Patent Publication Number: US-6666266-B2

Title: Screw-driven wellhead isolation tool

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to wellhead equipment, and more particularly to a wellhead isolation tool for isolating a wellhead from the high pressures and the abrasive and/or caustic substances used in well treatment procedures. 
     It is not unusual for oil and gas wells to require stimulation to restart, or to improve, a flow of hydrocarbons from a hydrocarbon bearing formation. Such stimulation typically involves pumping fluid mixtures into the formation at high pressures. Such fluid mixtures often comprise acidic solutions and/or proppants that can be caustic and/or abrasive. Hydraulic fracturing, one common form of stimulating a hydrocarbon bearing formation, forces liquids and/or gasses which may include proppants or other abrasives therein into the formation. Extremely high pressures and high flow rates must be employed in the hydraulic fracturing process so that the proppants will be forced into the hydrocarbon bearing formation. 
     Conventional wellheads, commonly called well trees, are not generally designed to withstand the pressures and/or the abrasive or caustic nature of the substances required to stimulate a formation. Generally, the wellhead is designed to withstand pressures of less than about 5,000 psi. The substances utilized to stimulate the formation will be pumped into the well at pressures greatly exceeding 5,000 psi and may be as much as 20,000 psi. 
     There are a number of existing wellhead isolation tools that provide for the reciprocation of a mandrel through the wellhead into the well so that the substance utilized to stimulate the well passes through the mandrel and into the well without damaging the wellhead. However, because of the potentially dangerous nature of the well stimulation operation, there is a continuing need to provide a wellhead isolation tool which can be easily connected to the wellhead and disconnected therefrom, which provides easy access for connection of lines to supply the treatment fluid, and which provides an efficient and safe method to stimulate the hydrocarbon bearing formation. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a wellhead isolation tool for injecting substances through a wellhead into a tubular element, such as a production tubing, in a well. The wellhead isolation tool includes a tubular mandrel adapted to be received through a longitudinal passage defined by the wellhead. The tubular mandrel defines a mandrel flow passage and has a nonthreaded outer surface. 
     The wellhead isolation tool further includes at least one high pressure valve connected to an upper end of the tubular mandrel. Treatment substances such as fracturing fluids containing proppants and other treatment fluids may be communicated through the high pressure valve into the mandrel flow passage when the at least one high pressure valve is in an open position. The tubular mandrel has an upper position in which the mandrel does not extend through the wellhead into the well and a lower position in which a lower end of the mandrel is sealingly received in the production tubing in the well. A threaded drive rod for reciprocating the tubular mandrel is vertically aligned with the tubular mandrel and is connected to the at least one valve. 
     The wellhead isolation tool further includes a drive mechanism for vertically displacing the threaded rod to urge the tubular mandrel downward through the wellhead and into the tubular element in the well. Once the tubular mandrel is sealingly engaged in the well, treatment fluids can be flowed into the well at extremely high pressures through the tubular mandrel without damaging the wellhead. The wellhead isolation tool further includes a support structure which provides for the easy location and connection of the wellhead isolation tool to the wellhead. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is perspective view of the wellhead isolation tool of the present invention. 
     FIGS. 2A-2C show a front elevational view of the wellhead isolation tool of the present invention. 
     FIGS. 3A-3B show a cross section of the wellhead taken from line  3 — 3  of FIGS. 2A-2C. 
     FIGS. 4A-4B are cross sections of the wellhead isolation tool of FIGS. 2A-2C showing the mandrel in a down position. The section is taken through the front legs and the mandrel. 
     FIG. 5 shows a prior art connection between a wellhead isolation tool and the wellhead. 
     FIG. 6 is a top view of a wellhead adapter sub of the present invention. 
     FIG. 7 is a side view of the wellhead adapter sub of the present invention. 
     FIG. 8 shows the lower end of the mandrel of the present invention sealingly engaged in production tubing in the well. 
