Study On Static And Dynamic Fracture Characteristics Of 3D Cracks On Drill String Surface

Drill pipe is an important part of drilling process, but drilling equipment is often one of the weakest link because of the sticking drill, limp drill, hop drill and other issues. In the failure accident of the drill string, the fracture of drill is the most important form, and it brings a significant loss of Oilfield. The crack propagation is the main form of fracture, while the semi-elliptical crack in outer surface is a quite dangerous one.This article makes use of abaqus to analysis the semi-elliptical crack in outer surface of5in drill string, and using30CrMo as the material and the fitted Ramberg-Osgood equation as constitutional relation,simulating the elastic-plastic fracture feature of the drill string which suffers the combined loading of axial force and torsion near the wellhead. It got the transition aspect ratios of the semi-elliptical surface cracks under different crack depth. The numerical studying indicates that the transition aspect ratio is smoother along with the extension of crack. When the relative crack depth is more than0.7, Jp increases rapidly. In addition, it verifies the applicability of the principle of superposition to the fracture performance of the outer surface semi-elliptical crack when the drill pipe suffered combined loading of axial force and torsion near the wellhead. By studying the rate of change of J integral with respect to tension and torque, we found that the fluctuations in torque contributes much on J compared with fluctuations in tension and the surface crack in drill string under torque fluctuations is more easily extended, so it is more dangerous to the drill string.When the depth of semi-elliptical crack in the outer-surface of the drill string gradually deepened, it becomes critical to penetrate crack. The J-integral of this type of crack near the deepest point will be much larger than in other regions. Thus, the deepest point of critical to penetrate crack will accelerate expansion. When the crack penetrates the wall thickness of drill string, the crack is still approximately half ellipse, and at this time the velocity of the crack propagation near the surface is larger than the points near the outer surface. When part of the penetrated crack continued expanding, the crack front will gradually become flat and it is approximately a straight line passing through the axis of the drill string.In terms of the dynamic characteristics of the fracture of the drill string, it determined a suitable time step for calculation based on an analysis of a rectangular plate with a surface crack, and according to the step, we calculated the dynamic characteristics when the outer-surface crack of the drill pipe is under stretching step load, torque step load and impact load. We found that in the case of stretching step load and torque step load, the maximum value of J’is respectively1.4and1.6. In addition, though calculating we found that the effect of the impact load in the joint of the drill string on the outer-surface crack is small.