Special thread sealing surface design method based on contact morphology and pressure distribution

Abstract

The invention belongs to the technical field of petroleum and natural gas development pipe columns, and particularly relates to a special thread sealing surface design method based on contact morphology and pressure distribution. A special thread sealing surface design method based on contact morphology and pressure distribution comprises the following steps: selecting a limit sample combination and preparing a limit tolerance sample for a designed air-tight seal special thread sealing structure; and carrying out upper shackle test evaluation and air sealing performance test evaluation, carrying out sealing surface dimensional tolerance fit analysis, upper shackle torque characteristic analysis, sealing surface contact morphology analysis and sealing surface contact pressure distribution analysis on the sample in sequence, and establishing a thread sealing surface contact morphology and pressure distribution criterion. According to the method, the contact pressure distribution and the contact length of the special thread sealing surface are analyzed through finite element simulation, the contact morphology and the air-tight sealing performance of the special thread sealing surface are evaluated and analyzed through a physical test, and the air-tight sealing thread failure criterion design method is established, so that the reliability of the sealing design of the special thread joint is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of oil and gas development pipe columns, and in particular relates to a special thread sealing surface design method based on contact morphology and pressure distribution. Background Art

[0002] The casing and tubing strings used in the development of oil and gas are subjected to harsh and complex downhole environments. Threaded connections are the weak link in the string, and the integrity of threaded seals is the decisive factor in ensuring safe and reliable downhole production. At present, the technical difficulty faced by high-temperature and high-pressure gas well completion strings is the serious annular pressure of the wellbore. 80% of the existing high-temperature and high-pressure gas wells in China's oil and gas fields have significant annular pressure, which seriously threatens downhole safety operations. The key factor causing the annular pressure of the wellbore is the lack and imperfection of the existing theoretical method for the design of special threaded gas seals.

[0003] The existing special thread gas seal design methods have the following main problems: 1. Basic method of gas seal design: API / ISO10400 standard proposes to calculate the maximum contact pressure of the main sealing surface based on contact mechanics theory and compare it with the internal pressure of the downhole working condition to determine whether it can be sealed. It is a single-point theoretical design method that does not consider the characteristics of the actual thread seal structure, the size of the seal diameter, the thread grease used on the sealing surface and the surface roughness. 2. Industrial method of gas seal design: 1) The sealing line load method based on axial contact of thread grease proposed by Tenaris was published in the Journal of Tribology in 2004. It did not consider the influence of the roughness of the sealing surface processing and the sealing surface diameter, that is, the outer diameter of the oil casing specification. 2) Canada C-FER considered the influence of the seal diameter based on the small sample ring on the flat plate test and proposed a new gas seal design method. The above-mentioned air seal design methods are all based on small samples and are designed under specific conditions. They are significantly different from the special thread seal design methods for oil casing and tubing in that the special thread seal contact is formed by the contact pressure and contact length of the sealing surface and the contact in the circumferential direction during the thread buckling process. That is to say, before the sealing surface finally forms a stable contact pressure and length, the sealing surface experiences friction and wear during the thread buckling process, which is significantly different from the premise established by the small sample test. This significant difference is one of the key factors leading to the failure of special thread seals for oil casing and tubing. In short, the existing special thread air seal design methods are all sealing performance under ideal conditions, and do not consider the impact of the special thread joint buckling process on the performance of the sealing surface. The final formation of the special thread seal requires the relative sliding of the inner and outer sealing surfaces during the thread buckling process, which causes friction loss on the sealing surface. Therefore, the existing design methods cannot truly evaluate the contact state and pressure distribution of the sealing surface. Summary of the invention

[0004] In view of the above problems, the object of the present invention is to provide a design method for a special thread sealing surface based on contact topography and pressure distribution. By finite element simulation analysis of the contact pressure distribution of the special thread sealing surface, and by means of physical test evaluation, the contact topography and gas sealing performance of the special thread sealing surface are analyzed. By comparing the finite element simulation results and the physical test evaluation results, a failure criterion design method for gas sealing threads is established, so as to effectively improve the reliability of the special thread joint sealing design and improve and supplement the deficiencies and defects of the existing metal sealing design method.

