This article provides guidelines for starting and operating an Electrical Submersible Pump (ESP) system.
A field engineer from the pump supplier should be present whenever an Electrical Submersible Pump is to be started for the first time. The field engineer should remain on the location until the well has stabilised and the Electrical Submersible Pump is operating properly. The field engineer should remain available for at least 12 hours after a pump is started.
2 Starting the Electrical Submersible Pump
An Electrical Submersible Pump (ESP) may be started and the well cleaned up using a soft starter or VSD, even when it is intended to operate the Electrical Submersible Pump on a fixed supply frequency. This will reduce the start-up current surge and allow initial clean up and production at a low flow rate, which can be increased as the well cleans up. In a new well the use of a VSD allows the well to be tested at multiple flow rates, permitting validation of the pump design. The inclusion of downhole pressure and temperature sensors to monitor the performance of new wells is recommended.
Electrical Submersible Pumps may be started with the flowline valve closed, to avoid excessive flow rates, or may be started with the flowline open. The risk of Electrical Submersible Pump damage due to excessive flowrates is normally small compared with the risk of pumping against a closed valve, but will depend on the well and Electrical Submersible Pump characteristics.
After the preliminary checks listed above have been carried out the Electrical Submersible Pump may be started. The supply voltage with no load connected should be observed and recorded. The voltmeter should remain connected for a load voltage check. After the start button has been pressed the Electrical Submersible Pump should start within 0.2 seconds.
3 Motor current
Use of a VSD or soft starter will reduce the starting current of the motor to a maximum of approximately 300% of the nameplate current for the first few seconds of operation, before falling back. If no VSD or softstarter is used the initial current may be up to 450% of the nameplate current depending on the cable characteristics.
The current drawn may initially be higher than the expected operating current, if the well is filled with high density brine or completion fluid. If no check valve is fitted the current may be lower than the normal operating current until the tubing is filled. Over- and underload trips levels may need to be temporarily reset to prevent disconnection of the power. The Electrical Submersible Pump assembly should be connected and run on a test bench or in a test well prior to installation. Data obtained in a test well allows accurate estimates of the start-up and operating currents to be made, and has been found to reduce the incidence of Electrical Submersible Pump failures occurring during the first days of operation. If no test data is available the normal operating current should be calculated from the Electrical Submersible Pump and motor specifications and compared with the values measured.
No more than three attempts should be made to start a Electrical Submersible Pump if abnormally high starting currents indicate that the Electrical Submersible Pump is stuck. Other techniques, such as reversal of two phases or acidising to remove scale should be tried before the Electrical Submersible Pump is pulled.
4 Sand production
If initial sand production is expected the likelihood of damage to the Electrical Submersible Pump will be reduced if flow rates are restricted until the well has cleaned up. This should preferably be done using a VSD as use of a surface choke will increase will increase downthrust while the sand is passing through the pump, causing rapid wear.
5 Stopping an Electrical Submersible Pump
Unless a potentially damaging condition such as unbalanced operation is detected a pump should not be stopped until any solids in the wellbore have been produced to surface. When a pump is stopped, fluid will drain back through the Electrical Submersible Pump and suspended solids may settle out in the pump preventing the pump from being restarted, unless a check valve is fitted.
6 Re-starting an Electrical Submersible Pump
If the pump is stopped for any reason fluid may drain back through the Electrical Submersible Pump causing the impellers to rotate in the reverse direction, or backspin. Sufficient time must be allowed for backspin to stop before attempting to restart a pump or damage to the Electrical Submersible Pump will result, such as breakage of the shaft. The use of a backspin relay to detect pump rotation is recommended.
7 Current and voltage measurements
The voltages and currents in each of the phases should be measured and recorded.
The voltages and currents in all three phases should remain within 5-10% of each other. Unbalanced currents or voltages may indicate a faulty power supply, motor or cable. If severely unbalanced conditions are observed at start-up the system should be shut down within a few seconds to prevent damage. Minor imbalances between the loading of the phases is common.
Unbalanced loading may be due to small differences in the impedance of the cable conductors and stator windings, or to differences in the supply voltages of the three phases. In some cases changing the phase connections may reduce the imbalance. Care should be taken not to reverse the direction of rotation of the motor when changing phase connections.
8 Over and underload settings
The uncertainties inherent in the design of an ESP system make accurate prediction of the motor current under operating conditions difficult. The current will depend on the frequency and voltage of the electrical supply. Unless the 'normal' values for these parameters are known the measurements at any instant are relatively meaningless. The upper and lower limits of current which will trigger a shut down must be set on the basis of the current actually measured following start-up or during normal operation of the system. To avoid damage to the motor in the event of closure of a SSSV or flowline valve the undercurrent trip must be set to approximately 85-90% (certainly above the idlemotor load) of the normal operating current. The overcurrent trip is normally set at 110-120% of the normal operating current. Since the current will be approximately proportional to the density of the fluid within the Electrical Submersible Pump the trip settings may need to be adjusted as the well cleans up, and whenever changes in the composition of the produced fluid occur. The vendor engineer should monitor the current during all start-up and initial running periods to enable the first setting of under/overload trips to be made.
The importance of the correct settings for over and underload trip settings is emphasised. The fall in motor current under pump-off or shut-in conditions is small and may not be detected if the undercurrent trip is incorrectly set, causing unnecessary damage to the motor and pump. Increases in fluid density or changes in supply voltage will increase the current drawn by a motor and may mask the decrease caused by pump-off or shut-in conditions.
Some controllers and CAO systems for Electrical Submersible Pump operation include facilities for automatic resetting of the over and undercurrent trips by continuously monitoring the current drawn and adjusting the trip levels to reflect the average current during a pre-set period. Gradual changes in current due to variations in fluid density can be discriminated from sudden changes due to pump-off or shutting in of the well.
The over and underload settings for a VSD may need to be adjusted in a similar manner to the settings for a fixed speed Electrical Submersible Pump. In addition the settings must be adjusted following any change in operating speed. The frequency at which a pump is operated should be monitored and recorded with the voltage, current and flowrates. The values of voltage, current, and flowrate are of little value unless the supply frequency is known.
In common with other 3-phase induction motors, ESP motors will rotate in either direction depending on the electrical phase sequence. The phase sequence can be changed by reversing any two of the three conductors, with the third remaining in its original position. This is usually done at the vented junction box.
Electrical Submersible Pumps are capable of producing large volumes of fluid when rotating in the wrong direction. Accurate measurements of the wellhead pressure and flowrate may be required to identify the correct direction of rotation. A pump will produce less fluid when running in reverse. The most reliable method for ensuring that a pump rotates in the correct direction is the use of phase rotation instruments before the Electrical Submersible Pump is run into the well.
11 Voltage adjustment
The voltage supplied to the downhole Electrical Submersible Pump can be adjusted by selection of different secondary tappings on the transformer. The exact voltage required will depend on the cable losses, which will be proportional to the current drawn by the motor. The optimum voltage can be determined either by calculation or, in case of doubt, by experimentation.
An ESP motor will draw an increased current if supplied with power at a voltage which is either higher or lower than the optimum. Measurement of the current at each of the voltages available from the transformer allows the optimum voltage to be determined. At the optimum voltage the current drawn will be at a minimum.
12 Initial testing
Following the initial start-up and stabilisation of an Electrical Submersible Pump installation the pressures and flow rates must be accurately monitored to enable an initial estimate of the pumps performance. If a pump is found to be operating outside its design range, remedial action should be considered. Remedial action might include:
·adjustment of surface choke;
·Electrical Submersible Pump replacement;
·installation of a VSD.