A Comparative Methodology for the POE Assessment of Corrosion vs Cracking

The probability of exceedance (POE) method refers to the likelihood that a defect exceeds a specific limit or threshold, such as pressure or depth. This concept is essential in risk-based pipeline management, as it helps operators to assess and mitigate potential failures before they occur. The approach also provides an advantage over integrity management decisions based on a deterministic criterion, where a limit threshold is based purely on a given defect size, rather than the probability that a defect could exceed a limit based on incorporating several variables beyond simply length and depth. POE is already commonly used for corrosion defects, partly because these have a longer history of evaluation, and in-line inspection (ILI) technologies can report metal loss with better sizing accuracy, providing uncertainty limitations that are manageable.

Ultrasonic crack detection tools provide reasonably accurate sizing data for crack-like defects. Meanwhile, magnetic flux leakage (MFL) technologies, when combined with stress measurement sensors, are rapidly improving and can provide data that can be assessed similarly, thereby minimizing uncertainties in defect sizing during inspection. The evaluation methods for corrosion-related defects, such as modified B31G or RSTRENG, are relatively simple compared to those for cracking defects, which require additional inputs, including material toughness, and the use of fracture mechanics algorithms to determine the fitness for purpose of the anomalies. Standard corrosion POE methodologies typically depend on linear corrosion growth rates. The issue of crack growth is significantly more complex, since fatigue crack growth rates are largely dependent upon pressure cycling. However, by approaching fatigue crack growth from the concept of the aggressiveness of benchmark cycles, an approximation can be provided for generalizing multiple scenarios in assessments.

This paper compares modern methodologies for determining the POE in pipelines for crack and corrosion defects and presents a case study to evaluate the data provided by an ultrasonic crack tool using ILI technology. While more involved, establishing a POE methodology for identifying crack defects using ILI can help operators estimate the likelihood of rupture and leak, ultimately supporting more risk-informed integrity management decisions for pipelines.