Abstract:
An apparatus for securing a mandrel of a well tool in an operative position in which the mandrel is packed off against a fixed-point in the well is described. The apparatus includes a mechanical lockdown mechanism to secure the tool to the wellhead and maintain the mandrel in proximity to the fixed-point for packoff, and a mechanical or a hydraulic mechanism to move the mandrel into the operative position while the mechanical lockdown mechanism is in a lockdown position. A second mechanical locking mechanism is provided to ensure the mandrel is maintained in the operative position in the event that hydraulic pressure is lost. The invention provides a mechanism to lock down well tools requiring fixed-point packoff in a well and advantageously improves the range of adjustment of the lockdown mechanism so that the length of a mandrel may be less precisely matched to a distance from a top of the wellhead to the fixed-point in the well.

Description:
TECHNICAL FIELD 
     The present invention relates to equipment for servicing oil and gas wells and, in particular, to an apparatus and method for securing a mandrel of a well tool in an operative position in which the mandrel is packed-off against a fixed-point in the well. 
     BACKGROUND OF THE INVENTION 
     Most oil and gas wells eventually require some form of stimulation to enhance hydrocarbon flow and make or keep them economically viable. The servicing of the oil and gas wells to stimulate production requires the pumping of fluids under high pressure. The fluids are generally corrosive and abrasive because they are frequently laden with corrosive acids and abrasive proppants such as sharp sand. Consequently, such fluids can cause irreparable damage to wellhead equipment if they are pumped directly through the spool and the various valves that make up the wellhead. To prevent such damage, wellhead isolation tools have been used and various configurations are well known in the art. 
     A general principle of wellhead isolation in the prior art is to insert a mandrel of the tools through the various valves and spools of the wellhead to isolate those components from the elevated pressures and the corrosive and abrasive fluids used in the well treatment to stimulate production. A top end of the mandrel is connected to one or more high pressure valves through which the stimulation fluids are pumped. A packoff assembly is usually provided at a bottom end of the mandrel for achieving a fluid seal against the inside of the production tubing or casing so that the wellhead is completely isolated from the stimulation fluids. The length of the mandrel need not be precise because the location of the packoff assembly in the production tubing or casing is immaterial so long as the mandrel is inserted into the production tubing or casing and a fluid tight seal is achieved between the production tubing or casing and the packoff assembly. 
     However, a packoff affixed to a bottom end of the mandrel which seals against the inside of the production tubing or casing, limits the internal diameter of the mandrel and, consequently, the flow rate at which stimulation fluids may be pumped into the well. To overcome this problem, applicant invented an improved mandrel for a wellhead isolation tool described in co-pending U.S. patent application Ser. No. 08/837,574 which was filed on Apr. 21, 1997 and entitled APPARATUS FOR INCREASING THE TRANSFER RATE OF PRODUCTION STIMULATION FLUIDS THROUGH THE WELLHEAD OF A HYDROCARBON WELL. The apparatus described in that patent application includes a mandrel for a wellhead isolation tool and a tubing hanger for use in conjunction with the mandrel. The mandrel includes an annular seal bonded to an outside wall above the bottom end of the mandrel. The annular seal cooperates with a sealing surface in the top end of the tubing hanger to isolate the wellhead equipment from high pressures and corrosive and abrasive materials pumped into the well during a well treatment to stimulate production. The novel construction of the mandrel and the tubing hanger eliminates the requirement for a packoff assembly attached to the bottom of the mandrel and thereby permits the mandrel to have a larger internal diameter for increasing the transfer rate of the production stimulation fluids through the wellhead. The axial length of the sealing surface in the tubing hanger available for packoff is limited and, therefore, the length of the mandrel is determined, to a large extent, by a distance from the top of the tubing hanger to the top of the wellhead. 
     Applicant describes another improved mandrel for a wellhead isolation tool in U.S. patent application Ser. No. 09/356,231 which was filed on Jul. 16, 1999 and entitled WELLHEAD ISOLATION TOOL AND METHOD OF USING SAME, which is incorporated herein by reference. The wellhead isolation tool includes a mandrel that is inserted into a wellhead. The mandrel is seated against an annular step above back pressure valve threads in a tubing hanger to isolate the pressure sensitive components of the wellhead from fluid pressure used in the well treatment and has a lower section extending past the back pressure valve threads and tubing threads into the tubing to protect the threads from washout. The annular step above the back pressure valve threads in the tubing hanger is a fixed-point for packoff of the mandrel and, therefore, a length of the mandrel is determined by the distance from the annular step to the top of the wellhead and a lockdown mechanism for securing the wellhead isolation tool to the wellhead preferably provides a range of adjustment to compensate for variations in the position of the top end of the mandrel when the mandrel is packed off in different wellheads. 