[0005] The technical solution of the present invention lies in: a design method for a special thread sealing surface based on contact topography and pressure distribution, and the specific process is as follows: S1: Design the casing specifications, sealing forms, sealing structures, sealing interference amounts, thread structures, thread interference amounts, and shoulder angle parameters of the gas sealing special thread sealing structure according to the performance requirements of the gas sealing special thread; S2: Select the limit specimen combination and prepare the limit tolerance specimen according to the gas sealing special thread sealing structure designed in S1; S3: Make an up-and-down torque test evaluation. According to the specified make-up torque, make-up speed, the same type and quantity of thread compound, conduct an up-and-down torque test, measure the sealing slip distance, and observe the sealing contact topography; S4: Make a gas sealing performance test evaluation. Conduct a gas sealing test evaluation on the specimen after up-and-down torque, and detect the sealing performance under different sealing surface contact topographies and contact pressure distributions; S5: Analyze the dimensional tolerance fit of the sealing surface, the make-up torque characteristics, the sealing surface contact topography, and the sealing surface contact pressure distribution of the specimen in turn; S6: According to the analysis results in step S5, compare the relationship between the sealing surface contact topography and the contact pressure, establish the contact topography and pressure distribution criteria for the thread sealing surface, judge whether the sealing performance of the gas sealing special thread sealing structure designed after make-up meets the requirements, and further judge whether the sealing structure and interference amount design are reasonable.

[0006] In the step S1, the sealing form is a metal seal, the sealing structure includes an outer sealing structure and an inner sealing structure, the outer sealing structure is a conical surface seal, the taper range of the conical surface is 1:8 to 1:10, the inner sealing structure is a spherical surface seal, the radius range of the spherical surface is 60 to 90 mm, the sealing interference amount range is 0.45 to 0.65 mm, the thread structure includes a thread taper, a pitch, and a tooth height, the thread taper range is 1:16 to 1:18, the pitch range is 5.08 mm or 6.35 mm, the tooth height range is 1.5 to 1.8 mm, the thread interference amount range is 0.1 to 0.3 mm, and the shoulder angle range is -10° to -15°.

[0007] In the step S2, the limit specimen combinations include four groups, namely high thread interference, low thread interference, high seal interference and low seal interference. The limit tolerance specimens are prepared with the same surface roughness and the same surface treatment.

[0008] In the evaluation of the gas sealing performance test in the step S4, for the specimens that have completed the step S3, the thread compound is baked at a high temperature of 180°C for 24 hours. First, a tensile load is applied to eliminate the interference of the contact between the front end of the seal and the thrust shoulder on the detection of the sealing performance of the main sealing surface. The tensile load is applied according to 95% of the tensile strength of the pipe body, and the tensile strength of the pipe body is determined according to the measured yield strength of the material and the measured geometric dimensions to ensure the separation of the contact between the sealing end face and the thrust shoulder. Subsequently, an internal pressure is applied. The internal pressure can be increased step by step to the maximum internal pressure. The maximum internal pressure is such that the combined stress of "tensile + internal pressure" is 95% of the yield strength of the material. The pressure is maintained for 15 minutes, and the seal leakage situation is observed.

[0009] In the step S5, the contact morphology of the seal during the make-up process is determined by the distribution of the internal thread sealing surface coating and the thread compound on the external sealing surface after the make-up. The main indicators include the uniformity, continuity and closure of the contact of the sealing surface, including circumferentially and axially; the sealing contact width includes the width of the thread compound and the width of the coating.

[0010] In the step S5, the finite element simulation is used to analyze the distribution of the contact pressure on the sealing surface under the conditions of make-up, tension and internal pressure of the special thread. The indicators of the contact pressure distribution on the sealing surface include: the magnitude of the contact pressure, the contact length, the distribution pattern of the contact pressure, and the distribution pattern under different load conditions.

[0011] The relationship between the contact morphology of the sealing surface and the contact pressure should satisfy: the range of the maximum contact pressure P of the sealing surface is 0.5σs to 1.1σs, where σs is the yield strength of the material; the sealing contact length ≥ 1%D, and D is the outer diameter of the casing.

[0012] The technical effect of the present invention is that: by finite element simulation analysis of the contact pressure distribution and contact length of the special thread sealing surface, and by the method of physical test evaluation, the contact morphology and gas sealing performance of the special thread sealing surface are analyzed. By comparing the finite element simulation results and the physical test evaluation results, a failure criterion design method for gas sealing threads is established, thereby effectively improving the reliability of the special thread joint seal design and improving and supplementing the deficiencies and defects of the existing metal seal design method.

