GB2513861A

GB2513861A - Pump for lifting fluid from a wellbore

Info

Publication number: GB2513861A
Application number: GB1308164.1A
Authority: GB
Inventors: Alexey Pavlov
Original assignee: Statoil Petroleum ASA
Current assignee: Equinor Energy AS
Priority date: 2013-05-07
Filing date: 2013-05-07
Publication date: 2014-11-12
Also published as: GB201308164D0

Abstract

A multistage centrifugal pump 40 for lifting fluid from a wellbore has at least one auto-balancer 30, 60 to reduce or eliminate radial vibration in at least a portion of the pump 40. The auto-balancer 30, 60 may comprise an annular race 64 containing freely movable balls 72 or a dense liquid (not shown), and may also contain a viscous fluid 80 to dampen movement of the balls 72 or dense liquid. The pump 40 includes a driving motor 46, and may be controlled to accelerate from rest according to a pre-determined speed profile for fast and optimum transition of the freely movable elements 72 in the auto-balancer 30, 60 to a steady-state position. The auto-balancer 30, 60 reduces vibration of the elongate pump shaft, and may be positioned close to the areas most sensitive to vibration e.g. a motor seal.

Description

Pump for Lifting Fluid from a Wellbore

Field of the Invention

This invention relates to a pump with a rotating shaft for lifting fluid from a wellbore.

Such pumps are commonly referred to as either electrical submersible pumps (ESP5) or hydraulic submersible pumps (HSPs) depending upon the nature of the drive mechanism. In particular, the invention relates to a pump configured to lift hydrocarbons or water from a well.

Background to the Invention

It is known to use artificial lift to boost production and increase the recovery of oil from a well when the reservoir pressure alone is not sufficient to flow fluids to the surface. In addition, water can build up in a well and this can hamper the production of hydrocarbons.

By placing a submersible pump in a wellbore, it is possible to lift fluids (typically oil or water) to the wellhead. This effectively lowers the pressure in the wellbore upstream of the pump making production flow viable and/or increasing the production rate.

Such artificial lift is typically performed by a centrifugal pump (e.g. ESP) which is placed in the well during completion. Due to their length and the need for high rotational speed, centrifugal pumps need to be placed in a relatively long and straight section of the well. In practice, this can be difficult to achieve due to well curvature. If the well is curved, the pump shaft will bend every time it rotates and this will result in vibration and wear which will lead to a short operational lifetime. Even if the pump is located in a straight well section, the high rotational speeds experienced by the pump shaft during use can result in undesirable vibrations, particularly radial vibrations due to imbalances along the shaft. It is therefore common to try to restrain the shaft with radial support bearings.

Large vibration of the rotating components is detrimental for the whole pump system.

In particular, it can wear and eventually destroy the bearings supporting the shaft. It can also lead to material fatigue of the shaft and affect the motor and the seal, which are vulnerable to vibration, leading to the premature failure of these components.

Current manufacturing techniques try to reduce or eliminate such vibration by ensuring that the rotational components of the pump are balanced when produced. As a result, it is commonly felt that vibration effects do not contribute significantly to the failure of such pumps.

It is an aim of the present invention to provide a pump for lifting fluid from a wellbore, which addresses at least some of the afore-mentioned problems.

Summary of the Invention

The Applicants have discovered that despite the tight manufacturing control and strict installation standards employed to balance the rotational components of an ESP, vibration is still problematic during use. Moreover, after the ESP has been installed, the only way limit vibration is not to run the ESP at rotational frequencies that lead to high vibration levels. In practice this means that the operational frequency of the ESP is kept below its optimal (designed) frequency in order to minimise vibration and keep it within a relatively safe range. However, the solution of limiting the operational frequency of the ESP to minimise vibration leads inevitably to lower production rates.

A potential solution to jump over the resonance frequency of the shaft (at which the radial vibrations have their largest amplitude) may solve the above problem only in part. It leaves a range of frequencies around the resonance frequency where the pump cannot be operated due to high vibration. This is not appropriate in complex production systems with multiple ESP lifted wells where it may be important, due to operational needs, to run the ESP in that particular frequency range.

There is also a desire to develop high speed ESPs which operate at frequencies greater than 100Hz as opposed to the current 60-80Hz. However, small imbalances satisfying the existing standards may cause large vibrations at such speeds and tightening the standards to compensate for this will make the production of such high speed ESAs significantly more expensive.

