Studies on the effect of flaw detection probability assumptions on risk reduction at non-destructive inspection

Reliability estimates of piping (and other structural components) are needed e.g. in probabilistic safety assessment (PSA) studies, risk-informed ISI applications, and other structural reliability assessments. The reliability of piping components is affected by ISI, since inspection results increase the knowledge of the state of the inspected components. Probabilistic fracture mechanics (PFM) models can account for the ISI effectiveness and interval, and allow sensitivity studies. A detailed modelling of ISI effectiveness is difficult, since it depends on several factors, and it would be very expensive to produce statistical data to estimate the probability of detection (POD) for various flaws. On the other hand, one could question the need of very detailed POD estimates in RI-ISI applications. If sufficient risk reduction can be shown by using simplified (conservative) POD estimates, more complex PODs are not needed. The paper summarises the results of a study of the effect of piping inspection reliability assumptions on failure probability using structural reliability models. The main interest was to investigate whether it is justifiable to use a simplified probability of detection (POD) curve. Further, the study compared various structural reliability calculation approaches for a set of cases. The results indicate that the use of a simplified POD could be justifiable in RI-ISI applications