Abstract:
A blowout preventer (BOP) protector featuring a low profile and a mechanical lockdown mechanism is described. The BOP protector includes a mandrel having an annular sealing body bonded to its bottom end for sealing engagement with a bit guide which protects a top of a casing of a well to be stimulated with acidifying or fracturing fluids. The mandrel is locked down with a mechanical lockdown mechanism having a broad range of adjustment. The advantages include a low profile, the security of a mechanical lockdown mechanism, and full access to the casing with downhole tools.

Description:
TECHNICAL FIELD 
     The present invention relates to equipment for servicing oil and gas wells and, in particular, to an apparatus and method for protecting blowout preventers from exposure to high pressures and abrasive or corrosive fluids during well fracturing and stimulation procedures. 
     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 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. 
     In some wells, stimulation to improve production can be accomplished at moderate pressure which may be safely contained by blowout preventers (BOPs) and, therefore, stimulation fluids may be pumped directly through a valve attached to the BOPs. This procedure is adopted to minimize expense and to permit full access to the well casing with downhole tools during the well servicing operation. It has been demonstrated that it is advantageous to have full access, or substantially full access, to a well casing during a well stimulation treatment. Full access to the casing permits use of downhole tools which are often required, or at least advantageously used during a stimulation treatment. 
     An apparatus for providing full access to the casing while permitting stimulation treatments at extreme pressures that approach a burst pressure rating of the casing 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 patent describes an apparatus for protecting BOPs during well treatments to stimulate production. The apparatus includes a hollow spool that has spaced apart inner and outer side walls that define an annular cavity. A mandrel is forcibly reciprocatable in the cavity. The mandrel includes an annular seal at the bottom end for sealingly engaging a bit guide attached to the top end of the casing. The apparatus is mounted above a BOP attached to a casing spool of the well before well stimulation procedures have begun. The mandrel is stroked down through the BOP to protect it from exposure to fluid pressure as well as to abrasive and corrosive well stimulation fluids, especially extreme pressure and abrasive proppants. The BOP protector provides a simple, easy to operate apparatus for protecting BOPs which provides full access to the well casing with well servicing tools to facilitate well stimulation at pressures approaching the burst pressure rating of the well casing. 
     The BOP protector has been readily accepted by the industry and has been proven to be an effective tool which reduces the cost of well stimulation treatments while enabling an ultimate choice of treatment options. However, further improvements are still desirable because the BOP protector described in U.S. Pat. No. 5,819,851 is a hydraulic unit which is mounted above the BOPs during an entire stimulation process. This raises the high pressure valve which controls the flow of stimulation fluids well above a top of the BOPs, which complicates access and reduces the run-in room for perforating gun strings, and other lengthy tools. Consequently, a low profile BOP protector would be advantageous to lower the position of the high pressure valve for easy access during stimulation processes. In addition, a mechanical lockdown mechanism for securing the BOP protector mandrel in an operative position is considered more reliable because a source of pressurized hydraulic fluid is not required. 
     An apparatus and method of isolating a well tree located on an oil or gas well from the effects of high pressure or corrosion caused by stimulation of a well is 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. The patent describes an apparatus to permit the injection of fluids, gases, solid particles or mixtures thereof through a well tree while protecting the well tree during well stimulation treatments. The apparatus includes a single hydraulic cylinder supported in an axial alignment over a well tree by at least two elongated support rods. The hydraulic cylinder support rods are connected between a base plate and a hydraulic cylinder support plate for supporting the hydraulic cylinder above the well tree at a distance approximately equal to the height of the production tree. The apparatus permits the insertion of a single length of high pressure tubing through any well tree regardless of its height. Once the high pressure tubing is seated in a well tubing or casing, the hydraulic cylinder, hydraulic cylinder plate and support rods are removed to provide 360° access to a high pressure valve attached to the top of the high pressure tubing. The bottom end of the high pressure tubing has a packoff nipple assembly which is inserted into the production tubing or casing and seals against the inner wall. The extent to which the high pressure tubing extends into the production tubing or casing is unimportant so long as the packoff nipple assembly is sealed against the inner wall. Consequently, variations in the length of the production tree are of no consequence and a lockdown mechanism with a short reach is adequate. Consequently, there exists a need for a mechanical lockdown mechanism that provides a broad range of adjustment to permit packoff with a fixed packoff surface in a wellhead. 
     SUMMARY OF THE INVENTION 
     It is a primary object of the invention to provide a BOP protector which isolates BOPs from well stimulation pressures and fluids while overcoming the shortcomings of the prior art. 
