Investigation and analysis of fracture failure of TS-110 coiled tubing used in oil and gas field in southwest China

TS-110 coiled tubings used in drilling operations rarely fracture and fail after only five cycles. In this study, a failed coiled tubing was analysed by conducting various experiments. The experiments included macroscopic observation, outside diameter and wall thickness measurement, physical and chemical inspection, magnetic particle inspection, Vickers hardness test, optical microscopy, scanning electron microscopy, and energy dispersive spectrometry. The results revealed a band structure composed of granular bainite and martensite at the centre of the wall thickness. In addition, the formation of martensite influenced the uniformity of local tensile properties. During coiled tubing operations, martensite with undesirable plasticity cracked and formed microscopic voids which continued to expand. Finally, when the coiled tubing was lifted to the tubing reel for bending deformation, the tensile stress on the outer arc surface reached its maximum value. Moreover, the cracks in this part first penetrated the wall thickness, which triggered cracking and tubing failure.