You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
FIELD OF THE INVENTION 
       [0001]    This invention relates in general to equipment for connecting a joint of casing to a string of casing suspended by spider at the rig floor, and in particular to a guide for guiding a casing gripper into the upper end of a joint of casing. 
       BACKGROUND OF THE INVENTION 
       [0002]    Casing comprises pipe that is used to line a wellbore and is cemented in place. The casing may extend all the way to the wellhead at the top of the well, or it may extend up only to the lower end of a next upper string of casing. In the latter instance, the casing is typically referred to as a liner. The casing may be installed in a portion of the wellbore that has been previously drilled by drill pipe. Alternately, the casing may itself be used as the drill string to drill portions of the well. 
         [0003]    In either event, the individual joints or sections of casing are secured to each other to make up a casing string being lowered into the well. When adding a new joint of casing to a string of casing, the string of casing will be supported by a spider at the rig floor. The spider has a set of slips that support the weight of the casing string. In one technique, the drilling rig has a top drive, which is a rotary power source that travels up and down the drilling rig. A casing gripper is secured to the quill or drive stem of the top drive. The casing gripper has radially moveable gripping elements that will grip either the inner diameter or outer diameter of the joint of casing. A set of links, also called bails, are mounted to the casing gripper to support a casing elevator below the lower end of the casing gripper. The elevator comprises a clamp that fits around the casing joint below the collar on the upper end of the casing joint. Hydraulic cylinders will pivot the bails outward to engage the next joint of casing, which may be spaced laterally from the spider and inclined on a ramp or V-door. 
         [0004]    After clamping the elevator around the joint of casing, the driller raises the top drive and allows the links to swing back into vertical alignment with the top of the string of casing. The operator then lowers the top drive and the joint of casing until it lands on and is supported by the string of casing. The operator continues to lower the top drive and the casing gripper while the joint of casing remains supported on top of the string of casing. The gripping elements of the casing gripper will slide into or over the upper end of the joint of casing. Once in place, the operator actuates the casing gripper to grip the joint of casing, then rotates the gripping element to rotate the joint of casing and make it up with the string of casing. 
         [0005]    In some instances, the elevator links are quite long because they must be able to pivot laterally outward to engage the next joint of casing as it is supported on the V-door. In large rigs, this lateral distance can be substantial. The operator may be able to adjust the length of the links or use longer links. However, longer links place the elevator several feet below the lower end of the casing gripper. This arrangement makes it difficult for the driller to stab the casing gripper into or over the upper end of the casing, particularly with small diameter casing. The upper end of the casing may be 35 to 40 feet above the driller when the stabbing has to occur, making it difficult to see. Having elevator a considerable distance below the casing gripper results in extra time required for making up a new joint of casing with the casing string. 
       SUMMARY OF THE INVENTION 
       [0006]    In this invention, a guide is mounted to the links between the elevator and the casing gripping assembly. The guide has a central opening sized for receiving an upper end of the joint of casing to be connected to the string of casing. Preferably, this central opening is flared at its lower end so as to guide the upper end of the joint of casing as the casing gripper is lowered into or over it. The upper end of the opening may also be flared. 
         [0007]    Optionally, a resilient centering device, such as bow springs or spring loaded roller balls, may be located in the guide opening to guide the upper end of the joint of casing. Optionally, a sensor may be mounted to or adjacent the guide for sensing when the gripper and the upper end of the joint of casing engage each other. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIGS. 1A and 1B  comprise a side elevational view, partially sectioned, of a casing gripper having a guide in accordance with the invention and shown suspending a joint of casing above a string of casing. 
           [0009]      FIGS. 2A and 2B  comprise a side elevational view, partially sectioned, of the casing gripper of  FIG. 1 , and showing the joint of casing being supported on but not yet secured to the string of casing. 
           [0010]      FIGS. 3A and 3B  comprise a side elevational view of the casing gripper of  FIG. 1 , showing the joint of casing being gripped by the casing gripper and being made up to the string of casing. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0011]    Referring to  FIG. 1 , a casing gripper  11  includes an actuator portion and a spear  13  extending below and having a longitudinal axis  14 . Several gripping elements  15  are spaced circumferentially around spear  13 . In this example, gripping elements  15  are on cam or ramp surfaces of spear  13 . When the actuator portion of casing gripper  11  strokes gripping elements  15 , they will move radially outward. Alternately, they could be mounted within a housing surrounding spear  13  for radial inward movement when stroked. A seal  17  is located on spear  13  below gripping elements  15  in this embodiment. Spear  13  has a passage through it with an opening in a nose  19  for discharging drilling fluid. 
