JPH0280886A - Threaded joint for oil well tube excellent in seal surface pressure retaining function - Google Patents

Abstract

PURPOSE:To improve airtightness and to reduce tensile stress by providing a joint for fitting a male screw disposed on the tube end portion to a female screw disposed on a cylinder and forming a clearance either between the thread ridge and the thread base or between both sides of the thread ridge. CONSTITUTION:In a tube joint for fitting a male screw 17 mounted on the end of a tube 11 to a female screw 19 in a cylinder 12, the taper male screw 17 is formed on the taper cylindrical portion 16 of the tube 11. The female screw 19 is threaded on the inner periphery of the joint end portion 18 of the cylinder 12 corresponding to the male screw 17 in such a manner as to decrease in diameter as it is directed to the innermost portion. The effective diameter of the tube 11 is made smaller for at least four screw ridges among all taper fitting screw trains, the effective diameter of the cylinder 12 is made larger, or a clearance is formed either between the screw ridge and the screw base or between both sides of the screw ridge. Accordingly, a decrease in surface pressure of a seal portion can be prevented, the circumferential tensile stress of the cylinder can be reduced, and stress corrosion cracking can be prevented.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention is directed to oil country pipes (casing pipes, chuhink pipes) used for well protection and oil extraction of oil (gas) wells.
The present invention relates to a joint for oil country tubular goods that has excellent airtightness and stress corrosion cracking resistance.

[Prior Art] As oil well environments become more and more severe, the requirements for airtightness and stress corrosion cracking resistance of joints for oil country tubular goods are becoming more sophisticated year by year. Current special joints have a metal contact airtight portion, and various inventions have been made regarding the shape of the airtight portion. Except for this airtight part shape, special joints are generally represented by the shape shown in FIG. 4, and have a non-threaded tapered part 39 at the tip of the male thread 31
A common feature is that the box 32 is provided with a central protrusion 40. This joint shape has the following two main effects. The first is the non-threaded tapered part 33.3
A high degree of airtightness can be obtained by tightly fitting 5 into the joint when fitting it into the joint. Second, the protrusion 40 of the box serves as a stopper for tightening the male screw, and the screw fitting portion 37.
38 and the airtight portions 33 and 35 are provided with a predetermined interference margin (male side outer diameter - corresponding female side inner diameter).

This is an extremely important point in designing the airtight pressure and controlling the tensile stress in the circumferential direction of the box. Especially 33
The goal of all special joints is to generate as high a contact surface pressure as possible between -35 and improve airtight performance.

In order to keep the contact pressure of the metal contact part between 33 and 35 at a high level, 1) Increase the interference margin between 33 and 35. 2) Use high strength material to suppress the yielding of the unthreaded taper part 39, and make it large. Maintain the tight fit effect even with interference 3) Loosen the fit of the screw near the seal part so that the tight fit effect of the screw fitting part 37J8 does not reduce the surface pressure of the metal contact seal part. Measures are currently being taken. The functions aimed at by the present invention are 3)
The prior art related to this effect will be explained below.

Figure 5 (a) shows a finite element method for quantifying the effect of excessive screw fitting on the metal contact surface pressure at the tip of the pin screw for the joint disclosed in Japanese Patent Publication No. 59-44552. This is a joint model. The interference allowance of the screw is GAP e in the figure.
element, and here the diameter difference is 0.2.0.1
We considered mm. Figure 5(b) shows the screw interference allowance O
It shows the surface pressure distribution of the metal seal part when mm. The maximum sealing surface pressure is about 60 kg/mm2 when not affected by the tightening action of the screw. On the other hand, 0.2 mm from the 2nd to 3rd thread counting from the tip of the pin screw 0
．． Figure 5(c) shows the surface pressure distribution of the metal seal portion when a screw interference margin of 1 mm is given. Maximum surface pressure is 20kg each
/mm2, which has decreased significantly to 40kg/mm2.

