side views of a bucket showing how a mixture of methanol or ethanol with ethylene glycol (monoethylene glycol -MEG -diethylene glycol -DEG -or triethylene glycol -TEG) may be used in the bucket to deal with hydrate slurry while being retained between the water level and the well fluids within the bucket.

Figures 1(a) and 1(b) show a team of two service vessels 10 holding a submerged inverted bucket 12 centrally between them, suspended in the water column by opposed chains or wires 14 of equal length. The service vessels 10 are located about 2000m (6560 feet) apart in this example and so are advantageously positioned safely away from the unsafe area on the surface that is directly above the well. They move the bucket 12 over a spilling wellhead 16 by travelling across the surface 18 in unison and in parallel. By paying out or pulling in the chains or wires 14 in unison using winches on each vessel 10, they control the depth of the bucket 12 beneath the surface 18. Atypical distance of lOm above the well could be expected.

Figure 1(a) shows a second team of two service vessels 10 on stand-by, waiting to take its turn over the wellhead 16 with a further bucket 12 when the first bucket 12 is full and is recovered for emptying. Depending on the flow rate expected and the capacity of the buckets 12, the number of teams of service vessels 10 and the frequency of their visits to the wellhead 16 may be adjusted.

If it is assumed that the deployment and removal of the bucket 12 will be a daily or twice-daily operation, the typical minimum size of the bucket 12 is the daily or half the daily volume of hydrocarbon released at the wellhead 16. For example, based on a daily release of 5000 BPD, the required volume is 800 m3. That volume can be achieved with a cylinder of circular cross-section 8m (26') in diameter and 1 6m (52') long. For a twice-daily operation the required volume would be for example 6.1 m (20') in diameter and 14m (45') long. These dimensions are typical of suction anchors used for anchoring drilling rigs or floating production facilities. If fabrication is necessary, this speeds production of the buckets 12; it also makes the buckets 12 easy for existing service vessels to handle.

Referring now to Figure 2 of the drawings, the design and construction of the bucket 12 are also similar to the design and construction of suction anchors. The bucket 12 comprises a hollow cylinder 20 of circular cross-section made of rolled, plated carbon steel and is fabricated by welding. The cylinder 20 has a flat, closed top 22. A frusto-conical skirt 24 flares from the open bottom of the cylinder 20 to capture a wider area of well fluid and lead it into the cylinder 20 in use. The angled skirt 24 of the bucket 12 reacts with lateral force against an impinging jet of well fluid in use and so helps to align the open bottom of the bucket 12 with the jet.

The flat top 22 of the bucket 12 is reinforced with steel beams 26. The top 22 is penetrated centrally by a pipe 28 equipped with a valve 30, typically an ROV-operable 8" (20cm) ball valve 30, and a connector 32. Foam blocks 34 atop the bucket 12 around the pipe 28 provide buoyancy to the bucket 12, their position raising the centre of buoyancy and so helping to keep the bucket 12 upright when it is deployed in the water. Alternatively the buoyancy can be connected by chain or wire. Optionally there is provided a link with a load cell to measure the vertical load as this load varies during the filling of the bucket 12 by well fluid.

The skirt 24 of the bucket 12 is surrounded by spars 36 radiating downwardly and outwardly from the bottom edge of the cylinder 20. The spars 36 help to support and stiffen the skirt 24. The spars 36 also extend beyond the skirt 24 to support an annular ring 38 that helps to stabilise the bucket 12 in use to minimise movement of the bucket 12 when over-passing the jet of well fluid at the wellhead 16. The weight and enlarged diameter of the stabilising ring 38 near the bottom of the bucket 12 advantageously lowers its centre of gravity and increases its inertial resistance to lateral movement.

The stabilising ring 38 provides a restoring moment additional to that which may be provided by impingement of the jet with the angled skirt 24 of the bucket 12.

