NO20100903A1 - A pressure control system for motor and pump barrier fluids with differential pressure control - Google Patents

Description

Pressure regulation system for motor and pump barrier fluids with differential pressure control

Field of the invention

The present invention generally relates to subsea equipment involved in the transport of process fluids that are generated in subsea hydrocarbon production. More specifically, the present invention relates to a system designed to handle barrier and lubricating fluid pressure in a submarine engine and pump module.

Background and known technique

A process fluid in subsea hydrocarbon production is typically a multiphase fluid comprising oil and gas and possibly solids, which is extracted from an underground reservoir. A motor/pump module is arranged on the seabed and configured to transport the process fluid from the reservoir to a host facility on the surface or on land. The motor/pump module is often exposed to significant variations in pressure in the pumped medium, in addition to significant load changes during start and stop sequences in pumping, for example. The average pressure on the suction side of the pump can be in the order of several hundreds of bars, which requires corresponding countermeasures in the motor/pump module to prevent process fluids and particulate matter from migrating from the inside of the pump into a motor casing via bearings and seals in the motor/pump module.

Screw rotor pumps are often used for the purpose of pumping multiphase fluid in subsea production. The screw rotor pump is a type of displacement pump, with two screw shafts that are rotated by cooperating gears, where a specific volume of fluid is displaced in the axial direction between the screws, from an inlet side of the pump to an outlet on the pressure side of the pump. The screws are stored in bearings in a pump housing and are drive connected with a motor arranged in a motor housing. In the case of a two-rotor screw pump, interlocking timing gears on the screw shafts ensure synchronization of the rotary motion. The motor housing is hydraulically separate from the pump housing by a sealing arrangement, the drive shaft being supported to extend to connect with the pump rotor shaft. The pump bearings are separated from the pump medium by sealing devices at both ends of the pump.

For lubrication and cooling purposes, as well as to prevent the penetration of seawater and pumped medium into the structures of a subsea engine and pump module, the engine/pump module must be supplied with hydraulic fluid. In this connection, barrier fluid and lubricating fluid are basically different definitions of the same type of fluid used to protect the motor/pump module internally.

Consequently, a motor housing that protects the motor from the surrounding sea and from the medium in the pump must be maintained at a pressure above the internal pressure in the pump, so that it also acts as a barrier that prevents the ingress of process fluid and particles into the motor housing via sealing and the storage facility. As a result of the pressure difference, it is unavoidable that a leakage flow of hydraulic fluid occurs along the drive shaft. The leakage rate is dependent on fluid properties, pressure difference, the alternating operating conditions of the pump and the tightness of the seal(s). The leakage is compensated by topping up the engine housing from an external supply of hydraulic fluid.

Likewise, hydraulic fluid is typically also supplied to the pump for lubrication of its internal structure such as pump rotor bearings, seals and register gears. The pressure in the lubricating fluid circuit of the pump must be kept greater than the pressure in the medium that is displaced through the pump, in order to prevent the penetration of process fluid and particles into pump bearings, seals and register gears. Leakage via the pump seals into the pumped medium is compensated by top-up from an external supply of hydraulic fluid.

Motor and pump can be drive connected inside the motor housing

or outside the motor housing. For example, motor and pump can have one and the same shaft in common, without any separate coupling connecting them in a drive context. In other designs, the pump shaft may be connected to the motor shaft inside the motor housing. In still other constructions, the motor and pump can be driven together by means of a coupling located in a coupling chamber located between the motor housing

and the pump. In all alternatives, however, it is desirable at all times to maintain a pressure difference across the interfaces, i.e. between the motor casing, the coupling chamber when present, and respectively the pump lubrication system and the pumped medium.

Usually, both an engine barrier fluid and a pump lubrication fluid are supplied via an umbilical cord, and leakage compensation and pressure regulation are taken care of using directional control valves on the surface installation. As an increasing number of subsea hydrocarbon production sites are installed and commissioned at ever greater depths and longer distances, response times and control requirements for lubrication and cooling systems will increase accordingly. Consequently, there is an increasing need for a pressure regulation system for motor and pump barrier fluids that operates with an immediate response to a change in pressure in the motor and pump module and that provides increased operational reliability.

