WO2016147222A1

WO2016147222A1 - Drill pipe with double shoulder tool joints

Info

Publication number: WO2016147222A1
Application number: PCT/JP2015/001527
Authority: WO
Inventors: Koji Sakura; Toshihiko Fukui; Tomoyuki NARIKAWA; Tomoya Inoue; Tsuyoshi Miyazaki; Masanori Kyo
Original assignee: Nkktubes; Japan Agency For Marine-Earth Science And Technology
Priority date: 2015-03-18
Filing date: 2015-03-18
Publication date: 2016-09-22
Also published as: EP3271631A4; JP6528292B2; JP2018509573A; EP3271631A1; CN107429861A

Abstract

A drill pipe with double shoulder tool joints by which a stress concentration can be relaxed and provided with an excellent fatigue characteristic without largely changing a predetermined threaded portion shape is provided. A drill pipe (1) with a double shoulder tool joint includes a pin (3) including a male threaded portion (30) having a predetermined shape, and a box (2) including a female threaded portion (20) to be screwed with the male threaded portion (30), and a stabbing flank (41) of each of three or less consecutive proximal thread ridges on a larger-diameter side of the male threaded portion (30) and/or three or less consecutive proximal thread ridges on a smaller-diameter side of the female threaded portion (20) includes a round processed portion.

Description

DRILL PIPE WITH DOUBLE SHOULDER TOOL JOINTS

The present invention relates to a drill pipe with double shoulder tool joints, more particularly, to a drill pipe with double shoulder tool joints with a stress relief function for improving fatigue characteristics, which is suitably applied to drill pipes for ground drilling, heavy weight drill pipes, landing string drill pipes, etc.

With respect to the fatigue of the drill pipe, there are some provisions and restrictions, and Spec 7G of API (American Petroleum Institute) standard, DS-1 and the like are used as the guideline. For example, the relationship between the load bending stress as an index of fatigue and the allowable number of repetitions as an index of possible rotational number is described in API Spec. 7G. Further, API Spec. 7G presents a drilling condition in which an inclination angle of a well is expressed in a parochial angle at every reach for 33 m and the drilling should be carried out by rotating the drill pipe when the inclination angle of the well is 6 degrees or less, and should be carried out by rotating only a bit tip portion when the inclination angle of the well is greater than 6 degrees.

In either case, however, the fatigue may become a problem in the drill pipe tube body with a thin wall thickness, but not tool joints formed at both ends of the drill pipe. Therefore, the means for solving the problem in application of a higher load on the tool joint along with the development of large depth-slope drilling has not been a major problem even for the tool joints (threaded joints) according to the API standard, especially in the double shoulder tool joints, as long as they are used with considering the static torsional strength and yield strength of the tube body. However, the drill pipes such as drill collars in which a wall thickness of the tube body is greater than a wall thickness of the tool joint(s), the tool joints become the bottleneck in the strength of the drill pipe, namely, the most fragile portion in the drill pipe. In such cases, it has been recommended to provide a neck parallel portion of the male threaded portion (external thread) with groove machining (API Spec. 7G).

In the prior art, as applied in API Spec. 7, it has been typically proposed to provide a stress relief groove (Hereinafter, referred to as “SRG”) machining at a pin neck portion, a significant change in the profile of thread ridge, or the increase in radius of a root over the entire length of the threaded portion.

As to the improvement in the fatigue strength of the threaded joint, the following techniques are also disclosed. Patent Literature 1 discloses a tool joint with high torque and high fatigue strength, in which a cylindrical ring-shaped portion (annular portion) arranged on a side of a box inner face of an API tool joint receives the make-up (tightening) torque to a certain extent. Patent Literature 2 discloses a technique of arranging a ring with high elastic deformability on a shoulder face, in order to maintain a sufficient contact of a shoulder portion during the use. Patent Literature 3, Patent Literature 4 and Patent Literature 5 disclose techniques for dispersing the reaction force from the shoulder to more thread ridges by gradually changing the height of the thread ridges. Furthermore, as techniques for dispersing the reaction force, Patent Literature 6 discloses a technique of reducing the rigidity of the thread ridges by providing notches in crests, while Patent Literature 7 discloses a technique of providing notches in roots of thread ridges.