     Another example of a well tool in an operative position in which the mandrel of the well tool is packed-off against a fixed-point in the well is described in Applicant&#39;s U.S. Pat. No. 5,819,851 which issued on Oct. 13, 1998 and is entitled BLOWOUT PREVENTER PROTECTOR FOR USE DURING HIGH PRESSURE OIL/GAS WELL STIMULATION. The blowout preventer protector described in that patent includes a mandrel that is forcibly reciprocatable in an annular cavity of a spool. The mandrel is stroked down through a blowout preventer and packed off at the bottom end against a bit guide that is attached to a top end of the casing to protect the blowout preventer from exposure to fluid pressure as well as abrasive and corrosive well stimulation fluids. The bit guide attached to the top end of the casing provides a fixed-point for packoff of the mandrel. 
     It is apparent from the examples described above that, as a result of new tools being invented and new technology being developed, there is a need for a lockdown mechanism for securing a well tool requiring a fixed-point packoff in an operative position in the well. 
     The blowout preventer protector described in U.S. Pat. No. 5,819,851 includes a mandrel that is integrally incorporated with a hydraulic setting tool. The mandrel is not separable from the hydraulic setting tool and the setting tool is used to hydraulically lock the mandrel in an operative position. The mandrel. can be secured at any location within the annular cavity by maintaining the hydraulic pressure in the annular cavity after the mandrel is packed-off against the bit guide. The stroke of the hydraulic setting tool is used for inserting the mandrel through the blowout preventer, and also provides compensation for variations in a distance from the bit guide to the top of the blowout preventer when the mandrel is inserted through different wellheads. The blowout preventer protector is widely accepted in the industry and the hydraulic setting tool is very convenient for securing a mandrel of a well tool in the operative position requiring fixed-point packoff in the well. However, the setting tool must be fairly long to provide sufficient stroke. Furthermore, the setting tool is not removable from the mandrel during a well treatment to stimulate production. Consequently, the blowout preventer protector has a high profile. A well tool with a high profile is not convenient because access to equipment mounted thereto, such as a high pressure valve, is impeded by the height of the valve above ground. In addition, a hydraulic lockdown mechanism is considered less secure than a mechanical lockdown mechanism. The hydraulic lockdown mechanism is dependent on maintenance of the hydraulic fluid pressure in the setting tool. Since fluid pressure may be lost for a variety of reasons, persons in the industry are generally less inclined to endorse or accept a hydraulic lockdown mechanism. 
     A mechanical lockdown mechanism having a range of adjustment is used for the well tools described in Applicant&#39;s co-pending U.S. patent application filed on Jun. 23, 1999 and the application filed on Jul. 16, 1999 referenced above. The mechanical lockdown mechanism described in the above two patent applications is for securing a mandrel of well tools in an operative position requiring fixed-point packoff in the well, and provides a broad range of adjustment to compensate for variations in the height of different wellheads to which the well tool is mounted. The mechanical lockdown mechanism includes a base member that is adapted to be mounted to a top of the wellhead, the base member having a central passage to permit the insertion and the removal of the mandrel. The passage is surrounded by an integral sleeve having an elongated spiral thread for engaging a lockdown nut that is adapted to secure the mandrel in the operative position. The spiral thread on the integral sleeve and the lockdown nut have a length adequate to ensure safe operation at well stimulation fluid pressures. At least one of the spiral threads on the integral sleeve and the lockdown nut has a length adequate to provide a significant range of adjustment to compensate for variation in a distance between the top of the wellhead and the fixed-point for packoff in the well when the tool is mounted to different wellheads. The mechanical lockdown mechanism is separated from the hydraulic setting tool and, therefore, permits the setting tool to be removed from the well tool after the mandrel is locked down in the operative position. The tools therefore provide a low profile to facilitate well stimulation operations. The advantages also include the security of a mechanical lockdown mechanism. Therefore, there exists a need for a lockdown mechanism for securing a mandrel of a well tool in an operative position requiring fixed-point packoff in the well which provides a broader range of adjustment while ensuring a secure mechanical lockdown for maximum security. 
