Abstract:
An apparatus configured to a downhole tubular to a pump. The apparatus comprises: a body portion having a bore; a locking assembly comprising at least one locking member pivotally mounted to the body portion so as to permit the locking member to be pivoted between an open position in which the downhole tubular can be inserted into the bore of the body portion, and a closed position in which the locking member engages the downhole tubular so as to lock the downhole tubular in the bore of the body portion. Fixing means is provided for fixing the looking member in the closed position. A method of drilling a wellbore using a borehole casing.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 11/609,797, filed Dec. 12, 2006, now U.S. Pat. No. 7,384,077, which is a continuation of Ser. No. 10/399,053, filed Oct. 2, 2003, now U.S. Pat. No. 7,147,254, which is a §371 application of PCT Application No. PCT/NZ01/00227, filed on Oct. 16, 2001, which claims benefit of New Zealand patent application number 507539, filed Oct. 16, 2000. Each of the aforementioned related patent applications is herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to coupling apparatus for coupling a downhole tubular (such as a borehole casing) to a pump, conventionally known as a circulating head. The invention also relates to a method of pumping fluid into a downhole tubular, and to a method of drilling. 
     2. Description of the Related Art 
     A conventional circulating head is described by LeFleur et al in U.S. Pat. Nos. 5,282,653, 5,152,554 and 5,348,351. As described by LeFleur et al, when casing is being run in connection with the drilling of an oil or gas well, it sometimes becomes necessary to connect surface pumping equipment to circulate drilling fluid down the well. Typically, this need arises when a tight spot is encountered and drilling fluid is circulated down the well to run the casing past the tight spot and avoid the need for removing the casing and redrilling the hole. 
     The circulating head described by LeFleur et al uses a segmented ring of eight dogs to lock a cylindrical member in place. The dogs are fixed in place by rotating a bottom end cap. 
     This arrangement suffers from a number of problems. Firstly the apparatus has a large number of working parts. Secondly it can be difficult and time consuming to rotate the bottom end cap. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to address these problems or at least to provide the public with a useful alternative. 
     According to a first aspect of the invention there is provided apparatus for coupling a downhole tubular to a pump, the apparatus comprising: a body portion having a bore; a locking assembly comprising at least one locking member pivotally mounted to the body portion so as to permit the locking member to be pivoted between an open position in which the downhole tubular can be inserted into the bore of the body portion, and a closed position in which the locking member engages the downhole tubular so as to lock the downhole tubular in the bore of the body portion; and fixing means for fixing the locking member in the closed position. 
     The invention provides a simple construction which enables the pump and tubular to be easily and quickly connected. 
     Preferably the locking assembly comprises a pair of locking members pivotally mounted to the body portion at a substantially common pivot point. 
     Typically the apparatus further comprises one or more pivot pins pivotally mounting the locking member(s) to the body portion. 
     Any suitable means may be provided for fixing the locking member(s) in the closed position. In a preferred embodiment the fixing means comprises a lever clamp mechanism. 
     Typically the body portion comprises an upwardly directed locking surface which engages a locking surface of the locking member(s). 
     In the arrangement of LeFleur et al the body of the apparatus has a radially outer ring with an upper surface. The dogs are secured to the ring by screws, which engage the upper surface of the ring. A problem with this arrangement is that the screws are not sufficiently strong to enable the apparatus to support a heavy weight. 
     In accordance with a second aspect of the invention there is provided apparatus for coupling a downhole tubular to a pump, the apparatus comprising: a body portion having a bore and a locking surface which is upwardly directed, when in use; a locking assembly comprising at least one unitary locking member having first and second locking surfaces, wherein the locking member is movably mounted to the body portion so as to permit the locking member to be moved between an open position in which the downhole tubular can be inserted into the bore of the body portion, and a closed position in which the first locking surface engages the downhole tubular so as to lock the downhole tubular in the bore of the body portion and the second locking surface engages the upwardly extending locking surface of the body portion so as to secure the locking member to the body portion; and fixing means for fixing the locking member in the closed position. 
     The upwardly directed locking surface directly engages the locking member, in contrast to the system of LeFleur in which the upwardly directed surface of the ring engages the screw. This provides a more secure connection. Preferably the connection is strong enough to enable part of the full weight of the downhole to be lifted by lifting the body portion. 
     Typically the locking surface of the body portion is substantially horizontal when in use (i.e. the surface is directed substantially vertically). In other words, the normal to the locking surface is substantially parallel to the direction of insertion of the downhole tubular. Alternatively the locking surface of the body portion may have a positive or negative camber. 
