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The Use of ICCP-Systems on Azimuth Propulsor Vessels
Problemfree Corrosion Protection?
SPECIFIC TECHNICAL PROBLEMS
IN VESSELS HAVING AZIMUTH PROPULSION
Corrosion Mechanism
- Propeller
corrosion starts to occur, when the voltage difference
(ca.100 mV) starts generate current
- The
current in the ICCP protection system is sometimes
very low, even zero, and sometimes quite high
(50-150 A )
- The
real current route from the anode back to the
DC power source is difficult
- The
propeller itself makes the best surface for the
route for electric current from the water and
also from the anode through the water, because
the steel and other structures have normally very
good surface protection
- The
location of anodes and voltage references is a
most important design decision
- Especially
in the short vessel like a tug, the distance from
the propeller to the anodes can vary a lot, depending
on the steering angle. The propeller rpm is also
an important factor.
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Current
Conducting Elements Inside the Propulsor
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In an azimuth thruster the connection back to
the DC source should be arranged over two oil
immersed rotating elements, where the resistance
should be kept low even with quite small currents
to keep the propeller protected
- The
resistance of the rotating elements depends in
many cases on the amount of the current. With
a higher current the resistance is normally lower
- There
are also other elements in azimuth propulsors,
which in certain situations provide a route for
the current of the cathodic system (e.g. bearings,
sealings, clutches...)
- When
the propeller rpm is 0, there could be good connection
through power train · When the propeller is running
with higher rpm, there could be isolating oil
film in the rotating conductive elements
- The
propeller is "outside" of the hull of the ship
and provides a good route for the current back
to the DC source
- The
grounding of the propulsor body is important.
Thus, there is no standard solution for this problem.
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Vessels
with Diesel-Electric Propulsion
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- The
propulsion motor can in some cases transfer current
through the power transmission elements and this
should be taken into account in the corrosion
system design
- The
current and magnetic flows are also distorted,
which causes different type of stray current in
different elements also providing a possible electromotive
force for corrosion.
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Other
Structures around the Vessel
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- Today
very heavy and powerful cathodic protections are
also used in different type of land structures
(onshore terminals, offshore constructions and
pipelines, docks, jetties, platforms, bridges
and other steel structures)
- These
may supply electromotive forces through a vessels
cathodic protection system and disturb its function
Resistivity
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