In August 2009, there was a 2.5 m long rupture in the longitudinal seam weld of a crude oil pipeline in France. The failure caused a spillage of approximately 2000 cubic metres in a protected area. This rupture caused the authorities to withdraw the permit to operate a 260 km long section of this pipeline.
The pipeline had a similar failure in August 1980. The 1980 failure was attributed to a fatigue crack initiating at the inner side of the longitudinal weld. There was evidence of ‘roof topping’ along this weld.
Penspen Ltd., UK were contracted by the pipeline operator Societe du Pipeline Sud Europeen (SPSE) to carry out an independent investigation into the cause of the 2009 failure, review and confirm the actions needed for safe short term operation to allow internal inspection, and determine a safe future life for the pipeline.
The failed pipe specimen was not immediately available for inspection. An initial analysis indicated that purely an analytical evaluation would not provide conclusive results, due to variability in material properties, geometry and loading. It was decided that a better understanding of the behaviour of defects in the pipe, and the fatigue performance, was required.
A detailed laboratory programme of burst and fatigue testing on a section of line-pipe was recommended. The tests were carried out on a combination of ‘defect-free’ ring specimens, and ring specimens with initial Electric Discharge Machined (EDM) slits to represent crack-like defects. These tests showed:
1. BURST TESTS: The failure pressures of the burst tests could be predicted using a recognised industry model. Roof topping, laminations and inclusions, and toughness variations were found to have no noticeable effect on the defect size at failure.
2. FATIGUE TESTS: The fatigue test results showed that defect-free rings had a fatigue life one order of magnitude longer than those containing an EDM slit, and that the fatigue life of a defect-free ring could be predicted using a standard S-N method. In addition, it was found that crack growth was conservatively predicted using standard fatigue fracture mechanics.
The fatigue test results showed that the 1980 and the 2009 failures were caused by a combination of cyclic pressure loading, roof topping, and a pre-existing weld defect (probably present when the pipeline went into service in 1972).
This paper provides an overview of the failure investigation, with a discussion of the ring testing, supporting tests, and fracture surface inspection.