There are many subsea oil and gas fields located around the world. Oil and gas is extracted from these fields and transferred to land, where it is refined. The first subsea oil and gas fields to be exploited were located in relatively shallow water, so that it was relatively easy for divers to reach apparatus located on the seabed to perform maintenance tasks or carry out repairs. As existing oil and gas fields become depleted, new oil and gas fields are being exploited. These newer fields tend to be in less favourable locations. It is now desired to exploit oil and gas fields located at a depth of around 1000m or more, for example the Ormen Lange field in the Norwegian Sea.

At around 1000m depth the temperature of seawater in the Norwegian Sea is around-2°C (this temperature is unusually cold). Hydrocarbon fluid, especially hydrocarbon gas, will undergo a temperature and pressure drop as it leaves the ground and passes to a pipeline or valve. The temperature and pressure drop is such that the hydrocarbon fluid may tend to freeze, a process which is known as hydrate formation.

Hydrate formation is a problem because the hydrate may block the pipeline through which the liquid is flowing, or may cause a valve to become jammed in a particular configuration.

One way in which hydrate formation is suppressed is to inject methyl ethylene glycol (MEG), which inhibits hydrate formation, into gas or oil extraction pipelines at the point at which the gas or oil leaves the seabed. The injection is via a constant slow release mechanism, the rate of injection being selected so as to be sufficient to inhibit hydrate formation. However, the precise nature of the conditions which cause hydrate formation are not well understood, and it is difficult to predict when hydrate formation will occur. Hydrate formation may occur if, for example, the rate of MEG injection is incorrectly selected, or if it is not possible to provide MEG at a rate sufficient to inhibit hydrate formation. In addition hydrate formation may occur due to an increase of the pressure drop experienced by the gas or oil as it leaves the ground, especially if the amount of injected MEG is calculated on the basis of a previously measured pressure drop.

Despite the steady release of MEG into pipelines on the seabed, hydrate formation may still occur. If hydrate formation occurs in a valve located on the seabed then the valve can become jammed. Oil or gas extraction must be interrupted in order to disconnect the valve from pipelines which it serves, and the valve is then raised to the surface. Since the water temperature at the surface is higher than at the seabed, it is usually the case that the hydrate melts by the time the valve has been raised to the surface. Removing hydrate from a valve in this way is disadvantageous because the interruption of oil extraction, which may last for some time, leads to a significant loss of revenue. In addition the pipeline will be contaminated with seawater.

It is an object of the present invention to provide a valve which overcomes or substantially mitigates the above disadvantage.

According to the present invention there is provided a valve comprising a ball located in a chamber defined by a housing, the ball being moveable between an open configuration which allows fluid to flow from an upstream pipe to a downstream pipe via the valve and a closed configuration which prevents the flow of fluid through the valve, wherein the valve is provided with means to selectively allow hydrate dissolving fluid to be delivered to the valve.

The invention is advantageous because it allows a build up of hydrate in the valve to be dissolved without removing the valve from its operational location (the seabed).

Preferably, the hydrate dissolving fluid delivery means comprises a dedicated conduit which passes through at least part of the housing and opens into the chamber.

The term'dedicated conduit'is intended to mean that the conduit carries hydrate dissolving fluid when it is required, but does not deliver fluid from the upstream pipe to the downstream pipe.

Preferably, the hydrate dissolving fluid delivery means comprises a conduit which passes through at least part of the housing and opens into the chamber.

Suitably, the conduit opens into the chamber at a location adjacent the ball.

Preferably, the ball is rotatable ball and is provided with a bore.

Suitably, the conduit opens into the chamber at a location within the bore of the rotatable ball.

Preferably, the rotatable ball is operated via a stem, and the conduit passes along at least part of the stem.

Preferably, the conduit is located within the stem and opens into an annular region defined between the stem and the housing.

Preferably, the annular region is in fluid communication with a further portion of the conduit, which passes to the exterior of the housing.

Suitably, the conduit comprises a bore which branches into two bores, a first bore opening into the chamber at a location adjacent the ball, and a second bore opening into the chamber at a location within the bore of the rotatable ball.

