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You are an expert at summarizing long articles. Proceed to summarize the following text: 
FIELD OF THE INVENTION 
     This invention relates in general to running casing into a well bore, and in particular a method of avoiding dropping the casing while it is being run. 
     BACKGROUND OF THE INVENTION 
     Most oil and gas wells are drilled by a drill string made up of drill pipe. At various depths and at the total depth, the operator will remove the drill pipe, then run in a string of casing. The casing lines the well bore and is cemented in place. In another technique, casing is employed as the drill string and when reaching a desired depth, it is cemented in place. 
     When running casing into a well either for casing the well or for casing drilling the well, normally a powered spider is employed at the drill rig floor. The powered spider is a device that encircles the hole in the rig floor. It has segments or slips that will slide down to a gripping position gripping the casing string suspended in the well bore. Fluid power, typically hydraulic, is employed to move the slips back to an upper position to allow the casing to be lowered into the well bore. 
     The spider will support the casing string while a new joint of casing is being made up to it. The new joint of casing will be hoisted by a casing lifting mechanism above the casing string suspended in the well. In one technique, the casing lifting mechanism comprises a casing gripper mounted to a top drive. The top drive runs up and down the derrick and also is capable of rotating a drill string or casing string. The casing gripper has gripping members that can be moved to a gripping position wherein they will engage a wall of the casing to support the casing. The grippers may engage either the inner diameter or the outer diameter of the casing. Typically the casing gripper is actionable by fluid power, such as hydraulic fluid, to move the grippers to their released position. 
     In another technique, rather than a casing gripper mounted to a top drive, the operator will employ a casing elevator, which is suspended by the blocks or a top drive in the derrick. The casing elevator is capable of gripping a string of casing and supporting the weight. The casing elevator is typically moved from the gripping position to the released position by fluid power. 
     After the operator makes up the new joint of casing with the casing string suspended by the spider, he will lift the entire casing string slightly, then release the spider to lower the casing string further into the well. When the upper end of the uppermost joint of the casing string nears the spider, the operator again engages the spider to support the casing string. The operator releases the casing lifting mechanism and repeats the process. 
     There are thus at least two valves that are controlled by personnel on the rig floor, one being to release the spider and the other being to release the casing gripping mechanism. If an operator accidentally moves the spider valve while the casing lifting mechanism is open, it is possible that the casing string could fall into the well bore. Normally the fluid release mechanism for the casing lifting mechanism is not sufficiently strong to release the casing lifting mechanism unless the weight of a joint of casing has been removed from it. However, many spiders have release mechanisms that will release a casing string while supporting it if the casing string weight is not very much. For example, that might occur when only a few joints of casing make up the casing string. It might also occur with a long casing string when this string is being run into a highly deviated well such as a horizontal well. It could also occur with under balanced drilling. There are some proposed solutions but improvements are desired. 
     SUMMARY OF THE INVENTION 
     In this invention, the spider is not released until a controller is assured that the casing lifting mechanism is supporting a minimum amount of weight. In one embodiment, that is performed by providing an axial load sensor for the casing gripper. The load sensor may be a sub mounted above the casing gripper. Preferably the casing gripper sensor will send a signal to the controller indicative of the amount of weight that it is sensing. If the weight is greater than a selected minimum amount, the controller will allow the spider to be released. If not, the controller prevents the spider from being released. If the casing gripper sensor is mounted to a sub above the casing gripper, the weight sensed by it has to be greater than the weight of the casing gripper plus a minimum amount before the controller will release the spider. 
     In another embodiment, the operator provides the spider with a load sensor. The load sensor determines the weight being supported by the spider. If the load sensor indicates that the spider is supporting more than a minimum amount of weight, the controller will not allow the manual control to release the spider. One manner of determining this weight is to place the spider on a weight measuring scale or sensor which senses the weight imposed on it. If the weight sensed by the spider sensor is greater than the weight of the spider, the controller will not allow the spider to release. For redundancy, both the casing gripper sensor and the spider sensor may be employed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view illustrating the safety method of this invention being employed by sensing weight on a casing gripper. 
         FIG. 2  is a schematic view of an alternative embodiment of this invention wherein a spider load sensor senses the weight supported by the spider. 
