Improving Safety Through Engineering Assessments for Change in Location Class

In Canada, when location class changes on a gas pipeline CSA Z662-15 requires operators to comply with design requirements of the new location class or perform an Engineering Assessment (EA). The compliance option is often perceived by regulators and the public as the better option compared to the EA option. This paper demonstrates that a well-executed EA that accounts for relevant threats and consequences, and provides explicit levels of reliability, can deliver improved pipeline safety. To comply with design requirements with respect to location factor, the two compliance options are to de-rate or replace the pipeline to achieve the lower operating stress level dictated by the new location factor. However, lower operating stress levels do not always address the higher risk levels or safety concerns caused by the change in class and ensuing potential increase in mechanical damage. For gas pipelines, where class location is applicable, ensuring human safety is the primary objective of pipeline integrity management. In this context, safety is defined as the control of recognized hazards to achieve an acceptable level of risk. To provide site-specific safety, an acceptable level of risk needs to be achieved by ensuring sufficiently low enough probabilities of failure for given site-specific consequence levels. Increased wall thickness via pipe replacement, can lead to lower probability of failure for a pipeline. However, as pipelines are subjected to many different combinations of threats, which depend on site specific conditions, the pipelines that are designed with thicker walled pipes for higher location classes do not always provide lower probabilities of failure. As the general design considerations do not account for the site specific threats and mitigation actions, complying with design requirements alone do not consistently provide lower probabilities of failure, especially in areas of potentially higher third-party activities. In TransCanada’s site-specific EAs, quantitative risk or reliability assessments consider verified population estimates, actual lethality zones and site-specific threats. Appropriate and site-specific mitigation actions address the actual risk. This enables providing an appropriate site specific reliability level. Case studies and comparison between methodologies are used to illustrate the importance of performing site-specific EAs using site-specific information to achieve safety levels that are greater than those achieved by strictly complying with the standard design requirements. Accounting for actual-site specific threats and the actual consequences ensures accurate assessment of risk and consequent appropriate mitigation and efficient risk reduction.

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