Abstract:
A method for cementing a casing in a wellbore, the method having the following steps: attaching a valve to a casing; locking the valve in an open configuration; running the casing and the valve into the wellbore; reverse circulating a cement composition down an annulus defined between the casing and the wellbore; injecting a plurality of plugs into the annulus; unlocking the valve with the plurality of plugs; and closing the valve.

Description:
BACKGROUND 
   This invention relates to reverse cementing operations. In particular, this invention relates to methods and apparatuses for floating the casing and controlling fluid flow through the casing shoe. 
   After a well for the production of oil and/or gas has been drilled, casing may be run into the wellbore and cemented. In conventional cementing operations, a cement composition is displaced down the inner diameter of the casing. The cement composition is displaced downwardly into the casing until it exits the bottom of the casing into the annular space between the outer diameter of the casing and the wellbore. It is then pumped up the annulus until a desired portion of the annulus is filled. 
   The casing may also be cemented into a wellbore by utilizing what is known as a reverse-cementing method. The reverse-cementing method comprises displacing a cement composition into the annulus at the surface. As the cement is pumped down the annulus, drilling fluids ahead of the cement composition around the lower end of the casing string are displaced up the inner diameter of the casing string and out at the surface. The fluids ahead of the cement composition may also be displaced upwardly through a work string that has been run into the inner diameter of the casing string and sealed off at its lower end. Because the work string by definition has a smaller inner diameter, fluid velocities in a work string configuration may be higher and may more efficiently transfer the cuttings washed out of the annulus during cementing operations. 
   The reverse circulation cementing process, as opposed to the conventional method, may provide a number of advantages. For example, cementing pressures may be much lower than those experienced with conventional methods. Cement composition introduced in the annulus falls down the annulus so as to produce little or no pressure on the formation. Fluids in the wellbore ahead of the cement composition may be bled off through the casing at the surface. When the reverse-circulating method is used, less fluid may be handled at the surface and cement retarders may be utilized more efficiently. 
   In reverse circulation methods, it may be desirable to stop the flow of the cement composition when the leading edge of the cement composition slurry is at or just inside the casing shoe. To know when to cease the reverse circulation fluid flow, the leading edge of the slurry is typically monitored to determine when it arrives at the casing shoe. Logging tools and tagged fluids (by density and/or radioactive sources) have been used monitor the position of the leading edge of the cement slurry. If significant volumes of the cement slurry enters the casing shoe, clean-out operations may need to be conducted to insure that cement inside the casing has not covered targeted production zones. Position information provided by tagged fluids is typically available to the operator only after a considerable delay. Thus, even with tagged fluids, the operator is unable to stop the flow of the cement slurry into the casing through the casing shoe until a significant volume of cement has entered the casing. Imprecise monitoring of the position of the leading edge of the cement slurry can result in a column of cement in the casing 100 feet to 500 feet long. This unwanted cement may then be drilled out of the casing at a significant cost. 
   SUMMARY 
   This invention relates to reverse cementing operations. In particular, this invention relates to methods and apparatuses for floating the casing and controlling fluid flow through the casing shoe. 
   According to one aspect of the invention, there is provided a method for cementing a casing in a wellbore, the method having the following steps: attaching a valve to a casing; locking the valve in an open configuration; running the casing and the valve into the wellbore; reverse circulating a cement composition down an annulus defined between the casing and the wellbore; injecting a plurality of plugs into the annulus; unlocking the valve with the plurality of plugs; and closing the valve. 
   A further aspect of the invention provides a valve having a variety of components including: a valve housing defining a valve seat; a closure element adjustably connected to the valve housing, wherein the closure element is configurable relative to the valve seat in open and closed configurations; a lock in mechanical communication with the closure element to lock the closure element in the open configuration when the lock is assembled in the valve housing, wherein the lock comprises a strainer; and a bias element in mechanical communication with the valve housing and the closure element, wherein the bias element biases the closure element to the closed configuration. 
   Another aspect of the invention provides a system for reverse-circulation cementing a casing in a wellbore, wherein the system has a valve with a hole and a plurality of plugs, wherein the plugs have a plug dimension larger than the hole dimension. The valve may have a valve housing defining a valve seat; a closure element adjustably connected to the valve housing, wherein the closure element is configurable relative to the valve seat in open and closed configurations; a lock in mechanical communication with the closure element to lock the closure element in the open configuration when the lock is assembled in the valve housing, wherein the lock comprises a strainer with holes comprising a hole dimension; and a bias element in mechanical communication with the valve housing and the closure element, wherein the bias element biases the closure element to the closed configuration. 
