JP5776222B2 - Threaded joints for steel pipes - Google Patents

Description

  TECHNICAL FIELD The present invention relates to a threaded joint for steel pipes, and in particular, oil well pipes including tubing and casings generally used for exploration and production of oil wells and gas wells, that is, OCTG (oil country tubular goods), riser pipes, line pipes, etc. The present invention relates to a threaded joint for steel pipes that is suitable for use in connecting steel pipes and has excellent sealing properties and compression resistance.

  Threaded joints are widely used to connect steel pipes used in oil industry equipment such as oil well pipes. Conventionally, a standard threaded joint specified in the API (American Petroleum Institute) standard has been typically used for connecting steel pipes used for oil and gas exploration and production. However, in recent years, the wells for crude oil and natural gas have been deepened, and the number of vertical wells, horizontal wells, inclined wells, etc. has increased, and the drilling and production environment has become severe. In addition, the demand for screw joints such as compression resistance, bending resistance, and external pressure seal performance (external pressure resistance) has diversified due to the increased development of wells in poor environments such as the ocean and polar regions. ing. For this reason, the use of high-performance special threaded joints called premium joints is increasing, and the demands on the performance are also increasing.

  A premium joint usually has a male screw member (hereinafter referred to as a pin) formed on a pipe end portion, each having a taper screw, a seal portion (specifically, a metal touch seal portion), and a shoulder portion (specifically, a torque shoulder portion). It is a coupling type joint in which a female screw member (hereinafter referred to as a box) for connecting pins to each other is coupled. The taper screw is important for firmly fixing the pipe joint, and the seal part plays a role of securing the sealing performance by metal contact between the box and the pin at this part, and the shoulder part is a stopper of the joint during tightening. It becomes the shoulder side that plays the role.

  FIG. 4 is a schematic explanatory view of a premium joint for an oil well pipe, and these are longitudinal sectional views of a threaded joint of a circular pipe. The threaded joint includes a pin 3 and a box 1 corresponding to the pin 3, and the pin 3 has a male screw portion 7 on the outer surface thereof and a screwless portion provided adjacent to the male screw portion 7 on the distal end side of the pin 3. It has a nose portion 8 (pin nose 8) which is a length portion. The nose portion 8 has a seal portion 11 on its outer peripheral surface and a torque shoulder portion 12 on its end surface. The opposing box 1 has a female screw part 5, a seal part 13, a part that can be screwed or contacted with the male screw part 7, the seal part 11, and the shoulder part 12 of the pin 3 on the inner surface thereof, respectively. A shoulder portion 14 is provided.

Patent document 1 is mentioned as a prior art regarding the said premium joint.
In the conventional example of FIG. 4, the seal portion is at the tip of the pin nose 8, and desired sealing performance can be realized by applying an appropriate tightening torque.
Since the tightening torque is affected by lubrication conditions, surface properties, etc., a design that does not depend greatly on this is a radial seal method in which the radial component of the seal contact pressure is relatively strong. For example, Patent Literature 1 discloses an example of a radial seal method having a large pin seal R shape and a small seal taper angle. However, the problem with the radial seal method with such a small seal taper angle is that goling is likely to occur during tightening, and in particular, the amount of seal interference for the purpose of ensuring seal performance and sealing stability. When a large value is set, the ease of occurrence of goling is remarkably increased.

Japanese Utility Model Publication No. 61-44068

The seal part 11, 13 in FIG. 4 or the seal part of the above-mentioned radial direction seal method (hereinafter also referred to as radial seal type) is subjected to a necessary and sufficient pressure, so that a healthy seal part that does not cause leaks or goling. Is formed.
However, according to a study by the inventors, in the radial seal type threaded joint for steel pipes, there is still room for improvement in securing the sealing performance in terms of the rigidity of the pin nose. In other words, the rigidity of the pin nose 8 is important for ensuring the sealing performance of the radial seal type, and the seal point sp (see FIG. 1; relative to the outer peripheral surface of the nose portion on the pin 3 side when screw coupling is performed. It is necessary to ensure a sufficient cross-sectional area at the inner peripheral surface on the box 1 side (hereinafter referred to as the seal surface) on the outer peripheral surface of the nose portion on the pin 3 side that first contacts. This is because when the axial compressive force is applied, a compressive load is applied according to the ratio of the cross-sectional area of the pin cross-sectional area at the seal point sp and the cross-sectional area of the unprocessed part (the equivalent of the raw pipe before threading). However, if the ratio of the cross-sectional area is not appropriate, plastic deformation accompanying axial compression force also occurs in the vicinity of the seal portion. This is because compression and bending deformation occurs in the pin nose 8, and the outer diameter of the pin 3 at the seal point sp is reduced. When this reduction is significant, the pressure of the seal portion 20 becomes insufficient and the sealing performance cannot be secured. There has been no knowledge of such issues and solutions.

