CA1165791A

CA1165791A - Threaded joint with high gas-leak-tightness for oil and gas well pipe

Info

Publication number: CA1165791A
Application number: CA000375211A
Authority: CA
Inventors: Kazushi Maruyama; Fuzimasa Kohyama; Sigeyuki Akase; Masayoshi Tanabe
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 1980-04-12
Filing date: 1981-04-10
Publication date: 1984-04-17
Also published as: FR2480400A1; DE3114651A1; IT8148243A0; JPS56143889A; FR2480400B1; JPS5750993B2; IT1170881B; GB2074685A; DE3114651C2; GB2074685B

Abstract

ABSTRACT OF THE DISCLOSURE:
A threaded joint with high gas-leak-tightness for oil and gas well pipe comprises a box having an internal female thread and a pin having an external male thread, the box and pin being screwed together in use. The external male thread on the pin starts from the pin end as a rounded triangular thread and follows into a buttress thread and the internal female thread 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 perfect sealing an excellent repeat make-up performance is obtained.

Description

In recent years, the -tre~d has been toward drilling deeper gas wells with higher pressuxe and as a resul-t demand has risen for a joint for oi.l and gas well pipes having performance properties superior to those employing ~PI
threads (round, buttress) and haviny high tensile joint strength and excellent gas-leak-tightness as well as resis-tance to corrosion and erosion. Among the desired properties of such joints, gas-leak-tightness is especially important.
This is because the leakage of an inflammable gas might lead 1~ to fire or explosion while the leakage of a poisonous gas such as hydrogen sulfide might cause a disas-trous accident.
As conventional ways to enhance the gas-leak-tightness of a threaded portion, it is known to make the clearance as small as possible by strictly controlling the machining tolerance, to plate the threaded portion with a soft metal such as Sn having high malleability, 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. ~lso, i-t is known to be important to control the torque and number of turns at the time of make-up. However7 even with these means, the leakage of gas cannot be perfectly prevented under the conditions of high temperature and high pressure currently being encountered in gas and oil wells. Because of this, special types of joints have been used to improve gas-leak-tightness. Some of these have a seal por-tion which employs metal-to-metal contact 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 joints having a seal portion, several varieties are known. To assure effective metal-to-metal contact in these known joints, however, it * trade mark for tetrafluoroethylene '7~

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 for make-up~but to do so causes uneven plastic deformation at the contact portion.
In particular, this deformation tends to degrade gas-leak-tightness in repeat make-up and is also a cause of 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 without causing any damage.
This invention relates to a joint for oil and gas well pipe having excellent seal tightness.
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 provides a revolutionary solution to the above-mentioned problem in the form of a joint which gives perfect sealing performance and has excel~
lent repeat make-up performance, and which was realized through an improvement in the screw thread portion achieved as the result of various studies which took into consideration the fact that conventional special joints all aim at enhancing gas-leak-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 forming the joint portion with a composite thread structure comprising a rounded triangular thread and a square thread 7~ 1 or a buttress thread.
~ s is widely lcnown, the tapered rounded triangular thread has excellent sealing function in itself, and is standardized as a taper thread for pipes under JIS (Japanese Industrial Standard) and is used widely in high pressure piping.
Now, the sealing mechanism of this tapered rounded triangular thread will be briefly described as follows.
The leakage route of the fluid within a made-up 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 threads are engaged, the clearance between them is concentrated between the crest and the root of the threads when the taper thread is tightned. 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, adopted as a standard for ~PI 5B but it cannot bc easily used to obtain both superhigh pressure sealiny and high joint strength at the same time.
More specifically, the sealing effect of the rounded triangular thread results from -the fact that the 7~1~