Suitably, the conduit comprises two separate bores which do not intersect, a first bore opening into the chamber at a location adjacent the ball, and a second bore opening into the chamber at a location within the bore of the rotatable ball.

Preferably, the conduit further comprises an additional bore which opens into the chamber at a location spaced away from the first bore and the second bore.

Preferably, the conduit is connected to an input valve which controls the delivery of hydrate dissolving fluid to the valve chamber.

Preferably, connection means are provided at an upstream side of the input valve to allow hydrate dissolving fluid to be delivered at pressure into the conduit.

Preferably, the connection means comprises a hot stab connector.

Preferably, the valve further comprising means to selectively allow hydrate dissolving fluid, and dissolved hydrate, to be removed from the valve.

Preferably, the removal means comprises a bore which passes from the chamber to an output valve.

Preferably, the output valve is connected to a pipe which is located downstream of the valve.

Preferably, the ball valve is provided with a double piston seal, which allows pressurisation of the valve cavity without cavity relief. This is advantageous because it aids functionality.

Preferably, the housing is provided with one or more grab rails to allow a remotely operated vehicle (ROV) to be attached to the housing.

The invention also provides a valve comprising a ball located in a chamber defined by a housing, the ball being moveable between an open configuration which allows fluid to flow from an upstream pipe to a downstream pipe via the valve and a closed configuration which prevents the flow of fluid through the valve, wherein the valve is provided with means to selectively allow fluid to be purged from the valve.

A specific embodiment of the invention will now be described by way of example only with reference to the accompanying figures, in which: Figure 1 is a cross-sectional drawing of a valve which embodies the invention; Figure 2 is a side view of the valve shown in figure 1 ; Figure 3 is a cross-sectional view of a double piston valve seat which forms part of the valve of figure 1 ; and Figure 4 is a cross-sectional view of a second valve which embodies the invention.

Referring to figure 1, a ball valve is generally indicated at 1. The ball valve 1 comprises a ball 2 provided with a central bore 3 and attached to a stem 4. A housing 5 defines a chamber 6 in which the ball 2 is held.

Referring to figure 2, the valve 1 is connected between first and second pipes 7,8, and controls the flow of fluid from the first pipe 7 (referred to hereafter as the upstream pipe 7) to the second pipe 8 (referred to hereafter as the downstream pipe 8) Referring again to figure 1, the valve is operated via rotation of the stem 4.

Rotation of the stem 4 moves the ball 2 between an open configuration in which the bore 3 is aligned with the pipes 7,8 to allow fluid to flow through the valve 1, and a closed configuration in which the bore 3 is transverse to the pipes 7,8 (i. e. the bore 3 is not in fluid communication with the pipes), thereby preventing fluid from flowing.

The closed configuration of the valve 1 is shown in figure 1.

The seating design of the ball valve is a double piston configuration. The double piston configuration is important to the operation of the valve, as is described further below in relation to figure 3.

The ball valve is configured to allow the injection of methyl ethylene glycol (MEG), or other suitable fluid, into the ball valve to melt hydrate within the ball valve. For this reason a bore 9 is provided in the housing 5. The bore 9 passes downwards through a branch 10 and opens into the chamber 6 in which the ball 2 is held. The bore 9 also connects with a bore 11 located in the valve stem 4, which opens into the bore 3 in the ball 2.

The connection between the bore 9 and the bore 11 provides fluid communication between the bores 9,11 irrespective of the orientation of the valve stem 4. This is achieved using a'gallery'which, referring to figure la, comprises an annular recess 101 together with a series of radially extending bores 102 which communicate with the bore 11. 0-ring seals 103 are provided between the stem 4 and the housing 5. In use, fluid which flows through the bore 9 will flow into the annular recess 101, and via the radially extending bores 102 into the bore 11. The flow of fluid will occur irrespective of the orientation of the valve stem 4., The MEG injection bore 9 is connected to an isolation ball valve 12, which is in turn connected to a hot stab connector 13. The isolation ball valve 12 is referred to hereafter as the input ball valve 12. Both the input ball valve 12 and the hot stab connector 13 are configured to allow operation by a remotely operated vehicle (ROV) which carries MEG, thereby allowing MEG to be injected into the bore 9. The hot stab connector 13 is provided with a dummy receptacle 14 which is secured in the hot stab connector to isolate the hot stab connector from the sea.

