Barrier fluid management system

Abstract

The present invention relates to a subsea drilling mud pump assembly with a liquid filled motor (2) with a sealed housing, an output shaft and at least one motor shaft seal (3) A motor pressure compensator (11) is in fluid connection with the sealed housing of the liquid filled motor (2). A pump has an input shaft and a fluid filled cavity between a plurality of pump shaft seals (21) including an upper seal (21a) and a lower seal (21b). A supply reservoir (22) is in fluid connection with the fluid filled cavity and the sealed housing of the liquid filled motor (2). The output shaft and the input shaft are connected in a mid-cavity (4). An excess fluid reservoir (8) is in fluid connection with at least one of: the mid-cavity (4) through a mid-cavity excess valve (27) and the sealed housing of the liquid filled motor through a motor housing valve. An excess fluid line allows excess fluid from the excess fluid reservoir (8) to be expelled.

Description

BARRIER FLUID MANAGEMENT SYSTEMTechnical Field

[0001] The present invention relates to a system that allows for zero leakage to the environment of barrier and motor fluid when using a subsea motor that is compensated to sea.Background Art

[0002] Most centrifugal pumps have some form of mechanical seal on the shaft to separate the process fluid from the environment.

[0003] Most liquid-filled motors have some form of mechanical seal on the shaft to separate the motor liquid from the environment.

[0004] There are motors and pumps with sealing solutions that do not involve mechanical seals. These are out with the current invention.

[0005] In many subsea applications for offshore oil and gas drilling it is desirable to ensure that there is zero leakage liquids to sea. This could be because the fluid in use is considered harmful to the environment, or simply because there is a zerodischarge policy.

[0006] In many applications it is desirable to have zero leakage of process fluid to the environment. This could be because the process fluid is considered harmful to the environment, or simply because there is a zero-discharge policy.

[0007] For subsea pumps, i.e., any pump that is submerged in water, typically the ocean, but also lakes and the like, there is an ambient pressure related to the water depth.

[0008] The motor and the pump must be designed to withstand the external and internal pressures they will be exposed to.

[0009] Mechanical seals will always have some degree of leakage across them when they are exposed to a differential pressure. The main direction of leakage is from the side with the highest pressure to the side with the lowest pressure.

[0010] Due to the osmotic effect, there could also be leakage from the low-pressure to the high-pressure side, depending on the properties of the involved liquids.

[0011] In many subsea process pumping applications, for example, for subsea hydrocarbon pumps, the motor and pump are close-coupled, and both the motor and the pump are contained within a common pressure envelope. A typical example is shown in figure 1 . The motor pressure casing has several disadvantages. It adds weight to the assembly. It adds cost to the assembly. The high wall-thickness lowers the conductivity of the casing compared to a casing with a lower wall thickness. This makes it harder to get rid of the heat that the motor generates. Such solutions therefore frequently require large external cooling circuits, which further adds to the cost and complexity. Also, such subsea pumping units often have challenges with the pressure compensation systems related to the mechanical seals, both for rapid changes in process pressure, but also for deployment and recovery of the systems. The person skilled in the art is familiar with these issues.

[0012] Since there is always a leakage of fluids across the mechanical seals, in a subsea pumping solution where the motor and pump are contained within a common pressure barrier, the pressures inside the motor / pump will typically be such that the leakage is towards the process, across the mechanical seal, as shown in figure 2. The fluids leak out of the system, but into the process, not into the sea.

[0013] Many pump / motor systems in operation today utilize motors that are pressure-compensated to ambient pressures and where only the pump part of the pumping system has a pressure casing. In this configuration, there is an area between the motor and the pump that is open to sea. For subsea motors in general, and particularly for electrical motors, one wants to avoid ingress of sea water. Such systems will therefore operate with an overpressure on the motor side, and there will be a leakage of motor fluid across the mechanical seal to the sea. On the pump side, the pump will typically have an internal pressure that is higher than the ambient pressure. For a pump with a single mechanical seal, there will be a leakage of process fluid into the sea across the mechanical seal. Such a system is shown in figure 3. For a motor with a single mechanical seal towards sea, due to the leakage over the seal, the system will need to be topped up with fluid from an external supply, typically a compensator, a line from surface or with some other re-fill option. The external supply will also typically account for volume changes of fluid in the motorassociated with temperature changes. Where the motor fluid is water, this leakage into the ocean will often be acceptable. Where this motor fluid is considered harmful to the environment, e.g., where it is oil, this discharge can be problematic, particularly with an increased environmental awareness in society.

