Page 154 - Reliability and Maintainability of In service Pipelines

140 Reliability and Maintainability of In-Service Pipelines

since most of them have considered only one failure mode when studying the reli-
ability analysis of pipes. Moreover, the interaction of corrosion, as an environ-
mental factor, and loading has been covered to introduce a novel and realistic fail-
ure assessment method. In addition, categorizing failure modes into serviceability
and ultimate strength allows for more practical use of the probability of service-
ability failure and/or the probability of ultimate failure.
On the other hand, applying the time-dependent corrosion formula to the para-
meters in each limit state can approximate the amount that each limit state contri-
butes to the overall reliability of a pipeline. Hence, this study continues previous
research in this field by adapting existing pipeline design codes to deal with the
uncertainty associated with the time-dependent process of corrosion.
Further research can be done to address the situation where the pipeline is
being protected by a coating. Coatings and other corrosion protection methods
decrease the overall rate of corrosion and eventually affect the probability of fail-
ure of pipeline due to corrosion.
In this case study, a system reliability analysis for corrosion affected buried
steel water pipelines has been developed. Using the violation of the limit state
design criteria as the failure modes, the effects of external loads, pipe material,
and corrosion etc. were considered. A nonlinear corrosion model was adopted in
order to model the loss of wall thickness due to corrosion. Consequently, a time-
dependent system reliability model considering all the involved limit states was
developed, so that the probability of the pipeline system failure within time was
estimated.
Using the aforementioned system failure analysis, the overall likelihood of
pipeline failure was then determined by using a Monte Carlo simulation approach.
The results highlighted that the studied pipeline is not safe and the appropriate
repair or replacement time must be chosen based on the accepted probability of
failure (P a ) and also time of failure (t c ). Time of repair and/or rehabilitation of
steel water pipelines is of important benefit to pipeline engineers and assess
managers.
From the results, it can be seen that the corrosion coefficients are the most
influential factors among all the studied factors. Therefore, it is crucial for the
designers to consider the impact of corrosion, as the most determining factor in
the failure of pipelines, in the design stage to improve the service life of the pipe-
lines. Additionally, the results of the proposed method, allows asset managers to
plan an accurate maintenance strategy for the pipeline. Since maintenance strate-
gies mainly involve finding a balance between probability of failure and the cost
of reducing the risk, an optimal maintenance strategy can be obtained using the
probability of failure estimation from the presented method and cost information
which is determined by stakeholders.