Strategy 5 (tl) shows the surface pressure distribution when the position where screw interference is applied is moved away from the 2nd and 3rd threads to the 4th and 5th threads. Although a similar screw interference margin of 0.2 mm was provided, the maximum surface pressure returned to the original state of the screw interference margin of 0 mm (FIG. 5(b)). Figure 5(e) focuses only on the maximum surface pressure,
This is a summary of the above results. Therefore, these figures are
The positive fitting of the screw extending from the tip of the pin to about the fourth thread is most important, and this shows that this reduces the critical metal contact surface pressure.

As mentioned above, it is very effective to provide the 37 functions to the joint. Patents that specifically incorporate this function into the joint structure include USP 2062407 (API EL casing joint) and JP-A-62-196488.

FIG. 6 shows the state of screw loosening near the metal contact seal portion when the EL casing joint is fitted, and the screw processing method to achieve this. Its feature is that when screwed into the fitting, the threaded line and threaded bottom line are made up of a complete threaded part where the thread is tapered to the tube axis, and an incompletely threaded part parallel to the tube axis. Also, clearance must be provided for both the pin thread of the incomplete thread and the box thread amount. On the other hand, in the fully threaded part, the bottom of the pin thread and the thread of the box are in firm contact, and a clearance is provided between the thread of the pin and the thread of the box. Although this method achieves the intended purpose of maintaining the surface pressure of the seal portion at a high level, it has the following two drawbacks.

1) The threads parallel to the tube axis near the sealing part of the box usually require an additional step of topping processing to process the thread corner R part in the process that usually ends with screw thread processing and thread processing. In addition, since the taper of the pin changes in the middle, if the thread is machined using a thread forming tool at the same pitch, the depth of cut will be shallower to the blade or the workpiece at the incompletely threaded part, so the thread groove will be smaller. becomes narrower. As it is, the corresponding female thread will not fit into the thread groove, so at the screw processing stage, the pitch of the incompletely threaded part and the fully threaded part is changed, and the thread side of the incompletely threaded part of the male thread is slightly shaved. You need to drop it and widen it. Otherwise, in order to machine the thread groove as designed, the entire incomplete thread and 1.2
A full-form cutting tool that completely covers the thread of the mountain is required.
The cost of chasers will be high. If the incomplete thread part is 5~7
If the number of mountains is increased, the overall shape of Chezer will be as wide as 7 to 9 mountains.

2) In terms of functionality, the incomplete part of the box screw near the seal is the part where the thread receives the strongest force, but if that part is an incomplete thread with an insufficient thread height, it will be a perfect thread a few threads ahead. The force of the screw thread at the beginning of the box screw increases, and the bottom cross section of the box screw is smaller than the incomplete screw thread, and the complete screw start portion becomes a dangerous cross section for tensile breakage, which sacrifices joint efficiency. On the other hand, in Japanese Patent Application Laid-Open No. 62-196488, an incomplete screw with a loose fit is provided in the vicinity of the sealing part, similar to the above-mentioned LISP. However, the above point is that the screw thread of that part is composed of the thread bottom line or the taper pin thread bottom that is the same as the fully threaded part, the box thread exit line and the taper pin thread that is steeper than the fully threaded part, and the box thread bottom line. It is different from the US patent. However, the processing and functional drawbacks are basically similar. That is, addition of pin topping processing, addition of variable pitch processing for aligning the thread width at the box taber switching part, and addition of incomplete thread topping processing. In terms of functionality, there is concern that the dangerous cross section of the box during tension will move from the imperfectly threaded part adjacent to the seal, where the thread effective cross section is large, to near the start of the complete thread, where the thread bottom effective cross section is small, resulting in a decrease in joint efficiency.

As described above, in the previous invention, an incomplete thread shape is arranged in the part where the screw fitting is loosened, and the taper is changed from the fully threaded part only in that part, so it takes a lot of processing time, and it is not functionally effective. There is room for improvement, including concerns about reduced joint efficiency.

[Problems to be Solved by the Invention] An object of the present invention is to provide a tapered threaded joint for oil country tubular goods equipped with a non-threaded metal contact seal portion at the tip of the pin, with uniform processing and functionality equivalent to a threaded joint. It is an object of the present invention to provide a method for partially adjusting the amount of tapered screw fitting of a fully threaded portion so that the substantial metal contact interference margin does not decrease due to the influence of screw fitting.

