JPH0611078A - Threaded joint for oil well pipe - Google Patents

Abstract

PURPOSE:To provide a special threaded joint for an oil well pipe of which reliability for leakproof property is remarkably improved. CONSTITUTION:The outer cicumference of the extreme end part of a steel pipe 1 is provided with a tapered part 14 toward the extreme end, external thread 3 is carved thereon, and a seal part 5 is formed on the extreme end. A coupling 2 is formed with internal thread on the inner circumferential face, and seal part 6 capable of being engaged with the seal part 5 of the steel pipe 1. The thread part of the steel pipe 1 and the coupling 2 is formed into trapezoidal screw thread, and in the threadedly engaged condition, a decided clearance 8 is generated between the thread ridge 3a of the steel pipe 1 and the thread groove 4b of the coupling 2. By changing the height 9 and the width 11 of the thread ridge 3a of the external thread 3, and the depth 10 and the width 12 of the thread groove 4a of the internal thread 4, the discharge resistance of compound is reduced so as to be in a range under 7000mm<-1>.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a threaded joint used for mining oil and gas, and more particularly to a threaded joint for oil country tubular goods characterized by the shape of the engraved screw.

[0002]

2. Description of the Related Art In recent years, with the depletion of resources or the progress of drilling technology, the development of oil wells and gas wells has been promoted under a higher temperature and higher pressure environment, and the demand for oil well pipes to be used therefor has been increased. The characteristics to be dealt with are becoming more severe. In the threaded shape of the threaded portion of the threaded joint that connects the oil country tubular goods, the conventional API (American Petro)
leum Institute: American Petroleum Institute) Standard API: A threaded joint with a sealing function on the threaded part, such as the round thread shape of Specification STD 5B (Figure 3 (b)) and the buttress thread shape (Figure 3 (a)). It has been widely used, but it has problems in leak resistance under high temperature and high pressure, joint tensile strength, and the occurrence of galling in the screw part due to repeated tightening and loosening, and is superior to the above API thread shape. Special threaded joints with characteristics tend to be used.

An example of the special screw joint for oil country tubular goods is disclosed in Japanese Patent Publication No. 12995/1990, and as shown in FIG. 1, the seal is a tip 5 of the steel pipe 1.
Based on the metal seal between the metal and the torque shoulder portion 6 of the coupling 2, the threaded portion 15 is determined not by the sealing function but by the characteristic of the required joint tensile strength. Of the two types of thread shapes defined by the API standard, the buttress thread shape has a better joint tension than the round thread shape, and therefore the special thread joint has a buttress thread shape, that is, The trapezoidal screw shape is widely adopted.

When the special screw joint is fastened, a grease called a compound, a lubricant containing zinc, lead or the like as a main component is previously added to the male screw part 3, the pipe tip 5, the female screw part 4, and the like. And the torque shoulder portion 6 is applied to the entire surface or a part thereof, and after the worker first manually tightens the steel pipe 1 and the coupling 2 (hereinafter referred to as hand tight), the tip 5 of the steel pipe 1 and the cup. The metal seal portion 7 formed with the torque shoulder portion 6 of the ring 2 is tightened to a predetermined torque by a fastener until a contact surface pressure required to secure a desired leak-proof performance is generated.

At this time, if the screw shape is exactly the same as the API standard buttress screw, the clearance between the screwed male screw 3 and female screw 4 is extremely small as shown in FIG. When the compound is tightened,
It accumulates between the male screw 3 and the female screw 4 and in the vicinity of the metal seal portion 7. As a result, a pressure called a dope pressure is generated in the metal seal portion 7 to adversely affect the sealing performance, and the tightening and loosening are repeated. Since galling occurs in the screw portion 15, in the conventional special threaded joint, the buttress shape is partially corrected as shown in FIG.
The screw shape is such that a predetermined clearance 8 is formed between the male screw 3 and the female screw 4 in the tightened state.

The above-mentioned fastening work is carried out not only in the drilling well for oil and gas but also in the case where the pipe is shipped with the coupling 2 attached to the end of the pipe at the manufacturing plant.

[0007]

As described above, during the fastening operation of the special screw joint, the lubricant called compound is applied and hand-tight, and then the fastener is tightened to a predetermined torque value. As shown in FIG. 5, the tightening torque is a torque called shoulder torque generated by the interference between the male screw 3 of the pipe 1 and the female screw 4 of the coupling 2, and a torque called a delta torque (after point A) higher than that. are categorized.