     Preferably the body portion has an outwardly directed projection (e.g. a flange) whose upper surface provides the upwardly directed locking surface, and the locking member is mounted about the projection. Typically the locking member has a recess which receives the projection. 
     According to a third aspect of the invention there is provided apparatus for coupling a downhole tubular to a pump, the apparatus comprising: a body portion having a bore; a locking assembly for locking the downhole tubular in the bore of the body portion; and an annular sealing member mounted within the bore of the body portion and having a lower surface which engages an upper surface of the downhole tubular when the downhole tubular is received in the bore of the body portion, wherein the body portion and an upper surface of the annular sealing member are arranged so as to at least partially define a chamber which receives high pressure fluid in use so as to force the annular sealing member into engagement against the upper surface of the downhole tubular. 
     The third aspect of the invention provides a secure seal which reduces the risk of fluid leakage during a pumping operation. A pressure difference is set up across the annular sealing member, when in use, so as to increase the integrity of the seal. 
     The sealing member may be rigidly fixed to the bore of the body portion, and may flex in response to the fluid pressure so as to force the annular sealing member against the upper surface of the downhole tubular. However preferably the sealing member is slidably mounted in the bore so as to permit the sealing member to translate into sealing engagement with the upper surface of the downhole tubular. 
     Preferably resilient means (for instance springs) are provided to resiliently bias the annular sealing member towards the upper surface of the downhole tubular. 
     Preferably the annular sealing member has an upwardly directed flange which further defines the chamber. 
     Preferably a resilient sealing member is provided to ensure a secure seal. The resilient sealing member may be provided as a separate item, or the annular sealing member may itself be formed of resilient material. The resilient sealing member may engage the top of the downhole tubular (providing a top face seal), the outside of the downhole tubular (providing an external seal), the internal bore of the downhole tubular (providing an internal seal), or all three. Where an internal or external seal is provided, the resilient sealing member preferably has an angled surface so as to provide a wedging action. 
     According to a fourth aspect of the invention there is provided apparatus for coupling a downhole tubular to a pump, the apparatus comprising: a body portion having a bore; a locking assembly for locking the downhole tubular in the bore of the body portion; and a resilient member arranged between an external surface of the downhole tubular and the bore of the body portion, wherein the resilient member is formed with a chamber for receiving fluid so as to inflate the resilient member into sealing engagement with the external surface of the downhole tubular and the bore of the body portion. 
     The fourth aspect of the invention provides an inflatable seal which can adapt to different diameter downhole tubulars, or at least ensure a reliable seal. The seal may be inflated hydraulically or pneumatically. 
     Preferably the chamber is toroidal in shape. 
     According to a fifth aspect of the invention there is provided apparatus for coupling a downhole tubular to a pump, the apparatus comprising: a body portion having a bore; a locking assembly comprising at least one unitary locking member having a locking surface and an elevator engagement surface, wherein the locking member is movably mounted to the body portion so as to permit the locking member to be moved between an open position in which the downhole tubular can be inserted into the bore of the body portion, and a closed position in which the locking surface engages the downhole tubular so as to lock the downhole tubular in the bore of the body portion; and fixing means for fixing the locking member in the closed position, wherein the elevator engagement surface is arranged so as to be externally accessible when the downhole tubular is locked in place whereby the elevator engagement surface can be engaged by an elevator which supports at least part of the weight of the downhole tubular. 
     The arrangement of the fifth aspect of the invention makes the locking member(s) externally accessible to enable the weight of the downhole tubular to be transferred to an elevator through the locking member(s). 
     Preferably the locking member is fitted on the outside of the body portion. 
     Preferably the locking member is externally accessible from below, and the elevator engagement surface is downwardly directed. 
     The locking member may be coupled to the body portion by screws, as in the arrangement described by LeFleur et al. However preferably the locking member is coupled as described in the second aspect of the invention. 
     According to a sixth aspect of the invention there is provided apparatus for coupling a downhole tubular to a pump, the apparatus comprising: a body portion having a bore; at least one locking member movably mounted to the body portion so as to permit the locking member to be moved between an open position in which the downhole tubular can be inserted into the bore of the body portion, and a closed position in which the locking member engages the downhole tubular so as to lock the downhole tubular in the bore of the body portion and takes up at least part of the weight of the downhole tubular; fixing means for fixing the locking member in the closed position; and one or more connectors for transferring the weight of the downhole tubular from the locking member to a support. 