In addition to improving airtightness, the present invention also aims to reduce the tensile stress in the circumferential direction of the box.

[Means for Solving the Problems] The present invention solves the conventional problems and advantageously achieves the objects of the present invention. The extension is equipped with a non-threaded metal contact seal part, and the cylinder (box) to which this male threaded tube end pin is fastened has a corresponding Taper male thread engraved on it, and the extension is provided with a corresponding Taper male thread. is a joint structure for oil country tubular goods that has a metal contact seal part of the unthreaded inner cylinder that corresponds to the unthreaded part of the pin, and a shoulder that the tip of the pin comes into contact with. On the tip half, the thread shape is a threaded thread.For a bottom contact type nutless screw, should the threaded bottom on the pin side be deep and the threaded thread on the pin side be made low, or conversely, should the threaded bottom on the box side be made deep and the threaded thread on the box side be made low? Alternatively, by making the screw bottoms of both pin boxes deeper or lowering the screw threads of both pin boxes, the interference in the radial direction between the female and male screws can be eliminated, and the hook or pin tip can be compressed only in this area. We took measures to prevent the so-called shrink-fitting effect of the tube from working, and the remaining parts, that is, the half of the pin base side of the fully threaded fitting part and the incomplete threaded part, were made with the desired thread interference distance to provide sufficient torque resistance and prevent loosening. This is a joint for oil country tubular goods that has a high sealing surface pressure retention function that does not reduce the substantial interference margin of the metal contact part due to screw fitting.

If the screw shape is a so-called in-blue buttress where the screw thread rotor flank surface and the staking flank surface contact each other, and there is clearance between the screw thread and the screw bottom, the half of the pin tip side of the fully screwed part will interfere in the radial direction. Either reduce the thread width on the pin side to prevent this from happening, or conversely reduce the thread width on the box side, or reduce both thread widths little by little, but the actual interference allowance for screw fitting or metal contact does not decrease. This is a pipe joint for oil country tubular goods that has a function of maintaining high seal surface pressure.

More recently, the pressure accumulation effect of the screw tightening lubricant that accumulates between the pin boxes has been studied (DC/5PE17208 to I).
19881ADC/SPE Drilling Con
ferenceFeb, 28-March 2.198
8) Invented a deformed buttress thread shape (designed so that the load flank and staking flank surfaces do not overlap), which intentionally designed the clearance between the box thread bottom and pin thread to a certain size so that grease can escape quickly. has been done. With such a thread shape, the threads are simply machined to provide clearance between the pin thread bottom of the pin tip half of the fully threaded fitting section and the box thread, resulting in a high seal that does not reduce the actual interference margin at the metal contact part due to threaded fitting. It can be improved into a pipe joint for oil country tubular goods that has a surface pressure retention function.

The present invention is applicable to special joints in which the tip of a male thread is threaded and a non-threaded metal contact seal is provided on the extension thereof, and is applicable to radial seal type threaded joints based on the shrink fit theory. It is possible. As mentioned above, the method for determining the thread shape is different, but the key is to devise a way to counteract the effect of the pinhook threads rubbing against each other in the radial direction without changing the taper of the complete thread part.

The important point is to eliminate radial interference of the partially threaded fitting adjacent to the metal contact seal.

FIG. 1 shows an overall view of the threaded joint of the present invention. This figure shows an example in which the present invention is applied to a threaded thread bottom contact type buttress screw. There are three ways to provide thread and thread bottom clearance in a complete thread row. An enlarged view is shown in Fig. 2.
The pin effective diameter offset type shown in (a) is machined by driving the pitch diameter of the threaded portion at the tip end of the pin to the inside of the ideal effective pitch diameter where clearance is desired. (
The Hox offset type b) is machined by cutting the pitch diameter at the back of the box where clearance is desired to be outside the ideal effective pitch diameter. The pin box deep group type shown in (C) is a processing method in which both the screw bottoms of the pin box are deepened and the pitch diameter is not offset.

