Comparing Depth Sizing of Complex Field SCC (NNpH and High pH) Between ILI, NDT and Laboratory Methods

Abstract Through-wall morphology of high-pH SCC (HpH SCC) cracks and near-neutral-pH SCC (NNpH SCC) cracks tend to be distinctly different due to the different driving mechanisms. NNpH SCC cracks tend to be dozens (if not hundreds) of micrometers wide throughout the whole crack depth, whereas HpH SCC cracks are much tighter, narrowing down to almost sub-micrometer width at the crack tip. Both types of cracks can display varying degrees of branching, and in the case of HpH SCC, some cracks can grow at angles inclined away from the perpendicular (i.e., away from the through-wall direction). All of these morphological complexities present great challenges for in-line inspection (ILI) tools (including those based on electromagnetic acoustic induced technology [EMAT]) for the purposes of detecting and sizing such complex anomalies. Recent EMAT inspections of operating 22″ and 24″ gas pipelines (API5L X52, both built in 1960) found NNpH and HpH SCC, respectively. Samples from each pipeline were cut out. Then, artificial EDM notches were machined into them and reinspected by EMAT pull-tests, with selected “natural” SCC and “artificial” EDM anomalies analysed further by PAUT and precise destructive (metallographic) methods. This paper presents the results of the comparison of assessed depths between the three methods, encompassing artificial EDM flaws as well as “natural” NNpH SCC and complex (inclined angle) HpH SCC anomalies.