     SUMMARY OF THE INVENTION 
     It is a primary object of the invention to provide a lockdown mechanism for securing a mandrel of a well tool in an operative position in which the mandrel is packed-off against a fixed-point in the well. 
     It is another object of the invention to provide a lockdown mechanism for securing a mandrel of the well tool in an operative position requiring fixed-point packoff in the well, the lockdown mechanism having a low profile for easy access to a high pressure valve during use while the tool is in the operative position. 
     It is a further object of the invention to provide a lockdown mechanism for securing a mandrel of the well tool in an operative position requiring fixed-point packoff in the well which is convenient to use. 
     It is yet a further object of the invention to provide a lockdown mechanism for securing a mandrel of a well tool in an operative position requiring fixed-point packoff in the well which combines a hydraulic lockdown mechanism with a mechanical lockdown mechanism. 
     In accordance with one aspect of the invention, there is provided an apparatus for securing a mandrel of a well tool in an operative position requiring fixed-point packoff in the well, comprising a first and a second lockdown mechanism arranged so that the mandrel is locked in the operative position only when both the first and the second lockdown mechanisms are in respective lockdown positions; the first lockdown mechanism adapted to detachably maintain the mandrel in proximity to the fixed-point packoff when in the lockdown position, the first lockdown mechanism including a base member for connection to a wellhead of the well and a locking member for detachably engaging the base member; and the second lockdown mechanism having a range of adjustment adequate to ensure that the mandrel can be moved into the operative position and locked down in the operative position while the first lockdown mechanism is in the lockdown position. 
     The second lockdown mechanism preferably comprises a first member connected to the mandrel and a second member connected to the locking member of the first lockdown mechanism, the first and second members being linked to permit movement with respect to each other within the range of adjustment. 
     In accordance with one embodiment of the invention, the second member of the second lockdown mechanism includes at least one threaded bolt connected at a fixed end to the locking member of the first lockdown mechanism and the first member of the second lockdown mechanism has at least one bore to permit the at least one threaded bolt to pass therethrough without resistance, the at least one threaded bolt being prevented from being withdrawn from the bore by a lock nut which is adapted to be rotated from a free end of the threaded bolt towards the fixed end to lock the second lockdown mechanism in the lockdown position. 
     In accordance with another embodiment of the invention, the first member of the second lockdown mechanism includes a piston fixed to the mandrel and the second member of the second lockdown mechanism includes a cylinder connected with the locking member of the first lockdown mechanism, the piston being adapted to be reciprocated within the cylinder using fluid pressure. 
     In accordance with another aspect of the invention, there is provided an apparatus used for securing a mandrel of a well tool in an operative position in which the mandrel is packed off against a fixed-point in the well, comprising a mechanical lockdown mechanism for detachably securing the well tool to a wellhead of the well and maintaining the mandrel in proximity to the fixed-point for packoff, the mechanical lockdown mechanism including a base member for connection of the wellhead and a locking member for detachably engaging the base member; a hydraulic mechanism including a cylinder and a piston which may be reciprocated within the cylinder using fluid pressure, the cylinder being connected to the locking member of the mechanical lockdown mechanism and the piston being fixed to the mandrel of the tool so that the mandrel may be moved to and maintained in the operative position by injecting fluid pressure into the cylinder while the mechanical lockdown mechanism is in a lockdown position. The hydraulic mechanism preferably comprises a mechanical locking mechanism to ensure the mandrel is maintained in the operative position in the event that the fluid pressure is lost. 
