In the current era where pipeline safety is of paramount interest, optimization of corrosion mitigation for vintage pipelines has a significant effect on the industry’s management systems and related costs. To that end, reliability-based Limit State Design and assessment (LSD) corrosion criteria have been developed for onshore pipelines as part of a joint industry project. These criteria have the unique characteristic of being calibrated to meet specific reliability targets within a certain tolerance but having a deterministic format that does not require probabilistic calculations for application.
The assessment criteria cover both burst and leak failure modes. Their formulations are characterized by three elements: the equations used to calculate the characteristic demand (e.g. operating pressure) and capacity (e.g. burst pressure resistance at a corrosion feature); the characteristic values of the key input parameters for these formulas (such as yield strength, pressure and feature depth); and the safety factors defining the characteristic demand as a ratio of characteristic capacity.
The number of corrosion repairs required by the LSD criteria was compared to those required by the CSA Z662 deterministic assessment criteria and the full probabilistic criteria developed by TransCanada Pipeline Ltd. (TCPL) to ensure that the criteria lead to practical decisions for real cases. The comparison was conducted using a comprehensive set of TCPL pipeline cases that cover a wide range of pipeline diameters (NPS 6 to 42), hoop stress-SMYS ratios (0.4 to 0.8) and corrosion severities (feature density from 0.625 to 6508 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 approach.
This comparison demonstrates that the LSD criteria provide a simple and deterministic procedure that capitalizes on the benefits of reliability analysis 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 optimize the cost by achieving maximum risk reduction for the dollar spent.