     This construction enables all or part of the weight of the downhole tubular to be transferred to a support, such as a pair of bails, bypassing the upper part of the body portion. 
     The locking member may slide or pivot to one side so as to permit the downhole tubular to be inserted into the bore of the body portion. Alternatively the locking member may comprise one or more slips which engage an external surface of the downhole tubular. 
     The locking member may be pivotally mounted to the body portion, for instance by a pivot pin and/or by two or more chains. 
     The connector typically comprises an aperture or laterally extending lug. 
     Typically the locking member and connector are sufficiently strong to support a weight in excess of 10,000 kg, preferably 100,000 kg. 
     According to a seventh aspect of the invention there is provided apparatus for coupling a downhole tubular to a pump and transmitting drilling torque to the downhole tubular, the apparatus comprising: a body portion having a bore; a locking assembly for locking the downhole tubular in the bore of the body portion; a gripping assembly for gripping an external surface of the downhole tubular; and means for transmitting drilling torque from the body portion to the gripping assembly. 
     Typically the means for transmitting drilling torque can transmit a torque greater than 30 ft lbs, preferably greater than 250 ft lbs. 
     In a preferred embodiment the means for transmitting drilling torque comprises a plurality of teeth. 
     Typically means for actuating the gripping assembly is provided, and is preferable actuable when no fluid is being pumped into the downhole tubular. 
     The following comments apply to all aspects of the invention, where applicable. 
     Preferably the downhole tubular has a terminal collar which is received in the bore of the body portion. The terminal collar may be integral with the downhole tubular or may be screwed on as a separate item. 
     Preferably the locking member is substantially C-shaped in cross-section. 
     Preferably at least two locking members are provided. The locking members may pivot or translate between the open and closed positions. 
     Preferably the locking member(s) form an annular ring when in the closed position. 
     Typically a resilient seal member is mounted in the bore of the body portion and seals against the tubular member when the tubular member is received in the bore. 
     Preferably the resilient seal member has a plurality of projections which engage the tubular member when the tubular member is received in the bore. Preferably the projections are angled in the direction of insertion of the cylindrical member. Typically the projections are in the form of circumferentially extending ribs. 
     According to an eighth aspect of the invention there is provided a joint comprising a downhole tubular received in a borehole; a pump tubular; and apparatus coupling the downhole tubular to the pump tubular, the apparatus comprising a body portion with a first bore receiving the downhole tubular and a second bore receiving the pump tubular, and a locking assembly locking the downhole tubular in the first bore of the body portion, wherein the apparatus is constructed so as to permit relative axial movement between the pump tubular and the downhole tubular. 
     The eighth aspect of the invention provides a flexible joint between a downhole tubular, and a pump tubular. When in use, the pump tubular directs fluid from a pump into the downhole tubular. By allowing relative axial movement between these two parts, we reduce the chance of breakage in the event that the downhole tubular sticks when it is being lowered into a borehole. 
     The pump tubular is preferable received in the downhole tubular. This reduces the chance of leakage. 
     In one embodiment the joint further comprises a resilient member coupling the pump tubular to the body portion and providing a resilient biasing force which acts along the length of the pump tubular. A variety of resilient members may be used, but in a preferred example the resilient member comprises a coil spring wrapped around the pump tubular and coupled at a first end to the body portion and at a second end to the pump tubular. 
     The resilient member is preferably housed at least partially in the first bore of the body portion. 
     The invention also extends to a method of pumping fluid into a downhole tubular, the method comprising: coupling the downhole tubular to a pump tubular using apparatus according to any aspect of the present invention; and pumping fluid from the pump tubular into the downhole tubular. 
     The tubulars are typically circular in cross-section, although it will be understood that other cross-sectional shapes may be possible. Therefore the expression ‘tubular’ should be construed broadly in this specification, covering any elongate member having a bore formed along its length. 
     In a typical application the downhole tubular is a drill pipe, casing or other tubing for a borehole such as an oil or gas well. 
     A ninth aspect of the invention provides a method of drilling comprising: engaging an external surface of a length of casing with a gripping assembly; drilling a hole by transmitting torque via the gripping assembly to a drilling bit mounted on an end of said casing; and sealing said casing in the drilled hole. 
     Typically pumping fluid is directed into the casing during drilling, and the casing is subsequently sealed in the borehole by pumping sealing fluid into the casing. 