A second isolation ball valve 15 is connected to a bore 16 which opens into the chamber 6. The second isolation ball valve is referred to hereafter as the output ball valve 15. The point at which the bore 16 opens into the chamber 6 is at an opposite end of the chamber 6 from the point at which the bore 10 opens into the chamber 6. There is a clearance of around 1 centimetre between the ball 4 and the housing 5, with the effect that different regions of the chamber 6 are in fluid communication with each other. The output ball valve 15 is configured to allow operation by a ROV. The output ball valve 15 is connected to the downstream pipe 8 (this connection is not shown in the figures).

Referring to figures 1 and 2, a grab rail 17 extends around the hot stab connector 13 and the input ball valve 12. The grab rail 16 is arranged to allow the ROV to secure itself to the valve 1, so that it can operate the hot stab connector 13 and input ball valve 12. A grab rail is also provided at the output ball valve 15, and at the valve stem 4, although these are not shown in figures 1 and 2.

In operation, a ROV (not shown in the figures) is controlled to swim down to the valve. The ROV removes the dummy receptacle 14 from the hot stab connector 13. The dummy receptacle 14 is retained by the ROV. The ROV then attaches a hot stab receptacle to the hot stab connector 13. The hot stab receptacle is connected to a pressurised supply of MEG which is held within the ROV. The input ball valve 12 is then opened and MEG is injected into the valve 1 via the bore 9.

The MEG is injected into the valve 1 at high pressure, typically between 3500 PSI and 5000 PSI. The high pressure MEG breaks down hydrate formations which have accumulated in the valve 1. Once MEG has been injected into the valve 1 under pressure, the input ball valve 12 is closed.

If the valve 1 is in the closed configuration (the configuration shown in figure 1), the MEG and dissolved hydrate will be held in the valve 1 at pressure. The output ball valve 15 is then opened by the ROV, to allow the dissolved hydrate and the MEG to leave the valve 1. The dissolved hydrate and MEG passes from the output ball valve 15 to the downstream pipe 8. The output ball valve is then closed by the ROV.

If the valve 1 is in the open configuration then the dissolved hydrate and MEG will flow from the valve 1 into the downstream pipe 8.

The hot stab receptacle is removed from the hot stab connector 13, and the dummy receptacle 14 is relocated in the hot stab connector 13. The ROV then returns to the surface.

Suitable chemicals other than MEG may be used to melt the hydrate, for example methanol. The methanol is of a higher purity than MEG, and is therefore more effective at breaking down hydrate formations.

The double piston configuration of the seating design of the valve is shown in figure 3. The ball 2, housing 5 and upstream pipe 7 correspond to those shown in figures 1 and 2. The valve seating comprises a first annulus 18 located against the ball 2, and a second annulus 19 located between the first annulus 18 and the upstream pipe 7. A helical spring 20 is located behind the second annulus 19, and resiliently biases the second annulus 19 towards the ball 2. The second annulus 19 is provided with a first seal 21 which acts between the first annulus 18 and the second annulus 19, and a second seal 22 which acts between the second annulus 19 and the upstream pipe 7. The first annulus 18 is provided with a lip 23 arranged to form a seal against the ball 2.

In use, when the valve is in the open configuration, as shown in figure 3, the first and second annuluses 18,19 will together be biased towards the ball 2 and the lip 23 of the first annulus 18 will form a seal against the ball 2. In the absence of any significant pressure in the pipe 7, the first and second annuluses are biased towards the ball 2 by the helical spring 20. When there is significant pressure in the pipe 7, the pressure acts on a rear surface 24 of the second annulus. The pressure also acts on the lip 23 of the first annulus. Since the area of the rear surface 24 is greater than the area of the lip 23, the resultant force pushes the first and second annuluses against the ball 2, thereby forming a seal against the ball. The first seal 21 is energised by the pressure in the pipe 7, thereby forming a seal between'the first annulus and the second annulus. The second seal 22 is energised by the pressure in the pipe 7, thereby forming a seal against the pipe 7.

