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CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional Application No. 60/567,031 filed Apr. 30, 2004, the disclosure of which is hereby incorporated by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates generally to threaded API casing connections, but more specifically to the couplings for API Buttress threaded casing to be used as a combination drill string and casing string—i.e., Drilling With Casing (DWC).  
         [0004]     2. Description of the Related Art  
         [0005]     Presently the conventional method for drilling oil and gas wells is to use drill pipe specifically designed for and dedicated to drilling the well bore. Upon drilling a well to completion, the drill pipe is pulled from the well and transferred to the next location for drilling another well. The drill pipe is thus used until it is worn out. The open hole left by the drill pipe is sealed off by running a string of casing pipes to the bottom of the hole and cementing the casing string in place.  
         [0006]     In contrast to the above procedure, it is the purpose of new technology to eliminate the use of the above-described drill pipe and instead use the casing string for both drilling the well and casing off the open hole. The procedure is commonly referred to as “Drilling With Casing” (DWC). This procedure has been tried in various parts of the world but with limited success. However DWC offers so much potential for reducing drilling costs that interest remains high throughout the industry and many new projects are aimed at advancing the technology.  
         [0007]     In conventional casing usage, the casing and its connections are subjected only to static loads comprising tension, torsion, compression, bending, pressure and any combination thereof. In DWC usage, the casing and connections are not only subject to all of the above static loads, but also to dynamic loads due to rotating the casing at 100 to 150 RPM while drilling the well bore. As the casing rotates and advances down the well bore, the casing string and particularly the connections, which have a larger outside diameter than the casing, are subject to cyclic fatigue loads, severe abrasion wear and impact loading, then finally to all the static loads mentioned above after the casing is set and cemented in the well.  
         [0008]     This invention is directed at one of the primary problems associated with DWC—the connections which join each length of casing, one to another. Experience to date with DWC has demonstrated a need for a more robust, yet economical casing connection to withstand the additional rigors of dynamic loading and frictional wear caused by rotating the string while drilling.  
       BRIEF SUMMARY OF THE INVENTION  
       [0009]     The invention comprises the following modifications of the standard API Buttress threaded coupling only, while maintaining it&#39;s compatibility with standard API Buttress threaded Pins: 
        1. The coupling threads only are modified with multiple tapers to reduce and equalize makeup stresses through the thinner cross-sections of both the pins and coupling, thereby minimizing the possibilities of thread galling during connection assembly.     2. The multiple thread taper reduces the bearing stresses, and therefore the localized stress risers in the run-out pin threads where the connections commonly fail in fatigue, under rotational cyclic loading.     3. The coupling is shortened to allow abutment of the two pins at the center of the coupling maximizing the torque capabilities of the connection.     4. The coupling can be formed with an integral, sacrificial wear-sleeve extension, protecting the coupling proper from frictional wear as the casing is rotated down the well bore.       
 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)  
       [0014]      FIG. 1  is a side view, partially in cross-section, of two pipes joined with a coupling of the prior art.  
         [0015]      FIG. 2  is a side view, partially in cross-section, of two pipes joined with a prior art coupling having a center section of increased thickness.  
         [0016]      FIG. 3  is a side view, partially in cross-section, of two pipes joined with a coupling according to the present invention.  
         [0017]      FIG. 4  is a side view, partially in cross-section, of two pipes joined using an alternative embodiment of the present invention.  
         [0018]      FIG. 5  is a side view, partially in cross-section, of two pipes joined using a third embodiment of the present invention.  
         [0019]      FIG. 6  is a side view, partially in cross-section, of two pipes joined using a fourth embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0000]     Casing Couplings of the Prior Art  
         [0020]     Referring now to the drawing in  FIG. 1 , a standard API Buttress Threaded casing string  10  and coupling  15  according to the prior art is illustrated. The casing string  10  includes two casing sections, or pipes,  11  and  12 , having pin ends  11 A and  12 A, interconnected with a coupling  15  according to the prior art.  FIG. 1  shows the connection fully assembled. Note the separation between pin ends  11 A and  12 A. This gap between the pins is commonly known as the “J” area.  
         [0021]     Still referring to  FIG. 1 , the casing members  11  and  12  include pin threads  13  and  14  on the outside end of each casing section, the threads mating with the threaded internal surface of the coupling  15 . The coupling  15  includes a first end  18  and a second end  19  with internal threaded surfaces  16  and  17 . The threads are preferably tapered API Buttress threads as are commonly used in the industry and in this application. However, other thread forms may be used.  
