WO2016048163A1

WO2016048163A1 - High pressure barrier fluid system for subsea pumps and compressors

Info

Publication number: WO2016048163A1
Application number: PCT/NO2015/050174
Authority: WO
Inventors: Gunder Homstvedt; Dag ELVEBAKKEN; Pål Birger BENDIKSEN
Original assignee: Aker Subsea As
Priority date: 2014-09-26
Filing date: 2015-09-25
Publication date: 2016-03-31

Abstract

The invention provides a barrier fluid system for lubricating, cooling and cleaning bearings and seals of subsea turbomachines, also in situations where the pressure can undergo large changes. The system comprises: a topsides barrier fluid supply facility; an umbilical connecting the topsides barrier fluid supply facility to a subsea turbomachine; and a barrier fluid pressure regulator for delivery of barrier fluid at a controlled first overpressure from the umbilical to bearings and seais in a motor cavity of the subsea turbomachine. The system is distinctive in that it further comprises: at least one highest pressure barrier fluid reservoir or supply faciiity: and a pressure regulation system for controlling the pressure of barrier fluid delivered to the pressure regulator to be at a second overpressure compared to the first overpressure.

Description

HIGH PRESSURE BARRIER FLUID SYSTEM FOR SUBSEA PUMPS AND COMPRESSORS Field of the invention

The present invention relates to subsea pressure boosting of oil, condensate, gas, multiphase fluid or water, by using a subsea turbomachine. More specifically, the invention relates to improvements to the barrier fluid system of the subsea turbomachines, providing increased reliability, prolonged service life and enhanced versatility with respect to dynamic pressure control in deep water and high pressure applications.

Background of the invention and prior art

The terms subsea turbomachines and subsea pressure boosters mean subsea pumps and compressors. The term pump comprises single-phase fluid pumps and multiphase fluid pumps. Persons skilled in the art of subsea pressure boosting will have knowledge of subsea pumps and compressors and may be aware of the numerous challenges involved. Reliability is the single most crucial factor for operation of pressure boosting turbomachines subsea, since failure can be very expensive and difficult to mitigate. For subsea pumps and compressors, the typical sources of problems are the bearings and seals. The bearings, typically at least two radial bearings and at least one axial bearing are operatively arranged to the shaft or shafts, the functionality of the bearings are vital for reliable operation of the pump or compressor. The seals include dynamic seals such as shaft seals.

During operation of the subsea pressure booster, a pressurized barrier fluid is circulated through vital bearings and seals for lubrication, cooling and removal of any contamination. The barrier fluid shall at all times be held at a pressure higher than or equal to the pump cavity or compressor cavity pressure, preferably higher, to ensure that leakage flow is in the direction from the relatively clean motor cavity to the relatively contaminated pump or compressor cavity. The barrier fluid overpressure in the motor is conveniently in the range 2- 30 bar higher than the pump or compressor cavity pressure, typically about 10 bar overpressure. Intentionally a low flow rate of barrier fluid leaks out from the barrier fluid system toward the pump or compressor cavity, to ensure sufficient lubrication, cooling and cleaning of the mechanical shaft seals. The leaked out barrier fluid must be replaced continuously or periodically. At steady state operation, said leakage is small, however, at transient operation, such as at change in reference pressure or barrier fluid temperature, said leakage or consumption is larger. During operation of the pump or compressor, the barrier fluid should be at overpressure at all times, it is only at shut down the barrier fluid at lowest can be equal to the pump or compressor cavity pressure, but preferably is higher.

Some arrangements for subsea barrier fluid supply and control exists. These include supply through an umbilical from a topsides barrier fluid bank, using a topsides barrier fluid pump or a subsea barrier fluid pump. Furthermore, having replaceable tanks with barrier fluid subsea, close to the subsea pressure booster, is also possible.

