Multiphase pump and method for operating such a pump

The invention relates to a multiphase pump for conveying a multiphase process fluid in accordance with the preamble of the independent apparatus claim. The invention further relates to a method for operating such a pump. Multiphase pumps are used in many different industries, where it is necessary to convey a process fluid which comprises a mixture of a plurality of phases, for example a liquid phase and a gaseous phase. An important example is the oil and gas processing industry where multiphase pumps are used for conveying hydrocarbon fluids, for example for extracting the crude oil from the oil field or for transportation of the oil/gas through pipelines or within refineries.

Fossil fuels are usually not present in pure form in oil fields or gas fields, but as a multiphase mixture which contains liquid components, gas components and possibly also solid components. This multiphase mixture of e.g. crude oil, natural gas and chemicals may also contain seawater and a not unsubstantial proportion of sand and has to be pumped from the oil field or gas field. For such a conveying of fossil fuels, multiphase pumps are used which are able to pump a liquid-gas mixture which may also contain solid components, sand for example. One of the challenges regarding the design of multiphase pumps is the fact that in many applications the composition of the multiphase process fluid is strongly varying during operation of the pump. For example, during

exploitation of an oil field the ratio of the gaseous phase (e.g. natural gas) and the liquid phase (e.g. crude oil) is strongly varying. These variations may occur very sudden and could cause a drop in pump efficiency, vibrations of the pump or other problems. The ratio of the gaseous phase in the multiphase mixture is commonly measured by the dimensionless gas volume fraction (GVF) designating the volume ratio of the gas in the multiphase process fluid. In applications in the oil and gas industry the GVF may vary between 0% and 100%. In order to handle such large variations in the GVF it is state of the art to provide a buffer tank upstream of the inlet of a multiphase pump. The multiphase process fluid to be pumped by the multiphase pump is first supplied to a buffer tank of suited volume and the outlet of the buffer tank is connected to the inlet of the pump. By this measure the strong variations of the GVF may be damped thereby improving the pump performance. Modern multiphase pumps in the oil and gas industry may handle multiphase process fluids having a GVF of up to 95% or even more. Furthermore to control the torque required to safely handle the fluids in various GVF, viscosity or other compostitional ranges, the speed of the pump must also be changed.

In view of an efficient exploitation of oil- and gas fields there is nowadays an increasing demand for pumps that may be installed directly on the sea ground in particular down to a depth of 100 m, down to 500 m or even down to more than 1 ,000 m beneath the water's surface. Needless to say that the design of such pumps is challenging, in particular because these pumps shall operate in a difficult subsea environment for a long time period with as little as possible maintenance and service work. This requires specific measurements to minimize the amount of equipment involved and to optimize the reliability of the pump.

One concept that is used in particular for subsea applications of a multiphase pump is the so-called seal-less design. A seal-less pump has no mechanical seals. A mechanical seal is normally used for the sealing of the rotating shaft of a centrifugal pump and shall prevent the leakage of the process fluid along the shaft of the pump. Typically, a mechanical seal comprises a stator and a rotor. The rotor is connected in a torque-proof manner with the shaft of the pump and the stator is fixed with respect to the pump housing such that the stator is secured against rotation. During rotation of the shaft the rotor is in sliding contact with the stator thus performing the sealing action. Although such mechanical seals are widely spread within the technology of centrifugal pumps they are somewhat problematic for subsea applications because they are quite complicated and usually require additional equipment, which is often considered as a drawback for subsea applications. Therefore, seal-less pumps have been designed, i.e. pumps that have no mechanical seal. In many cases this requires that the pump and the motor for driving the pump are flooded with the process fluid. The advantage of the seal-less pump concept is the simpler design of the pump. In addition, the process fluid itself may be used for cooling and lubricating components of the pump, e.g. the bearings of the pump shaft and the motor of the pump.

A seal-less multiphase pump that is in particular suited for subsea

applications is for example disclosed in WO 20014/095291 A1 the disclosure of which is included by reference. Although this multiphase pump performs very well and reliably in praxis there is still a demand for improvements or alternatives to such multiphase pumps.

Starting from this state of the art it is therefore an object of the invention to propose an improved or an alternative multiphase pump for conveying a multiphase process fluid that is in particular suited for subsea application. The pump shall have a low complexity with regard to the equipment and a high reliability in operation. In addition, it is an object of the invention to propose a method for operating such a multiphase pump.

