Technical Field The present invention relates to a subsea hydraulic power unit. Background of the Invention

Subsea control systems are used to actuate subsea valves on devices such as production lines, Christmas trees and complex manifold systems. The control systems often include means to measure and report the status of particular valves being controlled, fluid pressure and so on . Control systems may also be automated to ensure that subsea valves are automatically closed under certain predetermined conditions.

Electrical power is not typically used to operate subsea process valves, and so subsea control systems typically use hydraulic power. Hydraulic power may be provided directly from a surface location (either shore or platform-based). This is typically adequate where only a small amount of hydraulic power is required, and the distance between the source of the hydraulic power and the valves is relatively short. However, for systems such as Christmas trees which may require considerably more power, the cost of providing sufficient hydraulic power becomes high. Furthermore, owing to long distances between the source of the hydraulic power and the valves, higher diameter conduits are required to compensate for friction losses, leading to higher costs. I n addition, longer distances between the source of the hydraulic power and the valves greatly increase the cost in providing and maintaining the lines providing the hydraulic power. In these circumstances it is desirable to provide hydraulic power directly at the subsea location. Subsea Hydraulic Power Units (SHPUs) of various types have been proposed to provide hydraulic power close to the valves. SHPUs use mineral oils to provide hydraulic power. GB 2421 530 describes a hydraulic control assembly having a subsea intensifier and electric motor. WO 2009/0451 10 describes an electrically-driven hydraulic pump unit having an accumulator module for a subsea control system. A single stage pump in the form of a piston is described that uses a mineral oil as a hydraulic fluid.

A problem with existing SHPUs is that they require piston pumps to generate a required pressure and flow of hydraulic fluid. Piston pumps use mineral oils or other synthetic hydraulic fluids. However, over a long period of time the hydraulic fluids will be contaminated with fluids such as methane gas, and eventually mix and form an emulsion, degrading the performance of the SHPU or causing failure. As distances between a platform and subsea production sites increases, and more marginal reserves are developed, SHPUs become a viable alternative to providing hydraulic power from a platform or shore facility, but the risk of contamination of the hydraulic fluids with other fluids limits the lifetime of a SHPU.

It is known to use water-based hydraulic fluids, but these must be provided from a platform or the shore, as water based piston pumps that can provide adequate pressure to the hydraulic fluids use a separate mineral oil reservoir to handle lubrication of the crank and bearings. There is currently no existing concept for a water based SHPU.

Summary of the Invention

It is an object of the invention to provide a Subsea Hydraulic Power Unit that uses water based hydraulic fluid. According to a first aspect, there is provided a subsea hydraulic power unit (SH PU) comprising an electrically powered rotodynamically operated pump for powering a lower pressure first hydraulic circuit. A pressure amplifier is also provided that amplifies the lower pressure provided from the first hydraulic circuit. A higher pressure second hydraulic circuit powered by the pressure amplifier is also provided. This allows the much lower pressures typically generated by a rotodynamic pump (for example, a centrifugal pump) to be amplified to a useful pressure for subsea applications, and therefore allows the use of rotodynamic pumps which do not necessarily need to use mineral oils. The lower pressure first hydraulic circuit optionally operates at a pressure below 20 bars, and the higher pressure second hydraulic circuit optionally operates at a pressure between 200 and 350 bars. The SH PU is optionally provided with an outlet for the higher pressure second hydraulic circuit to provide hydraulic fluid for actuating valves in a subsea hydrocarbon well.

The hydraulic fluid used in any of the first and second hydraulic circuits optionally comprises a water-based fluid , for example a mixture of water and monoethylene glycol.