International patent publication WO 2014092581 A1 contains detailed information on a mechanical subsea pressure regulator (MSPR). The regulator is often termed a PVR, a Pressure Volume Regulator. A mechanical subsea pressure regulator can be considered more reliable than regulators requiring electrical power and control to be operable, since fewer components can fail, electrical components typically have lower reliability than mechanical components and an electrical control system blackout will not stop the mechanical subsea pressure regulator system from being operative. Related background art is described in the patent publications WO 2011161517 A1 , US 6059539 A and NO 333696 B1. However, even though the mechanical subsea pressure regulator of WO 2014092581 A1 is the state of the art for subsea control of barrier fluid pressure, the reliability can be improved. The design service life can still be improved, particularly for field applications with very large dynamic pressure range in operation. Some fields can have an extreme dynamic pressure range between flowing pressure and static shut in pressure. For example, deep low permeability reservoirs can have a well shut in pressure up to 1000 bar or even higher, whilst the flowing pressure during production is typically 200 - 50 bar or even lower.

The objective of the present invention is to improve the reliability of subsea turbomachines by improving the barrier fluid system even further - in particular when the subsea pressure controller has to operate under conditions of large pressure variations.

Summary of the invention

The invention meets the objective by providing a barrier fluid system for lubricating, cooling and cleaning bearings and seals of subsea turbomachines, also in situations where the pressure can undergo large changes, comprising: a topsides barrier fluid supply facility; an umbilical connecting the topsides barrier fluid supply facility to a subsea turbomachine; and a barrier fluid pressure regulator for delivery of barrier fluid at a controlled first overpressure, relative to the pressure in a pump or compressor cavity of the subsea turbomachine, to bearings and seals in a motor cavity of the subsea turbomachine.

The system is distinctive in that it further comprises:

at least one highest pressure barrier fluid reservoir or supply facility, which highest pressure at least equals the highest pump or compressor cavity pressure at well shut in,

and

a pressure regulation system for controlling the pressure of barrier fluid delivered to the pressure regulator to be at a second overpressure compared to the first overpressure,

wherein the pressure regulation system controls the delivery of barrier fluid at second overpressure to be at a level typically far lower than said highest pressure but at demand said second overpressure at least equals said highest pressure.

The term second pressure regulation system is used to describe the system that adjust the pressure into the first pressure regulator so that it reliably can maintain its function in a shut-in scenario or other cases where high pressure or large pressure variations is present in the pump/compressor.

The term overpressure means in the context of the invention a differential pressure unless otherwise is specified. More specifically, the first overpressure of barrier fluid delivered from the first barrier fluid pressure regulator (MSPR) to bearings and seals in the motor cavity is the absolute pressure of the barrier fluid as delivered to the motor cavity minus the absolute pressure in the pump cavity close to the mechanical seals. The second overpressure is the absolute pressure of the barrier fluid delivered to the barrier fluid regulator (MSRP) minus the absolute pressure in the pump cavity close to the mechanical seals. The second overpressure is larger than the first overpressure. One or both of the terms "bara" ("bar-absolute" - contrary to bar or barg - "bar-gauge") and

"absolute" are used when absolute pressure is referred to. The first pressure regulator is preferably a mechanical subsea pressure regulator, a MSPR, often termed a PVR or a Pressure Volume Regulator, as described and illustrated in the international patent application

PCT/NO2013/050211. More specifically, said pressure regulator for control of supply and pressure of barrier fluid for pumps and compressors subsea, comprises an inlet for barrier fluid, a first outlet and a second outlet. The inlet pressure is higher than the first outlet pressure that again is higher than the second outlet pressure. The pressure regulator is distinctive in that the inlet valve mechanism and the second outlet valve mechanism are interlocked by a movable longitudinal interlock member with valve elements operatively arranged at either end, the length of the interlock member plus said valve elements is shorter than the distance between an open-close valve mechanism of the inlet and an open-close valve mechanism of the second outlet.

