Title:
METHOD AND APPARATUS FOR PROVIDING CONTINUOUS FOAM CIRCULATION IN WELLS
United States Patent 3559739


Abstract:
A method and apparatus is disclosed for maintaining continuous circulation of foam in a well through a segmented tubing string while the tubing string is being made up or broken up. A chamber having a foam entry port is formed around the tubing string above the wellhead. A valve is provided above the foam entry port to close off the upper portion of the chamber when the tubing string is broken and the upper portion thereof raised above such valve. When it is desired to add or remove a tubing section from the tubing string, the tubing string is held by slips with its open end in the lower portion of the chamber. The upper tubing section is lifted in the chamber to above the valve. The valve is closed and foam is circulated in the chamber through the foam entry port to provide for continuous foam circulation while another section of tubing is added or removed from the tubing string.



Inventors:
HUTCHISON STANLEY O
Application Number:
04/835268
Publication Date:
02/02/1971
Filing Date:
06/20/1969
Assignee:
CHEVRON RESEARCH CO.
Primary Class:
Other Classes:
166/81.1, 166/380, 175/69, 175/207
International Classes:
E21B19/16; E21B21/02; E21B21/14; E21B33/068; E21B37/00; (IPC1-7): E21B33/02; E21B21/00
Field of Search:
166/77,75,77.5,84,81,311,315,312 175
View Patent Images:
US Patent References:
3463231GENERATION AND USE OF FOAMED WELL CIRCULATION FLUIDSAugust 1969Hutchison et al.
3130798Method for drilling bore holesApril 1964Schramm et al.
2994379Bleed-off assembly for well pipesAugust 1961Brown et al.
2670225Lubricator deviceFebruary 1954McKinney
2522444Well fluid controlSeptember 1950Grable
2096882Fluid saving deviceOctober 1937Chernosky
1586923Well-drilling equipmentJune 1926Townsend



Other References:

vail et al., "New Developments in air-gas drilling and completions,"World Oil, Gulf Publishing Co., Houston, Texas, Reprint from Nov. and Dec., 1963. Copy in 175-69.
Primary Examiner:
Novosad, Stephen J.
Claims:
I claim

1. A method of providing continuous circulation of fluids in a segmented tubing string during makeup of such string comprising flowing fluid in or out of a well through a tubing string, forming a fluid-tight chamber about the upper end of said tubing string, providing a port in said chamber for flow of fluid in the same mode as the fluid flowing in said tubing string, opening said tubing string in said chamber, flowing fluid through said port and the open end of said tubing string to maintain circulation of fluid in said tubing string, adding another section of tubing to said tubing string and flowing fluid through said tubing string including the section added thereto.

2. A method of maintaining continuous circulation in a segmented tubing string and well annulus system comprising the steps of connecting a foam source to the upper end of a tubing string, injecting foam from said foam source down said tubing string and into a well, flowing foam up the well annulus between the tubing string and the well wall and exhausting the foam at the earth's surface, forming a foam-tight chamber about the upper end of said tubing string, injecting foam from said foam source into the lower portion of said chamber while continuing to inject foam from said foam source down said tubing string, positioning said tubing string so that the connection between said foam source and said tubing string is positioned in said chamber, hanging said tubing string below said connection in tension in the well, disconnecting said connection in said chamber to open the upper end of the tubing string and the lower end of said foam source in said chamber, raising the lower end of said foam source to near the upper part of said chamber, closing off said chamber to flow between the lower end of said foam source and the entry for foam into the lower portion of said chamber, stopping foam flow through the lower end of said foam source, removing the lower end of said foam source from said chamber while continuing to maintain foam flow into the lower portion of said chamber and into the open end of said tubing string for injection down said well, connecting a new section of tubing to the lower end of said foam source, inserting said newly connected section of tubing into said chamber, connecting said newly connected section of tubing with the open end of said tubing string in said chamber and injecting foam from said foam source through said tubing string into said well.

