钻杆接头多轴疲劳寿命

在实际钻井过程中，钻杆接头以螺纹连接处的疲劳破坏为主，导致钻井成本显著增加。现有对钻杆接头的研究主要集中于钻杆接头的静力学特性分析，少有涉及钻杆接头的疲劳破坏的研究。基于虚功原理、Von Mises屈服准则及接触非线性理论，考虑螺纹升角的影响，建立了钻杆接头的三维有限元计算模型，分析了钻杆接头的上扣特性及其在复合载荷作用下的力学特性。基于材料S-N曲线，综合考虑尺寸系数、应力集中系数、表面加工系数及表面强化系数的影响，确定了钻杆接头的弯曲疲劳S-N曲线理论模型，通过对比验证建立了满足工程要求的钻杆接头疲劳有限元计算模型。运用多轴疲劳算法，计算了钻杆接头在复合交变载荷下的疲劳寿命，分析了残余应力和表面质量对疲劳寿命的影响。研究结果表明，在上扣扭矩或复合载荷作用下，钻杆接头最大Mises应力均出现在公扣第1有效啮合螺纹牙根处；弯曲井段API钻杆接头的疲劳寿命较小，建议开发适合超深井、水平井、大位移井及大斜度井的专用钻杆接头；残余应力与表面质量对钻杆接头的疲劳寿命影响较大，在钻杆接头螺纹根部引入可控的残余压应力和改善表面质量可以大幅提高其疲劳寿命。

Multiaxial fatigue life of drill pipe joint

Fatigue damage mainly occurs at the threaded joints of drill pipe in the actual drilling process, leading to a significant increase in the drilling cost. In recent years, domestic and foreign scholars have done a lot of research on drill pipe joint, but mainly focusing on the analysis of its static characteristics, rarely involving the fatigue failure of drill pipe joint. In view of the above problem, considering the influence of lead angle, a 3D numerical simulation model of drill pipe joint has been created based on virtual work principle, Von Mises yield criterion and nonlinear contact theory, so as to analyze the screw-on feature of drill pipe joint and its mechanical property under composite load. Based on the material S-N curve, a theoretical model on the bending fatigue S-N curve of drill pipe joint has been established by comprehensively considering the influence of size factor, stress concentration factor, surface processing coefficient and surface strengthening coefficient. Through comparison and verification, this study establishes the fatigue finite element calculation model of drill pipe joint meeting the engineering requirements. The fatigue life of drill pipe joint under composite alternating load has been calculated by multi-axis fatigue algorithm to further analyze the effects of residual stress and surface quality on fatigue life. The results show that under the action of the make-up torque or multi-axis load, the maximum Mises stress of drill pipe joint appears at the root of the first effective meshing thread of pin; the fatigue life of API drill pipe joint in curved portion is short. It is recommended to develop suitable special drill pipe joints for ultra-deep wells, horizontal wells, large-displacement wells and high-angle wells; residual stress and surface quality have a great impact on the fatigue life of drill pipe joints. The introduction of controllable residual compressive stresses at the roots of drill pipe joints and the improvement of the surface quality can greatly improve the fatigue life.