Page 37 - Reliability and Maintainability of In service Pipelines

26 Reliability and Maintainability of In-Service Pipelines

capacity of pipes, it can lead to damage and failure, presented in the form of “per-
formance limits” which include leaks, ruptures, erosion, stress corrosion cracking
issues (SCC), and fatigue.
Before understanding these performance limits it is important to recognize the
difference between what factors simply contribute to performance limits and what
factors cause and present an end result of performance limits. An example of this
would be the loads and stresses on pipes which define the conditions for perfor-
mance limits which will contribute to ring or longitudinal deformation of pipes.
As a result, these deformations will eventually lead to cracks, leaks, ruptures, and
collapsing of pipes, which are defined as performance limits.
However before gaining an understanding of this concept, it is useful to iden-
tify that this logical idea actually lies within the pipe soil interaction mechanism.
The pipe soil interaction mechanism is essential as soil is not only considered as
a load on pipes, but it is a component of the pipe structural system. Therefore,
this mechanism is what defines the factors that contribute to performance limits
and ultimately what causes them.
The structural performance depends on the pipe soil interaction. In this mech-
anism, the properties of soil play a significant role in keeping the pipe in shape
and in place. Both the stiffness of the soil and the pipe ring are defined as resis-
tance to deflection with soil stiffness defining most of the resistance to deflection
of flexible steel pipes and ring stiffness ensuring the pipe maintains its shape dur-
ing the implementation process of the embankment and compaction. However,
ring stability of pipes can undergo spontaneous deformation causing instability,
which becomes a performance limit. This ring instability can then cause pipes to
invert if the deflection of the pipe ring and soil slippage occurs simultaneously,
causing a change in structure and function of the pipe. Therefore, the mechanism
by which ring stability is maintained is through the pipe soil interaction.
As pipes are subject to external loads, surface loads, construction, and hydro-
static and soil pressures, having a required minimum soil cover is critical. The
minimum soil cover is important for preventing damage from the external loads
as well as damage from weather conditions. To further explore this issue, the
stress and strain relationship in the design of buried flexible pipes is analyzed. It
is understood that soil compression is defined as soil strain and in excess, causes
the shifting and deflection of pipes. To prevent this action, it is essential that the
embedment does not exceed the compression limit as it results in nonuniform
bedding. This causes adjoining pipes to deflect at a joint, resulting in leakage and
possibly rupture. The excessive compression also causes a flattening at the bottom
of the pipe, which alters the ring deflection ratio (Δ/D) of the flexible steel pipe.
This ratio states that the ring deflection of the pipe is almost equal to the vertical
strain of the side-fill embedment. Given that strain is a function of stress and its
relationship is nonelastic, it is observed that in pipes subject to any stress,