Threaded joint for steel pipes

Abstract

Provided is a threaded joint for a steel pipe that can be quickly and properly fastened. The threaded joint includes a steel pipe (20m), a steel pipe (20f), and a pipe joint (50) for joining the steel pipes (20m, 20f) to each other. The pipe body (21m, 21f) of the steel pipes (20m, 20f) respectively includes a ring-shaped mark groove (23m, 23f) formed on the outer periphery of the pipe body (21m, 21f).

Description

[0001] This application is a divisional application of PCT application No. 202080011492.8, entitled "Threaded Joint for Steel Pipes", filed by Nippon Steel Corporation on March 3, 2020, and which entered the national phase on July 29, 2021. Technical Field

[0002] This disclosure relates to threaded fittings for steel pipes, and more specifically, to combined threaded fittings for connecting two steel pipes to each other. Background Technology

[0003] For example, steel pipes known as well pipes are used in the exploration or production of oil wells, natural gas wells, etc. (hereinafter collectively referred to as "oil wells"), the development of unconventional resources such as oil sands and shale gas, the recovery and storage of carbon dioxide (CCS), geothermal power generation, or hot springs. Threaded joints are used to connect the steel pipes to each other.

[0004] These threaded fittings for steel pipes are broadly categorized into composite and integral types. In the composite type, one of the two pipes being connected is a steel pipe, and the other is a fitting. In this case, external threads are formed on the outer circumference of both ends of the steel pipe, and internal threads are formed on the inner circumference of both ends of the fitting. The external thread of the steel pipe is screwed into the internal thread of the fitting, thereby securing and connecting the two. In the integral type, both pipes being connected are steel pipes, and no additional fitting is used. In this case, external threads are formed on the outer circumference of one end of the steel pipe, and internal threads are formed on the inner circumference of the other end. The external thread of one steel pipe is screwed into the internal thread of the other steel pipe, thereby securing and connecting the two.

[0005] Typically, the fitting portion of a pipe end with external threads includes an element into which an internal thread is inserted, and is therefore called a "male thread." On the other hand, the fitting portion of a pipe end with internal threads includes an element to receive the external thread, and is therefore called a "female thread." The aforementioned male and female threads are the ends of pipes, and are therefore both tubular.

[0006] For oil wells, the drilling process involves reinforcing the well walls with tubing while simultaneously excavating to prevent wall collapse. This results in a multi-layered tubing configuration. In recent years, the development of deeper and ultra-deep-sea oil wells has been a trend. However, in such environments, to efficiently develop oil wells, tubing connections often utilize threaded joints where the inner and outer diameters of the fitting are the same as or slightly larger than those of the steel pipe. Using such threaded joints minimizes the gaps between the multiple tubing configurations, allowing for efficient well development even at greater depths without significantly increasing the well diameter. Given these inner and outer diameter constraints, threaded joints require excellent sealing performance against both internal and external fluid pressures (hereinafter referred to as "internal pressure"). Furthermore, in cases such as deep oil wells, the thermal expansion of the tubing imposes significant tensile and compressive loads on the threaded joints. In such an environment, threaded joints are required to have excellent sealing performance.

[0007] As a threaded joint used to ensure sealing performance, threaded joints with a metal-to-metal contact seal (hereinafter referred to as "metal seal") are known. A metal seal refers to a structure where the diameter of the sealing surface of the male thread is slightly larger than the diameter of the sealing surface of the female thread. When the threaded joint is tightened and the sealing surfaces interlock, the sealing surface of the male thread narrows while the sealing surface of the female thread widens. Due to the elastic restoring force of each sealing surface to return to its original diameter, contact pressure is generated on the sealing surfaces, resulting in a complete circumferential seal and achieving sealing performance. Furthermore, as a threaded joint used to ensure sealing performance, threaded joints with a structure that does not possess a metal seal or utilizes the threaded portion to achieve sealing performance in conjunction with a metal seal are also known. Specifically, this structure involves a region exceeding a predetermined length of the threaded portion in the tightened state where the gap between the threaded surfaces of the male and female threads is small, allowing a viscous lubricant, referred to as a coating, to be present in this gap. Furthermore, contact surface pressure is generated on the threaded surfaces due to the interference of the thread diameters of the male and female threads (hereinafter referred to as "thread seal (structure)"). Threaded joints that possess sealing properties against internal and external pressures by having such threaded seals are also known.

[0008] This specification references the following prior art documents.

[0009] Patent Document 1: WO2018 / 180218A1

[0010] Patent Document 2: WO2018 / 052141A1

[0011] Patent Document 3: US5233742A

[0012] Patent Document 4: WO2001 / 086185A1

[0013] Patent Document 5: WO2008 / 029957A1

[0014] Patent Document 6: WO2005 / 040657A1

[0015] Patent Document 7: US4641410A Summary of the Invention

[0016] The purpose of this disclosure is to provide a threaded fitting for steel pipes that can be tightened quickly and properly.