Fig. 3 does not show a method for eliminating radial screw interference in the case of an in-brushed buttress screw shape in which the rotor flank and stamping flank surfaces of the screw are flush against each other. When the female and male screws are screwed together,
To create a clearance between the pin tapping flank and the box tapping flank as shown in the diagram, in the final pass of pin screw machining, slightly move the pitch to the opposite side of the pin tip to make the pin tapping flank extra large. The thin threads that have been ground down are good. Alternatively, using a similar idea, the pitch may be slightly moved to the inner side of the box in the final pass of box screw machining, and the hox stamping flank surface may be ground down excessively. Either method eliminates the problem of the screw thread biting into the mating screw groove and causing the box to bulge or the pin to contract, and it is possible to ensure a substantial interference margin for the seal portion.

Regarding the deformed nutless screw with grease removal clearance, the difference is that a clearance is provided between the pin thread and the hook screw bottom in the area where there is radial interference in Fig. 2, and the other differences are as shown in Fig. 2. The improvement measures are the same as for threaded thread bottom contact type buttress screws.

[Function] The present invention, as represented by FIG. 1, has the following three excellent functions when used after the joint has been fastened.

FIG. 1 shows prevention of reduction in surface pressure of the metal seal portion. The effect in this respect is as demonstrated in FIG. 5. Second, since the remaining box sleeve is compressed into the screw-fitting part or the torque-resisting part, it acts to suppress the expansion of the box, reducing the box's maximum circumferential tensile stress, and The structure is resistant to stress corrosion cracking, which is a problem in the environment. Third, since the torque generation function of the original taper screw is inherited, the pin and box are firmly connected, and the joint structure does not wobble or loosen under combined loads such as tension, internal pressure, and external pressure. ing. Fourthly, there is no fear of deterioration in tension joint efficiency.

[Example] Below, a pipe joint for oil country tubular goods (hereinafter referred to as the present joint) according to the present invention will be described based on an example joint shown in FIG.

As shown in FIG. 1, the tapered cylindrical portion 16 at the distal end of the tube 11 has a diameter of 1/16 mm as it approaches the distal end (left side).
A male screw 17 based on an API standard heartless screw whose diameter decreases with a taper is provided. The male thread 17 has a fully threaded portion PS having a predetermined thread height h and a length L3 (pin fully threaded), and a non-tapered pipe extending from the end of the fully threaded portion Ps. An incompletely threaded portion I, in which the height of the thread does not reach a predetermined thread height h when reaching the outer circumferential surface 11a, is formed to have a length L4 (incomplete pin thread).

On the inner periphery of the joint end 18 in the box 12, a female thread 19 is provided with an L-thread to obtain a threaded engagement corresponding to the male thread 17.
The length L2 is carved so that the diameter decreases toward the inner depth (to the left) up to the middle point of 3, and at that depth, only the pitch diameter is smaller than the ideal position without changing the screw shape. 2mm
A complete screw OP3 offset to the outside is carved by a length Ll.

Continuing to the end of the OP, thread the torque shoulder 24. and the non-threaded portion of the 1/10 taper 25. in an area corresponding to 4 pitches. The innermost torque shoulder 25 is arranged in a stepped manner, and the inner circumferential surface of this non-threaded part or the inner circumferential sealing surface 14
It is a. On the other hand, the tube 11 also has a torque shoulder 27 and a non-threaded portion 28 with a 1710 taper at the tip of the male thread 17. The innermost torque shoulder 29 is arranged so as to correspond to the box, and the outer periphery of the 1/1O taper unthreaded lip portion 28 forms an outer periphery sealing surface 20a corresponding to the inner periphery sealing surface 14a. The diameter interference margin of the sealing surface (pin outer circumferential seal diameter D20a - corresponding hook inner circumferential seal diameter DI48) is usually designed in the range of 0.2 to 0.8 mm. Since the use of NC lathes is common for thread cutting of special joints, thread rows that require high-precision thread processing technology, such as the one in the present invention, can also be processed using a full-form thread cutting tool from the end of the hook to the inner depths. When proceeding, the depth of cut is gradually changed from the boundary of 1 to 2 peaks to 0.
Just add 2mm and finish cutting the L1 part. This can be done in the final finishing pass of the thread, so it does not reduce machining efficiency.