[0014] In many cases, it is unacceptable for process fluids from the pump to leak into the sea. One solution is to utilize a double mechanical seal on the pump, use a non-harmful fluid as a barrier fluid and pressurize the cavity to a pressure higher than the process pressure. With this set-up, shown in figure 4, the barrier fluid will leak into the process and out into the sea. As the pressure on the barrier fluid is higher than on the process-side one ensures that there is no leakage of process fluid into the sea.

[0015] In some existing applications, the cavity between the motor and the pump is fitted with a compensator collecting the fluid. The challenge with this design is that it has a limited volume capacity before it fills up. If the level exceeds full, there is, for safety reasons, a relief valve that dumps the fluid to sea, which is not desired.Summary of invention

[0016] With the present invention, which relates to subsea pumping of drilling mud using centrifugal pumps and motors, it is possible to have zero leakage of process and motor fluid to the sea, also when utilizing a motor with a housing that is compensated to sea, with a very low differential pressure rating, typically less than 10 bar.

[0017] Accordingly, the present invention defines a subsea drilling mud pump assembly comprising: a liquid filled motor with a sealed housing, an output shaft and at least one motor shaft seal. A motor pressure compensator is in fluid connection with the sealed housing of the liquid filled motor. A pump with an input shaft and a fluid filled cavity between a plurality of pump shaft seals includes an upper seal and a lower seal. A supply reservoir is in in fluid connection with the fluid filled cavity and the sealed housing of the liquid filled motor. Furthermore, the subsea drilling mud pump assembly includes a connection between the output shaft and the input shaft in a mid-cavity. An excess fluid reservoir is in fluid connection with at least one of a) the mid-cavity through a mid-cavity excess valve, and b) the sealed housing of the liquidfilled motor through a motor housing valve. An excess fluid line allows excess fluid from the excess fluid reservoir to be expelled.

[0018] The mid-cavity forming the opening between the motor and the pump, may be inside a casing. Both the pump side and the motor side are fitted with seals. The seals may be e.g., in the shape of a dual pump seal and a single motor seal, but can also be double dual seals, triple seals or any other seal combination. The supply reservoir may provide fluid at a pressure slightly above process pressure to the system.

[0019] The purpose of the fluid connection from the supply reservoir to the fluid filled cavity between the plurality of pump shaft seals, including the upper seal and a lower seal, is to create a fluid pressure barrier towards the process. The fluid connection from the supply reservoir to the motor allows fluid to be supplied to the motor. The supply line between the supply reservoir and the motor may have a pressure reducer to supply fluid at a lower pressure than to the pump seal. The pressure-compensated motor compensator is fluidly connected to the motor and may ensure that the fluid pressure in the motor is maintained in case the supply reservoir is disconnected.

[0020] The supply reservoir may be fitted with a level indicator. The level indicator may be a visual indicator requiring an ROV for observation, or a transmitter that can transmit the signal to surface. The supply reservoir will in many cases be a spring- loaded or similar compensator where there is a known relationship between the degree of filling and the pressure. The supply reservoir may also be fitted with a pressure transmitter to monitor pressure, which in turn can be used to estimate the level in the supply reservoir.

[0021] In some operational scenarios, the pressure on the process side can be lower than the ambient pressure. E.g., in pumped riser operations, the pressure on the process side at the pump inlet can drop down to close to vacuum. Therefore, the invention may have a feature to ensure that the system does not inadvertently empty and collapse when operating with a low pressure on the process side. This could be achieved in many ways such as e.g., by fitting a non-return valve with a crack-open pressure higher than that of the ambient pressure at the operating depth.