The base of the cylinder 20 above the skirt 24 is surrounded by a circular reinforcement 40 that also provides mounting points 42 for the chains or wires 14 that suspend the bucket in use. Such a reinforcement is not required for a regular suction pile which is held from the top. For optimum stability, the centre of buoyancy of the bucket 12 is above the mounting points 42, and above the centre of gravity of the bucket 12, even when the bucket is full of water. The centre of buoyancy will rise further as buoyant oil and gas displaces water downwardly from within the bucket 12.

The bucket 12 may be equipped with pressure transducers and load cells. The purpose is to monitor the amount of well fluid being received and possibly adjust the rate at which buckets 12 are brought to the wellhead 16 for filling.

Figure 3(a) shows well fluid comprising oil and gas flowing upwardly from a wellhead 16 on the seabed 44 in a fully turbulent jet 46. The jet 46 is initially confined within a narrow angle a, typically about 100. Once the jet 46 loses momentum, the well fluid rises buoyantly thereafter through the water column as a more dispersed plume with a wider angle. The chance of catching the well fluid is enhanced by lowering the bucket 12 close enough (typically lOm) to the wellhead 16 to receive the jet 46 through the open bottom of the bucket 12 as shown schematically in Figure 3(b), rather than keeping the bucket 12 above the jet 46 and hence waiting until the well fluid has dispersed into a plume.

The desirability of the bucket 12 encountering the jet 46 with its turbulent dynamic forces emphasises the importance of the stability of the bucket 12 in use. Here, the low position of the stabilising ring 38 is advantageous as this concentrates weight below the point at which the jet 46 impinges on the skirt 24 and the inside of the cylinder 20.

As the well fluid is collected from the well into the bucket 12, it separates into oil 48 and a gas phase 50. The gas phase 50 rises from the oil 48 within the bucket 12 and is trapped by the oil 48 in the closed top portion of the bucket 12. At 1500m water depth, the pressure is 150 bar (2000 psi) and the gas phase 50 occupies a correspondingly small volume as shown in Figure 3(c). The volume of the gas phase 50 increases with decreasing depth and pressure as the bucket 12 is recovered toward the surface, as shown in Figure 3(d). Here, the depth is 500m, pressure has reduced to 50 bar and the volume of the gas phase 50 has trebled over that at 1 500m in accordance with Boyle's Law, assuming constant temperature. More elaborate Pressure Volume Temperature (PVT) calculation can of course be used to confirm this order of magnitude estimate, which is valid for black oils. This expansion displaces the oil 48 toward the open bottom of the bucket 12.

The downward displacement of oil 48 will not be a problem if the bucket 12 is deep enough to prevent the oil 48 from escaping from the bucket 12 through the open bottom. However, if it were necessary under unexpected conditions to control the expanding volume of the gas phase 50 to prevent the escape of oil 48, it is possible to vent the gas phase 50 by opening and closing the valve 30 (either automatically, under remote control or using an ROV) periodically during the recovery procedure.

Referring to Figure 4, the bucket 12 is brought to the vicinity of a floating storage unit 52 such as a shuttle tanker, equipped with a separator 54 and a flare 56. For a flow rate of 5000 BPD, a standard 2000 psi test separator 54 should be suitable. The bucket 12 is connected to the floating storage unit 52 via a conventional flexible hose 58 connected to the valve 30 and connector 32 on the top of the bucket 12, whereupon oil 48 and gas phase 50 are released from the bucket 12 as water rises through the open bottom. The gas phase 50 is flared off and oil 48 is held in tanks in the storage unit 52 for subsequent unloading. Jetting or pumping could alternatively be used Variations are possible without departing from the inventive concept. Examples are shown in Figures 5 and 6, in which the bucket 12 is allowed to rise by virtue of its increased buoyancy when full and dead-man anchors (DMA5) are used to control the rise of the bucket 12.

In Figure 5, a single DMA 60 is suspended from each chain or wire 14 close to the bucket 12. When the bucket 12 is full as shown to the right of Figure 5, it rises due to its increased buoyancy but its rise is controlled by the DMAs 60.

