Page 92 - Reliability and Maintainability of In service Pipelines

Methods for Structural Reliability Analysis 81

service/performance requirements are measured by their reliabilities. The reliabil-
ity of a pipeline is defined as its probability of survival (Melchers 1999):
P s 5 1 2 P f ð3:1Þ
where

P s : probability of survival;
P f : probability of failure.
Failure can be defined in relation to different possible failure modes, com-
monly referred to as limit states. Reliability is considered to be the probability
that these limits will not be exceeded and is equal to the probability of survival.
Each of the limit state function variables is attributed to a probability density
function that presents its statistical properties.
To summarize, structural reliability analysis can be generally used for the fol-
lowing purposes:
Service life prediction of in-service pipelines, for funding allocation to most
critical parts of the pipeline or network.
Evaluation of the effect of repair, maintenance, and rehabilitation actions on
the service life of the pipeline (ability to examine the consequences of poten-
tial action or inaction relative to operational and maintenance procedures).
To be used at the conceptual design stage to evaluate various design choices
and to determine the impact that their implementation would have upon the
service lives.

To predict the service life of in-service pipeline, information is required on
the present condition of the pipeline, rates of degradation, past and future loading,
and definition of the failure of the pipeline. Based on remaining life predictions,
cost benefit analysis can also be made on whether or not a pipeline should be
repaired, rehabilitated, or replaced.

3.2 Theory of Reliability Analysis

In the past, the design of pipelines considered all loads and strengths as determin-
istic values. The strength of a pipeline was determined in such a way that it with-
stood the load within a certain margin. The ratio between the strength and the
load was denoted a safety factor.
This safety factor was considered as a measure of the reliability of the pipe-
line. However, uncertainties in the loads, strengths, and in the modeling of the
pipeline require that methods based on probabilistic techniques to be used. A
pipeline is usually required to have a satisfactory performance in the expected