JP2002069690A - Structure of screw joint for oil well pipe and electroplating method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve durability of sealing performance and seizing resistance, of a screw joint for an oil well pipe. SOLUTION: The method comprises blending a predetermined quantity of a brightener into an electrolytic solution, when plating copper to an inner side of a box 1 which is a screw joint. Thereby, metal crystal grains of a plated layer 5 are refined, and when a plated layer on a screw part or a sealed part is rubbed with a high bearing strength by repetition of makeup and breakout of an oil well pipe and a strong frictional force is generated, fine metal crystal grains in the most outer plated layer exfoliate in sequence grain by grain from the surface of the plated layer to prevent seizing generation due to cold-working hardening and degradation of the sealing performance.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a threaded joint structure for oil country tubular goods having excellent sealing performance and anti-seizure property by preventing cold work hardening of a thread portion and a sealing portion, and an electroplating method suitable therefor.

[0002]

[Prior art]

[0003] Oil well pipes used in oil fields and natural gas fields have a long length of several thousand meters, so that a stainless steel pipe having a unit length of tens of meters is used with a tubular screw joint called a short box. Excavation is proceeded while adding. FIG. 4 is a cross-sectional view showing one end of each of the pin 2 and the box 1. A male screw 3 a formed on the outer peripheral surface of the end of the pin 2 is replaced with a male screw formed on the inner peripheral surface of the end of the box 1. 3b and the two are joined. The oil country tubular goods are occasionally pulled up for periodic inspections and the like, and the pins 2 are removed from the box 1, so that the male screw 3a and the female screw 3b
Each of the sliding contact surfaces and the sealing portion 4a at the tip of the pin 2 and the sealing portion 4b on the box side where the tip sealing portion 4a abuts and abuts repeatedly receive strong friction. As described above, since the pin 2 and the box 1 are repeatedly connected and detached many times, if the durability is not sufficient, the seal portions 4a and 4b
Problems such as poor sealing and seizure at the screw portions 3a and 3b occur. Therefore, the function of the threaded joint for an oil country tubular good is to a) withstand not only the tensile force in the axial direction due to the weight of the connected pipe, but also b) withstand the pressure of the fluid inside and outside, and c) ten times or more. It is required that good sealability or seizure resistance be maintained even by repeated use of. In particular, in recent years, the depth of oil wells has tended to be further increased, and the frequency of use in harsh environments such as polar regions has increased, and the required quality has become increasingly severe.

[0004] Conventionally, as proposed in Japanese Patent Publication No. 1-12955 or the like, the screw portion 3a or 3b is plated with copper or subjected to a surface treatment such as a phosphate treatment, and the seizure resistance is further improved. Therefore, compound grease has been applied to the joint interface of the pin 2 or the box 1 during make-up.

[0005]

[Problems to be solved by the invention]

[0006] The threads of the pins and boxes are subjected to high surface pressures that exceed the yield stress of the material. That is, the taper of the metal seal is 1/10 to 30 °, the screw is generally 8 threads / inch to 4 threads / inch, and the metal seal interferes little by little as the screw rotates.
This is because the metal must be contacted by a long sliding distance.

That is, it must have a performance that does not cause seizure even under severe conditions of high surface pressure × sliding distance.

However, the copper plating layer is gradually cold-work hardened as make-up and break-out are repeated, and initially has a sufficient softness and exhibits good sealing properties. It was found that not only could the sealability not be maintained, but also the mating thread or seal was damaged.

Therefore, as another method for improving the sealing property and the seizure resistance, a method of applying a multilayer plating to the joint portion,
A method of dispersing and mixing a fluororesin powder in a plating layer,
A method of forming a lubricious protective film by sputtering,
Although a method using a solid lubricating film instead of compound grease has been proposed, none of them has achieved sufficient sealing properties or seizure resistance.

The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to prevent a cold work hardening of a copper plating layer in a portion where a pin and a box are relatively slid, and to provide a good seal using a soft metal. The purpose is to maintain the properties and seizure resistance for a long time.

