EVENT TREE ANALYSIS

EVENT  TREE  ANALYSIS

Event tree analysis evaluates potential accident outcomes that might result following an equipment failure or process upset known as an initiating event. It is a “forward-thinking” process, i.e. the analyst begins with an initiating event and develops the following sequences of events that describes potential accidents, accounting for both the successes and failures of the safety functions as the accident progresses

Steps to perform an event tree analysis:

1. Define the system: Define what needs to be involved or where to draw the boundaries.
2. Identify the accident scenarios: Perform a system assessment to find hazards or accident scenarios within the system design.
3. Identify the initiating events: Use a hazard analysis to define initiating events.
4. Identify intermediate events: Identify countermeasures associated with the specific scenario.
5. Build the event tree diagram
6. Obtain event failure probabilities: If the failure probability can not be obtained use fault tree analysis to calculate it.
7. Identify the outcome risk: Calculate the overall probability of the event paths and determine the risk.
8. Evaluate the outcome risk: Evaluate the risk of each path and determine its acceptability.
9. Recommend corrective action: If the outcome risk of a path is not acceptable develop design changes that change the risk.
10. Document the ETA: Document the entire process on the event tree diagrams and update for new information as needed Guideline

   

 Event Tree & Fault tree Analysis PDF

Event Tree Analysis In Risk Analysis

Event tree analysis can be used in risk assessment by determining the probability that is used to determine the risk when multiplied by the hazard of the event. Event Tree Analysis is a tool that makes easy to see what pathway is creating the greatest probability of failure for a specific system. It is common to find single point failures that do not have any intervening events between the initiating event and a failure. With Event Tree Analysis single point failure can be targeted to include an intervening step that will reduce the overall probability of failure and thus reducing the risk of the system. The idea of adding an intervening event can happen anywhere in the system for any pathway that generates too great of a risk, the added intermediate event can reduce the probability and thus reduce the risk.

Fire Protection System Fault tree Analysis  Example

1.          Identify an initiating event of interest.
2.          Identify the safety functions designed to
3.          deal with the initiating event.
4.          Construct the event tree.
5.          Describe the resulting accident event
6.          Sequence.
7.          Looks at system, beginning with an event
8.          Identifies (all) possible outcomes
9.          Useful for decision analysis (more later)
10.          Again, historically engineering, but broadly applicable

Advantages

• Enables the assessment of multiple, co-existing faults and failures
• Functions simultaneously in cases of failure and success 
• No need to anticipate end events
• Areas of single point failure, system vulnerability, and low payoff countermeasures may be identified and assessed to deploy resources properly
• paths in a system that lead to a failure can be identified and traced to display ineffective countermeasures
• Work can be computerized
• Can be performed on various levels of details
• Visual cause and effect relationship
• Relatively easy to learn and execute
• Models complex systems into an understandable manner
• Follows fault paths across system boundaries
• Combines hardware, software, environment, and human interaction
• Permits probability assessment

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