JP7747263B2 - Steel pipe threaded joints - Google Patents

Description

The present invention relates to a threaded joint for steel pipes.

Conventionally, when excavating a tunnel in soft ground, the AGF (All Ground Fasten) method, a long steel pipe forepiling method, has been used to prevent collapse. In this AGF method, multiple steel pipes are driven into the cross section of the tunnel face, with an outer diameter of 114.3 mm, a wall thickness of 6 mm, and a total length of 12.5 m. For example, steel pipes each about 3 m long are fastened together manually on a drilling machine commonly known as a jumbo drill, and the entire length is completed to 12.5 m while the tunnel is being excavated.
As for the steel pipes for piles used in the conventional AGF method as described above, from the viewpoint of ease of construction, steel pipe threaded joints are used at the joints of the steel pipe ends (see, for example, Patent Document 1).

The tip of these steel pipe members contains a drill bit for excavating the ground, and the rotational power of the drilling machine is transmitted to the drill bit via the inner bit and inner rod. To facilitate the insertion of the steel pipe member and to protect the inner bit, which transmits power to the drill bit, from the surrounding ground, a protective member called a casing shoe is placed between the drill bit and the steel pipe. The leading pipe and the steel pipes connected to it from the intermediate pipe onwards act as reinforcing members for the tunnel. Because the casing shoe is placed on the outside of the inner bit and the casing shoe and leading pipe are connected with a steel pipe threaded joint, the steel pipes from the leading pipe onwards also penetrate into the ground as the drill bit excavates, and are ultimately left in the ground as reinforcing members.

JP 2015-110994 A

However, conventional steel pipe threaded joints have the following problems.
In other words, casing shoes are generally manufactured from steel pipes and are connected to steel pipe members via steel pipe threaded joints, but there was a demand for threaded joints that further improved the general structural performance, such as tensile strength.
In particular, the steel pipe threaded joint that fastens the casing shoe and the head pipe is prone to breakage during excavation, causing excavation to be halted. This halt in excavation reduces construction efficiency, and there is room for improvement in this regard.

The present invention was made in consideration of the above-mentioned problems, and aims to provide a steel pipe threaded joint that can prevent a decline in construction efficiency by improving tensile strength.

In order to achieve the above object, the steel pipe threaded joint according to the present invention is a steel pipe threaded joint made between a steel pipe to be driven into the ground in a tunnel reinforcement method and a casing shoe to be placed between the steel pipe and a drill bit for ground excavation, the steel pipe threaded joint having a box with an internal thread machined on the inside of the steel pipe and a pin with an external thread machined on the outside of the casing shoe, the thread rows formed on the box and the pin are tapered with respect to the pipe axis of the steel pipe, the box has an incomplete thread portion in which the height of the internal thread gradually decreases toward the center of the steel pipe in the radial direction, at least a part of the incomplete thread portion is fitted with the external thread of the pin, and the material of the casing shoe The material tensile strength is set higher than the material tensile strength of the steel pipe, the drill bit is fixed to an inner bit arranged inside the steel pipe, the casing shoe has a casing body located at the drilling tip side in the pipe axial direction and the pin connected to the base end side of the casing body, the casing body has a thick pipe part in the center in the pipe axial direction that is thicker than the steel pipe, and the thick pipe part has a collision surface against which the inner bit collides from the base end side in the pipe axial direction, and the material tensile strength Tp and critical cross-sectional area Ap of the pin and the material tensile strength Tb and critical cross-sectional area Ab of the box satisfy the relationship expressions (1) and (2) .

According to the steel pipe threaded joint of the present invention, the tensile strength of the steel pipe threaded joint portion is higher than that of the steel pipe body, and the fracture position can be on the steel pipe side rather than the casing shoe, so fracture of the steel pipe threaded joint during construction of the tunnel reinforcement method can be suppressed.
In other words, the use of a steel pipe threaded joint consisting of tapered threads in this invention makes it easier to ensure the joint cross-sectional area, which is closely related to tensile strength. Furthermore, because the casing shoe is provided with a pin having an external thread and the steel pipe is provided with a box having an internal thread, this invention can prevent the so-called "zipper effect" that occurs when a box is provided on the casing shoe side and the pin is provided on the steel pipe side, in which the pin contracts radially inward when a tensile load acts in the pipe axial direction, causing the threads to come loose and resulting in early fracture.

Furthermore, in the present invention, the box has an incomplete thread portion, and the incomplete thread portion of the box and the complete thread portion of the pin fit together, thereby improving the tensile strength of the threaded joint and more effectively suppressing diameter reduction of the box.
Furthermore, in the present invention, since the tensile strength of the material of the casing shoe is higher than the tensile strength of the material of the steel pipe, the threaded joint can be broken from the dangerous cross section of the box, and the components arranged inside the casing shoe can be more reliably protected.

