Page 31 - Offshore Electrical Engineering Manual

18 CHAPTER 2 The Offshore Electrical Environment

6. Uncontaminated combustion, purging and normal ventilation air is available to
prime movers.
7. Uncontaminated air supplies are available to personnel and emergency generator
and fire pump prime movers and other essential service equipment are provided
with combustion and ventilation air in times of emergency.

Ventilation systems, especially those associated with switchrooms and generator
rooms, are discussed in more detail in PART 5 Chapter 5.
The three above-mentioned systems (i.e., fire and gas, emergency shutdown and
HVAC) are interconnected and are often required to work in concert. An example of
this would be if a fire occurred in a particular switchroom. The fire would be detected
by smoke or heat detectors and the central fire and gas system monitoring the room
would initiate the following actions:

1. Signals the ventilation system to seal the room by the closure of ventilation fire
dampers and switch off associated fans.
2. Sounds an alarm in the switchroom to warn personnel that escape is necessary
and that a fire extinguishant is to be released.
3. Depending on the system logic signals the emergency shutdown system to
isolate the switchboards in the switchroom by opening the appropriate feeder
circuit breakers, or even shuts down all main generators if the switchroom in
question contains the main switchboard.
4. Releases the fire extinguishant (CO , water mist or halon gas) into the switch-
2
room after a suitable time delay to allow for personnel to escape.

DISTRIBUTION CONFIGURATION
Figs 1.2.1–1.2.3 show various ways in which attempts have been made to obtain opti-
mum availability from the platform electrical power system within the limitations of
weight and conditions imposed by the situation.
Fig. 1.2.1 shows the system arrangement on an earlier installation, consist-
ing of a main 6.6 kV switchboard with two 15 MW gas turbine generators and
a smaller 6.6 kV switchboard with two 2.5 MW gas turbines. In this configura-
tion the smaller ‘utilities’ switchboard provides supplies for all cooling seawater
pumps and also feeds all low-voltage distribution transformers except those in a
water injection module which was added to the platform at a later date. Should a
fault occur affecting the whole of the utilities switchboard, for example, a fault
in the bus sections or sectioning switch leading to busbar damage, then only the
emergency generator is left available. This is because there is insufficient spare
capacity in the emergency generator to run the main generator auxiliaries, and
in this particular case the main generators require seawater for cooling, which
is normally available from the seawater lift pumps powered from the utilities
switchboard.