Design method of oil casing coupling for deep well

Abstract

The invention relates to the technical field of well drilling, in particular to a design method of an oil casing coupling for a deep well, which comprises the following steps of: S1, confirming an aluminum alloy material series of the coupling according to an aluminum alloy material of an oil casing; s2, strength calculation is conducted on the oil casing and couplings made of different series of aluminum alloy materials, and the wall thickness of a connector of the oil casing and the wall thickness of the couplings and the type of connecting threads are determined; s3, carrying out finite element simulation analysis on the oil casing and the coupling, simulating stress distribution when the oil casing and the coupling are loaded after being connected, and confirming thread parameters with optimal strength; and S4, according to a finite element analysis experiment result, the wall thickness of the coupling, the types and parameters of connecting threads of the oil casing and the coupling and the final material of the coupling are finally confirmed. According to the method, the galvanic corrosion phenomenon generated when the coupling makes contact with the oil casing can be avoided, meanwhile, the coupling structure meeting the strength can be designed according to a more comprehensive and accurate scheme, and the design reliability and the practical application safety of the oil casing and the coupling are improved.

Description

Technical Field

[0001] The invention relates to the technical field, in particular to a design method for a deep well oil casing coupling. Background Art

[0002] The exploration and development of deep oil and gas resources has important practical and strategic significance for strengthening energy security, and drilling is one of the key technologies in the exploration and development of deep oil and gas resources. It uses oil drilling equipment to drill through multiple formations along the designed trajectory from the ground to reach the predetermined target layer (oil and gas layer or potential oil and gas layer), forming an oil well that is convenient for oil and gas extraction, and then an oil pipe can be lowered into the oil well to export the oil and gas.

[0003] After the oil well is drilled, in order to stabilize the well wall, it is necessary to use a drilling rig to hang oil casing into the well so that the well wall is fixed and supported, to help the oil well separate the rock formation and prevent the oil well from collapsing, to ensure the stability of the drilling channel, and to ensure that the entire oil well can operate normally. Aluminum alloy can usually be used for deep well and ultra-deep well oil casing. On the one hand, it can reduce the load of the drilling rig hook hanging into the oil casing. On the other hand, aluminum alloy also has good corrosion resistance, which can prevent H2S and CO2 in the formation water from corroding the pipe wall.

[0004] Due to the deep depth of oil wells, multiple oil casings need to be spliced ​​and installed. Adjacent oil casings are provided with external threaded joints, and then the adjacent oil casings are spliced ​​and fixed with tubular couplings with internal threads to ensure the sealing between the oil casings. The environmental conditions during drilling operations are harsh. The strength and durability of the oil casings and their couplings are extremely important to ensure the safe operation of drilling projects, especially for deep wells (well depths of 4500 meters to 6000 meters) and ultra-deep wells (well depths of 6000 meters to 9000 meters). The depth of the oil well itself is deep, the stability conditions of the well wall are complex, and the temperature gradient and pressure gradient of the oil well are high. In order to ensure high reliability of the oil casing under such complex working conditions, it is necessary to put forward more stringent requirements on the design of the coupling connection structure of the oil casing to prevent safety accidents in drilling projects.

[0005] In view of the structural design of the oil casing coupling, the prior art "casing joint design method, coupling threaded joint and straight-connected threaded joint" (publication number: CN 118296753 A) proposes a casing coupling design method, which uses finite element analysis software to apply load to the casing model for simulation analysis, and obtains finite element data that can characterize the sealing level of the casing threaded joint and the coupling, and then continuously adjusts various parameters of the oil casing and the coupling until the data meets the industry standard, so as to obtain a coupling structure that meets the strength requirements. However, this prior art still has the following technical problems:

[0006] The casing coupling design method of the prior art measures the strength of the oil casing based on the sealing level of the adjacent oil casing threaded joints and the coupling connection. However, in actual drilling projects, the oil well environment is complex. If only the sealing of the oil casing coupling connection is considered for structural design, it is likely to lead to insufficient coupling strength in actual applications, resulting in strength failure at the oil casing coupling connection, leading to safety risks such as oil well collapse. Summary of the invention

[0007] The invention provides a design method for a deep well oil casing coupling, which can solve the problem that the oil casing coupling designed in the prior art is insufficient in strength and easily causes safety risks such as oil well pipe string breakage.

