Patent Publication Number: US-2009223661-A1

Title: Split non-welded casing cap for high temperature service

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from U.S. Provisional Patent Application No. 61/068,482 filed Mar. 7, 2008, which is incorporated by reference in its entirety herein to the extent that there is no inconsistency with the present disclosure. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a casing cap for use in high temperature wellhead applications. The invention also extends to a method of making the casing cap connection to inner and outer casings. 
     The typical fashion in which a well is drilled in the ground, for example for oil and gas, is to first drive or drill a shallow large diameter pipe, commonly called the conductor pipe or casing, into the ground, and to then drill a smaller and deeper hole inside the boundary defined by the conductor casing so that a smaller diameter and longer pipe, commonly called the surface casing, can be placed into the hole. The annular space between the surface casing and the conductor pipe is then filled with cement. If the well is of sufficient depth or due to geological requirements, multiple strings of casing may be required. Each casing string will be cemented in place. Further drilling beyond the depth of the surface casing is done to a sufficient depth that geological formations encountered may cause pressurized fluid to escape into the hole and travel to the surface. To control this fluid, and to prevent its escape into the atmosphere, the drilling is done through a sealed pressure vessel at the surface wellhead that is known as the blowout preventer stack. In addition, drilling at these depths requires the use of a weighted column of fluid, known as drilling mud, to control the well, to aid drilling by cooling the drilling bit, and to remove cut rock. A pressure vessel known as the casing head, attaches to and seals around the surface casing or production casing to provide a means for hooking up the blowout preventer stack and the drilling mud lines located thereabove. 
     A casing cap is sometimes needed to seal off the annulus formed between the conductor casing and the surface casing or between the surface casing and the production casing. In thermal applications the casing string strings are cemented to surface to limit the thermal growth of the casing. Due to the large temperature changes the well can be exposed to (650° F. to −50° F.), the inner and outer casing strings will be subject to differential thermal expansion and contraction. The casing cap ideally does not lock the two casing strings together constraining this movement as this differential expansion and contraction can induce large stresses on the casing and the casing cap. These large temperature changes and the movement of the casing can compromise the cement in the annulus between the casing strings. If the pressure integrity of the cement is compromised the pressure in the formation can escape to the surface where the casing cap is installed to control the pressure. Ideally this pressure is vented out a port in the casing cap in a controlled manner. 
     Once the casing head or wellhead is already in place on the surface casing and/or production casing, a conventional casing cap can no longer be installed. In this circumstance, a split type casing cap design is required, where the casing cap components are split in half with weld preparations to allow for installation of the halves around the surface and/or production casing, below the existing casing head. However, welding is not always convenient or permissible, for example on a remote wellhead where welding expertise is not available, or on a live well where welding is not recommended or safe. In Alberta, Canada a process known as Steam Assisted Gravity Drainage (SAGD) requires high thermal energy input through the wellhead to a heavy oil formation. Particularly when the well is in production and the cement (to the surface casing) may fail, split casing caps may need to be installed. Welding in these SAGD producing wells or steam injecting wells is not safe, so a split casing cap that can be installed without welding is desirable. 
     Thus, there is a need for a casing cap of a split type design for use in applications where the casing head or wellhead is already in place, and where welding is not recommended or available. 
     SUMMARY OF THE INVENTION 
     The split type casing cap of this invention provides a pressure barrier between an outer and an inner casing (ex. between a conductor casing and a surface casing, or between a surface casing and a production casing), preferably without the use of welding. The casing cap may be adapted for concentric as well as eccentric casing applications. The casing cap is used in applications where the wellhead is already in place and where pressure control between the casings is required. The preferred embodiments have particular application where elastomer seals may not be used due to elevated operating temperature (although the casing cap also works with elastomer seals). 
     Broadly stated, the invention provides a casing cap to seal the casing annulus formed between an upper end of an outer casing and an inner casing extending through and above the outer casing. The casing cap includes a generally annular casing cap housing formed in two or more split portions which when joined at their splits form the casing cap housing to cover and seal the casing annulus. The joined casing cap split portions form a side wall having an upper and a lower end. The upper end forms an annular top section adapted to be supported by the outer casing, to cover the casing annulus and to form an upper annular seal to the inner casing. The lower end is adapted to form a mechanical connection to the outer casing and to form a lower annular seal to the outer casing. Each casing cap split portion forms a sealing surface at each split for sealing together to form the annular casing cap housing. A sealing element is included for placement between opposing sealing surfaces. Connectors between the casing cap split portions clamp together the sealing surfaces of the casing cap split portions with the sealing element to form pressure-containing, non-welded seals at the splits. 