     The invention provides a lockdown mechanism with a greater range of adjustment for securing a mandrel of a well tool in an operative position requiring fixed-point packoff in the well, in comparison with prior art lockdown mechanisms. Consequently, the length of a mandrel may be less precisely matched to a distance from the fixed-point for packoff to the top of the wellhead. Other features and advantages will become apparent given the preferred embodiments which are described below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be further explained by way of example only and with reference to the following drawings, in which: 
     FIGS. 1 to  4  illustrate cross-sectional views of an apparatus in various working positions in accordance with a preferred embodiment of the invention; 
     FIGS. 5 to  7  illustrate cross-sectional views of an apparatus in various working positions in accordance with another preferred embodiment of the invention; 
     FIG. 8 is a schematic diagram of the apparatus shown in FIG. 5 mounted to a blowout preventer through which a mandrel is to be stroked and secured in an operative position in which the mandrel is packed off against a bit guide mounted to a top of a casing of the well; and 
     FIG. 9 is a schematic diagram of the apparatus shown in FIG. 1 mounted to a wellhead through which a mandrel is to be stroked and secured in an operative position in which the mandrel is packed off against an annular step in a tubing hanger of the wellhead. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows a cross-sectional view of a first lockdown mechanism  20  for securing a mandrel  22  of a well tool in an operative position in which the mandrel  22  is packed off against a fixed-point  24  in the well. The fixed-point for packoff may be a bit guide mounted to the top of a casing, as shown in FIG. 8, an annular step above back pressure valve threads of a tubing hanger, as shown in FIG. 9, or any other type of fixed-point location used for packoff in a wellhead, a casing, a tubing or a downhole tool. For the purpose of convenient description, the mandrel is assumed to be packed off against a fixed packoff point at the bottom of FIGS. 1 through 7. 
     The apparatus  20  includes a mandrel head  26  connected to a top end of the mandrel  22  and a base plate  28  mounted to a top of the wellhead, which is indicated by line  30 . The mandrel head  26  is separable from the base plate  28  to permit the mandrel  22 , which is connected to the mandrel head  26 , to be inserted through the base plate  28  into the wellhead until the mandrel  22  reaches a fixed-point  24  for packoff. In different wellheads, a distance “D” from the fixed-point  24  for packoff to the top of the wellhead may vary. Although a length “L” of the mandrel  22  may be adjusted by the insertion of extension sections, as described in Applicant&#39;s co-pending patent applications, it is not practical to provide a large number of extension sections, each having a different length to permit mandrels to be assembled to precisely match the distance “D” of each wellhead. A distance “d” from a top of the wellhead  30  to a top end of the mandrel  22  is a constant when the mandrel  22  is locked down to the base plate  28  by a first locking member  38 , as shown in FIG.  1 . Consequently, a range of adjustment “B” is provided by a second locking member such that “B” is greater than a distance “C” between a bottom end of the mandrel  22  and the fixed packoff point  24  when the mandrel  22  is locked down to the base plate  28 , as shown in FIG.  1 . 
     The base plate  28  is preferably a circular disc which includes an integral concentric sleeve  32  perpendicular to the base plate  28 . A spiral thread  34  on the exterior of the integral sleeve  32  mates with a complementary spiral thread  36  on an interior surface of a lockdown nut  38 . The base plate  28  and the integral sleeve  32  include a central passage  40  to permit the mandrel  22  to pass through. The lockdown nut  38  includes a top wall  42  for rotatably retaining a connector  44 . The connector  44  is a cylindrical body with an upper flange  46  and a lower flange  48  which engages the top wall  42  of the lockdown nut  38 . A central passage  50  through the connector  44  permits the mandrel  22  to fully pass through. The mandrel head  26  is a cylindrical body with an upper flange  52  for connection of equipment, such as a high pressure valve, and a lower flange  54  which is adjustably linked to the connector  44 . The adjustable link between the connector  44  and the mandrel head  26  is provided by at least two threaded bolts  56  which extends through at least two respective bores  58  in the lower flange  54 . The threaded bolts  56  are connected at their fixed ends to the upper flange  46  of the connector  44 . Nuts  60  at a free end of each bolt  56  prevents the bolt from being withdrawn from the flange  54 . The bore  58  has an internal diameter slightly larger than an external diameter of the bolt  56  to permit the bolt  56  to pass therethrough without resistance. The threaded bolt  56  has an adequate length to permit the range “B” of movement of the mandrel head  56  relative to the connector  44 . 
     When the lockdown nut  38  is locked to the integral sleeve  32  by the engagement of threads  34 ,  36  and the mandrel head  26  is moved towards or away from the connector  44  within the range “B”, the mandrel can be packed off against a fixed-point for packoff. Therefore, when the mandrel  22  may be used with wellheads having different configurations and the distance D from the fixed-point  24  for packoff to the top of the wellhead indicated by line  30  varies by a distance “C” that is not greater than the range of adjustment “B”, that is, 0≦C≦B, the apparatus  20  is adapted to be locked down in the operative position in which a bottom end of the mandrel  22  is packed off against the fixed-point  24  for packoff. 
