Casing connection structure and casing connection method

Abstract

To provide a casing connection structure and a casing connection method that can prevent the fastening of male and female threads at the connection points between casings from loosening, thereby preventing the borehole using the casing from deviating from the planned drilling path. [Solution] The casing connection structure (10) of the present invention holds a hollow cylindrical member (2: sleeve) inside the portion where the threads (S1) are formed at the above-ground end (1A) of the casing (1), and the sleeve (2) extends at least radially inward from the portion where the threads (S1) of the above-ground end (1A) and the threads (S2) of the subsequent underground end (1B-1) of the casing (1-1) are fastened, in order to improve the rigidity of the portion where the threads (S1) of the above-ground end (1A) and the threads (S2) of the subsequent underground end (1B-1) of the casing (1-1) are fastened.

Description

Technical Field

[0001] The present invention relates to the connection of a casing used in a drilling operation particularly carried out in a narrow place.

Background Art

[0002] In some cases, a casing used in a drilling operation is used by connecting an integral casing provided with a male screw at one end and a female screw at the other end. In the case of connecting integral casings, they are connected by fastening the screw portion at the underground side end of the subsequent casing to the screw portion at the ground side end of the preceding casing. In such a connection of casings, the number of connection points (the fastening points of the male screw and the female screw) increases in proportion to the length of the hole to be drilled (drilling length).

[0003] When carrying out a drilling operation in a narrow environment, it is often necessary to adopt a shortened casing. And when the drilling length is 25 m, even under the same conditions, if a casing with an effective length of 2.5 m is adopted, the number of connection points is 9, but for a casing with a hole length of 1 m, it is 24, for a casing with a hole length of 50 cm, it is 49, and for a casing with a hole length of 25 cm, the number of fastening points is 99. Here, as one of the conditions required for drilling, it is possible to drill along the planned path, in other words, it is possible to drill a borehole along the planned drilling path without so-called "hole deviation". However, it is well known to those skilled in the art that the degree of hole deviation increases in proportion to the number of connection points. Furthermore, depending on the geological layer, there may be significant resistance to drilling by pushing with the bit, reducing the bit's thrust. At that time, a force acts from the surrounding ground to restrain the casing, and the resulting rebound phenomenon makes it easy for gaps to form on the surfaces where the casings engage at the connection point between the male and female threads. This phenomenon of gap formation is one of the causes of loosening at the connection point between the male and female threads of adjacent casings. When the connection point between the male and female threads of adjacent casings loosens, the borehole deviates from the planned path, resulting in what is known as "hole curvature." This type of loosening of the connection point is particularly likely to occur when the above-ground end of the preceding casing is a male thread and the underground end of the following casing is a female thread. Regardless of the drilling length, as the number of connection points in the casing increases, the likelihood of the male and female threads at the connection points loosening naturally increases, leading to greater hole curvature.

[0004] As mentioned above, in response to the gap that occurs between the casing surfaces at the fastening points of the male and female threads due to forces acting from the surrounding ground, it is conceivable that deformation between the areas where the male and female threads are formed can be suppressed by increasing the rigidity of the fastening points of the male and female threads. The wall thickness (radial thickness dimension) at the fastening point between the male and female threads is about half that of other parts. For example, there is a technique that forms male threads at the above-ground end of the preceding casing and the underground end of the succeeding casing, provides a coupling with female threads on the outside of the casing, and screws the female threads of the coupling into the male threads formed at the ends of two adjacent casings, thereby connecting the casings and increasing the rigidity by increasing the radial thickness dimension of the connection point, thereby preventing the fastening between adjacent casings from loosening (so-called "external coupling"). However, the aforementioned coupling protrudes radially outward from the casing, and (couplings that protrude radially outward from the casing) are subjected to resistance from the ground during excavation, which further increases the likelihood of "hole curvature." Therefore, in drilling machines, members that protrude radially outward from the casing should not be used to prevent loosening of the connection points of adjacent casings.