     Preferably, the casing cap housing is formed in two split half portions with one or more sealing surfaces being formed at each left and right split of each casing cap split half portion. The sealing element is preferably a gasket seal or a ring seal adapted for sealing when clamped between the sealing surfaces, or a sealing compound such as epoxy or gasket compounds adapted for sealing when applied and clamped between the sealing surfaces. Preferred connectors used with sealing compounds are external clamp connectors provided at the top and side walls of the housing at the splits. Preferred connectors used with gasket seals are screw connectors extending through the side walls and the sealing surfaces. Particularly preferred are high temperature gasket seals formed from graphite sealing materials. 
     The casing cap housing preferably forms annular compression seals to the inner and outer casings which accommodate relative axial movement of the casings, for example with thermal expansion and contraction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded top perspective view of the components of the casing cap of the present invention, showing the components in horizontal alignment for installation on an outer conductor casing and an inner surface casing. 
         FIG. 2  is a top perspective view showing the split casing cap housing halves joined around the conductor and surface casings, with split surfaces mated with epoxy and bolting steps in progress. 
         FIGS. 3A and 3B  show side by side top and bottom perspective views showing the top and bottom split packing glands and compression packings vertically aligned for sealing the casing cap to the surface and conductor casings respectively. 
         FIGS. 4A and 4B  show side by side top and bottom perspective views of the completed casing cap, showing the top and bottom packing glands bolted in place to seal the surface and conductor casings respectively. 
         FIG. 5  is a top sectional view taken above the casing cap, showing the split casing cap housing halves bolted together and showing the top packing gland bolted in place. 
         FIG. 6  is a side sectional view taken along line A-A of  FIG. 5 , showing the casing cap and top and bottom packing glands bolted in place, and showing the casing cap retained on the outer conductor casing by a split retainer ring. 
         FIG. 7  is a partial side view of a preferred external clamping arrangement for the split casing cap housing halves, showing the clamping plate formed with bottom ridges which are received in V-grooves on the top section of the casing cap in order to transfer the vertical bolting force to a horizontal clamping force. 
         FIG. 8  is an exploded top perspective view of a second embodiment of the components of the casing cap, showing the components in horizontal alignment for installation on an inner and outer casing, with retention cap screws at the upper end of the outer casing replacing the retention ring of  FIGS. 1-7  for mechanical connection to the outer casing, and with a gasket seal at the split cap housing halves. 
         FIGS. 9A and 9B  show side by side top and bottom perspective views of the embodiment of  FIG. 8 , showing the top and bottom split packing glands and packings vertically aligned for sealing the casing cap to the inner and outer respectively. 
         FIGS. 10A and 10B  show side by side top and bottom perspective views of the completed casing cap of  FIG. 8 , showing the top and bottom packing glands bolted in place to seal the inner and outer casings respectively. 
         FIG. 11  is a top sectional view taken above the casing cap of  FIG. 8 , showing the split casing cap housing halves bolted together and showing the top packing gland bolted in place. 
         FIG. 12  is a side sectional view taken along line A-A of  FIG. 11 , showing the casing cap with the top and bottom packing glands and retainer rings bolted in place, and showing the split gasket seal connection at the sealing surfaces. 
         FIG. 13  is a side sectional view taken along line B-B of  FIG. 11 , showing the casing cap with top and bottom packing glands and retainer rings bolted in place, and showing the casing cap retained on the outer conductor casing by the retention cap screws. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention has application in sealing the casing annulus between concentric or eccentric casings at a wellhead where the inner casing is already connected to the upper wellhead components, such as a casing head. Generally, the casing cap is used to seal the casing annulus between inner and outer casings such as an outer conductor casing and a surface casing (as shown in  FIGS. 1-13 ), or between an outer surface casing and an inner production casing. The term “casing” as used herein and in the claims is meant to include any casing, tubing, pipe or other similar device located at the wellhead and known to persons skilled in the art. Thus the invention has application where a casing annulus between an inner and outer casing needs to be sealed at the upper terminated end of the outer casing. While the inner and outer casings will generally be vertical at a conventional wellhead, the casings might deviate from the vertical as in the case of directionally drilled wells where the wells are angled from vertical toward the horizontal. 