3. A method of maintaining continuous circulation in a segmented tubing string and well annulus system comprising the steps of injecting foam down a well annulus, flowing foam up a tubing string located in the well and exhausting the foam through a foam exhaust connected to such tubing string, forming a foam-tight chamber about the upper end of said tubing string, positioning said tubing string so that a connection between the foam exhaust and said tubing string is positioned in said chamber, hanging said tubing string below said connection in tension in the well, providing a port for exit of foam from said chamber, disconnecting said connection in said chamber to open the upper end of said tubing string in said chamber, raising the lower end of the foam exhaust to the upper portion of said chamber, closing off said chamber to flow between the lower end of said foam exhaust and the exit for foam from said chamber to cause foam from said tubing string to be exhausted from said chamber, through said port, removing the lower end of said foam exhaust from said chamber, connecting a new section of tubing to the lower end of said foam exhaust, inserting said newly connected section of tubing into said chamber, connecting said newly connected section of tubing with the opened end of said tubing string in said chamber and exhausting foam from said well through said newly connected section.

4. A method of refluxing foam in a well annulus formed between a well wall and a tubing string located therein comprising establishing a foam source at the earth's surface, forming a fluid-tight chamber about the upper end of a tubing string extending down a well providing a port in said chamber for flow of foam thereto, connecting said port with said foam source, connecting the well annulus with said foam source and alternately injecting foam down said tubing string and up said well annulus and injecting foam down said well annulus and up said tubing string.

5. Apparatus for use in a foam circulating system comprising a tubular shell of greater diameter than a tubing string to be used therewith, said tubular shell having an open upper end and an open lower end and a port for foam injection into the interior thereof or foam exhaust from the interior thereof, stripper rubber means in both the upper end and the lower end of said tubular shell, said stripper rubber means adapted to slidably engage tubing inserted through the ends of said tubular shell in a flow-tight manner to form with said shell a chamber about a tubing so inserted, valve means on said shell for closing off said shell to foam flow above said foam entry port and means for connecting said tubular shell in vertically spaced apart alignment with a wellhead.

6. Apparatus for use on a wellhead to provide continuous circulation in a tubing string extending into a well and the annulus formed between the exterior of the tubing string and the well casing comprising a tubing string, means for running said tubing string in a well, a foam source, conduit means for connecting said foam source to the upper end of said tubing string, a tubular shell arranged about the upper part of said tubing string and having a diameter greater than the diameter of said tubing string, said tubular shell having an open upper end and an open lower end, resilient stripper means fixedly connected in the ends of said tubular shell slidably engaged against the tubing string contained therein to cooperate with said tubing string to close off the ends of said tubular shell to form a closed annular chamber, means for flowing foam from said foam source into the lower portion of said annular chamber, valve means in said annular chamber above said foam entry means for closing off said annular chamber to longitudinal flow in the absence of a tubing section located adjacent thereto, a wellhead having tubing string hanging means and means for fixedly connecting said tubular shell to said wellhead in axially aligned, spaced apart relationship with the entry for the tubing string into said wellhead.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS

The present invention is related to the field of inventions in copending U.S. Pat. application Ser. No. 704,832, filed Feb. 12, 1968, now U.S. Pat. No. 3,463,231, Ser. No. 720,977, filed Apr. 12, 1968, now U.S. Pat. No. 3,486,560 and Ser. No. 807,766, filed Mar. 13, 1969, on all of which I am either an inventor or a coinventor.

BACKGROUND OF THE INVENTION

This invention relates to a method and apparatus for providing continuous foam circulation in a well during drilling or workover operations and, more particularly, this invention provides for maintaining continuous circulation in a segmented tubing string and the annulus between the tubing string and the well wall during makeup or breakup of the tubing string as it is lowered or raised in the well to accomplish well cleanout, drilling or the like.

Well cleanout, which includes the removal of drill chips, sand or other unconsolidated materials and fluids from a well has long been a problem. Thus the removal from a well of drilled solids produced while penetrating a formation with a drilling string and also the removal of the solids which flow into the well during oil production have both continued to be a problem to the oil industry. This is particularly true when formations penetrated during drilling or from which fluids are produced, are unconsolidated low-pressure formations. Various techniques, including the use of wire line bailers and various circulating fluids have heretofore been suggested for removing such material from wells.

