The present invention relates to an underwater pumping system for pumping fluid into or out of a wellbore.

Pumps for pumping fluids from subsea oil and gas wells are known. When an oil well is young, the pressure in the reservoir is large, due to the pressure from the weight of the sea and the rock pushing down on the reservoir. The reservoir pressure is typically 5000-10,000 pounds per square inch, and can be up to around 15,000 pounds per square inch. This pressure pushes the produced fluids up towards the surface. Often the pressure needs to be limited by a valve in the Christmas tree, so that the pressure of the produced fluids in the pipe leading to the surface is reduced to a level which the pipe can stand, e.g. around 5,000 pounds per square inch. However, once the oil well has been producing fluids for a while (e.g. a year or more) the reservoir pressure drops. This makes it harder to retrieve any further fluids from the well. The reservoir pressure must be enough to push the fluids from the reservoir to the seabed, and from the seabed to the oil rig on the surface. When the reservoir pressure is no longer sufficient to enable this to happen, no further produced fluids will reach the oil rig.

To enable continued production in this situation, it is known to remove the Christmas tree and to insert an elongate pump into the production bore, and then to replace the Christmas tree. However, this is a very expensive and complicated operation, which requires use of a pressure control system whilst the Christmas tree is removed and replaced. It is also very risky removing a Christmas tree, as the Christmas tree is preventing the produced fluids in the production bore of the well from leaking into the environment. Furthermore, often it is found that once the pump has been installed, it does not work properly, for example, a cable may have been accidentally severed in the installation process.

According to a first aspect of the present invention there is provided a wellbore-external underwater pumping system for pumping process fluid into or out of a wellbore, the underwater pumping system comprising: a piston and cylinder assembly for pumping process fluid, the piston and cylinder assembly comprising at least one piston and at least one cylinder, the piston being driveable by a drive fluid.

The term "process fluid" means the fluid to be pumped, as distinct from the drive fluid that drives the piston. The process fluid may comprise oil, water, gas, sand/other small particulates or any combination of these (i.e. multiphase fluids) . When used to pump fluids out of a well, the process fluid typically comprises the fluids produced from that well.

The use of a piston pump, as opposed to any other kind of pump, provides the advantage that the pumping system can be made small, light, and can generate high pressures. The piston pump can also be simpler and cheaper to produce than other kinds of pumps. Being wellbore-external means that the underwater pumping system of the invention can be retrofitted to existing Christmas trees without needing to remove the tree.

According to a further aspect of the present invention there is provided a piston and cylinder assembly; and a closed housing; wherein the piston and cylinder assembly comprises at least one piston and at least one cylinder; and wherein the piston and cylinder assembly is located within the housing. Some embodiments enable the pumping system to be used in remote locations, for example, subsea. In such embodiments, the housing can prevent any fluids from inside the pumping system from leaking into the environment, which is very important. However, the housing is not essential to the invention; the pumping system could be sealed in any way from its surrounding environment.

Being wellbore-external, the underwater pumping system does not require the removal of the Christmas tree for installation, therefore providing cheaper, less complicated and less risky systems. Produced fluids can exit the Christmas tree, or other subsea manifold, through the conventional fluid outlet, and these can be directed to the inlet of the pumping system of the invention. The pumping system can then pump these fluids into an outlet line, as will be more fully described below. The pumping system being encased in a housing covers embodiments in which the pumping system to be used in conjunction with a conventional tree, as a separate component.

The wellbore-external underwater pumping system may optionally be located on a Christmas tree or other subsea manifold. All references to a Christmas tree include references to any subsea manifold.

Optionally, the underwater pumping system can be used to pump fluids to or from a single wellbore. Such arrangements have the advantage that their size and weight can be minimised further. A further advantage lies in that if the underwater pumping system malfunctions, this would only affect one well.

Alternatively, a single underwater pumping system may be used to pump fluids to or from a plurality of wellbores.

The underwater pumping system may be located anywhere in the flowpath to or from a wellbore, and may be located at or near a junction connecting a plurality of wellbores. Thus, the underwater pumping system is not necessarily located at the wellbore itself and may be located at some distance from the or each wellbore.

Reservoirs are normally located at around the same depth relative to sea level, regardless of whether the well is in deep water or not. In a shallow- water well, the task of propelling the well fluids from the seabed to the surface is not so difficult. However, when wells are located in deep waters, the distance from the seabed to the surface tends to be much greater than the distance from the distance between the reservoir and the seabed. Thus, the task of propelling the fluids between the seabed and the surface is even more important. Therefore, some embodiments of the invention give even greater benefits when used in deep wells, as the pumping system can be used to propel the produced fluids over the large distance between the seabed and the surface. Preferably, the piston and cylinder assembly comprises two pumping chambers, and is arranged such that on each stroke of the at least one piston, fluid is expelled from one of the pumping chambers.

Typically, the piston and cylinder assembly comprises a double-acting piston and cylinder assembly. "Double-acting piston and cylinder assembly", means a single piston that is reciprocal ■ in a single cylinder and adapted to expel process fluid on each stroke of the piston.

However, a double-acting piston and cylinder assembly is not essential, e.g. in the Fig 6 embodiment, two pistons are provided, each located in respective cylinders which are connected by a flowpath to allow the exchange of drive fluid between the cylinders. This embodiment comprises two single-acting pistons and respective cylinders. "Single-acting piston and cylinder assembly" means a reciprocal piston that expels process fluid when moving in one direction, but not when moving in the opposite direction. In the Fig 6 embodiment, process fluids are expelled from one of the two cylinders at each stroke of the piston and cylinder assembly.

