Conditional Modifiers

About Conditional Modifiers

A conditional modifier is an action or event that can reduce the probability of an undesirable event. Details about a conditional modifier are stored in Consequence Modifier records, which are linked to LOPA records. Based on the modifier type that you selected when you create a Conditional Modifier, the probability of occurrence of the modifier is determined using the default probabilities defined in the Consequence Adjustment Probabilities record for the modifier type.

The product of probability of occurrence of conditional modifier and initiating event frequency is referred to as Unmitigated Consequence Frequency on the Conditional Modifier datasheet.

Example

Suppose that the SIL analysis team is conducting a LOPA to investigate the risk scenario illustrated in the following diagram, where each box represents a part of the scenario, and each label indicates the family that stores the relevant information:

When Valve A-1001 fails, flammable gas is released into the atmosphere. If the flammable gas reaches a potential ignition source (e.g., electric switch, motor), causing a vapor cloud explosion in the vicinity of the operator, it could cause a fatal injury.

In this risk scenario, the fatal injury is a consequence of the valve failure, and the following events or actions are the conditional modifiers:

  • The flame igniting
  • The vapor cloud exploding
  • The operator being in the vicinity of the explosion

Since these actions and events appear within the risk scenario, the probability associated with the consequence occurring is increased exponentially. In other words, if the operator was not in the vicinity of the blast, the probability of fatal injury would be less. By examining the granular events that are associated with a risk, the SIL analysis team can more accurately assess the SIL value for the safety system.

In this example, consider the following values:

  • Probability of the operator being present in the vicinity of blast = 0.5 (i.e., 50%)
  • Frequency of failure of Valve A-1001 = 0.2

For the above values, the Unmitigated consequence frequency of the scenario is calculated as follows:

  • Probability of the operator being present in the vicinity of blast x Frequency of failure of Valve A-1001 = 0.5 x 0.2 = 0.1

Access a Conditional Modifier

Before You Begin

Procedure

  1. Access the Layer of Protection Analysis (LOPA) that is linked to the Conditional Modifier that you want to access.
  2. In the left pane, select Conditional Modifiers.
    The Conditional Modifiers workspace appears, displaying a list of Conditional Modifiers linked to the LOPA.
  3. In the Consequence Modifier ID column, select the link for the Conditional Modifier that you want to access.
    The datasheet for the Conditional Modifier appears.
    Note: As needed, you can modify the values in the available fields, and then select to save your changes. You can modify values for a Conditional Modifier only if the associated LOPA datasheet is in the Planning state.

Create a Conditional Modifier

Before You Begin

Procedure

  1. Access the LOPA for which you want to create a Conditional Modifier.
  2. In the left pane, select Conditional Modifiers.
    The Conditional Modifiers workspace appears, displaying a list of Conditional Modifiers linked to the LOPA.
  3. In the upper-left corner of the workspace, select .
    A blank datasheet for the Conditional Modifier appears.
  4. As needed, enter values in the available fields.
  5. On the upper-right corner of the datasheet, select .
    The Conditional Modifier is created and associated with the LOPA.

Results

  • In the Conditional Modifiers workspace, the Conditional Modifier that you created appears in the list of records.
  • The unmitigated consequence frequency value and mitigated consequence frequency value for the LOPA are recalculated.
  • The summary of the LOPA is updated with the details of the Conditional Modifier.

Delete a Conditional Modifier

About This Task

You can delete a Conditional Modifier only if the associated LOPA record is in the Planning state.

Procedure

  1. Access the Layer of Protection Analysis (LOPA) that is associated with the Conditional Modifier that you want to delete.
  2. In the left pane, select Conditional Modifiers.
    The Conditional Modifiers workspace appears, displaying a list of Conditional Modifiers linked to the LOPA.
  3. Select the row containing the Conditional Modifier that you want to delete.
  4. In the upper-right corner of the Conditional Modifiers workspace, select .
    The Delete Modifier dialog box appears, asking you to confirm that you want to delete the Modifier.
  5. Select OK.
    The Conditional Modifier is deleted.

Results

  • The unmitigated consequence frequency value and mitigated consequence frequency value for the LOPA are recalculated to remove the effect of the deleted Conditional Modifier.
  • The details of the Conditional Modifier is removed from the summary of the LOPA.