Page 68 - Offshore Electrical Engineering Manual

Load Profiles 55

that gas at the wrong temperatures and pressures cannot reach the engine. This would
mean the automatic operation of isolation valves and shut down of generator set.
Occasionally, there may be fluctuations in the quantity of condensate that pass
through the process system and reach the engines. ‘Slugs’ of condensate, as discussed
earlier, are more likely to cause speed fluctuations in a gas turbine, but with a gas-ignition
engine, significant quantities of ‘incompressible’ liquid present in one or more cylinders
will invariably result in serious damage. ‘Knockout’ pots are essential for both types of
engine, but it is essential that for gas-ignition reciprocating engines a small separator or
knockout pot is installed close to the engine, which is designed to remove both slugs of
condensate and any liquid which has condensed on the walls of the fuel gas supply pipe.
High liquid level in the knockout pot should be arranged to shut down the engine.

LOAD PROFILES
The following topics need to be taken into consideration when selecting the number,
type and rating of generator sets on a particular installation.

PROJECTED DEMAND

Over the life of the platform, the generation requirements may double or even triple
as each new operational phase is reached. A typical demand profile is shown in the
following.

Operation Platform Power Demand (MW)
1. Drilling 3
2. Oil export 12
3. Gas compression/export 20
4. Artificial lift phase 25

VARIABILITY OF DEMAND OVER 24 HOURS
On a large oil production platform, the larger power users such as water injection
pumps, main oil line pumps and gas compressors constitute the majority of the elec-
trical demand. This will remain constant over 24 h unless some planned change of
plant is necessary or a breakdown occurs.
If drilling activities are powered from the main platform system, some quite large
but transient demands, possibly of the order of a few megawatts, can be expected
from the rotary table or drawworks when ‘difficult’ mineral formations are experi-
enced. On small installations where gas is exported without the need for compres-
sion, and where there are no ‘round-the-clock’ maintenance shifts, there will be a
distinct profile created by the use of galley equipment and electrical water heaters in
the accommodation areas.