Typically, the pumping system includes at least one drive fluid switch valve. Typically, the drive fluid switch valve is adapted to switch from a first position in which it connects a first combination of flowpaths, and a second position, in which it connects a second combination of flowpaths. Typically, the drive fluid switch valve is adapted to cause the drive fluids to flow in a first direction when in the first position, and in the opposite direction when in the second position, to control the direction of movement of the piston.

Optionally, a process fluid switch valve is provided to control the direction of flow of the process fluids when entering and/or leaving the piston and cylinder assembly. Alternatively, this is achieved by one or more one-way valves.

Optionally, the switch valve (s) is triggered to switch when the piston reaches the end of its cylinder. The trigger can be an electrical or mechanical trigger.

Optionally, first and second pistons and respective cylinders are provided. Optionally, a direction change of the first piston is triggered by the second piston.

Optionally, the first and second pistons each have a respective drive fluid switch valve. Optionally, a mechanical trigger is connected to the first piston, which triggers the drive fluid switch valve of the second piston, and vice versa, so that movement of one piston causes a change in direction of the other piston. Preferably, the mechanical trigger of the first piston is adapted to trigger the drive fluid switch valve of the second piston when the first piston reaches part way through its stroke, e.g. halfway, and vice versa .

Optionally, a source of pressurised drive fluid is provided, e.g. a drive means to pump the drive fluid. Preferably, the drive means is adapted to pressurise the drive fluid to a definable maximum pressure. Typically, the drive means comprises a hydraulic power unit. Preferably, the hydraulic power unit has a definable maximum pressure. This can make it impossible to over-pressurise the pumping system, which safeguards the pumping system and the environment. Typically, the hydraulic power unit comprises a centrifugal pump.

Other drive systems may be used instead of a hydraulic power unit.

Optionally, the drive fluid at least partially fills the volume bounded by the closed housing. Optionally, a drive fluid conduit (e.g. a make-up line) is provided which connects with the housing; drive fluid can be added through the drive fluid conduit. The drive fluid conduit can also be used to control the pressure of the drive fluid in the housing. Preferably, the pumping strength of the hydraulic power unit is selected such that the sum of the pressure of the drive fluids in the housing and the pressure added to the drive fluids by the pump is less than the pressure limits of the pumping system. In embodiments without a hydraulic power unit, the pressure of the drive fluids provided by the drive fluid inlet line should be less that the pressure limits of the pumping system.

Alternatively, in embodiments that do not include a housing, the drive fluid conduit may lead directly to a drive fluid inlet of the piston and cylinder assembly. In some embodiments, the drive fluids may be pressurised before entering the drive fluid conduit, and hence may not require to be pressurised by any drive means of the pumping system.

Preferably, the drive fluid comprises water, glycol, or a mixture of both. Such embodiments are particularly useful subsea, where there is a large quantity of water available, and where it is very important to reduce the risk of leaking anything toxic into the environment.

Typically, the piston has at least one head and the piston and cylinder assembly is provided with a seal to restrict fluid leakage around the piston head. The seal could comprise one "or more of a labyrinth seal and a physical seal e.g. an o-ring seal, a piston ring seal, a hydraulic seal, etc.

Optionally, the piston has two heads, one at either end of the piston rod. Preferably, the piston divides the cylinder into an inner region between the two heads, and two separate outer regions between the cylinder heads and their respective cylinder ends. Preferably, the piston heads are a close fit in the part of the bore of the cylinder in which they are located, to prevent significant fluid leakage around the piston heads.

Typically, the pumping system includes fluid conduits adapted to direct process fluid into the inner region or the outer region, and further fluid conduits adapted to direct drive fluid into the other region.

Preferably, the piston and cylinder assembly is pressured such that the fluid pressure in the drive fluid region of the cylinder is greater than the pressure in the process fluid region. Such embodiments can help ensure that if any leakage of fluid occurs around the piston heads, the leakage always occurs from the drive fluids into the process fluids. This can be useful, as this prevents process fluid from leaking into the housing or the environment.

Preferably, the at least one cylinder has a reduced diameter portion. Preferably, the piston is sealed in the cylinder at the reduced diameter portion to restrict or minimize (most preferably, prevent) fluid flow between opposite sides of the reduced diameter portion. Typically, the piston is pressure-sealed. Typically, the piston and cylinder assembly has a labyrinth seal. Typically, the labyrinth seal comprises a series of grooves in the reduced diameter portion. Optionally, the piston and cylinder assembly has at least one physical seal (e.g. an o-ring seal and/or a piston ring seal and/or a hydraulic seal) . One or more of the above types of seal, or any other type of seal may be used.

Optionally, at least one fluid conduit is provided, linking the inner region and the outer region, and the pumping system is adapted to exhaust the drive fluid into the process fluid for removal with the process fluid. Such embodiments do not require that a source of drive fluid is provided within the housing; the source of drive fluid may be located remotely (e.g. the Fig 3 embodiment) .

Alternatively, a source of pressurised drive fluid (e.g. a hydraulic power unit) is provided within the housing.

According to a second aspect of the present invention, there is provided the underwater use external to a wellbore of an underwater pumping system for pumping process fluid into or out of a wellbore, the underwater pumping system comprising: a piston and cylinder assembly for pumping the process fluid, the piston and cylinder assembly comprising at least one piston and at least one cylinder.

According to a further aspect of the invention there is provided a pumping system comprising: a piston and cylinder assembly; and a source of drive fluid; wherein the piston and cylinder assembly comprises at least one piston, at least one cylinder and two pumping chambers, and is arranged such that on each stroke of the at least one piston, fluid is expelled from one of the pumping chambers .

Preferably, the source of drive fluid comprises a hydraulic power unit. Preferably, the hydraulic power unit has a maximum pressure. Typically, the hydraulic power unit comprises a centrifugal pump.

