新型螺杆钻具连接螺纹抗疲劳性能

Anti fatigue performance of connecting thread of new type positive displacement motor

以油田现场使用的螺杆钻具壳体连接螺纹为基础,设计出了一种特锥连接螺纹。并利用有限元分析软件,分析了特锥螺纹在上扣扭矩、上扣扭矩+轴向载荷及上扣扭矩+轴向载荷+弯矩载荷下的螺纹受力情况,并通过应变测试证明了分析结果的准确性;利用疲劳强度分析软件分析了特锥连接螺纹在弯矩载荷和轴向载荷作用下的疲劳寿命,并通过下井试验证明了新设计螺纹的有效性。发现在上扣扭矩+轴向载荷+弯矩载荷极限复合载荷情况下,影响螺纹工作性能的顺序为上扣扭矩>弯矩>拉伸载荷,螺纹拧紧后,部分螺纹牙的Mises应力值已经超过屈服强度,材料发生塑性变形,但一定程度的拉力载荷会减轻螺纹负担,弯矩会使接头部分的螺纹结构承受更大的压力;在交变拉力载荷和交变弯矩载荷分别循环作用10000次的情况下,特锥螺纹仍具备一定的工作能力,在两种轴向循环载荷的作用下,公螺纹安全系数大于1.8的部位均超过99.45%,母螺纹安全系数大于1.8的部位均超过98.55%,现场下井试验表明,在纯钻时间达到近50 h后,未见异常,可继续下井使用,证明特锥螺纹的疲劳可靠性。

A special taper connecting thread was designed based on the connecting thread of positive displacement motor shell used in the oil field.Using the finite element analysis software,the thread stress of special taper thread under the make-up torque,make-up torque+axial load and make-up torque+axial load+bending moment load was analyzed,and the accuracy of the analysis results was proved by the strain test;the fatigue life of special connecting taper thread under the action of bending moment load and axial load was analyzed by fatigue strength analysis software,and the effectiveness of the new designed thread was proved by field well test.It is found that under the ultimate composite loads of make-up torque+axial load+bending moment load,the order of influence on the working performance of thread is make-up torque>bending moment>tensile load.After the thread is tightened,the Mises stress value of some thread teeth exceeds the yield strength,and the material has plastic deformation.But a certain degree of tensile load can reduce the thread load and bending moment can make the thread structure in joint part bear greater pressure.When the alternating tension load and the alternating bending moment load are applied for 10000 times respectively,the special taper thread still has certain working capacity.Under the action of two kinds of axial cyclic loads,the parts where the safety factor of male thread is more than 1.8 exceed99.45%,and the parts where the safety factor of female thread is more than 1.8 exceed 98.55%.The field well test shows that after pure drilling for nearly 50 h,no abnormality is found,and it can still be used in the well,which proves the fatigue reliability of special taper thread.