Non-linear finite element modeling of girth welded pipes under biaxial loading conditions

Offshore pipelines are often installed by the reeling method which introduces large plastic strain in them. These pipelines will also need to be able to withstand loading due to impact, restrained thermal expansion, high differential movements in pipe lines due to seismic ground motion are situations when high strain situations may occur during the operation lifetime of the pipelines. So, it is very important to assess the acceptability of the girth welded pipeline Further the steel pipes are joined by girth welding which may contain both surface and embedded cracks. The pipe lines should have adequate resistance against both crack extension by tearing and unstable fracture during installation as well as during operation. However, common flaw assessment procedures, e.g. BS 7910:2005 [10], are not explicitly developed for such situations with large plastic strains. The main aim of this study is the evolution of crack tip opening displacement (CTOD) of the pipeline with semi-elliptical surface and embedded crack in weld girth is investigated under tensile loading and bi-axial loading conditions of (tensile and internal pressure) using a three dimensional elastic-plastic finite element method. The effects of crack depth, ratio of semi-major axis to semi-minor axis and internal pressure are examined. The results show that at moderate levels of global strain, the variation of CTOD with global strain can be well approximated by a simple linear relationship under tensile loading as well as biaxial loading conditions. The result is compared with that of the current finite element method. A fracture criterion is given and a user-friend strain-based fracture assessment is made for the pipeline when the global strain is 3%.Bachelor of Engineering (Mechanical Engineering