[0013] The following will be further described in conjunction with the drawings. Brief Description of the Drawings

[0014] Figure 1 It is a flow chart of a special thread sealing surface design method based on contact morphology and pressure distribution according to an embodiment of the present invention.

[0015] Figure 2 Schematic diagram of the gas-tight special thread sealing structure according to an embodiment of the present invention.

[0016] Figure 3 Appearance diagram of the make-up and break-out threads according to an embodiment of the present invention.

[0017] Figure 4 Modeling diagram of the finite element analysis of the threads according to an embodiment of the present invention.

[0018] Figure 5 Comparison diagram of the appearance and contact pressure distribution of the sealing surface after the break-out according to an embodiment of the present invention. Specific implementation manners Embodiment 1

[0019] As Figure 1 , Figure 2 shown, a design method for a special thread sealing surface based on contact appearance and pressure distribution is as follows: S1: Design the casing specifications, sealing forms, sealing structures, sealing interference amounts, thread structures, thread interference amounts, and shoulder angle parameters of the gas-tight special thread sealing structure according to the performance requirements of the gas-tight special thread; S2: Select the limit specimen combination and prepare the limit tolerance specimens according to the gas-tight special thread sealing structure designed in S1; S3: Make-up and break-out test evaluation. Conduct the make-up and break-out tests according to the specified make-up torque, make-up speed, the same type and quantity of thread compound, measure the sealing slip distance, and observe the sealing contact appearance; S4: Gas-tight performance test evaluation. Conduct the gas-tight test evaluation on the specimens after make-up and break-out, and detect the sealing performance under different sealing surface contact appearances and contact pressure distributions; S5: Conduct the analysis of the dimensional tolerance fit of the sealing surface, the make-up and break-out torque characteristics analysis, the sealing surface contact appearance analysis, and the sealing surface contact pressure distribution analysis on the specimens in sequence; S6: According to the analysis results in step S5, compare the relationship between the sealing surface contact appearance and the contact pressure, establish the criteria for the thread sealing surface contact appearance and pressure distribution, judge whether the sealing performance of the designed gas-tight special thread sealing structure after make-up meets the requirements, and further judge whether the sealing structure and interference amount design are reasonable.

[0020] In the step S1, the sealing form is metal sealing. The sealing structure includes an outer sealing structure and an inner sealing structure. The outer sealing structure is a conical surface sealing, and the taper range of the conical surface is 1:8 to 1:10. The inner sealing structure is a spherical surface sealing, and the radius range of the spherical surface is 60 to 90 mm. The sealing interference range is 0.45 to 0.65 mm. The thread structure includes a thread taper, a pitch, and a tooth height. The thread taper range is 1:16 to 1:18. The pitch range is 5.08 mm or 6.35 mm. The tooth height range is 1.5 to 1.8 mm. The thread interference range is 0.1 to 0.3 mm. The shoulder angle range is -10° to -15°.

[0021] In the step S2, the extreme sample combinations include four groups, namely high thread interference, low thread interference, high sealing interference, and low sealing interference. The extreme tolerance samples are prepared with the same surface roughness and the same surface treatment.

[0022] In the evaluation of the gas sealing performance test in the step S4, for the samples completed in the step S3, the thread grease is baked dry at a high temperature of 180°C for 24 hours. First, a tensile load is applied to eliminate the interference of the contact between the front end of the seal and the thrust shoulder on the detection of the sealing performance of the main sealing surface. The tensile load is applied according to 95% of the pipe body tensile strength, and the pipe body tensile strength is determined according to the measured material yield strength and the measured geometric dimensions to ensure the separation of the contact between the sealing end face and the thrust shoulder. Then, an internal pressure is applied. The internal pressure can be increased step by step to the maximum internal pressure. The maximum internal pressure is that the "tensile + internal pressure" combined stress is 95% of the material yield strength. Keep the pressure for 15 minutes and observe the sealing leakage situation.

[0023] In the step S5, the contact morphology of the seal during the make-up process is determined by the distribution of the inner thread sealing surface coating and the thread grease bonded on the outer sealing surface after the make-up. The main indicators include the uniformity, continuity, and closure of the contact of the sealing surface, including circumferentially and axially. The sealing contact width includes the width of the thread grease and the width of the coating.

[0024] In the step S5, the finite element simulation is used to analyze the contact pressure distribution on the sealing surface under the conditions of make-up, tension, and internal pressure of the special thread. The indexes of the contact pressure distribution on the sealing surface include: the magnitude of the contact pressure, the contact length, the distribution pattern of the contact pressure, and the distribution pattern under different load conditions.