One such circulating fluid found successful in such operations is a stable, low density foam. Foam of this type has proven quite successful in removing solids and fluids from wells. However, it has been found that certain techniques must be utilized to overcome problems encountered in the use of foam as a circulating fluid for removing sand and the like from a well. One such technique is disclosed and claimed in the above noted application Ser. No. 807,766. In this technique, the foam in the tubing string is maintained for a period of time in a dynamic condition by certain one-way valve means provided in the tubing string. This method has proven successful. However, it has also been found that where delays may be encountered in changing tubing or if downtime is likely to occur it is desirable to provide for continuous circulation of foam during makeup or breakup of the tubing string.

One prior art technique for providing continuous circulation is disclosed in an article entitled "New Developments in Air-Gas Drilling and Completions" in WORLD OIL, Nov.--Dec. 1963. The procedure disclosed in this article used a special sub in each tubing joint having a hinged disc to provide for continuous circulation during tubing string makeup. The obvious problem with this procedure is that a special sub is required in each tubing section to obtain circulation.

The reason for the need for such special techniques required in the application of foam to wells will be better understood when it is realized that in a typical well cleanout operation utilizing foam as the circulating medium, a segmented tubing string is used to inject foam down a well to the position in the well from where it is desired to remove sand and/or produced fluids. The foam picks up the sand or fluids and carries them out of the well via the tubing-well wall annulus. A tubing string is made up of a plurality of individual pipe sections coupled together. A rig or hoist is used to lower each successive pipe section into the well to approximately its full length. After the pipe section has been lowered to such a position, it is gripped near its upper end by various means such as slips to suspend it in the well. The hoist is then released from the pipe section and a second pipe section is connected to the first section. The hoist is then connected to the second pipe section and the slips released from the hanging tubing string and it is lowered further into the well to near the full extent of the second pipe section when the slips are again applied and the above sequence repeated. As this sequence is repeated, successive pipe sections form a tubing string which progressively goes deeper and deeper into the well. Once the lower end of the tubing string has reached a level from where it is desired to remove sand or liquid a flexible tube is connected onto the top pipe section and foam is injected into the tubing string and circulated down the string out the bottom end and up the annulus between the tubing string and the well wall or well casing to the surface. As foam is circulated, it carries up the fluids and/or sand and chips from the well. The tubing string during this foaming period is progressively lowered deeper into the well. This necessitates, of course, adding additional pipe sections to the tubing string at the wellhead. When such pipe sections are added, the flexible foam tube must be disconnected from the top pipe section forming the uppermost joint of the tubing string and reconnected to the top of the newly connected pipe section. Thus for a short time foam circulation down the tubing is stopped. It has been found necessary, however, to prevent backflow of foam up the tubing string and consequent collapse of foam in the tubing string-casing annulus. If foam is allowed to backflow up the tubing, uncomfortable and dangerous conditions will exist at the wellhead. Even more seriously, if foam is allowed to collapse in the annulus, solids will settle out of the foam and bridge against the tubing string causing it to become stuck in the well. As noted above, the special techniques directed at preventing this have not been entirely satisfactory. The present invention is directed to solving these problems.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for continuously circulating foam in a well through a segmented tubing string while the tubing string is being made up or broken up. The invention permits maintaining continuous circulation during such makeup or breakup--normal circulation down the tubing string and up the well annulus or reverse circulation down the well annulus and up the tubing string. A foam source is provided with a conduit connecting it to a tubing string through an appropriate coupling known as a pup joint. A chamber is formed around the tubing string above the wellhead and is provided with a port through which foam can be injected into the chamber or, if circulation is reversed, from which foam can be removed from the chamber. The ends of the chamber permit sliding movement of the tubing string therethrough while preventing escape of foam from the chamber. The joint that is desired to disconnect is opened in the chamber while in the case of normal circulation foam is also injected into the chamber through the port provided for such purpose and then into the open end of the tubing string and then down the tubing string. For reverse circulation operation the port is opened to provide flow of foam from the chamber. The pup joint is lifted up the chamber above the valve means and the lower half of the chamber is then isolated. In reverse circulation operation the foam is flowed out of the chamber through the port until the new tubing section is connected. In normal circulation foam injection is stopped from said foam source through the pup joint and the pup joint is removed from the chamber. Meanwhile foam is continuously circulated through the foam entry port into the chamber and then into the open end of the tubing string and down the string into the well. A new section of tubing is connected to the pup joint and is inserted into the upper half of the chamber and foam is injected therein. The valve means is then opened to open the chamber and the new tubing is inserted and connected to the tubing string. After such connection is made, foam injection through the port in the chamber is stopped, the slips are released from the tubing and the tubing string is run further into the hole.

