There's one straightforward and reliable way to ensure that pipe, that's about to be re-reeled, can handle the strains..and that's by bending it.
Exposing weaknesses in the welding and pipe laying procedure early on is unquestionably the cheapest and environmentally safest way forward. Realistically it's the only way.
And it was for this reason that TWI was recently approached by Technip Offshore shortly before it embarked on a pipe recovery and re-installation operation.
It wanted TWI to assess the behaviour of gas tungsten arc and shielded metal arc welded main line and repair girth welds in 4.5 inch diameter thin walled pipe. Based on radiographic inspection acceptance criteria artificial surface flaws up to 2.5mm deep and 20mm long were placed at the girth weld cap. The pipe had already been installed by reeling but now needed to be moved using the same method.
To tackle the job TWI designed and built a pipe bending rig which enabled pipe reeling strains to be simulated. Full scale plastic strain tests on pipe specimens containing artificial flaws were carried out.
Each specimen contained a pair of flaws at six and twelve o'clock positions in the weld cap. Their tips were in the fusion boundary/heat affected zone of the weld. This corresponds to the positions of maximum strain on the pipe during installation.
To simulate exactly the strains experienced by the pipe offshore it was bent in the special rig in TWI's main engineering hall. This nominally created a strain of just over two percent. To compensate for local increases, possibly caused by misalignment and inconsistencies in pipe wall thicknesses and strengths, further tests to nearly four per cent strain were also conducted.
The trials were backed up by tensile tests, post-test metallography and fractography on the parent pipe and weld material to record notch tip microstructure and the extent of ductile crack extension as a result of the pipe bending.
As a direct consequence of the laboratory work TWI was able to draw conclusions about the relative behaviour of the two welding processes based on the pipe test results. These in turn gave Technip Offshore predictive information about the in-service pipeline's expected performance during recovery and re-installation.
For information about TWI's capabilities please contact us.