   The objects, features, and advantages of the present invention will be readily apparent to those skilled in the art upon a reading of the description of the exemplary embodiments which follows. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
     The present invention may be better understood by reading the following description of non-limitative embodiments with reference to the attached drawings wherein like parts of each of the several figures are identified by the same referenced characters, and which are briefly described as follows. 
       FIG. 1  is a cross-sectional, side view of a valve having a lock pin or orifice tube stung into a flapper seat to lock a flapper open. 
       FIG. 2A  is a cross-sectional, side view of a lock pin having a strainer section and a cylindrical stinger section. 
       FIG. 2B  is a side view of the lock pin of  FIG. 2A . 
       FIG. 2C  is a perspective view of the lock pin of  FIG. 2A . 
       FIG. 2D  is a bottom view from the stinger end of the lock pin of  FIG. 2A . 
       FIG. 3A  is a cross-sectional, side view of a valve having a lock pin stung into a flapper seat to lock open a flapper as a cement composition and plugs flow into the valve. 
       FIG. 3B  is a cross-sectional, side view of the valve of  FIG. 3A  wherein the lock pin is pumped out of the flapper seat and the valve is closed. 
       FIG. 4A  is a cross-sectional, side view of a valve having a lock pin stung in into a poppet valve to lock open the poppet as a cement composition and plugs flow into the valve. 
       FIG. 4B  is a cross-sectional, side view of the valve of  FIG. 4A  wherein the lock pin is pumped out of the poppet valve and the valve is closed. 
       FIG. 5  is a cross-sectional side view of a valve and casing run into a wellbore, wherein a cementing plug is installed in the casing above the valve. 
       FIG. 6A  is a cross-sectional, side view of a portion of a wall of a strainer section of a lock pin, wherein the wall has a cylindrical hole and a spherical plug is stuck in the hole. 
       FIG. 6B  is a cross-sectional, side view of a portion of a wall of a strainer section of a lock pin, wherein the wall has a cylindrical hole and an ellipsoidal plug is stuck in the hole. 
       FIG. 7A  is a cross-sectional, side view of a portion of a wall of a strainer section of a lock pin, wherein the wall has a conical hole and a spherical plug is stuck in the hole. 
       FIG. 7B  is a cross-sectional, side view of a portion of a wall of a strainer section of a lock pin, wherein the wall has a conical hole and an ellipsoidal plug is stuck in the hole. 
       FIG. 8A  is a cross-sectional, side view of a lock pin having a strainer section and a flanged stinger section. 
       FIG. 8B  is a side view of the lock pin of  FIG. 8A . 
       FIG. 8C  is a perspective view of the lock pin of  FIG. 8A . 
       FIG. 8D  is a bottom view from the stinger end of the lock pin of  FIG. 8A . 
   

   It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, as the invention may admit to other equally effective embodiments. 
   DETAILED DESCRIPTION 
   This invention relates to reverse cementing operations. In particular, this invention relates to methods and apparatuses for floating the casing and controlling fluid flow through the casing shoe. 
   Referring to  FIG. 1 , a cross-sectional side view of a valve is illustrated. This embodiment of the valve  1  has a flapper seat  2  and a flapper  3 . The flapper seat  2  is a cylindrical structure that is positioned within the inner diameter of a casing  4 . In particular, the flapper seat  2  may be assembled between  2  sections of the casing  4  as illustrated. A seal  5  closes the interface between the outer diameter of the flapper seat  2  and the inner diameter of the casing  4 . The flapper seat  2  has an inner bore  6  for passing fluid through the flapper seat  2 . At the mouth of the inner bore  6 , the flapper seat  2  has a conical lip  7  for receiving the flapper  3  when the flapper is in a closed position. The flapper  3  is connected to the flapper seat  2  by a hinge  8 . A spring  9  is assembled at the hinge  8  to bias the flapper  3  toward a closed position in the conical lip  7  of the flapper seat  2 . 