     Various embodiments of the present invention will now be described by way of example with reference to the accompanying drawings in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
         FIG. 1  is a cross-sectional side view of a top circulating head and borehole casing prior to connection; 
         FIG. 2  is a plan view of the apparatus of  FIG. 1 , with the doors in their open position; 
         FIG. 3  is a cross-sectional side view of an assembled joint; 
         FIG. 4  is a plan view of the assembled joint of  FIG. 3 ; 
         FIG. 5   a  is a side view of a lever clamp mechanism; 
         FIG. 5   b  shows the lever clamp mechanism of  FIG. 5   a  being used to clamp the locking members together; 
         FIG. 6  is a plan view of an alternative apparatus with the locking assembly in its open position; 
         FIG. 7  is a plan view of a further alternative apparatus with the locking assembly in its open position; 
         FIG. 8  is a cross-sectional view of an oil rig lowering casing into a borehole; 
         FIG. 9  is a cross-sectional side view of a top circulating head having a top face seal; 
         FIG. 10  is a view similar to  FIG. 9  showing a top circulating head with an external seal; 
         FIG. 11  is a view similar to  FIG. 9  showing a top circulating head with an internal seal; 
         FIG. 12  is a view similar to  FIG. 9  showing a top circulating head with an inflatable seal; 
         FIG. 13  shows an alternative top circulating head incorporating an elevator assembly attached; 
         FIG. 14  is a plan view of the side door elevator shown in  FIG. 13   
         FIG. 15  is a cross-sectional view of a further alternative arrangement incorporating a slips type elevator: 
         FIG. 16  is a cross-sectional view of an alternative top circulating head system viewed from the right-hand side; 
         FIG. 17  is a cross-sectional right side view of a casing drilling system; 
         FIG. 18  is a right side view of the system of  FIG. 17 , with some parts shown in silhouette; 
         FIG. 19  is a front view of the system of  FIG. 17 , with some parts shown in silhouette; 
         FIG. 20  is a plan view showing the locking assembly in its open position; and 
         FIG. 21  is a plan view of the hypergrip system. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , the top circulating head is designated by numeral  1 , and is used to couple a slick joint  2  with a bore hole casing  3 . 
     The assembly  1  comprises a main body portion  4  having a large cylindrical bore  5 , a small cylindrical bore  6  and a flange portion  7  with a pair of lifting lug holes  8 ,  9 . Shoulder portion  10  between bores  5  and  6  supports a bumper ring  11  made of rubber or a similar material. Resilient (e.g. rubber) external seals  12  and  13  are mounted in the bores  5  and  6 . A locking assembly  15  is mounted on the body portion  4  about an outwardly extending flange  14 . 
     Referring to  FIG. 2 , the locking assembly  15  has a first semicircular door  16  and a second semicircular door  17 . The doors  16 ,  17  each have respective hinge rings  30 ,  31  ( FIG. 1 ) with bores  32 ,  33  aligned with a bore  20  in the flange  7 . A pivot pin (not shown) passes through the bores  20 ,  32  and  33 . 
     The main body portion  4  has an outwardly extending flange  14  which is received in an inwardly facing recess  51  formed in the doors  16 ,  17 . The flange  14  has an upper locking surface  50  which engages a locking surface  52  provided by the upper wall of the recess  51 . 
     The casing  3  is screwed into a terminal collar  21 . Prior to insertion of the casing  3  and collar  21 , the doors  16 ,  17  (shown in their closed positions in  FIG. 1 ) are pivoted to the open positions shown in  FIG. 2 . This permits the insertion of the terminal collar  21  into the bore  5 . 
     Referring to  FIGS. 3 and 4 , when the terminal collar  21  has been fully inserted against the bumper plate  11 , the doors  16 ,  17  are pivoted to their closed positions shown in  FIGS. 3 and 4  and clamped together by a lever clamp mechanism  22 . The action of the mechanism  22  is shown schematically in  FIGS. 5   a  and  5   b . The mechanism comprises a lever arm  24  pivoted at one end to the door  16  and at the other end to a clip  25 . As shown in  FIG. 5   b , the clip  25  is hooked round a catch  26  on the door and snapped shut as indicated by arrow  27  in  FIG. 5   b . This forces the doors  16 , 17  together and ensures a secure connection. 