[0005] Other conventional technologies include techniques for recovering casing pipes during the installation of ground anchors (see Patent Document 1). However, this conventional technology (Patent Document 1) is intended to seal the inside of the casing even below the groundwater level, and does not prevent the fastening of the male and female threads of the casing or rod from loosening, or the borehole from bending (deviation) from the planned excavation path. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Patent No. 5414924 [Overview of the project] [Problems that the invention aims to solve]

[0007] This invention was proposed in view of the problems of the prior art described above, and aims to provide a casing connection structure and a casing connection method that can prevent the fastening of male and female threads at the points where casings are connected from loosening, thereby preventing the borehole using the casing from deviating from the planned drilling path. [Means for solving the problem]

[0008] The casing connection structure (10) of the present invention holds a hollow cylindrical member (2: sleeve) inside the portion where a thread (S1) is formed at the above-ground end (1A) of the casing (1), and the hollow cylindrical member (2) extends at least radially inward from the portion where the thread (S1) of the above-ground end (1A) and the thread (S2) of the subsequent underground end (1B-1) of the casing (1-1) are fastened, in order to improve the rigidity of the portion where the thread (S1) of the above-ground end (1A) and the thread (S2) of the subsequent underground end (1B-1) of the casing (1-1) are fastened. In this specification, the symbol "1" for "casing" can refer to casing in general, or to the preceding (underground) casing in Figure 1. When the symbol "1-1" is used, it refers to the subsequent (above-ground) casing in Figure 1.

[0009] The method for connecting casings (1) of the present invention involves preparing casings (1) to which the casing connection structure (casing connection structure of claim 1) is applied, and casings to which the casing connection structure is not applied. In areas where the reaction force of the ground in the region where the casings exist acts in a direction that would release the fastening of the threads on the above-ground end and the threads on the underground end of the subsequent casing, the casings (1) to which the casing connection structure is applied are connected. In areas where such a force does not act, the casings to which the casing connection structure is not applied are fastened together. When connecting two casings (1) to which the above-mentioned casing connection structure is applied, a hollow cylindrical member (2: sleeve) is inserted into the preceding casing (1) and is characterized by extending radially inward the area where at least the threads (S1) of the above-ground end (1A) and the threads (S2) of the underground end (1B-1) of the succeeding casing (1-1) are fastened together.

[0010] This invention can be applied to drilling operations performed in confined spaces. However, it can also be applied to drilling operations performed in non-confined work sites. Furthermore, in this invention, a shortened casing can be used as the casing. However, it can also be applied to the connection of a casing that is not shortened. Furthermore, in the present invention, the casings (1) are connected with a female thread (S2) formed on the underground end (1B) and a male thread (S1) formed on the above-ground end (1A), and the casings are connected by fastening the male thread (S1) on the above-ground end (1A) of the preceding casing (1) to the female thread (S2) on the underground end (1B-1) of the succeeding casing (1-1). However, the present invention can also be applied when the casings (1) are connected with a male thread (S1) formed on the underground end (1B) and a female thread (S2) formed on the above-ground end (1A), and the casings are connected by fastening the female thread (S2) on the above-ground end (1A) of the preceding casing (1) to the male thread (S1) on the underground end (1B-1) of the succeeding casing (1-1). Here, before assembling the hollow cylindrical member (2) (and its outer surface 2D) into the inner wall surface (1C) of the casing (1), it is desirable to reduce the tolerance of these gaps to 0.2 mm or less and to apply water-resistant molybdenum grease to prevent seizing and water ingress. [Effects of the Invention]

[0011] According to the present invention having the above-described configuration, a hollow cylindrical member (2) is arranged radially inward of the male screw (S1) at the above-ground end (1A) of the casing (1). Therefore, by fastening the thread (S1) of the above-ground end (1A) of the preceding casing (1) with the thread (S2) of the underground end (1B-1) of the succeeding casing (1-1), the fastening part becomes a triple structure consisting of the above-ground end (1A) of the preceding casing (1), the underground end (1B-1) of the succeeding casing (1-1), and the hollow cylindrical member (2), thus increasing its rigidity. As a result, even if a force acts to loosen the fastening of the threads (S1, S2) of the preceding casing (1) and the succeeding casing (1-1) due to the reaction force of the ground being drilled, the fastening of the male screw (S1) and the female screw (S2) is not released, and the fastening of the threads is maintained. As a result, the fastening of the screw threads (S1, S2) between the preceding casing (1) and the following casing (1-1) is maintained, preventing hole bending. Here, since the hollow cylindrical member (2) is positioned inside the casing (1), there are no members protruding radially outward from the casing (1), and hole bending caused by resistance from members protruding radially outward from the casing (1) can also be prevented.

