Titanium alloy threaded connection structure

Abstract

The present application relates to the technical field of oil casing, in particular to a titanium alloy threaded connection structure, comprising a pipe body and a coupling, the outer surface of the end of the pipe body is a male end, the male end has a male end thread and a male end sealing cylindrical surface, the inner surface of the coupling is a female end, the female end has a female end thread and a female end sealing cylindrical surface, the male end and the female end are connected through thread engagement, the stress uniformity, sealing performance and anti-sticking ability of the connection structure are improved through dynamic adjustment and optimization of each parameter of the thread, and safe connection and effective sealing of titanium alloy oil casing and oil pipes in extreme service environments are realized.

Description

Technical Field

[0001] This invention relates to the field of oil casing technology, specifically to a titanium alloy threaded connection structure, which is particularly suitable for high temperature, high pressure, and environments with high hydrogen sulfide and carbon dioxide content. Background Technology

[0002] During the service of oil and gas wells, traditional carbon steel casing and tubing are susceptible to corrosion, wear, and fatigue failure due to high temperatures, high pressures, and corrosive media (such as hydrogen sulfide and carbon dioxide), seriously threatening the safety of downhole equipment. Titanium alloys, due to their low density, high strength, corrosion resistance, and high temperature resistance, have become ideal materials for casing and tubing, but they have problems such as low elastic modulus, high Poisson's ratio, and a tendency to stick together.

[0003] To improve the anti-sticking performance of titanium alloy threads, Chinese patent CN106011807A discloses a surface treatment method for titanium alloy oil pipe threads, relying on the wear resistance of chemical nickel-phosphorus plating. However, this technical approach has inherent defects: the oxide film on the titanium alloy surface is stable, and the plating requires substrate destruction for activation, which easily leads to plating peeling; moreover, the plating process is complex (including sandblasting, pickling, electroplating, etc.), posing risks of hydrogen embrittlement and corrosion failure, resulting in poor long-term reliability. Chinese patent CN103527110A discloses a multi-stage sealing anti-sticking thread for titanium alloy oil casing, employing multi-stage sealing and a complex shoulder structure, attempting to improve performance by increasing the number of sealing layers. However, this introduces new problems: the elastic modulus of titanium alloy is only 50% of that of steel (approximately 110 GPa), the complex shoulder structure easily leads to stress concentration, and poor thermal matching; moreover, the multi-stage structure requires extremely high assembly precision (≤0.01 mm), and micro-defects can easily cause inter-stage leakage, resulting in high process and testing costs.

[0004] Therefore, a titanium alloy threaded connection structure is needed that is simple in structure, avoids stress concentration, has high stability, and has excellent sealing performance and anti-sticking ability. Summary of the Invention

[0005] To overcome the shortcomings of the prior art, the present invention provides a titanium alloy threaded connection structure. By comprehensively optimizing the parameters of the thread structure, the stress uniformity, sealing performance and anti-sticking ability of the connection structure are improved, so as to achieve safe connection and effective sealing of titanium alloy oil casing and oil pipe under extreme service environment.

[0006] This invention is achieved through the following technical solution:

[0007] This invention provides a titanium alloy threaded connection structure, including a pipe body and a coupling. The outer surface of the pipe body is a male end, which has a male thread and a male sealing cylindrical surface. The inner surface of the coupling is a female end, which has a female thread and a female sealing cylindrical surface. The male and female ends are connected by threaded engagement. The thread tooth length L-pin-thread of the male end satisfies the formula:

[0008]

[0009] After sorting, we get:

[0010]

[0011] Where D is the outer diameter of the tube, t is the nominal wall thickness of the titanium alloy tube, H-thread is the thread tooth height, E is the elastic modulus of the titanium alloy (approximately 110 GPa, note that both 110 GPa and 210 GPa use the same unit), and v is the Poisson's ratio of the titanium alloy.

[0012] The formula for calculating the thread profile length L-box-thread at the female end is:

[0013]

[0014] Where Tap is the taper of the thread tooth profile; L-pin-thread is the length of the male thread tooth profile, and H-thread is the height of the thread tooth profile;

[0015] Female end sealing cylinder length (L-box-seal): Same as male end sealing cylinder length, i.e.

[0016] L-box-seal = L-pin-seal,

[0017] The length of the male end sealing cylinder (L-pin-seal) ranges from D / 18 to D / 20 to ensure precise matching between the sealing surface and the threaded connection area.

[0018] The axial interference fit of the thread, L-zhouxiang, satisfies the formula: L-zhouxiang=L-pin-seal×ν.

[0019] This ensures sufficient contact pressure after the threads are fully engaged, achieving an effective elastic seal.

[0020] Furthermore, the outer diameter D of the tube is 60.32 mm to 339.72 mm.

[0021] Furthermore, the nominal wall thickness t of the titanium alloy tube is 5mm to 15mm.

[0022] Furthermore, the thread tooth height H-thread is 1.0mm to 3.0mm.

[0023] Furthermore, the elastic modulus E of the titanium alloy material is 80-140 GPa.

