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FIELD OF THE INVENTION 
       [0001]    This invention relates, in general, to equipment utilized in conjunction with operations performed in subterranean wells and, in particular, to an apparatus and method for disconnecting an upper part of a tubular string from a lower part of a tubular string during downhole operations. 
       BACKGROUND OF THE INVENTION 
       [0002]    Without limiting the scope of the present invention, its background is described with reference to safety joints, as an example. 
         [0003]    It is common practice in the wellbore drilling and completion arts to include a safety joint in a downhole tubular string to provide a point of separation in the tubular string such that the portion of the tubular string uphole of the safety joint can be retrieved to the surface while leaving the portion of the tubular string downhole of the safety joint in the wellbore. Safety joints are useful in a variety of circumstances. For example, safety joints are commonly used during the installation of certain tools, such as packers, in a wellbore. Similarly, safety joints are useful in allowing the recovery of a majority of the tubular string when an element of the tubular string below the safety joint become stuck or during fishing operations to recover a downhole element that was previously stuck in the wellbore, without the risk of sticking the entire recovery tubular string during the fishing operation. 
         [0004]    Conventional safety joints have been operated using a variety of complicated or risky techniques. For example, certain safety joints have been operated by reciprocating the tubular string up and down while maintaining right-hand torque on the tubular string. In this design, the tubular string reciprocation and right-hand torque backs off a left-hand exterior threaded nut within the housing, which nut prevents the mandrel of the safety joint from coming free from the housing during normal tubular string movement. It has been found, however, that while this type of safety joint may be acceptable in some circumstances, there are occasions when the amount of right-hand torque which can be applied to a tubular string while reciprocating the string is limited by the ability of tools in the tubular string to withstand the required torque. 
         [0005]    In another design, the safety joint is operated by neutralizing the weight of the tubular string at the location of the safety joint and rotating the tubular string to the right, which rotation backs off a left-hand exterior threaded nut within the housing. It has been found, however, that in certain wellbore configurations such as deep or deviated wellbores, torque does not transmit well along the tubular string such that the tubular string itself can be put under large amounts of force which can damage the tubular string. 
         [0006]    In still other designs, the safety joint includes a release sleeve coupled to a mandrel with one or more shearable elements which must be parted by the application of a predetermined tensile or compressive force on the tubular string to enable the desired separation. It has been found, however, that the application of the required tensile or compression force on the tubular string elongates or compresses the tubular string prior to parting the shearable elements. Upon parting of the shearable elements, the tubular string violently recoils within the wellbore, which may cause damage to components in the wellbore or within the tubular string. In addition, it has been found, that during the recoil of the tubular string, the maximum allowable flowrate through certain components of the tubular string may be exceeded. 
         [0007]    Accordingly, a need has arisen for an improved apparatus and method for disconnecting an upper part of a tubular string from a lower part of a tubular string during downhole operations. A need has also arisen for such an apparatus and method that enable separating the tubular string into two parts without the use of complicated or risky techniques. In addition, a need has arisen for such an apparatus and method that enable separating the tubular string into two parts without the need to perform rotations of the tubular string. Further, a need has arisen for such an apparatus and method that enable separating the tubular string into two parts without causing the tubular string to recoil due to tensile elongation. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention disclosed herein is directed to an improved apparatus and method for disconnecting an upper part of a tubular string from a lower part of a tubular string during downhole operations. The apparatus and method of the present invention enable separating the tubular string into two parts without the use of complicated or risky techniques. Also, the apparatus and method of the present invention enable separating the tubular string into two parts without the need to perform rotations of the tubular string. In addition, the apparatus and method of the present invention enable separating the tubular string into two parts without causing the tubular string to recoil due to tensile elongation. 
