There has been considerable recent interest in the toughness of the bond line of Electric Welded [1] (EW) pipe. This has been partly due to failures of old pipe, leading to an increased regulatory focus on the inspection and assessment of bond line features. However, other cases have involved modern pipe, apparently produced in conformity with current standards, which has subsequently been found to have a low toughness.

Fracture mechanics methods can be used in both of these situations to predict the behaviour of a feature in the bond line. These assessments require the fracture toughness as an input; the toughness can be obtained by direct measurement or estimated from the Charpy impact energy. A concern is that there is little validation of the assessment methods, either generic or industry specific, for EW pipe.
This paper reports the results of six full scale burst tests carried out by the European Pipeline Research Group (EPRG) on EW pipe with a low bond line toughness. Five tests used notches spark eroded into the bond line, whilst the final test contained a long natural bond line defect. The results were analysed using both generic and pipeline specific fracture assessment codes. The fracture toughness was obtained from conventional fracture toughness tests and estimated from the Charpy impact energy. The predicted failure pressures were generally conservative.

The work has confirmed that standard methods can be used to assess imperfections in the bond line of EW pipe provided appropriate inputs are used, particularly for the fracture toughness.

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[1] The description Electric Welded is used by current standards to describe any welding process where the “heat for welding is generated by the resistance to flow of electric current applied by conduction or induction”. It covers both electric resistance welding and low and high frequency induction welding.