Patent Publication Number: US-9416608-B2

Title: Slip, tangential slip system having slip, and method thereof

Description:
BACKGROUND 
     In the drilling and completion industry, the formation of boreholes for the purpose of production or injection of fluid is common. The boreholes are used for exploration or extraction of natural resources such as hydrocarbons, oil, gas, water, and alternatively for CO2 sequestration. It is often necessary to anchor equipment within a tubular structure such as a casing or tubing string. A common and long used apparatus for such duty is a set of slips with attendant support structure. In some embodiments, slips are utilized with conical structures that impart radially outwardly directed impetus on each slip as the slip is axially moved along the cone, usually under a compressive load. While such configurations have been extensively used, it is also known that this type of configuration can become stuck in the tubular structure in which it has been set, thereby rendering retrieval thereof difficult. 
     In another embodiment of a slip configuration, the slips are tangentially loaded to avoid the need for the conical portion. Depending upon the configuration of these tangentially loaded systems, there has been difficulty in retrieval or difficulty in creating acceptable holding strength. U.S. Pat. No. 7,614,449 provides slips that have different lengths to delay a tensile force being applied to the slips when retraction of the slip system is desired, which reduces the force necessary to retract the slip system. 
     The art would be receptive to alternative devices and methods for improving the retractability of slip systems. 
     SUMMARY 
     A slip for a tangentially loaded retrievable slip system, the slip includes a frame including an interior having first and second sides; and, a key having a first portion within the interior of the frame, the first portion having first and second sides, the first portion of the key movable within the interior of the frame; wherein the key is movable from a set condition with at least a contact point of the first and second sides of the first portion of the key pushing against at least a portion of the first and second sides of the interior of the frame in the set condition, respectively, to an unset condition with the first and second sides of the first portion of the key releasing pressure from the first and second sides of the interior of the frame. 
     A slip system includes a set of drive slips; a set of gripping slips operatively interengagable with the set of drive slips; a drive slip end ring in operable communication with the set of drive slips; and a gripping slip end ring in operable communication with the set of gripping slips, the end rings capable of transmitting a load applied in an axial direction of the system to the set of gripping slips and the set of drive slips to tangentially load the set of drive slips and the set of gripping slips against each other thereby increasing a radial dimension of the system; wherein at least one slip in the set of drive slips and the set of gripping slips is a keyed slip, the keyed slip including: a frame including an interior having first and second sides; and, a key having a first portion within the interior of the frame, the first portion having first and second sides, the first portion of the key movable within the interior of the frame; the key configured for movement from a set condition with at least a contact point of the first and second sides of the first portion of the key pushing against at least a portion of the first and second sides of the interior of the frame in the set condition, respectively, to an unset condition with the first and second sides of the first portion of the key releasing pressure from the first and second sides of the interior of the frame. 
     A method of managing a tangential load imparted by a tangential slip system, the tangential slip system including a set of drive slips and a set of gripping slips operatively interengagable with the set of drive slips, the method includes providing at least one keyed slip amongst the set of drive slips and the set of gripping slips, the at least one keyed slip having a frame and a first portion of a key movable within the frame; setting the at least one keyed slip by pushing at least one contact point of first and second sides of the first portion of the key against first and second sides of the frame; and, unsetting the at least one keyed slip by moving the key to release pressure from the first and second sides of the frame and at least partially remove a tangential load in the system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring now to the drawings wherein like elements are numbered alike in the several Figures: 
         FIG. 1  shows a front plan view of an exemplary embodiment of a slip for a tangentially loaded slip system; 
         FIG. 2  shows a perspective view of an exemplary embodiment of the slip system disclosed herein in a set position; 
         FIG. 3  shows a perspective view of an exemplary embodiment of the slip system disclosed herein in a retracted position; 
         FIG. 4  shows a perspective view of one of the slips from the illustration of  FIG. 2 ; 
         FIG. 5  shows a perspective view of another of the slips illustrated in  FIG. 1 ; and 
         FIG. 6  shows a perspective view of an alternate exemplary embodiment of a slip ring configured to unset the slip system. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows an exemplary embodiment of a tangential slip  100  usable in a tangential slip system  10  (see  FIG. 2 ). The slip  100  includes a slip frame  102  and a key  104  movably disposed within the slip frame  102 . Due to the key  104  within the frame  102 , the slip  100  may also be termed a “keyed slip.” The slip frame  102  includes a substantially trapezoid-shaped exterior  106 , and a substantially trapezoid-shaped interior  108 . The trapezoidal shape, or other tapered profile, of the exterior  106  is important because it facilitates radial expansion of the slip system  10 , with reference to  FIG. 2 , upon axial compression of the system  10  into a shorter axial dimension. Growth in the radial direction is of course important to a slip system because it is such radial growth that allows the system itself to become anchored into the receiving tubular structure. Because of the trapezoidal shape and positioning of that shape, each slip acts as a wedge (perimetrically) against its two neighboring slips. When the axial length of system  10  is increased, the radial dimension of the system  10  will necessarily and naturally decrease. 