     In the Figures, two exemplary embodiments of the invention are shown.  FIGS. 1-7  show an embodiment where the casing cap is retained on the outer casing by a retention ring and where epoxy is used as a sealing element to form a seal at the split sealing surfaces.  FIGS. 8-13  showing an embodiment where the casing cap is retained on the outer casing by a plurality of cap screws fastened around the upper end of the outer casing, and where a sealing element in the form of a gasket seal is used to form a seal at the split surfaces. Both embodiments show the casing cap and other components formed in split halves (left and right half portions). It will be recognized that a different plurality of split portions is possible, but split half portions is most preferred to minimize the number of sealing surfaces and thus the complexity of the overall casing cap. In the Figures, like components are labeled with the same reference numerals. It will further be recognized that alternate retention devices might be used with the outer casing, for example friction type connectors such as slip lock connectors. Exemplary slip lock connectors to a casing are shown in U.S. Pat. No. 6,834,718 issued Dec. 28, 2004 and U.S. Pat. No. 7,069,987 issued Jul. 4, 2006 to Kwasniewski et al., both owned by Stream-Flo Industries Ltd. (assignee for this patent application). The retention device to the outer casing might not be needed as a separate component in the event that a collar or similar limiting device is already in place on the outer casing at or near its upper terminated end. 
     Having reference to  FIGS. 1-7 , a first preferred embodiment of the split type casing cap of this invention is shown generally at  10 .  FIG. 1  shows the components in horizontal alignment for assembly.  FIGS. 2 ,  3  show components during assembly.  FIGS. 4-7  show the assembled casing cap  10 . Where components of the casing cap  10  are provided in split halves, the halves are identified in the figures by the same reference numerals, for assembly in left and right mating relationship around the outer conductor casing  12  and inner surface casing  14 . A pressure-containing casing head  15  is shown connected to the top of the inner surface casing  14  in  FIGS. 1-4 . The top flange  15   a  of the casing head  15  connects to the wellhead members (not shown) located thereabove, as is known in the art. The casings  12 ,  14  are shown as concentric in the Figures, but the present invention can accommodate eccentric casings as well, in which case measurements are taken to determine the eccentricity, and the casing cap dimensions are customized for the off-centre casing annulus. The casing cap  10  is adapted and sized to cover the annulus A formed between the casings  12 ,  14  at the upper end  27  of the outer casing  12 , and to seal to each of the outer surfaces of the casings  12 ,  14 . 
     The casing cap  10  is formed in split housing portions  16 , preferably two generally symmetrical left and right mating members ( 16   a ,  16   b ) which are connected together at vertical sealing surfaces  17  (preferably flat sealing surfaces or shaped for mating relationship) around the casings  12 ,  14  to form a completed annular cap housing  18 . Each housing portion (left housing half  16   a  and right housing half  16   b ) forms two split faces which, when joined in mating relationship, form left and right hand splits (shown as S in the Figures). The sealing surfaces  17  may comprise the entire area of the split faces (as in the Figures, where  17  represents both the split face and the sealing surface), or only a portion of the area of the split faces. As well, multiple sealing surfaces may be formed on a split face with mating multiple sealing surfaces being formed on the opposing split face. Each split housing portion  16  includes a vertical side wall  19  which surrounds the outer casing  12 , and an annular top section  20  which extends horizontally inwardly from the upper end  21  of the side wall  19  toward the inner casing  14  to cover and close the casing annulus A. An upper annular seal  22  (see  FIG. 6 ) is formed by the top section  20  of the cap housing  18  at the inner casing  14 . This seal  22  is preferably an annular compression seal such as a stuffing box type seal that allows for thermal expansion and contraction of the inner casing  14 , as occurs during high temperature applications. Although in some applications welding might be used to attach the casing cap  10  to the outer casing  12 , if welding is not feasible, it is generally preferable to provide a mechanical connection at the lower end  24  of the side wall  19  in order to retain the casing cap  10  on the outer casing  12 . It is also preferable to form an annular seal  26  at the lower end  24  to the outer casing  12 . The seal  26  is most preferably a lower annular compression seal. In alternate embodiments, the lower end  24  of the casing cap  10  may seal and connect to the inside surface of the outer casing  12 , in a manner known to persons skilled in the art. 
