Patent Grant
7703540
This invention involves an apparatus and method for installing pipe and control line in an earthen borehole. Specifically, this invention involves a spider having components that are adapted for being manipulated to facilitate securing of control line to a pipe string as it is being made up and run into a borehole.
Oil and gas wells may be equipped with control lines for mechanically, electrically, pneumatically, hydraulically or optically linking various downhole devices to the surface. Control lines may be used to receive data from downhole instruments or to operate downhole devices such as valves, switches, sensors, relays or other devices. Control lines may be used to open, close or adjust downhole valves in order to selectively produce or isolate formations at locations deep in the well. A control line may transmit data gathered downhole to the surface or communicate commands to downhole devices to take samples, readings, or to stroke valves. Control lines may comprise electrically conductive wires or cables, optical fibers, or fluid conduits for pneumatically or hydraulically controlling downhole devices or transmitting data.
Control lines are generally of a small diameter relative to the diameter of the pipe string to which they are secured, and are generally between 0.5 and 6 cm in diameter. A plurality of control lines may be aggregated to form an umbilical having a diameter of up to 10 cm or more. Control lines are generally secured along the length of the outer surface of a pipe string, generally parallel to the center axis of the bore of the pipe string. Continuous control lines are secured to the pipe string and installed in the well as joints of pipe are made up into a pipe string and run into a well.
Control lines secured to pipe string are subject to being damaged and being rendered useless if they are pinched or crushed by the pipe slips used to grip and support the pipe string while it is being made up and run into the well. This presents a challenge in securing the control lines to the pipe string as it is made up and run into the borehole. Depending on the diameter, length and pipe thickness, the pipe string may weigh more than four hundred thousand pounds. A pipe-gripping tool called a spider is required to grip and support the pipe string at or near the rig floor. The spider generally comprises a tapered bowl having a bore with an axis that is generally aligned with the borehole. The pipe string passes through the tapered bowl, and the tapered bowl receives a generally circumferential arrangement of radially inwardly movable slips that surround and engage the pipe string within the tapered bowl. The generally wedge-shaped slips are adapted for engaging the outer curved surface of the pipe string and bearing against the tapered inner surface of the bowl to provide generally radially distributed support in a self-tightening manner.
It is important that the pipe slips in the spider generally uniformly grip and support the pipe string in order to minimize localized stress and loads on the pipe that may crush or damage the pipe string. The radially inwardly disposed gripping surfaces of the slips are concave in order to contact the pipe over a radially large area to minimize localized stresses. When control lines are being secured to the pipe and run into the borehole, it is important to prevent the control lines from being pinched or trapped between the spider slips and the outer surface of the pipe string, or between adjacent slips as they move radially inwardly to grip and support the pipe string. If a control line is trapped between the slips and the pipe string or between two adjacent slips, the control line may be damaged with a resulting loss or impairment of surface control of, or communication with, downhole devices or instruments that are linked to other devices or to the surface using control line(s). It is important that control lines be secured to the pipe string in a manner that will prevent control line damage.
One method of installing control lines involves extending the control lines along the portion of the pipe string that is gripped and supported within the tapered bowl of the spider. A control line may be aligned and positioned along the length of the exterior surface of the pipe string to radially coincide with and pass through a gap or recess between adjacent slips. This method may be unsatisfactory where multiple control lines are being secured to the pipe string because more of the circumference of the pipe string is required to accommodate the control lines, leaving less contact circumference for the slips to engage and support the pipe string.
The growing appreciation for the advantages and benefits of controllable downhole tools and devices and for receiving data from downhole instruments has resulted in the development of new tools and methods for installing control lines in a well. One approach involves the use of a table-elevated spider constructed on the rig floor to support the spider and the pipe string, thereby creating and maintaining a clamping zone between the table and the rig floor. This “clamping zone” provides access to a portion of the pipe string beneath the spider for introducing and securing control lines along the length of the pipe string. The control lines are fed to the pipe string at a location underneath the table that supports the spider, secured to the pipe string, and then fed into the borehole along with the pipe string as it is made up and lowered into the borehole. While the table-elevated spider prevents slip damage to control lines at the spider, the legs supporting the table must be strong enough to support the entire pipe string, the spider, and the table, which is a work platform for machines and personnel. The required strength of the legs and the space restrictions of the table present significant expense and safety concerns.
