A serrated thread joint for oil casing

Abstract

The present invention relates to a sawtooth thread joint for oil casing, comprising: a pipe (1) and a coupling (2); the outer surface of the end of the pipe (1) forms a male thread (101); the inner surface of the end of the coupling (2) forms a female thread (202); the end of the pipe (1) is inserted into the end of the coupling (2); a sealed connection is achieved by screwing the male thread (101) and the female thread (202); the sawtooth thread profile parameters of the male thread (101) and the female thread (202) are: tooth profile angle α=60°, load side angle 3°, lead-in side angle 57°, tooth height h=1.427 mm, pitch p: 8 teeth / in, taper T=1:16; the overall length L4 of the male thread is 13.5*D 2 / 5 ≤L4≤14*D 2 / 5 Calculate and then round up, where D is the outer diameter of the tube; the tightening allowance A=f*D 3 / 4 , where D is the outer diameter of the tube, f is a coefficient, and f is 0.10 to 0.12. The overall size of the threaded joint obtained in this way can meet the requirements of tensile strength, sealing performance, and anti-galling performance of the threaded joint.

Description

Technical Field

[0001] The invention relates to a threaded joint for oil casing, and more particularly to a serration threaded joint. Background Art

[0002] Oil and gas extraction engineering, in simple terms, is the engineering technology of transporting oil and gas stored in the formation to the ground through pipelines. The pipe string composed of oil well pipes (and related components) as the main structure of oil and gas wells is such a transportation pipeline. Pipe strings that are hundreds of meters or even thousands of meters long are generally made up of multiple single pipes connected by threads. As the weakest link in the pipeline, the connection strength and sealing reliability of the threaded joint play a vital role in the construction and development of oil and gas wells. Even the failure of a threaded joint may cause immeasurable and significant losses.

[0003] For a long time, the performance defects of oil well pipe threads (API threads for short) that meet the standards of the American Petroleum Institute (API) have not been effectively solved in the field of oil well pipe manufacturing. API round threads have low tensile strength and poor anti-galling performance; while API stepped threads have high connection strength, but poor sealing performance of threaded joints, and although their anti-galling performance is better than that of round threads, it is still not very ideal.

[0004] On the other hand, for the oil pipes used in oil and gas wells deeper than 3,000 meters, API EUE is generally used to meet the required connection strength and sealing requirements. For oil pipe pipe companies, the "pipe end roughing" and heat treatment required to produce API EUE thickened oil pipes will extend the production cycle and increase energy consumption, and the repair cost of API EUE thickened oil pipes with unqualified thread processing is higher.

[0005] Sawtooth thread is different from hook thread. Sawtooth thread only has the lead-in side and the load-bearing side, while hook thread also has the tooth bottom and tooth top. Sawtooth thread is a deformed round thread, while hook thread is a deformed trapezoidal thread. Sawtooth thread joints adhere to the design concept of "simple structure and excellent performance", adopt the flat end non-thickening design of serrated thread, and achieve the best combination of product performance and product production cost through detailed optimization of thread parameters, striving to achieve the following two overall goals:

[0006] (1) Inherit the advantages of API threads and overcome their disadvantages. Break through the technical bottleneck of low tensile strength of API round threads and avoid API EUE thickened oil pipes with high production costs, long production cycles and difficult repairs.

[0007] (2) No additional metal sealing structure is added to the serrated thread joint, thereby avoiding the complexity of the sealing structure of the thread joint. Summary of the invention

[0008] The object of the present invention is to provide a serrated thread joint for oil casing, which involves the calculation of the overall length L4 of the male thread and the tightening allowance A (also referred to as the tightening allowance in this article) of the joint structure dimensions.

[0009] Joint structure dimensions: The calculation of the overall length of the male thread L4 and the tightening allowance A is based on the thread profile. The basic parameters of the thread profile are as follows:

[0010] Thread profile angle α = 60°, load side angle 3°, lead-in side angle 57°, thread profile angle = load side angle + lead-in side angle;

[0011] Thread tooth height h = 1.427mm;

[0012] Thread pitch p = 3.175 mm, i.e. 8 threads / in;

[0013] Thread taper T = 1:16, i.e. 0.0625 in / in;

[0014] The oil pipes of different steel grades and specifications are further designed with joint structural dimensions based on the tooth profile of the above parameters, so as to obtain specific threaded joint structures matching the steel grades and specifications: full length of male thread L4, make-up machine tightening allowance A, coupling length NL, etc. This application specifically relates to the calculation method of full length of male thread L4 and make-up machine tightening allowance A. The calculation method is more suitable for pipes with a nominal outer diameter D: 2.375in~5.5in, that is, pipes with a nominal outer diameter D: 60.32mm~139.7mm.

