In a recent geometry inspection project Inspipe Integrity Ltd. encountered a phenomenon that has been known to the pipeline industry for many years. The subject of Pressure Reversals has been studied by many researchers over the years and even today it can still pose a challenge to pipeline operators and inspection companies. It is a complex topic and we will return to it in more detail in future media releases from Inspipe Integrity.
A Pressure Reversal can be defined as:
“… the occurrence of a failure of a defect at a pressure level that is less than the pressure level that the defect has previously survived.”
A pressure reversal is usually due to defect growth produced by the previous higher pressurization and possible subsequent damage upon depressurization.
In the case of the geometry inspection survey conducted by Inspipe Integrity Ltd., the pressure reversal occurred in a dented region of the pipe where the radius of curvature of the wall differed from the average radius of the pipe. Because Inspipe Integrity Ltd is an integrity specialist rather than only a conventional inspection company we were able to understand the true nature of the pressure reversal and its implications, underlining why inline inspection must always be driven by integrity management principles and expertise.
Dents can occur during manufacture, shipping, installation or in-service operation of the pipe. Those occurring during in-service operation are of special concern because they are often caused by third party impact events and may not be brought to the attention of the pipeline operator for inspection and remediation.
A considerable amount of research and analysis on dents has been carried out. Studies show that for dents in pressurized pipes up to d/D = 24% (d = depth of the dent root; D = pipe diameter) there is only a negligible effect on the pressure to rupture the pipe. This applies mainly to ductile pipes that fail by plastic yielding. Therefore the presence of a smooth, plain dent in an otherwise undamaged, ductile pipe should not require any reduction in pipeline maximum operating pressure. However, since the stress and strain needed to create a dent are quite large, embrittled pipes, corroded pipe, and old electric resistance welded (ERW) seams may fail during the denting process.
Of even greater concern than instantaneous failure is the possibility that the dent might make the pipe more susceptible to fatigue failure. Under certain circumstances, pressure cycling of dents can initiate cracks and grow them to failure. This is because the dent can locally magnify the stress to levels well beyond the maximum operating pressure-induced stress.
It should be noted that such stress magnification effects are usually confined to small diameter pipes with large D/t ratios. In addition, the pressure cycling is usually such that re-rounding of the dent does not occur. If re-rounding occurs the stress magnification of the dent is no longer effective.
By Derek Storey
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