[0008] The present application provides the following technical solutions: a method for designing a deep well oil casing coupling, comprising the following steps: S1, determining the aluminum alloy material of the oil casing according to the depth of the oil and gas well, and then confirming the aluminum alloy material series of the coupling according to the aluminum alloy material of the oil casing; S2, performing strength calculations on the oil casing and couplings of different series of aluminum alloy materials, confirming the wall thickness of the joint of the oil casing and the coupling, and the type of connecting thread; S3, performing finite element simulation analysis on the oil casing and the coupling, simulating the stress distribution when the oil casing and the coupling are loaded after being connected, and confirming the thread parameters with the best strength; S4, based on the finite element analysis experimental results, finally confirming the wall thickness of the coupling, the type and parameters of the connecting thread between the oil casing and the coupling, and the final material of the coupling.

[0009] Invention concept: During the drilling process of deep wells and ultra-deep wells, the oil casing needs to be hoisted into the oil well by the big hook of the drilling rig for installation. In order to ensure the strength and sealing of the spliced ​​joints of each oil casing, the existing couplings usually use high-strength steel couplings to connect the aluminum alloy oil casing. However, the inventor found that although the strength of the steel coupling is high, the standard electrode potential of aluminum is lower than that of steel. Under the action of the downhole medium, the steel coupling is easy to form galvanic corrosion with the aluminum alloy oil casing, thereby endangering the safety of the oil well pipe string. Therefore, the inventor wants to improve this situation. The simplest way is to directly Changing the oil casing to steel means that both the oil casing and the coupling are made of steel, and galvanic corrosion will not occur. However, this will increase the load on the drilling rig hook and increase the cost, which is not the best solution. Therefore, the inventor started with the coupling and conducted research and analysis, considering using aluminum alloy as the material of the coupling to eliminate the galvanic corrosion problem. However, the strength of the aluminum alloy coupling is lower than that of the steel coupling, which will reduce the overall connection strength of the oil casing. Therefore, the inventor selected the material of the coupling and the type of thread connecting the coupling and the oil casing to ensure that galvanic corrosion is avoided while also solving the coupling strength problem.

[0010] Beneficial effects:

[0011] Improve the reliability of the design of oil casing and couplings, and ensure the safety of the actual application of oil casing and couplings. According to the design method of the present invention, the material of the coupling is selected as aluminum alloy, which can not only avoid the galvanic corrosion phenomenon caused by the steel coupling when it comes into contact with the aluminum alloy oil casing in an electrolyte environment, but also improve the service life and safety of the drill string; at the same time, the overall weight of the aluminum alloy is light, which can reduce the load of the drilling rig and increase the drilling rig's downhole depth when the oil casing is hoisted; and in order to solve the strength problem of the aluminum alloy coupling, the yield strength, tensile strength, internal and external pressure strength of the connection between the oil casing and the coupling are accurately calculated to confirm the wall thickness of the oil casing and the coupling, and then by comparing the effects of various different thread structures on the connection strength, the most suitable one can be confirmed. The invention has a thread structure that meets the tightening and sealing effect, and then uses finite element analysis to simulate the stress distribution when the oil casing and the coupling are connected under load, and analyzes the stress condition of each tooth of the thread one by one to confirm the thread parameters with optimal strength. Compared with the prior art, the invention can design a coupling structure that meets the strength with a more comprehensive and accurate solution, prevent the oil casing from slipping and failing in engineering applications or rupturing and failing under the action of internal and external pressures, effectively ensure the connection strength and stability of the oil casing and the coupling, and improve the safety and reliability of the oil casing and the coupling in actual drilling engineering applications, thereby avoiding safety accidents such as oil well collapse.

[0012] Furthermore, in step S2, it is confirmed that the material of the coupling includes Al-Cu-Mg aluminum alloy and Al-Zn-Mg aluminum alloy.

[0013] Beneficial effects: Al-Cu-Mg aluminum alloy and Al-Zn-Mg aluminum alloy, Al-Zn-Mg has higher strength, but poorer heat resistance. It can be considered to be used at the wellhead end of the oil and gas well during design. The Al-Cu-Mg series is lower in strength than the Al-Zn-Mg series, but has better heat resistance, and can be used in the lower part of the oil and gas well. It can adapt to the complex environmental conditions under the oil well and ensure reliability.

