Fracture causes of S135 steel drill pipe in a well

A 101.6 mm (outside diameter) drill pipe at the bottom of a well fractured about 1.1 m from the shoulder of the common joint platform. The fracture causes of the drill pipe were analyzed by means of macroscopic observation, chemical composition analysis, mechanical property test, metallographic examination, fracture analysis, drill pipe force calculation and finite element simulation analysis. The results show that the yield strength of the fractured drill pipe did not meet the requirements of API Spec 5DP-2009 standards. When the drill pipe passed through an irregular borehole, it had serious bending and flattening deformation, resulting in the instability and fracture of the drill pipe. In the process of unlocking, the repeated tensile and compressive loads near the breaking point caused the 'Bauschinger effect' in the region, which led to the further decrease of yield strength and accelerated the fracture of drill pipe.