Page 11 - LOPC Survey

A more general picture, when a “deficient condition” is different from an “incipient failure,” can be
illustrated as follows:

All criteria within Deficient
acceptable range condition Incipient failure Failure (LOPC)

For example, gradual wall thinning to below the minimum wall thickness would put the equipment into
a deficient condition, and if the thinning continues, it may result in an incipient failure (minor leak) before
total failure (LOPC). There are two points in this process at which inspections, tests, observations,
condition monitoring, etc. might detect the situation in time to avert a failure: first as a deficient
condition and then as an incipient failure. The PHA initiating cause for this situation is still the LOPC
equipment failure. If the initiating cause frequency is not estimated directly from generic or historical
data, then it may be able to be estimated as follows, in general form:

Probability of
Probability of
not detecting
Deficient not detecting Initiating
and correcting
condition and correcting cause (LOPC)
frequency x deficient x incipient failure = frequency
condition in time
(per year) in time to avoid (per year)
to avoid incipient
LOPC
failure

Evaluating the situation in this way may result in a better estimate of the LOPC frequency when the
damage mechanism under study involves gradual deterioration in the integrity of the containment
system over time. Of course, if an incipient failure cannot be detected or the transition from an incipient
failure to LOPC happens rapidly, then the “Probability of not detecting and correcting incipient failure in
time to avoid LOPC” would be equal to 1.

Conclusions

The survey results and this monograph have shown that several approaches are available for addressing
potential incident scenarios that are not as easily assessed with traditional PHA techniques. For these
scenarios, the initiating cause directly results in loss of containment, with few or no practical preventive
safeguards. For such scenarios, most risk management strategies focus on measures to keep the
initiating cause from occurring, mitigate the loss-of-containment impact, or both. There is no
intermediate option to react to the initiating event and prevent it from propagating to a LOPC event.
Organizations need to have a defined process for identifying and addressing such loss-of-containment
scenarios. Whether handled in the normal course of a PHA, included as a part of an overall facility siting
study, or addressed in a stand-alone analysis, a process for identifying, assessing, and addressing the risk
from such scenarios is a necessary part of overall risk management. Based on a company’s individual
experience and culture, a multi-pronged approach may be best.

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