A rapid quantitative evaluation method for the integrity of special threaded joints of oil well pipes

Abstract

A method for rapid quantitative evaluation of the integrity of special threaded joints of oil well pipes, characterized in that the preload theoretical model of the special threaded joints produced by the tightening torque is mainly created based on the API6A standard, a torque loading method of the axisymmetric two-dimensional model of the special threaded joint using the virtual displacement principle and the inversion of the contact offset of the thread stop shoulder is proposed, an axisymmetric two-dimensional mechanical model of the special threaded joint under normal working conditions and the severe working conditions of single tooth biting is established using ANSYS commercial software, and the hot spot peak equivalent stress, stress amplitude coefficient SAF and main sealing surface sealing energy multiple of the threaded joint under normal working conditions and the severe working conditions of single tooth biting under alternating loads are simulated and calculated based on the stress amplitude coefficient method, the fatigue resistance of the threaded joint is rapidly quantitatively evaluated by comparing the SAF under normal working conditions and the severe working conditions, and the sealing performance of the threaded joint is rapidly quantitatively evaluated by using the sealing energy multiple of the main sealing surface. The present invention is applicable to the technical field of oil and gas drilling and production engineering.

Description

[0001] The invention relates to the technical field of oil and natural gas drilling and production engineering, and in particular to a method for quickly quantifying the integrity of a special threaded joint of an oil and gas well. Background Art

[0002] The special threaded connection for oil well pipe gas sealing has excellent structure and sealing integrity and is widely used in deep and ultra-deep high temperature and high pressure gas wells. At present, the main basis for domestic and foreign oil companies to test whether their sealing performance is superior is the international standard ISO 13679 (i.e. API RP 5C5) oil casing threaded connection test evaluation procedure for the petroleum and natural gas industry. The main feature and advantage of this method is that it uses a full-scale physical test to evaluate the sealing performance of oil well pipe threaded joints under simulated working conditions. The evaluation results are really reliable. However, the full-scale oil well pipe test evaluation method is not only expensive, but also has a long time period, and it is difficult to evaluate the fatigue resistance of threaded joints under alternating loads. To this end, many scholars at home and abroad have carried out simulation analysis of thread sealing based on the ISO13679 test evaluation procedure, and believe that simulation evaluation can greatly save time and economic costs, and can replace the physical test evaluation of special buckles to a certain extent.

[0003] As we all know, the key to simulating the sealing performance of threaded joints using the finite element method is how to apply torque scientifically and reasonably. To solve the problem of torque loading, scholars use commercial tools such as ABAQUS and ANSYS to establish true three-dimensional finite element mechanical models, and directly apply a rotation angle on the top of the threaded joint to achieve torque loading. However, this method is not only complex to model, but also consumes a lot of simulation resources, has low calculation efficiency, and is difficult to converge.

[0004] Therefore, in order to solve the problem of high three-dimensional time and economic costs and to achieve rapid evaluation of the sealing performance of threaded joints, scholars have chosen a pseudo-three-dimensional, i.e., an axisymmetric two-dimensional mechanical model, to perform finite element simulation on the sealing performance of threaded joints. However, in the 2D axisymmetric finite element simulation, torque cannot be directly applied to the threaded joint. Usually, the preload can only be applied by inverting the torque of the threaded shoulder, and then artificially changing the thread structure (i.e., artificially changing the interference) to apply the preload. However, this method not only does not take into account the torque generated between the thread pairs, but also artificially changes the thread structure and interference, which destroys the structure of the thread itself and causes changes in the structural stiffness. The accuracy of the calculation results will be affected, and more importantly, it cannot reflect the actual threading and tightening mechanical behavior of the threaded joint.

[0005] In addition, the experimental evaluation procedures and finite element methods provided by the existing international standard ISO 13679 are mainly aimed at evaluating the sealing performance of threaded joints under normal working conditions, and do not consider the impact of single-tooth joints caused by processing and manufacturing errors, thread inclusions, and eccentricity of threads and oil well pipes due to misalignment of operations on the integrity of threaded joints, especially special threaded joints for deep and ultra-deep oil well pipes. In addition, special threaded joints for deep and ultra-deep oil well pipes will face alternating loads and are prone to fatigue failure. At present, scholars mainly use stress peak and stress concentration factor SCF to evaluate the fatigue performance of threaded joints. There are few reports on the rapid quantitative evaluation of thread fatigue resistance based on stress amplitude factor SAF. Studies have shown that fatigue damage is a function of stress change amplitude rather than stress peak. Therefore, SAF has a more direct and accurate effect on fatigue than SCF. It should be noted that: (1) severe working conditions are defined as the situation where the first tooth of the thread close to the main sealing surface is loaded due to processing and manufacturing errors, thread inclusions, misaligned operation, etc. (2) normal working conditions are defined as the situation where the thread is fully engaged under service conditions; (3) If the difference between the thread SAF under severe working conditions and normal working conditions is too large, it means that the severe working conditions have a great impact on the fatigue performance of the structure and should be avoided.

