Patent Publication Number: US-6662868-B1

Title: Clamping well casings

Description:
This is a continuation-in-part of U.S. patent application No. 09/563,959, filed on May 3, 2000. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to the clamping of concentric well casings, where an inner well casing is to be clamped in position relative to an outer well casing, to achieve a desired relative axial position between the casings, for operational reasons axial fixing between coaxial well casings over a range of positions may be required at various times during drilling and/or production from wells, and the present invention makes it possible to clamp one or more casings within another at any desired position and subsequently to unclamp the casings for disassembly, or to change their relative positions and then reclamp the casings in a new relative position. As the clamping mechanism is preinstalled and can be externally activated, the blowout preventers can remain in place throughout the installation, clamping or release of the subsequently installed casing. 
     BACKGROUND OF THE INVENTION 
     In oil and gas wells, it is conventional to pass a number of concentric tubes or casings down the well. An outermost casing is fixed in the ground, and the inner casings are each supported in the wellhead or in the next outer casing by casing or tubing hangers. 
     These casing hangers may take the form of a body with interengaging internal shoulders on the outer casing and a body with external shoulders on the inner casing hangers, located at fixed positions on each previously installed casing. 
     There are however applications where a fixed position casing hanger is unsatisfactory, because the hang-off point of one casing on another may require to be adjustable. 
     The invention has particular application for such casing and tubing hangers, which require adjustment. 
     Where drilling or production wellheads have to accommodate a casing or tubing without predetermined hang-off point, it has been known to use casing slip-type support mechanisms. 
     It is also known from European patent number EP251595B2 to use an adjustable landing ring on a surface casing hanger to accommodate a space-out requirement. 
     It is furthermore known that where production wellheads have to accommodate casing or tubing with a tension load greater than the running weight, retractable shoulders or internal wedge mechanisms have been used to allow passing of the casing or tubing hanger, and re-tensioning to a predetermined point. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention, there is provided a pre-installed clamping arrangement for clamping a subsequently installed tubular well casing of a first diameter within a previously installed tubular casing of larger internal diameter, the arrangement comprising a sleeve associated with the large diameter casing, the sleeve having a collar at one end which has an external tapered surface, the arrangement also including an annular component with an internal tapered surface, the sleeve and annular component being relatively axially moveable between a first position in which the tapered surface of the annular component exerts minimal or no radial force on the collar and a second position in which the tapered surface of the annular component exerts sufficient radial force to distort the collar into the bore of the larger diameter casing, to grip the well casing of smaller diameter, the arrangement also including a removable device for maintaining the surfaces in the first position, and separate means for urging the annular component axially against the collar. 
     The sleeve may be of one piece with the large diameter casing, but more probably will be a separate component which could either be threaded onto the casing or be located in a suitable locating and receiving area on the casing. 
     The clamping arrangement preferably also provides a sealing function across the interface between the tapered surfaces, either through the metal/metal contact between the tapered surfaces, or through a separate seal body. Where the sleeve is a separate component from the larger diameter casing, there may be a metal/metal seal between the tapered surfaces and, in addition, a separate seal between the sleeve and the casing. 
     The device for maintaining the surfaces in the first position is preferably a spacer ring. The spacer ring may be removable, or may be a ring which can be moved axially by rotating it on a thread. 
     The sleeve can be formed as part of a casing hanger used for supporting a casing in a well. 
     The annular component can be a wellhead spool, and means can be provided to move the annular component axially in a direction away from the sleeve. This means for moving can comprise a chamber between the sleeve and the annular component, and the chamber can be pressurized to urge the wellhead component away from the sleeve. 
     The means for urging the annular component axially against the collar can comprise radially extending bolts extending through threaded bores in the annular component and each ending in a tapered dog, and recesses around the larger diameter casing, the recesses having inclined flanks and being positioned so that when the bolts are screwed in, the dogs enter the recesses and make contact with the inclined flanks, and as the bolts are screwed further in, the annular component is drawn further towards the sleeve. 
     The internal bore of the larger diameter casing may have a constant internal diameter, and the sleeve can be located between the larger diameter casing and the annular component, and when the arrangement is in use, the sleeve is in abutment with the larger diameter casing. 
     According to a second aspect of the present invention, there is provided a pre-installed clamping arrangement for clamping a subsequently installed tubular well casing of a first diameter within a previously installed tubular casing of larger internal diameter, wherein the larger diameter casing has a wall thickness which is sufficiently thin to allow the casing wall to be distorted inwards to grip the smaller diameter casing, the arrangement also comprising a compression unit which includes a compression collar surrounding the larger diameter casing, a compression ring axially movable relative to the collar and means for producing relative axial movement between the ring and the collar, the compression ring and compression collar having oppositely directed axially tapered annular surfaces, so that relative axial movement between the collar and ring produces a reduction in the internal diameter of the unit to distort the larger diameter casing inwards to grip the smaller diameter casing. 
     The tubular annular walls of oil well casings have to withstand substantial pressures, and it is this requirement to withstand certain pressures which generally determines the wall thickness of the casings. In most cases, casing walls will be too thick to allow inward deflection to grip an internal component. However by making the walls thin enough to allow such deflection, it becomes possible to achieve the advantageous clamping arrangement of the invention. It will be a matter of trial and error, or of relatively straightforward calculation, to determine the appropriate casing wall thickness for any particular application. Factors which have to be taken into account are the gap between the larger and smaller diameter casings (this gap has to be bridged when the clamp is tightened), the overall diameter of the casings and the material of which they are made. It is desirable to maintain deflection of the casing wall in the elastic range, thereby allowing the casing to expand to it&#39;s original diameter once the clamping or compression force has been removed. This allows the clamping arrangement to be reversed or disengaged relatively quickly and easily without any permanent alteration to the casing. The clamping system can, of course, plastically deflect the casing should such be required by a particular application. One such application would be clamping more than one inner casing. In such a case, it is likely that the outer casing would be plastically deformed because of the greater clamping force required to adequately grip the most inner casing. 
