Page 21 - Offshore Electrical Engineering Manual

8 CHAPTER 1 The Offshore Electrical Dimension

always available is usually much more important offshore. This is because although
every effort is put into keeping it to a minimum, there is much more interdependence
between systems offshore.
A few examples of small low-voltage supplies which are vital to the safe and
continuous operation of the installation are

1. safe area pressurisation fans,
2. hazardous area pressurisation fans,
3. generator auxiliaries,
4. large pump auxiliaries,
5. large compressor auxiliaries,
6. galley and sanitation utilities for personnel accommodation,
7. uninterruptible power supply systems for process control and fire and gas
monitoring,
8. seawater ballast systems (on tension leg platforms and semisubmersibles).

The topics of maintenance and availability are covered in PART 8 Chapter 1.

EMERGENCY OR BASIC SERVICES SWITCHBOARD
As a statutory requirement, every installation must have a small generator to pro-
vide enough power to maintain vital services such as communications, helideck and
escape lighting, independent of any other installation utility or service. In the event
of a cloud of gas enveloping the platform due to a serious leak, even this will need to
be shut down as a possible ignition source.

FIRE PUMPS
Again as a statutory requirement, every installation must be provided with at least
sufficient fire pumps with enough capacity to provide adequate water flow rates for
fighting the most serious wellhead, pipeline riser or process fires. The numbers and
capacities of these pumps have to take into account the unavailability due to rou-
tine maintenance and failure. These pumps may be submersible electrical, hydraulic
powered or directly shaft driven from a diesel engine.
Typically, one pump arrangement could have an electrically driven 100% capacity
pump supplied from a dedicated diesel generator set which is directly cabled to the
pump, i.e., with no intervening switching or isolating devices, and has the advantages
of increased reliability because of fewer components and ‘soft’ starting of the motor.
This kind of pump runs up to operating speed with the generator, in the same manner
as a diesel-electrical railway locomotive would accelerate from start. The second 100%
capacity pump could again be electrical but supplied from the platform distribution sys-
tem in the conventional way. The purpose of this arrangement is to avoid failure of both
pumps due to a common operational element, i.e., common mode failure. A third 100%
capacity pump would be required to allow for maintenance down time. Details on the
electrical design of diesel–electrical fire pump packages are given in PART 2 Chapter 12.