Svetsaren 1/2008 - Esab

typically for cryogenic applications down to
-196°C. Here the liquefied natural gas, arriving in
LNG tankers, is stored and held at a temperature
of -163°C at a pressure slightly above atmospheric.
For distribution, the LNG is re-gasified by heat
exchange with sea water, odorised and transported
through the pipeline network at a pressure of
70-100 bar.
The thickness of the tank bottom plates is 6mm,
and the stringer plate 10mm. The metal plates for
the primary tank - in contact with the liquid - vary
in thickness from 16.6mm (at the bottom) to
12mm (at the top), compensating for the
hydrostatic pressure from the stored liquid, which
increases gradually towards the bottom. Figure 1,
representing the tank cross section, gives an idea
of the complexity and types of materials involved
in the construction.
Materials and welding
The main component is the primary tank
designed to contain the liquid natural gas. The
primary tank is surrounded by a corner protection
system - lower in height and designed to offer
additional safety in the case of liquid leakage from
the primary tank. These components are made
from 9%Ni steel, resistant to temperatures down
to -196°C (coloured red in Figure 1). Further protection,
known as the vapour barrier and covering
the whole tank internally, is made of carbon steel
plates and serves to hold the gas in equilibrium
with the liquid.
According to Bentini’s Welding Engineer in
charge, Mr. Emanuele Ceroni, “The welding was
of vital importance – from the initial welding
process qualifications right through to the on-site
management and monitoring of the human
resources - due to the importance of the
construction and the potentially associated risks.
The welding processes used for construction of
the tank are MMA, SAW and semi-automatic
GMAW. The latter process was used for welding
the suspended aluminium roof with ESAB OK
Autrod 5183 wire. For the vertical joints of the
carbon steel vapour barrier, about 7500m for
each tank, Saipem had proposed uphill MMA.
The two phases in construction of the suspended
roof and vapour barrier did not overlap. I, therefore,
had the idea to also use the semi-automatic
Figure 1. Cross section of the LNG storage tank.
GMAW process for the vapour barrier, where it
involved a fillet weld. After obtaining approval from
Saipem we searched for the right consumable.
ESAB advised us to use the vertical downhill
technique with 1.2mm diameter Tubrod 14.12
wire. In production, this wire allows an appreciable
increase in productivity and consequent saving of
time, as well as limited deformation.”
As previously mentioned, various materials are
involved in the construction, starting with the
most strategic component, X8Ni9 steel (EN
10028-4), with 9% nickel. It is steel typically used
for cryogenic applications and has been widely
used in this type of plant. However, as it is highly
sensitive to magnetic fields, it could create
potential problems associated with welding. This is
the reason why the ESAB OK 92.55 electrode was
chosen as it can also be used with AC to minimise
the risk of magnetic arc blow. Most of the welding
of the 9% nickel steel was done with these electrodes
at a consumption of around 32 tons.
1 – Piping from 114.3 to 762.0mm
(mat. EN 10028-7 X2CrNi 18/9)
2 – Compression ring
(mat. EN 10028-3 P 355 NL1)
3 – Outer reinforced concrete wall
4 – Carbon steel vapour barrier
(mat. EN 10028-3 P 275 NL1)
5 – Main tank (mat. EN 10028-4 X8Ni9)
6 – Insulation with perlite
7 – Corner protection system
(mat. EN 10028-4 X8Ni9)
8 – Resilient blanket
9 – Bottom, carbon steel vapour barrier
(mat. EN 10028-3 P 275 NL1)
10 – Bottom, secondary tank
(mat. EN 10028-4 X8Ni9)
11 – Bottom, primary tank
(mat. EN 10028-4 X8Ni9)
12 – Foundation
13 – Suspended aluminium roof
(mat. ASTM B 209 ALLOY 5083)
14 – Stringer plate
(mat. EN 10028-4 X8Ni9)
A smaller quantity of wire and flux was used for
submerged arc welding of the bottom plate with a
suitable tractor and for circumferential welding
with the ESAB Circomatic system, using
ERNiCrMo-4 wire.
In construction, carbon steel was used for the
metal plates of the outer lining, P275NL1 steel
(EN 10028-3) for the base and S 275 J2G3 (EN
10025) for the entire roof structure. Also, part of
the piping was made of ASTM A106 Gr. B steel.
A total of 21 tons of ESAB Citobasico electrodes,
2600kg of ESAB OK Tubrod 14.12 cored wire
and modest quantities of OK Tigrod 12.60 for
certain TIG welding operations were used.
In addition, a high quantity of stainless steel
piping in X2CrNi 18/9 (EN 10028-7) was welded
with ESAB OK 61.35. ESAB OK 67.60 (309L)
electrodes were used for the dissimilar joints
between stainless steel and carbon steel, as well
as for the joints between pipes and metal plates
16 - Svetsaren no. 1 - 2008