As an option, the pressure amplifier of the SHPU comprises a piston rod and a first piston attached to the piston rod to drive the piston rod using a low pressure hydraulic fluid in a first hydraulic circuit. An inductive body is connected to the piston rod. Aa first sensor and a second sensor are provided, and the proximity of the inductive body to the first sensor causes application of a hydraulic pressure in the first hydraulic circuit in a first direction, whereas a proximity of the inductive body to the second sensor causes application of a hydraulic pressure in the first hydraulic circuit in a second direction opposite to the first direction. This causes a reciprocating motion of the piston rod. The hydraulic pressure amplifier further comprises a second piston attached to the piston rod, wherein a motion of the piston rod drives the second piston to produce a high pressure fluid flow in a second hydraulic circuit. As an option, the SHPU further comprises a microprocessor for receiving signals from the first and second sensor. A control valve is provided for controlling a direction of pressure in the first hydraulic circuit. The microprocessor is arranged to control the control valve in response to signals received from the first and second sensor. According to a second aspect, there is provided a method of providing subsea hydraulic power An electrically powered rotodynamically operated pump is operated to power a lower pressure first hydraulic circuit. The lower pressure is amplified using a pressure amplifier and use to operate a higher pressure second hydraulic circuit powered by the pressure amplifier. Accord ing to a third aspect, there is provided a hydraulic pressure amplifier for converting a low pressure fluid flow to a high pressure fluid flow. A piston rod and a first piston attached to the piston rod to drive the piston rod using a low pressure hydraulic fluid in a first hydraulic circuit are provided. An inductive body is connected to the piston rod, and a first sensor and a second sensor are provided. A proximity of the inductive body to the first sensor causes application of a hydraulic pressure in the first hydraulic circuit in a first direction, and a proximity of the inductive body to the second sensor causes application of a hydraulic pressure in the first hydraulic circuit in a second direction opposite to the first direction. This leads to a reciprocating motion of the piston rod. The hydraulic pressure amplifier further comprises a second piston attached to the piston rod, wherein the reciprocating motion of the piston rod drives the second piston to produce a high pressure fluid flow in a second hydraulic circuit. As an option, there is also provided a microprocessor for receiving signals from the first and second sensors. A control valve controls a direction of pressure in the first hydraulic circuit and the microprocessor is arranged to control the control valve in response to signals received from the first and second sensors. The low pressure hydraulic fluid is optionally pressurized by a centrifugal pump.

As an option, the second hydraulic circuit provides high pressure hydraulic fluid for actuating valves in a subsea hydrocarbon well. According to a fourth aspect, there is provided a method of controlling a pressure amplifier as follows:

a) a piston rod is driven by a piston operatively connected to a low pressure first hydraulic circuit, the piston rod being connected to an inductive body;

b) low pressure hydraulic fluid is pressurized in a first direction in a first hydraulic circuit to cause a movement of the piston rod in a first direction;

c) a signal is received from a first sensor indicating a proximity of the inductive body to the first sensor;

d) the application of hydraulic pressure in the first hydraulic circuit is reversed; e) a signal is received from a second sensor indicating a proximity of the inductive body to the second sensor;

f) an application of hydraulic pressure in the first hydraulic circuit is reversed, and steps c to f are repeated to cause a reciprocating motion of the piston rod,

g) a second piston is attached to the piston rod, such that a reciprocating motion of the piston rod drives the second piston to produce a high pressure fluid flow in a second hydraulic circuit.

According to a fifth aspect, there is provided a program for controlling a computer device to perform the method as described above in the fourth aspect.

According to a sixth aspect, there is provide a computer readable medium in the form of a memory, wherein the program described above in the fifth aspect is stored on the memory.

Brief Description of the Drawings

Figure 1 illustrates schematically in a block diagram the components of an embodiment of the invention;

Figure 2 illustrates schematically in a block diagram a subsea hydraulic power unit according to an embodiment of the invention;

Figure 3 is a graph illustrating pressure against flow rate for a rotodynamic pump operating at different speeds;

Figure 4 illustrates schematically in a block diagram a hydraulic circuit according to an embodiment of the invention; Figure 5 is a flow diagram showing steps of an embodiment of the invention; and

Figure 6 illustrates schematically in a block diagram a computer device according to an embodiment of the invention. Detailed Description of the Invention

There is provided an SHPU that uses a water-based hydraulic fluid that can replace conventional hydraulic supply lines and distribution systems, reducing the need to fit and maintain supply lines. The SHPU generates hydraulic power at the production site, rather than on the platform, or the shore.

Figure 1 illustrates an SHPU according to an embodiment of the invention. There is provided an electrical power supply 1 and a regulator 2 that supplies a three-phase electric motor 12 with power having variable voltage and frequency, known as a variable speed drive (VSD). A microprocessor 3 is provided for controlling the SHPU. An interface circuit 4 is connected by wires 6, 7 to position sensors 8, 9. A modem 5 is connected via cables 10, 1 1 to a central control station 21 , 22 that may be located on a platform or shore facility.