         FIG. 3  is a schematic view of another embodiment of this invention, employing both the casing gripper sensor of  FIG. 1  and the spider sensor of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , a drilling rig has a rig floor  11  with an opening  13  aligned with a well (not shown). Opening  13  may be a rotary table. A casing string  15  is shown extending into the well through opening  13 . Casing string  15  is made up of pipe intended to line the well bore and be cemented in place. Casing string  15  also may be employed to perform drilling of the well bore before it is cemented in place. Alternately, the well bore may have already been drilled by drill pipe and casing string  15  is being run into the well bore. Although the term “casing string” is employed, the pipe could either be what is conventionally referred to as casing or what is conventionally referred to as liner. The pipe for both casing and liner may be the same; however casing extends all from the bottom of the well bore all the way to the well head at the top of the well. A liner is also cemented in place but it typically extends only to a short distance above the lower end of a preceding string of casing installed in the well. The term “casing string” is thus meant to include both casing and liner strings. 
     A spider  17  at rig floor  11  supports the weight of casing string  15 . Casing string  15  may comprise only one or two joints of casing or it may comprise several hundred joints of casing. Spider  17  has segments or slips  19  that slide down a ramp surface to grip and support the weight of casing string  15 . Slips  19  may be moved back up to a released position by imposing fluid pressure to a piston incorporated within spider  17 . The fluid pressure may be pneumatic or hydraulic and comes from a fluid line  21 . The operator is able to move slips  19  back up to the released position by closing a valve  25  that is connected into fluid line  21  between a fluid pressure source  27  and spider  17 . Although spider  17  is shown extending above rig floor  11 , it could alternately be located recessed so that its upper end is substantially flush with rig floor  11 . 
     The drilling rig includes a top drive  29  in this example. Top drive  29  is a conventional member that may be raised and lowered in the derrick by a hook (not shown). Top drive  29  has a quill  31  that it rotates. In this embodiment, a sensing sub  33  is mounted to quill  31 . Sensing sub  33  has at least one sensor  35  that will sense the axial load passing through sensing sub  33 . For example, sensor  35  may be a strain gauge. Sensor  35  is connected to a transmitter  37  mounted to sensing sub  33 . Transmitter  37  is battery powered and will send a radio frequency signal  38  indicative of the weight being sensed by sensing sub  33 . Sensing sub  33  may have other sensors, as well, such as one to measure torque. 
     A casing lifting mechanism comprising a casing gripper  39  has a rotary mandrel  40  that secures to the lower end of sensing sub  33 . Casing gripper  39  may be of various types and in this example, it includes a spear  41  that extends downward from and is rotated by mandrel  40 . Grippers  43  are mounted on spear  41 . Casing gripper  39  has an internal mechanism for sliding grippers  43  along a ramp surface to move radially outward and grip the inner diameter of a casing joint  45 . Alternately, grippers  43  may be positioned on the outer side of casing joint  45  for moving radially inward to grip the exterior of casing joint  45 . Casing gripper  39  normally has a spring and a piston (not shown). The spring urges grippers  43  toward the gripping position. Fluid pressure applied to the piston urges grippers  43  back to the released position. Mandrel  40  and spear  41  are rotatable in unison with each other while the actuator portion of casing gripper  39  remains stationary. Typically, an anti-rotation member (not shown) extends downward from the stationary part of top drive  29  into engagement with the stationary portion of casing gripper  39  to prevent its rotation. 
     The fluid pressure to release casing gripper  39  may be either pneumatic or hydraulic. In this example, the fluid pressure is supplied by a fluid line  47  from a fluid source  49 . Fluid source  49  may be the same as source  27 . A casing gripper valve  51  is operated by personnel on the rig floor to supply fluid pressure to cause casing gripper  39  to move to the released position. 
     It is important that an operator not cause spider  17  to release casing string  15  unless casing string  15  is being supported by casing gripper  39 . Normally, the weight of a single casing joint  45  is sufficient to prevent casing gripper  39  from being released even if casing gripper valve  51  is closed. However, if the weight of casing string  13  is not very heavy, it is possible that the fluid pressure in line  21  and the piston mechanism of spider  17  are sufficient to release casing string  15  from spider  17  if spider valve  25  is closed. A controller  59  is employed to prevent this occurrence. 