     Referring to  FIG. 3 , in their closed positions the doors  16 ,  17  engage a shoulder  23  of terminal collar  21  so as to lock the casing  3  in place. The terminal collar  21  also engages resilient external seal  13  so as to provide a fluid tight seal. The seal  13  has circumferential ribs  28 ,  29  etc which are angled in the direction of insertion of the casing  3 . Similarly, the slick joint  2  engages a resilient external seal  12  with reverse-directed ribs for a fluid tight seal. The slick joint  2  has a support flange  24  which engages the bumper ring  11  when the slick joint  2  is used to lift the casing  3  or to lower the casing  3  into a borehole. Alternatively, the slick joint  2  can be pushed downwards into the casing  3 . 
     The seal  12  is mounted in the bore  6  between a pair of phosphor-bronze bushes. The bushes and slick joint are highly polished in order to minimize friction. A lubricant may also be provided. 
     The coupling apparatus  1  provides a fluid-tight seal between the slick joint  2  and casing  3 , permitting fluid to be pumped at high pressure into the casing  3 . After the pumping operation is finished, the clamping mechanism  22  is released and the doors  16 ,  17  are pivoted to their open positions. The assembly  1  is then lifted up by the slick joint  2  or by the lug holes  8 ,  9 . 
     Part of the weight of the casing  3  can be supported by the slick joint  2 , due to the secure connection provided by the substantially horizontal locking surface  50  which supports the opposed substantially horizontal surface  52  of the doors  16 ,  17 . Although a horizontal locking surface  50  is provided on the flange  14 , the surface  50  may have a positive or negative camber. If a positive camber lie sloping to the outside) is provided, then a stronger clamping mechanism  22  will be required to keep the two members  16 ,  17  together. 
     Although part of the weight of the casing can be supported by the slick joint  2  as discussed above, the majority of the weight of the casing  3  is supported by a side door elevator  49  which has a flange  48  with an upper surface  47  which engages the bottom surfaces of the members  16 ,  17 . 
     The elevator  49  is coupled to a rig (not shown) in the manner shown in detail in  FIG. 8 . 
     The assembly  1  may be rotated with respect to the slick joint  2 . 
     In an alternative arrangement the elevator  49  may be omitted and the weight of the casing  3  transferred to a rig (not shown) by bails (also not shown) attached to the lug holes  8 ,  9 . In this case the locking assembly  15  (which effectively functions as an elevator) may need to be reinforced so as to support the weight of the casing  3  (which may be many hundreds of tons). Instead of transferring the weight via the lug holes  8 ,  9 , the assembly  15  may have lug holes or rings for attachment to the bails. 
     An alternative assembly is shown schematically in  FIG. 6 . In this case the locking assembly comprises a pair of members  40 ,  41  which are identical to the doors  16 ,  17  in cross-section (apart from the omission of the hinge rings  30 ,  31 ). The members  40 ,  41  are mounted on rams  42 ,  43  which are driven by hydraulic cylinders  44 ,  45  so as to translate the members between the open position shown in  FIG. 6 , and a closed position (not shown). 
     In a further alternative shown schematically in  FIG. 7 , the cylinders  44 ′,  45 ′ are attached to member  41  and the rams  42 ′,  43 ′ are attached to the opposite member  40 . It can be seen that the system of  FIG. 7  is fully self contained, in the sense that no external mounting is required. In contrast, in the system of  FIG. 6 , the cylinders  44 ,  45  must be mounted on external supports. 
     A cross-section through an oil rig is shown in  FIG. 8 . In an initial drilling operation the rig is used to drill a borehole with a drill pipe (not shown) which is rotated by a top drive  50  or a rotary table  51 . Following drilling, a casing pipe  3  is passed along the length of the borehole. 
     Starting from the top of  FIG. 8 , a set of suspension wires  55  are coupled to a support structure (not shown). The wires  55  carry a block  56  with a hook  61  which support the top drive  50 . The top drive  50  has a drive shaft  59  and is prevented from rotating by a pair of rails  62  mounted on a derrick (not shown). 
     A slick joint  2  is coupled to the drive shaft  59 . A mud supply pipe  79  is provided to pass mud at high pressure through the slick joint  2 . 
     In contrast to  FIG. 3 , the weight of the casing is supported by a slip type elevator  63  (instead of a side door elevator) which has slips  64  which grip the sides of the casing  3 . The slip type elevator  63  has a pair of side lugs  46  which are supported by the bails  58 . Thus the weight of the casing  3  is transferred to the block  56  by the bails  58  via the elevator  63 , bypassing the slick joint  2 . Part of the weight of the casing  3  can also be supported, if necessary, by the slick joint  2 . The slick joint  2  is coupled to the drive shaft  59  by a frangible coupling  60  incorporating shear pins which break if the load carried by the slick joint  2  exceeds a set threshold. 
