Investigation of the influence of the location of the main plane of the thread on the stress-strain state of the locking threaded connection of the elements of the drill string

Abstract

Today it is impossible to imagine drilling wells without the use of threaded connections. Despite continuous improvement of these threaded connections, their accident rate does not decrease. The peculiarity of operation of threaded joints of drill string elements is the impact of significant torsional, bending moments and tensile and compression loads on them. That is why the research of their stress-strain state is still important. To determine the stress-strain state of a threaded connection, in which there are deviations of one of the most important thread parameters, namely, the location of the main plane, a three-dimensional model of the NC50 connection has been constructed for study using the finite element method. As a result of the simulation of the thread connection, the main plane of which is placed in accordance with the requirements of regulatory documents, the distribution of stresses on the thread hollows of the nipple has been established, according to which the maximum stresses occur in the hollows of the coils placed closer to the supporting face, and then gradually decrease. This distribution of stresses obtained by simulation coincides with the known theories. Performed simulations, in which the placement of the main plane shifted by 2 mm toward the supporting face of the nipple showed a change in the distribution and growth of the value of stresses on the thread troughs of the nipple and the change in the value of the contact pressure at the supporting ends of the nipple and socket. It is worth noting that such changes will adversely affect the operation of the thread connection. The study of a threaded connection in which the main plane is displaced by 2 mm away from the bearing end of the nipple showed an increase in stresses along the troughs of the threads of the nipple and a sharp decrease in the values ​​of contact pressure at the bearing ends of the nipple and coupling. Under these conditions, the threaded connection will fail in a small number of operating cycles.