[0022] In a first aspect of the invention the excess fluid reservoir is fluidly connected to the process. There may be a pump pumping fluid from the excess reservoir into the process.

[0023] In a second aspect of the invention the excess fluid reservoir is fluidly connected to a supply reservoir. This supply reservoir is used to supply the dual seal / motor with fluid. There may be a pump pumping fluid from the excess reservoir back to the supply reservoir. There may be a filter inline to ensure that clean fluid is returned to the supply reservoir.

[0024] In a third aspect of the invention the excess fluid reservoir is fluidly connected to a line in the same umbilical that that is used to power the pump. This umbilical goes back to the surface. There may be a pump pumping fluid from the excess reservoir to surface through the umbilical. There may be a filter inline to reduce the risk of clogging in the umbilical.

[0025] In a fourth aspect of the invention, the setup is similar to the third aspect of the invention. The difference being that there is no pump present, instead one uses the lower density of the fluid compared to the sea water as the driving force to lift the fluid upwards in the umbilical.

[0026] In a fifth aspect of the invention, the excess reservoir is fluidly connected to a subsea ROV receptacle. Once the return reservoir has been filled beyond a certain threshold, an ROV can be deployed to connect to the ROV receptacle and empty the return reservoir. This could be done in many ways, such as by operating a pump on the ROV that pumps to a tank, or by fluidly connecting a compensator on the ROV, with a lower spring setting than the return reservoir. The ROV will then bring the liquid to surface where it can be disposed of in a safe manner.

[0027] For aspects 1 -5 of this invention, it is critical that the process fluid does not end up in the sea. In this system, the process differential pressure to ambient will typically be higher than the differential pressure rating of the motor. Mechanical seals are known to fail. Therefore, there needs to be means in place to monitor the state of the mechanical seals.

[0028] In a sixth aspect of the invention the system allows for identification of which of the pump seal surfaces would be damaged in case of malfunction. By setting upvalves in a certain way and reading levels in supply and excess reservoirs, it can be identified which of the seal surfaces is leaking.

[0029] In a seventh aspect of the invention the system allows for testing of the integrity of the motor seal. By setting up valves in a certain way and reading levels in supply and excess reservoirs, it can be verified that the motor seal integrity is undamaged.

[0030] The present invention concerns a subsea drilling mud pump assembly. The mud pump assembly includes a liquid filled motor with a sealed housing, an output shaft and at least one motor shaft seal. A motor pressure compensator is in fluid connection with the sealed housing of the liquid filled motor. A pump with an input shaft includes a fluid filled cavity between a plurality of pump shaft seals including an upper seal and a lower seal. A supply reservoir is in fluid connection with the fluid filled cavity and the sealed housing of the liquid filled motor. The output shaft and the input shaft are connected in a mid-cavity. An excess fluid reservoir is in fluid connection with at least one of: a) the mid-cavity through a mid-cavity excess valve; and b) the sealed housing of the liquid filled motor through a motor housing valve. An excess fluid line allows excess fluid from the excess fluid reservoir to be expelled.

[0031] The excess fluid line may be in fluid contact with a feed pump feeding the excess fluid from the excess fluid reservoir to the supply reservoir.

[0032] The excess fluid line may be in fluid contact with an umbilical.

[0033] The excess fluid line may be in fluid contact with a feed pump feeding the excess fluid from the excess fluid reservoir to the umbilical.

[0034] The excess fluid line may be in fluid contact with a feed pump feeding the excess fluid from the excess fluid line is into the process flow through a process return line.

[0035] The excess fluid line may be in fluid contact with an ROV stab receptacle allowing the excess fluid to be evacuated by an ROV.

[0036] The supply reservoir may include a supply reservoir fluid pressure transmitter and a supply reservoir fluid level transmitter.

[0037] The subsea drilling mud pump assembly may further include a mid-cavity pressure transmitter connected to the mid-cavity.

[0038] The pump shaft and the motor shaft may be vertical, and the motor may be located above the pump.

[0039] The motor pressure compensator may include a compensator fluid level transmitter.

[0040] The excess fluid reservoir may include an excess fluid level transmitter.

