Research On Mechanical Properties And Structural Optimization Of New Special Threaded Tubing Joints

With the progress of petroleum mining technology and the frequent exploitation of high-pressure gas wells,ultra-deep wells,extended-reach wells and horizontal wells,the service conditions of the tubing string are getting worse and worse.As the connection part of the tubing string,the thread connections bear various complex loads together with the string.The frequent failure accidents causes huge economic losses to the oil and gas fields,so improving the strength and the sealing performance of the thread connections is the key to ensure the safety exploitation of the oil and gas fields.At present,the scholars at home and abroad mainly use the two-dimensional axisymmetric model to analyze tubing mechanics problems and their main research object is the single sealing pipe joint.Rare research has been reported on the combined primary sealing pipe joint.In addition,their research methods is mainly finite element method and seldom carry out full-scale test resulting in lacking of a comprehensive demonstration of the safety of threaded joints,which restricts the development and application of new thread joints.In view of the above problems,the connection strength and sealing performance of the newly proposed premium thread tubing joint is studied by finite element analysis and the strength and sealing reliability of the joint is verified by full size test.The detailed analysis methods and research results are as follows:(1)A 3D numerical simulation model of the tubing thread connection have been established based on the Mises yield criterion and nonlinear contact theory.It also considers the influence of asymmetrical load at the same time.A new premium thread joint structure is proposed and the five key parameters of the joint thread:pitch,tooth height,bearing surface angle,guiding surface angle and taper are optimized by the orthogonal optimization.In addition,the sensitivity of the key parameter of the sealing structure:cone angle,spherical radius,shoulder angle are analyzed.The optimal combination of thread parameters are taper 1:24,pitch 6 teeth/inch,guiding surface angle 10°,bearing surface angle-5°,tooth height 1.375mm and sequence for impact of each parameter on tensile strength are as follows:taper,bearing surface,tooth height,pitch,guiding surface.The optimal combination of sealing surface parameters are cone taper 1:6,spherical radius 12mm,shoulder angle-18°.(2)The bearing performance of the new premium joint and the API thread joint is contrastively calculated under tensile load,compression load and bending load.The research results show that compared with the API thread joint,the tensile properties of the new special thread joint increase 17.77%,the compression performance increase 10.67%and the bending strength increase 3.94%.(3)The mechanical properties and connection security of the new premium thread joint are analyzed under tensile load,compression load,internal pressure,external pressure,bending load,Tension+ bending + internal pressure compound load,compression+ bending+internal pressure compound load.The research results show that tensile load can increase the equivalent force of the bearing surface,significantly reduce the sealing performance of the shoulder surface and cone/cone structure but has no obvious effect on the sealing performance of the cylinder/sphere structure.Compressive load can increase the equivalent stress of the guiding surface,improve the sealing performance of the shoulder and conical surface but increase the risk of yield failure,reduce the sealing performance of the cylinder/sphere structure.Both internal and external pressure can promote the sealing performance of the main sealing structure.The internal pressure will reduce the sealing performance of the shoulder surface but the external pressure produce the opposite effect.The bending load increase the stress of the middle section of the thread obviously and reduce the sealing performance of the shoulder and the main sealing surface.The joint has good connection strength and sealing performance under the extreme load.(4)The tensile test,compression test,internal pressure sealing test,external pressure sealing test and compound load test are conducted by foll-scale test method.The results of the experiments verify the correctness of the results of the finite element analysis and they show that the joint has good connection strength and sealing performance under the extreme load.