The subject matter of the invention satisfying these objects is characterized by the features of the respective independent claim.

Thus, according to the invention a multiphase pump for conveying a multiphase process fluid is proposed, having a common housing surrounding a pump unit and a drive unit, wherein the housing comprises a pump inlet and a pump outlet for the process fluid, wherein the pump unit comprises at least one impeller for conveying the process fluid from the pump inlet to the pump outlet, and a pump shaft on which each impeller is mounted, and wherein the drive unit comprises a drive shaft for driving the pump shaft, and an electric motor for rotating the drive shaft about an axial direction, and wherein a hydrodynamic torque converter is provided for hydrodynamically coupling the drive shaft to the pump shaft, the torque converter having a casing for receiving a transmission fluid, a pump wheel connected to the drive shaft, a turbine wheel connected to the pump shaft, and a stator for guiding the transmission fluid to the turbine wheel. Using a hydrodynamic coupling between the drive unit and the impeller of the pump instead of a mechanical coupling has the considerable advantage that especially sudden changes in the phase composition of the multiphase process fluid may be absorbed - at least partially- by the torque convertor. Thus, the negative impact of a slug flow is considerably reduced because the torque convertor may at least partially compensate sudden load changes to the pump impeller which are caused for example by large gas bubbles in the process fluid.

This results in several possibilities for reducing the complexity of the equipment of a multiphase pump installation. For example, the buffer tank that is used upstream of the pump inlet for damping variations in the phase composition of the process fluid in known multiphase pumps may be dispensed with. Especially in view of subsea applications this constitutes a considerable advantage because the complexity of the pump is reduced and the installation of the pump requires less work, thus reducing also the costs caused by the subsea installation of the pump.

Since the torque convertor may be used for the speed control of the pump or for a self-regulation of the rotational speed of the pump impeller, it is also possible to replace the variable frequency drive (VFD) with a less expensive drive, e.g. a single speed electric drive, or to use a VFD that has a narrower range for the variation of the frequency of the drive shaft rotation. Both measures constitute a considerable advantage from the economical perspective.

According to a particularly preferred embodiment the multiphase pump is designed as a seal-less pump without a mechanical seal, because the seal- less design concept has proven successful especially for applications, where the multiphase pump is installed at locations which are difficult to access, e.g. on the sea ground. According to a preferred embodiment the multiphase pump comprises at least one separating system for at least partly separating at least one phase- enriched component from the multiphase process fluid. The separated phase-enriched component, in particular a liquid-enriched liquid component and/or a gas-enriched gas component, can be used for a lubrication and/or a cooling of the pump unit and/or of the motor unit.

It is an advantageous measure that the multiphase pump comprises a first cooling circuit for cooling and lubricating the pump, wherein the first cooling circuit is designed to receive the process fluid or at least one phase-enriched component of the process fluid as coolant. Thus, the process fluid itself is used for cooling and lubricating components of the pump which reduces the complexity of the pump.

Preferably, the electric motor is designed as a canned motor having a motor stator, a rotor, an annular gap between the rotor and the motor stator, and a can between the rotor and the motor stator for sealing the motor stator hermetically with respect to the rotor, and wherein an inlet opening as well as an outlet opening is provided for circulating a cooling fluid of a second cooling circuit through the motor stator. By this measure the electric motor may be flooded with the process fluid or components thereof without jeopardizing the reliability or the safe operation of the motor. The motor stator is cooled by means of a second cooling circuit in which an electrically non-conducting second cooling fluid, for example a dielectric oil is used.

In order to achieve an effective cooling of the electric motor it is preferred that the gap between the rotor and the motor stator is in fluid communication with the first cooling circuit. Thus, the electric motor can be cooled by means of the process fluid, in particular with a phase-enriched component of the process fluid. Particularly preferred the gap is cooled with a gas-enriched gas component of the process fluid.

From the point of view of efficiency of the pump it is preferred that the electric motor is designed as a permanent magnet motor or as an induction motor. In view of controlling the output torque of the torque convertor to a greater effect or a greater extend it may be advantageous when the electric motor is designed as a variable frequency drive. However, depending on the specific application it may be also preferred to avoid a VFD and to use a simpler electric motor that can be operated at a single frequency only.

