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BACKGROUND 
       [0001]    Lock assemblies are ubiquitous in the downhole drilling and completions industry. One common type of lock assembly involves locking a plug, choke, pressure holding device, tool, etc. in place by radially extending a plurality of dogs into engagement with corresponding features of a radially disposed tubular. In order to accommodate the dogs, windows must be formed in a mandrel or other component of the tubular string, with relatively narrow struts located between each window presenting likely failure points when the string experiences high pressure situations. This results in the need to balance the width of the dogs and the width of the struts, as making either too small can result in failure of the system (e.g., inability of the dogs to lock the string in place and/or fracture of the struts due to heavy loading). In view of these issues and the prevalence of dog type locking systems in the industry, advances and alternatives in the field of lock assemblies are always well received. 
       BRIEF DESCRIPTION 
       [0002]    A lock assembly including a mandrel having at least one window therein, at least one dog corresponding to the at least one window and radially extendable therethrough, wherein the at least one dog is operatively arranged for engaging at least one feature of a tubular radially disposed with the mandrel, and at least one seal element arranged for sealing the mandrel with respect to the tubular, the at least one seal operatively positioned with respect to the at least one dog for maintaining the mandrel solely in compression against the at least one dog during fluid pressure loading on the mandrel in either longitudinal direction. 
         [0003]    A method of locking an assembly including engaging at least one dog of an assembly with a profile of a tubular radially disposed with the assembly, loading the assembly in either longitudinal direction by pressurizing a fluid, the assembly including at least one seal element for sealing the assembly with respect to the tubular, and maintaining the assembly solely in compression against the at least one dog regardless of the longitudinal direction of the loading. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    The following descriptions should not be considered limiting in any way. With reference to the accompanying drawings, like elements are numbered alike: 
           [0005]      FIG. 1  is a quarter-sectional view of a lock assembly being loaded in a first direction; 
           [0006]      FIG. 2  is a quarter-sectional view of the lock assembly of  FIG. 1  being loaded in a second direction opposite to the first direction; and 
           [0007]      FIG. 3  is a quarter-sectional view of a lock assembly according to another embodiment described herein. 
       
    
    