     FIG. 9 shows a perspective view of the roller assembly of the present invention. 
     FIG. 10 is a view from line  10 — 10  of FIG.  3 A. 
     FIG. 11 shows a view from line  11 — 11  of FIG.  10 . 
     FIG. 12 is a detail showing a roller plate of the present invention. 
     FIG. 13 is a detail showing a support plate of the present invention. 
     FIG. 14 is a front view of an alternative embodiment of a support structure of the present invention. 
     FIG. 15 is a left-side elevation view of the support structure of FIG.  14 . 
     FIG. 16 is a right-side elevation view of the support structure of FIG.  14 . 
     FIG. 17 is a view from line  17 — 17  of FIG. 14, rotated 180°. 
     FIG. 18 is a view from line  18 — 18  of FIG.  14 . 
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
     Referring now to the drawings and more particularly to FIGS. 1-3, a wellhead isolation tool designated by the number  10  is shown. Wellhead isolation tool  10  has a front  12  and a rear  14 . FIG. 2C shows wellhead isolation tool  10  connected to a wellhead  15  defining a longitudinal passage  17 , positioned over a well  20 . Well  20  may comprise a wellbore  25  having a casing  30  cemented therein. Well  20  has a tubular element, which may be a production tubing  32  disposed therein. 
     Wellhead isolation tool  10  comprises a drive rod  34  which is preferably an externally threaded drive rod. Drive rod  34  preferably has a regular screw thread on the exterior thereof. Drive rod  34  has a longitudinal central axis  35 , an upper end  36  and a lower end  38 . As will be explained in more detail herein, drive rod  34  may be moved vertically by a type of screw drive mechanism. For example, a machine screwjack  39  driven by a motor  40  of a type known in the art may be utilized. The details of each are not depicted, but are shown schematically in the figures. Motor  40  is not shown in FIG. 1, but is schematically represented in FIG.  2 . Screwjack  39  may be, for example, a machine screwjack model 50 MSJ, available from Nook Industries, Inc. An outer protective tube  41  may be disposed about drive rod  34  above screwjack  39 . Drive rod  34  has internal threads  42  at the lower end  38  thereof. A coupling  44  is threadedly connected at its upper end  46  to drive rod  34  at internal threads  42 . Coupling  44  has a lower end  48  with a wing nut assembly  50   a  disposed thereabout. Wing nut assembly  50   a  is connected at a threaded connection  52  to a valve  54  which may be referred to as an upper valve  54 . Wing nut assembly  50   a  is connected to upper valve  54  at the upper end  56  thereof. Drive rod  34  is thus connected to upper valve  54  with coupling  44  and wing nut assembly  50   a.  As will be explained in more detail hereinbelow, wing nut assembly  50   a  may be easily disconnected from upper valve  54  so that a fluid line may be connected thereto. Upper valve  54  is depicted as a remote control valve which is movable between open and closed positions. In the open position, fluid may be displaced through a passage  58 , and in the closed position no fluid is allowed to pass therethrough. 
     Upper valve  54  has a lower end  60 . A wing nut assembly  50   b  is disposed about the lower end  60  of upper valve  54 , and connects upper valve  54  with a lower valve  62 . There are a plurality of wing nuts  50  utilized with the wellhead isolation tool  10 . Wing nuts  50  are referred to as wing nuts, or wing nut assemblies  50   a - 50   e  for ease of identification. Wing nuts  50   a - 50   e  may be identical to one another. 
     Lower valve  62  is depicted as a manually operated valve which defines a passage  64 . Lower valve  62  is movable between an open position wherein fluid may be displaced through passage  64  and a closed position wherein fluid flow therethrough is prevented. Although the wellhead isolation tool  10  of the present invention shows two valves, namely upper and lower valves  54  and  62 , respectively, it is understood that wellhead isolation tool  10  will have at least one valve and preferably a plurality of valves. In the embodiment shown, a remote control valve is shown as upper valve  54  and a manually operated valve is shown for lower valve  62 . The positions of the valves may be switched or both may be manual or remote control. 