It is a further preferred measure, when the torque convertor comprises at least one adjustable guide vane. A torque convertor with adjustable guide vanes has the advantage that the output torque of the torque convertor or the rotational speed of the pump shaft may be controlled or adjusted by means of the adjustable guide vanes of the torque convertor without modifying the rotational speed of the electric motor.

According to a preferred embodiment of the invention the torque convertor is designed for receiving the cooling fluid of the second cooling circuit as transmission fluid. By this measure the amount of different liquids or fluids for operating the multiphase pump can be reduced.

Regarding this embodiment it is an advantageous measure, when the casing of the torque convertor is integrated into the second cooling circuit. By this measure the torque convertor is used as an additional driving force to circulate the cooling fluid in the second cooling circuit. Thereby the circulation in the second cooling circuit is enhanced resulting in an improved cooling of the motor stator.

For using the cooling fluid of the second cooling circuit as transmission fluid of the torque convertor or for integrating the casing of the torque convertor into the second cooling circuit, respectively, it is a preferred measure when the casing of the torque convertor comprises an intake for supplying the cooling fluid of the second cooling circuit to the casing and a discharge for the drain of the cooling fluid of the second cooling circuit, wherein a connecting line is provided connecting the intake or the discharge of the casing with the motor stator. In a preferred embodiment the multiphase pump is designed for subsea oil and gas conveyance. In particular, the multiphase pump may be designed for installation on the sea ground.

In addition, according to the invention a method is proposed for operating a multiphase pump designed according to the invention. The method is characterized in that the multiphase process fluid is directly supplied to the pump inlet without passing a buffer device upstream of the pump inlet. By avoiding a buffer device for damping the variations of the phase composition of the multiphase process fluid, for example a buffer tank the necessary equipment for the multiphase pump is considerably reduced. Thereby, the costs for the multiphase pump and the weight as well as the work for installation of the pump, e.g. on the sea ground, may be advantageously reduced.

Further advantageous measures and embodiments of the invention will become apparent from the dependent claims. The invention will be explained in more detail hereinafter with reference to the drawings. There are shown in a schematic representation:

Fig. 1 : a cross-sectional view of an embodiment of a multiphase pump according to the invention, and Fig.2: a cross-sectional view of an embodiment of a torque convertor.

Fig. 1 shows a cross-sectional view of an embodiment of a multiphase pump according to the invention which is designated in its entity with reference numeral 1 . The multiphase pump 1 is designed as a centrifugal pump for conveying a multiphase process fluid and has a common housing 2, a pump unit 3 and a drive unit 4. Both the pump unit 3 and the drive unit 4 are arranged within the common housing 2. The common housing 2 is designed as a pressure housing, which is able to withstand the pressure generated by the pump 1 as well as the pressure exerted on the pump 1 by the

environment. As shown in Fig. 1 the common housing 2 may comprise several housing parts, here two housings parts, which are connected to each other to form the common housing 2 surrounding the pump unit 3 and the drive unit 4.

In the following description reference is made by way of example to the important application that the multiphase pump 1 is designed and adapted for being used as a subsea pump in the oil and gas industry. In particular, the multiphase pump 1 is configured for installation on the sea ground, i.e. for use beneath the water-surface, in particular down to a depth of 100 m, down to 500 m or even down to more than 1000 m beneath the water-surface of the sea. In such applications the multiphase process fluid is typically a

hydrocarbons containing mixture that has to be pumped from an oilfield for example to a processing unit beneath or on the water-surface or on the shore. The multiphase mixture constituting the process fluid to be conveyed can include a liquid phase, a gaseous phase and a solid phase, wherein the liquid phase can include crude oil, seawater and chemicals, the gas phase can include methane, natural gas or the like and the solid phase can include sand, sludge and smaller stones without the multiphase pump 1 being damaged on the pumping of the multiphase mixture.

It goes without saying that the invention is not restricted to this specific example but is related to multiphase pumps in general. The invention may be used in a lot of different applications, especially in such applications where the multiphase pump 1 is installed at locations which are difficult to access.

