RU2616950C2 - Double-shouldered threaded joint of drill pipes - Google Patents

Abstract

FIELD: machine engineering.

SUBSTANCE: double-shouldered threaded joint of the drill pipes consists of a nipple and a coupling having inner and outer bearings joints. The gap in the thrust junction, in screwed but loose connection, is determined, depending on the threaded connection parameters and the correlation of the forces required to create a set of stresses in the dangerous nipple section and in the inner thrust junction, by the following relationship: δ=(1-Q_itj/Q_dns)Δl_n+Δl_c, where: Q_itj and Q_dns - the forces required to create a set of stresses respectively in the inner thrust junction and the dangerous nipple section; Δl_n and Δl_c - lengthening and shortening the threaded section of the nipple and the coupling, respectively, when tightening the connection.

EFFECT: ensuring strength and integrity of the double-shouldered threaded joint of the drill pipes.

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Description

The invention relates to drill pipes, i.e. to casing, drill and tubing used in exploration and deep drilling.

A threaded connection with a double-threaded thread in the practice of exploration, including domestic, drilling is used, for example, in the closest analogue, a drill pipe adopted as the prototype of the proposed one and included in the core drill complex with a removable core receiver (USSR standard - GOST 26250). In addition, it is also used in casing exploration pipes (American standard DCDMA) [1].

Recently, it has found application in domestic drill pipes with welded joints for deep drilling for oil and gas, although not standardized.

Such a connection allows to increase the tightness of casing, tubing and drill strings, as well as the torque of the latter during the drilling process.

However, the experience of using drill pipes with such a connection, for example, as part of complexes with a removable core receiver, showed its instability in terms of failure-freeness, expressed in the fact that the breaks of the drill string along threaded joints were poorly correlated with their production time in the well.

This was confirmed by the results of bench tests for alternating bending of drill pipe samples with a diameter of 55 mm with a wall thickness of 4.8 mm and a trapezoidal thread with a taper of 1:32 of the SSK-59 complex (GOST 26250). The cyclic durability at the same bending moment (750 nm) and the test base of 5⋅10 ⁶ loading cycles differed by several orders of magnitude [1].

The instability of the cyclic durability of such a connection is explained by the mismatch of the gap in the internal thrust joint to the ratio of the forces necessary to create the specified stresses in the internal thrust joint and in the dangerous section of the nipple.

According to GOST 26250, the distance between the thrust ends and ledges of the external thread (conditionally nipple) and internal thread (conditionally coupling) for three pipe diameters 43; 55 and 70 mm are set equal and equal to 41 _{± 0.05} mm.

Given the specified tolerance, we have, after full make-up, the following clearance values in the internal thrust joint: two zero, one +0.10 and one -0.10 mm. The last of them means the presence of a gap not in the inner, but in the outer joint.

Such a wide range of the gap in the inner joint does not provide the required loads and stresses after tightening the threads, corresponding to the areas of the dangerous section of the nipple and the internal thrust joint.

The latter case, when after tightening the joint, the gap may turn out to be not in the inner but in the outer joint, will lead to an immediate break in the drill string.

As you can see, the range of variation in the values of the gap in the inner joint is consistent with the difference in the cyclic durability of the tested samples.

A double-threaded threaded connection [2] is known, characterized in that a slotted spring is installed between the thrust shoulder of the coupling and the thrust end of the nipple, due to which, after tightening the connection, a strictly defined axial load on the outer joint is provided, as the difference between the specified total tightening force of the threaded connection and the force created by the spring on the internal thrust joint. However, the disadvantage of such a connection is that in thin-walled pipes, to which the pipe belongs according to GOST 26250, it is practically impossible to install a slotted spring with specified geometric parameters, due to limited space.

The objective of the invention is that, in order to ensure the strength and tightness of the double-threaded threaded joint of the drill pipe, the gap in the internal thrust joint of the joint provides after tightening the specified loads and the corresponding stresses in the dangerous section of the nipple and in the internal thrust joint.

