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BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a mechanism for releasing a wireline cable from a cable head. 
     2. Background Art 
     In oil and gas wireline operations, downhole tools, e.g., logging tools, are conveyed downhole within a wellbore using a wireline cable. The downhole tools are typically tubular members that are threaded together to form a “tool string.” A cable head couples the wireline cable to the tool string. Occasionally, during operation, the tool string may become stuck in the wellbore. When the tool string gets stuck, a high tension is usually applied to the tool string to try to free the tool string from its stuck position. This high tension is applied to the wireline cable at the surface, and the wireline cable transmits the applied tension to the cable head. The cable head in turn transmits the tension to the tool string. The amount of tension available to free the tool string from its stuck position depends on the breaking strength of the wireline cable, the profile and coefficient of friction of the wellbore, the position of the tool string inside the wellbore, and various other parameters, in particular the weight of the cable in the wellbore. 
     The connection between the cable head and the wireline cable typically includes a “weak point.” A weak point is a link designed to break when a predetermined amount of tension is applied to it. Normally, the weak point has the lowest breaking strength in the tensile string. The weak point allows the wireline cable to be separated from the cable head in the event that enough tension cannot be applied to free the tool string. The operator first latches onto the cable head or tool string using a fishing tool coupled to one end of a drill pipe and then applies tension to the wireline cable to break the weak point and release the wireline cable from the cable head. The wireline cable is first removed from the wellbore, and hen the cable head and the tool string are pulled out of the wellbore by removing the drill pipe. 
     The weak point is usually designed for the worst case scenario. In other words, the breaking strength of the weak point must be lower than the minimum tension that the wireline cable can transmit to the desired maximum depth of descent into the wellbore. Otherwise, if the tool string gets stuck at a depth where the amount of tension that can be transmitted safely through the wireline cable is less than the breaking strength of the weak point, it will be impossible to break the weak point. The strength of the weak point must also be greater than the weight of the tool string plus a safety factor. These requirements sometimes limit the depth to which the tool string can safely descend inside the wellbore. 
     However, the tool string will not always get stuck at the maximum depth of descent into the wellbore. If the tool string gets stuck at a point above the maximum depth of descent into the wellbore, the maximum tension that can be transmitted to the cable head without breaking the wireline cable will be much greater than what is needed to break the weak point. If this maximum tension is transmitted to the cable head, the weak point will break before the fully available tension can be used to try to free the tool string. 
     The different tool sticking conditions give rise to the need for a weak point having two distinct breaking strengths. In one mode, the breaking strength of the weak point is greater than the breaking strength of the wireline cable so that all the tension capable of being transmitted to the cable head can be applied to freeing the tool string from its stuck position. In another mode, the weak point can be broken without exceeding the breaking strength of the wireline cable at any depth of descent. 
     U.S. Pat. No. 6,032,733 issued to Ludwig et al. discloses a latch assembly for releasably connecting a wireline cable to a cable head which operates in two modes. The latch assembly includes an anchor sub keyed within an inner housing. The anchor sub has a tensile strength greater than the safe pull of the wireline, where “safe pull” of the wireline is defined as a tension that does not exceed one-half the breaking strength of the wireline. The anchor sub has a neck portion and a bore extending from the neck portion to the body of the anchor sub. A latch housing is threaded to the neck portion of the anchor sub, and a chamber is defined within the latch housing. A latch shaft extends through the chamber. One end of the latch shaft is coupled to the wireline. The latch shaft has an enlarged portion which divides the chamber into two sub chambers. In the latched position, there is an interference fit between the latch shaft and the latch housing, the upper sub chamber contains a fusible material, and the volume of the lower sub chamber is substantially zero. The latch assembly also includes heaters for heating the fusible material. 
     During normal operation, the latch assembly couples the wireline to the cable head housing. When it is desired to release the wireline from the cable head, the operator sends a command to a switching circuit which then directs current to the heaters. The heaters, which are in contact with the latch housing, heat the metal of the latch housing, causing the latch housing and the enlarged portion of the latch shaft to expand. The latch housing has a higher coefficient of expansion than the enlarged portion of the latch shaft. Thus, a gap is formed between the latch housing and the enlarged portion of the latch shaft as the latch housing is heated. The heated latch housing also causes the fusible material in the upper chamber to melt. The melted fusible material flows into the lower sub chamber through the gap formed between the latch housing and the enlarged portion of the latch shaft. As the fusible material flows into the lower sub chamber, the tension applied to the latch shaft by the wireline cable causes the latch shaft to move upwardly. This causes the latch shaft to move to the unlatched position. 
