In the past, major industrial accidents tended to be seen as
just another part of the overall risk of doing business. It
was felt that it was just too expensive (if not impossible)
to analyse industrial systems in enough detail to be able to
manage the risks with any credibility.
In more recent times, reliability professionals have developed powerful tools (such as probabilistic or quantitative
risk assessment) to assess the cumulative probabilities of
failure and associated overall levels of risk inherent in
complex systems.
However, one limitation of these techniques, especially
when applied to protected systems, has been a tendency to
regard the probability of failure of the protected function
and protective device as fixed. This leads to the belief that
the only way to change the probability of multiple failures
associated with such systems is to change the hardware (in
other words, to modify the system), perhaps by adding more
protection or by replacing existing components with ones
which are thought to be more reliable.
In fact, it is now apparent that it is possible to vary both
the probability of failure of a protected function and (especially) the downtime of the protective device by adopting
suitable maintenance and operating policies. As a result, it
is also possible to reduce the probability of multiple failures
to almost any desired level within reason by adopting such
policies. (Zero is of course an unattainable ideal.)
The probability which is considered acceptable for any
multiple failure depends on its consequences. Sometimes
levels of acceptability are specified by regulatory authorities, but in the vast majority of cases the assessment has to be
made by the users of the asset. Since these consequences
vary hugely from system to system, what is deemed to be
acceptable varies equally widely. This means that there no
universal standards of risk that can be applied to all systems of a particular type (at least, not yet).
But someone has to make a decision as to what level of
risk is acceptable before it is possible to decide what must
be done to design, operate and maintain protected systems. (In fact, merely persuading senior people to accept
that this is a manageable variable which they must therefore manage is currently one of the biggest challenges
facing maintenance professionals.)