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Risk assessment and inspection for integrity

Fig.3. Partial fault tree for outage probability.
methods uses a formula that includes various factors related to expected
pipeline condition and consequences of failures. The calculated index or
ranking of pipeline sections indicates the priority for inspection. The formula
used by each company reflects aspects of the materials, system configuration
and business situation that are specific to its pipeline system.
We recognized that none of the formulae used by other companies could
be directly applied to our system, and we have adopted fault-tree analysis as
a tool for risk assessment. It provides a logical structure that helps us to
understand the component of outage probability and risk, to perform the
analysis required to quantify them, and to communicate the results. The
analysis method also is useful to describe the reduction of risk accomplished
by pipeline integrity projects that reduce outage probability.

Outage probability

The probability of an outage caused by a structural failure, which is one of
the key factors required to assess safety and economic risks, is estimated using
the fault tree illustrated in Fig.3. The data to estimate outage probabilities are
derived largely from our own engineering studies, data on pipeline character-
istics and failure statistics, supplemented by industry data and experience.
The details of the analysis are beyond the scope of this paper; however, it
is important to understand that for each pipeline segment the method allows
the contribution, to the total outage probability, of each significant failure
cause to be estimated separately. This helps to recognize how pipeline

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