Abstract
Crack or crack-like anomaly is one of the major threats to the safety and structural integrity of oil and gas transmission pipelines. The crack threat is usually managed by hydrostatic test or regular in-line inspection (ILI). For a given crack ILI pipeline tally, operators need to identify critical anomalies, determine appropriate response time, and whether pressure restriction (derate) is required. Traditionally, a deterministic approach is used to determine the mitigation plan based on characteristic values of pipe properties, conservative crack sizing and crack growth rate, and further considering a minimum required safety factor.
This study introduces a reliability-based approach to make the mitigation decision with full details. The annual probability of failure (POF) is evaluated for each reported crack anomaly by Monte Carlo simulation technique considering all the uncertainties associated with pipe geometry, material properties, crack size measurement, and assessment model error explicitly. Both environmental and cyclic fatigue load driven crack growth are considered following API RP 1176. A reliability-based mitigation threshold is proposed and calibrated against the deterministic minimum required safety factor for maintaining the system to a consistent safety level.
Two case studies were conducted to demonstrate the advantages of the reliability-based assessment approach in this paper. Case Study #1 considers an NPS 20 transmission gas pipeline with more than 2000 ILI reported crack anomalies. Compared to the deterministic approach using conservative characteristic values, the reliability-based approach can reduce the number of required mitigation activities significantly without compromising safety. Case Study #2 assumes a severe crack defect on an NPS 34 liquid pipeline to demonstrate the impact of fatigue driven crack growth on POF. The POF of crack anomalies can be compared and combined with other threats, e.g. external corrosion, to evaluate the quantitative risk throughout pipeline systems. The proposed framework in this study could be used by operators to improve the crack assessment programs.
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