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You are an expert at summarizing long articles. Proceed to summarize the following text: 
RELATED APPLICATIONS 
     This application claims the benefit of provisional application Ser. No. 60/120,270 filed Feb. 16, 1999. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to removal of rigid liners from within cylindrical cavities, and, more particularly, to the use of inflated liners to contact the cylindrical cavity while removing rigid liners. 
     BACKGROUND OF THE INVENTION 
     The installation of flexible supporting and sealing liners is described in U.S. Pat. Nos. 5,176,207, 5,803,666, and 6,026,900. In some cases, it is desirable to temporarily insert a rigid liner within a cylindrical cavity, such as a borehole, and to then replace the rigid liner with a flexible inflated liner with contact between the inflated liner and the cylindrical cavity during the replacement procedure. 
     However, if one inflates a flexible liner inside a rigid pipe of the same or less diameter, the flexible liner can not be easily withdrawn from the pipe, nor can the pipe be easily removed from off the flexible liner because of the excessive drag of the inflated flexible liner on the rigid liner. The pressure of the inflated flexible liner against the pipe wall and the large surface area of the pipe combine to produce very large drag resistance to sliding the pipe off the flexible liner. For example, a differential pressure of 1 psi in a 10 ft. long flexible liner inside a 4 in. diameter pipe, with a drag coefficient of 1.0, requires over 1500 lbs of force to pull the inflated liner out of the pipe. Such a large force may tear the flexible liner. As the pipe length increases to that of common well depths, the larger drag force is certain to tear the flexible liner. 
     It is common practice to emplace a rigid liner (e.g., a pipe) to support a drill hole wall while drilling a well. It may then be useful to replace the rigid liner support of the hole wall with the support of an inflated flexible liner. However, if the inflated liner is anchored to the bottom of the hole, and one pulls upward on the rigid liner, the drag of the rigid liner on the inflated liner prevents the rigid liner from being removed from the hole without a potentially destructive drag force on the flexible liner. 
     The present invention eliminates most of the drag force of a rigid liner on the inflated liner without the use of a lubricant to reduce the friction coefficient. 
     Various objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. 
     SUMMARY OF THE INVENTION 
     In accordance with the purposes of the present invention, as embodied and broadly described herein, the present invention includes apparatus for continuously contacting a cylindrical cavity while removing a rigid liner from within the borehole. The apparatus includes a flexible liner insertable within the rigid liner, having an expanded diameter greater than a diameter of the borehole, and having a top end and closed bottom end. A hollow conduit is attached to the bottom end of the flexible liner and has an opening adjacent the bottom end for introducing a fluid to expand the flexible liner against the cylindrical cavity and contact the cylindrical cavityl as the rigid liner is withdrawn from about the flexible liner. 
     In another characterization of the present invention, a cylindrical cavity is continuously contacted while removing a rigid liner from within the cavity. A flexible liner is inserted within the rigid liner, the flexible liner having an inflated diameter greater than the cavity diameter and forming an annulus with the rigid liner. The flexible liner is first filled with a fluid. The annulus between the rigid liner and the flexible liner is then filled with a fluid to equalize with the pressure of the fluid within the flexible liner. The rigid liner is withdrawn from the cavity while adding fluid to maintain the fluid level within the flexible liner as the flexible liner expands against the cavity as the rigid liner is withdrawn. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and form a part of the specification, illustrate the embodiments of the present invention and, together with the description, serve to explain the principles of the invention. In the drawings: 
     FIGS. 1A and 1B show a side cross-section and a top cross-section, respectively of a rigid liner filled with a liquid inflated flexible liner. 
     FIG. 2 is a side cross-section of the liner shown in FIG. 1A with the introduction of a liquid between the rigid liner and the inflated liner. 
     FIG. 3 is a side cross-section to illustrate the flexible liner partially extended beneath the rigid liner and expanded to anchor the flexible liner to the hole wall. 
     FIG. 4 is an expanded cross-section of a preferred shape of the bottom end of the rigid liner. 
     FIGS. 5A and 5B are cross-sections of an embodiment using a sheath to compress the flexible liner in slender pipes. 
     FIG. 6 is a cross-section of an embodiment using a central support for large diameter pipes. 
     FIG. 7 is a cross-section of an embodiment using air as the fluid in the system. 