[0017] The threaded fitting for steel pipes disclosed herein includes a first steel pipe, a second steel pipe, and a fitting for connecting the first and second steel pipes to each other. The first steel pipe includes a first pipe body and a tubular first male thread. The first male thread is continuously formed with the first pipe body and is formed at the top end of the first steel pipe. The second steel pipe includes a second pipe body and a tubular second male thread. The second male thread is continuously formed with the second pipe body and is formed at the top end of the second steel pipe. The fitting includes a tubular first female thread and a tubular second female thread. The first female thread is inserted into and secured to the first male thread. The second female thread is formed on the opposite side of the first female thread and is inserted into and secured to the second male thread. The first and second male threads each include external threads formed on the outer periphery of the male thread. The first and second female threads each include internal threads corresponding to the external threads and formed on the inner periphery of the female thread. The external and internal threads are trapezoidal threads and tapered threads. In the tightened state, at least a portion of the external and internal threads constitute a thread seal. The external thread includes an external thread crest face, an external thread groove bottom face, an external thread insert facet formed on the side closer to the tip of the male thread, and an external thread load facet formed on the side farther from the tip of the male thread. The external thread insert facet has a first external thread insert facet section and a second external thread insert facet section. The first external thread insert facet section is formed on the side farther from the pipe axis and has an insert facet angle of -10 degrees to 15 degrees. The second external thread insert facet section is formed on the side closer to the pipe axis and has an insert facet angle of 20 degrees to 60 degrees. The second external thread insert facet section has a height of 20% to 60% of the external thread height. The internal thread includes an internal thread crest face opposite to the external thread groove bottom face, an internal thread groove bottom face opposite to the external thread crest face, an internal thread insert facet opposite to the external thread insert facet, and an internal thread load facet opposite to the external thread load facet. The internal thread insert tooth flank has a first internal thread insert tooth flank section and a second internal thread insert tooth flank section. The first internal thread insert tooth flank section is formed on the side farther from the pipe axis and has the same insert tooth flank angle as the first external thread insert tooth flank section. The second internal thread insert tooth flank section is formed on the side closer to the pipe axis and has the same insert tooth flank angle as the second external thread insert tooth flank section. The first male thread also includes a first male thread shoulder surface formed at the top of the first male thread. The second male thread also includes a second male thread shoulder surface formed at the top of the second male thread and in contact with the first male thread shoulder surface in the tightened state. The first pipe body includes an annular first marking groove formed on the outer periphery of the first pipe body. Attached Figure Description

[0018] Figure 1 This is a longitudinal sectional view of the threaded joint for steel pipe according to the embodiment, along the pipe axis.

[0019] Figure 2 They represent respectively Figure 1Enlarged longitudinal sectional view showing the shape of the external and internal threads.

[0020] Figure 3 yes Figure 1 Enlarged longitudinal sectional views of the external and internal threads shown. Detailed Implementation

[0021] The threaded connector for steel pipes according to this embodiment includes a first steel pipe, a second steel pipe, and a pipe fitting for connecting the first and second steel pipes to each other. The first steel pipe includes a first pipe body and a tubular first male thread. The first male thread is continuously formed with the first pipe body and is formed at the top end of the first steel pipe. The second steel pipe includes a second pipe body and a tubular second male thread. The second male thread is continuously formed with the second pipe body and is formed at the top end of the second steel pipe. The pipe fitting includes a tubular first female thread and a tubular second female thread. The first female thread is inserted into and secured to the first male thread. The second female thread is formed on the opposite side of the first female thread and is inserted into and secured to the second male thread. The first and second male threads each include external threads formed on the outer periphery of the male thread. The first and second female threads each include internal threads corresponding to the external threads and formed on the inner periphery of the female thread. The external and internal threads are trapezoidal threads and tapered threads. In the tightened state, at least a portion of the external and internal threads constitute a thread seal. The external thread includes an external thread crest face, an external thread groove bottom face, an external thread insert facet formed on the side closer to the tip of the male thread, and an external thread load facet formed on the side farther from the tip of the male thread. The external thread insert facet has a first external thread insert facet section and a second external thread insert facet section. The first external thread insert facet section is formed on the side farther from the pipe axis and has an insert facet angle of -10 degrees to 15 degrees. The second external thread insert facet section is formed on the side closer to the pipe axis and has an insert facet angle of 20 degrees to 60 degrees. The second external thread insert facet section has a height of 20% to 60% of the external thread height. The internal thread includes an internal thread crest face opposite to the external thread groove bottom face, an internal thread groove bottom face opposite to the external thread crest face, an internal thread insert facet opposite to the external thread insert facet, and an internal thread load facet opposite to the external thread load facet. The internal thread insert tooth flank has a first internal thread insert tooth flank section and a second internal thread insert tooth flank section. The first internal thread insert tooth flank section is formed on the side farther from the pipe axis and has the same insert tooth flank angle as the first external thread insert tooth flank section. The second internal thread insert tooth flank section is formed on the side closer to the pipe axis and has the same insert tooth flank angle as the second external thread insert tooth flank section. The first male thread also includes a first male thread shoulder surface formed at the top of the first male thread. The second male thread also includes a second male thread shoulder surface formed at the top of the second male thread and in contact with the first male thread shoulder surface in the tightened state. The first pipe body includes an annular first marking groove formed on the outer periphery of the first pipe body.