[0015] The technical solution of the present invention is:

[0016] 1. Calculation of the full length L4 of the male thread

[0017] Male thread length L4 is 13.5*D 2 / 5 ≤L4≤14*D 2 / 5 The calculation is performed and then rounded, where D is the outer diameter of the tube.

[0018] If the total length L4 of the male thread is too long, the end face of the pipe will be too narrow, the rigidity will be low, and deformation will be easy to occur; if the total length L4 of the male thread is too short, the number of thread teeth involved in the engagement will be too small, the connection strength will be too low, and slippage will occur easily.

[0019] 2. Calculation of the tightening allowance A of the upper fastening machine

[0020] The tightening allowance of pipe and coupling is A=f*D 3 / 4 , where D is the outer diameter of the tube, f is a coefficient, f is between 0.10 and 0.12, and a more preferred value of f is 0.1025.

[0021] The tightening allowance A is related to the tensile strength, sealing performance and make-up and make-up performance of the threaded joint. During the tightening process of the pipe and coupling threads, as the make-up torque increases, the male and female threads will undergo a matching process from "loose" to "tight". After being made to the designed position, a tight interference fit will be formed, and contact stress will appear on the surface of the male and female threads. The tightening allowance A affects the overall tensile capacity, sealing performance and make-up and make-up performance of the joint. If the tightening allowance A is designed to be too large, it is easy to cause excessive contact stress when the threads are engaged, and cold welding or sticking is easy to occur; if the tightening allowance A is designed to be too small, the threads will be too loose, the pipe and coupling will easily fall off, the sealing will be poor, and leakage is easy to occur.

[0022] Since it is impossible to directly establish a formula for the tightening allowance A and the tensile strength, sealing performance and make-up and make-up performance of the joint, the calculation formula for the make-up tightening allowance A in this application is an empirical formula that comprehensively considers the tensile strength, sealing performance and make-up and make-up performance.

[0023] Compared with the prior art, the advantages of the present invention are: this application aims at the design requirements of oil pipe threaded joints in the oil and gas production field, and on the basis of the basic parameters of the serrated thread tooth shape, provides the calculation formulas for the structural dimensions of the threaded joints: the total length L4 of the male thread and the tightening allowance A of the upper thread. The overall dimensions of the threaded joints obtained in this way can meet the tensile strength, sealing performance and anti-galling performance requirements of the threaded joints in the oil pipe field. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is an axial cross-sectional view of the serrated thread connection joint between a pipe and a coupling of the present invention;

[0025] Figure 2 The tooth profile of the box thread and the pin thread of the present invention;

[0026] Figure 3 It is a matching diagram of the male thread and the female thread of the present invention;

[0027] Figure 4 It is a matching diagram of the serrated thread connection joint of the present invention;

[0028] In the figure, pipe 1, coupling 2, pin thread 101, box thread 202, load-bearing side angle α1, and introduction side angle α2. DETAILED DESCRIPTION

[0029] The present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. The embodiments are exemplary and intended to be used to explain the present invention, but should not be understood as limiting the present invention.

[0030] Serration thread joints for oil casing

[0031] 1. Specification range:

[0032] The joint structure is processed on the pipe body with a nominal outer diameter D: 2.375in (60.32mm) to 5.5in (139.7mm).

[0033] Plain end non-upset threaded connection.

[0034] Thread direction: right-hand thread.

[0035] Thread surface treatment: External thread of oil pipe: no treatment or sandblasting; Internal thread of coupling: phosphating or copper plating.

[0036] 2. Technical objectives:

[0037] (1) Connection strength: The tensile strength of the threaded joint shall not be less than the tensile yield strength of the pipe body (coupling).

