THREADED CONNECTION

Abstract

Provided is a threaded joint having excellent sealing performance even at extremely low temperatures. This threaded joint is for a steel pipe, and comprises a tubular pin and a tubular box, the pin and the box being fastened together as the pin is screwed into the box, wherein the pin includes a pin seal surface and a male thread, the box includes a box seal surface corresponding to the pin seal surface, and a female thread corresponding to the male thread, and the following formula (1) is satisfied. The variables in formula (1) are defined in the specification.

Description

Threaded joints

[0001] The present disclosure relates to a threaded joint, and more particularly to a threaded joint used to connect steel pipes.

[0002] Conventionally, steel pipes known as oil well tubular goods (OCTGs) have been used in oil wells, natural gas wells, and the like (hereinafter collectively referred to as "oil wells"), and the oil well tubular goods are connected to each other by threaded joints and installed in the oil wells.

[0003] Generally, the end of an oil country tubular good having a male thread is called a pin because it includes an element that is inserted into the female thread, and the end of an oil country tubular good or a coupling having a female thread is called a box because it includes an element that receives the male thread.

[0004] Carbon dioxide Capture and Storage (CCS) technology is expected to be a key technology for achieving a sustainable, carbon-neutral society. The design of CCS wells requires consideration of the behavior of carbon dioxide during operation, which requires knowledge different from that of conventional oil and gas wells. For example, the impact of sudden temperature drops that may occur during operation on oil well equipment must be considered. Non-Patent Document 1 reports that carbon dioxide injection tubing can be cooled to -20°C in a short period of time during transient operations (closing-in and restarting), and to -70°C in the event of problems such as leaks, blowouts, or releases.

[0005] If the inner surface of the tubing cools suddenly, there is concern that the sealing performance of the threaded joint that fastens the tubing may change. In other words, threaded joints known as premium joints (PJs) exhibit extremely high sealing performance due to the interfering metal seals, but if the inner surface of the PJ pin cools, it is expected that the seal contact force will decrease due to thermal contraction of the pin's metal seal surface. However, the impact of cooling the inner surface of the pin on the contact force of the PJ's metal seal, and PJ design that takes this impact into account, have not yet been fully explored.

[0006] Republished WO2016 / 113790 Patent Publication No. 2002-295747 Patent Publication No. 2002-22070 Republished WO2002 / 075195 Republished WO2015-141159 Republished WO2012-060474

[0007] Luis Acevedo and Ajay Chopra, "Influence of phase behavior in the well design of CO2 injectors", Energy Procedia 114(2017)5083-5099

[0008] An object of the present disclosure is to provide a threaded joint that has excellent sealing performance even at extremely low temperatures.

[0009] A threaded joint according to the present disclosure is a threaded joint for steel pipes that is composed of a tubular pin and a tubular box, and that is fastened to the box by screwing the pin into the box, wherein the pin has a pin seal surface and a male thread, and the box has a box seal surface that corresponds to the pin seal surface and a female thread that corresponds to the male thread, and satisfies the following formula (1):

[0010]

[0011] where:

[0012] D: Outer diameter of the pipe body [mm] D i : Inner diameter of pipe body [mm] t : Wall thickness of pipe body = (DD i ) / 2 [mm] D BO :Outer diameter of the box [mm] D sp : Pin seal diameter [mm] D sb : Box seal diameter [mm] δ s : Seal interference amount = D sp -D sb [mm] d s : Heat shrinkage of pin seal diameter [mm] δ s ’ : Seal interference amount after cooling [mm] T L: Temperature of the inner surface of the pin [℃] T s T: Temperature of pin seal surface [℃] H :Outer surface temperature of the box [℃] α :Thermal expansion coefficient of the tube body [1 / ℃] T at room temperature L =T H = 25℃ After cooling, T L = -80℃, T H =25℃

[0013] According to the present disclosure, it is possible to obtain a threaded joint that has excellent sealing performance even when the inner surface of the pin is cooled to an extremely low temperature and the pin is thermally shrunk.