         [0009]    In one aspect, the present invention is directed to an apparatus for separating a downhole tubular string into two parts. The apparatus includes a receptacle having a profiled surface that is operably associated with a first part of the downhole tubular string. A mandrel is operably associated with a second part of the downhole tubular string. The mandrel is slidably positioned relative to the receptacle. A first sleeve having a profile is slidably positioned between the receptacle and the mandrel. The first sleeve is operable to be securably coupled to the mandrel in first and second positions relative to the mandrel. A second sleeve having a profile is also positioned between the receptacle and the mandrel. A first ring is slidably positioned between the profile of the first sleeve and the profiled surface of the receptacle. A second ring is slidably positioned between the profile of the second sleeve and the profiled surface of the receptacle. The first and second rings initially limit longitudinal movement of the receptacle relative to the mandrel in first and second directions. 
         [0010]    Upon application of a predetermined longitudinal force between the mandrel and the receptacle, the receptacle engages the second ring such that the second ring engages the first ring which shifts the first sleeve from the first position to the second position and allows the second ring to slide off the profile of the second sleeve such that the receptacle engages the first ring, thereby limiting longitudinal movement of the receptacle in the first direction. Upon subsequent movement of the receptacle in the second direction relative to the mandrel, the receptacle engages the first ring such that the first ring slides off the profile of the first sleeve, thereby allowing longitudinal movement of the receptacle relative to the mandrel in both the first and second directions. 
         [0011]    In one embodiment, the profiled surface of the receptacle may be formed in a radially increased portion of an inner surface of the receptacle. In another embodiment, the mandrel may include a radially reduced portion that is operable to receive the first and second sleeves. In yet another embodiment, the receptacle may be positioned to the exterior of the mandrel. In this and other embodiments, at least one seal may be positioned between the receptacle and the mandrel. In one embodiment, the first sleeve may be securably coupled to the mandrel in the first position with at least one shear pin and may be securably coupled to the mandrel in the second position with a retaining ring. In this and other embodiments, the first and second rings may be split rings that are radially outwardly biased when positioned respectively on the profiles of the first and second sleeves. 
         [0012]    In another aspect, the present invention is directed to an apparatus for separating a downhole tubular string into two parts. The apparatus includes a receptacle having a profiled surface that is operably associated with a first part of the downhole tubular string. A mandrel is operably associated with a second part of the downhole tubular string. The mandrel is slidably positioned relative to the receptacle. First and second sleeves are slidably positioned between the receptacle and the mandrel and are operable to be securably coupled to the mandrel in first and second positions relative to the mandrel. The first and second sleeves each have a profile. First and second rings are slidably positioned respectively between the profiles of the first and second sleeves and the profiled surface of the receptacle. The first and second rings initially limit longitudinal movement of the receptacle relative to the mandrel in first and second directions. 
         [0013]    Upon application of a predetermined longitudinal force between the mandrel and the receptacle, the receptacle engages the second ring such that the second ring engages the first ring which shifts the first sleeve from the first position to the second position and allows the second ring to slide off the profile of the second sleeve such that the receptacle engages the first ring, thereby limiting longitudinal movement of the receptacle in the first direction. Upon subsequent movement of the receptacle in the second direction relative to the mandrel, the receptacle engages the first ring such that the first ring slides off the profile of the first sleeve, thereby allowing longitudinal movement of the receptacle relative to the mandrel in both the first and second directions. 
         [0014]    In a further aspect, the present invention is direct to a method for separating a downhole tubular string into two parts. The method includes operably associating a receptacle with a first part of the tubular string and a mandrel with a second part of the tubular string; initially limiting movement in first and second longitudinal directions of the receptacle relative to the mandrel within a range between first and second positions; applying a predetermined longitudinal force between the mandrel and the receptacle; shifting the receptacle in the first direction relative to the mandrel to a third position that is outside of the range; limiting further movement of the receptacle relative to the mandrel in the first direction beyond the third position; releasing the predetermined longitudinal force between the mandrel and the receptacle; and shifting the receptacle in the second direction relative to the mandrel to a fourth position that is within the range to enable longitudinal movement of the receptacle in both the first and second directions. 