     The casing may be divided axially into different sections, and it can then be appropriate to make the section of the casing which is to be distorted inwards out of a high value/high strength material, in order to assist that section in withstanding high internal pressures and the effects of corrosion. 
     If it is not possible to achieve the necessary pressure resistance whilst allowing the necessary distortion for clamping to take place, then the thin walled tube may be externally reinforced to enable it to resist the hoop stresses arising when there is a high internal pressure. 
     The reinforcements may take the form of annular bands around the casing section, and these bands can provide the necessary thickness of material to allow a valve or valves to be fitted to the casing in the area where the casing wall is relatively thin. 
     The casing section where the compression unit is located may be readily separated from the rest of the casing, so that it can be replaced when necessary. 
     The compression unit preferably has a compression ring which is in contact with the outer surface of the large diameter casing and a compression collar which surrounds the ring and is axially movable relative to the ring. The ring may be split at one or more points around its circumference to assist assembly, and reduce radial stiffness. 
     The ring and the collar may each have one tapered annular surface. Alternatively, and preferably, the ring has two tapered surfaces, tapering in opposite axial directions, and the collar is split into two sections with opposite axial tapers and the means for producing the relative movement acts between the two sections of the collar to move the sections in opposite directions over the ring. 
     Preferably the ring has its region of greatest diameter between its two ends, and the two collar sections are drawn towards one another, for example by bolts through both sections, to compress the ring and thus to clamp the larger diameter casing onto the smaller diameter casing. Although a preferred method of drawing the two sections together is by use of a bolt, it should be understood that any suitable mechanism may be used bring the two sections together. 
     In addition to mechanical actuators, any suitable actuator may be used to create sufficient deflection in the outer casing. For instance, hydraulic pressure exerted on the exterior of the outer casing could be used or thermal energy could be used to create expansion and/or contraction of the outer casing thereby manipulating it&#39;s internal diameter. 
     The clamping arrangement described here can be used, as described, to clamp a plain walled tube. In some circumstances however (particularly for small diameter casings) it may be expedient to provide a small hanger shoulder to take a part of the casing load and/or to locate the tubular casings in a desired axial position before applying a clamping arrangement as described here to clamp the casings in position. 
     According to a third aspect of the invention, there is provided a pre-installed clamping arrangement for clamping a subsequently installed tubular casing of a first diameter within a previously installed tubular casing of larger internal diameter, the arrangement comprising first and second compression rings having oppositely tapered external surfaces, an annular compression actuator having an internal tapered surface surrounding the first compression ring and an external tapered surface radially outside its internal tapered surface, and an annular component having two tapered surfaces, one of said surfaces mating with the second compression ring, and the other of said surfaces mating with the external tapered surface of the compression actuator, and means for moving the annular component axially relative to the compression rings and the compression actuator between a first position in which the tapered surfaces of the annular component exert no radial force on the compression rings or the compression actuator and a second position in which the tapered surfaces of the annular component exert sufficient radial force to distort the compression rings into the bore of the larger diameter casing, to grip the casing of smaller diameter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be further described, by way of example, with reference to the accompanying drawings, in which: 
     FIG. 1 shows a cross section through a surface wellhead arrangement incorporating a first embodiment of a clamping arrangement in accordance with the present invention; 
     FIG. 2 shows a second embodiment of a clamping arrangement; 
     FIG. 3 shows a detail, on a larger scale, a third embodiment of a clamping arrangement 
     FIG. 4 shows, on a larger scale, a cross-section through a fourth embodiment of a clamping arrangement; 
     FIGS. 5 and 6 show two alternative arrangements incorporating the clamping arrangement of FIG. 4; 
     FIG. 7 shows a fifth embodiment of a clamping arrangement; 
     FIG. 8 shows a sixth embodiment of a clamping arrangement; 
     FIG. 9 shows a seventh embodiment of a clamping arrangement; 
     FIG. 10 shows a hydraulically actuated clamping arrangement; 
     FIG. 11 shows a thermally actuated clamping arrangement; 
     FIG. 12 shows a clamping arrangement having an adjustable locking mechanism; 
     FIG. 13 shows a clamping arrangement which includes slip segments; 
     FIG. 14 shows a clamping arrangement which includes sensors; 
     FIG. 15 shows a moveable clamping arrangement; 
     FIG. 16 shows a clamping system for adjustment of tension and/or compression in a string; 
     FIG. 17 shows a clamping arrangement for use at a subsea wellhead; 
     FIG. 18 shows a riser spaceout system which employs the clamping system of the present invention; 
     FIG. 19 shows an eighth embodiment of the clamping arrangement; 
     FIG. 20 shows a cross sectional view of a slotted casing which may be used in the invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The adjustable wellhead shown in FIG. 1 has a surface casing starting head  1  mounted on a casing section  30 . An intermediate casing  32  is located in the casing  30  and has a casing hanger  2  by means of which the casing is landed on a shoulder  34 . The hanger  2  has an extended upper neck  6  which has a tapered external profile. 
