Abstract:
A threaded, coupled connection having tapered, buttress-type threads, the threaded portions of the connection having a thread pitch of seven threads per inch. Preferably the pin threads of the coupled connection are pull-out threads.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to threaded connections and, more particularly, to coupled, threaded connections for use on tubing used in the completion and workover of oil and gas wells. 
       BACKGROUND OF THE INVENTION 
       [0002]    In the completion and production of an oil/gas well, it is often sometimes necessary to drill out a plug or other down hole obstruction which was used in the construction of the well. An example of this is so called fracing plugs used in fracing operations that are commonly conducted in shale formations. The fracing plugs are typically used to isolate lateral or horizontal sections of the well bore so that successive, isolated sections can be fraced to stimulate production. However, the fracing plugs must be drilled out so that the oil from the formation can flow to the well head for recovery. 
         [0003]    In these types of workover or intervention activities, it is common to use a tubing string to drill out the plugs. In the past, this was conventionally done with work strings of tubing employing so called eight-round or LTC connections. However, in highly deviated or horizontal wells where most fracing occurs, connections employing eight-round threaded components are not sufficiently rugged enough to withstand the changing tension and compression loads that the work/tubing string undergoes. 
         [0004]    Recognizing this problem, many operators elect to use a two-step or dual step threaded connection with a metal to metal radial seal which is an integral connection i.e., there is no coupling between the sections of tubing. While two-step, integral connections for tubing work strings are better than coupled eight-round threaded tubing strings, they are not without disadvantages. For one thing, two-step threaded connections are more expensive to manufacture and more expensive to rethread in the event of damage. 
         [0005]    Ideally, a tubing string used in the activities described above e.g., drilling out of plugs and other well intervention techniques, would be capable of withstanding high make-up torque and could be made-up and broken-out multiple times e.g., 20 or more times, without any significant reduction in break-out torque. Such a connection would last longer and while in use would be more rugged and able to withstand the tension compression and bending loads placed on the connection especially, for example, in more acute bending modes e.g. 20 degrees per 100 feet. In particular, such a tubing string to would be resistant to the threaded connections backing-off to the point where the string separates. 
       SUMMARY OF THE INVENTION 
       [0006]    In one aspect there is provided a coupled threaded connection which can withstand high make-up torque. 
         [0007]    In another aspect there is provided a threaded, coupled connection which exhibits a break-out torque only slightly less than the make-up torque after repeated makes and breaks. 
         [0008]    In still a further aspect there is provided a threaded, coupled connection comprised of a coupling having first and second boxes and first and second pins which are upset. 
         [0009]    These and further features and advantages will become apparent from the following detailed description, wherein reference is made to the figures in the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a longitudinal sectional view showing schematically the characteristics of a threaded connection having a run-out thread on a standard generally uniform O.D. pipe. 
           [0011]      FIG. 2  is a view similar to  FIG. 1  but showing the characteristics of a threaded connection having a pull-out thread on a section of a pipe having an upset O.D. 
           [0012]      FIG. 3  is a partial, longitudinal section of one embodiment of the present invention showing a coupled threaded connection with mechanical stops or torque shoulders. 
           [0013]      FIG. 4  is a view similar to  FIG. 3  of another embodiment of a coupled, threaded connection of the present invention. 
           [0014]      FIG. 5  is a partial, cross-sectional view of the thread form used in the threaded connections shown in  FIGS. 3 and 4 . 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0015]    The thread form used in the threaded connections of the present invention, described more fully hereafter, is particularly useful in coupled, tubing strings employed as work strings. The terms “box”, “box connection(s)” and similar terms refers to a tubular member having an internally threaded section. The terms “pin”, “pin connection(s)” or similar terms refers to a tubular member having an externally threaded section. It is a feature of the thread form and the threaded connections of the present invention that the threads are tapered having a taper of one inch per foot and a thread pitch of seven threads per inch. The threads used in the threaded connections of the present invention have a thread height of from about 0.042 to about 0.046 inches, particularly about 0.044 inches. Furthermore, the pin or pin connections used in the threaded connections of the present invention are “upset” meaning they have an OD adjacent the threaded sections which is greater than the OD of the pipe to which the pin connection is attached. 
