When AND When NOT to Use Enabling Conditions and Conditional Modifiers

Enabling conditions and conditional modifiers are not used in every LOPA. They only warrant being used when they support the objectives of the LOPA and are consistent with the risk criteria employed.

• Guidance on when to use enabling conditions
• Guidance on when not to use enabling conditions
• Guidance on when to use conditional modifiers
• Guidance on when not to use conditional modifiers

Guidance on when to use enabling conditions

The following are typical situations where enabling conditions might be used in LOPAs:

• The event sequence would only be realized if the unit was in a particular state of operation (e.g., in recycle mode or feeding directly from a transport container), where the unit being in that state is necessary to realize a consequence of concern but independent of the rest of the event sequence.
• The consequence would only be realized if the unit was using a particular raw material or catalyst or processing a particular formulation, where the situation was necessary to achieve the consequence of concern but independent of the rest of the event sequence.
• The event sequence would only be realized if other circumstances such as a low or high ambient temperature existed at the time an initiating event occurred.

In each case, the purpose of employing the enabling condition is to take into account conditions that are necessary for an abnormal situation to proceed to the consequence of concern.  The capabilities of the LOPA analyst(s), the established company or facility LOPA methodology and the availability of relevant data would all need to support the use of enabling conditions.

A full description of LOPA enabling conditions, along with worked examples, is given in Chapter 2.  Enabling conditions may be associated with short-duration situations with severe potential consequences.  The elevated risk exposure during these brief situations, termed “peak risk,” should be considered and managed appropriately.

Guidance on when NOT to use enabling conditions

The following are typical situations where a LOPA team should avoid the use of enabling conditions:

• The LOPA analyst(s) have insufficient knowledge of enabling conditions to employ them correctly
• Insufficient data or information is available to assess the probability to be assigned to an enabling condition.
• The company’s or facility’s established LOPA procedure specifically indicates that enabling conditions are not to be used in its LOPAs, for whatever reason.
• A potential enabling condition does not meet company criteria.  For example, established procedures may specify that an enabling condition is valid only if it gives a full order of magnitude reduction in frequency, but the potential enabling condition does not meet this.
• The company or facility does not have the resources, capability or practices in place to properly assess and document the use of enabling conditions and maintain their ongoing validity.  (See Section 2.6 for guidance on maintaining the validity of enabling conditions over time.)

In addition, situations where one particular type of enabling condition, namely a “time-at-risk” enabling condition, should not be used are listed in Section 2.3.

See enabling conditions | See conditional modifiers

Guidance on when to use conditional modifiers

A typical situation where conditional modifiers might be used in LOPAs is when a company’s risk criteria are based on best-estimate risk values rather than conservative bounding estimates.  In this case, not using conditional modifiers may result in risk estimates inconsistent with the company’s risk criteria.  For example, in a situation where the probability of igniting a given release is expected to be much less than 100%, then using a probability of ignition of 100% would give a conservative risk estimate but not a best-estimate value.  Basing risk management decisions on an overly conservative risk estimate could result in a misallocation of risk-reduction resources.  For scenarios where more than one conditional modifier is pertinent, this over-conservatism could be even further amplified.

The capabilities of the LOPA analyst(s), the established company or facility LOPA methodology and the availability of relevant data would all need to support the use of conditional modifiers.

Guidance on when not to use conditional modifiers

Many organizations decide not to use conditional modifiers in LOPAs for various reasons that might include one or more of the following:

• If the organization’s approach is used to implicitly include conditional modifier probabilities when selecting a consequence severity category, then their explicit use in a LOPA would be double-counting these factors.
• If the organization judges that the uncertainties or complexities involved in incorporating conditional modifier values in LOPAs are too great to warrant their use.
• If the organization considers that the difficulties inherent in validating conditional modifier values are considered to be too great, recognizing that conditional modifiers cannot generally be audited or functionally tested in the same way as for initiating events and independent protection layers.
• If the organization chooses to use a conservative approach that considers conditional modifier factors such as likelihood of ignition to always have a probability of 1.

In addition, the following are typical situations where conditional modifiers should be avoided or limited in LOPAs:

• The facility’s risk criteria for evaluating LOPA scenarios uses severity categories based on the size of a material or energy release (for example, a 10,000 lb flammable liquid release may be one severity category) rather than the potential consequences of the release such as fire or  vapor cloud explosion, fatalities or environmental impacts.
• The LOPA analyst(s) have insufficient knowledge of conditional modifiers to employ them correctly.
• Insufficient data or information is available to assess the probability to be assigned to a conditional modifier.
• The company’s or facility’s established LOPA procedure specifically indicates that conditional modifiers are not to be used in its LOPAs, for whatever reason.
• The company’s or facility’s established LOPA procedure is to not use conditional modifiers unless they provide a full order-of-magnitude effect on the risk calculation.
• The company or facility does not have the resources, capabilities or practices in place to properly document the use of conditional modifiers and maintain their ongoing validity.  (See Section 3.8 for guidance on maintaining the validity of conditional modifiers over time.)
• Even when an organization’s approach is to use conditional modifiers in LOPAs, situations exist where specific conditional modifiers should not be used when evaluating specific scenarios, as discussed in Chapter 3 for the various types of conditional modifiers.

See enabling conditions | See conditional modifiers

It is hoped that the current Guidelines will aid in the decision process as to the appropriate use of enabling conditions and/or conditional modifiers in LOPAs and other risk evaluations, and to provide the understanding needed for their proper usage.