     FIG. 8 is a cross-section of an embodiment using a covering for enhancing the friction reduction. 
    
    
     DETAILED DESCRIPTION 
     In accordance with the present invention, a rigid liner can be withdrawn from about an inflated flexible liner that expands to fill the space occupied by the rigid liner. FIGS. 1A and 1B show flexible liner  10  located inside of rigid liner  12 . Rigid liner  12  is shown within a cylindrical cavity, e.g., a drill hole  14  in earth  16  for clarity of the application. In FIG. 1, flexible liner  10  is inflated with water  18 . 
     Preferably, central tube  24 , shown FIGS. 1A and 1B, is for the injection of a fluid, generally water, through bottom end  26  of central tube  24  as explained next. Fluid could be added from the top of flexible liner  10 , but central tube  24  is convenient for introducing the fluid. The pressure  22  of water  18  against flexible liner  10  wall forces flexible liner  10  against the inside surface of rigid liner  12 . If rigid liner  12  were lifted from hole  14 , the drag of flexible liner  10  on rigid liner  12  is so large that the entire flexible water filled liner  10  would be lifted with rigid liner  12  or would be torn. 
     If flexible liner  10  were suitably anchored at its bottom end to the bottom of hole  14 , e.g., by the weight of the fluid in liner  10 , rigid liner  12  can be lifted from the hole only by pulling with a force greater than the drag friction, which is equal to P*C*F*L, where P is the average pressure, C the circumference of rigid liner  12 , F the friction coefficient of flexible liner  10  on rigid liner  12 , and L the length of the contact between the liners. This drag force is often greater than the tensile strength of flexible liner  10 . Therefore, rigid liner  12  can not be drawn from the hole without damage to flexible liner  10 , if flexible liner  10  is anchored in hole  14 . 
     FIG. 2 shows the process of this invention. A small quantity of water is added to annular space  34  between flexible liner  10  and rigid liner  12 . The gravity driven flow of the water downward in space  34  between flexible liner  10  and rigid liner  12  causes a fluid pressure to be applied to the outside of flexible liner  10 . In some instances, flexible liner  10  may be raised and stretched to aid the downward flow of the annular water. The annular fluid pressure becomes equal to the interior pressure of flexible liner  10  or flexible liner  10  will be displaced until the fluid pressures equilibrate. The water levels in flexible liner  10  and in annular space  34  become equal. The effect is to eliminate the differential pressure that was forcing flexible liner  10  against rigid liner  12 . Since flexible liner  10  is no longer pressing against rigid liner  12 , the drag of flexible liner  10  against rigid liner  12  becomes very small. The only drag remaining is due to the confinement of flexible liner  10  within rigid liner  12 . This may be further reduced as discussed below. 
     Once the differential pressure in flexible liner  10  is small, rigid liner  12  may be lifted from the hole, leaving flexible liner  10  in place. However, when flexible liner  10  is resting against the wall of hole  14 , which is uncovered as rigid liner  12  is raised, the fluid in the annular space  34  is lost to the formation and the full differential pressure of the water head in flexible liner  10  forces flexible liner  10  against the hole wall. The force of flexible liner  10  against the hole wall is a very strong stabilizing force, preventing hole collapse. The strong force of flexible liner  10  against the hole wall also causes flexible liner  10  to drag against the hole wall, and tends to anchor flexible liner  10  in hole  14 , further resisting the lifting of flexible liner  10  as rigid liner  12  is raised. 
     FIG. 3 shows rising rigid liner  12  with the dilation  30  of flexible liner  10  against the hole wall as rigid liner  12  exposes the hole wall. This mechanism is very useful to the emplacement of flexible liners inside unstable holes. As rigid liner  12  is lifted, water is added to the interior of flexible liner  10  via central tube  24  to compensate for the increasing volume  30  of flexible liner  10  as it dilates against the wall. The water level in annulus  38  is maintained as necessary. Often, rigid liner  12  and flexible liner  10  are filled near the top of rigid liner  12 . In practice, the top section of rigid liner  12  is removed as rigid liner  12  is raised. This allows continued access to the top of flexible liner  10  and central tube  24 . 
     The drag reduction of this technique can be more than a thousand-fold in rigid liners of 100 ft. lengths. 
