(69a) Consideration of Escalation Risks in Facility Siting

A small fire when uncontrolled, can potentially lead to failure of an equipment containing large inventory of hydrocarbon or a structure holding hydrocarbon bearing equipment within the same process unit, resulting in an escalation of a minor event to a much larger event with catastrophic consequences. Similarly, a major fire or an explosion in a process unit may escalate to cause failure of equipment and piping in adjoining units or facilities, thereby causing more severe damage. Identifying and quantifying the escalation risk is the subject of this paper in terms of summarizing the current approaches and how adopting this in a rigorous manner can help minimize losses to asset and production in addition to saving lives. A few case studies are presented to illustrate the benefits of incorporating mitigation measures to minimize escalation risks.

Various accidents in the past such as the Formosa Plastics propylene explosion in 2005 show that small leaks or fires can escalate to cause damage to the unit and also escalate to cause damage to adjoining units. Inadequate separation between units, inadequate active and passive protection of structures and equipment, particularly for jet fires, inadequate isolation and blowdown measures, inadequate specification of safety critical elements such as ESDVs and flare headers are some of the causes for escalation. To minimize asset loss and injuries, facility siting and plant layout need to incorporate a systematic assessment of escalation risk to ensure mitigation measures are identified and incorporated.

Escalation risks can be assessed following a consequence-based approach or a risk-based approach incorporating both frequency and consequence estimation. While consequence-based analysis is a simple and easy tool to use, the selection of credible failure scenarios can become a subject of debate - many projects have adopted a generic 1” or 2” hole size as the basis for analysis whereas some have considered larger hole sizes. Given the increased size of piping and equipment due to larger throughputs, a 1” or 2” hole size may be considered as insufficient. The alternative is to follow a risk based approach wherein all leak sizes are modelled together with mitigation system failure, directionality and ignition/explosion probability to determine the cumulative likelihood of a damaging fire or explosion event. Such an approach would typically be based on a target frequency threshold of 1E-4 per year. This method is subject to the usual uncertainties associated with a frequency assessment including those associated with failure frequency data and the various event probabilities.

Case studies for onshore installations handling hazardous substances are presented considering both risk-based and consequence-based approaches for comparison. The case studies also illustrate how mitigation measures can be designed based on such an analysis to reduce the risk of escalation. Examples of mitigation measures considered include increased spacing between process units, increased spacing between sections of a large process unit by segregated fire zones, spill containment, fire and gas detection, isolation and blowdown, and active and passive fire protection. The case study results are also compared with typical deterministic spacing tables, which are commonly used for plot plan development, whereby recommendations are provided to minimize the risk of escalation early on during the initial plot plan development stage of the project.