NO345592B1

NO345592B1 - Subsea motor and pump assembly and its use in a subsea desalination plant

Info

Publication number: NO345592B1
Application number: NO20200154A
Authority: NO
Inventors: Tom Gunnar Omberg; Morten Hana; Jo Jernsletten; Davoud Tayebi
Original assignee: Waterise As
Priority date: 2020-02-06
Filing date: 2020-02-06
Publication date: 2021-05-03
Also published as: WO2021158118A1; NO20200154A1

Description

It has become feasible during the recent decades to operate pumps located subsea at the seabed at significant depths. The reliability of subsea pumps has increased with increased operating experience of subsea pumps. The hydraulic part of the pump (in contact with the product liquid) can be designed to operate at any capacity and head, if the motor can provide the power. Pumps are typically of the multi-stageimpeller centrifugal type and can provide up to hundreds of bar differential pressure.

The trend is moving towards all electrical subsea systems and it has become feasible to use electrical motors to operate subsea pumps.

The power is transferred with a rotating shaft from the motor to the pump. An advantage of a direct shaft connection is that there is no practical limitation in the power that can be transferred from the motor to the pump impeller(s). The disadvantage is that the motor housing enclosure will not be hermetically sealed because the shaft seals do not seal completely, and hence leaks occur. The motor housing is filled with a barrier liquid under sufficiently high pressure to assure no ingress of unwanted liquids that threaten the integrity of the motor. The motor barrier liquid may also be of a type that does not short-circuit the electrical components, or the motor's electrical parts may be covered with insulating materials. The motor barrier liquid is also used as a cooling media in the subsea motor's cooling system.

The shaft torque transfer design creates a leak path from the motor enclosure to the pump casing and the pump impeller(s) along the rotating shaft. The pressure of the barrier liquid on the motor side of the shaft seals must always be above the pressure at the other side in order avoid ingress of sea water or pumped product liquid into the motor enclosure. The barrier liquid leak rate is limited by the narrow gaps that are present in the seal system. Hence, the motor liquids will leak in the direction of the the pump casing with impeller(s), and into the pumped liquid.

The leak flow of barrier liquid can be prevented if the motor is housed in a hermetically sealed enclosure where the torque from the motor is transferred with an in-motor shaft to the motor side of a magnetic coupling. The other side of the magnetic coupling is outside the hermetically sealed motor enclosure, and the transfer of the torque to the pump impellers is done with an in-pump shaft.

An advantage is that the motor can be gas/air-filled and completely isolated against the ambient subsea environment. Another advantage is that less equipment is required above sea surface to serve the pump with chemicals. The disadvantage is that there is a limit to the power that can be transferred through the magnetic coupling, relative to the optimal design flow rate of the pump for some applications. Hence, the hermetically sealed solution is not yet feasible for very large subsea pumps with power of above say 1-2 MW, unless the power limit of the magnetic coupling is increased.

Larger pumps than say 1-2 MW power are necessary in order to fully optimize the CAPEX and OPEX of a subsea desalination plant. A small subsea desalination plant with a capacity of say 50000 cubic meter per day will need pumping power of say 3 MW to bring the product to shore. This may be done with two pumps with hermetically sealed motors and magnetic couplings, or with one larger pump with torque transfer with a single shaft. For larger subsea desalination plants, it is more efficient with respect to CAPEX and OPEX to use larger pumps.

US 2014241907 discloses a high pressure water injection pump system with at least two water injection pump units connected in series. Each pump unit includes a motor in a motor compartment operatively connected to drive a pump in a pump compartment. The pump units following the first pump includes a choke compartment arranged between the pump and motor compartments. The motor compartment of the pump units following the first pump unit has lower pressure class than the operatively connected pump compartment.

US 4224009 discloses a submersible pump provided with an improved labyrinth seal to avoid undesirable leakage of a pump fluid into a hydraulic motor of the pump and to avoid leakages from the hydraulic motor into the pump.

WO 2007055589 discloses a subsea compression module with an electric motor in a first pressure housing for being pressurized with a pressure (Pm) and a compressor in a second pressure housing with a suction pressure. A third intermediate pressure housing is provided between the motor and the compressor. A seal for the output shaft of the motor, seals between the pressure housing of the motor and the third intermediate pressure housing. A seal for the input shaft of the compressor seals between the intermediate pressure housing and the compressor. A connection is provided between the motor and the compressor in the third intermediate pressure housing.