JP-A 06-281060 JP-A 07-260054 JP-A 02-35208 JP-A 01-48988 JP-A 04-157283 JP-A 2005-221038 JP-A 04-66483

In the prior art, the SRG machining of the neck parallel portion of the male threaded portion of the tool joint, the machining of the threaded portion and the like have been proposed independently from each other. According to such separate means for solving the problem, it has been difficult to design the device for the purpose of the precise relief of stress concentration, thereby caused the extension of machining time and increase in machining cost due to the unnecessary machining, as well as the reduction in strength reduction due to the reduction in strength elements of the structure due to the unnecessary machining. In particular, the change in the shape of the threaded portion is not recommended because it will sacrifice the critical cross-sectional area derived from the performance of the tool joint.

As countermeasures other than the means as described above, the overall design changes or the improvement in strength of the material per se may be contemplated. In the former case, it is necessary to prepare specialized inspection devices since the versatility of the product is lost, which may cause the increase in cost. In addition, the sales promotion will be impaired because a complicated quality control is required. In the latter case, it is not recommendable to further increase the strength of the material for the tool joint with considering the strength balance in friction welding that has been used to the bonding with the pipe. For example, such configuration may cause the problems in mechanical test values (high hardness and less toughness lead to brittle.). Therefore, it is necessary to improve only the fatigue strength without deteriorating the performance of the original drill pipe according to the API standard or the like by taking into account the many constraints, such as limitations on readjustment of the strength, variation of the shape, and the like. In this case, it is the most important to design the drill pipe without reducing the critical cross-sectional area of the portion which may become the bottleneck.

According to the technique disclosed in Patent Literature 1, it is possible to expect the improvement in torque gain and fatigue strength in the tool joints according to the API standard by arranging a cylindrical ring-shaped part. However, the above effect cannot be expected in the double shoulder tool joint having a shoulder on the inner face originally. For the elastic ring described in Patent Literature 2, it is necessary to use the material different from the material of the tool joint, so that it is likely to generate bimetallic corrosion. In the technique described in

Patent Literatures

3, 4, and 5, it is difficult to adequately disperse the reaction force of the shoulder only by changing the height of the thread ridge, so that a pressure receiving area of a flank of the thread ridge is reduced. Accordingly, the improvement in fatigue strength cannot be expected. According to the techniques described in Patent Literatures 6 and 7, the shape of the threaded portion becomes very complicated, so that the machining becomes very difficult, which may cause the increase in costs and the increase in machining time.

Accordingly, it is an object of the present invention to provide a drill pipe with double shoulder tool joints having excellent fatigue characteristics, which enables the relief of the stress concentration without considerably changing the predetermined shape of the threaded portion.

[1] So as to achieve the aforementioned object, a feature of the present invention provides a drill pipe with double shoulder tool joints comprising:
a pin comprising a male threaded portion having a predetermined shape; and
a box comprising a female threaded portion to be screwed with the male threaded portion,
wherein a stabbing flank of each of three or less consecutive proximal thread ridges on a larger-diameter side of the male threaded portion and/or each of three or less consecutive proximal thread ridges on a smaller-diameter side of the female threaded portion comprises a round machined portion.
[2] In the drill pipe with double shoulder tool joints according to [1], the pin may comprise another round machined portion having at least two curvatures at a neck portion of the pin.
[3] In the drill pipe with double shoulder tool joints according to [1] or [2], a minimum cross-sectional area at the root of the round machined portion may be equal to a critical cross-sectional area of the root of the male threaded portion or the female threaded portion.
[4] The drill pipe with double shoulder tool joints according to any one of [1] to [3], the predetermined shape may comprise a threaded portion shape according to API standard and the round machined portion is formed by post-machining.
[5] Another feature of the invention provides a drill pipe with double shoulder tool joints, comprising:
a pin comprising a male threaded portion having a predetermined shape; and
a box comprising a female threaded portion to be screwed with the male threaded portion,
wherein an imperfect thread ridge of either of the male threaded portion and the female threaded portion is configured to be screwed with a perfect thread ridge of a counter part of the either of the male threaded portion and the female threaded portion,
wherein a stabbing flank of the perfect thread ridge comprises a round machined portion.
[6] In the drill pipe with double shoulder tool joints according to [5], the pin may comprise another round machined portion having at least two curvatures at a neck portion of the pin.
[7] In the drill pipe with double shoulder tool joints according to [5] or [6], a minimum cross-sectional area at the root of the round machined portion may be equal to a critical cross-sectional area at the root of the male threaded portion or the female threaded portion.
[8] In the drill pipe with double shoulder tool joints according to any one of [5] to [7], the predetermined shape may comprise a threaded portion shape according to API standard and the round machined portion is formed by post-machining.

