Layer of Protection Analysis (LOPA) and Safety Integrity Level (SIL) Assignment
Risk Management Professionals conducts Safety Integrity Level (SIL) Reviews using a Layer of Protection Analysis (LOPA) to assign SIL values to the Safety Instrumented Systems. This approach meets the requirements of the IEC (International Electrotechnical Commission) standards, IEC 61508 and IEC 61511. The SIL Review is a method to establish a “fit-for-purpose” design of (instrumented) safety measures, which are able to mitigate process hazards with respect to safety, environmental consequences, and economic loss.
The primary purpose of LOPA is to determine if there are sufficient layers of protection against an accident scenario (can the risk be tolerated?). A scenario may require one or more Independent Protection Layers (IPLs) depending on the process complexity and potential worst-case severity of a consequence. Note that for a given scenario, only one layer must work successfully for the consequence to be prevented. However, since no layer is completely reliable, sufficient protection layers must be provided to render the risk of the accident tolerable.
Additionally, LOPA is useful because it provides a consistent basis for judging whether there are sufficient IPLs to control the risk of an accident for a given scenario. When the estimated risk of a scenario is not acceptable, additional IPLs are added. Alternatives encompassing inherently safer design are evaluated as well.
The specific LOPA methodology uses order of magnitude categories for initiating event frequency, consequence severity, and the likelihood of failure of IPLs to approximate the risk of a scenario. The key steps used to conduct the LOPA are listed below:
- Select Hazard Scenario from HAZOP based on its severity level.
- Select Target Frequency (TF).
- Identify Initiating Cause(s) from the HAZOP and quantify Initiating Cause Likelihood (ICL).
- Determine Intermittent Hazard(s) credit if applicable.
- Identify Independent Protection Layers (IPLs) from HAZOP safeguards.
- Quantify the Probability of Failure on Demand (PFD) for each IPL.
- Identify and quantify Conditional Modifiers/Vulnerability Factors if applicable.
- Calculate the LOPA Ratio for each Safety, Environmental, and Commercial consequence category as applicable.
- Using the LOPA Ratio, determine Integrity Level (IL) requirements for the existing Safety Instrumented System (SIS), if required, or identify requirements for additional IPL or SIS and evaluate SIS integrity level.