     DETAILED DESCRIPTION 
       [0008]    A detailed description of one or more embodiments of the disclosed apparatus and method are presented herein by way of exemplification and not limitation with reference to the Figures. 
         [0009]    Referring now to  FIG. 1 , a lock assembly  10  is shown having a mandrel  12  radially disposed with a tubular  14 . The mandrel  12  includes at least one first dog  16  and at least one second dog  18  axially displaced from each other. For simplicity, the dogs  16  and  18  may be referred to in plural although only one of each is needed in the assembly  10 . The dogs  16  and  18  are, e.g., arranged in windows  20  of the mandrel, with struts  22  adjacent the windows  20  and connecting between opposite axial sides of the dogs  16  and  18 . The struts  22  are shown in hidden lines because one strut is located behind each dog  16  and  18  (with multiple struts  22  alternatingly arranged with the windows  20  about the mandrel  12 , with one of the dogs  16  and/or  18  in each window  20 ). Due to the reduced widths of struts and the stress concentrations that result from presence of windows, fracture of struts presents a likely mode of failure during high pressure loading of current dog-type lock assemblies. This problem is alleviated by the current invention as discussed in more detail below. 
         [0010]    The dogs  16  and  18  are arranged to be extended from the mandrel  12  for engagement with corresponding recesses  24  and  26  in the tubular  14  in order to lock the assembly  10 , and therefore the string of which it is a part, with respect to the tubular  14 . The dogs  16  and  18  could be radially extendable according to any known arrangement. For example, in the embodiment of  FIG. 1  the dogs  16  and  18  are arranged to climb ramps  28   a  and  28   b  located between relatively movable components  12   a,    12   b,  and  12   c  of the mandrel  12 , which could be, e.g., threaded together, with relative movement therebetween prevented until the dogs  16  and  18  are aligned with their respective recesses  24  and  26 . 
         [0011]    In  FIG. 1 , the assembly  10  is shown loaded in a direction indicated by an arrow  30 . For example, the loading indicated by the arrow  30  could be the result of a hydraulic or some other fluid pressure event, e.g., acting against an up-hole side of the assembly  10 . Accordingly, the assembly  10  includes a seal element  32  for sealing the mandrel  12  against the tubular  14 , such that pressure on the mandrel  12  in the direction of the arrow  30  acts to move the assembly  12 , forcing the dogs  16  to engage against a profile or face  24   a  of the recess  20 , where the tubular  14  picks up the loading. In response to picking up the loading, the tubular  14  exerts a reaction force on the dogs  16 , which is then exerted by the dogs  16  on the mandrel  12 , as indicated by the direction of an arrow  34 . The mandrel  12  is put into compression between the two forces indicated by the arrows  30  and  34 . The dogs  18  are not in contact with either of a set of profiles  26   a  and  26   b  of the recess  26  when the dogs  16  are engaged against the profile  24   a.    
         [0012]    When loaded in the opposite direction as indicated by an arrow  36  in  FIG. 2 , e.g., the result of a pressure event acting against a downhole side of the assembly  10 , the assembly  10  is shifted until the dogs  18  contact the profile  26   a  of the recess  26 . In response to the tubular  14  picking up the loading at the profile  26   a,  the tubular exerts a reaction force on the dog  18 , which is then exerted by the dog  18  on the mandrel  12 , as indicated by the direction of an arrow  38 . Thus, the mandrel  12  is put into compression between the two forces indicated by the arrows  36  and  38 . The dog  16  is not in contact with either of a set of profiles  24   a  or  24   b  of the recess  24  when the dog  18  is engaged against the profile  26   a.    
         [0013]    Advantageously, as described above, the mandrel  12  is put into compression only, which is more readily handled by the aforementioned dog-type mandrels, particularly the struts of the mandrels, as opposed to tension (e.g., due to the alternating window-strut arrangement described above). That is, for example, a seal element installed upstream from the dog and/or window, with respect to the direction of loading, will prevent a portion of the mandrel being put into tension. By positioning a single seal element between a pair of dogs, the mandrel is sufficiently sealed in both axial loading directions. It is additionally noted that the assembly  10  can be further strengthened by closely setting the tolerances of the dogs  16  and  18  in their respective recesses. That is, with the close tolerances, after the mandrel  12  compresses or buckles some relatively nominal degree (e.g.,  0 . 01  inches), the non-engaged one of the dogs will engage a profile of its corresponding recess for supporting the assembly  10  by enabling the tubular  14  to pick up the loading. 
         [0014]    A lock assembly  40  according to another embodiment, but providing the same benefits noted above, is shown in  FIG. 3 . The lock assembly  40  resembles the assembly  10  in that it includes a mandrel  42  run with a tubular  44 . At least one dog  46  corresponds to and is extendable through corresponding windows  48  in the mandrel  42 . Struts  50  are similarly alternatingly arranged with the windows  48 . The mandrel  42  is formed from two relatively movable components  42   a  and  42   b,  e.g., threaded or ratcheted together, for enabling each dog  46 , initially housed in a notch  52 , to be extended into a corresponding recess  54  in the tubular  44 . The component  42   b,  could be or include, for example, a fishing neck for the mandrel  42 . 
         [0015]    In  FIG. 3 , the assembly  40  is loaded in the direction indicated by an arrow  56  (e.g., an up-hole pressure event). This forces the dogs  46  against a profile or face  54   a  of the recess  54 , where the tubular  44  picks up the loading. Similar to the above, a reaction force is exerted on the dogs  46 , and in turn on the mandrel  42 , in the direction indicated by an arrow  58 . One of ordinary skill will recognize that the system works similarly when loaded in the opposite direction, but with the dogs  46  instead engaging a profile  54   b  of the recess  54 . A pair of seal elements  60  and  62  is disposed with one seal element on each opposite axial side of the dogs  46 . If only the seal element  62  were provided and the mandrel  42  was loaded in the direction of the arrow  56 , for example, the portion of the mandrel generally between the dogs  46  and the seal element  62 , including the struts  50 , would be put in tension. A similar outcome would result if only using the seal element  60  and loading in the direction opposite to the arrow  56 . Thus, by positioning the seal elements  60  and  62  on both axial sides of the dogs  46 , the mandrel  42  is advantageously maintained in compression regardless of the loading direction. 
         [0016]    While the invention has been described with reference to an exemplary embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the claims. Also, in the drawings and the description, there have been disclosed exemplary embodiments of the invention and, although specific terms may have been employed, they are unless otherwise stated used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention therefore not being so limited. Moreover, the use of the terms first, second, etc. do not denote any order or importance, but rather the terms first, second, etc. are used to distinguish one element from another. Furthermore, the use of the terms a, an, etc. do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item.

Summary:
A lock assembly including a mandrel having at least one window therein. At least one dog corresponds to the at least one window and is radially extendable therethrough. The at least one dog is operatively arranged for engaging at least one feature of a tubular radially disposed with the mandrel. At least one seal element is arranged for sealing the mandrel with respect to the tubular and operatively positioned with respect to the at least one dog for maintaining the mandrel solely in compression against the at least one dog during fluid pressure loading on the mandrel in either longitudinal direction with respect to the lock assembly.