In Figure 6, an additional DMA 62 is suspended from each chain or wire 14 further from the bucket 12, at a depth just deeper than the limit of hydrate stability 64. Allowing for the length of chain or wire 14 between the bucket 12 and the additional DMA 62, the additional DMA 62 holds the bucket 12 at an intermediate depth just above the limit of hydrate stability 64. Here, any hydrate slurry in the bucket 12 will decompose to release gas, which is trapped in the bucket 12 above the layer of oil within. Holding the bucket 12 at this depth before the chain or wire 14 is pulled in allows the hydrate slurry to decompose in a controlled manner without disrupting the stability of the bucket 12 or risking an escape of oil.

For ease of illustration, Figure 6 shows the bucket 12 above the wellhead 16 when at the intermediate depth. In reality, while rising to the intermediate depth, the bucket 12 will be towed laterally toward a floating storage unit for eventual unloading as shown in Figure 4. This lateral movement saves time and clears space for a further bucket 12 to be lowered over the wellhead 16 in the aforementioned noria' arrangement.

The depth of the limit of hydrate stability 64 depends upon the composition of the gas hydrate and the temperature of the water. The temperature of polar seas is close to zero Celsius and fluctuates very little with depth. The temperature of equatorial seas rarely exceeds five Celsius at l000m depth and below 2000m, it remains practically constant within the range of one to three Celsius. However, nearing the surface, the temperature of equatorial seas rises rapidly.

The depth of the limit of hydrate stability 64 therefore varies around the world. In the subtropics, for example, hydrates of methane can be stable in depths below 500m, whereas hydrates of a natural gas with a relative density of 0.6 can be stable in depths below 300m. In the Arctic, by contrast, the limit of hydrate stability 64 comes closer to the surface: hydrates of methane can be stable in depths below 300m; hydrates of a natural gas with a relative density of 0.6 can be stable in depths below lOOm.

Figure 7 shows the principle of a gas release system that allows gas to escape through the valve 30 in the top 22 of the bucket 12 but prevents oil escaping through that valve too. In this drawing, the valve 30 is an ROV-operable ball valve fitted to a flange 64 on the top 22. When opened, the valve 30 defines a vent path 66 through which gas inside the top of the bucket 12 may be vented into the sea. However as gas is vented, the oil level within the bucket 12 will rise and there is therefore a risk that oil could escape from the bucket 12 through the valve 30.

To prevent oil escaping through the valve 30, the bottom of the vent path 66 is fitted with a cylindrical ball cage 68 within which a ball 70 is constrained to rise and fall with the oil level, the ball 70 being less dense than the oil and hence floating on its surface.

The ball cage 68 is perforated, in this case with longitudinal slots 72, whereby gas above the oil can escape through the slots 72 and out of the valve 30 along the vent path 66, whenever the valve 30 is opened. The oil level rises as gas escapes, raising the ball 70 within the ball cage 68 until the ball 70 seals against an annular valve seat 74 extending around the top of the ball cage 68. The valve seat 74 is above the top of the slots 72, whereby the ball 70 then prevents oil entering the vent path 66 and hence possibly escaping through the valve 30.

Referring finally to Figure 8, this illustrates a scheme for dealing with hydrate slurry in the bucket 12 using a mixture of either methanol or ethanol with ethylene glycol in the form of monoethylene glycol (MEG), diethylene glycol (DEG) or triethylene glycol (TEG). These thermodynamic inhibitors help to prevent formation of gas hydrates (also known as clathrates), and to disperse hydrates when formed, by shifting the hydrate equilibrium conditions towards lower temperatures and higher pressures or by decreasing the rate of hydrate formation. For the purposes of the invention, methanol and ethanol have similar properties and may be used interchangeably; MEG, DEG and TEG also have similar properties and may be used interchangeably.