Other similar regulators can also be used, including e.g. electro mechanical with control loops closed subsea or topsides. Alternatively, a pressure controlled subsea barrier fluid pump can be used as the first pressure regulator.

The first overpressure is controlled relative to the pump or compressor cavity pressure, so that a first overpressure over said pump or compressor cavity pressure can be maintained at all times. The first overpressure can be in the range from just above zero up to well above 30 bar. The difference is

maintained as the absolute pressure in the pump cavity varies at different modes of pump operation. A first overpressure of about 10-30 bar is convenient. At normal operation, the absolute pressure in pump- or compressor cavity is typically 50 - 200 bara. However, at shut in conditions, the static well pressure can be acting directly on the pump or compressor cavity. This pressure can be up to 15 kpsia or 1035 bara or even higher, in which situation the first

overpressure preferably still shall be higher, such as about 10 bar higher. The second overpressure is higher than the first overpressure. The second overpressure, which conveniently can be about 50 bar, can be in the range from just above the first overpressure, at static conditions only, and higher. The absolute pressure of the barrier fluid establishing the second overpressure corresponds at least to the highest well shut in pressure. Accordingly, the full supply chain up to the topsides barrier fluid supply facility must be designed to said highest shut in pressure plus the second overpressure. At the highest shut in pressure, the pump or compressor is not operating, for which reason the first and second overpressures can be reduced down to almost zero. Since the pump or compressor cavity pressure and also the motor cavity pressure will be reduced when the shut-in situation is cleared and pressure boosting resume, the MSPR adjust the required pressure difference across the mechanical seals to the pre-set value. The corresponding absolute pressure of the highest expected second overpressure is termed the "highest pressure", and the system of the invention comprises at least one highest pressure barrier fluid reservoir, supply facility or accumulator. The shut in pressure depends on the well and reservoir characteristics and will be known, and the highest pressure barrier fluid reservoir or accumulator is preferably at somewhat higher pressure than the highest shut in pressure, for example 50 bar higher at the pump or compressor subsea.

The second overpressure shall be sufficient for responsive and reliable operation of the pressure regulator (e.g. MSPR). Typically, the second overpressure is 20 - 200 bar, more preferably 30 - 100 bar, most preferably about 50 bar. In any case, the second overpressure should include or compensate for static height, friction loss, elasticity in tubing, fluid compressibility and thermal expansion or contraction of the barrier fluid. Only at static condition can the second overpressure be reduced toward, but never below the first overpressure, which only at static condition can be reduced toward zero. Said overpressures may be increased at increasing motor speed, to ensure sufficient lubrication, cooling and cleaning.

The second overpressure is dynamic and will be set according to the specific application, but never below a lower limit as governed by the design and required pressure differential over the first pressure regulator, and never below the first overpressure. About 50 bar nominal is a typical second overpressure, however, for example 10-200 bar nominal can be feasible dependent on water depth, umbilical length, umbilical elasticity, thermal expansion/contraction and other factors.

The absolute pressure required to create the second overpressure has to be dynamic and adjustable depending on the pressure in the subsea pressure booster pump/compressor. In normal operation it is lower than in a high pressure shut in case. Such arrangement will ensure good operating conditions and long service life of the first overpressure regulator (MSPR) since the pressure adjustment is limited.

In a first preferable embodiment, the regulation system for controlling the second overpressure comprises:

a subsea pressure reduction valve (1 ) with constant pressure reduction level (constant dP) arranged operatively in the supply from the umbilical,

at least one remotely operated valve (2a, 2b), arranged in parallel to the pressure reduction valve (1 ),

at least one pressure sensor (4), arranged on the barrier fluid supply side of the pressure reduction valve and coupled to a topsides barrier fluid supply facility (13) being capable of delivering barrier fluid at absolute pressure up to a highest pressure, typically corresponding to 15 kpsi (1035 bara) subsea, and the highest pressure barrier fluid reservoir (3) is arranged subsea to the fluid feed from the umbilical. at least one pressure sensor (21 ), sensing pressure in the pump cavity (8) and connected to a controller that operates the remotely operated valves (2a, 2b). Control of the valves (2a, 2b) may also be done manually by an operator based on information from pump cavity pressure sensor (21 ).