     A wellhead spool  3  is shown above the casing hanger  2 . The wellhead spool has a tapered internal profile  7  which mates with the tapered external profile of the neck  6  and, in the position shown in FIG. 1, the spool is supported above the hanger  2  on a spacer ring  12 . An annular seal ring  5  fitted with O-rings  10  provides a seal between the spool  3 , the starting head  1  and the casing hanger  2 . 
     A chamber  5   a  is present above the seal ring  5 . This chamber can be pressurized, through a passage  22 , to raise the spool  3  above the starting head  1 , and such raising action will have the effect of unloading the weight of the wellhead spool  3  from the spacer ring  12 . 
     The spacer ring  12  is axially movable (possibly removable) and is fitted between the starter head  1  and the spool  3 , and, when present, this spacer ring controls the extent to which the spool  3  can be lowered onto the starter head  1 . In one position of the spacer ring, it prevents any effective contact between the tapered surfaces  6  and  7 . In another position, it does not obstruct engagement of the tapered surfaces. 
     The ring can be moved axially by rotating it on a thread  12   a  so that it moves up and down along the string, on the thread. Alternatively, the ring can be simply removed to remove any obstruction to engagement of the surfaces  6  and  7 . 
     Bolts  9  (only one of which can be seen in FIG.  1 ), which each end in a tapered dog  8  which enters a tapered annular recess  11 , allow the spool  3  to be drawn down onto the starter head  1 . By screwing in the bolts, the dogs  8  bear against the tapered side wall of the recess  11  and the spool is pulled down by the camming action of the dogs. In practice, the spool will be drawn down by tightening each bolt around the string circumference, by a specified tightening extent, in turn. Working around the string circumference in this way will gradually pull the tapered surfaces  6  and  7  against one another to effect a clamping action. 
     A production casing  36  is run into the well on an adjustable surface casing hanger  4 . The casing  36  is threadedly engaged at  38  with the hanger  4 . The casing is slotted at  13  for flow-by, and the hanger is positioned so that part of its length is surrounded by the extended upper neck  6  of the casing hanger  2 . 
     During running of the casing  36 , the internal diameter of the neck  6  is such that the casing hanger  4  can move axially past the neck. At this stage, the spacer ring  12  is in its first position where it prevents engagement between the surfaces  6  and  7 , the extended upper neck  6  is unstressed and so the casing hanger  4  is able to move freely past the neck  6 . 
     However when the casing hanger  4  has reached a position within the intermediate casing hanger at which it is to be clamped, the annular chamber  5   a  is pressurized to lift the wellhead  3  and to allow the spacer ring  12  to be lowered or removed. Once this has happened, the annular seal  5  is relieved of pressure and both gravity and tightening of the tapered bolts  9 , results in the wellhead  3  being lowered onto the casing hanger  2  so that the tapered surfaces  6 ,  7  come into contact with one another. On further tightening of the bolts  9 , the wedging effect of the surfaces  6  and  7  results in the extended upper neck  6  being distorted into the path of the production casing hanger  4 , to a position where it grips the casing hanger which therefore becomes clamped in the well at that point. 
     If the position in the well of the production casing then has to be altered, for example after tensioning the production casing, then this can be done by releasing the bolts  9  and then pressurizing the seal ring  5  to raise the wellhead. This relieves the wedging force acting on and distorting the extended upper neck. The neck then returns to its unstressed position where the casing hanger  4  can move freely axially past the neck. 
     When the casing components have taken up their new positions, for example after tensioning the production casing, then they can be clamped relative to one another by once again lowering the wellhead using the procedure described above to activate the clamp. 
     The bolts  9  with their clamping dogs  8  must be retracted before the wellhead can be fully lifted, and have to be fully inserted in order to apply maximum clamping force to the casing hanger  4 . 
     FIG. 2 shows a second embodiment which is largely similar to the embodiment of FIG. 1 except that the internal bore represented by the intermediate casing  16  and its casing hangar  15  now has a uniform internal diameter, so that the production casing hangar  14  can pass completely through the intermediate casing  16  and its hangar  15  when the clamp is not operated. 
     However, the production casing hangar  14  can be gripped and clamped in the intermediate casing  15 , by a mechanism the same as that shown in FIG. 1, and corresponding parts in FIG.  2  carry the same reference numerals as they carry in FIG.  1 . 
     FIG. 3 shows a wellhead housing  40  with a tubing head  17  installed above it. An extended upper neck  21  is formed on an annular component  18  which has an internal diameter just slightly greater than the external diameter of the tubing hanger  20 . The tubing head has a tapered surface  23  which mates with the neck  21 . 
     As described in relation to FIGS. 1 and 2, the surface of the neck  21  and the surface  23  of the tubing head  17  mate to form, when the tubing head is lowered onto the component  18 , a clamp which clamps the tubing hanger  20  relative to the wellhead housing  40 . 
     FIG. 4 shows a well casing tube  50  within which a casing hanger  52  is positioned. The casing hanger  52  is a close fit within the internal wall of the tube  50 , and a casing  54  is suspended from the lower end of the hanger  52 . At the top of the hanger  52  is a socket  56  which can be used to connect a running tool to the hanger. The outer surface of the hanger, at  58 , is relieved by a flow-by passage  60 , and this passage is, when necessary, closed by an annular seal  62 . 
     On the outside surface of the casing or wellhead housing  50 , and alongside the position of the casing hanger  52 , a compression ring  64  is fitted. This compression ring extends right the way around the casing  50 , but may be split at one point around its circumference to allow it to be compressed and reduced in diameter. The ring  64  has two oppositely directed tapered surfaces  66 ,  68 , and the point of greatest diameter of the ring is midway between its ends. 