     SUMMARY OF THE INVENTION 
     In one aspect, the invention relates to a cable release apparatus which comprises a housing and a latch mounted at one end of the housing. The latch has a central opening and a plurality of projecting members extending into the housing. A releasable connector is mounted inside the housing. An actuator has one end disposed in the central opening in the latch and another end in contact with the releasable connector. The actuator is movable between a first position prior to activation of the releasable connector and a second position wherein the releasable connector is activated. Prior to activation of the releasable connector, the latch is held in place by an interference fit between the projecting members and the housing. When the releasable connector is activated, the projecting members are deflected by applying tension to the latch, thereby releasing the latch from the housing. 
     In some embodiments, the projecting members comprise outer wedged surfaces for engagement with an inner wedged surface on the housing. In some embodiments, the projecting members comprise inner wedged surfaces for engagement with an outer wedged surface on the actuator. In some embodiments, the releasable connector comprises a plurality of connector segments held together by a spring and a heater for heating a solder joint in the spring so as to enable expansion of the spring. In some embodiments, a spring is provided to apply a force on the actuator such that the actuator moves in the direction of the releasable connector when the releasable connector is activated. 
     In another aspect, the invention relates to a cable head which comprises a head housing and a cable release housing mounted inside the head housing. The cable head further comprises a latch mounted at one end of the cable release housing. The latch has a central opening and a plurality of projecting members extending into the cable release housing. The cable head further comprises a cable connector coupled to the latch. A releasable connector is mounted inside the cable release housing and an actuator has one end disposed in the central opening in the latch and another end in contact with the releasable connector. The actuator is movable between a first position prior to activation of the releasable connector and a second position wherein the releasable connector is activated. Prior to activation of the releasable connector, the latch is held in place by an interference fit between the projecting members and the housing. When the releasable connector is activated, the projecting members are deflected by applying tension to the latch, thereby releasing the latch from the housing. 
     In another aspect, the invention relates to a logging tool which comprises a head housing and a cable release housing disposed inside the head housing. The cable head further comprises a latch mounted at one end of the cable release housing. The latch has a central opening and a plurality of projecting members extending into the cable release housing. The cable head further comprises a cable connector coupled to the latch. A releasable connector is mounted inside the cable release housing and an actuator has one end disposed in the central opening in the latch and another end in contact with the releasable connector. The actuator is movable between a first position prior to activation of the releasable connector and a second position wherein the releasable connector is activated. A downhole tool is coupled to the head housing and the cable release housing. Prior to activation of the releasable connector, the latch is held in place by an interference fit between the projecting members and the cable release housing. When the releasable connector is activated, the projecting members are deflected by applying tension to the latch. 
     Other aspects and advantages of the invention will be apparent from the following description and the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an electrically controlled release device according to an embodiment of the invention. 
     FIG. 2 is a cross-section of the release device of FIG.  1 . 
     FIG. 3 shows the release device of FIG. 1 located in a logging head. 
     FIG. 4 shows a logging tool suspended in a wellbore on the end. of a wireline cable. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the invention provide an electrically controlled release device for a downhole cable head, e.g., a downhole logging head. The electrically controlled release device has two modes of operation. In the first mode of operation, the electrically controlled release device transmits the tension applied to the cable head by a wireline cable to the downhole tools coupled to the cable head without releasing the wireline cable from the cable head. In the second mode of operation, the electrically controlled device releases the wireline cable from the cable head when a low tension is applied to the cable head. The electrically controlled release device can be activated to release the wireline cable regardless of the tensile load it is transmitting. 
     Various embodiments of the invention will now be described with reference to the accompanying drawings. FIG. 1 shows an electrically controlled release device  2  in accordance with one embodiment of the invention. The release device  2  comprises an upper housing body  4  and a lower housing body  6 . The upper housing body  4  is coupled to the lower housing body  6  by a threaded connection  8 , for example. An o-ring  7  provides a seal between the upper housing body  4  and the lower housing body  6 . The upper housing body  4  is provided with a central opening  10 , and the lower housing body  6  is provided with a central chamber  12 . A latch  14  is mounted on the upper housing body  4 . The latch  14  has a latching head  16  and fingers  18  which extend from the latching head  16 . The fingers  18  extend into the central opening  10  in the upper housing body  4 . The fingers  18  have wedge-shaped surfaces  20  which are adapted to engage with a wedge-shaped surface  22  in the inner wall of the upper housing body  4 . An o-ring  24  provides a seal between the latching head  16  and the upper housing body  4 . An actuator  26  is disposed within a central opening  28  in the latch  14 . A lower portion  30  of the actuator  26  extends through the upper housing body  4  into the central chamber  12  in the lower housing body  6 . The actuator  26  has a tapered surface  32  which engages with tapered surfaces  34  on the fingers  18 . An o-ring  35  provides a seal between the latching head  16  and the actuator  26 . 