   The valve  1  also has a lock pin  10  stung into the inner bore  6  of the flapper seat  2 . The lock pin  10  has a stinger section  11  and a strainer section  12 . In the illustrated embodiment, the stinger section  11  has a cylindrical structure having an outside diameter only slightly smaller than the inside diameter of the inner bore  6  of the flapper seat  2 . Along its longitudinal axis, the stinger section  11  has a flow conduit  13  extending all the way through the stinger section  11 . The strainer section  12  is connected to one end of the stinger section  11 . In this embodiment, the strainer section  12  has a hemisphere-shaped structure with a plurality of holes  14 . 
   When the lock pin  10  is inserted into the flapper seat  2  of the valve  1 , as illustrated in  FIG. 1 , the flapper  3  is locked in an open configuration. With the stinger section  11  fully inserted into the inner bore  6  of the flapper seat  2 , the stinger section  11  extends from the inner bore  6  and beyond the conical lip  7  to hold the flapper  3  open. The lock pin  10  may be retained in the flapper seat  2  by a pin or pins  15 . 
     FIG. 2A  is a cross-sectional side view of a lock pin  10  of the present invention taken along plane  100  identified in  FIG. 2D , discussed below. The lock pin  10  has a stinger section  11  connected to a strainer section  12 . The stinger section  11  has a flow conduit  13  that extends the entire length of the stinger section  11 . In this embodiment, the flow conduit  13  has a neck  16  where the flow conduit  13  opens into the interior of the strainer section  12 . The strainer section is a dome with mushroom-shape such that the interior of the dome faces the open end of the flow conduit  13  at the neck  16 . The strainer section  12  has a plurality of holes  14  that extend through its curved walls. In various embodiments of the lock pin  10 , the cumulative flow area through the holes  14  is equal to or greater than the flow area through the flow conduit  13  and/or neck  16 . A shoulder  17  extends radially outward between the stinger section  11  and the strainer section  12  so as to fit into a corresponding counter-bore  18  in the flapper seat  2  (see  FIG. 1 ). 
     FIGS. 2B and 2C  illustrate side and perspective views, respectively, of the lock pin  10  of  FIG. 2A . As noted previously, the lock pin  10  has a stinger section  11  and a strainer section  12 , wherein the strainer section  12  has a plurality of holes  14  that extends through its walls. The holes  14  are arranged in a radial pattern around the curved walls of the strainer section  12 . The shoulder  17  extends radially outward between the stinger section  11  and the strainer section  12 . 
     FIG. 2D  illustrates a bottom view from the stinger end of the lock pin  10  of  FIGS. 2A through 2C . Concentric rings indicate wall surfaces of the various structures of the lock pin  10 . The neck  16  has the smallest inner diameter followed by the flow conduit  13 . The flow conduit  13 , of course, is defined by the stinger section  11 . The shoulder  17  extends between the outer rim of the strainer section  12  and the stinger section  11 . Portions of the holes  14  are visible on the interior side of the strainer section  12  through the neck  16 . 
     FIG. 8A  is a cross-sectional side view of an alternative lock pin  10  of the present invention taken along plane  200  identified in  FIG. 8D , discussed below. The lock pin  10  has a stinger section  11  connected to a strainer section  12 . The stinger section  11  has four flanges extending the entire length of the stinger section  11 , wherein the flanges extend radially outwardly from a central axis where the flanges are connected. In this embodiment, the flow conduit  13  opens into the interior of the strainer section  12  through the shoulder  17  (see  FIG. 8D ). The flanges of the stinger section  11  extend into the flow conduit  13  so as to be connected to the interior surfaces of the flow conduit  13  at the four points where the flanges merge with the flow conduit  13 . The strainer section  12  is a dome with mushroom-shape such that the interior of the dome faces the open end of the flow conduit  13 . The strainer section  12  has a plurality of holes  14  that extend through its curved walls. The shoulder  17  extends radially outward between the stinger section  11  and the strainer section  12  so as to fit into a corresponding counter-bore  18  in the flapper seat  2  (see  FIG. 1 ). 
     FIGS. 8B and 8C  illustrate side and perspective views, respectively, of the lock pin  10  of  FIG. 8A . As noted previously, the lock pin  10  has a stinger section  11  and a strainer section  12 , wherein the strainer section  12  has a plurality of holes  14  that extend through its walls. In  FIG. 8B , two of the flanges extend to the left and the right from the center portion of the stinger section  11 , while a third flange is shown extending out of the figure toward the viewer. Similarly,  FIG. 8C  illustrates two of the flanges extending mostly left and right, respective, while a third flange extends mostly toward the front. The fourth flange is hidden from view in the back. 
