Page 329 - Offshore Electrical Engineering Manual

316 CHAPTER 4 Hazardous Area Installation

case, such high-powered transmitters should be shut down on a gas detection alarm.
Guidance on which transmitters may produce an ignition risk is given in the above
reference document (PD CLC/TR 50427).
Where ships are required to make a close approach to an installation (e.g., supply
vessels), radar antenna on the bigger ships may be level with hazardous areas on the
platform. Therefore, such vessel radar sets considered a risk should be temporarily
shut down. GPS for dynamic positioning should not be affected.

HAZARD SOURCE SCHEDULES
The process design must be scoured for every possible point where flammable substances
are likely to leak. Only continuously welded pipes and vessels are not regarded as poten-
tial points of release. This task is best done by a team of engineers which includes a
chemical or process engineer, an instrument/systems engineer and an electrical engineer.
It is recommended that a hazop study be carried out concurrently with this task.
The resulting hazardous source schedule should give details of each source of
hazard, its location, risk of occurrence, extent and type of hazardous zone produced
and remarks regarding any environmental aspects affecting the hazard such as dilu-
tion, ventilation and airlocks. The schedule should also refer to a relevant and up-to-
date hazardous area boundary drawing. If the installation already exists and is due
to be modified or expanded, it will require to be thoroughly surveyed to ensure that
the existing source schedules reflect the current situation. A typical hazardous source
schedule is shown in Fig. 5.4.4.

DEFINING BOUNDARIES
The extent of the hazardous zone round a source of release is usually determined in
accordance with the British Institute of Petroleum Model Code of Safe Practice or
equivalent American API Code. This gives details illustrated by diagrams of typical
situations with distances of the zone boundary from the point of release.

VENTILATION
The degree of ventilation around the source of release will affect the classification of
the hazard, unless one of the three following situations apply:

1. Open spaces with no structure or equipment restricting substantially free circu-
lation of air, vertically and horizontally.
2. A module with a roof and not more than one side closed, free from obstruction
to natural passage of air through it, vertically and horizontally.
3. Any enclosed or partly enclosed space provided with artificial ventilation to a
degree equivalent to natural ventilation under low wind velocity conditions, and
having adequate safeguards against failure of the ventilation equipment.