     It is another object of the invention to provide a BOP protector which has a low profile for easy access to a high pressure valve during a stimulation treatment. 
     It is a further object of the invention to provide a BOP protector which is locked down in its operative position by a mechanical lockdown mechanism. 
     It is yet a further object of the invention to provide a BOP protector which has a mandrel that can be separated from a tool used for setting the mandrel. 
     It is still a further object of the invention to provide a BOP protector which is economical to manufacture and maintain. 
     In accordance with one aspect of the invention there is provided an apparatus for protecting a blowout preventer from exposure to fluid pressures, abrasives and corrosive fluids used in a well treatment to stimulate production. The apparatus comprises a mandrel adapted to be inserted down through the blowout preventer to an operative position. The mandrel has a mandrel top end and a mandrel bottom end, the mandrel bottom end including an annular sealing body for sealing engagement with a top of a casing of the well when the mandrel is in the operate position. A mechanical lockdown mechanism detachably secures the mandrel to the blowout preventer, the lockdown mechanism being adapted to ensure that the annular sealing body is securely seated against the top of the casing when the mandrel is in the operative position. 
     The mechanical lockdown mechanism preferably includes a base member that is adapted to be mounted to a top of the blowout preventer, the base member having a central passage to permit the insertion and 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 (10,000-15,000 psi). At least one of the spiral thread on the integral sleeve and the lockdown nut has a length adequate to provide a significant range of adjustment, preferably at least about 5″ (12.5 cm), to compensate for variations in a distance between a top of the BOP and a bit guide in the tubing hanger spool where the mandrel packs off. The mandrel may be inserted through the blowout preventer using any type of insertion tool used for the insertion of well tree savers or casing savers. Once inserted, the mandrel is securely locked in its operative position by the mechanical lockdown mechanism. 
     In more specific terms, the invention provides an apparatus for protecting a blowout preventer from exposure to fluid pressures, abrasive and corrosive fluids during a well treatment to stimulate production. The apparatus comprises a mandrel adapted to be inserted down through the blowout preventer, the mandrel having a mandrel top end adapted to protrude above the blowout preventer and a mandrel bottom end that includes an annular sealing body for sealing engagement with a bit guide at a top of a casing of the well when the mandrel is in an operative position. A hydraulic cylinder is conveniently used for inserting the mandrel into and removing the mandrel from the blowout preventer. The hydraulic cylinder is supported by at least two elongated hydraulic cylinder support rods fixed relative to the blowout preventer for supporting the hydraulic cylinder in vertical and axial alignment with the blowout preventer, the support rods and the cylinder being removable when the mandrel is operatively inserted through the blowout preventer and the annular sealing body of the mandrel bottom end is seated against the bit guide. A mechanical lockdown mechanism detachably secures the mandrel to the blowout preventer when the mandrel is in the operative position. 
     A primary advantage of the invention is the low profile which provides easy access to a high pressure valve mounted to the top end of the mandrel to control fluid flow during a well stimulation treatment. A further advantage is the security provided by a mechanical lockdown mechanism, which eliminates concern respecting hydraulic fluid pressure losses in the hydraulic system used to lock down Applicant&#39;s prior art BOP protector. Furthermore, the separable insertion tool reduces manufacturing and maintenance costs of the apparatus because a single setting tool can be used to set a plurality of mandrels and a damaged or washed-out mandrel is easily replaced without dismantling the tool. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a longitudinal cross-sectional view of a blowout preventer (BOP) protector in accordance with the invention, showing the mandrel in an exploded view; 
     FIG. 2 is a longitudinal cross-sectional view of the BOP protector shown in FIG. 1, illustrating the lockdown nut disengaged from the base plate. 
