TWI's approach involved several of its in-house disciplines. Both destructive and non destructive tests were going to be required to confirm the material's properties, identify the damage mechanisms and to characterise the extent of the damage. From these tests, and the results of in-line inspection (pigging) carried out by KOC, the team would be able to draw conclusions about the fitness for purpose of the pipeline as a whole and make recommendations for future operation, inspection and integrity management.
The work began with a detailed review of the pipeline operating history, and the available in-line inspection data. This was followed by non destructive tests on the supplied materials using ultrasonic testing and magnetic particle inspection. Lastly destructive testing including tensile, hardness, and toughness tests, together with chemical analysis and metallography, were carried out in order to characterise the materials and the damage.
The operating history and in-line inspection reports revealed that the line had suffered a failure in 1976, with 'laminations and blistering' reported at the failure site. The pipeline had also been briefly co-opted to transport sea water for fire-fighting in 1991, following the Gulf War, before being returned to high pressure gas transport service. Other than this brief service with sea water, the known history of the pipeline only involved the transport of clean, dry gas.
An inspection report was available from 1981, recommending approximately 6km of repairs, although there were no records indicating whether this report had been actioned. The in-line inspection showed that although the line was predominantly long-seam welded, there was approximately 7km of spiral welded pipe, predominantly in 12m lengths, distributed throughout the pipeline. Internal corrosion damage was almost completely absent from the spiral welded pipe, but was widespread in the long-seam welded pipe.