     A split bobbin assembly  36  is disposed in the central chamber  12  in the lower housing body  6 . As shown in FIG. 2, the split bobbin assembly  36  includes quartered bobbin pieces  44  and a resistive heater  46  arranged in a ring structure. The bobbin pieces  44  are preferably made of a heat-resistant material. A metal spring or coil  45  is tightly wound around the bobbin pieces  44  and the resistive heater  46  and soldered in place, as shown at solder joint  47 . In this way, the bobbin pieces  44  are held together. In one embodiment, the metal spring  45  is made of a heat-resistant conductive material such as beryllium-nickel alloy. As will be further discussed below, the purpose of the resistive heater  46  is to melt the solder joint  47  so that the metal spring  45  expands. When the metal spring  45  expands, the bobbin pieces  44  become separated. 
     Returning to FIG. 1, the lower housing body  6  includes two apertures  48 ,  50  for receiving insulating electrical feed-throughs  52 ,  54 , respectively.  0 -rings  49 ,  51  provide seals between the lower housing body  6 , and feed throughs  52 ,  54  respectively. The electrical feed-throughs  52 ,  54  provide the electrical current needed to power the resistive heater  46  (shown in FIG.  2 ). A plate  38  made of insulating material is arranged between the split bobbin assembly  36  and the lower housing body  6 . A nose portion  40  of the actuator  26  is in contact with the split bobbin assembly  36 . A spring  42  disposed between the actuator  26  and the upper housing body  4  applies a biasing force to the actuator  26  such that the nose portion  40  of the actuator  26  is held against the bobbin pieces  44  (shown in FIG. 2) in the split bobbin assembly  36 . 
     The release device  2  has two modes of operation. In mode one, the release device transmits tension applied to the latch  14  without the fingers  18  separating from the upper housing body  4 . In mode two, the fingers  18  can be separated from the upper housing body  4  with a small tension applied to the latch  14 . In mode one, a tensile load may be applied to the latch  14  through the surface  55  of the latch  14 . The tension applied to the latch  14  is transmitted to the upper housing body  4  through the surfaces  20 ,  22 . The wedging effect of the surfaces  20 ,  22  tends to cause the fingers  18  to deflect, causing a compressive force to be applied to the actuator  26  through the surfaces  32 ,  34 . The wedging effect created by the interface of the fingers  18  and the actuator  26  at surfaces  32 ,  34  tends to push the actuator  26  against the split bobbin assembly  36 , causing a compressive load at the interface  57  between the nose portion  40  of the actuator  26  and the split bobbin assembly  36 . The split bobbin assembly  36  in turn applies a compressive load to the insulating plate  38 , and the insulating plate  38  in turn applies a compressive load to the lower housing body  6  through interface  61 . 
     The lower housing body  6  is coupled to the upper housing body  4  such that a compressive load is reacted from the fingers  18  through surfaces  20 ,  22 , through surfaces  32 ,  34 , through the interface  57  between the actuator  26  and the split bobbin assembly  36 , through the interface  59  between the split bobbin assembly  36  and the insulating plate  38 , and through the interface  61  between the insulating plate  38  and the lower housing body  6 . As long as the compressive loop is reacted, the fingers  18  cannot deflect, and they are held in place relative to the upper housing body  4  via an interference fit. Thus, a tensile load can be transmitted from the fingers  18  to the upper housing body  4  without separating the fingers  18  from the upper housing body  4 . The tensile load transmitted to the upper housing  4  is then transmitted to the lower housing body  6  through the connection  8  between the upper housing body  4  and the lower housing body  6 . 
     In mode one, the bobbin pieces  44  (shown in FIG. 2) are held together by the metal spring  45  and the fingers  18  are held in place relative to the upper housing body  4 . In mode two, the fingers  18  can be separated from the upper housing body  4  with a small tension applied to the latch  14 . To switch the release device  2  from mode one to mode two, a command is sent to a switching circuit (not shown) to power the resistive heater  46  (shown in FIG.  2 ). The switching circuit (not shown) directs current to the resistive heater  46  (shown in FIG. 2) through the electrical feed-throughs  52 ,  54 . The resistive heater  46  (shown in FIG. 2) melts the solder joint  47  in the metal spring  45 , as previously described, thus allowing the metal spring  45  to expand and the bobbin pieces  44  to become separated. When the bobbin pieces  44  separate, the actuator  26  moves downwardly. The force which causes the actuator  26  to move downwardly comes from the spring  42  and the wedging effect between the surfaces  20 ,  22  and  32 ,  34  created by the tension applied to the latch  14 . In this state, the compressive loop described above can no longer be reacted and a small tension applied to the upper housing body  4  will separate the fingers  18  from the upper housing body  4 . Once the fingers  18  are separated from the upper housing body  4 , the latch  14  can be removed from the release device  2 . 