As defined above, the corresponding absolute pressure of the highest expected second overpressure is termed the "highest pressure".

In a second preferable embodiment, the regulation system for controlling the second overpressure comprises:

at least one remotely operated valve (2a, 2b), arranged between the umbilical and the highest pressure subsea barrier fluid reservoir (3),

at least one pressure sensor arranged to the umbilical (9) and coupled to a topsides barrier fluid supply facility, such pump being capable of delivering barrier fluid at absolute pressure up to a highest pressure, typically

corresponding to 15 kpsi (1035 bara) subsea, but at normal operation the delivered pressure preferably corresponds to a second overpressure in the range (50 - 200) bar measured subsea.

at least one pressure sensor (21 ), sensing pressure in the pump cavity (8) and connected to a controller that operates the remotely operated valves (2a, 2b).

Fast provision of the highest pressure in a shut-in situation can be achieved by opening at least one of the valves (2a, 2b). The highest pressure subsea barrier fluid reservoir (3) is filled through the umbilical by temporarily raising the pressure in the umbilical to the highest pressure. The fluid in the reservoir 3 is stored until at the highest pressure until needed.

In a third preferable embodiment, the regulation system for controlling the second overpressure comprises:

at least one subsea pressure reduction valve (1 a) with pressure reduction level set with reference in the pump/compressor cavity (8), arranged operatively in the barrier fluid flow from the umbilical,

a pilot line (pressure reference line) arranged for the pressure reduction valve from the pump or compressor cavity (8) (i.e. the process fluid pressure close to the mechanical seal(s) ) of the subsea turbomachine to the pressure reduction valve (1 a and 1 b),

at least one pressure sensor (4) arranged on the barrier fluid supply side of the pressure reduction valve and coupled to a barrier fluid pump of the topsides barrier fluid supply facility, the pump being capable of delivering barrier fluid at absolute pressure up to a highest pressure measured subsea, typically 15 kpsi (1035 bara), and the at least one highest pressure barrier fluid reservoir (3) is arranged subsea to the fluid feed from the umbilical.

In a fourth preferable embodiment, the regulation system for controlling the second overpressure comprises:

at least one remotely operated isolation valve (2a, 2b), arranged between the umbilical and the highest pressure barrier fluid reservoir (3), which valve and highest pressure reservoir are arranged topsides,

at least one pressure sensor (4) arranged to monitor the pressure on the highest pressure barrier fluid reservoir side of the isolation valve,

for which embodiment the umbilical is at an absolute pressure corresponding to the second overpressure.

at least one pressure sensor (21 ), sensing pressure in the pump cavity

(8) and connected to a regulator that operates the remotely operated valves

(2a, 2b).

The system preferably comprises at least one pressure sensor (9) coupled to the inlet side of the first pressure regulator (7), an operational pressure reservoir (11 ) and the supply facility, isolation valves, further instrumentation and a control system for monitoring the pressures of the system and delivering barrier fluid from the barrier fluid supply facility, or in emergency from the highest pressure reservoir. Fast provision of the highest pressure will be achieved by opening at least one of the valves (2a, 2b).

In a fifth preferable embodiment, the second pressure regulation system comprises: two separate lines (6a and 6b) in the umbilical connected to the subsea regulator (7) inlet, the first line (6a) providing fluid under normal operation pressure from a topsides located pump (17) and the second one (6b) providing fluid at the highest pressure from a high pressure topsides pump (18) typically corresponding to 15 kpsi (1035 bara) subsea

at least one remotely operated valve (2a, 2b), arranged between the second umbilical line and the regulator inlet,

at least one check valve (15) at the supply side of the normal pressure line in the umbilical

at least one pressure sensor (9) arranged to the inlet of the subsea regulator

at least one pressure sensor (21 ), sensing pressure in the pump cavity (8) and connected to a regulator that operates the remotely operated valves (2a, 2b).