Furthermore, in this case, by satisfying the relationships of equations (1) and (2), the tensile strength of the steel pipe threaded joint can be reliably increased compared to the steel pipe, and the fracture position when a tensile load is applied can be determined to be at the position of the steel pipe body rather than at the steel pipe threaded joint.

Furthermore, the steel pipe threaded joint according to the present invention may be characterized in that the height of the male thread of the pin has an incomplete thread portion that gradually decreases radially outward.

By using this configuration, the presence of incomplete threads on both the box and the pin improves impact resistance and reduces fracture of the steel pipe threaded joint during tunnel reinforcement construction. Therefore, the present invention can achieve a steel pipe threaded joint with excellent durability against the impact load applied to the casing shoe as the drill bit rotates. This prevents the steel pipe threaded joint connecting the casing shoe and steel pipe from fracture during excavation, which would otherwise require excavation to be interrupted, and reduces the decline in construction efficiency due to interruptions in excavation.

In addition, in the steel pipe threaded joint according to the present invention, the collision surface may be characterized in that it is located closer to the drilling tip of the casing shoe in the pipe axial direction than the shoulder portion of the pin that faces the tip of the box.

With this configuration, the impact surface of the casing shoe that strikes the inner bit is positioned closer to the tip than the shoulder of the pin, improving impact resistance and preventing fracture of the steel pipe threaded joint during tunnel reinforcement construction. Therefore, the present invention can achieve a steel pipe threaded joint that has excellent durability against the impact load applied to the casing shoe as the drill bit rotates. This prevents the steel pipe threaded joint that fastens the casing shoe and steel pipe from fracturing during excavation, which would otherwise require excavation to be interrupted, and prevents a decrease in construction efficiency due to interruptions in excavation.

The steel pipe threaded joint of the present invention improves tensile strength, thereby preventing a decline in construction efficiency.

FIG. 1 is a longitudinal cross-sectional view showing a tunnel reinforcement member equipped with a steel pipe threaded joint according to an embodiment of the present invention. FIG. 2 is an enlarged view of the steel pipe threaded joint shown in FIG. 1. FIG. 10 is a longitudinal sectional view showing the configuration of a steel pipe threaded joint according to a modified example. 1A and 1B are diagrams illustrating an analytical model according to a first embodiment. FIG. 2 is a diagram showing an analytical model according to the first embodiment. FIG. 10 is a diagram showing an analysis result according to the first embodiment. FIG. 10 is a diagram showing the analysis results according to the first embodiment, illustrating a relative comparison of equivalent strain for each analysis case. FIG. 10 is a side view showing a schematic configuration of a testing device according to a third embodiment.

Below, a steel pipe threaded joint according to an embodiment of the present invention will be described with reference to the drawings.

The steel pipe threaded joint 1 according to this embodiment, shown in Figure 1, is applied to a tunnel reinforcement member 10 that is driven into the ground during tunnel reinforcement construction and includes a steel pipe 2 for reinforcing the ground. The tunnel reinforcement member 10 of this embodiment is installed by driving it approximately horizontally (diagonally upward) into the ground above the mirror surface to prevent the mirror surface of the tunnel face from collapsing during tunnel excavation, and is used in the long mirror bolt construction method, which requires both adhesion to the ground and tensile strength, and the long forepile construction method, which prevents crown collapse. Each steel pipe 2 of the tunnel reinforcement member 10 is, for example, approximately 3 m long, and multiple pipes are connected in series with connecting joints.

Here, in the tunnel reinforcement member 10, the direction along the central axis (pipe axis O) of the steel pipe 2 is referred to as the pipe axis direction X, the direction perpendicular to the pipe axis O is referred to as the radial direction, and the direction going around the pipe axis O as viewed from the pipe axis direction X is referred to as the circumferential direction. In the pipe axis direction X, the forward side of the tunnel reinforcement member 10 being driven is referred to as the front side, forward, or tip side X1, and the opposite side is referred to as the rear side, rear, or base side X2. In the following description, the pipe axis O side in the radial direction is referred to as the inside, and the side opposite the inside and away from the pipe axis O is referred to as the outside.

The tunnel reinforcement member 10 comprises the above-mentioned steel pipe 2 that is driven into the ground using the tunnel reinforcement method, a drill bit 3 for excavating the ground with the steel pipe 2, a casing shoe 4 that is positioned between the steel pipe 2 and the drill bit 3, and an inner bit 5 that is positioned inside the steel pipe 2 and secures the drill bit 3 from the rear. The steel pipe threaded joint 1 of this embodiment is made of the steel pipe 2 and the casing shoe 4.