     FIG. 4 shows the preferred rounded shape of the bottom edge  13  of rigid liner  12  to prevent scarring or cutting of flexible liner  10  as it dilates against the hole wall. If the differential pressure inside flexible liner  10  is plotted, it is essentially zero over most of the length of rigid liner  12 . However, at the bottom end  13  of rigid liner  12 , the differential pressure must be a continuous, monotonic distribution between zero, inside rigid liner  12  and annulus  34 , and the full differential pressure, outside rigid liner  12 . The length of this transition region has a direct effect on the drag that must be associated with any non-zero differential pressure. The longer the transition, the greater the drag. The shape of bottom  13  of rigid liner  12  can reduce the risk of damage, but it can also reduce the length of the transition by effecting a partial seal against leakage of the annular water out of rigid liner  12 . This procedure works best when there is less loss of the annular water  34  as rigid liner  12  is withdrawn. 
     FIGS. 5A and 5B are a side cross-section and a top cross-section, respectively, of an embodiment using a compressive sheath  40  for installation of flexible liner  10  inside a very small diameter rigid liner  12  (e.g., less than 2″i.d.). The emplacement of flexible liner  10  into the interior of rigid liner  12  is an essential part of the procedure of supporting the hole with a flexible liner or for other similar situations described under “Applications.” Compressive sheath  40  allows flexible liner  10  to be lowered or pushed inside rigid liner  12 . Compressive sheath  40  is a tubular sleeve of suitable flexible material (e.g., tubular plastic film or woven fabric). Sheath  40  is pulled over flexible liner  10  before flexible liner  10  is lowered into rigid liner  12 . Sheath  40  is typically sewn with a weak seam  42  so that after flexible liner  10  is placed in rigid liner  12 , sheath  40  is split by applying an internal fluid pressure within flexible liner  10  via central tube  24 . Seam  42  separates to liberate flexible liner  10 , which expands against the wall of hole  14 , forming a good anchor of flexible liner  10  in hole  14  as rigid liner  12  is withdrawn, as described for FIGS. 1-3. 
     FIG. 6 illustrates the application of the present invention to large diameter rigid liners  50 . Flexible liner  52  does not stand in rigid liner  50  without support as it does in a small diameter rigid liner. For a large diameter rigid liner  50 , a device consisting of a slender pipe  54  is lowered into flexible liner  52 . The top of flexible liner  52  is attached to the top of slender pipe  54 . Pipe  54  holds liner  52  upright in rigid liner  50  like a “tent pole”. In this case, no additional central tube is needed as pipe  54  serves as the conduit of fluid to the bottom of flexible liner  52 . A hole  58  at the bottom of pipe  54  allows the fluid to flow into the interior of flexible liner  52 . As rigid liner  52  is raised, the fluid must still be added to the interior of flexible liner  52 . 
     FIG. 7 shows the geometry of the flexible/rigid liner system when air is used as the driving fluid instead of a liquid. In this case, flexible liner  70  is pressurized with air, or any other suitable gas, to develop the desired supporting pressure desired of flexible liner  70  against the hole wall  72 . The air pressure develops a differential pressure and associated drag against rigid liner  74 . Cap  76  is placed on top of rigid liner  74  and air is injected  78  into the top end of rigid liner  74  at a pressure equal to the pressure in flexible liner  70  (or slightly higher). The injected air flows into annular space  80  between flexible liner  70  and rigid liner  74  wall to eliminate the differential pressure in flexible liner  70 . This allows rigid liner  74  to be lifted out of the hole and off of flexible liner  70  with relative ease and with limited drag on flexible liner  70 . In practice, the drag can be so small that rigid liner  74  is forced off flexible liner  70  by the pressure against the top end cap  76  of rigid liner  74 , much like a hydraulic cylinder and piston. 