Furthermore, it has been suggested that magnetic coupling should be employed between the motor shaft and the pump shaft in the design of high speed ESPs. It is conceivable that a high vibration level of the pump shaft will make this coupling less efficient (since the distance between the shafts in the coupling can slightly increase leading to reduced magnetic forces) thus resulting in slippage between the shafts with resulting loss of speed as well as coupling. Accordingly, reducing the vibration in the vicinity of the coupling may be required.

Not only can imbalances arise during installation (e.g. in a slightly curved well) but they can also develop during operation as a result of scale formation or wear of the impellers. In which case, the imbalance is uncertain and largely uncontrollable.

There is therefore a desire to reduce vibration without limiting production.

According to a first aspect of the present invention there is provided a pump for lifting fluid from a wellbore, the pump comprising: a motor having a drive shaft coupled to a rotatable pump shaft; the pump shaft comprising a plurality of impellers; and at least one auto-balancer arranged to reduce or eliminate radial vibration in at least a portion of the pump when in use.

Thus, embodiments of the present invention provide a pump which can be used to generate artificial lift (e.g. of hydrocarbons or water) in a well and which, due to the presence of at least one auto-balancer, is less susceptible to vibration and the associated implications of increased wear, reduced lifetime or lower production rates.

More specifically, a reduction in vibration is achieved through use of at least one auto-balancer and this in turn results in less wear of the pump components leading to an extended pump lifetime. Moreover, the pump does not require its speed to be limited or adjusted to avoid resonant frequencies in order to keep vibrations to an acceptable level and consequently, higher and more optimal production rates can be achieved.

The present invention may also reduce the need to impose very strict balancing standards for the manufacture of high speed pumps, making them a more viable option for the future.

The pump can be considered to comprise a coupled shaft which comprises the drive shaft and the pump shaft when coupled together.

The term auto-balancer will be understood to mean a device configured to dynamically compensate for imbalances in a rotating system. It will be understood that the auto-balancer will exploit mechanical forces to dynamically compensate for such imbalances which may otherwise cause radial vibration of the pump during use. In particular, if the rotational frequency of the pump is essentially higher than a first resonance frequency of the coupled shaft vibration, then due to centrifugal forces, weights in the auto-balancer will automatically position themselves to counteract the imbalance.

Auto-balancers have previously been applied to systems having shod shafts (such as in DVD drives and grinding machines), where the imbalance and the auto-balancer are essentially co-planar. However, they do not appear to have previously been applied to systems employing long slender and somewhat flexible shafts, such as those encountered in downhole pumps. Instead, as explained above, the problems associated with vibration in such pumps have, until now, been tackled in other ways (e.g. by balancing the components during production and avoiding the resonant frequency). However, the applicants have surprising found that auto-balancers can be successfully incorporated into such pumps and their simulation results show that a significant reduction in vibration can be achieved.

The main challenge in the current application compared to the previous applications of auto-balancers is that the coupled shaft is long. Thus the imbalance and the auto-balancer may be located relatively far away from each other. Moreover, the coupled shaft may be flexible (primarily due to its length) and there may therefore be dynamic interplay between the imbalanced parts, which may either be constituted by the auto-balancer itself or another imbalance. This interplay is not present in other applications.

According to simulations carried out by the applicant, when the shaft is long, the auto-balancer substantially cancels the vibration at the location of the auto-balancer; and near the auto-balancer the vibration is also reduced; but the reduction disappears as we move along the shaft away from the auto-balancer. The applicants therefore suggest that embodiments of the invention can exploit this property by locating the auto-balancer close to parts of the pump that are most vulnerable to vibration.

Embodiments of the present invention are particularly suited to use in wellbore applications, both on-shore and off-shore, including subsea wells. As artificial lift is especially desirable in deep water applications and those involving heavy oil, the present pump may be configured for these applications. Moreover, the present pump could be configured for topside circulation or arranged on or adjacent the seabed so as to pump several wells together.

The pump may comprise a seal (sometimes also known as a protector) configured to protect the motor and prevent it from being contaminated with well fluids. The applicants believe that in traditional ESP systems the seal is probably the most vulnerable to vibration, since it comprises an elastomer material tightly sealed around the drive shaft. Thus large shaft vibration will lead to 1) fatigue of the seal material leading to its failure and 2) small leakages caused by non-perfect sealing of the seal around the shaft because of vibration. These small leakages, when accumulated, may cause contamination of the motor by well fluids, leading to motor failure.

