Assessment Summary (LOPA)

The different Consequence Categories (People, Assets, Community, and Environment) together with their associated Severity, and Tolerability Criteria per year (TC) are shown. For each Consequence category a Safety Integrity Level (SIL) is calculated and shown. The Overall SIL, which is the driving SIL from the four categories, can then be used to evaluate the adequacy of the existing or proposed Barriers against the Hazard Scenario.

The Consequence is often copied from a HAZOP. If so, it will also be synced with the HAZOP. Thus, changes made in the LOPA will be copied to the HAZOP and vice versa. See Syncing HAZOPs with connected LOPAs.

The Assessment Calculations

Severity

The Severity for each Consequence Category is copied from the HAZOP, if applicable. Otherwise, it will show “N/A”. If needed, the Severity can be changed. To do so click Edit and select a different Severity from the drop-down. The available Severities will depend on if "Tolerability 2017" or "Tolerability 2020" was selected (see image below). This is also copied from the HAZOP. Take note that Consequences that have a Top event with Hazards cannot be changed in a LOPA, since in these cases the Severity will depend on the predefined Hazards. Note: It is possible that a different RAM matrix has been defined for your site. This would mean that different Severities and Tolerabilities could apply for you. To learn more about flexible RAM see Flexible Risk Assessment Matrix (RAM).

For the Asset Severity, it is possible to do a “downtime cost calculation”. To do so click the calculate button. A popup will appear. Select the applicable Production Loss Equation (PLE) and fill in the hours and miscellaneous cost. Note: PLEs can be defined is the IMS settings.

TC

The Tolerability Criteria (TC) depends on the selected Severity as well as the selected Tolerability option. When harm to the general public is expected, you can select Public, which will add extra safeguard with regards to the People Assessment. The People TC will decrease with 1E-01.

DR and Res

Using LOPA principles, IMS determines the Demand Rate (DR) and Residual Risk (Res) values for each Consequence category from the Initiating Event Frequencies and the Barrier PFDs.

RRF

IMS uses the Res, together with the TC, to determine a Risk Reduction Factor (RRF) for each Consequence category. RRF = Res/TC.

PFD

The Probability of Failure on Demand (PFD) is derived from the RRF. If RRF>1, PFD = 1/RRF else PFD = 1.

SIL

Based on the IEC 61511 criteria, a Safety Integrity Level (SIL) for each Consequence category is derived from the PFD. It can show “X”, “SIL 4”, “SIL 3”, “SIL 2”, “SIL 1”, “A”, or “-”. “X” is the highest (worst). “A” is Acceptable and “-” is neglectable. (See SIF Analysis Overview.)

Overall

Lastly an Overall RRF, Overall PFD, and Overall SIL is determined. This will be the driving RRF, driving PFD and driving SIL (i.e., the highest SIL level from all Consequence categories). This can be used to assess the adequacy of the existing or proposed Barriers against the Hazard Scenario.

Updating the Calculation

When a calculation is required, i.e., when data has changed, this calculate button will be red. When you click it, the calculations will update. When the calculation is up to date, the calculate button will show in blue.

For more information on the calculations see also Calculating and evaluating the Overall SIL and Calculating the SIF Analysis Target PFD.