Another approach to securing control lines to a pipe string as it is being made up and run into a well involves a spider adapted for being received in a retainer that can be vertically reciprocated from and to its retracted position within or near the floor of the rig. This invention eliminates the need for an elevated table with legs strong enough to support the spider, table and pipe string. After the weight of the pipe string is transferred to the elevator, the retainer and spider are raised from the floor position to create a temporary clamping zone between the raised spider and the rig floor. The control line may be directed over roller guides or sheaves secured on or adjacent to the retainer that supports the spider so that the control line will conveniently align along the exterior length of the pipe string within the temporary clamping zone. After the control line is secured to the pipe string in the clamping zone, the pipe string and the control line are lowered into the borehole and the retainer and the spider are returned to their original position in or near the rig floor for again receiving and supporting the pipe string while an additional pipe segment is made up into the pipe string.
While vertically reciprocating the spider in this manner eliminates the expense and safety concerns associated with the table-elevated spider, there remains a need to optimize the equipment and the methods for securing control line to a pipe string. What is needed is a method of securing a control line to a pipe string that does not require the repeated movement of the entire spider to establish a clamping zone. What is needed is an apparatus that permits the repeated movement of select components of the spider in order to create a clamping zone for securing control line to the pipe string.
The present invention utilizes a spider having slips for being received within a tapered bowl of the spider, and a vertically reciprocating control line guide for engaging and then imparting a desired configuration or pathway to a control line. The control line guide rollably or slidably engages a control line, and moves between a retracted position and a raised position. The control line guide remains in a retracted position when the spider engages and supports the pipe string. The retracted position of the control line guide is characterized as having at least a portion of the control line guide beneath the top surface of the tapered bowl of the spider. When the control line guide is in its retracted position, the lowermost point on the control line guide, or the “exit,” is positioned below the bottom of the slips and adjacent to the pipe string. The raised position of the control line guide is characterized as having the control line guide sufficiently raised above the top surface of the tapered bowl of the spider to provide a clamping zone in which the control line is positioned along at least a portion of the pipe string between the raised control line guide and the rig floor. The clamping zone provides access to the portion of the pipe string and the control line for application of a clamp or fastener for securing the control line to the pipe string.
The spider is adapted for repeated manipulation or removal of one or more components of the spider to create an unobstructed pathway for raising the control line guide from its retracted position to its raised position. In one embodiment, the slips are the spider component that are adapted for being repeatedly unseated from their engaged position within the tapered bowl of the spider and removed from the tapered bowl at least to an extent sufficient to clear a pathway for the control line guide to elevate along a portion of the length of the pipe string near the spider. In this embodiment, the spider may comprise a tapered bowl and a set of three slips that includes a center, manipulated slip and two following slips, each hinged or movably coupled to the manipulated slip. The slips surround, engage and support the pipe string when received in their engaged position within the tapered bowl of the spider. The tapered bowl comprises a slot in which the control line guide moves between its retracted and raised positions. The slot may be positioned to coincide with a gap between adjacent following slips when the set of slips is engaged with the pipe string within the tapered bowl. Optionally, the slot may be positioned generally opposite the manipulated slip, which will generally align the slot between the adjacent following slips. The slips may be upset from their engaged position by application of a lifting force to the manipulated slip, primarily in a vertical direction at first, and the set of slips may be completely or just partially removed from the engaged position within the tapered bowl to clear a pathway above the slot to permit raising of the control line guide.
The control line guide may be raised to create a clamping zone when the weight of the pipe string is supported by the elevator and the set of slips are sufficiently removed from their engaged position to clear a pathway for raising the control line guide. The control line guide may be coupled to a jack or to a winch for vertically raising the control line guide above the slot to create a clamping zone. In one embodiment, the slot in the spider may be closable using a plug-in door adapted for being generally vertically received within the slot in an interlocking fashion so that the plug-in door provides added load bearing capacity to the tapered bowl. In one embodiment, the plug-in door may be secured to the same jack that raises and lowers the control line guide. In this embodiment, after the control line guide and the plug-in door are raised to create a clamping zone and the control line is secured to the pipe string, the pipe string and the control line may be lowered into the borehole, and the control line guide and the plug-in door may then be lowered to their retracted and engaged positions, respectively. The slips are then restored to their set position within the tapered bowl to engage and support the pipe string while another pipe segment is threadably coupled to the proximal end of the pipe string. The plug-in door may be interlockably received into a slot that is positioned above a “half door” that resembles a conventional side door of a spider, but occupies only a portion of the full vertical height of the spider. The control line guide may penetrate the wall of the spider between the half door and the plug-in door when in its retracted position so that it may be raised along with the plug-in door to create a clamping zone without opening of the half door.