The at least one auto-balancer may be located adjacent, at, within or close to the seal and/or close to the motor (e.g. on the drive shaft) to reduce vibration in these particularly vulnerable areas thereby preserving the most critical components. In which case, the vibration in the vicinity of the seal may be reduced but the vibration further away from the auto-balancer may not be reduced and, in some instances, it may even be increased. To solve this problem, a further auto-balancer may be provided at a location further away from (i.e. remote from) the seal and/or motor to reduce vibration there as well.

The at least one auto-balancer may be located at or close to a coupling between the drive shaft and the pump shaft to increase its resistance to vibration. The coupling may be mechanical and/or magnetic.

The at least one auto-balancer may comprise a hollow annulus having at least one freely moveable element therein, which is capable of moving around the annulus to oppose an imbalance on the coupled shaft.

The at least one freely moveable element may comprise at least two weights (e.g. balls).

The annulus may comprise a viscous fluid to dampen the movement of the at least one freely moveable element. The fluid may be pressurised and the annulus sealed so as to withstand high pressure conditions at a downhole pump location. Ideally, the fluid will be pressurised such that it is at a higher pressure than the well pressure to prevent ingress of well fluids into the auto-balancer.

In an alternative embodiment, the at least one auto-balancer may comprise a hollow annulus partially filled with a relatively heavy fluid (i.e. constituting the freely moveable element), and partially (e.g. the remaining volume) filled with a relatively light fluid. The relatively light fluid may comprise water and the relatively heavy fluid may comprise mercury or another relatively heavy material that will be liquid under the operating conditions of the pump. For example, the temperature downhole may be high and a material which is not liquid at the surface may be liquid downhole. The relatively heavy fluid and the relatively light fluid may each have a high viscosity to dampen small relative movements.

The at least one auto-balancer may be provided on the pump such that the annulus is in a plane perpendicular to the rotational axis of the coupled shaft, the centre of the annulus coincides with the centre of rotation of the coupled shaft and the annulus is attached to the shaft such that it rotates at the same frequency as the coupled shaft.

The at least one auto-balancer may be permitted to move along the coupled shaft as it rotates at the same speed as the coupled shaft.

In particular embodiments, two or more auto-balancers may be employed along the coupled shaft. However, the number and location of auto-balancers employed in a given pump should be optimised since too many auto-balancers may interact with each other and actually induce vibrations, even when there is no initial imbalance. The applicants have, however, found that the presence of a viscous fluid in the annulus may provide a solution which avoids dynamic interaction between two or more auto-balancers.

The pump may be configured as an ESA. Alternatively, the pump may be configured as an HSP.

The pump may be configured as a multistage centrifugal pump (typically comprising 60 to 80 identical stages forming the pump shaft).

The at least one auto-balancer may be configured as a pump stage having a mechanical interface permitting inclusion in a pump string. Alternatively, the at least one auto-balancer may be provided with an interface identical to that of the impellers.

A control system may be provided which operable from a topside or subsea location to send appropriate signals to operate the pump as desired. For example, the control system may be configured to alter the speed of operation of the pump (e.g. by means of a variable speed drive). The control system may be configured to generate a pre-determined speed profile to speed up the pump from rest to a desired speed. The speed profile may be configured for fast and optimal transition of the at least one freely movable element in the auto-balancer to its steady-state (i.e. balanced) position.

Without such a speed profile, the pump may experience relatively large vibration levels during start up.

The pump length is not limited. However, a typical coupled shaft may be approximately 5-12m in length.

It will be understood that the number, size, configuration (e.g. mass, number of weights, viscosity of any fluid) and location of the at least one auto-balancer will need to be determined based on the pump parameters in order to provide an optimal reduction in vibration and to avoid any significant vibration creation.

According to a second aspect of the present invention there is provided an auto-balancer for a pump for lifting fluid from a wellbore, the auto-balancer being configured as a pump stage having a mechanical interface permitting inclusion in a pump string.

An advantage of this aspect of the invention is that no re-design or alteration of the pump itself is required in order to incorporate one or more auto-balancers since an existing pump stage can simply be replaced by an auto-balancer stage.

In other embodiments, the auto-balancer may be configured to be incorporated into the pump by another means and/or at another location.

Accordingly to a third aspect of the invention, a system comprising two or more pumps according to the first aspect may be provided to lift fluids from a wellbore.