[0022] According to this embodiment, the first steel pipe and the second steel pipe can be quickly and properly fastened to the pipe joint.

[0023] Alternatively, the second tube body may include an annular second marking groove formed on the outer periphery of the second tube body. Alternatively, the first marking groove may have a width narrower than the width of the second marking groove.

[0024] In this case, it is easier to properly fasten the first steel pipe to the pipe joint, and the second steel pipe can be quickly fastened to the pipe joint.

[0025] Alternatively, the first female buckle may also include a tubular first recess. The first recess is formed at the open end of the first female buckle and has an inner circumferential surface that is opposite to and separate from the outer circumferential surface of the first tube body. Alternatively, the second female buckle may also include a tubular second recess. The second recess is formed at the open end of the second female buckle and has an inner circumferential surface that is opposite to and separate from the outer circumferential surface of the second tube body.

[0026] In this case, the paint will not overflow from the open end of the pipe fitting and reach the marking groove.

[0027] Alternatively, the portion of the external thread of the second male thread and the internal thread of the second female thread that constitutes the thread seal may have a smaller thread diameter interference than the portion of the external thread of the first male thread and the internal thread of the first female thread that constitutes the thread seal.

[0028] In this case, it is possible to suppress the rotation of the first male thread caused by the common rotation during the screwing-in process of the second steel pipe.

[0029] Alternatively, the external thread of the first male thread and the internal thread of the first female thread may include a complete thread and an incomplete thread formed between the first pipe body and the complete thread. Alternatively, the first male thread and the first female thread may be fully or partially bonded together in the incomplete thread in the tightened state.

[0030] In this case, it is possible to suppress the rotation of the first male thread caused by the common rotation during the screwing-in process of the second steel pipe.

[0031] Alternatively, the steel pipe may have an outer diameter exceeding 16 inches.

[0032] The pipe assembly of this embodiment includes a first steel pipe and a pipe fitting for connecting the first steel pipe and a second steel pipe to each other. The first steel pipe includes a first pipe body and a tubular first male thread. The first male thread is continuously formed with the first pipe body and is formed at the top end of the first steel pipe. The pipe fitting includes a tubular first female thread and a tubular second female thread. The first female thread is inserted into and fastened to the first male thread. The second female thread is formed on the opposite side of the first female thread and is inserted into and fastened to the second male thread of the second steel pipe. The first male thread includes an external thread formed on the outer periphery of the male thread. The first female thread includes an internal thread corresponding to the external thread and formed on the inner periphery of the female thread. The external thread and the internal thread are trapezoidal threads and are tapered threads. In the tightened state, at least a portion of the external thread and the internal thread constitute a thread seal. The external thread includes an external thread crest face, an external thread groove bottom face, an external thread insert facet formed on the side closer to the tip of the male thread, and an external thread load facet formed on the side farther from the tip of the male thread. The external thread insert facet has a first external thread insert facet section and a second external thread insert facet section. The first external thread insert facet section is formed on the side farther from the pipe axis and has an insert facet angle of -10 degrees to 15 degrees. The second external thread insert facet section is formed on the side closer to the pipe axis and has an insert facet angle of 20 degrees to 60 degrees. The second external thread insert facet section has a height of 20% to 60% of the external thread height. The internal thread includes an internal thread crest face opposite to the external thread groove bottom face, an internal thread groove bottom face opposite to the external thread crest face, an internal thread insert facet opposite to the external thread insert facet, and an internal thread load facet opposite to the external thread load facet. The internal thread insert tooth flank has a first internal thread insert tooth flank section and a second internal thread insert tooth flank section. The first internal thread insert tooth flank section is formed on the side farther from the pipe axis and has the same insert tooth flank angle as the first external thread insert tooth flank section. The second internal thread insert tooth flank section is formed on the side closer to the pipe axis and has the same insert tooth flank angle as the second external thread insert tooth flank section. The first male thread also includes a first male thread shoulder surface formed at the top of the first male thread. The first pipe body includes an annular first marking groove formed on the outer periphery of the first pipe body. The open end of the first female thread is positioned within the width of the first marking groove.

