Analysis of flaws in North Cormorant jacket saves Shell £45m
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North Cormorant during successful float-out
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At a late stage in fabricating Shell's North Cormorant offshore platform, extensive chevron cracking (weld metal hydrogen cracking) was found in submerged-arc longitudinal seam welds (welds A) in the tubulars used to make the
nodes. There was a crack about every 50mm along the welds. The tubulars, which were 100mm thick, had been post-weld heat treated before delivery to site. The chevron cracking was first found during UT of one of the circumferential
erection welds (welds B), when the NDT technician had to probe through the end of the longitudinal node seam weld (weld A), where this intersected the erection weld. Further UT probing revealed that some 20% of the seam welds were
affected. Repair of the cracks would have had horrendous consequences for the construction programme. The structure would have had to be dismantled, because in-situ PWHT of the repair welds was impossible. TWI carried out a fatigue and
fracture analysis, which showed the flaws to be quite harmless. It was decided, with the concurrence of the certifying authority, to allow construction to proceed. This decision enabled the platform to be installed on time and is thought
to have saved Shell at least £45m.
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