     As will be understood by those skilled in the art, in order to safely restrain fluid pressure during a well treatment to stimulate production, the number of the threaded bolts  56 , nuts  60  and bores  58  is generally more than two. The bolts  56  are circumferentially spaced from each other, the number of each being dictated by the fluid pressures to be restrained and the quality of materials used. The periphery of the lower flange  54  of the mandrel head  26  extends beyond the flange  52  of the mandrel head  26  so that the upper flange  52  does not interfere with the threaded bolts  56  as the mandrel head  26  is moved towards the connector  44 . The mandrel head  26  has a central passage  62  in fluid communication with the mandrel  22 . The passage  62  has a diameter not smaller than the internal diameter of the mandrel  22  for a full access to the mandrel. A spiral thread is provided at the lower end of the central passage  62  for connection of the threaded top end of the mandrel  22 . A sealing mechanism (not shown) is provided in the threaded connection between the top end of the mandrel  22  and the mandrel head  26  to prevent well fluids from escaping to atmosphere. The central passage  40  through the base plate  28  has a recessed lower region for receiving a steel spacer  64  and packing rings  66  preferably constructed of brass, rubber and fabric. The steel spacer  64  and packing rings  66  define a passage of the same diameter as the periphery of the mandrel  22 . The steel spacer  64  and the packing rings  66  are removable and may be interchanged to accommodate different sizes of mandrel  22 . The steel spacer  64  and the packing rings  66  are retained in the recessed region by a retainer nut  68 . The combination of the steel spacer  64 , packing rings  66  and the retainer nut  68  provide a fluid seal to prevent the passage to atmosphere of well fluids between the exterior of the mandrel  22  and the interior of the wellhead when the mandrel  22  is inserted into the wellhead. 
     FIG. 2 shows a cross-sectional view of the apparatus  20  in a working position in which the nuts  60  are at the free end of the threaded bolts  56  and the lockdown nut  38  is disengaged from the base plate  28 . In this condition, the base plate  28  can be mounted on the top of the wellhead while the other parts of the apparatus  20  are connected to the top end of mandrel  22  and are moved with the mandrel  22  when the mandrel  22  is inserted into the wellhead by a setting tool, which will be described in more detail with reference to FIGS. 8 and 9. When the apparatus  20 , except for the base plate  28 , is moved downwardly as the mandrel  22  is inserted through the wellhead, the upper flange  46  of the connector  44  is spaced from the lower flange  54  of the mandrel head  26 , as shown in FIG.  2 . For safe engagement to restrain the high fluid pressures during a well treatment to stimulate production, threads  34 - 36  are engaged a distance “A” by rotating the lockdown nut  38 . At this stage, the bottom end of the mandrel  22  is still above the fixed-point  24  for packoff by the distance “C”, as shown in FIG.  1 . After the lockdown nut  38  is fully engaged as shown in FIG. 3, the mandrel  22  is further stroked down until the bottom end of the mandrel  22  packs off against the fixed-point  24 . The nuts  60  are then rotated down against the lower flange  54  of the mandrel head  26  to prevent a fluid seal on the lower end of mandrel  22  (not shown) from being forced away from the fixed-point  24  for packoff after the setting tool is removed from the wellhead and pressurized fluids are injected into the well. 
     Alternatively, the mandrel  22  with connected mandrel head  26  may be stroked downwardly without engaging the lockdown nut  38  with the base plate  28  as shown in FIG. 4 until the bottom end of the mandrel  22  is packed off in an operative position against the fixed-point  24  for packoff. The lockdown nut  38  is then rotated to engage the threads  34  on the integral sleeve  32 . The final locked position is the same as shown in FIG.  3 . Therefore, the nuts  60  are turned down against the lower flange  54  of the mandrel head  26  to lock the apparatus  20  in the operative position. 
