In the first part of quantitative risk analysis techniques, we learnt about crucial step of the analysis which are hazard identification and cause and frequency analysis. In second part, we learnt continuation of the first part, which are fault tree analysis or event tree analysis as an approach to estimate frequency. After that we learnt about consequence modeling and several potential consequences.
In this post we will learn about risk results and risk reduction and mitigation.
Risk Results
There are two graphic forms that are usually used to present risk result: individual risk contours and societal risk F-N curves (Frequency-Number of fatalities).
Individual risk is the risk to a person in the vicinity of a hazard. This includes nature of the injury to the individual, likelihood of injury occurring, and time period of injury occur.
Societal risk is a measure of risk to a group of people. It is mostly expressed as frequency distribution of multiple casualty events.
Graphs below show example of individual risk contour and societal risk graph (F-N graph) in LNG receiving terminal in Hong Kong.
Individual risk is plotted in a map. It will show if specific building or facility is affected at the risk level.
Societal risk is expressed in F-N graph. If the graph is below the criteria lines, the risk is considered tolerable.
Risk Reduction and Mitigation
Purpose of mitigation is to reduce consequence or to reduce frequency. There are several typical mitigation measures used: fire barriers, fast detection and response, moving the source away from sensitive area. For LNG industry, these are several mitigation.
Mitigation for Leaks in LNG Transfer Line
- Install powered emergency release connection (PERC) for loading arms
Mitigation for Process Leaks
- Minimize flange connection
- Install ESD system
- Install flammable gas leak detection system
Mitigation for Storage Tank Rollover
- Maintain circulation with pump loop
- Provide stratification sensors
- Sample incoming LNG to avoid stratification tendencies
Reference: LNG risk based safety – modeling and consequence analysis – Woodward and Pitblado