[0011]

[Means for Solving the Problems]

The present inventor has conducted various experiments to achieve the above object. As a result, it is effective to add a brightening agent to the electrolytic solution when electroplating the relative sliding contact portion between the pin and the box. I found that. This brightener is conventionally used for plating of electronic parts and the like as necessary to improve the smoothness of the plating layer surface. When soft metal plating is applied to the surface of the box, metal crystal grains are refined and precipitated, and when these fine metal crystal grains are rubbed with high contact pressure at the screw portion and the seal portion at the time of make-up and breakout, Conventionally, cold work hardening causes damage or seizure of the mating metal surface in the relative sliding contact part, but before it reaches it, it gradually falls off and separates from the surface of the plating layer in particle units It was confirmed that. Such detachment and detachment of the plating layer metal crystal particles in the unit of particles under such high surface pressure friction was first discovered by the present inventor, and this caused the conventional problem in the relative sliding contact portion between the pin and the box. The damage and seizure caused by the cold work hardening are prevented, and the fine metal crystal grains that have fallen off and peeled also function as a solid lubricant in the screw portion, so that make-up and breakout operations can be performed smoothly.

[0013] That is, a threaded joint structure for oil country tubular goods according to the present invention for solving the above-mentioned problems is a threaded joint structure for oil country tubular goods for mutually connecting unit length oil country tubular goods by a screw joint. An electroplating layer of fine metal crystal grains made of a soft metal using an electrolytic solution containing a brightener is formed on a surface of the oil country tubular goods including a thread part.

The electrolytic solution may be either an acidic electrolytic solution or an alkaline electrolytic solution. The brightener may be an inorganic brightener containing a sulfur compound such as potassium thiocyanate or a metal compound such as sodium selenite, or triethanol. Any organic brightener containing an amine or benzoic acid may be used, and as the soft metal, for example, copper, zinc or tin can be used.

Further, in the method for electroplating a threaded joint structure for an oil country tubular good according to the present invention, a brightener is added to an electrolytic solution having a soft metal as one electrode, and the soft metal is placed on the opposite side under the action of the brightener. It is characterized in that it is deposited as an electroplating layer of fine metal crystal grains on the surface of the electrode including the threaded joint for oil country tubular goods or the threaded part of the unit long oil country tubular goods.

The electrolytic solution may be either an acidic electrolytic solution or an alkaline electrolytic solution. The brightener may be an inorganic brightener containing a sulfur compound such as potassium thiocyanate or a metal compound such as sodium selenite, or triethanol. Any organic brightener containing an amine or benzoic acid can be used.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to the drawings. In the threaded joint structure of the present invention, FIG.
As shown in (B), a copper plating layer 5 is formed on the inner peripheral surface of the box 1 which is a threaded joint by electroplating.
The plating layer 5 is formed by adding a predetermined amount of potassium thiocyanate as a brightening agent to an acidic electrolytic solution according to a conventional method, and is shown in the scanning electron micrographs of FIGS. 3A and 3B at a magnification of 30,000. As shown in FIG. 3B, the metal crystal grains of the plated copper without addition of the brightening agent are extremely finer than those in FIG. 3B, so that the plating layer is formed in the threaded portion 3a or the threaded portion 3a or the sealing portion 4b. When it was rubbed by the seal portion 4a, it was confirmed that the fine crystal grains of copper were separated and peeled in units of particles as shown in FIGS. 2 (A) and 2 (B).
When the electrolytic solution was changed to an alkaline electrolytic solution, when the brightener was changed to an inorganic brightener containing a sulfur compound other than potassium thiocyanate or a metal compound such as sodium selenite, or when triethanolamine or benzoate was used. Similar results were obtained when the organic brightener containing an acid or the like was used.
Generally, as the addition ratio of the brightener increases, the refinement proceeds, so that the addition ratio may be appropriately adjusted according to the desired level of crystal grain refinement according to the type and use of the threaded joint. In addition, various conditions for performing electroplating, such as the temperature of the electrolytic solution, the amount of electricity and the time of electricity, are appropriately adjusted according to a standard method.
In order to confirm the effectiveness of the plating method according to the present invention, the present inventors applied copper plating to a 13Cr stainless steel box having a screw diameter of 7 inches by the method of the present invention, and applied a pin of the same material to the female screw portion of the box. An experiment was conducted to repeatedly make up and break out the external thread. In the experiment, make-up and break-out were continuously repeated up to 13 times, but no seizure occurred. The standard of the seizure resistance is that no seizure is caused by make-up and breakout more than 10 times, so that the method of the present invention has a sufficient seizure resistance. On the other hand, in a repeated experiment of make-up and breakout using the same box and pin plated with copper by the conventional method, seizure occurred as early as the fifth time.