Fast provision of the highest pressure can be achieved by opening at least one of the valves (2a, 2b).

For the first and third preferable embodiments described above, the normal barrier fluid delivery pressure from the topsides barrier fluid supply is equal or higher, preferably higher, than the highest absolute pressure in the motor or compressor cavity in a shut-in situation or other high pressure cases. As a limit condition for all embodiments, the absolute pressure of the supplied barrier fluid corresponding to the second overpressure and the highest pressure, respectively, is equal to the highest shut in pump or compressor pressure, meaning that the first and second overpressures can both be reduced to zero, but only at static condition. However, the first overpressure is most preferably about 10 - 30 bar and the second overpressure is most preferably about 50 bar as measured subsea but can conveniently be up to 3-400 bar.

The 3^rd embodiment is the only version that operate without an electric controlled valve to generate the highest pressure.

Figures The invention is illustrated with five figures, of which:

Figure 1 illustrates a first embodiment of a system of the invention,

Figure 2 illustrates a second embodiment of a system of the invention,

Figure 3 illustrates a third embodiment of a system of the invention,

Figure 4 illustrates a fourth embodiment of a system of the invention, and Figure 5 illustrates a fifth embodiment of a system of the invention.

Detailed description

Reference is made to Fig. 1 , illustrating a first embodiment of a system 10 of the invention. More specifically, at the pressure booster end of the umbilical 6, an MSPR (7) (that is a barrier fluid pressure regulator) is coupled to a subsea pump, with a first line 5 at pressure P1 to the motor cavity and a second line 8 at pressure P2 = pump cavity inlet reference pressure. P1 = P2 + 10 bar (typically). Thus, P1 is the absolute pressure corresponding to the first overpressure, whilst P2 is the reference pressure sensed via the second line 8, which also functions as a dump line. Accordingly, the first overpressure is P1 minus P2, which typically is 10-30 bar and is 10 bar in the illustrated

embodiment. The typical reference pressure P2 is 50 - 200 bar. The inlet to the MSPR is coupled to the delivery side of the umbilical via the pressure reduction valve 1 , at a typical pressure P0 of 250 - 500 bar. For further information on the MSPR, please refer to the international patent application

PCT/NO2013/050211.

At the delivery side of the umbilical, close to the MSPR, a set of pressure transmitters (at least one) 9 are arranged to the umbilical for sensing and transmitting of pressure information. A normal pressure accumulator bank 11 is included for quick delivery of barrier fluid, at normal operation pressure, in situations with special demand. Such situation can be transient pump operation at start, stop or large load change, when barrier fluid expansion, contraction or leakage may require extra supply. These accumulators will also be a barrier fluid supply during installation or act as a redundant supply. Neither the normal barrier fluid pressure accumulator 11 nor the pressure transmitters 9 are obligatory features. Further upstream on the delivery side of the umbilical, a subsea pressure reduction valve (1 ) with a constant pressure reduction level is arranged operatively in the barrier fluid path. One, or several, remotely operated isolation valves (2a, 2b), each typically normally closed, fail open, each typically operable (for override) by ROV, are arranged in parallel to the pressure reduction valve (1 ). At least one pressure sensor (21 ), sensing pressure in the pump cavity (8) and connected to a controller that operates the remotely operated valves (2a, 2b) is included.