The drill bit 3 is mounted in front of the casing shoe 4 and is rotatable relative to the casing shoe 4 around the pipe axis O. The drill bit 3 is circular when viewed from the front and has multiple cutting bits 31 along the outer periphery of its tip. The drill bit 3 is fixed to the tip of the inner bit 5.

The inner bit 5 is for transmitting the rotational driving force to the drill bit 3 when driving the tunnel reinforcement member 10 into the ground, and is provided integrally with the drill bit 3. The inner bit 5 is arranged inside the steel pipe 2 and the casing shoe 4. The inner bit 5 includes a fixing part 51 that fixes and supports the drill bit 3 from the rear, and an inner rod 52 that connects the fixing part 51 to a rear rotational driving source (here, for example, the driving device of a drill jumbo). The inner rod 52 has an outer diameter smaller than that of the steel pipe 2, so that it does not come into contact with the steel pipe 2 even when rotated. As the steel pipe 2 is extended, the inner rod 52 is also extended.
The drill bit 3 and the inner bit 5 rotate integrally while being separated from the steel pipe 2 and the casing shoe 4 .

2, the fixing portion 51 of the inner bit 5 has a large diameter portion 51A whose rear portion has a larger diameter than a front portion 51B. A collision receiving surface 51a that faces the collision surface 40a (described later) of the casing shoe 4 is provided on the front end surface on the outer circumferential side of the large diameter portion 51A. The collision receiving surface 51a is formed as a tapered surface that gradually becomes forward from the outside toward the inside in the radial direction.
When the drill bit 3 rotates while drilling the ground, it pulsates back and forth against the hard rock in the pipe axis direction X. During this process, the impact surface 51 a of the inner bit 5 repeatedly collides with the impact surface 40 a of the casing shoe 4.

The steel pipe 2 of the tunnel reinforcement member 10 has a box 21 with a female thread 22 machined into the inner surface at the tip side X1. The steel pipe 2 can be, for example, a general structural carbon steel pipe specified in JIS G3444 or a carbon steel pipe for building construction. The steel pipe 2 has an outer diameter of, for example, 76.3 mm to 114.3 mm.

The thread row of the female thread 22 formed in the box 21 is tapered with respect to the pipe axis O of the steel pipe 2. The taper of the box 21 is formed so that it gradually slopes from the radially inner side to the radially outer side as it approaches the tip side X1. The taper of the thread row of the female thread 22 is set, for example, in the range of 1/6 to 1/24.

The box 21 has an incomplete thread portion 22A, in which the height of the female thread gradually decreases toward the radial center (inward). At least a portion of the incomplete thread portion 22A is engaged with the male thread 43 (described below) of the pin 41.

As mentioned above, multiple steel pipes 2 used as tunnel reinforcement members 10 are connected in series in the pipe axis direction X using joints consisting of male and female threads. Since standard joints can be used to connect the steel pipes 2, a detailed explanation will be omitted here.

The casing shoe 4 is non-rotatably connected to the tip end of the steel pipe 2 by the steel pipe threaded joint 1. The casing shoe 4 has a casing body 40 on the tip end side X1 and a pin 41 on the base end side X2 behind the casing body 40. The casing body 40 has a thickened pipe portion 40C whose pipe thickness increases in the central portion in the pipe axis direction X and whose inner diameter is smaller than that of the front portion 40A and the rear portion 40B. The inner diameter of the rear portion 40B of the casing body 40 is the same as that of the pin 41, and the inner surfaces of the rear portion 40B and the pin 41 are connected flush with each other.
At the boundary between the rear portion 40B on the outer surface side of the casing body 40 and the pin 41, there is a shoulder portion 42 that forms a step with the outer surface of the pin 41.

Furthermore, on the inner surface of the casing body 40, at the boundary between the rear portion 40B and the thick pipe portion 40C, a collision surface 40a is provided around the entire circumference of the casing body 40, against which a portion of the outer surface of the inner bit 5 (collision surface 51a, described below) collides from behind. The collision surface 40a is located closer to the tip side X1 than the shoulder portion 42 in the pipe axis direction X. The collision surface 40a forms a tapered surface that gradually slopes radially from the outside to the inside toward the tip side X1. By providing such a collision surface 40a, movement of the inner bit 5 toward the tip side X1 is restricted, preventing the inner bit 5 and drill bit 3 from protruding toward the tip side X1 relative to the casing shoe 4.

