NUMERICAL AND EXPERIMENTAL STUDY OF ELLIPTICAL CRACKS IN THE CRITICAL POSITIONS OF PIPELINE

Abstract

Hydrocarbon’s transportation is a very significant aspect of the economic where it must be used with much efficacy. Pipelines are used to transport the aggressive fluids with large volumes over great distance. The structural integrity of piping system is very essential in order to guarantee the transportation continue of energy resources from production fields to their locations of exportation. Pipe elbow is a main component in the piping networks which considers as a critical and sensitive tool due to their stress intensification and the effect of bend curvature. They are more exposed to different corrosion failure modes than straight pipes. The aim of this thesis was to investigate the causes of cracks and defects that occur in the critical positions of piping systems. This work was divided in two axes; numerical investigation and inspection analysis. The numerical axis was performed in two sections; (i) effect of semi-elliptical cracks on the degradation of pipe elbow, and (ii) assessment validity of the international mathematical standards on the pipe bends. A rectangular corrosion defect at the intrados section was done and compared to different codes for calculating limit pressure. Moreover, the area with the corrosion cracks and defects with different relative defect depth to wall thickness ratios was FEM modeled at the intrados section of the pipe elbow where the highest hoop stress exists. The second axis was focused on a real case of pipe elbow which occurred to the erosion-corrosion phenomenon. Different inspection methods such as; (i) optical visualization, (ii) scanning electron microscope (SEM), (iii) X-ray diffraction (XRD) and (iv) X-ray fluorescence (XRF) test, were used to discover the main factors which were responsible for this failure. The obtained results proved that the critical position can be located in different zones according to the boundary and additional service conditions. The numerical results proved that the critical position was found at 72° of intrados side based on the value of Von-Mises stress and SIFs. In fact, the critical position could change with the presence of butterfly valve before the elbow. In addition, the increase percentage of Silicon (Si) on the corroded structure explained that the sand particles were responsible to the erosion-corrosion phenomenon.