BRIEF DESCRIPTION OF THE INVENTION

Briefly, the present invention provides for maintaining continuous foam circulation while connecting or disconnecting a pipe section to the tubing string. A chamber is formed above the wellhead around the tubing string and the ends of the chamber are provided with resilient means which allow passage of the tubing string in both an up and down direction therethrough. The resilient means, however, frictionally engage against the tubing in a manner to prevent flow of foam from the chamber. The chamber is provided with a foam port and a valve is provided for closing off the chamber to flow above such port. The connection to be broken is positioned in the chamber below such valve and foam is injected through the foam entry port into the chamber at least the injection pressure of the foam circulating down the tubing. The connection is then broken by suitable means such as a power swivel or power tongs, and the top section of pipe is raised up to above the valve. The valve is closed to close off the upper portion of the chamber and foam injection through the top section of pipe is stopped. This top section may then be removed from the chamber and a new pipe section added thereto. Meanwhile foam is continuously circulated from the chamber down the tubing. As noted above, if reverse circulation is desired the foam leaves the chamber through the entry port while the tubing section is being made up. When the new section of pipe is connected to the top section, its lower end is inserted into the chamber and foam is injected therethrough. The valve is opened to open the chamber and the new pipe section is lowered and connected into the tubing string. After such connection is made, foam injection into the chamber is stopped and the tubing string is lowered further into the hole while foam injection continues down the tubing.

The apparatus of the present invention provides continuous circulation through a tubing string extending into a well and the annulus formed between the exterior of the tubing string and the well casing during makeup of the tubing string. The apparatus includes a tubular shell having a diameter greater than the diameter of tubing run in the well. The tubular shell has an open upper end and an open lower end. Resilient stripper means are fixedly connected in the ends of the tubular shell to slidably engage against the tubing string contained therein to cooperate with said tubing to close off the ends of said tubular shell to form a closed annular chamber. Means for flowing foam into or out of the lower portion of the annular chamber are provided. Gate valve means are located in the annular chamber above the foam entry means for closing off the annular chamber to longitudinal flow in the absence of tubing therein. A wellhead having tubing hanging means and means for fixedly connecting the tubular shell to the wellhead in axially aligned, spaced apart relationship with the entry for tubing into said wellhead are also provided in accordance with the invention.

OBJECTS OF THE INVENTION

A particular object of the present invention is to provide for continuous circulation of foam in a well during foam cleanout or the like while a tubing string is being made up or broken up. Further objects and advantages of the present invention will become apparent from the following detailed description read in light of the accompanying drawings which are made a part of this specification.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view, with parts broken away for clarity of presentation, of the preferred embodiment of apparatus assembled in accordance with the invention in which foam is injected through a tubing string into a well, and FIG. 2 is a view, with parts broken away for clarity of presentation, of the preferred embodiment of apparatus in which foam is injected into the foam chamber and thence into the tubing string while a new section of tubing is being added.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Refer now to the drawings and to FIG. 1 in particular where the preferred embodiment of apparatus assembled in accordance with the present invention is illustrated. The preferred embodiment is described with regard to maintaining normal circulation in the well. As noted, normal circulation exists when fluid is injected down the tubing string and up the well annulus. As shown, foam is being injected from a foam source 18 down a string of tubing 20 to a location in a well indicated generally by a number 22 from where it is desired to remove produced fluids and/or solids. This might be desirable, for example, in a well cleanout operation. The circulating medium such as foam is circulated down the segmented tubing string 20 and up the well annulus 24 formed between the outside of the tubing string 20 and the well casing 26. The foam carries the solids and fluids up the well annulus and out blooie line 28 to a suitable sump (not shown) at the surface. A valve 30 is located on the blooie line 28 and is normally left open.