The rear portion 40B of the casing shoe 4 is provided with a pin 41 that screws into the box 21 of the steel pipe 2 and has external threads 43 machined on its outer surface. The thread row of the external threads 43 formed on the pin 41 is tapered with respect to the pipe axis O of the steel pipe 2. The taper of the pin 41 gradually inclines radially from the outside to the inside as it moves from the tip side X1 to the base side X2. The taper of the thread row of the external threads 43 is set, for example, in the range of 1/6 to 1/24.

The pin 41 has an incomplete thread portion 43A in which the height of the male thread gradually decreases radially outward. At least a portion of the incomplete thread portion 43A is engaged with the female thread 22 of the box 21.

In addition, the material tensile strength of the casing shoe 4 is set to be higher than the material tensile strength of the steel pipe 2.

In the steel pipe threaded joint 1 according to this embodiment, the material tensile strength Tp and critical cross-sectional area Ap of the pin 41, and the material tensile strength Tb and critical cross-sectional area Ab of the box 21 are set to satisfy the relationship between equations (1) and (2). Here, the critical cross-sectional area Ap of the pin 41 is the cross-sectional area at the critical cross section indicated by symbol Q1 in Figure 2. The critical cross-sectional area Ab of the box 21 is the cross-sectional area at the critical cross section indicated by symbol Q2 in Figure 2.

The critical cross section Q2 of the box 21 and the critical cross section Q1 of the pin 41 are machined in the steel pipe threaded joint 1, but considering the normal dimensional tolerances (intermediate) for machined products specified in JIS B0405, the thickness of the critical cross sections Q1 and Q2 can vary by -0.1 to +0.1 mm. If we consider the case where the thicknesses of the critical cross sections Q1 and Q2 of both the box 21 and the pin 41 are as close to the original steel pipe 2 as possible, the minimum wall thickness of a general structural steel pipe with an outer diameter of 76.3 to 114.3 mm is 2.8 mm, so the wall thicknesses of the critical cross sections Q1 and Q2 can vary from 2.7 to 2.9 mm. In this case, the respective critical cross-sectional areas Ap and Ab are approximately 668 ^mm2 at a wall thickness of 2.7 mm and approximately 624.3 ^mm2 at a wall thickness of 2.9 mm, a difference of 7%. Since a difference of 7% in the critical cross-sectional area can inevitably occur, 10% is secured to stably satisfy the relationship of the above formula (1), and Tp>1.1Tb shown in formula (2) is set.

In addition, because the inner bit 5 passes through the inner surface of the pin 41 of the casing shoe 4, increasing the critical cross-sectional area (Ap above) cannot be done on the inner diameter side, so it is necessary to increase the thread length. Therefore, if the critical cross-sectional area Ap of the pin 41 is to be increased, the length of the casing shoe 4 must be increased, which increases the productivity and cost of the joint. Therefore, it is preferable to keep the critical cross-sectional area Ap of the pin 41 as small as possible while still ensuring tensile strength.

Next, the operation of the above-described steel pipe threaded joint 1 will be explained in detail with reference to the drawings.
As shown in Figure 2, with the steel pipe threaded joint 1 according to this embodiment, the tensile strength of the steel pipe threaded joint portion is higher than that of the body of the steel pipe 2, and the fracture position can be on the steel pipe 2 side rather than the casing shoe 4, so fracture of the steel pipe threaded joint 1 during construction of the tunnel reinforcement method can be suppressed.

In other words, in this embodiment, the use of a steel pipe threaded joint 1 consisting of tapered threads makes it easier to ensure the joint cross-sectional area, which is closely related to tensile strength. Furthermore, in this embodiment, the casing shoe 4 is provided with a pin 41 having a male thread 43, and the steel pipe 2 is provided with a box 21 having a female thread 22. This prevents the so-called "zipper effect," which occurs when a tensile load acts in the pipe axis direction X, causing the pin to contract radially inward, causing the threads to come loose and resulting in premature fracture, as occurs when a box is provided on the casing shoe 4 side and a pin is provided on the steel pipe 2 side.

Furthermore, in this embodiment, the box 21 has an incomplete thread portion 22A, and the incomplete thread portion 22A of the box 21 and the complete thread portion 43B of the pin 41 are fitted together, thereby improving the tensile strength of the steel pipe threaded joint 1 and more effectively suppressing diameter reduction of the box 21.
Furthermore, in this embodiment, since the material tensile strength of the casing shoe 4 is higher than the material tensile strength of the steel pipe 2, the steel pipe threaded joint can be broken from the dangerous cross section Q2 of the box 21, and the components arranged inside the casing shoe 4 can be more reliably protected.

