Risk assessment is the process of risk analysis and evaluation. It is a required component of pipeline integrity management programs (IMP) and is generally the first step in most IMPs.
For the risk assessment of natural gas pipelines, the primary concern is the safety of population near the pipeline right of way (ROW). TC Energy’s SWRA uses a quantitative risk assessment model that considers the effect of the thermal radiation due to ignited pipeline rupture and evaluate the consequence on the surrounding population. The overall risk is then evaluated using two specific risk criteria: societal risk and individual risk, with the societal risk measuring the overall level of risk to a community or a group of people and the individual risk measuring the level of risk to specific individuals who are present within the pipeline hazard zone.
Natural gas pipeline systems often extend hundreds or even thousands of miles. As such, societal risk criteria for pipelines are typically defined based on a given length of pipeline segment, usually in 1 km or 1 mile (1.6 km). To assess the societal risk of actual pipelines, different approaches are taken on how the risk along the length of a pipeline should be aggregated and compared to the criteria. For example, the PD8010-3 standard in the UK recommends the societal risk of a pipeline through a community to be aggregated and then normalized to the unit length to be compared with criteria; whereas the Dutch regulation requires societal risk at the worst location to be used.
In the current SWRA, the societal risk along the length of a pipeline going through development areas or communities is aggregated following the recommendation of the UK PD8010-3, where the risk is aggregated and normalized to the pipeline length. Due to the vast scale of the pipeline system, it is impractical to manually review all development along the pipelines for conducting societal risk assessment on a system wide basis. As such, extent of communities and development areas is determined by a computer program using a simple set of rules. It was found to have led to unsatisfying granularity in the societal risk assessment in certain situations, with some interaction lengths being too long and thus failing to identify the more critical section within the interaction length, and certain development lengths being too short and thus not very meaningful from a societal risk perspective.
To overcome issues with the current societal risk assessment method in SWRA, an alternative method largely following the direction of the Dutch approach is introduced in this paper. In this alternative approach, the societal risk is evaluated continuously along a pipeline with a predefined a sliding length, and thus variations in the societal risk levels along the entire length of a pipeline, including the locations with the highest societal risk levels, can be identified. Implantation details and computational efficiency were discussed. The results from the alternative method were compared to that from the current method. The sensitivity of the sliding length method to the predefined sliding length was also investigated. The study showed that this alternative method improves the accuracy and granularity of the societal risk assessment in the SWRA, and, although it is relatively computational commanding, with an efficient implementation, is still practical even for very large gas transmission systems.