Page 65 - Offshore Electrical Engineering Manual

52 CHAPTER 4 Prime Mover Selection Criteria

2. The effects of any process flares must be taken into consideration, both for hot
gas and radiation.
3. The effect of wind on all the various platform exhausts must also be considered.
Although the prevailing wind is the most important consideration, the turbine
must be able to develop sufficient power in any wind condition, and with any
combination of other engines installed on the platform, working at their normal
outputs. If the exhaust plume from another engine causes the engine in question
to govern down, this may be overcome by the use of extra ducting or a water
curtain installed around the exhaust of the other engine.

Airflow through the engine, and hence cooling, may be improved by cleaning the
compressor section of the engine after a few months of operation. The improvement
in output power after routine cleaning is usually significant and can be as much as
10% of its rated power. The selectors of the prime mover must take this into account
in their rating calculations.
If it is necessary to install a load shedding system because of engine power
limitations or increasing electrical demand, as mentioned earlier the system should
take turbine exhaust temperature into account. If the load shedding system tripping
level is based purely on monitoring electrical load for a fixed worst-case value,
production operations may be unnecessarily curtailed. In colder weather condi-
tions, as much as 15% of extra engine capacity would still be available. This may
amount to several megawatts with a 25 MW generator set. The converse is also
possible when exceptionally warm weather conditions may reduce engine capacity
to below the load shedding system tripping point causing the generator set to trip
on underfrequency.

GAS INGESTION FROM LEAKS
A large gas turbine generator set may take in the region of 30 s from the initiation
of an emergency stop signal to slow down to a speed at which reacceleration is no
longer viable without cranking. During this period of deceleration, it is vulnerable
to ingestion of gas from serious process gas leaks on the platform. Such ingestion of
gas in sufficient quantity, bearing in mind that all electrical loads would have prob-
ably been removed by the emergency shutdown system by this time, may cause the
machine to reaccelerate and overspeed to destruction.
The risk of this occurring must be minimised by the following:

1. Carefully siting gas detectors to ensure that serious gas leaks are detected as
soon as possible. A ‘2-out-of-N’ voting system should be used to minimise
spurious generator trips due to faulty detectors.
2. Governor response to an emergency engine stop signal should be as fast as pos-
sible within the metallurgical limitations imposed by turbine blade cooling rates
etc.