[0025] The relationship between the contact morphology of the sealing surface and the contact pressure should satisfy: the range of the maximum contact pressure P on the sealing surface is 0.5σs to 1.1σs, where σs is the material yield strength; the sealing contact length ≥ 1%D, and D is the outer diameter of the casing. Example 2

[0026] Adopt a special thread sealing surface design method based on contact topography and pressure distribution as in Example 1 to carry out special thread gas sealing design for a high corrosion-resistant alloy super 13Cr casing. The specific process is as follows: Step 1: Design of the gas sealing special thread sealing structure; The design of the gas sealing special thread sealing structure for the high corrosion-resistant alloy super 13Cr casing is shown in Table 1: Table 1 Thread structure for gas sealing test of high corrosion-resistant alloy super 13Cr casing

[0027] Step 2: Combination and preparation of gas sealing thread ultimate specimens; According to the size requirements for specimen preparation, the measured parameters of the external thread specimens on the pipe body are shown in Table 2, the measured parameters of the internal thread specimens of the coupling are shown in Table 3, the specimen combinations are shown in Table 4, and the interference of the measured specimen parameters is shown in Table 5; Table 2 Measured parameters of external thread specimens

[0028] Table 3 Measured parameters of internal thread specimens

[0029] Table 4 Specimen pairing numbers

[0030] Table 5 Interference of measured thread parameters of specimens

[0031] Step 3: Evaluation of make-up and breakout test; Test conditions: Test environment is at room temperature, make-up speed (≤7 revolutions per minute), thread compound type (Bestolife2000), and 25 g of thread compound is evenly applied to both the internal and external thread surfaces at each end. Test results: No sticking occurred during the make-up and breakout tests of specimens 1# to 4#, and the make-up / breakout test data of the specimens are shown in Table 6, and the breakout morphology of the threads is shown in Figure 3 ; Table 6 Make-up / breakout test data

[0032] Step 4: Evaluation of gas sealing performance test; For specimens 1# to 4# that have completed the make-up and breakout test evaluation in Step 2, bake the thread compound at 180 °C for 24 hours, and then conduct the test detection of gas sealing performance. Based on the measured yield strength (758 MPa) and the measured wall thickness of the casing material, calculate the internal pressure at which the thread reaches 95% of the measured yield strength of the material under tension + internal pressure as shown in Table 7, and check whether the thread leaks. The test results are shown in Table 8; Table 7 Test load

[0033] Table 8 Test Results

[0034] Step 5: Analysis of the dimensional tolerance fit of the sealing surface; The threads have the same processing technology (copper plating, sandblasting, the same sealing structure, surface roughness Ra3.2, and the same thread compound). The factor causing seal leakage is analyzed as the dimensional tolerance fit of the sealing structure. The comparison of the dimensional tolerance fit of the thread sealing structures of Samples 1# to 4# is shown in Table 9; Table 9 Comparison of the interference of the sample thread seals Unit (mm)

[0035] Based on the gas seal test results, no leakage occurred in Sample 1#, and the sealing parameters of Sample 1# were used as a reference to compare Samples 2# to 4# with leakage. Sample 1# has the smallest seal interference (0.39 mm), the smallest average seal interference (0.41 mm), and the largest seal interference ellipticity (0.05 mm), indicating that the minimum average seal interference (0.41 mm) can seal the internal pressure (71.9 MPa); a sealing surface ellipticity ≤ 0.05 mm has no effect on sealing. Sample 2# has the same sealing parameters at end A but has serious leakage; Samples 3# and 4# have the largest seal interference (0.59 mm) and an ellipticity ≤ 0.05 mm, but still have serious leakage; the test results show that the seal interference and ellipticity are not the main factors causing leakage; Step 6: Analysis of the make-up torque characteristics; The comparison and analysis of the make-up torques of the gas seal leakage ends of Sample 1# and Samples 2# to 4# are shown in Table 10. The comparison of the first make-up torque at end A of Samples 2 to 4# with the minimum value at end B of Sample 1#: The total torque is 17 - 26% higher; the shoulder torque inflection points of Samples 3# and 4# are twice as high, and that of Sample 2# is 33% lower; the incremental torque of Sample 2# is 11.8% higher and has the largest shoulder interference, while Samples 3# and 4# are 28.4% lower and have the smallest shoulder interference. The make-up torque analysis shows that the differences in incremental torque and shoulder torque inflection points are not the main reasons for seal leakage; Table 10 Comparison of the make-up torques of the samples