Of these torques, the latter delta torque contributes to the leak resistance of the special screw joint. The delta torque and the contact surface pressure generated in the metal seal portion monotonically increase as shown in FIG. Since a desired contact surface pressure can be obtained by keeping the delta torque within an appropriate range, efforts have been made to suppress variations in shoulder torque so that the delta torque falls within a certain range. .

The factors that cause the shoulder torque to fluctuate greatly are variations in the screw interference amount within the product tolerance, the type of compound used, the surface treatment conditions for the screw surface, and the machined roughness of the screw surface. Is widely known, and conventionally, the following measures have been taken. (1) Optimization of tolerance range of pipe and coupling screw dimensions (2) Change of fastening torque value according to compound Friction Factor (3) Standardization of surface treatment conditions (4) Surface roughening of screws by standardization of thread cutting conditions However, even if the above measures are taken, it has been difficult in the past to keep the variation in shoulder torque during tightening of the special screw joint within a certain range. The shoulder torque may fluctuate significantly, so that the desired delta torque may not be secured, and there is a risk that leakage may occur from the joint portion due to insufficient contact surface pressure of the metal seal portion 7 in a severe environment. There was a problem.

The present invention has been made in view of the above problems, and provides a special threaded joint for oil country tubular goods which suppresses the variation of the shoulder torque and remarkably improves the reliability of leak resistance. Is intended.

[0011]

To achieve the above object, an oil well pipe threaded joint of the present invention is an oil well pipe threaded joint having a metal seal portion and a threaded portion following the metal seal portion,
When the above-mentioned threaded portion is fastened to the pipe and the coupling, the fitting of the threaded portion with respect to the cross-sectional area along the axis of the gap formed between the complete threaded portion of the male screw and the female screw It is characterized in that the ratio of the screw thread lengths measured spirally along the screw thread in length is 7,000 mm ^-1 or less.

[0012]

As shown in FIG. 4, a buttress screw having the same thread shape as the API standard buttress screw (FIG. 4 (a)) and a buttress screw partially processed to remove the influence of dope pressure at the time of fastening are used. Predetermined clearance 8 between screw 3 and female screw 4
With the conventional special threaded joint with the screw shape (Fig. 4 (b)) formed with the same conditions, the thread interference amount, the machined roughness of the thread surface, the surface treatment, the compound to be used, etc. are all set under the same conditions. A torque-turn curve at the time of fastening these screw joints having metal seal portions is obtained as shown in FIGS. 7 and 8.

Incidentally, FIG. 7 shows a threaded joint having a thread shape conforming to the API standard, and FIG. 8 shows a conventional special threaded joint. As you can see from this figure,
When it is changed in the range of 4 rpm to 20 rpm, the API standard threaded joint has a substantially constant shoulder torque value with almost no effect on the tightening speed, whereas the conventional special screw joint has a tightening speed. It can be seen that the shoulder torque value becomes smaller as the tightening speed becomes faster.

As described above, even with similar trapezoidal screw shapes, the influence on the shoulder torque value with respect to the tightening speed remarkably differs due to the difference in design, and in particular, the modified buttress shape widely used for special screw joints. It can be seen that when the screw part of is adopted, it is strongly affected. Incidentally, these facts are due to the above-mentioned conventional (1)-
Even if the measure of (4) is taken, it is considered to be a major cause for the shoulder torque to change significantly.

In spite of the fact that all screw dimensions, contact surface properties, etc. are the same as described above, in a special screw joint,
The shoulder torque value changes depending on the tightening speed only because the friction coefficient at the contact surface between the male screw of the pipe and the female screw of the coupling changes depending on the tightening conditions. The coefficient of friction depends on how much of the compound used as the lubricant remains between the contact areas.

That is, when the clearance between the male screw and the female screw is extremely small, as in the API standard screw joint,
It is difficult for the compound existing between the screw surfaces in the handtight state to be discharged to the outside during tightening, so the lubrication condition is constant and the shoulder torque value is stable regardless of the tightening speed. When a certain clearance is formed between the male and female threads as in the conventional special threaded joint, the compound that originally exists on the thread surface in the hand tight state will be removed from the contact surface of the screw during tightening to the outside. This is because it is easy to be discharged to. Then, if the tightening speed is slow, the tightening time becomes long, so that the amount of the compound discharged to the outside increases and the friction coefficient at the contact surface increases, so that the value of the shoulder torque increases. On the other hand, when the tightening speed is fast, the tightening time is shortened, so that it can be estimated that the discharge of the compound is reduced, the friction coefficient on the contact surface is reduced, and the shoulder torque value is reduced.