The wellhead 32 includes a suitable tubing head 34 having a collar 36 which contains a suitable fitting 38 for the blooie line and a fitting 40 for an emergency foam line 42 which is connected through valve 44. The emergency foam line is located below the wellhead equipment and is intended for emergency use such as might be caused by a downhole fire. A wellhead base plate 46 having a central opening for the tubing string therethrough is connected to the top of the collar 36. A stripper rubber 48 is located in the collar 36 to frictionally engage the tubing string therein in a liquid-tight manner to prevent flow of foam through the wellhead collar past the tubing string. Tubing hanging slips 50 are arranged around the tubing string 20 above the base plate 46. The tubing slips are used to hang the tubing on the wellhead while it is disconnected from the pup joint 70 and the power swivel. A chamber is formed about the tubing string 20 above the wellhead by a tubular shell 52 which is larger in diameter than the tubing string 20. The shell 52 is positioned around the tubing string and located above the slips 50 on the wellhead. The tubular shell 52 thus forms a chamber around the tubing string above the wellhead. The ends 53 and 55 of the tubular shell 52 contain stripper rubbers 54 and 56 which slidably engage the tubing string so as to prevent foam from leaving the chamber by passing along the outside of the tubing string. Thus a fluid-tight chamber is formed about the tubing string by the cooperation of the shell and the sealed ends. A foam entry is provided into the chamber by port 58 in the shell 52. The entry port 58 is located near the lower end portion of the tubular shell 52. A valve indicated generally by the number 60 is arranged intermediate the ends of the tubular shell 52 and has the capability of closing off flow up the chamber in the absence of the tubing string. The chamber may thus be divided into two separate compartments. The foam entry port 58 is connected to the foam source 18 by means of a suitable conduit 64. Valve 66 controls flow of foam from the foam source 18 through line 64 to foam entry port 58. By controlling the valves foam may be injected as desired into the chamber formed about the tubing string.

The wellhead 32 and the tubular shell 52 which forms the chamber about the tubing string are connected in vertically aligned, spaced apart relationship by suitable means such as a plurality of centralizing and supporting arms 68. The tubular shell 52 must be securely anchored in position while foam is injected therein under high pressure. The centralizing and supporting arms 68 serve to align and to support the tubular shell 52 in such a manner by connecting the supporting arms 68 to the wellhead. In this mode the weight of the tubing string 20 hanging from the wellhead serves as an anchor for the tubular shell 52. It is usually preferred to have three spaced apart supporting arms 68 to connect the tubular shell to the wellhead.

In accordance with the invention foam is provided for injection down tubing string 20 by a suitable foam source 18. The foam source 18 is connected to a pup joint 70 located in power swivel 72 by means of a suitable flexible conduit 74. Valve 75 controls foam flow through conduit 74. A bleed down line 76 having valve 78 is provided on the conduit 74. In the embodiment shown in FIG. 1 the foam in injected from the foam source 18 into the tubing string 20 down the well and up the annulus to the surface. As noted above, the tubing string 20 is progressively lowered into the well as the cleanout goes on to a point where the upper end of the tubing string 20 which is connected to the pup joint 70 at connection 80 is located in the lower portion of tubular shell 52. Obviously this connection 80 was at one time located well above the upper end 53 of the shell 52 and as the tubing string 20 was lowered by lowering the power swivel 72 the connection 80 was lowered to its present position. When the connection 80 has reached such a position, it is time to disconnect the pup joint 70 from the tubing string and to add a new joint of tubing to the up pup joint 70 for connection to the top of the tubing string 20. As is evident, however, foam injection through the pup joint and thence into the tubing string 20 during this period must be stopped. Utilizing the apparatus of the present invention, however, foam injection through the tubing string 20 need not be halted during this period.

