The Layer of Protection Analysis (LOPA) method is a Process Hazard Analysis tool. The method utilizes the hazardous events, event severity, initiating causes and initiating likelihood data developed during the Hazard and Operability analysis (HAZOP). The LOPA method allows the user to determine the risk associated with the various hazardous events by utilizing their severity and the likelihood of the events being initiated. Using corporate risk standards, the user can determine the total amount of risk reduction required and analyze the risk reduction that can be achieved from various layers of protection. If additional risk reduction is required after the reduction provided by process design, the basic process control system (BPCS), alarms and associated operator actions, pressure relief valves, etc., a Safety Instrumented Function (SIF) may be required. The safety integrity level (SIL) of the SIF can be determined directly from the additional risk reduction required.

The Layer of Protection Analysis (LOPA) method is a Process Hazard Analysis tool. The methodutilizes the hazardous events, event severity, initiating causes and initiating likelihood data developed during the Hazard and Operability analysis (HAZOP). The LOPA method allows the user to determine the risk associated with the various hazardous events by utilizing their severity and the likelihood of the events being initiated. Using corporate risk standards, the user can determine the total amount of risk reduction required and analyze the risk reduction that can be achieved from various layers of protection. If additional risk reduction is required after the reduction provided by process design, the basic process control system (BPCS), alarms and associated operator actions, pressure relief valves, etc., a Safety Instrumented Function (SIF) may be required. The safety integrity level (SIL) of the SIF can be determined directly from the additional risk reduction required.

Annex F – Layer of Protection Analysis from the Draft IEC 61511 Part 3 Standard is attached. The IEC 61511 is the process industry specific safety standard based on the IEC 61508 standard and is titled « Functional Safety of Safety Instrumented Systems for the Process Industry Sector ». IEC 61511 Part 3 is informative and provides guidance for the determination of safety integrity levels.
Annex F illustrates the general principles involved in the LOPA method and provides a number of references to more detailed information on the methodology. It should be noted that Annex F is derived from a CDV version of the IEC 61511 Part 3 Standard dated 24 November 2000. The CDV version of the standard is for National Committee comments and vote on the draft. The draft will be subject to change based upon comments received from various National Committees around the world. I have included a few editorial comments received from members of the ISA SP84 Committee that were submitted to the IEC with a positive vote for the IEC 61511 Part 3 standard.

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