Advantageous Effects of Invention

FIG.1 is a diagram showing a total configuration of a drill pipe with double shoulder tool joints and a state in which the drill pipes are joined with each other in one embodiment according to the present invention. FIG.2A is a partial cross-sectional view taken along a pipe axis as shown in FIG.1 in a neck portion of a male threaded portion of the drill pipe with double shoulder tool joints in the first embodiment according to the present invention. FIG.2B is a detailed cross-sectional view of a portion A in FIG.2A. FIG.3 is a partial cross-sectional view taken along a pipe axis as shown in FIG.1 in the neck portion of the male threaded portion of the drill pipe with double shoulder tool joints for explaining the method of round machining (SRG machining) in the neck portion of the male threaded portion as shown in FIG.2A. FIG.4 is a partial cross-sectional view taken along the pipe axis as shown in FIG.1 in a root portion of a female threaded portion of the drill pipe with double shoulder tool joints in the first embodiment according to the present invention. FIG.5A is a stress analysis diagram by Finite Element Analysis (FEA) of the neck portion of the male threaded portion with a threaded portion shape according to API standard in the drill pipe with double shoulder tool joints. FIG.5B is a stress analysis diagram by the FEA of the neck portion of the male threaded portion with the threaded portion shape according to API standard for which SRB machining is performed in the drill pipe with double shoulder tool joints in the first embodiment according to the present invention. FIG.6 is a partial cross-sectional view taken along the pipe axis as shown in FIG.1 in the neck portion of the male threaded portion of the drill pipe with double shoulder tool joints in the second embodiment according to the present invention. FIG. 7 is a partial cross-sectional view taken along the pipe axis as shown in FIG.1 in the neck portion of the male threaded portion of the drill pipe, for explaining the method of round machining (SRG machining and SRG machining) of the neck portion of the male threaded portion as shown in FIG.6. FIG.8A is a stress analysis diagram by the FEA of the neck portion of the male threaded portion with the threaded portion shape according to API standard in the drill pipe with double shoulder tool joints. FIG.8B is a stress analysis diagram by the FEA of the neck portion of the male threaded portion with the threaded portion shape according to API standard for which SRB machining is performed in the drill pipe with double shoulder tool joints in the second embodiment according to the present invention. FIG.9A is a diagram showing a fatigue testing machine for a tool joint of the drill pipe with double shoulder tool joints in the embodiments according to the present invention. FIG.9B is a diagram showing a broken specimen of the tool joint in the drill pipe with double shoulder tool joints in Comparative example.

(Total configuration of a tool joint and a drill pipe)
FIG.1 is a diagram showing a total configuration of a drill pipe 1 with double shoulder tool joints and a state in which the drill pipes 1 are joined with each other in one embodiment according to the present invention. This drill pipe 1 with double shoulder tool joints (Hereinafter referred to as “drill pipe 1”) comprises a drill pipe tube body 4 comprising both ends each of which is provided with a box 2 with a female threaded portion (i.e. internal thread) 20 and a pin 3 with a male threaded portion (i.e. external thread) 30. The box 2 and the pin 3 constitute each tool joint in a drill pipe 1 with double shoulder tool joints. The drill pipes 1 are joined with each other by connecting the box 2 of one drill pipe 1 and the pin 3 of the other drill pipe 1 with screwing the male threaded portion 30 of the other drill pipe 1 into the female threaded portion 20 of one drill pipe 1, as shown in FIG.1. The connections of the drill pipes 1 may be increased for a required number.

In the present specification, “SRG machining” refers to machining, especially round machining, for providing a stress relief groove (SRG) at a corner portion between a neck portion (a proximal end) on a larger-diameter side of the male threaded portion and a pin shoulder face. Further, “SRB machining” refers to machining, especially round machining, for providing a stress relief bottom (Hereinafter referred to as “SRB”) over a stabbing flank starting from a root (a root point is indicated by P) of the thread. Here, the round machining (rounding process) refers to machining for providing the object with a roundness, a predetermined curvature, or the combination thereof.

(First embodiment)
FIG.2A is a partial cross-sectional view taken along a pipe axis CL as shown in FIG.1 in the neck portion 33 of the male threaded portion 30 of the drill pipe 1 with double shoulder tool joints in the first embodiment according to the present invention. FIG.2B is a detailed cross-sectional view of the portion A in FIG.2A.

