• escalation
• event tree analysis
• physical protection systems
• probabilistic assessment
• security risk

Intentional attacks towards industrial facilities are credible and potentially catastrophic. However, in Europe, no consolidated guidelines are available for the assessment of this specific risk. The aim of the present work is to provide a methodology that can be implemented in quantitative security risk analysis. The methodology is aimed at creating a set of “enhanced event trees” that allow considering the role of safety and security barriers (PPS) in preventing and/or mitigating the risks arising from intentional attacks. Both safety and security barriers (physical protection system (PPS)) have been implemented in the event tree. The potential of the method is related to the fact that the role of barriers has been expressed considering their presence and their performance.
In literature, studies related to “advanced” event trees, developed considering safety barriers, are available. Starting from these studies, the method has been tailored to scenarios arising from intentional attacks, also considering the role of PPS.
The core of the thesis is the development of integrated event trees. The barriers have been introduced using logical operators (“gates”), which allow considering the barrier activation and performance, expressed in terms of availability and effectiveness. The barriers and their performances have been characterized through literature data or by developing specific analyses (e.g., fault tree analysis).
The developed method has been implemented into a risk assessment framework and applied to a case study; this step allowed to underline the strengths and the weaknesses of the methodology. The main critical aspects of the method are related to the frequency assessment depending on the barrier performance. This element is often influenced by the human component and by the degradation of barriers which leads to decrease their performance over time. Nevertheless, the method allows evaluating final scenarios and the role of barriers.