Abstract:
The present invention provides an unsymmetrical thread profile used to connect stator housings of a positive displacement motor. The unsymmetrical thread profile comprises one or more non-load bearing flanks angled with respect to the longitudinal axis of the thread profile such that the root radius is greater than the root radius of a symmetrical thread profile having substantially the same pitch and first angle.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates to mud driven motors used in the drilling of oil wells. More particularly, the invention relates to an unsymmetrical thread profile for connection of a positive displacement motor (PDM) housing.  
       BACKGROUND OF THE INVENTION  
       [0002]     Positive Displacement Motors (PDMs) are known in the art and are commonly used to drill wells in earth formations. PDMs operate according to a reverse mechanical application of the Moineau principle wherein pressurized fluid is forced though a series of channels formed on a rotor and a stator. The channels are generally helical in shape and may extend the entire length of the rotor and stator. The passage of the pressurized fluid generally causes the rotor to rotate within the stator. For example, a substantially continuous seal may be formed between the rotor and the stator, and the pressurized fluid may act against the rotor proximate the sealing surfaces so as to impart rotational motion on the rotor as the pressurized fluid passes through the helical channels.  
         [0003]      FIG. 1  shows a conventional downhole motor assembly known in the art. As shown, the assembly  10  includes a rotatable drill bit  12 , a bearing/stabilizer section  14 , a transmission section  16  which may include an adjustable bent housing (for directional drilling), a motor power section  18 , and a motor dump valve  20 . The bent housing  16  and the dump valve  20  are not essential parts of the downhole motor. As mentioned above, the bent housing can be used as a means of directional drilling. The dump valve can be used to allow drilling fluid to enter the motor as it is lowered into the borehole and to allow drilling fluid to exit the motor when it is pulled out of the borehole. The dump valve also shuts the motor off when the drilling fluid flow rate drops below a threshold. During operation, drilling fluid pumped through the drill string (not shown) from the drilling rig at the earth&#39;s surface enters through the dump valve  20 , passes through the motor power section  18  and exits the assembly  10  through the drill bit  12 .  
         [0004]     Prior art  FIGS. 2 and 3  show details of the power section  18  of the downhole motor. The power section  18  generally includes a housing  22  which houses a motor stator  24  within which a motor rotor  26  is rotationally mounted. The power section  18  converts hydraulic energy into rotational energy by reverse application of the Moineau pump principle. The stator  24  has a plurality of helical lobes,  24   a - 24   e , which define a corresponding number of helical cavities,  24   a ′- 24   e ′. The rotor  26  has a plurality of lobes,  26   a - 26   d , which typically number one fewer than the stator lobes and which define a corresponding plurality of helical cavities  26   a ′- 26   d ′. Generally, the greater the number of lobes on the rotor and stator, the greater the torque generated by the motor. Fewer lobes will generate less torque but will permit the rotor to rotate at a higher speed. The torque output by the motor is also dependent on the number of “stages” of the motor, a “stage” being one complete spiral of the stator helix.  
         [0005]     Stator housings of a PDM are typically connected with threaded connections. The threaded connections are particularly susceptible to bending stress fatigue failures. In part, such failures result because the wall thickness of these housings is typically smaller than that commonly found on the connections of drill collars.  
         [0006]     Accordingly, there is a need for a thread profile that reduces the stator housings&#39; susceptibility to bending stress fatigue failures.  
       SUMMARY OF INVENTION  
       [0007]     In one aspect, the present invention comprises an unsymmetrical thread profile used to connect stator housings of a positive displacement motor. The unsymmetrical thread profile comprises one or more load bearing flanks angled with respect to the longitudinal axis of the thread profile at a first angle. The unsymmetrical thread profile further comprises one or more non-load bearing flanks angled with respect to the longitudinal axis of the thread profile at a second angle. The unsymmetrical thread profile further comprises a root radius between the non-load bearing blanks and the load bearing flanks. The second angle is less than the first angle such that the root radius is greater than the root radius of a symmetrical thread profile having substantially the same pitch and first angle.  
         [0008]     In another aspect, the present invention provides a method of providing a larger root radius for a thread profile. The method comprises providing a load bearing flank having a first angle and providing a non-load bearing flank having a second angle less than the first angle.  
         [0009]     In another aspect, the present invention provides a method of reducing the fatigue experienced by a threaded connection comprising reducing the angle of the non-load bearing flank to increase the root radius.  