[0014] Fig. 1 is a longitudinal sectional view showing a threaded joint according to an embodiment. Fig. 2 is a sectional view for explaining the seal interference amount. Fig. 3 is a sectional view for explaining another seal interference amount. Fig. 4 is a sectional view for explaining the parameter δ in the first embodiment. s ’ 5 is a graph showing the relationship between the parameter δ / (t / D) and the seal contact force. s_act ’ 10 is a graph showing the relationship between / (t / D) and seal contact force.

[0015] <Overview of Embodiments> A threaded joint according to a first embodiment is a threaded joint for steel pipes that is composed of a tubular pin and a tubular box, and in which the pin is screwed into the box to fasten the pin and the box, wherein the pin has a pin seal surface and a male thread, and the box has a box seal surface that corresponds to the pin seal surface and a female thread that corresponds to the male thread, and satisfies the following formula (1).

[0016]

[0017] where:

[0018] D: Outer diameter of the pipe body [mm] D i : Inner diameter of pipe body [mm] t : Wall thickness of pipe body = (DD i ) / 2 [mm] D BO :Outer diameter of the box [mm] D sp: Pin seal diameter [mm] D sb : Box seal diameter [mm] δ s : Seal interference amount = D sp -D sb [mm] d s : Heat shrinkage of pin seal diameter [mm] δ s ’ : Seal interference amount after cooling [mm] T L : Inner surface temperature of pin [℃] T s T: Temperature of pin seal surface [℃] H : Outer surface temperature of the box [℃] α : Thermal expansion coefficient of the tube body [1 / ℃] T at room temperature L =T H = 25℃ After cooling, T L = -80℃, T H =25℃

[0019] A threaded joint according to the second embodiment is a threaded joint for steel pipes that is composed of a tubular pin and a tubular box, and in which the pin is screwed into the box to fasten the pin and the box, wherein the pin has a pin seal surface and a male thread, and the box has a box seal surface that corresponds to the pin seal surface and a female thread that corresponds to the male thread, and satisfies the following formula (2):

[0020]

[0021] where:

[0022] D: Outer diameter of the pipe body [mm] D i : Inner diameter of pipe body [mm] t : Wall thickness of pipe body = (DD i ) / 2 [mm] D BO :Outer diameter of the box [mm] D sp : Pin seal diameter [mm] D sb : Box seal diameter [mm] δ s : Seal interference amount = D sp -D sb [mm] δth : Thread interference amount [mm] d s : Heat shrinkage of pin seal diameter [mm] d th : Amount of thermal contraction of the outer diameter of the male thread [mm] δ s ’ : Seal interference amount after cooling [mm] δ th ’ : Thread interference after cooling [mm] δ s_act ’ : Actual seal interference after cooling [mm] T L : Temperature of the inner surface of the pin [℃] T s T: Temperature of pin seal surface [℃] th : Outer surface temperature of male screw [℃] T H :Outer surface temperature of the box [℃] α :Thermal expansion coefficient of the tube body [1 / ℃] T at room temperature L =T H = 25℃ After cooling, T L = -80℃, T H =25℃

[0023] The threaded joint according to the second embodiment preferably satisfies the following formula (3).

[0024] Here, T L =T H = 25℃, after cooling T L = -80℃, T H = 25°C, but the room temperature and cooling temperature will vary depending on the environment in which the threaded joint is actually used, so there are no limitations on the temperature that is actually used.

[0025] <Details of the embodiment> Hereinafter, the embodiment will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are designated by the same reference numerals, and the same description will not be repeated.

[0026] 1 , a threaded joint 10 according to an embodiment is composed of a tubular pin 20 and a tubular box 30. The pin 20 is screwed into the box 30 to fasten the pin 20 and the box 30 together. The pin 20 has a pin seal surface 21 and a male thread 22. The box 30 has a box seal surface 31 that corresponds to the pin seal surface 21, and a female thread 32 that corresponds to the male thread 22.