[0041] The subsea drilling mud pump assembly may furthermore include an excess header line connecting the excess fluid reservoir with the mid-cavity and the sealed housing of the liquid filled motor.

[0042] The subsea drilling mud pump assembly may furthermore include a pressure reducer in the fluid connection between the supply reservoir and the sealed housing of the liquid filled motor.

[0043] A motor supply valve may be located in the flow path between the supply reservoir, and the sealed housing of the liquid filled motor.

[0044] A mid-cavity supply valve may be located in the flow path between the supply reservoir, and the fluid filled cavity between the plurality of pump shaft seals.

[0045] The supply reservoir may be at least one of a subsea supply reservoir and a topside supply reservoir connected to the fluid filled cavity and the sealed housing of the liquid filled motor with an umbilical.A seal cavity pressure transmitter may be connected to the fluid filled cavity between the plurality of pump shaft seals.Brief description of drawings

[0046] Figures 1 -6 describe the prior art;

[0047] Figure 1 is a schematic representation of a motor and a pump, contained within a common pressure envelope;

[0048] Figure 2 is a schematic representation of a motor and a pump, showing the direction of leakage from motor towards process fluid;

[0049] Figure 3 is a schematic representation of a motor and a pump, where they are not contained within a common pressure envelope. The motor being compensated to sea. Both motor and pump have single seals;

[0050] Figure 4 is a schematic representation of a motor and a pump, where they are not contained within a common pressure envelope. The motor being compensated to sea. The motor has a single seal, and the pump has a dual seal;

[0051] Figure 5 is a schematic representation of a motor and a pump, where the cavity fluid is collected in an excess reservoir;

[0052] Figure 6 is a schematic representation of a pumped riser system;

[0053] Figures 7-12 describe the invention;

[0054] Figure 7 is a schematic representation of the various aspects of the invention;

[0055] Figure 8 shows aspect 1 of the invention;

[0056] Figure 9 shows aspect 2 of the invention;

[0057] Figure 10 shows aspect 3 of the invention;

[0058] Figure 11 shows aspect 4 of the invention;

[0059] Figure 12 shows aspect 5 of the invention; and

[0060] Figure 13 corresponds to figure 10 apart from disclosing an umbilical to the supply reservoir.Detailed description of drawings

[0061] Figure 1 is a schematic representation of a motor and a pump, contained within a common pressure envelope towards the sea 50. The motor 2 is fluidly connected to the pump 1 with a mechanical seal 18 in between. Arrows show the direction of the process fluid through the pump 1 . The motor is located above the pump and the motor and pump shafts are vertical.

[0062] Figure 2 is a further detailing of figure 1 to emphasize that normally it is desired to have a controlled leakage in the direction from the motor towards the pump / process.

[0063] Figure 3 is a schematic representation showing a motor 2 and a pump 1 , where the motor 2 is compensated to sea via a motor compensator 11 . A single motor shaft seal 3 seals between the motor fluid and the motor shaft 19 towards thesea 50. A single pump shaft seal 5 seals between the process fluid in the pump 1 and the motor shaft 19 towards the sea 50.

[0064] Figure 4 is a version of figure 3 where the pump shaft seal is a dual pump seal 21 . The dual pump seal 21 comprises an upper or outer shaft seal 21 a and an inner or lower shaft seal 21 b. The outer shaft seal is further away from the pump than the inner shaft seal. In between the upper shaft seal 21 a and the lower shaft seal 21 b there is a fluid filled cavity which is connected to a supply reservoir 22.

[0065] Figure 5 is a version of figure 4 where the return fluid from the mid-cavity 4 is collected in an excess reservoir 8. A pressure relief valve 43 ensures that in case the excess reservoir 8 is full, fluid is dumped to sea 50 to not damage the excess reservoir 8.

[0066] Figure 6 shows schematically a pumped riser system. A pump 1 is connected to a marine riser 44, via a pump inlet 46. In some operational scenarios the fluid level 45 in the riser is very low, close to the pump inlet 46 that the pressure in the pump 1 is close to vacuum.