The common housing 2 of the multiphase pump 1 comprises a pump inlet 5 through which the multiphase process fluid enters the pump 1 and a pump outlet 6 for discharging the process fluid with an increased pressure as compared to the pressure of the process fluid at the pump inlet 5. Typically the pump outlet 6 is connected to a pipe (not shown) for delivering the process fluid to another location. The pressure of the process fluid at the pump outlet 6 is referred to as 'high pressure' whereas the pressure of the process fluid at the pump inlet 5 is referred to as 'low pressure'. A typical value for the difference between the high pressure and the low pressure is for example 50 to 200 bar. The pump unit 3 further comprises at least one impeller 7, but typically as shown in the embodiment in Fig. 1 a plurality of impellers 7 for conveying the process fluid from the pump inlet 5 to the pump outlet 6 and thereby

increasing the pressure of the process fluid. The impellers 7 are mounted in series on a pump shaft 8 in a torque proof manner. By means of the pump shaft 8 the impellers 7 are driven during operation of the pump 1 for a rotation about an axial direction A that is defined by the longitudinal axis of the pump shaft 8.

The drive unit 4 comprises a drive shaft 9 for driving the pump shaft 8 and an electric motor 10 for rotating the drive shaft about the axial direction A.

The multiphase pump 1 is configured as a vertical pump, meaning that during operation the pump shaft 8 is extending in the vertical direction which is the direction of gravity. The drive shaft 9, too, extends in the vertical direction. Thus, the axial direction A coincides with the vertical direction. Referring to the usual orientation during operation that is shown in Fig. 1 the drive unit 4 is located above the pump unit 3. However in another embodiment the pump unit 3 may be located on top of the drive unit 4.

According to the invention the drive shaft 9 is operatively coupled to the pump shaft 8 by means of a hydrodynamic torque converter 20 that is only schematically represented in Fig. 1 . Details of the torque converter 20 will be explained later on with reference to Fig. 2.

The multiphase pump 1 is designed as a seal-less pump. A seal-less multiphase pump which has been disclosed for example already in

WO 2014/095291 A1 is a pump that has no mechanical seals. Mechanical seals are usually used for the sealing of the rotating shaft of a pump and comprise a rotor fixed to the shaft and rotating with the shaft as well as a stationary stator fixed with respect to pump housing. During operation the rotor and the stator are sliding along each other - usually with a liquid there between - for providing a sealing action to prevent the process fluid from escaping to the environment or entering the drive of the pump. The seal-less pump 1 shown in Fig. 1 has no such mechanical seals. The process fluid or parts thereof is/are deliberately allowed to enter the drive unit 4 and is/are used for cooling and lubricating the multiphase pump 1 or parts thereof, respectively.

The electric motor 10 of the drive unit 4 is designed as a canned motor having an inwardly disposed rotor 1 1 as well as an outwardly disposed motor stator 12 surrounding the rotor 1 1 with an annular gap 14 between the rotor 1 1 and the stator 12. The gap 14 is radially outwardly delimited by a can 13 that seals the motor stator 12 hermetically with respect to the rotor 1 1 and the gap 14. Thus, process fluid flowing through the gap 14 cannot enter the motor stator 12. The rotor 1 1 constitutes a part of the drive shaft 9 or is rotationally fixedly connected to the drive shaft 9, so that the rotation of the rotor 1 1 drivers the drive shaft 9.

Preferably, the electric motor 10 is a permanent magnet motor or an induction motor. To supply the electric motor 10 with energy, a power penetrator 15 is provided at the common housing 2 for receiving a power cable (not shown) that supplies the motor 10 with power.

The electric motor 10 may be designed to operate with a variable frequency drive (VFD) in which the speed of the drive, i.e. the frequency of the rotation is adjustable by varying the frequency and/or the voltage supplied to the motor. However it is also possible that the electric motor 10 is configured differently, for example as a single speed or single frequency drive.

The multiphase pump 1 further comprises two motor bearings 16, 17 for supporting the drive shaft 9 radially and axially as well as two pump bearings 18, 19 for radially and axially supporting the pump shaft 8. During operation the multiphase pump 1 is cooled and lubricated by means of the multiphase process fluid. In order to achieve an efficient cooling and an efficient lubrication, it is advantageous to generate from the multiphase process fluid one or more phase-enriched component(s). A phase-enriched component is a component that contains the respective phase with a higher percentage or with a higher concentration than the multiphase process fluid entering the pump 1 through the pump inlet 5. For example, by removing at least partially the solid phase from the multiphase process fluid a liquid- and gas -enriched component can be generated.

