(25c) The Interaction of Earthquakes with Process Equipment in the Framework of Risk Assessment

Large parts of European and American territories are affected by significant natural hazard. Furthermore, recent advances in regulatory hazard assessments and also risk perception by public opinion, have lead to update and enlarge boundaries of ?hazardous areas?. This circumstance makes difficult the planning and management of urban areas as well as risk assessment of industrial plants. If seismic hazards are of concern, risk analysis has to take properly account of seismic vulnerability of industrial components and of the related effects. To this aims, fragility curves for structural facilities in terms of mechanical damage and intensity of loss of containment are needed, in order to cross them with the outcomes of Probabilistic Seismic Hazard Analysis, which is a sound methodology for this type of analysis. Once the seismic failure probabilities have been quantified, consequence analysis can be then performed for those events which may be triggered by the loss of containment following seismic action. Results can be then combined by means of specific developed codes in terms of risk contour plots and compared with analogous reports obtained by considering only process-related top events. With specific reference to the prevention and mitigation of earthquakes, Early Warning System (EWS), i.e. a set of actions that can be taken from the moment when a seismic event is triggered with a significant reliability to the moment the quake strikes in a given location, seems to be a very valuable tool. More specifically, EWS can activate any preventing countermeasure aimed at limiting the probability of occurrence of catastrophic accidental scenarios, which in turns are triggered by the release of relevant amount of gas or liquid flammable or toxic substances from damaged equipment. The EWS can assessed in terms of an a-dimensional number which depends on: i) the characteristic time for the seismic wave to travel from the fault to the installation; ii) the intensity of earthquake expressed in terms of peak ground acceleration (PGA) and the related probability of occurrence; iii) the ability of structures (equipment) to resist to any PGA; and iv) the ability of safety system (e.g. safety interlock systems) to prevent and mitigate the release of substances. In the paper, results are given as plots for anchored and unanchored atmospheric tanks and for pressurised cylindrical horizontal vessel at different fill levels. Furthermore, PGA threshold values for the earthquake intensity for the structural damage and accidental scenario are defined and discussed in order to identify cry-wolf issues.