[0038] (2) Sealing performance: The sealing ability of the threaded joint reaches the internal yield strength of the pipe body (coupling): The sealing ability of the threaded joint is the internal pressure that the threaded joint can withstand without leakage after the female thread and the male thread are tightened and screwed to the specified position. The internal yield pressure value of the pipe body is: 2*f*Yp*t / D, refer to the API standard, where: f is the coefficient, representing the minimum wall thickness as a percentage of the nominal wall thickness, and f=0.875 for general oil casing; Yp is the yield strength of the pipe body, t is the wall thickness of the pipe, and D is the outer diameter of the pipe. When the internal yield pressure value reaches the internal yield strength, the sealing requirement is met.

[0039] (3) Anti-galling performance: The serrated thread joint meets the 9-in / 9-out performance, which is better than API thread.

[0040] 3. Tooth profile parameters of serrated thread, such as Figure 2 , 3

[0041] The thread profile angle α=60°. In this embodiment, the load-bearing side angle α1: 3°, the lead-in side angle α2: 57°, and α, α1, and α2 satisfy α=α1+α2;

[0042] Thread tooth height h = 1.427 (mm);

[0043] Thread pitch p = 3.175 mm (8 threads / in);

[0044] Thread taper T = 1:16 (0.0625 in / in);

[0045] Tooth top fillet radius R: 0.33mm

[0046] Tooth bottom fillet radius R': 0.3mm.

[0047] 4. Thread structure dimensions of serrated thread joints

[0048] (4.1) Male thread length L4

[0049] According to 13.5*D 2 / 5 ≤L4≤14*D 2 / 5 Perform the calculation and then round.

[0050] (4.2) Tightening allowance A

[0051] A=f*D 3 / 4

[0052] In the formula, the coefficient f ranges from 0.10 to 0.12, and 0.1025 is one of the preferred values.

[0053] V. Implementation

[0054] The structural parameters of the five groups of serrated thread joints are shown in Table 1

[0055] Table 1

[0056]

[0057] In the table, the thread median diameter E7=Dh, D: outer diameter of the pipe, h: thread tooth height.

[0058] According to the example, detailed calculations are performed:

[0059] (5.1) Calculation of tensile strength of threaded joints (quantitative): thread profile bearing area ≥ dangerous cross-sectional area of ​​pipe body (coupling). The calculation meets the requirements.

[0060] Tube cross-sectional area S 管体 =π Wt(D-Wt);

[0061] Coupling dangerous cross-sectional area S 接箍 = π / 4{W 2 -[D-(L4-g+A-1) / T] 2} Where g is the incomplete thread length, g=h*2 / T, h is the tooth height, and T is the thread taper;

[0062] Thread profile bearing area S 螺纹 = ((L4-g) / p+g / p / 3)(Dh)π 0.756, where p is the pitch;

[0063] When checking the tensile strength of threaded joints, it is sufficient to compare only the bearing (cross-sectional) areas of the threads, pipe body, and coupling.

[0064] The tensile strength of a thread depends on the sum of the tooth profile bearing areas over the entire length of the helix.

[0065] In the calculation formula of the thread profile bearing area, 0.756mm is the straight segment matching height of the thread profile, and the arc matching of the tooth bottom and tooth top is not included in the strength contribution. Take 1 / 3 of the number of incomplete thread teeth to be included in the strength contribution (g / p / 3). Considering the influence of chamfer, the number of teeth of the complete thread is taken to be included in the strength contribution (L4-g) / p. The helical length of the thread is approximately calculated by multiplying the number of thread teeth by the circumference of the thread at the mid-diameter.

[0066] Check the tensile strength of the thread of the serrated thread joint, see Table 2

[0067] Table 2

[0068]

[0069] (5.2) Sealing capability and anti-galling of serrated threads

[0070] The sealing performance is related to the tooth profile design and the tightening allowance A. After the male and female threads are engaged, the size of the gap between the engaged threads determines the sealing ability.

[0071] Anti-galling performance is also related to the tooth profile design and the tightening allowance A. After the male and female parts are engaged, the larger the tightening allowance A, the greater the contact stress of the meshing threads, and the more likely galling will occur.

