JP2008185204A - Special screw joint for oil well pipe - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a special screw joint for an oil well pipe, having a sufficient effect to suppress generation of a gas leak path caused by surface properties on the side of a pin, facilitating a surface modifying process, and further improving the sealing property. <P>SOLUTION: This special screw joint for the oil well pipe includes the pin (1) formed at the end of a steel pipe and having a male screw part and a corner made to serve as a seal part (4) by metal-metal contact, and a coupling (2) having, on the side (2A) of a box, a female screw part screwed around the male screw part and an annular recessed part in metal-metal contact with the corner. Leastways, the surface roughness of the corner as the seal part (4) before fastening is set to 1.0 to 3.0 μm in Ra, and a white metal layer with thickness of 5 to 20 μm is formed on the basis material of the side (2A) of the box. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a special threaded joint for oil well pipes for connecting oil well pipes.

  In recent years, oil wells and gas wells that are easy to develop have decreased, and wells at higher depths and higher temperatures and pressures have increased. In addition, with the progress of drilling technology, wells with more complicated shapes such as inclined wells and horizontal wells are increasing. Along with this, requirements for threaded joints of oil well pipes, such as airtightness under complex loads such as tension, compression, internal / external pressure, bending, etc., are becoming increasingly sophisticated (Non-Patent Document 1).

Some special threaded joints for oil country tubular goods have a structure as shown in FIG.
This special threaded joint is composed of a pin 1 having a male threaded portion and a coupling 2 having a female threaded portion that is screw-coupled to the male threaded portion, and becomes a seal portion 4 by metal-metal contact in order to improve sealing performance. A corner is provided on the pin 1 and an annular recess in metal-metal contact with the corner is provided on the box side 2A of the coupling 2. Further, the shoulder portion 5 connected to the seal portion 4 is also in metal-metal contact to form a gas seal portion.

The male screw portion and the female screw portion of this special threaded joint are tapered screws. The male screw portion provided on the pin 1 is referred to as a pin screw portion, and the female screw portion provided on the box side 2A of the coupling 2 is referred to as a box screw portion.
1B is a cross-sectional view of the main part showing the state of the screw coupling portion 3, and FIG. 1C is a contact state of the seal portion 4 and the shoulder portion 5 formed by metal-metal contact. It is principal part sectional drawing which shows these.

The part to be a gas seal part other than the screw coupling part 3 is generally cut at a feed rate of about 0.1 mm / rotation by a rhombus tool called a point cut tool having a tip of about R0.8 mm, and the surface roughness after the processing is Finished to Ra ≦ 1.6 μm.
Further, for high alloy steel oil well pipes containing 9% or more of Cr, it is known that galling called goling is likely to occur in the screw coupling portion 3, the seal portion 4, and the shoulder portion 5 when the joint is fastened. . In order to prevent goling, the surface of the pin side of the box side 2A is usually subjected to a blasting process using shot grains made of glass, alumina, ceramic, stainless steel, etc. in the steel pipe manufacturing process. Is subjected to a plating treatment such as copper plating (Patent Document 1).

In addition, the surface roughness of the pin screw portion after the blasting rises to about Ra 1.0 to 3.0 μm. When the joint is fastened, it is considered that the minute recess on the surface of the pin screw portion becomes a kind of oil reservoir, and the goling can be prevented by the lubricating action of the oil accumulated in the recess.
JFE Technical Report No. 9 (August 2005) p.46-50 JP-A-2003-74763

However, if minute dents are continuously present on the surface of the pin screw portion and the seal portion, a gas leak path after fastening of the joint is formed, which causes a decrease in sealing performance.
The substrate surface on the other box side 2A is plated with Cu plating, Cu-Sn plating or the like having a thickness of about 5 to 20 μm. The plating for preventing goling is hard plating that does not expose the substrate even if the tightening-rewinding operation is repeated when the joint is fastened. Although this hard plating has the effect of suppressing the occurrence of gas leak paths caused by goling, it does not have the effect of suppressing gas leak paths caused by surface properties on the pin side.

  Therefore, the present applicant has a surface roughness before fastening on the pin side of Ra 1.0 to 3.0 [mu] m and a thickness of 5 to 20 [mu] m on the box side substrate, and further a thickness on the plating layer. A special threaded joint for oil well pipes having a soft plating layer of 1 μm or more was proposed (Japanese Patent Application No. 2006-234774: August 31, 2006). However, there are two plating layers and the formation of the plating layer is not easy It was.