[0033] Hereinafter, with reference to the accompanying drawings, one embodiment of a threaded fitting for steel pipes will be described. The same or equivalent parts in the drawings will be labeled with the same reference numerals, and identical descriptions will not be repeated.

[0034] Reference Figure 1The threaded connector 10 is a combination type, comprising a steel pipe 20m, a steel pipe 20f, and a connector 50 for connecting the steel pipes 20m and 20f to each other. The steel pipes 20m and 20f are not particularly limited, but may have an outer diameter exceeding 16 inches. The steel pipe 20m comprises a pipe body 21m and a tubular male thread 30m. The male thread 30m is continuously formed with the pipe body 21m at the top end 22m of the steel pipe 20m. The steel pipe 20f comprises a pipe body 21f and a tubular male thread 30f. The male thread 30f is continuously formed with the pipe body 21f at the top end 22f of the steel pipe 20f. The connector 50 comprises a tubular female thread 40m and a tubular female thread 40f. The female thread 40m is inserted into and fastened to the male thread 30m. Female thread 40f is formed on the opposite side of female thread 40m, for male thread 30f to be inserted and fastened to male thread 30f. Male threads 30m and 30f respectively include external threads 31m and 31f formed on the outer periphery of male threads 30m and 30f. Female threads 40m and 40f respectively include internal threads 41m and 41f corresponding to external threads 31m and 31f and formed on the inner periphery of female threads 40m and 40f. External threads 31m and 31f and internal threads 41m and 41f are trapezoidal threads and are tapered threads.

[0035] The male 30m thread is pre-fastened to the female 40m thread in the factory, hence it is called the "mill end". The male 30f thread is fastened to the female 40f thread in the well, hence it is called the "field end".

[0036] External threads 31m and 31f are helical on the outer circumference of male threads 30m and 30f, with the diameter of the helix decreasing as it approaches the tip (shoulder surface 24m and 24f) of the male threads 30m and 30f. Internal threads 41m and 41f are helical on the inner circumference of female threads 40m and 40f, with the diameter of the helix increasing as it approaches the open end 51m and 51f of the female threads 40m and 40f. The preferred taper ratio of the tapered threads is 6.0 to 18.0%. The taper ratio is designed to obtain an appropriate thread length based on the relationship with the wall thickness of the steel pipe 20m and 20f. The taper ratio can also be constant, but preferably the taper ratio of the external threads 31m and 31f decreases as it moves away from the tip of the male threads 30m and 30f.

[0037] In the tightened state, at least a portion of the external threads 31m, 31f and the internal threads 41m, 41f constitute a thread seal. The portion of the external threads 31m, 31f and the internal threads 41m, 41f constituting the thread seal has a length in the pipe axial direction that is more than three times the wall thickness of the steel pipe 20m, 20f. The external threads 31m, 31f and the internal threads 41m, 41f constituting the thread seal are complete threads. The longer the thread seal, the better the sealing performance. On the other hand, if the thread seal is too long, in addition to the cost and effort of thread cutting, there is also the possibility of wear during tightening. The length of the thread seal is preferably less than eight times the wall thickness. The threaded joint 10 has such a thread seal but does not have a metal seal. However, a Teflon (registered trademark) sealing ring may also be used in conjunction with the thread seal. A coating is sandwiched between the thread sealing surfaces. The sealing performance is improved by sandwiching the coating.

[0038] The portion constituting the thread seal in the male 30m external thread 31m and the female 40m internal thread 41m (hereinafter referred to as the "thread seal portion") has a smaller thread diameter interference than the portion constituting the thread seal in the male 30f external thread 31f and the female 40f internal thread 41f. The thread diameter interference refers to the difference between the outer diameter of the thread seal portion of the male 30m and 30f and the inner diameter of the corresponding thread seal portion of the female 40m and 40f. In the thread seal portion, the outer diameter of the male 30m and 30f external threads 31m and 31f is larger than the inner diameter of the corresponding internal threads 41m and 41f of the female 40m and 40f. That is, the thread diameter interference is positive throughout the entire region of the thread seal portion.

[0039] The male thread 30m also includes a male thread shoulder surface 24m formed at the top of the male thread 30m. The male thread 30f also includes a male thread shoulder surface 24f, which is formed at the top of the male thread 30f and contacts the male thread shoulder surface 24m in the tightened state. The threaded connector 10 has a so-called Pin to Pin structure. In the threaded connector 10 of this embodiment, the male thread 30m at the factory end is typically tightened to the female thread 40m first. Then, the male thread 30f at the well site end in the oil well is tightened to the female thread 40f. As a result, the male thread shoulder surface 24f of the male thread 30f contacts the male thread shoulder surface 24m of the male thread 30m.