[0012] When arranging the hollow cylindrical member (2) inside the casing (1), for example, a slight step (1E) can be provided on the underground side of the inner wall surface (1C) of the casing (1) to prevent the hollow cylindrical member (2) from moving underground, and a snap ring (3) can be fitted to the above-ground end (1) of the casing (1) to prevent the hollow cylindrical member (2) from moving above ground. Here, before assembling the hollow cylindrical member (2) (and its outer surface 2D) into the inner wall surface (1C) of the casing (1), if the tolerance of these gaps is made to 0.2 mm or less and water-resistant molybdenum grease is applied, seizing and water ingress can be prevented. [Brief explanation of the drawing]

[0013] [Figure 1] This is a cross-sectional view showing the structure of the connection point of the casing according to an embodiment of the present invention. [Figure 2] It is a cross-sectional view showing a short casing used in an embodiment. [Figure 3] It is a cross-sectional view showing a sleeve used in an embodiment. [Figure 4] It is a flowchart showing the procedure of a casing connection method according to an embodiment of the present invention.

Mode for Carrying Out the Invention

[0014] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. Here, in the illustrated embodiment, the case of carrying out drilling work at a narrow work site is exemplified. In such a narrow work site, when connecting casings, the end with the female thread of the casing may be used as the underground end, and the end with the male thread may be used as the above-ground end. By doing so, the coupling between the most underground casing and the core can be omitted by fastening the female thread on the underground side of the most underground casing and the male thread on the above-ground end of the core.

[0015] In FIG. 1, in the two connected casings, the casing 1 (underground casing) on the right side in FIG. 1 is the leading casing, and the casing on the left side is the subsequent casing 1-1 (above-ground casing). Each casing 1 is shown in FIG. 2. FIG. 1 shows a casing connection structure composed of the leading casing 1 (underground casing), the subsequent casing 1-1 (above-ground casing), and a sleeve 2 (hollow cylindrical member) inserted at the connection portion of the casings 1 and 1-1, and the connection structure is indicated by reference numeral 10. The connection structure 10 shows the casing connection structure according to the illustrated embodiment in an inclusive manner. The illustrated embodiment exemplifies the case of carrying out drilling work at a narrow work site, and as the casings 1 and 1-1, for example, short casings with a total length of 375 mm and a diameter of φ165.2 mm are used.

[0016] In FIG. 1, both the preceding casing 1 and the subsequent casing 1-1 have the same shape and specifications. In FIGS. 1 and 2, on the outer radius of the ground-side end 1A (the left-side end in FIGS. 1 and 2) of the casings 1 and 1-1, a male thread S1 is formed, and on the inner radius of the underground-side end 1B (the right-side end in FIGS. 1 and 2), a female thread S2 is formed. On the inner peripheral surface of the casings 1 and 1-1, a groove 1F (FIG. 2) for fitting a snap ring 3 (FIG. 1) is formed. Also, on the outer periphery of the casings 1 and 1-1, a groove 1G (or groove 1H) as a wear gauge line is formed. Also, a groove 1H (or groove 1G) is formed on the outer periphery of the casings 1 and 1-1, and the groove 1H (or groove 1G) also functions as a mark indicating that the casing is one to which the connection structure of the illustrated embodiment is applied. On the inner wall 1C (FIGS. 1 and 2) of the casings 1 and 1-1, a shoulder portion 1E (step portion: FIGS. 1 and 2) is formed, which prevents the sleeve 2 from moving underground.

[0017] As described above, in the illustrated embodiment, the end of the casings 1 and 1-1 where the female thread S2 is formed is used as the underground-side end 1B, and the end where the male thread S1 is formed is used as the ground-side end 1A for connection. Therefore, in FIG. 1, the male thread S1 of the ground-side end 1A of the preceding casing 1 is fastened to the female thread S2 of the underground-side end 1B-1 of the subsequent casing 1-1, and thus, the preceding casing 1 and the subsequent casing 1-1 are connected.