clamping action of the tapered male thread into the female thread is such as to generate high contact surface pressure at the thread flank, the crest and the root. Thus, the wedge effect of the rounded triangular thread works advan~ageously insofar as sealing performance is concerned. However, since the flank, which bears a part of the load, is 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 the jump-out phenomenon, the phenomenon of the pin being pulled out of the box without breakage of 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.
The object of the present invention is to provide 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.
Accordingly, what is broadly claimed herein is an improvement in a threaded joint for oil and gas well pipe wherein a box having an internal female thread and a pin having an external male thread are screwed together, the improvement being that the threaded joint has high gas-leak-tightness wherein said pin is formed at its end with a number of turns of rounded triangular thread which follow into a buttress or square thread, and said box is provided with a thread structure correspondlng to sald threads provided on said pln whereby a composite thread arrangement is provided in a single thread train in both said pin and said box.

7~.1 Brief description o the drawings:
Further objec-ts and additional features of the present invention will become apparent from the following detailed description, given in conjunction with the accom-panying drawings, wherein:
Pig. 1 is a longitudinal sectional view of a coupling type joint provided with the sealing structure of the present invention.
Fig. 2 is a longitudinal sectional view showing one part of an embodiment in which the rounded triangular thread at the end of a pin is inclined toward the side of the pin root.

- 4a -3 ~ 7'~
*

Fig. 3 is a vie~ sho~iny o~e example o -the sh~pe of the flank and -the cr~st of a rounded triangul~r thread having a wedge eEEect.
Fig. 4*is an enlarged view o-f an inverse rounded triangular thread practically furnished and extended into a buttress female thread.
Fig. 5 and Fig. 6 are views showing examples of application of the joint according to the present invention.
Description of the preferred embodiments:

Fig. 1 shows a joint wherein a tapered male thread of a pipe body is engaged with a tapered female thread of a box couplings 1. The threads consist of rounded trian-gular threads 3 at the end of a pin 2 and buttress threads 4.
The rounded triangular threads 3 serve mainly to provide sealing while the buttress threads 4 serve to sustain the high axial load. The important feature of this thread arran-gement is that there is a clear division of function between threads of different shapes, with the threads of one shape being used for sealing and those of the other shape to ~0 provide joint strength. Namely, since the joint comprises both rounded triangular threads and buttress threads, it not only gives improved seal tightness but also provides enhanced reliability of joint strenght. Thus, with the arrangement according to this invention, since there is no need to confer sealing capability on the buttress thread portion at the time of screw tightening, it is no necessary to give the pin and box an excessive interference allowance.
As a result, the circumferential tensLle stress arising in the coupling during make~up, a major problem in conjuction with conventlonal -tapered threads, can be reduced to a low level.
Moreover, differently from the conven-tional joint * first sheet of drawings which has a metallic contact portion at the end of its pin and is unable to assure stable gas-leak-tightness because of its easy deformation under the combination of internal pressure with secondary and tertiary external forces (tension, bending etc.), the threaded joint accordint to 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 gas-leak-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 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 of 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, wherein gas-leak-tightness and high joint strength are provided by rounded triangular threads and buttress threads arranged in the same thread train displays a very large effect as an oil and gas well pipe joint solely as a result of the structural features described so far. To obtain a joint evcn better suitcd to actual practice, however, it is ad-visable to addi-tionally employ the following measures for imparting cven highcr sealincJ effcct to thc rounded triangular threads:
~1) L'rovi.sion of a shoul~lcr portioll 8 on tllc ccntral internal surface of the box 1 and, similarly, of a non-threaded portion at the end of the pin 2, this non-threaded portion having an end surface 11 corres-ponding to the shoulder portion 8 so that these t~>7~l two portions 8, 11 make surface-to-surface contact a~ter make-up. sy 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 utilized 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) Giving the rounded triangular thread at the pin end such 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 utilize 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." ) Namely, when a tensile load and internal pressure act on the joint at the same time, the force which the male thread at thc pin cnd exerts on the female thread on the internal surface of the box 1 is as shown in Fig. 2, that represented by the vector 19, the resultant of the internal pressure 18 acting on the internal surface 15 and the tensile force 17.