Preferably, the piston and cylinder assembly comprises at least one double-acting piston.

Optionally, the pumping system comprises two piston and cylinder assemblies, which move out of phase with each other. Preferably, movement of one piston triggers a change of direction of the other piston.

Optionally, the closed housing is filled with fluid. Typically, the closed housing is filled with drive fluid.

According to a further aspect of the present invention, there is provided a pumping system comprising: a piston and cylinder assembly; and a hydraulic power unit; wherein the piston and cylinder assembly comprises at least one piston, at least one cylinder and two pumping chambers, and is arranged such that on each stroke of the at least one piston, fluid is expelled from one of the pumping chambers.

According to a third aspect of the present invention there is provided a method of pumping process fluids into or out of a wellbore using an underwater pumping system located underwater but external to the wellbore, the underwater pumping system comprising a piston and cylinder assembly, the piston and cylinder assembly comprising at least one piston and at least one cylinder, the at least one piston being driven by a drive fluid.

According to a further aspect of the invention, there is provided a method of pumping fluids using a pumping system comprising a piston and cylinder assembly provided in a closed housing, the piston and cylinder assembly comprising at least one piston and at least one cylinder.

Optionally, the piston and cylinder assembly includes two pumping chambers, and the method includes the step of expelling fluid from one of the pumping chambers on each piston stroke.

Preferably, the method includes the step of isolating the drive fluid from the process fluid. Preferably, the drive fluid and the process fluid are directed to different regions of the piston and cylinder assembly, which are sealed from each other, by a pressure seal and/or by a physical seal. Optionally, the piston is double-headed, and one of the drive fluid and the process fluid is directed between the heads, and the other is directed on the outer sides of the heads.

Optionally, the method includes the step of exhausting the drive fluid into the process fluid for recovery with the process . fluid.

Alternatively, the method includes the step of re- circulating the drive fluid.

Optionally, the piston and cylinder assembly is provided in a housing. Preferably, the housing is sealed. Optionally, the housing is at least partially filled with drive fluid.

Preferably, the piston is moveable in two directions within the cylinder and the method includes the step of changing the direction of movement of the piston. Typically, the change in direction of the piston is accomplished by a drive fluid switch valve, the switch valve being moveable to create new flowpath connections which reverse at least some of the fluid flow directions in the pumping system to change the direction of the piston. Typically, the switch valve is triggered when the piston reaches a cylinder end.

Optionally, first and second pistons are provided in respective cylinders, and the method includes the step of synchronising the movement of the pistons. Preferably, the pistons are maintained out of phase with one another. Preferably, the phase difference is a quarter of a period. Such embodiments can provide an overall more continuous pumping action.

Preferably, the pistons are synchronised using a mechanical trigger system. Some such embodiments avoid the need for a conventional, electrically powered timer, and can also provide reliable synchronisation over long time scales. Preferably, each piston is provided with at least one drive fluid switch valve, which is moveable to make different flowpath connections. Typically, the drive fluid switch valve of the first piston is triggered by movement of the second piston and vice versa. Typically, the drive fluid switch valve of the first piston is triggered when the second piston reaches halfway or part way along its cylinder and vice versa.

Preferably, the cylinder is divided into a drive fluid region and a process fluid region, and the method includes the step of maintaining the drive fluid region at a higher pressure than the process fluid region. Typically, the cylinder is divided into two regions by the piston (therefore, these regions are not fixed, but change location or size with the movement of the piston) .

According to a further aspect of the present invention there is provided a pumping system comprising: a piston and cylinder assembly comprising at least one piston and at least one cylinder; and a closed housing in which the piston and cylinder assembly is located; wherein the pumping system further comprises a pump having a maximum pumping pressure, the maximum pumping pressure being selected such that the pressure of fluids leaving the pump is always less than the maximum containment pressure of the housing.

Preferably, the pump comprises a centrifugal pump .

Embodiments of the invention therefore provide a pumping system which physically cannot be over- pressurised to break the housing, or any other component of the pumping system, providing a very safe pumping system.

Embodiments of the invention will now be described, by way of example only, and with reference to the following drawings, in which:-

Fig 1 shows a pumping system according to the invention; Fig 2 shows a pumping system similar to the Fig 1 embodiment, wherein the Fig 1 valve system has been replaced by a switch valve; Fig 3 shows a further embodiment of a pumping system, wherein the drive fluid is exhausted into an outlet line; Fig 4 shows a further embodiment, where the locations of the drive fluids and the process fluids in the cylinder have been swapped as compared to the Fig 1 embodiment; Fig 5 shows an embodiment similar to that shown in Fig 4, wherein the switch valve is replaced by four one-way valves; Fig 6 shows an embodiment having two cylinders and two pistons, which are interconnected; Fig 7 shows the Fig 5 embodiment with additional components; and Fig 8 shows a further embodiment, having two pistons in respective chambers, which can be synchronised with each other.

The following description will include two different sets of fluids; fluids to be pumped, which will be referred to as "process fluids" and drive fluids, which drive the pumping system. The process fluids can be single or multiphase fluids, e.g. one or more of the following: oil, water, gas, particulates (e.g. sand) . The process fluids may be being pumped from a wellbore, or being pumped into a wellbore. The drive fluid, may comprise water or a water- glycol mixture.

Fig 1 shows a pumping system having a double piston 10 which has a rod 12 and a head 14R, 14L at either end of the rod 12. The piston 10 is in a cylinder 16, which has right and left ends, RE, LE. Hence, Fig 1 shows a double-acting piston and cylinder assembly. The cylinder 16 is connected to a drive means adapted to drive a drive fluid. The drive means comprises a hydraulic power unit 18.