US 6059539 discloses a pressure compensator for a sub-sea pumping system for pumping an effluent from a deep-sea wellhead with a topside module, a sub-sea module, and an umbilical connection between the topside and sub-sea modules. Hydraulic fluid is circulated through the sub-sea module for cooling and lubricating the motor and the pump. The hydraulic cooling and lubricating fluid are preferably a single hydraulic fluid that is compatible with the effluent being pumped. The hydraulic fluid is circulated through a submerged pressure compensator which may include a bellows assembly.

US 6655932 discloses device for cooling and lubricating a subsea device which includes an electric rotary machine and a fluid-rotation machine drive-connected thereto. The electric rotary machine has a lubrication/cooling circuit and a cooler exposed to the surrounding seawater. The fluid-rotation machine has a fluid pressure at the end facing towards the electric rotary machine. The lubrication/cooling circuit is pressure-impacted by the fluid pressure at one point in the lubrication/cooling circuit outside the fluid-rotation machine.

In a subsea desalination system will a large subsea pump draw down the upstream absolute pressure in order to provide the differential pressure over the upstream RO-membrane cartridges. Hence, for a subsea desalination system upstream of a short pipeline or riser extending to an onshore receiving facility close to a coastline or on a floating vessel, the pressure at the pump's outlet is not much different from the pressure of the ambient sea water surrounding the subsea pump. The pressure at the pump's inlet is significantly lower than the pressure of the ambient sea water surrounding the subsea pump.

The pressure of the pump motor's barrier liquid is above the ambient sea water pressure and above the pump's outlet pressure. Vendors of subsea pumps have developed control systems to ensure that the barrier liquid / cooling liquid pressure is assured in order to protect the integrity of the subsea motor. For subsea desalination pumping, the required upstream pressure of the motor barrier liquid is predictable because the maximum pressure inside the pump is essentially the same as the ambient sea water pressure.

The invention provides a solution where the larger subsea pumps are feasible while maintaining strict demands on the pumped product's quality.

The present invention thus relates to a subsea motor and pump assembly with a motor assembly with a rotor, a stator, and a sealed enclosure with a barrier liquid feed inlet. A main pump assembly includes a main impeller and a pump casing. A shaft extending through a mid-chamber assembly connects the rotor of the motor assembly and the impeller of the main pump assembly. A pumping device is adapted to pump leaked liquids out of the mid-chamber assembly and to maintain a pressure in the mid-chamber below a pressure in a pump outlet and below a pressure in the sealed enclosure of the motor assembly.

The pumping device pumping leaked liquids out of the mid-chamber assembly may include a separate pump and motor assembly with a leaked liquid pump driven by a separate motor.

The pumping device pumping leaked liquids out of the mid-chamber assembly may include includes an impeller pump plate.

The impeller pump plate may be mounted on the main impeller.

The impeller pump plate may be mounted on the shaft inside the mid-chamber assembly.

The pumping device may be formed as a part of the impeller.

The subsea motor and pump assembly may further include a liquid level sensor in the mid-chamber, and the pumping device may be controlled in relation to a liquid level in the mid-chamber.

A pressure sensor may be located in the mid-chamber, and the pumping device may be controlled in relation to the pressure in the mid-chamber.

The subsea motor and pump assembly may further include a pressure sensor in the sealed enclosure of the motor assembly and a pressure sensor in the pump casing, and the pumping device may furthermore be controlled in relation to the pressure in the sealed enclosure and in the pump casing.

The pump assembly may be adapted to pump drinking water.

The pumping device may be adapted to pump the leaked liquids out of the midchamber assembly and to a third location selected from the group: another pumped flow in a neighbouring system, a separator to treat the leaked flow, a storage device for leaked flow, including a system to transport the volume of leaked liquid, a conduit line in an umbilical to a topside facility either offshore or onshore, a hot stab connection for a downcomer conduit line from a vessel, and an outlet to the surrounding seawater in the ambient environment surrounding the pump module.

The shaft connection mechanically connecting the pump shaft and the motor shaft may be flexible and located inside the mid-chamber.

The mid-chamber assembly with a mid-chamber for leaked liquids may include a motor shaft opening and a pump shaft opening.

Finally, the invention relates to use of a subsea motor and pump assembly as described above in a subsea desalination plant. The subsea motor and pump assembly is used to pump the desalinated water without contaminating the desalinated water in the subsea desalination plant. The subsea motor and pump assembly may also be used on the seawater side to prevent contamination in particular from the liquid filled motor from entering the subsea desalination plant.