[0067] Figure 7 shows the various aspects of the invention in one drawing. A pump 1 and a motor 2 may be fluidly connected via e.g., a dual pump seal 21 , a mid-cavity 4 and a single motor seal 3. The seal configuration may be different from what is shown in Figure 7, such as a triple seal, quadruple seal or any other configuration of seals. A pressure transmitter 14 may be connected to the mid-cavity 4. A supply reservoir 22 may be connected to a dual pump seal 21 via a dual pump seal supply line 42, and to the motor 2 via a motor supply line 41 . The supply reservoir 22 may be a subsea reservoir but can also be e.g., an umbilical from topside. The functionality is to supply fluid to the motor 2 and the dual pump seal 21 at desired pressure. Each of the lines from the supply reservoir may be fitted with valves 31 and 32. The motor supply line may be fitted with a pressure reducer 34 as the fluid pressure supplied to the motor differs from the pressure supplied to the dual pump seal 21 in addition to the motor supply line 31 . The supply reservoir 22 may have a supply reservoir pressure transmitter 16 showing pressure and a supply reservoir level transmitter 17 showing fluid level in addition to the mid-cavity supply valve 32. A seal cavity pressure transmitter 20 may be connected to the cavity in the dual pump seal 21 . The mid-cavity 4 may be fitted with a cavity excess line 10 including a cavityexcess line valve 27. The motor 2 may be fitted with a motor excess line 12 including a motor excess line valve 30. Both excess lines may lead to an excess header line 13. There may be an excess reservoir 8 with a level transmitter 35 to measure fluid level. This excess reservoir would typically be compensated to ambient pressure in the sea at the motor depth. Fluid evacuated from the cavity / motor may be collected in the excess reservoir which acts as an excess fluid buffer. The excess header line 13 may be connected via an ROV excess line 36 to an ROV stab receptacle 7. On the excess header line 13 there may be a non-return valve 28, typically with a set crackopen pressure of 2 bar, connected to sea and a feed pump 6. This non-return valve may also be fitted anywhere else upstream the feed pump 6. The feed pump 6 may be a piston pump which may be powered separately or from the existing subsea control system. The non-return valve 28 ensures that the pressure can never fall below the ambient pressure in the excess header line, if the pressure in the excess header line is sufficiently reduced, e.g., 2 bar below ambient pressure, the non-return valve opens and lets sea water into the excess header line. Downstream the pump there may be a process return line 38 going into the process, a reservoir return line 39 going back to the supply reservoir and an umbilical return line 40 going to a topside facility via an umbilical 25. The process return line 38 may be fitted with a valve 15, the reservoir return line 39 may be fitted with a valve 29 and the umbilical return line 40 may be fitted with a valve 33. These valves may be set up to achieve the desired excess fluid return path. There may be a filter 24 on the line going to the supply reservoir and the umbilical. The filter 24 ensures that the fluid going to the supply reservoir 22 or topside via an umbilical 25, is clean enough to be re-used or to reduce the risk of clogging in the umbilical 25. A motor compensator 11 with a level transmitter 23 may be connected to the motor 2 ensuring constant supply of fluid.

[0068] In Figures 8-12 the valves 27 and 30 may be configured as desired depending on whether one wishes to route the excess fluid to exit from the cavity (open 27 and close 30) or from the motor (open 30 and close 27). The invention may also exclude any of the lines 10, 12, 41 or 42 with respective valves 27, 30, 31 and 32, as long as there is at least one line going from the supply reservoir 22 into the mid-cavity 4 or to the motor 2, and at least one line going from the excess header line 13 into the mid-cavity 4 or the motor 2. If both valves 27 and 30 are present in the setup, valve 30 must only be operated if the pressure in the excess reservoir 8 islower than in the motor compensator 11 . This can be controlled by emptying the excess reservoir 8 to reduce the pressure. If only one of the valves 27 or 30 is present in the setup, this valve will always be open during operation. It may be closed e.g., when the feed pump 6 is used to empty the excess reservoir 8. In such a scenario pressure in the mid-cavity 4 would be monitored on pressure transmitter 14. The supply reservoir 22 has a supply reservoir pressure transmitter 16 showing pressure and a supply reservoir level transmitter 17 showing fluid level in addition to the mid-cavity supply valve 32.