[0036] Step 7: Analysis of the contact morphology of the gas seal thread sealing surface; The analysis of the contact morphology of the external thread sealing surface after unthreading after gas seal test leakage is shown in Table 11; Table 11 Comparison of the contact morphology and contact pressure distribution of the sealing surface

[0037] Step 8: Sealing contact pressure simulation analysis; The sealing ellipticity of the specimen machining tolerance is not the key factor causing leakage. Therefore, a finite element two-dimensional axisymmetric model can be used to analyze the sealing contact pressure distribution of different dimensional tolerance fits and make-up torques. The sealing contact pressure is analyzed by combining finite element simulation analysis and experimental analysis. The contact state of the sealing surface after the connection is broken and the sealing contact pressure distribution states under three load conditions of make-up, tension, and tension + internal pressure are compared and analyzed. The finite element modeling of the B / A ends of specimens 1# to 4# is shown in Figure 4 , and the critical contact pressure for the pressure to penetrate the sealing surface is set as the internal pressure of the seal, 71.9 MPa. The comparison between the sealing contact pressure and length distribution of the threads under the load conditions of make-up, tension, and tension + internal pressure and the morphology of the specimen sealing surface is shown in Figure 5 , and the sealing performance analysis is shown in Table 12; Table 12 Thread sealing performance analysis of specimen B / A ends

[0038] By comparing and analyzing the thread sealing performance of the B / A ends of 4 groups of specimens, the 1# specimen has the minimum contact pressure, contact length, and contact density at the B end under the load conditions of make-up, tension, and tension + internal pressure. The contact pressures, contact lengths, and contact densities of the 2#, 3#, and 4# specimens are all greater than those of the 1# specimen. All of the 1# to 4# specimens meet the sealing criterion, and the sealing values of the 2# to 4# specimens are all greater than those of the 1# specimen. According to this analysis result, the 1# specimen did not leak, and the rest of the specimens should not leak either. However, leakage occurred at the A ends of the 2# to 4# specimens. Therefore, the sealing criterion obtained from the tests of small specimens with specific sealing structures is not applicable to the diversity of the sealing structures of full-size physical gas-tight threads; Step 9: Establish the contact morphology and pressure distribution criteria for the thread sealing surface; The comparative analysis of the contact morphology of the sealing surface and the sealing surface contact pressure distribution state after the connection of the gas-tight special thread is broken is shown in Table 13; Table 13 Comparison of the contact morphology of the sealing surface and the contact pressure distribution

[0039] From the comparison between the morphology of the external thread connection break of the full-size physical test sample and the finite element sealing contact analysis, it can be seen that: (1) Uniform and continuous contact at the position of the maximum contact pressure on the sealing surface along the circumferential direction is a necessary condition for sealing; (2) The reason for the discontinuous contact at the position of the maximum contact pressure on the sealing surface is that the axial contact pressure along the sealing surface shows discontinuous double-peak contact after make-up; (3) The discontinuous double-peak contact along the axial direction causes the sealing contact pressure to be distributed in a spiral shape along the circumferential direction, and the position of the maximum contact pressure is not closed and discontinuous along the circumferential direction; (4)During the make-up process, the sliding length of the sealing surface along the circumference is long, the friction and wear are serious, and the traces of the copper plating layer adhered to the sealing surface are unevenly distributed.

[0040] According to the analysis results, the sealing structure of the gas-tight special thread of the high corrosion-resistant alloy super 13Cr casing is optimized as shown in Table 14. At the same time, physical tests and finite element simulation analyses are carried out according to S1~S9. After make-up, the axial sealing contact pressure is continuous, the sliding length of the sealing surface is less than 1 / 2 of the circumference, and the wear is slight. The maximum contact pressure in the finite element simulation analysis is closed and continuous along the circumferential direction. The test results show that no leakage occurred in all specimens, and no sealing failure accident occurred during the application process.

[0041] Table 14 Gas-tight test thread structure of high corrosion-resistant alloy super 13Cr casing

[0042] The above is only the preferred specific implementation mode of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention should be covered by the protection scope of the present invention.

