Page 401 - Offshore Electrical Engineering Manual

388 CHAPTER 1 Reliability

obtain, and therefore, for the purposes of this introduction, it will be assumed that
every component with the same function has the same reliability properties, i.e., its
failure rate and its mean time to repair (MTTR) is the same as any other with that
particular function. Nevertheless, it is advisable to keep records of failure and times
to repair of equipment installed, as analysis of this data in itself may indicate some
problem when the records are compared with available generic data.
Very significant reliability improvements can be made in systems, usually much
more significant than from simply replacing components, by reconfiguring them in
some way. Some examples of this are given in PART 1 Chapter 2. The basic principle
for any reconfiguration is to provide redundancy in such a way that during a com-
ponent’s outage, its function will be maintained by one or more other components
of similar function, whether this outage is planned or unplanned. In most cases the
‘component’ is a system in its own right, such as a generator package with its associ-
ated auxiliaries or a subsea cable supply from another platform.

FAILURE MODE, EFFECTS AND CRITICALITY ANALYSIS

This procedure, like the hazard and operability analysis (Hazops) which although is
really beyond the scope of this book, is worth applying to the more complex systems
as a method of identifying all modes of failure, analysing their effects and where pos-
sible, evaluating the frequency of their occurrence.
The basic failure mode, effects and criticality analysis (FMECA) process is the
completion of a table with column headings similar to Fig. 8.1.1.
However, before the columns can be completed, it is necessary to draw a block
schematic diagram of the system to be analysed at the level of detail required. This
level should be low at the initial stage and as it becomes more obvious which compo-
nents or subsystems are critical, these particular areas can be broken down into more
detail by producing a more detailed block diagram with more components identified
and analysed.

CIRCUIT BREAKER ILLUSTRATION

If we take a circuit breaker as an example item on an FMECA, this would appear
as a block in the diagram as shown in Fig. 8.1.2. A circuit breaker is a particularly
difficult component to describe in an FMECA, as it has several functions and several
failure modes.

CIRCUIT BREAKER FUNCTIONS
(i) Connects and isolates an electrical circuit as required.
(ii) In the event of a fault, automatically in conjunction with a sensing device, inter-
rupts the flow of fault current.