Seismic risk assessment for oil and gas pipelines - Seismic Risk Analysis and Management of Civil Infrastructure Systems - page 700

Prediction of the anticipated geotechnical hazard and liquefaction-
induced permanent ground displacements are critical considerations in the
design of remedial measures. These predictions are often undertaken using
approaches combining both numerical and empirical methods.
25.10 Challenges and issues
25.10.1 Pipeline strain versus size of rupture
Establishing pipeline strain criteria that relate pipeline strain to a probabil-
ity for loss of pressure integrity is an ongoing area of active research (PRCI,
2011). For new pipelines, establishing acceptable tension strain limits is
often based upon the results of full-scale or wide-plate testing of girth welds
with or without machined fl aws to represent defects that may be missed by
weld inspection procedures. However, the number of tests performed is
generally not suffi cient to provide a statistical relationship between tension
strain and failure probability. There are no generally accepted methods for
relating pipeline strain to the severity of the pipeline failure mode. As a
result, establishing strain criteria must be based upon expert judgment.
25.10.2 Geotechnical constraints
The available data on soil and groundwater conditions along pipeline align-
ments are typically limited. As a result, characterization of sites for soil-pipe
interaction often becomes a challenging task. In addition, high variability
of soil conditions along pipeline alignments combined with complexities
associated with the mechanical behavior of soil (e.g., stress and strain level
dependence, effects of particle fabric, effects of loading paths.) present
additional constraints to the level of accuracy attainable in seismic perfor-
mance evaluations. With respect to lateral spread displacement hazards,
some of the basic uncertainties in defi ning the hazard are illustrated in
Fig. 25.5. Sensitivity of the geotechnical hazard estimates to the above con-
siderations along with the uncertainties arising due to the diffi culties in
accurately defi ning earthquake shaking (i.e., ground acceleration time his-
tories) should be kept in mind when interpreting outcomes from hazard
assessments.
25.10.3 Condition of older pipelines
A key diffi culty in assessing the vulnerability of existing pipelines is the
uncertainty in estimating the tension strain capacity of the existing girth
welds. Compression strain capacity is governed by buckling of the pipe wall
and is not highly dependent upon girth weld quality. For older pipelines,