Page 30 - Reliability and Maintainability of In service Pipelines

Introduction 19

failure. Although the addition of pipe coatings and linings can slow down
pipe deformations, small cracks still tend to appear in these coatings and lin-
ings. However, these cracks are not critical, providing they are small, as moist
environments close the cracks by the formation of calcium carbonate
(CaCO 3 ) in the cement. The size of the cracks is determined by a comparison
to the thickness of a dime, with cracks being no bigger than a dime consid-
ered small.
The mechanism, in which the cracks are closed, is due to the pressure in the
pipe, which causes the pipe to deflect, forming a more rounded shape and ulti-
mately “healing” the cracks.
However, low pressures and gravity flows inside pipes can result in permanent
ring deformation of the pipe causing the cracks to expand in size. This can cause
water exposure to the steel, thereby resulting in corrosion of the pipe.
Cracks wider in size are known to occur in the tensile zones of pipes, however
in the case of coatings, the cracks typically occur at the spring line, as shown in
Fig. 1.2. Cracks in the linings of pipes are known to occur at the crown and
invert.
Cracks in pipes are determined by taking the width (δ) of the widest single
crack and a calculation based on the location of the crack via the equations below
(Whidden, 2009):
δ 1 1
5 2 coating ð1:7Þ
2w c r min r

Figure 1.2 Crack width, δ, at the springline in a cross-section of a pipe.