Page 26 - Reliability and Maintainability of In service Pipelines

Introduction 15

accidental damage during installation, and disturbance, including unexpected
imposed loads, subjected to the pipes during traffic loads. This is supported by
the fact that clay pipes are rigid, and are not particularly prone to flattening and
deflection under loads.
One of the characteristics of clay pipes is the ability to maintain consistency in
terms of structure and function due to its physical properties. This is further
highlighted by the impermeability of clay pipes to the surrounding environment,
including a reduction in the risk of the leakage of pipe contents into the surrounding
soil. Therefore it is safe to suggest that vitrified clay can be resistant to aggressive
ground conditions including those that contain chemicals and aggressive soils.
Furthermore, vitrified clay pipe was proven to be the only type of pipe that is
resistant (over centuries of usage) to sulfide-based corrosion and corrosion trig-
gered by aggressive ground conditions. It is safe to suggest that the vitrification
process of a clay pipe has been proven chemically to act as a built-in protection
mechanism against any deterioration and strength loss. This concludes that vitri-
fied clay pipes do not need a protective coating for protection against corrosion,
as these pipes have a long service life (Gladding McBean, 2017).

1.4 Design of Buried Pipelines

The design of buried pipes constitutes a wide ranging and complex field of engi-
neering, which has been the subject of extensive study and research in the world
for many years. There are two main stages for designing of pipes: (1) hydraulic
design, and (2) structural design. In the hydraulic design stage, the focus is on
determination of the demand of the system for collecting and/or conveying the
flow. Based on this, the diameter of the pipe is estimated. In the second stage,
focus is on determination of structural capacity or strength, including details like
wall thickness and/or reinforcement. This section discusses the structural design
of buried pipes. It introduces and compares different existing design methods.
The structural properties of the pipe are analyzed to ensure the pipe can safely
sustain external and internal loads during its service lifetime, without loss of its
function and without detriment to the environment.
A set of performance criteria must be met when the pipe is subjected to loads.
As for other structures, there are two categories of performance criteria for under-
ground pipes: ultimate limit state, and serviceability limit state.
The ultimate limit state is represented by the strength of the pipe and is
reached when the pipe collapses or fails in general. Flexural and shear failures are
the two main ultimate limit states that are considered in design and assessment of
pipelines (ASCE 15-98, 2000). Serviceability limit states may be measured by