[0014] Furthermore, in step S2, the strength, internal pressure strength and external pressure strength of the oil casing and coupling are calculated according to the standard values ​​of the yield strength and tensile strength of the aluminum alloy material to confirm the joint of the oil casing and the wall thickness of the coupling.

[0015] Beneficial effects: By calculating the tensile strength of the oil casing and coupling, it can be ensured that no breakage or permanent deformation will occur when subjected to the maximum expected axial force. The internal pressure strength calculation can ensure the strength state of the inside of the oil casing under high pressure conditions to prevent the pipe body from bursting due to excessive internal pressure. The external pressure strength calculation can ensure that the pipe body will not undergo plastic deformation or instability when the external pressure is high. The above calculations can be used to confirm the wall thickness of the oil casing joints and couplings that meet the strength requirements to ensure reliability.

[0016] Further, in step S2, the types of connecting threads include long round threads and special threads.

[0017] Beneficial effect: A variety of threads are selected for strength calculation to facilitate matching of aluminum alloy oil casing and couplings, thereby ensuring that the oil casing and couplings can obtain the best connection strength and sealing performance.

[0018] Further, in step S3, the finite element analysis includes:

[0019] a. Test the stress-curve of Al-Cu-Mg and Al-Zn-Mg aluminum alloy materials and establish the material constitutive model;

[0020] b. Test the friction coefficient of Al-Cu-Mg aluminum alloy;

[0021] c. Determine the basis for envelope calculation, and calculate the envelope of long round threads, special threaded oil casing joints and couplings;

[0022] d. Study the influence of friction coefficient on the maximum stress of connecting thread based on long round thread; (add an enter key)

[0023] e. Finite element models of long round threads and special threads were established to calculate the stress distribution under corresponding working conditions.

[0024] Beneficial effects: Through finite element analysis, the stress distribution of couplings and casing after connection with different materials and different thread specifications is simulated, so as to identify the specific parts where the strength may be insufficient, and then adjust the structural dimensions of the corresponding parts of the thread according to the stress distribution, so as to design the coupling with the optimal structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a schematic diagram of the connection between the middle coupling 2 and the oil casing 1 of the present invention.

[0026] Figure 2 The figure is a comparison chart of the tensile strength of the oil casing and its joints, and couplings made of different materials at different wall thicknesses in the present invention.

[0027] Figure 3 It is a comparison chart of the internal pressure strength of the oil casing and its joints, and couplings made of different materials at different wall thicknesses in the present invention.

[0028] Figure 4 The figure is a comparison chart of the internal pressure strength of couplings made of different materials at different wall thicknesses in the present invention.

[0029] Figure 5 It is a comparison chart of the connection strength of the oblong thread and the special thread in the present invention.

[0030] Figure 6 1 is a diagram of the external thread structure parameters of the oil casing of the first embodiment.

[0031] Figure 7 This is a diagram of the internal thread structure parameters of the coupling of the first embodiment. DETAILED DESCRIPTION

[0032] The following is further described in detail through specific implementation methods:

[0033] The symbols in the drawings of the specification include: oil casing 1, coupling 2.

[0034] Embodiment 1

[0035] like Figure 1 As shown, this embodiment takes an oil casing 1 with a length of 5331 m in an oil well with a formation temperature below 160° C. as an example to illustrate the design of the coupling 2 , wherein the outer diameter of the casing is 177.8 mm and the wall thickness is 14 mm.

[0036] A method for designing a casing coupling for a deep well comprises the following steps:

[0037] S1. Determine the aluminum alloy material of the oil casing according to the depth of the oil and gas well, and then confirm the aluminum alloy material series of the coupling according to the aluminum alloy material of the oil casing; in this embodiment, in the formation with a temperature below 120°C, the material of the oil casing is Al-Zn-Mg aluminum alloy, and in the formation with a temperature above 120°C and below 160°C, the material of the oil casing is Al-Cu-Mg series aluminum alloy. This embodiment takes the material of the oil casing as Al-Cu-Mg series aluminum alloy as an example. Considering the environment of deep wells and ultra-deep wells, the material of the coupling can be selected from Al-Cu-Mg series aluminum alloy or Al-Zn-Mg series aluminum alloy. This series of aluminum alloys has the characteristics of high strength and hardness, good toughness, and corrosion resistance, and can adapt to the complex environmental conditions under the oil well.

[0038] S2. Perform strength calculations on tubing and casing and couplings made of different series of aluminum alloys to confirm the wall thickness of the joints between the couplings and tubing, as well as the type of connecting threads.

