Seismic risk assessment for oil and gas pipelines - Seismic Risk Analysis and Management of Civil Infrastructure Systems - page 684

The typically seismic hazards to pipelines include:
• wave propagation;
• permanent ground deformation hazards:
• differential movements at topographic discontinuities or faults,
• slope failures,
• liquefaction-induced permanent ground deformations;
• volcanic hazards;
• tsunami inundation.
25.4.1 Wave propagation
Seismic wave propagation is a ground motion phenomenon that relates to
the passage of body waves, including compression waves and shear waves,
radially from the source of earthquake energy release (hypocenter) into the
surrounding rock and soil medium. Compression waves cause compressive
and tensile strains in the ground in a radial direction away from the hypo-
center. Shear waves cause shear strains in the ground perpendicular to these
radial lines. When the compression waves and shear waves are refl ected by
interaction with the ground surface, surface waves (Love waves and Ray-
leigh waves) are generated. Except at large distances from the epicenter,
the amplitudes of surface waves are much less than body waves. An earth-
quake at its source generates only compression and shear waves, and propa-
gation of its radiated waves can be evaluated using ray theory (Pujol, 2003).
Since the amplitude of shear waves is signifi cantly larger than compression
waves and thus generates greater strains in a pipeline, the examination of
wave propagation can be limited to the effects of shear waves. Also, the
strongest component of ground shaking in strong motion instrument records
used to derive attenuation relationships is typically from shear waves (Bolt
and Abrahamson, 2003).
A pipeline buried in soil that is subject to the passage of these seismic
waves will incur longitudinal and bending strains as it conforms to the
associated ground strains. In most cases, these strains are relatively small,
and welded pipelines in good condition typically do not incur damage.
Propagating seismic waves also give rise to hoop membrane strains and
shearing strains in buried pipelines, but these strains are small and may be
neglected.
25.4.2 Permanent ground deformation
Earthquake-induced permanent ground deformations have been recog-
nized as one of the major causes of system damage and associated service
disruption to lifeline facilities during past earthquakes (Hamada and
O'Rourke, 1992). Essentially, large soil loads arising from permanent ground