In order to generate the at least one phase-enriched component from the multiphase process fluid the multiphase pump 1 comprises at least one separating system 40 arranged at the lower end of the pump 1 (according to the representation in Fig. 1 ). During the operation of the multiphase pump 1 , some of the pumped multiphase process fluid is removed from the high- pressure side and supplied to the separation system 40. The separation system 40 separates the multiphase process fluid into three components, a gas-enriched component, a liquid-enriched component and a solid component which substantially includes the solid portion from the removed portion of the multiphase process fluid. For the sake of simplicity hereinafter the gas- enriched component is referred to as "process gas" and the liquid-enriched phase is referred to as "process liquid" although the gas-enriched component is typically not a pure gas phase and the liquid-enriched component is typically not a pure liquid phase. Regarding the design and the operating mode of the separation system 40 reference is made by way of example to WO 2014/095291 A1 where a detailed explanation of the separation system

40 can be found. The separation system 40 is in fluid communication with the low pressure side of the pump 1 , in particular with the pump inlet 5, by means of a balance line

41 through which the process fluid or parts thereof is/are channeled back to the pump inlet 5. In particular, the solid component that is separated by the separation system 40 is recycled to the pump inlet 5 through the balance line 41 and again introduced into the stream of the process fluid.

As it is also disclosed and described in detail for example in WO 2014/095291 A1 the bearings 16, 17, 18 which are supplied with the process liquid as lubricant and coolant are designed and used as additional separation systems in particular to separate at least partially the gaseous component of the process liquid from the process liquid.

Thus, the multiphase process fluid or components thereof are used as coolant and lubricant in a first cooling circuit 50 that will now be described. The first cooling circuit 50 is in fluid communication with a discharge port 42 of the separating system 40 through which a phase-enriched component, in particular the process liquid, is supplied to the cooling circuit 50. A line 51 connects the discharge port 42 with a heat exchanger 52 for cooling the process liquid circulating in the first cooling circuit 50. From the heat exchanger 52 the process liquid is channeled to and introduced into the motor bearing 16 through a line 53, into the second motor bearing 17 through a line 54, and into the pump bearing 18 through a line 55 for respectively cooling and lubricating the bearings 16, 17 and 18. The process liquid is discharged from the bearings 16, 17, 18 through respective lines 56, 57, 58. The lines 56, 57, 58 merge into a common return line 59 which in turn is connected to a supply port 43 of the separating system 40 through which the process liquid is recycled to the separating system 40.

In addition, the gap 14 between the rotor 1 1 and the motor stator 12 of the electric motor 10 is in fluid communication with the first cooling circuit 50. Preferably the gap 14 is flooded with a gas-enriched component of the process fluid, for example with the process gas. As already said, the pump bearing 18 and the second motor bearing 17 also function as additional separation systems in particular to separate at least partially the gaseous component of the process liquid from the process liquid. This gas-enriched component escapes from the pump bearing 18 and from the second motor bearing 17 as indicated by the arrows with the reference numeral G in Fig. 1 and flows as coolant through the gap 14 of the motor 10. The gas-enriched component is then recycled through the line 56. In order to cool the motor stator 12 of the electric motor 10 a second cooling circuit 60 is provided. In the second cooling circuit 60 an electrically

nonconductive fluid is circulated as cooling fluid because this cooling fluid has to be moved through the motor stator 12. The cooling fluid of the second cooling circuit 60 is for example a dielectric liquid such as a dielectric oil.

Since the motor stator 12 is hermetically sealed by the can 13 the process fluid of the first cooling circuit 50 flowing through the gap 14 cannot enter the second cooling circuit 60. An inlet opening 61 is provided through which the cooling fluid of the second cooling circuit is introduced into the motor stator 12. In addition, an outlet opening 62 is provided through which the cooling fluid of the second cooling circuit 60 is discharged from the motor stator 12. The outlet opening 62 or the inlet opening 61 may be provided at the power penetrator 15 in order to additionally cool the power penetrator 15. The outlet opening 62 is connected by a line 64 to a second heat exchanger 63 for leading off heat from the cooling fluid. The second heat exchanger 63 is connected by a line 65 to the inlet opening 61 to supply the cooling fluid to the motor stator 12. A pressure compensation device 70 is provided between the first cooling circuit 50 and the second cooling circuit 60 for balancing the pressure between the two circuits 50 and 60, respectively. By means of the pressure compensation device 70 the pressure within the motor stator 12 is matched to the pressure outside the motor stator 12. It goes without saying that the pressure compensation device 70 does not constitute a flow connection or a fluid communication between the first cooling circuit 50 and the second cooling circuit 60.