         [0012]    A pair of bails or links  21  is mounted to casing gripper  11 . Links  21  have upper ends  23  that have cylindrical co-axial apertures for receiving axles (not shown) extending outward from opposite sides of casing gripper  11 . Hydraulic cylinders (not shown) will pivot links  21  about their upper ends  23 . Upper ends  23  and casing gripper  11  are preferably constructed as in U.S. Pat. No.  7 , 140 , 443 , so that each link  21  rotates in a single plane. Referring still to  FIG. 1A , links  21  may have a fixed length. Alternately, the lengths of links  21  can be adjusted, then secured to a selected new length. In this embodiment, links  21  are adjusted by sliding portions relative to each other, then securing the portions by fasteners or pins  22 . 
         [0013]    An elevator  25  is mounted to the lower ends of links  21 . Elevator  25  is a clamp that is radially opened and closed, either manually or hydraulically. Elevator  25  has an opening sized to loosely receive a joint of casing  27 . Joint  27  has a collar  29  on its upper end that has a larger diameter than the opening in elevator  25 , so that elevator  25  will lift casing joint  27 , but is also able to slide downward on casing joint  27  if the casing joint is stationarily supported. Casing joint  27  has a lower end that normally will have external threads  31  as shown in  FIG. 1B . 
         [0014]    Referring still to  FIG. 1B , threaded end  31  is adapted to stab and be rotated into threaded engagement with a collar  33  located at the upper end of the uppermost casing joint of a casing string  35 . Casing string  35  is made up of joints of casing secured in the same manner as will be subsequently described. Casing string  35  is supported by a spider  37  located either flush with or on a rig floor  39  of a drilling rig. Spider  37  has slips that will grip the side wall of casing string  35  to support its weight. 
         [0015]    A threaded stem  41  is located on the upper end of casing gripper  11  for rotating spear  13  relative to links  21 . Threaded stem  41  extends through the housing of casing gripper  11  and is supported by bearings so that it will rotate relative to the housing of casing gripper  11 . An anti-rotation device (not shown) prevents rotation of the housing of casing gripper  11  and links  21 . Threaded stem  41  secures to a drive stem or quill  43  of a top drive  45  ( FIG. 2A ). Top drive  45  is moveable up and down the derrick along one or more rails (not shown). Top drive  45  comprises a motor that is either hydraulically or electrically driven for rotating quill  43 . 
         [0016]    A guide  47  is mounted to links  21  above elevator  25  and a short distance below nose  19  of gripper  11  when links  21  are vertical. Guide  47  extends between links  21  and preferably comprises at least two halves of a body that are clamped together by bolts  49 . Optionally, guide  47  may have a lower clamp  51  that is located below the body and separately clamped to links  21 . An opening  53  extends through the body of guide  47 . When links  21  are in the vertical position, the axis of the opening of elevator  25  is coaxial with axis  14  of spear  13 . 
         [0017]    Opening  53  may have a flared upper portion  55 . In this embodiment, upper portion  55  is conical and has an increasing diameter in an upward direction. Similarly, opening  53  may have a flared lower portion  57  that increases in diameter in a downward direction. In this embodiment, lower flared portion  57  has a greater axial length than upper flared portion  55  and a greater diameter at its lower end than the upper end of upper flared portion  55 . In this example, lower flared portion  57  is defined by a plurality of blades or segments  52  spaced in a circular array around axis  14  with gaps between each segment  52 . The inner edges of segments  52  circumscribe or define flared lower portion  57  of opening  53 . The lower ends of segments  52  may be attached, such as by welding, to lower guide clamps  51 . The upper ends of segments  52  are also secured, such as by welding, to the body of guide  47 . Rather than blades or segments  52 , the lower flared portion  57  could be a conical bore formed by two mating halves of a body in the same manner as upper flared portion  55 . 
         [0018]    Optionally, a resilient centering device or devices  59  may be mounted within the central portion of opening  53 . Centering devices  59  may comprise devices such as bow springs or roller balls that are biased by springs radially inward toward the axis of spear  13 . 