In another embodiment, the tapered bowl of the spider is adapted for removal from its aligned position with the borehole to clear a pathway for raising a control line guide. In this embodiment, the slips are adapted for being upset from their engaged position within the tapered bowl of the spider, but not necessarily for being completely removed from the tapered bowl. Instead, the tapered bowl comprises a generally vertical slot that allows the tapered bowl to be laterally moved to a remote position away from its aligned position with the borehole when the weight of the pipe string is supported by the elevator. The slot in the tapered bowl of the spider may be closable by a conventional door having interdigitated hinges or by a plug-in door that is generally vertically received in an interlocking fashion to close the slot of the tapered bowl and raised from its seated position to open the slot of the tapered bowl. Opening of the slot of the tapered bowl using a plug-in door or a conventional door, or both, provides for lateral movement of the tapered bowl away from its aligned position with the borehole to clear a pathway for the control line guide. The generally horizontal movement of the tapered bowl and slips to the remote position clears the pathway of the control line guide to allow the control line guide to be raised to create a clamping zone above the rig floor and below the raised control line guide.
After the control line is secured to the pipe string at one or more locations within the clamping zone, the pipe string and control line may be lowered into the borehole and the control line guide may be restored to its retracted position. The tapered bowl is laterally restored to its position aligned with the borehole so that it generally surrounds the pipe string, the door is repositioned to close the tapered bowl, and the slips are received within the tapered bowl to engage and support the pipe string.
In another embodiment of the present invention, the tapered bowl comprises a slot for permitting vertical reciprocation of the control line guide, and the slips are adapted for being secured to a jack and vertically raised from the tapered bowl by raising the jack. A slot in the tapered bowl may permit the spider to be received around and removed from the pipe string. The door for closing the slot in the tapered bowl may be a half door of the conventional interdigitated hinged type or it may be a slidably received plug-in door, or a combination of the two. The slips may be vertically reciprocated using the jack toward and away from the tapered bowl. Slips and/or the plug in door may be reciprocated using the same jack that reciprocates the control line guide through the pathway cleared by removal of the plug-in door. After the weight of the pipe string is transferred to the elevator, the jack is moved into position to engage the slips and the plug-in door. The plug-in door may be coupled to the control line guide so that coupling the jack to the plug-in door also couples the jack to the control line guide. Upon raising the jack, the slips, plug-in door and the control line guide are vertically moved from their positions within the tapered bowl to create a clamping zone between the control line guide and the tapered bowl. After the control line is secured to the pipe string at one or more locations within the clamping zone, the pipe string and the control line are lowered into the borehole, the control line guide and the plug-in door are restored to their retracted positions with at least a portion of the control line guide being beneath the top surface of the tapered bowl, and the slips are received within the tapered bowl to engage and support the weight of the pipe string.
In another embodiment of the present invention, the slot in the tapered bowl is openable to allow the tapered bowl to be laterally removed from its position aligned with the borehole when the weight of the pipe string is supported by the elevator. The slips may remain within the tapered bowl as it is laterally removed from its aligned position with the borehole, or the slips may be securable to a jack that raises the slips to a raised position generally above the tapered bowl before the tapered bowl is moved, and also lowers the slips toward their engaged position within the tapered bowl when the tapered bowl is restored to its aligned position with the borehole. Similarly, the control line guide may be secured to a plug-in door, and the plug-in door may be in turn secured to a jack that raises the control line guide and the plug-in door to a raised position to create a clamping zone between the control line guide and the rig floor. The slips may be secured to the same jack that raises the control line guide and the plug-in door so that the slips are vertically raised away from their engaged position within the tapered bowl as the control line guide is raised to create a clamping zone.
After the control line is secured to the pipe string at one or more locations within the clamping zone, the pipe string and the control line may be lowered into the borehole and the tapered bowl may be restored to its aligned position with the borehole. Once the tapered bowl is restored to its aligned position with the borehole, the control line guide, plug-in door and slips may be lowered by the jack so that the control line guide can be received into its retracted position with at least a portion of the control line guide being beneath the top surface of the tapered bowl, the plug-in door may be vertically slidably received into the slot to strengthen the tapered bowl for supporting the pipe string, and the slips may be received in the tapered bowl to engage and support the pipe string.
