Analysis of the Failure of a Coiled Tubing under the Influence of Pitting Corrosion and Fatigue at the Weld Zone

This study investigates the causes of failure in mobile casing pipes, focusing on pitting corrosion and its impact on the welded areas. The results from microscopic analyses and mechanical tests indicate that pitting corrosion, particularly in the weld region, is the primary factor responsible for the reduction in strength and the initiation of fatigue cracks in the pipes. This type of corrosion occurs on the internal surface of the pipes, especially in the welded regions, under the influence of factors such as chloride ions, carbon dioxide (CO₂), and other corrosive pollutants. Pitting corrosion leads to a reduction in the pipe thickness and the concentration of localized stresses in the damaged areas, which can accelerate the fatigue process and eventually cause pipe failure. Microscopic examination of the fracture surface, including Scanning Electron Microscopy (SEM), revealed that cracks typically originate from corroded areas and gradually propagate along the pipe. The weld region is particularly vulnerable to pitting corrosion due to structural inhomogeneity and the presence of residual stresses, often acting as the primary weak point in mobile casing pipes. Based on the obtained results, the use of protective methods such as coating, selection of corrosion-resistant materials, optimization of welding processes, and control of residual stresses can effectively prevent failures due to corrosion and fatigue, thus extending the service life of the pipes

کلیدواژه ها:

Pitting corrosion ، Weld region ، Fatigue crack ، Casing pipe failure