Page 155 - Reliability and Maintainability of In service Pipelines

Case Studies on the Application of Structural Reliability 141

5.3 Case Study 3-Cast Iron Water Mains in the UK

In this section, the applicability of the gamma process concept introduced in
Section 4.3 for time-dependent reliability analysis is tested on a case study
involving cast iron water pipes in the UK.
Both internal and external corrosion are considered and the results are com-
pared and discussed. For a more realistic and reliable outcome both strength fail-
ure and fracture failure, in hoop and axial planes, are considered.
A cast iron pipe of 254 mm diameter and effective length of 6.5 m located in
South East London is considered as the case to illustrate the proposed method.
Other data for the cast iron pipe required for calculation is presented in Table 5.6
(Mahmoodian and Li 2018). In total 15 random variables are involved in the
problem and their statistical data have been presented in Table 5.6.

5.3.1 PROBLEM FORMULATION

5.3.1.1 Definition of the Limit State Functions
Cast iron pipes can fail in many modes which in general can be summarized
into two categories: loss of strength due to the reduction of wall thickness of the
pipes, and loss of toughness due to the stress concentration at the tips of cracks
or defects. Even in one category there can be many mechanisms that cause fail-
ure. The strength failure can be caused by hoop stress or axial stress in the
pipes. A review of recent research literature (Sadiq et al., 2004; Moglia et al.,
2008; Yamini, 2009; Clair and Sinha, 2012) suggests that current research on
pipe failures focuses more on loss of strength than loss of toughness. As was
mentioned in Section 3.3.7(b), the literature review also revealed that in most
reliability analyses for buried pipes, multifailure modes are rarely considered
although in practice this is the reality. Therefore the aim of this section is to
consider multifailure modes in reliability analysis and service life prediction for
cast iron pipes. Both loss of strength and toughness of the pipe are considered.
A system reliability method is employed in calculating the probability of pipe
failure over time, based on which the service life of the pipe can be estimated.
Sensitivity analysis is also carried out to identify those factors that affect the
pipe behavior most.
Buried pipes are not only subjected to mechanical actions (loads) but also
environmental actions that cause the corrosion of pipes. Corrosion related defects
would subsequently cause fracture of cast iron pipes. In the presence of corrosion
pit, failure of a pipe can be attributed to two mechanisms: (i) the stresses in the
pipe exceed the corresponding strength; or (ii) the stress intensity exceeds fracture