Page 144 - Offshore Electrical Engineering Manual

Main Oil Line and Water Injection Pumps 131

DOWNHOLE PUMPS

Downhole pumps are used in the latter stages of well life to provide ‘artificial lift’
when wellhead pressure or crude oil flow rates need to be improved. Downhole
pumps are extremely rugged devices, having to work in a ‘hole’ less than 0.25 m
in diameter, thousands of feet below the sea bed and at pressures and tempera-
tures near the design limits. The pump–motor string although of very small diam-
eter, is often in excess of 10 m long. The pumps are expensive proprietary devices,
with their manufacturers’ closely guarding the design secrets. The installation of
a downhole pump may cost in excess of £4 million and as usually a minimal war-
ranty is provided once the pump is installed; however, that sum may need to be
spent again a few days after installation if nothing happens when the start button
is pressed.
However, much can be done at the surface to improve the reliability of the pump.
Notably, the use of variable-frequency convertors to provide a very ‘soft’ start for the
motor has been proved to be very successful in the past few years. Downhole pump
motor nominal surface voltages are usually around 2 kV to allow for voltage drop
in the cable and to trade off copper and insulation cross sections to minimise motor
dimensions. As the pump may be capable of lifting in excess of 20,000 barrels per
day of oil and water from the well, the frequency convertor and transformer units
tend to be large.

MAIN OIL LINE AND WATER INJECTION PUMPS
Apart from the hazardous area requirements, the motor drivers for these pumps will
be very similar to those used onshore. Because of the pressures involved (typically
200–400 bar g), care needs to be taken in the design of controls to ensure that no
undue stresses are put on the pump or pipework, particularly during start-up. It
should not be possible to start the motor if the pump motor set is running backwards
because wrong valves are being inadvertently opened. As well as causing shock to
the pipework, such maloperation may draw starting currents of excessive magnitude
from the system, possibly causing damage to the motor windings and/or power
system instability.