     Various different types of top circulating head configuration are shown in  FIGS. 9   12 . Components with a similar function are given the same reference numerals as the equivalent components in  FIGS. 1 and 3 . 
     The bumper ring  11  shown in  FIG. 1  is replaced in  FIGS. 9   12  with a sliding piston-type bumper ring  65  with a flange  66  which slides up and down the bore  5  sealed by resilient seals  67 . The ring  65  is coupled to the shoulder portion  10  and biased downwards by a coil spring  68 . 
     In the embodiment of  FIG. 9 , the ring  65  carries a resilient annulus of material  70  on its lower face to form a top face seal which engages the top of the collar  21 . 
     When mud is pumped into the casing  3 , mud at high pressure (up to 2500 psi) fills the chamber  90  defined by the upper face of the bumper ring  65 , internal and upper faces of flange  66 , bore  5  and shoulder  10 . In contrast, the chamber  71  below the ring  65  is free of mud. This sets up a fluid pressure differential which forces the top face seal  70  against the collar  21  and ensures a tight seal. 
     In an alternative arrangement shown in  FIG. 10 , the top face seal is replaced with an external seal formed by an annulus of resilient material  72  with a frustoconical inner surface  73  which provides a wedging action when the bumper ring  65  forces the annulus  72  down into the chamber  71 . 
     In a further alternative arrangement shown in  FIG. 11 , the top face seal is replaced with an internal seal formed by an annulus of resilient material  74  with a frustoconical outer surface  75  which engages the internal bore  76  of the collar  21  and provides a wedging action when the bumper ring  65  forces the annulus  74  down into the collar  21 . 
     In a further alternative arrangement shown in  FIG. 12 , the seal  13  ( FIG. 1 ) is replaced by a hydraulic seal, comprising a hollow torus of rubber (or similar) material  80  with a toroidal chamber  81  which is supplied with hydraulic fluid from a line  82 . Anti extrusion rings  83 ,  84  are provided to prevent the seal  80  from extruding up or down when inflated. The seal  80  engages the collar  21  and bore  5  when inflated. This enables collars  21  with differing diameters to be accommodated. 
     Referring to  FIG. 13 , an alternative top circulating head is shown, comprising a body portion  90  with a seal  13  of the type described in  FIG. 1 . A casing tube  91  is received in the bore  5  of the body portion  90  and a seal is effected by the seal  13 . A side door elevator  105  is suspended from the bottom of the body portion  90  by a pair of chains  92 ,  93 . For purposes of clarity, the chains  92 ,  93  are showed in  FIG. 13  with only four links. However, in practice a larger number of links will be required. 
     In contrast to the embodiments of  FIGS. 1   12 , the tubular shaft extending down from the top drive  50  is rigidly connected (for example by welding) to the body portion  90 . 
     As shown in  FIG. 14 , the elevator  105  comprises a main body portion  94  attached to a door  95  via a hinge  96 . The door  95  is locked in place by a connector  97 . The connector  97  can be released to permit the door  95  to swing back to the open position shown in dotted lines in  FIG. 14 . Prior to insertion of the casing  91 , the elevator  105  (with door  95  in its open position) is swung away from the mouth of the bore  5  on the chains  92 ,  93 . 
     Once the casing  91  has been fully inserted into the bore  5 , the elevator  105  is swung back and the casing  91  is received in the bore  98  of the elevator. The door  95  is then swung back into place and locked by connector  97 . The casing  91  has a shoulder  99  which is engaged by the elevator  105  to support the weight of the casing  91 . The weight of the casing  91  is transferred to a rig (not shown) via lugs  46  and bails (not shown). This prevents the weight of the casing  91  being transferred through the chains  92  and  93 . 
     In the alternative embodiment of  FIG. 15 , the side door elevator  105  of  FIG. 13  is replaced with a slip type elevator. In this case, the body portion of the top circulating head comprises a first portion  100  with a bore  5 , and a second, elevator portion  101  (formed as a single piece with the portion  100 , or attached e.g. by welding). Slips  102 , in the form of up to eight blades or wedges, are mounted inside elevator portion  101  and are slid upwards into the bore  5  to permit the casing  103  to pass through the elevator portion  101  into the bore  5 . The slips may be lifted by a hand lever, pneumatic or hydraulic cylinder (not shown). The slips  102  then retract downwards as they take the weight of the casing  103 . 
