Page 66 - Offshore Electrical Engineering Manual

Reciprocating Engines 53

RECIPROCATING ENGINES

DIESEL ENGINES
By far the most common engine for small- to medium-power requirements
offshore, as well as prime movers for alternators, is the diesel engine and will
be found directly driving anything from cranes to fire pumps. Diesel engines
larger than 2 MW are rarely found on fixed platforms, however, for the following
reasons:

1. Weight and vibration problems may be encountered in the platform structure.
2. If the engine is driving an alternator which provides normal production, i.e.,
nonessential supplies, and gas of sufficient quantity and quality is available
during production, then importing large quantities of relatively expensive diesel
oil is likely to be commercially unattractive. It would also require heavy storage
tanks located in a site that would not constitute a fire hazard, which could well
be a problem in the case of a steel structure.

It is possible to reduce the quantity of diesel oil consumed by burning
a mixture of diesel oil and fuel gas. The ratio of gas to diesel is limited to
approximately 90%, the limitation is because of the amount of diesel required
to be injected as a pilot fuel to preserve the compression ignition action of the
engine. This limitation has the benefit that a governed diesel engine is less
likely to overspeed because of the ingestion of gas from a process gas leak. The
gas supply has to be at very high pressure to enable injection at diesel compres-
sion cycle pressures, and therefore, fuel gas booster compressors are required
for this duty.

GAS-IGNITION ENGINES
If it is required that the engine should run on fuel gas only, then an ignition system
will be required similar to that found on petrol engine. It is normal, however, in
order to improve reliability, to use a low-tension distribution system with indi-
vidual coils mounted over the spark plugs on each cylinder, known as a ‘shielded
system’.
Reliability is improved by the following:

1. Reduced line loss in the coil secondary circuit, as it is very short. Plugs and
coils screw together to form an integrated Ex certified unit.
2. The probability of earth faults occurring in the secondary circuit is substantially
reduced, as there are no high-tension cables routed close together.
3. Common routing of high-tension cables can also lead to transformer effects
which lead to ignition voltages appearing in the wrong cable at the wrong time,
engine misfiring and loss of power.