Investigation of fracture behaviour of pipelines containing flaws subjected to plastic straining in the presence of yield discontinuity

Abstract

An increase in the demand of fossil resources has driven the construction and operation of oil and gas pipelines in remote and harsh environments, such as seismically-active, Arctic and deep-water regions. Pipelines installed and/or operated in these regions are subjected to large plastic straining, which poses a threat to the integrity of pipelines containing crack-like flaws. Pipeline steels often exhibit a yield discontinuity, known as the Lüders plateau, which further complicates analyses and assessments of pipelines containing flaws. This thesis aims to investigate the fracture behaviour of pipelines subjected to plastic straining, and to provide guidance for analysis and assessment of flawed pipes in the presence of yield discontinuity. This work contained both experiments and numerical analyses, including uniaxial tensile tests with and without notches, and single edged notch tension (SENT) tests, with the utilisation of Digital Image Correlation (DIC) techniques for strain measurement. The fracture behaviour of pipelines containing flaws with Lüders bands were further examined numerically. Finite element analysis (FEA) of pipelines containing circumferential flaws subjected to both uniaxial and biaxial loadings were conducted, respectively. Main contribution to the knowledge includes optimised material models identified by comparing numerical results against the published full-scale test results, and thereafter the influence of various flaw sizes and the pressure-induced stress biaxiality on the development of Lüders bands and the crack driving force of the flawed pipes. A clear understanding is achieved for the significant effects of the deforming and fracture behaviour of pipelines containing flaws subjected to both uniaxial and biaxial loading conditions, and in turn, of the flaw sizes and stress biaxility on the formation and development of Lüders behaviour. The findings provide enhanced design and operation guidance for pipelines with improved structural integrity.