     Controller  59  has a receiver to receive signal  38  from casing gripper sub  33 . Controller  59  has circuitry that will determine whether that signal indicates a minimum weight is being supported by casing gripper  39 . Preferably, the minimum weight is equal to the weight of casing gripper  39  plus the approximate weight of an average casing joint  45 . In the position shown in  FIG. 1 , sensor  33  would be sensing the weight of casing gripper  39  and casing joint  45  but no more. Consequently, controller  59  would not send a signal to a safety valve  61 . Safety valve  61  is normally open and is connected into line  21  between source  27  and spider  17 . If the weight sensed by sensor  33  is greater than the weight of casing gripper  39  and casing joint  45 , the signal may be sent by controller  59  to safety valve  61 , either by electrical wire or wireless. In order for fluid power to be supplied to spider  17  over line  21 , safety valve  61  must be closed, and it will not close until it receives a signal from controller  59  indicating that the weight observed by sensor  33  is above the minimum selected weight. If the operator closes valve  25  when the weight is not above the minimum, controller  59  will not close safety valve  61 . Instead, it preferably sounds a warning that may be audible and/or visible. 
     Controller  59  may have circuitry and a panel that allow the operator to zero out the weight of casing gripper  39  and one joint of casing  45  when casing gripper  39  is first installed. In that event, sensor sub  33  would provide a signal  38  of zero weight if casing gripper  39  is supporting only casing joint  45  before it is connected to casing string  15 . Alternately, sensor sub  33  would provide no signal at all until the weight exceeds the weight of casing gripper  39  and casing joint  45 . The operator could also set a minimum value that is somewhat above that level. However, the value selected above the weight of casing gripper  39  and casing joint  45  would normally not be very high and is preferably less than the weight of one more joint of casing. During the initial stages of casing running, casing string  15  may comprise only a single joint, and if casing joint  45  is properly connected on its lower end to the single joint of casing string  15  and on its upper end to casing gripper  39 , the operator should be free to release spider  17  to lower casing joint  45  and casing string  15 . The release signal from controller  59  to safety valve  61  should be sent even though the weight sensed by sensor  35  is only the weight of two joints of casing plus the weight of casing gripper  39 . 
     In the operation of the embodiment of  FIG. 1 , the operator will have assembled one or more joints of casing to make up casing string  15  and will have it supported by spider  17 . For example, if there is only one joint of casing in casing string  15 , the operator will simply lower it then actuate spider  17  to grip casing string  15 . A bypass may be provided to allow an operator to bypass safety valve  61  to enable the first joint of casing in casing string  15  to be lowered into spider  17 . The operator then picks up a new joint of casing, represented by casing joint  45 . Normally the operator will pick it up with a set of elevators (not shown) attached to links, which in turn are attached to casing gripper  39 . In one technique, the operator then rests new casing joint  45  on casing string  15  and lowers spear  41  while casing gripper valve  51  is closed, which places the grippers  43  in a released position. Once spear  41  is inserted, the operator opens casing gripper valve  51  to cause grippers  43  to grip casing joint  45 . The operator then raises top drive  29  a short distance to assure that grippers  43  are gripping casing  45 .  FIG. 1  shows this position, with lower end  55  of new casing joint  45  a short distance above casing collar  57 , which is at the upper end of casing string  15 . 
     The operator then lowers lower end  55  into engagement with the threads of casing collar  57 . The operator rotates top drive quill  31 , sensing sub  33  and spear  41 , causing new casing joint  45  to rotate. With spider  17  or another mechanism, the operator prevents rotation of casing string  15  until the threads have properly made up. The operator then raises top drive  29  a short distance to remove the weight being supported by spider slips  19 . Sensing sub  33  at that point will be sensing the weight of casing gripper  39 , new casing joint  45  and casing string  15 . Since this weight is over the selected minimum, controller  59  will close safety valve  61 . The operator then closes spider valve  25  to open spider slips  19 . The closure of safety valve  61  and spider valve  25  allows fluid pressure from source  27  to move slips  19  upward and away from casing string  15 . When in the upper position, adequate clearance will be provided for casing collar  57  to pass downward through spider  17 . If the weight sensed by sensor  35  is not over the minimum when the operator closes spider valve  25 , controller  59  provides a warning and will not close safety valve  61 . 