Summary of the Invention

The present invention therefore aims to provide a pressure regulation system for motor and pump barrier fluids for a submarine motor and pump module which avoids the problems with hitherto known systems, and specifically such problems which are linked to large distances and great ocean depths.

The present invention specifically aims to provide a pressure regulation system for motor and pump barrier fluids for a subsea motor and pump module, where the system has a built-in ability to compensate for loss of hydraulic fluid resulting from leakage via seals and bearings in the motor and pump module. A further aim of the present invention is to provide a pressure regulation system for motor and pump barrier fluids where a preset pressure difference between a motor barrier fluid circuit and a pump barrier fluid circuit is automatically maintained at all times.

As used in this context, the engine barrier fluid circuit shall be understood to include the fluid volumes in the engine casing cavity and its associated components and flow lines involved in regulating the fluid pressure in the engine casing cavity. Correspondingly, the pump barrier fluid circuit must be understood so that it at least includes the fluid volumes in the coupling chamber cavity and its associated components and flow lines which are involved in order to regulate the fluid pressure in the coupling chamber cavity. In a more general sense, the pump barrier fluid circuit may also include fluid volumes in other cavities or passages in the pump.

The aim of the present invention has been achieved by providing a pressure regulation system for motor and pump barrier fluids for a subsea motor and pump module as the invention is defined in the attached patent claims.

In short: a pressure control system for handling motor and pump barrier fluids in a subsea motor/pump application comprises: • a barrier fluid circuit that provides fluid flow communication from a hydraulic fluid supply to a first cavity in the motor and

the pump module,

• a pressure-regulated flow control valve which regulates the supply of hydraulic fluid to the barrier fluid circuit, • a second cavity adapted to receive hydraulic fluid from the hydraulic fluid supply via the first cavity, and • a pilot pressure circuit comprising means for detecting and returning to the pressure-regulated flow control valve a pressure differential across a flow restrictor which connects first and second cavities.

An automatic pressure regulation system has thus been procured locally at the subsea motor and pump module and is characterized by its fast response and high reliability.

The system may be manifolded to maintain independent and constant pressure differentials between fluids in: an engine casing cavity (first cavity) and a cavity/passage (second cavity) in a pump structure; • an engine housing cavity (first cavity) and a cavity (second cavity) in a clutch chamber; • a cavity (first cavity) in a coupling chamber and a cavity/passage (second cavity) in the pump structure; or • a cavity/passage (first cavity) in the pump structure and the cavity (second cavity) in the pump through which the pumped medium is displaced during operation.

In other words, in relation to the first reproduced implementation, a flow control valve is used to supply fluid to the motor housing to maintain a constant pressure differential across a flow restrictor, so that in this way a constant excess pressure is maintained in the motor barrier circuit relative to the pump barrier circuit, and thus over the motor shaft seal. The flow rate through the flow control valve is controlled by the pressure differential across a flow restrictor. The result is that the flow rate in the fluid supply is always identical to the leakage rate at the defined pressure difference, regardless of variations in system pressure. If the leakage rate in the seal should increase due to wear for example, the valve will automatically increase the flow rate as needed to maintain the pressure differential across the seal When the pump pressure increases under normal operating conditions, the valve will automatically adjust the flow rate as needed to maintain a constant pressure differential across the seal. When the pump pressure is reduced under normal operating conditions, the valve will automatically adjust the flow rate to maintain a constant pressure differential across the seal.

An alternative and advantageous embodiment can be envisaged, namely that pump barrier fluid is received from the motor barrier fluid circuit via a flow restrictor with a fixed aperture diameter (orifice diameter) arranged outside the motor and pump module, the pressure difference across the flow restrictor being detected and returned to the pressure-regulated on/off valve which leads hydraulic fluid into the motor and pump barrier fluid circuits in response to detected deviations from a predefined, desired pressure differential.