     It can be seen in  FIG. 15  that the use of a slip type elevator enables casing  103  with no terminal collar to be supported. However, the slips  102  can be moved apart if necessary sufficiently to enable casing with a terminal collar (such as the terminal collar  12  of  FIG. 1 ) to be inserted. 
     An alternative top circulating head assembly is shown in  FIG. 16 . A top drive connection  110  has an internally threaded bore  111  which receives a top drive shaft (not shown). The internal bore  111  enables mud to be pumped through the top drive connection  110 . The top drive connection  110  has a pair of bores  112 ,  113  which receive shear bolts (not shown). The bores  112 ,  113  are aligned with bores  114 ,  115  in a connector  116 . The shear bolts pass through the bores  114 ,  115  and provide a frangible connection between the connector  116  and top drive connection  110 . The connector  116  has an internally threaded bore  117  which receives a threaded end of a slick joint  118  to rigidly connect the slick joint  118  to the connector  116 . The connector  116  has a flange  120  which supports an external saver sub assembly  121 . 
     A bell designated generally at  122  is formed by a tube  123  which is welded to a cap  124  and a flange  125 . A keeper plate  127  is bolted to the cap  124 . The slick joint  118  engages a pair of phosphor-bronze bearings  128 ,  129  and a seal  130 , which enable the slick joint  118  to slide up and down. 
     The slick joint  118  is attached at its lower end to a stinger adjustment sub casing  131  which is attached in turn to a stinger  132  with a non drip valve  133  at its lower end. The stinger is received inside casing  134  and is maintained in a central position by an internal guide  135 . 
     A coil spring  136  (shown in its compressed loaded configuration) is mounted between cap  124  and a spring support flange  137  welded to the slick joint  118 . 
     The casing  134  has a terminal collar  138  which is clamped in place with a locking assembly  139  similar to the locking assembly  15  shown in  FIG. 1 . 
     A seal locking ring  141  and external seal  142  provide a fluid-tight seal between the components. 
     The weight of the casing is supported as shown in  FIG. 8 , that is by a slip elevator  63  and pair of bails  58  (both omitted in  FIG. 16  for clarity). The spring  136  allows a small movement between the slips  64  and the casing  134 , without the full weight of the casing being transferred through the top circulating head to the connector  116 , which would cause the shear bolts to shear, resulting in down-time. Also, if an operator tries to pick up the casing with the slips disengaged, then the spring  136  will compress further under this load as the slick joint  118  is pulled out of the bell  122 . The surface of the slick joint  118  is coated in a visible color (for example black or red) below the level where the slick joint  118  is normally visible. As the pull continues, more and more colored slick joint will appear, until the shear bolts shear. This color change should alert the operator that something is wrong. 
     The external saver sub assembly  121  is a hinged collar that fits onto the flange of connector  116 . When the casing  134  is lowered into a hole, it may stick sufficiently to hold the entire weight of the casing. If the operator is not monitoring the load indicator, and continues lowering the top drive, then the slick joint  118  will slide down into the bell  122 . As this is happening, the slips  64  will release automatically and slide down the casing. 
     At this point, the slick joint  118  will have slid all the way down into the bell until the external saver sub assembly  121  engages the keeper plate  127 . Now if the operator lifts the top drive, the slips  64  will engage and start lifting the casing  134 . 
     It takes about 50 70 mm of downward movement to release the slips  64 . This is no longer possible because the external saver sub assembly  121  is engaging the keeper plate  127 . In order to release the slips, the external saver sub assembly  121  is opened. This enables the top drive to be lowered sufficiently to release the slips, and the slick joint to be pulled up to its operating position. The external saver sub assembly  121  can then be closed. As an alternative, an internal saver sub assembly (comprising an inflated torus  143 ) can be deflated to permit the terminal collar  138  to move upwards inside the bell  122  sufficiently to release the slips. 
     A casing drill-in system is shown in  FIGS. 17   21 . Casing  200  has a drilling tool (not shown) mounted on its end. A suitable type of drilling tool is described in WO/0146550. The system can lift, rotate and push down on the casing  200  during a drilling operation. Lubricating mud can also be directed under pressure down the casing  200  through the top drive connection, as in the previously described embodiments. 