[0018] As shown in FIG. 1, inside the radius of the connection part where the male thread S1 of the ground-side end 1A of the preceding casing 1 and the female thread S2 of the underground-side end 1B-1 of the subsequent casing 1-1 are fastened, a sleeve 2 (hollow cylindrical member) separate from the casings 1 and 1-1 is inserted. The structure of the sleeve 3 alone is shown in FIG. 3. In FIG. 1, the sleeve 2 extends at least inside the radius of the region of the perforated length (effective thread length) of the part where the male thread S1 and the female thread S2 are fastened in the connection part.

[0019] In Figure 1, the underground end 2B of sleeve 2 is in contact with the shoulder portion 1E (step portion) formed on the inner wall 1C of casing 1, 1-1, and does not move beyond the shoulder portion 1E into the ground. On the other hand, the above-ground end 2A of sleeve 2 is in contact with the snap ring 3 (Figure 1), and does not move beyond the snap ring 3 into the ground. Therefore, it is prevented that sleeve 3 will move within casing 1, 1-1 in the longitudinal direction (central axis direction: left-right direction in Figure 3). When positioning sleeve 2, it is inserted and positioned radially inward from the male thread S1 at the above-ground end 1A of the preceding casing 1. By fastening the thread S1 of the preceding casing 1 with the thread S2 of the underground end 1B-1 of the succeeding casing 1-1, a triple structure is created at the fastening point, consisting of the above-ground end 1A of the preceding casing 1, the underground end 1B-1 of the succeeding casing 1-1, and sleeve 2, resulting in strong rigidity.

[0020] In Figure 1, two snap rings 3 are fitted into grooves 1F (see also Figure 2) formed near the ground-side end 1A of the casing 1. In the illustrated embodiment, the groove 1F is formed such that its ground-side edge is 13 mm from, for example, the outermost edge 1AA of the ground-side end 1A of the casing 1. If the groove 1F into which the snap ring 3 fits is located too far to the ground (left side in Figure 1), the snap ring 3 may come off due to vibrations when it is struck with a friction packer or the like during drilling. On the other hand, if the groove 1F into which the snap ring 3 fits is positioned too far underground (to the right in Figure 1), the above-ground end 2A of the sleeve 2 will be located underground further than the above-ground end of the fastening point between the male screw S1 and the female screw S2. As a result, the sleeve 2 will not extend radially inward in the region between the above-ground end 2A of the sleeve 2 and the above-ground end of the fastening point between the male screw S1 and the female screw S2. Consequently, the sleeve 2 will not be able to cover the region of the threaded hole length, and a region with a small composite area will exist at the fastening point between the male screw S1 and the female screw S2. In addition, if the position of groove 1F into which the snap ring 3 fits is too far underground, the efficiency of fitting the snap ring 3 into groove 1F will decrease, and if there are many casings 1 and 1-1 to connect, the work time will increase.

[0021] Here, it is preferable to have multiple snap rings 3 fitted into groove 1F rather than just one, and in particular, it is preferable to fit two (double) snap rings into groove 1F as shown in the illustrated embodiment. Here, it is possible to have three or more snap rings 3, but if the number of snap rings 3 becomes too large, the efficiency of fitting the snap rings 3 into groove 1F will decrease, and especially if there are many casings 1, 1-1 to connect, it will cause the work time to increase.

[0022] In Figure 3, the inner diameter D2 of sleeve 2 is set to a size that is necessary and sufficient for inserting and removing the tendon (not shown) of the friction packer. If it is discovered during the connection work of casing 1 that the inner diameter D2 of sleeve 2 is smaller than the size required for inserting the tendon, the inner wall (inner circumferential surface 2C) of sleeve 2 can be machined to enlarge the inner diameter of sleeve 2 to the required size.