The hydraulic power unit 18 typically comprises a centrifugal pump, which works by rotating a rod to spin the drive fluid radially outwards in a chamber to raise the pressure of the drive fluid. The centrifugal pump may include several stages; at each stage the pressure is raised successively. Other types of pumps can be used in the hydraulic power unit 18.

The cylinder 16 is connected to the hydraulic power unit 18 by a drive fluid flowpath. The drive fluid flowpath comprises a flowpath 20a from the hydraulic power unit 18 to a switch valve 22, and a return flowpath 20b from the switch valve 22 to the hydraulic power unit 18.

The switch valve 22 is connected to a reduced diameter portion 17 at the mid-point of the cylinder 16. The piston heads 14L, 14R are located on opposite sides of the reduced diameter portion 17. The cylinder walls project inwardly at the reduced diameter portion 17 and the inner diameter of the reduced diameter portion bears against the outer diameter of the rod 12. This splits the cylinder into right and left-hand portions, R, L, which are not in direct fluid communication with each other. The portions R, L may be sealed from each other. The switch valve 22 is connected to the cylinder by two further flowpaths 24a, 24b, which are formed in the wall of the reduced diameter portion 17.

Flowpaths 24a, 24b have outlets, facing in opposite axial directions, into the cylinder 16. The flowpath 24a is in communication with the right hand portion R and the flowpath 24b is in communication with the left-hand portion L.

The switch valve 22 is switchable from a first position in which it connects flowpath 20a with flowpath 24a, and flowpath 20b with flowpath 24b, and a second position in which the connections are reversed. The first position is shown in Fig 1.

Each end of the cylinder RE, LE is connected to an inlet line I and an outlet line 0 by respective flowpaths. The end RE is connected to the inlet line I via flowpath 261 and one-way valve Vl, and to the outlet line 0 via flowpath 260 and one-way valve V2. The end LE is connected to the inlet line via flowpath 281 and one-way valve V3, and to the outlet line 0 via flowpath 280 and one-way valve V4.

Each of the one-way valves Vl, V2, V3, V4 in the flowpaths 261, 260, 281, 280 is adapted to open in the direction of fluid flow, i.e. the valves Vl, V3 in the inlet flowpaths 261, 281 are adapted to open when the pressure from the inlet fluid is greater than the pressure on the other side of these valves, and they open so as to allow fluid to pass from the inlet line I to the cylinder. The valves V2, V4 in the outlet flowpaths 26O, 280 are adapted to open when the pressure from the process fluid in the cylinder ends, LE, RE, is greater than the pressure in the outlet line 0, and they open so as to allow process fluid to pass from the cylinder ends LE, RE, into the outlet line 0.

The inlet line I carries process fluid to be pumped (e.g. hydrocarbons), and the outlet line 0 carries process fluid which has been pumped. In use, the hydraulic power unit 18 pumps drive fluid along the flowpath 20a. When in the position shown in Fig 1, the switch valve 22 directs this fluid into flowpath 24a, from where it enters the right hand portion R of the cylinder 16. This builds up pressure behind the cylinder head 14R, which moves the piston 10 to the right, as shown by the arrow X in Fig 1.

The piston head 14R pushes on the process fluid in the end RE of the cylinder 16. This increases the pressure on the valve V2, which opens the valve V2 to allow the process fluid to enter the outlet flowpath 260 and to exit via the outlet line 0. The build-up of pressure in RE does not open the valve Vl, but merely seats the ball more firmly in the socket.

The movement of the whole piston 10 to the right also causes a vacuum to build up behind the piston head 14L. This pulls valve V3 open, which allows some more process fluid to enter the cylinder 16 from the inlet line I, via the inlet flowpath 281. This fluid enters the left-hand portion L.

The movement of the piston 10 to the right also compresses the drive fluid in the left-hand portion of the cylinder between the piston head 14L and the reduced diameter portion 17. Therefore, this exhausts some of the drive fluid in the portion L out through the flowpath 24b. The switch valve 22 in the first positions shown in Fig 1 directs it into the flowpath 20b, from where it is returned to the hydraulic power unit 18 and recycled into the right hand portion R of the cylinder, as described above.

When the piston 10 reaches the right hand end RE of the cylinder 16, the switch valve is switched (typically automatically) over to the second position, where the flowpath 20a is connected to the flowpath 24b and the flowpath 20b is connected to the flowpath 24a. This directs the flow of drive fluid behind the head 14L, which causes the piston 10 to move to the left, and pushes the process fluid in the left hand portion L out of the valve V4 into the outlet line O. A vacuum is created behind the piston head 14R, which pulls valve Vl open to cause more process fluid to enter the right hand portion R from the inlet line I. The drive fluid from the right hand end RE is exhausted through the flowpaths 24a and 20b. When the piston 10 reaches the left- hand end LE, the switch valve 22 is automatically switched back to the first position, and the cycle is repeated.

Fig 2 shows a pumping system similar to the Fig 1 pumping system, and like parts are numbered the same. In this embodiment, the four flowpaths, 261, 260, 281, 280 and their respective valves Vl to V4 are replaced by a further switch valve 34, and two further flowpaths 30, 32 connecting the left and right hand ends LE, RE respectively to the switch valve 34. The switch valve 34 is also connected to the inlet line I and the outlet line 0. The switch valve 34 is adapted to move from a first position (see Fig 2) in which the flowpath 30 is connected to the inlet line I and the flowpath 32 is connected to the outlet line 0, and a second position, in which these connections are reversed.