[0040] The pipe body 21m includes an annular marking groove 23m. The marking groove 23m is formed on the outer periphery of the pipe body 21m. The pipe body 21f includes an annular marking groove 23f. The marking groove 23f is formed on the outer periphery of the pipe body 21f. The marking groove 23m has a width wm that is narrower than the width wf of the marking groove 23f (wm < wf). The marking grooves 23m and 23f are formed by cutting the pipe bodies 21m and 21f with a cutting tool while rotating the steel pipes 20m and 20f on a lathe. Therefore, the marking grooves 23m and 23f extend circumferentially and surround the pipe bodies 21m and 21f.

[0041] More specifically, at the well site end, the marking groove 23f is located ±α (e.g., α = 0.75 mm) centered on the opening end 51f of the female thread 40f. The width wf of the marking groove 23f is 2α (e.g., 1.5 mm). On the other hand, at the plant end, the marking groove 23m is located +(α - β) towards the side farther from the top and -α towards the side closer to the top, centered on the opening end 51m of the female thread 40m. Here, β ≤ 3 / 4α (e.g., 0.5 mm). The width wm of the marking groove 23m is 2α - β (e.g., 1.0 mm). Therefore, the opening end 51m of the female thread 40m is located within the width wm of the marking groove 23m.

[0042] The female buckle 40m also includes a tubular recess 52m. The recess 52m is formed at the open end of the female buckle 40m and has an inner circumferential surface that is opposite to and separate from the outer circumferential surface of the tube body 21m. The female buckle 40f also includes a tubular recess 52f. The recess 52f is formed at the open end of the female buckle 40f and has an inner circumferential surface that is opposite to and separate from the outer circumferential surface of the tube body 21f. The recesses 52m and 52f each have a length of at least 5 mm, for example, 15 mm, in the axial direction of the tube.

[0043] Reference Figure 2 and Figure 3 The external threads 31m and 31f (hereinafter collectively referred to as "31") include an external thread crest surface 32, an external thread groove bottom surface 33, an external thread insertion facet 34, and an external thread load facet 35. The external thread insertion facet 34 is formed on the side closer to the tip of the male thread 30m and 30f. The external thread load facet 35 is formed on the side farther from the tip of the male thread 30m and 30f.

[0044] The external thread insert facet 34 has two external thread insert facet sections 341 and 342. External thread insert facet section 341 is formed on the side farther from the pipe axis X of the steel pipe 20 and has an insert facet angle α1. External thread insert facet section 342 is formed on the side closer to the pipe axis X and has an insert facet angle α2. Insert facet angles α1 and α2 are the angles of inclination of the external thread insert facet 34 (external thread insert facet sections 341 and 342) relative to a plane Y perpendicular to the pipe axis X. When the insert facet 34 is overhanging, the insert facet angle α1 is negative. The insert facet angle α2 is larger than the insert facet angle α1 (α2 > α1). The insert facet angle α1 is -10 to 15 degrees. The upper limit of the insert facet angle α1 is preferably 14 degrees, more preferably 13 degrees, and even more preferably 12 degrees. The smaller α1 is, the higher the compression resistance. The lower limit of the insert facet angle α1 is preferably 0 degrees, more preferably 8 degrees. On the other hand, the larger α1 is, the easier the machining process is when forming the threaded portion. The insert tooth flank angle α1 is, for example, about 10 degrees. The insert tooth flank angle α2 is 20 to 60 degrees. The upper limit of the insert tooth flank angle α2 is preferably 50 degrees, more preferably 40 degrees, and even more preferably 32 degrees. The smaller α2 is, the higher the compression resistance. The lower limit of the insert tooth flank angle α2 is preferably 23 degrees, more preferably 26 degrees, and even more preferably 28 degrees. The larger α2 is, the less likely mis-threading will occur. The insert tooth flank angle α2 is, for example, about 30 degrees. Therefore, the insert tooth flank 34 of the external thread is concave in the middle of the web.

[0045] The height of the external thread insert flank 342 (the length from the bottom surface 33 of the external thread groove to the boundary between the external thread insert flanks 341 and 342) is 25% to 60% of the height of the external thread, for example, 35%.

[0046] The internal threads 41m and 41f (hereinafter collectively referred to as "41") include an internal thread crest face 42, an internal thread groove bottom face 43, an internal thread insert face 44, and an internal thread load face 45. The internal thread crest face 42 is opposite to the external thread groove bottom face 33. The internal thread groove bottom face 43 is opposite to the external thread crest face 32. The internal thread insert face 44 is opposite to the external thread insert face 34. The internal thread load face 45 is opposite to the external thread load face 35.