In order to confirm the results of the present invention, two sets of pins and hooks with a tube outer diameter of 177.8 mm, a wall thickness of 8.05 mm, a box outer diameter of 194.46 mm, and a tensile strength of 70 kg/mm2 were manufactured. One side has a screw interference distance (IT) of 0.8m.
m.

Seal interference distance (Is) 0.2mm, the other is I'r, I
The special processing of the present invention was performed without changing s. Gas leak performance was determined according to the API RP37 standard, with an internal pressure of 80% of the tube yield internal pressure and an axial force of 80% of the tube yield axial force.
The airtightness of the joint was confirmed in heat medium oil whose temperature changed from 70°C to 170t every minute. Leakage was confirmed by filling the pipe with N2 gas and detecting N2 gas leaking from the bubble detection hole shown in FIG. As a result of the experiment, a leak of N2 gas was detected at the 20th temperature cycle in a joint in which no clearance was provided in the thread adjacent to the seal part. On the other hand, in the threaded joint to which the present invention was applied, no N2 leakage was observed until the 100th temperature cycle, so the test was stopped at that stage.

[Brief explanation of the drawing]

Figure 1 is a detailed view of a threaded joint adopted as an example of the present invention, and Figures 2 (a), (b), and (c) are for partially eliminating taper screw interference with the API buttress thread shape. The screw row adjacent to the seal at the tip of the male screw, or the screw row adjacent to the seal at the innermost part of the female screw, or both, has a clearance in the thread thread, the bottom of the screw, and the crank surface of one screw. Fig. 3 is an explanatory diagram of the method of providing the same, and Figure 3 is a diagram to explain the processing ingenuity to provide the desired clearance similar to the above for the modified API threadless thread shape (clearance between the thread bottoms, contact with the thread flank surface biting). , Fig. 4 is a diagram showing a typical special joint equipped with a current tapered thread, a tapered metal contact sealing surface, and a stopper shoulder, and Fig. 5 (a) is a diagram for analyzing the extent to which the amount of thread interference affects the amount of seal interference. The created finite element method model diagram (b). (c) and (d) are diagrams quantifying the influence of the position and amount of screw interference on the seal surface pressure distribution, and (e) shows the influence of the position and amount of screw interference on the seal maximum surface pressure. Figures 6(a) and 6(b), which are quantification figures, are diagrams of the screw fitting of the EL casing in which a clearance is provided at the incompletely threaded portion adjacent to the seal. 11... Pipe 11a... Pipe outer peripheral surface 1
2...Box 14a...Inner peripheral sealing surface 16...Tapered cylindrical portion 17...Male thread 18
...Joint end 19...Acid screw 24...
・Torque shoulder 25...No screw part 26...
・Innermost torque shoulder 27...Torque shoulder 28...No screw part 29
...Innermost torque shoulder 2 (la...outer seal Menta l station V/I/-/'-

Claims (1)

[Scope of Claims] 1. A tapered male thread is carved into the pipe end (pin), and an extension thereof is provided with a non-threaded metal contact seal part, while a cylinder (box) that is screwed into this is provided. For a joint with a corresponding tapered female thread and a pinless cylindrical metal contact seal part on the inner extension of the joint, there is a total diameter with a positive interference margin in the radial direction when the pin and box are connected. Among the taper mating screw rows, reduce the effective diameter of the pin without changing the taper of only some screw rows with at least 4 threads or the maximum full thread length adjacent to the metal contact seal part, or reduce the effective diameter of the box. A threaded joint for oil country pipes with an excellent ability to maintain seal surface pressure, which is characterized by providing clearance between the threads, between the thread bottoms, and on one of both sides of the threads by increasing the diameter of the thread or by hollowing out the thread bottom.