         [0022]     Now referring to  FIG. 2 , which shows another oil well casing connection in accordance with U.S. Pat. No. 5,015,017, a casing string is illustrated generally at  20 . The casing string  20  includes two casing sections  21  and  22  interconnected with coupling  25 , and the casing sections include external API Buttress pin threads  23  and  24  which mate with matching coupling threads  26  and  27 .  
         [0023]     Essentially  FIG. 2  is the same as  FIG. 1  with the exception that the “J” area between pin ends  23  &amp;  24  contains an integral reinforcing cross-section  30  of the coupling  25 . This heavy cross-section  30  substantially improves the strength of the coupling by converting the structural/mechanical behavior of the coupling from a simple beam to a cantilever beam. The visual contrast is readily noted by comparing the cross-sections of coupling  15  ( FIG. 1 ) and coupling  25  ( FIG. 2 ). Both connections preferably use the Standard API Buttress threads and are interchangeable with each other.  
         [0000]     Casing Couplings According to the Present Invention  
         [0024]     With reference to  FIG. 3  and this invention, a casing string is illustrated with two casing sections  41  and  42  interconnected with coupling  45  and the casing sections include externally threaded pins  43  and  44 , which mate with internal coupling threads  60 . Threaded pins  43  and  44  contain standard API Buttress Threads with a constant taper. The faces of the two pin ends  65  are square cut to furnish maximum bearing face when butted together at center  50  of coupling  45 .  
         [0025]     Again referring to  FIG. 3  and this invention, it is standard with the API Buttress Thread Form for the coupling threads  60  and pin threads  43 ,  44  to have identical thread tapers so as to produce uniform radial thread interference through the full length of the thread profile. When the connection is assembled, it is this thread interference that creates the contact pressure and therefore the sealing capabilities of the mating threads.  
         [0026]     It will be further noted in  FIG. 3  that the thread tapers of the pin and coupling members result in variable cross-sections along the thread profile of each member, with a thinner cross-section at pin ends  65  and similar thinning cross-sections at coupling ends  53 . When the connection is assembled it is seen that the thinner cross-sections of the respective pin and coupling members are opposite the heavier cross-sections of the mating member. The cross-sections therefore are unbalanced at the thinner ends of both members. When the connection is assembled, it is this imbalance between these cross-sections and the resulting excessive hoop stresses in the thinner cross-sections, that this invention addresses.  
         [0027]     Referring again to  FIG. 3 , it is evident that a uniform taper between the pin and coupling threads results in uniform interference along the thread profile. However, as pointed out above, the cross-sections of the mating members vary along the thread profile. Therefore, if the interference between the threads is uniform, but the cross-sections behind the threads are variable, then the resulting hoop stresses created in the cross-sections must also be variable; graduating from low stresses in the thicker part of the cross-section to high stresses in the thinner part. Indeed it has been found through Finite Element Analysis (FEA) that, after assembly, the hoop stresses in the thinner cross-sections of both the pins and coupling can exceed the yield strength of the steel. In addition to the negative impact on the yield strength, in the thinner portions of the mating members this differential yielding at the thin vs. thick cross-sections also causes differential movement between the threads at these same high stress points. This differential movement, at the high stress points, in turn results in thread galling in both coupling and pins at  65  and  66 . It is also anticipated that these same high stressed areas, particularly at the run-out threads  66  of the pin members  43  and  44 , could result in fatigue failure when the connections are used in the drilling mode (DWC).  
         [0028]     Referring again to  FIG. 3 . there is shown a cross-sectional view of a shortened API Buttress Threaded coupling  45  connecting two Buttress Threaded Pins  43  and  44  that abut at the center of the coupling  45 .  FIG. 3  illustrates this shortened and multiple tapered coupling designed to: 
        1) Moderate the concentrated high stresses previously outlined     2) Minimize the thread galling in the areas of high stress     3) Maintain compatibility with standard API Buttress threaded pin members     4) Create a high torque connection by butting the pin ends at the center of the coupling        
 
         [0033]     To accomplish these objectives the thread tapers in only the coupling member  45  are modified at the areas of high stress; i.e., the areas of cross-sectional imbalances at coupling ends  53  and pin ends  65 . As shown in  FIG. 3  the thread taper in coupling  45  is segmented into sections S1, S2 and S3. In the present invention the taper in section S2 is maintained at the API standard taper. The taper in section S1 is greater than the taper in section S2, and the taper in section S2 is greater than the taper in section S3. The variable tapers in coupling  45  relative to the uniform tapers of pins  43  and  44  reduce bearing pressures in the mating thread elements in areas of the connection with unbalanced cross-sections; i.e.  65  on the pin ends and  53  of the coupling ends. The employment of multiple tapers reduces the contact pressure in the overstressed areas S1 and S3 and thus mitigates the problems of high stresses, thread galling and fatigue failure.  