  Therefore, in view of the above problems, the present invention provides a special threaded joint for oil well pipes that has a sufficient effect of suppressing a gas leak path due to the surface properties on the pin side, and that can be easily improved in surface modification and can further improve sealing performance. The purpose is to provide.

  That is, the present invention includes a pin having a corner portion that becomes a seal portion by a metal-metal contact with a male screw portion at a pipe end of a steel pipe, and a female screw portion that is screw-coupled to the male screw portion on the box side, the corner portion, and a metal-metal. In a special threaded joint for oil country tubular goods composed of a coupling having an annular concave portion in contact with each other, the surface roughness before fastening of at least the corner portion serving as the seal portion is Ra 1.0 to 3.0 μm, and the box side A special threaded joint for oil country tubular goods, wherein a white metal layer having a thickness of 5 to 20 μm is formed on a substrate.

  According to the present invention, there is a sufficient effect of suppressing a gas leak path caused by the surface properties on the pin side, and the surface modification treatment is easy and the gas sealing property can be further improved. For this reason, a special threaded joint for oil well pipes can be obtained that can stably exhibit excellent sealing performance even if variations occur in the surface properties on the pin side by cutting or blasting in the manufacturing process of the steel pipe.

In the present invention, at least the corner portion on the pin side that becomes the seal portion 4 is blasted by metal-metal contact, the surface roughness before fastening is Ra 1.0 to 3.0 μm, and the box side substrate is thick. A white metal layer having a thickness of 5 to 20 μm is formed.
In the special threaded joint for oil country tubular goods according to the present invention, a portion other than the corner portion of the pin 1, that is, the pin screw portion, etc. may be subjected to blasting in the steel pipe manufacturing process. On the other hand, in the process of manufacturing the coupling 2, a white metal layer having a thickness of 5 to 20 μm is formed on the substrate side of the box side 2A. In order to form the white metal layer, first, machining is performed on the member of the coupling 2 to form a box screw portion, an annular recess, and a shoulder portion 5 to expose the base of the box side 2A. When the joint is fastened, it is possible to form a white metal layer that does not cause goling even if the tightening-rewinding operation is repeated.

FIG. 2 schematically shows the white metal layer 6 on the substrate side of the box side 2A thus formed. t indicates the thickness of the white metal layer 6.
According to the special threaded joint for oil country tubular goods according to the present invention, when the joint is fastened, the white metal layer 6 provided on the ground surface of the one box side 2A causes a minute corner portion that becomes the seal portion 4 on the other pin 1 side. As a result, there is almost no minute dent continuously in the portion to be the seal portion 4, and as a result, the sealing performance is improved.

In addition, according to the present invention, as shown in FIG. 3, the galling prevention plating layer 7 having a thickness t _{1 of 5} to 20 μm is formed on the substrate side of the box side 2A, and the soft plating layer 8 having a thickness t ₂ of 1 μm or more thereon. Compared with the case of having a plating layer, it is easier to form a plating layer. FIG. 3 shows the surface structure of the box side 2A of the special threaded joint for oil well pipes proposed in Japanese Patent Application No. 2006-234774.
Here, when the thickness t of the white metal layer 6 is less than 5 μm, the galling resistance is poor, so that the thickness t is set to 5 μm or more. However, as the thickness t of the white metal layer 6 provided on the substrate side of the box side 2A increases, the tightening torque required for screw coupling increases when the joint is fastened.

  Normally, when the final tightening torque is within the specified range, it is judged that the joint has been tightened. Therefore, if the torque required for screw connection rises, the tightening torque at the other seal parts becomes insufficient. As a result, the seal is insufficient. In order to prevent such a high shoulder phenomenon, the thickness t of the white metal layer 6 provided on the substrate side of the box side 2A is set to 20 μm or less. Note that a grease compound containing an appropriate amount of Cu, Pb, or the like is usually used as a lubricant when performing the tightening / returning operation of the joint.

FIG. 4 shows the turn and the tightening torque of the rotation speed around the pin 1 when the failure is caused by the high shoulder phenomenon (broken line: large screw coupling torque) and when not (solid line: small screw coupling torque). Illustrated in the relationship.
In the figure, a section between points A and B where only the screw connection proceeds, a section between points B and C where the seal portion 4 is formed by metal-metal contact, and a point C after the point where the tightening torque suddenly increases are the shoulder portion 5. Is a section that becomes a gas seal portion, and T _f within the range of the maximum torque and the minimum torque is the final tightening torque.

