(48z) Sil Determination: Shortcomings with the Use of LOPA

Proposed Topic Area: Managing Risks

For a number of years, industry has been trying to apply the principles outlined in IEC 61508.  In the process sector, this has resulted in a focus on the requirements of IEC 61511 (2003) and ANSI/ISA 84.00.01 (2004).   These are risk based standards.  They involve the identification of potential hazardous events, the causes that could lead to each event and the necessary risk reduction for each event.  Where the risk reduction involves an instrumented function (e.g. a trip or an interlock), the required performance of the instrumented function can described in terms of the average probability of failure on demand.  In the standards, the performance for such instrumented functions is assigned to one of four Safety Integrity Levels.  The assessment of what performance level is needed in respect to a specific hazardous event is referred to as SIL Determination.  In the field of SIL Determination, IEC 61511 provides examples of a number of different approaches – these are shown in IEC 61511 Part 3.  These include Event Trees, Risk Graphs, Safety Matrices, Fault Tree Analysis and Layer of Protection Analysis (LOPA).

In the first few years, following the publication of IEC 61508 Edition 1 in 2000, industry generally embraced the use of Risk Graphs as the method of choice.  However, over the subsequent few years, many companies using this method found that it was (a) giving higher safety integrity level targets than really needed and (b) was less flexible than other methods and consequently more difficult to use in many real situations.  This has resulted in a general move to other methods and in particular to the use of Layer of Protection Analysis.  Indeed, LOPA may now be the predominant “Method of Choice” for SIL Determination.  As a method, LOPA gives the impression of simplicity and appears quite straightforward.  It almost invites anyone with a minimum of understanding of the method to be a practitioner.  However, there are a number of shortcomings in the use of the LOPA method that have become evident through its use across the globe.

This paper will review the application of LOPA and provide insights into what may be seen as the proper use of the method and its core principles.  It will also highlight various shortcomings in use that have been found during expert reviews carried out of a number of existing studies.  The paper will be of benefit both to those who are new to the LOPA method, but also of value to those who have been using the method for some time.