     Preferably, the seal provided by  0 -ring seal  35  is broken when the bobbin pieces  44  separate and as the actuator  26  moves downwardly. This allows the release device  2  to be flooded with wellbore fluid so that pressure balance is created between the interior and the exterior of the release device  2 . This is necessary because the interior of the release device  2  is initially at atmospheric pressure and the release device  2  may need to be separated at ambient external pressures as high as 20,000 psi. If the release device  2  were not pressure balanced, the pressure forces holding the latch  14  and the upper housing body  4  would be too great to allow the fingers  18  to be separated from the upper housing body  4 . The flooding of the release device  2  also provides additional force for moving the actuator  26  downwardly. In addition, the wedge shape of the surfaces  32  of the actuator  26  allows the release device  2  to be separated while tension is being transmitted by the release device  2 . If the surface  32  were parallel to the axis of the release device  2 , frictional forces would keep the actuator  26  from moving while the release device  2  is transmitting tension, even if the bobbin pieces  44  are separated. 
     FIG. 3 shows a well logging cable head  60 . It should be noted that the cable head  60  is not shown in its entirety to avoid obscuring the invention. In operation, the lower end of the cable head  60  would be coupled to a logging tool assembly (not shown). The cable head  60  includes an outer housing  62 . The electronically controlled release device  2  (previously shown in FIG. 1) is mounted inside the outer housing  62 . A fishing neck  66  is mounted at the upper end of the outer housing  62 . The fishing neck  66  has a central bore  68  for receiving a shell  70 . The lower end of the shell  70  is secured to the latching head  16  of the release device  2 . A housing  73  is attached to the upper end of the shell  70 . Inside the housing  73  is a rope socket  72  which has an aperture  74  for receiving a wireline cable (not shown). A conductor sleeve  76  is mounted inside the shell  70 . The conductor sleeve  76  connects the terminal ends of conductors in the wireline cable (not shown) to a connector  78  in the shell  70 . The connector  78  is in turn connected to the rest of the tool by electrical wiring  80 . In this way, signals can be transmitted to and from the surface through the wireline cable (not shown). 
     FIG. 4 shows the cable head  60  suspended in a wellbore  82  on the end of a wireline cable  84 . The wireline cable  84  is payed from a surface winch  86 . In operation, tension from the surface winch  86  is transmitted down to the cable head  60  via the wireline cable  84 . The tension transmitted to the cable head  60  is then transmitted to the logging tool assembly  64  attached to the cable head  60  through the release device  2  in the cable head  60  (see FIG.  3 ). During normal logging, the release device  2  in the cable head  60  (see FIG. 3) is in mode one in which it will transmit high tensions without separating. When desired, the release device  2  in the cable head  60  (see FIG. 3) is actuated to mode two and will separate with only a small tensile force applied to it. 
     In mode two, the operator first sends a command to the downhole switching circuit (not shown) to power the resistive heater  46  (shown in FIG.  2 ). The resistive heater  46  (shown in FIG. 2) melts the solder joint  47  in the metal spring  45  (shown in FIG.  2 ), allowing the bobbin pieces  44  (shown in FIG. 2) to separate. Once the bobbin pieces  44  (shown in FIG. 2) separate, a small tensile force applied to the release device  2  will separate the fingers  18  (shown in FIG. 1) from the upper housing body  4  (shown in FIG.  1 ). When the fingers  18  (shown in FIG. 1) are separated from the upper housing body  4  (shown in FIG.  1 ), the latch  14  (shown in FIG. 1) and the other components coupled to the latch  14 , e.g., the receiving sleeve  70 , can be released from the cable head  60  and pulled to the surface. The rest of the cable head  60  and the logging tool assembly  64  can then be pulled out of the wellbore  82  using a fishing tool (not shown). 
     The invention is advantageous in that it provides an electronically controlled weak point that will release regardless of the tension it is transmitting. The release device operates in one of two modes. In mode one, the release device will not separate while transmitting tension. In this mode, the weak point is then the wireline cable. In mode two, the release device will separate with a small applied tension. The release device will separate regardless of the tension it is transmitting. The release device can be located in a cable head, as shown in FIG. 3, or in any cable head in general. 
     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.

Summary:
A cable release apparatus includes a housing and latch mounted at one end of the housing. The latch has a central opening and a plurality of projecting members extending into the housing. A releasable connector is mounted inside the housing. An actuator has one end disposed in the central opening in the latch and another end in contact with the releasable connector. The actuator is movable between a first position prior to activation of the releaseable connector and a second position wherein the releasable connector is activated. Prior to activation of the releasable connector, the latch is held in place by an interference fit between the projecting members and the housing. When the releasable connector is activated, the projecting members are deflected by applying tension to the latch, thereby releasing the latch from the housing.