     FIG. 3 is a front elevational view, partially in cross-section, of the BOP protector in accordance with the invention mounted to a wellhead with the mandrel inserted through the BOP and seated in its operative position; 
     FIG. 4 is an alternate embodiment of the lockdown mechanism used in the BOP protector shown in FIG. 1; 
     FIG. 5 is another alternate embodiment of the lockdown mechanism used in the BOP protector shown in FIG. 1; 
     FIG. 6 is a partial cross-sectional view of a first embodiment of an annular sealing body for sealing against a bit guide mounted to a top of a casing of the well; and 
     FIG. 7 is a partial cross-sectional view of an alternate preferred embodiment of an annular sealing body for sealing against a bit guide mounted to the top of a casing of the well. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows a cross-sectional view of the apparatus for protecting the BOPs (hereinafter referred to as a BOP protector) in accordance with the invention, generally indicated by reference numeral  10 . The apparatus includes a lockdown mechanism  12  which includes a base plate  14 , a mandrel head  16  and a lockdown nut  18  which detachably interconnects the base plate  14  and the mandrel head  16 . The base plate  14  is preferably a circular disc that includes an integral sleeve  20  which is perpendicular to the base plate  14 . A spiral thread  22  is provided on an exterior of the integral sleeve  20 . The spiral thread  22  is engageable with a complementary spiral thread  24  on an interior surface of the lockdown nut  18 . The base plate  14  and the integral sleeve  20  provides a passage  26  to permit a mandrel  28  to pass therethrough. The mandrel head  16  is an annular flange, having a central passage  30  defined by an interior wall  32 . A top flange  34  is adapted for connection of equipment, such as a high pressure valve, which will be described below in more detail. A lower flange  36  retains a top flange  38  of the lockdown nut  18 . The lockdown nut  18  secures the mandrel head  16  from movement with respect to the base plate  14  when the lockdown nut  18  engages the spiral thread  22  of the integral sleeve  20 . The mandrel head  16  with its upper and lower flanges  34 ,  36  and the lockdown nut  18  with its top flange  38  are illustrated in FIG. 1 as an integral unit assembled, for example, by welding or the like. However, persons skilled in the art will understand that either one of the mandrel head  16  and the lockdown nut  18  be constructed to permit disassembly to enable the mandrel head  16  or the lockdown nut  18  to be independently replaced. 
     The mandrel  28  has a mandrel top end  40  and a mandrel bottom end  42 . Complementary spiral threads  43  are provided on the exterior of the mandrel top end  40  and on a lower end of the interior wall  32  of the mandrel head  16 , so that the mandrel top end  40  may be securely attached to the mandrel head  16 . One or more O-rings (not shown) provide a fluid tight seal between the mandrel head  34  and the mandrel  28 . The passage  26  through the base plate  14  has a recessed region on the lower end for receiving a steel spacer  44  and packing rings  46  preferably constructed of brass, rubber and fabric. The steel spacer  44  and packing rings  46  define a passage of the same diameter as the periphery of the mandrel  28 . The steel spacer  44  and packing rings  46  are removable and may be interchanged to accommodate different sizes of mandrel  28 . The steel spacer  44  and packing rings  46  are retained in the passage  26  by a retainer nut  48 . The combination of the steel spacer  44 , packing rings  46  and the retainer nut  48  provide a fluid seal to prevent passage to atmosphere of well fluids between the exterior of the mandrel  28  and the interior of the BOP when the mandrel  28  is inserted into the BOP, which will be described with reference to FIG.  3 . 
     An internal threaded connector  50  on the mandrel bottom end  42  is adapted for the connection of mandrel extension sections of the same diameter. The extension sections permit the mandrel  28  to be lengthened as required by different wellhead configurations. An optional mandrel extension  52  has a threaded connector  54  at a top end  56  adapted to be threadedly connected to the mandrel bottom end  42 . An extension bottom end  58  includes a threaded connector  60  that is used to connect a mandrel packoff assembly  62 , which will be described below in more detail. High pressure O-ring seals  64 , well known in the art, provide a high pressure fluid seal in the threaded connectors between the mandrel  28 , the optional mandrel extension(s)  52  and the mandrel packoff assembly  62 . The mandrel  28 , the mandrel extension  52  and the mandrel packoff assembly  62  are preferably each made from 4140 steel, a high-strength steel which is commercially available. 4140 steel has a high tensile strength and a Burnell hardness of about 300. Consequently, the assembled mandrel  28  is adequately robust to contain extreme fluid pressures of up to 15,000 psi, which approaches the burst pressure of the well casing. In order to support a packoff gasket  66 , however, the walls of the mandrel packoff assembly  62  are preferably about 1.75 inches (4.45 cm) thick. As will be explained with reference to FIG. 3, it is preferable that the wall thickness of the mandrel packoff assembly  62  be such that it fits closely within the tubing head spool of a well being treated. 
