Page 30 - Offshore Electrical Engineering Manual

Hydrocarbon Hazards 17

On normally manned installations the control point needs to be located in or
adjacent to the accommodation area and may be in or adjacent to the Offshore
Installation Manager’s office and radio room. This control point also requires pub-
lic address facilities to be close at hand, and as a Temporary Safe Refuge, it would
normally be the last area to be vacated in an emergency; the room in which it is
contained would be H120 fire rated, with a dedicated ventilation system (see PART 5
Chapter 5).
In the larger platforms, this is a limited repeat of a far more sophisticated moni-
toring system located in or near the process control room.
On normally unmanned platforms the basic control point may be located on an
adjacent normally manned installation or even at a control centre onshore.

EMERGENCY SHUTDOWN SYSTEM

As in the case of a nuclear reactor or similar complex system, the continuing safe
condition of the platform cannot be left solely to the human operators, as they would
not always have sufficient time to investigate each abnormality and respond with the
appropriate sequence of corrective actions in every case. Because of this, it is neces-
sary to provide a system which either initiates the correct sequence of actions itself
or provides a series of simple options (levels of shutdown) that the operator may take
when a particular event occurs.
Every installation has start-up and shutdown systems of varying sophistication,
which attempt to provide the greatest possible safety for personnel and equipment.
These systems are interrelated, with the process control system being subordinate to
the emergency shutdown system. The various levels of shutdown and their effects on
the electrical system are discussed in PART 7 Chapter 6.

SAFE AND HAZARDOUS AREA VENTILATION
Most of the heating, ventilation and air-conditioning (HVAC) systems must run
continuously during normal platform operation to ensure the following:

1. Acceptable working environments are maintained in process modules contain-
ing equipment or pipework which may leak hydrocarbon gas.
2. Comfortable environmental conditions are maintained within the accommoda-
tion modules and normally manned nonhazardous areas, and an acceptable
working environment is provided in normally unmanned modules.
3. Positive pressurisation with respect to adjacent hazardous areas or the outside
atmosphere is maintained in nonhazardous modules or rooms.
4. Potentially hazardous concentrations of explosive gas mixtures are diluted in, or
removed from, hazardous area modules.
5. When fires occur or dangerous concentrations of gas are detected, individual
areas are sealed from ventilation and the associated fans shut down in accor-
dance with the logic of the emergency shutdown system.