     The interior  108  of the frame  102  has a first end  110  and a second end  112 , wider than the first end  110 , and first and second sides  114 ,  116  that connect the second end  112  to the first end  110 . The second end  112  serves as a stop shoulder for the key  104 . The frame  102  also includes a neck-shaped opening  118  extending from the second end  112  and sized to receive a portion of the key  104 , and a tapered receiving portion  120  tapering outwardly from the neck-shaped opening  118 . An exterior front surface  122  of the frame  102  is provided with wickers  124  that may extend substantially perpendicular with respect to a longitudinal axis  126  of the slip  100 . Unlike the frame  102 , an exterior front surface  128  of the key  104  is not provided with wickers, and may be substantially smooth. 
     The key  104  of the slip  100  may include a substantially trapezoidally shaped first portion  130  sized for arrangement within the interior  108  of the frame  102 . Alternatively, the key  104  and the interior  108  may include substantially triangular shaped profiles, substantially complementary tapered profiles, profiles having substantially the same or similar angles from the longitudinal axis  126  of the slip  100 , or any profile having one or more contact points on either the interior of the frame  102  or first portion  130  of the key  104  that contact each other in a set condition of the slip  100 . As illustrated, the first portion  130  includes a first end  132 , a second end  134  wider than the first end  132 , and first and second sides  136 ,  138  that connect the first end  132  to the second end  134 . Adjacent the second end  134  of the first portion  130  of the key  104 , the first and second sides  136 ,  138  may be truncated, as shown by truncated portions  140 ,  142 . A longitudinal length L1 of the first portion  130  is less than a longitudinal length L2 of the interior  108  of the frame  102  such that the key  104  is slidable within the frame  102  by a distance D1 of the gap  144  from the second end  134  of the first portion  130  to the second end  112  of the interior  108  when the first portion  130  of the key  104  is pushed as far as possible in direction A into the interior  108 . When the first end  132  of the first portion  130  of the key  104  abuts with the first end  110  of the interior  108  of the frame  102 , a width from the first side  136  to the second side  138  of the first portion  130  of the key  104 , at at least one contact point of the first and second sides  136 ,  138 , is greater than a corresponding width from the first side  114  to the second side  116  of the interior  108  of the frame  102 . Thus, when the key  104  is moved as far as possible into the frame  102 , such as with the first end  132  of the key  104  abutting the first end  110  of the interior  108  of the frame  102 , such as in the set position shown in  FIG. 1 , at least a contact point of the first and second sides  136 ,  138  of the key  104  push outwardly on the first and second sides  114 ,  116  of the frame  102 , which in turn may push the first and second sides of the exterior  106  of the frame  102  at least slightly outwardly. While the contact point of the first and second sides  136 ,  138  of the key  104  is illustrated along a substantially linear segment of the sides  136 ,  138  of the key  104 , in alternative embodiments, the first and second sides  136 ,  138  of the key  104  or the interior  108  of the frame  102  may include one or more protrusions, such as via an undulating or castellated surface, that provide one or more contact points between the key  104  and the interior  108  of the frame  102  in the set condition. Thus, the term “contact point” with respect to the key  104  is meant to encompass any feature of the key  104  that engages with the interior  108  of the frame  102  for the purpose of applying the above-described pressure in the set condition. When the first end  132  of the first portion  130  of the key  104  is pulled away from the first end  132  of the interior  108  of the frame  102 , such as shown in  FIG. 3 , pressure is removed from the first and second sides  114 ,  116 , releasing the outwardly directed pressure experienced by the first and second sides of the frame  102 . This action is used advantageously for retrieval purposes as will be described further below. 
     A second portion  146  of the key  104  includes an outwardly extending shoulder, such as a T-shaped structure, sized for accommodation within and engagement with a slot  20 ,  22 , or  24  in a drive slip ring  16  or gripping slip ring  18 , shown in  FIG. 2 . The first portion  130  is connected to the second portion  146  by a third portion  148 . The third portion  148  includes a first section  150  that is narrower than a width of the second end  134  of the first portion  130 , and a second section  152  that expands outwardly towards the second portion  146 . The first section  150  of the third portion  148  is sized for sliding within the neck-shaped opening  118  of the frame  102 , and the second section  152  of the third portion  148  is sized to substantially abut with and nest within the tapered receiving portion  120  of the frame  102  in the set condition of the slip  100 . For sliding the key  104  relative to the frame  102 , at least one longitudinally extending rod  154 , extending substantially parallel to the longitudinal axis  126 , is fixedly mounted within the frame  102  and supports the key  104  for slidable movement thereon within the frame  102 . Two rods  154  are shown in the illustrated embodiment of the slip  100  in  FIG. 1 , one on each side of the longitudinal axis  126  of the slip  100  for evenly supporting the key  104  within the frame  102 . 