     Lower valve  62  is connected at its upper end  66  to upper valve  54  with wing nut  50   b,  and is connected at its lower end  68  with a wing nut  50   c  to a mandrel assembly  70 . Mandrel assembly  70  has a longitudinal central axis  71  and comprises an upper mandrel  72  and a lower mandrel  74 . Longitudinal central axis  71  and longitudinal central axis  35  are collinear, such that drive rod  34  and mandrel assembly  70  have a common longitudinal axis and are thus coaxial. Upper mandrel  72  is connected at its upper end  76  to wing nut  50   c  at the lower end  68  of lower valve  62 . A wing nut  50   d  is disposed about lower end  78  of upper mandrel  72 . Mandrel assembly  70  may be reciprocated between an upper position  80  as shown in FIGS. 2A-2C, to a lower position  82  as shown in FIGS. 4A-4B. In upper position  80 , wing nut  50   d  disposed about the lower end of  78  of upper mandrel  72  is not connected to any other part. In lower position  82 , however, wing nut  50   d  is connected to a lower central support, as will be explained in more detail hereinbelow. 
     Lower mandrel  74  has an upper end  84  connected to upper mandrel  72  at threaded connection  86 . An O-ring seal  88  may be disposed above threaded connection  86  to provide a sealed connection between the upper and lower mandrels  72  and  74 . Lower mandrel  74  has a lower end  90 . Lower end  90  is connected to a sealing device  91  as shown in FIG.  8 . Sealing device  91  may comprise a cup mandrel  92  with a cup seal  93  connected thereto. A shoe  94  is disposed about cup mandrel  92 . A diffuser, like that shown in U.S. Pat. No. 4,262,743, the details of which are incorporated herein by reference, may also be used. Mandrel assembly  70  defines a passageway  96  for the flow of substances such as treating fluids or other fluids therethrough. Passages  58 ,  64  and passageway  96  thus define a longitudinal central flow passage  98  which provides for flow therethrough into production tubing  32 . Lower mandrel  74  has an outer surface  100 . 
     Wellhead isolation tool  10  further includes upper and lower central supports  110  and  112  respectively, which may also be referred to as upper and lower hubs  110  and  112 . Upper central support  110  comprises a body portion  114  and upper and lower flanges  116  and  118 , respectively, extending outwardly from body portion  114 . Upper and lower flanges  116  and  118  define a space  120  therebetween. Upper and lower flanges  116  and  118  are preferably circular flanges. Screwjack  39  and motor  40 , or other means for vertically displacing drive rod  34 , may be mounted by any means known in the art to upper flange  116 . Upper and lower flanges  116  and  118  define pin receiving holes  122  and  124 , respectively. Pin receiving holes  122  and  124  are aligned with one another. Upper central support  110  defines an opening  126  therethrough which allows drive rod  34  to reciprocate between its upper and lower positions  80  and  82 , respectively. 
     Wellhead isolation tool  10  also comprises lower central support  112 . Lower central support  112  has an upper end  132  and a lower end  134 . Upper end  132  preferably has a threaded outer surface so that in the lower position  82  of mandrel assembly  70 , wing nut  50   d  may be threadedly connected thereto to connect upper mandrel  72  to lower central support  112 , and fix mandrel assembly  70  in lower position  82 . Lower central support  112  includes a body portion  136  and has upper and lower flanges  138  and  140  extending radially outwardly therefrom. Upper and lower flanges  138  and  140  define a space  142  therebetween. Upper flange  138  has a plurality of pin receiving holes  144  defined therethrough and lower flange  140  has a plurality of pin receiving holes  146  defined therethrough. Pin receiving holes  144  and  146  are aligned with one another. Lower central support  112  includes a sleeve  148 . Lower central support  112  defines a central opening  150  to allow lower mandrel  74  to pass therethrough and be moved vertically in and out of wellhead  15 . A wellhead adapter  152  is connected to lower central support  112  at its lower end  134 . Wellhead adapter  152  is connected to lower central support  112  with a clamp  154  which may comprise two clamp portions  155  bolted together, or which may comprise any type of clamp known in the art. 
