[Univ of Cambridge] [Dept of Engineering]

Behaviour of pressurised pipes containing dents and gouges

 E.R. Lancaster


Mechanical damage to oil and gas pipelines is considered to be the most serious cause of in-service failures in real pipelines. Damage in the form of combined dents and gouges may be of arbitrary shape and size and has from time to time resulted in failures at very low pressures. Full size experiments on pipes with dents and gouges have identified dent size, gouge size and pipe material fracture toughness as the main factors which influence failure. These tests have however exhibited much scatter.

This experimental study has focused on understanding the problem through small-size testing on model pipes. An experimental model material was selected and pipe dimensions were carefully scaled to ensure that the small-size pipe was an exact model of a real pipeline. Damage was also appropriately scaled but cracks were specifically excluded.

Model testing concentrated on 'short dents'. The gouge length, gouge depth, dent depth, gouge position and gouge orientation were varied extensively. A few tests were carried out with dents formed under non-zero pressure. Tests showed that the position of the gouge relative to the dent, and the gouge depth are the most relevant parameters affecting failure pressures. Zones of high straining were identified using photoelasticity and strain gauging. The off-centre and long gouges which failed at low pressures were observed to pass through these regions of high straining. Nonetheless, failures occurred at pressures generally higher than those observed in full-size tests and very little scatter was observed.

From comparison of the full-size and small-size tests, the importance of initial cracks was established. Cracks are now believed to be responsible for the transition from a stable yielding type failure to a fracture dependent type failure in combined dent and gouge defects resulting in low pressure failures.

The Battelle Flow Stress Dependent Model for gouges of infinite length has been shown to be the best method for predicting failure in pipes with short central axial gouges. Model testing, using an equivalent pipeline steel and the correct D/t ratio, is the cheapest known method of determining burst pressures and understanding the behaviour of pipes containing dents and gouges.

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