Layer of Protection Analysis also known as LOPA is a risk management technique commonly used in the chemical process industry that can provide a more detailed, semi-quantitative assessment of the risks and layers of protection associated with hazard scenarios. LOPA is a means to identify the scenarios that present the most significant risk and determine if the consequences could be reduced by the application of inherently safer design principles. LOPA can also be used to identify the need for safety instrumented systems (SIS) or other protection layers to improve process safety.
LOPA is a risk assessment methodology which uses simplified, conservative rules to define risk as a function of both frequency and potential consequence severity. LOPA is defined as a simplified risk assessment of a one cause – one consequence pair.
Conceptually, LOPA is used to understand how a process deviation can lead to a hazardous consequence if not interrupted by the successful operation of a safeguard called an independent protection layer (IPL). An IPL is a safeguard that can prevent a scenario from propagating to a consequence of concern without being adversely affected by either the initiating event or by the action (or inaction) of any other protection layer in the same scenario.
Examples of IPLs are as follows:
- Standard operating procedures,
- Basic process control systems,
- Alarms with defined operator response,
- Safety instrumented systems (SIS),
- Pressure relief devices,
- Blast walls and dikes,
- Fire and gas systems, and
- Deluge systems.
LOPA can be used at any point in the lifecycle of a project or process, but it is most cost effective when implemented during front-end loading when process flow diagrams are complete and the P&IDs are under development. For existing processes, LOPA should be used during or after the HAZOP review or revalidation. LOPA is typically applied after a qualitative hazards analysis has been completed, which provides the LOPA team with a listing of hazard scenarios with associated consequence description and potential safeguards for consideration.
The six major steps to the LOPA process are as follows:
- Record all reference documentation
- Document the process deviation and hazard scenario under consideration by the team
- Identify all of the initiating causes for the process deviation and determine the frequency of each initiating cause
- Determine the consequence of the hazard scenario
- List the IPLs that can completely mitigate all listed initiating causes
- Provide specific implementable recommendations
There are four primary benefits to implementing LOPA over other SIL assignment methodologies procedures.
- Due to its scenario-related focus on the process risk, LOPA often reveals process safety issues that were not identified in previous qualitative hazards analysis.
- Process hazards are directly connected to the safety actions that must take place, providing clear identification of the safety instrumented systems and associated SIL.
- It has been proven effective in resolving disagreements related to qualitative hazards analysis findings.
- LOPA often identifies acceptable alternatives to the SIS, such as adding other layers of protection, modifying the process, or changing procedures. This provides options for the project team to evaluate using cost/benefit analysis, allowing the most cost effective means of risk reduction to be selected.
In conclusion, LOPA allows the safety review team an opportunity to discover weaknesses and strengths in the safety systems used to protect employees, the plant, and the public.