When the valve is in a closed configuration, the first and second annuluses 18, 19 will both be biased towards the ball 2 when the pressure in the pipe 7 exceeds the pressure in the valve cavity 6. However, when the pressure in the valve cavity 6 exceeds the pressure in the pipe 7 then the first annulus 18 will be biased towards the ball 2 but the second annulus 19 will be biased towards the pipe 7. This occurs because there is fluid communication between the valve cavity 6 and an interface area 25 between the first and second annuluses 18,19. Pressure from the valve cavity 6 enters the interface area 25, pushes the second annulus 19 towards pipe 7, and pushes the first annulus 18 towards the ball 2. The seal 21 prevents pressure from the interface area 25 entering the pipe 7, irrespective of the relative locations of the first and second annuluses. The effect of the first and second annuluses is to allow the pressure within the valve cavity 6 to rise above the pressure in the pipe 7 without pressure leaking from the valve cavity 6 to the pipe 7. This is advantageous because it allows the injection of MEG (or methanol) into the valve cavity under pressure, irrespective of the pressure in the pipe 7. The MEG (or methanol) is retained under pressure in the valve cavity 6 until it is released either by opening the output ball valve 15 (see figure 1) or opening the valve 1 itself.

Figure 4 shows a second embodiment of the invention. Figure 4 corresponds in large part to figure 1, and equivalent features of the embodiment shown in figure 4 are labelled with equivalent reference numerals. The second embodiment is configured to allow an operator to select whether-EG is injected directly into the bore 3 of the ball valve 1, or injected into the chamber 6.

A hot stab connector 13 and an input ball valve 12 are connected to a bore 9 in the housing 5. The bore 9 does not branch into two bores, but instead turns downwards to open into the cavity 6.

A second isolation ball valve 27 (hereafter referred to as the second input ball valve 27) and a second hot stab connector 28 are provided at an opposite side of the housing 5. The second input ball valve 27 is connected to a bore 29 which passes to the stem 4 of the valve 1. The stem 4 is provided with a bore 30 opens into the bore 3 of the ball 2 The second embodiment of the invention is advantageous because it allows an operator to choose whether to inject MEG into the bore 3 at the centre of the ball 2 or to inject MEG into the chamber 6. The embodiment of the invention shown in figure 1 allows only the simultaneous injection of MEG into both these regions. An operator may for example choose to inject a small amount of MEG at intervals into the chamber 6 via the bore 9 in order to inhibit hydrate formation, when the valve is in the open configuration. The operator may choose to inject MEG into the chamber 6 via the bore 9 immediately after moving the valve to the closed configuration, thereby filling the chamber 6 with MEG and inhibiting hydrate formation over time. If a plug of hydrate has formed in the valve, the operator may choose to inject directly into the bore 3 at the centre of the ball 2, via the bore 29,30. The operator may choose to inject methanol rather than MEG, as this is likely to melt the hydrate plug more easily.

The second embodiment of the invention is operated using a ROV in the same way as the first embodiment of the invention. The ROV may be configured to allow simultaneous injection of MEG via the first hot stab connector 13 and the second hot stab connector 28.

In a further embodiment of the invention (not shown) a bore may be provided which allows MEG to be injected at a midpoint of the chamber 6. The apparatus which would allow this to be done is equivalent to that shown in figures 1 and 4, i. e. an isolation ball valve and a hot stab connector. A suitable location for injection is indicated in figures 1 and 2 by dotted lines 31.

As an alternative to the use of a ROV, a tank of MEG held under pressure may be permanently connected to one or more input ball valves 12, 27. Where this is done means may be provided to allow remote operation of the input ball valves 12,27 and the output ball valve 15 in the absence of a ROV.

The embodiments of the invention described above, in addition to allowing the injection of MEG or other fluid into the valve 1, may be used to draw fluid out of the pipeline 7,8. This may be done for example by connecting a pump (not shown) to the hot stab connector 13 (the pump may be carried by an ROV), then moving the valve 1 so to the open configuration and pumping fluid from the valve via the bore (11,30).

The pipeline may be purged or vacuum dried in this way.

It may be useful to draw fluid out of the valve 1 prior to commissioning of the pipeline, or after commissioning of the pipeline.