In some applications, very precise control of the fluid flow rate is required in a lower flow rate range, while high maximum flow rates will be required under certain operating conditions. Both requirements are satisfied in an embodiment where two or more valves that have different flow rate ranges are connected in parallel in the fluid supply to the motor barrier circuit. More specifically, this embodiment comprises a set of valves, including at least a first and a second valve, which are arranged in parallel and supply hydraulic fluid to the engine barrier fluid circuit, and which each respond to the pressure differential across the flow restrictor, and further in that said at least two pressure-regulated flow control valves individually respond to their respective areas of pressure differential across the flow restrictor.

In a set of first and second pressure controlled flow control valves, the first valve is preferably set to open for hydraulic fluid flow into the engine barrier fluid circuit as a result of a pressure differential equal to or less than a first predetermined pressure differential across the flow restrictor, and the second valve is preferably set to open to allow hydraulic fluid flow into the motor barrier fluid circuit as a result of a pressure differential equal to or less than a second predetermined pressure differential lower than the first predetermined pressure differential.

In many subsea hydrocarbon production applications, the first predetermined pressure differential may be set at about 5 bar, and the second predetermined pressure differential may be set at about 4.5 bar.

In the set of first and second pressure controlled flow control valves arranged in parallel to feed hydraulic fluid into the motor barrier fluid circuit, the first valve may be sized for lower flow rates and the second valve may be sized for higher flow rates. For example, the first valve may be sized for flow rates in the range down to about 0.3 l/min and the second valve may be sized for flow rates in the range up to about 100 l/min or more.

If required, an on/off valve can be connected in series with the second pressure regulated flow control valve with a higher flow rate, to regulate any internal leakage in this valve.

For safety reasons, a pressure-regulated safety relief valve may be arranged in parallel with the flow restrictor to drain hydraulic fluid from the engine barrier fluid circuit into the pump.

The pressure regulation system for motor and pump barrier fluids i

present invention can advantageously be used in a subsea motor and pump module which comprises a pump motor placed in a motor housing, a pump placed in a pump housing with a pump inlet on a suction side and a pump outlet on an outlet side of the pump, as well as a

pump-rotor assembly with a double screw arranged between them and stored in bearings in the pump casing. The pump-rotor assembly is drive-coupled to the motor by an extending drive shaft

between the motor and the pump via a sealing arrangement and is configured to displace a fluid medium from the pump inlet to discharge via the pump outlet.

Brief description of the drawing figures

Embodiments of the invention will be described in more detail below with reference to the drawings. The drawing figures show: Figure 1 is a diagram illustrating a pressure regulation system for motor and pump barrier fluids in a first embodiment, and Figure 2 is a corresponding diagram illustrating a second embodiment of the pressure regulation system.

Detailed description of preferred designs

In the drawings, a subsea motor and pump module comprises a motor/pump assembly generally labeled 1, to which motor barrier fluid and pump barrier fluid are supplied from an external hydraulic fluid supply.

Because the invention is not limited to any specific type or model of motor/pump assembly, but is actually applicable to various motor/pump configurations involved in transporting a process fluid from subsea hydrocarbon production, configurations that will be known to those skilled in the art, the details of motor- /pump assembly not to be reviewed in detail here. In general, the motor/pump assembly comprises a motor enclosed in a pressurized, watertight container or motor housing 2, in addition to a pump-rotor assembly enclosed in a pump housing 3. The motor driving the pump is typically an electric motor, although other drive units such as hydraulic motors or turbines alternatively can be used.

The pump rotor is configured for displacement of a pumped medium that enters the pump via a pump inlet 4 to discharge via a pump outlet 5, as illustrated by an arrow F. The pump rotor is drive-connected to the motor in a coupling chamber 6, which is internally hydraulically separated from the pressurized (typically oil-filled) the motor housing by means of a sealing device 7 which seals against the outside of a rotating shaft with which the pump rotor is connected to the motor. The pump rotor is further stored in bearing devices in the pump casing 3. Internally in the pump, pump barrier fluid is typically circulated to lubricate internal structures in the pump, such as bearings, seals, any register gears, etc., which also provides a barrier against the medium that passes through the pump.

