Sawtooth-threaded joint for oil casing pipe

Abstract

The present invention relates to a sawtooth-threaded structure for an oil casing pipe, the sawtooth-threaded structure comprising a pipe (1) and a coupling (2), wherein a male fastener thread (101) is formed on the outer surface of an end of the pipe (1), and a female fastener thread (202) is formed on the inner surface of an end of the coupling (2); and the end of the pipe (1) is inserted into the end of the coupling (2), and a sealed connection is achieved by means of engagement of the male fastener thread (101) and the female fastener thread (202). Sawtooth thread profile parameters of the male fastener thread (101) and the female fastener thread (202) include the thread angle α=60°, the load-bearing flank angle = 3°, the trailing flank angle = 57°, the thread height h=1.427 mm, the pitch p=8 threads / in, and the taper T=1:16. The total length L4 of the male fastener thread is calculated according to 13.5*D2 / 5≤L4≤14*D2 / 5, and rounding is then performed, where D is the outer diameter of the pipe. The make-up machine tightening allowance A=f*D3 / 4, where D is the outer diameter of the pipe, and f is a coefficient with a value ranging from 0.10 to 0.12. The overall dimensions of the threaded joint obtained thereby can meet requirements for the tensile strength, sealing performance and anti-galling performance of the threaded joint.

Description

A sawtooth thread joint for oil casing TECHNICAL FIELD

[0001] The present application relates to a thread joint for oil casing, more particularly a sawtooth thread joint. BACKGROUND

[0002] Oil and gas production engineering, simply speaking, is the engineering technology of transporting oil and gas stored in the formation to the ground through a pipeline, and the tubular column composed of the oil well pipe (and related components) as the main structure of the oil and gas well is such a transport pipeline. A tubular column hundreds of meters or even thousands of meters long is generally connected by multiple single pipes with threads. The connection strength and sealing reliability of the thread joint as the weakest link of the pipeline play a crucial role in the process of well construction and oil and gas development. Even if one thread joint fails, it may cause immeasurable and significant losses.

[0003] For a long time, the performance defects of the oil well pipe thread conforming to the American Petroleum Institute (API) standard (referred to as API thread) have not been effectively solved in the oil well pipe manufacturing field. The tensile strength of the API round thread is relatively low, and the anti-stuck performance is relatively poor; while the connection strength of the API buttress thread is relatively high, but the sealing performance of the thread joint is relatively poor, and the anti-stuck performance is better than that of the round thread, but it is still not ideal.

[0004] On the other hand, for the oil pipe used in relatively deep oil and gas wells above 3000 meters, API extra-ultimate strength oil pipe (API EUE) is generally used to meet the required connection strength and sealing requirements. For oil pipe manufacturing enterprises, the "pipe end thickening" and heat treatment required for producing API EUE thickened oil pipe will cause the production cycle to be prolonged and the energy consumption to be increased, and for the API EUE thickened oil pipe with unqualified thread processing, the repair cost is even higher.

[0005] Unlike the hook-type thread, the sawtooth thread only has a lead-in side and a load-bearing side, while the hook-type thread also has a tooth bottom and a tooth top. The sawtooth thread is a deformed round thread, and the hook-type thread is a deformed buttress thread. The sawtooth thread joint adheres to the design concept of "simple structure and excellent performance", adopts a flat-end non-thickened design of sawtooth thread, and realizes 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 thread and overcome its shortcomings. Break through the technical bottleneck of low tensile strength of API round thread, and avoid the API EUE thickened oil pipe with high production cost, long production cycle and difficult repair.

[0007] (2) without additional metal sealing structure, avoiding the sealing structure of the threaded joint being complicated. SUMMARY

[0008] The present application relates to the calculation of the pin thread full length L4 and the make-up allowance A in the joint structure size.