[0047] The internal thread insert flank 44 has two internal thread insert flank sections 441 and 442. Internal thread insert flank section 441 is formed on the side farther from the pipe axis X, and has the same insert flank angle α1 as the external thread insert flank section 341. Internal thread insert flank section 442 is formed on the side closer to the pipe axis X, and has the same insert flank angle α2 as the external thread insert flank section 342. Therefore, the internal thread insert flank 44 bulges out in the middle. The insert flank angles α1 and α2 of the external thread insert flank sections 341 and 342 and the insert flank angles α1 and α2 of the internal thread insert flank sections 441 and 442 may not be completely identical, as long as they are substantially the same. That is, there may be cases where the insert flank angles α1 and α2 have errors associated with the cutting process.

[0048] Preferably, the internal thread insert flank 442 has the same height as the external thread insert flank 342. This prevents the gap between the male and female thread surfaces from increasing excessively, allowing for good sealing performance using the threaded sealing structure. The heights of the external thread insert flank 342 and the internal thread insert flank 442 do not necessarily have to be exactly the same, as long as they are substantially identical. That is, these heights can be subject to errors arising from machining processes.

[0049] The external thread 31 also includes external thread surfaces 36 to 39. External thread surface 36 is formed at the angle between the external thread crest surface 32 and the external thread insertion surface 34. External thread surface 37 is formed at the angle between the external thread crest surface 32 and the external thread load surface 35. External thread surface 38 is formed at the angle between the external thread groove bottom surface 33 and the external thread insertion surface 34. External thread surface 39 is formed at the angle between the external thread groove bottom surface 33 and the external thread load surface 35.

[0050] The internal thread 41 includes internal thread surfaces 46 to 49. Internal thread surface 46 is formed at the angle between the internal thread crest surface 42 and the internal thread insert facet 44. Internal thread surface 47 is formed at the angle between the internal thread crest surface 42 and the internal thread load facet 45. Internal thread surface 48 is formed at the angle between the internal thread groove bottom surface 43 and the internal thread insert facet 44. Internal thread surface 49 is formed at the angle between the internal thread groove bottom surface 43 and the internal thread load facet 45.

[0051] Surfaces 36–39 and 46–49 are so-called R-surfaces (rounded chamfer surfaces) with a predetermined radius of curvature. The radius of curvature is 0.1–1.2 mm, preferably 0.3–0.8 mm.

[0052] The external thread load flank 35 has a load flank angle β. The load flank angle β is the angle at which the external thread load flank 35 is inclined relative to a plane Y perpendicular to the pipe axis X. When the load flank 35 is overhanging, the load flank angle β is negative. The load flank angle β is -10 to 3 degrees, preferably -5 to -1 degree, for example, about -3 degrees. The internal thread load flank 45 has the same load flank angle β as the external thread load flank 35. The load flank angle β of the external thread load flank 35 and the load flank angle β of the internal thread load flank 45 may not be exactly the same, as long as they are substantially the same. That is, there may be cases where the load flank angle β has errors associated with the cutting process.

[0053] The external thread crest surface 32, the external thread groove bottom surface 33, the internal thread crest surface 42, and the internal thread groove bottom surface 43 are formed parallel to the pipe axis X. Specifically, the lines of surfaces 32, 33, 42, and 43, as shown in a longitudinal section containing the pipe axis X, are parallel to the pipe axis X.

[0054] like Figure 3 As shown, the external thread insertion tooth flank 34 and the internal thread insertion tooth flank 44 have a gap of 60–120 μm between them in the tightened state. Additionally, the external thread crest surface 32 and the internal thread groove bottom surface 43 have a gap of 0–50 μm between them in the tightened state. The external thread groove bottom surface 33 and the internal thread crest surface 42 also have a gap of 0–50 μm between them in the tightened state.

[0055] The following describes an example of the manufacturing method of the threaded joint 10, namely, the method of fastening two steel pipes 20m and 20f using the pipe fitting 50.

[0056] In the factory, the male thread 30m of the 20m steel pipe at the factory end is screwed into the female thread 40m. At this time, the male thread 30m is screwed in such a way that the open end 51m of the female thread 40m enters the width wm of the marking groove 23m. Thus, the 20m steel pipe at the factory end is secured to the pipe fitting 50. The 20m steel pipe at the factory end and the pipe fitting 50 constitute a pipe assembly, which is sent from the factory toward the oil well.

[0057] Then, in the well, the male thread 30f of the well site end of steel pipe 20f is screwed into the female thread 40f. At this time, without holding the pipe fitting 50, the male thread 30f is screwed in while holding steel pipe 20m and steel pipe 20f. In addition, the male thread 30f is screwed in such a way that the open end 51f of the female thread 40f enters the width wf of the marking groove 23f. Thus, steel pipe 20f at the well site end is secured to pipe fitting 50. In this way, the two steel pipes 20m and 20f are connected to each other by pipe fitting 50, resulting in the creation of threaded joint 10.