     Sleeve  148  extends downwardly into a central opening  156  defined by wellhead adapter  152 . Sleeve  148  has a flange  158  positioned between lower central support  112  and wellhead adapter  152 . A gap  159  thus exists between wellhead adapter  152  and lower central support  112 . Sleeve  148  is held in place in lower central support  112  by wellhead adapter  152  which is connected to lower central support  112  with clamp  154 . Sleeve  148  is a removable sleeve such that sleeves having any desired inner diameter may be utilized to accommodate mandrels of different outer diameters. Wellhead adapter  152  comprises a wellhead adapter housing  160  having a wing nut  50   e  connected to a lower end  162  thereof. 
     Wellhead isolation tool  10  may further comprise a wellhead adapter sub  164  as shown in FIGS. 6 and 7. Wellhead adapter sub  164 , which may be also referred to as mounting sub  164 , comprises a mounting plate  166  having a plurality of bolt or pin holes  168  therethrough. A threaded neck  170  extends upwardly from mounting plate  166 . A prior art wellhead  15  is depicted in FIG. 5 and, as shown therein has a plurality of bolts  172  extending upwardly therefrom in a defined bolt pattern. Bolt holes  168  in wellhead adapter sub  164  are adapted to match the pattern of bolts  172  so that wellhead adapter sub  164  may be mounted to wellhead  15  simply by placing wellhead adapter sub  164  thereon and threading nuts onto the bolts  172  extending upwardly on wellhead  15 . As shown in FIG. 2C, wing nut  50   e  may be connected to wellhead adapter sub  164  simply by threading wing nut  50   e  thereon, thus connecting wellhead adapter  152  to wellhead  15 . Prior art wellhead isolation tools included a wellhead adapter  174  that had mating holes to connect directly to the bolts  172  as shown in FIG.  5 . 
     The wellhead isolation tool  10  of the present invention provides for easier connection and disconnection of the wellhead isolation tool  10  on any wellhead  15 , including those with differing bolt patterns, since a plurality of wellhead adapter subs  164  may be designed having different bolt patterns to match the bolt patterns on different wellheads. The wellhead adapter sub  164  can be placed on the wellhead  15  prior to the time the wellhead isolation tool  10  is to be connected thereto which will provide for easier connections that can be made in less time, since the wellhead adapter  152  can simply be threaded to wellhead adapter sub  164 . 
     Sleeve  148  has openings  176  therethrough. An annulus  178  is defined between outer surface  100  of lower mandrel  74  and wellhead adapter  152 . Likewise, an annulus  180  exists between the outer surface  100  of lower mandrel  74  and an inner diameter  182  defined by sleeve  148 . Inner diameter  182  defines a portion of central opening  150  in lower central support  112 . Annulus  178  and annulus  180  define a fluid path  183  that is communicated with an annulus  184  through openings  176 . Annulus  184  is defined between an outer diameter  186  of sleeve  148  and inner diameter  188  of body portion  136  of lower central support  112 . When lower mandrel  74  is inserted through wellhead  15  into well  20 , fluid can pass from a relief valve  190 , through openings  176 , annulus  178  and annulus  180  to urge cup seal  93  inwardly so that it will not engage production tubing  32  as it is lowered therethrough. Once the wellhead isolation tool  10  reaches its desired location, fluid flow through relief valve  190  ceases, and in operation, the cup seal  93  will expand to engage production tubing  32  as shown in FIG.  8 . Although in the embodiment shown, the sealing device  91  engages production tubing  32 , the mandrel assembly  70  and sealing device  91  can be used to seal other tubular elements, such as casing  30  in the well  20 . 
