Collapse of internally corroded pipe under external pressure
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This thesis is concerned with the effect of internal corrosion or erosion defects on the collapse of pipelines under external pressure. Experiments are conducted on SS-304 tubes with D/ts of 21.0 and 18.7 with axially uniform grooves of several thicknesses and angular spans of 6 − 60°. All specimens exhibited the classical ovalization mode of collapse with the grooved side being more deformed. For groove angles between 6° and 20° the collapse pressure decreases as the angle increases but remains essentially constant for angles between 20° and 60°. The collapse pressure was found to decrease with groove depth, reducing by nearly 50% when the groove depth reached 50% of the pipe wall thickness. The experiments were simulated numerically using both the custom computer code BEPTICO as well as an axially uniform finite element model. The predictions were found to follow the trends of the experimental results and to be usually somewhat lower than the measured collapse pressures. Using a 3-D finite element model that includes the seals used at the ends of the test specimens it was confirmed that the seal provided some rotational constraint that increased the measured collapse pressure. The three-dimensionality of grooves was examined using appropriate finite element models. It was found that grooves shorter than about 8D constitute local imperfections and accurate prediction of their effect on collapse pressure requires fully 3-D modeling. By contrast, grooves 10D long or longer behave essentially as "long" grooves and can be analyzed using 2-D models.