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Pipe and Pipe Installation Considerations 137

4.4.2 Failure Modes in Flexible Pipe
Flexible pipe failures occur when the performance limits for the pipe
material are reached. Performance limits for flexible pipes refer to
(Moser and Folkman, 2008):

• Wall crushing
• Wall buckling
• Deflection
• Strain limit
• Longitudinal stress
• Wall loss (due to corrosion)

Wall Crushing
Wall crushing occurs when the in-wall compressive stresses reach the
yield stress or the ultimate stress of the pipe material. This type of
failure occurs when pipe is embedded in very stiff material and sub-
jected to very high loads (Massicotte, 2000). It is characterized by
localized yielding at the springline of the pipe and usually occurs in
smaller-diameter pipes with thicker walls. Figure 4.15 provides an
illustration of this type of failure. Normally ring compression is the
influential stress causing this type of failure, although bending
stresses may also cause wall crushing (Moser and Folkman, 2008).
Wall Buckling
This type of failure occurs in pipes with low stiffness embedded in
low stiffness soils. It is usually caused due to the pressure around the
pipe or creation of vacuum inside (Moser and Folkman, 2008). Design
of large-diameter pipes may be governed by buckling, especially in
conditions of high soil pressure and low stiffness soil (Reddy, 2002).
Figure 4.16 gives an illustration of this phenomenon. Buckling is pri-
marily dependant on the stiffness of the pipe, which in turn depends

FIGURE 4.15 Wall crushing of fl exible pipes. (Moser and Folkman, 2008.)