Crack Failure Analysis of a Spiral Drill Collar

Cracks were found at the step root of elevator groove of a batch of spiral drill collars before underground service. Configuration dimension detection, nondestructive testing, chemical composition analysis, mechanical properties testing, metallographic examination, XRD, Scanning electron microscope (SEM) were used to comprehensively investigate the failure reason of the spiral drill collar. The results show that configuration dimension, chemical composition, tensile properties, Charpy impact, and hardness test results of the failed spiral drill collar meet the requirements of the spiral drill collar ordering technical standard. The energy spectrum test results of the crack position show that the cracks are not caused by environmental corrosion. The finite element analysis (FEA) method was used to analyze the stress concentration factor of elevator groove and simulated crack location. The FEA calculation results show that the stress concentration factors at the bevel and crack of elevator groove are greater than 2.9 and 10, respectively. The step root of the elevator groove was subjected to complex loads during the underground service, including periodic alternating axial tensile compressive stress, torque, bending and other loads, as well as local uneven friction. The maximum borehole diameter of the well is 444.5 mm, while the diameter of the drill collar is 203.2 mm. There is a large gap between the borehole wall and the drill collar, which is easy to bend the drill string at high speed, accelerating the fatigue crack under the operation of alternating bending stress. Therefore, the cracks at the step root of the elevator groove of this batch of spiral drill collars belonged to fatigue failure originated from the concentration of stresses which was caused by structural mutation at the step root of elevator groove of spiral drill collar and cyclic alternating load action in drilling operation. Suggestions to prevent such failures are proposed.