     The casing cap  10  is shown resting on the upper end  27  of the outer casing  12 , although the method of attachment to the outer casing  12  so as to resist upward movement of the casing cap, may vary using techniques generally known in the art. In the Figures, the top section  20  of the housing is shown resting on this upper end  27  of the casing  12 . The mechanical connection to the outer casing  12  is shown in two embodiments in the Figures, in which an outwardly extending circumferential extension or limit device is attached to the outer casing  12  to anchor the casing cap  10  to the outer casing  12 . Alternatively, the casing cap  10  may attach and seal on the inside surface of the outer casing  12 , in which case the mechanical connection to the outer casing  12  includes an inwardly extending circumferential extension or limiting device. 
     The preferred embodiments of the mechanical connection to the outer casing  12  are best seen in cross sectional detail in  FIG. 6  and  FIG. 13 . In  FIG. 6 , a split retention ring  28  is fastened with retention cap screws  30  and bolts  31  extending through drill holes  32  formed around the upper end  27  of the outer casing  12 . This retention ring  28  thus provides an outwardly extending circumferential extension to the outer casing  12  to act as an anchor for the casing cap  10  to resist upward movement. The side wall  19  is formed with a C-channel recess  34  (or alternate shaped recess) to accommodate the retention ring  28 . This C-channel recess  34  with the retention ring  28  provides a lower end anchor which mechanically connects the lower end of the casing cap to the outer casing to resist upward movement. In  FIG. 13  the retention ring  28  is not present, and the heads  35  of the retention cap screws  30  provide the outwardly extending circumferential extension to the outer casing  12 , sufficient to function with the C-channel  34  as the lower end anchor. The C-channel  34  is preferably discontinuous at the split sealing surfaces  17  (see section at the split surfaces in  FIG. 12 ) to improve the seal at the splits. Alternatives to the C-channel recess  34  will be evident to one skilled in the art, for example an inwardly projecting bottom lip might be formed at the lower end  24  of the side wall  19 . Alternatives to the retention ring  28  or retention cap screws  30  will be evident to one skilled in the art. For instance, in applications where welding is permitted, a ring, collar or stop lugs might be welded to the outer casing  12 . In some instances a casing collar or lip might already be present on the outer casing  12 , which might be used to anchor and attach the lower end  24  of the casing cap  10  to the outer casing  12 . Still alternatively, the casing cap housing might accommodate a friction type mechanical connection such as a slip-lock connector (not shown) its lower end  24  to attach to the outer casing, as is known in the art with other wellhead members. A friction type connection such as a slip lock connector is particularly advantageous in applications where neither welding nor machining (such as drilling holes) is permitted, such as exist with flammable gas emissions. 
     In the embodiment of  FIGS. 1-7 , the facing sealing surfaces  17  of the split housing portions  16  are sealed together with the sealing element being a sealing compound such as an epoxy or gasket type compound. A metal high temperature epoxy compound is preferred. The sealing surfaces  17  are then clamped together at the left and right splits S using an external clamping system (i.e., clamps integral with, connected to, or separate from the cap housing which apply a horizontal clamping force to clamp together the half portions  16  at their splits). Epoxy is a binary (two component) bonding agent that is inert in its unmixed state, but which hardens when mixed. Epoxy is applied on at least a portion of the sealing surfaces  17  at the splits S of the casing cap half housings  16  and cures once the half portions  16  are clamped together. The casing cap housing portions  16  are formed with vertical compression plates  36  carried by the outside surface of the side wall  19  proximate to, but stepped back from, each of the sealing surfaces  17 . The plates  36  are located to be parallel facing in spaced apart relationship when the casing cap housing portions  16  are joined. Aligned bolt holes  37  are formed in opposite plates  36 , to receive stud and nut connectors  38  to bolt the casing cap housing portions  16  together. Preferably, a clamping plate  40  is bolted into the top section  20  over the joint or split S of the casing cap housing portions  16 , with stud and bolt connectors  42  being used on each side of the adjoining casing cap housing portions  16 . Generally V-shaped ridges  44  are formed on the bottom of the clamping plate  40  to be received in V-shaped radial grooves  46  formed in the top surface of the top section  20  adjacent each of the sealing surfaces  17 . The staggered relationship of the V-angled grooves  46  (relative to the compression plates  36 ) allow the forces applied by vertical bolting down the clamping plate  40  to act perpendicular to the direction of the split, applying horizontal clamping force across the split S. For large differences in casing diameters, additional clamping devices can be used on the top section  20 . 