The table of density in Figure 8 contrasts the typical densities of methanol or ethanol (800 kg/m3), MEG, DEG or TEG (1125 kg/m3), sea water (1030 kg/m3), oil (850 kg/m3), hydrate slurry (950 kg/m3) and a 1:1 (i.e. 50/50) mix of methanol and MEG (970 kg/m3).

Ethanol may be substituted for methanol and DEG or TEG may be substituted for MEG without significantly changing the density of the 1:1 mixture.

The practical implications of these relative densities are illustrated in Figure 8, which shows buckets 12 containing well fluid in full water depth under hydrate formation conditions. In a bucket 12, oil 48 will float above hydrate slurry 76, which in turn will float above sea water 78. If methanol or ethanol 80 is added to the bucket 12, it will float on the oil 48, which then separates the methanol or ethanol 80 from the hydrate slurry layer 76. Conversely if MEG, DEG or TEG 82 is added to the bucket 12 it will simply sink out of the bottom of the bucket 12 through the sea water 78. Consequently, neither pure methanol or ethanol 80 nor pure MEG, DEG or TEG 82 will remain at the interface with the hydrate slurry layer 76 and so those compounds will be ineffective to inhibit hydrate formation or to disperse hydrates.

Where a 1:1 mix 84 of methanol and MEG is used, however, that mix 84 floats on the sea water 78 to interface with the hydrate slurry layer 76. The methanol/MEG mix 84 may therefore promote decomposition of the hydrate slurry 76. As the hydrate slurry 76 decomposes to release gas 50, that gas 50 rises through the oil 48 and is trapped in the top of the bucket 12. The methanol/MEG mix layer 84 rises as the hydrate slurry layer 76 diminishes until eventually the methanol/MEG mix layer 84 interfaces with the oil layer 48. In doing so, the methanol/MEG mix layer 84 separates the oil 48 from the sea water 78; the methanol/MEG mix layer 84 also remains trapped within the bucket 12 to be recovered and recycled if desired, or may be left in the bucket 12 for another run to the wellhead 16 when the oil 48 has been removed from the bucket 12.

The density of the hydrate slurry 76 may of course vary in practice, in which case the mix ratio of the methanol/MEG mix 84 may also be varied to ensure that the mix layer 84 interfaces effectively with the slurry layer 76.

In other variations, the closed top of the bucket could comprise a dished head, but the time needed to fabricate that feature may not be compatible with the urgent delivery schedule required in an emergency situation.

Provision may also be made for anchoring the bucket temporarily to the wellhead or to the seabed to keep the bucket in position above the jet during the collection of well fluid. It would also be possible to manoeuvre the bucket using ROVs to position the bucket above the jet and to resist drift while the bucket remains on station, collecting the well fluid.

