Page 283 - Offshore Electrical Engineering Manual

270 CHAPTER 8 Power Management

duplicated or triplicated so that errors may be detected and the system ‘frozen’ in a
safe state so that operators may take over the function manually. The software used
must also be extremely reliable, with built-in error trapping routines and means for
ensuring a fast return to normal operation after a system crash. Memory devices
should be read-only where possible; where read-write memory is necessary, this
must be well protected from electromagnetic or other harmful effects. Any volatile
memory must be permanently battery backed. Most microprocessor systems incor-
porate watchdog and visual indication in the occurrence of a fault.
Such a system, when properly programmed, can provide a whole range of useful
facilities to make the operator’s life easier, as follows:

1. Automatic starting and paralleling of generators as demanded by rising load and
engine intake temperature according to a prearranged programme.
2. Automatic shutdown of surplus generators on falling load requirement or ambi-
ent temperature according to a prearranged programme.
3. Staged removal of noncritical loads from the system in the order of priority, the
number of stages being dependent on the severity of the overload condition.
4. Automatic start-up and paralleling of another generator during serious fault or
failure of a running generator. This facility can easily be provided in a system
with only two machines but requires a programmable device where more than
two machines are installed.
5. On a dynamically positioned vessel, backups to auxiliary machinery can also be
programmed in; for example, additional cooling water pumps, lube oil pumps or
fuel supply pumps can be started automatically.

POWER MANAGEMENT SYSTEM DESIGN
The principle documents involved in designing a suitable power management (PM)
system are listed in the following.

1. Functional design specification
This document sets out the detailed requirements of the system and has to be
agreed between the manufacturer and the client usually before any design work
starting.
2. Failure modes, effects and criticality analysis
The purpose of the failure modes, effects and criticality analysis (FMECA) is to
give a description of the different failure modes of the equipment when referred
to its functional task. Most failure modes identified by this analysis will require
to be covered by redundancy in some form if feasible and economic. If not,
some operating procedure will need to be developed to provide a solution or
mitigation.
3. Cause and effect matrix
Once the detailed function of the system is fully described, the system logic can
be developed in the form of a cause and effect matrix. It may also be necessary
to use flow charts to clarify the logic in some cases.