A least one pressure sensor (4) is arranged on the barrier fluid supply side of the pressure reduction valve. The system is pressurized by a topsides barrier fluid supply facility 13 being capable of delivering barrier fluid at absolute pressure corresponding at least up to said highest pressure, typically

corresponding to 15 kpsi (1035 bara) subsea. At least one highest pressure barrier fluid accumulator 3 (reservoir) is arranged subsea to the high pressure side of the umbilical. A check valve 12 and typically an isolation valve 14, normally open, is arranged in the umbilical between the highest pressure reservoir 3 and the supply facility. The barrier fluid system is with the first embodiment divided into a high pressure side and a normal pressure side. The high pressure side is on the supply facility side of the subsea pressure reduction valve 1. The pressure of the high pressure side is typically 1035 bara or higher. The normal pressure side of the barrier fluid system is on the downstream side of the pressure reduction valve 1. The supply pressure P0 is typically 250 - 500 bar when the system is operating in normal pressure mode. This is the absolute pressure corresponding to the second overpressure (P0 minus P2). When the system is operating in high pressure mode, one of the valves 2a or 2b is open and the P0 pressure becomes equal to P3 pressure. Reference is made to Figure 2, illustrating a second embodiment of a system of the invention, where the barrier fluid delivered from the topsides barrier fluid supply facility has a normal supply pressure resulting in the normal supply pressure subsea at the MSPR (7) being 250 - 500 bara. For the second and further embodiments, features identical or similar to features of the first embodiment are not necessarily described. For this embodiment, there is no subsea pressure reduction valve. Accordingly, the normal pressure regulation to the second overpressure is from topsides. Absolute pressure for the highest pressure is stored in an accumulator bank 3 and monitored by pressure transmitters 4. This highest pressure is normally isolated from the MSPR (7) inlet by the (one or several) normally closedvalves 2a and 2b, arranged in parallel for redundancy. This accumulator bank (3) is filled and pressurized from the barrier fluid pump of the topsides barrier fluid supply facility. This is typically done in a stationary and/or shut-in situation when the pressure booster is not in operation.

A second check-valve 15 is included to prevent back-flow into the normal pressure accumulator bank (11 ) and the umbilical, when one of the valves 2a and 2b are open. At well shut in and in other cases where there is a need for high barrier fluid pressure to the pressure booster, the highest pressure accumulator (3) can, by opening valve 2a and/or 2b, be coupled to deliver barrier fluid at highest pressure. The barrier fluid pump topsides being capable of delivering barrier fluid at absolute pressure corresponding to said second overpressure, typically 15 kpsi (1035 bar) subsea, but at normal operation the delivered pressure, that is the absolute pressure corresponding to the second overpressure, is in the range 250 - 500 bara.

Reference is made to Figure 3, illustrating a third embodiment of a system of the invention, where two pressure reduction valves 1 a and 1 b with pilot line to the pump cavity and pressure P2, are arranged in parallel for redundancy. The two reduction valves 1a and 1 b are connected to the inlet of the MSPR (7) by the two respective isolation valves 2a and 2b. One of the valves (2a or 2b) is normally open, the other is normally closed but can be opened as backup. The reduction valves 1 a and 1 b are set to reduce the supply pressure to typically 60 bar above the reference pressure P2. For this embodiment, the barrier fluid pressure is supplied from the supply facility at absolute pressure 1035 bar or higher. The regulated working pressure P0 to the MSPR is therefore typically P2 + 60 bar. Thus, the second overpressure is 60 bar. This arrangement does not require operation of any subsea valves to generate the highest absolute pressure corresponding to the second overpressure.