Fig. 1 is a schematic representation of a prior art system;

Fig. 2 is a schematic representation of the consequence of a prior art system; Fig. 3 is a schematic representation presenting the purpose of the invention;

Fig. 4 is a schematic representation presenting the main features of an embodiment of the invention with a pumping device within the mid-chamber assembly;

Fig. 5 is a cross section of a prior art system;

Fig. 6 is a cross section of another prior art system;

Fig. 7 is a cross section presenting an embodiment of the invention;

Fig. 8 is a cross section presenting another embodiment of the invention;

Fig. 9 is a cross section presenting yet another embodiment of the invention;

Fig. 10 is a cross section presenting yet another embodiment of the invention; Fig. 11 is a cross section presenting yet another embodiment of the invention; Fig. 12 is a cross section presenting yet another embodiment of the invention; Fig. 13 is a cross section presenting yet another embodiment of the invention; Fig. 14 is a schematic representation presenting the main features of another embodiment of the invention;

Fig. 15 is a schematic representation presenting the main features of yet another embodiment of the invention;

Fig. 16 a-e are schematic representations of a few examples of a third location where the leaked liquid outlet flow is discharged; and

Fig. 17 a, bare schematic representations of a subsea periodic evacuation system for liquids that will be collected periodically with an offshore vessel.

The refence numerals are similar in the all figures. The entire text can thus be referred to, to find a description of each reference numeral.

Figure 1 presents a prior art pump module 1 that is located subsea submerged in sea water 2. The pump module 1 has a motor assembly 3 and a main pump assembly 4. The product inlet flow 10 enters the main pump assembly 4 through the pump inlet 6. Power is transferred from the rotor inside the motor assembly 3 to the impeller inside the main pump assembly 4. The impeller inside the main pump assembly 4 increases the product outlet flow 11 pressure at the pump outlet 7.

The motor assembly 3 has a barrier / cooling liquid inlet 8. The barrier / cooling liquid flow 12 enters the motor assembly through the inlet 8 at a higher pressure than of the product flows 10 and 11 and at a higher pressure than the pressure in the ambient sea water 2, in order to protect the motor assembly 3 from ingress of the product liquids flowing between the inlet 6 and the outlet 7, and protect the motor assembly 3 from ingress of the ambient sea water 2 surrounding the pump module 1.

Figure 2 presents the consequence of the prior art pump module in Figure 1. Power is transferred from the rotor inside the motor assembly 3 to the impeller(s) inside the main pump assembly 4 with a rotating shaft. There will be a leak path 25 of barrier / cooling liquid from the motor assembly 3 along the rotating shaft into the main pump assembly 4. The leaked barrier / cooling liquid will exit the pump module through the outlet 7 and be commingled into the product flow 11.

The commingling of the barrier / cooling liquid leaked along the leak path 25 with the product flow 11 at the pump outlet 7 deteriorates the quality of the product flow 11.

The deterioration of the quality of the product flow 11 may be of negligible consequence for a subsea pump if the product flow 11 is oil. The deterioration is likely to be unacceptable if the product flow 11 is drinking water.

Figure 3 presents an embodiment of the invention where a mid-chamber assembly 5 with a mid-chamber inside a mid-chamber assembly 5 located between the motor assembly 3 and the main pump assembly 4. The purpose of the mid-chamber assembly 5 is to collect the leaked barrier / cooling liquid along the leak path 25 and the leaked product flow along the leak path 26 and send the commingled leak flow 13 out of the pump module 1 through the outlet 9 of the mid-chamber assembly 5.

Figure 4 presents the embodiment of the invention presented in Figure 3 in more detail. The barrier / cooling liquid flow 12 is pumped into the motor assembly 3 through the inlet 8 at a higher pressure than the ambient pressure of the sea water 2, and at a higher pressure than the product flow inside the main pump assembly 4 between the inlet 6 and the outlet 7.

The inlet flow 10 of the product liquid enters the main pump assembly 4 at the inlet 6. The pump discharge flow 11 exits the main pump assembly 4 at the outlet 7. The pump impeller(s) inside the pump casing inside the main pump assembly 4 generate(s) the pressure difference between the outlet flow 11 and the inlet flow 10. As the pressure downstream of the outlet 7 is fixed and substantially the same as the pressure of the ambient sea water 2 surrounding the pump module 1, the pressure of the product inlet flow 10 upstream of the inlet 6 is decreased.

