Yards Moving Forward - GL Group

OFFSHORE INDUSTRY
Welding Techniques
on the Testbench
In the offshore installations of the oil and gas industry, new types of steel
are coming into use. To ensure the required durability of the installations, the
existing welding techniques have to be adapted.
The demands are becoming tougher and
tougher. Oil and gas companies are drilling
for the black gold in increasingly deep
waters nowadays, because, in view of high prices
on the world market, production may still be profitable
under the most adverse of conditions. At
depths of more than 500 metres, the engineers of
the offshore installations encounter conditions
which call for a change in the material selection.
The risk of damage and repairs must be reduced
drastically, because it would be much too expensive
to tear the pipelines and pumps out of the
seabed for an overhaul ashore. Up until now, the
oil and gas companies have used “C steels” (carbon
steels) wherever possible. However, in strong
concentrations of CO 2 , chlorides and acid gas as
well as at high pressures and temperatures ranging
up to 200 °C – an operating environment which
occurs frequently in deep water – this material is
too susceptible to corrosion. For this reason,
duplex steels must increasingly be used in future
for production and downhole equipment,
pipelines and pressure vessels, as they guarantee
high strength and corrosion resistance at the
same time. Furthermore, these steels must
exhibit balanced grain structure: comprising both
austenite, the main constituent of many stainless
steels, and the higher-tensile ferrite. Today, the
proportion of duplex steels in use is only about
two percent, but “with a strong upwards trend”,
according to Johann Taferner. As a specialized
welding engineer and former design manager for
piping and tank construction, he is now responsible
at Germanischer Lloyd Oil and Gas for the area
of materials, welding and corrosion. In his opinion,
the oil companies, plant construction firms
and welding workshops are inadequately prepared
for the application of duplex steels. “Very often,
austenitic-ferritic duplex steels and austenitic
steels are just lumped together and the literature
references are simply transferred from one materi-
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al to the other – with catastrophic results,” says
Taferner. The weak points of the duplex pipelines
are generally the welding seams, “where many
errors are possible in the production process,” the
expert points out. In order that the mechanical
properties and the corrosion resistance of weld
material, heat-affected zone and base material can
agree, the welding consumables must be specially
dosed and a watchful eye must be kept on the temperature
control. Here it is sometimes wrongly
assumed that the duplex steels should be treated
like austenitic steels as regards the welding technology.
At high cooling rates, as is desired for
austenites, a grain structure with a high ferrite
component may be produced, leading to insufficient
corrosion resistance and tenacity. On the
other hand, if cooling rates are too low, then low
strength values and poor resistance to stress corrosion
cracking must be expected. To attain the
desired material properties, a ferrite/austenite
ratio of 50/50 is needed. In the root layer and final
pass, a ferrite content of between 30 and 70 percent
is still acceptable, in Taferner’s view.
In addition, the tarnishes are of particular
importance in respect of corrosion. During welding,
the passive layer of chrome oxide covering the
duplex material around the weld is destroyed and
replaced by other layers rich in iron oxide. These
layers, which are also referred to as heat tinting,
impair the corrosion resistance of the steels.
Through appropriate use of inert gases, these discolourations
can be suppressed to a large degree.
It is not possible to inhibit the tarnish completely,
however. For certain applications, only weak yellow
colours, which arise at temperatures up to
about 400 degrees, may be tolerated. “All other tarnishes
which occur at higher temperatures must
be removed,” Taferner advises. ■ MPH
For furthor information: Johann Taferner, Germanischer Lloyd Oil and Gas,
Phone: +49 40 36149-7739, johann.taferner@gl-group.com