Further reference is made to Figure 4, illustrating a fourth embodiment of a system of the invention where the highest pressure reservoir (3) of the barrier fluid is arranged topsides. The two isolation valves (2a, 2b) and a set of pressure sensors (4) are arranged between the umbilical feed entry and the highest pressure barrier fluid reservoir (3), also located topsides. The pressure sensors (4) are arranged to monitor the pressure on the highest pressure barrier fluid reservoir side of the isolation valves, for which embodiment the umbilical is at the absolute pressure corresponding to the second overpressure subsea. Normally such absolute pressure are 250 - 500 bara generated by the "normal pressure HPU" (17), while such absolute pressure up to 1035 bara or higher at shut down is generated by a "high pressure HPU" (18). Both HPU's are located topsides and may be just one unit with capability of delivering both pressure levels. The normal pressure HPU (17) is feeding the normal pressure barrier fluid reservoir 11 and pressure is monitored by sensor 9. The normal pressure HPU (17) is feeding the umbilical through check valve 12a and isolation valves 14 and 16, normally open. The high pressure HPU (18) is feeding the high pressure barrier fluid reservoir (3) through check valve 12b and pressure is fed to the umbilical only when one of the isolation valves 2a or 2b are open. Check valve 12a prevents backflow to the normal pressure reservoir (17).

Further reference is made to Figure 5, illustrating a fifth embodiment of a system of the invention, comprising two lines in the umbilical (6a and 6b) feeding barrier fluid at different pressures to the subsea pressure booster from two HPU's located topsides - the normal pressure barrier fluid HPU (17) and the high pressure HPU (18). The extra umbilical conduit (6b) has been arranged as a high pressure reserve, with a highest pressure accumulator (3) located subsea, for assurance of pressure control of the subsea booster in a high pressure well shut in scenario.

Numerous other embodiments are possible. For example, arranging two MSPR's or PVR's in series is a possible embodiment, one of said regulators can be arranged topsides. For longer umbilical lengths, that is longer than 20 - 40 km, the embodiments 1 and 3 as described above are supposed to be most preferable. For shorter umbilical lengths, control of the second overpressure normally from topsides, as for embodiments 4 and 5, may be more preferable.

In another embodiment of the invention, only one pressure transmitter is absolutely required, sensing the pressure of either the motor cavity or the pump or compressor cavity or the pressure regulator inlet, and operatively arranged to the second pressure regulation system for providing a feasible second overpressure at all operating conditions.

The pressure reservoirs, accumulator banks 3 and 11 are charged to have its operational range at the pressures that are required. The normal pressure reservoir (3) will have its typical operational range from 250 - 500 bar, while the high pressure reservoir (11 ) is operational in the shut-in pressure range and can be as high as 800 to 1035 bar or even higher. These settings will be adjusted to fit the actual application.

All embodiments, except version 3, will require a topside control system and/or safety system that senses pressure in the pump cavity, by pressure sensor (21 ), and activates one of the isolation valves 2a or 2b to achieve the highest absolute pressure corresponding to the second overpressure.

The barrier fluid system of the invention may comprise any feature or step as here described or illustrated, in any operative combination, each such

combination is an embodiment of the invention. In addition, related methods and uses, with similar or identical steps or features, are embodiments of the invention.

The barrier fluid system of the invention provides prolonged service life and reliability for subsea pressure boosters, improved flow or production assurance, and may allow production to continue even at platform or control system black out or failures, and allow production to resume faster after a well shut in.

Claims

1.

Barrier fluid system (1) for lubricating, cooiing and cleaning bearings and seals of subsea turbomachines, also in situations where the pressure can undergo large changes, comprising: a topsides barrier fluid supply facility; an umbilical connecting the topsides barrier fluid supply facility to a subsea turbomachine; and a barrier fluid pressure regulator for delivery of barrier fluid at a controlled first overpressure, relative to the pressure in a pump or compressor cavity of the subsea turbomachine, to bearings and seals in a motor cavity of the subsea turbomachine , c h a r a c t e r i s e d i n that the system further comprises: at least one highest pressure barrier fluid reservoir or supply facility, which highest pressure at least equals the highest pump or compressor cavity pressure at well shut in,

and

2.

System according to claim 1 , wherein the second overpressure Is sufficient for responsive and reliable operation of the first pressure regulator, typically 20-200 bar second overpressure, more preferably 30 - 100 bar most preferably about 50 bar.

3.

System according to claim 1 or 2, wherein the second overpressure is constant or dynamic, but never below a lower limit as governed by the design and required pressure differential over the first pressure regulator.

