Layer of Protection Analysis (LOPA)

Layer of Protection Analysis is a semi quantitative assessment, providing a numerical estimate of risk using a simpler methodology that QRA.

The methodology involves pairing an initiating event (something that could lead to a hazardous scenario), with a particular hazardous consequence or level of harm. Starting with the frequency with which the initiating event occurs, each protective layer that that prevents the accident sequence proceeding reduces the frequency of the adverse consequence occurring. The combination of the level of harm and the final frequency is the risk level associated with the particular event. The reduction achieved by each layer of protection is known as a Probability of Failure on Demand (PFD). It represents how effective each protective layer is (it is the probability the layer will fail each time it is needed to prevent the risk scenario, and the lower the PFD, the better.

Other factors such as mitigating factors and enabling conditions are also used to adjust the final frequency. These are typically chances that certain conditions such as a given wind direction may occur.

LOPA is a good transparent form of risk assessment that clearly identifies why a given risk level has been assigned. For this reason, many regulators responsible for administering legislation that require safety cases have a preference for LOPA. It is one of the assessment techniques referred to in AS / IEC 61511 for Functional Safety SIL allocation. Unfortunately, it is not easily adapted for risks that are not associated with specific initiating events, for example, risks associated with the ongoing corrosion of containment systems.

Arriscar uses the commercially available software package PHA –Pro v8 to record most LOPA.