In certain embodiments of the present invention, the tapered bowl of the spider is adapted for slidably receiving and surrendering a plug-in door to complete and strengthen the tapered bowl. Unlike the more conventional side door that couples to the tapered bowl with pins inserted through interdigitated hinges disposed on each end of the door, a plug-in door may be vertically slidably received in an interlocking fashion within a slot in the side of the tapered bowl. The plug-in door may comprise a door with a pair of generally vertical and downwardly disposed elongated posts, each coupled at their top end to a support plate and each receivable into a receptacle or port in the tapered bowl. Another type of plug-in door comprises a pair of outwardly disposed opposing T-shaped keys adapted for being vertically slidably received into mating T-shaped slots disposed on either side of the slot of the tapered bowl into which the plug-in door seats.
The control line guide which, depending on the embodiment, may or may not be coupled to a plug-in door, may be shaped to impart a desired pathway to a portion of the control line that is received thereon. The control line guide is adapted to gradually bend and redirect a portion of the control line into position adjacent to and along the portion of the pipe string that extends from below the raised control line guide and into the borehole. The control line approaches the control line guide from a position radially outwardly from the pipe string. Sheaves, rollers or guides may be used to strategically position and direct the control line to the receiving portion of the control line guide. The control line guide may be shaped or adjustable for accommodating differing control line sizes or approach angles depending on the configuration of the rig, but generally it is preferred to have the control line approach the control line guide from a position lateral to and above the control line guide in order to prevent tripping hazards or obstacles to movement by personnel working on the rig floor around the spider.
A control line guide usable for each of the above-referenced embodiments is adapted for slidably or rollably contacting a portion of the control line that is reeved through the control line guide. The pathway imposed by the control line guide on the control line is contoured to prevent unwanted kinking or excessive localized bending of the control line that might permanently impair the function or capacity of the control line. The control line guide may comprise a series of slides, rollers, guides or combinations of these, secured in a fixed or in an adjustable relationship one to the others. The control line guide may be adapted for continuous feed of a lubricant, coating or adhesive to the exterior jacket of the control line as it passes through or over the control line guide, and the control line guide may be adapted for accommodating instruments for inspection or testing of the control line as it passes through or over the control line guide.
In addition to raising and lowering the control line guide and other components, machines may also be adapted to manipulate components of the spider to clear a pathway for the vertical reciprocation of the control line guide. For example, after the weight of the pipe string is transferred to the elevator, the slips may be engaged and upset from their set position within the tapered bowl, and then partially lifted and partially removed from their aligned position with the borehole, all using a pneumatically or hydraulically-powered mechanism. A mechanical linkage may be coupled to a latching portion at or near the top of the manipulated slip to displace it initially upwardly and then radially outwardly away from the pipe string thereby causing the following slips to each rotate relative to the manipulated slip to clear the pathway for the vertically reciprocating control line guide. In some embodiments, this movement of the slips also allows passage through the tapered bowl of the clamp that secures the control line to the pipe string. Alternately, a mechanical linkage may be coupled to the manipulated slip to displace it initially upwardly and radially outwardly away from the pipe string to cause each of the following slips to rotate relative to the manipulated slip and to clear a pathway for the withdrawal of the pipe string from the tapered bowl with lateral movement of the tapered bowl away from its position aligned with the borehole. Depending on the embodiment of the invention used, the mechanism used to manipulate the slips may remove the slips completely from the tapered bowl or it may only partially remove the slips from their set position within tapered bowl, depending on the extent to which the slips must be moved. The extent of movement of the slips may be minimal for releasing the pipe string, more for reciprocation of the control line guide, and still more for providing clearance for the control line clamp to pass through the tapered bowl.
For the embodiments of the present invention adapted for removal of the tapered bowl to a remote position and restoration of the tapered bowl back to its position aligned with the borehole, a runway may be adapted for slidably or rollably receiving and supporting the tapered bowl as it moves laterally away from and then back to its position aligned with the pipe string. The runway acts as a support platform for the tapered bowl to facilitate movement to one or more remote positions to clear a pathway for reciprocation of the control line guide. The runway may be selectively radially positionable at two or more positions about the borehole, but is preferably aligned opposite the slot of the tapered bowl.
