JP2002174375A - Evaluation method for sealing performance of oil country tubular goods threaded joints - Google Patents

Abstract

(57) [Summary] [Problem] To provide a sealing performance evaluation method for oil country tubular goods threaded joints, which can estimate the sealing performance closer to the actual one. SOLUTION: In the seal performance evaluation method for an oil country tubular good threaded joint in which the contact between a pin 1 and a coupling 2 is determined by elasto-plastic FEM analysis and the seal performance is evaluated based on the result, the screw is used for the elasto-plastic FEM analysis. Is approximated by an axisymmetric ring, the screw portion of the coupling and the main body are individually defined as regions, the fastening process is divided into a plurality of steps, and the screw portion of the coupling is slid in the order of these steps,
The contact between the pin and the coupling is determined.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of evaluating the sealing performance of a threaded oil country tubular goods joint, and more particularly to a method of evaluating the sealing performance of a threaded oil country tubular goods joint by elasto-plastic FEM analysis.

[0002]

2. Description of the Related Art One of the important management items of oil country tubular goods is the sealing performance of threaded joints. Since the sealing performance of a threaded joint depends on the shape of the threaded joint, in order to obtain a predetermined sealing performance, information on the correspondence between the shape of the threaded joint and the sealing performance is required. As a means for obtaining this information, there is a method of evaluating sealing performance by elasto-plastic FEM analysis. In this method, as shown in FIG. 5, the stress and the stress when the pin 1 and the coupling 2 are deformed from the state of overlap 3 (a) to the appropriate surface contact state (b) in one step are obtained.
The strain distribution is determined, and the sealing performance is evaluated using the results. Since the contact determination as to whether or not a certain contact interface is in an appropriate surface contact state is made by determining whether the balance of the forces on both sides of the contact interface is successful, this analysis includes:
A convergence calculation for searching for a contact interface that satisfies the balancing condition is included.

[0003]

However, in the above-mentioned conventional method, the deformation is caused in one step in the analysis, which is different from the actual deformation process in which the contact state between the pin and the coupling gradually changes. However, when the effect of the plastic strain history during the deformation cannot be ignored, there is a high possibility that the sealing performance different from the actual one is estimated. Therefore, there is a problem that the validity of the result must be confirmed by an experiment. Further, since the calculation is performed in one step, especially when the amount of interference is large, the convergence is poor and a solution may not be obtained in some cases.

Accordingly, an object of the present invention is to provide a method for evaluating the sealing performance of an oil country tubular good threaded joint, which can stably estimate a more realistic sealing performance.

[0005]

SUMMARY OF THE INVENTION The present invention provides an elasto-plastic FEM.
In the method for evaluating the sealing performance of a threaded joint for oil country tubular goods, in which the contact between the pin and the coupling is determined by analysis and the sealing performance is evaluated based on the result, in the elasto-plastic FEM analysis, the screw is approximated by an axisymmetric ring. The screw thread portion of the ring and the body portion are separately defined, the fastening process is divided into a plurality of steps, and the screw portion of the coupling is slid in the order of these steps to determine the contact between the pin and the coupling. This is a method for evaluating the sealing performance of oil country tubular goods threaded joints.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, commercially available stress analysis software such as ABAQUS can be preferably used as the elasto-plastic FEM analysis means. These softwares have an automatic surface-to-surface automatic contact determination function and stable convergence. The elasto-plastic condition and the multipoint constraint condition can be arbitrarily set by an attached user subroutine. The result of the contact determination is
Since the output is output as an appropriate contact state (contact interface position and stress / strain distribution) between the pin and the coupling found by the convergence calculation, the sealing performance can be evaluated based on the output data.

[0007] The thread of the oil country tubular good threaded joint actually forms a three-dimensional spiral, but in the present invention, this is approximated by an axisymmetric (two-dimensional) ring shape (Fig. 2). As a result, two-dimensional handling becomes possible, and the calculation load can be greatly reduced. Note that sufficient approximation accuracy can be obtained even with such approximation.
In the conventional analysis, as shown in FIG. 1 (a), the screw portion of the coupling 2 and the main body portion are integrally defined as an area. Cannot be reproduced. Therefore, in the present invention, as shown in FIG. 1 (b), the screw portion and the main body portion of the coupling 2 are individually defined as regions, and the screw portion can be slid independently of the main body portion. Thus, mutual interference between the pin and the threaded portion of the coupling can be avoided, and the fastening process can be reproduced.

In the conventional analysis, contact determination is made assuming one-step deformation. In contrast, in the present invention, contact determination is performed by dividing the fastening process into a plurality of steps and performing the slide in the order of steps. As a result, the deformation state in the process of fastening can be reproduced, and evaluation can be performed in consideration of the influence of the plastic strain history. In actual sealing performance evaluation tests, the sealing surface pressure after repeated tightening → loosening → tightening is often evaluated, but according to the present invention, the fastening process is divided into a plurality of steps. Thus, it is possible to easily reproduce the actual repetition process.