         [0016]    The connections of the present invention are also characterized by the fact that the pins have “pull-out” threads rather than “run-out” threads. For purposes of illustration and comparison between those two types of thread forms, reference is made to  FIGS. 1 and 2 . Referring first then to  FIG. 1  which schematically depicts a run-out thread, it can be seen that the pin, shown generally as  10 , is formed on one end of a pipe or tubular section  11  and has a threaded section shown generally as  12 , the threaded section  12  extending generally from approximately the pin nose  14  to a location, indicated by the line  16  at which point the threads run-out of the pipe body  11 . As is well known, a run-out thread maintains the same taper per foot (angle) with a thread height decreasing generally after about the last fully engaged full height thread at which point the thread height starts uniformly decreasing. In short, a run-out thread maintains the same taper per foot (angle) with a thread height decreasing as the thread runs out of the body. As can be seen with respect to  FIG. 1 , there are a series of lines and surfaces which collectively describe a run-out thread. Line  13  is an imaginary line which is parallel to the long axis of the pin  10  i.e., it is coincident with the ID of the pin  10 . Dotted line  22  depicts the root of the threads while surface line  24  depicts the crests of the threads before the thread height begins to decrease which is generally in an area depicted by line  20 , line  26  depicting the crests of the decreasing thread height threads until the run-out point indicated by line  16 . Line  18  depicts the length of the portion of threaded section  12  where the threads are generally at full height and fully engaged, line  19  generally depicting the length of the threaded portion  12  in which the heights of the threads are decreasing. Thus as can be discerned the thread height of the threads generally between the nose  14  of the pin  10  and line  20  is the difference between dotted line  22  and surface line  24 . The thread height of the decreasing thread height threads is the difference between dotted line  22  and surface line  26 . Thus while the threaded section  12  extends from about the pin nose  14  to about line  16 , the fully engaged full height portion of the threads extends generally from the pin nose  14  to line  20  while the decreasing thread height threads extend from about line  20  to about line  16 .  FIG. 1  also shows that the angle A-A of 2.39° is the thread taper which stays constant from about the nose  14  of the pin to the point  16  on the surface of the pin  10  where the threaded section  12  runs-out. 
         [0017]    Referring now to  FIG. 2 , there is shown a “pull-out thread”. In general, pull-out threads are characterized by the fact that they maintain the same thread height until the threaded portion reaches a set distance from the nose of the pin at which point the threads pull-out of the pipe body on a different, greater angle. With reference to  FIG. 2 , the pin shown generally as  30  is formed on a pipe body  32  and has a pin nose  34 . There is a threaded section indicated generally as  35  which extends generally form the pin nose  34  to a point indicated by the line  36  at which point the threads pull-out of the body. As can be seen, the root of the threads of the threaded portion  35  is on two angles one being a constant angle of 2.39° (relative to the long axis of the pin  30 ) extending generally from pin nose  34  to a point on the pin indicated by line  40 . Commencing at about line  40  the angle or taper of the threads (again relative to the long axis of the pin  30 ) is 4.67° as indicated by angle B-B. The height of the threads of the threaded section extending from about pin nose  34  to about line  40  is the difference between the thread root line  38  and a surface line  42  which defines the crests of the threads of that threaded portion. In short, the angle of the threads between the line  40  and the line  36  increases rather sharply from 2.39° to 4.67° as the thread pulls-out of the body  32 . 
         [0018]    As can be seen by comparing line  10 A of  FIG. 1  with line  30 A of  FIG. 2  which is a depiction of the wall thickness of the respective pins  10  and  30 , pin  30  has a greater wall thickness than pin  10 . Further, as shown hereafter the minimum diameter of the last engaged thread of the threaded section  35  of pin  30  is greater than the OD of the pipe body on which of pin  10  is formed. 
