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Risk Graph and LOPA methods have been used to determine the required Safety Integrity Level. Both are most popular methods compared to other methods outlined in the SIS standard for process sector IEC 61511. Risk graph is a structured means to determine ILs using semi quantitative judgment based on series of parameters that are relevant to a risk. LOPA is one of process hazard analysis methods, which are also used to determine ILs. LOPA starts with the identification of initiating cause and protection layer of hazard mitigation. Both Risk Graph and LOPA are suitable tools to determine ILs based on complexity of process being assessed. In general, Risk Graph is particularly appropriate for screening application, while more detail study can be done using LOPA. This paper presents an overview of Risk Graph and LOPA. Results of applying those methods to existing offshore facilities, required documents and other aspects towards more effective workshop are also presented. It can be concluded that LOPA may give lower ILs compared to the Risk graph, but the shortcoming of LOPA is that it is more complex and time consuming.

Oleh : Mefredi, CFSE

Risk Graph and LOPA methods have been used to determine the required Safety Integrity Level. Both are most popular methods compared to other methods outlined in the SIS standard for process sector IEC 61511. Risk graph is a structured means to determine ILs using semi quantitative judgment based on series of parameters that are relevant to a risk. LOPA is one of process hazard analysis methods, which are also used to determine ILs. LOPA starts with the identification of initiating cause and protection layer of hazard mitigation. Both Risk Graph and LOPA are suitable tools to determine ILs based on complexity of process being assessed. In general, Risk Graph is particularly appropriate for screening application, while more detail study can be done using LOPA. This paper presents an overview of Risk Graph and LOPA. Results of applying those methods to existing offshore facilities, required documents and other aspects towards more effective workshop are also presented. It can be concluded that LOPA may give lower ILs compared to the Risk graph, but the shortcoming of LOPA is that it is more complex and time consuming.

Keywords: Risk Graph, LOPA, Safety Integrity Level, Safety Instrumented System

1. Introduction

Safety Instrumented System has been used for many years to perform safety instrumented function in the process industries. If instrumentation is to be effectively used for safety-instrumented functions, it is essential that this instrumentation achieve certain minimum standard and performance levels. The international standard addressing the application of safety-instrumented system in the process industries is IEC 61511, which was finally published in 2003. IEC 61511 is process sector under the framework of IEC 61508 that covers all safety-related system. Within this standard, there are two fundamental concepts pertaining to their application, namely safety lifecycle and safety integrity level. IEC 61511 sets out an approach for safety life-cycle activities in order to achieve the performance standard. To achieve this objective, the IEC 61511:

􀂃 requires the hazard and risk assessment to be carried out in order to identify the overall safety requirements;

􀂃 requires the allocation of safety requirements for safety instrumented system(s) to be carried out;

􀂃 works within the framework which is applicable to all instrumented methods to achieve functional safety;

􀂃 discusses the use of specified activities, such as safety management, that may be applicable to all methods for achieving functional safety.

IEC 61511 also deals with guidance to determine Integrity Level (IL) in hazard and risk analysis processes. Within this standard, information is given to provide broad overview of the methods used in hazard and risk analysis process. There are many methods to determine the Integrity Level required, and they are addressed in IEC 61511 standard:

􀂃 Qualitative Safety Matrix

􀂃 Qualitative Risk Graph

􀂃 Semi Qualitative (Calibrated) Risk Graph

􀂃 Semi Quantitative using LOPA (Layer of Protection Analysis)

In normal approach for risk-based standard document, i.e. IEC 61511, the guidance provided contains insufficient details for direct implementation. All methods to illustrate underlying principle have been subjected to simplification. The user will need to explore the real practice on how to implement the guidance to fit with each corporate or organization guideline.
Many companies feel that the qualitative methods are not consistent with their risk criteria. In this regard, Calibrated Risk Graph and LOPA are able to provide alignment with the risk criteria of a company.

In present paper, author would like to discuss about the workshop-based experiences using Calibrated Risk graph and LOPA method to determine ILs for either newly designed or existing-modified facilities. This paper aims to give reader a brief overview of Safety Integrity Level (SIL) determination workshop, available methods, required documents, and team involvement to achieve effective results.

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