What is Event Tree analysis in Health and Safety? Is Event tree analysis a Qualitative or Quantitative Risk assessment?

What is Event Tree analysis in Health and Safety? Is Event tree analysis a Qualitative or Quantitative Risk assessment?

Risk assessment is of two type .i.e. Quantitative and Qualitative. Quantitative risk assessment means prioritizing the risk through its quantification. Like in 5 X 5 risk assessment matrix, risk prioritized on the basis of Severity and Probability. We multiply both severity and probability and higher the number higher the risk. This ranking or numbering gives us an idea about dealing the high number risk prior to low number risk. This is called as Quantitative risk assessment and Event tree analysis is one of the quantitative risk assessment techniques.

Qualitative risk assessment means no quantity and hazard, risk identified from the activity. After identification of hazard and risk we apply control. There is no priority, no ranking. This is called as qualitative risk assessment. Example of qualitative risk assessment is Job safety analysis (JSA).

Event Tree analysis

Facts about Event tree analysis are as below:

1-      Rarely used technique

2-      Forward logic based

3-      Reasoning based (inductive approach)

4-      Mathematical technique

5-      Based on Boolean algebra (Use of 0 and 1)

Steps to perform an event tree analysis:

Define the system: Define what needs to be involved or where to draw the boundaries.

Identify the accident scenarios: Perform a system assessment to find hazards or accident scenarios within the system design.

Identify the initiating events: Use a hazard analysis to define initiating events.

Identify intermediate events: Identify countermeasures associated with the specific scenario.

Build the event tree diagram

Obtain event failure probabilities: If the failure probability can’t be obtained use fault tree analysis to calculate it.

Identify the outcome risk: Calculate the overall probability of the event paths and determine the risk.

Evaluate the outcome risk: Evaluate the risk of each path and determine its acceptability.

Recommend corrective action: If the outcome risk of a path is not acceptable develop design changes that change the risk.

Document the ETA: Document the entire process on the event tree diagrams and update for new information as needed.

Mathematical concepts

1 = (probability of success) + (probability of failure)

The probability of success can be derived from the probability of failure.

Overall path probability = (probability of event 1) X (probability of event 2) X (probability of event n....)

Event tree analysis done to realize the overall reliability of the system and impacts of failure. So, it is a quantitative risk assessment technique. Initial or initiating events in event tree analysis are failures .i.e. Failure in assembly of equipments, Environment failure like Earthquake, lightening strike etc, Failure of Power system, failure of protective system etc. Unsafe conditions like localized stress in the equipment lead to breakage. All these are initial or initiating events.

Let’s take an example of Chemical reactor catastrophic failure. Here initial event is Cooling Jacket failure and final outcome is catastrophic failure. Now let’s do an Event tree analysis:

N means function not worked. Like Temperature indicator not worked, Pressure gauge not worked, operator not responded.

Y means function worked.

Probabilities of both not worked and worked have been given in the picture.

Red line indicates the path to catastrophic failure. This will happen when initial event cooling jackets fail to cool, concurrently Temperature gauge does not show rise in temperature of Chemical reactor, and concurrently pressure gauge does not show rise in pressure on internals shell, concurrently safety relief valve does not work and same time operator does not respond. If these all happen which is shown in figure with red colour line then catastrophic failure in chemical reactor is probable.

Also in case any of the safety interlocks if work like temperature indicators works then system will fail but catastrophic failure or rupture will not happen. It means either shutdown or breakdown will happen.

Probability of failure will calculate by multiplication following below methodology for given example:

Catastrophic failure probability = Initiating event probability X Failure event 1 probability X Failure                                                 event 2 probability X Failure event 3 probability X Failure event 4 probability

Catastrophic failure = 1 X 0.2 X 0.3 X 0.4 X0.2

Catastrophic failure probability = 0.0048 which means Failure probability in 235 years or so for given example.

Reference: https://en.wikipedia.org/wiki/Event_tree_analysis