[0023] The above-ground end 2A and the underground end 2B of sleeve 2 each have a taper 2T. When using a friction packer during drilling, for example, the friction packer is inserted into and withdrawn from the casing 1. If the above-ground end 2A and the underground end 2B of sleeve 2 do not have a taper 2T, they will form shoulders (steps) against the inner wall 1C of the casing. As a result, when inserting the friction packer, the step formed by the above-ground end 2A of sleeve 2 will hinder insertion, and when withdrawing the friction packer, the step formed by the underground end 2B of sleeve 2 will hinder withdrawal. By providing a taper 2T at the above-ground end 2A and the underground end 2B of sleeve 2, the friction packer can be inserted and withdrawn smoothly. In Figure 3, the taper 2T of sleeve 2 has, for example, a taper angle α = 30°, and the boundary between taper 2T and the inner surface 2C of the sleeve is formed by a circular arc with a radius of curvature R = 50 mm. However, these values ​​can be changed depending on the work content, the equipment used, etc.

[0024] As shown in Figure 1, a sleeve 2 is provided that extends radially inward to at least the perforated length (effective thread length) of the fastened portion between the male thread S1 of the above-ground end 1A of the preceding casing 1 and the female thread S2 of the underground end 1B-1 of the succeeding casing 1-1. Therefore, even if the thickness of the above-ground end 1A of the preceding casing 1 and the underground end 1B-1 of the succeeding casing 1-1 is about half that of the other parts, the radial thickness dimension of the fastened portion between the male thread S1 and the female thread S2 increases by the amount of the sleeve 2, improving rigidity. If the rigidity of the fastening portion between the male screw S1 and the female screw S2 is improved, as shown by the symbol F in Figure 1, even if a force acting in the direction of loosening the fastening between the male screw S1 and the female screw S2 is applied based on the reaction force of the hard ground being drilled, the improved rigidity will prevent deformation of the ends of the casing 1, 1-1 even when the applied force F is applied, and the fastened state of the male screw S1 and the female screw S2 can be maintained. Furthermore, since the loosening of the fastening between the male screw S1 and the female screw S2 is one of the important factors causing "hole bending," according to the illustrated embodiment, "hole bending" can also be prevented. Furthermore, since the sleeve 2 is positioned inside the casing 1, there are no members protruding radially outward from the casing 1, thus preventing hole bending caused by resistance from members protruding radially outward from the casing 1.

[0025] In Figure 1, the sleeve 2 extends underground (to the right in Figure 1) beyond the fastening point between the male thread S1 and the female thread S2. That is, the sleeve 2 is configured to have a length in the longitudinal direction of the casing that is longer than the length required to cover the inside of the area where rigidity is increased. As a result of various studies, the inventor found that if the length of the sleeve 2 is short (for example, about 125 mm), when the friction packer is inserted and the casing 1 is removed, the friction packer will come into contact with the sleeve 2 and move towards the ground. Furthermore, the inventor found that if the length of the sleeve 2 is 200 mm or more, the resistance between the sleeve 2 and the friction packer decreases, and the friction packer does not move towards the ground when the casing 1 is removed. In the illustrated embodiment, if the sleeve length is 200 mm or more, the sleeve 2 extends underground beyond the point where the male thread S1 at the above-ground end 1A of the preceding casing 1 is fastened to the female thread S2 at the underground end 1B-1 of the succeeding casing 1-1. Therefore, the longitudinal length of the sleeve 2 in the casing direction (length in the left-right direction in Figure 1) becomes longer than the longitudinal length of the casing at the point where the male thread S1 at the above-ground end 1A of the preceding casing 1 is fastened to the female thread S2 at the underground end 1B-1 of the succeeding casing 1-1.

[0026] In the illustrated embodiment, when inserting and positioning the sleeve 2 into the casing 1, the tolerance between the inner wall surface 1C of the casing 1 and the outer circumferential surface 2D of the sleeve 2 is set to 0.2 mm or less, and grease (e.g., molybdenum grease) is applied to the outer circumferential surface 2D of the sleeve 2. This prevents foreign matter, water, etc. from entering the boundary with the inner wall surface 1C of the casing, and also prevents the sleeve 2 from digging into various components even if it is inclined with respect to the longitudinal centerline of the casing 1. In addition, seizing can also be prevented. Furthermore, due to the viscosity of the grease, the sleeve 2 is held integrally with the inner wall surface 1C of the casing. Note that the grease is not limited to molybdenum grease. Any grease other than water-soluble grease can be used.