Furthermore, in this embodiment, by satisfying the relationships of the above-mentioned formulas (1) and (2), the tensile strength of the steel pipe threaded joint 1 can be reliably increased compared to the steel pipe 2, and the fracture position when a tensile load is applied can be determined at the position of the steel pipe body 2, rather than at the position of the steel pipe threaded joint 1.

In addition, in this embodiment, by providing incomplete thread portions 22A, 43A on both the box 21 and the pin 41, impact resistance can be improved, and fracture of the steel pipe threaded joint 1 during construction using the tunnel reinforcement method can be suppressed. As a result, this embodiment achieves a steel pipe threaded joint 1 with excellent durability against the impact load applied to the casing shoe 4 as the drill bit 3 rotates. This prevents the steel pipe threaded joint 1, which fastens the casing shoe 4 and steel pipe 2, from fracture during excavation, which would otherwise require the excavation to be interrupted, and reduces a decrease in construction efficiency due to interruptions in excavation.

In addition, in this embodiment, the impact surface 40a of the casing shoe 4 that strikes the inner bit 5 is positioned closer to the tip than the shoulder portion 42 of the pin, thereby improving impact resistance and preventing fracture of the steel pipe threaded joint during tunnel reinforcement construction. As a result, this embodiment achieves a steel pipe threaded joint 1 that has excellent durability against the impact load applied to the casing shoe 4 as the drill bit 3 rotates. This prevents the steel pipe threaded joint 1 that fastens the casing shoe 4 and steel pipe 2 from fracturing during excavation, which would otherwise require the excavation to be interrupted, and prevents a decrease in construction efficiency due to interruptions in excavation.

In addition, in this embodiment, the collision surface 40a between the casing shoe 4 and the inner bit 5 is located closer to the tip of the pin 41 than the shoulder portion 42 of the pin 41, thereby improving impact resistance.

In the steel pipe threaded joint according to the present embodiment described above, the tensile strength is improved, thereby preventing a decline in construction efficiency.

Next, the modified steel pipe threaded joint 1A shown in Figure 3 has a modified collision surface 41a of the casing shoe 4 in a different position. Specifically, the collision surface 41a of the modified example is struck from behind by the struck surface 51a of the inner bit 5, is located closer to the base end X2 of the casing shoe 4 in the pipe axis direction X than the shoulder portion 42 of the casing shoe 4, and is positioned at the middle of the inner surface of the pin 41 in the pipe axis direction X. In this case, the portion of the pin 41 on the tip side X1 of the collision surface 41a is thicker so that it is radially inward than the portion on the base end X2, and the inner surface of the tip side X1 of the pin 41 is flush with the inner surface of the rear portion 40B of the casing main body 40.

In the steel pipe threaded joint 1A according to the modified example, stress and strain concentration can be further suppressed near the critical cross section of the male thread 43 of the casing shoe 4. That is, in the range of the casing shoe 4 through which the inner bit 5 passes, the inner diameter of the casing shoe 4 must be larger than the outer diameter of the inner bit 5. Therefore, there are restrictions on the thickness of the pin 41, but by removing the critical cross section Q1 of the pin 41 from the range through which the casing shoe 4 passes, as in this modified example, it is possible to ensure a thicker critical cross section Q1 of the pin compared to the steel pipe threaded joint 1 shown in Figure 2, which has the collision surface 40a of the above-described embodiment. In this way, the steel pipe threaded joint 1A according to the modified example can achieve improved impact resistance.
Furthermore, in the steel pipe threaded joint 1A according to this modification, the overall length of the casing shoe 4 can be made shorter than in the steel pipe threaded joint 1 of the above-described embodiment. Therefore, the casing shoe 4 can be manufactured at lower cost.

Next, we will explain examples conducted to demonstrate the effectiveness of the steel pipe threaded joint according to the above-described embodiment.

(First Example)
In the first example, a numerical simulation analysis (finite element analysis) was used to apply a tensile load in the axial direction of the steel pipe threaded joint, and the state of stress and strain concentration in the steel pipe threaded joint was confirmed and its impact resistance was evaluated. That is, when drilling ground, the rotation of the drill bit causes the inner bit to repeatedly come into impact contact with the casing shoe, which may cause the casing shoe to break near the critical cross section. To prevent this, it is necessary to suppress the stress and strain concentration near the critical cross section (the portion marked Q1 in Figure 1), and this was verified in the first example.