         [0010]     Other aspects and advantages of the invention will be apparent from the following description and the appended claims. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0011]      FIG. 1  is a side elevation view of a prior art drilling motor assembly;  
         [0012]      FIG. 2  is a cutaway view of the motor of  FIG. 1  showing the rotor and the stator;  
         [0013]      FIG. 3  is a cross section taken along the line  3 - 3  in  FIG. 2 ;  
         [0014]      FIG. 4  shows a prior art thread profile used to connect stator housings of a positive displacement motor.  
         [0015]      FIG. 5  shows an enlarged view of a prior art stator housing threaded connection.  
         [0016]      FIG. 6  shows an embodiment of the thread profile of the “pin-side” threaded connection of the present invention.  
         [0017]      FIG. 7  shows an enlarged view of an embodiment of the thread profile of the present invention.  
         [0018]      FIG. 8  shows an embodiment of the thread profile of the “box-side” threaded connection of the present invention.  
         [0019]      FIG. 9  shows an enlarged view of an embodiment of the thread profile of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0020]      FIGS. 4 and 5  show a prior art conventional thread profile  30  that is used to connect stator housings of a positive displacement motor (PDM).  FIG. 4  shows the threaded connection  32  on the pin-side  34  of the stator housing  36 , while  FIG. 5  shows an enlarged view of the threaded connection  32 . As used in this application, the term “pin-side” refers to the end  34  of the stator housing  36  that is threadably received by the “box-side”  38  of the adjacent housing/drill string component  40 .  
         [0021]     The illustrated conventional thread profile  30  of the threaded connection  32  is a tapered thread with a symmetrical V-profile and a symmetrical root radius  42 . The thread root radius  42  governs the stress concentration and the level of fatigue resulting from the bending and axial stresses experienced in the threaded connection  32  during the drilling process. For a given pitch diameter and profile angle, the root radius  42  is limited by the useful mating area and the thread pitch P.  
         [0022]      FIGS. 6 and 7  show an embodiment of the thread profile  44  of a threaded connection  46  of the present invention. The threaded connection  46  is adapted for the connection of the stator/adapter housing  48  of a PDM.  FIG. 6  shows the threaded connection  46  on the pin-side  50  of the stator/adapter housing  48 , while  FIG. 7  shows an enlarged view of the threaded connection  46 . As best seen in  FIG. 7 , the embodiment of the thread profile  44  of the present invention is shown in solid lines that overlay the conventional thread profile  30  that is indicated by the dashed lines.  
         [0023]     As shown, the thread profile  44  of the present invention is unsymmetrical. The resulting profile  44  provides for a larger thread root radius  52  than the root radius  42  provided by the conventional prior art design. Because the thread root radius  52  governs the stress concentration and the level of fatigue of the threaded connection  46  experienced under bending and axial stresses, the larger root radius  52  acts to reduce the susceptibility to fatigue failure of the threaded connection  46 . The same is accomplished by reducing the angle of the non-load bearing flank  54  of the threaded connection  46 . In this way, the load bearing area and pitch P of the threaded connection  46  is maintained while a larger root radius  52  is provided. Accordingly, the make-up parameters, such as torque carrying capacity of the threaded connection  46 , remain unaffected while the fatigue failure susceptibility is reduced.  
         [0024]      FIGS. 8 and 9  show an embodiment of the thread profile  56  of the threaded connection  58  of the present invention wherein the thread profile  56  is adapted for use on the box-side  60  of the stator/adapter housing  62 .  FIG. 8  shows the box-side  60  of the stator/adapter housing  62 , while  FIG. 9  shows an enlarged view of the threaded connection  58 . As best seen in  FIG. 9 , the embodiment of the thread profile  56  of the present invention is shown in solid lines that overlay the prior art conventional thread profile  30  that is indicated by the dashed lines.  
         [0025]     Similar to the above discussion regarding the thread profile  44  for the threaded connection  46  on the pin-side  50  of the stator/adapter housing  48 , the thread profile  56  on the box-side  60  is unsymmetrical. The resulting profile  56  provides for a larger thread root radius  64  than the root radius  42  provided by the conventional prior art design. Because the thread root radius  64  governs the stress concentration and the level of fatigue of the threaded connection  58  experienced under bending and axial stresses, the larger root radius  64  acts to reduce the susceptibility to fatigue failure of the threaded connection  58 . The same is accomplished by reducing the angle of the non load-bearing flank  66  of the threaded connection  58 . In this way, the load bearing area and pitch P of the threaded connection  58  is maintained while a larger root radius  64  is provided. Accordingly, the make-up parameters, such as torque carrying capacity of the threaded connection  58 , remain unaffected while the fatigue failure susceptibility is reduced.  
         [0026]     While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.