     The first, second, and third portions  130 ,  146 ,  148  of the key  104  may all be integrally connected and uniformly manufactured in a single piece. Likewise, the frame  102  may be an integrally constructed unit. One exemplary method of fabricating the key  104  and the frame  102  includes electrical discharge machining (“EDM”). EDM, otherwise known as spark machining, is a manufacturing process of obtaining a desired shape by using electrical discharges to remove material from a workpiece by a series of rapidly recurring current discharges between two electrodes separated by a dielectric liquid and subject to an electric voltage. EDM is advantageously useful in forming the key  104  and frame  102  of the slip  100  due to the unique shapes and close tolerances that can be obtained with EDM. In other exemplary embodiments, however, the slip  100  may be formed using other machining techniques, so long as the requirements for shape and tolerances are met in the slip system  10 . 
     The slip  100  may be used in place of one or more of a drive slip  12  and a grip slip  14 .  FIG. 2  shows the slip system  10  including a configuration of a set of drive slips  12  and a set of grip slips  14  that together cooperate in a way that promotes tangential loading of the slips against one another to radially expand. The slip system  10  is similar to the tangentially-loaded high-load retrievable slip system disclosed in U.S. Pat. No. 7,614,449, herein incorporated by reference in its entirety, however the slip system  10  includes at least one tangential slip  100  having key  104 . For exemplary purposes only, the slip  100  is shown in place of one of the drive slips  12  and one of the grip slips  14 , however the slip  100  may also replace more than one of the drive slips  12 , and/or one or more of the grip slips  14 , or may only replace only one or all of the drive slips  12  or only one or all of the grip slips  14 . Radial expansion of the slip system  10  is necessary to set the system  10  by driving certain portions of the wicker threads (numerically introduced and discussed hereunder) into a receiving tubular structure (not shown). System  10  further includes a drive slip ring  16  and a grip slip ring  18 . Ring  16  is endowed with interengagement (for example, T-shaped) slots  20  about a perimeter thereof, each of the slots  20  being substantially the same shape and set of dimensions as each other. Ring  18  on the other hand, in one embodiment, may include a plurality of interengagement (for example, T-shaped) slots  22  disposed about a periphery thereof having a first set of dimensions and a plurality of interengagement (for example, T-shaped) slots  24  having another set of dimensions. In the illustrated embodiment of  FIG. 1 , slots  22  and  24  alternate (single alternating) around the perimeter of ring  18 . It is to be understood, however, that more of slot  22  or slot  24  could be grouped together in alternate embodiments such as, for example, two slot  22 &#39;s next to one another and two slot  24 &#39;s next to one another alternating with the  22 &#39;s (double alternating). Further, there is no requirement that there be any particular number of a certain type of slot  22  or  24 , for example, there may only be one slot  24  or two slots  24 , etc. or each slot could be unique as desired (random alternating). While an embodiment of ring  18  will be described having slots  22 ,  24  with different dimensions, for the reasons described below, due to the key  104  of the slip  100  as described herein, another exemplary embodiment of the ring  18  may also include slots having the same dimensions, e.g. slots  24 , for all of the grip slips  14  and or keyed slips  100  disposed therein. 
     In each of the rings  16  and  18 , the position of slots  20 ,  22  or  24  are such, relative to each other, that slips  12  and  14 , and slip  100  where utilized, are alternately positioned when engaged with adjacent T-shaped slots in each ring. The alternate positioning of slips  12  and  14  is easily seen in  FIGS. 1 and 2 . 
     It is to be noted that the radial expansion of system  10  is affected entirely by tangential application of force through the slips  12 ,  14 ,  100 ; this means that the ID of the slip system  10  can remain completely open and that conical structures previously used to radially displace slips are not necessary. 