As indicated above, the connection 80 is now located in the shell 80 in a position where it should be broken. However, before breaking the connection 70 foam under pressure must be supplied to the chamber formed by shell 52 about the upper end of the tubing string. To accomplish this the bleed down valve 82 on the alternate foam conduit 64 is closed. Then alternate foam valve 66 is opened to cause foam to flow from foam source 18 through conduit 64 and entry port 58 into the chamber formed by shell 58 about the tubing string 20. Bleed valve 84 on chamber bleed line 86 is also closed at this time. The tubing hang slips 50 are set so as to be ready to hang the tubing string 20 on the wellhead when it is disconnected from the power swivel and hoist. The connection 80 between the upper end of tubing string 20 and the lower end of the pup joint 70 is now broken by suitable means such as reversing the power swivel to rotate the pup joint and unscrew the connection. The power swivel 72 is then raised by a hoist means to lift the bottom of the pup joint 70 up the chamber to a position above gate valve 60. This position is illustrated in FIG. 2. The end of the pup joint 70 is maintained in the chamber between gate valve 60 and the stripper rubber 54 in the upper end of tubular shell 52. Valve 60 is now closed as indicated in FIG. 2 to close off the upper part of the chamber to the flow of foam. Foam circulation in the well is maintained through the upper end of the tubing string 20 by the foam entering shell 52 through line 64. At this time valve 75 is closed to stop foam flow in the main foam line and pressure is bled from the pup joint system 70 through lines 76 and 86 by opening valves 78 and 84. The dual bleed lines give additional assurance that complete bleed down will occur. The pup joint 70 is now removed from the chamber formed by shell 52 and a new joint of tubing is connected to the lower end of the pup joint. The lower end of this new joint of tubing is then inserted through the upper stripper rubber 54 into the upper portion of the chamber formed by shell 52. Bleed valves 78 and 84 are closed and main foam valve 75 is opened to inject foam into the upper portion of the chamber to equalized the pressure across gate valve 60. After the pressure has been so equalized, gate valve 60, opened and the new lower end of the tubing joint is lowered to contact the open upper end of the tubing string 20 and the power swivel is rotated to connect the new tubing joint into the tubing string. As this connection is made foam again flows down the tubing string 20 directly from the foam source. At this time foam injection through the alternate foam injection line 64 is stopped by closing valve 66. Pressure is bled from the alternate foam conduit by opening valve 82. The slips 50 are now released and the tubing string 20 is run another joint down into the well to continue circulating undesired debris therefrom.

The apparatus of the present invention is also effective in providing continuous circulation when the mode of operation requires reverse circulation. Reverse circulation requires that foam be injected down the well annulus and up the tubing. Thus with respect to FIGS. 1 and 2 foam is injected in line 42 through valve 44 and down well annulus 24 to the lower end 22 of tubing string 20. The foam then goes up tubing string 20 through pup joint 70 and is exhausted through conduit 74 which serves as the blooie line. Obviously the foam source 18 and the valving therefore would be connected to line 42 and the blooie line 28 for normal circulation would be closed. When it is desired to add a section of tubing to the tubing string the connection 80 which is to be broken is positioned as shown in the lower portion of shell 52 and the tubing slips 50 are set to hang the tubing string 20 on the wellhead 32. Valve 82 is opened and the connection 80 is then broken and the pup joint lifted above the gate valve 60. When gate valve 60 is closed, foam is exhausted from shell 52 via line 64. A new tubing section is added to the pup joint and it is inserted back into the top of shell 52. The gate valve is opened and the new tubing section is connected into the tubing string and operations are resumed.

As is also evident, circulation may be sequentially changed or refluxed from normal to reversed while the open tubing string is being held in the lower portion of the shell 52 by the tubing slips. Thus by the manipulation of the exhaust and injection valves circulation may be refluxed. Refluxing of the foam is useful for many purposes. It is particularly useful when cleaning a well liner with a cleaning fluid such as solvent, acid or the like. A slug of such a cleaning fluid is positioned in the well adjacent a liner and a column of foam is established in both the annulus and the tubing on both sides of the slug. The tubing string 20 is hung in the shell 52 forming the closed chamber as shown in FIG. 2. By alternatively injecting foam down the tubing string 20 and bleeding it from the well annulus 24 and injecting foam down the annulus 24 and bleeding it from the tubing string 20, the cleaning slug is reciprocated up and down the well liner to provide a more thorough cleaning action. This operation is accomplished by injecting foam from the foam source 18 through valve 66 and line 64 into the shell 52 and then down the open tubing string 20 to the annulus 24. The foam in the annulus 24 above the cleaning slug is removed from the annulus through blooie line 28. When it is desired to reflux the cleaning slug valve 30 on conduit 28 is closed as is valve 66 on the conduit from the foam source to the shell. Valve 82 is opened to provide a vent to atmosphere through conduit 64. Valve 44 on conduit 42 which is connected to the foam source is opened to cause foam to be injected down the well annulus 24 to reflux the cleaning slug while the foam which moves up the tubing string 20 is bled from shell 52 through conduit 64 and valve 82.

While preferred embodiments of the invention have been shown and described, it is evident that modifications thereof can be made by one skilled in the art without departing from the scope of the invention.