In order to quantitatively evaluate these phenomena,
In the present invention, the cross-sectional area S along the axial direction of the gap portion formed by the complete screw portion of the male screw and the complete screw portion of the female screw after fastening.
Attention was paid to the ratio of the thread length Lc measured in a spiral shape along the thread at the fitting portion to c. It is considered that this ratio can be used to evaluate the degree of resistance of the compound existing in the fitting portion in the hand-tight state to the outside during tightening. This ratio is referred to as the compound discharge resistance (Lc / Sc) and In other words, it can be expected that the compound is more difficult to be discharged as the screw structure having a larger discharge resistance (Lc / Sc), that is, the smaller the cross-sectional area Sc or the longer the screw thread length Lc is.

Actually, the screw shape is changed, the cross-sectional area and the thread length are changed, and the change in the shoulder torque value when tightening to a predetermined torque value at several tightening speeds is obtained. Then, the results shown in FIG. 9 were obtained.
In FIG. 9, the vertical axis represents the amount of change in shoulder torque value at the tightening speeds of 4 rpm and 20 rpm, and the horizontal axis represents the discharge resistance (Lc / Sc) of the compound.

As can be seen from this figure, as the discharge resistance (Lc / Sc) increases, the amount of change in shoulder torque increases to a certain level, and when it exceeds a certain value, it decreases and eventually decreases. Is a value close to zero. Further, the amount of change in the shoulder torque increases up to a certain level as the discharge resistance (Lc / Sc) decreases, but decreases below a certain value, and finally decreases to a value close to zero. Has become.

The discharge resistance (Lc / Sc) is 3
The position (B) where the value is 10,000 indicates the threaded state of the API standard screw joint. The above results indicate that when the discharge resistance (Lc / Sc) is extremely large, the compound is difficult to be discharged to the outside regardless of the tightening speed and the shoulder torque becomes a constant value, but the discharge resistance (Lc / Sc) is extremely large. On the contrary, if it is small, it is likely to be always discharged, and in this case as well, it means that the value is almost constant regardless of the tightening speed.

It was also confirmed that this tendency can be treated uniformly by the discharge resistance (Lc / Sc) value, without depending on the outer diameter, the wall thickness, etc. of the threaded joint. Here, as described above, the screw shape with extremely large discharge resistance (Lc / Sc), that is, A
In the PI standard buttress screw shape, there is a problem of adverse effects on the sealing performance due to the occurrence of goring on the threaded portion at the time of tightening and the dope pressure near the metal seal portion after fastening. From the result of No. 9, the discharge resistance (Lc / Sc) is small and the fluctuation of the shoulder torque is at the position (A) close to zero (A).
/ Sc) is formed in a screw shape so that it is set to a value of 7,000 mm ^-1 or less.

In the special screw joint having the screw portion designed as described above, a stable shoulder torque and a desired delta torque can be obtained without being affected by the tightening speed due to the difference in the type of fastening machine. Can be concluded. In the special screw joint of the present invention,
Although the compound between the screw surfaces in the hand-tight state is easily discharged during tightening, the discharged compound is excessive for the original lubricity and has no adverse effect on the occurrence of galling in the screw portion.

[0023]

Embodiments of the present invention will be described with reference to the drawings.
First, the structure will be described. As shown in FIG. 1, the basic structure is the same as that of the conventional special threaded joint, and the outer circumference of the tip end portion of the steel pipe 1 has a taper 14 having a smaller diameter toward the tip end. Are engraved to form a threaded portion 15, and a seal portion 5 on the side of the pipe 1 following the threaded portion 15 is formed at the tip thereof.

The coupling 2 has an inner peripheral surface formed with a female screw 4 which can be screwed into the male screw 3 of the steel pipe 1, and the coupling 2 side which is engageable with the seal portion 5 of the steel pipe 1. The seal portion 6 is formed, and the seal portions 5 and 6 of both are formed.
The metal seal portion is constituted by. Further, the screw shapes of the steel pipe 1 and the coupling 2 are trapezoidal screws as shown in FIG. 2, and in a screwed state, a predetermined distance is provided between the thread 3a of the steel pipe 1 and the screw groove 4b of the coupling 2. There is a clearance 8 of.

