Analysis of failure causes of S135 drill pipe

Abstract

The drill pipes must feature sufficient strength to prevent its longitudinal bending under the action of axial loads, centrifugal forces, and torque. In the lower zone, where the stress is compressive most frequent occurrence of fatigue cracks in drill pipes. The task of the study is to investigate the reason for the premature failure of the grade S135 steel drill pipes, used for drilling oil and natural gas, which failure before reaching the permissible load cycles at a depth of about 4500 m. For this purpose, macrostructural and microstructural analysis and mechanical tests of tensile strength, impact toughness, and fatigue strength were conducted.From the performed microstructural analysis the presence of metallurgical defects (micro and mesopores) has been established, which significantly reduces the steel quality and contribute to the crack source creation leading to premature failure of drill pipes.After the mechanical tests conducted and the subsequent microstructural analysis the presence of macro defects due to pitting corrosion has been established. The active interaction of the dispersed soil primer with the macropores creates conditions for the development of a cracks on the outer surface and is also a reason for the premature failure of drill pipes.This problem can only be solved at the production stage- to be specify the casting technological parameters of billets from which the drill pipes are produced.