In use, the pumping system functions generally in the same way as the Fig 1 embodiment. The drive fluid flows in exactly the same way. When the piston 10 is caused to move to the right as shown in Fig 2, process fluid is pushed out of the end RE, into the flowpath 32 and the outlet line 0. At the same time, a vacuum is created in the left-hand end LE, which pulls further process fluid into the end LE via the inlet line I, the switch valve 34 and the flowpath 30. Both switch valves 22, 32 are switched (typically automatically) when the piston 10 reaches the limit of its travel in both directions. This reverses the flow directions so in the next stroke, the piston 10 moves to the left. Fig 3 shows a further modified embodiment, in which the drive fluid is exhausted into the outlet line O. Most components are identical to the corresponding Fig 1 components, and the same reference numbers are used. In this embodiment, drive fluid enters the pumping system by a drive fluid inlet line 19. The drive fluid inlet line 19 is connected to a source of drive fluids, typically a high pressure source, which may be located proximate to the pumping system or far away. The drive fluid inlet line 19 is optionally connected to a hydraulic power unit. The source of drive fluids is optionally located on a rig. Alternatively, the source of drive fluids may be produced fluids from a further oil well in the vicinity. A young oil well which still has a high reservoir pressure can provide a high pressure source which is nearby (not requiring a long pipeline from the surface) . These produced fluids are then recovered in the outlet line 0, as explained above.

A modified switch valve 42 is adapted to connect the drive fluid inlet line 19 to one of the flowpaths 24a or 24b, while blocking the other. In the position shown in Fig 3, the drive fluid inlet line 19 is connected to the flowpath 24a, which causes the piston 10 to move to the right.

A further switch valve 52 of the same design is connected to the reduced diameter portion 17 and to two further flowpaths 54a, 54b provided in the reduced diameter portion 17, each having an outlet into opposite portions R, L of the cylinder 16. Two flowpaths 56, 58 connect the switch valve 52 to the ends RE, LE of the cylinder 16. The switch valve 52 is switchable between a first position (shown) in which the flowpath 54b is connected to the flowpath 58 and the flowpath 54a is not connected to any flowpath, and a second position, in which the flowpath 54a is connected to the flowpath 56, and the flowpath 54b is not connected.

In use, this embodiment functions largely in the same way as the Fig 1 embodiment, except for the route of the exhausted drive fluid. The valve 42 switches to alternately allow the drive fluid to enter the cylinder via flowpaths 24a, 24b to move the piston 10 in alternate directions. When the piston 10 moves to the right, the drive fluid in the left hand portion L of the cylinder 16 is compressed, and flows through the switch valve 52, through flowpath 58 into the left hand end LE. At the same time, the volume to the left of the piston head 14L is enlarged, which creates a vacuum effect, which pulls open the valve V3 to draws process fluid into the left hand end LE from the inlet I via the valve V3.

Process fluids in the right end RE are exhausted through the valve V2 and the outlet line 0. The pressure build-up behind the piston head 14R does not drive fluid through the switch valve 52, as the flowpath 54a is closed. When the piston 10 has reached the limit of travel to the right, the valves 54 and 42 are switched, connecting the inlet line 19 to the flowpath 24b to drive the piston 10 to the left. A vacuum effect is created in the right hand end RE behind the piston head 14R, which opens the valve Vl and draws further inlet fluids into the right hand end RE from the inlet line I. The process fluids in the left hand end LE are then exhausted through the valve V4 and the flowpath 280, and at the same time, drive fluid behind the piston 14R from the right hand portion R flows into the right hand end RE via the flowpath 56, to be exhausted via the valve V2 on the piston's next stroke to the right. Both valves 42, 52 are switched at the end of each stroke to the right or left.

Fig 4 shows an embodiment very similar to the Fig 2 embodiment, and like numbers are used to designate like parts. However, in Fig 4, the outlet line 0 and the inlet line I are connected to the switch valve 22 and the flowpaths 20a, 20b are connected to the switch valve 34.

Thus, process fluid is directed into the portion of the cylinder 16 between the piston heads 14R, 14L, and the drive fluid is delivered to the cylinder ends RE, LE, contrary to the Fig 2 system.

Drive fluid leaving the hydraulic power unit 18 is directed along the flowpath 30 into the left-hand end LE, and pushes the piston 10 to the right. This draws fluid into the space behind the piston head 14R and expels process fluid into the outlet line 0 from the space behind the piston head 14L. Drive fluid in the right hand end RE is pushed by the piston 10 into the flowpath 32, via the switch valve 34, and into the flowpath 20b back to the hydraulic power unit 18. When the piston 10 reaches the end of its stroke, the switch valves 22, 34 switch, so that the piston 10 will next move to the left, as in the other embodiments.

The Fig 5 is similar to the Fig 4 embodiment, in that this embodiment directs the drive fluid to the ends RE, LE of the cylinder 16, whilst directing the process fluid into the centre of the cylinder 16. The connections between the hydraulic power unit 18 and the ends LE, RE are exactly the same as in the Fig 4 embodiment. However, instead of the switch valve 22 being used to direct the process fluid to/from the centre of the cylinder 16, a valve system is used.

The inlet line I divides into two portions II, 12, each having a respective one-way valve V5, V6, which permit flow in the direction from the inlet line I only. The reduced diameter portion 17 includes two flowpaths 60a, 60b, leading to the space between the reduced diameter portion 17 and the left hand piston head 14L and the space between the reduced diameter portion 17 and the right hand piston head 14R respectively. The inlet line portion Il is connected to the flowpath 60a and the inlet line portion 12 is connected to the flowpath 60b.

Similarly, the outlet line O divides into two portions 01, 02, each having a respective one-way valve V7, V8, which permits flow in the direction towards the outlet line only. The reduced diameter portion 17 includes two further flowpaths 62a, 62b, leading from the space between the reduced diameter portion 17 and the left hand piston head 14L and the space between the reduced diameter portion 17 and the right hand piston head 14R respectively. The outlet line portion Ol is connected to the flowpath 62a and the outlet line portion 02 is connected to the flowpath 62b.

