- Likelihood is the change that something might happen. Likelihood can be defined, determined, or measured objectively or subjectively and can be expressed either qualitatively or quantitatively using mathematics.
- It is highly dependent on a range of site-specific factors such as the number of equipment items, its condition, activity frequencies, the quality of the management system and human error levels.
- Likelihood analysis flows directly from the preceding process of assessing the nature of hazards and accidents. Further evaluation of information generated in these processes may be carried out to derive an understanding of likelihood.
- Checklists offer straightforward and effective ways of ensuring that basic types of events are considered. Following are the techniques for checklists:
- Failure Modes Effects Analysis (FMEA)
- FMEA is a process for hazard identification where all conceivable failure modes of components or features of a system are considered in turn and undesired outcomes are analysed. This technique is quite specialised and may require expert assistance.
- Failure Modes Effects and Criticality Analysis (FMECA)
- FMECA is a highly structured technique that is usually applied to a complex item of mechanical or electrical equipment.
- The overall system is described as a set of sub-systems and each of these as a set of smaller sub-systems down to component level.
- Individual system, sub-system and component failures are systematically analysed to identify their causes (which are failures at the next lower-level system), and to determine their possible outcomes, which are potential causes of failure in the next higher-level system. This technique is quite specialised and usually requires expert assistance.
- Fault Tree and Event Tree Analysis
- Fault Trees describe loss of containment events in terms of the combinations of underlying failures that can cause them, such as a control system upset combined with failure of alarm or shutdown and relief systems.
- Event trees describe the possible outcomes of a hazardous event, in terms of the failure or success of control measures such as isolation and fire-fighting systems.
- Fault tree and event tree analysis is time-consuming, and it may not be practicable to use these methods for more than a small number of incidents.
- Historical records of incidents
- Databases of incidents and near misses that have occurred are a useful reference because they give a very clear indication of how incidents can occur.
- Employers should consider site history, company history, and industry history and possibly even wider sources of historical information for this purpose.
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