[0027] Consider the thermal contraction when the inner surface of the pin in a CCS well is cooled to a cryogenic temperature. The variables used hereafter are defined as follows: D: Outer diameter of the pipe body [mm] D i : Inner diameter of pipe body [mm] t : Wall thickness of pipe body = (DD i ) / 2 [mm] D BO :Outer diameter of the box [mm] D sp : Pin seal diameter [mm] D sb : Box seal diameter [mm] δ s : Seal interference amount = D sp -D sb [mm] δ th : Thread interference amount [mm] d s : Heat shrinkage of pin seal diameter [mm] d th : Amount of thermal contraction of the outer diameter of the male thread [mm] δ s ’ : Seal interference amount after cooling [mm] δ th ’ : Thread interference after cooling [mm] δ s_act ’ : Actual seal interference after cooling [mm] T L : Inner surface temperature of pin [℃] T s T: Temperature of pin seal surface [℃] th : Outer surface temperature of male screw [℃] T H : Outer surface temperature of the box [℃] α : Thermal expansion coefficient of the pipe body [1 / ℃]

[0028] First, the temperature T H After the threaded joint 10 fastened in the above manner is buried in the CCS well, the inner surface temperature of the pipe body reaches T LAt this time, the outer surface temperature of the box 30 is T H In addition, if we ignore the thermal resistance and heat dissipation at the contact points and assume that the temperature distribution is linear in the radial direction, the outer surface temperature T th The temperature Ts of the pin seal surface 21 is expressed by the following equation.

[0029]

[0030] At this time, if the thermal expansion coefficient of the pin 20 is α [1 / °C], the amount of thermal contraction of the outer diameter of the male screw d th , and the amount of thermal shrinkage of the pin seal diameter d s is expressed by the following formula:

[0031]

[0032] When the inner surface of the pin 20 is cooled and thermally contracted, the thread interference δ th and seal interference amount δ s decreases.

[0033] In the case of a threaded joint in which the thread root surface of the pin and the thread crest surface of the box make interference contact, the amount of thread interference refers to the difference between the diameter of the design male thread root surface before interference at any position in the direction of the pipe axis CL and the diameter of the design female thread crest surface before interference at that position.In the case of a threaded joint in which the thread crest surface of the pin and the thread root surface of the box make interference contact, the amount of thread interference refers to the difference between the diameter of the design male thread crest surface before interference at any position in the direction of the pipe axis CL and the diameter of the design female thread root surface before interference at that position.

[0034] As shown in Figure 2, in the longitudinal section of the threaded joint, the curve of the pin seal surface 21 and the curve of the box seal surface 31 intersect at two points Pi1 and Pi2. The two intersection points Pi1 and Pi2 are connected by a straight line Li. Dsp is the diameter of the design pin seal surface 21 before interference at the midpoint Pi3 of the straight line Li. Dsb is the diameter of the design box seal surface 31 before interference at the midpoint Pi3 of the straight line Li. The amount of seal interference is the difference between Dsp and Dsb.

[0035] As shown in Figure 3, in a longitudinal cross section of a threaded joint in which the pin seal surface 21 and the pin shoulder surface 23 are adjacent, if the curve of the pin seal surface 21 and the curve of the box seal surface 31 intersect at a point Pi2 and the curve of the pin shoulder surface 23 and the curve of the box seal surface 31 intersect at Pi1, the two intersection points Pi1 and Pi2 are connected by a straight line Li. Dsp is the diameter of the design pin seal surface 21 before interference at the midpoint Pi3 of the straight line Li. Dsb is the diameter of the design box seal surface 31 before interference at the midpoint Pi3 of the straight line Li. The amount of seal interference is the difference between Dsp and Dsb.

[0036] Thread interference after heat shrinkage δ th ’ and seal interference amount δ s ’ are expressed by the following formulas, respectively.

[0037]

[0038] <First embodiment> In a threaded joint 10, the greater the seal interference, the greater the contact pressure of the metal seal, improving sealing performance. Furthermore, the internal pressure load imposed on the threaded joint 10 generally increases in proportion to the ratio t / D of the wall thickness to the outer diameter of the pipe body. Here, the seal interference δ after cooling s ’ Consider the parameter divided by t / D.

