Page 56 - Offshore Electrical Engineering Manual

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CHAPTER
3
Emergency Generators

EMERGENCY GENERATION
Most offshore production installations have three or four main levels of operation
which are reflected in control systems such as the emergency shutdown (ESD) sys-
tem (PART 2 Chapter 15). If, however, there is a very large gas leak such that the
installation is enveloped in a gas cloud, it would be necessary to isolate all forms of
electrical power capable of igniting the gas, including in some instances, the direct
current secure supply batteries. Assuming this dire situation (usually called Level
Zero) has not occurred, the first level of operation is on battery power only and is
considered in the ESD section in PART 2 Chapter 15.
The next level of operation is with the emergency generator only running.
The provision of an emergency generator is a statutory safety requirement and
as such should be designed to provide reliable power for statutory communica-
tions equipment, navigational aids, fire and gas monitoring, ballast systems for
floating units and, although not a statutory requirement, accommodation cook-
ing, drinking water, laundry and sanitation facilities. As this generator must not
be dependent on the platform production processes for fuel, it is invariably die-
sel driven. Storage of petrol or propane on the platform would be considered a
hazard, which would rule out the use of a spark ignition engine for this purpose.
Emergency generators are usually designed to be automatically started on failure
of other larger generators in the installation, by use of ‘dead bus’ relays. Again,
there is a statutory requirement that the starting equipment for this generator is
capable of at least six start attempts, frequently with a second bank of batteries
on a manual changeover switch.
This generator should be located in a ‘safe’ area, close to the accommodation,
radio room and process control room. A ‘day tank’ is required near the generator, big
enough to run the machine for the time specified in the relevant statutory regulation.
The time will vary depending on other installation conditions such as whether it is
regarded as a ‘manned installation’ or ‘normally unmanned installation’ but may be
24, 48 or even 96 h.
The following points are often overlooked in specifications for emergency gen-
erator sets:

1. Despite the small size of the prime mover, air intakes must still be provided with
spark-arresting devices and overspeed flap valves to prevent ingestion of gas,
and exhausts with spark arrestors.

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