GB2074685A

GB2074685A - Joint for oil or gas well pipes

Info

Publication number: GB2074685A
Application number: GB8111557A
Authority: GB
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 1980-04-12
Filing date: 1981-04-13
Publication date: 1981-11-04
Also published as: GB2074685B; DE3114651C2; JPS56143889A; IT8148243A0; DE3114651A1; FR2480400B1; CA1165791A; FR2480400A1; IT1170881B; JPS5750993B2

Abstract

A fluid tight threaded joint for oil or gas well pipes comprises a box (1) having an internal female thread and a pin (2) having an external male thread, the box (1) and pin (2) being screwed together in use. The external male thread on the pin starts from the pin end as a rounded triangular thread (3) and become a buttress thread (4) and the internal female thread on the box (1) is formed in a corresponding manner, thus forming a composite thread in a single thread train in each of the box and the pin, whereby a threaded joint with excellent sealing and repeat make-up performance is obtained. <IMAGE>

Description

SPECIFICATION A fluid tight joint for oil or gas well pipes This invention relates to a joint for a gas or oil well pipe having good sealing properties against leakage.

In recent years, the trend has been toward drilling deeper gas wells involving higher pressures and as a result demand has risen for a joint for oil and gas well pipes whose performance properties are superior to those employing normal API threads (round, buttress), the joint having a high tensile strength and leak-tightness as well as good resistance to corrosion and erosion. Among the desired properties of the such joints, fluid-tightness is especially important. This is because the leakage of an inflammable gas might lead to fire or an explosion whereas the leakage of a poisonous gas such as hydrogen sulfide might cause a disastrous accident.

One way to enhance the fluid tightness of a threaded portion of a pipe is to make the clearance between the cooperating threads as small as possible by strictly controlling the machining tolerance, to plate the threaded portion with a soft highly malleable metal such as tin, and to apply to the threaded portion a compound grease mixed with a very fine metallic powder and a mineral oil thickened with a calcium stearate or a lithium stearate. Also, it is known to be important to control the torque and number of turns when the pipe is made up. However, even with these means, the leakage of gas cannot be completely prevented at the high temperatures and pressures currently being encountered in gas and oil wells. Because of this, special types of joints have had to be used to improve fluid tightness.Some of these have a seal portion which employs a metal-to-metal intact between a pin (steel pipe) and a box (coupling) forming the joint, while others have an annular groove with a Teflon ring inserted therein.

Among the special type of joints having a seal portion, several varieties are known. To assure effective metal-to-metal contact in these known joints, however, it is important that there be no clearance in said contact portion even when the steel pipe is subjected to tensile force. Therefore, in this kind of joint, it is necessary to use a large torque when connecting up the pipes but this causes uneven plastic deformation at the contact portion.

In particular, this deformation tends to degrade the fluid tightness of the joint after repeated connections and it can also cause galling at the screw thread portion.

In general, it can be assumed that repeat make-up will be effected three times at the most in a casing string and ten times at the most in a tubing string, and in either case, a high degree of seal tightness is required with out causing any damage.

According to the invention, there is pro vided a fluid tight threaded joint for oil. or gas well pipes comprising one part having an internal female thread which cooperates with another part having an external male thread thereon, said other part being formed as its end with a plurality of turns of a rounded triangular shaped thread which lead into a buttress or square shaped thread, said one part being provided with a thread structure corresponding to that of the other part whereby a composite thread arrangement is provided in a single thread train in both said one and said other parts.

In the present specification, the word "box" means a coupling with a female thread on the internal surface thereof, and the word "pin" means a pipe end with a male thread on the external surface thereof.

The present invention overcomes the above mentioned problems by providing a joint which has excellent sealing properties and good repeat make-up performance. The novel thread used in the joint of the invention was achieved as a result of various studies which took into consideration the fact that conven tional special joints all aim at enhancing fluid tightness by the sole expedient of specially contrived metal-to-metal contact portions. The improvement of the screw thread portion in accordance with this invention involves form ing the joint portion with a composite thread structure comprising a rounded triangular thread and a square thread or a buttress thread.