     Wellhead isolation tool  10  includes a support structure  200 . Support structure  200  includes an upper support plate  202 , a lower support plate  204  and a plurality of vertical support members, such as support legs  206 . In the embodiment shown, support structure  200  includes four support legs  206 . 
     Upper support plate  202  has forward and rear edges  208  and  210  respectively and side edges  212 . Upper support plate  202  has a cutout  214  which may be referred to as a semicircular or generally U-shaped cutout  214  on the forward edge  208  thereof. The U-shaped cutout  214  is adapted to be received in space  120  about body portion  114  of upper central support  110 . Upper support plate  202  has a plurality of openings  216  defined therethrough. Openings  216  are positioned to align with pin receiving holes  122  and  124  so that connectors, such as pins  218 , may be inserted therethrough to mount upper central support  110  to upper support plate  202 . One or more pins  218  may be utilized. Support legs  206  are connected to upper support strips  220 , preferably by welding or other means known in the art, at upper end  222  thereof. Support legs  206  likewise have a lower end  224 . Support legs  206  are connected by welding or other means known in the art at lower end  224  to lower support strips  226 , which may be identical to upper support strips  220 . Support legs  206  have tension rods  228  disposed therein. In the embodiment shown, each of four support legs  206  has a tension rod  228  therein. If desired, tension rods  228  may be included only in the two support legs  206  at the front  12  of wellhead isolation tool  10 . Tension rods  228  have an upper end  230  and a lower end  232 . Upper end  230  is threadedly connected to a tension rod connecter  234  which extends upwardly in support legs  206  through upper support strips  220  and openings  235  defined in upper support plate  202 . Tension rod connectors  234  have an upper end  236  which is threaded so that nuts  238  may be threaded thereon thus connecting support legs  206  and tension rods  228  to upper support plate  202 . A nut and bolt arrangement  239  may be utilized to further connect upper support strips  220  to upper support plate  202 . 
     Lower end  232  of tension rod  228  is threaded. Tension rods  228  extend through openings defined in lower support strips  226  and through openings  242  defined in lower support plate  204 . Nuts  243  are threaded on lower ends  232  of tension rods  228  to connect tension rods  228  and thus support legs  206  to lower support plate  204 . 
     Lower support plate  204  has a forward edge  244 , a rear edge  246  and side edges  248 . Forward edge  244  has a semicircular or generally U-shaped cutout  250  so that lower support plate  204  may be received in space  142  between the upper and lower flanges  138  and  140  of lower central support  112 . Upper and lower support plates  202  and  204  may be identical and thus interchangeable. Lower support plate  204  has a plurality of openings  251  positioned to align with pin receiving holes  144  in upper flange  138  and pin receiving holes  146  in lower flange  140  so that pins  252  may be inserted therethrough to mount lower support plate  204  to lower central support  112 . One or more pins  252  may be utilized and may be held in place with a cotter pin or by other means known in the art. 
     Support structure  200  has a forward, or front  253 , and a back or rear  254 , corresponding to the front and rear  12  and  14  of wellhead isolation tool  10 , and has sides  256 . Support structure  200  further includes a plurality of rear cross braces  260 . The embodiment shown includes three rear cross braces  260  that extend between two support legs  206  at the rear  254  of support structure  200 . Rear cross braces  260  may be connected by welding or by other means known in the art. 
     Wellhead isolation tool  10  may also include side cross braces  262  at the sides  256  of the well isolation tool  10 . Side cross braces  262  may be connected by welding or otherwise and extend from the support legs  206  at the front  253  of support structure  200  to the support legs  206  at the rear  254  of support structure  200 . Support structure  200  may also include rear angle braces and side angle braces  264  and  266 , respectively, at locations where rear and side cross braces  260  and  262  are mounted. Rear and side angle braces  264  and  266  may be welded or otherwise connected to support legs  206  and to the rear and side cross braces  260  and  262  respectively. 
