Far East fitness-for-purpose check
Connect, no.76, May 1996
McDermott SEA Pte Ltd recently called in TWI to carry out a fitness-for-purpose assessment. The subject was a small root undercut detected in an X65 pipeline girth MMA weld made for the Cariogly field off the Malaysian coast.
Initial assessment was carried out using conservative estimates of fracture toughness value and results were reported within one day of receipt of the necessary information. Following discussions with McDermott's clients, the calculations were updated using improved data and measured fracture toughness (CTOD) values.
The work done using the minimum measured CTOD value showed that surface cracks of approximately 2mm deep x 105mm long are not critical. Safety margins of more than 2 on flaw depth and of more than 3.5 (including misalignment effects) on applied stress were determined.
This inexpensive and fast turnaround project demonstrated that the mechanical integrity of the pipeline is unaffected by the presence of a small root undercut throughout its design life.
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