Flaw assessment saves repair

Measuring CTOD

The pressure to maintain production rates on laybarges can force inspection personnel to interpret girth weld radiographs under less than optimum conditions, sometimes when films are still wet. On the pipeline in question, a post installation review of weld radiographs revealed that a volumetric weld defect, which had been overlooked during installation, was outside code acceptance criteria. As the pipeline then lay on the seabed in more than 50m of water, the cost of an underwater repair gave rise to considerable concern.

Some of the original pipe material and welding consumables were still available, and a girth weld was made using the original weld procedure. TWI then initiated a series of crack tip opening displacement (CTOD) tests to characterise the fracture toughness of the girth weld at typical operating temperatures.

A defect assessment using BSI PD6493:1980 techniques (PD6493 has since been superseded by BS 7910) was carried with design stress data. This was able to demonstrate that the defect in the pipeline girth weld would remain stable with respect to brittle fracture and plastic collapse under extreme operating conditions.

With the integrity of the flowline thus assured, there was no need for a very expensive underwater repair.

Copyright © 2001, TWI Ltd