In order to solve the above-mentioned problems, the inventors have found a guideline for making the ratio of the cross-sectional area of the pin at the seal point sp and the cross-sectional area of the unprocessed portion appropriate, through various experimental studies.
That is, the present invention is as follows.
(1)
A pin having a male threaded portion with a load flank angle of −10 degrees to 0 degrees, a nose portion extending from the male threaded portion to the tube end side, and a shoulder portion provided at the tip of the nose portion;
A female screw part that is screw-coupled with the male screw part to form a screw part, a seal surface that faces the outer peripheral surface of the nose part of the pin, and a box that has a shoulder part that contacts the shoulder part of the pin, A screw joint for a steel pipe in which the pin and the box are coupled by screw coupling, and the outer peripheral surface of the pin and the seal surface of the box are in metal-metal contact, and the contact portion forms a seal portion,
The pin side nose part outer peripheral surface is a taper shape, the box side seal surface is an outwardly convex curved shape,
The cross-sectional area of the pin at the seal point, which is a portion on the outer peripheral surface of the nose portion on the pin side that first comes into contact with the seal surface on the box side during the screw connection, is the cross-sectional area of the raw pipe portion that is the unprocessed portion of the pin 35% or more,
The seal point position index x / L defined by the ratio of the distance x from the screw tip to the seal point to the length L of the nose from the screw tip to the nose tip is 0.2 to 0.8. A threaded joint for steel pipes, characterized by being.

  According to the present invention, the rigidity of the pin nose is improved, and even when a large axial compression force is applied, the pin outer diameter is hardly reduced due to compression and bending deformation, and a threaded joint for steel pipe that can ensure excellent sealing performance is realized.

It is sectional drawing which shows embodiment of this invention. FIG. 2 is an enlarged cross-sectional view illustrating the shape of the seal portion of FIG. (A) is a cross-sectional view showing the definition of the load flank angle β, the stubbing flank angle γ, and the gap G of the threaded portion, and (b) is a cross-sectional view showing the definition of the position index x / L of the seal point sp. . It is sectional drawing which shows the conventional threaded joint for steel pipes, (a) is whole sectional drawing, (b) is an expanded sectional view which shows the screw part in (a), (c) is an enlarged drawing which shows the pin nose vicinity in (a) It is sectional drawing.

  The threaded joint for steel pipes according to the present invention includes, as shown in FIGS. 1 and 2, for example, a male screw portion 7, a nose portion 8 extending from the male screw portion 7 toward the pipe end side, and a tip of the nose portion 8. A pin 3 having a shoulder portion 12 provided; a female screw portion 5 screwed to the male screw portion 7 to form a screw portion; a seal surface facing the outer peripheral surface of the nose portion of the pin 3; A box 1 having a shoulder portion 14 abutting against the shoulder portion 12, and the pin 3 and the box 1 are connected by the screw connection so that the outer peripheral surface of the nose portion of the pin 3 and the seal surface of the box 1 are metal. -It was assumed that this was a steel pipe threaded joint with metal contact and the contact part forming the seal part 20 (a threaded joint equivalent to a conventional radial seal type premium joint).

In the threaded joint based on the above premise, the load flank angle β, stubbing flank angle γ, and thread gap G as defined in FIG. 3A are used as the thread shape parameters.
The load flank angle β is an angle β formed by the load flank surface 18 with respect to an orthogonal line to the threaded joint axis, and when the orthogonal line passes through the lower end (end on the inner diameter side of the pin) of the load flank surface, the orthogonal line On the other hand, when the upper end (end on the pin outer diameter side) of the load flank surface is located on the pin front end side, it is positive, and when it is located on the pin rear end side, it is negative. Usually, β = −10 degrees to 0 degrees.

  The stubbing flank angle γ is an angle γ made by the stubbing flank surface 19 with respect to the orthogonal line to the threaded joint axis, and when the orthogonal line passes through the lower end of the stubbing flank surface (end on the inner diameter side of the pin), When the upper end (end on the pin outer diameter side) of the stubbing flank is located on the tip end side of the pin with respect to the orthogonal line, it is negative. Usually, γ = 10 degrees to 30 degrees.

The gap G of the thread portion is a gap G between the thread 7a of the male thread and the thread groove 5a of the female thread that meshes with the thread 7a. Usually, G = 0.03 to 0.15 mm.
In the present invention, based on the above assumption, as shown in FIG. 2, the outer peripheral surface of the nose portion on the pin 3 side is a tapered shape, and the seal surface on the side of the box 1 is a curved shape protruding outward. The taper angle θ of the taper surface forming the taper shape is defined as an angle θ formed by the taper surface with respect to the threaded joint axis. Further, the seal point sp is a portion on the outer peripheral surface of the nose portion on the pin 3 side that first comes into contact with the seal surface on the box 1 side during screw connection.

