Page 94 - Offshore Electrical Engineering Manual

Process Area Distribution 81

LIVING QUARTERS SUPPLIES

Electrical supplies for living accommodation are important for the well-being of the
offshore staff. Failure will not only bring discomfort, but will present a serious health
hazard, when frozen food begins to thaw, toilets cannot be flushed etc.
As mentioned in PART 1 Chapter 2, domestic pipework should be kept out of
switchrooms where possible. Where pipes do have to pass through switchrooms,
possible sources of leaks such as flanges, valves etc., should be kept well away from
switchboards or avoided altogether.
Most offshore accommodation is sealed and positively pressurised to pre-
vent ingress of gas, should any process leaks occur, and so the operation of air
conditioning is also vital. Should some serious incident result in a gas cloud
developing, leading to ingestion of gas by the accommodation module HVAC,
all generation is usually shut down and doors and fire dampers closed as auto-
matic actions of the emergency shutdown (ESD) system and fire and gas systems.
The actions taken however will depend on many design and operational factors
associated with the particular installation. When the incident is over, certified
hazardous area fans fed from another source may be used to purge the remaining
gas from the module. Some operators prefer to allow natural ventilation to clear
the gas, as purging fan systems have been known to ignite the gas and cause an
explosion.
A serious fire outside the accommodation module may lead to its envelopment in
dense smoke. Smoke detectors located at the ventilation air intakes should signal the
fire and gas system to close down the ventilation system automatically, shutting fire
dampers and doors to seal the module.

PROCESS AREA DISTRIBUTION
By far the largest power consumers on any offshore oil installation are the process
modules. Apart from the large process drives, power is required for level control and
circulating pumps, agitators, centrifuges, compressors and ventilation fans. Lighting
and instrumentation power must also be provided, and for maintenance, sockets for
welding transformers and other temporary equipment. The production switchboards
located in safe area switchrooms provide the majority of controlgear for this smaller
type of equipment. Welding sockets and portable tool sockets are equipped with iso-
lators. A popular type of socket unit is certified Ex ‘e’ but an Explosion-proof cas-
sette containing the isolator contacts is housed within the Ex ‘e’ enclosure. All such
sockets are fed from shutdown contactor feeders in the production switchboards, so
that should gas be detected, all portable equipment in the area may be immediately
isolated.
Lighting distribution is discussed in PART 2 Chapter 14.