         [0015]    The operation of applying a predetermined longitudinal force between the mandrel and the receptacle may also include parting at least one shearable element, applying a tensile force between the mandrel and the receptacle or applying a compressive force between the mandrel and the receptacle. 
         [0016]    In an additional aspect, the present invention is direct to a method for separating a downhole tubular string into two parts. The method includes operably associating a receptacle with a first part of the tubular string and a mandrel with a second part of the tubular string, the receptacle and the mandrel having first and second sleeves disposed therebetween; initially limiting longitudinal movement of the receptacle relative to the mandrel in first and second directions with first and second rings respectively positioned between profiles of the first and second sleeves and a profiled surface of the receptacle; applying a predetermined longitudinal force between the mandrel and the receptacle such as a tensile force or a compressive force; engaging the second ring with the receptacle to shift the first sleeve from a first position to a second position relative to the mandrel and to slide the second ring off the profile of the second sleeve such that the receptacle engages the first ring which limits longitudinal movement of the receptacle in the first direction; and moving the receptacle in the second direction relative to the mandrel to slide the first ring off the profile of the first sleeve, thereby allowing longitudinal movement of the receptacle in both the first and second directions. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    For a more complete understanding of the features and advantages of the present invention, reference is now made to the detailed description of the invention along with the accompanying figures in which corresponding numerals in the different figures refer to corresponding parts and in which: 
           [0018]      FIG. 1  is a schematic illustration of an offshore oil and gas platform operating an apparatus for disconnecting an upper part of a tubular string from a lower part of a tubular string during downhole operations according to an embodiment of the present invention; 
           [0019]      FIG. 2  is a cross sectional view of an apparatus for disconnecting an upper part of a tubular string from a lower part of a tubular string during downhole operations in a first configuration according to an embodiment of the present invention; 
           [0020]      FIG. 3  is a cross sectional view of an apparatus for disconnecting an upper part of a tubular string from a lower part of a tubular string during downhole operations in a second configuration according to an embodiment of the present invention; 
           [0021]      FIG. 4  is a cross sectional view of an apparatus for disconnecting an upper part of a tubular string from a lower part of a tubular string during downhole operations in a third configuration according to an embodiment of the present invention; 
           [0022]      FIG. 5  is a cross sectional view of an apparatus for disconnecting an upper part of a tubular string from a lower part of a tubular string during downhole operations in a fourth configuration according to an embodiment of the present invention; 
           [0023]      FIG. 6  is a cross sectional view of an apparatus for disconnecting an upper part of a tubular string from a lower part of a tubular string during downhole operations in a fifth configuration according to an embodiment of the present invention; 
           [0024]      FIG. 7  is a cross sectional view of an apparatus for disconnecting an upper part of a tubular string from a lower part of a tubular string during downhole operations in a sixth configuration according to an embodiment of the present invention; and 
           [0025]      FIG. 8  is a cross sectional view of an apparatus for disconnecting an upper part of a tubular string from a lower part of a tubular string during downhole operations in a seventh configuration according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]    While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts, which can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention, and do not delimit the scope of the invention. 
         [0027]    Referring initially to  FIG. 1 , an apparatus for disconnecting an upper part of a tubular string from a lower part of a tubular string being deployed from an offshore platform is schematically illustrated and generally designated  10 . A semi-submersible platform  12  is centered over submerged oil and gas formation  14  located below sea floor  16 . A subsea conduit  18  extends from deck  20  of platform  12  to wellhead installation  22 , including blowout preventers  24 . Platform  12  has a hoisting apparatus  26 , a derrick  28 , a travel block  30 , a hook  32  and a swivel  34  for raising and lowering pipe strings, such as a tubular string  36 . 