     FIG. 5 is a cross-sectional view of an apparatus  70  in accordance with another preferred embodiment of the invention. The apparatus  70  includes a mandrel head  26  threadedly connected to a top end of the mandrel  72  and a base plate  28  adapted to be mounted to the top of the wellhead, indicated by line  30 . The mandrel head  26 , base plate  28  and other parts indicated by reference numerals corresponding to those shown in FIG. 1 are respectively identical to the corresponding parts of the apparatus  70 . The principal difference is that the apparatus  70  includes an integral hydraulic cylinder  74  in place of the connector  44  of the apparatus  20 . The hydraulic cylinder  74  includes upper and lower walls  76 ,  78  which respectively surround the mandrel  72 . The cylinder  74  further includes a sidewall  80  which defines an annular cavity  82 . A piston  84  is fixed to the mandrel  72 . O-ring seals  86  are provided respectively between the piston  84  and sidewall  80 , upper wall  76  and the lower wall  78  and the exterior surface of a mandrel  72  to permit introduction of pressurized hydraulic fluid into the annular cavity  82  to induce movement of the piston  84 . The hydraulic fluid is injected, as required, through an upper port  88  and drained through a lower port  90 , and vice versa. The piston preferably has a stroke about equal to the distance “B”, to match the functional length of the threaded bolts  56 . The threaded bolts  56  are connected at their fixed ends to the upper wall  76 . The cylinder  74  further includes a connecting flange  92  connected to but spaced from the lower wall  78  for rotatable engagement with the top wall  42  of the lockdown nut  38 . 
     FIG. 6 is a cross-sectional view of the apparatus  70  shown in FIG. 5 with the piston  84  at the top of cylinder  74 , and the lockdown nut  38  disengaged from the integral sleeve  32  of the base plate  28 . As described above, the base plate  28  is mounted to the top of the wellhead before the mandrel  72  is inserted into the wellhead. The mandrel  72  is stroked down under a force P 1  exerted by a setting tool, as will be described below with reference to FIGS. 8 and 9. The piston  84  is maintained at a top of the hydraulic cylinder by a force P 2  exerted by pressurized hydraulic fluid trapped in the cylinder. The lockdown nut  38  is turned down to its locked position as shown in FIG.  5 . The bottom end of the mandrel  70  is then a distance “C” above the fixed-point  24  for packoff. 
     After the lockdown nut  38  is fully engaged with the base plate  28 , the setting tool is removed from the wellhead and the well tool is left unobstructed for access. Pressurized hydraulic fluid is injected into the upper port  88  of the cylinder  74  while the hydraulic fluid below the piston  84  is drained from the lower port  90  so that the mandrel  72  is forced downwardly to packoff against the fixed-point  24  under a force P 2  exerted on the piston  84  by the pressurized hydraulic fluid, as shown in FIG.  7 . The mandrel head  26  is thus forced downwardly over the distance “C” so that the space between the mandrel head  26  and the upper wall  76  of the cylinder  74  is reduced to B-C. The mandrel  72  is locked down in its operative position by the hydraulic force P 2 . To ensure that the mandrel is secured in the operative position, the nuts  60  are turned down against the lower flange  54  of the mandrel head  26 . 
     FIG. 8 shows an example of the use of the apparatus  70  shown in FIG. 5, using a hydraulic setting tool  93  to insert the mandrel  72  to an operative position for a well treatment to stimulate production. In this example, the mandrel  72  is used to protect a blowout preventer  100  and includes a packoff assembly  94  that is packed-off against a top of a bit guide  96  mounted to a top of a casing  98 , as described in Applicant&#39;s co-pending patent application filed Jun. 23, 1999. The hydraulic setting tool  93  illustrated in FIG. 8 is also described in Applicant&#39;s U.S. Pat. No. 4,867,243 which issued on Sep. 19, 1989 and is entitled WELLHEAD ISOLATION TOOL AND SETTING TOOL AND METHOD OF USING SAME, which is incorporated herein by reference. The blowout preventer  100  is connected to the well casing  98  by various spools, such as a tubing head spool  102 , for example. The blowout preventer  100  and the tubing head spool  102  are wellhead equipment that is well known in the art and their construction and function do not form a part of this invention. The blowout preventer  100  and the tubing head spool  102  are, therefore, not described. The apparatus  70  is supported on a top of the blowout preventer by mounting the base plate  28  in a fluid tight relationship to the top flange of the blowout preventer  100 . Mounted above the apparatus  70 , is a high pressure valve  104  which is used for fluid flow control during a well treatment to stimulate production and, also used to prevent well fluids from escaping to the atmosphere from the top of the mandrel  72 . The high pressure valve  104  is typically a hydraulic valve well known in the art. The hydraulic setting tool  93  includes a hydraulic cylinder  106  which is mounted to a support plate  108 . The support plate  108  includes a central passage (not shown) to permit a piston rod  114  of the hydraulic cylinder  106  to pass through the support plate  108 . The support  108  also includes at least two attachment points  110  for attachment of respective hydraulic cylinder support rods  112 . The spaced apart attachment points  110  are preferably equally spaced from the central passage to ensure that the hydraulic cylinder  106  and the piston rod  114  align with the blowout preventer  100 . The hydraulic cylinder support rods  112  are respectively attached at their lower ends to corresponding attachment points  116  on the base plate  28 . As is apparent, the base plate  28  and the support plate  108  have a periphery that extends beyond the wellhead to provide enough radial offset of the cylinder support rods  112  to accommodate the high pressure valve  104 , the mandrel head  26  and the cylinder  74 . The support rods  112  are identical in length. The support rods  112  are attached to the respective spaced apart attachment points  110 ,  116  on the support plates  108  and the base plate  28  by means of threaded fasteners or pins (not illustrated). The piston rod  114  is attached to the top of the high pressure valve  104  by a connector  118  so that a force can be applied to stroke the mandrel  72  down through the wellhead. 