     FIG. 8D  illustrates a bottom view from the stinger end of the lock pin  10  of  FIGS. 8A through 8C . An outermost portion of the underside of the strainer section  12  is shown extending beyond the shoulder  17 . The flow conduit  13  extends through the middle of the shoulder  17  and opens into the interior of the strainer section  12 . The flanges of the stinger section  11  divide the flow conduit  13  into four pie-shaped sections. Some of the holes  14  are visible from within the strainer section  12  through the flow conduit  13 . When this lock pin  10 , illustrated in  FIG. 8D , is inserted into flapper seat  2  of  FIG. 1 , the stinger section  11  extends beyond the conical lip  7  to hold the flapper  3  in an open position. In alternative lock pin embodiments, the stinger section may have any number of flanges. 
     FIGS. 3A and 3B  illustrate cross-sectional side views of a valve similar to that illustrated in  FIG. 1 , wherein  FIG. 3A  shows the valve in a locked, open configuration and  FIG. 3B  shows the valve in an unlocked, closed configuration. In  FIG. 3A , the lock pin  10  is stung into the flapper seat  2  so as to hold the flapper  3  in an open position. Pins  15  retain the lock pin  10  in the flapper seat  2 . In  FIG. 3B , the lock pin  10  is unstung from the flapper seat  2  and the flapper  3  is positioned within the conical lip  7  of the flapper seat  2  to close the valve  1 . 
   A reverse cementing process of the present invention is described with reference to  FIGS. 3A and 3B . The valve  1  is run into the wellbore in the configuration shown in  FIG. 3A . With the flapper  3  held in the open position, fluid from the wellbore is allowed to flow freely up through the casing  4 , wherein it passes through the flow conduit  13  of the stinger section  11  and through the holes  14  of the strainer section  12 . As the casing  4  is run into the wellbore, the wellbore fluids flow through the open valve  1  to fill the inner diameter of the casing  4  above the valve  1 . After the casing  4  is run into the wellbore to its target depth, a cement operation may be performed on the wellbore. In particular, a cement composition slurry may be pumped in the reverse-circulation direction, down the annulus defined between the casing  4  and the wellbore. Returns from the inner diameter of the casing  4  may be taken at the surface. The wellbore fluid enters the casing  4  at its lower end below the valve  1  illustrated in  3 A and flows up through the valve  1  as the cement composition flows down the annulus. 
   Plugs  20  may be used to close the valve  1 , when the leading edge  21  of the cement composition  22  reaches the valve  1 . Plugs  20  may be inserted at the leading edge  21  of the cement composition  22  when the cement composition is injected into the annulus at the surface. As shown in  FIG. 3A , the plugs  20  may be pumped at the leading edge  21  of the cement composition  22  until the leading edge  21  passes through the flow conduit  13  of the lock pin  10  of the valve  1 . When the leading edge  21  of the cement composition  22  passes through strainer section  12  of the lock pin  10 , the plugs  20  become trapped in the holes  14 . As more and more of the plugs  20  stop fluid flow through the holes  14 , the flow of the cement composition  22  becomes restricted through the valve  1 . Because the cement composition  22  is being pumped down the annulus or the weight of the fluid column in the annulus generates higher fluid pressure, fluid pressure below the valve  1  increases relative to the fluid pressure in the inner diameter of the casing  4  above the valve  1 . This relative pressure differential induces a driving force on the lock pin  10  tending to drive the lock pin  10  upwardly relative to the flapper seat  2 . Eventually the relative pressure differential becomes great enough to overcome the retaining force of the pin or pins  15 . When the pin or pins  15  fail, the lock pin  10  is released from the flapper seat  2 . The released lock pin  10  is pumped upwardly in the flapper seat  2  so that the stinger section  11  no longer extends beyond the conical lip  7 .  FIG. 3B  illustrates the configuration of the valve  1  after the stinger section  11  has been pumped out of the inner bore  6  of the flapper seat  2 . Once the lock pin  10  no longer locks the flapper  3  in the open position, the spring  9  rotates the flapper  3  around the hinge  8  to a closed position in the conical lip  7  to close the valve  1 . The closed valve  1  prevents the cement composition  22  from flowing up through the valve  1  into the inner diameter of the casing  4  above the valve  1 . 