In use, when the switch valve 34 is in the position shown in Fig 5, drive fluid is directed into the left hand end LE by the hydraulic power unit 18, which causes the piston to move to the right. This increases the volume between the reduced diameter portion 17 and the piston head 14R, which causes a vacuum, which pulls the valve V6 open to allow process fluid to enter that space via the flowpath 60b. Likewise, the volume between the reduced diameter portion 17 and the piston head 14L is reduced, causing the valve V7 to open and process fluid to be exhausted into the outlet line 0 via the flowpath 62a.

Drive fluid is exhausted from the right hand end RE back via the flowpath 32 to the hydraulic power supply 18. Like the other embodiments, the switch valve 34 is adapted to switch at the end of each pumping stroke, so that on the next stroke, the flow directions will be reversed and the piston 10 will move to the left.

Fig 6 shows a further embodiment, where the piston 10 is replaced by two individual pistons 70, 80, each of which is located in a respective cylinder 72, 82. Hence, the Fig 6 embodiment comprises two single-acting pistons and respective cylinders. In this embodiment, the cylinders 72, 82 have large diameter portions 72a, 82a at their right hand ends, and small diameter portions 72b, 82b at their left- hand ends; however in alternative embodiments, the diameters could be uniform. The cylinders 72, 82 also have reduced diameter portions 77, 87, in which the piston rods are a close fit. Seals S are provided in these reduced diameter portions to prevent any fluid mixing between fluids from either side of the reduced diameter portions 77, 87.

The pistons 70, 80 each have a central rod and respective piston heads 74R, 74L and 84R, 84L. Each piston head is adapted to fit closely within the part of the bore of the cylinder in which it is located (e.g. the left hand piston head 74L fits closely within the bore of the cylinder 72 at the small diameter portion 72b) .

The small diameter portions 72b, 82b are fluidly connected via a flowpath 75, which optionally includes a tank (not shown) . The large diameter portions 72a, 82a, are connected to the inlet line I via one-way valves VlO, VIl respectively, and are connected to the outlet line O via one-way valves V9 and V12 respectively.

A hydraulic power unit assembly 68 comprises a hydraulic power unit 79 and an optionally integrated switch valve 78. In other embodiments, the switch valve 78 and the hydraulic power unit 79 may be separate components. Flowpaths 86, 88 lead from the switch valve 78 to the cylinders 82, 72 respectively. The flowpaths 86, 88 enter the cylinders 82, 72 on the left hand sides of, and close to, the reduced diameter portions 87, 77.

The switch valve 78 is adapted to change the connections of the hydraulic power unit 79 to the flowpaths 86, 88, between a first position (shown) where the drive fluid leaving the hydraulic power unit 79 is connected to the flowpath 86 and the flowpath 88 is connected to the return path for the drive fluids, and a second position, in which these connections are reversed. A further connection 81 connects the cylinders 72, 82 at a point between the reduced diameter portions 77, 87 and their respective heads 74R ,84R. Typically, drive fluid is in these portions. A drive fluid make-up line 71 is provided to supply additional drive fluid to replace any leakage of drive fluid. In use, when the switch valve 78 is in the position shown in Fig 6, drive fluid from the hydraulic power unit 79 enters the cylinder 82 between the reduced diameter portion 87 and the piston head 84L. This pushes the piston 80 to the left as shown in the drawing, which expels drive fluid from the end of the cylinder portion 82b via the flowpath 75 and into the portion 72b of the first cylinder 70. This drive fluid pushes the piston 70 to the right in the drawing.

The movement of the piston 80 to the left causes a vacuum in the right hand portion 82a of the cylinder 80, which pulls the valve VIl open, which allows process fluid from the inlet line I to enter this portion 82a. The movement of the piston 70 to the right pushes process fluid out of the right hand portion 72a of the cylinder 70, opening the valve V9, allowing this fluid to pass into the outlet line O. As the piston 80 moves to the left, drive fluid in the large diameter portion 82a is expelled through the connection 81 into the cylinder 72, where it fills the vacuum caused by the piston 70 moving to the right.

When the pistons 70, 80 have reached the end of their respective strokes, the switch valve 78 switches, so that the drive fluids flow in the opposite directions, causing the pistons 70, 80 to move in opposite directions in their next strokes. The seals S prevent any mixing between fluids on opposite sides of the reduced diameter portions 77, 87. All of the embodiments of the invention may be provided with a seal or a plurality of seals. The seals S may be rubber o-ring seals, but any type of seal may be used, e.g. piston seals, hydraulic seals, etc.

In some embodiments, labyrinth seals are provided in addition to, or instead of physical seals. The labyrinth seals may take the form of grooves in the reduced diameter portion 17. If fluid leaks between the left and right sides L, R of the cylinder 16, it has to pass these grooves. On entering the grooves, turbulence is created, which restricts the flow. Therefore, labyrinth seals are pressure-seals, which can maintain different locations at different pressures, even if they do not totally restrict flow between these locations.

Labyrinth seals can be advantageous for long-term use, for example, for subsea .use, where pumping systems are required to work for periods of four of five years or more without servicing. In such long- term use, physical seals might wear out after the first year or so. In contrast, the labyrinth seals will not wear out and will continue to provide a pressure-sealing effect. Some embodiments may use a combination of labyrinth seals and piston rings.

