X80 Pipelines in Arctic Environment: Prediction of the Long-Distance Ductile Fracture Propagation/Arrest
Part of the proceedings of
Several pilot lots of heavy-wall 1420 mm OD X80 steel line pipes supplied by a number of world-leading mills were subjected to full-scale burst test according to the Gazprom technical requirements for “Bovanenkovo-Uhta” pipeline project. The test is passed if the crack is arrested within first three pipes in each direction. Despite of the fact that all the supplied materials have similar standard mechanical properties, not every lot stood the full-scale burst test. Analysis of the full-scale burst test results shows that the crack arrestability of steel depends on straining of the material adjacent to the running crack tip as it increases the specific energy of fracture propagation. It was noticed that low arrestability was accompanied by high intensity of fracture surface separation (splitting). Separation lowers the volume of the strained material (and thus the arrestability) due to “multiple necking”. Fracture mechanics and microstructure studies were carried out to develop new laboratory methods of the ductile crack arrestability prediction and to reveal metallurgical reasons for excessive separation, respectively. Two new laboratory test types giving results which correlate with the results of the full-scale burst test are proposed. Microstructure parameters leading to high separation intensity are quantified basing on X-ray diffraction and EBSD measurements with respect to the position across the wall thickness.
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