     The mandrel packoff assembly  62  includes a packoff upper end  68  and a packoff lower end  70 . The packoff upper end  68  includes a threaded connector  72  which engages the threaded connector  50  on the lower end of the mandrel  28  or the threaded connector  60  on the extension bottom end  58  of the optional mandrel extension  52 . The packoff lower end  70  includes the annular seal  66  which sealingly engages a top of the well casing as will be described below with reference to FIG.  3 . The annular seal  66  is preferably a thermal plastic or a synthetic rubber seal that is bonded directly to the lower end  70  of the mandrel packoff assembly  62 . The packoff lower end  70  is preferably machined to provide a bearing surface to which the annular seal  66  may be bonded. The annular seal  66  is preferably made from a polyurethane or a nitryl rubber. The annular seal  66  should have a hardness of about 80 to about 100 durometer. The internal diameter of the mandrel packoff assembly  62  is at least as large as the internal diameter of the casing, e.g., 5 inches (12.7 cm). The extension and the packoff assembly are more completely described in U.S. Pat. No. 5,819,851, which is incorporated herein by reference. 
     FIG. 2 illustrates the apparatus  10 , shown in FIG. 1, prior to being mounted above a BOP for a well stimulation treatment. The lockdown nut  18  is disengaged from the integral sleeve  20  of the base plate  14  and the mandrel head  16  is connected to the top end  40  of the mandrel  28  which includes any required extension section(s)  52  and the packoff assembly  62  to provide a total length required for a particular wellhead. The base plate  14  is mounted on the top end of the BOP and the combination of the lockdown nut  18 , the mandrel head  16  and the mandrel  28 , inserted from the top into the BOP using any one of several insertion tools known in the industry. 
     FIG. 3 illustrates an example of the use of the BOP protector  10 , shown in FIG. 1, using a hydraulic setting tool to insert the BOP protector  10  to an operative position for a well treatment to stimulate production. The hydraulic setting tool illustrated in FIG. 3 is described in U.S. Pat. No. 4,867,243, which is incorporated herein by reference. A BOP  74  is connected to a well casing  76  by various casing hangers, well known in the art such as a tubing head spool  78 , for example. The BOP  74  is a piece of wellhead equipment that is also well known in the art and its construction and function do not form a part of this invention. The BOP  74  and the tubing head spool  78  are, therefore, not described. Mounted above the BOP protector  10 , is a high pressure valve  80  which is used for fluid flow control during a well treatment to stimulate production and, also, used to prevent well fluids from escaping to atmosphere from the top of the mandrel  28  during the insertion and removal of the mandrel  28 . The high pressure valve  80  is typically a hydraulic valve well known in the art. The hydraulic setting tool includes a hydraulic cylinder  82  which is mounted to support plate  84 . The support plate  84  includes a passage (not shown) to permit a piston rod  85  of the hydraulic cylinder  82  to pass through the support plate  84 . The support plate  84  also includes at least two attachment points  86  for attachment of respective hydraulic cylinder support rods  88 . The spaced apart attachment points  86  are preferably equally spaced from the central bore to ensure that the hydraulic cylinder  82  and the piston rod  86  align with the BOP  74  to which the hydraulic cylinder  82  is mounted. The hydraulic cylinder support rods  88  are respectively attached on their lower ends to corresponding attachment points  90  on the base plate  14 , which is mounted to the top of the BOP  74 . As is apparent, the base plate  14  and the support plate  84  have a periphery that extends beyond the wellhead to provide enough radial offset of the cylinder support rods  88  to accommodate the high pressure valve  80 . The cylinder support rods  88  are identical in length. The support rods  88  are attached to the respective spaced apart attachment points  86 ,  90  on the support plate  84  and the base plate  14  by means of threaded fasteners or pins (not illustrated). The piston rod  85  is attached to the top of the high pressure valve  80  by a connector  92  so that mechanical force can be applied to the BOP protector  10  and the attached high pressure valve  80  to stroke them in and out of the wellhead. 
     When the BOP protector  10  is in the operative position shown in FIG. 3, the bottom end of the packoff assembly  62  is in sealing contact with a bit guide  94  attached to a top of the casing  76 . The bit guide  94  caps the casing  76  to protect the top end of the casing  76  and to provide a seal between the casing  76  and the tubing head spool  78  in a manner well known in the art. As noted above, the extension section(s)  52  is optional and of variable length so that the assembled mandrel  28 , including the packoff assembly  62 , has adequate length to ensure that the top end  40  of the mandrel  28  extends above the top of BOP  74  just enough to enable the mandrel to be secured by the lockdown assembly  12  described above when the packoff assembly  62  is seated against the bit guide  94 . However, the distance from the top of the bit guide  94  to the top of the BOP  74  may vary to some extent in different wellheads. This variation cannot be accommodated by a conventional lockdown mechanism such as taught in Applicant&#39;s U.S. Pat. No. 4,867,243. 