Hence, the inlet flow 10 will have a substantially lower pressure than the outlet flow 11. The outlet flow 11 has a pressure substantially equal to, or slightly above the pressure of the ambient sea water 2 when the conduit downstream of outlet 7 ends close to the sea level. The pressure difference between the product flow 11 at the outlet 7 and the pressure of the ambient sea water 2 is significantly less than the pressure difference between the product flow 11 at outlet 7 and the product flow 10 at the inlet 6.

The shaft 16 that transmits the power from the motor rotor inside the motor assembly 3 to the pump impeller inside the main pump assembly 4 forms the leak paths 25, 26 along the shaft 16.

The leak rates along the shaft 16 through the leak paths 25 and 26 are low due to the seals surrounding the shaft 16. The leak rates along the leak paths 25 and 26 can however not be completely eliminated as the motor assembly 3 and the main pump assembly 4 not are hermetically sealed.

In order to prevent the situation illustrated in Figure 2 where the product flow is commingled with leak flow from the motor assembly, the embodiment of the invention in Figure 4 includes a pumping device 28 located in the mid-chamber housing of the mid-chamber assembly 5 between the motor assembly 3 and the main pump assembly 4.

The leak flow through the leak path 25 along the shaft 16 of barrier / cooling liquid from the motor assembly 3 will enter the mid-chamber of the mid chamber assembly 5 upstream of the pump device 28. The leak flow through the leak path 26 along the shaft 16 of product liquid from the main pump assembly 4 will enter the mid-chamber upstream of the pump device 28.

The pump device 28 provides the pressure difference and reduce the pressure in the volume inside the mid-chamber 5 upstream of the pump device 28 to maintain a lower pressure inside the mid-chamber than the pressures in the motor assembly 3 and the pressure in the main pump assembly 4 respectively. The pressure difference provided by the pumping device 28 is also sufficient to pump the commingled flow 13 of barrier / cooling liquid and product liquid out of the midchamber 5 through the outlet 9.

The commingled flow 13 flows through a conduit 14 to a third location 15 in the system. The third location 15 can be another process flow in the overall process system where the ingress of flow 13 is acceptable. The third location 15 may also be a collector device for downstream treatment, separation and/or disposal. The third location 15 may also be an outlet to the ambient sea water 2.

Figure 5 presents more details of the prior art embodiment example presented in Figure 2.

The prior art pump module 1 is submerged in sea water 2. The pump module has a motor assembly 3 and a main pump assembly 4. The product inlet flow enters the main pump assembly 4 through the pump inlet 6. Power is transferred from the rotor 20 inside the motor assembly 3 to the main impeller 23 inside the casing 24 of the main pump assembly 4. The inlet side of the main impeller 23 is facing upwards. The main impeller 23 inside the main pump assembly 4 increases the pressure of the product flow at the pump outlet 7.

Barrier / cooling liquid flows into the sealed enclosure 19 inside the motor assembly 3 where the motor stator 21 and motor rotor 20 are located. In Figure 5 the rotating shaft has a motor shaft 16a and an impeller shaft 16c connected with a flexible shaft connection 16b. The motor shaft 16a is supported by journal bearings 17a and thrust bearings 17b. The pump shaft 16c is also supported by journal bearings 17a and thrust bearings 17b.

The pressure of the barrier / cooling liquid at inlet 8 is above the pressure of the surrounding sea water 2 and the pressure of the pumped product liquid at any location between the pump inlet 6 and the pump outlet 7.

The main pump assembly 4 includes the stationary pump casing 24 that surrounds the pump main impeller 23. There are seals 18 between the pump casing 24 and the main impeller 23 in order to minimize internal leak flow between the main impeller 23 and the casing 24.

Seals 18 are located between the stationary motor assembly 3 and the rotating motor shaft 16a. Seals 18 are also located between the stationary main pump assembly 4 and the rotating pump shaft 16c. The seals 18 cannot fully prevent leakage of liquid from the high-pressure side inside the motor enclosure 19 in the motor assembly 3 to the lower pressure side inside the pump enclosure 22 inside the main pump assembly 4. The path of barrier / cooling liquid is illustrated with the curved arrows representing the leak path 25.

