Feature 3 | Finland’s Marine industries
Figure 3. Results probability of 87.49% for cruiseships with
of the failure Azipod propulsion and 84.82% for shaft-
comparison driven vessels.
between pod The reliability statistics for the different
propulsion and systems is shown in figure 4. In this
conventional shaft comparison it is assumed that the time
propulsion. between regular drydockings do not
differ between cruiseships with different
systems.
‘According to the results, Azipod is
on the same level with shaft lines with
regards reliability, but our goal is naturally
to further improve,’ Mr Hokkanen said.
‘We are by no means seeking excuses for
our performance not to meet with our
customers’ expectations. We have learnt
a lot and have improved in all areas, in
design, production, and also in operations.
In addition, our tools and methods for pro-
active maintenance have much improved.’
Figure 5 shows the development trend
of the reliability of Azipod propulsion. The
availability statistics show an improving
trend over time. The influence of faults
Figure 4. Reliability can be seen to be distorted, at a time when
comparison. only a few Azipod vessels were on the
market. Today, Azipod propulsion has
accumulated some 3.3 million operating
before a specific time (horizontal The results in figure 3 show that the hours, increasing with some 63,000 each
scale). mean time between failures (MTBF) for month. There are currently some 138 units
cruiseships with shaft drive systems is in operation on 64 vessels.
Pods versus shafts 91,097 hours, ie some 10.4 years. Here the Mr Hokkanen pointed out that it was
Sample cases were assessed of cruiseships shaft systems investigated included failures important to determine the definitions
with both shaft and podded drives, of the propellers, rudders, and eventual correctly when investigating system
minimising the effects of differences stern thruster, shaft bearings, reduction operation with probabilistic methods.
in service routines. Information was gears, and related auxiliary systems. Mean time to a failure (MTTF) is too
mainly gleaned from Internet sources. The similar duration for the Azipod easily presumed as a time until a technical
The reliability of the information on system with auxiliaries was 112,192 hours, application may be operated ‘trouble free’.
failures of shaft systems was verified by ie 12.8 years. Being an average MTTF is the time until
comparing the information provided on When looking at the statistics on approximately half of the population has
failures on pod systems with ABB’s own availability, the figure shows 99.95% failed. Depending on the shape of the
documented information on failures for shaft drives and 99.86% for Azipod probability distribution curve, the failures
of the Azipod system. The comparison drives. This means that of 100 ships with may either concentrate around the MTTF
verified the reliability of the source shaft drives, operating for 100 days, one or they may be scattered over a wider
material. In all, 50 cruiseships with vessel experiences five downtime days, or area of a time span. ‘As shown in this
shaft-driven propulsion were statistically five vessels one off-hire day, whereas the example, MTTF may be calculated from
compared with 38 cruiseships with corresponding amount of days is 14 for field data that is from real applications.
electrically driven pods. vessels with Azipod propulsion. There are mathematical tools that also
Downtime was calculated for both The results indicate that there are fewer take into consideration the cases where
systems over a total period of five years, faults leading to downtime for Azipod a part has been replaced before it failed
from 2002 to 2006. In the calculations propulsion, as the MTBF is longer, but by estimating that parts life time with
of downtime, days spent at a repair yard when there is a failure, the Azipod requires proven and feasible statistical methods,’
and days spent in port forced by a failure more time to be repaired. Mr Hokkanen noted.
were calculated. Also, one missed port The figure also shows that a vessel ABB uses the Kaplan-Meier and
call was considered as corresponding to operates without a failure during a Maximum Likelihood method to estimate
one day in a drydock. drydocking period of 2.5 years with a the survivor function, or the reliability, from
70 The Naval Architect February 2008
NA Feb 08 - p68+70+
71.indd 70 05/02/2008 09:48:14
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