[0009] The calculation of the joint structure size, i.e. the pin thread full length L4 and the make-up allowance A, is based on the thread profile, the basic parameters of which are as follows:

[0010] Thread profile angle α = 60°, load flank angle 3°, stab flank angle 57°, profile angle = load flank angle + stab flank angle;

[0011] Thread height h = 1.427 mm;

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

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

[0014] The joint structure size is further designed based on the thread profile of the above parameters for oil pipes of different steel grades and specifications, so as to obtain specific threaded joint structures matching the steel grade and specification, i.e. the pin thread full length L4, the make-up allowance A and the coupling length NL, and the present application specifically relates to the calculation method of the pin thread full length L4 and the make-up allowance A. The calculation method is more suitable for pipes with nominal outer diameter D of 2.375 inch to 5.5 inch, i.e. pipes with nominal outer diameter D of 60.32 mm to 139.7 mm.

[0015] The technical solution of the present application is as follows:

[0016] I. Calculation of the pin thread full length L4

[0017] The pin thread full length L4 is calculated according to 13.5*D 2 / 5 ≤ L4 ≤ 14*D 2 / 5 and then rounded, wherein D is the outer diameter of the pipe.

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

[0019] II. Calculation of the make-up allowance A

[0020] The make-up allowance A of the pipe and the coupling = f*D 3 / 4Wherein D is the outer diameter of the pipe, and f is a coefficient, f is 0.10-0.12, and more preferably f is 0.1025.

[0021] The make-up allowance A of the make-up machine is related to the tensile strength, sealing performance and make-up and demake-up performance of the threaded joint. During the screwing process of the pipe and the coupling, with the increase of the make-up torque, the pin thread and the box thread will experience a matching process from "loose" to "tight", and a tight interference fit will be formed after the make-up to the design position, and the contact stress will appear on the surface of the pin thread and the box thread. The make-up allowance A affects the tensile capacity, sealing performance and make-up and demake-up performance of the whole joint. If the make-up allowance A is designed to be too large, the contact stress will be too large when the threads are engaged, and cold welding, i.e. galling, will easily occur. If the make-up allowance A is designed to be too small, the threads will be too loose, the pipe and the coupling will easily fall off, the sealing performance will be poor, and leakage will easily occur.

[0022] Since the formula for directly establishing the relationship between the make-up allowance A and the tensile strength, sealing performance and make-up and demake-up performance of the joint cannot be established, the calculation formula of the make-up allowance A of the present application is an empirical formula considering the tensile strength, sealing performance and make-up and demake-up performance.

[0023] Compared with the prior art, the present application has the advantages that: the present application is aimed at the design requirements of the threaded joint of the oil pipe in the oil and gas exploitation field, and the calculation formula of the structure size of the threaded joint, i.e. the full length L4 of the pin thread and the make-up allowance A of the make-up machine, is given on the basis of the basic parameters of the sawtooth thread profile, and the overall size of the threaded joint obtained according to the formula can meet the requirements of the tensile strength, sealing performance and anti-galling performance of the threaded joint in the oil pipe field. BRIEF DESCRIPTION OF DRAWINGS

[0024] Fig. 1 is an axial sectional view of the sawtooth threaded connection joint of the pipe and the coupling according to the present application;

[0025] Fig. 2 is the profile of the box thread and the pin thread according to the present application;

[0026] Fig. 3 is a matching diagram of the pin thread and the box thread according to the present application;

[0027] Fig. 4 is a matching diagram of the sawtooth threaded connection joint according to the present application.

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

[0029] The present application will be further described in detail below in combination with the drawings and examples, which are exemplary and intended to explain the present application, and cannot be understood as a limitation of the present application.

[0030] Sawtooth threaded joint of oil pipe

[0031] I. Specification range:

[0032] The joint structure is machined on pipes with a nominal outer diameter D of 2.375in (60.32mm) to 5.5in (139.7mm).

[0033] Flat-end, non-thickened threaded connection.

[0034] Thread direction: Right-hand thread.

[0035] Thread surface treatment: Oil pipe external threads: no treatment or sandblasting; coupling internal threads: phosphated or copper plated.