7~

And, for the purpose of obtaining a rn~re effective wedge effect, the direction of said vector 19 and that of the crest of the rounded triangular thread are preferably 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 is understandable 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 sur-face and forming the corresponding part of -the pin end with an arched surface or, otherwise, forming these with linear conical surfaces. ~ith such an arrangement, 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 strengthened.
The flanks of a rounded triangular thread may be either curved or flat. However, as shown in Fig. 3, it is preferable from the viewpoint of machining for improvement of accuracy in finishing 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 foregoing is a description of sealing structure and mechanism of the joint according to the present invention.
Its rcsistancc to strcss corrosion cracklng ~nd erosion is, moreover, very high since, similarly to those dcscribcd in publishcd litcraturc (Patcnt Publication 18096/
1970 and Laid-Open Patent Application 17125/1973) the internal surface of the joint are smooth and are subjected to compres-l~tj5~1 sive stress ~ene~ated at the time of make-up.
Next, an embodiment of the present in~ention will be explained with reference to Fig. l 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 force of reaction on the rounded triangular thread portion (b) when it comes into pressing contact with the surface of the box shoulder portion 8, whereby the non-threaded surface 9 of the pin end does not come into contact with the non-threaded portion surface 12 of the box.
The rounded triangular thread lO is given an in-verse 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 minimize interference between the male and female threads at the time of make-up. An angle of inclination of lO - 30 wi-th respect to a line normal to the pipe axis is preferable in consideration of machinability.
The thread train consisting of two diEfering thread shapes provided continuously according to this invention can be machined either by a method wherein the boundary portion between the two thread shapes ((c) in Fig. 2) is first turned smaller than 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 '7~1 therea~ter machined for 3 - 4 threads a-t the pi~ e~d. Like the pin, the box has a non-threaded internal sux~ace portion 7, a rounded triangular thread portio~ 6, a buttress thread portion 5 and a central shoulder portion 14~ The non-threaded surface 12 is on an extension 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 Eemale thread. The angle ~ between the pipe axis 41 and the bisector 42 of the flank angle is 63 (incli-nation angle 27), the height of the thread is about two times that of the API buttress thread and the flank angle of the thread is 55. Fig. 4 merely shows an example 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 ls small.
The joint according to the present invention can be applied in various shapes. Namely, when it is used for the internal surface of a joint, it will act as a leakage preventing mechanism for fluid from thc inside to the outside of the joint, and when it is used for the cxtcrnal surface of the joint, it will display a leakage preventing function for fluid from the outside to the inside of the joint.
Fig. 5 and Fig. 6 show concrete examples of these applications.

Fig. 5 shows a joint in which pins 54 with male threads machined thereon are clamped by a coupling 53. Portions 51 within the circles have sealing function from inside to it~ l outside, and po~tions 52 within the circles have sea~ing function from outside to inside. It will be understood that even with only one or the other of these cases, seali~g function from inside to 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 perfection. 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 without using a coupling. Here, similar to the joint in Fig. 5, the portions within circles 61, 62 are round thread portions having seal tightness and the intermediate portion 63 is provided with a buttress thread having joint strength. In this case, either one of the round thread portions 61, 62 can be omitted and ~ buttress thread provided in its place without causing any problem in actual use of the joint.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a threaded joint for oil and gas well pipe wherein a box having an internal female thread and a pin having an external male thread are screwed together, the improved threaded joint having high gas-leak-tightness wherein said pin is formed at its end with a number of turns of rounded triangular thread which follow into a buttress or square thread, and said box is provided with a thread structure corresponding to said threads provided on said pin whereby a composite thread arrangement is provided in a single thread train in both said pin and said box.

2. A screw joint according to claim 1, characterized in that the central portion of said box has a non-threaded internal surface and a shoulder portion, the pin end has a non-threaded external surface corresponding to said non-threaded internal surface and a nose abutting on the shoulder portion of said box.

3. A screw joint according to claim 1 or 2, charac-terized in that 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.