     In the embodiment of  FIGS. 8-13 , the facing sealing surfaces  17  of the split housing portions  16   a ,  16   b  are sealed together with gaskets  50 , such as high temperature graphite gaskets used as the sealing element. The gasket is placed at split covering at least a portion of the sealing surfaces  17 . The left and right splits S are connected with a screw connection system in which threaded type connectors extend through the side wall  19  and through the gasket  50  and sealing surfaces  17  at the splits S in a manner that applies a horizontal clamping force to clamp together the half portions  16  with the sealing element at the splits. The graphite gaskets  50  may be formed from one or more expanded graphite sheets cut to cover at least a portion of the sealing surfaces  17 . The gaskets  50  are held in place for assembly purposes by cap screws  52  received in tapped holes  54  formed on the surfaces  17  of housing portion  16   b , with matching drilled holes being formed on the opposing face surfaces  17  of housing portion  16   a  to accept the heads of the cap screws  52 . An exemplary and preferred graphite gasket is SIGRAFLEX™ BSSC (a graphite gasket with a stainless steel inner sheet made by SGL Group, Germany), but other high temperature gasket sheet or foil materials might be used. Depending on the application, alternate sealing elements may include other gasket seals such as graphoil seals, ring seals such as elastomeric O-rings, sealing compounds such as epoxy or gasket compounds (as shown above), or thermoplastic Teflon™ type seals. The cap housing portions  16  are clamped together using vertically aligned cap screws  56  (six shown) extending through recessed holes  58  formed on either side of the left half housing portion  16   a  ( FIG. 8 ) proximate the outer peripheries of the side wall  19  and received in mating threaded holes  60  formed in the facing surface  17  of the right half housing portion  16   b  ( FIG. 8 ). As well, threaded studs  62  extend through recessed holes  64  formed on either side of the left half housing  16   a  through the side wall  19  and top section  20  proximate the inner periphery of the housing  18  and are received in mating threaded holes  66  formed in the facing surface  17  of the right half housing portion  16   b . The cap screws  56  and bolts  68  on the ends of the studs  62  are tightened against the side wall  19  to squeeze the gaskets  50  evenly, for example with about 400 ft-lb torque bolting to make up the high temperature gasket seal at the sealing surfaces  17 . Any extruded edges of the gaskets  50  within the sealing areas for the upper and lower annular compression seals  22 ,  24  are removed prior to forming those seals to the casings  12 ,  14 , as set out below. 
     Alternate non-welded sealing techniques such as O-ring seals secured by for example external C-clamp connectors may be used to clamp the seal element between the sealing surfaces  17 . 
     In preferred embodiments the casing cap  10  forms upper and lower annular compression seals  22 ,  26  to the inner and outer casings  14 ,  12  respectively. A static form of stuffing box type seals (i.e., using compression packings and packing glands) is shown in the Figures at both of these locations, but other annular seals might be formed as will be evident to persons skilled in the art. These annular seals allow the inner casing  14  to move axially relative to the outer casing  12  (ex. thermal expansion/contraction) while maintaining the seals to the casings. In installation, the lower annular compression seal  26  is preferably formed before the upper annular compression seal  22 . In the Figures, the lower annular compression seal  26  is shown below the mechanical connection to the outer casing  12  (shown below C-channel  34 ). However, with alternate mechanical connections such as slip lock connectors the lower annular compression seal might be located above the mechanical connection. 