In Figure 5 the conduit 52 in the main pump assembly 4 makes a liquid connection between the high-pressure side of the main impeller 23 and a location near the pump shaft 16c and the flexible shaft connection or coupling 16b. The reason is that the absolute pressure of the product flow at the pump outlet 7 is substantially equal to or somewhat higher than the absolute pressure of the ambient sea water 2 while the absolute pressure of the product flow at the inlet 6 is significantly lower that the pressure of the ambient sea water 2. The liquid connection 52 decreases the driving pressure of barrier / cooling liquid from the motor enclosure 19 to the pump enclosure 22 and hence also decrease the leak rate along the leak path 25 illustrated with the curved arrows.

The deterioration of the quality of the product flow at the pump outlet 7 due to ingress of barrier / cooling liquid along the leak path 25 between the rotating shaft 16a, 16c and the seals 18 is likely to be unacceptable if the product flow is drinking water.

Figure 6 shows a prior art embodiment of the pump where the main difference relative to Figure 5 is that the inlet side of the main impeller 23 is facing downwards.

In order to protect the internal system inside the motor assembly 3, the absolute pressure of the barrier / cooling liquid that enters the motor assembly through inlet 8 is higher than the hydrostatic pressure of the sea water 2, and also higher than the product flow pressure at any location between the pump inlet 6 and the pump outlet 7.

Seals 18 are located between the stationary motor assembly 3 and the rotating motor shaft 16a. Seals 18 are also located between the stationary main pump assembly 4 and the rotating pump shaft 16c. The seals 18 cannot fully prevent leakage of liquid from the high-pressure side inside the motor enclosure 19 to the pump enclosure 22. The leak path 25 of barrier / cooling liquid is illustrated with the curved arrows 25.

The pump may have additional impellers 23 mounted on the pump shaft 16c that are not shown in Figure 5. In this case additional pump casings 24 with impellers 23 are mounted on top of each other where the outlet from the upstream side is directed into the inlet of the downstream side.

Figure 7 presents an embodiment of the invention that is applied on the prior art embodiment presented in Figure 5. Figure 7 includes the mid-chamber assembly 5 mounted between the motor assembly 3 and the main pump assembly 4 shown in Figure 5.

The purpose of the mid-chamber is to collect and pump the leaked flows 13 out of outlet 9 through conduit 14 to a third location 15 as shown in Figure 4, and hence to prevent ingress of product flow from the main pump assembly 4 into the sealed enclosure 19 of the motor assembly 3, and also to prevent ingress of barrier / cooling liquid from the motor assembly 3 into the pump enclosure 22 in the pump assembly 4.

The mid-chamber assembly 5 in Figure 7 has a pump device 28b driven by a leak pump motor 29. The pump device 28b sets up a pressure difference between the liquid inside the mid-chamber and the leak liquid outlet 9. The commingled leak flows of barrier / cooling liquid along leak path 25 and product flow along leak path 26 is pumped out of the pump module through the conduit 27 to the outlet 9 of the mid-chamber.

In this embodiment, the purpose of the pressure balancing line 52 is to decrease the necessary design pressure of the pumping device 28b. This is advantageous when the pressure of the product flow at the inlet 6 is significantly below the pressure of the product flow at the outlet 7 and the pressure of the product flow at the inlet 6 is also significantly below the pressure of the sea water 2 surrounding the pump module 1.

The leak pump motor 29 can be energized during operation of the pump when the motor inside the motor assembly 3 is running, and the leak pump motor 29 can also be energized when the main motor inside the motor assembly 3 is stopped.

Figure 8 presents an embodiment of the invention that is applied to the prior art embodiment presented in Figure 6 where the low-pressure inlet side of the impeller is facing down. Figure 8 includes the mid-chamber assembly 5 mounted between the motor assembly 3 and the main pump assembly 4.

The purpose of the mid-chamber 5 is to pump the commingled leak flow through the conduit 27 and then out of outlet 9. The commingled leak flow 13 the flows through conduit 14 to a third location 15 as shown in Figure 4, and hence assure no ingress of product flow from the main pump assembly 4 into the sealed enclosure 19 of the motor assembly 4, and also assure no ingress of barrier / cooling liquid from the motor assembly 3 into the pump enclosure 22 in the main pump assembly 4.

