Research On The Mechanical Behavior And Gluing Failure Process In Oil Pipe Threaded Connection

In oil and natural gas industry, oil pipe is the important part and the application amount is very large. At present, it has already occupied nearly50%of the total steel in petroleum industry. More than80%of thread failures occur on the joint of threaded connection, which show the joint is the weakest point in oil tubing and casing. The connection of pin and box thread is a complicated contact and deformation process of spatial helical surface under external forces. The key point to investigate the mechanical behavior in oil pipe threaded connection is how to calculate the load and deformation distribution in threaded connection accurately and determine the load and deformation on each thread tooth as well. In oil pipe threaded connection, the load and deformation on each thread tooth is a procedure of transmission and accumulation, and the load and deformation between the former tooth and the latter tooth are closely related. In the previous analytical modeling of threaded connection, researchers usually consider the whole deformation and neglect the single deformation on each thread tooth, which leads to the inaccuracy on load distribution and could not make analysis and calculation in oil pipe threaded connection effectively. In addition, thread gluing is a main failure in threaded connection, but the researches on the failure process of gluing failure at present are still not sufficient. Based on the investigations and summaries in related literatures in the world, the mechanical behavior in oil pipe threaded connection under tightening and tension load is analyzed and the failure process of thread gluing is also observed by the experiment. The main research contents and achievements in this dissertation are described as follows:(1) The mechanical behaviors of cylindrical pipe threaded connection in tightened and tensioned conditions are investigated respectively. The equations of axial deformation on pin and box thread are derivated and the calculating equations of load distribution on thread surface in tightened and tensioned conditions are developed. Take some parameters of P-110S oil pipe threaded connction for calculating example, the load distributions on contact surface in tightened and tensioned conditions are calculated respectively.(2) The mechanical behavior of conic threaded connection with interference fit and tension load are investigated respectively. The equations of radial deformation on pin and box thread are established and the equation of contact pressure on contact surface with interference fit is obtained. By the modification of established cylindrical pipe threaded connection model, the equation of load distribution on conic threaded connection with tension load is derivated and the equation of contact pressure on conic threaded connection with interference fit and tension load is obtained. Take P-110S conic threaded connction for calculating example, the load distributions on contact surface in different conditions, such as intereference fit, tension load and combined loads, are calculated respectively. The difference between thicken and non-thicken threaded connection is also investigated. (3) Take the joint of P-110S oil pipe threaded connection for research object and using the non-linear finite element software ABAQUS, the mechanical property of oil pipe threaded connection under make up, inner pressure, axial load and combined loads is analyzed and several regular curves are obtained. The results of finite element analysis to some extent verified the proposed analytical models in this dissertation.(4) Based on the thick wall theory, the calculation model of required tightening torque is established. Take P-110S oil pipe threaded connection for calculating example, the tightening torque of oil pipe threaded connection under make up procedure is calculated. Compared the obtained tightening torques from analytical method, finite element analysis and the practical used tightening torque on Wuxi Seamless Oil Pipe Co., Ltd, the results indicate that the proposed calculation model on tightening torque in this dissertation is reasonable.(5) Considering the existence of tooth shape error and tooth height error, the equation of load distribution includes these errors in oil pipe threaded connection is established and the influences of tooth shape error and tooth height error on load distribution are analyzed respectively.(6) The analytical model in oil pipe threaded connection that includes the effect of tooth modification cut is established. This model can calculate the axial load and load distributions with tooth modification cut respectively. It can also be used to investigate the influence of tooth modification cut on load distribution effectively. Applying this proposed model, the influences of several factors, such as different tooth modification cut, materials of pin and box thread, taper angle and wall thickness, are also investigated, which provides the guidance for choosing parameters of tooth modification cut, material, taper angle and wall thickness.(7) The experimental equipment that can simulate the contact of threaded connection is developed. Take P-110S oil pipe threaded connection for test example, the variation process on thread surface with different load and friction is observed and the influence of contact stress on gluing failure is investigated. Applying the practical tightening contact stress to the thread surface, the surface topography with gluing failure is observed and the maximum times of make up and disassembly is obtained. This obtained times number is identical to the practical used number in industry.The research contents and achievements obtained in this dissertation will be helpful for the application in petroleum industry. This research will contribute to the further researches on the mechanical behavior in oil pipe threaded connection and also offers theoretical supports to the investigation and prevention of thread gluing.