     A compression collar  70  is made up of two collar sections  72  and  74  which can be drawn towards one another by tightening one or both of nuts  76  at opposite ends of the bolt  78 . The collar sections  72  and  74  (which are each annular) have inwardly directed tapered faces  80  and  82  which match the tapered faces  66 ,  68  on the ring  64 . 
     When the nuts  76  are tightened, the sections  72 ,  74  are drawn towards one another and they ride up the ramps  66 ,  68  with the result that the ring  64  is squeezed and reduced in diameter. This reduction in diameter is transmitted to the part of the casing  50  immediately within the ring  64 , and the casing  50  will be compressed inwards to squeeze the casing against the outer surface  58  of the hanger  52 . 
     It will be appreciated that there will be bolts  78  with nuts  76  arranged at regular intervals around the circumference of the compression unit  70 , and to tighten the compression unit to produce clamping, it will be necessary that the bolts be tightened sequentially around the circumference until the correct clamping force has been achieved. 
     It will be clear that the clamping can only be effective if there is sufficient deformability within the casing tube  50 . To achieve this deformation, it is likely that the tube  50  will have to be thinner than it would otherwise be. It is not however anticipated that a skilled man would find it difficult to design a tube which would have the necessary deformability for a particular application of this invention. 
     Instead of bolts  78  and nuts  76 , it may be possible to use an alternative mechanism which draws the two collar sections  72 ,  74  together. 
     FIG. 5 shows the arrangement of FIG. 4, but on a smaller scale with other ancillary components also being in view. 
     Because the casing tube  50  is thinner than it would be expected to be (in the absence of the clamping arrangement described here) other devices are fitted around the tube, to strengthen the tube and to assist the tube in resisting hoop stresses caused by high internal pressures. 
     Below the pressure unit  70 , there is a reinforcing ring  84  which is put in place by sliding it over the top of the casing  50 . The ring  84  is annular in form to support the whole of the circumference of the tube  50 . 
     At one or several points around the circumference of the casing tube  50  there is an outlet port  86 , and the reinforcement ring  84  has a corresponding passage in which a threaded insert  88  is fitted. A valve flange  90  then is bolted onto the reinforcement ring  84 . The threaded insert  88  is made up into the mouth of the opening  86 , to form a metal-to-metal seal. On the side of the valve flange, the threaded insert  88  is fitted with an annular groove  89  into which a seal ring is fitted to effect a seal between the flange and the valve body. 
     The flange  90  will be the flange plate of a conventional valve (the valve itself is not shown here), so that when the assembly is completed, the valve can be opened or closed to open or close communication between the interior of the casing and the exterior through the passage  86 . 
     Above the compression unit  70 , there is an upper collar  92  which will be slid onto or threaded onto the external surface of the tube  50 , to provide reinforcement in this upper area. 
     It will be seen in FIG. 5 that the casing tube  50  is part of one piece of material with the wellhead spool  3 . However in FIG. 6, where the same parts carry the same reference numerals, the tube  50  is a separate component from the wellhead spool  3 , with the two components being sealed to one another along a thread line  96 , with the interposition of O-ring seals  98 . This construction makes it possible to manufacture the deformable tube  50  from a material different from the spool  3 . It also makes it possible to replace the tube part  50  independently of the spool  3 . The material and manufacturing of each of these parts can therefore be optimized for the particular function and an internally damaged wellhead can be refurbished by exchanging the tube  50  only. 
     FIGS. 5 and 6 also show a drilling riser connector  100  to which a drilling riser can be connected. The connector  100  is fitted to the reinforcing ring  92  and is secured on the ring by means of axially directed dogs  102 , in a manner which is in itself known. A metal sealing ring  104  provides the necessary seal. 
     In order to effect the clamping more easily, tube  50  can be relatively thin, and can be economically made of a high value material. It may need to be a material particularly resistant to corrosion, and of course it has to be capable of the distortion necessary to achieve clamping. However because the tube is backed up around most, if not all, of its external circumference it does not need great mechanical strength. The mechanical strength can be provided by the surrounding components. In some cases, it may be necessary to provide a clamping force that will deform the casing tube  50  beyond it&#39;s elastic limit. For instance, where more than one inner casing must be clamped or gripped, the force required to clamp the innermost casing may likely cause plastic deflection of the outermost casing. In certain instances, it may be desirable to provide an outer casing that has more flexibility. FIG. 20 shows such a casing. The slotted casing  880  has a plurality of outer or exterior slot  882  located at intervals along it&#39;s outer surface  886 . Similarly, the slotted casing  880  also may have a plurality of inner or interior slots  884  formed at intervals along it&#39;s inner surface  888 . These plurality of slots  882 ,  884  allow the casing  880  to flex or compress to a greater degree than non-slotted casing within the elastic range or for a given activation force. The slots also help to alleviate hoop stresses which may occur during compression of the casing. In a preferred embodiment, the slots are approximately 0.25 inches in width and are positioned every 7.5 degrees around the interior surface  888  and/or exterior surface  886  of the casing  880 . 
     One particular advantage of the embodiment shown in FIGS. 4-6 is that there is no discontinuity in the wall of the casing, and therefore no potential leak path for the leakage of pressure. 
     It is thus possible to close off an annulus in an oil or gas well, with the closure and the seal being arranged at any convenient position along the length of the casing string. 