[0024] Furthermore, the Poisson's ratio v is 0.29-0.39.

[0025] Furthermore, the thread tooth taper (Tap) is 1 / 4 to 1 / 18. The thread tooth taper is used to adjust the taper angle of the thread and improve the uniformity of force distribution.

[0026] This invention abandons the "structural stacking" approach and coating route, and uses parametric design to dynamically compensate for the characteristics of titanium alloys: the deformation is corrected by the "E / 210" term in the formula, and an equivalent sealing effect (sealing pressure ≥100MPa) is achieved by using a single-stage cylindrical seal + axial interference. This not only simplifies the structure by 60%, but also drives the optimization of thread parameters with the elastic modulus (E) and Poisson's ratio (ν), fundamentally avoiding local stress concentration. The thread length and interference are directly optimized by the formula, without relying on the coating, completely preserving the integrity of the titanium alloy matrix, eliminating new risks such as hydrogen embrittlement and coating embrittlement, greatly reducing process complexity, adapting to the low elastic modulus characteristics of titanium alloys, and significantly improving long-term reliability.

[0027] In summary, this invention replaces "surface modification" and "structural complexity" with "parameter optimization," improving core indicators such as the adaptability of titanium alloys, process complexity, and long-term reliability. In particular, it demonstrates fundamental innovative advantages in solving the technical problems caused by the low elastic modulus and easy sticking of titanium alloys.

[0028] The advantages and beneficial effects of this invention are:

[0029] 1. By dynamically adjusting the thread tooth height and tooth length through parametric design, the stress distribution is ensured to be uniform when the threaded connection is under load, and fatigue failure caused by local stress concentration is prevented.

[0030] 2. The elastic modulus E and Poisson's ratio ν are introduced to correct the elastic deformation of titanium alloys under high temperature and high pressure conditions and optimize the matching relationship between thread length and pipe wall thickness.

[0031] 3. The dimensions of the male and female sealing cylinders are set according to fluid sealing theory to ensure a stable and continuous sealing effect at the contact surfaces after the threads are fully engaged.

[0032] 4. By rationally designing the thread tooth profile and taper, the difficulties in loading and unloading caused by material adhesion during the connection process are reduced, thereby improving maintenance efficiency. Attached Figure Description

[0033] Figure 1 This is a schematic diagram of the pipe body and coupling of the titanium alloy threaded connection structure of the present invention. Detailed Implementation

[0034] The present invention will be further described in detail below through specific embodiments. The following embodiments are merely descriptive and not limiting, and should not be used to limit the scope of protection of the present invention.

[0035] In practical implementation, based on the above design parameters and theoretical foundation, the optimal design parameters can be determined through numerical calculation and finite element analysis to meet the connection and sealing requirements of titanium alloy oil casing or tubing under different working conditions. The following examples illustrate this in detail.

[0036] Example 1

[0037] A titanium alloy threaded connection structure based on parametric design includes a pipe body and a coupling. The outer surface of the pipe body is the male end, which has a male thread and a male sealing cylindrical surface. The inner surface of the coupling is the female end, which has a female thread and a female sealing cylindrical surface. The male and female ends are connected by threaded engagement, with the following parameters:

[0038] The outer diameter of the tube is D = 244.48 mm, and the wall thickness is t = 11.99 mm.

[0039] Thread tooth profile height H-thread = 2.0 (typical value)

[0040] The taper of the thread tooth is 1 / 8.

[0041] The elastic modulus of titanium alloy is E = 110 GPa (which is the same unit as 210 GPa, so E / 210≈0.5238).

[0042] The Poisson's ratio of titanium alloy is ν = 0.34 (therefore, 1 - ν = 0.66).

[0043] Calculation of male thread tooth length (L-pin-thread):

[0044] From the formula

[0045]

[0046] have to:

[0047] The length of the male end sealing cylinder, L-pin-seal, is taken as D / 18-D / 20; D / 18≈13.58mm, D / 20≈12.22mm, and the median value is selected as approximately 12.9mm.

[0048] Female end thread tooth length:

[0049]

[0050] in

[0051] Therefore, L-box-thread = 29.53 mm.

[0052] The length of the female end sealing cylinder, L-box-seal, is approximately 12.9 mm.

[0053] The axial interference of the thread, L-zhouxiang, satisfies the formula:

[0054] The above parameters have been theoretically calculated and optimized to ensure the mechanical properties and sealing effect of the casing under high temperature, high pressure and corrosive media environments. At the same time, the anti-sticking performance has been improved by appropriate thread parameter design, which meets the design requirements of large titanium alloy oil casing.

[0055] Example 2

[0056] A titanium alloy threaded connection structure based on parametric design includes a pipe body and a coupling. The outer surface of the pipe body is the male end, which has a male end thread and a male end sealing cylindrical surface. The inner surface of the coupling is the female end, which has a female end thread and a female end sealing cylindrical surface. The male end and the female end are connected by threaded engagement.

[0057] The parameters are as follows:

[0058] The outer diameter of the tube is D = 88.9 mm, and the wall thickness is t = 6.45 mm.