         [0028]    A wellbore  38  extends through the various earth strata including formation  14 . Wellbore  38  includes casing that is cemented within wellbore  38  by cement  42 . Disposed within the lower portion of wellbore  38  as part of tubular string  36  is a tool string  44  including a variety of tools such as safety devices, flow control devices, sand control screens, packers and the like that are used to complete the well. In addition, tubular string  36  includes a safety joint  46  that provides a point of separation in tubular string  36  such that an upper portion  48  of the tubular string  36  can be retrieved to the surface while leaving a lower portion  50  of tubular string  36  downhole. Safety joint  46  may be used to disconnect upper portion  48  from lower portion  50  after the installation of tool string  44  or in the event a tool within tool string  44  become stuck in wellbore  38  prior proper installation. In either case, safety joint  46  may be operated using a combination of compressive and tensile forces to disconnect upper portion from lower portion  50  as described in greater detail below. 
         [0029]    Even though  FIG. 1  depicts a deviated wellbore, it should be understood by those skilled in the art that the apparatus for disconnecting an upper part of a tubular string from a lower part of a tubular string of the present invention is equally well suited for use in wellbores having other directional orientations including vertical wellbores, horizontal wellbores, multilateral wellbores or the like. Accordingly, it should be understood by those skilled in the art that the use of directional terms such as above, below, upper, lower, upward, downward, uphole, downhole and the like are used in relation to the illustrative embodiments as they are depicted in the figures, the uphole direction being toward the top or the left of the corresponding figure and the downhole direction being toward the bottom or the right of the corresponding figure. Also, even though  FIG. 1  depicts an offshore operation, it should be understood by those skilled in the art that the apparatus for disconnecting an upper part of a tubular string from a lower part of a tubular string of the present invention is equally well suited for use in onshore operations. 
         [0030]    Referring next to  FIG. 2 , therein is depicted an apparatus for disconnecting an upper part of a tubular string from a lower part of a tubular string or safety joint  100  according to the present invention. Safety joint  100  includes a longitudinally extending, generally tubular receptacle  102 . Receptacle  102  includes a profiled surface  104  that is depicted as a radially increased annular portion  106  of the inner surface of receptacle  102  that defines an upper shoulder  108  and a lower shoulder  110 . Preferably, receptacle  102  is operably associated with the lower portion of the downhole tubular string in which safety joint  100  is a part. Even though  FIG. 2  depicts receptacle  102  as a single tubular member, those skilled in the art will recognize that receptacle  102  could alternatively be formed from a plurality of tubular sections that are threadably or otherwise secured together. 
         [0031]    Safety joint  100  includes a longitudinally extending, generally tubular mandrel  112 . Mandrel  112  includes a radially reduced annular portion  114  that defines an upper shoulder  116  and a lower shoulder  118 . Mandrel  112  includes an upper shear pin receiving groove  120  and a lower shear pin receiving groove  122 . Alternatively, mandrel  112  may have discrete shear pin receiving openings that may be individually threaded to receive shear screws therein. Mandrel  112  also includes an upper retainer ring receiving groove  124  and a lower retainer ring receiving groove  126 . Preferably, mandrel  112  is operably associated with the upper portion of the downhole tubular string in which safety joint  100  is a part. Even though  FIG. 2  depicts mandrel  112  as a single tubular member, those skilled in the art will recognize that mandrel  112  could alternatively be formed from a plurality of tubular sections that are threadably or otherwise secured together. 
         [0032]    Mandrel  112  includes a gland groove  128  that is operable to receive a sealing array  130  therein that provides a seal between mandrel  112  and receptacle  102 . In the illustrated embodiment, sealing array  130  includes a pair of oppositely disposed adaptor members  132 ,  134 , a pair of upper back up rings depicted as V-rings  136 ,  138 , a pair of lower back up rings depicted as V-rings  140 ,  142 , and an energizing element depicted as O-ring seal  144 . It should be understood by those skilled in the art that the material or materials selected for the V-rings and O-ring is based upon factors such as chemical compatibility, application temperature, sealing pressure and the like. In addition, even though a particular sealing array has been depicted and described, those skilled in the art will understand that other sealing systems having a greater number of seal elements or a lesser number of seal elements could alternatively be used in conjunction with the present invention. Further, in certain embodiments of the present invention, no sealing array or seal is required. 