[0058] When screwing the male thread 30m of the 20m steel pipe into the female thread 40m at the factory end, the torque gradually increases as screwing progresses, but without a sharp increase. Therefore, unlike the screwing at the well site end described later, it is difficult to determine the appropriate tightening position based on the change in torque. However, according to the threaded connector 10, since a marking groove 23m is formed, the positional relationship between the marking groove 23m and the open end 51m of the female thread 40m can be visually confirmed at the factory end, and the steel pipe 20m can be tightened to the pipe connector 50 at the desired position.

[0059] At the well site end, when steel pipe 20f is screwed in, the male thread shoulder 24f of male thread 30f abuts against the male thread shoulder 24m at the factory end. At this time, the tightening torque increases sharply. Therefore, if steel pipe 20f is screwed in while monitoring this torque change, steel pipe 20f can be tightened to pipe fitting 50 with an appropriate torque. However, to confirm this, monitoring of the torque increase needs to be performed after each tightening operation.

[0060] In contrast, if a marking groove 23f is also pre-formed on the steel pipe 20f, then after tightening the steel pipe 20f with a pre-set appropriate torque, the positional relationship between the open end 51f of the female thread 40f and the marking groove 23f can be visually confirmed. Therefore, even without strictly monitoring torque changes as described above, the steel pipe 20f can be quickly and appropriately tightened to the pipe fitting 50.

[0061] Here, whether the steel pipe 20f can be properly tightened depends mainly on whether the previously tightened steel pipe 20m has been properly tightened. Therefore, if the width wm of the marking groove 23m at the factory end is made narrower than the width wf of the marking groove 23f at the well site end beforehand, the tightening of the steel pipe 20m at the factory end can be managed more strictly.

[0062] As a result of the above, tightening can be performed in oil well sites without strictly monitoring the tightening torque. Therefore, equipment for monitoring the tightening torque is not required, making operation easier and more efficient.

[0063] Furthermore, recesses 52m and 52f are formed at the open ends of the female threads 40m and 40f. Therefore, even if paint seeps out from the gap between the threaded sealing surfaces, it will accumulate in the gap between the recesses 52m and 52f and the pipe bodies 21m and 21f. Consequently, paint will not overflow from the open ends 51m and 51f of the pipe fitting 50 and reach the marking grooves 23m and 23f. As a result, the positional relationship between the marking grooves 23m and 23f and the open ends 51m and 51f can always be visually observed.

[0064] Furthermore, the thread diameter interference at the well site end is smaller than that at the factory end. Therefore, during the screwing-in process of the steel pipe 20f, the contact surface pressure of the male thread 30f and female thread 40f until the male thread shoulder surface 24f contacts the male thread shoulder surface 24m is smaller than that of the male thread 30m and female thread 40m. Therefore, the rotation amount at the factory end caused by joint rotation during tightening at the well site end can be zero or less than an allowable amount. "Joint rotation" refers to the phenomenon that when the steel pipe 20f at the well site end is rotated and the male thread 30f is screwed into the female thread 40f, the pipe fitting 50 rotates together with the steel pipe 20f, that is, it rotates relative to the steel pipe 20m at the factory end. To prevent sintering, the thread diameter interference at the factory end is preferably below a predetermined value. To improve the sealing performance of the thread seal, the thread diameter interference at the well site end is preferably above a predetermined value.

[0065] Furthermore, the male thread 30m and female thread 40m at the factory end sequentially encompass regions Z1 to Z3 from the top of the male thread 30m. The external thread 31m of the male thread 30m and the internal thread 41m of the female thread 40m at the factory end may also include a complete thread and an incomplete thread formed between the pipe body 21m and the complete thread. The complete thread is formed in region Z2. The incomplete thread is formed in regions Z1 and Z3. The male thread 30m and female thread 40m at the factory end may also be bonded in the incomplete thread of region Z3 in a tightened state. Specifically, the male thread 30m and female thread 40m may also be bonded in whole or in part in the incomplete thread of region Z3. In addition, the male thread 30m and female thread 40m may also be bonded in part in the complete thread adjacent to the incomplete thread. When the male thread 30m and female thread 40m at the factory end are bonded, during the screwing of the steel pipe 20f at the well site end, the amount of rotation at the factory end caused by common rotation can be zero or less.

[0066] During factory-end fastening, adhesive is pre-applied to area Z3, where the male thread 30m and female thread 40m are to be bonded, and coating is pre-applied to area Z2. This facilitates prevention of sintering and ensures a tight seal. The adhesive can be applied to the entire area Z3 or only a portion of it. Alternatively, it can be applied in excess to a portion of area Z2 adjacent to area Z3. Furthermore, the adhesive can be applied only to the external thread 31m, only to the internal thread 41m, or both.

[0067] The above describes the implementation methods, but the present invention is not limited to the above implementation methods. Various modifications can be made as long as they do not depart from the main idea.