[0027] In Figure 2, two grooves 1G and 1H are provided in the area of ​​the casing 1 near the male thread formation portion, on the underground side. One of the grooves 1G and 1H (for example, groove 1G) is a wear gauge line. When the area of ​​the casing 1 near the male thread formation portion wears down and the wear gauge line (for example, 1G) is about to disappear, the worker can recognize that the casing 1 has worn down and reached its limit of use. When fastening the male thread S1 and the female thread S2, the outer surface of the female thread S2, which is located outside the male thread S1, is in contact with the ground on the outside and is therefore more prone to wear. Thus, the depth dimension of the wear gauge line (for example, 1G) is set to correspond to the wear limit of the female thread S2. The other groove (e.g., groove 1H) is a marker indicating that the casing 1 is to which the connection structure according to the illustrated embodiment is applied. When implementing the illustrated embodiment, it is not necessary for all casings used to be casings 1 to which the connection structure according to the embodiment is applied. Both casings to which the connection structure according to the illustrated embodiment is applied and casings to which it is not applied are used. The operator can recognize that a casing with two grooves (1G and 1H) is casing 1 to which the connection structure according to the illustrated embodiment is applied, and that a casing with only one groove (1G or 1H) is casing to which the connection structure according to the illustrated embodiment is not applied. As a result, it is possible to easily and accurately distinguish between casing 1 to which the connection structure according to the illustrated embodiment is applied and casing to which it is not applied.

[0028] Referring to Figure 4, the connection method according to the illustrated embodiment will be described. When connecting casing 1 using the casing connection structure shown in Figure 1, as described above, it is not necessary for all casings used to be casing 1 to which the connection structure according to the embodiment is applied. For example, in areas where a force acts in a direction that would release the fastening between the thread S1 at the above-ground end 1A of casing 1 and the thread S2 at the underground end 1B-1 of the subsequent casing 1-1 due to the reaction force of the ground, that is, in areas where loosening of the connection and bending of the hole are expected, casings 1 should be connected using the connection structure according to the illustrated embodiment. On the other hand, in areas where such forces do not act, that is, in areas where loosening of the connection part or bending of the hole is not expected, it is not necessary to apply the connection structure according to the illustrated embodiment, and it is sufficient to use a normal casing that does not apply the illustrated connection structure.

[0029] In step S1 of Figure 4, a casing 1 with the connection structure according to the illustrated embodiment and a casing without the connection structure according to the illustrated embodiment are prepared, and the areas where the casing 1 with the connection structure according to the illustrated embodiment should be used (for example, the area near the drilling bit) and areas where the casing with the connection structure according to the illustrated embodiment does not need to be used (for example, the area close to the ground) are predicted. In doing so, the length of the borehole, the casing specifications (total length, etc.), and other conditions are taken into consideration when making the prediction. Then, for each of the casings to be connected, we broadly categorize whether to use casing 1 to which the connection structure according to the illustrated embodiment is applied, or to use a casing to which the connection structure according to the illustrated embodiment is not applied. As described above, in the case of casing 1 to which the connection structure according to the illustrated embodiment is applied, two grooves 1G and 1H (Figure 2) are formed in the area of ​​casing 1 near the male threaded portion and on the underground side. In the case of a normal casing to which the connection structure according to the illustrated embodiment is not applied, only one of grooves 1G and 1H is provided. Therefore, workers can easily and accurately distinguish between the two.

[0030] In the next step S2, during the casing connection work, it is determined whether or not it is necessary to use casing 1 to which the connection structure according to the illustrated embodiment is applied (based on the result of step S1) for each casing to be connected. If the determination in step S2 indicates that it is necessary to use the casing 1 to which the connection structure according to the illustrated embodiment is applied (step S2 is "Yes"), proceed to step S3. If it is not necessary to use the casing 1 to which the connection structure according to the illustrated embodiment is applied (step S2 is "No"), proceed to step S4.

