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Electric submersible pumps

Electric submersible pumps description/claims

This application is a divisional of U.S. patent application Ser. No. 10/562,255 (Atty. Dock. No. MRKS/0141), which is the National Stage of International App. No. PCT/GB04/02667, filed Jun. 21, 2004, which claims priority to GB 0314553.9, filed Jun. 21, 2003, which are herein incorporated by reference in their entireties.

1. Field of the Invention

This invention relates to motors and electronic drives for electric submersible pumps and compressors, and is concerned more, but not exclusively, with centrifugal pumps.

2. Description of the Related Art

Submersible pumping is a well-established technique for extracting hydrocarbons from deep boreholes, where the natural pressure in the reservoir is insufficient to lift the fluid or gas to surface. The technique is also used in water production.

Typically the production requirement is to lift large volumes of liquid against a pressure difference related to the depth of the well in which the pump is installed. For very heavy crude oils, slow-speed positive displacement pumps are suitable. These are usually rotated by a motor at the surface connected to the pump by a long flexible rod system. Centrifugal pumps have been found most suitable for normal crude oils, gas and water. These pumps are rotated by a submerged motor connected directly to the pump, with electric power being delivered from the surface by a long cable. Also, the use of electric cables makes installation possible in deep or long horizontal wells which would otherwise not be possible with the use of rotating rods.

The electric motors used for driving the centrifugal pumps are very elongated, sometimes of a length of more than one hundred times their diameter. The resulting complexity of such a device, the difficulty of its manufacture and the quantity of the degradable insulation materials it employs all reduce the system reliability.

Electric motor shaft power output is defined as the product of rotation speed and torque. For a given physical size and type of motor there is a limit to the level of torque that can be produced, typically due to self-heating. A high-speed motor therefore provides a means for obtaining more power from the same length of motor, or the same power from a shorter length.

The output of a pump is normally given in terms of its hydraulic power, which is the product of flow rate and lifting pressure (in rationalised units). Centrifugal pump technology is characterised by the power output being proportional to the cube of the rotational speed. This known relationship, sometimes termed the “affinity law”, means that a relatively small increase in the rotational speed can give rise to a substantial power increase.

Centrifugal pumps are frequently made with hundreds of impellers threaded on a common shaft, each impeller adding a little to the lifting pressure. Reducing the number of impellers by increasing the speed would therefore afford an improvement in reliability.

The above demonstrates that a high-speed motor and pump would, by being shorter for a given power, present direct advantages in reliability due to reduced complexity, or alternatively yield a higher output for a similar size. A large proportion of boreholes are deviated from the vertical and commonly even to the horizontal. A much shorter motor/pump combination would also lead to a reduction in damage caused by mishandling and bending during deployment through the curved sections of the borehole. Furthermore, the much-shortened length would allow motor/pump combinations to be assembled and tested in ideal conditions at the manufacturer's plant prior to being transported to the borehole location.

As will be described more fully below, innate limitations in the established motor and motor controller technology used in the electric submersible pumping industry have prevented the objective of higher speed being recognised or addressed.

Historically, electric submersible motors used for centrifugal pumping have been of the asynchronous, or induction, type. The stator is made of steel laminations and copper windings, and the rotor of steel laminations with copper bars forming the winding known as a squirrel cage. The rotor laminations are keyed to a shaft, this shaft providing the means of transmitting output torque. The rotor poles are produced by induction or transformer action between the stator and the rotor, using a portion of the stator current. The stator, in addition, produces a rotating stator field due to the alternating current in its windings. Since the transformer coupling to the rotor requires an alternating field in the rotor, the rotor must turn at a different (lower) speed than synchronous speed, producing a so-called slip frequency for induction. Electric submersible motors are made with two poles in order to achieve the maximum rotating speed from a standard 60 Hz utility supply. This speed is typically 3500 rpm, slightly less than the unattainable synchronous speed of 60 Hz×1 pole pair×60 s/min=3600 rpm.

It has become common to use variable speed drives to power these motors, rather than direct connection to the utility supply. Variable speed drives first convert utility AC power, typically at 60 Hz, to DC, and then by electronic switching convert the DC to a variable frequency alternating voltage. The use of a variable speed drive confers advantages during starting when it can limit the motor current to a safe level, and during production when it can be used to manage flow rates. The latter is important when the changing characteristics of a reservoir are considered over its producing life. Although variable speed drives, by creating an artificial supply of 70 Hz or more, can operate the motor at higher speed than when directly connected to the utility supply, this is a limited capability. Firstly the elongated induction motor is not suited to high-speed operations due to internal losses and small mechanical clearances, and secondly at the medium voltages used (often several thousand volts rms) drive losses become very high. Performance is generally limited up to 80 Hz or about 4500 rpm.

In order to maximise the induced rotor pole strength it is necessary to minimise the gap between the rotor and the stator. Unless very hot, the oil in the gap is sheared by the rotor turning yet remains in laminar flow. As a result the friction absorbs several percent of motor power. Motor efficiencies above 90% are sought, and this is an important source of loss in existing motors. The internal heating caused by these losses, and the copper losses in the squirrel cage, reduce motor life by aging the insulation materials.

The small gap is also a cause of premature failure due to mechanical causes. The limited diameter of boreholes is a natural disadvantage to both motors and pumps, and as a result their design is very elongated. A pump and induction motor assembly for producing 250 HP may be 20 metres long. This slender assembly is difficult to handle and particularly subject to damage when being deployed into deviated or horizontal wells, since small deflections of the motor housing can cause the rotor to impact on the stator. Rotor vibration due to bearing wear or imbalance also increases the chance of rotor impact.

The requirement for the rotor to be made of laminations and the limited overall motor diameter act together to constrain the diameter of the inner torque-carrying shaft. It is common practice, for example, to couple two 250 HP motors of 5.62 inch diameter together so as to make a longer 500 HP motor. Shaft strength limitation prevents this being increased to 750 HP or 1000 HP.

To provide a high-speed electric pumping system, it is desirable to increase the rotor clearance, and to reduce the internal sources of power loss that increase with speed. It is also necessary to use a drive technology which remains efficient at high speed and at the different operating voltage levels needed for different motor speeds required during the life of the well.

A further requirement of any high or low speed electric submersible system using variable speed drives is to minimise the deleterious effects of the electrical switching used to produce the alternating output voltages. Switching events on the long cables used in submersible cable propagate as wave fronts that reflect at connections and most particularly at the motor terminals. These reflections cause voltage transients that can approach twice the original voltage, and hence destroy insulation to earth. Commonly the motor voltage is presumed to be proportionally distributed through the turns of the stator winding, and the inter-turn insulation is less than that of the winding to earth. However a wave front impinging on a motor terminal must travel through the winding turn by turn before settling to its final value. Therefore there are short periods in which one turn of a winding carries the wave front at full voltage and an adjacent turn is unexcited. This internal voltage difference can exceed the inter-turn insulation rating, again causing premature failure. Increasing the insulation level to overcome these problems reduces the space available for the copper in the winding and also reduces the heat transfer from the copper, so that the motor specification is reduced.

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