     FIG. 7 shows two adjacent casing sections  203  and  230 . The upper casing section  203  has a tapered internal profile  207  which mates with a tapered external profile  206  of a clamp component  218 . The component  218  is threaded to the lower casing section  230  at  220 , and seals  222  provide the necessary sealing function. 
     Around the exterior of the casing section  230 , an anchoring ring  224  is fitted, the ring being connected to, and adjustable relative to, the casing section on a thread  226 . The ring  224  has a series of threaded bores  228  arranged around the circumference. Only one of these bores is visible in the figure. 
     The upper casing section  203  has a shoulder  232  which has a series of through bores  234  each of which registers with one of the threaded blind bores  228  in the ring  224 . Threaded studs  236  are fitted in each of the bores. 
     Each stud  236  has a lower end which screws into one of the blind bores  228 . A nut  238  is threaded onto the stud, and a thrust plate  240  with a washer  249  lies above the nut. The upper casing section  203  is then placed over the upstanding part of the stud, and a further nut  242  is threaded onto the top of the stud. 
     In use, the nuts  242  can be tightened to draw the casing section  203  and its tapered surface  207  down onto the tapered surface  206  of the clamp component  218 , to clamp a tube  300 . It will be seen from FIG. 7 that the positions of the nuts  238  determine the extent to which the tapered surface  207  of the casing section  203  can be drawn down onto the tapered section  206  of the clamp component  218 , and thus determines the clamping force which can be applied to the tube  300 . However the positions of the nuts  238  relative to the lower casing section  230  can be altered by turning the nuts on the threads of the studs  236 . 
     The nuts  238  can also be used to release the clamp. To do this, the upper nuts  242  are slackened off, and a tool is used to turn the nuts  238  so that they lift the upper casing section  203  to reduce the engagement between the surfaces  206  and  207 . The presence of the thrust plate  240  and washers  249  makes it possible to turn the nuts  238  when they are under load. 
     FIG. 7 also shows a fixed end stop  244 , which provides the ultimate limit to relative axial movement between the tapered surfaces, and annular seals  246  and  248  between the separate clamp component  218  and the upper casing section  203 . 
     FIG. 8 shows the arrangement of FIG. 7, but with the clamp fully tightened up to the stop  244 . It will be seen that there is clamping contact between the component  218  and the casing  300  at  252 . The view shown in FIG. 8 is taken at a different point around the casing circumference, and shows a monitoring port  250  which communicates with the gap between the clamp component  218  and the uppercasing section  203 . 
     FIG. 9 shows an embodiment which combines features from earlier described embodiments. 
     In FIG. 9, a casing hanger  352  is to be clamped within an upper casing section  303  and a lower casing section  330 . The hanger  352  has a flow-by passage  360 , and has a casing  354  threaded to its lower end. 
     Two compression rings  364  (each similar to one half of the ring  64  of FIG. 4) separated by a plain ring  365  are retained within a correspondingly shaped annular, internal recess formed by the upper and lower section  303 ,  330 . Also within this recess is an annular sleeve  301 . The sleeve  301  is threaded at  302  onto a corresponding internal thread on the section  330 . Seals  304  are provided to seal between the sleeve and the section  330 . 
     The sleeve  301  has an upper region  305  which has both an internally tapered surface  306  and an externally tapered surface. The upper section  303  has an upper internally tapered surface  308  and a lower internally tapered surface  307 . 
     When the components are assembled as shown in FIG. 9, tightening of the nuts  342  (of which there will be several around the circumference) draws the upper section  303  towards the lower section  330 . This will cause all the tapering surfaces to ride over one another. 
     The surface  308  of the upper section  303  will ride over the upper compression ring  364  and will compress the ring inwardly. 
     The surface  307  of the upper section  303  will ride over the upper part of the sleeve  301  and will compress the sleeve inwardly. 
     At the same time, the upper part of the sleeve  301  will be driven into the tapering gap between the lower one of the compression rings  364  and the upper section  303 , and this will cause the lower compression ring to be compressed radially inwards, to grip the casing hanger  352 , at whatever part of the hanger lies within the circumference of the rings  364 . 
     In this embodiment, metal/metal seals exist between the surfaces of the upper and lower sections, the compression rings  364  and the sleeve  301 . The surfaces of the compression rings which will make contact with the hanger  352  can be ribbed or serrated, in order to enhance the grip of the rings on the hanger. The compression ring could be made from a single component with two oppositely tapered surfaces, instead of the construction described above. 
     The clamping/clamping system described here is easy and simple to operate and allows the parts of the clamp to be held apart, against gravitational influences, until the components to be clamped are in their correct relative positions. It also allows the clamp to be easily opened and closed to allow adjustment of relative axial positions. 
     As previously described, the clamping arrangement of the present invention uses mechanical force to create a gripping force on the pipe. Specifically, a plurality of nuts are typically tightened to draw a corresponding pair of tapers together thereby causing deflection of the outer casing and subsequent gripping of the inner casing. It should be understood that any mechanism or method may be used to cause deflection of the outer casing sufficient to create a gripping or clamping force to be exerted on the inner casing. 
