Risk Assessment of a Propane Storage Sphere: Maintain or Decommission?

Ten years ago, the authors were asked to assist a utility with a risk management decision. Specifically, the utility was considering whether they should maintain or decommission an 800,000 gallon propane storage sphere. The 30-year old sphere was the fuel storage vessel for a peak-shaving power plant. One of their major concerns was the shrinking buffer zone around their facility. Although the facility was originally located in a remote rural area, residential development had begun to encroach upon the facility's buffer zone. The original analysis was based on a consideration of the offsite consequences for a worst-case scenario. This was done at the request of the client as it reflected their degree of risk acceptance. The consequences of two hazard scenarios were analyzed: an unconfined vapor cloud explosion and a BLEVE. To support this analysis, the mechanical integrity of the sphere was evaluated deterministically by considering the pressure vessel design documents, the limited nondestructive test and evaluation data available, and published data on typical corrosion rates. Additional factors incorporated into the analysis were the capabilities of the existing fire protection system, the facility security system, and the propane unloading station for tank car deliveries. Finally, the economics of maintaining the sphere versus decommissioning it were explored. Ultimately, the utility decided to decommission the sphere.

In this paper we conduct a retrospective study using a risk-based approach. We identify a set of hazard scenarios for the propane storage sphere, calculate the leak frequency for each scenario, compute the associated source term, and evaluate the consequences. The results are compared with the original consequence analysis and other published hazard studies. The implications for risk management are discussed.

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