   Referring to  FIGS. 4A and 4B , cross-sectional, side views of an alternative valve of the present invention are illustrated. In this embodiment, the valve is a poppet valve. In  FIG. 4A , the poppet valve is in a locked, open configuration and in  FIG. 4B , the poppet valve is in an unlocked, closed configuration. 
   Referring to  FIG. 4A , a valve housing  52  is positioned within a valve casing  54  by a valve block  53 . The valve housing  52  is further supported by cement  55  between the valve housing  52  and the valve casing  54 . The valve housing  52  defines a conical lip  47  for receiving the poppet  43 . A poppet holder  48  extends from the valve housing  52  into the open central portion within the valve housing  52 . A poppet shaft  50  is mounted in the poppet holder  48  so as to allow the poppet shaft  50  to slide along the longitudinal central axis of the valve housing  52 . The poppet  43  is attached to one end of the poppet shaft  50 . A spring block  51  is attached to the opposite end of the poppet shaft  50 . A spring  49  is positioned around the poppet shaft  50  between the spring block  51  and the poppet holder  48 . Thus, the spring  49  exerts a force on the spring block  51  to push the spring block  51  away from the poppet holder  48 , thereby pulling the poppet shaft  50  through the poppet holder  48 . In so doing, the spring  49  biases the poppet  43  to a closed position in the conical lip  47 . 
   The valve  1 , illustrated in  FIGS. 4A and 4B , also has a lock pin  10 . In this embodiment of the invention, the lock pin  10  has a stinger section  11  and a strainer section  12 . The stinger section  11  is a cylindrical structure having an outside diameter slightly smaller than the inside diameter of the valve housing  52 . The stinger section  11  also has a flow conduit  13  which extends along the longitudinal direction through the stinger section  11 . The strainer section  12  is connected to one open end of the stinger section  11 . The strainer section  12  has a plurality of holes  14 . The lock pin  10  also has a lock rod  19  that extends from the strainer section  12  along the longitudinal central axis of the lock pin  10 . As shown in  FIG. 4A , when the lock pin  10  is stung into the valve housing  52 , the lock rod  19  presses firmly against the spring block  51 . The lock pin  10  is held in the valve housing  52  by pins  15 . In this position, the lock rod  19  pushes on the spring block  51  to compress the spring  19  against the poppet holder  48 . Thus, when the lock pin  10  is stung into the valve housing  52 , the lock pin  10  locks the poppet  43  in an open configuration. 
   Referring to  FIG. 4B , the valve  1  is shown in an unlocked, closed configuration. The lock pin  10  is unstung from the valve housing  52 . With the lock pin  10  gone from the valve housing  52 , the lock rod  19  no longer presses against the spring block  51  to hold the poppet  43  in an open configuration. The spring  49  is free to work against the spring block  51  to drive the poppet shaft  51  up through the poppet holder  48  to pull the poppet  43  into engagement with the conical lip  47 . Thereby, the valve  1  is closed to restrict fluid flow the wellbore up through the valve  1  into the inner diameter of the casing  44 . 
   In an alternative embodiment, the lock pin  10  illustrated in  FIGS. 8A through 8D  may be used with the poppet valve  1  illustrated in  FIGS. 4A and 4B . In this embodiment, because the stinger section  11  has four flanges that are joined along the longitudinal, central axis of the stinger section  11 , there is no need for a lock rod  19 . Rather, the distal ends of the flanges simply butt against the spring block  51  to lock the valve in an open configuration. In further alternative designs, the poppet valve is on the bottom. In still further designs, the poppet valve is on the top where the poppet moves down during flow or has a ball valve. 
   Similar to that previously described with reference to  FIGS. 3A and 3B , a reverse circulation cementing operation may be conducted through the valve illustrated in  FIGS. 4A and 4B . In particular, plugs  20  may be injected into a leading edge  21  of a cement composition  22  for circulation down an annulus while returns are taken from the inner diameter of the casing  4 . As the leading edge  21  of the cement composition  22  begins to flow through the valve  1 , the plugs  20  become trapped in the holes  14  of the strainer section  12  to restrict fluid flow through the lock pin  10 . Increased relative pressure behind the lock pin  10  works to drive the lock pin  10  upwardly relative to the valve housing  52 . Eventually, the pins  15  are no longer able to retain the lock pin  10  so that the lock pin  10  is pumped out of the valve housing  52 . Thus, the plugs  20  function to unlock the valve  1 , and allow the poppet  43  to moved to a closed configuration in the conical lip  47  (see  FIG. 4B ). 