[0039]

[0040] Seal interference amount after cooling δ s ’ The larger the value of this parameter, and the smaller the internal pressure load on the metal seal, the larger the value of this parameter. Therefore, it is expected that the larger this parameter is, the better the sealing performance will be.

[0041] As will be described in detail in the Examples below, the inventors have conducted research and found that excellent sealing performance can be obtained when this parameter exceeds 4.09.

[0042] <Second embodiment> In a threaded joint 10, the greater the seal interference, the greater the contact pressure of the metal seal, improving sealing performance. In addition, if the threads 22, 32 also have interference, the thread interference causes the diameter of the pin 20 to decrease and the diameter of the box 30 to increase, which reduces the effective seal interference. In other words, when considering the sealing performance of the seal, it is effective to consider the effective interference, which is the seal interference minus the thread interference. Here, the effective seal interference δ after the inner surface of the pin 20 has cooled is s_act ’ is expressed by the following formula:

[0043]

[0044] Furthermore, the pressure load applied to the threaded joint 10 generally increases in proportion to the ratio t / D of the wall thickness to the outer diameter of the pipe body. s_act ’ Consider the parameter divided by t / D.

[0045]

[0046] Seal interference amount after cooling δ s_act ’ The larger the value of this parameter, and the smaller the pressure load applied to the metal seal, the larger the value of this parameter. Therefore, it is expected that the larger this parameter is, the better the sealing performance will be.

[0047] As will be described in detail in the Examples below, the inventors have conducted research and found that excellent sealing performance can be obtained when this parameter is 2.0 or more.

[0048] On the other hand, if the thread interference becomes 0 after cooling, the flow of heat from the high-temperature box 30 to the low-temperature pin 20 through the threads will be small. In other words, the temperature of the pin 20 will be lower than the temperature assumed in equation (1) or (2), which may increase the amount of thermal contraction and further reduce the seal interference after cooling. Therefore, as shown in the following equation, the thread interference δ after thermal contraction th ’ is preferably greater than zero.

[0049]

[0050] Furthermore, the present disclosure is not limited to the above-described embodiments, and various modifications are possible without departing from the scope of the present disclosure.

[0051] In order to confirm the effects of the steel pipe threaded joint according to the present disclosure, a numerical analysis simulation was carried out using the elastic-plastic finite element method.

[0052]

[0053] (1) Steel Pipe Dimensions: As shown in Table 1. (2) Steel Pipe Grade: 25% Cr steel OCTG material with a nominal yield stress of 80 ksi was used. For the finite element analysis, the material was assumed to be an isotropically hardened elastic-plastic body with a yield strength of 80 ksi (551.6 MPa) at Young's modulus E = 210 GPa, Poisson's ratio ν = 0.3, and 0.2% proof stress. (3) Joint Geometry: The pin has a shoulder, nose, metal seal, and male thread from the tip. (4) Thread Geometry: The buttress-type threads are in contact with the thread crest of the box when fully tightened, and the pin's load surface and the box's load surface also come into contact. There are gaps between the thread crest of the pin and the thread root of the box, and between the pin's stabbing surface and the box's stabbing surface. (5) Thermal Expansion Coefficient: 1.31 x 10 -6 1 / ℃ (6) Temperature Condition 1: Outside of box 25℃, inside of pin 25℃ Condition 2: Outside of box 25℃, inside of pin -80℃ The temperature inside of the pin was set to the lowest temperature that could be expected in a CCS well.

[0054] <Evaluation method> After analyzing the screw tightening of each test specimen, a combined load cycle simulating the Series A test of the API 5C5 standard was applied. The seal contact force per unit length in the circumferential direction was used to evaluate sealing performance. A steady-state analysis of thermal-stress coupling was performed for each joint model under the temperature conditions shown in Conditions 1 and 2, and the minimum value of the seal contact force under the load cycle was obtained. The minimum value obtained under Condition 2 (at extremely low temperature) was then divided by Condition 1 (at room temperature) to obtain the seal contact force ratio. The higher this value, the better the sealing performance.

[0055] <Results (First Embodiment)> As shown in FIG. 4, for specimens #1, #4, and #7, the parameter δ s ’ / (t / D) is 4.08 or less, and the seal interference amount δ after thermal contraction of the pin against the applied internal pressure load s ’ Therefore, the sealing performance after cooling was low.

