A METHODOLOGY FOR THE QUANTITATIVE ASSESSMENT OF THE RISK INCREASE IN PROCESS PLANTS DUE TO SEISMIC ACTIVITY

摘要

If a process plant is settled in a seismic area, the likely of a loss of containment (LOC) may increase because of an earthquake. This component of the overall risk due to major accidents may be indicated as the "Seismic Risk" of an industrial site having a relevant inventory of hazardous substances. In the framework of quantitative risk analysis of process plants, only the consequences of process equipment failures due to the earthquake are of concern, while the collapse of civil buildings are not considered since they are outside the scope of the analysis. Thus, an earthquake may be considered as a particular initiating event leading to an escalation sequence starting with one or multiple LOCs and ending with one ore more than one accidental scenario. The assessment of the "seismic risk" component in industrial plants is of utmost importance, in particular for emergency planning and land use control in seismic zones. The present study aims at the development of a procedure for the quantitative assessment of the seismic risk contribution to industrial risk. The first step for assessing seismic risk is to evaluate the expected frequencies and consequences (magnitude) of primary events that are in this case the earthquakes. Both terms are often available on the basis of historical data. Usually the expected frequency (e.g. on yearly basis) of a generic earthquake in a given location is known. More difficult is to obtain data on the expected frequencies of a seismic event with a given magnitude, based on the Richter scale and on the ground acceleration. The second step in quantitative risk assessment is to quantify the increase in the risk indexes caused by seismic activity in the area. This may be performed by the knowledge of the equipment-dependant failure probabilities (vulnerability or fragility curves). The equipment vulnerability of fragility models may be derived from historical data for different equipment classes or from the structural analysis of single equipment items. When a seismic event occurs, usually a LOC may occur in more than a single unit, so many different alternative scenarios may be possible. A specific procedure was developed in order to identify all the possible combinations of events resulting from the LOCs, and to assess the expected frequencies and consequences of these combinations of events. The procedure was implemented in a GIS-based software tool in order to manage the assessment of the high number of event sequences. The software also allows the calculation and the representation of the individual and societal risk curves due to seismic risk contribution. The software tool allowed the application of the procedure to several case-studies, that evidenced the validity of the approach. The increase in the industrial risk values are highly dependent on the expected frequencies of earthquakes, but in all the case-studies seismic risk resulted in relevant contribution to the overall values of the industrial risk indexes.