Investigation on Cracking Cause of Drill Pipe Joint Used in an Ultra Deep Well

Abstract

The drill pipe joint cracked during service, resulting in joint failure. The geometric dimensions of thread, chemical composition and microstructure of the drill pipe joint were tested. The geometric dimensions, chemical composition and microstructure were found to comply with the standard requirements. The whole macroscopic fracture didn’t show obvious corrosion product. The tensile properties, Charpy impact properties, and Brinell hardness met the standard requirements. Microstructure of fracture was analyzed, which showed a typical erosion micromorphology with parallel arranged fatigue strips. Through simulation analysis, it was found that the location of cracking was the stress concentration area. The failure mechanism of the drill pipe joint was revealed. The stress concentration led to fatigue cracking at the drill pipe joint. After the fatigue crack penetrated through the wall thickness, the drilling fluid leaked from the inner wall to the outer wall. The flow rate of the leaking drilling fluid is significantly increased. And it is sprayingly leaked, which further leads to fan-shaped erosion damage of the cracked surface from the inner wall to the outer wall.