Patent Publication Number: US-10767446-B2

Title: Float valve sub

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage of International Application No. PCT/JP2016/071390 filed Jul. 21, 2016, claiming priority based on Japanese Patent Application No. 2015-144300 filed Jul. 21, 2015, the contents of all of which are incorporated herein by reference in their entirety. 
     TECHNICAL FIELD 
     The present invention relates to a float valve sub used for a drill string, and especially to a float valve sub in which a float valve assembly is detachably attached to the inside of bottomhole assembly. 
     BACKGROUND ART 
     The technique is known that collects a sample core of stratum. For example, in the technique, a drill bit is provided onto the circumference at the end of a cylindrical structure which is called a drill string. The drill string is rotated to invade the inside of stratum. After that, the drill string is extracted from the stratum, and a stratum sample of a column-like shape is collected from the inside of drill string. It becomes possible to know the physical characteristics such as the structure and space percentage of the stratum in detail by analyzing the stratum sample collected in this way. For example, this technique is expected to contribute to the oil and gas layer evaluation and the research of earthquake. 
     When the stratum is drilled with the drill string, there is a possibility that the phenomenon called kick or blowout occurs by stratum fluid flowing into a well. For example, when there are liquid layers such as an underground water vein and an oil and gas layer in a region where the tip of the drill string has reached, a possibility could be considered in which the stratum fluid flows backward from the drill bit depending on pore pressure in the stratum, passes through the inside of drill string, and blows out onto a ship or to the ground, so as to make the continuation of drilling difficult. When such kick and blowout seem to occur, it is desirable to take a well control measure in which a blowout preventing device is provided inside the drill string previously to prevent the kick and the blowout appropriately. 
     In relation to the above, inventions of a flapper-type float valve are disclosed in Patent Literature 1 and Patent Literature 2. In these inventions, a float valve using a flapper-type lid is provided inside the drill string in any case. 
     CITATION LIST 
     Patent Literature 
     
         
         [Patent Literature 1] U.S. Pat. No. 2,162,578 
         [Patent Literature 2] U.S. Pat. No. 3,066,693 
       
    
     SUMMARY OF THE INVENTION 
     From the viewpoint of drilling work, it is desirable that the outer diameter of the drill string should be smaller. On the other hand, from the viewpoint of stratum analysis, it is desirable the inner diameter of the drill string should be larger to collect a stratum sample with a larger diameter. However, in the flapper-type float valve in the prior art, the flapper-type lid is provided inside the drill string so that a ratio of the minimum inner diameter of the float valve to the maximum outer diameter of a part of the drill string where the float valve is installed decreases significantly. This problem becomes more conspicuous in the drill string of a 2-layer structure to be mentioned later. Other subject matters and new features will become clear from the description of this Specification and the attached drawings. 
     According to one embodiment, a float valve sub  2  includes: an outer barrel assembly and a float valve assembly. Here, the float valve assembly is arranged detachably in the inside of outer barrel assembly. The float valve assembly includes: a first end section, a second end section, a float valve middle section, and a lid section. Here, each of the first end section and the second end section has a cylindrical shape. The float valve middle section is arranged between the first end section and the second end section. The lid section is attached to the first end section, and moves turnably between a first position and a second position. Here, the lid section closes a passage of the first end section in the first position and opens a float valve first opening section as the passage of the first end section in the second position. The float valve middle section has a side opening through which a part of the lid section is possible to pass. The outer barrel assembly includes: a first section, a second section, and an outer barrel middle section. 
     Here, the first section has an inner circumference surface complementary to an outer circumference surface of the first end section to receive the first end section. The second section has an inner circumference surface complementary to an outer circumference surface of the second end section to receive the second end section. The outer barrel middle section is arranged between the first section and the second section. The outer barrel middle section has a concave section possible to receive the lid section in the second position. A minimum inner diameter of the first section is larger than that of the second section, and an inner diameter of the concave section is larger than the minimum inner diameter of the first section. 
     According to the one embodiment, when the flapper-type float valve is installed in the drill string, the ratio of the minimum inner diameter of the float valve to the maximum outside diameter of the part of the drill string where the float valve is installed can be improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing a configuration example of a coring system (a kind of bottomhole assembly) which drills the seafloor. 
         FIG. 2A  is a partial sectional view showing a configuration example of a core barrel provided for a distal end of the drill string used in a wire line recovery system. 
         FIG. 2B  is a sectional view showing a configuration example of an outer barrel section of the core barrel shown in  FIG. 2A . 
         FIG. 2C  is a sectional view showing a configuration example of an inner barrel section of the core barrel shown in  FIG. 2A  in detail. 
         FIG. 3A  is a diagram showing a first step of an example of core collecting technique using the core barrel shown in  FIG. 2A  to  FIG. 2C . 
         FIG. 3B  is a diagram showing a second step of the example of core collecting technique. 
         FIG. 3C  is a diagram showing a third step of the example of core collecting technique. 
         FIG. 4  is a diagram showing an example of core collecting technique in which a casing pipe is combined with the drill string shown in  FIG. 2A  to  FIG. 2C . 
         FIG. 5A  is a sectional view showing a configuration example of the core barrel using a float valve sub according to one embodiment. 
         FIG. 5B  is a partial sectional view showing a connection relation of the float valve sub according to the embodiment with another sub. 
         FIG. 6  is a sectional view showing a configuration example of an outer barrel assembly of the float valve sub according to the embodiment. 
         FIG. 7A  is a diagram showing a state of a configuration example of a float valve assembly according to the embodiment when a flapper lid is in a first position. 
