Page 20 - Offshore Electrical Engineering Manual

Description of a Typical System 7

consumer as seawater injection is underestimated for at the time of construction. In
general, however, it is better to concentrate switchrooms in one area of the platform
to avoid complications with hazardous areas and ventilation etc., as discussed in
PART 1 Chapter 2.
With such relatively high generation capacities and heavy-power users within
the limited confines of an offshore platform, calculated prospective fault currents
are often close to or beyond the short circuit capabilities of the millivolt switch-
gear designs available at the platform topside design phase. Currently fault ratings of
1000 MVA are available, and with careful study of generator decrement curves etc.,
it is usually possible to overcome the problem without resorting to costly and heavy
reactors. All the available types of millivolt switchgear are in use offshore. The use
of bulk oil types, however, is questionable because of the greater inherent fire risk.
Unlike land-based switchboards, there has been found to be a significant risk of earth
faults occurring on the busbars of offshore switchboards and so some form of earth
fault protection should be included for this. The platform distribution at medium
voltage normally consists of transformer feeders plus motor circuit breaker or con-
tactor feeders for main oil line pumps, seawater lift and water injection pumps and
gas export and reinjection compressors. Depending on the process cooling require-
ments, the cooling medium pumps may also be driven by medium-voltage motors.
Operating such large motors on an offshore structure (i.e., three quarters of the
way up a high steel or concrete tower) can lead to peculiar forms of failure due to
the associated vibration and mechanical shock, almost unheard of with machines
securely concreted to the ground. This has led to offshore platform machines being
fitted with more sophisticated condition monitoring than that usually found on simi-
lar machines onshore.
Another problem, which will be discussed in more detail later in PART 4, is the
transient effect on the output voltage and frequency of the platform generators with
such large motors in the event of a motor fault or, for that matter, during normal large
motor switching operations. Computer simulation of the system must be carried out
to ensure stability at such times, both at initial design stage and also when any addi-
tional large motor is installed. Facilities such as fast load shedding and automatic
load sharing may be installed to both improve stability and make the operator’s task
easier. This subject is discussed in PART 2 Chapters 12 and 15 and 15.

LOW-VOLTAGE DISTRIBUTION

Using conventional oil- or resin-filled transformers, power is fed to the low-voltage
switchboards via flame-retardant plastic-insulated cables. Cabling topics are covered
in PART 2 Chapter 8. Bus trunking is often used for incoming low-voltage supplies
from transformers. Because of the competition for space, this is just as likely to
be due to bending radius as to current rating limitations of cables, as bus ducting
may have right angle bends. The type of motor control centre switchboard used off-
shore would be very familiar to the onshore engineer. However, the configuration
of the low-voltage distribution system to ensure that alternative paths of supply are