Brief Description of the Drawings

Specific embodiments of the present invention will now be described with reference to

B

the accompanying drawings, in which: Figure 1 shows a schematic cross-sectional view of a part of a pump in accordance with a first embodiment of the present invention, with the motor, seal and auto-balancer not shown; Figure 2 shows a graph illustrating the flow rate obtained using a first known ESP at a lower than optimal frequency in order to minimise vibrations; Figure 3 shows a graph illustrating the flow rate obtained using a second known ESP at a lower than optimal frequency in order to minimise vibrations; Figure 4 shows a perspective view of a known auto-balancer with its lid detached; Figure 5 shows a schematic view of a pump in accordance with a second embodiment of the present invention, with some preferred locations for the auto-balancer indicated; Figure 6 shows a schematic view of a simulation of a long pump shaft having two auto-balancers and two imbalances; Figure 7 shows a graph showing the simulated vibration amplitude obtained at several positions along the shaft of Figure 6, both with an empty auto-balancer and with an auto-balancer including counter-weight balls; Figure BA shows a cross-sectional view of an auto-balancer configured for use in a pump in accordance with a third embodiment of the invention; Figure 8B shows a plan view of the auto-balancer of Figure BA; Figure 9A shows a part cross-sectional view of four stages of a pump for use in a fourth embodiment of the invention; and Figure YB shows a view similar to that of Figure 9A but wherein one stage has been replaced by an auto-balancer in accordance with the fourth embodiment of the invention.

Detailed Description of Certain Embodiments

With reference to Figure 1, there is illustrated a portion of a pump 10 for lifting fluid from a wellbore in accordance with a first embodiment of the present invention. The pump 10 comprises a motor (not shown) having a drive shaft (not shown) coupled to a rotatable pump shaft 14 which is fitted with a number of impellers 16. The motor (not shown) will be connected to the pump shaft 14 either at a top or at a bottom end of the pump shaft 14 and a seal (not shown) will be provided between the motor and the pump shaft 14. Diffusers 1 B are provided above each set of impellers 16. The pump 10 is configured as a multistage centrifugal pump and each impeller 16 and diffuser 16 are provided together in a stage 20. The multiple stages 20 are joined together to form a continuous housing 22. Below the impellers 16 there is an intake for channelling well fluids into the pump 10. In a particular embodiment, the motor (not shown) is provided below the intake 24 and the seal (not shown) is provided between the motor and the intake 24 to seal against the drive shaft (not shown) and to protect the motor and prevent it from being contaminated with well fluids.

Although not shown in Figure 1, at least one auto-balancer is provided in the pump 10 to reduce or eliminate vibration in at least a portion of the coupled shaft (comprising the pump shaft 14 and the drive shaft) when in use. For example, an auto-balancer may be provided in one or more of the following locations: in region of the seal; between the motor and the seal; and in the region of the coupling between the drive shaft and the pump shaft 14. These positions are shown in Figure 5 and described in more detail below. In addition, an auto-balancer may be provided further along the pump shaft 14, for example, at position A and/or position B as indicated in Figure 1.

When employed downhole, a power cable (not shown) will be provided to connect the pump 10 to a topside control system (not shown), which is configured to operate the pump 10 at a desired frequency.

Figures 2 and 3 show graphs illustrating the flow rate obtained using two different known ESPs (similar to those of Figure 1 but without any auto-balancers). More specifically, Figure 2 illustrates a well where production was started on 9th April and vibration was observed through sensors mounted on the pump from 30tb April onwards.

In order to reduce the vibrations, the operating frequency of the ESP was limited to 54Hz giving a production flow rate of 1400m3 per day. This is in contrast to a target operating frequency of 60Hz which would result in a flow rate of 1900 m3 per day, thereby representing a production loss of greater than 20%.

Figure 3 illustrates a well where production was started on 24th August and vibration was observed through sensors mounted on the pump from 25th August onwards. In order to reduce the vibrations in this case, the operating frequency of the ESA was limited to 52Hz giving a production flow rate of 2000m3 per day. This is in contrast to a potential operating frequency of 65Hz which would result in a flow rate of 2700 m3 per day, thereby also representing a production loss of greater than 20%.

Reasons for the different delay in the onset of vibrations in each case may include how well the ESP was balanced prior to installation and whether installation itself (e.g. in a curved well) may have caused an imbalance. The balancing may also have deteriorated due to progressive wear of the components or due to uneven formation of scale on the impellers.

Typically production is started with an ESP operating at a relatively low frequency (e.g. 40Hz) and gradually the frequency is increased to the desired operating frequency.

However, when vibrations start the operating frequency may be limited so as to minimise the impact of the vibrations on the lifetime of the ESP. As illustrated above, this has a negative impact on production flow rates and it is an aim of the present invention to provide an alternative solution to such vibrations.