Each of the female threaded portion 20 and the male threaded portion 30 as shown in FIGS.2A and 2B has a thread shape of the standard threaded joint of the drill pipe as defined in the API standard. In the present embodiment, as the male threaded portion 30, a taper thread having the neck portion 33 on the larger-diameter side (i.e. at a proximal end) and a pin nose end face 32 on the smaller-diameter side (i.e. at a proximal end) (e.g. API Spec. 7) is used.

As shown in FIG. 2A, the female threaded portion 20 of the box 2 and the male threaded portion 30 of the pin 3 are screwed to be engaged with each other (i.e. screw-mating). By the engagement, a box end face 22 of the box 2 contacts (abuts) a pin shoulder face 31 of the pin 3 to constitute a double shoulder configuration. Further, a box shoulder face 21 of the box 2 contacts (abuts) a pin nose end face 32 of the pin 3 as shown in FIG. 4 to be described later. Namely, in the double shoulder tool joint, an external shoulder (a contact portion between the pin shoulder face 31 and the box end face 22) and an internal shoulder (a contact portion between the box shoulder face 21 and the pin nose end face 32) contact (abut) with each other by engagement of the female threaded portion 20 and the male threaded portion 30, to provide a sealing function and transmit a rotational torque of the drill pipe.

Here, as shown in FIGS.2A and 2B, in the engagement of the female threaded portion 20 and the male threaded portion 30, one flank (a load flank 40) of a thread ridge of the female threaded portion 20 is in contact (abutting contact) with one flank of a thread ridge of the male threaded portion 30, while the other flank (a stabbing flank 41) of the thread ridge of the female threaded portion 20 is not in contact (abutting contact) with one flank of the next thread ridge of the male threaded portion 30.

By the engagement of the female threaded portion 20 of the box 2 and the male threaded portion 30 of the pin 3 as shown in FIG.2A, the pin shoulder face 31 comes into contact (abutting contact) with the box end face 22. As shown in FIG.4 to be described later, the pin nose end face 32 comes into contact (abutting contact) with the box shoulder face 21. In the threaded portions (the female threaded portion 20 and the male threaded portion 30), the load flank 40 functions as a load face in which one flank of the thread ridge of the female threaded portion 20 is in contact (abutting contact) with one flank of the thread ridge of the male threaded portion 30.

(SRB machined portion of the male threaded portion)
In the double shoulder tool joint, high surface pressure is generated at the external shoulder and the internal shoulder by making-up (i.e. tightening) the tool joints, and high fatigue characteristics to the bending stress are exhibited by virtue of the surface pressure at the external shoulder in particular. Three consecutive proximal thread ridges on the larger-diameter side of the male threaded portion 30 bear the reaction force of the face pressure of the shoulder portion, so that the stress concentration occurs at the roots of the threads until the three consecutive thread ridges, the neck portion of the male threaded portion, or the imperfect threads, which will cause a large tensile stress. Therefore, these portions are the most fragile portions to the fatigue fracture. For solving this problem, the stabbing flanks 41 of three or less consecutive proximal thread ridges on the larger-diameter side of the male threaded portions 30 are machined by the SRB machining with a curvature having a curvature radius larger than an original root radius. Namely, the number of the proximal thread ridges subject to the SRB machining is three at maximum, and may be one or two.

An SRB machined portions 37 of the male threaded portion 30 is a round machined portion on the stabbing flank 41 of each of three or less consecutive proximal thread ridges on the larger-diameter side of the male threaded portions 30 of the pin 3. The SRB machined portion 37 is a round machined portion for stress relief and formed only at the root of the thread on the side of the stabbing flank 41.

As shown in FIGS.2A and 2B, the SRB machined portions 37 of the male threaded portion 30 are formed by performing the SRB machining on the stabbing flanks 41 of the three or less consecutive proximal thread ridges including imperfect thread ridge(s) on the larger-diameter side of the neck portion (i.e. the proximal end) 33 of the male threaded portions 30. In other words, the SRB machining is performed on the stabbing flanks 41 of perfect thread ridges of the male threaded portion 30 to be screwed with imperfect thread ridges of the female threaded portion 20. The round machined portions defined by the expressions in two different manners are substantially the same and provide the effects to be described later in any case.