The mid-chamber assembly 5 in Figure 8 includes a pump device 28b driven by a leak pump motor 29. The pump device 28b sets up a head difference between the mid-chamber and the leak liquid outlet 9. The commingled leak flows of barrier / cooling liquid along leak path 25 and product flow along leak path 26 is pumped out of the pump module through the conduit 27 to the outlet 9 of the mid-chamber.

The main difference between Figure 8 and Figure 7 is the orientation of the pump main impeller 23. In Figure 8 the low pressure inlet side of the pump main impeller 23 is faced down. There is no need for a pressure balancing line (as shown in Figure 7) to decrease driving pressure from the motor assembly 3 to the pump assembly 4.

The leak pump motor 29 can be energized during operation of the pump when the main motor inside the motor assembly 3 is running, and the leak pump motor 29 can also be energized when the main motor inside the motor assembly 3 is stopped.

Figure 9 presents an alternative embodiment to the invention shown in Figure 7. The pumping of the flows of barrier / cooling liquid along leak path 25 and product flow along leak path 26 is performed with a pump plate 28a mounted on the pump shaft 16c.

The motor shaft 16a and pump shaft 16c are rotating with the rotor 20 when the main motor inside the motor assembly 3 is energized. The pump plate 28a acts as an impeller that sets up a head difference between the mid-chamber nearby the pump shaft 16c and the entrance of the conduit 27 of the mid-chamber. The pressure difference is proportional to the diameter of the pump plate 28a and proportional to the square of the rotational speed of the pump plate 28a.

The pump plate will pump the commingled leak flow through the conduit 27 and out of the mid-chamber outlet 9 when the main motor inside the motor assembly 3 is running.

Figure 10 presents an alternative embodiment to the invention shown in Figure 8. The pumping of the flows of barrier / cooling liquid along leak path 25 and product flow along leak path 26 is performed with a pump plate 28a mounted on the pump shaft 16c.

The motor shaft 16a and pump shaft 16c are rotating with the rotor 20 when the main motor inside the motor assembly 3 is energized. The pump plate 28a acts as an impeller that sets up a head difference between the mid-chamber nearby the pump shaft 16c and the entrance of the conduit 27 of the mid-chamber assembly 5. The head difference is proportional to the diameter of the pump plate 28a and proportional to the square of the rotational speed of the pump plate 28a.

Figure 11 presents an alternative embodiment of the invention as presented in Figure 7. The mid-chamber assembly 5 in Figure 11 leads the leaked flows into the pump device 28b driven by the leak pump motor 29 located in the mid-chamber assembly 5. The pump device 28b sets up a pressure difference between the domain nearby the pump shaft 16c inside the mid-chamber and the conduit 27 out of the mid-chamber. The commingled leak flows of barrier / cooling liquid along leak path 25 and product flow along leak path 26 is pumped out of the pump module through the conduit 27 to the outlet 9 of the mid-chamber.

The pump device 28b with pump motor may also be located outside of the midchamber assembly 5 downstream of the outlet 9 of the mid-chamber assembly 5 (not shown in Figure 11). See the schematic representation in Figure 14 where another alternative location of the pump device 28b is shown.

Figure 12 presents an alternative embodiment of the invention as presented in Figure 8. The mid-chamber assembly 5 in Figure 12 collect the leaked flows and the pump device 28b driven by the leak pump motor 29 located in the mid-chamber assembly 5 pumps the leaked flows out of the mid-chamber 5 through the outlet 9. The pump device 28b sets up a pressure difference between the area surrounding the pump shaft 16c inside the mid-chamber and the conduit 27 out of the midchamber to the outlet 9.

The pump device 28b with pump motor may also be located outside of the midchamber downstream of the outlet 9 of the mid-chamber assembly 5 (not shown in Figure 12). See the schematic representation in Figure 14 where an alternative location of the pump device 28b is shown.

Figure 13 presents an embodiment of the invention where the mid-chamber assembly 5 do not includes any pump device 28.

In Figure 13, a pump plate 28c is mounted onto the main impeller 23 to be located inside the volume between the main impeller 23 and the pump casing 24. The purpose of the pump plate 28c is to increase the pressure of the leaked product liquid upstream the interface between the main pump assembly 4 and the midchamber assembly 5. An additional pump device may be located downstream of the outlet 9 from the mid-chamber assembly. See Figure 15.

Figure 14 presents an alternative embodiment of the invention. The pump device 28b is located downstream of the outlet 9 from the mid-chamber assembly 5. There is no pump device inside the mid-chamber assembly 5.