[0059] Thread tooth height H-thread = 1.5 (typical value)

[0060] The taper of the thread tooth profile, Tap, is 1 / 8.

[0061] The elastic modulus of titanium alloy is E = 110 GPa (which is the same unit as 210 GPa, so E / 210≈0.5238).

[0062] The Poisson's ratio of titanium alloy is ν = 0.34 (therefore 1 - ν = 0.66).

[0063] The formula for calculating the male end thread tooth length (L-pin-thread) is:

[0064]

[0065] The calculated L-pin-thread length is 18.4 mm.

[0066] The length of the male end sealing cylinder, L-pin-seal, is taken as D / 18-D / 20; D / 18≈4.94mm, D / 20≈4.45mm, and the median value is selected as approximately 4.7mm.

[0067] Female end thread tooth length:

[0068] Therefore, L-box-thread = 20.8mm.

[0069] Length of the female end sealing cylinder: L-box-seal = L-pin-seal ≈ 4.7mm.

[0070] Thread axial interference: L-zhouxiang=L-pin-seal×ν≈4.7×0.34≈1.60mm.

[0071] To evaluate the sealing effect, a composite load testing machine is used to apply internal pressure, external pressure, axial tensile load, and axial compressive load to the thread structure and sealing structure to verify the sealing effect of the thread, or the entire tubing string is lowered into the corresponding high-pressure well to test whether the tubing string can guarantee the sealing effect.

[0072] For the titanium alloy special thread structure designed and developed in Example 1, with an outer diameter of 88.9 mm and a wall thickness of 7.34 mm, a composite load evaluation test was conducted in a high-temperature and high-pressure downhole environment. The test was conducted at a temperature of 160°C, a maximum internal pressure of 93 MPa, and an external pressure of 90 MPa. No leakage was observed during the test, and the tubing string maintained excellent sealing performance. The tubing could complete more than 10 uncoupling cycles without sticking. In a high carbon dioxide and hydrogen sulfide environment, no corrosive media flowed out, successfully sealing corrosive media such as hydrogen sulfide and carbon dioxide within the formation inside the tubing. The tubing could complete more than 10 uncoupling cycles without sticking.

[0073] A composite load evaluation test was conducted on the titanium alloy special thread structure designed and developed in Example 2, with an outer diameter of 88.9 mm and a wall thickness of 6.45 mm, under an internal pressure of 93.26 MPa, an external pressure of 96.3 MPa, and a high temperature of 180°C. The test lasted 30 days. After completion, the sealing effect was good, and the overall tubing structure did not experience structural failure or leakage. The tubing also completed more than 10 uncoupling cycles without sticking.

[0074] This invention, through parameter optimization design, greatly improves the long-term stability of the special threaded structure of titanium alloy, significantly enhances the anti-sticking ability and sealing performance, reduces the frequency of threaded structure replacement, and improves usage efficiency.

[0075] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several modifications and improvements can be made without departing from the inventive concept, and these all fall within the protection scope of the present invention.

Claims

1. A titanium alloy threaded connection structure, comprising a pipe body and a coupling, wherein the outer surface of the pipe body is a male end, the male end having a male end thread and a male end sealing cylindrical surface, and the inner surface of the coupling is a female end, the female end having a female end thread and a female end sealing cylindrical surface, the male end and the female end being connected by threaded engagement, characterized in that, The length L-pin-thread of the male end thread profile satisfies the formula: Where D is the outer diameter of the tube; t is the nominal wall thickness of the titanium alloy tube; H-thread is the thread tooth height; E is the elastic modulus of the titanium alloy material; and ν is Poisson's ratio. The length L-box-thread of the female end thread profile satisfies the following formula: Where Tap is the taper of the thread tooth profile; L-pin-thread is the length of the male thread tooth profile, and H-thread is the height of the thread tooth profile; The axial interference of the thread, L-zhouxiang, satisfies the formula: L-zhouxiang=L-pin-seal×v, Where L-pin-seal is the length of the male end sealing cylinder, and ν is Poisson's ratio.

2. The titanium alloy threaded connection structure according to claim 1, characterized in that, The length of the male end sealing cylinder is equal to the length of the female end sealing cylinder, which is D / 18-D / 20.

3. The titanium alloy threaded connection structure according to claim 1, characterized in that, The outer diameter of the tube is 60.32 mm to 339.72 mm.

4. The titanium alloy threaded connection structure according to claim 1, characterized in that, The nominal wall thickness of the titanium alloy tube is 5mm to 15mm.

5. The titanium alloy threaded connection structure according to claim 1, characterized in that, The thread tooth height is 1.0mm to 3.0mm.

6. The titanium alloy threaded connection structure according to claim 1, characterized in that, The elastic modulus of the titanium alloy material is 80-140 GPa.

7. The titanium alloy threaded connection structure according to claim 1, characterized in that, The Poisson's ratio is 0.29-0.

39.

8. The titanium alloy threaded connection structure according to claim 1, characterized in that, The taper of the thread tooth profile is 1 / 4 to 1 / 18.

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