Finite element analysis of improving designs for a drill string joint

Most drill string failures occur in the last-engaged threads of the pin and box. This type of failure is mainly caused by stress concentration at the root of the threads. Some improving design schemes to decrease stress concentration are proposed, which include using SST thread, cutting the last-engaged threads of pin and box and adjusting the taper of pin. Using the method of finite element contact analysis, the effect of each improving design scheme on the stress concentration at the root of last-engaged threads is studied and compared with that of the original NC46 thread of the drill string joint. The results show that every improving design scheme can decrease stress concentration in the last-engaged threads, and the scheme of adjusting the taper of pin has the highest efficiency, and the scheme of cutting the last-engaged threads is better than the scheme of using SST thread.