Risk assessment ignition probability

Crawley and Grant (1997) have developed a risk assessment tool for new offshore installations. They have examined typical leak frequencies of equipment items and the ignition probability of these leaks in four pressure bands. With this information it was possible to define leak size and frequency for any piece of equipment and the ignited leak frequency. In off-shore installations gas separation vessels were found to have ten times higher ignited event frequency than oil pumps. 

Assume that a quantified risk assessment has predicted a probability of 20% that failure, involving overpressure, will lead to subsequent pipe rupture and ignition. Furthermore it is predicted that, due to the high population density, fatality is 50% likely. 

As the first step of determining fire load, the initial fire risk assessment is performed to assign risk ranking to potential cases by combining leak frequency, ignition probability, segment inventories and fire types. Representative cases with the highest potential risk are selected for further detailed consequence analysis. 

Evaluation of potential ignition sources to review causes and to determine suitable probability values to be used in the risk assessment. 

The fault tree is used to estimate the risk of gas migrating into a building and accumulating to an explosive concentration. This is the probability of a hazard occurring. Once the probability of the hazard occurring is known then the frequency of exposure to the hazard (usually the frequency of ignition in a methane assessment) must be assessed. The overall risk is then given by ... 

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