(12) United States Patent
`Dallas
`
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`US006179053Bl
`US 6,179,053 Bl
`Jan.30,2001
`
`(10) Patent No.:
`(45) Date of Patent:
`
`(54) LOCKDOWN MECHANISM FOR WELL
`TOOLS REQUIRING FIXED-POINT
`PACK OFF
`
`(76)
`
`Inventor: L. Murray Dallas, 790 River Oaks Dr.,
`Fairview, TX (US) 75069
`
`5,247,997 * 9/1993 Puccio .................................. 166/348
`5,540,282 * 7/1996 Dallas ....... ... ......... .... ........... 166/379
`10/1998 Dallas .
`5,819,851
`* cited by examiner
`Primary Examiner-Frank Tsay
`(74) Attorney, Agent, or Firm-Dority & Manning, P.A.
`
`( *) Notice:
`
`Under 35 U.S.C. 154(b), the term of this
`patent shall be extended for 0 days.
`
`(57)
`
`ABSTRACT
`
`(21) Appl. No.: 09/373,418
`
`(22) Filed:
`
`Aug. 12, 1999
`
`Int. Cl.7 ...................................................... E21B 23/03
`(51)
`(52) U.S. Cl. ........................ 166/77.51; 166/341; 166/382
`(58) Field of Search ................................. 166/77.51, 348,
`166/341, 338, 351, 381, 383, 382
`
`(56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`3,924,678 * 12/1975 Ashlstone ............................. 166/120
`4,241,786
`12/1980 Bullen .
`4,632,183
`12/1986 McLeod .
`4,867,243
`9/1989 Garner et al. .
`5,069,288 * 12/1991 Singeetham .......................... 166/348
`5,080,174 * 1/1992 Hynes ............. ... ......... ... ...... 166/382
`5,145,006 * 9/1992 June ..................................... 166/341
`
`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 pro(cid:173)
`vided 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 mecha(cid:173)
`nism 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.
`
`27 Claims, 9 Drawing Sheets
`
`62
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`52
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`98
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`Jan. 30, 2001
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`Jan. 30, 2001
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`US 6,179,053 Bl
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`1
`LOCKDOWN MECHANISM FOR WELL
`TOOLS REQUIRING FIXED-POINT
`PACK OFF
`
`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(cid:173)
`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 com- 35
`pletely 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 40
`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, appli(cid:173)
`cant 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 require(cid:173)
`ment 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
`
`2
`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 well(cid:173)
`head 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.
`10 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
`15 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
`20 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 prefer(cid:173)
`ably provides a range of adjustment to compensate for
`variations in the position of the top end of the mandrel when
`25 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's U.S. Pat.
`No. 5,819,851 which issued on Oct. 13, 1998 and is entitled
`30 BLOWOUT PREVENTER PROTECTOR FOR USE DUR(cid:173)
`ING 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 stimu(cid:173)
`lation 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
`45 operative position in the well.
`The blowout preventer protector described in U.S. Pat.
`No. 5,819,851 includes a mandrel that is integrally incor(cid:173)
`porated with a hydraulic setting tool. The mandrel is not
`separable from the hydraulic setting tool and the setting tool
`50 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
`55 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 blow(cid:173)
`out preventer protector is widely accepted in the industry
`60 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
`65 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
`
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`US 6,179,053 Bl
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`3
`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 mainte(cid:173)
`nance 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'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 adjust(cid:173)
`ment to compensate for variations in the height of different
`wellheads to which the well tool is mounted. The mechani-
`cal 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 engag(cid:173)
`ing 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 30
`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 sepa(cid:173)
`rated 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 40
`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.
`
`35
`
`4
`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
`10 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 mem(cid:173)
`ber; and the second lockdown mechanism having a range of
`adjustment adequate to ensure that the mandrel can be
`15 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
`20 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
`25 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 recip(cid:173)
`rocated 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
`45 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
`50 mechanical lockdown mechanism including a base member
`for connection of the wellhead and a locking member for
`detachably engaging the base member; a hydraulic mecha(cid:173)
`nism including a cylinder and a piston which may be
`reciprocated within the cylinder using fluid pressure, the
`55 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
`60 mechanism is in a lockdown position. The hydraulic mecha(cid:173)
`nism 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 mecha-
`
`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 lock(cid:173)
`down 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 65
`well which combines a hydraulic lockdown mechanism with
`a mechanical lockdown mechanism.
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`5
`nisms. 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 appa(cid:173)
`ratus in various working positions in accordance with a
`preferred embodiment of the invention;
`FIGS. 5 to 7 illustrate cross-sectional views of an appa(cid:173)
`ratus 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'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
`
`6
`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
`10 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
`15 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
`20 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
`25 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
`30 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
`35 is, O~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
`40 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
`45 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
`50 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
`55 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
`60 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
`65 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
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`US 6,179,053 Bl
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`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 disen(cid:173)
`gaged 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 35
`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 40
`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 inven(cid:173)
`tion. The apparatus 70 includes a mandrel head 26 thread- 45
`edly 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 50
`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 55
`72. The cylinder 74 further includes a sidewall 80 which
`defines an annular cavity 82. A piston 84 is fixed to the
`mandrel 72. 0-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 60
`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
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`8
`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
`10 inserted into the wellhead. The mandrel 72 is stroked down
`under a force Pl 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
`P2 exerted by pressurized hydraulic fluid trapped in the
`15 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
`20 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
`25 forced downwardly to packoff against the fixed-point 24
`under a force P2 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
`30 the cylinder 74 is reduced to B-C. The mandrel 72 is locked
`down in its operative position by the hydraulic force P2. To
`ensure that the mandrel is secured in the operative posi