从钻铤事故谈钻铤螺纹的选型与加工

Selection and machining of drill collar threads

两起事故钻铤的断口均属钻铤连接螺纹处弯曲强度比不足引发的疲劳折断。通过对钻铤连接螺纹区域的工作应力定性分析，导出了内、外螺纹上各自的危险截面，提出了对钻铤平衡连接的理解。认为随意改变钻铤螺纹的规格和类型，不随钻铤外径磨损及时根据弯曲强度比减小钻铤螺纹的规格，是现场两种常见的因弯曲强度比不足导致钻铤早期疲劳失效的原因。经对数字型（NC）螺纹和正规型（REG）、贯眼型（FH）、内平型（IF）螺纹各自的应力集中系数Kf的比较，建议使用NC螺纹以降低应力集中，提高钻铤连接螺纹的疲劳寿命。还对影响Kf值的螺纹表面形状精度、粗糙度、表面组织进行了讨论，提出了提高螺纹表面质量的相应措施，以延长钻铤使用寿命。

The fatigue breaking of the drill collar was caused by insufficient flexural strength ratio at the connection threads. By a qualitative analysis of the working stress on the connection threads, the critical sections on the inside and outside threads are deduced, and the recognition of the balance connection of the drill collar is presented. It is indicated that varying the size and style of drill collar threads casually and not decreasing the size of the collar threads according to the flexural strength ration are the two main reasons of the premature fatigue failure of the collar. By a comparison between the stress concentration factors K_f of NC style, REG style, FH style and IF style threads, it is recommended to use the NC style threads to improve the fatigue life of the collar connection threads. The factors influencing the value of K_f are investigated, and it is proposed to extend the service life of the collar by improving the surface quality of the connection threads.