The discharge conduit 14 for the commingled leak flow 13 out of outlet 9 makes a liquid connection to another location 15, for example to another location in the pump module 1 or to another location at another module.

In Figure 14, the pump device 28b collects the commingled leak flow 13 from a single pump module 1 and pumps the combined leak flows 13 to the third location 15 through the downstream discharge conduit 14.

In an alternative embodiment, the pump device 28b pumps commingled leak flows 13 through multiple discharge conduits 14 in liquid connection with multiple upstream mid-chamber assembly 5 outlets 9 of multiple pump modules 1 to the pump device 28b. The pump device 28b pumps the combined multiple leak flow through the downstream discharge conduit 14 to the third location 15.

The third location 15 can for example be: Another pumped flow in a neighbouring system; Separator to treat the leak flow; Storage device of leak flow, including system to allow transport of the leaked volume; Conduit line in an umbilical to topside facility either offshore or onshore; Hot stab connection for periodic emptying to a vessel through a downcomer conduit; Outlet to the sea water 2 in the ambient environment surrounding the pump module 1;

Figure 15 presents the location of a pump device 28b downstream of the outlet 9 in the mid-chamber assembly 5 for the system shown in Figure 13. An additional pumping device with a pump plate 28c is located inside the main pump assembly 4, mounted on the impeller as shown in Figure 13.

The pump plate 28c shown in the Figures 13 and 15 will in this case boost the pressure of the product flow. The pump plate 28c may be connected to the impeller and located between the impeller and the pump casing, and the pump plate 28c may also be an integrated part of the main impeller 23 as shown in in Figure 13.

Figure 16 presents a few examples of the third location 15 where the leak liquid outlet flow 13 is discharged. Figure 16a presents a simplified sketch of Figure 15, where details inside the pump module 1 are hidden in order to simplify the sketch. Additionally, the motor 29 of the pump device 28b is included for the case where there is a pump device downstream outlet 9. The pump device and motor may as an alternative instead be located inside the pump module 1 as shown in Figure 4. The third location 15 is illustrated with the dashed line box 15 in Figure 16a.

Figure 16b presents a case where the third location 15 consists of an outlet 30 downstream the discharge conduit 14 where the leak liquid outlet flow 13 is discharged to the sea water 2 of the ambient surrounding environment.

Figure 16c presents a case where the third location 15 is a receiving facility 36 located either onshore or floating on the sea surface on a vessel. Here the leak liquid outlet flow 13 is conducted through a liquid conduit 14 inside an umbilical or flowline between the pump module and the receiving facility 36.

Figure 16d presents a case where the third location consists of a separator 31 downstream the discharge conduit 14 in which the leak liquid outlet flow 13 is directed through the inlet 32. The separator separates the mixture that is the leak liquid outlet flow 13 into the two liquid phases of barrier / cooling liquid 34 and process liquid 35 respectively. The separated phases are discharged through the two outlets 33 respectively of the separator 31. The streams exiting the outlets 33 can be directed further downstream in the system to neighboring locations 39 and 40 respectively.

Figure 16e presents the case where the third location 15 is a tee-connection where the leak liquid outlet flow 13 is conducted through a liquid conduit 14 and then commingled with another process flow 37 flowing through another process conduit 38. The process flow 37 is flowing from a neighboring process location 39 upstream of the tee-connection to another neighboring process location 40 downstream of the tee-connection. These locations 39 and 40 in Figure 16e are not necessarily the same as the locations 39 and 40 in Figure 16d. The downstream location 40 in Figure 16e may be an outlet similar to the outlet 30 shown in Figure 16b.

The flows discharged from the one or both outlets 33 of the separator 31 in Figure 16d may be discharged to an outlet similar to the outlet 30 shown in Figure 16b, or a process facility 36 as shown in Figure 16c, or another tee-connection as shown in Figure 16e. The flows discharged from the one or both outlets 33 of the separator 31 in Figure 16d may also be discharged to subsea periodic evacuation systems 41 as shown in Figure 17.

Figure 17a presents a sketch of a subsea periodic evacuation system 41 for liquids that will be collected periodically with an offshore vessel 48. The incoming liquid flow 42 is conducted through a conductor 43 into the inlet 46 of the storage device 44. The storage device 44 has a storage tank, bladder, or accumulator 45 with sufficiently large volume to collect the total volume of the incoming liquid flow 42 during the interval periods between emptying operations. The hot stab connector 47 will be used during the periodic emptying operation.

