Feature 3 | Finland’s Marine industries
Refined thinking on Azipod reliability
The reliability of the operation of Azipod propulsion depends on many
factors - design and production, how the Azipod is operated, and also on
operating conditions, service routines, and schedules.
Figure 1. Definitions
I
n the process of refining the
Azipod, ABB Marine last year
of failure statistics
conducted a study aiming at
and operating
verifying the differences between failure
times.
and downtime of vessels using electrical
podded propulsion with shaft-driven
propulsion. Probabilistic methods based
on failure statistics were used.
‘It is very important to gather
statistics on failures, as our clients want
to optimise their service routines and
schedules,’ said Markku Hokkanen, ABB
Marine sales manager, cruise vessels.
‘With these types of investigations by
our reliability engineering team we can
provide answers to questions on failure
statistics, based on existing real material.
The more information we gather over
time, the better is the reliability of the
analysis.’
Figure 2a and 2b.
Mr Hokkanen said definitions are
Empirical data of
often misinterpreted in practice, say
failure statistics
in the example ‘time between failures’
over time.
(TBF), and ‘time to failure’ (TTF)
as defined in figure 1. ‘If the mean
time between failures is two years,
for example, one should understand
that the statistical spread defines the
probability to operate a system for that
time without failure. The bigger the
spread, the more probable is a failure
already after a shorter time in operation.
Spread tends to become bigger when the
sample covers a wider range of different
operation conditions and operators.’
Figure 2 shows empirical data of
failure statistics for a mechanical
application over time. Figure 2a shows
the probability density function. The
proportion of the area under the clock-
formed curve between, for example
time A and time B to total area under
the curve, gives the probability that
the failure takes place in that time
period. Figure 2b shows the cumulative
distribution function. The rising curve
tells what the probability is that a fault
will take place, (vertical scale) on or
68 The Naval Architect February 2008
NA Feb 08 - p68+70+71.indd 68 05/02/2008 09:48:12
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