In Example 1, analytical models were created for three analysis cases (Analysis Case 1, Analysis Case 2, and Analysis Case 3), and numerical simulation analysis was performed. In Example 1, Analysis Case 2 has a configuration equivalent to the steel pipe threaded joint 1 of the embodiment described above. Analysis Case 1 has a configuration in which the pin shape is changed from Analysis Case 2, and Analysis Case 3 is a comparative example to Analysis Cases 1 and 2. The specific configurations of Analysis Cases 1 to 3 are as follows:

Analysis case 1 shown in Figure 4 is an example in which the critical cross section Q1 of the male thread 43 of the pin 41 of the casing shoe 4 in the tapered steel pipe threaded joint 1 shown in the embodiment described above is a complete thread portion. In this case, the female thread 22 of the box 21 is an incomplete thread portion 22A (see Figure 2).
In analysis case 2, in the tapered steel pipe threaded joint 1 shown in the embodiment described above, the critical cross section Q1 in the male thread 43 of the pin 41 of the casing shoe 4 is an incomplete thread portion 43A, as shown in Figure 2. In this case, the female thread 22 of the box 21 is an incomplete thread portion 22A (see Figure 2).
Analysis case 3 shown in FIG. 5 is a configuration in which a general parallel thread is used as the threaded joint between the box 21 and the pin 41.

In the first example, the equivalent stress was determined by analysis when a tensile load equivalent to 50% of the pipe yield strength was applied to each of the steel pipe threaded joints in analysis cases 1 to 3. Figure 6 shows the distribution of equivalent stress from the analysis results of the analysis models for each of analysis cases 1 to 3 as a contour diagram.
As shown in Fig. 6, it is confirmed that the area of the high stress portion decreases in the order of analysis case 3, analysis case 1, and analysis case 2, and it is understood that the stress is alleviated in that order. Here, in Fig. 6, the high stress portions in analysis cases 1, 2, and 3 are denoted by the symbols K1, K2, and K3, respectively.

In analysis case 3, it can be seen that the high stress portion K3 expands over a wide area on the shoulder portion 42 side of the pin 41. Specifically, it was confirmed that the high stress portion K3 is large on the tip end side X1 of the pin, and that the stress increases over a wide area of the pin from the tip end side X1 toward the base end side X2.
In analysis case 1, a high stress portion K1 occurs at the terminal end (tip) of the fully threaded portion 43B of the pin 41. It was confirmed that the high stress portion K1 is a region where stress is high in the range of the thread root of the male thread at the critical cross section Q1 of the pin 41.
In addition, in analysis case 2, it was confirmed that a high stress portion K2 occurred only at the contact portion between the incomplete thread portion 43A of the pin 41 and the female thread 22 of the box 21.

Furthermore, Figure 7 shows the results of a comparison of the magnitude of plastic strain in analysis cases 1, 2, and 3 in Example 1. Figure 7 compares the ratio of the magnitude of maximum equivalent plastic strain that occurs in steel pipe threaded joints in analysis cases 1, 2, and 3 when a tensile load equivalent to the pipe yield strength is applied to the threaded joints, between analysis cases 1 and 2, where analysis case 3, which uses a general parallel thread, is set to 100%.
As a result, it is clear that the amount of strain is most mitigated for the steel pipe threaded joint in analysis case 1 at approximately 75%, and for the steel pipe threaded joint in analysis case 2 at 20% or less.

In the first example, in all of analysis cases 1 to 3, large stresses and strains occur at the thread root, but by providing an incomplete thread portion on the pin, the thread root is made shallower, and it is clear that the concentration of stress and strain at the thread root near the critical cross section can be suppressed.

(Second Example)
In the second example, the tensile strength of the above-described steel pipe threaded joint was confirmed by tensile testing. In the second example, tensile tests were conducted on test specimens prepared under the conditions shown below for each of the six test cases of Examples 1 to 3 and Comparative Examples 1 to 3, as shown in Table 1. Table 1 shows the test specimen conditions and tensile test results for each of the six tests used in the second example.

As shown in Table 1, the steel pipes used in the tests for all six test specimens of Examples 1 to 3 and Comparative Examples 1 to 3 had a diameter of 114.3 mm and a wall thickness of 6 mm.
The test specimens of Examples 1 to 3 are steel pipe threaded joints with tapered threads, and the box has an incomplete thread portion. Furthermore, the test specimens of Examples 1 to 3 satisfy the relationship of the above-mentioned formula (1) (i.e., pin material tensile strength Tp × critical cross-sectional area Ap > box 21 material tensile strength Tb × critical cross-sectional area Ab). Here, in Table 1, "◯" indicates that formula (1) is satisfied, and "×" indicates that formula (1) is not satisfied. Test specimens of steel pipe threaded joints under the same conditions were prepared and tested for each of Examples 1 to 3.

