ETA, also known as event tree analysis, is a systematic method used when trying to analyze accidents. This is done by breaking them down into a series of events.
When doing ETA, you must start with an event called a “initiating event”. These will often refer to equipment failure, human error etc. From this initiating event different paths will branch out resembling a tree in the end, where each branch will represent a different scenario that can occur because of the initiating event.
By doing this the ETA will be able to provide the user with different probabilities for each scenario and outcome, aiding in the assessment of the overall risk. ETA will also consider the safety barriers that are in place such as alarms, backups, emergency procedures and if they function as intended (or fail). The ETA functions at its best when used after the completion of preliminary hazard analysis such as HAZOPs or others.
When using the ETA method there are 7 key steps one must follow.
- Identify and define a realistic and relevant initial event that can lead to additional unwanted consequences. When doing this you need to determine what kind of event it is, where it takes place and when it occurs. This is followed up by identifying system dependencies and conditional system responses.
- Identify all barriers designed to address the initial event. These barriers must be listed in the activation sequences. Barriers include automatic detection systems, automatic safety systems, alarms, procedures and personnel actions and finally mitigating barriers. These barriers must then be described by a negative statement such as “function failure”.
- Build the event tree
- Identify and describe potential sequences resulting from the initial event. These can also be called additional events and must always be described by worst case scenario. These can include gas ignition if the initial event is gas leakage.
- Determine frequency of initial event and the probability of each of the branches in the tree
- Calculate probability/frequency of identified outcomes
- Compile results
ETA IS great at visualizing event chains following a initial event, barriers and the sequence of activation. It also provides a good foundation to base evaluations of procedures and safety functions and whether they must be improved.
However, there are certain shortcomings when utilizing the ETA. For starters it is not possible to study multiple initial events in one analysis, which forces you to make more than one. It can also be easy to overlook certain dependencies.
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