         [0033]    Safety joint  100  includes a longitudinally extending, generally tubular upper sleeve  146  and a longitudinally extending, generally tubular lower sleeve  148 . Upper and lower sleeves  146 ,  148  are slidably received around radially reduced annular portion  114  of mandrel  112 . As illustrated, upper sleeve  146  is secured to mandrel  112  via a plurality of shearable elements depicted as shear pins  150  that may be threadably received within a like number of openings that extend through the wall of upper sleeve  146 . Likewise, lower sleeve  148  is secured to mandrel  112  via a plurality of shearable elements depicted as shear pins  152  that may be threadably received within a like number of openings that extend through the wall of lower sleeve  148 . Upper sleeve  146  includes a retainer ring groove  154  that houses a retainer ring  156 . Lower sleeve  148  includes a retainer ring groove  158  that houses a retainer ring  160 . Upper sleeve  146  includes an annular profile  162  that defines an annular shoulder  164 . Lower sleeve  148  includes an annular profile  166  that defines an annular shoulder  168 . 
         [0034]    Safety joint  100  includes an upper ring  170  and a lower ring  172 . Upper ring  170  is positioned on profile  162  of upper sleeve  146 . Lower ring  172  is positioned on profile  166  of lower sleeve  148 . Preferably, upper ring  170  and lower ring  172  are in the form of C-rings or split rings and are biased outwardly when positioned on profile  162  and profile  166 , respectively and have a free configuration that is sized to be tight around mandrel  112  for the reasons discussed below. As illustrated in  FIG. 2 , upper ring  170  and lower ring  172  are also received within profiled surface  104  of receptacle  102 . In this configuration, upper ring  170  and lower ring  172  limit the longitudinal travel of receptacle  102  relative to mandrel  112  within a predetermined range. Specifically, receptacle  102  can travel between the point at which shoulder  108  contacts upper ring  170  and the point at which shoulder  110  contacts lower ring  172 . In the illustrated embodiment, further downward travel of receptacle  102  relative to mandrel  112  is prevented by shear pins  152  due to contact between upper ring  170  and lower ring  172 . Likewise, further upward travel of receptacle  102  relative to mandrel  112  is prevented by shear pins  150  due to contact between lower ring  172  and upper ring  170 . 
         [0035]    A first operating mode of safety joint  100  will now be described with reference to  FIGS. 3-5 . As illustrated in  FIG. 3 , weight is being applied from the surface which has caused mandrel  112  to move downhole relative to receptacle  102  such that lower ring  172  has come in contact with or been engaged by shoulder  110  of receptacle  102 . In this configuration, application of a predetermined compressive longitudinal force between mandrel  112  and receptacle  102  causes shear pins  150  to part due to the force of lower ring  172  on upper ring  170 . When shear pins  150  part, upper sleeve  146  is shifted in the uphole direction until upper sleeve  146  contacts shoulder  116 , as best seen in  FIG. 4 . In this position, retainer ring  156  snaps into retainer ring receiving groove  124  which prevents subsequent longitudinal movement of upper sleeve  146 . 
         [0036]    During the initial movement of upper sleeve  146 , lower ring  172  slides along profile  166  until support thereunder is lost. Lower ring  172  then snaps into contact with mandrel  112 . In this configuration, lower ring  172  no longer limits longitudinal travel of receptacle  102  relative to mandrel  112 . The longitudinal travel of receptacle  102  relative to mandrel  112  in the uphole direction outside of the initial range is allowed but the extent of the travel is now limited by upper ring  170  due to the engagement of shoulder  110  of receptacle  102  with upper ring  170 . In this manner, any compression in the tubular string during the operation of safety joint  100  will not result in recoil of the tubular string as the distance receptacle  102  can travel relative to mandrel  112  is limited. 