     Top drive connection  201  is coupled to a top drive  202 . The top drive connection  201  is welded directly to bell tube  203 . The structure of the top circulating head is similar to the structure shown in  FIG. 16 . The casing is locked into the bell by a pair of doors  210 ,  211  shown in their open position in  FIG. 20 . The doors  210 ,  211  are clamped shut by a clamp mechanism  212 . Door  211  is mounted on a top hinge plate  213  and door  210  is mounted on a bottom hinge plate  214 . For clarity, the bottom hinge plate  214  is omitted from  FIG. 20 . The hinge plates rotate about a hinge pin  215  which is coupled to the bell  203  by a mounting member  216 . Each door  210 ,  211  is formed with a number of gear teeth  205  which are received in recesses  206  formed in the bell flange  209 , as shown in the plan view of  FIG. 20 . 
     The arms  210 ,  211  each are connected to a respective jaw of a hypergrip system  207  of the kind shown in detail in  FIG. 21 . The connection is made by approximately ten extended mounting bolts which each carry a pair of springs, with the hypergrip system  207  mounted between the springs. The system of  FIG. 21 , and alternative gripping systems, are described in detail in WO 01/21933, the contents of which are incorporated herein by reference. The system  207  has a pair of jaws  217 ,  218  which are connected to respective hinge plates  219 ,  220  which rotate about the pivot pin  215 . The jaws  217 ,  218  are clamped in place by a clamping mechanism  221 . 
     Jaw  217  includes a semi-circular cage  315  containing rollers  316 . Jaw  218  includes a semi-circular cage  317  containing rollers  318 . The inner faces of jaws  217  and  218  adjacent cage assemblies  315  and  317  have recesses formed therein which have ramp surfaces for wedging the rollers against the casing  200 . 
     The jaws  217  and  218  may pivot away from each other so that the jaws may open. This enables the casing  200  to be axially introduced between the jaws and the jaws closed to retain the casing. 
     When cages  315  and  317  are in their initial positions, rollers  316  and  318  are positioned adjacent the apexes of the recesses formed in the jaws. This allows the casing  200  to be rotated in either direction. When it is desired to grip the casing  200 , cages  315  and  317  are rotated relative to jaws  217  and  218  in the direction in which rotation is to be restrained. This brings rollers  316  and  318  into engagement with their respective ramp surfaces so as to wedge the rollers  316  and  318  between the outer surface of casing  200  and jaws  217  and  218 . To release casing  200  it may be rotated in the opposite direction and locking mechanism  212  is released to open the jaws. 
     The majority of the weight of the casing  200  is transferred to the top drive  202  via the doors  210 ,  211 , bell flange  209 , bell tube  203  and top drive connection  201 . Torque can also be transferred from the top drive  202  to the casing  200  via the same elements (including the gear teeth  205  and recesses  206 ). 
     The system of  FIG. 17  is intended to be used to drill relatively shallow wells, thus restricting the string weight to a maximum of about 100 tons (224,000 lbs). A circulating pressure of about 2500 psi is also envisaged. Torque values in the range of 0 30,000 ft lbs are also envisaged. 
     The teeth  205  and recesses  206  can transmit torque up to approximately 30,000 ft lbs to the hypergrip system  207  via the mounting bolts. In the event of an overload, the hinge pin  215  is able to carry the excess. 
     During drilling, mud is pumped down the casing and passes up the well on the outside of the casing, bringing drilling cuttings up to the surface. When a desired depth has been reached, cement is pumped down the inside of the casing. The cement may be pumped through the circulating head shown in  FIGS. 17   21 , or using a different pump connection apparatus. A plug is then driven down, which forces the cement up the outside of the casing. The cement then dries and seals the casing in place. 
     If further depth is required, then casing of a smaller diameter can be passed down inside the existing casing, with a smaller drill bit mounted on its end. The drill-bit can then drill through the existing drill-bit and the process is repeated. 
     If the casing  200  is lifted while the hypergrip system  207  is transferring torque, then the lower springs supporting the hypergrip system will compress. Similarly, the upper springs will compress if a downward push is made on the casing  200 . This prevents the rollers  316 ,  318  from taking up axial load, which would cause deformation of the cages  315 ,  317  (which are designed to take up the weight of the rollers only). 
     It should be noted that the casing is engaged on its external surface by the hypergrip system  207  and the top circulating head assembly. As a result, if any of these components breaks up, parts will not fall down the interior of the casing  200 . 
     Also the hypergrip system  207  is actuated independently of the mud circulation system. 
     Although this invention has been described by way of example and with reference to possible embodiments thereof, it is to be understood that modifications and improvements may be made without departing from the spirit or scope of the invention.