LOPA is a semi-quantitative risk analysis technique. Therefore it is a midway approach between 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). During a process hazards analysis (PHA), a team is responsible for assessing the process risk from various process deviations by determining the potential frequency and consequence of the incident.
Plants are protected by various 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
LOPA can be used at any point in the lifecycle of a project or process, but it is most cost effective when implemented at the detailed design stage when process flow diagrams are complete and the P&IDs are under development. For existing processes, LOPA is typically applied as part of a PHA or after the PHA has been completed. The PHA provides the LOPA team with a listing of hazard scenarios with associated consequence description and potential safeguards for consideration.
LOPA evaluates risk in order of magnitude of selected accident scenarios. There are five basic steps in LOPA:
- Identify the scenarios
- Select an accident scenario
- Identify the initiating event of the scenario and determine the initiating event frequency (events per year)
- Identify the Independent Protection Layers (IPL) and estimate the probability of failure on demand of each IPL
- Estimate the risk of scenario
Important task is to understand of the frequency and consequence of the potential hazardous event. Then the process risk is compared to either a risk matrix or a specific tolerable risk frequency to determine whether the risk is acceptable or whether Independent Protection Layer (IPL) is required for further risk reduction.
