Page 27 - Offshore Electrical Engineering Manual

14 CHAPTER 2 The Offshore Electrical Environment

3.3 kV latched contactors are supplied from the same low-voltage switchboard.
A spurious gas alarm occurs in the vicinity of the low-voltage switchroom and
the low-voltage switchboard incoming circuit breakers are opened by the emer-
gency shutdown system. The lube oil pumps then stop but the compressor control
system is unable to open the main drive motor contactors and the motor runs to
destruction.
Example 2 A platform has two low-voltage switchboards dedicated to provide
the safe and hazardous ventilation necessary for continued safe operation of the
platform. Depressurisation of any module would lead to process shutdown. The
particular platform is a pumping station for oil from other platforms including
those of other companies and therefore a considerable amount of oil revenue is
at stake if the platform is shut down. Unfortunately, each switchboard is fed by
a single incomer and the ventilation fan motor starters are distributed so that the
majority of supply fans are on one switchboard and the majority of extract fans are
on the other. This arrangement resulted in the export of oil from a number of large
North Sea installations being dependent on the continuous operation of two small
low-voltage switchboards.
The subject of reliability is dealt with in greater detail in PART 8 Chapter 1.

ENVIRONMENT

This topic is covered in greater detail in PART 5 Chapter 4 and is covered exhaus-
tively in all the relevant standards, recommendations and codes of practice (see
Bibliography). However, it is important to be very clear as to the fundamental
reasoning behind all the regulations governing electrical installation offshore.
Because both the safety and cost of an installation are highly sensitive to equip-
ment selection, it is also important to have a clear understanding of the rea-
sons behind the classification of hazardous areas and of the different methods
employed by equipment manufacturers to make their equipment suitable for
particular environments. Where this is practicable, electrical equipment is best
installed in an environmentally controlled room located in an area that is not
classified as hazardous with respect to hydrocarbon gas ignition risk, effectively
sealed from the outside atmosphere and provided with a recirculating air condi-
tioning system. Of course, this optimum scheme cannot be considered for equip-
ment which

1. has to be located outside (such as navigational aids),
2. has to be located under or near water (such as seawater lift pump motors), or
3. is associated with some other equipment which may occasionally or does nor-
mally leak hydrocarbon gases (such as gas compressor drive motors).

Often the electrical equipment installed has to cater safely for a combination of
all three of these situations and may also be required to operate at elevated pressures
and temperatures.