         FIG. 7B  is a diagram showing a state of the configuration example of the float valve assembly according to the embodiment when the flapper lid is in a second position. 
         FIG. 7C  is a side view of the float valve assembly in the state shown in  FIG. 7B . 
         FIG. 8A  is a diagram showing the flapper lid of the float valve assembly shown in  FIG. 7A  and  FIG. 7B . 
         FIG. 8B  is a sectional view of the flapper lid shown in  FIG. 8A  along the sectional line A-A. 
         FIG. 8C  is a sectional view of the flapper lid shown in  FIG. 8A  along the sectional line B-B. 
         FIG. 9  is a diagram showing a geometrical relation between the inner diameter of an annular concave section according to the embodiment and the outer diameter of a float valve first opening section. 
         FIG. 10  is a diagram showing a configuration example of the coring system using a riser drilling system. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Embodiments of a float valve sub will be described below with reference to the attached drawings. 
     A coring technique to collect a sample from the strata of seafloor will be described as an example.  FIG. 1  is a diagram showing a configuration example of a coring system which drills the seafloor. Here, the coring system is a kind of bottomhole assembly  1 . In an example shown in  FIG. 1 , first, a drilling rig  12  is prepared on the sea surface  14  straightly above the seafloor  15  to be drilled. It is desirable that the drilling rig  12  continues to keep the position the drilling rig  12  straightly above a drilling position by using GPS (global positioning system) satellite  18  and so on. The drilling rig  12  is used to repeat a process of elongating a drill string  11  and listing down the elongated drill string  11  into the sea  13 . When a tip of the drill string  11  (to be referred to as a drilling hole  17 ) reaches the stratum  16  of seafloor  15 , the drilling rig  12  controls the drill string  11  to collect a stratum sample core from the stratum  16  of seafloor  15 . 
     However, there is a case where the distance from the sea surface  14  to the seafloor  15  is thousands of meters. In such a case, if the whole of drill string  11  is lifted up and down every time one core is collected, the work efficiency is very low. Therefore, a technique is known in which the cores are continuously recovered in the drilling rig  12  by use of an inner barrel section inserted inside an outer barrel section of the drill string  11  without lifting up the drill string  11  after the drilling by use of the drill string  11  is once started. As one of such techniques, a wire line recovery system is known. 
     Note that the coring system shown in  FIG. 1  is usable in the stratum drilling on a land in addition to the seafloor drilling. 
       FIG. 2A  is a sectional view showing a configuration example of a core barrel  10  which is provided in a distal end of the drill string  11  used in the wire line recovery system. The core barrel  10  shown in  FIG. 2A  has a 2-layer structure, and an outer barrel section  30  is provided outside and an inner barrel section  50  is provided inside. The outer barrel section  30  has a cylindrical shape, and the inner barrel section  50  is movable inside the outer barrel section  30  in a longitudinal direction of the core barrel  10  (the direction of ±Z in the coordinates shown in  FIG. 2A ) in the inside of outer barrel section. 
     Note that in  FIG. 2A , a sectional view of the outer barrel section  30  is shown and a side view of the inner barrel section  50  is shown. The outer barrel section  30  is provided in the distal end of the drill string  11 . 
       FIG. 2B  is a sectional view showing a configuration example of the outer barrel section  30  of the core barrel  10  shown in  FIG. 2A . The outer barrel section  30  shown in  FIG. 2B  has a core bit  31 , a near bit sub  32 , a drill collar sub  33 , a landing sub  34 , a head sub  35 , a landing ring  36  and a latching section  37 . 
     The core bit  31  is provided in the distal end of the outer barrel section  30 . The near bit sub  32  is connected with an upper end section of the core bit  31 . The drill collar sub  33  is connected with an upper end section of the near bit sub  32 . The landing sub  34  is connected with an upper end section of the drill collar sub  33 . The head sub  35  is connected with an upper end section of the landing sub  34 . The landing ring  36  is provided near an upper end opening section on an inner wall of the drill collar sub  33 . The latching section  37  contains a space formed by boring an inner wall of the landing sub  34 . 
       FIG. 2C  is a diagram showing a configuration example of the inner barrel section  50  of the core barrel  10  shown in  FIG. 2A  in detail. The inner barrel section  50  shown in  FIG. 2C  has an inner tube  51 , a core liner  52 , a length control mechanism  53 , a swivel mechanism  54 , a landing mechanism  55 , a latching mechanism  56  and a fishing neck  57 . Here, the length control mechanism  53  has lock bolts  58 . 
     The inner tube  51  is provided in the distal end of the inner barrel section  50 . The core liner  52  is provided inside the inner tube  51 . The length control mechanism  53  is provided on an upper-end side from the inner tube  51 . The swivel mechanism  54  is provided on an upper-end side from the length control mechanism  53 . The landing mechanism  55  is provided on an upper-end side of the swivel mechanism  54 . The latching mechanism  56  is provided on an upper-end side from the landing mechanism  55 . The fishing neck  57  is provided on an upper-end side from the latching mechanism  56 . 
     The fishing neck  57  is used to detachably connect the inner barrel section  50  with a wire line extending from the drilling rig  12 . The drilling rig  12  controls the wire line to be attached to or detached from the fishing neck  57 . 
     The latching mechanism  56  is engaged with the latching section  37  to fix the inner barrel section  50  to the outer barrel section  30 . When the latching mechanism  56  is fixed to the outer barrel section  30 , the rotation of the outer barrel section  30  is transferred to the inner barrel section  50 . Also, the engagement of the latching mechanism  56  with the latching section  37  is canceled when the inner barrel section  50  is to be recovered to the drilling rig  12  through the wire line. 
