Session: 03-03-02 Assessment Model Enhancements Other Threats - Part III

Paper Number: 87259

87259 - A More Efficient Effective Area Method Algorithm for Corrosion Assessment 

Corrosion is one of the major threats to the safety and structural integrity of oil and gas transmission pipelines. The corrosion threat is usually managed by regular in-line inspection (ILI) using magnetic flux leakage (MFL) or ultrasonic tools. After ILI, operators need to identify the critical anomalies, determine the time of mitigation, and whether pressure restriction (derate) is required. For critical anomalies, after excavation, the corrosion size is usually measured by field laser scan before operators make repair decisions. For all the integrity decisions mentioned above, it is required to convert the measured corrosion size to predicted burst pressure using some assessment model.  

The effective area method is the most popular corrosion assessment model commonly used by regulators, operators and ILI vendors, which was originally developed by Kiefner and Vieth, and adopted by ASME B31G and CSA Z662. As it was originally implemented through a software application, RSTRENG (Remaining STRENGth), the effective area method is also known as RSTRENG model. Given detailed corrosion measurement profiles, the effective area method involves an iterative process to find the minimum burst pressure. As stated in ASME B31G, “for a corroded profile defined by n measurements of depth of corrosion including the end points at nominally full wall thickness, n!/2(n − 2)! iterations are required to examine all possible combinations of local metal loss with respect to surrounding remaining material”, the widely used effective area algorithm has an order of n-square time complexity (O(n^2)). As the corrosion becomes longer or more complex (n increases), the computation time increases nonlinearly.

This paper reviewed the traditional effective area algorithm first, and demonstrated that it is not necessary to always loop through and check all the combinations. Because some combinations with shallower and shorter corrosion size are clearly not the final critical combination corresponding to the minimum burst pressure we want. A more efficient algorithm is proposed and presented in this paper, which can reduce the algorithm time complexity to the order of n, i.e., the computation time is linearly increased as the number of measurement points n becomes larger.

When conduct deterministic assessment using modern computer or cloud-based server, the effective area algorithm will not impact the performance or user experience significantly. But when perform reliability-based probabilistic assessment using Monte Carlo method, millions of simulations are required. TC Energy has developed a new corrosion assessment model, i.e., plausible profiles (Psqr) model, published in IPC 2018 and 2020, and reviewed by industry experts through PRCI. Psqr model needs to generate thousands of profiles for one corrosion anomaly and repeatedly uses the effective area algorithm. When perform probabilistic assessment, or use Psqr model, or even perform probabilistic assessment using Psqr model, the more efficient effective area algorithm becomes the key component.  A case study is conducted in this paper comparing the computing time, when use different effective area algorithms. For large corrosion clusters, with high resolution ultrasonic or laser scan measurements, the more efficient algorithm could be hundreds time faster. 

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.