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) using electromagnetic acoustic transducer (EMAT) or ultrasonic crack‐detection (UTCD) tools. For a given crack ILI pipeline tally, operators need to identify the critical anomalies, determine the time of mitigation, 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 considering a minimum required safety factor (e.g. 1.25).

 

This study introduces a reliability-based approach to make the mitigation decision. The annual conditional probability of failure (POF) is evaluated for each reported crack anomaly by Monte Carlo simulation technique considering all the uncertainties associated with the 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 advantage of 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 by approximately 20% without compromising safety.  Case Study 2 considers an NPS 30 liquid pipeline to illustrate the impact of environmental and cyclic fatigue load driven crack growth on the POF. The POF for crack anomalies can be compared and combined with other threats, e.g. external corrosion, to evaluate the quantitative risk of the pipeline system. The proposed framework in this study can also be used by other operators to improve their crack assessment programs.


Reliability-Based Crack Threat Assessment and Management

Division / Sponsors: 
Pipeline Systems Division

Paper Proceedings: https://asmedigitalcollection.asme.org/IPC/IPC2020/volume/84447

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