Referring now in particular to Fig. 2 the hydrodynamic torque convertor 20 that operatively couples the drive shaft 9 to the pump shaft 8 in a

hydrodynamic manner will be described.

The torque convertor 20 comprises a casing 21 for receiving a transmission fluid, a pump wheel 22 connected torque proof to the drive shaft 9 of the drive unit 4, a turbine wheel 23 connected torque proof to the pump shaft 8 of the pump unit 3, and a stator 24 for guiding the transmission fluid. The stator 24 is connected to the casing 21 that comprises an upper casing part 21 1 and a lower casing part 212 being connected to each other to form the liquid tight casing 21 in which the transmission fluid is contained. During operation the drive shaft 9 rotates the pump wheel 22 which acts on and accelerates the transmission fluid thereby transforming mechanical energy in flow energy. Being guided and diverted by the stator 24 the transmission fluid transfers the energy to the turbine wheel 23 that transforms the flow energy back to mechanical energy and drives the pump shaft 8. In the embodiment shown in Fig. 2 the torque converter 20 comprises a plurality of adjustable guide vanes 25 with which the streaming of the transmission fluid can be changed. The position of the guide vanes 25 can be changed and maintained by means of an adjustment device 251 . By pivoting the guide vanes 25 the incident flow of the guide vanes 25 can be changed, whereby the flow of the transmission fluid in the casing 21 is modified.

Thereby the ratio of the torque transmitted to the pump shaft 8 and the torque transmitted from the drive shaft 9 is controllable.

It has to be noted that the adjustable guide vanes 25 are not necessary with respect to the invention. It is also possible that the torque convertor 20 has no adjustable guide vanes 25. For example, the torque convertor may be embodied with only the pump wheel 22, the turbine wheel 23 and the stator 24, i.e. without any adjustable guide vanes.

Providing the torque convertor 20 for coupling the dive shaft 9 with the pump shaft 8 instead of e.g. a mechanical coupling of the drive shaft 9 and the pump shaft 8 has several advantages.

The composition of a multiphase process fluid that shall be pumped by the multiphase pump 1 usually varies strongly in time. For example in the oil and gas industry, during exploitation of an oilfield the ratio of the gaseous phase (e.g. natural gas) and the liquid phase (e.g. crude oil) is strongly and suddenly varying. The ratio of the gaseous phase in the multiphase mixture is commonly measured by the dimensionless gas volume fraction (GVF) designating the volume ratio of the gas in the multiphase process fluid. In applications in the oil and gas industry the GVF may basically vary between 0% and 100%. These variations could result in a drop in pump efficiency, occurrence of surge, vibrations of the pump or other problems. To handle such large variations in the GVF or more generally in the composition of the multiphase process fluid it is state of the art to provide a buffer tank upstream of the inlet of a multiphase pump. The buffer tank at least damps such variations considerably, especially sudden and strong variations and thus protects the multiphase pump. In such a known arrangement the pump inlet is connected to a buffer tank of suited volume, so that the process fluid cannot enter the pump directly but has to pass the buffer tank. The multiphase process fluid to be pumped by the multiphase pump firstly enters the buffer tank and the outlet of the buffer tank is connected to the inlet of the pump for supplying the process fluid from the buffer tank to the multiphase pump.

The multiphase pump 1 according to the invention renders a buffer tank upstream of the pump inlet 5 superfluous. The torque convertor 20 coupling the pump shaft 8 to the drive shaft 9 is able to compensate the effects caused by variations in the composition of the process fluid at least to a sufficient extend to maintain an efficient and proper operation of the multiphase pump 1 . Accordingly, the multiphase pump 1 according to the invention is preferably operated without a buffer tank upstream of the pump inlet 5, i.e. the

multiphase process fluid is directly supplied to the pump inlet 5 of the pump 1 without passing a buffer tank or any other buffer device upstream of the pump inlet 5.

