Layers of Protection Analysis (LOPA) is another risk assessment technique, more commonly used to determine the required Safety Integrity Level (SIL) for Safety Instrumented Systems (SIS) under specific standards (i.e. BS EN 61508 and BS EN 61511).
The methodology essentially differentiates between installed risk reduction measures. The outcome from most risk assessment techniques records all the risk reduction measures. These measures can include items of plant and equipment, human actions and procedures, etc. LOPA defines certain measures as special, and calls them Independent Protection Layers (IPLs). IPLs are defined as measures which are:
All the identified risk reduction measures are screened and non-IPLs removed, before deciding if the remaining IPLs reduce the level of risk to an acceptable Target Risk Level. The level of risk reduction achieved or credit for each identified IPL is determined using a LOPA Rule Book (which specifies how risk reduction credits can be awarded for various situations).
If, after assigning the risk credits for the installed IPLs, the Target Risk Level is not attained, then the standards assume that the remaining risk gap will be filled by the installation of a Safety Instrumented System (SIS). The size of the risk gap determines the Safety Integrity Level (SIL) of the SIS required.
We are actively involved in helping operators to determine whether or not proposed and existing safety related systems can provide the necessary level of risk reduction or SIL, consistent with the requirements of the standards.
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