Marine Maintenance Technology International - April 2016

NDT (^) ⠿
MARINE MAINTENANCE TECHNOLOGY INTERNATIONAL | APRIL 2016 ⠿ 79
enables quick, non-intrusive and safe testing
of switchboards and switches, and allows
detection of electric problems at a very early
stage and well before the failing component
will start to generate heat.
The only thing an ultrasonic operator has
to do is give the electrical components or
system a quick scan (by sweeping over the
electrical arrangements) to discover partial
discharges or problems. This diagnosis
with ultrasound is of crucial importance to
confirm the system is safe, well maintained
and properly installed.
Rotating parts
Rotating parts can also be monitored. While
vibration analysis generally requires high
RPM, ultrasounds do remarkably well at low
speeds, even on parts that rotate as slowly
as 0.25rpm.
A slewing bearing can be considered
a slowly rotating part, and were therefore
included in our tests. Although no defects
were detected, the measurements obtained
represent a baseline for future checks.
Many cranes will have electric motors
as well and these can also be checked for
electrical failures and bearing problems.
On ships that are equipped with heavy lift
cranes, or those on offshore vessels (often
with lifting capacities of 1,000 metric tons
or more), problems with sheaves and their
bearings can result in extremely expensive
claims and delays.
A recent survey of an offshore crane over
1,000 metric tons capacity revealed that its
sheave bearings produced a cracking sound
when they were operated as low as 1rpm.
Both static and dynamic measurements
were taken at the forward (boom tip) and aft
sheave banks, with the intention to use the
measurement for further analysis.
While the survey clearly showed that it is
possible to monitor the bearing condition of
sheaves, it also showed the benefit of having
data at hand on the operational rotation
speeds for further analysis by comparing
harmonics to determine which of the sheave
bearings are defective.
Seal tightness testing
Another type of testing is assessing the
tightness of access hatches, which are
often located in the pedestal of a crane and
provide access to the cargo holds, together
with the access that leads to the crane
driver’s cabin. In case the cargo holds are
fumigated, it is of paramount importance
that the access hatch lids are airtight to
prevent any highly toxic gases from entering
the pedestal and crane driver’s cabin.
There have been cases where people
entered the crane pedestal and climbed up to
the crane cabin for maintenance works when
the ship was at sea. They were overcome by
the fumigation gas and subsequently died.
With ultrasound testing, access hatches like
the one above and many others can be easily
and quickly checked, and serious injuries
and fatalities prevented, with the added
benefit that high-quality fumigation of the
cargo holds is guaranteed.
Conclusion
Cranes are hugely important to enable a
ship to fulfill its commercial and operational
obligations. Whether it be a cargo, provision,
hose or bunker crane, they all need to be
reliable as operational failures will inevitably
result in unsafe conditions, accidents, delays
and off-hire claims. \
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Sheave bearings can be
easily checked for
problems using ultrasonic
test equipment