POWER GENERATION
Winded
Wind turbines are
typically designed to
last for 25 to 30 years.
During this service
life they will require
regular maintenance. As subsidies
and power purchase guarantees have
been removed, the wind industry faces
significant pressure to reduce costs.
Operators have been forced to reduce
maintenance services and in some cases
to end full maintenance contracts.
For wind turbines, as with any
machinery, condition monitoring and
preventative maintenance can prevent
failures that could lead to secondary
damage, excessive down time and higher
costs. Any site maintenance of wind
turbines is complicated by their remote
location and height. This means that
remote condition monitoring takes on
even greater importance. This article
examines the methods and technologies
used to minimise the maintenance cost
for these valuable but inaccessible assets.
Wind turbine manufacturers normally
require an annual service of their product
as a condition of warranty. This involves
basic machine maintenance such as
cleaning, lubrication, tightening bolts,
changing filters and taking oil samples.
Most of the maintenance takes place
in the nacelle where the generator and,
if present, the gearbox are housed.
However, anchor bolts on the baseplate
may also need to be tightened and turbine
blades inspected for damage. Large
Not only are offshore wind turbines a long way from shore, their rotating
machinery is housed tens of metres above the surface of the water. The
financial incentive to minimise the need to perform inspections and carry out
maintenance has encouraged development of technological solutions
By Jody Muelaner
turbines may even require internal blade
maintenance.
Avoiding unscheduled maintenance
requires estimates for the probabilities
of component failures so that decisions
can be taken on when to schedule
maintenance to avoid an outage. The
German Wind Energy Measurement
Programme tracked the performance
of 1,500 wind turbines in Germany
between 1997 and 2006, providing
detailed information on component failure
probabilities and outage durations. At
that time, on average, electrical faults
caused stoppages every two machineyears
and resulted in about 1.5 hours of
downtime. Gearbox failures happened
less frequently, about once every seven
machine-years, but they resulted in
outages of about six days. More recent
studies have found that outage durations
are increasing significantly as machines
get larger.
The probability of a given component
failing can be combined with cost of the
outage which would result from its failure.
The probability multiplied by the incident
cost gives an expected cost. If this
expected cost is greater than the cost of
the preventative maintenance, then the
maintenance should be carried out.
Offshore wind turbines are typically
reached by boat. The boat will push on
to the transition piece, the lower part of
the turbine with an external ladder. At this
stage, wind turbine technicians must wear
a climbing harness and life jacket, as well as
an immersion suit if the sea temperature
is below 10°C. After climbing to a platform
on the transition piece, technicians enter
the tower to climb to the nacelle.
EVEN MORE CHALLENGING
Inspecting and repairing turbine blades
can be even more challenging, as the
blades are suspended at significant height
with no access fixtures. Manual work is
carried out by a rope access technician
(RAT) who moves down a blade from the
nacelle. Each blade must be orientated at
a slight angle from the vertical, extending
downwards from the nacelle to allow
access, one blade at a time. Ropes
looped around the blade allow the RAT
to maintain contact with the blade while
working. Visual inspection may also be
carried out by drone, and robotic repair
solutions have been developed.
72 www.operationsengineer.org.uk Autumn 2020
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