Seismic risk assessment for oil and gas pipelines - Seismic Risk Analysis and Management of Civil Infrastructure Systems - page 691

located in generally fl at areas, these settlements need careful attention in
some situations. For example, plant structures, compressor stations, etc.,
located in soft/loose soil zones may require pile-supported foundations.
Under such conditions, there is a potential for post-liquefaction settle-
ments leading to major differential settlements between pile-supported
structures and pipelines. Thus, provision should be made for adequate fl ex-
ibility of pipeline to minimize potential pipeline breakage due to differen-
tial settlements.
Studies conducted to evaluate volume change after cyclic loading have
shown that in general, the factors controlling post-cyclic settlements in
sands are the degree of excess pore water pressure generation and the
induced cyclic shear strain. Owing to the direct connection with excess pore
water pressure development, potential for volumetric strains has been often
linked with the fi eld density. Several simplifi ed methods have been pro-
posed to estimate settlements of soils knowing the fi eld penetration resis-
tance (i.e., standard penetration resistance N -value or cone penetration
testing resistance) or laboratory cyclic shear resistance and the cyclic stress
ratio corresponding to the level of ground shaking being considered (e.g.,
Tokimatsu and Seed, 1987; Wu, 2002; Wijewickreme and Sanin, 2010).
Much larger vertical movements are expected at river crossings, in the
vicinity of dykes, ditches, road embankments, etc., due to distortion of the
soil mass. Estimation of such vertical deformations would require rigorous
site-specifi c analyses.
25.7 Pipelineresponsetoearthquakehazards
The analytical assessment of buried pipeline response to permanent ground
displacement dates back to the mid-1970s. Current recommendations on
the methodology for assessing pipelines for ground displacement are con-
tained in PRCI guidelines (Honegger and Nyman, 2004; PRCI, 2009). Non-
linear fi nite element techniques are the only practical means available to
analyze all but the most simple of problems. Methods relating generic
ground displacement values to rates of pipeline damage, such as those
incorporated into HAZUS risk assessment software (FEMA, 2011) and
presented in American Lifelines Alliance guidelines (ALA, 2001b), are
totally inappropriate for assessing oil and gas pipeline damage.
Finite element approaches provide a means to investigate the effects of
changes in backfi ll characteristics, pipeline material, wall thickness, and
pipeline alignment. The mechanics of implementing a fi nite element analy-
sis have remained largely the same over the last 30 to 40 years. The primary
advancement in performing such analyses is the availability of powerful
desktop personal computers and compatible nonlinear analysis software
that accounts for material yielding and large deformations.