[0039] S2.1 The strength calculation is performed according to the standard values ​​of the yield strength and tensile strength of the aluminum alloy material. The outer diameter of the coupling is determined according to the outer diameter of the casing and tubing according to the industry standard. In this embodiment, the outer diameter of the coupling is 200.03 mm. The standard values ​​of the yield strength and tensile strength are as follows:

[0040] Al-Cu-Mg aluminum alloy: yield strength σs = 325MPa, tensile strength σb = 460MPa;

[0041] Al-Zn-Mg aluminum alloy: yield strength σs = 480 MPa, tensile strength σb = 530 MPa;

[0042] First, the tensile strength index of the pipe body, joint, and coupling is calculated without considering the threaded connection. In this embodiment, the tensile strength index of the pipe body, joint, and coupling of the oil casing is not less than 1900KN. Under the condition of known outer diameter and standard tensile strength σb and standard yield strength σs, the minimum wall thickness of the oil casing joint and coupling that meets the minimum tensile strength index of 1900KN in this embodiment can be calculated by the tensile strength mechanics formula, such as Figure 2 As shown, the minimum wall thickness of the oil casing joints must be ≥12mm, and the minimum wall thickness of the Al-Cu-Mg aluminum alloy couplings and Al-Zn-Mg aluminum alloy couplings must be ≥11mm. Then the internal pressure strength of the pipe body, joints, and couplings without considering the threaded connection is calculated: In this embodiment, the internal pressure strength index that the pipe body, joints, and couplings of the oil casing should be able to bear is not less than 55MPa. When the outer diameter is known, according to the downhole pressure and internal pressure strength calculation formula, the minimum wall thickness that can withstand an internal pressure strength of not less than 55MPa in this embodiment can be obtained, as shown in FIG. Figure 3 As shown, it can be confirmed that the minimum wall thickness of the oil casing joint must be ≥15mm, the minimum wall thickness of the Al-Cu-Mg series aluminum alloy coupling must be ≥16mm, and the minimum wall thickness of the Al-Zn-Mg series aluminum alloy coupling must be ≥15mm.

[0043] Yield pressure in non-threaded pipes, joints and couplings:

[0044]

[0045] Where P is the minimum internal yield pressure in pounds per square inch (psi), rounded to the nearest 10 (psi); Y P The minimum yield strength is specified for the material in pounds per square inch (psi); t is the nominal wall thickness in inches (in); and D is the nominal outside diameter in inches (in).

[0046] Finally, the external pressure strength calculation without considering the threaded connection is carried out: the external pressure is calculated according to GB / T20657-2006P4 formula 8, and the external pressure strength is preferably the maximum value; Figure 4 As shown, under the condition of equal wall thickness, the compressive strength of Al-Zn-Mg series aluminum alloy coupling is greater than that of Al-Cu-Mg series aluminum alloy coupling.

[0047] External yield pressure of non-threaded pipes, joints and couplings under axial force:

[0048]

[0049] Where S a is the axial stress (tensile is positive), in pounds per square inch (psi), Y PY is the minimum yield strength of the pipe material in pounds per square inch (psi). pa The yield strength is the equivalent value of axial stress, expressed in pounds per square inch (psi).

[0050] S2.2 Analyze the connection strength and internal pressure strength of long round threads and special threads based on the thread strength calculation formula.

[0051] Calculation of threaded connection strength:

[0052] When the outer diameter and wall thickness of the oil casing and the outer diameter and wall thickness of the coupling are known, the appropriate long round thread and special thread can be preliminarily selected according to the industry standard, and then the connection strength of the long round thread and special thread can be compared according to the thread strength calculation formula, such as Figure 5 As shown, the wall thickness of the oil casing reaches 15mm, and the connection strength of the special thread is significantly higher.