         [0019]    In addition, a sensor  61  may be mounted to or adjacent guide  47 . Sensor  61  will detect the presence of collar  29  and provide a signal to the driller. Sensor  61  could be an optical device, such as one employing a laser beam that is interrupted by the presence of one of the collars  29 . Sensor  61  may include a transmitter for making a wireless transmission to a receiver located near or on the driller&#39;s control panel. 
         [0020]    In operation, the operator picks up casing joint  27  in a conventional manner. Initially, casing joint  27  may be located laterally from spider  37  ( FIG. 1B ) and supported at an inclination by a V-door of the rig. The operator will tilt links  21  about upper ends  23  and relative to axis  14  and secure elevator  25  around casing joint  27 . The operator then lifts top drive  45  while allowing links  21  to pivot back to a vertical orientation, placing casing joint  27  in the position shown in  FIG. 1A . The lower threaded end  31  of casing joint  27  will be spaced above collar  33  of the uppermost casing joint of casing string  35 . The distance from nose  19  to collar  29  on casing joint  27  may be several feet. 
         [0021]    The operator then lowers top drive  45  until casing joint threaded end  31  lands in casing collar  33 , as shown in  FIG. 2B . The portion of casing string  35  above spider  37  will support the weight of casing joint  27  at this point, but threads  31  are not yet made up to the internal threads in casing collar  33 . The operator continues lowering top drive  45 , which causes guide  47  to approach and receive casing joint collar  29 , as shown in  FIG. 2A . Flared lower portion  57  will center casing collar  29  on axis  14  as guide  47  slides downward over casing collar  29 . At the point shown in  FIG. 2A , nose  19  has begun to enter casing collar  29 . Guide  47  is positioned such that it will move over at least a part of the casing joint collar  29  before casing gripper nose  19  begins to enter casing joint  27 . 
         [0022]    If sensor  61  is employed, it detects the presence of collar  29  as guide  47  moves below collar  29 . Sensor then informs the driller that nose  19  is now entering the bore of casing joint  27 . The driller continues lowering casing gripper  11  a short distance, at which time gripping elements  15  will be fully enclosed within casing joint  27  as shown in  FIG. 3A . Optionally, the upper end of collar  29  will abut a stop when gripping elements  15  ( FIG. 2A ) are fully located within casing joint  27 . 
         [0023]    The operator then supplies power to the actuator of casing gripper  11 , which causes gripping elements  15  ( FIG. 2A ) to move radially outward into gripping engagement with the inner diameter of casing joint  27 . The operator then supplies power to top drive  45  to rotate quill  43 , which in turn causes casing joint  27  to rotate. This results in threads  31  ( FIG. 2B ) making up to a desired torque with the threads in casing collar  33 , as shown in  FIG. 3B . 
         [0024]    At this point, collar  29  of casing joint  27  will be spaced several feet above guide  47 , and elevator  25  will be spaced several feet below casing joint collar  29 . The operator then lifts top drive  45  a short distance and releases spider  37 . Once released, the operator lowers top drive  45 , which lowers casing joint  27  and casing string  35 . When the upper end of casing joint  27  is near spider  37 , the operator actuates spider  37  to engage casing joint  27 , which is now the uppermost joint or section of casing string  35 . The operator releases elevator  25 , releases gripping elements  15  ( FIG. 2A ) and lifts top drive  45  while casing joint  27  is supported by spider  37 . Guide  47  will slide up past collar  29 , with flared upper portion  55  centering guide  47  relative to collar  29  to prevent damage to the lower edge of collar  29 . Once guide  47  is above collar  29 , the operator may then pivot links  21  outward to engage the next joint of casing. 
         [0025]    The guide is particularly useful when the links are quite long, in that it centers the upper end of the casing joint with the casing gripper. The guide may be employed when running casing into a previously drilled wellbore and also when drilling with casing. Although shown in connection with an internal gripping mechanism, the same is applicable to an external casing gripper. 
         [0026]    While 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 is susceptible to various changes without departing from the scope of the invention.

Summary:
A system for running a string of casing into a well utilizes a casing gripping assembly. The casing gripping assembly connects to a top drive and has radially movable gripping elements. A pair of links have upper ends pivotally connected to the casing gripping assembly. A casing elevator is mounted below the casing gripping assembly to lower ends of the links. A guide is mounted to the links between the elevator and the casing gripping assembly. The guide has a vertically extending central opening that has a lower portion that defines a flared entrance to the opening.