The test specimen for Comparative Example 1 is a steel pipe threaded joint with tapered threads, and the box has an incomplete thread, but does not satisfy the relationship in formula (1) (marked "X" in Table 1). The test specimen for Comparative Example 2 is a steel pipe threaded joint with parallel threads, and the box does not have an incomplete thread, so formula (1) is satisfied. The test specimen for Comparative Example 3 is a test specimen consisting only of a pipe body without a thread, and uses a steel pipe with the same material specifications as the steel pipe used in Examples 1-3 and Comparative Examples 1 and 2.

The tensile results for each tensile test were evaluated by calculating the ratio of Examples 1 to 3 and Comparative Examples 1 and 2, with Comparative Example 3 being set at 1.00. In other words, if this ratio exceeds 1.00, it is determined that the tensile strength is greater than that of Comparative Example 3. Furthermore, in this test, the fracture location of the joint was confirmed visually.

The test results for Example 2 shown in Table 1 show that Examples 1 to 3 all had a tensile strength ratio of 1.07, which exceeded that of Comparative Example 3, i.e., the tube body. On the other hand, Comparative Example 1 had a tensile strength ratio of 0.96, and Comparative Example 2 had a tensile strength ratio of 0.56, meaning that both Comparative Examples 1 and 2 had lower tensile strengths than Comparative Example 3. Furthermore, while Examples 1 to 3 all failed at the tube body, Comparative Example 1 failed at the pin and Comparative Example 2 failed at the box in each test.

In this way, in Comparative Example 1, formula (1) was not satisfied, and therefore the tensile strength of the threaded joint was lower than that of the steel pipe body, and the fracture position was at the position of the pin. Therefore, it was confirmed that by satisfying formula (1) as in Examples 1 to 3, the fracture position could be located at the pipe body.
Furthermore, from the results of Comparative Example 2, it was confirmed that even if formula (1) is satisfied, when the box does not have an incomplete thread with a parallel thread, the tensile strength is reduced to nearly half of that of the tube of Comparative Example 3, and the fracture position is also at the box.
From these results, it was found that in a tapered thread steel pipe threaded joint that meets the conditions of Examples 1 to 3, by having an incomplete thread in the box and satisfying the condition of formula (1), it is possible to realize a threaded joint with high tensile strength, with the fracture position being in the pipe body.

(Third Example)
In the third example, the impact resistance of the above-mentioned steel pipe threaded joint was confirmed using the testing apparatus 100 shown in Figure 8. In the third example, impact resistance tests were carried out by creating test specimens under the conditions shown below for the three test cases of Examples 1 and 2 and the Comparative Example, as shown in Table 2. Table 2 shows the test specimen conditions and impact resistance test results for each of the three tests used in the third example.

As shown in Table 2, the steel pipes used in the tests had a diameter of 114.3 mm and a wall thickness of 6 mm for the specimens of Example 1 and Comparative Example 1, and a diameter of 76.3 mm and a wall thickness of 5.2 mm for the specimen of Example 2.
The test specimens of Examples 1 and 2 are threaded joints for steel pipes with tapered threads, in which the box has an incomplete thread portion and the pin has an incomplete thread portion. Furthermore, the test specimens of Examples 1 and 2 satisfy the relationship of formula (1) described above (i.e., pin material tensile strength Tp × critical cross-sectional area Ap > box 21 material tensile strength Tb × critical cross-sectional area Ab). In Table 2, a "◯" indicates that formula (1) is satisfied. Furthermore, the position of the impact surface of the casing shoe in the test specimens of Examples 1 and 2 is midway between the pin shoulder and the tip (the position shown in FIG. 2 of the above-described embodiment).

The test specimen for Comparative Example 1 was a steel pipe threaded joint with parallel threads, and the box and pin had no incomplete threads and satisfied formula (1). Furthermore, the test specimen for Comparative Example 1 did not have a casing shoe impact surface.

As shown in Figure 8, the test device 100 for testing the impact resistance of the third embodiment employs a machine used in actual tunnel construction, and a member (referred to as a reinforcing member 10) equivalent to the tunnel reinforcing member equipped with a drill bit 3, a casing shoe 4, and an inner bit on a steel pipe 2, which is the test specimen of the above-mentioned Examples 1 and 2 and Comparative Example 1, is oriented horizontally, and the base end side X2 of the reinforcing member 10 in the pipe axis direction X is held by a power machine 101. The power machine 101 applies a rotational force to the held reinforcing member 10 while moving it to the tip end side X1. A concrete wall 102, through which the reinforcing member 10 will drill, is provided at the tip end side X1 of the reinforcing member 10.
In this test, a steel pipe 2 with a casing shoe 4 or the like attached to its tip was rotated by a power machine 101 and repeatedly brought into contact with a concrete wall 102, thereby applying an impact load to the steel pipe threaded joint. The number of contacts with the concrete wall 102 was 1500 to 2000 times per minute.
The test results were the test time (minutes) and the state of the joint after the test time had elapsed, which were visually confirmed.