     In accordance with the invention, the mechanical lockdown mechanism  12  is configured to provide a broad range of adjustment to compensate for variations in the distance from the top of the BOP  74  to the top end  40  of the mandrel  28 . The complementary spiral threads  22 ,  24  (FIG. 1) on the respective integral sleeve  20  and lockdown nut  18  having a length adequate to provide the required compensation. Preferably, the respective threads  22 ,  24  are at least about 9″ (22.86 cm) in axial length. A minimum engagement for safely containing the elevated fluid pressures acting on the BOP protector  10  during a well treatment to stimulate production is represented by a section labelled “A” (FIG.  1 ). Sections “B” represent the adjustment available to compensate for variations in the distance from the top of the BOP  74  to the top end  40  of the mandrel  28 . A spiral thread with about 9″ of axial length provides about 5″ of adjustment while ensuring that a minimum engagement of the lockdown nut  18  is maintained. 
     FIGS. 4 and 5 illustrate two alternate embodiments of the mechanical lockdown mechanism  12  in accordance with the invention. In FIG. 4, the spiral thread  24  on the lockdown nut  18  has an axial extent “A” adequate to ensure the minimum engagement required for safety, and the thread  22  on the integral sleeve  20  of the base plate  14  has a full length spiral thread, which includes the “A” section for the minimum engagement and the “B” section for adjustment. The mechanical lockdown mechanism  12  illustrated in FIG. 5 provides a similar adjustable lockdown with length “A” for minimum safe threaded engagement on the integral sleeve  20  and length “B” for adjustment on the lockdown nut  18 . 
     FIGS. 6 and 7 illustrate the packoff assembly  62  in accordance with alternate embodiments of the invention. Field experience has shown that the bit guides of used wellheads tend to become deformed by small chips, dents, or scratches after a period of running in and out with production tubing and downhole tools. In such cases, the annualar seal used in the embodiment of FIG. 1 sometimes permits pressure leakage at high stimulation pressures and the packoff assembly  62  illustrated in FIGS. 6 and 7 may be used for the BOP protector  10  to improve performance, as described in Applicant&#39;s co-pending U.S. patent application Ser. No. 09/299,551, filed on Apr. 26, 1999 and entitled HIGH PRESSURE FLUID SEAL FOR SEALING AGAINST A BIT GUIDE IN A WELLHEAD AND METHOD OF USING, which is incorporated herein by reference. In FIG. 6, a high pressure fluid seal  98  is an elastomeric material preferably made from a plastic material such as polyethylene or a rubber compound such as nitryl rubber. The elastomeric material preferably has a hardness of about 80 to about 100 durometer. The high pressure fluid seal  10  is bonded directly to the bottom end of the packoff assembly  62 . The bottom end of the packoff assembly  62  includes at least one downwardly protruding annular ridge  100  which provides an area of increased compression of the high pressure fluid seal  98  in an area preferably adjacent an outer wall  102  of the packoff assembly  62 . The annular ridge  100  not only provides an area of increased compression, it also inhibits extrusion of the high pressure fluid seal  98  from a space between the packoff assembly  62  and the bit guide  94  when the mandrel  28  is exposed to extreme fluid pressures. The annular ridge  100  likewise helps ensure that the high pressure fluid seal  98  securely seats against the bit guide  94 , even if the bit guide  94  is worn due to impact and abrasion resulting from the movement of the production tubing or well tools into or out of the casing  76 . A pair of O-rings  104  are preferably provided as back-up seals to further ensure wellhead components are isolated from pressurized stimulation fluids. 
     The packoff assembly  62  illustrated in FIG. 7 has a thicker wall, and an inner wall  106  which extends downwardly past the bit guide  94  and a top edge of the casing  76  into an annulus of the casing  76 . High pressure fluid seal  108  is particularly useful in wellheads where the bit guide  94  does not closely conform to the top edge of the casing  76 , leaving a gap  110  in at least one area of circumference of a joint between the casing  76  and the bit guide  94 . The gap makes the top edge of the casing  76  susceptible to erosion called “wash-out” if large volumes of abrasives are injected into the well during a well stimulation process. The packoff assembly  62  in accordance with this embodiment of the invention covers any gaps at the top end of the casing  76  to prevent wash-out. The length of the inner wall  106  is a matter of design choice. As noted above, the high pressure fluid seal  108  is bonded directly to the end  112  of the packoff assembly  62  using techniques well known in the art. The high pressure fluid seal  108  covers an outer wall portion  120  of the inner wall  106 . It also covers a portion of an outer wall  122  located above the end  112 . A bottom edge of the outer wall  122  of the packoff assembly  62  protrudes downwardly in an annular ridge  124  as described above to provide extra compression of the high pressure fluid seal  108  to ensure that the high pressure fluid seal  108  is not extruded from a space between the packoff assembly  62  and the bit guide  94  when the high pressure fluid seal  108  is securely seated against the top surface of the bit guide  94 . 
