Risk Assessment: Tools, Techniques, and Their Applications

12.1 Event Trees

An event tree is a graphical representation of a series of possible events in an accident sequence. Using this approach assumes that as each event occurs, there are only two outcomes, failure or success. A success ends the accident sequence and the postulated outcome is either the accident sequence terminated successfully or was mitigated successfully. For instance, a fire starts in a plant. This is the initiating event. Then the fire suppression system is challenged. If the system actuates, the fire is extinguished or suppressed and the event sequence ends. If the fire suppression system fails then the fire is not extinguished or suppressed and the accident sequence progresses. Table 12.1 shows this postulated accident sequence. Figure 12.1 shows this accident sequence in an event tree.

Figure 12.1 Event tree.

Table 12.1 Accident Sequence

Event	Description	Possible outcomes
Fire	This is the initiating event	—
Fire suppression system actuates	The fire suppression system detects the fire and it actuates	Success—system actuates and controls the fire Failure—system fails to control the fire
Fire alarm system actuates	Fire alarm system detects the fire and sends a signal to the appropriate fire department. Fire department arrives in time to extinguish the fire before major damage occurs	Success—fire is extinguished prior to major damage occurring Failure—fire ...