The test results for Example 3 shown in Table 2 show that in the case of the threaded joint for steel pipes with parallel threads of Comparative Example 1, fracture occurred at the threads when an impact load was applied for 35 minutes. In contrast, in the case of the threaded joints for steel pipes with tapered threads of Examples 1 and 2, no damage to the threads was observed even when an impact load was applied for 48 and 69 minutes, respectively.
From these results, it was confirmed that in a tapered thread steel pipe threaded joint, which satisfies the conditions of Examples 1 and 2, impact resistance can be improved by having incomplete threads on the box and pin, satisfying the condition of formula (1), and further providing a collision surface midway between the shoulder and tip of the pin.

The above describes an embodiment of a steel pipe threaded joint according to the present invention, but the present invention is not limited to the above embodiment and can be modified as appropriate without departing from the spirit of the invention.

For example, in this embodiment, the material tensile strength Tp and critical cross-sectional area Ap of the pin 41, and the material tensile strength Tb and critical cross-sectional area Ab of the box 21 must satisfy the relationship between equations (1) and (2) described above, but they are not limited to satisfying the relationship between equations (1) and (2).

In addition, in this embodiment, the pin 41 is configured to have an incomplete thread portion 43A. However, while the incomplete thread portion 22A of the box 21 is essential, the incomplete thread portion 43A of the pin 41 can be omitted.

Furthermore, in this embodiment, the inner surface of the casing shoe 4 is provided with a collision surface 40a against which the inner bit 5 collides from behind, but this collision surface 40a can be omitted. Also, in this embodiment, the collision surface 40a is located on the tip side X1 of the shoulder portion 42, but the position of the collision surface is not limited to this position. For example, as in the modified example described above, the collision surface may be located in the middle portion of the pin 41 in the axial direction X (positioned on the base side X2 of the shoulder portion 42).

In addition, it is possible to replace the components in the above-described embodiments with well-known components as appropriate, without departing from the spirit of the present invention.

REFERENCE SIGNS LIST 1, 1A Steel pipe threaded joint 2 Steel pipe 3 Drill bit 4 Casing shoe 5 Inner bit 21 Box 22 Female thread 22A Incomplete thread portion of box 40 Casing body 40a Collision surface 41 Pin 41a Collision surface 42 Shoulder portion 43 Male thread 43A Incomplete thread portion of pin 43B Complete thread portion of pin 51a Collision surface 10 Tunnel reinforcement member Q1 Dangerous cross section of pin Q2 Dangerous cross section of box X Pipe axis direction X1 Tip side X2 Base side

Claims (3)

A steel pipe threaded joint processed into a steel pipe driven into the ground by a tunnel reinforcement method and a casing shoe placed between the steel pipe and a drill bit for ground excavation,
A box having an internal thread machined on the inside of the steel pipe;
a pin having an external thread on the outside of the casing shoe;
The thread rows formed on the box and the pin are tapered with respect to the pipe axis of the steel pipe,
The box has an incomplete thread portion in which the height of the female thread gradually decreases toward the radial center of the steel pipe,
At least a portion of the incomplete thread portion is engaged with the male thread of the pin,
The tensile strength of the material of the casing shoe is set higher than the tensile strength of the material of the steel pipe,
The drill bit is fixed to an inner bit disposed inside the steel pipe,
The casing shoe has a casing body located on the excavation tip side in the pipe axial direction, and the pin connected to the base end side of the casing body,
The casing body has a thick pipe portion at the center in the pipe axis direction, the thick pipe portion being thicker than the steel pipe,
a collision surface with which the inner bit collides from the base end side in the pipe axial direction is formed in the pipe thick portion;
A steel pipe threaded joint characterized in that the material tensile strength Tp and critical cross-sectional area Ap of the pin, and the material tensile strength Tb and critical cross-sectional area Ab of the box satisfy the relationship expressions (1) and (2) .
2. A threaded joint for steel pipes according to claim 1 , characterized in that the height of the male thread of the pin has an incomplete thread portion that gradually decreases radially outward. A steel pipe threaded joint according to claim 1 or 2 , characterized in that the collision surface is located closer to the drilling tip of the casing shoe in the pipe axial direction than a shoulder portion of the pin that faces the tip of the box.