A reliability-based Limit State Design (LSD) method for assessment of corrosion and crack features has been developed for onshore transmission pipelines as part of a joint industry project. The rule-based LSD approach is a simplified form of the reliability-based approach that reduces the latter to a set of deterministic checks. The LSD corrosion assessment method and a comparison of its performance against one operator’s reliability approach were published in previous IPC papers. This paper describes the application of the LSD corrosion and crack assessment methods to four Enbridge liquid pipelines and provides a comparison of the results to those of Enbridge’s internally developed Level I reliability analysis method. Enbridge’s reliability analysis (published in a previous IPC paper) is staged into three levels, where Level I analysis is also a simplified form of the reliability-based approach where the probability of failures of predefined features’ sizes are precalculated using conservative assumptions for a fast turnaround screening analysis of the entire pipeline system in order to identify areas requiring more in-depth Level II or III analysis.    

The comparative analysis was based on the number of feature repairs required according to each method. Out of the four pipelines considered, two have corrosion and the other two have cracks as the dominant threat. The results show that there are significant differences between the output of the two methods. The root cause of these differences was investigated by conducting a sensitivity analysis on the input parameters, including: pipeline segmentation procedure, segment reliability target, wall thickness distribution, feature depth distribution, feature length distribution, and the usage of model error (which is used in the LSD method but not in Enbridge’s Level I method). For cracks, differences between the Modified Ln-Secant model used in the LSD method and the CorLAS model used by Enbridge were also considered.

It was observed that the discrepancies in the required repairs resulting from the two methods can be mostly attributed to the inclusion of model error in the LSD method and the method used to define the wall thickness and feature depth distributions. This observation was confirmed by comparing the LSD results with the results of Enbridge’s more in-depth Level II reliability analysis. For cracks, the burst pressure model selection also had a significant impact on the results. Comments are provided on the implications of using each approach to define the key inputs for the assessment, and recommendations are made for continued development of a viable and practical reliability-based feature assessment approach.


Demonstration of Limit States Design Method for Assessment of Corrosion and Crack Features

Division / Sponsors: 
Pipeline Systems Division

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

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