Machines may be adapted for movement of the tapered bowl, for opening or closing of the side door of the tapered bowl, and for removal or restoring the plug-in door to its position in the slot of the tapered bowl. For example, the embodiments requiring manipulation of the slips and the tapered bowl to clear a pathway for reciprocating the control line guide may require a first linkage for unseating the slips from their engaged position after the weight of the pipe string is transferred to the elevator. If the spider comprises a tapered bowl having a hinged side door, the door must be unlatched and opened to enable removal of the pipe string from the tapered bowl. A sliding latch mechanism may couple to the tapered bowl and move it along the runway adjacent to the borehole to its remote location. After the control line guide is raised to create a clamping zone and the control line is secured, the pipe string and the control line are lowered into the borehole, and the sliding latch mechanism may move the tapered bowl back along the runway to restore the tapered bowl to its position aligned with the borehole, and other mechanisms may restore the side door or plug-in door to close the slot in the tapered bowl, and also to restore the slips to engage and support the pipe string.
The mechanical linkage for moving the tapered bowl along the runway may cooperate with the control line guide such that a position sensor on the mechanical linkage enables the powered jack to begin raising the control line guide only after the tapered bowl reaches a certain distance from the pipe string. Similarly, a tapered bowl position sensor on the control line jack may enable the linkage to begin returning the tapered bowl along the runway towards its aligned position with the borehole when the control line guide has been lowered to a certain position or when it has been returned to its fully retracted position.
In the embodiments of the present invention adapted for vertical displacement of the slips, the slips may be raised using the same or a different jack that raises the control line guide and/or the plug-in door to a raised position. Because the initial movement of the slips from engagement is necessarily up and then radially outwardly away from the pipe string, a jack for raising the slips may be adapted for providing an initial upward unseating movement of the slips, followed by a raising of the slips and/or the control line guide. Similarly, a mechanical linkage may be adapted for providing lateral movement of the slips away from the pipe string. For example, in the embodiment adapted for lateral movement of the slips from the tapered bowl, the slips may be initially raised from their engaged position within the tapered bowl to a vertical position sufficient to clear the top surface of the tapered bowl, and then the slips may be moved radially outwardly away from their aligned position with the borehole.
In the embodiments adapted for removal of both the slips and the tapered bowl from their aligned positions with the borehole, the slips need only be unseated from their engaged position, and then raised to a substantially shorter vertical distance sufficient to disengage them from the pipe string and to permit the following slips to rotate slightly relative to the manipulated slip. This limited movement of the slips suffices to clear the pathway of the control line guide without necessarily displacing the slips to a position above the top surface of the tapered bowl.
In each embodiment of the present invention, after the control line guide is raised and the control line clamp installed to secure the control line to the pipe string, the pipe string and the control line may be lowered into the borehole as the control line is fed to the control line guide. The control line guide is retracted, the tapered bowl and the slips restored to their positions aligned with the borehole, the side door and/or plug-in door is restored to close the slot and strengthen the tapered bowl, and the slips are then disposed to their set position in the tapered bowl so that the weight of the pipe string can be transferred to the spider. After a new pipe segment is threadably coupled to the proximal end of the pipe string and torqued to a predetermined torque, the weight of the pipe string is transferred to the elevator and the process is repeated.
“Jack,” as that term is used herein, includes but is not limited to jacks, winches, lifts and other powered devices for generally one-dimensional displacement of an object. A jack may be powered pneumatically, hydraulically, electrically or mechanically, and it may include a rotating screw drive, cylinder, scissor extension, track and pinion or other devices.
“Elevator,” as that term is used herein, includes but is not limited to a side door elevator, an elevator comprising internal or external slips and all other devices used for gripping and supporting a pipe string from above the spider, including those supported by a top drive or draw works.
The terms “comprising,” “including,” and “having,” as used in the claims and specification herein, shall indicate an open group that may include other elements not specified. The term “consisting essentially of,” as used in the claims and specification herein, shall indicate a partially open group that may include other elements not specified, so long as those other elements do not materially alter the basic and novel characteristics of the claimed invention.
The terms “a,” “an,” and the singular forms of words shall be taken to include the plural form of the same words, such that the terms mean that one or more of something is provided. For example, the phrase “an apparatus having a drive motor” should be read to describe an apparatus having one or more drive motors. The term “one” or “single” shall be used to indicate that one and only one of something is intended. Similarly, other specific integer values, such as “two,” are used when a specific number of things is intended.
The terms “preferably,” “preferred,” “prefer,” “optionally,” “may,” and similar terms are used in the specification to indicate that an item, condition or step being referred to is an optional (not required) feature of the invention.
While a preferred form of the present invention has been described herein, various modifications of the apparatus and method of the invention may be made without departing from the spirit and scope of the invention, which is more fully defined in the following claims.
The foregoing, as well as other, objects, features, and advantages of the present invention will be more fully appreciated and understood by reference to the following drawings, specification and claims.
