FREQUENCY OCCURRENCE ESTIMATION
Event tree analysis (ETA) graphically shows the sequence of accidents starting from an initiating event [1, 3, 4]. ETA includes the responses of safety systems and operators to the initiating events. ETA requires a knowledge of the potential initiating failures or system upsets (that can potentially cause a nondesired event), as well as the safety system operation or the emergency procedures, which can potentially mitigate the consequences of an initiating event. ETA depends on the complexity of the initiating events and safety systems. A Delphi technique is sometimes used to solve ETAs [1, 3, 6]. In the study, we construct a quantitative graphical model of ETA, as shown Figure 1. From the PHA results, it is possible to select undesired event scenarios to build an ETA. In this case, ETA considers the event sequence of a natural gas leakage from the pipeline because it is considered as a moderate risk in the PHA. Frequency estimates result in jet fire or flash fire probabilities of 3.3E-7 occurrences/yr and dispersion of 3.2E-05 occurrences/yr, respectively.