The three-phase electric motor 12 powers a centrifugal pump 13 or other type of pump having a rotodynamic action. A mechanical coupling 14 is used to transmit force from the electric motor 12 to the centrifugal pump 13. The centrifugal pump 13 is directly connected to the hydraulic system, which includes a hydraulic sump 15. A control valve 16 is provided between the centrifugal pump 13 and a pressure amplifier 17. An example of a valve that can be used is a 4/2 valve, which has four ports and two positions. Hydraulic power is output via a high pressure port 20. A hydraulic accumulator 18 and a check valve 19 are located between the centrifugal pump 13 and the pressure amplifier 17. An example of a suitable hydraulic accumulator 18 that may be used is a nitrogen-precharged hydraulic accumulator.

The electric motor 12 drives the centrifugal pump 1 3, which pumps a water-based hydraulic fluid. A limitation of the centrifugal pump 13 is that it generates a high flow of hydraulic fluid at low pressure. This combination of flow and pressure is unsuitable for subsea hydraulic control. The pressure amplifier 17 is therefore provided to convert the fluid flow from high flow, low pressure to low flow, high pressure, which is suitable for subsea hydraulic control. A typical requirement for the high pressure hydraulic circuit serving a single well is 345 bars at less than 10 litres per minute. The output flow of fluid may be stored in the hydraulic accumulator 18. For deep water applications where nitrogen pre-charged accumulators are inefficient, a portion of the energy storage may be provided by means of an electrical battery.

The SHPU may be used for new field development projects (often referred to as green fields), or as a retrofit in older installations (often referred to as brown fields). These different applications may have different requirements for the SHPU.

For green field development projects, the SHPU may require a small, low cost, trickle charge hydraulic conduit in the controls umbilicals such that minor losses of fluid may be compensated. Fluid returned from downhole functions may be discharged to the ambient sea such that methane gas often found in the hydraulic lines is dumped in the sea and not in the hydraulic sump. The amount of gas discharged to the ambient sea is very small, but such a small amount of gas would nevertheless have a significant negative impact on the hydraulic circuitry.

For brownfield projects, a trickle charge line is rarely available. All return fluid must be returned to the sump 15 to prevent loss of fluid. Such HPUs will normally suffer from a gas contamination of the fluid. Techniques are under development to remove such gas, but these are out of the scope of the present discussion.

As described above, existing SHPUs generate around 200 - 350 bar pressure in a single stage by means of a piston pump operating using mineral oil or synthetic fluid. Piston pumps are also available for water based fluids, but require an isolated volume of a lubricating fluid for the crank and bearing parts. The inventors have realised that a limiting factor on the life of an SHPU is that it is difficult to keep the hydraulic fluid and surrounding fluids (from the sea or downhole environment) apart over long periods of operation, and so eventually the fluids will form an emulsion, which has a negative impact on the performance of the SHPU. The present SHPU uses one type of fluid. An example of such a fluid is a mixture of water and monoethylene glycol (MEG). Instead of piston-based pumps, it is proposed to use a centrifugal (or other type of rotodynamic) pump in order to use a water-based hydraulic fluid. Such pumps can make use of hydrodynamic or hydrostatic bearings. However, as described above, centrifugal pumps produce a low pressure at high flow, while a subsea control system requires high pressure and low flow. The pressure amplifier 17 is therefore used to transform high flow low pressure (for example 200 litres/minute at 10 bar) to a low flow at high pressure (for example, 10 litres/minute at 200 bar). Pressure amplifiers are typically based on linear, reciprocating devices based on a differential piston effect. These types of pressure amplifier have the advantage that almost any type of hydraulic fluid can be used.

A reciprocating pressure amplifier consists of three types of components:

1 . A system of differential pistons where the low pressure fluid acting on the large piston surfaces (engines) moves smaller pistons (pumps) generate high pressure; 2. Check valves to isolate the volumes of fluid at different pressure levels; and 3. A mechanism to provide a turning movement.

The first two parameters are simple to implement, but the mechanism to provide a turning movement of the pump can be problematic when associated with inversion at extremely low speeds. Complex mechanical devices are used to handle inversion at low speeds, but such devices can fail over the lifetime of the SHPU as they have many moving parts in contact with one another.

In order to address the problem of complex mechanical devices to provide a turning movement in the pressure amplifier 17, it is proposed to instead use inductive position sensors 8, 9. Turning to Figure 2, there is illustrated a pressure amplifier 17in a SHPU. The pressure amplifier is provided with a piston rod 24 attached at one end to a metal mass 23. The metal mass 23 is located adjacent to position sensors 8, 9. Piston rod 24 is connected at the other end to a piston 25 (there are two pistons, only one is numbered for clarity) located in a hydraulic cylinder 27 (there is a further hydraulic cylinder 26). Check valves 30, 31 , 32, 33, 40, 41 , 42, 43 are provided.