In the SRB machined portion 37, the minimum cross-sectional area at a root of the round machined portion is set to be equal to or greater than the critical cross-sectional area of the thread root of the male threaded portion 30. Here, for both the female threaded portion 20 and the male threaded portion 30, the critical cross-section is a cross-section of a portion out of the engaging portion in the threaded portion. A male threaded portion critical cross-sectional area is a critical cross-sectional area at a root of the last engaged thread on the larger-diameter side (i.e. the most proximal engaged thread) in a thread engaging portion of the male threaded portion 30. A female threaded portion critical cross-sectional area is a critical cross-sectional area at a root of the last engaged thread on the smaller-diameter side (i.e. the most proximal engaged thread) in a thread engaging portion of the female threaded portion 20. Although a critical cross section is a cross section of an engaging portion to be engaged with an imperfect thread next to the last engaged thread (i.e. the imperfect thread closer to the proximal end or the proximal end), a cross-sectional area of the last engaged thread is determined as the critical cross-sectional area of the last engaging portion for the reason of the safety, since a thread groove is spirally provided.

In the drill pipe 1 with double shoulder tool joints shown in FIGS.2A and 2B, a root P of the male threaded portion 30 is used for determining a diameter D for defining the male threaded portion critical cross-sectional area. The SRB machined portion 37 is formed on the stabbing flank 41 by round machining with the use of the diameter D as the smallest machining diameter.

FIG.3 is a partial cross-sectional view taken along the pipe axis CL as shown in FIG.1 in the neck portion 33 of the male threaded portion 30 of the drill pipe 1 with double shoulder tool joints for explaining the method of round machining (SRB machining) in the neck portion 33 of the male threaded portion 30 as shown in FIG.2A. In FIG.3, the drill pipe 1 is turned around the pipe axis CL, and subjected to the round machining (SRB machining) by a turning tool BT1. The turning tool BT1 is a forming tool in which a nose radius is formed to be equal to a machining radius R1 of the SRB machined portion, wherein the machining radius R1 is greater than the root radius R which has been already machined according to the API standard.

As shown in FIG.3, the threaded portion shape according to the API standard is subjected to the cutting as post-machining (additional machining). The cutting is performed on the stabbing flanks 41 of the three or less consecutive proximal thread ridges including imperfect thread ridge(s) on the larger-diameter side of the neck portion 33 of the male threaded portions 30. In other words, the cutting is performed on the stabbing flanks 41 of the perfect thread ridges of the male threaded portion 30 to be screwed with the imperfect thread ridges of the female threaded portion 20.

The feeding pitch of the turning tool BT1 is equal to a thread pitch. Further, when the diameter of the thread root P of the male threaded portion 30 as shown in FIG.3 is used to determining the diameter D for defining the male threaded portion critical cross-sectional area, the depth of cutting is set in such a manner that the diameter D is set as a minimum machining diameter.

Detailed machining dimensions are determined under the conditions for minimizing the machining time loss within the range in which the stress relief can be expected from the results of the FEA to be described later. As to the thread root, the stress concentration is suppressed by connecting the root and the stabbing flank 41 with a round machined portion having a slightly large curvature radius of about 1 mm to 1.3 mm, only for the stabbing flank 41 which reduces the stress concentration from the surface receiving the reaction force of the shoulder, without changing a surface roughness Ra of the thread root.

(SRB machined portion of the female threaded portion)
An SRB machined portion 27 of the female threaded portion 20 is a round machined portion on the stabbing flank 41 of each of three or less consecutive proximal thread ridges on the smaller-diameter side of the female threaded portions 20 of the pin 2. The SRB machined portion 27 is a round machined portion for stress relief and formed only on the stabbing flank 41 at the root of the thread.

As shown in FIG.4, the SRB machined portions 27 of the female threaded portion 20 are formed by performing the SRB (round) machining on the stabbing flanks 41 of the three or less consecutive proximal thread ridges including imperfect thread ridge(s) on the smaller-diameter side of the neck portion 23 of the female threaded portions 20. In other words, the SRB (round) machining is performed on the stabbing flanks 41 of the perfect thread ridges of the female threaded portion 20 to be screwed with the imperfect thread ridges of the male threaded portion 30.

In the SRB machined portion 37, the minimum cross-sectional area of a round machined root of the round machined portion is set to be equal to or greater than the critical cross-sectional area of the root of the female threaded portion 20.