4,

System according to any one of claim 1-3, wherein the regulation system for controlling the second overpressure comprises;

a subsea pressure reduction vaive (1) with constant pressure reduction level (constant dP) arranged operatively in the supply from the umbilical connected to a topsides barrier fluid supply facility (13) being capable of delivering barrier fluid at absolute pressure up to a highest pressure, typically corresponding to 15 kpsi (1035 bara) subsea, and the highest pressure barrier fluid reservoir (3) is arranged subsea to the fluid feed from the umbilical.

at least one remotely operated valve (2a, 2b), arranged in parallel to the pressure reduction vaive (1),

at least one pressure sensor (21), sensing pressure in the pump cavity

(8),

a regulator to operate one of the valves (2a, 2b) when the pump pressure sensor (21) indicate values higher than normal, the regulator can be located either topsides or subsea.

5.

System according to any one of claim 1-3, wherein the regulation system for controlling the second overpressure comprises:

at least one remotely operated valve (2a, 2b), arranged between the umbilical (9) and the highest pressure subsea barrier fluid reservoir (3),

at least one pressure sensor (4) arranged to the highest pressure subsea barrier fluid reservoir (3),

an umbilical (9) coupled to a topsides barrier fluid supply facility, such being capable of delivering barrier fluid at absolute pressure up to a highest pressure, typically corresponding to 15 kpsi (1035 bara) subsea, but at normal operation the delivered pressure preferably corresponds to a second

overpressure in the range (50 - 200) bar measured subsea,

at least one pressure sensor (21), sensing pressure in the pump cavity

(8),

a regulator to operate one of the valves (2a, 2b) when the pump pressure sensor (21) indicate values higher than normal, the regulator will also operate one of the valves (2a, 2b) when the highest pressure reservoir sensor (4) indicate pressure below the highest pressure to allow recharging the reservoir from topsides, the regulator can be located either topsides or subsea.

6.

System according to any one of claim 1-3, the regulation system for controlling the second overpressure comprises:

at least one subsea pressure reduction vaive (1a) with pressure reduction level set with reference in the pump/compressor cavity (8), arranged operatively in the barrier fluid flow from the umbilical,

a pilot Sine (pressure reference line) arranged for the pressure reduction valve from the pump or compressor cavity (8) (i.e. the process fluid pressure close to the mechanical seal(s) ) of the subsea turbomachine to the pressure reduction valve (1a and 1b),

7.

at least one pressure sensor (21) arranged subsea to monitor the pump cavity pressure

a regulator operating one of the valves (2a or 2b) when the pump pressure (21) is higher than normal, which regulator is arranged topsides, for which embodiment the umbilical is at an absolute pressure corresponding to the second overpressure.

8. System according to any one of claim 1-3, the regulation system for controlling the second overpressure comprises;

at least two hydraulic lines (6a and 6b) transferring pressure from sources located topsides, one transferring normal pressure to the normal pressure reservoir (11), located subsea, and the other transferring the highest pressure to the highest pressure barrier fluid reservoir (3), located subsea

at least one remotely operated valve (2a, 2b) between the highest pressure barrier fluid reservoir (3) and the first pressure regulator (7), which valve and highest pressure reservoir are arranged subsea,

at least one pressure sensor (21 ) arranged subsea to monitor the pressure at the pump

9.

System according to claim 1-8, wherein it comprises pressure sensors coupled to the norma! pressure reservoir (11) and the highest pressure reservoir (3), a check valve between the subsea system and the supply facility, isolation valves, further instrumentation and a control and/or safety system for monitoring the pressures of the system and delivering barrier fluid from the barrier fluid supply facility, or in emergency from the highest pressure reservoir, at the second overpressure or higher pressure.

10.

System according to claim 5 and 8, wherein it comprises a check valve (15) between the inlet side of the first pressure regulator (7) and the normal pressure reservoir (11).