     In one embodiment, a hydraulic force may be used to create a clamping force. FIG. 10, shows a preferred hydraulic clamping arrangement  600 . The hydraulic clamping arrangement  600  includes a hydraulic chamber  602 , inlet port  604  and seals  606 . The chamber  602  is configured such that one side  608  of the chamber is formed by the outer casing  610 . The remaining sides of the chamber are formed such that the chamber forms a continuous loop or circle around the riser. A hydraulic fluid may be introduced into the chamber through port  604 . The seals  606  prevent escape or leakage of the fluid once it has been introduced into the chamber and thereby maintain pressure within the chamber. As the pressure in the chamber is increased, either by introduction of an increased volume of fluid or through expansion of a substance present in the chamber, the side  608  is deflected to create a gripping or clamping force between it and the inner casing or hanger  612 . By selection of the hydraulic fluid and use of monitoring equipment, such as pressure gauges, strain gauges and the like, the specific force exerted by the fluid on the outer casing  610  may be precisely controlled to create a specific amount or degree of deflection. Any suitable fluid may be used, however, concrete, plastic, or a similar fluid is preferable because it can be supplied at a relatively low pressure into the chamber, at which point it may expand to create a hydraulic force to cause deflection of the outer casing. Additionally, concrete hardens over time, thereby maintain pressure in the chamber without reliance on the seals, which otherwise would require replacement over time. 
     In another embodiment and as shown in FIG. 11, the force required for clamping or gripping may be formed through the use of thermal expansion and/or contraction. A heating system  700  may incorporate a heating unit or body  702  which may contain any number of heating elements  704 . The heating elements may be of any suitable type, but are preferably heating coils  706  or heating conduits  708 . As with the other clamping arrangements, previously described, the heating system  700  is positioned around and in close proximity or in contact with the outer casing  712 . Once in position, and prior to insertion of the inner casing or hanger  714 , the outer casing  712  is heated using thermal energy provided via the heating elements  706  or the heating conduits  708 . Any suitable method or mechanism may be used to provide the thermal energy. Typically, the heating elements  706  convert electrical power into thermal energy, thereby providing the heat needed. Any number of suitable fluids may be used to supply thermal energy via the heating conduits, for example, steam or a heated liquid may be passed through the conduits. As the thermal energy is passed from the system  700  into the outer casing, the outer casing expands. After sufficient expansion, the inner casing or hanger  714  may be passed through the outer casing  712  and properly positioned. Once the hanger is in place, the system is allowed to cool, either through simple dissipation of heat in the system or by passing a cooling medium through the system, such as through the conduits. As the outer casing cools, it shrinks toward it&#39;s original diameter. By selecting a hanger with an external diameter slightly larger the inner diameter of the outer casing at ambient temperature, a gripping or clamping force may be created by the outer casing on the hanger. 
     The clamping system herein described may be used in a relatively large number of applications, some of which may include additional material or mechanisms or may include alternate configurations of the clamping system. It should be understood that any number of combinations and uses may be found for the present invention. 
     For instance and as shown in FIG. 12, the clamping system previously described may further include a locking system to prevent substantial movement of the hanger in the event that the clamping system loses grip. Typically, such a locking system is provided as a safety device, although other uses are possible. The locking system  400  includes an outer member  402  capable of engaging an inner member  404 . The outer member  402  has a lower threaded portion  406  for engaging a similarly threaded upper threaded portion  408  of the inner member  404 . The outer member may also include an upper, running profile portion  403  for enabling the installation of the locking system  400  by the engagement of a running tool not shown. The lower portion  406  of the outer member preferably has a tapered shoulder  410  for engaging a similarly tapered shoulder  412  on the inner surface  414  of the outer casing or riser  416 . The inner member has a threaded upper portion  408  for engaging the outer member and a threaded lower portion  418  for engaging a similarly threaded portion on the hangar  420 . By including threaded portions on both the inner and outer members, as well as on the hanger, the locking system may be axially adjusted as needed. Typically, the outer member will be adjusted so that the shoulder  410  is in contact or immediately above the riser shoulder  412 . Thus, if the gripping system should fail or otherwise loose grip, the two shoulder areas will engage to prevent axial downward movement of the hanger. Although the locking system is typically located above the gripping system, as shown in FIG. 12, it may be positioned in any suitable location. 
     In yet another embodiment, slip segments may be used in conjunction with the clamping arrangement to provide additional safety or backup features. As shown in FIG. 13, the clamping system  800  includes an upper clamping ring or component  802  and a lower clamping ring or component  804  which are drawn together using a threaded bolt, for instance, or any other suitable method or mechanism, as has been previously described. A notch or groove  806  may be formed in the outer casing  810  or alternatively, in the hanger  810  (groove in hanger not shown), the groove having a angled or substantially triangular shape, such that a substantially shorter side  814  is angled outward from the inner surface  816  of the outer casing  808 . A second substantially longer side  818  is angled downward and inward. An annular wedge or slip segment  812  may be disposed in the groove  806 . The slip segment  812  is typically triangular or wedge shaped, corresponding generally to the shape of the groove, but being somewhat smaller overall. Due to the greater size of the groove  806 , the slip segment  812  may slide axially in the groove between an initial or installed position and a safety or engaged position, as shown in FIG.  13 . The slip segment is installed into the groove prior to moving the hanger into place. The slip segment is typically maintained in an upper, initial position by a shear pin or similar structure. Once the hanger is installed and clamped or gripping, the slip segment is maintained in the initial position until there is a loss of clamping force. Once the hanger  806  begins to move axially downward in relation to the outer casing  808 , the shear pin will separate, leaving the slip segment free to move axially downward. Due to the inwardly sloped side  818  of the groove  806 , the slip segment  812  is forced inward and into contact with the inner casing  810 , forming a mechanical gripping force there between, which increases with greater axial movement of the inner casing  810  and slip segment  812 . Preferably, the inner surface  820  of the slip segment is provided with notches or teeth to better grip the inner casing. Although described with particularity, it should be understood that the groove and slip segment may be of any suitable size and shape. 