     After the mandrel  72  is stroked downwardly to an extent that the packoff assembly  94  is in proximity to the bit guide  96 , and the lockdown nut  38  is turned down to its locked position, as illustrated in FIG. 8, the setting tool  93  including the hydraulic cylinder  106 , support plate  108 , cylinder support rods  112  and the connector  118  are removed. The packoff assembly  94  on the bottom of the mandrel  72  is then stroked further down until it is packed off against the bit guide  96  by injecting pressurized fluid into the top port  88  of the hydraulic cylinder  74 , as illustrated in FIG.  7 . 
     FIG. 9 shows an example of the use of the apparatus  20 , shown in FIG. 1, using the hydraulic setting tool  93  to insert the mandrel  22  to an operative position for a well treatment to stimulate production. In this example, the wellhead is constructed in a well known manner from a series of valves and related flanges. The wellhead schematically illustrated in FIG. 9 includes a tubing spool  120  which receives and supports a tubing hanger  122 . Connected by flange connections to the top of the tubing spool  120 , are valves  124  and  126 . The purpose of the two valves  124  and  126  is to control the flow of hydrocarbons from the well. The apparatus  20  is mounted above the wellhead, that is, atop the valve  126 . The mandrel  22  is inserted through the wellhead into the operative position in which an elastomeric seal (not shown) on a sealing shoulder  128  is seated against an annular step  130  located above back pressure valve threads  132  of the tubing hanger  122  while a lower section of the mandrel  22  enters the top of the tubing  134  to protect the back pressure valve threads  132  and tubing threads  136 , as described in Applicant&#39;s co-pending patent application filed Jul. 16, 1999. The annular step  130  of the tubing hanger  122  is the fixed-point  24  for packoff in the well. The distance from the annular step  130  to the top of the valve  126  may vary in different wellheads and, therefore, the apparatus  20  is used to provide a broad range of adjustment to compensate for variations to ensure that the mandrel  22  can be locked down in the operative position. After the setting tool  93  is mounted to the base plate  28  in the same way as described with reference to FIG. 8, the steps described with reference to FIGS. 1 to  4  are followed to lock the mandrel  22  in the operative position in which the elastomeric seal on the sealing shoulder  128  of the mandrel  22  is packed-off against the annular step  130 . 
     The two examples described with reference to FIGS. 8 and 9 are for the purpose of illustration of the invention only and do not limit the applications of the invention. For example, the two embodiments described above may be used interchangeably. Likewise, other setting tool known in the art may be used in conjunction with the apparatus  20  or  70  for inserting the mandrel through the wellhead into proximity of the operative position. For example, a setting tool described by McLeod in U.S. Pat. No. 4,632,183 and entitled INSERTION DRIVE SYSTEM FOR TREE SAVERS which issued on Dec. 5, 1984, the entire specification which is incorporated herein by reference, may be used. Another type of setting tool which may also be used to insert the mandrel in proximity to the operative position is described by Bullen in U.S. Pat. No. 4,241,786, entitled WELLTREE SAVER which issued on May 2, 1979 and is also incorporated herein by reference. 
     It should also be understood that the apparatus described above can be used to lock down other types of tools which must be packed-off against a fixed-point in a well and is not limited to use with the mandrels described above. 
     Modifications and improvements to the above-described embodiments of the invention may become apparent to those skilled in the art. The foregoing description is intended to be exemplary rather than limiting. The scope of the invention is therefore intended to be limited solely by the scope of the appended claims.