LAYER OF PROTECTION ANALYSIS(LOPA)

LOPA is a semi-quantitative risk analysis technique. It lies in between a HAZOP and a quantitative risk assessment (QRA) in terms of its rigorousness. This technique evaluates risks by orders of magnitude of the selected accident scenarios and builds on the information developed in qualitative hazard evaluation. (e.g. PHA)

Layers of Protection

Plants are protected by various layers of protection:

• Pressure Relief Devices
• Flare Systems
• Fire Suppression Systems
• Safety Instrumented System (SIS) or Emergency Shutdown System
• Automatic action safety interlock system
• Basic controls, critical alarms
• Community emergency response
• Inherently safe design features
• Operator intervention
• Plant emergency response

When should you use LOPA?

LOPA is applied when a scenario is too complex or the consequence is too severe for the HAZOP team to make a sound judgment based solely upon the qualitative information.

LOPA helps you to answer the following questions about your facility:

• What's the likelihood of undesired events / scenarios?
• What's the risk associated with the scenarios?
• Are there sufficient risk mitigation measures?

Benefits of using LOPA

• Requires less time and resources than a QRA but is more rigorous than HAZOP.
• Many process safety systems are over-engineered for safety with additional costs and have unnecessary complexities. LOPA helps focus the resources on the most critical safety systems.
• Acts as a decision making tool, helps make judgments quicker, resolves conflicts and provides a common base for discussing risks of a scenario.
• Removes subjectivity while providing clarity and consistency to risk assessment.
• Improves scenario identification by pairing of the cause and consequence from PHA studies.
• Helps to compare risks based on a common ground if it is used throughout a plant.
• Helps decide if the risk is As Low As Reasonably Possible (ALARP) for regulatory reasons.
• Identifies operations, practices, systems and processes that do not have adequate safeguards.
• Provides basis for specification of IPLs as per ANSI/ISA S84.01, IEC 61508 and IEC 61511.
• Helps to decide which safeguards to focus on during operation, maintenance and related training.
• Support compliance with process safety regulations 

LOPA evaluates risk in order of magnitude of selected accident scenarios. There are five basic steps in LOPA:

• 
     1. Identify the scenarios
     2. Select an accident scenario
     3. Identify the initiating event of the scenario and determine the initiating event frequency (events per year)
     4. Identify the Independent Protection Layers (IPL) and estimate the probability of failure on demand of each IPL
     5. Estimate the risk of scenario