Motor barrier fluid and pump barrier fluid are supplied to the subsea motor and pump module from an external supply (on the surface offshore or on land) of hydraulic fluid via the hydraulic fluid supply line 8. All other components of the system are installed subsea. Engine barrier fluid is supplied to the engine casing via a flow control valve 9 which regulates the flow of hydraulic fluid into an engine barrier fluid circuit 10 which leads into the engine casing.

Pump barrier fluid is received in the coupling chamber 6 from the engine housing 2 which communicates with the coupling chamber via the seal means 7. This flow communication over the seal means 7 is a leakage of hydraulic fluid through the seal, the seal means 7 in this context representing a restrictor with a fixed opening diameter, apart from the wear of the sealing rafts over time.

In order to compensate for leakage flow via the flow restrictor 7, the flow control valve 9 is pressure regulated and controlled in accordance with the pressure difference between the motor barrier fluid on the upstream side of the flow restrictor 7 and the pump barrier fluid on the downstream side thereof, i.e. the pressure difference between the motor housing and the coupling chamber. The pressure difference across the flow restrictor 7 is continuously detected and returned to the flow control valve 9 by means of a pilot pressure circuit 11. In the drawings, the pilot pressure circuit 11 is illustrated with thin or dashed lines.

In the alternative embodiment illustrated in Figure 2, pump barrier fluid is supplied to the pump/clutch chamber 6 from the motor barrier fluid circuit 10 via the pump barrier fluid circuit 12. A fixed orifice diameter flow restrictor 13 is provided in the pump barrier fluid circuit outside the motor/pump module, restricting the flow of hydraulic fluid from the motor barrier fluid circuit 10 into the pump barrier fluid circuit 12.

Accordingly, the valve 9 is a pilot operated flow control valve which cooperates with an internal (7) or an external (13) restrictor to maintain a constant pressure differential between the motor barrier and pump barrier fluid circuits by adjusting the flow through the flow control valve. The flow through the restrictor is only a small flow that enters the pump via the pump barrier fluid circuit, while the main flow is taken to the motor via the motor barrier circuit.

In normal practice and scale, it is considered sufficient to include flow rates in the order of 0-100 l/min of supplied hydraulic fluid. In order to handle a wider range of flow rates than a single valve 9 can deliver, however, at least one additional pressure-regulated flow control valve 14 may optionally be arranged in parallel with the first flow control valve 9 to deliver fluid into the engine barrier fluid circuit 10. The flow control valves 9 and 14 each reacts separately to the pressure in the pilot pressure circuit 11. As previously explained, the flow control valves 9 and 14 can react individually to their own areas of the pressure difference across the flow restrictor, and they can also be sized for different flow rates. If necessary, a set of more than two flow control valves can be connected in parallel to supply hydraulic fluid into

the engine barrier fluid circuit.

With further reference to the drawing, reference number 15 refers to a pressure relief valve arranged to allow motor barrier fluid into the pump barrier fluid circuit via a bypass line 16 which has an outlet in the pump barrier fluid circuit downstream of the flow restrictor 7 or 13. Pressure relief valve 15 handles and regulates larger pressure differentials during start-up and as a result of thermal expansion of the liquid volumes in the motor and pump. The pressure relief valve 15 also acts as a safety valve if there is an unexpected rise in the fluid pressure to too high a level. A one-way valve 17 is preferably arranged to prevent backflow from the pump into the engine barrier circuit 10. A pressurized accumulator 18 may be arranged in the fluid supply line to provide sufficient pressure in the supplied fluid to the pressure-regulated flow control valve(s) 9, 14.

