Svetsaren 1/2008 - Esab

Figure 8. FCAW of a lifting trunnion.
Figure 9. SAW on a lifting trunnion.
Foundation piles
The foundation piles are pre-fabricated by Sif
Group bv, arriving in 83-93 m lengths. To achieve
their final length of 170 –190 m, they need to be
welded together (Figure 10 and 11). This is again
done by FCAW with PZ6138, but here it is
mechanised welding with ESAB Railtrac equipment.
The joint configuration is adapted to this
method -an unsymmetrical X-joint with most of
the weld volume on the outside. The inside part is
welded manually, vertically-up. The root pass is
deposited on ceramic backing. After removal of
the fit-up plates, the majority of the weld volume
is mechanised welded from the outside, verticallyup
from 6 to 12 ‘o clock, mostly with a slight
weaving motion. Welding parameters are adapted
to the several clock positions by the operators.
Dimensional control and weld finish
Normal offshore fabrication, eg, a jacket on top of
which the deck and operational facilities are placed,
is naturally, subject to strict dimensions, but it is
more forgiving than in the case of the Tombua
Landana project. The fact is that three
substructures, fabricated by three different yards,
are stacked on top of each other in almost 400 m
deep waters - and simply have to fit. This places
extremely high demands on the dimensional control
– roughly 3 times as high as normally required.
This is as equally valid for the fabrication of the
TBT’s components – the pile sleeve clusters and
the rows – as it is for the assembly of the super
structure. To exemplify the dimensional control
and its implications for welding, we return to the
fabrication of the pile sleeve clusters, shown in
Figure 12.
This image shows the completed pile sleeve cluster
and the nominal distances between the centre
lines of the pile sleeves and the centre line of the
main leg (5172.5 and 5173 mm). The maximum
acceptable tolerance on these distances is 3 mm.
A similar small tolerance is valid on the distances
between the pile sleeves themselves, in the X and
Y directions and on the mutual distances into the
Z direction. This dimensional control is the key
requirement, and everything else is subject to it.
Ideally, the pre-fabricated yoke plates, including
K-bevel, fit exactly, so that there is a constant
root gap between the pile sleeves/main leg and
Figure 10. Welding tents made from shrink foil protect the weld area from wind and rain.
the yoke plate’s K-bevel. In practice, this is
extremely difficult to achieve. Practically always, the
root gap appears to be more or less eccentric. This
must be corrected by grinding on the narrow side
and buttering & grinding on the wider side
– an extremely time consuming exercise.
Measuring was performed by three parties;
Heerema Vlissingen, Passe-Partout (independent
contractor) and Chevron, who worked
independently according to agreed measuring
principles. Chevron were responsible for final
measuring and reporting.
Another time-consuming aspect was Class C and
Class A grinding of weld surfaces. Grinding is
done with an aluminium oxide based disc.
Class C grinding is required for the TKY joints
between the braces and the dummy leg (middle
of the row) and between the braces and the main
leg of the pile sleeve cluster. It is performed to
correct excessive convexity, notches or undercut
at the toes of the weld. The grinding of the toes
of the cap must be performed to the point where
a 1 mm diameter wire cannot pass between the
disc and the plate (Figure 13).
Class A grinding is performed on the welds connecting
the lower yoke plates to the main legs of
the pile sleeve clusters – at both sides of the
K-joint. Class A means that weld profile is ground
back to the theoretical radius. This is checked by
using a template with a 45 mm radius, with a gap
12 - Svetsaren no. 1 - 2008