[0053] Special threaded pipe connection strength: P j =0.95A p U p [1.008-0.0396(1.083-Y P / U p )D]

[0054] Special threaded coupling connection strength: P j =0.95A C U C

[0055] Where P j is the minimum connection strength, in pounds (Ib); Y P is the minimum yield strength of the pipe material in pounds per square inch (psi), U p is the minimum tensile strength of the pipe material, in pounds per square inch (psi), U C A is the minimum tensile strength of the coupling in pounds per square inch (psi). p is the cross-sectional area of ​​the plain end pipe in square inches (in 2 );A C is the cross-sectional area of ​​the coupling in square inches (in 2 ), D is the outside diameter of the pipe, in inches (in);

[0056] Long round thread connection strength:

[0057] Breaking strength: P j =0.95A jp U p

[0058] Slip strength:

[0059] Where P j is the minimum connection strength, in pounds (Ib); A jp The cross-sectional area of ​​the pipe at the last complete thread, in square inches. 2 );Y P is the minimum yield strength of the pipe material in pounds per square inch (psi), U p is the minimum tensile strength of the pipe material, in pounds per square inch (psi); D is the outside diameter of the pipe, in inches (in); and L is the thread engagement length, in inches (in).

[0060] The internal pressure strength of threaded joints and couplings is the lower value of the yield pressure of the unthreaded pipe and the yield pressure of the coupling. The calculation formula is as follows:

[0061] The yield pressure of the non-threaded joint is based on GB / T20657-2006 P13 formula 31:

[0062]

[0063] Yield pressure inside threaded coupling:

[0064]

[0065] Where P is the minimum internal yield pressure in pounds per square inch (psi), rounded to the nearest 10 (psi); Y P The minimum yield strength of the material is specified in pounds per square inch (psi); t is the nominal wall thickness in inches (in); D is the nominal outside diameter in inches (in); d1 is the diameter of the coupling thread root corresponding to the external thread end face in the machine-tight state, in inches (in); W is the outside diameter of the coupling in inches (in).

[0066] For round threaded casing and tubing:

[0067] d1=E1-(L1+A)T+H-2S m )

[0068] Where E1 is the finger-tight face median diameter in inches (API Spec 5B).

[0069] For trapezoidal threads:

[0070] d1=E7-(L7+I)T+0.062)

[0071] Where, E7 is the pitch diameter in inches (in) (API Spec 5B), and L7 is the full thread length in inches (in) (API Spec 5B). The values ​​of I and T are selected according to the following table:

[0072]

[0073] S2.3 Preliminary confirmation is that the coupling material is Al-Zn-Mg aluminum alloy, the wall thickness is ≥15mm, and the thread is a special thread.

[0074] S3. Use finite element analysis software to perform finite element simulation analysis on the tubing, casing and coupling, simulate the stress distribution of the tubing, casing and coupling when they are loaded, and confirm the thread parameters with the best strength; the specific steps are as follows:

[0075] a. Test the stress-curve of Al-Cu-Mg aluminum alloy and Al-Zn-Mg aluminum alloy materials and establish the material constitutive model;

[0076] b. Test the friction coefficient of Al-Cu-Mg aluminum alloy;

[0077] c. Determine the basis for envelope calculation, calculate the envelope of long round thread, special thread oil casing joints and couplings; through the calculation of envelope, determine the load combination value (including axial load and internal pressure load) of finite element simulation of long round thread, short round thread and special thread coupling, so as to facilitate the evaluation of the impact of multiple loads on the structure and ensure the safety and reliability of the design;

[0078] d. Based on the long round thread and special thread, the influence of friction coefficient on the maximum stress of the connecting thread is studied; if the friction coefficient is too small, the bolt is prone to self-loosening; if the friction coefficient is too large, the conversion efficiency of the clamping torque is low and the stick-slip effect is prone to occur. Therefore, the friction coefficient should be controlled within a reasonable range; the preload dispersion error can be reduced.

[0079] e. Establish finite element models of long round threads and special threads, and simulate and calculate the stress distribution of each surface of each tooth of the thread under the corresponding working conditions. Under the premise of meeting the strength requirements, adjust the parameters of the connecting thread and the wall thickness of the coupling according to the stress distribution feedback. The thread parameters include the taper of the thread cone, pitch, thread height, tooth front rake angle, tooth back rake angle, tooth top front rounding, and tooth top back rounding.

[0080] S4. According to the finite element analysis test results, the wall thickness of the coupling, the type and parameters of the thread connecting the oil casing and the coupling, and the final material of the coupling are finally confirmed.

[0081] S4.1 Based on the finite element analysis test results, determine the most suitable coupling specifications:

[0082] For the Al-Cu-Mg aluminum alloy casing with an outer diameter of 177.8mm and a wall thickness of 15mm, the coupling with an outer diameter of 200mm, a wall thickness of 25.1mm and a material of Al-Zn-Mg aluminum alloy was finally selected.

