Implementation of Reliability-Based Criteria for Corrosion Assessment

Abstract

In an era where pipeline safety is of paramount interest, vintage pipelines with corrosion have to be managed responsibly. Optimization of corrosion mitigation for these pipelines has a significant effect on the industry’s management systems and related costs. To help optimize the corrosion management process, reliability-based limit state design (LSD) corrosion assessment criteria have been developed for onshore pipeline as part of a joint industry project. The LSD approach is a simplified form of the reliability-based approach. It achieves risk or safety consistency within a certain tolerance, while utilizing a deterministic procedure that is easier to apply. The overall methodology and development of the criteria are described in a companion paper. This paper describes the application of the LSD corrosion criteria to real pipeline cases and evaluation of the results. The performance of the LSD criteria, as determined by the number of corrosion repairs required, was compared to that of the CSA Z662 deterministic assessment criteria and the full probabilistic criteria used by TransCanada Pipelines Ltd. (TCPL) to determine if the criteria lead to practical solutions for real cases. The CSA criteria use safety factors that are not directly based on the risk level associated with the pipeline, while the TCPL criteria utilize pipeline-specific reliability targets. The comparison was conducted using a comprehensive set of TCPL pipeline cases that covered a wide range of diameters (NPS 6 to 42), hoop stress-to-SMYS ratios (0.4 to 0.8) and corrosion densities (0.625 to 6508 features per km). The results show that the LSD criteria perform similarly to the TCPL reliability-based criteria, and that both are generally less conservative than the CSA deterministic criteria. The results demonstrate that the LSD criteria provide a simple and deterministic procedure that capitalizes on the benefits of more complex reliability analyses in eliminating unnecessary conservatism and focusing on the repairs required to achieve consistent safety levels for all cases. Thus, these criteria will enable operators to maximize risk reduction for the dollar spent.