Page 331 - Offshore Electrical Engineering Manual

318 CHAPTER 4 Hazardous Area Installation

should shut down and all non-certified electrical equipment in it should be isolated.
Often, a loss of pressurisation will only give an alarm; the decision to shut down
the plant will be left with the operator, and prevention of gas ingress will depend
on an airlock door system. Apart from the serious ignition hazard that all the live
non-certified equipment poses in this situation, the control operator’s only means
of escape is via the compression module where the gas leak has occurred. Owing
to the amount of work involved in providing automatic isolation, with shutdown
contactors for all the electrical supplies, and separating the instrument and control
functions so that the main installation control room is unaffected, it could well
be more beneficial to relocate the compressor control room to a safe area or pos-
sibly include its functions in an extended central installation control room. The last
option would be more in line with the Cullen Report, as it would remove an extra
manned control room. Both relocation options eliminate the need to provide an
escape route through the compression module protected to some degree from fire
and explosion.
The ventilation of engine enclosures is discussed in PART 5 Chapter 4.
A detailed discourse on the ventilation of offshore installations is beyond the
scope of this book. However, the following is a list of basic criteria with which such
ventilation systems should comply:

1. the positive pressurisation of non-hazardous areas with respect to adjacent haz-
ardous areas or the external atmosphere;
2. the containment, dilution and removal of potentially hazardous concentra-
tions of explosive gaseous mixtures in hazardous modules, and the adequate
segregation of hazardous area ventilation exhausts from ventilation and engine
intakes;
3. the provision of comfortable environmental conditions in accommodation and
normally manned non-hazardous areas, and acceptable working conditions in
normally unmanned areas;
4. the provision of acceptable working conditions within hazardous modules;
5. the isolation of individual areas and control of ventilation in emergency condi-
tions, in accordance with the shutdown logic of the installation’s ESD, fire, gas
and alarm systems;
6. the provision of combustion air for essential prime movers, ventilation air for
escaping, firefighting and rescue personnel and purging air services required to
operate effectively during an emergency.

To avoid depressurisation and potential release of explosive mixtures, all exits to
hazardous areas should be airlocked with self-closing doors. If single doors are used,
a hazardous zone will extend outside the compartment.
Safe area control rooms may be located within hazardous modules provided they
are kept pressurised with air obtained from a non-hazardous external area. Purging,
alarm and timed shutdown facilities are required, similar to those for a pressurised
motor. However, location of control rooms in safe areas is preferred for reasons given
above.