Similarly to the SRB machined portions 37 of the male threaded portion 30, the SRB machined portions 27 of the female threaded portion 20 are formed by performing the SRB (round) machining on the root of the thread of the female threaded portion 20 on the side of the stabbing flank 41 by the turning tool BT1, in which a nose radius is formed to be equal to a machining radius R1 of the SRB machined portion 27, wherein the machining radius R1 is greater than the root radius R of the thread which has been already machined according to the API standard. The detailed machining dimensions are also the same as those of the SRB machined portion 37 of the male threaded portion 30.

FIG.5A is a stress analysis diagram by Finite Element Analysis (FEA) of the neck portion 33 of the male threaded portion 30 with the threaded portion shape according to the API standard in the drill pipe with double shoulder tool joints. FIG.5B is a stress analysis diagram by the FEA of the neck portion 33 of the male threaded portion 30 with the threaded portion shape according to the API standard for which SRB machining is performed in the drill pipe 1 with double shoulder tool joints in the first embodiment according to the present invention.

FIGS.5A and 5B show the stress distribution by the gray-scale in which the mesh arrangement is dense for the crest and the root of the thread ridges of each threaded portion. According to this analysis result, in the original threaded portion shape according to the API standard as shown in FIG.5A, a high stress region exists in the roots of three consecutive proximal threads of the male threaded portion 30, so that the stress concentration occurs. In contrast, in the first embodiment according to the present invention as shown in FIG.5B, the SRB machined portions 37 are provided at the roots of three consecutive proximal thread ridges of the male threaded portion 30. It is confirmed that the stress concentration at the roots of the three consecutive proximal thread ridges of the male threaded portion 30 is relieved based on the comparison of the stress values and stress concentration region with those in FIG.5A.

(Second embodiment)
The second embodiment of the present invention is different from the first embodiment of the present invention in that the SRG machining is performed as round machining on the neck portion 33 of the male threaded portion 30 of the pin 3 in addition to the SRB machining shown in the first embodiment.

(SRG machined portion)
FIG.6 is a partial cross-sectional view taken along the pipe axis CL as shown in FIG.1 in the neck portion 33 of the male threaded portion 30 of the drill pipe 1 with double shoulder tool joints in the second embodiment according to the present invention. An SRG machined portion 35 is a round machined portion comprising at least two curvatures at the neck portion 33 of the male threaded portion 30 of the pin 3. The SRG machined portion 35 is a round machined portion for stress relief.

By performing the SRG machining on the portion including the imperfect thread of the neck portion 33 of the male threaded portion 30, unnecessary thread root is removed. Further, a synergistic effect of reducing the stress concentration at a root between firstly-adjacent thread ridges can be achieved by increasing a depth of the SRG machined portion 35 as much as possible. However, the increase in depth of the SRG machined portion 35 more than necessary may reduce the static tensile strength of the joint. Therefore, the minimum cross-sectional area of the root with the SRG machined portion 35, which is most effective, is set to be equal to the critical cross-sectional area of the root by which the strength of the joint can be determined.

FIG.6 shows an example of performing the round machining for providing a round machined portion comprising at least two curvatures, in which the SRG machined portion 35 comprises three curvatures. Similarly to the SRB machined portion 37, the SRG machined portion 35 comprises a curvature R2, which is set such that a minimum cross-sectional area at the round machined root is maximum to the extent that does not reduce the diameter D of the root of the last thread ridge as a critical cross-sectional area of the root of the male threaded portion 30, a curvature R3 which smoothly connects between the curvature R2 and the pin shoulder face 31, and a curvature R4 which smoothly connects between the curvature R2 and the portion leading to the last thread ridge. As shown in FIG.6, the diameter D at the root of the last thread ridge as the critical cross-sectional area of the root of the male threaded portion 30 as described above is the same as the diameter D for defining the critical cross-sectional area of the SRB machined portion 37 of the male threaded portion 30.

FIG.7 is a partial cross-sectional view taken along the pipe axis CL as shown in FIG.1 in the neck portion 33 of the male threaded portion 30 of the drill pipe 1, for explaining the method of round machining (SRG machining and SRG machining) of the neck portion 33 of the male threaded portion 30 as shown in FIG.6. Since the SRB machining in the second embodiment is the same as the SRB machining in the first embodiment, only the SRG machining will be explained below.

