Study of temperature distribution in the locked threaded connection of drill pipes

Abstract

The effect of temperature on threaded connections is one of the important factors affecting their strength, tightness, and durability during operation. At elevated temperatures, thread materials undergo thermal expansion, which changes their geometric parameters and fits, affecting the tightening force and tightness of the connection. High temperatures contribute to plastic deformation of the surface layers of the thread, accelerated wear, and the formation of backlash, which reduces the reliability of the connection. The lock threaded connection (LTC) used in drill strings for oil and gas well construction must meet the requirements for strength, fatigue life, and tightness. For the study, a three-dimensional model of an SC with NC50 thread was constructed in accordance with the technical conditions, and its simulation modeling was performed using the finite element method (FEM) to determine the temperature distribution in the pipe body and SC at a drilling fluid temperature of 20 °C inside the pipe and 100 °C outside the pipe. It was found that the temperature distribution in the pipe body and the ZRZ is uneven. Considering the temperature distribution in the ZRZ, there is a decrease in temperature from the thrust end of the nipple to the coupling body from 70 °C to 47 °C. The obtained temperature gradient in the pipe and the ZRZ was used as input data for further analysis of the stress-strain state of the ZRZ. The distribution of stresses in the ZRZ under the action of the screw-in torque and the screw-in torque in combination with the temperature effect was established. The results of simulation modeling showed that temperature reduces the equivalent stresses in the recesses of the nipple threads by 15–19%, especially in the recesses of the first two nipple threads. This reduction in stress can have a positive effect on the fatigue strength and service life of the connection. Therefore, it is important to take temperature effects into account when designing, manufacturing, and evaluating the reliability of threaded connections, especially when drilling pipes are operated in variable temperature conditions. The next step is to study the fatigue strength of threaded connections, taking into account temperature effects and adjusting the calculated tightening torque.