     The landing mechanism  55  controls a position relation in the longitudinal direction of the drill string  11  between the inner barrel section  50  and the outer barrel section  30 . In an example shown in  FIG. 2C , the outer diameter of the landing mechanism  55  is larger than the inner diameter of the landing ring  36 . The position relation between the inner barrel section  50  and the outer barrel section  30  may be determined by putting a lower surface of the landing mechanism  55  on the upper surface of the landing ring  36 , when the inner barrel section  50  is lifted down from the drilling rig  12  to the end section of the outer barrel section  30 . 
     The swivel mechanism  54  is provided to prevent the core liner  52  arranged in a distal end of the swivel mechanism from being rotated following the rotation of the outer barrel section  30  so that the core on the way of collection is not twisted to the drilled stratum. In an example shown in  FIG. 2C , the swivel mechanism  54  has an outer section connected to an upper end side of the inner barrel section  50 , an inner section connected to a lower end side thereof and a bearing provided between the outer section and the inner section. The rotation of the outer barrel section  30  is not transferred to the components of the inner barrel section  50  which are arranged on the lower side from the swivel mechanism  54 . 
     The length control mechanism  53  is provided to control the full length of the inner barrel section  50 . The length control mechanism  53  has an inner section connected with an upper-end side of the inner barrel section  50 , an outer section connected with a lower-end side thereof and lock bolts  58  connecting the inner section and the outer section. Each of the inner section and the outer section has a plurality of holes through which the lock bolts  58  pass. The length of inner barrel section  50  can be controlled by selecting the holes through which the lock bolts  58  pass suitably, in each of the inner section and the outer section of the length control mechanism  53 . 
     The inner tube  51  supports the core liner  52  in its inside. The core liner  52  stores the collected core. It is desirable that the inner tube  51  has a core catcher and a core lifter which are not illustrated. Here, the core catcher and the core lifter separate the core to be collected from the stratum. Also, they support the core detached from the stratum  16  to prevent the core from falling down. Note that any one of the core catcher and the core lifter may be used. 
     Note that the outer barrel section  30  and the inner barrel section  50  shown in  FIG. 2A  to  FIG. 2C  are called a rotary core barrel, and is used when the stratum  16  to be collected is comparatively hard. When the stratum  16  to be collected is comparatively soft, the inner barrel section  50  having another configuration may be used. 
       FIG. 3A  to  FIG. 3C  are diagrams showing steps of an example of collecting the core by using the drill string  11  shown in  FIG. 2A  to  FIG. 2C . 
     At a first step shown in  FIG. 3A , the drill string  11  is extended toward the stratum  16  for a sample to be collected, and the inner barrel section  50  is lifted down toward the outer barrel section  30  through the inside of drill string  11 . At this time, it is desirable that an opening of core bit  31  provided in a tip of outer barrel section  30  and an opening of core liner  52  provided in a tip of inner barrel section  50  overlap in the drilling proceeding direction of the drill string  11 . Note that the drilling proceeding direction of the drill string  11  coincides with the longitudinal direction of the drill string  11 , and also coincides with the direction of rotation axis of the drill string  11 . 
     At a second step shown in  FIG. 3B , the drill string  11  is rotated around the rotation axis and drills the stratum  16 . At this time, a part of the stratum  16  is supplied to the inside of core liner  52  through the opening of core bit  31 . 
     At a third step shown in  FIG. 3C , the drilling rig  12  extends the wire line into the inside of outer barrel section  30  so as to be connected with the fishing neck  57  of the inner barrel section  50 , and to lift up the inner barrel section  50  together with the wire line. At this time, the part of stratum  16  stored in the core liner  52  is separated from the stratum  16  and supported by the core catcher and the core lifter and then is collected as a core  40  of a stratum sample. 
     After that, after the core  40  is taken out from the inner barrel section  50  which has been lifted up to the drilling rig  12 , the steps from the first step to the third step are repeated. In this way, the cores  40  can be continuously recovered without lifting up the drill string  11  which contains the outer barrel section  30 . 
     A technique using a casing pipe is known to carry out the drilling more deeply. When the stratum  16  is drilled by use of the drill string  11 , there is a possibility that the stratum surrounding the drilling hole  17  collapses so that the rotation of the drill string  11  and the drilling are hindered and the continuation of the drilling becomes difficult. To prevent these situations, it could be considered that the inner wall of the drilling hole  17  is reinforced with the casing pipe protecting the drill string  11 , after the drilling to a depth of some degree is carried out. 
     Because the outer diameter of the casing pipe allowing insertion into the drilling hole  17  is equal to the outer diameter of the core bit  31  having drilled this drilling hole  17 , another core bit  31  with the smaller outer diameter becomes necessary to further drill the drilling hole  17  reinforced with the casing pipe. Also, the drilling hole  17  which has been drilled with the core bit  31  with the smaller outer diameter is reinforced with another thinner casing pipe. The deeper drilling becomes possible by repeating such steps. 
       FIG. 4  is a partial sectional view showing an example of the coring technique in which the casing pipe and the drill string  11  shown in  FIG. 2A  to  FIG. 2C  are combined. 
     The sectional view shown in  FIG. 4  contains the stratum  16 , the drill string  11 , a first casing pipe  71 , a second casing pipe  72 , a third casing pipe  73  and a fourth casing pipe  74 . 