[0006] In order to avoid the integrity failure of special threaded joints of oil well pipes in deep and ultra-deep high-temperature and high-pressure gas wells, it is necessary to quickly and quantitatively evaluate their sealing performance, fatigue resistance and integrity, especially the integrity under such harsh working conditions as single-tooth joint bearing. To this end, the present invention proposes a method for quickly and quantitatively evaluating the integrity of special threaded joints of oil well pipes from the two perspectives of sealing performance and fatigue resistance of special threaded joints. Summary of the invention

[0007] The purpose of the present invention is to provide a method for rapid quantitative evaluation of the integrity of special threaded joints of oil well pipes, so as to solve the technical problems of complex three-dimensional modeling of special threaded joints, high calculation time cost and difficulty in convergence, and difficulty in accurately loading the fastening / tightening torque on the axisymmetric two-dimensional mechanical model of special threaded joints. At the same time, the influence of the harsh working condition of single-tooth engagement caused by eccentricity of threads and oil well pipes (caused by processing and manufacturing errors, thread inclusions, operation misalignment, etc.) on the integrity of special threaded joints is considered, and a technical basis is provided for the structural optimization design of special threaded joints, fatigue resistance and rapid quantitative evaluation of failure trends.

[0008] To achieve the above object, the present invention adopts the following technical solutions:

[0009] The present invention proposes a method for rapid quantitative evaluation of the integrity of special threaded joints of oil well pipes, which is characterized in that: the method is mainly based on API The 6A standard creates a theoretical model of the preload force generated by the tightening / tightening torque of special threaded joints, and proposes a torque loading method for the axisymmetric two-dimensional model of special threaded joints using the virtual displacement principle and the inversion of the contact offset of the thread stop shoulder. The axially symmetric two-dimensional mechanical model of the special threaded joint under normal working conditions and the severe working conditions of single-tooth biting is established using ANSYS commercial software. The peak equivalent stress, variation amplitude, stress amplitude coefficient SAF and contact pressure and sealing energy multiples of the hot spots of the special threaded joint under normal working conditions and the severe working conditions of single-tooth biting under the action of alternating loads are simulated and calculated based on the stress amplitude coefficient SAF calculation method provided by the ISO13625 standard. The fatigue resistance of the special threaded joint is quickly and quantitatively evaluated by comparing the SAF under normal working conditions and the severe working conditions. The sealing performance of the special threaded joint is quickly and quantitatively evaluated by using the contact pressure and sealing energy multiples on the main sealing surface. The integrity of the special threaded joint of the oil well pipe is quickly and quantitatively evaluated by combining the fatigue resistance and sealing performance. The specific technical solutions adopted are as follows:

[0010] Step 1: Create a theoretical model f0(M) of preload force generated by make-up / tightening torque of special threaded joints based on API 6A standard i ,F Mi ), f0(M i ,F Mi )=2M i [πd m -fPsec(α)] / {d m [P+πfd m sec(α)]+kf s D s [πd m -fPsec(α)]}, where M i F is the tightening torque of the threaded joint, Nm; Mi is the axial force of the threaded joint, i.e. the preload, N; d m is the major diameter of the thread, mm; P is the thread pitch, mm; f is the friction coefficient between the thread pairs, 0.1; α is the thread half angle, 20°; fs is the friction coefficient of the stop shoulder 11 at the box buckle 13, 0.1; Ds is the maximum outer diameter of the stop shoulder 11 at the box buckle 13, mm; k is the centering coefficient of the stop shoulder 11, taking the ideal state, that is, the threaded joint is not eccentric, k=1, and the eccentric state k<1.