         [0019]    Referring now to  FIG. 3  there is shown one embodiment of the present invention comprising a coupled, shouldered connection. The connection, shown generally as  50  comprises a coupling body shown generally as  52 , only one end of which is shown, it being understood that the other end is the same. Coupling body  52  has an internal, annular, radially inwardly projecting rib  54 , there being a first internal, annular axially facing shoulder  56  formed on rib  54 . A first inwardly projecting, annular thread relief  58  is formed adjacent shoulder  56 . There is a first coupling body end face  60 , and a first internally threaded portion having threads  62 . Accordingly there is formed a first box connection  52 A generally bounded by shoulder  56  and first end face  60 . 
         [0020]    Coupling body  52  also forms a second box connection  52 B having a second annular, axially facing shoulder  66  formed on rib  54  opposite shoulder  56 , and a radially inwardly extending annular thread relief  68  adjacent shoulder  66 , second box connection  52 B being generally bounded by shoulder  66  and a second coupling body end face (not shown). 
         [0021]    Coupled connection  50  also comprises a first pin  70  connected to a first pipe body  71  and a second pin  72  connected to a second pipe (not shown), pins  70  and  72  having first and second pin noses  70 A and  72 A, respectively. First pin  70  has a first upset portion  74  while second pin  72  has a second upset portion (not shown). When made-up as shown in  FIG. 3 , first pin  70  and second pin  72  one threadedly received in first box  52 A and second box  52 B, respectively, pin noses  72 A and  70 A being made-up to a desired torque against shoulders  56  and  66 , respectively. Connection  50 , as seen, has a generally flush ID. 
         [0022]    As seen in  FIG. 3 , pipe body  71  has an OD of D indicated by arrow C. As also seen there is an arrow D showing the minimum diameter of the last engaged thread of pin connection  70 . As can be seen from the dotted line  80 , the minimum diameter of the last engaged thread indicated by the arrow D is greater than the OD of the pipe body  71  indicated by the arrow C. 
         [0023]    The pull-out feature of threaded connection  50  can be readily appreciated by looking at the threaded area of the pin  70  bounded by the arrows E and F. As can also be seen, there is no thread of first pin  70  which extends beyond the first end face  60  of coupling body  52 , a like situation existing with respect to second pin  72 . 
         [0024]    Referring now to  FIG. 4  there is shown another embodiment of the present invention which comprises a coupled connection, shown generally as  90 . Coupled connection  90  comprises a coupling body  92  having a centerline indicated by arrow G which is perpendicular to a longitudinally extending, product axis (not shown), concentric with coupling body  92 , coupling body  92  having a first end  94  and a second end (not shown). Coupling body  92  forms a first box  96  which extends generally from first end face  94  to the center line G of coupling body  92 . A female, threaded section having threads  98  extends between first end face  94  and center line G. There is also a second box  91  which is generally formed between the second end (not shown) extending generally to center line G. 
         [0025]    Coupled connection  90  also includes a first pin  98  having an externally threaded section, comprised of threads  100 . First pin  98  has a first pin nose  102 . Coupled connection  90  also includes a second pin  104 , comprised of threads  105 . Second pin  104  has a second pin nose  106 . As seen in  FIG. 4 , when the coupled connection  90  is fully made-up, pin noses  102  and  106  are in abutting relationship. Accordingly, the coupled connection  90  has a substantially flush internal ID. 
         [0026]    First and second pins  98  and  104  are connected to first and second pipe bodies, only first pipe body  103  being shown. As in the case of the embodiments shown on  FIG. 3  the minimum diameter of the last engaged thread of the pin connections  98  and  104  are greater than the OD of the pipe bodies e.g., pipe body  103 . Also as in the case of the embodiment shown in  FIG. 3 , it can also be seen that the pins  98  and  104  of the coupled connection  90  are pull-out threads as described above. Likewise, there is no thread on the pins  98  and  104  which extends beyond the respective first and second end faces of coupling body  92 . 