     Support structure  200  likewise includes a guide beam  268 . Guide beam  268  essentially comprises an I-beam having a center section  270 , rear flanges  272  and forward flanges  274  extending from center section  270 . Guide beam  268  is mounted to rear cross braces  260  by welding or other means known in the art. Guide beam  268  has an upper end  276  and a lower end  278 . Guide beam  268  has a pair of forward flanges  274  which may be referred to as first and second forward flanges  280  and  282  respectively. 
     Wellhead isolation tool  10  further includes a roller assembly  284 . A perspective view of roller assembly  284  is shown in FIG.  9 . Roller assembly  284  includes a base  286  which is preferably a circular base, having a cap  288  rigidly connected thereto by welding or other means known in the art as shown in FIG.  10 . Roller assembly  284  also includes a mounting plate  290 , which is rotatably mounted to cap  288 , with a nut and bolt arrangement  292 , and can be rotatably mounted thereto by any means known in the art. For example a hexagon socket head shouldered screw may be utilized to provide the necessary rotation. FIG. 11 shows a bolt  293  with a shoulder  294  extending through cap  288 . Threads  295 , which are smaller than shoulder  294 , extend through mounting plate  290 , and a nut  296  is threaded thereon. Such an arrangement will allow rotation of cap  288  and base  286 , relative to mounting plate  290 . Roller assembly  284  includes a pair of roller plates  298 , a detail of which is shown in FIG.  12 . Roller plates  298  have first and second sides  300  and  302  and front and rear edges  304  and  306 . A tang  308  extends from front edge  304  and may be received in corresponding slots (not shown) defined in mounting plate  290 , so that roller plates  298  may be welded or otherwise affixed to mounting plate  290 . 
     A plurality of rollers, and in the embodiment shown four rollers  312  are mounted to each roller plate  298 . Rollers  312  may be of any type known in the art, such as for example a cam follower with bearings, and are mounted by any manner known in the art. Each roller plate  298  has a pair of forward rollers  314  and rear rollers  316 . A space  318  is defined between front and rear rollers  314  and  316 , respectively. First and second forward flanges  280  and  282  of guide beam  268  are received between forward and rear rollers  314  and  316 . A bolt  319  with an arm  320  is likewise attached to each roller plate  298 . Bolt  319  can be threaded through roller plates  298 . 
     Roller assembly  284  may be connected to one of upper or lower valves  54  or  62  and in the embodiment shown is connected to both of upper and lower valves  54  and  62 . An adjustable nut  330  of a type known in the art may be threaded into a threaded cavity  332  in the rear side of upper valve  54  and lower valve  62 . Adjustable nut  330  has a head portion  334  that extends from cavity  332 . Head portion  334  has openings  336  defined therethrough. Openings  336  are aligned with openings  338  in base  286  of roller assembly  284 . Pins  340  may be inserted through openings  336  and  338  to connect upper and lower valves  54  and  62  to roller assembly  284 . 
     Roller assembly  284  will initially be positioned so that first and second forward flanges  280  and  282  on guide beam  268  are positioned between the forward and rear rollers  314  and  316 , respectively. Base  286  can be rotated so that openings  338  will align with openings  336  to allow pins  340  to be inserted therethrough. 
     The operation of the well isolation tool may be described as follows. Well isolation tool  10  is first positioned over a wellhead  15 . A wellhead adapter sub  164  is connected to the upper end of wellhead  15 . Wellhead isolation tool  10  is lowered with the mandrel assembly  70  in its upper position  80 , so that wing nut  50   e  may be threaded onto wellhead adapter sub  164 . Prior to the time wellhead isolation tool  10  is connected to wellhead  15 , valves  342  and  344  on wellhead  15  are closed. Likewise, upper and lower valves  54  and  62  on wellhead isolation tool  10  are closed to prevent flow therethrough. Once the wellhead isolation tool  10  is connected to wellhead  15 , valves  342  and  344  are opened. Motor  40  can then be actuated to urge drive rod  34  downwardly which in turn moves mandrel assembly  70  downwardly. Lower mandrel  74  is thus moved downwardly through longitudinal passage  17  in wellhead  15  and into well  20 . More specifically, sealing device  91  connected to lower mandrel  74  is sealingly received in a tubular element in well  20 , which is preferably production tubing  32 , but which may be a casing. 
