Page 140 - Offshore Electrical Engineering Manual

Seawater Lift Pumps 127

Typically, the machines must be capable of accepting a voltage variation of
0–750 V DC and continuous load currents of 1600 A. For mud pump duty, two
motors will run in parallel on one silicon controlled rectifier (SCR) bridge, whilst
for the drawworks and rotary table, one SCR bridge will be assigned to each motor
with appropriate current limiting devices in operation. The motors must also be
capable of producing around 600 kW continuously and 750 kW intermittently at
1100 rpm. These machines are shunt-wound machines with class H insulation,
derived from railway locomotive designs. However, as they operate in hazardous
areas, the construction is closed air circuit, water cooled to restrict surface tem-
peratures, with the enclosures being pressurised to prevent ingress of explosive gas
mixtures.
This type of motor is often provided without a terminal box, the winding tails
passing through a sealed and insulated gland to a separate flameproof terminal box.
It is recommended, however, that motor-mounted terminal boxes should be used, as
the exposed winding tails are difficult to protect mechanically.

POWER SWIVELS
On some drilling rigs the drill string is powered by the swivel instead of the rotary
table. The system consists of the swivel powered by a hydraulic motor fed by hoses
from a hydraulic power pack which is located in a pressurised room. Power pack
consists of a swash plate pump driven by a medium-voltage motor. The motor is a
conventional squirrel-cage type, as described earlier.

SEAWATER LIFT PUMPS
In floating installations or those with hollow concrete legs, conventional pumps may
be used to obtain source water for cooling because the pump may be located at or
near sea level.
However, on steel jacket construction platforms, it is necessary to draw seawater
for cooling and firefighting up to the topsides using seawater lift pumps.
There are three basic types of drives for seawater lift pumps:

a. submersible electric
b. submersible hydraulic
c. electric shaft driven

The last two types use standard forms of electric motor as the power source, in
which the power is transmitted mechanically or hydraulically to the location of the
pump (see Fig. 2.12.3).
Therefore, only the first type will be discussed as a special application.
Submersible pumps for this duty are of small diameter and usually very long, con-
sisting essentially of a series of small induction motors all mounted on the same
shaft. The motor-pump string, having been connected to a special flexible cable, is