Fig 7 shows the Fig 5 pumping system, with some additional components. Like parts are designated with like numbers. The Fig 5 assembly is contained within a pressure containment housing H; such embodiments can be used in different environments, for example, subsea. In subsea use, the housing H ensures that hydrocarbons being pumped do not leak into the environment. All operational equipment is typically contained within,the housing H. Where breaks in the housing occur, to allow piping connections for the process fluid to enter and leave, etc, these must be strictly sealed and controlled to ensure than no leakage can occur. The housing H typically has the form of a series of large, equipment-containing pipes interconnected with smaller process fluid exchange pipes.

The housing H is filled with the drive fluid at a pressure P3. The drive fluid is typically water, or a mixture of water and glycol. The hydraulic power unit 18 takes drive fluid from the general volume inside the housing H and returns the exhausted drive fluid to this volume.

It is preferable that any leaks occur in the direction from the drive fluids on the outer sides of the piston heads 14R, 14L to the process fluids on the inner sides of the piston heads 14R, 14L. In this way, the leaked drive fluids will simply pass into the outlet line 0 with the process fluids. If the reverse were true, the process fluids would leak into the drive fluids filling the housing H, increasing the pressure, which may increase the risk of a leak into the environment. To ensure this, the pressure of the drive fluids is typically held higher than or equal to the pressure of the process fluids.

The pressures in the system will now be explained for the Fig 7 embodiment with the switch valve 34 in the illustrated position (piston 10 moving to the right) .

On the left of the piston head 14L, the pressure of the drive fluids is Pd, which is the pressure of the drive fluids on leaving the hydraulic power unit 18. These drive fluids are pushing the piston head 14L to the right. On the right of the piston head 14L, the process fluids are resisting the movement of the piston 10 to the right. The process fluids are flowing into the output O, so their pressure is the process outlet pressure P0.

Likewise, on the left of the piston head 14R, the process fluids from the inlet line I are pushing the piston head 14R to the right. These process fluids have a pressure Pi, which is the process fluid inlet pressure. Resisting movement of the piston 10 are the drive fluids on the right of the piston head 14R. These drive fluids are flowing into the volume inside the housing H, so these drive fluids have the same pressure as the drive fluids in the housing H, which is P3.

For any leakage to be from the drive fluids to the process fluids: Pd≥P0 and Pt≥P, However, there is also another relationship between the pressures, because these are not independent pressures, due to the need for equilibrium forces on the piston.

The force equation is:

Force = pressure x area

For equilibrium, the overall force on the piston 10 must be equal. It is useful if the system is close to equilibrium, because the piston 10 must be able to move in both directions depending on the orientation of the switch valve 34. If the piston 10 were heavily biased one way or the other, movement in the opposite direction would be extremely difficult and the system would not be so efficient.

The areas on the outer sides of the piston heads 14R, 14L (on which the drive fluids act) are AbOEe- The areas on the inner sides of the piston heads 14R, 14L (on which the process fluids act) are Aannuius, which is smaller than Abore by an amount equal to the area of the piston rod. 1 The total force to the right is caused by the drive 2 fluids to the left of the piston 14L and the process 3 fluids to the left of the piston 14R, which is: 4 EL P Λ a. P A ^ o cumulus s boi e 6 7 and likewise, the total force to the right is: 8

0 1 Setting these forces equal to each other (for 2 equilibrium) and substituting Pd = P0, then: 3

+ 5 6 and rearranging for P3 gives : 7

9 0 Therefore, starting from Pd = P0^ for an overall 1 movement to the right, then P3 must be less than or 2 equal to the right hand side of the above equation. 3 For subsea applications, P1 is constant, as this is 4 the well pressure, so to ensure that Ps is suitably 5 low, Pd (=P0) can be increased. 6 7 As explained above, Pd should preferably be greater 8 than or equal to P0, to ensure that any leakage 9 occurs from the drive fluid into the process fluids. 0 However, Pd should preferably not be very much larger than P0, otherwise a large amount of leakage will occur. Typically, in use, Ps is increased until Pd is just larger than P0, to provide the ideal working conditions.

Alternatively, if the substitution P3 = Pi is made and a similar rearrangement is performed:

P _ D I ■ "n.a. nnuhu ( p τ> \ Λbore

The above equations all apply to the embodiments where the drive fluids are on the outer sides of the piston heads 14R, 14L and the process fluids are on the inner sides. In the embodiments where the reverse is true (e.g. Fig 1), the equations will be:

For Pd = P0

and for P3 = P±

Pd = i> + -^-(P0_i>) anmiliis using the same principals.

It is advantageous if the hydraulic power unit 18 is of such a design that it is impossible to over- pressurise the housing H.

The pressure at the pump discharge, Pd, is equal to the housing pressure P3 plus the pressure increase generated by the hydraulic power unit (call this

'

Therefore, if:

Ps + PHPU (MaxCont Pr essureHouse

(where MaxContPressureHouse means the maximum containment pressure of the housing) there is no danger of the pump generating enough pressure to overcome the maximum containment pressure of the housing H, and the system is very safe, with a greatly reduced risk of hydrocarbons leaking into the environment.

This can be ensured by selecting a hydraulic power unit 18 with which it is physically impossible to generate pressures above a certain value. The strength of the hydraulic power unit and the required housing pressure Ps can both be selected to satisfy this condition.

An example of a suitable hydraulic power unit 18 that can only generate up to a maximum pressure is a centrifugal pump. Therefore, embodiments of the invention provide a safe system suitable for subsea use and in other situations where it is vital not to over-pressurise the housing H. Since PHpo, Ps and any other relevant parameters can be controlled at the design stage, embodiments of the invention provide an inherently safe pumping system that can be operated in remote locations with a minimum of monitoring.