The torque at point C at which the tightening torque begins to rise rapidly is referred to as shoulder torque. When the shoulder torque exceeds a certain determination value T ₁ (broken line), it is determined that the tightening torque is insufficient due to the high shoulder phenomenon, and when the determination value T ₁ does not exceed the solid value (solid line), the tightening torque is sufficient. Is determined. For example, a determination value T ₁ = 0.75 × T ₂ is set for the shoulder torque. When the high shoulder phenomenon occurs, the sealing performance except for the screw coupling portion 3 becomes insufficient.

As a white metal, among Pb-Sn-Sb white metals (JIS 5401) generally used as a bearing alloy, Sb content: 5 to 20 mass%, Cu content: 3 to 10 mass%, the balance Pb, Sn Inevitable impurities are not peeled even when the joint fastening operation is repeated, and are suitable for preventing the high shoulder phenomenon.
Further, if the surface roughness of the corner portion that becomes the seal portion 4 on the pin 1 side is too small, the effect of preventing goling becomes insufficient. Conversely, if the surface roughness is too large, it is difficult to block the gas leak path. 3.0 μm.

A gas seal test was performed on a sample with a maximum torque of 24877 N · m in a special threaded joint for 110 ksi grade oil well pipes made of 13% Cr steel and having an outer diameter of 5.5 ″ and a wall thickness of 0.415 ″. Table 1 shows the general specifications of the samples of the inventive examples, comparative examples, and conventional examples subjected to this test.
In both the inventive example and the comparative example, the white metal layer was formed on the base of the box side 2A by exposing the base of the box side 2A which is the inside of the coupling 2 by machining. In the conventional example, a Cu plating layer was formed on the substrate side of the box side 2A.

ガスシール試験に供した特殊ねじ継手は、発明例、比較例とも各１０組とした。そのうち各５組は、１回目の継手の締結時、最終締付けを行った（グループＡとする）。残りの５組は、３回目の継手の締結時、最終締付けを行った（グループＢとする）。

Special screw joints subjected to the gas seal test were 10 sets each for invention examples and comparative examples. Of these, each of the five groups was subjected to final tightening (referred to as group A) when the first joint was fastened. The remaining five groups were subjected to final tightening (referred to as group B) when the third joint was fastened.

  Group A: Gas seal test results when the final tightening is performed at the time of the first coupling, and Table B shows the gas seal test results when the final tightening is performed at the time of the third coupling. It is shown in Table 3.

この結果から、従来例では、グループＡにおいて、図３に示す９５％ＶＭＥ（フォンミーゼス[Von Mises]の相当応力）曲線上の点ＬＰ２（ＩＳＯのロードポイント‘２’）でリークが発生した。

From this result, in the conventional example, in group A, a leak occurred at point LP2 (ISO load point '2') on the 95% VME (Von Mises equivalent stress) curve shown in FIG.

  On the other hand, in the inventive examples, in groups A and B, no goling occurred in all the samples, and the samples passed without leakage. In the comparative example, a leak occurred due to the high shoulder phenomenon.

It is a partial section schematic diagram showing an example of a special threaded joint for oil country pipes. It is a cross-sectional schematic diagram which shows the plating layer structure by the side of the box concerning this invention. It is a cross-sectional schematic diagram which shows the plating layer structure of the box side proposed by Japanese Patent Application No. 2006-234774. It is a characteristic view at the time of fastening of a joint. It is a conceptual diagram of the load condition in a gas seal test.

Explanation of symbols

W oil well pipe (steel pipe)
t, t ₁ , t ₂ thickness 1 Pin 2 Coupling 2A Box side 3 Screw coupling part 4 Seal part 5 Shoulder part 6 White metal layer 7 Anti-goling plating layer 8 Soft plating layer

Claims (1)

  A pin (1) having a corner portion that becomes a seal portion (4) by metal-metal contact with a male screw portion at the pipe end of the steel pipe, and a female screw portion and a corner portion that are screw-coupled to the male screw portion on the box side (2A) In a special threaded joint for oil well pipes composed of a coupling (2) having an annular recess that makes metal-metal contact, the surface roughness before fastening of at least the corner portion that becomes the seal portion (4) is Ra 1.0 to 3. A special threaded joint for oil country tubular goods, characterized in that a white metal layer having a thickness of 5 to 20 μm is formed on the substrate side of the box side (2A).

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