As shown in FIG.7, the drill pipe 1 is turned around the pipe axis CL to be subjected to the round machining (SRG machining) with the use of a forming tool BT2. The forming tool BT2 is formed to have a machining radius smaller than a nose radius of the curvature R3 which is the smallest curvature of the three curvatures (R2, R3, R4). By appropriately setting the feeding pitch and the cutting depth of the forming tool BT2, the round machining (SRG machining) is performed such that the diameter D of the root of the last thread ridge, in which the minimum cross-sectional area corresponds to the critical cross-sectional area of the root of the male threaded portion 30 is equal to or greater than the diameter D for defining the critical cross-sectional area of the SRG machined portion 35 of the male threaded portion 30.

FIG.8A is a stress analysis diagram by the FEA of the neck portion 33 of the male threaded portion 30 with a threaded portion shape according to API standard in the drill pipe with double shoulder tool joints. FIG.8B is a stress analysis diagram by the FEA of the neck portion 33 of the male threaded portion 30 with the threaded portion shape according to the API standard for which the SRB machining is performed in the drill pipe 1 with double shoulder tool joints in the second embodiment according to the present invention.

By comparing the stress distributions and stress values in FIGS.8A and FIG. 8B, it is confirmed that the stress concentration is relieved at the root of the last thread ridge on the side of the neck portion 33 of the male threaded portion 30 and the roots of three consecutive proximal thread ridges of the male threaded portion 30.

(Comparative Example)
FIG.9A is a diagram showing a fatigue testing machine 100 for a tool joint of the drill pipe 1 with double shoulder tool joints in the embodiments according to the present invention. FIG.9B is a diagram showing a broken specimen 110 of the tool joint in the drill pipe with double shoulder tool joints in comparative example.

In the fatigue testing, tension-compression under tension fatigue test was carried out on two specimens (N=2) for each of Example and Comparative Example with the use of real pipes (drill pipes with double shoulder tool joints), and the effect thereof was observed.
As the testing condition, two specimens of conventional drill pipes (DSTJ-NC31 made by NKKTubes) were prepared for Comparative example. Two specimens of drill pipes were prepared by performing both the SRG machining and the SRB machining on the conventional drill pipes (DSTJ-NC31 made by NKKTubes) for Example.

Table 1 shows the test results.

In the test results as shown in Table 1, both the specimens 1-1 and 1-2 provided with neither SRG machined portion nor SRB machined portion in Comparative Example were broken at the threaded portion. In contrast, both the specimens 2-1 and 2-2 provided with both the SRG machined portions and the SRG machined portions in Example were not broken at the threaded portions but at the chuck portion (tube body). In the drill pipe with double shoulder tool joints subjected to both the SRG machining and the SRB machining in the embodiments according to the present invention, it was confirmed that the lifetime of the threaded portions was at least twice as compared with the drill pipe with double shoulder tool joints which has not been subjected to the SRG machining and the SRB machining. It is assumed that the fatigue resistance was improved by the relief of the stress concentration at the threaded portions.

(Effects of the embodiments)
According to the embodiments of the present invention, following effects will be achieved.
(1) In the first embodiment, the round machined portion (SRB machined portion 37) is provided on the stabbing flank 41 of each of the three or less consecutive proximal thread ridges including the imperfect thread ridge on the larger-diameter side of the neck portion 33 of the pin 3. In other words, the round machined portion (SRB machined portion 27) is provided on the stabbing flank 41 of each of the perfect thread ridges of the male threaded portion 30 to be screwed with the imperfect thread ridges of the female threaded portion 20. According to this configuration, the stress concentration at the root is relaxed, so that the fatigue resistance is improved.
(2) The round machined portions are provided on the stabbing flanks 41 of three or less consecutive proximal thread ridges including the imperfect thread ridge. However, since the round portions are not provided at the load flanks 40, the bending strength with respect to the load applied to the thread ridges will not be lowered, so that the strength of joints (tool joints) will not be lowered.
(3) In the second embodiment, the SRG machined portion 35 comprising a round machined portion comprising at least two curvatures are provided at the neck portion 33 of the male threaded portion 30 of the pin 3. According to this configuration, the stress concentration at the root can be relaxed, so that the fatigue resistance can be improved.
(4) In the round machined portions (SRB machined portion 37, SRB machined portion 27, SRG machined portion 35), the maximum cross-sectional area of the round machined root is set to be equal to or greater than the critical cross-sectional area of the female threaded portion 20 or the male threaded portion 30. According to this configuration, it is possible to relax the stress concentration thereby improving the fatigue resistance without reducing the critical cross-sectional area of the most fragile portion of the tool joint.
(5) The round machined portions (SRB machined portion 37, SRB machined portion 27, SRG machined portion 35) can be provided by the post-machining (additional machining) to a predetermined shape such as the threaded portion shape according to the API standard. According to this configuration, it is possible to provide the novel drill pipe with double shoulder tool joints by utilizing the conventional drill pipe with double shoulder tool joints based on the API standard.
(6) Since the basic configuration of the male and female threaded portions are substantially the same as those defined in the API standard, it is possible to use the inspection devices generically used for the inspection of the conventional products according to the API standard. Further, it is possible to achieve a sufficient quality control by applying the conventional method for inspecting the products according to the API standard.
(7) In the embodiments, since the round machined portion is provided at the threaded joints of the double shoulder tool joints of the drill pipe, it is possible to accurately relax the stress concentration due to the bending stress during drilling at the engaging portions at the pin and the box of the tool joints for which the make-up and break-out operations are repeated. Further, the tool joint having the threads as described above effectively functions, particularly in the heavy weight drill pipe, the landing string drill pipe or the like, which has a large wall thickness and is subject to a high stress to tool joints.
(8) According to the above configurations of the invention, it is possible to respond to the relative reduction in strength of the tool joint due to the increase in strength or the thickening in wall thickness of the drill pipe. In accordance with the improvement in the fatigue strength, the possibility of drilling use of the heavy wall drill pipe can be enhanced, thereby achieving the drilling of deeper wells.

