CONICET | Buscador de Institutos y Recursos Humanos

This paper describes the fractographic and metallographic characterization of circumferentially welded pipes, failed under a biaxial stress state of longitudinal traction and internal pressure. The pipes are samples from pipelines that were recently constructed to transport natural gas and natural gas liquids from the Amazon basin, over deep forest and the Andes. Girth welds were done with a SMAW procedure utilizing cellulosic consumables for the root, fill and cap. Since installation of these lines, geotechnical instability have given rise to a number of failures by crack propagation along Heat Affected Zone of (in-field) girth welds. In response to these failures, comprehensive Root Cause Analyses were carried out, including small-scale and full-scale tensile strain investigations. Analysis of data from construction and filed inspection, assessment of previous failure analyses, mechanical models and experimental tests allowed recreation and understanding of most of the conditions that led to the failures. Root causes are related to events during construction and early service, mainly due to soil movement Pressurized strain to failure tests were conducted on welded joints. As a result of this study, it is shown that welded pipelines have a very ductile behaviour, although most crack characteristics of real failures were duplicated and explained by experiments, little stable cracking was obtained experimentally. Laboratory testing was able to duplicate the through the thickness cracks from outer weld toes, but did not duplicate the staged cracking mechanism seen in some of the failures. The mechanism of this staged cracking is still matter of speculation. It is now believed that staged crack growth, as seen in some of the analyzed failures, is only possible under conditions of very slow strain rates. This is due to the different behaviours of coarse grain HAZ and the rest of the tube material.