By changing the height 9 and the width 11 of the screw thread 3a of the male screw 3 and the depth 10 and the width 12 of the screw groove 4a of the female screw 4, the fitting length L between the complete screw portions is changed. The cross-sectional area Sc of the clearance 8 along the axis is increased more than ever before. Alternatively, by changing the screw pitch 13 or the screw taper 14, the screw thread length Lc measured spirally along the screw thread at the fitting length L is reduced as compared with the conventional case.

As a result, the compound discharge resistance (Lc / Sc) is reduced to the above range of 7,000 mm ^-1 or less. Actually API standard L80, outer diameter 139.7
When a threaded joint for an oil well pipe having a thickness of mm and a wall thickness of 7.72 mm was processed and tightened under the same conditions, the results shown in Table 1 were obtained.

[0027]

[Table 1]

Table 1 shows that the screw joints in which the dimensions of the male screw 3 and the female screw 4 are changed and the discharge resistance (Lc / Sc) of the compound is variously changed are tightened under the same conditions. Yes, the shoulder torque change amount in the figure is
It shows the difference in shoulder torque values when tightening at 4 rpm and 20 rpm. In the figure, the joint No. No. 1 is a threaded joint of API standard, and the discharge clearance (Lc / Sc) is the largest at 29000 mm ⁻¹ because the clearance cross-sectional area Sc of the screw part is as small as 0.153 mm ² . In this joint, galling occurred in a part of the screw portion 15 during repeated tightening.

Further, the joint No. 2 to 19 in the table,
The shaded values are compound discharge resistance (Lc /
Sc) in the direction of decreasing the joint No. It is an item in which individual dimensions or multiple dimensions are changed with respect to No. 1. 16, 18 and 19 are threaded joints formed according to the present invention, the compound discharge resistance (Lc / Sc) is 7,000 mm ^-1 or less, and the shoulder torque is constant regardless of the tightening speed. Can be confirmed.

In other threaded joints, the compound discharge resistance (Lc / Sc) value is approximately 1000.
It is in the range of 0 to 20000 mm ^-1 , and it can be seen that the shoulder torque greatly varies depending on the tightening speed. Furthermore, when a gas leak test was conducted on all of the above threaded joints under the conditions according to the API standard, the joint No. 2,
In Nos. 3, 5, 8, 9 and 11, leaks occurred during the test only when tightened at a relatively low speed, but in joints Nos. 16, 18, and 19 of the examples based on the present invention, It was confirmed that no leak occurred during the test regardless of the tightening speed, and the effectiveness of the threaded joint according to the present invention was confirmed.

[0031]

As described above, according to the present invention,
It becomes possible to supply the oil well pipe threaded joint that can always obtain a substantially constant shoulder torque value while suppressing the influence of the tightening speed, which could not be strictly controlled in the past. As a result, the specified delta torque can always be applied regardless of the tightening speed, so the contact surface pressure that is the minimum required for leak resistance is always secured in the seal part, and the reliability in using the oil country tubular good is maintained. The effect of improving the degree is obtained.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing a screw joint fastening state in an example according to the present invention and a conventional example.

FIG. 2 is a cross-sectional view showing a screw shape of an embodiment according to the present invention.

FIG. 3 is a cross-sectional view showing a screw shape of an API standard screw joint.

FIG. 4 is a sectional view showing a trapezoidal screw shape.

FIG. 5 is a diagram showing a relationship between a tightening torque and a turn in a special screw joint.

FIG. 6 is a diagram showing a relationship between delta torque and seal portion surface pressure.

FIG. 7 is a diagram showing the effect of tightening speed on the relationship between tightening torque and turn in the API standard screw joint.

FIG. 8 is a diagram showing the effect of tightening speed on the relationship between tightening torque and turn in a special screw joint.

FIG. 9 is a diagram showing a relationship between a compound discharge resistance and a shoulder torque change amount.

[Explanation of symbols]

 1 Steel pipe 2 Coupling 3 Male screw 3a Screw thread 4 Female screw 5 Pipe tip 6 Torque shoulder 7 Metal seal part 8 Clearance 15 Screw part L Fit length

Claims (1)

[Claims] 1. A threaded joint for oil country tubular goods comprising a metal seal portion and a threaded portion following the metal seal portion, wherein when the pipe and the coupling are fastened to each other, the threaded portion is screwed into a male screw and a female screw. The ratio of the thread length measured spirally along the thread at the fitting length of the thread portion to the cross-sectional area along the axis of the gap formed between the complete thread portions is 700.
A threaded joint for oil country tubular goods characterized by having a diameter of ⁰ mm ^-1 or less.