The Bowtie for Risk Assessment

The Bowtie Risk Assessment Diagram is simply a pictorial visualization tool to assess complex risk events. The creation of the diagram begins with these steps:

1. Identify the Risky Action.
2. Identify the significant Top Event that could occur.
3. Based on the Top Event, brainstorm the possible Causes and/or Threats.
4. List possible barriers that would help prevent the cause from contributing to the Top Event.
5. Based on the event, brainstorm the possible Consequences. 
6. List possible Barriers that could mitigate the specific Consequence. 

7. Evaluate Threats and Consequences by suggesting barriers that may be added to both sides to mitigate or even eliminate the possibility of the Top Event or mitigate the specified Consequences.

Steps in Example:

1. In the example, the routine action is refueling an aircraft.
2. What could occur? In this case the Top Event hazard is the contamination of the fuel.
3. Brainstorming events yield possible causes. These causes range from Wrong Fuel to Contaminated Storage Tank. 
4. What could happen if the fuel was contaminated? Brainstorming revealed Results or Consequences. These range from Repair to Engine Shutdown which could lead to an accident.  
5. Now list barriers that mitigate possible causes. In our example, wrong fuel used could be mitigated through training and color coded hoses. 
6. Brainstorming of barriers to mitigate possible consequences show repairs could be mitigated by inspection. Engine shutdown could be caught before take off by an engine run-up that might reveal problems causes by contaminated fuel. 
7. Brainstorm new barriers to mitigate the results of the event to lessen the impact. For example: addition of fuel cutoff valves may lessen the effect of contaminated fuel on the engines themselves and could prevent the spread of fire. 

NOTE: The Bowtie Diagram illustrated here is the basic tool. The diagram can be expanded to include “holes” in the barriers. These are referred to as “Escalation Factors.” Here is an example:

A hole identified in training is if the training is inadequate. This will make difficult for the worker to identify wrong fuel. Also, we have identified that color coded hoses will not work if there is no procedure on what the colors indicate. 

You can even add Barriers to the escalation factors. These barriers would mitigate the barrier hole.

In this example: the Inadequate training result can be mitigated by the use of a post course exam.  Your thoughts.........