     FIG. 8 shows the use of an impermeable covering  94  to further reduce the friction between flexible liner  90  and rigid liner  92 . The mechanism of the drag reduction is to shorten the transition length, discussed above, and to decrease the friction coefficient between flexible liner  90  and rigid liner  92 . Covering  94  prevents the annular fluid from leaking out of rigid liner  92  in the folds of flexible liner  90  (see FIG.  1 B). This reduces the length of the transition of the differential pressure from the full pressure to zero, because the permeability of the flow path out of the annulus is decreased by the covering. The reduced permeability steepens the gradient for a given pressure drop, hence the reduction of the transition length. In fact, if the covering is not sealed at the top of the flexible liner, the friction is not reduced. The transition length inferred from drag measurements is reduced to a couple of inches instead of the more common 2-3 ft. Another effect of the covering is to reduce the friction coefficient in the transition region, since cover  94  can be a material with a low coefficient of friction, such as a plastic film (nylon, polyethylene, Teflon). With covering  94  on flexible liner  90 , the measured drag was reduced a hundred-fold. This is in addition to the thousand-fold reduction due to the annular fluid addition described above. 
     The logical reverse of this method also has many applications. Instead of applying a fluid pressure to the annular space between a flexible and rigid liner, one can apply a vacuum. The effect is to increase the friction instead of reducing the friction. This can be used to anchor a flexible liner inside of a rigid liner such as a pipe. The drag friction can be much stronger than other kinds of connectors such as a vacuum coupling. Hence a quick and very strong connection can be effected to lift or pull extremely heavy loads. The highest friction can be obtained by a pressure in flexible liner  90  and a vacuum in rigid liner  92  (see FIG. 7 with a vacuum drawn through the end fitting  78 . 
     Examples of the application 
     The application for which this technique was invented was to allow a flexible liner to be inserted into a rod that has been pushed 50-100 ft. into the earth. The liner is inserted in a central hole in the rod, the rod raised, the water added to the interior and exterior of flexible liner and the rod is then removed in sections from the hole. The flexible liner dilates in the hole as it is exposed by the rod removal, but the flexible liner does not dilate significantly in the rod. Without annular water addition, the liner is torn off because of the friction in the rod. In the application to small diameter rigid liners, the flexible liner may be compressed with a sheath to allow it to be emplaced in the rod. 
     This mechanism is also useful for the emplacement of flexible liners with an outer covering that reacts to the presence of contaminants in the ground water. The flexible liner is again emplaced in the rod, the rod removed, the reaction occurs. Thereafter, the flexible liner is inverted from the hole so as to prevent the contact of the reactive covering with any other portion of the hole. On the surface, the reactive covering is surveyed for the location of subsurface deposits of solvents and other contaminants that react with the covering. 
     Larger liners can be emplaced in larger rigid liners, like sonic driven casing, a common drilling method. The rigid liner is driven to support the hole during the drilling procedure. The flexible liner is emplaced and the rigid liner removed. The flexible liner can be fitted with multi-level water sampling hardware and other instruments. The internal pressure of the flexible liner prevents hole wall collapse and assures a good seal against vertical water transport in the hole. This emplacement may require the use of a central support with the flexible liner. 
     A rigid liner can be lowered into the interior of a hollow stem auger drilling device. The flexible liner can be installed in the rigid liner inside the hollow stem of the auger. As the hollow stem auger is removed, the rigid liner can be raised with the auger, leaving the flexible liner in place below the auger to support the hole wall and to emplace a variety of devices. Using a low friction covering to drastically reduce the friction, and using air to provide the annular fluid, the rigid liner may actually rise off the flexible liner and press against the top end of the rising auger. This greatly reduces the complexity of installing flexible liners in hollow stem augered holes. 
     There are many other flexible liner installations possible with this technique where the flexible liner (e.g., a cure-in-place sewer liner) can be carried into position by a rigid liner inside a sewer or other pipe and the rigid emplacement liner can be removed by this means of eliminating the differential pressure of an internally pressurized liner. The cure-in-place liner can be pressurized to force the cure-in-place liner against the sewer pipe wall as the rigid liner is removed. The advantage of this approach is that the resin of a cure-in-place liner can be maintained, uniformly distributed in the matrix material until it is positioned against the wall of the passage to be relined or reinforced. 
     The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.

Summary:
A cylindrical cavity is continuously contacted while removing a rigid liner from within the cavity. The supporting apparatus includes a flexible liner insertable within the rigid liner, having an expanded diameter greater than a diameter of the cavity, and having a top end and closed bottom end. A hollow conduit is attached to the bottom end of the flexible liner and has an opening adjacent the bottom end for introducing a fluid to expand the flexible liner against the cavity and contact the cavity as the rigid liner is withdrawn from about the flexible liner.