[0083] S4.2 According to the finite element analysis test results, it is confirmed that the most suitable thread type is the conical trapezoidal thread:

[0084] The thread parameters of the oil casing joint are as follows: Figure 6 As shown: the reference thread length is 191.36mm; the cone taper is 1:16, the pitch is 5.08mm (tooth groove width 2.54mm + tooth width 2.54mm), the thread height is 1.676mm (tooth bottom and center diameter height 0.838mm + tooth top and center meridian height 0.838mm), the front rake of the tooth is 3° and the rear rake of the tooth is 10°, the front chamfer of the tooth top is R0.3 and the rear chamfer of R0.7, the front chamfer of the tooth bottom is R0.2 and the rear chamfer of R0.3; the angle of the joint end face is 15°, the inclination angle of the sealing surface is 20°, and the tip diameter is φ163.66mm.

[0085] Coupling thread: Aluminum alloy coupling with a length of 435.00 mm and a minimum thread length of 183.36 mm. The structural dimensions of the aluminum alloy casing coupling are as follows: Figure 7 As shown; the parameters of the conical trapezoidal thread are: conical taper 1:16, pitch 5.08mm (tooth groove width 2.44mm + tooth width 2.64mm), thread height 1.776mm (tooth bottom and mid-diameter height 0.938mm + tooth top and mid-diameter height 0.838mm), front face inclination of the tooth 3° and rear face inclination 10°, front chamfer R0.4 and rear chamfer R0.2 at the tooth top, front chamfer R0.6 and rear chamfer R0.3 at the tooth bottom; the angle of the joint end face is 15°, the inclination angle of the sealing surface is 20°, and the tip diameter is φ163.22mm.

[0086] The above are only embodiments of the present invention. The invention is not limited to the field involved in this implementation case. The common knowledge such as the known specific structure and characteristics in the scheme is not described in detail here. It should be pointed out that for those skilled in the art, several deformations and improvements can be made without departing from the structure of the present invention, which should also be regarded as the protection scope of the present invention, and these will not affect the effect of the implementation of the present invention and the practicality of the patent. The scope of protection required by this application shall be based on the content of its claims, and the specific implementation methods and other records in the specification can be used to interpret the content of the claims.

Claims

1. A design method for a deep well oil casing coupling, characterized in that: The method comprises the following steps: S1, determining the aluminum alloy material of the oil casing according to the depth of the oil and gas well, and then confirming the aluminum alloy material series of the coupling according to the aluminum alloy material of the oil casing; S2, performing strength calculation on the oil casing and couplings of different series of aluminum alloy materials, confirming the wall thickness of the joint of the oil casing and coupling, and the type of connecting thread; S3, performing finite element simulation analysis on the oil casing and coupling, simulating the stress distribution when the oil casing and coupling are connected and loaded, and confirming the thread parameters with the best strength; S4. According to the finite element analysis test results, the wall thickness of the coupling, the type and parameters of the thread connecting the oil casing and the coupling, and the final material of the coupling are finally confirmed.

2. The method for designing a deep well casing coupling according to claim 1, characterized in that: In step S2, it is confirmed that the coupling material includes Al-Cu-Mg aluminum alloy and Al-Zn-Mg aluminum alloy.

3. The design method of a deep well oil casing coupling according to claim 2 is characterized in that: In step S2, the strength, internal pressure strength and external pressure strength of the oil casing and coupling are calculated according to the standard values ​​of the yield strength and tensile strength of the aluminum alloy material, and the joint of the oil casing and the wall thickness of the coupling are confirmed.

4. The method for designing a deep well oil casing coupling according to claim 3 is characterized in that: In step S2, the types of connecting threads include long round threads and special threads.

5. The method for designing a deep well casing coupling according to claim 4, characterized in that: In step S3, the finite element analysis includes: a. Test the stress-curve of Al-Cu-Mg and Al-Zn-Mg aluminum alloy materials and establish the material constitutive model; b. Test the friction coefficient of Al-Cu-Mg aluminum alloy; c. Determine the basis for envelope calculation, and calculate the envelope of long round threads, special threaded oil casing joints and couplings; d. Study the influence of friction coefficient on the maximum stress of connecting thread based on long round thread; e. Finite element models of long round threads and special threads were established to calculate the stress distribution under corresponding working conditions.

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