[0011] Step 2: According to the designed special threaded joint tightening torque range, the preload theoretical model f0(M,F) is used to calculate the theoretical preload F generated by the threaded joint under different tightening torques Mi. Mi (i>10);

[0012] Step 3: Without considering dynamic loads such as vibration, impact, and inertia, the ANSYS commercial software was used to establish an axisymmetric two-dimensional mechanical model of the oil well pipe special buckle threaded joint under normal working conditions and severe working conditions of single-tooth biting, and hot spots were set at the root, variable cross-section, and both ends of the oil well pipe threaded joint. The hot spots on the two ends of the body were defined as reference hot spots;

[0013] Step 4: Apply different contact offsets to the thread stop shoulder in the axisymmetric two-dimensional mechanical model using the virtual displacement principle, and use ANSYS software to simulate and invert to obtain the axial force F generated by the special threaded joint of the oil well pipe under different contact offsets. Si , get F Si =F Mi The corresponding stop shoulder contact offset Si;

[0014] Step 5: Fitting to obtain the contact offset S at the stop shoulder i With preload F Mi The functional relationship is f1(F Mi ,S i );

[0015] Step 6: Based on the functional relationship f1(F Mi ,S i ) Establish the functional relationship between the contact offset at the stop shoulder and the tightening torque of the special threaded joint f2(M i ,S i );

[0016] Step 7: Based on the functional relationship f2(M i ,S i ) and preload theoretical model f0(M i ,F Mi ) can obtain the nominal axial preload generated by the special threaded joint under any tightening torque Mi and the minimum thread contact offset S required at the corresponding stop shoulder i ;

[0017] Step 8: Determine the rated tensile strength T of the oil well pipe special threaded joint R And tightening torque M R , and using the functional relationship f0(M i ,F Mi )、f1(F Mi ,S i ) and f2(M i ,S i ) Calculate the nominal axial preload F N And the corresponding minimum thread contact offset Si;

[0018] Step 9: Apply alternating loads composed of axial load and preload to special threaded joints under normal working conditions and severe working conditions: The first load step L S1 :Apply tightening torque M R The nominal axial preload F generated N ; The second load step L S2 :Apply tightening torque M R Nominal axial preload F N and 0.2T R Additional axial tension or pressure; third load step L S3 :Apply tightening torque M R The nominal axial preload F generated N and 0.4T R Additional axial tension or pressure; fourth load step L S4 :Apply tightening torque M R The nominal axial preload F generated N and 0.6T R Additional axial tension or pressure; fifth load step L S5 :Apply tightening torque M R The nominal axial preload F generated N and 0.8T R Additional axial tension or pressure;

[0019] Step 10: Provide an evaluation method based on the stress amplitude coefficient SAF, that is, SAF = △σ HP / △σ Ref , △σ HP is the peak stress σ at the root of the special threaded joint and the hot spot of the variable section HP The change relative to the first load step, △σ Ref is the reference equivalent stress σ at the reference hot spot on the two ends of the special buckle Ref Changes relative to the first load step;

[0020] Step 11: Simulate and calculate the peak equivalent stress σ at the root and variable cross-section of the special threaded joint under normal and severe working conditions at each load step HP , contact pressure on the main sealing surface σ (l) , reference peak equivalent stress at reference hot spot σ Ref , σ Ref and σ HP Variation amplitude and stress amplitude factor SAF;

[0021] Step 12: Change the tightening torque M R And the corresponding nominal axial preload F NRepeat step nine to simulate and calculate the peak equivalent stress σ at the root and variable cross-section of the special threaded joint under normal working conditions and severe working conditions at each load step HP , contact pressure on the main sealing surface σ (l) , reference peak equivalent stress at reference hot spot σ Ref , σ Ref and σ HP Variation amplitude and stress amplitude factor SAF;

[0022] Step 13: Peak equivalent stress σ obtained based on step 12 HP , reference point equivalent stress σ Ref , σ Ref and σ HP Change amplitude, use the SAF calculation method given in step 10 to obtain the stress amplitude coefficient SAF of different hot spots under each load step;

[0023] Step 14: Based on the contact pressure σ on the main sealing surface obtained in step 11 (l) , using formula S n =∫ l σ 1.95 (l) dl / K(2R) m Q n P calculates the sealing performance multiple S of the main sealing surface under each load step n , where l is the seal contact length, mm; K is the seal constant, 24MPa 0.95 mm 1-m (ml / min) -n ; m is the influence coefficient of surface treatment; n is the influence coefficient of surface roughness; P is the effective sealing pressure, MPa; Q is the leakage rate, 0.9 cm 3 / 15min; R is the sealing radius, mm. It should be noted that the sealing energy multiple S n Greater than 1, indicating that it can effectively seal the effective pressure inside and outside the oil well pipe P; S n Less than or equal to 1, indicating that the effective pressure P inside and outside the oil well pipe cannot be effectively sealed; S n The larger the value, the better the sealing performance of the oil well pipe.