[0056] For specimens #2, #3, #5, and #6, the parameter δ s ’ / (t / D) is greater than 4.08, and the seal interference δ is sufficient against the internal pressure load even after the pin has been thermally shrunk. s ’ Therefore, it had excellent sealing performance after cooling.

[0057] <Results (Second Embodiment)> As shown in FIG. 5, for specimens #1, #4, #6, and #7, the parameter δ s_act ’ / (t / D) is less than 2.0, and the actual seal interference amount δ after thermal contraction of the pin is s_act ’ Therefore, the sealing performance after cooling was low.

[0058] For specimens #2, #3, and #5, the parameter δ s_act ’ / (t / D) is 2.0 or more, and the actual seal interference amount δ is sufficient against the pressure load applied even after the pin has been thermally shrunk. s_act ’ Therefore, it had excellent sealing performance after cooling.

[0059] In specimens #2, #3, and #5, the thread interference was greater than 0 even after cooling, so heat flowed from the high-temperature box to the low-temperature pin, preventing excessive cooling of the pin, and therefore exhibited excellent sealing performance.

[0060] 10: Threaded joint 20: Pin 21: Pin seal surface 22: Male thread 30: Box 31: Box seal surface 32: Female thread

Claims

1. A threaded joint for steel pipes comprising a tubular nipple and a tubular coupling, designed to be screwed together when the nipple is screwed into the coupling, wherein the nipple includes a sealing surface of the nipple and an external thread, the coupling includes a coupling sealing surface corresponding to the sealing surface of the nipple, and an internal thread corresponding to the external thread, in this case, the threaded connection satisfies the following expression (1): [Math. formula 1] (1), Where [Math. formula 2] And D: outer diameter of pipe body [mm] D i : inner diameter of the pipe body [mm] t: pipe body wall thickness=(DD i ) / 2 [mm] D BO : outer diameter of coupling [mm] D sp: nipple seal diameter [mm] D sb : coupling seal diameter [mm] δ s : seal interference value = D sp - D sb [mm] d s : thermal contraction value of the nipple seal diameter [mm] δ s ': seal interference value after cooling [mm] T L : temperature of the inner surface of the nipple [°C] T s : temperature of the nipple sealing surface [°C] T H : temperature of the outer surface of the coupling [°C] α: coefficient of thermal expansion of the pipe body [1 / °C] T L =T H =25°C at room temperature T L =-80°C and T H =25°C after cooling 2. A threaded joint for steel pipes comprising a tubular nipple and a tubular coupling, configured to be screwed together when the nipple is screwed into the coupling, wherein the nipple includes a sealing surface of the nipple and an external thread, the coupling includes a coupling sealing surface corresponding to the sealing surface of the nipple, and an internal thread corresponding to the external thread, where the threaded connection satisfies the following expression (2): [Math. formula 3] (2), Where [Math. formula 4] And D: outer diameter of pipe body [mm] D i : inner diameter of the pipe body [mm] t: pipe body wall thickness=(DD i ) / 2 [mm] D BO : outer diameter of coupling [mm] D sp : nipple seal diameter [mm] D sb: coupling seal diameter [mm] δ s : seal interference value = D sp -D sb [mm] δ th : thread tension value d s : thermal contraction value of the nipple seal diameter [mm] d th : the value of thermal contraction of the diameter of the outer surface of the external thread [mm] δ s ': the value of the seal interference after cooling δ' th : thread tension value after cooling [mm] δ' s_act : actual value of seal interference after cooling [mm] T L : temperature of the inner surface of the nipple [°C] T s : temperature of the nipple sealing surface [°C] T th : external surface temperature of external thread [°C] T H : temperature of the outer surface of the coupling [°C] α: coefficient of thermal expansion of the pipe body [1 / °C] TL =T H =25°C at room temperature T L =-80°C and T H =25°C after cooling 3. A threaded connection according to clause 2, which satisfies the following expression (3): [Math. formula 5] (3).

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