[0068] Explanation of reference numerals in the attached figures

[0069] 10. Threaded connector; 20m, 20f, steel pipe; 21m, 21f, pipe body; 23m, 23f, marking groove; 24m, 24f, male thread shoulder face; 30m, 30f, male thread; 31, 31m, 31f, external thread; 32, external thread crest face; 33, external thread groove bottom face; 34, external thread insert side face; 341, 342, external thread insert side face; 35, external thread load side face; 40m, 40f, female thread; 41, 41m, 41f, internal thread; 42, internal thread crest face; 43, internal thread groove bottom face; 44, internal thread insert side face; 441, 442, internal thread insert side face; 45, internal thread load side face; 50. Pipe connector; 51m, 51f, open end; 52m, 52f, recess.

Claims

1. A threaded joint, wherein the threaded joint comprises: a first steel pipe; a second steel pipe; and a pipe joint for connecting the first steel pipe and the second steel pipe to each other, the first steel pipe comprises: a first pipe body; and a tubular first pin which is continuously formed with the first pipe body, formed at a top end portion of the first steel pipe, the second steel pipe comprises: a second pipe body; and a tubular second pin which is continuously formed with the second pipe body, formed at a top end portion of the second steel pipe, the pipe joint comprises: a tubular first box into which the first pin is inserted and fastened with the first pin; and a tubular second box which is formed at an opposite side of the first box, into which the second pin is inserted and fastened with the second pin, the first pin and the second pin each comprise an external thread formed at an outer periphery of the pin, the first box and the second box each comprise an internal thread which corresponds to the external thread and is formed at an inner periphery of the box, the external thread and the internal thread are trapezoidal threads and tapered threads, in a fastened state, at least a part of the external thread and the internal thread constitute a thread seal, the external thread comprises: an external thread crest face; an external thread root face; an external thread stabbing flank formed at a side closer to a top end of the pin; and an external thread load flank formed at a side farther from the top end of the pin, the external thread stabbing flank has: a first external thread stabbing flank section formed at a side farther from a pipe axis of the steel pipe, having a stabbing flank angle of -10 degrees to 15 degrees; and a second external thread stabbing flank section formed at a side closer to the pipe axis, having a stabbing flank angle of 20 degrees to 60 degrees, the second external thread stabbing flank section has a height of 20 to 60% of a height of the external thread, the internal thread comprises: an internal thread crest face which is opposite to the external thread root face; an internal thread root face which is opposite to the external thread crest face; an internal thread stabbing flank which is opposite to the external thread stabbing flank; and an internal thread load flank which is opposite to the external thread load flank, the internal thread stabbing flank has: a first internal thread stabbing flank section formed at a side farther from the pipe axis, having a stabbing flank angle which is the same as that of the first external thread stabbing flank section; and a second internal thread stabbing flank section formed at a side closer to the pipe axis, having a stabbing flank angle which is the same as that of the second external thread stabbing flank section, the first pin further comprises a first pin shoulder face formed at a top end of the first pin, the second pin further comprises a second pin shoulder face formed at a top end of the second pin and which is in contact with the first pin shoulder face in a fastened state, the first pipe body comprises a ring-shaped first marker groove formed at an outer periphery of the first pipe body, the second pipe body comprises a ring-shaped second marker groove formed at an outer periphery of the second pipe body, the first marker groove has a width which is narrower than that of the second marker groove.

2. The threaded joint according to claim 1, wherein the first box further includes a tubular first recess formed at the open end of the first box, having an inner peripheral surface opposite to and separated from the outer peripheral surface of the first tubular body, the second box further includes a tubular second recess formed at the open end of the second box, having an inner peripheral surface opposite to and separated from the outer peripheral surface of the second tubular body.

3. The threaded joint according to claim 1 or 2, wherein the portion of the external thread of the second pin and the internal thread of the second box that constitutes the thread seal has a smaller thread diameter interference than the portion of the external thread of the first pin and the internal thread of the first box that constitutes the thread seal.

4. The threaded joint according to claim 1 or 2, wherein the external thread of the first pin and the internal thread of the first box include a full thread and an incomplete thread formed between the first tubular body and the full thread, the first pin and the first box are bonded at all or part of the incomplete thread in the fastened state.

5. The threaded joint according to claim 3, wherein the external thread of the first pin and the internal thread of the first box include a full thread and an incomplete thread formed between the first tubular body and the full thread, the first pin and the first box are bonded at all or part of the incomplete thread in the fastened state.

6. The threaded joint according to claim 1 or 2, wherein the first and second steel pipes have an outer diameter exceeding 16 inches.

7. The threaded joint according to claim 3, wherein the first and second steel pipes have an outer diameter exceeding 16 inches.

8. The threaded joint according to claim 4, wherein the first and second steel pipes have an outer diameter exceeding 16 inches.

9. The threaded joint according to claim 5, wherein the first and second steel pipes have an outer diameter exceeding 16 inches.

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