     The first casing pipe  71  to the fourth casing pipe  74  are structures having a circular cylinder shape different from each other in thickness and length. The outer diameter of the first casing pipe  71  is the thickest, the outer diameter of the second casing pipe  72  is next thicker, the outer diameter of the third casing pipe  73  is next thicker, and the outer diameter of the fourth casing pipe  74  is the thinnest. Also, the first casing pipe  71  is the shortest, the second casing pipe  72  is next shorter, the third casing pipe  73  is next shorter, and the fourth casing pipe  74  is the longest. 
     The first casing pipe  71  to the fourth casing pipe  74  are arranged concentrically when seeing from a directly upper position of the drilling hole  17 , and are buried in the stratum  16 . The upper end of each of the first casing pipe  71  to the fourth casing pipe  74  may be situated on the surface of the stratum  16 . 
     The drilling depth possible to drill is improved by using a plurality of casing pipes although the thickness (the outer diameter) of usable core bit  31  become thinner in a step-by-step manner. Therefore, the outer diameter and inner diameter of usable drill string  11  become smaller in the step-by-step manner. 
     Moreover, when a float valve is provided inside the drill string  11  to prevent a blowout, a partial inner diameter of the drill string  11  is decreased more. Therefore, a ratio of the inner diameter to the outer diameter in the drill string  11  is decreased more. 
     First Embodiment 
     In a first embodiment, a structure is proposed in which the decrease of the ratio of the inner diameter to the outer diameter can be restrained even if the float valve is provided inside the drill string. 
       FIG. 5A  is a sectional view showing a configuration example of the outer barrel section  30  which uses the float valve sub according to this embodiment. The structure of the outer barrel section  30  shown in  FIG. 5A  is the same as the structure in which the float valve sub  2  according to this embodiment is added to the outer barrel section  30  shown in  FIG. 2A  to  FIG. 2C . The float valve sub  2  is arranged between the drill collar sub  33  and the near bit sub  32 , and the inner tube  51  passes inside the float valve sub  2 . 
     Note that the float valve sub  2  according to this embodiment may be arranged in another position of the outer barrel section  30 . For example, the float valve sub  2  according to this embodiment may be arranged between the core bit  31  and the near bit sub  32 . Or, the float valve sub  2  may be arranged between the landing sub  34  and the drill collar sub  33 . As a further modification example, the float valve sub  2  according to this embodiment can be provided for a rotary core barrel of a so-called conventional type in which the inner tube  51  is not removed. 
     The other components contained in the outer barrel section  30  shown in  FIG. 5A  are same as those in case of  FIG. 2A  to  FIG. 2C . Therefore, further detailed description is omitted. 
     Note that it is desirable that the shape of each of the subs including the float valve sub  2  has a rotation symmetry as high as possible with respect to the rotation axis of the drill string  11  in order for the drill string  11  to rotate stably. Also, it is desirable that each sub has a higher rotation symmetry, if possible, to realize the shaping and processing more preciously and more easily. For these reasons, the rotating bodies such as a circle, a disk, a column, and a circular cylinder appear in various portions of the following description. Here, these rotating bodies are not limited to the circle, the disk, the column, the circular cylinder and so on which are strictly geometrically defined. These rotating bodies may contain modifications in actual ranges of an extent not hinder the stable rotation of the drill string  1 , and the assembling of subs and so on. 
       FIG. 5B  is a partial sectional view showing connection relation of the float valve sub  2  according to the present invention to another sub. 
     The float valve sub  2  shown in  FIG. 5B  has an outer barrel assembly  100  as an outer cylinder assembly and a float valve assembly  200 . The float valve assembly  200  is arranged inside the outer barrel assembly  100 .  FIG. 5B  is a sectional view of the outer barrel assembly  100 . 
     The float valve sub  2  shown in  FIG. 5B  is connected on its upper-end side with an upper side sub  300 . Also, the float valve sub  2  shown in  FIG. 5B  is connected on its lower-end side with a lower side sub  500 . It is desirable to use tapered screws which are excellent in water-tightness, for the connection of the upper side sub  300  and the float valve sub  2  and the connection of the float valve sub  2  and the lower side sub  500 . 
       FIG. 6  is a sectional view showing a configuration example of the outer barrel assembly  100  of the float valve sub  2  according to this embodiment. 
     It is desirable that the outer barrel assembly  100  as an outer cylinder assembly is formed of a single member from the viewpoint of strength and water-tightness. The outer barrel assembly  100  has an outer barrel first section  110 , an outer barrel second section  120  and an outer barrel middle section  130 . 
     The outer barrel first section  110  is a proximal end of the outer barrel assembly  100 . The outer barrel second section  120  is a distal end of the outer barrel assembly  100 . The outer barrel middle section  130  is arranged between the outer barrel first section  110  and the outer barrel second section  120 . 
     The outer barrel first section  110  has an outer barrel first connection section  111  and an outer barrel first receiving section  112 . Also, a space inside the outer barrel first section  110  is called an outer barrel first opening section  101 . 
     The outer barrel first connection section  111  is connected with a lower-end-side connection section of the upper side sub  300 . In the configuration example shown in  FIG. 5B  and  FIG. 6 , a tapered female screw is formed inside the outer barrel first connection section  111 , and is engaged with a tapered male screw formed outside the lower-end-side connection section of the upper side sub  300 . 
     The inner circumference surface of the outer barrel first receiving section  112  has a complementary shape to the upper-side end section (an outer circumference surface of the upper-side end section) of the float valve assembly  200 , and receives and supports the upper-side end section of the float valve assembly  200 . Note that the outer barrel first receiving section  112  has a shape by which the whole float valve assembly  200  can pass to an outer barrel second receiving section  122  when the float valve assembly  200  is attached to the outer barrel assembly  100 . 