As is widely known, a tapered rounded triangular thread has excellent sealing proper ties in itself, and is standardised as a taper thread for pipes under JIS (Japanese Indus trial Standard) and is used widely in high pressure piping.

The sealing effect of this tapered rounded triangular thread will be briefly described as follows.

The leakage route of the fluid within a made-joint is generally considered to follow two passages in the threaded portion. The first of these is in the axial direction of the pipe and the second is in the helical direction along the thread locus. As for the leakage in the axial direction of the pipe, in a tapered thread, high contact surface pressure is generated due to the wedge effect of the triangular thread and, since a number of turns of the thread are provided in the axial direction of the pipe, the leakage cutoff function in this direction is superior to any other sealing mechanism. The rounded triangular thread is also superior in preventing helical leakage.

When the threads are engaged, the clearance between them is concentrated between the crest and the root of the threads when the taper thread is tightened. Thus, leakage can be prevented by filling the clearance with compound and then compressing the compound by the "wedge" effect of the rounded triangular thread to eliminate all voids.

This rounded triangular thread is, of course, adapted as a standard for API 5B but it cannot be easily used to obtain both superhigh pressure sealing and high joint strength at the same time.

More specifically, the sealing effects of the rounded triangular thread results from the fact that the clamping action of the tapered male thread into the female thread is such as to generate high contact surface pressure at the threaded flank, the crest and root. Thus, the wedge effect of the rounded triangular thread works advantageously insofar as sealing performance is concerned. However, since the flank, which bears a part of the load, is preferably inclined at 30 to the pipe axis, the axial load on the pipe proper is transmitted from the box to the pin not only as tensile force but also as a pipe expanding force which may give rise to "jump-out" i.e. when the pin is pulled out of the box without bearking the thread.Therefore, the strength of the joint is much inferior to one employing a buttress thread which has its flank practically perpendicular to the pipe axis.

From the foregoing, it will be seen that the present invention provides a joint which derives its seal performance and strength from its threaded engagement and which takes positive advantage of the sealing mechanism (wedge effect) peculiar to the rounded triangular thread without reducing the strength of the joint.

Further objects and additional features of the present invention will become apparent from the following detailed description, given by way of example only in conjunction with the accompanying drawings, wherein: Figure 1 is a longitudinal sectional view of a coupling joint of the present invention; Figure 2 is a longitudinal sectional view showing part of a coupling joint in which the rounded triangular thread at the end of a pin is inclined toward the side of the pin root; Figure 3 is a view showing one example of the shape of the flank and the crest of the rounded triangular thread of the joints of Figs.

1 and 2; Figure 4 is an enlarged view of an inverse rounded triangular thread which extends into a buttress female thread; and Figure 5 and Figure 6 are views showing other applications of the joint of the present invention.

Fig. 1 shows a joint in which a tapered male thread of a pipe body engages with a tapered female thread of a box coupling 1.

The threads consist of rounded triangular shaped threads 3 at the end of a pin 2 and buttress threads 4. The rounded triangular threads 3 serve mainly to provide the required sealing whereas the buttress threads 4 serve to sustain the required high axial load. An important feature of this thread arrangement is that there is a clear division of function between the threads of different shapes, the threads of one shape being used for sealing and those of the other shape to provide joint strength. Since the joint comprises both rounded triangular threads and buttress threads, it not only gives improved fluid tightness but it also provides improved reliability of joint strength. Accordingly, since there is no need to confer a sealing capability on the buttress thread portion during tightening of the joint, it is not necessary to give the pin and box an excessive interference allowance.

As a result, the circumferential tensile stress arising in the coupling during tightening can be reduced to a level which is a major problem in conventional tapered threads.

Moreover, unlike a conventional joint which has a metal contact portion at the end of its pin which is unable to ensure stable fluidtightness because of its easy deformation under the combination of internal pressure with secondary and tertiary external forces (tension, bending etc.), the threaded joint of the present invention uses rounded triangular threads to provide sealing so that the restraining force is strong and the fit between the threads excellent, giving stable fluid-tightness even under exposure to such external forces.

