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92 Reliability and Maintainability of In-Service Pipelines

for CI pipes using the assessment of residual tensile strength based on pit charac-
teristics and fracture toughness. Results illustrated that the proposed models using
tensile strength and fracture toughness of CI pipes successfully estimated the
residual life of water pipes. Analysis of the results showed that the determination
of fracture toughness may be more reliable than considering only the pit depth.

3.6.2 PROBABILISTIC MODES

Probabilistic models are often used when historical failure or inspection data is
limited or unavailable. These models specifically analyze the effective parameters
on pipe performance rather than evaluating the previous pipe failure history.
Uncertainties are involved by considering random variables. Usually, these meth-
ods are applied to pipes where the process of deterioration and factors for failure
are well understood.
Various frameworks have been proposed to model the behavior of under-
ground pipelines for different types of material, using the reliability-based con-
cept. Ahammed and Melchers (1994, 1995, 1997) reported a comprehensive and
continuous study on the reliability analysis of underground steel pipelines. To
consider uncertainty associated with the rate of corrosion and the uncertain loca-
tion of its occurrence, they used a probabilistic approach (first-order second-
moment reliability method, FOSM) for the analysis of pipeline reliability.
In 1994, they defined the failure mode as exceeding the sum of total stresses
from the maximum allowable stress (yield strength of the pipe). They considered
three types of existing stresses caused by internal pressure and external pressure:
The circumferential stress due to internal fluid pressure.
The bending stress in the circumferential direction produced in the pipe wall
by external soil loading.
The circumferential bending stresses produced in the pipe wall due to external
traffic loads.
Taking into account an empirical time-dependent corrosion model, resulted in
a nonlinear limit state function that required an iterative solution technique for
the calculation of reliability index and for sensitivity analysis.
In 1995 they modeled the growth of corrosion pits to assess the service life of
liquid carrying metallic pipelines. In the pitting (localized corrosion) model, two
corrosion related parameters (including pipeline dimension and liquid flow) were
treated as probabilistic variables.
The limit state function was defined as the difference between the allowable
fluid loss and the estimated fluid loss through the pit holes.