     For the upper annular compression seal  22  (best seen in  FIG. 13 ), the inner surface of the top section  20  forms an annular upper seal pocket  70  to be located adjacent the inner casing  14 . An inwardly extending lip or step  71  is formed at the base of the seal pocket  70  to retain the seals  22 . A split top compression packing  72 , preferably in multiple rings, is provided in the seal pocket  70 . Preferably compression packing materials include multiple rings (or rope windings) of expanded graphite packing materials, which might be wired reinforced in one or more of the rings. Exemplary material are ROBCO™ 1220 and ROBCO™ 1200 packings (Robco Inc., LaSalle, Quebec, Canada). Alternate high temperature compression packing materials which retain pressure when compressed might be used. The splits of each adjacent ring are preferably offset one from the other, for example by 120°, to improve sealing. One or more split metal washers  74  may be included below and/or above the graphite packings  72  to limit extrusion of the packings  72 . In  FIGS. 1-7 , an annular split top packing gland  76  is shown bolted to the top section  20  with stud and nut connectors  80  to compress the top compression packings  72 . In  FIGS. 8-12 , an annular split retaining ring  78  is bolted to the top packing gland  76  with cap screws  81  to form a more rigid packing gland, with the splits of the packing gland  76  being offset, preferably 90°, from the splits of the retaining ring  78 . The assembled packing gland  76  and retaining ring  78  is bolted to the top section  20  with the stud and nut connectors  80  to compress the top compression packings  72 . The splits of the top packing gland  76  are offset, preferably by 90°, from the splits at the casing cap housing half portions  16 . These offsets improve sealing and more evenly compress the compression packings  72 . Set screws  82  (see  FIG. 8 ) may be used for temporary placement of the gland  76  against the casing  14  during installation. 
     The lower annular compression seal  26  (best seen in  FIG. 13 ) is similar to the upper annular compression seal  22 . The inner surface of the side wall  19  forms an annular lower seal pocket  84  to be located adjacent the outer casing  12 . An inwardly extending lip or step  86  is formed at the base of the seal pocket  84  to retain the seal  26 . A split bottom compression packing  87 , preferably in multiple rings, is provided in the seal pocket  84 . Preferred compression packing materials are as indicated above for the upper seal  22 , with splits of adjacent rings being offset as above. One or more split metal washers  88  may be included below and/or above the graphite packings  87 . An annular split bottom packing gland  90  and an annular split retaining ring  92  are bolted together with cap screws  93 , and the assembled packing gland  90  and retaining ring  92  are bolted to the lower end  24  of the side wall  19  with stud and nut connectors  94  to compress the bottom compression packings  87 . The splits of the packing gland  90 , casing cap housing half portions  16 , and the retaining ring  92  are offset as set out above for the upper annular compression seal  22 . Set screws  96  may be used for temporary placement of the assembled packing gland and retaining ring  90 ,  92  against the casing  12  during installation. 
     As best seen in  FIG. 13 , a vent  98  may be formed to provide access to the annulus A. The vent  98  is shown to form an angled port through the top section  20 , with a threaded outlet  99  to connect to gauges or containment equipment (not shown) as known in the art. Alternatively, a flanged outlet might be provided for the vent. 
     The top section  20  preferably includes a plurality of lifting eye hooks  100  for ease of installation of the casing cap at the wellhead. 
     As used herein and in the claims, the word “comprising” is used in its non-limiting sense to mean that items following the word in the sentence are included and that items not specifically mentioned are not excluded. The use of the indefinite article “a” in the claims before an element means that one of the elements is specified, but does not specifically exclude others of the elements being present, unless the context clearly requires that there be one and only one of the elements. 
     All references mentioned in this specification are indicative of the level of skill in the art of this invention. All references are herein incorporated by reference in their entirety to the same extent as if each reference was specifically and individually indicated to be incorporated by reference. However, if any inconsistency arises between a cited reference and the present disclosure, the present disclosure takes precedence. Some references provided herein are incorporated by reference herein to provide details concerning the state of the art prior to the filing of this application, other references may be cited to provide additional or alternative device elements, additional or alternative materials, additional or alternative methods of analysis or application of the invention. 
     The terms and expressions used are, unless otherwise defined herein, used as terms of description and not limitation. There is no intention, in using such terms and expressions, of excluding equivalents of the features illustrated and described, it being recognized that the scope of the invention is defined and limited only by the claims which follow. Although the description herein contains many specifics, these should not be construed as limiting the scope of the invention, but as merely providing illustrations of some of the embodiments of the invention. 
     One of ordinary skill in the art will appreciate that elements and materials other than those specifically exemplified can be employed in the practice of the invention without resort to undue experimentation. All art-known functional equivalents, of any such elements and materials are intended to be included in this invention. The invention illustratively described herein suitably may be practiced in the absence of any element or elements, limitation or limitations which is not specifically disclosed herein.