Mechanical Performance Analysis Of Variable Taper Drill String Connection Threads

As the weakest link to the whole drill string system,the drill string joint needs to bear a lot of complex load.Failure accidents often occur to the drilling process of the drill string,resulting in huge economic losses.Therefore,improving the strength performance of the drill string joint and selecting the appropriate drilling parameters according to the bearing capacity of the drill string joint is the keys to ensure the efficient and safe drilling operation.The stress levels of API drill string joints are unevenly distributed among complex down hole loads.In order to improve the stress distribution of drill string joints and increase the load-bearing performance of the joint thread,a variable taper drill strings joint thread is proposed,and the working mechanical performance analysis of the variable taper drill strings joint thread is in-depth studied,in order to design and develop special drill string joint threads.Provide theoretical basis and reference.First of all,this paper studies the specifications,model parameters,characteristics of API standard drill string joint threads and their development and use status.The research and development status of special threaded joints are investigated,and the design features of the current special threaded joints are summarized from the perspective of overall contour shape and size optimization of drill string joints,and thread parameter optimization.The common failure modes of drill string joints under complex loads are summarized.At the same time,the reasons for the failure of drill string joints are analyzed,and the research direction of the mechanical properties of drill string joints under complex load conditions is clarified.Secondly,the control equation of the drill string thread connection and the basic method of analysis of related mechanical characteristics are determined,the basic assumption of nonlinear contact,the boundary conditions of the nonlinear contact surface and the constraint method of the contact problem are put forward,and the theory of the thread contact finite element problem is clarified.Finally,based on the conventional drill string connecting thread,the specific structural parameters of the single shoulder and double shoulder variable taper drill string connecting thread are determined,and the two-dimensional axial symmetry model of the variable taper drill strings joint thread is established.The analysis is calculated using the ABAQUS finite element software.The effect of the make-up torque and comprehensive loads with the thread mechanical properties of the variable taper drill string joints with single shoulder variable taper drill string joints and double shoulder variable taper drill string joints,and the comparison with the mechanical properties of the drill string connection thread under the standard taper,the following conclusions were reached:(1)The equivalent stress distribution of the single-shoulder standard drill strings connection thread under the action of the make-up torque is extremely uneven,showing a stress distribution state of "large on one side and small on the other",among which the effective meshing thread near the main shoulder Bearing the maximum equivalent stress,the maximum equivalent stress value gradually decreases with the distance of the main shoulder.Single shouldered variable tapers drill string connecting thread reduces the uneven stress distribution to a certain extent.Single-shoulder variable tapers type II has the best load-bearing performance under the action of make-up torque.The specific taper is female taper 1:6,The taper of the male buckle is 1:6.2,and the maximum equivalent stress value is reduced by 56% compared with the standard drill string joint.(2)The equivalent stress distribution of the double-shoulder drill string connection thread under the action of the make-up torque is extremely uneven,showing a stress distribution state of "small in the middle and large on both sides",which is closest to the main shoulder and the auxiliary shoulder.The effective meshing thread tooth bears a larger equivalent stress,and the middle effective meshing thread tooth stress has little fluctuation.Double-shoulder variable tapers drill strings connection threaded significantly to relieve stress unevenness under the action of make-up torque.Under the action of make-up torque,double shoulder variable tapers type I has the best load-bearing performance.The specific taper is female taper 1:6,male buckle taper1:6.1,the maximum equivalent stress value is reduced by 34% compared with conventional drill string joints.(3)The mechanical analysis results of the variable taper drill string connection thread under tensile load show that the variable taper type II drill string joint thread has the best tensile performance in a single shoulder,which is better than the standard API The tensile performance of the single shoulder drill string joint is increased by 18.96%;the double shoulder medium tapers type II drill string joint has the best thread tensile performance,which is 6.98% higher than that of the conventional double shoulder drill string joint;(4)The mechanical analysis results of the variable taper drill string connection thread under compressive load show that the variable taper type II drill string joint has the best thread compression performance in a single shoulder,which is better than the standard API single set The tensile performance of the shoulder drill string joint is increased by 36.60%;the double shoulder medium taper type II drill string joint has the best thread tensile performance,which is 34.23% higher than that of the standard API single shoulder drill string joint.(5)The mechanical analysis results from the variable taper drill string connection thread under the internal and external pressure difference show that the maximum equivalent stress value of the API single shoulder drill string joint does not change under the change of the internal and external pressure difference.The compression performance of single shoulder variable tapers drill string joint thread type Ⅰ,Ⅱ,Ⅲ,and Ⅳ drill string joints is better than that of API single shoulder drill string joints,and the compression performance is increased by 5.6%,49.21%,51.68%,49.21%respectively.Under the change of internal and external pressure difference,the maximum equivalent stress value of the double shoulder drilling string joint remains unchanged.The double shoulder variable taper drilling string joint threads type Ⅰ and type Ⅱ compressive performance is better than the API single shoulder drill string joint.Increased by 17.06% and 16.93% respectively.