| 2 | 2010 - Schiff & Hafen

SHIPBUILDING & EQUIPMENT | PIPING SYSTEMS
Tungum-tube pipework for
offshore construction
ANTI-CORROSIVE TECHNO-
LOGY | Tungum is an Aluminium-Nickel-Silicon-Brass
alloy
which was fi rst discovered by
metallurgist Sidney Tungay in
the 1920’s trying to fi nd a decorative
alloy resembling of 22
carat gold when polished. This
copper based alloy has, in fact,
excellent qualities to be used
for pipework, especially for offshore
applications.
Tungum alloy is a cryogenic
material, suitable for chemical
engineering and low temperature
processes. Its corrosion
resistance often enables its use
in conveying fl uids and gasses
containing corrosive elements.
Highly resistant to sea water
and its atmosphere, Tungum
resists both stress and crevice
corrosion to offer outstanding
serviceability, even at intermittent
duty in the highly corrosive
‘splash’ zone. Non-magnetic
and non-sparking properties
make Tungum invaluable in
piping high pressure gases, particularly
oxygen where its thermal
conductivity/ defusivity
characteristics virtually eliminate
the potential dangers present
when lesser materials are
employed.
In salt-laden marine atmospheres,
‘316’ stainless steel is
highly susceptible to crevice
corrosion and chloride pitting.
After just a few years of salt
40 Ship & Offshore | 2010 | N o 2
Tungum pipes are installed on Toisa Paladin Photo: Sealion Shipping Ltd.
spray exposure, it may still look
bright from a distance, but closer
inspection reveals telltale
signs of imminent failure to
hold pressure.
Tungum alloy, however, possesses
a natural protection mechanism
whereby, on exposure
to salt spray, a very thin oxide
coating is generated over the
exposed surface, no more than
two thousandths of an inch
thick, when complete. The tube
becomes discoloured, it may
even have a verdigris coating,
but under the oxide layer the
tube material is perfect and will
remain so for a very long time.
Tungum Alloy tube remains
unscathed despite more than 10
years of marine exposure on a
semi-submersible support vessel.
In comparison, the stainless
steel section from a southern
North Sea gas platform, shows
both crevice corrosion and
chloride pitting after barely fi ve
years in the same environment,
in lines under pipe clamps.
The special corrosion resisting
characteristics of Tungum alloy
tubing, carefully developed for
use in the hydraulics systems
of marine aircraft remains just
as valid in today’s polluted sea
waters. An examination of the
development of the oxide coating
shows time plotted against
a minute weight loss during its
formation. After 1000 hours
the weight has virtually stabilised
indicating that the protective
coating is already almost
complete.
The strength to weight ratio
of Tungum alloy compares favourably
with other materials.
In tubes, this often affords the
opportunity to employ smaller,
lighter sections, hence reducing
the size and cost of fi ttings and
supports. As would be expected
of a material originally developed
for use in the hydraulic
control systems of aircraft, Tungum
alloy has excellent fatigue
resisting properties.
In practice, tubing is often used
after bending. In this operation
the outer wall of the tube
becomes thinner and the inner
wall thicker. The severity of this
depends on the radius of curvature
and the angle encompassed
by the bend. The tube
also becomes oval due to the
forming operations. The radius
of the bend, the angle of the
bend, the ovality of the tube
and obviously properties of the
tubing material, all infl uence
its fatigue life. The relationship
between the maximum stress,
calculated for straight and circular
Tungum alloy tubing,
and the number of stress repetitions
to cause failure is shown
below.
Although initially more expensive
than stainless steel, the
proven life expectancy of Tungum
make the long-term operating
costs far more attractive,
not to mention the costs involved
in rig shut down and ‘old’
tube disposal.
Jens Olberts,
Sales Manager
MGI-Imhäuser GmbH,
Olpe, Germany