[0036] II. 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 capacity of the threaded joint reaches the internal yield strength of the pipe body (coupling): The sealing capacity of the threaded joint is the ability of the threaded joint to withstand the internal pressure without leakage after the female and male threads are tightened and engaged and screwed to the designated position. The internal yield pressure value of the pipe body is: 2*f*Yp*t / D, refer to the API standard, where: f is a coefficient, representing the minimum wall thickness as a percentage of the nominal wall thickness, generally f = 0.875 for 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. The sealing requirement is met when the internal yield pressure value reaches the internal yield strength.

[0039] (3) Anti-gluing performance: The sawtooth threaded joint meets the 9-on / 9-off standard, which is superior to API thread.

[0040] III. Tooth profile parameters of sawtooth threads, as shown in Figures 2 and 3.

[0041] The thread profile angle α = 60°. In this embodiment, the bearing side angle α1: 3°, and the guide side angle α2: 57°; 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 crest fillet radius R: 0.33mm

[0046] The radius of the fillet at the tooth root, R', is 0.3 mm.

[0047] IV. Sawtooth Thread Connector Thread Structure Dimensions

[0048] (4.1) Total length L4 of male thread

[0049] Press 13.5*D 2 / 5 ≤L4≤14*D 2 / 5 Perform the calculation, then round down.

[0050] (4.2) Upper buckle tightening allowance A

[0051] A = f * D 3 / 4

[0052] In the formula, the coefficient f takes values ​​from 0.10 to 0.12, with 0.1025 being one of the preferred values.

[0053] V. Examples

[0054] The structural parameters of the sawtooth threaded joints in the five sets of embodiments are shown in Table 1.

[0055] Table 1

[0056] In the table, the thread pitch diameter E7 = Dh, where D is the outer diameter of the pipe and h is the thread tooth height.

[0057] Detailed calculations and demonstrations are performed based on the embodiments:

[0058] (5.1) Calculation of tensile strength of threaded joint (quantitative): The bearing area of ​​the thread profile ≥ the dangerous cross-sectional area of ​​the pipe body (coupling). The calculation meets the requirements.

[0059] Pipe cross-sectional area S 管体 =πWt(D-Wt);

[0060] dangerous cross-sectional area S of coupling 接箍 =π / 4{W 2 -[D-(L4-g+A-1) / T] 2 In the formula, g is the incomplete thread length, g = h * 2 / T, where h is the tooth height and T is the thread taper;

[0061] Thread tooth profile bearing area S 螺纹 =((L4-g) / p+g / p / 3)(Dh)π0.756, where p is the pitch;

[0062] When checking the tensile strength of a threaded joint, only the bearing (cross-sectional) areas of the thread, pipe body, and coupling need to be compared.

[0063] The tensile strength of a thread depends on the total bearing area of ​​the tooth profile over the entire length of the helix.

[0064] In the formula for calculating the bearing area of ​​a thread profile, 0.756 mm is the straight segment fit height of the thread profile; the root and tip circular arc fits are not included in the strength contribution. One-third of the number of teeth in an incomplete thread is included in the strength contribution (g / p / 3). Considering the chamfer effect, the number of teeth in a complete thread is included in the strength contribution (L4-g) / p. The helix length of the thread is approximated by multiplying the number of threads by the circumference at the thread pitch diameter.

[0065] See Table 2 for the tensile strength check of the sawtooth thread joint thread.

[0066] Table 2

[0067] (5.2) Description of the sealing ability and anti-sticking properties of the sawtooth thread

[0068] Sealing performance is related to tooth profile design and machine tightening allowance A. After the male and female threads mesh, the size of the gap between the meshing threads determines the sealing capacity.

[0069] Anti-gluing performance is also related to tooth profile design and tightening allowance A. After the male and female threads are engaged, the larger the tightening allowance A, the greater the contact stress of the engaged threads, and the easier it is for them to engage.