     In use of the BOP protector  10 , the base plate  14  is secured to the top of the BOP  74  with the lockdown nut  18  disengaged from the integral sleeve  20  of the base plate  14 , as shown in FIG.  2 . The combination of the mandrel  28 , mandrel head  16  and the lockdown nut  18  may be supported by a rig or other insertion tool. The high pressure valve  80  is mounted to the upper flange  34  of the mandrel head before inserting the mandrel  28  into the BOP  74 . The high pressure valve  80  is closed to prevent well fluids from escaping from the top end  40  of the mandrel head  28  when the mandrel  28  is inserted into the well. The BOP  74  is fully opened to permit the insertion of the mandrel  28 . The mandrel  28  may be inserted through the BOP  74  using the hydraulic cylinder setting tool illustrated in FIG.  3 . If so, the hydraulic cylinder  82 , support plate  84  and the cylinder support rods  88  are mounted on the top of the wellhead in such a manner that the hydraulic cylinder  82  is supported in vertical and axial alignment with the BOP  74  with the piston rod  86  connected by the connector  92  to the top of the high pressure valve  80  and the cylinder support rods  88  attached at their lower ends to the respective attachment points  90  on the base plate  14 . During insertion of the mandrel  28 , well fluids are prevented from escaping to atmosphere by the packing rings  46  located between the mandrel top end  40  and the interior  32  of the mandrel head  16 , which were described above with reference to FIG.  1 . When the mandrel  28  is inserted to its operative position, the lockdown nut  18  is engaged with the threaded integral sleeve  26  of the base plate  14 . The mandrel  28  is inserted into the BOP  74  until the annular seal  66  sealingly contacts the top of the bit guide  94  and the lockdown nut  18  is rotated down to its locking position so that the mandrel  28  is securely held in the operative position during the entire well treatment to stimulate production. 
     After the mandrel  28  is inserted into the operative position, the insertion tool is removed from the wellhead. The insertion tool is remounted to the wellhead after the well treatment to stimulate production is completed. The insertion tool is then operated to stroke the mandrel  28  upward out of the BOP  74 . The BOP  74  is closed before the bottom end of the mandrel  28  is completely withdrawn from the base plate  14  to prevent well fluids from escaping to atmosphere. After the BOP  74  is closed, the entire assembly of the BOP protector  10  and the high pressure valve  80  as well as the hydraulic setting tool is removed from the top of the BOP  74 . The sequence of the steps described above may be changed to adapt to specific circumstances, as will be apparent to persons skilled in the art. 
     Although a hydraulic setting tool as described above with reference to FIG. 3 has been used to illustrate the use of the preferred embodiment of the invention, other types of setting tool may be used for inserting the mandrel  28  through the BOP  74  to the operative position. For example, a setting tool described by McLeod in U.S. Pat. No. 4,632,183 entitled INSERTION DRIVE SYSTEM FOR TREE SAVERS which issued on Dec. 5, 1984, the entire specification of which is incorporated herein by reference, may be used. Another type of setting tool which may also be used to insert the mandrel  28  is described by Bullen in U.S. Pat. No. 4,241,786 entitled WELL TREE SAVER which issued on May 2, 1979 and is also incorporated herein by reference in its entirety. Each of these patents describe an insertion tool in which the force applied to the top of the mandrel is applied by a pair of horizontally oriented beams which are parallel and spaced apart. The lower beam is attached to the top of the BOP, while the upper beam is attached to the mandrel head. A pair of jacks are operatively coupled between the upper and lower beams at respective ends to lower or raise the upper beam with respect to the lower beam so that a force is applied on the mandrel to insert the mandrel into or withdraw the mandrel from the BOP. Other setting tools or rigs known in the art may also be used to insert or remove the BOP protector in accordance with the invention. 
     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.