Page 69 - Offshore Electrical Engineering Manual

56 CHAPTER 4 Prime Mover Selection Criteria

LOW LOADING PROBLEMS
No internal combustion engine will run efficiently at loads much below 50% of its
rated full power output. This is particularly a problem with reciprocating engines,
as below about 30% of full power, combustion products such as soot and gum will
begin to collect inside the engine in amounts sufficient to substantially reduce the
power available from the engine when the load demands it. Turbochargers are par-
ticularly susceptible and will be stopped by deposits after only a few hours running
at low loads. If the load profile dips dangerously low for an hour or so, followed by
a period where the load is substantially higher, i.e., greater than 50% of full power,
it is likely that the increased combustion will clear the engine of the build-up in
these deposits.
As with a fuel gas analysis, it is important that a cyclic load profile is presented
to the engine manufacturer and a statement obtained from him to the effect that the
engine will continue to run without deterioration with the load profile submitted.
Should such guarantees not be forthcoming, it will be necessary to reconsider some
of the other engine options and/or whether the most suitable number and rating of
generator sets has been selected.

CHOICE OF FUEL
The choice of fuel is usually governed by the following:

1. The quality and quantity of gas available from the field being exploited. Where
the reservoir produces a preponderance of oil and there is a likelihood that there
will not always be sufficient gas for fuel, then some other fuel will be required,
either continuously or as a standby.
2. Availability of well gas at the times when the engine is required to run. If the
engine is the prime mover for an emergency generator, it will need to run when
the production process is shut down and no gas supply is available.
3. Logistical costs associated with the transport of diesel oil to the platform con-
cerned. The use of diesel oil requires that sufficient quantities of diesel can be
stored on or in the structure to allow for periods of bad weather when refuelling
is not possible. As this fuel will constitute a considerable fire risk, the storage
location will need to be carefully considered.
4. Comparative costs of connection to a suitable power generation complex. This
may be a nearby platform with spare capacity or an onshore supply system.
The costs may also need to include those for the purchase and installation of
a suitable motor generator set to cater for platform supply frequency and/or to
effectively reduce the supply system impedance.

MAIN GENERATION
Having established all the limitations, such as weight and dimensions, imposed by
the platform structure, the supply and operating constraints can be tackled.