Claims

1. A special thread sealing surface design method based on contact topography and pressure distribution, characterized in that: It includes the following steps: S1: Design the casing specifications, sealing form, sealing structure, sealing interference, thread structure, thread interference and shoulder angle parameters of the gas-tight special thread sealing structure according to the performance requirements of the gas-tight special thread. S2: Select the extreme specimen combination and prepare the extreme tolerance specimens according to the gas-tight special thread sealing structure designed in S1. S3: Make an evaluation of the make-up and break-out test. Conduct the make-up and break-out test according to the specified make-up torque, make-up speed, the same type and quantity of thread compound, measure the sealing slip distance, and observe the sealing contact morphology. S4: Make an evaluation of the gas-tight performance test. Conduct the gas-tight performance test on the specimens after make-up and break-out, and detect the sealing performance under different sealing surface contact morphologies and contact pressure distributions. S5: Analyze the dimensional tolerance fit of the sealing surface, the make-up and break-out torque characteristics, the sealing surface contact morphology and the sealing surface contact pressure distribution of the specimens in sequence. S6: According to the analysis results in step S5, compare the relationship between the sealing surface contact morphology and the contact pressure, establish the criteria for the thread sealing surface contact morphology and pressure distribution, judge whether the sealing performance of the designed gas-tight special thread sealing structure after make-up meets the requirements, and further judge whether the sealing structure and interference design are reasonable.

2. The special thread sealing surface design method based on contact topography and pressure distribution according to claim 1, characterized in that: In the said step S1, the sealing form is metal sealing. The sealing structure includes an outer sealing structure and an inner sealing structure. The outer sealing structure is a conical surface sealing, and the taper range of the conical surface is 1:8~1:

10. The inner sealing structure is a spherical surface sealing, and the radius range of the spherical surface is 60~90mm. The sealing interference range is 0.45~0.65mm. The thread structure includes thread taper, pitch and tooth height. The thread taper range is 1:16~1:

18. The pitch range is 5.08mm or 6.35mm. The tooth height range is 1.5~1.8mm. The thread interference range is 0.1~0.3mm. The shoulder angle range is -10°~-15°.

3. The special thread sealing surface design method based on contact topography and pressure distribution according to claim 1, characterized in that: In the said step S2, the extreme specimen combination includes four groups, namely high thread interference, low thread interference, high sealing interference and low sealing interference. The prepared extreme tolerance specimens have the same surface roughness and the same surface treatment.

4. A special thread sealing surface design method based on contact topography and pressure distribution according to claim 1, characterized in that: In the gas-tight performance test evaluation of step S4, bake the thread compound of the specimens completed in step S3 at 180°C for 24 hours at high temperature. First, apply a tensile load to eliminate the interference of the contact between the front end of the seal and the thrust shoulder on the detection of the sealing performance of the main sealing surface. The tensile load is applied according to 95% of the body tensile strength of the pipe, and the body tensile strength of the pipe is determined according to the measured material yield strength and the measured geometric dimensions to ensure the separation of the sealing end face and the thrust shoulder contact. Subsequently, apply internal pressure. The internal pressure can be increased step by step to the maximum internal pressure. The maximum internal pressure is that the "tensile + internal pressure" combined stress is 95% of the material yield strength. Keep the pressure for 15 minutes and observe the sealing leakage situation.

5. The special thread sealing surface design method based on contact topography and pressure distribution according to claim 1, characterized in that: In step S5, the contact morphology during the make-up process is determined by the distribution of the internal thread sealing surface coating and thread compound bonded to the outer sealing surface of the shackle. The main indicators include the uniformity, continuity, and closure of the sealing surface contact, including the circumferential and axial directions; the sealing contact width includes the width of the thread compound and the coating width.

6. The special thread sealing surface design method based on contact topography and pressure distribution according to claim 1, characterized in that: In step S5, finite element simulation is used to analyze the contact pressure distribution on the sealing surface during the make-up, tension, and internal pressure application of special threads. The indicators of the contact pressure distribution on the sealing surface include: the magnitude of the contact pressure, the contact length, the contact pressure distribution pattern, and the distribution pattern under different load conditions.

7. A special thread sealing surface design method based on contact topography and pressure distribution according to claim 1, characterized in that: The relationship between the contact morphology of the sealing surface and the contact pressure should satisfy: the range of the maximum contact pressure P on the sealing surface is 0.5σs to 1.1σs, where σs is the material yield strength; the sealing contact length ≥ 1%D, where D is the outer diameter of the casing.

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