Further, even if the fitting allowance of the rounded triangular thread portion is increased in order to obtain an adequate gas-leak-sealing effect, insofar as the increase is not so large as to cause the pin end to yield, there is no need to worry about breaking the oil film or of galling due to excessive contact between smooth surfaces since the clearance between crest and root of the threads is always filled with compound grease.

A joint of the type just described in which fluid-tightness and high joint strength are provided for by the rounded triangular threads and buttress threads arranged in the same thread train is already a substantial improve-' ment over prior art joints simply as a result of the structural features described so far. However, to obtain a joint even better suited to actual practice, it is advisable to employ additionally the following measures for imparting an even better sealing effect to the rounded triangular threads: 1) The provision of a shoulder portion 8 on the central internal surface of the box 1 and, similarly, the provision of a non-threaded portion at the end of the pin 2, this nonthreaded portion having an end surface 11 corresponding to the shoulder portion 8 so that these two portions 8, 11 make surface-tosurface contact after tightening of the joint.

By the incorporation of this structural feature, the interference allowance of the rounded triangular thread portion can be controlled to a fixed value and, at the same time, the force of reaction from the shoulder portion 8 can be utilised to compress the end of the pin 2, so as to cause the rounded triangular thread of the pin to press strongly onto the rounded triangular thread of the box, thus reinforcing the seal tightness of the rounded triangular threads.

2) Making the rounded triangular thread at the pin end such a shape that it is inclined toward the side of the pin root (generally called "inverse angle") (See Fig. 2).

There are two reasons for adopting this inverse angle. One is to more effectively utilise the wedge effect of the rounded triangular threads when internal pressure, axial load or bending load act on the joint, and the other is to prevent the pin from opening to the inside and the box to the outside at the rounded triangular thread portion of the pin end, the most important point as regards sealing effect.

(This type of opening is generally called the "unzipper effect").

When a tensile load and internal pressure act on the joint at the same time, the force which the male thead at the pin end exerts on the female thread on the internal surface of the box 1 is as shown in Fig. 2, by the vector 19, i.e. the resultant of the internal pressure 18 acting on the internal surface 15 and the tensile force 1 7.

In order to obtain a more effective wedging effect, the direction of the vector 19 and that of the crest of the rounded triangular thread should preferably be coincident. As for the unzipper effect, assuming that in Fig. 2 the non-threaded external surface 9 of the pin end and the non-threaded internal surface 12 of the box are respectively subjected to perpendicular forces, it will be appreciated that the thread with the inverse angle as shown in Fig.

2 will have stronger resistance than a round thread and that the fitting portion at the pin end of the former will be less openable than the latter.

3) Forming the shoulder portion with a convex conical surface and forming the corresponding part of the pin end with a curved surface or, otherwise, forming these with linear conical surfaces. With such an arangement, the pin end will be pushed in the radial direction along the shoulder portion of the box, and as a reaction to this, the engagement between the round thread portions will be improved.

The flanks of an individual triangular thread may be either curved or flat. However, as shown in Fig. 3, it is preferable, from the machining view point to improve finishing accuracy, that the flanks 31 be made flat and the shape of thread crest 32 conform to the arc of a circle tangent to the flanks 31.

The resistance of the joint just described to stress corrosion cracking and erosion is very high since its internal surfaces are smooth and are subjected to compressive stress generated at the time of make-up.

One embodiment of the present invention will now be explained with reference to Fig. 1 and Fig. 2.

The pin 2 has a non-threaded end portion (a), a rounded triangular thread portion (b) and a buttress thread portion (c). The end surface 11 of the pin end is formed so as to exert a reactive force on the rounded triangu lar thread portion (b) when it comes into pressing contact with the surface of the box shoulder portion 8, the non-threaded surface 9 of the pin end not contacting the non threaded portion surface 12 of the box.