To solve this problem, in a double-threaded threaded connection of drill pipes, including external and internal thrust joints, depending on its parameters and the ratio of forces required to create specified stresses in the dangerous section of the nipple and in the internal thrust joint, the gap in the latter, when screwed, but loose connection is determined by the following relationship:

Where:

Q _vus and Q _main - the forces necessary to create the specified stresses respectively in the internal thrust joint and a dangerous section of the nipple;

Δl _n and Δl _m - lengthening and shortening of the threaded section, respectively, of the nipple and coupling when tightening the connection.

The claimed invention allows to solve the problem.

Indeed, we first consider the loads and deformations that occur in a conventional threaded connection with one external stop joint. The tightening of such a connection is intended to fulfill two conditions:

1. Creating the optimal tensile stress of the nipple to ensure the maximum endurance limit of the connection of this design; this voltage is determined by the following relationship

Where:

σ _t - yield strength of the compound material;

K _s is the tightening coefficient, which for the exploration drill pipe joints is taken equal to 0.66, which corresponds to the safety factor ϑ = 1.5 [3].

2. Creating a tight joint in the outer joint; calculations and practical experience have shown that, taking into account the maximum tensile and bending loads during operation, this condition is satisfied if the first condition is provided, i.e. the axial tightening force, which creates stress according to expression (2), is sufficient to simultaneously provide the necessary tightness of the connection [4].

When these two conditions are met, the nipple of the usual connection, without having to tighten the obstacles from its end, is stretched by Δl _n , and the coupling is compressed by Δl _m .

In conjunction with two thrust joints, it is necessary to fulfill the third condition — the creation of such a load in the inner thrust joint that would provide the maximum increase in torque from the introduction of this second thrust joint in comparison with a conventional joint. Therefore, the force acting on the internal thrust joint must create a stress in it close to the yield strength of the joint material (safety factor ϑ = 1.1-1.2).

Moreover, here all three conditions must be fulfilled simultaneously in the course of one tightening operation. Since the fulfillment of the first condition provides at the same time the second, the final task is to simultaneously fulfill the first and third conditions.

For their implementation, it is necessary to establish a gap formed by screwing in by hand in the internal thrust joint, taking into account the forces acting upon tightening and the resulting deformations of the nipple and the coupling.

If the forces necessary to create the considered stresses in the cross section of the internal thrust joint Q _vus and in the dangerous section of the nipple Q _main are equal, then the load must be created in these sections simultaneously.

Therefore, from the moment of tightening, the internal thrust joint with the closed but unstretched external thrust joint must also be closed, i.e. the gap in the internal thrust joint must be equal to zero (δ = 0). But with a closed external thrust joint, the movement of the end of the nipple as a result of stretching of its threaded section is limited, and the clutch is compressed by Δl _m . Therefore, in order to ensure complete deformation of the nipple, in this case, a gap equal to this strain value, i.e. with Q _вyc = Q _main

However, depending on the parameters of the threaded connection, most often represented by a tapered thread, the force required to create a given voltage in the dangerous section of the nipple is greater than the force necessary to create a given voltage in the internal thrust joint. Therefore, in such a connection, the action of the force in the dangerous section of the nipple should be ahead of the action of the force in the internal thrust joint by an amount corresponding to the ratio of these forces, which is taken into account by the coefficient

Given this coefficient, the gap in the internal thrust joint in general form can be represented by the following expression:

or

Where:

δ is the gap in the internal thrust joint in the threaded connection with a closed, but unloaded, external thrust joint.

In expression (6), the first term is the part of the gap in the internal thrust joint, corresponding to the part of the elongation of the nipple, determined by the ratio of the forces acting in the internal thrust joint and in the dangerous section of the nipple.

An example implementation of the invention

As an example, we take three characteristic types of threaded joints of drill pipes to calculate the gap δ in their internal thrust joint according to expression (5). The calculation procedure is given by the example of a threaded connection of a drill pipe with a diameter of 70 mm for shells with removable core receivers (GOST 26250).

1. The force Q _DOS necessary to create voltage in a dangerous section of the nipple

Where:

F _DOS - the area of the dangerous section of the nipple, mm ² ;

σ _t - yield strength of the material of the compound, N / mm ² ;

K _s - the tightening coefficient of the threaded connection.