The control line guide 80 comprises a plurality of generally spaced-apart rollers 82, each having a generally horizontal axis of rotation, and each retained in a generally fixed relationship relative to the other rollers. The control line guide receives the control line 92 from, or surrenders the control line to, a spool (not shown). The control line 92 may be reeved over sheaves (not shown) to strategically direct the control line to the control line guide from above or, when the control line is being removed from the borehole, to strategically direct the control line to a spool (not shown) for storage.
The tapered bowl 20 comprises a door 24 received to close a slot 25. The door 24 is secured to the tapered bowl 20 with a pair of hinges 36 adapted for receiving a pair of pins 27. Removal of either closure pin 27 enables the door to hingedly swing open for removal of the tapered bowl 20 from its aligned position with the borehole (as shown in
A runway 28 is positioned adjacent to the rotary adapter 22 for slidably or rollably supporting the tapered bowl when the door 24 is opened and the tapered bowl is moved laterally away from the pipe string 30 (as shown in
The plug-in door 81 is coupled to the traveling end of the hydraulically powered telescoping jack legs 86, and the control line guide 80 is pivotally supported beneath the lift plate 84 using support links 82. The inserts 84A of the plug-in door are vertically aligned with the receptacles 84B in the tapered bowl so that the inserts are received into the receptacles upon retraction of the hydraulically telescoping jack legs 86 and lowering of the plug-in door 81 and the control line guide 80.
As shown in
The slips 60, 62 are adapted for being removed from their engaged position within the tapered bowl 20 to a remote position as shown in
The pathway for raising the plug-in door 81, the control line guide 80 and the slips 60, 62 from the tapered bowl 20 is determined by the A-frame 70. The A-frame 70 comprises a pair of generally vertical rails 72, each slidably receiving a pair of sleeves 73 each coupled to the lift plate 75. The lift plate 75 is coupled to a winch cable 94 that raises the lift plate 75, the control line guide 80 and the slips 60, 62 to a raised position. Upon actuation of the winch (not shown), the sleeves 73 slide along the vertical length of the rails 72, and the vertical path of the plug-in door 81 and control line guide 80 conforms to the pathway provided by the sliding movement of the sleeves 73 on the rails 72 positioned adjacent to the pipe string 30. After the winch is actuated to raise the plug-in door and control line guide to their raised position to create the clamping zone 100, clamps (not shown) may be applied to secure the control line 92 to the pipe string 30. After the pipe string and control line are lowered into the borehole, the winch rotation is reversed to lower the control line guide back to its retracted position through the slot of the tapered bowl. The A-frame 70 may be rollably removable from the vicinity of the borehole on a set of wheels 76 when control line is not being run into the well.
A pair of opposing plug-in door supports 85 are coupled to and extend outwardly from lift plate 74 to pivotably engage and couple to the plug-in door 81 which supports the control line guide 80. The plug-in door supports 85 are rotatable about pivots 85A to permit the generally arcuate plug-in door supports to substantially surround the pipe string 30 and engage, support and raise the plug-in door 81 and the attached control line guide 80 to position a portion of the control line 92 along the pipe string in the clamping zone 100.
The tapered bowl 20 further comprises a pair of generally opposed T-slots 102A and 102B disposed on opposite sides of the slot 25 for receiving a pair of generally T-shaped keys (see
The slot 25 that receives the plug-in door (see
A variety of interlocking configurations can be utilized for slidably and vertically receiving the plug-in door 81 to circumferentially interlock with the tapered bowl 20 to provide enhanced hoop strength to the tapered bowl. The T-slotted plug-in door 81 shown in
As shown in
While a preferred form of the present invention has been described herein, various modifications of the apparatus and method of the invention may be made without departing from the spirit and scope of the invention, which is more fully defined in the following claims.
This application is a continuation-in-part of U.S. patent application Ser. No. 10/995,905 filed on Nov. 24, 2004, now U.S. Pat. No. 7,222,677 issued on May 29, 2007, which is a continuation-in-part of U.S. patent application Ser. No. 10/315,617 filed Dec. 10, 2002, now U.S. Pat. No. 6,920,931, issued on Jul. 26, 2005. This application is also a continuation-in-part of U.S. patent application Ser. No. 10/995,907 filed on Nov. 24, 2004, now U.S. Pat. No. 7,216,716 issued on May 15, 2007, which is a continuation-in-part of U.S. patent application Ser. No. 10/315,617 filed Dec. 10, 2002, now U.S. Pat. No. 6,920,931, issued on Jul. 26, 2005.
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