Avoiding a buffer tank upstream of the pump inlet 5 of the multiphase pump 1 considerably reduces the amount of equipment involved, thus reducing the costs, the weight, and the necessary effort to install the multiphase pump 1 . This is in particular advantageous regarding subsea applications, e.g. when the multiphase pump 1 is installed on the sea ground. Avoiding the buffer tank reduces the weight and the amount of the equipment that has to be

transported to the sea ground as well as the time and the effort for the installation of the pump 1 .

It is a further advantage of the multiphase pump 1 according to the invention that the torque convertor 20 may also be used for the speed control of the pump 1 or for a self-regulation of the pumping process. This is in particular applicable when the torque convertor 20 is designed with adjustable guide vanes 25, but also applicable when the torque convertor 20 has only fixed guiding elements - like the stator 24 - for the transmission fluid and no adjustable guide vanes. Using the torque converter 20 for the speed control of the pump shaft 8 renders possible to operate the pump 1 without a VFD but with a less complex and less expensive electric motor 10. Alternatively, it is possible to use a VFD with a considerably narrower range for the variation of the frequency. Both measures, configuring the pump 1 without a VFD or configuring the pump with a VFD having a narrower frequency range, reduce the costs and the complexity of the pump 1 .

A further preferred measure is to design the torque convertor 20 for receiving the cooling fluid of the second cooling circuit 60 as transmission fluid.

According to this measure the cooling fluid of the second cooling circuit 60, i.e. the cooling circuit for the motor stator 12 is additionally used as the transmission fluid for the torque convertor 20.

To this end, the casing 21 of the torque convertor 20 (see Fig. 1 ) is integrated into the second cooling circuit 60. In the embodiment shown in Fig. 1 or Fig. 2, respectively, the casing 21 of the torque converter 20 comprises an intake 26 for supplying the cooling fluid of the second cooling circuit 60, e.g. the dielectric oil, to the casing 21 of the torque convertor 20 and a discharge 27 for the drain of the cooling fluid of the second cooling circuit 60 from the torque convertor 20. The second cooling circuit 60 comprises a first

connection line 66 that connects the line 65 with the intake 26 of the torque convertor 20 and a second connecting line 67 that connects the discharge 27 of the torque convertor 20 with the motor stator 12. During operation, the cooling fluid of the second cooling circuit is additionally circulated through the torque convertor 20 and functions as the transmission fluid of the torque convertor 20. Of course, it is also possible to design the second cooling circuit 60 without the inlet opening 61 at the motor stator 12, such that the line 65 only leads in the first connection line 66 but not directly in the motor stator 12. Accordingly, the cooling fluid of the second cooling circuit 60 flows from the second heat exchanger 63 through line 65, the first connection line 66 and the intake 26 into the torque convertor 20, leaves the torque convertor 20 through the discharge 27, is channeled through the second connecting line 67 to the motor stator 12, flows through the motor stator 12, leaves the motor stator 12 through the outlet opening 62 and flows back to the second heat exchanger 63 through line 64. Using the cooling fluid of the second cooling circuit 60 as the transmission fluid and integrating the torque convertor 20 into the second cooling circuit 60 has the considerable advantage that the torque convertor 20 generates the or an additional driving force for circulating the cooling fluid through the second cooling circuit 60. Thereby, the circulation of the cooling fluid through the second cooling circuit 60 is remarkably enhanced which results in an improvement especially of the cooling of the motor stator. The increased flow of the cooling fluid through the motor stator 12 renders possible to lead away a larger amount of heat from the motor stator 12.

Each of the lines or connecting lines 51 , 53, 54, 55, 56, 57, 58, 59, 64, 65, 66, 67 of the first cooling circuit 50 and the second cooling circuit 60 may be designed as an internal channel, as a pipe or as a combination of an internal channel and a pipe. According to another embodiment it is also possible that the cooling fluid of the second cooling circuit 60 is not circulated through the torque convertor 20 but only through the motor stator 12. In such embodiments the casing 21 of the torque convertor 20 has neither an intake 26 nor a discharge 27 and there is no fluid communication between the torque convertor 20 and the motor stator 12 or the second cooling circuit 60. The casing 21 of the torque convertor 20 is filled with a transmission fluid that cannot escape from the torque convertor 20.