[0031] In step S3 (when it is necessary to use casing 1 to which the connection structure according to the illustrated embodiment is applied), the casings 1 are connected to each other using casing 1 to which the connection structure according to the illustrated embodiment is applied. When connecting two casings 1 to which the connection structure according to the illustrated embodiment is applied, a sleeve 2 is inserted into the preceding casing 1, and the underground end of the sleeve 2 is brought into contact with a shoulder portion 1E (step portion) provided on the underground side of the inner wall 1C of the casing. Then, a snap ring 3 is fitted into a groove 1F provided on the above-ground end 1A of the casing 1 to hold the sleeve 2 in the longitudinal direction of the casing 1. In this configuration, a sleeve 2 is integrally positioned radially inward of the male thread S1 at the above-ground end 1A of the casing 1. Then, in a conventionally known manner, the female thread S2 at the underground end 1B-1 of the subsequent casing 1-1 is fastened to the male thread S1 at the above-ground end 1A of the preceding casing 1. As a result, the fastening portion between the male thread S1 and the female thread S2 has a triple structure in its radial direction, consisting of the above-ground end 1A of the preceding casing 1, the underground end 1B-1 of the subsequent casing 1-1, and the sleeve 2, which increases rigidity and makes it less susceptible to deformation. Therefore, even if a reaction force from the ground acts, the male thread S1 and the female thread S2 are prevented from loosening, and hole bending is prevented. Therefore, even if a force indicated by symbol F in Figure 1 (a force that loosens the fastening between the male screw S1 and the female screw S2) acts due to the reaction force of the excavated ground, the fastening between the male screw S1 and the female screw S2 is maintained, and as a result of maintaining the fastening between the male screw S1 and the female screw S2, hole bending is also prevented. Then proceed to step S5.

[0032] On the other hand, in step S4 (where it is not necessary to use casing 1 to which the connection structure according to the illustrated embodiment is applied), a normal casing is used as the casing to be connected, and the casings are connected to each other. Then proceed to step S5.

[0033] Step S5 determines whether the connection of the casing used in the excavation work has been completed. If, as a result of the determination in step S5, all connections of casing 1 are completed (step S5 is "Yes"), the connection method according to the illustrated embodiment is terminated. On the other hand, if the casing connection is not complete (step S5 is "No"), return to step S2 and continue with the connection method according to the illustrated embodiment.

[0034] The illustrated embodiment describes a case in which shortened casings are connected to perform drilling work in a confined work site, with the end of the casing with female threads facing underground and the end with male threads facing above ground, and the shortened casings connected to each other. However, this invention is not limited to drilling work in confined work sites; it can also be used to connect casings other than shortened casings. Furthermore, it can be applied when connecting casings with the female threaded end facing above ground and the male threaded end facing underground.

[0035] The illustrated embodiments are for illustrative purposes only and are not intended to limit the technical scope of the present invention. [Explanation of symbols]

[0036] 1. Casing (preceding casing) 1-1... Subsequent casing 1A... Ground-side end of the casing 1B...Underground end of the casing 1C...Inner wall surface of the casing 2. Sleeve (hollow cylindrical member) 2A... Ground-side end of the sleeve 2B...Underground end of sleeve 2C...Inner wall (inner surface) of the sleeve 2D... Outer surface of the sleeve 3. Snap ring 10. Casing connection structure S1... Screw thread (male screw) S2... Screw thread (female thread)

Claims

1. A casing connection structure characterized in that a hollow cylindrical member is held inside the portion where the threads are formed at the above-ground end of the casing, and the hollow cylindrical member extends at least radially inward from the portion where the threads at the above-ground end and the threads at the subsequent underground end of the casing are fastened, in order to improve the rigidity of the portion where the threads at the above-ground end and the threads at the subsequent underground end of the casing are fastened.

2. Prepare a casing to which the casing connection structure described in claim 1 is applied, and a casing to which the casing connection structure described in claim 1 is not applied. If the reaction force of the ground in the area where the casings are located acts in a direction that would release the fastening of the threads on the above-ground end and the threads on the underground end of the subsequent casing, connect the casings to which the casing connection structure is applied. If no such force acts in the area, fasten the casings to which the casing connection structure is not applied together. A method for connecting casings to which the above-mentioned casing connection structure is applied, characterized in that a hollow cylindrical member is inserted into the preceding casing and extends radially inward to the area where at least the threads on the above-ground end and the threads on the underground end of the succeeding casing are fastened together.

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