In accordance with embodiments of the present invention, at least one auto-balancer 30 as shown in Figure 4 is provided on a pump 10 such as that of Figure 1. The auto-balancer 30 comprises a hollow annulus 32 and six balls (weights) 34 capable of moving around the annulus 32 to oppose any imbalances on the pump. In this embodiment, the annulus 32 is formed of a U-shaped channel 36 and a planer lid 38.

In other embodiments, more or fewer balls 34 may be provided.

Although not shown, the annulus 32 preferably comprises a viscous fluid to dampen the movement of the balls 34. In practice, the fluid is pressurised and the annulus 32 is sealed so as to withstand the high pressure conditions that will be experienced at a downhole pump location.

In another embodiment (not shown), the balls may be replaced by a heavy liquid (e.g. a liquid metal) which is configured to move around the annulus so as to balance any imbalances in the coupled shaft of the pump.

Figure 5 shows a complete pump 40 in accordance with a second embodiment of the present invention, fitted with an auto-balancer 30 similar to that shown in Figure 4, and installed in a well 41. The complete punip 40 include a pump section 10 similar to that shown in Figure 1 and so like reference numerals will be used as appropriate. Below the pump 10 there is provided an optional gas separator 42, a seal 44 (i.e. protector) and a motor 46. As illustrated, the auto-balancer 30 is located between the motor 46 and the seal 44. In other embodiments an auto-balancer may be located in region of the seal 44 or in the region of the coupling between the drive shaft and the pump shaft.

It will be understood that all of the components from the motor 46 to the pump 10 have coupled rotating shafts such that auto-balancers 30 can be installed on any part of the combined shaft string.

As shown in Figure 5, the complete pump 40 is coupled to a wellhead 47, a junction box 48 and a switchboard 49 which may house or be connected to a control system configured to send appropriate signals to operate the pump 40 as desired.

Figure 6 shows a schematic view of a simulation of a long pump shaft 14 fitted with two auto-balancers 30 (denoted 2 and 5) and two imbalances 50 (denoted 3 and 4). Each end of the shaft 14 is denoted 1 and 6.

Figure 7 shows a graph showing the simulated vibration amplitudes obtained at each of the locations 1 through 6 along the shaft 14 of Figure 6, both with empty auto-balancers 30 and with auto-balancers 30 including balls 34 as per Figure 4. The results clearly show that in the location of the auto-balancers 30, vibrations are all but eliminated when the auto-balancers include the balls 34. In addition, although the presence of the balls 34 has a reduction in vibrations along the entire shaft 14 length, the reduction is less at locations away from the auto-balancers 30.

Figures 8A and SB show a further auto-balancer 60, configured for use in a pump in accordance with a third embodiment of the invention. The auto-balancer 60 is essentially in the form of a wheel having a central annular hub 62 for location around the coupled shaft of the pump and an outer annulus 64 connected to the hub 62 via series of spokes 66. It will be understood that this arrangement allows well fluids to flow between the spokes 66 during use.

The outer annulus 64 comprises a U-shaped channel 68, the sides of which include hook-like grooves 70. One or more balls 72 is located in the U-shaped channel 68 and the annulus is sealed using a further U-shaped annulus 74 which is sized to fit within the U-shaped channel 68 and which includes protruding hooks 76 on its outer side surfaces to mate with and lock onto the hook-like grooves 70. When the U-shaped annulus 74 is press-fitted into the U-shaped channel 68, the balls 72 are contained therein. Advantageously, the hook-like grooves 70 and co-operating hooks 76 will be configured to retain the parts together (e.g. by their directionality) and when the pressure inside the annulus 64 is increased, the pressure will serve to reinforce the sealing between the hooks 76 and the hook-like grooves 70. As illustrated in Figure 8A, the inner surface of the outer wall 78 of the U-shaped annulus 74 has a concavely curved profile so that the balls 72 will tend to centre in a vertical direction as the auto-balancer 60 rotates.

A viscous damping fluid 80 is added to the annulus 64 using a one way valve 82 (for example a spring loaded ball and seat valve). In practice, an equivalent mass or second valve (not shown) will be provided on an opposite side of the annulus 64 for balancing reasons. The fluid will be chosen such that it has the desired viscosity at the temperature downhole.

Figure 9A shows a part cross-sectional view of four stages 20 of a pump 90 for use in a fourth embodiment of the invention. As described in relation to Figure 1, each stage 20 comprises an impeller 16 and a diffuser 18. In addition, a series of thrust washers 92 are illustrated adjacent the impellers 16.

