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中文摘要：
在钻井作业过程中，井口附近钻杆受到的拉力和扭矩往往达到整个钻柱的最大值，尤其是在上提钻柱解卡、震击解卡以及蹩钻、卡钻情况发生时，井口附近钻杆受力更为恶劣。针对井口附近钻杆的断裂问题，以某超深直井的实际工况载荷为例，将钻杆外表面的缺陷简化为横向半椭圆裂纹，以30CrMo作为钻杆材料并通过实验拟合得到其Ramberg-Osgood（R-O）本构模型，研究了拉力和扭矩组合作用下裂纹的弹塑性断裂特性。得到钻进工况下钻柱外表面裂纹扩展至不同深度的临界形状比以及塑性成分在J积分中所占的比例；定量分析了拉力波动和扭矩波动对井口附近钻杆弹塑性断裂性能的影响，发现扭矩波动对裂纹扩展的贡献远大于拉力波动，避免钻柱受到扭转冲击对延长钻柱寿命有重要意义。
英文摘要：
The drill pipe near the surface stands the largest tension and torsion in the whole drillstring especially during drill pipe sticking or bit bouncing. As an example,a 5 inch drill pipe made of 30CrMo that was near the well head of an ultra-deep straight well was analyzed here. Simplifying the crack of the drill pipe as circumferential semi-elliptical surface crack and using the fitted Ramberg-Osgood equation as constitutional relation,we simulated elastic-plastic fracture feature of the drill string under combined loading of axial force and torsion corresponding to its drilling behavior. The transition aspect ratios are determined for the given combined loading and the crack shape could be determined approximately corresponding to different crack depths. The contribution of the plastic J-integral is estimated and the effect of material nonlinearity on the crack propagation is described. Variations of J-integral as a function of axial force and torsional moment respectively are compared, and the dangers from tension and torque fluctuation are evaluated quantitatively. Compared with tension,fluctuation in torque contributes much more to crack propagation. It is an important significance for increasing drill string′s service life to avoid the drill string from severe torsion impact.
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