API RP 1171, which was recently incorporated by reference in US regulation 49 CFR 192.12, and the recent changes to the California Code of Regulations have mandated risk assessments be performed for underground storage wells. A quantitative risk assessment framework has been developed for SoCalGas’s underground gas storage sites in California. This framework includes quantitative models for accidental impact threats to the wellhead and lateral piping, including damage from excavations, vehicle collisions, lifting operations, and aircraft crashes (for first, second, and third-parties). In an industry survey of underground storage operators conducted by the Gas Research Institute in 2004, 15% of the recorded significant release incidents were due to accidental impact threats. Additionally, since there are likely individuals in the immediate vicinity when the incidents occur, releases due to accidental impact threats may carry more significant consequences. In this work, models were adapted from existing industry models for pipelines, nuclear facilities, and aboveground piping stations by updating the inputs to reflect an underground gas storage context and calibrating the overall results to match historical underground gas storage incident data. The models incorporate well-specific attributes and measurements as well as subject matter expert (SME) experience and judgement, with key variables for each model including:

• Excavation damage: excavation frequency (estimated from site-specific one-call data), material and operational properties of buried lateral piping, depth of cover

• Vehicle collisions: vehicle traffic near the well (estimated from modelling the site road network and all site activities involving vehicles), presence of any barriers, the layout of any aboveground lateral piping

• Lifting operations: lifting operation frequency near the well (estimated from modelling the future workover activities at all nearby wells), material and operational properties of the wellhead, and any aboveground lateral piping

• Aircraft crashes: proximity and orientation relative to nearby airports, amount and type of traffic at the nearby airports, the layout of the lateral piping

This paper illustrates the results for field-wide assessments of these accidental impact threats, showing how the risk is differentiated by the specific well context. In contrast to using a single historical incident rate, the models capture the differences in risk between wells within and across fields and can help estimate the effect of performing mitigation activities on specific wells.

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