Figure 9B shows a view similar to that of Figure 9A but wherein one stage has been replaced by an auto-balancer stage 42 in accordance with the fourth embodiment of the invention. The auto-balancer stage 42 includes an auto-balancer 60 (similar to that shown in Figures 8A and 8B) and has the same mechanical interface as the other stages 20 so that it can be easily incorporated in the pump string. In other embodiments, two or more stages 20 may be replaced by auto-balancer stages 42.

The auto-balancer 60 will be configured so that the flow path for well fluid through the pump 90 is not obstructed and excessive friction is avoided.

As mentioned previously, pumps in accordance with embodiments of the present invention can be used to generate artificial lift in wells and, due to the presence of at least one auto-balancer, such pumps can be configured to be less susceptible to vibration and the associated implications of increased wear, reduced lifetime and lower production rates.

It will be appreciated by persons skilled in the art that various modifications may be made to the above embodiments without departing from the scope of the present invention, as defined by the claims. In addition, features described in relation to one embodiment may be incorporated into other embodiments and vice versa.

Claims

CLAIMS: 1. A pump for lifting fluid from a wellbore, the pump comprising: a motor having a drive shaft coupled to a rotatable pump shaft; the pump shaft comprising a plurality of impellers; and at least one auto-balancer arranged to reduce or eliminate radial vibration in at least a portion of the pump when in use.
2. The pump according to claim 1 wherein a coupled shaft comprises the drive shaft and the pump shaft when coupled together.
3. The pump according to claim 1 or claim 2 comprising a seal between the motor and the pump shaft to protect the motor and prevent it from being contaminated with well fluids.
4. The pump according to claim 3 wherein the at least one auto-balancer is located adjacent, at, within or close to the seal.
5. The pump according to any preceding claim wherein the at least one auto-balancer is located close to the motor.
6. The pump according to claim 4 or claim 5 wherein a further auto-balancer is provided at a location remote from the seal and/or motor.
7. The pump according to any preceding claim wherein the at least one auto-balancer is located at or close to a coupling between the motor and the pump shaft.
8. The pump according to claim 2 wherein the at least one auto-balancer comprises a hollow annulus having at least one freely moveable element therein, which is capable of moving around the annulus to oppose an imbalance on the coupled shaft.
9. The pump according to claim 8 wherein the at least one freely moveable element comprises at least two weights or balls.
10. The pump according to claim 8 wherein the at least one freely moveable element comprises a relatively heavy fluid and a reminder of the annulus comprises a relatively light fluid.
11. The pump according to any one of claims 8 to 10 wherein the annulus comprises a viscous fluid to dampen the movement of the at least one freely moveable element.
12. The pump according to claim 11 wherein the fluid is pressurised and the annulus sealed so as to withstand high pressure conditions at a downhole pump location.
13. The pump according to any one of claims 8 to 12 wherein the at least one auto-balancer is provided on the pump such that the annulus is in a plane perpendicular to the rotational axis of the coupled shaft, the centre of the annulus coincides with the centre of rotation of the shaft and the annulus is attached to the coupled shaft such that it rotates at the same frequency as the coupled shaft.
14. The pump according to claim 2 wherein two or more auto-balancers are employed along the coupled shaft.
15. The pump according to any preceding claim configured as an ESP or an HSP.
16. The pump according to any preceding claim configured as a multistage centrifugal pump.
17. The pump according to claim 16 wherein the at least one auto-balancer is configured as a pump stage having a mechanical interface permitting inclusion in a pump string.
18. The pump according to any preceding claim wherein the at least one auto-balancer may be provided with an interface identical to that of the impellers.
19. The pump according to any preceding claim comprising a control system which operable from a topside or subsea location to send appropriate signals to operate the pump as desired.
20. The pump according to claim 19 wherein the control system is configured to generate a pre-determined speed profile to speed up the pump from rest to a desired speed; the speed profile being configured for fast and optimal transition of a freely movable element in the at least one auto-balancer to a steady-state position.
21. A pump substantially as hereinbefore described with reference to the accompany drawings.
22. A system comprising two or more pumps according to any preceding claim.
23. An auto-balancer for a pump for lifting fluid from a wellbore, the auto-balancer being configured as a pump stage having a mechanical interface permitting inclusion in a pump string.
24. The auto-balancer according to claim 23 configured for a pump according to any one of claims ito 21.
25. An auto-balancer substantially as hereinbefore described with reference to the accompany drawings.