Cause Analysis on Piercing of the φ101.6 mm G105 Drill Pipe in a Well

Piercing failure accident happened to a φ101.6 mm×8.38 mm G105 steel grade drill pipe during drilling operation in a company. The chemical compositions, mechanical properties, microstructure and fracture surface of the pierced drill pipe were analyzed by direct reading spectrometer, tensile machine, optical microscope, scanning electron microscope and energy spectrometer. And combined with the actual well condition during the drilling operation, the piercing corrosion mechanism of the drill pipe and the finite element analysis of the thickened transition zone were analyzed. The results show that the piercing of the drill pipe was an early corrosion fatigue failure, and the fatigue cracks originated from the bottom of the corrosion pits at the inner wall of the drill pipe where the thickened transition zone disappeared. The failure drill pipe had no inner coating, and under the action of the alternating load, corrosion fatigue cracks originated from the bottom of the corrosion pits of the stress concentrated thickened transition zone disappearing position. The cracks continued to expand and eventually caused the piercing failure. Finally, according to the reasons of the early corrosion fatigue of the drill pipe, the corresponding improvement suggestions were put forward.