Other features shown in the Fig 7 embodiment include a drive fluid conduit in the form of make-up line 89. The make-up line 89 typically comprises a glycol make-up line 89 or a glycol/water mix make-up line. As explained above, the drive fluids may leak into the process fluids and be removed from the housing H through the outlet line O. If this happens, it is desirable to replenish these fluids, and this is done by the make-up line 89. The make- up line 89 is very well sealed where it enters the housing H, so that no leakage from the housing H will occur. The make-up line 89 can be used to control the pressure P3 in the housing H. Optionally, a valve (not shown) is provided to control the housing pressure P3 between certain threshold values.

A rupture disc vessel 90 is also provided inside the housing H. The rupture disc vessel 90 comprises a hollow tube, sealed at one end; the opposite end has a rupture disc, which will burst at a set pressure. A pressure sensor is located within the tube, which is sealed at atmospheric pressure. If the pressure within the housing H exceeds the rupture disc "set" pressure, the disc will fail, allowing fluids into , the rupture disc vessel 90. The pressure sensor would then sense a pressure increase, allowing corrective action to be taken. Some embodiments do not include a rupture disc vessel 90. An inflow damper 92, an outflow damper 94, and a pump flow damper 96 are also provided coupled to the inlet line I, the outlet line 0 and the hydraulic power unit 18 respectively. These devices can temporarily store a quantity of fluid (e.g. from a surge) and release this fluid back into the system in a controlled manner. Suitable dampers 92, 94, 96 include hydraulic accumulators, but other dampers may also be used.

A pump relief valve 97 is optionally provided in the flowpaths 20a, 20b to and from the hydraulic power unit 18. The pump relief valve 97 has a first position in which allows flow through the flowpaths 20a, 20b as normal (shown in Fig 7) and a second position in which the flow from the hydraulic power unit 18 is short circuited back to the hydraulic power unit 18. The pump relief valve 97 prevents the drive pump generating excessive pressure. Once a certain pressure is reached, the pump relief valve 97 moves, dumping the pump flow back into the housing H. The pump relief valve 97 is reset by an external signal. Even if the hydraulic power unit 18 is chosen such that is cannot produce dangerously high pressures, the hydraulic power unit 18 may still overheat if allowed to operate continuously at high pressures. Therefore, the pump relief valve 97 can be used to prevent this from happening.

A process flow valve 98 is optionally provided in the outlet line O. The process flow valve 98 receives a signal from the hydraulic power unit outlet. The process flow valve 98 has a first position in which flow to the outlet line 0 is allowed, and a second position (shown) in which flow to the outlet line 0 is prevented. The process flow valve 98 allows flow of process fluids to the outlet line 0 (the first position) only when a certain threshold pressure is detected at the hydraulic power unit outlet line. The process flow valve 98 is also connected to the pump relief valve 97 and can control/be controlled by the pump relief valve 97.

A thermal relief valve 100 is optionally provided, which allows a small quantity of fluid to escape into the outlet line O if the housing H is over- pressurised due to temperature effects. The thermal relief valve 100 is not needed in some embodiments.

Any of the assemblies shown in any of Figs 1 to 6 and Fig 8 may be provided with a housing H and any of the additional equipment shown in Fig 7.

Fig 8 shows a further embodiment including first and second pistons 200, 300. Each of the piston arrangements is similar to the Fig 2 piston arrangement, and like parts are given similar reference numbers. A hydraulic power unit 218 is connected to and drives fluid into both the switch valves 222, 322. The first piston 200 is provided with a trigger switch 240, which is connected to a similar trigger switch 340 associated with the second piston 300. The trigger switch 240 is operatively connected to the switch valves 322 and 334.

The trigger switch 240 can typically be activated by a mechanical lever connected to the first piston 200. The trigger switch 240 is switchable between two positions. For example, the trigger switch 240 may be triggered when the piston 200 has travelled half way along its cylinder 216.

Movement of the trigger switch 240 switches the positions of the switch valves 322, 334 to change the flow directions associated with the second piston 330, and accordingly change the direction of travel of the second piston 330.

In the same manner, the trigger switch 340 is operatively connected to the switch valves 222, 234 of the first piston 200. This trigger switch 340 switches the switch valves 222, 234 when the second piston 300 reaches halfway along its cylinder. This alters the flow directions of the first piston arrangement, hence altering the direction of travel of the first piston 200.

In this way, one of the pistons will always be a quarter of a cycle behind the other piston. This means that at each quarter of a cycle, one of the pistons is halfway along its cylinder, and the other is at one of the ends. The halfway piston triggers the other piston at its end to change direction. Therefore, some embodiments of the invention provide two pistons that are permanently out of phase with each other, being triggered by each other, so that they can never become in phase with one another. This can produce an overall more continuous pumping effect, as when one piston is temporarily stationary at one end of the cylinder (in that instant not moving any process fluids), the other piston is still mid-stroke, pumping strongly.

In some embodiments, the triggering system can be provided with a timer instead of a trigger switch system, such as that described above. However, for uses in remote locations, such as for subsea use, it may be advantageous to use a purely mechanical switching system that does not rely on a timer. Timers can become inaccurate over time, so the pistons may become in phase with one another due to a timer error. Moreover, timers can break down, which may be exceedingly inconvenient if this occurs hundreds or thousands of metres beneath sea level, and furthermore, timers require electrical connections, which could themselves fail, and are at best an additional undesirable complication.

Modifications and improvements can be incorporated without departing from the scope of the invention. For example, different numbers of cylinders and pistons can be used, e.g. 3, 4, 5, or 6, or some other number, using the principles herein described. As explained above, a hydraulic power unit is not an essential element of the invention, and the drive system may not even be hydraulically powered.

In a modification to the Fig 8 embodiment, the triggering does not occur at the half way point; in alternative embodiments, the triggering could occur at any point along the piston's path, or at the cylinder ends .