The effect of the NDE performance on the integrity level of girth welds

Nowadays one can derive allowable defect dimensions for a specific pipeline based on a deterministic engineering critical assessment (ECA). The question is whether these defect dimensions can be used as non-destructive examination (NDE) acceptance criteria (AC) for a specific pipeline or how do the allowable defect dimensions relate to the performance of the NDE technique. The allowable defect dimensions (the acceptance criteria) should depend on the performance of the NDE technique(s) used, i.e. the probability of detection and the sizing accuracy. The effect of the NDE technique on the probability of failure can only be assessed by probabilistic fracture mechanics analyses. To avoid performing probabilistic analyses for every configuration, a methodology has been developed to derive acceptance criteria for pipeline girth welds such that a high integrity level (low probability of failure) is maintained. In this methodology the acceptance criteria for defects are not fixed but ‘flexible’. The acceptance criteria are based on the allowable defect dimensions determined by an ECA and the performance of the NDE technique. The performance of the NDE technique determines the safety factor. The safety factor is to be applied on the allowable defect height. A ‘good’ performance of a NDE technique allows for a smaller safety factor on the defect dimensions, resulting in fewer repairs but without a loss in the integrity level. However a ‘bad’ NDE performance results in a larger safety factor resulting in smaller allowable defects