[0072] The thread will block the leakage channel under the action of contact pressure: the buttock thread is a triangular thread. The thread tooth shape in the axial section is like a "wedge", and under the action of force, the "wedges" are "wedged" against each other. The spiral gap zone (i.e. the leakage gap) formed between the top and bottom of the male and female threads gradually shrinks and disappears under the action of the tightening torque, thereby forming a tight interference fit and finally achieving the sealing effect of the threaded connection.

[0073] Explain the thread sealing mechanism from the perspective of material deformation: elastic strain will be generated on both sides of the thread tooth shape under the action of force (contact stress), and the relationship between stress and strain should follow Hooke's law ε=σ / E. Due to the incompressibility of the material volume, the deformation will fill the leakage gap, thereby achieving the sealing effect of the thread.

[0074] The buttock thread adopts a 60° tooth angle asymmetric self-locking design, with a load-bearing side angle of 3° and an introduction side angle of 57°. The threaded joint is broken under tensile load. Under tensile load, there will be no gap between the tooth bottom and the tooth top, and the sealing performance is reliable. Figure 3 The small gap design between the tooth top and the tooth bottom (0.03mm), that is, the difference between the tooth top fillet radius and the tooth bottom fillet radius in this embodiment is 0.03mm, which improves the sealing performance.

[0075] (5.3) Radial contact stress between the tightening allowance A and the thread mating surface

[0076] Solving the contact stress between thread mating surfaces is very complicated. For simplicity, the thread mating is assumed to be an interference fit between two thin-walled cylinders. The mating contact stress between the two cylinders obtained by elastic mechanics is used to approximate the radial contact stress of the male and female thread mating surfaces, and then a quantitative force analysis of the thread tooth shape is performed.

[0077] like Figure 4 As shown in the figure, J is the distance between the pipe end and the center of the coupling after mechanical tightening; L7 is the complete thread length of the male thread; g is the incomplete thread length; the total length of the male thread L4=L7+g.

[0078] The radial contact stress Py between the male and female threads caused by the make-up is obtained from the elastic mechanics formula as follows:

[0079]

[0080] In the formula,

[0081] E: elastic modulus of the material E=2*10 5 MPa

[0082] δ: Thread interference A / T

[0083] W: outer diameter of coupling

[0084] E7: thread diameter

[0085] d: inner diameter of the tube, D-2*Wt

[0086] Total radial contact pressure = contact area * radial contact stress Py)

[0087]

[0088] Where, L: thread equivalent fit length L = L4-g / 2

[0089] The radial contact stress Py and total radial contact pressure F of the buttress thread joint are shown in Table 3.

[0090] Table 3

[0091]

[0092] Taking the 88.9*6.45mm sawtooth thread as an example, Py=35.4MPa, F=555.9KN. The API rough pipe with the same outer diameter and wall thickness has Py=72.3Mpa, F=1149.4KN. The radial contact stress Py of the sawtooth thread is lower and the anti-galling ability is better.

[0093] In addition to the above embodiments, the present invention also includes other implementation modes. Any technical solutions formed by equivalent transformation or equivalent replacement should fall within the protection scope of the claims of the present invention.

Claims

1. A serrated thread joint for oil casing, comprising: A pipe (1) and a coupling (2), wherein the outer surface of the end of the pipe (1) forms a male thread (101), and the inner surface of the end of the coupling (2) forms a female thread (202); the end of the pipe (1) is inserted into the end of the coupling (2), and a sealed connection is achieved by screwing the male thread (101) and the female thread (202); the sawtooth thread profile parameters of the male thread (101) and the female thread (202) are: tooth profile angle α=60°, load side angle 3°, lead-in side angle 57°, tooth height h=1.427 mm, thread pitch p: 8 teeth / in, and taper T=1:16; The feature is that the full length of the male thread L4 is 13.5*D 2 / 5 ≤L4≤14*D 2 / 5 Calculate and then round up, where D is the outer diameter of the tube; The tightening allowance of pipe and coupling is A=f*D 3 / 4 , where D is the outer diameter of the tube, f is the coefficient, and f takes a value of 0.10 to 0.

12.

2. The serrated thread joint for oil casing according to claim 1, characterized in that: The coefficient f is 0.1025.

3. The serration threaded joint for oil casing according to claim 1, characterized in that: The outer diameter D of the tube (1) ranges from 2.375 in to 5.5 in.

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