Seismic risk assessment for oil and gas pipelines - Seismic Risk Analysis and Management of Civil Infrastructure Systems - page 696

initiating event (e.g., slope failure) leading to a fatality. Using an event tree
or fault tree approach requires assigning likelihoods to each branch of the
tree along the event path of interest, as shown in Fig. 25.3.

25.8.1 Relating computed strain to loss
of pressure integrity
For a seismic risk assessment, pipeline failure is related to the exceedance
of a specifi ed longitudinal strain limit, typically 2-4% for relatively modern
(post-1960) welded steel oil and gas pipelines. One or more failure branches
may be necessary to differentiate the consequences from seismic hazards
that can produce different probabilities of leak versus rupture. For example,
damage from large fault displacement or lateral spread displacement is
much more likely to result in rupture than small displacements associated
with liquefaction settlement.

25.8.2 Leak versus rupture

In general, opinions on pipeline failure of full bore pipeline rupture given
exceedance of an ultimate strain limit range from 10% to 40%. For seismic
risk assessment, the potential for full rupture should not be taken below
20% and be biased toward a higher likelihood based upon the degree to
which the seismic ground displacements exceed the estimated pipeline dis-
placement capacity.

25.8.3 Ignition likelihood

The likelihood for ignition is largely a function of the built environment
near the pipeline. Ignition is more likely for pipelines located within the
right-of-way of high-voltage transmission pipelines or urban areas with
above-ground electrical lines. Opinions of ignition range from 10% to over
30%, depending upon these factors and the amount of fl ammable that can
potentially be released.

25.8.4 Fire hazard from leak

The exposure to a fl ash fi re from a pipeline leak is dependent upon the size
of the leak which is generally assumed to be equivalent to a hole with a
diameter of 25-50 mm. In addition to the rate of material release from a
leak, the orientation of the leak can signifi cantly alter the exposure for
high-pressure pipelines. Caution is warranted when using simple programs
such as ALOHA which assume vertical release from pipelines. Proprietary