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72 NAVY ENGINEERING BULLETIN MARCH 2003
maintainability work up to that
time and shows how this work
has altered his design. The
contractor must also demonstrate
that design changes introduced
for performance reasons are
being considered for their effects
on reliability and maintainability.
Design changes are, after all,
sometimes proposed for reasons
other than improving reliability
and maintainability! However,
such changes must be “swept up”
by the reliability program.
Acceptance Must be
Demonstrable
Engineers have found that a clear
and unequivocal acceptance
criterion is essential, particularly
now that most equipment is
purchased on a fixed price basis.
The importance of defining
“failure”, for example, has already
been mentioned.
It is possible to demonstrate, with
a desired degree of statistical
confidence, that a prototype has
met quantified reliability and
maintainability targets, failures
and running hours recorded
during test can be related to
target MTBF. However, this can be
very time consuming (and hence
expensive) for items with large
desired MTBFs such as is found
in marine engineering. A single
main engine may reasonably be
expected to last a three month
mission without failure and would
need, typically an MTBF of 2 200
hours to achieve this. Even if the
prototype encountered no failures
during test it would take 66 000
hours – that is 7.5 years of
continuous testing – to
demonstrate achievement of the
MTBF with 95% statistical
confidence. It is for this reason,
as much as anything else, that
engineers have to be involved in
how a contractor conducts his
business; it is rarely practical, in
marine engineering, physically to
demonstrate achievement of
numerical RAM targets.
One possibility, which has been
successful in the procurement of
naval weapons, is to extend the
contractor’s responsibility to
include acceptance in the Fleet.
Design improvement continues at
the manufacturer’s expense
during the initial period after
installation. Failures and running
hours are monitored – again, at
the contractor’s expense – and
used to demonstrate achievement
of the requirement before formal
acceptance. This acceptance
testing has the advantages of
running machines in their own
true environment, together with
accumulating running hours fairly
quickly from the equipment “on
test”. Consideration should be
given to introducing this
approach for acceptance in
marine engineering systems and
equipment.
Concluding Remarks
Advancements in technology,
combined with escalating costs,
mean that consideration of
reliability and maintainability can
no longer be left to good
engineering judgement in the
course of a design; they must be
given specific attention. The
difficulties of demonstrating
achievement of quantified
reliability and maintainability
targets in mechanical equipment,
mean that owners must obtain
additional assurance by
monitoring the actual design
process without, however,
relieving a designer of his
contractual responsibilities.
The importance of reliability and
maintainability in modern naval
procurement does not mean,
however, that unlimited funds are
made available for every
conceivable type of reliability and
maintainability analyses. One
does not design motor cars to
have the same reliability as a
nuclear power station, despite the
fact that cars are far more likely
to kill! The emphasis today is to
obtain value for money. Within
the overall cost, the achievement
of specified availability and
performance must be accorded
equal consideration. The
reliability and maintainability
program is chosen carefully to
ensure that a good balance
between this work and
performance is obtained.
As the Fleet grows smaller, more
is being demanded of our ships
and men than ever before. They
deserve better equipment than in
the past and our aim should be
to ensure that everything is being
done to achieve this.