It should be noted that the present invention is not limited to the embodiments as described above and can be modified in various ways within the scope which does not go beyond or depart from the technical idea of the present invention. For example, the present invention can be applied to the drill pipe with single shoulder tool joints having only an external shoulder comprising a pin shoulder face 31 and a box end face 22.

The present invention provides a drill pipe with double shoulder tool joints with a stress relief function for improving fatigue characteristics. Further, the drill pipe with double shoulder tool joints in the present embodiments can be applied to a variety of drillings, such as ground drilling pipes, heavy weight drill pipes, landing string drill pipes, etc., and particularly suitable to the drill pipe for oil drilling.

Reference Sins List

1 Drill pipe with double shoulder tool joints
2 Box
3 Pin
4 Drill pipe tube body
20 Female threaded portion
21 Box shoulder face
22 Box end face
23 Neck portion of Box
27 SRB machined portion
30 Male threaded portion
31 Pin shoulder face
32 Pin nose end face
33 Neck portion of Pin
35 SRG machined portion
37 SRB machined portion
40 Load flank
41 Stabbing flank
100 Fatigue testing machine
110 Specimen

Claims

A drill pipe with double shoulder tool joints, comprising:
a pin comprising a male threaded portion having a predetermined shape; and
a box comprising a female threaded portion to be screwed with the male threaded portion,
wherein a stabbing flank of each of three or less consecutive proximal thread ridges on a larger-diameter side of the male threaded portion and/or each of three or less consecutive proximal thread ridges on a smaller-diameter side of the female threaded portion comprises a round machined portion.
The drill pipe with double shoulder tool joints according to claim 1, wherein the pin comprises another round machined portion having at least two curvatures at a neck portion of the pin.
The drill pipe with double shoulder tool joints according to claim 1 or 2, a minimum cross-sectional area at the root of the round machined portion is equal to a critical cross-sectional area of the root of the male threaded portion or the female threaded portion.
The drill pipe with double shoulder tool joints according to any one of claims 1 to 3, the predetermined shape comprises a threaded portion shape according to API standard and the round machined portion is formed by post-machining.
A drill pipe with double shoulder tool joints, comprising:
a pin comprising a male threaded portion having a predetermined shape; and
a box comprising a female threaded portion to be screwed with the male threaded portion,
wherein an imperfect thread ridge of either of the male threaded portion and the female threaded portion is configured to be screwed with a perfect thread ridge of a counter part of the either of the male threaded portion and the female threaded portion,
wherein a stabbing flank of the perfect thread ridge comprises a round machined portion.
The drill pipe with double shoulder tool joints according to claim 5, the pin comprises another round machined portion having at least two curvatures at a neck portion of the pin.
The drill pipe with double shoulder tool joints according to claim 5 or 6, a minimum cross-sectional area at the root of the round machined portion is equal to a critical cross-sectional area at the root of the male threaded portion or the female threaded portion.
The drill pipe with double shoulder tool joints according to any one of claims 5 to 7, the predetermined shape comprises a threaded portion shape according to API standard and the round machined portion is formed by post-machining.