Session: 07-04-02 Crack Management

Paper Number: 87252

87252 - Reliability-Based Self-Imposed Pressure Restriction/ Derate Pressure Estimation for Corrosion and Crack 

Corrosion and crack (or crack-like) anomalies are the major threats to the safety and structural integrity of oil and gas transmission pipelines. Pipeline operators commonly manage the corrosion and crack threats by regular in-line inspection (ILI) using magnetic flux leakage (MFL), electromagnetic acoustic transducer (EMAT) and other inspection tools. 

For a given ILI tally, operators need to identify critical anomalies and schedule the mitigation. Traditionally, a deterministic approach is used to assess anomalies using characteristic values of pipe properties, anomaly size, growth rate, and considering a minimum required safety factor (e.g. 1.25). TC Energy has used a full reliability-based method to determine the mitigation plan for corrosion and stress corrosion cracking (SCC) anomalies considering all the uncertainties associated with the pipe geometry, material properties, anomaly sizing, growth rate and assessment model error explicitly. This method enables TC Energy to maintain the annual probability of failure of all known anomalies in the same location class not exceeding a consistent threshold (e.g. 1E-3 per feature per year). 

Anomalies that do not meet the minimum safety margin (e.g. deterministic safety factor or reliability-based threshold) and cannot be mitigated timely, are usually managed by applying a short-term self-imposed pressure restriction (derate) until the mitigation completed. Derate pressure is typically calculated deterministically with conservative anomaly size, growth rate and a global safety factor. Applying conservative values to account for the potential variations often results in over conservative derate pressure. There is inconsistency between a full reliability-based mitigation plan and a deterministic short-term derate plan.

This study introduces a new efficient reliability-based approach using Monte Carlo simulation technique to determine the derate schedule (e.g. the minimum required derate pressure for each month in one year) to maintain the system to a consistent safety level. Two case studies were conducted in this paper, one MFL tally with critical EC anomalies and one EMAT tally with critical crack anomalies, to demonstrate the advantage of fully reliability-based derate approach. The optimized derate plan would minimize the economical business impact to operators. The proposed framework in this study can be used to improve derate programs.

Presenting Author: Jason Yan TC Energy

Presenting Author Biography: Jason is a pipeline risk and integrity engineer working at TC Energy for 8 years. He is in TC technical center, Innovation &amp; Decision Optimization team. Mainly works on the external corrosion and stress corrosion cracking threat feature assessment and management program strategy, including development of assessment model, assessment procedure, response criteria, compliance study etc.<br/><br/>Will provide details later, if necessary.