   Referring to  FIG. 5 , a cross-sectional side view of a valve similar to that illustrated in  FIGS. 4A and 4B  is illustrated. The valve  1  and casing  4  are shown in a wellbore  31 , wherein an annulus  32  is defined between the casing  4  and the wellbore  31 . In this embodiment, a standard cementing plug  30  is run into the inner diameter of the casing  4  to a position immediately above the valve  1 . The cementing plug  30  straddles the valve  1  and is a bottom plug pumped down as a contingency if the job was changed from a reverse cementing job to a standard job at the last minute. When a job is changed from reverse to standard, a top plug (not shown) is pumped down to land on the bottom plug. Pressure is then locked in at the top of the casing to prevent the cement from u-tubing back into the casing. In some embodiments, a top plug is pumped down to crush the mushroom head of the valve so that a bottom plug is not needed. 
     FIGS. 6A and 6B  illustrate cross-sectional, side views of a portion of the strainer section  12  of the lock pin  10 . In particular, a hole  14  is shown extending through the wall of the strainer section  12 . In this embodiment, the hole  14  is cylindrical. In  FIG. 6A , the illustrated plug  20  is a sphere having an outside diameter slightly larger than the diameter of the hole  14 . The plug  20  plugs the hole  14  when a portion of the plug  20  is pushed into the hole  14  as fluid flows through the hole  14 . In  FIG. 6B , the illustrated plug  20  is an ellipsoid wherein the greatest outside circular diameter is slightly larger than the diameter of the hole  14 . The ellipsoidal plug  20  plugs the hole  14  when a portion of the plug  20  is pushed into the hole  14  as fluid flows through the hole  14 . 
     FIGS. 7A and 7B  illustrate cross-sectional, side views of a portion of the strainer section  12  of the lock pin  10 . In particular, a hole  14  is shown extending through the wall of the strainer section  12 . In this embodiment, the hole  14  is conical. In  FIG. 7A , the illustrated plug  20  is a sphere having an outside diameter slightly smaller than the diameter of the conical hole  14  at the interior surface  25  of the strainer section  12  and slightly larger than the diameter of the conical hole  14  at the exterior surface  26  of the strainer section  12 . The spherical plug  20  plugs the hole  14  when at least a portion of the plug  20  is pushed into the hole  14  as fluid flows through the hole  14 . In  FIG. 7B , the illustrated plug  20  is an ellipsoid wherein the greatest outside circular diameter is slightly smaller than the diameter of the conical hole  14  at the interior surface  25  of the strainer section  12  and slightly larger than the diameter of the conical hole  14  at the exterior surface  26  of the strainer section  12 . The ellipsoidal plug  20  plugs the conical hole  14  when at least a portion of the plug  20  is pushed into the hole  14  as fluid flows through the hole  14 . 
   In one embodiment of the invention, the valve  1  is made, at least in part, of the same material as the casing  4 , with the same outside diameter dimensions. Alternative materials such as steel, composites, iron, plastic, cement and aluminum may also be used for the valve so long as the construction is rugged to endure the run-in procedure and environmental conditions of the wellbore. 
   According to one embodiment of the invention, the plugs  20  have an outside diameter of between about 0.30 inches to about 0.45 inches, and preferably about 0.375 inches so that the plugs  20  may clear the annular clearance of the casing collar and wellbore (6.33 inches×5 inches for example). However, in most embodiments, the plug outside diameter is large enough to bridge the holes  14  in the strainer section  12  of the lock pin  10 . The composition of the plugs may be of sufficient structural integrity so that downhole pressures and temperatures do not cause the plugs to deform and pass through the holes  14 . The plugs may be constructed of plastic, rubber, steel, neoprene plastics, rubber coated steel, or any other material known to persons of skill. 
   Therefore, the present invention is well adapted to carry out the objects and attain the ends and advantages mentioned as well as those that are inherent therein. While the invention has been depicted and described with reference to embodiments of the invention, such a reference does not imply a limitation on the invention, and no such limitation is to be inferred. The invention is capable of considerable modification, alternation, and equivalents in form and function, as will occur to those ordinarily skilled in the pertinent arts and having the benefit of this disclosure. The depicted and described embodiments of the invention are exemplary only, and are not exhaustive of the scope of the invention. Consequently, the invention is intended to be limited only by the spirit and scope of the appended claims, giving full cognizance to equivalents in all respects.