LOPA
The Layers of Protection Analysis method
Traditional process hazard analysis identifies the need for additional safeguards - but is based on subjective engineering judgement. This invariably leads to differences of opinion, which can result in risk reduction measures that are ineffectual. The Layers of Protection Analysis (LOPA) provides an objective methodology for identifying and recommending safeguards on a justifiable basis.
LOPA is a semi-quantitative method of risk analysis. The consequence evaluation is qualitative and the evaluation of the event likelihood is quantitative, based upon "order of magnitude" estimates of the initiating event frequency and the availability of the protection layers. The basic concept of LOPA is summarized in the following steps:
- Identify impact events, determine the types of impact, and classify event severity (Target Mitigated Event Likelihood - TMEL)
- List the causes for each impact event
- Estimate the frequency of each initiating cause (CF)
- List the Protection Layers (PLs) for each cause-consequence pair and assign Probabilities of Failure on Demand (PFD)
- Determine the PFD for the system using the equation PFD(System) = TMEL / [CF(Sum) * PFD(n)]
- Calculate the mitigated event frequency for each cause-consequence pair
- Sum the frequencies for each cause-consequence pair that will place a demand on the Safety Instrumented System (SIS)
- Compare the total mitigated event likelihood to the acceptability criteria for the associated event severity classification
- Determine Safety Integrity Level (SIL) based upon required risk reduction or identify other risk reduction measures, if required to meet the risk acceptability criteria.
LOPA fits well with the real world as it takes into account the Basic Process Control System (BPCS), operator response, mechanical design, etc. It allows some credit to be taken for all protection layers and sets an acceptable risk target for hazards by severity level. A variety of protection layers (safeguards) are normally designed to provide additional defence against major or catastrophic incidents. These safeguards should be able to prevent a scenario escalating into an undesirable event and will comprise of devices, systems or actions. Examples of such safeguards include:
- Emergency response plans for site and surrounds
- Relief devices (i.e. physical protection)
- Suppression systems for flammable or toxic releases
- Safety Instrumented Systems (SIS)
- Inherent safety design features.
How safe is safe enough?
The LOPA methodology will only be effective with risk tolerance criteria in place as the typical human response would be to keep adding safeguards even after a point where additional safeguards are unnecessary. The risk criteria help focus resources on the critical safeguards required to achieve tolerable risk. Thus, risk tolerance criteria need to be established for LOPA to answer the 'how safe is safe enough' question.
LOPA will also assist in determining what level of risk reduction is required and the number of protection layers that should be implemented. However, it does not assist in the selection of which specific independent protection layers should be used.
How does LOPA fit in with hazard analysis and QRA?
LOPA is seen as an additional screening step between the findings of a Hazard analysis and the commencement of a QRA. The hazard analysis should be tailored to facilitate the subsequent LOPA study by a variety of means including clarifying initiating events, expressing consequences in a LOPA format, identifying and logging safeguards etc.
Once the LOPA study has been completed a QRA can be carried out on the targeted processes and the scenario risk results should be lower than the more conservative LOPA method.
Fur further info on staff training and actually carrying out LOPA please contact us.