LOPA (Layer of Protection Analysis)
Layer of Protection Analysis (LOPA) is used to determine the reliability requirements of the instrumented safety functions based on the severity of the hazards, in combination with other risk reducing measures. An example of independent layers of protection is illustrated in the figure below.
LOPA is a semi-quantitative analysis, and is by several major companies the preferred method to define SIL requirements for instrumented safety functions designed to prevent unintended incidents.
The LOPA method conforms to industry standard IEC 61511.
In order to evaluate a safety instrumented function using the LOPA methodology, both the initiating events causing a demand on the function, and the consequences in case of failure of the function have to be assessed. A LOPA shall be performed in a multidisciplinary workshop and may be quite a comprehensive task.