     It is useful in many cases where the clamping arrangement of the present invention is used to be able to monitor on a real time basis the amount of clamping force being provided. This is important initially to prevent the use of an excessive amount of force which may plastically deflect the outer casing, where such action is not desired, for instance. It is also useful in determining if the system is losing clamping force while in operation. As shown in FIG. 14, a plurality of strain gauges  902  may be used to monitor the clamping force exerted by the clamping arrangement  900  at any given time. Preferably, each strain gauge provides data to a monitoring system which then allows operators to determine the status of the system. Typically, the strain gauges are located on the inner surface  904  of the hanger  906 . By placing gauges  902  at intervals along the length of the clamping area, operators can monitor not only the overall gripping force being exerted, but also the force being exerted at a particular location. In an alternate embodiment, one or a plurality of strain gauges may be located on the outside of the clamping arrangement. Regardless of the location, the gauges may prevent inadvertent or unknowing failure of the clamping arrangement by detecting decreases in gripping strength prior to loss of grip between the outer casing and the hanger. 
     A significant problem in some wells, especially deeper wells, is the inability of operators to predict the exact location of the hanger after the casing has been run. Due to production tolerances and the relatively large number of joints required for deep wells, space out becomes a problem. The present invention may be modified to alleviate this problem. As shown in FIG.  15 , the clamping arrangement  500  may be mounted in such a way to allow it to move or slide along the length of the outer casing  502 . Any suitable mechanism may be employed to allow movement, in one embodiment, a plurality of hooks or loops  504  may be attached to the clamping arrangement so that it can be raised and lowered using rope, chain or cable  506 . By allowing the clamping arrangement to move axially, the inner casing is more easily spaced out because precise location of the hanger  508  is not required. Clamping force may be provided as previously described and is not affected by movement of the arrangement. 
     Yet another problem, especially in deep water wells, is that either tension or compression may build up along the concentric strings, thereby causing problems in the capacity of a system limiting its capability in respect of the length of the concentric risers. The present invention may be used to alleviate such problems. Similarly, it may be desirable in certain instances to create tension or compression between inner and outer strings or a part thereof. As shown in FIG. 16, a plurality of clamping arrangements  980  may be employed to accomplish these goals. The clamping arrangements  980  may be arranged or positioned at intervals along the length of the string. The area or segment  986  between each clamping arrangement may then be separately controlled and changed for the specific needs of the particular string by clamping or gripping the casing at a predetermined location. The tension or compression in a particular segment  986  can this way be incrementally controlled. Typically at least some number of clamping arrangements will be located subsurface, however, the arrangements may be located in any suitable position. Typically, a clamping arrangement  980  is positioned around the riser or outer casing  982 , as previously described. The inner casing  984  may be of any suitable type for use in the present invention, but is preferably a special segment having thickened walls, such as found in hangers. 
     A common problem encountered in completing a subsea wellhead is the inability to properly control the tension in the casing after the subsea casing hanger has been landed on its shoulder in the Subsea wellhead. Currently, the casing tension is provided only by the residual load between the Subsea casing hanger and its running tool just prior to landing in the subsea wellhead. This method is relatively imprecise because the casing weight can be significantly or sometimes totally dissipated by differential sticking of the casing string in the hole. By incorporating a clamping arrangement of the present invention into the subset wellhead, a precise amount of tension may be added to the casing. As shown in FIG. 17, the present invention may be modified for use in a subset wellhead  900 . Typically, an outer casing or wellhead  902  is anchored to the sea floor. Preferably, the clamping arrangement  904  is incorporated into the outer casing or wellhead  902  itself, as shown in FIG. 17, however, it may be a separate assembly, as previously described. An externally tapered annular sleeve  906  is located around the outer casing or wellhead  902 . An annular component  908  having a plurality of bores or holes  914  there through is positioned substantially around and outside the sleeve  906 . Each bore has a shoulder  916  formed therein, for the purpose creating a piston area between seal sets  928 , which is used to hydraulically tension the system and ultimately retaining a fastener. The annular component  908  has a corresponding externally tapered surface for engaging the sleeve  906 . The outer casing or wellhead is preferably adapted for receiving threaded bolts, screws or other suitable fasteners  910  into correspondingly threaded bores  912  formed therein. The bolts preferably have a flange  918  capable of engaging the shoulder  916  of the holes  914 . The hole  914  corresponds generally to the threaded bore  912  such that the bolt  910  may pass through the holes  914  and engage the bore  912 . As the bolt  910  engages the threaded bore  912 , an annular pocket area is created between the flange  918  and shoulder  916 . When hydraulic fluid is introduced in the annular pocket area under sufficient pressure, the effect is that the annular component  908  is pushed downward. This downward movement causes the corresponding tapered surfaces of the annular component and the sleeve to engage and create a clamping force which is directed inwardly and which will deflect the outer casing or wellhead bore inwardly as well. Simultaneously to deflecting the outer casing or wellhead bore inwards the lower section  930  of the annular component  908  flares a lower rim  932  on outer casing or wellhead  902  outwards so as to friction lock the outer casing or wellhead  902  into the conductor receptacle  920 . As the inner casing  922  and hanger  924  are moved into location, prior to any clamping force being exerted, the casing  922  is typically cemented or otherwise affixed in the well bore. It is often desirable to have a certain degree of tension in the inner casing after it has been cemented. Using the prior art methods of shoulders or slips, is impossible to achieve in subsea wellhead applications. The present invention, however, allows the inner casing  922  to be positioned below it&#39;s final position, in a wider section of the outer casing or wellhead  902 , where the fluid returns generated, while the inner casing  922  is cemented in, can flow past the casing hanger  924 . Once the cement has set the casing  922  can be stretched upwards into position using the casing running string. This creates tension in the casing  922 . By pulling the casing  922  up a predetermined distance, or by using suitable measuring equipment, tension in the casing  922  may be set relatively precisely. Once the casing  922  and hanger  924  are pulled into position, hydraulic pressure is remotely introduced in the annular pockets between the seal sets  928 , thereby generating a clamping force between the bore of the outer casing  902  and the outer surface of the casing hanger  924 , for maintaining the position of the hanger  924  and casing  922 . A secondary effect of the gripping action of the outer casing or wellhead bore on the casing hanger is to compress a number of annular seals  926 , so as to seal the casing hanger outer diameter against the inner bore of the outer casing or wellhead  902 . 
