Page 167 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

7/144 Leak Impact Factor

Contributing &Material type
Factors
Stress
Crack formation
Cycles
Material
Crack growth Temperature

Ductile
Brittle
Transition

Pressure
Failure Types Internal loading
Fatigue
Pinhole
Puncture External loading
Tear Stress Various
Crack
Rate of stressing
Ovality
Geometry Gouge
Stress concentrator

Lamination
+
Manufacture t
Crack
defect Detection
methods
Puncture.
Initiator Outside Tear W Depth
force Gouge Geometry
/see Dent

Corrosion

Surface, general
Figure 7.5 Sample of factors that influence failure hole size
Stresses
consequently, leak size. For relatively incompressible fluids,
High stress levels in a pipe wall are one of the most impor- decompression wave speed will usually exceed crack propaga-
tant contributing factors to a catastrophic failure. High stress tion speed and hence will not promote large crack growth. In
levels are a function of internal pressure, external loadings, other words, on initiation of the leak, the pipeline depressures
wall thickness, and exact pipe geometry. Mechanical pipe quickly with an incompressible fluid. This means that usually
damage (e.g., dents, gouges, buckles) and improper use of insufficient energy is remaining at the failure point to support
some pipe fittings (e.g., sleeve taps, branch connections) can continued crack propagation.
dramatically impact stress levels by causing stress concentra- The use of crack arrestors can also impact the risk picture.
tion points.. A crack arrestor is designed to slow the crack propagation suf-
The energy source should also be considered here. A com- ficiently to allow the depressurization wave to pass. Once past
pressed gas, due to the higher energy potential ofthe compress- the crack area, the reduced pressure can no longer drive crack
ible fluid can promote significantly larger crack growth and, growth. More ductile or thicker material (stress levels are