[0069] Figure 8 shows how to achieve aspect 1 of the invention. When desired, the valve 15 is opened and the feed pump 6 is started. Fluid from the excess reservoir 8 is emptied into the process flow through the process return line 38.

[0070] Figure 9 shows how to achieve aspect 2 of the invention. When desired, the valve 29 is opened and the feed pump 6 is started. Fluid from the excess reservoir 8 is emptied into the supply reservoir 22 through the reservoir return line 39.

[0071] Figure 10 shows how to achieve aspect 3 of the invention. When desired, the valve 33 is opened and the feed pump 6 is started. Fluid from the excess reservoir 8 is emptied into the umbilical 25 through the umbilical return line 40.

[0072] Figure 11 shows how to achieve aspect 4 of the invention. Similar to aspect 3 of the invention, fluid is emptied through the umbilical 25. The difference being that there is no pump present to evacuate the fluid. The evacuation of the fluid is conveyed by the density differential between the fluid to be evacuated and the sea water density, where the fluid to be evacuated has a lower density, pushing the fluid upwards in the umbilical.

[0073] Figure 12 shows how to achieve aspect 5 of the invention. When desired, an ROV engages with the ROV stab receptacle 7. The fluid can then be evacuated by the ROV. This could be done by operating a pump on the ROV that pumps to a tank, or by fluidly connecting a compensator on the ROV, with a lower pressure setting than the return reservoir. The ROV will then bring the fluid to surface where it will be disposed of in a safe manner.

[0074] In aspect 6 of the invention the system enables detection of which mechanical seal element has failed when there is a pump mechanical seal malfunction. Such a pump mechanical seal malfunction can be detected as anincrease in the loss rate from supply reservoir 22. In a system as shown in Figure 7 where there typically is a dual pump seal 21 , comprising an upper seal 21 a and a lower seal 21 b, and a single motor seal 3, one can detect a possible malfunction in the dual pump seal 21 by having a setup as follows: close valves 30, 31 and open valves 27, 32. Monitor level in the supply reservoir 22, level in the excess reservoir 8 and level in the motor compensator 11 . Calculate the combined drop in level in the motor compensator 11 and the supply reservoir 22 and compare this combined drop with the increase in level in the excess reservoir 8. If these values match (typically 80% or more) the leak is on seal surface 21 a. If these values do not match (typically 20% or less), the leak is on seal surface 21 b. Prior to such an event, the user may have monitored the loss rates in normal operation from the supply reservoir 22 and the motor compensator 11 . These loss rates can then be used to remove the effect of loss rate from the motor compensator 11 when doing the calculations on level increase in the excess reservoir 8.

[0075] In aspect 7 of the invention the system enables detection of motor seal malfunction. In a system as shown in Figure 7 where there typically is a dual pump seal 21 and a single motor seal 3, one can detect a possible malfunction in the single motor seal 3 by having a setup as follows: close valves 30, 32 and open valves 27, 31 . Monitor levels in the supply reservoir 22, in the excess reservoir 8 and in the motor compensator 11 . Calculate the combined drop in level in the motor compensator 11 and the supply reservoir 22 and compare this combined drop with the increase in level in excess reservoir 8. If the level in the excess reservoir 8 increases, there is a leak on the single motor seal 3.

[0076] Figure 13 corresponds to figure 10, with the only difference being that the supply reservoir 22 a topside supply reservoir connected to the fluid filled cavity and the sealed housing of the liquid filled motor 2 with an umbilical 25b, allowing the supply reservoir 22 to be situated at any suitable location.

[0077] Clearly may the different embodiments shown above be combined, such that for instance an umbilical 25b can be used between the supply reservoir 22 connected to the fluid filled cavity and the sealed housing in all the embodiments.

[0078] The umbilical 25b for the supply reservoir 22 may clearly also be combined with the umbilical return line 40 going to a topside facility via an umbilical 25.