The rounded triangular thread 10 is formed at an inverse angle and is provided on the pin end as a perfect thread of 3 - 4 turns. The pitch diameter of the rounded triangular thread is an extension of the pitch diameter of the buttress thread. The shape of the thread is determined by inscribing the two-dot chain line shown in Fig. 2 within the imaginary buttress female thread 13. This shape is used to minimise interference between the male and female threads at the time of make-up.

An angle of inclination of 10 - 30 with respect to a line normal to the pipe axis is preferable from the machining point of view.

The thread train consisting of two differing thread shapes provided continuously can be machined either by a method wherein the boundary portion between the two thread shapes ((c) in Fig. 2) is first turned smaller that the diameter of the thread root by about one pitch and the rounded triangular thread and the buttress thread are thereafter cut separately, or by a method wherein the entire thread train is first machined as a buttress thread and the rounded triangular thread is thereafter machined for 3 - 4 threads at the pin end. Like the pin, the box has a non threaded internal surface portion 7, a rounded triangular thread portion 6, a buttress thread portion 5 and a central shoulder portion 14.

The non-threaded surface 12 is on an exten sion of the crest of the rounded triangular thread. The rounded triangular thread and the buttress thread are shaped to correspond to the shape of the pin and the thread clearance is set according to API standards.

Fig. 4 is an enlarged view of an inverse rounded triangular male thread which is nearly internally tangent to a buttress female thread. The angle 8 between the pipe axis 41 and the bisector 42 of the flank angle is 63 (inclination angle 27 ), the height of the thread is approximately twice that of the API buttress thread and the flank angle of the thread is 55 . Fig. 4 merely shows an exam ple of the inverse rounded triangular thread and other shapes may be used instead. In short, it is sufficient if the shape of the flanks and the tip which cause the wedge effect are round and the clearance between the female and male threads is small.

The thread of the invention can be applied in various ways. When it is used for the internal surface of a joint, it will act as a mechanism preventing leakage of fluid from the inside to the outside of the joint, and when it is used for the external surface of the joint, it will serve to prevent leakage of fluid from the outside to the inside of the joint. Fig.

5 and Fig. 6 show specific examples of such applications.

Fig. 5 shows a joint in which pins 54 with male threads machined therein are clamped by a coupling 53. Portions 51 within the circles have a sealing function preventing fluid leakage from the inside to the outside, portions 52 within the circles having a sealing function preventing fluid leakage from the outside to the inside. It will be understood that even with only one or the other of these cases, the sealing function from the inside to the outside and vice versa can be attained.

However, in this example the sealing portions are provided at both inlets of fluid permeation with the aim of attaining the best results. The portions 55 intermediate of the total length of the pin-coupling are provided with buttress threads to assure strength.

Fig. 6 shows a type of joint for clamping a pin 64 and a box 65 together without using a coupling. Here, similar to the joint in Fig. 5, the portions within circles 61, 62 are round sealing thread portions and the intermediate portion 63 is provided with a buttress thread to give the joint strength. In this case, either one of the round thread portions 61, 62 can be omitted and a buttress thread provided in its place without causing any problem in actual use of the joint.

Claims

1. A fluid tight threaded joint for oil or gas well pipes comprising one part having an internal female thread which cooperates with another part having an external male thread thereon, said other part being formed at its end with a plurality of turns of a rounded triangular shaped thread which lead into a buttress or square shaped thread, said one part being provided with a thread structure corresponding to that on the other part whereby a composite thread arrangement is provided in a single thread train in both said one and said other parts.

2. A threaded joint according to claim 1, wherein the central portion of said one part has a non-threaded internal surface formed with a shoulder portion, the end of the other part having a non-threaded external surface corresponding to said non-threaded internal surface and a nose for abutting on the shoulder portion of said first part.

3. A threaded joint according to claim 1 or 2, wherein the bisector of the flanks of the rounded triangular thread is inclined at an obtuse angle relative to the direction of tightening of the pin.

4. A threaded jont according to any one of claims 1 to 3, wherein said one part is a box and said other part is a pin.

5. A threaded joint substantially as herein described with reference to any of Figs. 1 to 6 of the accompanying drawings.