Page 134 - Reliability and Maintainability of In service Pipelines

Case Studies on the Application of Structural Reliability 121

In Canada, for example, there are 34,000 km of oil pipelines and 26,000 km of
gas pipelines where the prevention of corrosion-related failures at reasonable
costs is the main concern (Sinha and Pandey, 2002). According to The World
Factbook (2010), the US has approximately 800,000 km and Russia has
252,000 km of pipes transporting products like crude oil, natural gas, and petro-
leum products. The statistics for the UK and Australia are 20,000 km and
32,000 km, respectively. In the USA, corrosion has caused 23% and 39% of fail-
ures of oil and gas pipelines, respectively (Anon., 2002). One of the incidents
caused by external corrosion was the leakage of an oil pipeline which spilled
more than 140,000 gallons of crude on the Santa Barbara coast in May 2015.
In this section a case study on reliability analysis of oil and gas pipelines will
be reviewed (Mahmoodian and Li, 2017). A stochastic model for pipe strength
loss is developed which relates to key factors that affect the residual strength of a
corroded pipe. A pipeline system fails when its residual strength falls below its
operating pressure. The first passage probability method explained in Section 4.2
is employed to quantify the probability of failure due to corrosion so that the time
for the pipeline to be failed, and hence require repairs, can be determined with
confidence. To deal with the assessment of pipelines with more than one corro-
sion pit, system reliability analysis method is employed. Monte Carlo simulation
technique is applied to verify the results of the analytical method. For an exten-
sive reliability analysis, evaluation of the effect of various parameters on the pipe-
line structural reliability also is carried out via a parametric sensitivity analysis.
The proposed methodology provides a rational and consistent approach to make
quantitative assessment of pipeline failures.
A 2.5-kilometer length of crude oil pipeline in the US, made of grade X60
steel, is considered with 762 mm outer diameter (DN750) and 7.92 mm wall
thickness. The maximum allowable operating pressure is 5.7 MPa. Inspection
results have shown three pits caused by corrosion. The pipe characteristics, corro-
sion information, and the geometric information of the corrosion pits are pre-
sented in Tables 5.1 and 5.2.

5.1.1 PROBLEM FORMULATION

The practical failure criterion for corroded pipes is that residual strength falls
below the operating pressure. As discussed in Section 3.2, this criterion can be
expressed in the form of a failure function as follows:

GQ; P o ; tð Þ 5 QtðÞ 2 P o ð5:1Þ
where QðtÞ is the residual strength (structural resistance) at time t and P o is the
operating pressure (load effect). The residual strength QðtÞ decreases with time