2. The force required to create tension in the internal thrust joint

Where:

F _vus - the area of the internal thrust joint, mm ² ;

ϑ - margin of safety.

3. Extension of the nipple when tightening the connection

Where:

l _n - the length of the threaded section of the nipple, mm;

E is the elastic modulus of the material of the nipple, 2.1⋅10 ⁵ N / mm ² ;

F _psn - the cross-sectional area of the threaded section of the nipple in the middle of its length, mm ² .

4. Shortening the coupling when tightening the connection

Where:

l _m - the length of the threaded section of the coupling, mm;

F _psm - the cross-sectional area of the threaded section of the coupling in the middle of its length, mm ² .

5. According to expression (6), the gap in the internal thrust joint with a closed but loose joint of a given drill pipe is:

δ = 0.083 (1-157200 / 182400) + 0.059 = 0.083 (1-0.86) + 0.059 = 0.07 mm.

Similar calculations were performed for a double-threaded threaded joint of a prospecting casing with a diameter of 219 mm (DSDMA standard) and a lock ZP-162-95-2 (GOST 27834) of an oil drill pipe PK 127 × 9 (GOST R 50278). In the latter case, an internal thrust joint is introduced into the serial design of the lock.

The calculation results are given in table. one

According to the calculations, the gap 8 increases with increasing wall thickness of the threaded connection (p. 10, table. 1); the maximum design deviations from the nominal value for the presented compounds are respectively 14, 12 and 4% (see clause 12).

These deviations do not lead to significant changes in the stress state of the connection, i.e. specified safety margins in the inner joint and in the dangerous section of the nipple.

As for the gaps arising from the standard requirements, here the deviations from the calculated values are very significant - more than 100% down and up.

For example, in the connection of the drill pipe with a diameter of 70 mm and the lock ZP-162-95-2, the gap 8 instead of a positive one has a negative value (-0.1 mm), and, as a result, tightening the joint will lead to permanent deformations in the internal thrust joint and insufficient prestressing in a dangerous section of the nipple and even to the appearance of a gap in the external thrust joint and to fatigue failure of the connection in the well.

On the contrary, an increase in the positive gap by more than 40-50% will lead to the option of a conventional connection without an internal stop joint, but with overload in the dangerous section of the nipple, since the torque is designed to overcome the frictional forces in the outer and inner joints, but in fact it will act only on the external thrust joint.

In the proposed double-threaded threaded connection of drill pipes, such stress-hazardous fluctuations in the stress state of its elements are excluded. This is achieved by the fact that the gap in the internal thrust joint provides, after tightening the connection, the specified loads and the corresponding stresses in the dangerous section of the nipple and in the inner joint, and, therefore, the specified values of strength and tightness.

Literature

1. Lachinyan L.A. The work of the drill string. - 2nd ed., Revised. and additional - M: Nedra, 1992 .-- 214 p. silt.

2. Lachinyan L.A. Lachinyan N.L. Pat. 2521123 RF, IPC Е21В 17/042. Threaded connection of drill pipe. No. 2012121505/03. Claim 05/25/2012. Publ. 06/27/2014. Bull. Number 18.

3. Lachinyan L.A., Ugarov S.A. Design, calculation and operation of drill exploration pipes and their compounds. M., "Nedra", 1975, 232 pp.

4. Saroyan A.E. Theory and practice of the drill string. - M .: Nedra, 1990 .-- 263 s: ill.

5. Instructions for the assembly and operation of drill pipes with welded joints “TMK TDS”, “RE PS 02-010-2013 (second edition)”. https // www.grou.

Claims (5)

Two-stop threaded connection of drill pipes, consisting of a nipple and a coupling, having external and internal thrust joints, depending on its parameters and the ratio of forces required to create specified stresses in the dangerous section of the nipple and in the internal thrust joint, the gap in the latter, when screwed, but loose connection is determined by the following relationship: δ = (1-Q _vus / Q _main ) Δ l _n + Δl _m , Where: Q _vus and Q _main - the forces necessary to create the specified stresses respectively in the internal thrust joint and a dangerous section of the nipple; Δl _n and Δl _m - lengthening and shortening of the threaded section, respectively, of the nipple and coupling when tightening the connection.