[0024] Step 15: Use the method of comparing SAF under normal working conditions and severe working conditions to quickly and quantitatively evaluate the fatigue resistance and failure trend of special threaded joints. If the SAF under normal working conditions and severe working conditions differ greatly, it indicates that the fatigue resistance of special threaded joints under severe working conditions is poor and fatigue fracture failure may occur easily. The threaded joint structure needs to be optimized before it can be used. On the contrary, if the fatigue resistance is good, it can be used directly.

[0025] Step 16: Use the sealing energy multiple S of the hot spot on the main sealing surface nEvaluate the sealing performance of special threaded joints. If the sealing performance under severe working conditions is multiple S n The sealing energy multiple is less than or equal to 1 or much less than the normal working condition, indicating that the special threaded joint is prone to sealing failure under harsh working conditions, and the threaded joint structure needs to be optimized before it can be used; similarly, the sealing energy multiple S under normal working conditions is n Less than or equal to 1, the threaded joint structure still needs to be optimized before it can be used. On the contrary, the sealing performance is good and it can be used directly;

[0026] Step 17: Combined with the fatigue resistance and sealing performance of the special threaded joints under severe working conditions, comprehensively evaluate the integrity of the special threaded joints of oil well pipes.

[0027] The advantages of the present invention are:

[0028] (1) It can quickly and quantitatively evaluate the fatigue resistance, sealing performance and integrity of special threaded joints of oil well pipes under harsh working conditions;

[0029] (2) It can effectively guide the optimization design and selection of special threaded joint structures for oil well pipes in ultra-deep wells of 10,000 meters and high-temperature and high-pressure gas wells, thereby avoiding accidents such as gas channeling, annular pressure, and wellhead lifting caused by failure of thread integrity. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is the connection diagram of the special threaded joint for oil well pipe.

[0031] Figure 2 This is a diagram of a single-tooth nozzle with special thread for oil well pipes under harsh working conditions.

[0032] Figure 3 This is a composite diagram of a full-tooth nozzle with special thread for oil well pipe under normal working conditions.

[0033] Figure 4 Comparison of SAF coefficients of special buckles of oil well pipes under normal working conditions and severe working conditions.

[0034] Figure 5 The sealing performance of special buckles of oil well pipes under normal and severe working conditions is multiple S n contrast. DETAILED DESCRIPTION

[0035] The present invention is described in detail below in conjunction with the accompanying drawings and embodiments.

[0036] Referring to the accompanying drawings, the present invention proposes a method for rapid quantitative evaluation of the integrity of a special threaded joint in an oil and gas well. The method is given by taking the rapid quantitative evaluation of the integrity of a special threaded joint under the harsh working conditions of a single-tooth joint as an example. The method mainly includes the following steps:

[0037] Step 1: Create a theoretical model f0(M) of preload force generated by make-up / tightening torque of special threaded joints based on API 6A standard i ,F Mi ), f0(M i ,F Mi )=2M i [πd m -fPsec(α)] / {d m [P+πfd m sec(α)]+Kf s D s [πd m -fPsec(α)]}, where M i F is the tightening torque of the threaded joint, Nm; Mi is the axial force of the threaded joint, i.e. the preload, N; d m is the major diameter of the thread, mm; P is the thread pitch, mm; f is the friction coefficient between the thread pairs, 0.1; α is the thread half angle, 20°; fs is the friction coefficient of the stop shoulder 11 of the box buckle 13, 0.1; Ds is the maximum outer diameter of the stop shoulder 11 of the box buckle 13, mm; k is the centering coefficient of the stop shoulder 11, taking the ideal state, that is, the threaded joint is not eccentric, k=1.