     As with the inner casing, it is often difficult to predict the precise terminating location of the outer riser at the surface, especially in deep water wells. Due to production tolerances in the joints and the fact that the riser it typically landed on a fixed shoulder at the subset wellhead, shakeout of the outer riser typically involves having to cut the riser to the desired length once it has been completely installed. This procedure is time consuming and expensive. FIG. 18 shows a first riser segment  950  which incorporates a receiving area  952  having an increased inner diameter. A second riser segment  954  has a lower end  956  which has an outer diameter slightly smaller than the inner diameter of the receiving area  952  of the first riser segment  950 . Additionally a larger diameter pipe section  962  is shown onto which inverted air cans  960  are affixed. The air cans are used to impart upward tension to the first riser segment  954 , to counter act the weight of the riser segment which reaches all the way to the ocean floor. The, so called, tensioner riser  960  and  962  is at its upper end permanently affixed to a clamping arrangement  958 . The relative positions, with respect of the deck  968 , of the tensioner riser clamping arrangement  958 , as indicated by X  964  and of the top of the terminating wellhead  970 , as indicated by Y  966 , are important to the eventual layout of the production facility. The present invention may be used to more quickly and easily space out the clamping assembly  958  and the wellhead  970 . Following installation and temporary suspension of the tensioner riser  962  and  960  from the deck  968 , the first riser segment  950  is installed through the temporary suspended clamp assembly  958 . The receiving area  952  is configured of sufficient length to insure that at least a section of the receiving area  952 , of equivalent length to the claiming assembly  958  is located in the clamping assembly, this notwithstanding the fact that production tolerances will cause the first riser section  950  to be of uncertain length. Subsequently, the lower end  956  of the second riser segment  954  may be positioned into the receiving area  952  of the first riser segment  950 . Depending on the length of the lower end, a relatively large degree of adjustability may be achieved. Once the lower end  956  and the receiving end  952  are properly aligned, the clamping arrangement  958  disposed around the receiving area  952  may be used to provide a clamping force against the receiving end, thereby attaching the tensioned riser  960  and  962  to the outer diameter of the first riser segment  950  and clamping the lower end  956  of the second riser segment  954  in the receiving end  952 . The lower end  956  of the second riser segment  954  may have thickened walls to provide extra strength and to enable application of sufficient friction force. 
     Although the previous embodiments of the present invention show a clamping arrangement positioned externally of an outer pipe or tubular member, it should be understood that the clamping arrangement may be located in any suitable position for clamping or gripping the inner pipe. For instance, and as shown in FIG. 19, the clamping arrangement  1000  may comprise a housing  1002  which may be integrated into an outer pipe or casing  1004 . An inner annular, tapered clamping segment or ring  1006  is positioned in contact with an inner casing hanger  1008  or similar structure which must be clamped or gripped. The clamping ring preferably has an inner surface  1010  which is substantially parallel to the outer surface  1012  of the hanger  1008  and a tapered outer surface  1014 . An outer annular tapered clamping segment or ring  1016  is positioned radially outwardly of the first ring  1006  and has a tapered inner surface  1018  which corresponds to and is in contact with the tapered outer surface of the first ring  1014  and an outer surface  1020  which is preferably substantially parallel to the hanger  1008 . The housing  1002  is formed such that the lower or bottom surface  1022  forms the upper surface or roof of a pressure chamber  1024 . The pressure chamber includes a port  1026  for introducing a hydraulic fluid into the chamber  1024 . Preferably, the chamber also includes a bolt or screw  1026  extending through the floor  1028  of the chamber  1024 . As fluid is introduced into the chamber  1024 , the outer tapered ring  1016  is pushed axially upward. As the outer ring  1016  moves, the corresponding tapered inner surface  1018  and tapered outer surface  1014  cause an inward deflection of the inner ring  1006  thereby causing a gripping or clamping force to be exerted by the inner ring  1006  against the hanger  1008 . A retaining structure or ring  1030 , which is preferably prevents outward movement of the outer ring  1016 . Once the outer ring  1016  has been moved far enough to create a sufficient clamping force, the bolt  1026  may be threaded through the floor  1028  of the chamber  1024  and positioned against the bottom surface  1022  of the outer ring  1016 , thereby creating a mechanical stop or lock to prevent downward axial movement of the outer ring, even if pressure in the chamber is lost or decreased. The inner ring  1008  may be formed from a plurality of independent sections or may be a single, continuous ring. Where the inner ring  1008  is formed of sections, a band or O-ring  1030  may located around the outer surface  1014  of each sections to aid in retaining the sections in place during use.