     Similarly, the outer barrel second section  120  has an outer barrel second connection section  121  and an outer barrel second receiving section  122 . Also, a space inside the outer barrel second section  120  is called an outer barrel second opening section  102 . 
     The outer barrel second connection section  121  is connected with the upper-end-side connection section of the lower side sub  500 . In the configuration example shown in  FIG. 5B  and  FIG. 6 , a tapered male screw is formed outside the outer barrel second connection section  121 , and is engaged with a tapered female screw formed inside an upper-end-side connection section of the lower side sub  500 . 
     The inner circumference surface of the outer barrel second receiving section  122  has a shape which is complementary to the lower-side end section of the float valve assembly  200  (the outer circumference surface of the lower-side end section), and receives and supports the lower-side end section of the float valve assembly  200 . Note that the outer barrel second receiving section  122  has a shape by which the float valve assembly  200  does not fall on the side of the lower side sub of the outer barrel assembly  100 . As an example of such a shape, in a configuration example shown in  FIG. 6 , a minimum inner diameter DO 2  of the outer barrel second section  120  is smaller than the minimum inner diameter DO 1  of the outer barrel first section  110 . Note that this minimum inner diameter DO 2  is smaller than the maximum outer diameter of the float valve assembly  200  although the inner tube  51  can pass through the inside of the outer barrel second section  120 . 
     There is a space to receive the float valve assembly  200  inside the outer barrel middle section  130 . The outer barrel first opening section  101  is connected with the upper-side of this space. The outer barrel second opening section  102  is connected with the lower-side of this space. There is an annular concave section  103  on the outer circumference surface of this space and on the inner circumference surface of the outer barrel middle section  130 . The annular concave section  103  may be formed by boring the inner wall of the outer barrel assembly  100 . The annular concave section  103  is provided to receive a flapper lid which protrudes out of a cylindrical shape section of the float valve assembly  200  as mentioned later. The inner diameter DO 3  of the annular concave section  103  is larger than the minimum inner diameter DO 1  of the outer barrel first section  110 . 
     The inner diameter of the upper side of the outer barrel middle section  130  in the boundary with the outer barrel first section  110  is equal to the inner diameter D 01  of the outer barrel first receiving section  112 . The inner diameter DO 3  of the annular concave section  103  of the outer barrel middle section  130  is larger than the inner diameter DO 1  of the outer barrel first receiving section  112 . Here, there may be a region where the inner diameter continuously changes from DO 1  to DO 3  in the upper-side inner circumference of the outer barrel middle section  130 . 
     Similarly, the inner diameter of the lower-side from the outer barrel middle section  130  in a boundary with the outer barrel second section  120  is equal to the inner diameter D 01  of the outer barrel second receiving section  122 . The inner diameter DO 3  of the annular concave section  103  of the outer barrel middle section  130  is larger than the inner diameter DO 1  of the outer barrel second receiving section  122 . Here, there may be a region where the inner diameter changes continuously from DO 3  to DO 1  in the lower-side inner circumference surface of the outer barrel middle section  130 . 
     Note that in this embodiment, the inner diameter of the lower-side of the outer barrel middle section  130  in the boundary with the outer barrel second section  120  is equal to the inner diameter D 01  of the outer barrel first receiving section  112 . However, the former can be made smaller than latter. 
     It is assumed that the minimum thickness of the drill pipe of the drill string  11  is Tmin. Here, the drill pipe is an outer wall portion of the drill string  11  which is on the side of the drilling rig  12  from the rotary core barrel, and has a function of transferring a rotation motion to the rotary core barrel from the drilling rig  12  and so on. Because the outer diameter of the outer barrel assembly  100  is generally larger than the outer diameter of the drill pipe, the strength which is required to the outer barrel assembly  100  as a part of the structure configuring the drill string  11  is secured even in any portion of the outer barrel assembly  100  if the thickness is above the minimum thickness Tmin. Therefore, it is desirable that the thickness T is above the minimum thickness Tmin, assuming that the thickness of annular concave section  103  of the side wall of the outer barrel assembly  100  is T. Note that when a value above the minimum thickness Tmin cannot be secured as the thickness T, the material of the outer barrel assembly  100  may be changed to a stronger material. 
       FIG. 7A  is a diagram showing the state of a configuration example of the float valve assembly  200  according to this embodiment when a flapper lid  230  is in a first position.  FIG. 7B  is a diagram showing a state of the configuration example of the float valve assembly  200  according to this embodiment when the flapper lid  230  is in a second position.  FIG. 7C  is a side view of the float valve assembly  200  in the state shown in  FIG. 7B . To describe the inside of float valve assembly  200 , a part of the outer wall is shown as a sectional view in  FIG. 7A  and  FIG. 7B . 
       FIG. 8A  is a diagram showing the flapper lid  230  of the float valve assembly  200  shown in  FIG. 7A  to  FIG. 7C .  FIG. 8B  is a sectional view of the flapper lid  230  shown in  FIG. 8A  along the line A-A.  FIG. 8C  is a sectional view of the flapper lid  230  shown in  FIG. 8A  along the line B-B. 
     The float valve assembly  200  shown in  FIG. 7A  to  FIG. 7C  has a float valve body  210 , the flapper lid  230 , a hinge  240 , a biasing member  250 , a closing sealing member  224 , a retainer  225  and fixing sealing members  211  and  212 . 