Inductive position sensors 8 , 9 a re electronic proximity sensors, which detect the proximity of the metal mass 23. Because the sensors 8, 9 sense the proximity of the metal mass 23, there are no problems relating to lubricating moving parts, handling issues arising from friction, and parts wearing out.

Control valve 16 pumps high flow, low pressure fluid from the centrifugal pump 13 in a first direction towards piston 26. This causes the piston rod 24 to move the metal mass 23 towards sensor 9. When sensor 9 detects the proximity of the metal mass 23, it and sends a "stop" signal to the microprocessor 3. The microprocessor 3 will then instruct the control valve 16 to pump high flow low pressure fluid flow from the centrifugal pump in an opposite direction and towards piston 27 in order to reverse the direction of motion of the piston rod 24.

Piston rod 24 is now moving in an opposite direction to its original direction, such that the metal mass 23 moves towards sensor 8. Sensor 8 detects the proximity of the metal mass 23 and sends a "stop" signal to the microprocessor 3. The microprocessor 3 instructs the control valve 16 reverse the direction of motion of the piston rod 24 such that the metal mass 23 moves towards sensor 9.

In this way, the sensors 8, 9 detect the proximity of the metal mass 23 and the piston rod 24 is moved in a reciprocating motion. This in turn drives a smaller piston 44 in a reciprocating motion. As the smaller piston has a smaller area than the larger pistons 26, 27, but is moved with the same force, it can provide a higher pressure, lower flow fluid hydraulic fluid. In this way, hydraulic fluid acted on by the small piston 44 will have a higher pressure than the hydraulic fluid from the centrifugal pump 13, and so the pressure is amplified.

Control signals from the microprocessor 3 are used to activate solenoids 38, 39 to actuate valves 36, 37 respectively. Valves 36, 37 control the direction of flow of high flow, low pressure fluid from the centrifugal pump 13 towards the piston rod 24. Note that Lp in Figure 2 is the high pressure side of the pressure amplifier (typically around 207 to 345 bar). I n the context of a SHPU, this is referred to as Lp, low pressure, to distinguish it from Hp, high pressure which may be up to 690 bar or higher and used for the operation of down hole safety valves (DHSVs). However, it should be I remembered that this is the increased pressure output from the pressure amplifier. Typically, in control systems for subsea oil and gas production, low pressure is either 207 bars or 345 bars, and high pressure is between 345 and 690 bars. In known systems, both low pressure and high pressure are generated topside (either on a rig or shore) and transferred subsea where needed. It is possible to generate high pressure locally from the low pressure using all-mechanical pressure intensifiers. However, it is difficult to generate the low pressure (207 or 345 bars locally), and so this still needs to be provided via umbilicals. The present concept allows the generation subsea of pressures of typically 1 0 to 1 5 bars in a single stage centrigual (or other type of rotodynamic) pump. A pressure amplifier is then used to generate 207 or 345 bars from the input 10-15 bars. The pressure generated by the centrifugal pump is therefore much lower than conventional "low pressure", and may be thought of as "ultralow pressure". I n Figure 2, the pressure amplifier receives hydraulic fluid from the centrifugal pump 13 at ultralow pressure and amplifies it to low pressure. In use, the electric motor 12 drives the centrifugal pump 13 to suck the water-based hydraulic fluid from sump 15. Check valve 19 ensures that the hydraulic fluid can only move from the pump outlet and into the accumulator 18 (which assists in equalizing pressure surges). Control valve 1 6 is part of an electro-hydraulic monitoring and control system for the SHPU, and contributes to direct hydraulic fluid to the side of the pressure amplifier 17 that is used to move valves.

The pressure amplifier 17 described above is by way of example only, and it will be appreciated that other types of pressure amplifier and valve arrangements may be used. For example, Figure 2 shows a double-acting unit with two motors and two pumps. It will be apparent that a single-acting device could be used. Likewise, the exemplary control valve 16 may be a 4/2 valve (4 ports, 2 positions), but other types of valve, such as a 3/2 valve, may be used.