Claims

Claims 1. A method of recovering well fluid from a spilling undersea well, the method comprising: positioning a container over the well, the container having an open bottom and a closed top and initially containing water; allowing well fluid to enter the container through the open bottom to be trapped by the closed top while displacing water from the container; and recovering well fluid from the container.
2. The method of Claim 1, comprising raising the container to at or near the surface before recovering well fluid from the container.
3. The method of Claim 1 or Claim 2, comprising allowing the container to rise under buoyancy imparted to the container by the well fluid.
4. The method of Claim 3, comprising restraining the buoyant rise of the container by one or more anchors.
5. The method of Claim 4, comprising suspending the container from a surface vessel on a chain, wire or the like from which at least two dead-man anchors are suspended at locations spaced along the chain, wire or the like.
6. The method of any preceding claim, comprising positioning the container over a jet of well fluid emanating upwardly from the well.
7. The method of Claim 6, wherein the container is positioned such that the jet enters the open bottom of the container.
8. The method of Claim 6 or Claim 7, comprising aligning the container with the jet by impingement of the jet against the container.
9. The method of any preceding claim, comprising allowing gas in the well fluid to expand within the container as the container rises toward the surface.
10. The method of Claim 9, wherein the gas is vented from the container to control the volume of gas in the container as the container rises toward the surface.
11. The method of Claim 10, comprising blocking oil from exiting the container with the vented gas.
12. The method of any preceding claim, comprising positioning a hydrate inhibitor in the container in contact with gas hydrates in the container.
13. The method of Claim 12, wherein the hydrate inhibitor has a density selected to lie between a gas hydrate layer and sea water in the bottom of the container.
14. The method of any preceding claim, comprising positioning a hydrate inhibitor in the container between oil and sea water in the container.
15. The method of Claim 13 or Claim 14, wherein the hydrate inhibitor is a mixture of methanol and/or ethanol with monoethylene glycol, diethylene glycol and/or triethylene glycol.
16. The method of any preceding claim, comprising holding the container at an intermediate depth to allow gas hydrates in the container to decompose.
17. The method of any preceding claim, wherein the container is suspended between a plurality of surface vessels that move the container with respect to the well.
18. The method of any preceding claim and comprising successively positioning a plurality of containers over the well, each successive container taking over from the preceding container when the preceding container is removed from the well.
19. The method of any preceding claim, comprising monitoring filling of the container by the use of one or more load cells.
20. A system for recovering well fluid from a spilling undersea well, the system comprising: a container positionable over the well, the container having a chamber with an open bottom to admit well fluid into the chamber and a closed top to trap well fluid in the chamber; and a container support operable to move the container from the well for periodic recovery of well fluid from the container; wherein when filled with water and submerged, the container has a centre of gravity substantially below its centre of buoyancy.
21. The system of Claim 20, wherein the container has attachment points for chains, wires or the like of the container support, the attachment points being below the centre of buoyancy.
22. The system of Claim 21, wherein the attachment points are above the centre of gravity.
23. The system of any of Claims 20 to 22, comprising one or more anchors acting against buoyancy of the container.
24. The system of Claim 23 and comprising at least two dead-man anchors suspended at locations spaced along chains, wires or the like of the container support.
25. The system of any of Claims 20 to 24, comprising buoyancy means at or near to an upper end of the container.
26. The system of any of Claims 20 to 25, wherein the container is weighted at or near to a lower end.
27. The system of Claim 26, wherein the container has an annular stabilising ring at its lower end.
28. The system of Claim 27, wherein the stabilising ring is supported by spars extending downwardly from the chamber of the container.
29. The system of Claim 27 or Claim 28, wherein the stabilising ring is of substantially greater diameter than the chamber of the container.
30. The system of any of Claims 20 to 29, wherein the container comprises a flared skirt around its open bottom.
31. The system of any of Claims 20 to 30, wherein the chamber is defined by an upright elongate hollow cylinder having added buoyancy at an upper end and added weight at a lower end.
32. The system of any of Claims 20 to 31, further comprising one or more load cells for monitoring filling of the container.
33. The system of any of Claims 20 to 32, wherein the container support system comprises chains, wires or the like suspending the container between a plurality of surface vessels that together determine the depth and position of the container in the water.
34. The system of Claim 33, wherein the container is equi-spaced between the support vessels.
35. The system of any of Claims 20 to 34, and comprising a plurality of containers positionable in turn over the well.
36. The system of any of Claims 20 to 35, wherein the container comprises a vent for releasing gas, the vent comprising a valve for blocking oil from exiting the container with the vented gas.
37. The system of Claim 36, wherein the valve comprises a float for floating on the oil, arranged to close the valve if the oil level approaches the vent.
38. The system of Claim 37, wherein the float is constrained for movement in a cage, through which gas may flow to the vent when the valve is open.
39. The system of any of Claims 20 to 38 and comprising a hydrate inhibitor positioned in the container.
40. The system of Claim 39, wherein the hydrate inhibitor has a density selected to lie between a gas hydrate layer and sea water in the bottom of the container.
41. The system of Claim 40, wherein the hydrate inhibitor is a mixture of methanol and/or ethanol with monoethylene glycol, diethylene glycol and/or triethylene glycol.