[0038] Step 2: According to the designed special threaded joint tightening torque range, the theoretical preload force F generated by the threaded joint under different tightening torques Mi is calculated using the preload theoretical model f0(M,F). Mi (i>10);

[0039] Step 3: Without considering dynamic loads such as vibration, impact, and inertia, an axisymmetric two-dimensional mechanical model of a special threaded joint of an oil well pipe under harsh working conditions of a hot spot single-tooth joint is established using ANSYS commercial software, and hot spots are set at the root, variable cross-section, and both ends of the oil well pipe threaded joint. The hot spots on the two ends of the body are defined as reference hot spots, wherein the male buckle 12 is distributed with reference hot spot 1, hot spot 2, hot spot 5, and hot spot 7, and the female buckle 13 is distributed with hot spot 3, reference hot spot 4, hot spot 6, and hot spot 8. The single tooth 10 in the single-tooth joint is the first tooth to contact after the main sealing surface 5;

[0040] Step 4: Apply different contact offsets to the thread stop shoulder 11 in the axisymmetric two-dimensional mechanical model using the virtual displacement principle, and use ANSYS software to simulate and invert to obtain the axial force F generated by the oil well pipe special buckle threaded joint under different contact offsets. Si , get F Si =F Mi The corresponding contact offset Si of the stop shoulder 11;

[0041] Step 5: Fitting to obtain the contact offset S at the stop shoulder 11i With preload F Mi The functional relationship is f1(F Mi ,S i );

[0042] Step 6: Based on the functional relationship f1(F Mi ,S i ) Establish the functional relationship between the contact offset at the stop shoulder 11 and the tightening torque of the special threaded joint f2(M i ,S i );

[0043] Step 7: Based on the functional relationship f2(M i ,S i ) and preload theoretical model f0(M i ,F Mi ) can obtain the nominal axial preload generated by the special threaded joint under any tightening torque Mi and the minimum thread contact offset S required at the corresponding stop shoulder 11 i ;

[0044] Step 8: Determine the rated tensile strength T of the oil well pipe special threaded joint R And tightening torque M R , and using the functional relationship f0(M i ,F Mi )、f1(F Mi ,S i ) and f2(M i ,S i ) Calculate the nominal axial preload F N And the corresponding minimum thread contact offset Si;

[0045] Step 9: Apply an alternating load consisting of axial load and preload to the special threaded joint under harsh working conditions: The first load step L S1 :Apply tightening torque M R The nominal axial preload F generated N ; The second load step L S2 :Apply tightening torque M R Nominal axial preload F N and 0.2T R Additional axial tension or pressure; third load step L S3 :Apply tightening torque M R The nominal axial preload F generated N and 0.4T R Additional axial tension or pressure; fourth load step L S4 :Apply tightening torque M R The nominal axial preload F generated N and 0.6TR Additional axial tension or pressure; fifth load step L S5 :Apply tightening torque M R The nominal axial preload F generated N and 0.8T R Additional axial tension or pressure;

[0046] Step 10: Provide an evaluation method based on the stress amplitude coefficient SAF, that is, SAF = △σ HP / △σ Ref , △σ HP The peak stress σ is the hot spot 5, hot spot 6, hot spot 7, hot spot 8 at the root of the special threaded joint and the hot spot 2, hot spot 3 on the body of the two ends of the special threaded joint. HP The change relative to the first load step, △σ Ref is the reference peak equivalent stress σ at the reference hotspots 1 and 4 on the two ends of the special buckle Ref Changes relative to the first load step;

[0047] Step 11: Simulate and calculate the peak stress σ at hot spots 5, 6, 7, 8 at the root of the special threaded joint and hot spots 2 and 3 on the body of the special threaded joint at both ends under normal and severe working conditions. HP , contact pressure σ on the main sealing surface 10 (l) , Reference peak equivalent stress σ at reference hotspot 1 and reference hotspot 4 on the two ends of the special buckle Ref , △σ HP , △σ Ref and the stress amplitude factor SAF, such as Figure 4 As shown;

[0048] Step 12: Similarly, change the tightening torque M R And the corresponding nominal axial preload F N Repeat step nine to simulate and calculate the peak stress σ at hotspots 5, 6, 7, 8 and hotspots 2 and 3 on the body of the special buckle at each load step under severe working conditions. HP , contact pressure σ on the main sealing surface 10 (l) , Reference peak equivalent stress σ at reference hotspot 1 and reference hotspot 4 on the two ends of the special buckle Ref , △σ HP , △σ Ref and stress amplitude factor SAF;

[0049] Step 13: Peak equivalent stress σ obtained based on step 12 HP , reference peak equivalent stress σ Ref , σ Ref and σHP Change amplitude, use the SAF calculation method given in step 10 to obtain the stress amplitude coefficient SAF of different hot spots under each load step;

[0050] Step 14: Based on the contact pressure σ at the main sealing surface 10 obtained in step 11 (l) , using the formula ∫ l σ 1.95 (l) dl / K(2R) m Q n P calculates the sealing performance multiple Sn of the hot spot on the main sealing surface 10 under each load step, such as Figure 5 shown.