         [0037]    Once safety joint  100  is in the configuration depicted in  FIG. 4 , the weight from the surface can be reduced in a controlled fashion to allow decompression of the tubular string. Continued movement of mandrel  112  in the uphole direction now causes shoulder  108  of receptacle  102  to come in contact with or engage upper ring  170  once receptacle  102  returns to a position within its initial longitudinal range relative to mandrel  112 . This contact causes upper ring  170  to slide along profile  162  until support thereunder is lost. Upper ring  170  then snaps into contact with mandrel  112 , as best seen in  FIG. 5 . In this configuration, neither upper ring  170  nor lower ring  172  limits longitudinal travel of receptacle  102  relative to mandrel  112 . As such, mandrel  112  and the upper portion of the tubular string can be retrieved uphole or to the surface while leaving receptacle  102  and the lower portion of the tubular string in position in the wellbore. 
         [0038]    A second operating mode of safety joint  100  will now be described with reference to  FIGS. 6-8 . As illustrated in  FIG. 6 , the upper portion of tubular string is being raised from the surface which has caused mandrel  112  to move uphole relative to receptacle  102  such that upper ring  170  has come in contact with or been engaged by shoulder  108  of receptacle  102 . In this configuration, application of a predetermined tensile longitudinal force between mandrel  112  and receptacle  102  causes shear pins  152  to part due to the force of upper ring  170  on lower ring  172 . When shear pins  152  part, lower sleeve  148  is shifted in the downhole direction until lower sleeve  148  contacts shoulder  118 , as best seen in  FIG. 7 . In this position, retainer ring  160  snaps into retainer ring receiving groove  126  which prevents subsequent longitudinal movement of lower sleeve  148 . 
         [0039]    During the initial movement of lower sleeve  148 , upper ring  170  slides along profile  162  until support thereunder is lost. Upper ring  170  then snaps into contact with mandrel  112 . In this configuration, upper ring  170  no longer limits longitudinal travel of receptacle  102  relative to mandrel  112 . The longitudinal travel of receptacle  102  relative to mandrel  112  in the downhole direction outside of the initial range is allowed but the extent of the travel is now limited by lower ring  172  due to the engagement of shoulder  108  of receptacle  102  with lower ring  172 . In this manner, any tensile elongation of receptacle  102  and the portion of the tubular string therebelow during the operation of safety joint  100  will not result in recoil of the tubular string as the distance receptacle  102  can travel relative to mandrel  112  is limited. 
         [0040]    Once safety joint  100  is in the configuration depicted in  FIG. 7 , the tensile force from the surface can be reduced in a controlled fashion to allow the tubular string to return to its unstressed state. Continued movement of mandrel  112  in the downhole direction now causes shoulder  110  of receptacle  102  to come in contact with or engage lower ring  172  once receptacle  102  returns to a position within its initial longitudinal range relative to mandrel  112 . This contact causes lower ring  172  to slide along profile  166  until support thereunder is lost. Lower ring  172  then snaps into contact with mandrel  112 , as best seen in  FIG. 8 . In this configuration, neither upper ring  170  nor lower ring  172  limits longitudinal travel of receptacle  102  relative to mandrel  112 . As such, mandrel  112  and the upper portion of the tubular string can be retrieved uphole or to the surface while leaving receptacle  102  and the lower portion of the tubular string in position in the wellbore. 
         [0041]    While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments as well as other embodiments of the invention will be apparent to persons skilled in the art upon reference to the description. It is, therefore, intended that the appended claims encompass any such modifications or embodiments.

Summary:
An apparatus for separating a downhole tubular string into two parts. The apparatus includes a receptacle operably associated with a first part of the downhole tubular string and a mandrel operably associated with a second part of the downhole tubular string. First and second sleeves each having a profile are slidably positioned between the receptacle and the mandrel and are operable to be securably coupled to the mandrel in first and second positions relative to the mandrel. First and second rings are respectively positioned between the profiles of the first and second sleeves and the profiled surface of the receptacle. The first and second rings are operable to initially limit longitudinal movement of the receptacle relative to the mandrel, to prevent tubular string recoil during operation and to allow longitudinal movement of the receptacle relative to the mandrel after operation.