As described above, one embodiment of the present invention has been described. However, the present invention is not limited to the above embodiment, and various modifications can be made. For example, in the above embodiment, the inner peripheral surface of the box which is a threaded joint is plated with copper, but may be plated with other soft metal other than copper, such as zinc or tin. Also,
As a plating object, a box much smaller than a long pin is more reasonable in relation to plating workability and cost, but both ends of the pin may be plated by the method of the present invention. However, if both the pin and the box are plated, seizure is more likely to occur at the screw portion and the like, so it is preferable to plate only one of them.

[0019]

As described above, the present invention provides a method of forming fine metal crystal grains of a soft metal using an electrolytic solution containing a brightener on the surface of a threaded joint or a unit long oil country tubular good in a threaded joint structure for an oil country tubular good. Since the electroplated layer was formed, the outermost fine metal crystal grains of the plated layer were removed at the stage where the plated layer of the threaded part and seal part was rubbed by high surface pressure due to make-up and breakout and a strong frictional force was generated. The particles are sequentially dropped and peeled from the surface of the plating layer in units of particles, and the deterioration of the sealability and the occurrence of seizure caused by cold work hardening accompanying repetition of makeup and breakout are prevented.

[Brief description of the drawings]

FIG. 1A is a sectional view of a pin tip and a box,
(B) is a partially enlarged view of FIG.

FIG. 2A is an enlarged sectional view of a seal portion of a box,
(B) is an enlarged sectional view showing a state of detachment of crystal grains at a seal portion.

FIG. 3 (A) is a scanning electron micrograph of copper grains of electroplating by a method of the present invention using a brightener, and FIG. 3 (B) is a scanning electron microscope of copper grains of conventional electroplating without adding a brightener. Photo.

FIG. 4 is a cross-sectional view of each thread portion of a pin and a box.

[Explanation of symbols]

 1 Box 2 Pin 3a Male thread 3b Female thread 4a, 4b Seal 5 Plating layer 6 Fine metal crystal grains

Claims (6)

[Claims] 1. A threaded joint structure for an oil country tubular good for connecting unit long oil country tubular goods to each other by a threaded joint,
A threaded joint structure for oil country tubular goods, wherein an electroplating layer of fine metal crystal grains is formed of a soft metal using an electrolytic solution containing a brightener. 2. The method according to claim 1, wherein the electrolytic solution is an acidic electrolytic solution or an alkaline electrolytic solution, and the brightener is an inorganic brightener containing a sulfur compound such as potassium thiocyanate or a metal compound such as sodium selenite, The threaded joint structure for an oil country tubular good according to claim 1, wherein the threaded joint structure is any one of organic brighteners containing ethanolamine, benzoic acid, and the like. 3. The threaded joint structure for oil country tubular goods according to claim 1, wherein the soft metal is copper. 4. A brightening agent is added to an electrolytic solution having a soft metal as one electrode, and under the action of the brightening agent, the soft metal is supplied to a threaded joint for an oil well pipe of the opposite electrode or a threaded portion of a unit long oil well pipe. A method for electroplating a threaded joint structure for oil country tubular goods, comprising depositing a fine metal crystal grain as an electroplating layer on a surface containing chromium. 5. The electrolytic solution is either an acidic electrolytic solution or an alkaline electrolytic solution, and the brightener is an inorganic brightener containing a sulfur compound such as potassium thiocyanate or a metal compound such as sodium selenite; The electroplating method for a threaded joint structure for oil country tubular goods according to claim 4, wherein the method is any one of organic brighteners containing ethanolamine, benzoic acid, and the like. 6. The method for electroplating a threaded joint structure for oil country tubular goods according to claim 4, wherein said soft metal is copper.