[0051] Step 15: Use the SAF comparison method of normal working conditions and severe working conditions to quickly and quantitatively evaluate the fatigue resistance and failure trend of special threaded joints. Figure 4 It shows that the SAF under severe working conditions is much greater than that under normal working conditions, indicating that the fatigue resistance of special threaded joints under severe working conditions is poor and fatigue fracture failure may occur easily. The structure of threaded joints needs to be optimized before they can be used.

[0052] Step 16: Use the sealing energy multiple S of the main sealing surface 10 n Evaluate the sealing performance of special threaded joints, Figure 5 Displays the sealing energy multiple S under severe working conditions n Less than 1, and much less than the sealing energy multiple under normal working conditions, indicating that special threaded joints are prone to sealing failure under harsh working conditions, and the threaded joint structure needs to be optimized before it can be used; on the contrary, the sealing energy multiple S under normal working conditions is n Much larger than 1, the threaded joint has good sealing performance and can be used directly;

[0053] Step 17: Comprehensive analysis of the fatigue resistance and sealing performance of special threaded joints under severe working conditions shows that the integrity of special threaded joints of oil well pipes is prone to failure.

Claims

1. A rapid quantitative evaluation method for the integrity of special threaded joints of oil well pipes, characterized in that: The method comprises the following steps: Step 1: Create a theoretical model f0(M) of preload force generated by make-up / tightening torque of special threaded joints based on API 6A standard i ,F Mi ), f0(M i ,F Mi )=2M i [πd m -fPsec(α)] / {d m [P+πfd m sec(α)]+Kf s D s [πd m -fPsec(α)]}, where M i F is the tightening torque of the threaded joint, Nm; Mi is the axial force of the threaded joint, i.e. the preload, N; d m is the major diameter of the thread, mm; P is the thread pitch, mm; f is the friction coefficient between the thread pairs, 0.1; α is the thread half angle, 20°; fs is the friction coefficient of the stop shoulder (11) of the box buckle (13), 0.1; Ds is the maximum outer diameter of the stop shoulder (11) of the box buckle (13), mm; k is the centering coefficient of the stop shoulder (11), taking the ideal state, that is, the threaded joint is not eccentric, k = 1; Step 2: According to the designed special threaded joint tightening torque range, the preload theoretical model f0(M,F) is used to calculate the theoretical preload F generated by the threaded joint under different tightening torques Mi. Mi (i>10); Step 3: Ignoring the three dynamic loads of vibration, impact and inertia, the ANSYS commercial software was used to establish an axisymmetric two-dimensional mechanical model of the oil well pipe special threaded joint under normal working conditions and severe working conditions of single-tooth biting, and hot spots were set at the root, variable cross-section and both ends of the oil well pipe threaded joint, and the hot spots on the two ends of the body were defined as reference hot spots; Step 4: Apply different contact offsets to the thread stop shoulder (11) in the axisymmetric two-dimensional mechanical model using the virtual displacement principle, and use ANSYS software to simulate and invert to obtain the axial force F generated by the special threaded joint of the oil well pipe under different contact offsets. Si , get F Si =F Mi The corresponding contact offset Si of the stop shoulder (11) is: Step 5: Fitting to obtain the contact offset S at the stop shoulder (11) i With preload F Mi The functional relationship is f1(F Mi ,S i ); Step 6: Based on the functional relationship f1(F Mi ,S i ) establishes the functional relationship between the contact offset at the stop shoulder (11) and the tightening torque of the special threaded joint f2(M i ,S i ); Step 7: Based on the functional relationship f2(M i ,S i ) and preload theoretical model f0(M i ,F Mi ) can obtain the nominal axial preload generated by the special threaded joint under any tightening torque Mi and the minimum thread contact offset S required at the corresponding stop shoulder i ; Step 8: Determine the rated tensile strength T of the oil well pipe special threaded joint R And tightening torque M R , and using the functional relationship f0(M i ,F Mi )、f1(F Mi ,S i ) and f2(M i ,S i ) Calculate the nominal axial preload F N And the corresponding minimum thread contact offset Si; Step 9: Apply alternating loads composed of axial load and preload to special threaded joints under normal working conditions and severe