Developments in the LOPA method

The LOPA method is a simplified way of estimating the frequency of occurrence of a scenario by valuing the effect of layers of protection. The estimated frequency is an order of magnitude obtained through powers of 10 used to quantify the frequencies of initiating events and the probabilities of failure of barriers that meet the definition of independent layers of protection (IPL – independent protection layer).

The LOPA method can be considered as a semi-quantified method in the sense that it does not use precise values specific to the case studied. It is generally based on a risk analysis such as HAZOP, with the aim of quantifying the accident scenarios identified and defining LOPA objectives to reduce risks sufficiently to make them "acceptable" or "tolerable".

The LOPA method is a "seductive" approach, because at first sight it is relatively simple to implement, and can be set up by engineers who are not necessarily specialists in industrial risk analysis.

In its original concept, underneath its relative simplicity, this method can quickly lead to an erroneous estimate of the frequency of an accident scenario. Indeed, since the values used to quantify a scenario are all orders of magnitude generally expressed as powers of 10 (e.g. 0.1 or 0.01), the final result (scenario frequency) is necessarily not very accurate. The cumulative effect of the approximations made to the various parameters used in the calculation (frequency of initiators, probability of barrier failure, etc.) can then rapidly lead to an irrelevant final result.

In the first section, the article reviews the main concepts and foundations of the LOPA method. The second section is devoted to an analysis of the errors most frequently made when implementing the method. Finally, the last section of the article is devoted to the advantages of a quantified LOPA approach over a traditional (semi-quantified) approach. This last section is illustrated by an application example that highlights the benefits of using a quantified approach.