     Once sealing device  91  sealingly engages production tubing  32 , wing nut  50   d  is connected to upper end  132  of lower central support  112 , and support structure  200  can be removed. All that is required is to simply disconnect wing nut  50   a  from upper valve  54  and to remove pins  252  and pins  340 . Prior to removing pins  340 , bolts  319  can be rotated to engage first and second forward flanges  280  and  282  to hold roller assemblies  284  in place on guide beam  268  when the support structure  200  is removed. Lower support plate  204  can then simply be removed from between upper and lower flanges  138  and  140  on lower central support  112  and the support structure  200  can be moved as a unit. 
     A flow line of a type known in the art (not shown) can then be connected to upper valve  54 . Upper valve  54  and lower valve  62  can be opened to allow fluids or other substances to be flowed therethrough at high pressures through lower mandrel  74  into production tubing  32  and into a hydrocarbon containing formation therebelow. When the fracturing or other treatment is complete, wellhead isolation tool  10  provides for easy removal. Upper and lower valves  54  and  62  are closed after the treatment is complete. The fluid line is then disconnected from upper valve  54  and the support structure  200  is reconnected simply by positioning lower support plate  204  in space  142  and reconnecting wing nut  50   a  to upper valve  54 . Pins  252  are reinserted, as are pins  340  in roller assembly  284 , and wing nut  50   d  is disconnected from lower central support  112 . Motor  40  can then be actuated to cause drive rod  34  to move upwardly which will, because the wing nut  50   a  has been reconnected to upper valve  54 , cause mandrel assembly  70  to be lifted upwardly until the lower end  90  of lower mandrel  74  and sealing device  91  are moved above valves  342  and  344 . Valves  342  and  344  are closed after lower mandrel  74  is removed therefrom. Once valves  342  and  344  are closed, the mandrel assembly can be moved to its upper position  80 . Wellhead isolation tool  10  can be removed simply by disconnecting wing nut  50   e  and moving wellhead isolation tool  10  as a unit away from wellhead  15 . 
     In the embodiment shown in FIGS. 1-13, wellhead isolation tool  10  includes a support structure  200 . An alternative embodiment of a support structure generally designated by the numeral  400  is shown in FIGS. 14-18. Support structure  400  includes upper support plate  402  and lower support plate  404 . The upper and lower support plates  402  and  404 , respectively, may be identical to upper and lower support plates  202  and  204 , respectively. Upper plate  402 , therefore, has left and right sides, or edges  406  and  408  and front and rear edges  410  and  412 . Front edge  410  defines a generally U-shaped or semicircular cutout  414 . Lower support plate  404  has a left edge  416 , a right edge  418  and a rear edge  420 . Lower support plate  404  has a front edge  422  defining a generally U-shaped or semicircular cutout  424 . 
     A vertical support member or support frame  426  has an upper end  428  and a lower end  430 . Support frame  426  has a left side  432 , a right side  434 , a rear side or rear panel  436 , and a front  438 . Left side  432  has a plurality of access openings  440  defined therein. Likewise, right side  434  has a plurality of access openings  442  defined therein. Access openings  440  and  442  may be of any configuration and may be arranged in any desired patterns so as to allow access to wing nut assemblies  50 , upper and lower valves  54  and  62 , mandrel assembly  70  and any other parts of the wellhead isolation tool  10  to which access is desired. Rear side  436  may also have a plurality of access openings  444  defined therein. Front  438  of support frame  426  preferably defines an opening  445  extending from the upper end  428  to the lower end  430  thereof, which will also provide access to upper and lower valves  54  and  62  and other parts of the wellhead isolation tool  10 . Thus, a cross section of support frame  426  defines a generally rectangular periphery. 