[0070] Under contact pressure, the thread will seal the leakage path: sawtooth threads are a type of triangular thread. The thread profile in the axial section is like a series of "wedges," which are "wedged" together under the action of force. This causes the spiral gap (i.e., leakage gap) between the crest and root of the male and female threads to gradually shrink and disappear under the action of tightening torque, thus forming a tight interference fit and ultimately achieving the sealing effect of the threaded connection.

[0071] The thread sealing mechanism can be explained from the perspective of material deformation: Under the action of force (contact stress), elastic strain will be generated on both sides of the thread profile. 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.

[0072] The sawtooth thread adopts an asymmetric self-locking design with a 60° tooth profile angle, a 3° angle on the bearing side, and a 57° angle on the guide side. The failure mode of the threaded joint under tensile load is fracture. Under tensile load, no gap will appear at the tooth root and tooth crest, ensuring reliable sealing performance. As shown in Figure 3, the tooth crest and tooth root have a small gap design (0.03mm), meaning that the difference between the tooth crest fillet radius and the tooth root fillet radius in this embodiment is 0.03mm, which improves sealing performance.

[0073] (5.3) Radial contact stress between the machine tightening allowance A and the threaded mating surface

[0074] Solving for the contact stress between the thread mating surfaces is very complex. To simplify this, the thread mating is assumed to be an interference fit between two thin-walled cylinders. The contact stress between the two cylinders obtained by the method of elasticity is used to approximate the radial contact stress on the mating surfaces of the male and female threads, and then a quantitative stress analysis of the thread profile is performed.

[0075] As shown in Figure 4, J represents the distance between the pipe end and the center of the coupling after mechanical tightening; L7 represents the complete thread length of the male thread; g represents the incomplete thread length; and the total length of the male thread is L4 = L7 + g.

[0076] The radial contact stress Py between the male and female threads caused by the upper thread is obtained from the elasticity formula as follows:

[0077] In the formula,

[0078] E: Elastic modulus of the material, E = 2 * 10 5 MPa

[0079] δ: Thread interference fit A / T

[0080] W: Outer diameter of coupling

[0081] E7: Thread pitch diameter

[0082] d: Pipe inner diameter, D-2*Wt

[0083] Total radial contact pressure = contact area * radial contact stress (Py) F = πE7LP y

[0084] In the formula, L: equivalent thread fit length L = L4 - g / 2

[0085] The radial contact stress Py and total radial contact pressure F of the sawtooth threaded joint are shown in Table 3.

[0086] Table 3

[0087] Taking a sawtooth thread with a specification of 88.9*6.45mm as an example, Py=35.4MPa, F=555.9KN. For the same outer diameter and wall thickness, the API-advanced oil pipe has Py=72.3Mpa and F=1149.4KN. The radial contact stress Py of the sawtooth thread is lower and the anti-galling ability is better.

[0088] In addition to the above embodiments, the present invention also includes other embodiments. All technical solutions formed by equivalent transformation or equivalent substitution should fall within the protection scope of the claims of the present invention.

Claims

1. A sawtooth thread structure for oil casing, comprising: The tube (1) and the coupling (2) are connected by a male thread (101) on the outer surface of the end of the tube (1) and a female thread (202) on the inner surface of the end of the coupling (2). The end of the tube (1) is inserted into the end of the coupling (2), and a sealed connection is achieved by the engagement of the male thread (101) and the female thread (202). The tooth profile parameters of the male thread (101) and the female thread (202) are as follows: tooth angle α = 60°, bearing side angle 3°, guide side angle 57°, tooth height h = 1.427 mm, pitch p: 8 teeth / in, taper T = 1:

16. Its characteristic is that the total length L4 of the male thread is 13.5 * D. 2 / 5 ≤L4≤14*D 2 / 5 Perform the calculation and then round down, where D is the outer diameter of the pipe; The tightening allowance for the upper fastening of pipes and couplings is A = f * D. 3 / 4 In the formula, D is the outer diameter of the pipe, and f is a coefficient with a value of 0.10 to 0.

12.

2. The sawtooth thread structure for oil casing according to claim 1, characterized in that: The coefficient f takes the value 0.1025.

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

Images