         [0027]      FIG. 5  shows the thread form of the threads in the embodiments shown in  FIGS. 3 and 4 . Referring thus to  FIG. 5 , the thread form, shown generally as  210 , is shown with respect to the threads  212  of a pin  214  threadedly engaged with the threads  216  of a box  218 . Pin threads  212  have a stab flank  220 , a load flank  222 , a root  224  and a crest  226 . Box threads  216  have a stab flank  228 , a load flank  230 , a root  232  and a crest  234 . As seen, in the fully made-up position depicted in  FIG. 5  the load flanks  222 ,  230  of the pin, box, respectively, are engaged, the respective crests and roots of the pin threads  212  and the box threads  216 , are engaged and there is a clearance, depicted by the arrows X-X, between the stab flanks of the pin threads  212  and the box threads  216 . 
         [0028]    As can also be seen from  FIG. 5 , the stab flanks  220  of the pin  214  and stab flanks  228  of the box  218  are at a positive angle designated as Y-Y on  FIG. 5  relative to a line passing transversely through the pin/box connection and perpendicular to the product axis  242 . In this regard product axis  242  passes longitudinally through the center line of the pin/box connection and is generally concentric with the OD of the box  218  and the ID of the pin  214 . Generally speaking, the angle Y-Y is from about 8° to about 12°, particularly about 10°. The load flanks  222  of the pin  214  and the load flank  230  of the box  218  when in the fully made-up position as shown in  FIG. 5  are at a positive angle Z-Z of from about 2° to about 4° especially about 3° again with respect to the product axis  242 . 
         [0029]    When the pin/box connection of  FIG. 5  is made-up, the clearance X-X is from about 0.002 to about 0.004 inches, particularly about 0.003 inches. 
         [0030]    The threaded connections of the present invention using the thread form of  FIG. 5  provide unexpected results in terms of a reduced degree of galling typically experienced by Standard API (8 round or LTC) threads. It is known that such Standard API threads typically undergo galling after two to three make-ups but in any event after about five make-ups. In tests conducted on threaded tubing connections made in accordance with the thread form of the present invention, and it has been found that the tubing connections can undergo up to ten or more make-and-breaks without any significant galling. This is a significant advantage since it dramatically increases the usable life of the tubing before it must be reworked or replaced altogether. Furthermore, when in use, this reduced degree of galling ensures pressure integrity. 
         [0031]    The coupled connections of the present invention also have many advantages compared with integral/two-step connections commonly used in tubing work strings. To begin with, a typical two-step connection generally has minimal thread interference, applied torque being borne almost entirely by the load bearing shoulders, which in any event, have low break out torque compared to the make-up torque. The result is that when the connection is made-up, only the torque shoulders and metal-to-metal seals are in contact. The free running threads make virtually no contact at all and accordingly substantially all of the torque is limited to shouldering torque, the metal-to-metal seals primarily acting to hold the connection together. This is to be contrasted with the connections of the present invention which after tests involving 22 makes and breaks, showed no appreciable loss in break-out torque. This flows from the fact that when the connections of the present invention are made-up to full torque values, there is not only torque on the coupling shoulders or the pin to pin ends, there is also torque in the threads which resist backing-out. 
         [0032]    In an actual test on a threaded connection according to the present invention the connection was initially made-up to a torque value of approximately 2,300 foot pounds and an initial break-out torque of 1,800 foot pounds. After the connection had been made-up and broken-out 22 times the final break-out torque was 1,766 foot pounds. In other words the additional stored energy which can be placed into the threaded connections of the present invention when they are fully made-up ensure only minimal loss of break-out torque after repeated makes and breaks. Tests have shown this is not the case with a typical two-step threaded connection. The threaded connection of the present invention is thus characterized, in part, by its repeatability in terms of multiple makes, and breaks with little loss in break-out torque which flows from the fact that over and above the engaged threads bearing load, the stored energy in the shoulders of the connection virtually precludes any backing out even when tubing strings employing the threaded connections of the present invention are used in highly deviated e.g., horizontal wells. 
         [0033]    Although specific embodiments of the invention have been described herein in some detail, this has been done solely for the purposes of explaining the various aspects of the invention, and is not intended to limit the scope of the invention as defined in the claims which follow. Those skilled in the art will understand that the embodiment shown and described is exemplary, and various other substitutions, alterations and modifications, including but not limited to those design alternatives specifically discussed herein, may be made in the practice of the invention without departing from its scope.