Claims

AMENDED CLAIMS received by the International Bureau on 27 November 2024 (27.11.2024)1 . A subsea drilling mud pump assembly comprising: a liquid filled motor (2) with a sealed housing, an output shaft and at least one motor shaft seal (3); a motor pressure compensator (1 1 ) in fluid connection with the sealed housing of the liquid filled motor (2); a pump with an input shaft and a fluid filled cavity between a plurality of pump shaft seals (21 ) including an upper seal (21 a) and a lower seal (21 b); a supply reservoir (22) in fluid connection with the fluid filled cavity and the sealed housing of the liquid filled motor (2); a connection between the output shaft and the input shaft in a mid-cavity (4); an excess fluid reservoir (8) in fluid connection with at least one of: a) the mid-cavity (4) through a mid-cavity excess valve (27); and b) the sealed housing of the liquid filled motor (2) through a motor housing valve (30); and an excess fluid line allowing excess fluid from the excess fluid reservoir (8) to be expelled.

2. The subsea drilling mud pump assembly of claim 1 wherein the excess fluid line is in fluid contact with a feed pump (6) feeding the excess fluid from the excess fluid reservoir (8) to the supply reservoir (22).

3. The subsea drilling mud pump assembly of claim 1 wherein the excess fluid line is in fluid contact with an umbilical (25).

4. The subsea drilling mud pump assembly of claim 3, wherein the excess fluid line is in fluid contact with a feed pump (6) feeding the excess fluid from the excess fluid reservoir (8) to the umbilical (25).

5. The subsea drilling mud pump assembly of claim 2, wherein the excess fluid line is in fluid contact with a feed pump (6) feeding the excess fluid from the excess fluid line is into the process flow through a process return line (38).

6. The subsea drilling mud pump assembly of claim 1 , wherein the excess fluid line is in fluid contact with an ROV stab receptacle (7) allowing the excess fluid to be evacuated by an ROV.

7. The subsea drilling mud pump assembly of one of the claims 1 -6, wherein the supply reservoir (22) includes a supply reservoir fluid pressure transmitter (16) and a supply reservoir fluid level transmitter (17).

8. The subsea drilling mud pump assembly of one of the claims 1 -7, further including a mid-cavity pressure transmitter (14) connected to the mid-cavity (4).

9. The subsea drilling mud pump assembly of one of the claims 1 -8, wherein the pump shaft and the motor shaft are vertical and wherein the motor is located above the pump.

10. The subsea drilling mud pump assembly of one of the claims 1 -9 wherein the motor pressure compensator (11 ) includes a compensator fluid level transmitter (23).11 . The subsea drilling mud pump assembly of one of the claims 1 - 10, wherein the excess fluid reservoir (8) includes an excess fluid level transmitter (35).

12. The subsea drilling mud pump assembly of one of the claims 1 - 11 further including an excess header line (13) connecting the excess fluid reservoir (8) with the mid-cavity (4) and the sealed housing of the liquid filled motor (2).

13. The subsea drilling mud pump assembly of one of the claims 1 - 12, further including a pressure reducer (34) in the fluid connection between the supply reservoir (22) and the sealed housing of the liquid filled motor (2).

14. The subsea drilling mud pump assembly of one of the claims 1 - 13, wherein a motor supply valve (31 ) is located in the flow path between the supply reservoir (22), and the sealed housing of the liquid filled motor (2).

15. The subsea drilling mud pump assembly of one of the claims 1 - 14, wherein a mid-cavity supply valve (32) is located in the flow path between the supply reservoir (22), and the fluid filled cavity between the plurality of pump shaft seals (21 ).

16. The subsea drilling mud pump assembly of one of the claims 1 - 14, wherein a seal cavity pressure transmitter (20) is connected to the fluid filled cavity between the plurality of pump shaft seals (21 ).

17. The subsea drilling mud pump assembly of one of the claims 1 - 15, wherein the supply reservoir (22) is at least one of a subsea supply reservoir and a topside supply reservoir connected to the fluid filled cavity and the sealed housing of the liquid filled motor (2) with an umbilical.

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