(106f) A Three Level Approach for the Assessment of Domino Effect

Accidental scenarios due to domino effects are among the more severe that may take place in the process industry. However, the assessment of the risk caused by these events is a complex task due to the wide number of possible accidental scenarios that should be considered in the analysis and to huge amount of time and of computational resources required by a complete detailed assessment of accidental scenarios involving escalation events. The present study focused on the development of a multistage procedure for the quantitative assessment of domino effect. The procedure was aimed to the optimization of the resources needed for the assessment of domino effect by the integration of the three different levels of assessment. The starting point of the procedure (level 1 assessment), based on threshold criteria, is aimed to the identification of possible escalation targets, in order to exclude from the assessment the areas where escalation is unlikely or equipment are unlikely to be involved in escalation events. Detailed threshold criteria were recently developed to this aim. The further step of the procedure (level 2) is the quantitative assessment of the contribution to individual and societal risk of domino scenarios. The development of a specific procedure and of a software package allows the calculation of the risk increment due to domino scenarios in the areas of the plant where escalation was identified as possible in level 1 assessment. The results of the risk assessment allow the identification by a systematic procedure of critical areas and/or of critical targets of escalation. The areas identified in level 2 should be then considered for the application of a detailed and deterministic analysis of escalation events (level 3 assessment). The analysis of critical areas should be based on distributed parameter modeling of the primary scenarios (e.g. Computational Fluid Dynamics codes based on Reynold Average Navier-Stokes equation) and, for the evaluation of damage to target equipment, by means of structural analysis (e.g. finite elements codes) or, more likely and usefully in terms of time and cost, by means of simplified damage correlations based on peak overpressure. The methodology has been applied to a depressurising unit of a hydrocracker plant t. The possibility of escalation triggered by VCEs within the unit was considered. The blast wave parameters were evaluated by the Baker-Strehlow semi-empirical model and by Computational Fluid Dynamics (CFD). Similarly, escalation possibility and probability for the area surrounding the unit were assessed by threshold criteria (level 1) and by more complex approaches (level 2).