     The float valve body  210  shown in  FIG. 7A  to  FIG. 7C  has an upper-side float valve first end section  201 , a lower-side float valve second end section  202  and a float valve middle section  203 . Here, the float valve middle section  203  is arranged between the float valve first end section  201  and float valve second end section  202 . 
     Note that the float valve body  210  may be formed by assembling the float valve first end section  201 , the float valve second end section  202  and the float valve middle section  203  which are separately formed. 
     The float valve first end section  201  has a body-side hinge supporting section  213 , a body-side biasing member supporting section  214  and a float valve first opening section  221 . The float valve second end section  202  has a float valve second opening section  222 . The float valve middle section  203  has a side opening  223 . 
     The flapper lid  230  shown in  FIG. 7A  to  FIG. 7C  and  FIG. 8A  to  FIG. 8C  has a plane section  231 , an inner tube guide  232 , a side end section  233 , a lid-side hinge supporting section  234  and a lid-side biasing member supporting section  235 . 
     A connection relation of components shown in  FIG. 7A  to  FIG. 7C  and  FIG. 8A  to  FIG. 8C  will be described. 
     The float valve first end section  201  has a cylindrical shape. The retainer  225  also has a cylindrical shape and is engaged with the inside of float valve first end section  201 . The closing sealing member  224  is formed of an elastic material and has an annular shape, and is arranged between the float valve first end section  201  and the retainer  225 . However, an annular end surface of the closing sealing member  224  is exposed to the space inside the float valve body  210 . An aggregate of the float valve first end section  201 , the retainer  225  and the closing sealing member  224  has a cylindrical shape, and a space inside the aggregate is called the float valve first opening section  221 . At this time, the exposed part of the closing sealing member  224  is arranged to surround the opening surface on the lower side of the float valve first opening section  221 . Note that float valve first opening section  221  has a shape and a size in which the inner tube  51  can pass through the opening section. Here, it is assumed that the inner diameter of the float valve first opening section  221  is DF 2 . Note that in this embodiment, DF 2  is strictly equal to the inner diameter of the retainer  225 . 
     The fixing sealing members  211  and  212  each have an annular shape, and are arranged to surround the outer circumference of the float valve first end section  201 . Here, the float valve first end section  201  may have ditches on the outer circumference to position the fixing sealing members  211  and  212 . 
     Note that the fixing sealing members  211  and  212  are feasible with the configuration different from the above. For example, the ditch is provided for the surface of the float valve first end section  201  which comes in contact with the upper side sub  300 , and the closing sealing member  224  may be arranged in this ditch. 
     The float valve second end section  202  has a cylindrical shape and the space thereinside is called the float valve second opening section  222 . The float valve second opening section  222  has a size and shape such that the inner tube  51  can pass through the inside space. Here, in this embodiment, the inner diameter of the float valve second opening section  222  is assumed to be DF 2  which is the same as that of the float valve first opening section  221 . Note that the inner diameter of the float valve first opening section  221  is not necessary to be the same as that of the float valve second opening section  222 , if the inner tube  51  can pass. 
     The float valve middle section  203  has a cylindrical shape, and is connected at its upper-side end section with the float valve first end section  201  and at its lower-side end section with the float valve second end section  202 . The space inside the float valve middle section  203  is communicated at its upper-side with the float valve first opening section  221  and at the lower-side with the float valve second opening section  222 . 
     A side opening  223  is provided on the side surface of the float valve middle section  203 . The side opening  223  is wide so that the flapper lid  230  can pass through it when the flapper lid  230  moves between the first position and the second position. 
     The float valve first end section  201  and the flapper lid  230  are connected through the hinge  240 . The hinge  240  has a column shape, and pierces a body-side hinge supporting section  213 , and a lid-side hinge supporting section  234  in the longitudinal direction of the hinge itself. Here, it is desirable that the hinge  240  is fixed on the body-side hinge supporting section  213  or the lid-side hinge supporting section  234  with screws. 
     The flapper lid  230  can turn around a rotation axis which is set in the longitudinal direction of the hinge  240  to move between the first position and the second position. Here, when the flapper lid  230  is in the the first position, the flapper lid  230  tightly fits with the closing sealing member  224  to close the float valve first opening section  221  (a passage). Also, when the flapper lid  230  is in the second position, the flapper lid  230  opens the passage of the float valve first opening section  221 . At this time, the flapper lid  230  does not interfere with inner tube  51  which passes through the float valve assembly, and the second position is an evacuation position. The second position is a position where the whole flapper lid  230  does not overlap with the float valve first opening section  221  (or the float valve second opening section  222 ), viewing the outer barrel assembly  100  in the longitudinal direction. 
     The biasing member  250  biases the flapper lid  230  for the first position. In this embodiment, the biasing member  250  is a coil-like torsion spring, and the coil portion is arranged around the hinge  240 . The one end of the biasing member is in contact with a body-side biasing member supporting section  214 , and the other end thereof is in contact with a lid-side biasing member supporting section  235 . Here, the body-side biasing member supporting section  214  in this embodiment is deflected in a direction of a load toward the float valve body  210  from the biasing member  250  such that one end of the biasing member  250  does not come off. In the same way, the lid-side biasing member supporting section  235  in this embodiment is deflected in a direction of a load toward the flapper lid  230  from the biasing member  250 , to a part of flapper lid  230  such that the other end of the biasing member  250  does not come off. 
     An assembling operation of the float valve sub  2  in this embodiment will be described below. 
