Seismic risk assessment for oil and gas pipelines - Seismic Risk Analysis and Management of Civil Infrastructure Systems - page 697

software capable of more refi ned modeling of leak conditions may be neces-
sary to capture the effects of impeded fl ow or infl uence of jet direction on
thermal exposure.

25.9 Mitigation approaches to reduce risk to pipelines

Upon identifi cation of the geotechnical hazards and the resulting vulnera-
bility of a given structure, a combination of structural retrofi tting and/or
geotechnical remediation (ground improvement) is often considered in the
design of mitigation measures. In general, there are four options to improve
the performance of a given pipeline against an identifi ed geotechnical
hazard: (a) avoid the hazard by relocation; (b) isolate the pipeline from the
hazard; (c) accommodate the hazard by strengthening the pipeline or
increasing fl exibility; and (d) mitigate the hazard using ground improve-
ment. Although avoiding the hazard by relocation is the most effective
approach, this option is often not attractive because of prohibitive costs
associated with acquisition of pipeline right-of-way for realignment.
The potential for pipeline failure can be reduced by reducing exposure
of the pipeline to seismic hazard. Other than rerouting the pipeline to avoid
the hazard, there are two methods to reduce pipeline exposure. Horizontal
directional drilling techniques can be used to locate the pipeline below the
zone of ground displacement. This technique is most commonly used to
avoid lateral spread hazards at river crossings. In rare instances, aerial cross-
ings can be used to avoid hazards of limited size. Isolation of pipeline from
geotechnical hazard is also considered favorable in certain situations. Use
of isolation culverts, or above-ground supports, provide effective means of
isolating pipelines from ground movement hazards. The idea herein is to
provide a mechanism for the ground to 'slide past' or 'slide below' the
pipeline using a sliding support system. The above-ground isolation struc-
ture specifi cally designed to protect the Trans-Alaska Pipeline crossing of
the Denali fault performed successfully during the 2002 M7.9 Denali fault
earthquake, confi rming the suitability of isolation measures against geo-
technical hazards. Moreover, soil restraint acting on the pipelines can be
reduced by careful selection of pipeline trench geometry and backfi ll mate-
rial, low-friction pipeline coatings, and wrapping pipeline with two layers
of geotextile fabric, or placing a portion of the pipeline on the ground
surface.
The potential for pipeline failure can also be reduced by increasing the
pipeline wall thickness and material strength and also modifying the pipe-
line alignment to provide a more benefi cial angle between the direction of
ground displacement and the pipeline axis. The latter option is generally
only possible during the design of new pipelines because of extreme diffi cul-
ties in obtaining new right-of-way for existing pipelines.