working conditions: The first load step L S1 :Apply tightening torque M R The nominal axial preload F generated N ; The second load step L S2 :Apply tightening torque M R Nominal axial preload F N and 0.2T R Additional axial tension or pressure; third load step L S3 :Apply tightening torque M R The nominal axial preload F generated N and 0.4T R Additional axial tension or pressure; fourth load step L S4 :Apply tightening torque M R The nominal axial preload F generated N and 0.6T R Additional axial tension or pressure; fifth load step L S5 :Apply tightening torque M R The nominal axial preload F generated N and 0.8T R Additional axial tension or pressure; Step 10: Provide an evaluation method based on the stress amplitude coefficient SAF, that is, SAF = △σ HP / △σ Ref , △σ HP is the peak stress σ at the root of the special threaded joint and the hot spot of the variable section HP The change relative to the first load step, △σ Ref is the reference equivalent stress σ at the reference hot spot on the two ends of the special buckle Ref Changes relative to the first load step; Step 11: Simulate and calculate the peak equivalent stress σ at the root and variable cross-section of the special threaded joint under normal and severe working conditions at each load step HP , contact pressure σ on the main sealing surface (10) (l) , reference peak equivalent stress at reference hot spot σ Ref , σ Ref and σ HP Variation amplitude and stress amplitude factor SAF; Step 12: Change the tightening torque M R And the corresponding nominal axial preload F N Repeat step nine to simulate and calculate the peak equivalent stress σ at the root and variable cross-section of the special threaded joint under normal working conditions and severe working conditions at each load step HP , contact pressure σ on the main sealing surface (10) (l) , reference peak equivalent stress at reference hot spot σ Ref , σ Ref and σ HP Variation amplitude and stress amplitude factor SAF; Step 13: Peak equivalent stress σ obtained based on step 12 HP , reference point equivalent stress σ Ref , σ Ref and σ HP Change amplitude, use the SAF calculation method given in step 10 to obtain the stress amplitude coefficient SAF of different hot spots under each load step; Step 14: Based on the contact pressure σ on the main sealing surface (10) obtained in step 11 (l) , using formula S n =∫ l σ 1.95 (l) dl / K(2R) m Q n P calculates the sealing performance multiple S of the main sealing surface (10) under each load step n , sealing energy multiple S n If it is greater than 1, it means that the effective pressure P inside and outside the oil well pipe can be effectively sealed; S n Less than or equal to 1, indicating that the effective pressure P inside and outside the oil well pipe cannot be effectively sealed; S n The larger the value, the better the sealing performance of the oil well pipe; where l is the sealing contact length, mm; K is the sealing constant, 24MPa 0.95 mm 1-m (ml / min) -n ; m is the influence coefficient of surface treatment; n is the influence coefficient of surface roughness; P is the effective sealing pressure, MPa; Q is the leakage rate, 0.9 cm 3 / 15min; R is the sealing radius, mm; Step 15: Use the method of comparing SAF under normal working conditions and severe working conditions to quickly and quantitatively evaluate the fatigue resistance and failure trend of special threaded joints. If the SAF under normal working conditions and severe working conditions differ greatly, it indicates that the fatigue resistance of special threaded joints under severe working conditions is poor and fatigue fracture failure may occur easily. The threaded joint structure needs to be optimized before it can be used. On the contrary, if the fatigue resistance is good, it can be used directly. Step 16: Use the sealing energy multiple S of the hot spot on the main sealing surface n Evaluate the sealing performance of special threaded joints. If the sealing performance under severe working conditions is multiple S n The sealing energy multiple is less than or equal to 1 or much less than the normal working condition, indicating that the special threaded joint is prone to sealing failure under harsh working conditions, and the threaded joint structure needs to be optimized before it can be used; similarly, the sealing energy multiple S under normal working conditions is n Less than or equal to 1, the threaded joint structure still needs to be optimized before it can be used; Step 17: Combined with the fatigue resistance and sealing performance of the special threaded joints under severe working conditions, comprehensively evaluate the integrity of the special threaded joints of oil well pipes.

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