suggest that suitable procedures based on this process could
be developed for the longitudinal weld seams of split tee
assemblies.
There are several benefits of using a semi-mechanised
welding process over manual SMAW, which include:
)
High deposition rates.
)
Improved slag removal characteristics.
)
Higher quality bead appearance and weld profile compared
to manual welds.
)
Greater control of welding speed and the delivery of more
consistent heat input.
)
A reduction in stop/starts, which lowers the risk of defects
associated with these regions.
)
A reduction in the number of welders required.
)
Prevention of welder fatigue effects through reduced arc
time.
The primary aims of this project were to qualify a semi-
mechanised GSFCAW process and procedures for this
application and to additionally qualify repair procedures
using manual manipulation of the GSFCAW process. These
procedures could then be incorporated into the National
Grid specification for welding onto pressurised pipelines. A
comprehensive welding and test programme was performed
using 50 mm BS EN 10028-3 Grade 460 NL2 material
selected with a carbon equivalent value, which at 0.49%
was towards the upper end of the permitted range. Welding
was performed using an orbital welding system adapted for
welding in the longitudinal configuration in three positions,
PA/1G (downhand), PE/4G (overhead) and PC/2G (horizontal
vertical) to cover the full range of welding positions associated
with either horizontal or vertical branch off-take connections.
An example of test plate welding is shown in Figure 1.
Equipment
The welding system used was the Firefly system supplied by
IPWL Ltd., which used the proven Lincoln Electric S350 power
wave technology and a game console type welder control
unit that provides both a familiar and ergonomic interface to
the user. The use of existing off the shelf consumables, filler
metal and shielding gas also ensured that once the concept had
been developed there would be little additional development
required for these and the system could be utilised by the end
user if required.
In parallel to the welding development work, the incorporation
of specific induction heating equipment was also assessed. This was
based on the Miller Proheat 35 equipment and allowed the
minimum preheat to be maintained during all welding operations.
The integrated thermocouple arrangement delivered additional
heat only when required in specific regions of the test plates as
well as providing a means of logging interpass temperature for
weld qualification purposes. In addition to the high level of
control over welding parameters and resultant heat input, this
allowed the overall thermal cycle of the welds to be managed to
deliver consistent mechanical properties.
Equipment operation manuals and procedures for both the
welding and induction heating equipment along with associated
risk assessments allowed the necessary reviews to be carried
out, ensuring that all safety aspects were addressed. As with
all development work for an end user such as National Grid,
the safety implications of such projects are of paramount
importance and any operational or policy changes required must
be highlighted where deemed appropriate.
All welding was monitored by qualified personnel using ALX
weld process logging equipment from The Validation Centre,
although the Lincoln Electric system comes with the ability to
also log welding parameters for production control and quality
assurance.
Welding
During welding, the specific requirements of the governing
specifications were satisfied by ensuring procedural controls
were acknowledged and implemented. This included actions
such as welding initial passes from the centre of the test plate to
the outer edges in order to balance distortion. The requirement
to ensure start stop regions of each weld were staggered along
the length of the weld do not apply as the continuous electrode
characteristics of the GSFCAW process allow each weld pass to
be deposited for the entire length of the seam, and such regions
are therefore naturally contained on the run on and run off
plates (other than for the initial passes as mentioned above).
Figure 2.
Charpy impact test results for PA WPQR.
Figure 1.
Welding of 50 mm plate
52
World Pipelines
/
JANUARY 2015