The illustrated embodiment fulfills the purpose of the invention, namely to maintain a constant pressure difference between two fluid-filled cavities which are hydraulically interconnected in the motor and pump module. Fluid is supplied at a controlled flow rate to the cavity that has the highest pressure, so that the pressure difference is maintained constant and independent of the leakage rate between the cavities. The invention is also effective for compensating flow rates due to compression and thermal expansion of the fluids in the cavities.

Although the invention has been described with reference to a system for regulating barrier fluid pressure across an interface between an engine casing and a switch chamber, it will be apparent that a similar arrangement can be used to regulate barrier fluid pressure at other interfaces in a subsea level engine and pump module , where fluid barriers are required for separation or to prevent penetration from the surrounding medium. It is specified that in addition to the interfaces listed in the abstract, the invention can be used at any separating interface in the engine or pump, such as seals and bearings that form part of a pump-lubrication system, or for example seals and bearings that form parts of an engine lubrication or cooling system.

The invention is of course not limited in any way to the embodiments described above. On the contrary, many possibilities for modifications of the designs will be obvious to a person with normal knowledge of the technique, without deviating from the basic idea of the invention as defined in the attached patent claims.

Claims (9)

1. Pressure regulation system for motor and pump barrier fluids for a subsea motor and pump module (1), the pressure regulation system comprising: - a barrier fluid circuit (10) which ensures fluid flow communication from a hydraulic fluid supply (8) to a first cavity in the motor and pump module , - a pressure-regulated flow control valve (9) which regulates the supply of hydraulic fluid to the barrier fluid circuit (10), - a second cavity arranged to receive hydraulic fluid from the hydraulic fluid supply via the first cavity, and - a pilot pressure circuit (11) comprising means for detecting and returning to the pressure-regulated flow control valve (9) a pressure difference across a flow restrictor (7; 13) connecting the first and second cavities. 2. Regulation system according to claim 1, in that a pump barrier fluid circuit (12) is arranged to receive hydraulic fluid from a motor barrier fluid circuit (10) via a flow restrictor (13) with a fixed orifice diameter arranged externally to the motor and pump module (1), and above which the pressure differential is detected and returned to the pressure-regulated flow control valve (9) which feeds hydraulic fluid into the motor and pump barrier fluid circuits in response to deviations from a predefined, desired pressure differential. 3. Regulation system according to claim 1 or 2, wherein said pressure-regulated flow regulation valve (9) is one of a set of valves, including at least a first and a second valve (9, 14), which are arranged in parallel to introduce hydraulic fluid in the engine-barrier fluid circuit (10), and each of which reacts to the pressure difference across the flow restrictor (7; 13), and further in that said at least two pressure-regulated flow control valves each individually react to separate areas of the pressure difference across the flow restrictor. 4. Control system according to claim 3, in that in a set of first and second pressure-controlled flow control valves, the first valve (9) is set to open for hydraulic fluid flow into the motor barrier fluid circuit (10) as a result of a pressure difference equal to or lower than a first predetermined pressure differential across the flow restrictor (7; 13), and the second valve (14) is set to open for hydraulic fluid flow into the engine-barrier fluid circuit (10) as a result of a pressure differential equal to or lower than a second predetermined pressure differential lower than the first predetermined pressure difference. 5. Regulation system according to claim 4, in that the first predetermined pressure difference is approximately equal to 5 bar, and the second predetermined pressure difference is approximately equal to 4.5 bar. 6. Regulation system according to any one of claims 2-5, in that in a set of first and second pressure-regulated flow control valves, the first valve (9) is dimensioned for low flow rates, and the second valve (14) is dimensioned for higher flow rates . 7. Regulation system according to claim 6, in that the first valve (9) is designed for flow rates down to about 0.3 l/min, and the second valve (14) is designed for flow rates up to about 100 l/min or more. 8. A control system according to any one of claims 3-7, wherein an on/off valve is connected in series with the second pressure controlled flow control valve (14) having a higher flow rate. 9. A control system according to any preceding claim, wherein a pressure controlled pressure relief valve (15) is arranged in parallel with the flow restrictor (7; 13) to discharge hydraulic fluid from the engine barrier fluid circuit (10) into the pump.