Chemical facilities may be targets of deliberate acts of interference triggering major accidents (fires, explosion, toxic dispersions) in process and storage units. Standard methodologies for vulnerability assessment are based on qualitative or semi-quantitative tools, currently not tailored for this type of facilities and not accounting for the role of physical protection systems. In the present study, a quantitative approach to the probabilistic assessment of vulnerability to external attacks is presented, based on the application of a dedicated Bayesian Network (BN). BN allowed the representation of interactions among attack impact vectors and resistance of process units, which determine the final outcomes of an attack. A specific assessment of protection systems, based on experts’ elicitation of performance data, allowed providing a knowledge support to the evaluation of probabilities. The application to an industrial case study allowed the assessment of the potentialities of the approach, which may support both the evaluation of the vulnerability of a given facility, and the performance assessment of the security physical protection system in place.

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Malicious acts of interference and terroristic attacks may affect chemical facilities causing severe accidents, with potential catastrophic consequences for people, environment and asset. The lack of a consolidated practice in the risk-informed implementation of security countermeasures goes hand in hand with the existence of very few rigorous approaches to the effectiveness assessment of security systems, and in particular of Physical Protection Systems (PPS). The present study addresses the issue of quantitatively assessing the effectiveness of PPS adopted to protect chemical facilities, recognized as high-consequence loss physical assets. An extended set of quantitative expert judgment based data that take into account performance variations due to operative conditions was derived. The application of the results to the quantitative assessment of the contribution of security controls in reducing vulnerability was also addressed.@en