The relative position of the seal joint sp in the axial direction of the threaded joint is relative to the length (nose length) L of the nose portion 8 from the tip of the screw portion (the rear end of the nose portion) to the tip of the nose portion, as shown in FIG. The seal point position index x / L is defined by the ratio of the distance x from the tip of the threaded portion (the rear end of the nose portion) to the seal point sp.
In FIG. 2, δ is an interference amount in the seal portion 20, and this interference amount δ is the shrinkage when the seal point sp is reduced in diameter by the box 1 regarded as a rigid body at the time of screw connection. This is the diameter.

In the present invention, as shown in FIG. 1, the cross-sectional area S ₁ of the pin 3 at the seal point sp is 35% or more of the cross-sectional area S ₀ of the raw pipe portion which is the unmachined portion, that is, the cross-sectional area ratio S _1. / S ₀ * 100 ≧ 35 (%).
Since the cross-sectional area ratio S ₁ / S ₀ * 100 ≧ 35 (%), the compression resistance of the seal portion 20 is improved, and it is difficult to reduce the outer diameter of the pin even when subjected to a large axial compression force. Sealing performance can be secured. In a radial seal type threaded joint, if the cross-sectional area S ₁ is less than 35%, it is impossible to ensure a required level of contact area pressure (an index obtained by integrating surface pressure over the contact length). In order to obtain more sufficient compression resistance, the cross-sectional area ratio S ₁ / S ₀ * 100 ≧ 40 (%) is preferable.

The upper limit of the cross-sectional area ratio S ₁ / S ₀ * 100 is not particularly limited, but in consideration of the taper screw machining allowance and the seal taper allowance, a thick material having a wall thickness / outer diameter of about 7 to 8%. But the upper limit is about 70%.
In the present invention, it is preferable that the seal taper angle θ of the pin is 10 degrees or less, more preferably 5 degrees or less because deformation of the pin in the radial direction can be more effectively suppressed.

  In the present invention, the seal point position index x / L is preferably 0.2 to 0.8. That is, from the viewpoint of separating the seal part 20 from the nose tip to improve external pressure resistance and tensile resistance, and giving stable performance to the screw, 0.8 or less is preferable. If it is less than 0.2, interference between the seal portion and the screw portion is likely to occur at the time of fastening at the time of screw connection (make up; MU for short). Furthermore, 0.3 or more is more preferable for safety.

  Further, the load flank angle β, the stubbing flank angle γ, and the gap G of the threaded portion are not particularly limited, and the usual values can be employed.

  A finite element analysis (FEA) is performed on a threaded joint consisting of a steel pipe end machined with an outer diameter of 9 + 5/8 inch and a wall thickness of 0.545 inch, and a box corresponding to this pin. Samples were produced with the same area pressure, and ISO 13679 series A tests were conducted. In carrying out this experiment, the experiment was conducted at various levels shown in Table 1. Further, as experimental conditions not shown in Table 1, the seal point position index x / L = 0.7, the load flank angle β = −5 degrees, the stubbing flank angle γ = 25 degrees, common to all levels, The thread gap G was 0.06 mm, and the maximum tensile rate (yield stress ratio) was 95%.

In the condition of Comparative Example 2 where a very large amount of interference δ is required to obtain a predetermined contact area pressure, goling occurred during the Make & Break test. The experiment was conducted under the conditions of Comparative Example 2A, which was lowered to the same level as in Example 1.
As shown in Table 1, it is clear that the examples of the present invention have excellent sealing properties.

1 box
3 pin
5 Female thread
5a Female thread groove
7 Male thread
7a Male thread thread
8 Nose (pin nose)
11,13,20 Seal (Details are metal touch seal)
12,14 Shoulder (For details, Torque shoulder)
18 Road flank surface
19 Stubbing flank surface

Claims (1)

A pin having a male threaded portion with a load flank angle of −10 degrees to 0 degrees, a nose portion extending from the male threaded portion to the tube end side, and a shoulder portion provided at the tip of the nose portion;
A female screw part that is screw-coupled with the male screw part to form a screw part, a seal surface that faces the outer peripheral surface of the nose part of the pin, and a box that has a shoulder part that contacts the shoulder part of the pin, A screw joint for a steel pipe in which the pin and the box are coupled by screw coupling, and the outer peripheral surface of the pin and the seal surface of the box are in metal-metal contact, and the contact portion forms a seal portion,
The pin side nose part outer peripheral surface is a taper shape, the box side seal surface is an outwardly convex curved shape,
The cross-sectional area of the pin at the seal point, which is a portion on the outer peripheral surface of the nose portion on the pin side that first comes into contact with the seal surface on the box side during the screw connection, is the cross-sectional area of the raw pipe portion that is the unprocessed portion of the pin 35% or more,
The seal point position index x / L defined by the ratio of the distance x from the screw tip to the seal point to the length L of the nose from the screw tip to the nose tip is 0.2 to 0.8. A threaded joint for steel pipes, characterized by being.

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