Referring to Figure 3 herein, it is noted that operation of a centrifugal pump is more complicated than operation of a fixed displacement pump. Figure 3 shows pressure P against flow rate Q for a centrifugal pump operating at different speeds. The centrifugal pump should be operated to the right of the line 60 in Figure 3 in order to ensure stable operation. If th e pump were operated to the left of the line, the relationship between pressure and flow is not necessarily unique; two different flow rates may correspond to the same pressure level, leading to oscillations. Lines 61 , 62, 62, 62 are examples of P/Q characteristics obtained by operating the centrifugal pump at different speeds (rpm). Most centrifugal pumps fail under conditions of zero flow and higher pressure, typically caused by a blocked outlet. This can set certain requirements for procedures and equipment for the start of the pump when used as part of an SHPU. Referring to Figure 4, it may be desirable to have a bypass valve 76 when the pump is started to secure a flow from the pump initially. This bypass valve 76 can then be closed after a few seconds to build up pressure in the discharge. A hydraulic restriction 77 may be provided to allow pressure build-up. Techniques such as this, to ensure initiation of a hydraulic circuit, are well known and so are not described further.

It is important to ensure that the liquid passing through the SHPU remains clean, as filters have a finite capacity. Clean liquid ensures that the filters do not become clogged or require changing as often. Filters must be optimized in order to achieve long times between necessary interventions to clean or change the filters.

An inherent advantage of the proposed SHPU over known types of SHPU is that a centrifugal pump can generate small metallic particles compared to a high pressure piston pump used in known SHPUs, as piston pumps have more areas with direct metal contact. A centrifugal pump will therefore maintain clean liquid for a much longer period than a piston pump. A low-pressure circuit including centrifugal pump 13, piston engines 25, 26 and control valve 16 can work for long periods of time with relatively coarse filtration, while the bearings for the motor and pump (not shown) need fairly clean water. It may therefore be suitable to provide a separate circuit with a finer filter 70 to the hydrodynamic / hydrostatic bearings and valve control, such as 3 micron fineness. On the other hand, filter 73 for supplying fluid to the load 74 (i.e. the actuators) needs to be physically coarse

Turning to Figure 5, a flow diagram illustrates steps of an embodiment of the invention, with the following numbering corresponding to that of Figure 4: S1 . Low pressure hydraulic fluid is provided in a first hydraulic circuit by the centrifugal pump 13 to the control valve 16. S2. The control valve moves the piston rod 24 in a first direction so that the metal mass 23 moves towards sensor 9. This in turn drives the smaller piston 44 to provide high pressure hydraulic fluid in a second hydraulic circuit.

53. The metal mass 23 moves into proximity with sensor 9.

54. Sensor 9 detects the metal mass 23 and sends a signal to the microprocessor 3, which sends a signal to the control valve 16 to reverse the application of pressure in the low pressure first hydraulic circuit. S5. The piston rod 24 moves in a second direction opposite to that of the first direction so that the metal mass 23 moves towards sensor 8.

56. This in turn drives the smaller piston 44 to provide high pressure hydraulic fluid in the second hydraulic circuit.

57. The metal mass 23 moves into proximity with sensor 8.

58. Sensor 8 detects the metal mass 23 and sends a signal to the microprocessor 3, which sends a signal to the control valve 16 to reverse the application of pressure in the low pressure first hydraulic circuit, and the process reverts to step S2.

Turning now to Figure 6, there is illustrated a computer device 65. The computer device is provided with one or more receivers 66 to receive signals from the sensors 8, 9 indicating the proximity of the metal mass 23. The microprocessor 3 receives the signals and sends control signals using a transmitter 67 to the control valve 16 to control the direction of the application of pressure in the low pressure hydraulic fluid in the first hydraulic circuit. A memory 68 in the form of a computer readable medium may also be provided. In this case, a program 69 may be stored at the memory 68. The program 69, when executed by the microprocessor 3, causes the microprocessor 3 to implement the method described above with reference to Figure 5.

A SHPU that uses the pressure amplifier described above can be used for retrofits and repairs without the need to retrieve Umbilical Termination Heads or manifolds, and more importantly can be used to replace expensive shore or platform based hydraulic conduits in a control umbilical. The use of a centrifugal pump and a pressure amplifier allows water-based hydraulic fluids to be used, which avoids the problems associated with piston pumps that use mineral oils.

It will be appreciated by the person of skill in the art that various modifications may be made to the above described embodiment without departing from the scope of the present invention. For example, two pistons are shown as being driven by the control valve to drive the piston rod and, in turn, drive the smaller piston for developing high pressure hydraulic fluid. However, it will be appreciated that the same effect may be achieved using only one piston. Furthermore, while the pressure amplifier allows the use of a centrifugal pump, and water-based hydraulic fluids, it can be use with other types of rotary or linear pumps, and hydraulic fluids based on fluids other than water.