     Support structure  400  may include an upper mounting plate  450 . Upper mounting plate  450  may include side mounting strips  452  connected by a rear mounting strip  454 . Upper mounting plate  450  may further include ears  456  extending from side mounting strips  452 . Upper mounting plate  450  is connected to upper support plate  402  with bolts or other connectors known in the art. Screwjack  39  and motor  40 , or other mechanism to move drive rod  34  may be mounted to upper support plate  402 . 
     Upper mounting plate  450  may have notches  458  for receiving tangs  460  at the upper end of support frame  426 . Support frame  426  is preferably welded at the upper end  428  thereof to upper mounting plate  450 . 
     Support structure  400  may also include a lower mounting plate  462  which includes side mounting strips  464  and a rear mounting strip  466  extending between and connecting side mounting strips  464 . Lower mounting plate  462  may be connected to lower support plate  404  with bolts or other connectors known in the art. 
     Lower mounting plate  462  may have notches or grooves  468  defined therein for receiving tangs  470  defined at the lower end  430  of support frame  426 . Upper support plate  402  has a pair of openings  472  positioned identically to openings  124  in upper support plate  202 . Lower support plate  404  has a pair of openings  474  positioned identically to openings  251  in lower support plate  204 . Thus, support structure  400  maybe pinned to upper hub  110  and lower hub  112  in the same manner as support structure  200 . In other words, pins  218  may be inserted through openings  472  and pin receiving holes  122  and  124 . Likewise, pins  252  may be inserted through openings  474  and pin receiving holes  144  and  146 . 
     Support frame  426  may comprise support frame portions or support frame halves  480 . Support frame portions  480  may be referred to as first and second or left and right frame portions  482  and  484 , respectively, for ease of identification. As is apparent from the drawings, first and second frame portions  482  and  484  have identical cross sections but may have access openings of different sizes and in different locations. 
     Left frame portion  482  may comprise a side panel  486  and a rear panel  488 . Side panel  486  has a rear end  490  and a forward end  492 . An L-shaped flange  494  extends inwardly from forward end  492 . Likewise, an L-shaped flange  496  extends inwardly from an inner edge  498 . L-shaped flange  496  comprises a foot portion  500  and a leg portion  502 . 
     Right frame portion  484  comprises a side panel  504  having a forward end  506  and a rear end  508 . A rear panel  510  extends from the rear end  508  of side panel  504 . Rear panel  510  has an inner end  512 . An L-shaped flange  514  is connected to and extends inwardly from forward end  506  of side panel  504 . An L-shaped flange  516  is connected to and extends inwardly from inner end  512  of rear panel  510 . L-shaped flange  516  has a leg portion  518  and a foot portion  520 . 
     Bolts may be utilized to connect the first and second frame portions  482  and  484  through leg portions  502  and  518  of L-shaped flanges  496  and  516 , respectively. As is apparent from the drawings, the two L-shaped flanges  496  and  516  define an I-section such that foot portions  500  and  520  may be referred to as forward flanges  500  and  520  like first and second forward flanges  280  and  282  defined by guide beam  268 . Thus, the two L-shaped flanges  496  and  516  may be said to define a guide beam  522  with first and second forward flanges  500  and  520  which will be engaged by forward and rear rollers  314  and  316  in the same manner as first and second forward flanges  280  and  282 . Thus, wellhead isolation tool  10  may include either support structure  200  or support structure  400 . The operation of the wellhead isolation tool  10  is as described herein with both embodiments of the support structures described. 
     While numerous changes to the apparatus and methods can be made by those skilled in the art, such changes are encompassed within the spirit of this invention as defined by the appended claims.