“The easiest way to solve a problem is to pick an easy one.”  — Franklin P. Jones

We love having choices. We hate making choices. What if we pick wrong? There is no shortage of people ready to tell us.

It is always helpful to have criteria for choosing, or to be honest, to justify the choices we want to make anyway. Picking the scenarios to undergo Layer of Protection Analysis is no different.

You Can’t Do Everything

LOPA is an incredible tool for performing risk assessments. It’s simple, it’s easy to understand the results, and it leads to clear-cut decisions. It’s not free. Every LOPA scenario takes time to complete, and no one has enough time. Given that there is simply not enough time to do everything, it is really important to spend it doing things that matter. There are three criteria that will help you sort the scenarios that will benefit from the time you will spend on a LOPA:

  • High risk scenarios
  • High consequence scenarios
  • Scenarios where a new instrumented function is being considered

High Risk Scenarios

Begin with scenarios from your Process Hazard Analysis (PHA) that have high risk, scenarios that end up in the Red Zone of the risk matrix. The criteria will capture scenarios with terrible consequences. It will also capture scenarios with less severe consequences, but that happen way too often to be ignored.  Since LOPA is a form of likelihood analysis, it helps a team develop strategies for making the less severe consequences happen less often, lowering their frequency to something that is tolerable.

High Consequence Scenarios

Next, include the scenarios that have high consequences. We recommend using the consequence categories that includes a workplace fatality or worse. Most of these will already be captured in the first sort, for risk. But it is possible for a scenario to have a terrible consequence—death—and still not be high risk.


When a team convinces itself that the likelihood is really low, the risk will not be high, regardless of the consequences.  Think of asteroids. But as good as teams are at estimating the consequences of a scenario, they are terrible at estimating likelihood. But since LOPA is a form of likelihood analysis, it will replace a team’s feelings about likelihood with some rigor.

The likelihood analysis is not purely objective, but the LOPA will do a very good job of documenting the assumptions on which that likelihood analysis is based.

New Instrumented Functions

When a new instrumented function is under consideration, there should be two concerns:

  • Is the new function truly independent from the existing functions and the anticipated cause of the scenario?
  • Does the new function need to be installed in a Safety Instrumented System (SIS) as a Safety Instrumented Function (SIF) with a Safety Integrity Level (SIL), and if so, what SIL should it have.

As a tool, LOPA compels a team to consider the question of independence.

When its done, LOPA identifies the amount of risk reduction that is still required for the scenario to achieve a tolerable level of risk. In doing that, it also identifies the required SIL of the new function.

You Sure Know How to Pick ‘Em

Establishing the criteria for making choices takes most of the anxiety out of actually making the choices. There will still be those that disagree with the choices (there always are) but their disagreements will really be with the criteria, not with the choices. The choices won’t be mysterious or based on feelings, and discussions about criteria are much easier discussions to have.


  • Mike Schmidt

    With a career in the CPI that began in 1977 with Union Carbide, Mike was profoundly impacted by the 1984 tragedy in Bhopal and has been working on process safety ever since.