Page 70 - Offshore Electrical Engineering Manual

Load Profiles 57

NUMBER REQUIRED
From the reliability point of view, three generators, each rated for the full platform
load, is the optimum. This allows for one generator running, one standby and one
undergoing routine maintenance. If space is in particularly short supply, it may
be necessary to dispense with the third machine. If point loading on the platform
structure is a problem, it may be necessary to use a greater number of smaller rated
machines.

SIZE

The types of prime mover available have already been discussed in this chapter. One
of the benefits of using large machines, each capable of supplying the whole sys-
tem load, is that of improved motor starting capability and greater stability during
power system disturbances. However, some care will have to be exercised to ensure
that alternator subtransient reactances on such large machines are not so high that
switchgear of sufficient fault capability cannot be obtained without going outside
the normal ranges of switchgear in standard production. Few manufacturers produce
standard switchgear at voltages in the 11–15 kV region above 1000 MVA. Cables
capable of withstanding the associated prospective fault currents would have to be
sized very much over their current ratings and would be expensive, heavy and more
difficult to install. Therefore, at present, 1000 MVA is considered the practical top
limit for system fault levels.

LOCATION
Generator modules should be located in an area classified as ‘safe’ when the release of
flammable gases is considered. This is necessary to reduce the risk that flammable gas
might be drawn into the engine enclosure and be ignited on hot parts of the engine. If a
small quantity of gas is drawn into the engine intake, this should not cause a significant
increase in engine speed as the governor should correct for the presence of this extra
‘fuel’. Large concentrations of gas, however, may cause overspeed in gas turbines and,
in all engines, may interfere with combustion and, if no precautions were taken, may
be ignited in the engine air intake, leading to fire and/or explosion.
In general, the following precautions must be taken with gas-fuelled engine
enclosures.

1. Any part of the engine, including ancillaries such as turbochargers, exhaust sys-
tems and anything else in the enclosure which may have a surface temperature in
excess of 80% of the ignition temperature of the actual gas/air mixture (200°C in
the case of North Sea gas), must not under any circumstances be allowed to come
in contact with such an explosive gas/air mixture. This can be avoided by

a. providing sufficient ventilation to prevent gas accumulation,

b. enveloping the hot areas in a water-cooling jacket (note that it is fairly
impractical to do this with a turbocharger),