         [0034]     In one preferred embodiment, the threaded section on each side of a 7-inch API Buttress coupling is divided into three sections as previously described. The lengths and tapers for each section in this preferred embodiment are:  
                                                     Section   Length (in)   Taper (in./in.)                                1   1.784   0.07525       2   1.716   0.06250       3   1.125   0.05556                  
 
         [0035]     The section lengths and tapers employed in this invention are designed to reduce contact pressure in areas of the connection where cross-sections are unbalanced.  
         [0036]     It might be noted that other taper profiles, such as elliptical or curved, might be used, but the segmented profile is preferred because it maximizes the length of section S2 which has the same taper on both members therefore maximizing sealing integrity. It is emphasized that only the coupling tapers need be modified.  
         [0037]     It is also emphasized that the pin threads  43  and  44  should be made to standard API Buttress specifications with no modifications to length or taper. This allows the casing pipes to be threaded by the many API licensed machine shops or mills in the world. By contrast there are only a few coupling manufacturers in the world and most have the modern equipment to machine the modifications required by this invention. Also couplings are easily transportable as opposed to 40′ lengths of pipe.  
         [0038]     Again in  FIG. 3 , the coupling is shortened by approximately ¾ inch, removing what is commonly known as the “J” area between the two pin members as previously pointed out in  FIG. 1 . In this invention, elimination of the “J” area allows the two pins to butt one another at the coupling center thereby maximizing the torque capabilities of the connection and its use for DWC.  
         [0039]     Now referring to  FIG. 4 , an optional unthreaded extension  70  can be integrally machined on one end of the coupling  55 . The purpose of the extension is to provide a sacrificial wear sleeve to protect the main body of the coupling as the casing is rotated down the well bore. The wear sleeve would have the same outside diameter as the coupling with the inside diameter being slightly larger than the casing so as to slip over the casing when the connection is assembled. As an option, the wear sleeve can be hard banded if excessive abrasion is anticipated.  
         [0040]     Again at  FIG. 4  the inside diameter of the wear sleeve is uniform from the face  72  for a specific distance toward the center of the coupling, then is tapered outward toward the coupling OD  73 . This design detail is provided to permit the threading tool to cut perfect (full-formed) threads over the entire coupling thread length without cutting into the ID of the sacrificial wear sleeve. Elimination of machining marks in the wear sleeve near the coupling threads reduces the possibility of fatigue failures in the sacrificial wear sleeve extension.  
         [0041]     In an alternative embodiment illustrated in  FIGS. 5 and 6 , an API Buttress coupling has an internal reinforcing cross-section  80  at the center or in the “J” area. In this embodiment, each pin engages an internal square shoulder  82  at the heavy cross-section thereby maximizing the torque capability of the connection for its use for DWC.  
         [0042]     As in the embodiment shown in  FIG. 4 , an optional unthreaded extension  70  can be integrally machined on one end of the coupling  55 . This feature is illustrated in  FIG. 6 . The purpose of the extension  70  is to provide a sacrificial wear sleeve to protect the main body of the coupling as the casing is rotated down the well bore. The wear sleeve would have the same outside diameter as the coupling with the inside diameter being slightly larger than the casing so as to slip over the casing when the connection is assembled. As an option, the wear sleeve can be hard banded in area  74  if excessive abrasion is anticipated.  
         [0043]     Again, as in the embodiment shown in  FIG. 4 , the inside diameter of the wear sleeve is uniform from the face  72  for a specific distance toward the center of the coupling, then is tapered outward toward the coupling OD  73 . This design detail is provided to permit the threading tool to cut perfect (full-formed) threads over the entire coupling thread length without cutting into the ID of the sacrificial wear sleeve. Elimination of machining marks in the wear sleeve near the coupling threads reduces the possibility of fatigue failures in the sacrificial wear sleeve extension.  
         [0044]     It should be noted and anticipated that certain changes may be made in the present invention without departing from the overall concept described here and it is intended that all matter contained in the foregoing shall be interpreted as illustrative rather than in a limiting sense.

Summary:
A modified API Buttress threaded casing connection is disclosed for use in drilling oil and gas wells in lieu of using conventional drill pipe. The coupling threads only are multiple tapered yet can be mated with standard API Buttress pin threads having a single taper. The coupling is also shortened to permit abutment of the mating pins at the center of the coupling and can be further enhanced with an integral, sacrificial wear sleeve on one end of the coupling.