     When the float valve assembly  200  is attached on the inside of the outer barrel assembly  100 , the float valve first end section  201  is received by the outer barrel first receiving section  112  and is fixed. Also, the float valve second end section  202  is received by the outer barrel second receiving section  122  and is supported. At this time, the float valve second opening section  222  is communicated with the outer barrel second opening section  102 . 
     When the float valve assembly  200  is attached on the inside of the outer barrel assembly  100 , the fixing sealing members  211  and  212  seal the float valve first end section  201  and the outer barrel first receiving section  112  in liquid-tightness. Here, when the outer diameter of the float valve assembly  200  is DF 1 , DF 1  is approximately equal to the minimum inner diameter DO 1  of the outer barrel first section  110 . Here, it is desirable that DF 1  is equal to or less than DO 1 . 
     When the upper side sub  300  is attached to the outer barrel assembly  100  to which the float valve assembly  200  has been attached, the float valve first opening section  221  is communicated with the lower-side opening section of the upper side sub  300 . At this time, the float valve assembly  200  is fixed at its upper side by being connected with the upper side sub  300 . In this state, the inner tube  51  can pass through the upper side sub  300 , the float valve first opening section  221 , the float valve second opening section  222  and the outer barrel second opening section  102 . 
     An operation when the flapper lid  230  of the float valve sub  2  in this embodiment closes the float valve first opening section  221  will be described. 
     If the inner tube  51  exits from the inside of float valve body  210  when the inner barrel section  50  is lifted up by the drilling rig  12 , the flapper lid  230  moves to the first position shows in  FIG. 7A  by an operation of the biasing member  250 . 
     The flapper lid  230  has a flat plane section  231  in the peripheral area of its main surface at least. The plane section  231  comes into contact with the closing sealing member  224  when the flapper lid  230  moves to the first position. Thus, the flapper lid  230  closes the passage communicated with the float valve first opening section  221 . As a result, an upper portion and a lower portion with respect to the float valve sub  2  in the inner space of the drill string  1  are isolated by the flapper lid  230 . In this state, even if fluid flows into the inside of drill string  1  from the lower portion of the float valve sub  2 , the fluid does not leak out to the upper portion of the float valve sub  2  so that the blowout can be prevented. 
     The inventors confirmed that the float valve sub  2  having the inner diameter of 98.5 mm had durability upto the pressure of about 20 megapascals, as the result that an experiment was carried out in which water pressure was applied from the float valve second opening section in the first position of the flapper lid of the float valve sub  2  in this embodiment. Note that this experiment result is merely an example and does not limit the scope of the present invention. 
     An operation of the float valve sub  2  in this embodiment when the flapper lid  230  releases the float valve first opening section  221  will be described. 
     When the inner barrel section  50  is inserted to the end section of outer barrel section  30  so that the inner tube  51  enters the inside of float valve body  210 , the flapper lid  230  moves to the second position shown in  FIG. 7B  if the force of the inner tube  51  pushing the flapper lid  230  exceeds the force of the biasing member  250 . 
     When the flapper lid  230  moves to the second position, a part of the flapper lid  230  protrudes out of the float valve body  210  through the side opening  223 . This protruding part is called a side end section  233  to make an explanation easy. As shown in  FIG. 8C , the side end section  233  has a size and shape to be accommodated inside the annular concave section  103  of the outer barrel assembly  100 . 
     As shown in  FIG. 8C , the inner tube guide  232  of the flapper lid  230  has a curved surface similar to the outer circumference surface of the float valve second opening section  222  (the inner circumference surface of the float valve second end section  202 ). Since the inner circumference surface of the float valve second end section  202  has a curved surface complementary to the side surface of the inner tube  51  as a column-like member, the curved surface of the inner tube guide  232 , too, has a shape complementary to the side surface of the inner tube  51 . By this curved surface, the inner tube guide  232  makes it possible for the inner tube  51  to be inserted and extracted more stably when the flapper lid  230  is in the second position. 
     The inner diameter of the annular concave section  103  in this embodiment will be described. 
       FIG. 9  is a diagram showing geometrical relation of the inner diameter DO 3  of the annular concave section  103  and the diameter DF 2  of the float valve first opening section  221  in this embodiment. At first, when paying attention to the plane section  231  of the flapper lid  230 , the size of the plane section  231  must be equal to or more than the diameter DF 2  of the float valve first opening section  221  at least. Next, to store the flapper lid  230  moved to the second position in the annular concave section  103  so as not to interfere with the inner tube  51 , it is necessary that the inner diameter DO 3  of the annular concave section  103  is longer than a diagonal line of the square circumscribing a circle of the diameter DF 2 . That is, the length DO 3  must be equal to or more than a value obtained by multiplying the length DF 2  by the square root of 2. In other words, the square of length DO 3  must be equal to or more than twice the square of length DF 2 . Strictly, since the thickness of hinge  240 , the thickness of flapper lid  230  and so on must be taken into account, the above-mentioned relation between the length DO 3  and the length DF 2  can be shown by the following equations:
 
 DO 3&gt; DF 2×√{square root over (2)}
 
Or
 
 DO 3× DO 3&gt;2× DF 2× DF 2
 
     The minimum value of the length DO 3  is determined as mentioned above. Note that the maximum value of the length DO 3  depends on the strength required to the float valve sub  2  as mentioned above. That is, assuming that the minimum value of thickness of the side wall of the outer barrel assembly  100  which satisfies the required strength is Tmin1, and the outer diameter of outer barrel assembly  100  is DO 4 , the maximum value of the length DO 3  is equal to the difference between DO 4  and twice of Tmin1. Therefore, the numerical limitation of the length DO 3  can be shown by the following equation:
 
 DO 4−2× T min1≥ DO 3&gt; DF 2×√{square root over (2)}
 
     Second Embodiment 
     In the first embodiment, the explanation has been carried out, presupposing the so-called riserless drilling system, to simplify the structure. Here, the explanation will be made, presupposing the so-called riser drilling system in a second embodiment. 