Figure 17b presents a sketch of an evacuation system 41 during the emptying operation. A vessel 48 will be located on the sea surface 51 above the evacuation system 41. The downcomer 49 from the vessel 48 is connected to the hot stab connector 47. The outgoing flow 50 through the conduit inside the downcomer 49 empties the tank, bladder, or accumulator 45 into the vessel 48. When the the tank, bladder, or accumulator 45 is empty, the downcomer 49 is disconnected from the hot stab connector 47.

The incoming flow 42 in Figure 17 may for example be the flow 34 from the separator 31 in Figure 16d. In this case the location 39 in Figure 16d is the subsea periodic evacuation system 41 in Figure 17.

The sealed enclosure 19 of the motor assembly, the pump casing 24 of the main pump assembly 4 and mid-chamber for leaked liquid of the mid-chamber assembly 5 may be separate elements or may be formed as integrated chambers in one or more assemblies.

Claims

1. A subsea motor and pump assembly comprising a motor assembly (3) with a rotor (20), a stator (21), a sealed enclosure (19) with a barrier liquid feed inlet (8); a main pump assembly (4) with a main impeller (23) and a pump casing (24); a mid-chamber assembly (5) with a mid-chamber for leaked liquids;

a shaft (16) extending through the mid-chamber assembly (5), connecting the rotor (20) of the motor assembly (3) and the impeller (23) of the main pump assembly (4); and

a pumping device (28) adapted to pump leaked liquids out of the mid-chamber assembly (5) and to maintain a pressure in the mid-chamber below a pressure in a pump outlet and below a pressure in the sealed enclosure (19) of the motor assembly (3).

2. The subsea motor and pump assembly of claim 1, wherein the pumping device (28) pumping leaked liquids out of the mid-chamber assembly (5) includes a separate pump and motor assembly including a leaked liquid pump device (28b) driven by a separate motor (29).

3. The subsea motor and pump assembly of claim 1, wherein the pumping device (28) pumping leaked liquids out of the mid-chamber assembly (5) includes a pump plate (28a).

4. The subsea motor and pump assembly of claim 3, wherein the pump plate (28c) is mounted on the main impeller (23)

5. The subsea motor and pump assembly of claim 3, wherein the pump plate (28a) is mounted on the shaft (16) inside the mid-chamber assembly (5).

6. The subsea motor and pump assembly of claim 1, wherein the pump plate (28c) is formed as a part of the impeller (23)

7. The subsea motor and pump assembly of any of the previous claims, further including a liquid level sensor in the mid-chamber, and wherein the pumping device (28) is controlled in relation to the liquid level sensor in the mid-chamber.

8. The subsea motor and pump assembly of claim 1, further including a pressure sensor in the mid-chamber, and wherein the pumping device (28) is controlled in relation to the pressure in the mid-chamber.

9. The subsea motor and pump assembly of claim 8, further including a pressure sensor in the sealed enclosure (19) of the motor assembly (3) and a pressure sensor in the pump casing (24), and wherein the pumping device (28) furthermore is controlled in relation to the pressure in the sealed enclosure (19) and in the pump casing (24)

10. The subsea motor and pump assembly of any of the preceding claims, wherein the pump assembly is adapted to pump drinking water.

11. The subsea motor and pump assembly of any of the preceding claims, wherein the pumping device (28) pumps the leaked liquids (13) out of the mid-chamber assembly (5) and to a third location (15) selected from the group:

another pumped flow (37) in a neighbouring system;

a separator (31) to treat the leaked flow;

a storage device (44) of leak flow, including a system to transport the leaked volume; a conduit line in an umbilical to a topside facility (36) either offshore or onshore; a conduit in a downcomer (49) to a topside vessel (48); and

an outlet (30) to the surrounding seawater (2) in the ambient environment surrounding the pump module (1).

12. The subsea motor and pump assembly of any of the preceding claims, wherein the shaft (16) extending through the mid-chamber assembly (5) includes a flexible shaft connection (16b) located inside the mid-chamber, mechanically connecting a pump shaft (16c) and a motor shaft (16a).

13. The subsea motor and pump assembly of any of the preceding claims, wherein the mid-chamber assembly (5) with a mid-chamber for leaked liquids includes a motor shaft (16a) opening and a pump shaft (16c) opening.

14. Use of a subsea motor and pump assembly of claim 1 in a subsea desalination plant.