Further, since the actual thread of the oil country tubular good threaded joint is a tapered thread, sliding the thread of the coupling in the tightening direction interferes with the main body. In order to avoid this, the sliding is performed along the thread taper direction.
However, the sliding in the taper direction was performed for the shape before deformation (initial shape = state of zero stress), and the shape after deformation showed that the sliding along the inner surface profile of the main body determined by the contact judgment was obtained. Can be

[0010] By determining the contact by sliding the coupling screw portion for each step, the positional relationship between the screw portion and the main body portion changes. The mutual positional relationship after the execution of the step is, for example, as follows. Can be set. Now, at the first step start time t = 0, the position vector p
And the position vectors p _i−1 , p _i , p _{i + 1} , p
_{i + 2} Togazu located interrelated such as 3 (a), the threaded portion 1 point in the first step by just sliding displacement vector u _a contact judgment, as a result, the first step end time t = the displacement body part 4 points each vector _{u i-1, u i,} u i + 1, u i + 2 by a displacement to the position vector p _'i-1 as in t FIG. 3 (b), the p ' _i , p' _{i + 1} ,
p If _{'i + 2} to change, and one point the threaded portion that position vector displaced by the displacement vector u _b with the main body portion p' and those that vary the total displacement of the threaded portion 1 point The vector u is represented by u = u _a + u _b (1). The (1) displacement vector u _a is because the value per set value are known in formula the displacement vector u _b
Can be determined, the value of the total displacement vector u is determined, and the mutual positional relationship between the screw portion and the main body after the execution of the step is set.

The displacement vector u_bIs determined as follows:
Can be determined. First, after one point of the screw part slides,
The position vector p as shown in (b)_i, P _{i + 1}The body represented by
The line segment connecting the two points is defined as a ratio (1-α): α (where 0 ≦ α ≦
If it is shifted to the position of the point internally divided in 1), the displacement vector
Tol u_aIs u_a= (1-α) p_i+ Αp_{i + 1}−p (2) Since i and α satisfy the equation (2), i and α are obtained.

Assuming that the ratio of the distance between one point of the screw and one of the two points of the body sandwiching the screw and the distance between the other does not change before and after the contact determination, the displacement vector u _b
Because represented by _{u b = (1-α)} u i + αu i + 1 ...... (3), the (3) and i obtained in the previous (2) to the equation alpha
Is applied to determine its value.

A displacement vector u _a relating to a slide at a certain step is represented by a z component (screw axial direction component) u _az
And r component (screw radial component) u _ar , and these values indicate the amount of tightening (or loosening) at that step by Δ
Assuming that z, the profile of the inner surface of the coupling body at the start of the step is r (z), it can be determined by the following equation. u _az = Δz (4) u _ar = r (z + Δz) −r (z) (5) If r (z) can be approximated by a linear equation with a slope η, u is replaced by equation (5). _ar = ηΔz (5 ′) may be used.

[0014]

[Example] For a prototype of 3-1 / 2 "oil country tubular goods thread joint, 1
Each tightening process was divided into 22 steps according to the present invention and analyzed in the order of the steps, and the calculated values of the obtained strain and torque were compared with the actually measured values. Elasto-plastic FEM analysis was performed by ABAQUS. Elastic-plastic conditions adopt Mises' yield condition,
The stress-strain relationship was set based on the tensile test data. The strain comparison site was one site on the inner surface side of the tip of the pin, and the measured data was collected by attaching a strain gauge to the site. The actual torque was measured using a load cell incorporated in a screw tightening machine. The friction coefficient of the contact interface was set to 0.04 for the screw portion and the seal portion, and 0.07 for the torque shoulder portion.

As a result, as shown in FIG.
Both the strain and the torque almost coincided with the actually measured values over the entire process from the start to the end of the tightening, and the effectiveness of the present invention was verified.

[0016]

As described above, according to the present invention, it is possible to reproduce the sealing performance of an oil country tubular good thread joint in an almost actual form, and the reliability of the estimation is improved, so that a new product having excellent sealing performance is developed. It has an excellent effect of being accelerated.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a coupling region definition method according to the present invention.

FIG. 2 is an explanatory diagram of a thread shape approximation method according to the present invention.

FIG. 3 is an explanatory diagram showing an example of a method for setting a mutual positional relationship between a screw portion and a main body portion of the coupling after executing a step.

FIG. 4 is a graph showing calculated values of strain (a) and torque (b) generated in the screw joint tightening process according to the present invention in comparison with actually measured values.

FIG. 5 is an explanatory diagram of a conventional analysis method.

[Explanation of symbols]

 1 pin 2 coupling 3 overlap

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Akira Ito 1-1, Kawasaki-cho, Handa-shi, Aichi Prefecture Kawasaki Steel Corporation Chita Works (72) Inventor Hiroyuki Matsui 1-1-1, Kawasaki-cho, Handa-shi, Aichi Prefecture Kawasaki F-term in Chita Works (Reference) 2F051 AA06 AB00 BA07 3H013 GA01

Claims (1)

[Claims] 1. A method for evaluating the sealing performance of a threaded joint for oil country tubular goods, wherein the contact between a pin and a coupling is determined by elasto-plastic FEM analysis, and the sealing performance is evaluated based on the result.
In the elasto-plastic FEM analysis, the screw is approximated by an axisymmetric ring, the screw portion of the coupling and the main body are individually defined as regions, the fastening process is divided into a plurality of steps, and the screw portion of the coupling is performed in this order. A method for evaluating the sealing performance of a threaded oil country tubular good, characterized in that the contact between the pin and the coupling is determined by sliding the slide.