     In the riser drilling system, by filling the drilling hole  17  with muddy water to satisfy the condition of “stratum pressure&lt;muddy water pressure&lt;stratum destruction pressure”, it is prevented that the inner wall surrounding the drilling hole  17  collapses. 
     Note that the condition of “stratum pressure&lt;muddy water pressure&lt;stratum destruction pressure” is met in an open hole without any casing. However, because the stratum pressure rises as the drilling proceeds, the muddy water pressure must be raised according to this. When the drilling is continued without casing, the muddy water pressure exceeds stratum destruction pressure in an upper portion of the open hole, so that muddy water destroys the stratum and the stratum collapses. In the riser drilling system, the collapse is prevented by carrying out the casing before such a situation. As a result, in the riser drilling system, a depth possible to drill is improved drastically, as compared with the so-called riserless drilling system in which such a technique is not used. 
     The muddy water used in the riser drilling system needs a suitable adjustment physically and chemically. The muddy water adjusted in this way is generated by drawing up muds produced by the drilling of the stratum  16  and seawater in the periphery to the drilling rig  12  by the riser pipe, and adjusting the characteristics by a muddy water adjustment device loaded in the drilling rig  12 . The adjusted muddy water is sent to the bottom of drilling hole  17  through the inside of drill string  11 . 
       FIG. 10  is a diagram showing a configuration example of the coring system using the riser drilling system.  FIG. 10  shows a system equivalent to a coring system using the riserless drilling system shown in  FIG. 1  which is added with a riser pipe  19  and a blowout prevention device  20 . Note that illustration of the muddy water adjustment device loaded in the drilling rig  12  is omitted. 
     In the configuration example shown in  FIG. 10 , the riser pipe  19  is provided around the drill string  11  and extends from the seafloor  15  to the drilling rig  12 . Note that the blowout prevention device  20  which connects the drilling hole  17  and the riser pipe  19  is provided on the seafloor  15 . 
     The muddy water passes through the inside of drill string  11  in which the inner barrel section  50  is not inserted, when the muddy water moves from the drilling rig  12  to the bottom of drilling hole  17 . At this time, the muddy water passes through the float valve sub  2 , too. Because the flapper lid  230  can move to the direction in which the float valve first opening section  221  is released, if pressed by the muddy water entering from the upper-side, the flapper lid  230  does not hinder the passage of muddy water. 
     In this way, the float valve sub  2  according to the present invention is possible to apply to the riser drilling system easily. 
     As described above, the embodiments of the invention have been described specifically. However, the present invention is not limited to the embodiments, and various changes and modifications are possible in a range which does not deviate from the gift of the the present invention. Also, the features described in the embodiments can be freely combined in a range of no technical contradiction. 
     For example, the annular concave section  103  has been described above in case that its shape is a rotating body rotating around a rotation axis of the drill string  11 . This description is based on consideration of a possibility that the annular concave section  103  is formed by lathe processing or the float valve assembly  200  rotates to the outer barrel assembly  100  when the upper side sub  300  is attached or removed. However, as mentioned above, the annular concave section  103  is not always necessary to have a rotation body shape if a function can be accomplished of receiving the flapper lid  230  protruding from a cylindrical shape of the float valve assembly  200 . The annular concave section  103  may be a concave section having another shape such as an optional extension section, a grinding section, a cutting section, and so on. 
     EXPLANATION OF THE CODE 
     
         
           1  bottomhole assembly 
           2  float valve sub 
           10  core barrel 
           11  drill string 
           12  drilling rig 
           13  sea 
           14  sea surface 
           15  seafloor 
           16  stratum 
           17  drilling hole 
           18  GPS satellite 
           19  riser pipe 
           20  blowout prevention device 
           30  outer barrel section 
           31  core bit 
           32  near bit sub 
           33  drill collar sub 
           34  landing sub 
           35  head sub 
           36  landing ring 
           37  latching section 
           40  core 
           50  inner barrel section 
           51  inner tube 
           52  core liner 
           53  length control mechanism 
           54  swivel mechanism 
           55  landing mechanism 
           56  latching mechanism 
           57  fishing neck 
           58  lock bolt 
           71  first casing pipe 
           72  second casing pipe 
           73  third casing pipe 
           74  fourth casing pipe 
           100  outer barrel assembly 
           101  outer barrel first opening section 
           102  outer barrel second opening section 
           103  annular concave section 
           110  outer barrel first section 
           111  outer barrel first connection section 
           112  outer barrel first receiving section 
           120  outer barrel second section 
           121  outer barrel second connection section 
           122  outer barrel second receiving section 
           130  outer barrel middle section 
           200  float valve assembly 
           201  float valve first end section 
           202  float valve second end section 
           203  float valve middle section 
           210  float valve body 
           211  fixing sealing member 
           212  fixing sealing member 
           213  body-side hinge supporting section 
           214  body-side biasing member supporting section 
           221  float valve first opening section 
           222  float valve second opening section 
           223  side opening 
           224  closing sealing member 
           225  retainer 
           230  flapper lid 
           231  plane section 
           232  inner tube guide 
           233  side end section 
           234  lid-side hinge supporting section 
           235  lid-side biasing member supporting section 
           240  hinge 
           250  biasing member 
           300  upper side sub 
           500  lower side sub