     With spider slips  19  open, the operator lowers top drive  29 , casing string  15  and new casing joint  45 , which is now part of casing string  15 . When the upper end of new casing joint  45  is a short distance above spider  17 , the operator opens spider valve  25 , which causes slips  19  to move downward back into a gripping position. The operator then repeats the cycle until the entire casing string  15  is run. When running casing string  15 , the operator could rotate casing string  15  for drilling or reaming. Furthermore, drilling fluid would be pumped down through casing gripper  39  and casing string  15  if drilling is occurring. Spear  41  normally has a seal that seals to the inner diameter of casing string  15 . As the casing string  15  lengthens, the weight being sensed by sensor  35  may be quite high when casing gripper  39  is supporting the entire weight of casing string  15 . It is not necessary that an accurate weight be measured by sensor  35  once the amount is just over the minimum of the weight of casing gripper  39  plus one casing joint  45 . 
       FIG. 2  shows an alternate embodiment. The components that are the same use the same numerals as in  FIG. 1 . In this embodiment, casing gripper sensing sub  33  ( FIG. 1 ) may be eliminated as illustrated. Alternately, it could be employed but used only to supply torque information to controller  59 . In the embodiment of  FIG. 2 , a spider sensor unit  63  is employed for sensing the weight being supported by spider  17 . Spider sensing unit  63  comprises a load cell that is a flat weight measuring scale with a central hole through it for the passage of casing string  15 . Sensing unit  63  is capable of measuring a selected amount of weight. The selected amount of weight would at least be equal to the weight of spider  17 , which may weigh hundreds of pounds. Controller  63  may have a panel and circuitry that will enable the operator to zero out the weight of spider  17  on spider sensor unit  63 , so that it provides a signal indicating no weight if it is only sensing the weight of spider  17 . Alternately, spider sensor unit  63  could be calibrated to send a signal only if the weight is greater than the weight of spider  17 . When spider  17  is gripping a long string of casing  15  in a vertical well, a very large weight will be imposed on spider  17 . However, spider sensor unit  63  need not be capable of measuring any accurate amounts of weight beyond much more than the weight of spider  17 . There is no need for accuracy beyond a relatively low selected weight. Spider sensor unit  63  also will send a signal  65  to controller  59 . The signal may be wireless or it may be via an electrical wire. 
     In the operation of the embodiment of  FIG. 2 , controller  59  will not allow hydraulic fluid pressure to move spider slips  19  to the released position unless the weight sensed by spider sensor unit  63  is no greater than the selected amount.  FIG. 2  shows a position where new casing joint  45  has been made up to casing string  15 . The operator must now raise top drive  29  a short distance to release the weight imposed on spider slips  19  by casing string  15 . Until the operator lifts casing string  15  with top drive  29 , the weight observed by spider sensor unit  63  will be over the selected amount because it will still be observing at least part of the weight of casing string  15 . Consequently, controller  59  will not close safety valve  61 . Closing spider valve  25  by the operator will have no effect unless the weight observed by spider sensor unit  63  is at or less than the selected amount. 
       FIG. 3  illustrates both embodiments of  FIGS. 1 and 2  coupled together as redundant safety systems. Controller  59  now must receive two satisfactory signals  38  and  65  before it will close safety valve  61 . The signal that it must receive from spider sensor unit  63  is that there is no more than a selected weight being supported by spider  17  at that moment. The signal that it must receive from casing gripper sensing sub  33  is that the weight that it senses is greater than a selected minimum. When these two events occur, closing spider valve  25  will cause slips  19  to move up to the retracted position, which is the position shown in  FIG. 3 . The operator now is free to lower casing string  15  into the well. 
     Although the invention has been shown in only one of its forms, it should be apparent to those skilled in the art that it is not so limited but susceptible to various changes without departing from the scope of the invention. For example, although spider sensor unit  63  is shown as a separate unit mounted below spider  17 , it could alternately include weight sensors mounted directed to and incorporated with spider  17 . The spider sensing system of  FIG. 2  could be employed with casing running operations that employ casing lifting mechanisms other than a casing gripper suspended from a top drive.

Summary:
A method of running casing into a well utilizes a load sensor to avoid dropping the casing string accidentally. The rig has a spider at the rig floor that suspends a casing string in the well when in a gripping position. A casing lifting mechanism will place a new joint of casing on the casing string suspended in the spider. The new joint of casing is rotated to make up with the casing string. After makeup, the casing lifting mechanism lifts the new joint of casing and the casing string. The operator releases the spider to allow the casing string to be lowered further into the well. Before releasing the spider, a load sensor will send a signal indicating that the casing lifting mechanism is supporting a minimum amount of weight.