       FIGS. 4-5  show an exemplary drive slip  12  and grip slip  14 , respectively, for use in the slip system  10 . If not all of the slips  12 ,  14  are replaced with the slips  100 , then these exemplary slips  12 ,  14  can be used in the slip system  10 . Also, features of the slips  12 ,  14  that are described in detail below can also be incorporated into the slip. Referring now to  FIG. 4 , one of the drive slips  12  is illustrated in perspective view and enlarged from the  FIGS. 2 and 3  views. In the  FIG. 4  view there is visible interlocking members provided in each of the slips in order to keep them engaged as a single unit while simultaneously allowing them to slide relative to each other. Each one of the slips includes a keyed flange  26 , which in the embodiment illustrated, is of L-shape but may be of any shape that allows sliding motion while inhibiting disassociation of each slip from its neighboring slip. On an opposite side of slip  12  is a complementary flange keyhole  28 , one end of which is visible. It will be understood that the flange keyhole  28  extends the length of slip  12  as does keyed flange  26 . If one were to obtain an opposing slip (i.e. slip  14 ) one would notice that the keyed flange  26  and the flange keyhole  28  can be engaged as the slips  12  and  14  slid axially relative to one another. Sliding movement is thus enabled while lateral disassociation is prevented or at least inhibited. It should be further noted that the frame  102  of slip  100  is also provided with keyed flange  26  and flange keyhole  28  for complementarily fitting with interengaged slips  12  and  14 . 
     It should also be noted in passing that an angle of the mating surfaces  30 , on each slip  12  and  14 , is dictated by a radius extending from the axis of system  10 . This angle ensures smooth and distributed contact along each face  30  to improve overall efficiency and strength of system  10 . 
     Still referring to  FIG. 4 , an exemplary embodiment of drive slips  12  may possess a number of wickers  32 , a substantial number of which are truncated. In the illustrated embodiment, all of the wickers  32  are truncated, but it is to be appreciated that merely a substantial number of the wickers can be truncated to achieve the benefit of distribution of stresses in the receiving tubular structure. It is possible to add pointed wickers without departing from the scope of the invention. Truncation  34  removes what would otherwise be a sharper point of a slip gripping wicker. In one embodiment the truncation amount is of a dimension that is about the same as the amount of a sharp wicker that would be embedded in the material of the receiving tubular structure. Slips  12  are so configured to enhance retrievability of the slip system  10  as well as assist in the distribution of stresses in the receiving tubular structure. Due to the key  104  of the slip  100 , the slips  12  need not necessarily include all truncated wickers  32 , however exemplary embodiments may include any number of truncated wickers  32 . Also, the wickers  32  are also employable on the frame  102  of the slip  100 , in place of wickers  124 . 
     Each one of the wickers  32  that is truncated, is so truncated to an extent about equal to the amount of penetration into the receiving tubular structure that is anticipated for pointed wickers on the gripping slips  14 . The reason for this is so that when the pointed wickers are maximally embedded in the receiving tubular structure, the wickers  32  will be radially loaded against the receiving tubular structure without penetrating it into. This distributes the stresses of the receiving tubular structure more evenly about the tubular structure consistent with contact around the entirety of the slip system  10 . One further benefit of the configuration of slips  12  is realized in the case of paraffin or other debris lining the inside dimension of the receiving tubular structure. Because wickers  32  are still above the surface of slips  12 , those wickers are able to penetrate debris at the inside dimension of the receiving tubular structure and still ensure contact of truncation  34  with the inside dimension surface of the receiving tubular structure forming a frictional engagement therewith. 
     Each wicker  32 , of course, possesses a pair of flanks  36 , which in one embodiment, are positioned at 45°. It is to be understood that other angles are possible. It is also noted that in the system  10 , it is not necessary to harden wickers  32 , as they are not intended to bite into the receiving tubular structure. This is not to say that it is undesirable to harden wickers  32  but merely that it is not necessary to do so. 
     Referring to  FIG. 5 , an exemplary embodiment of the gripping slips  14  is illustrated. It will be noted that there are two distinguishing features of gripping slip  14  over driving slip  12  as illustrated in  FIG. 4 . These are a length  40  of a T-upright  42 , and a configuration of wickers  44  and  46 . Addressing the wickers first, it will be apparent that in the illustrated embodiment, every other wicker is sharp pointed (wicker  44 ) while the intervening wickers  46  are truncated (single alternating). In this embodiment, the degree of truncation of wickers  46  is roughly equal to the expected penetration of wickers  44  into the receiving tubular structure (not shown). Again the purpose for this construction, like that of the drive slip illustrated in  FIG. 4 , is to distribute the load on the receiving tubular structure imparted by radial motion of slip system  10 . More specifically, upon full penetration of wickers  44  into the receiving tubular structure, wickers  46  come into contact with the inside diameter of the receiving tubular structure thereby distributing stress in that structure. It is to be appreciated that only one embodiment of the slip system contemplated is shown in  FIG. 5 . It is also possible for numbers of wickers  44  and  46  to be grouped such as two wickers  44  alternating with two wickers  46  (double alternating) or three wickers  44  alternating with three wickers  46  (triple alternating) or even a number of sharp wickers  44  alternating with a different number of truncated wickers  46  (random alternating). The overall point of alternating sharp and truncated wickers is to distribute stress otherwise imparted in an undistributed way to the receiving tubular structure. It is further possible to retain all of the wickers on slips  14  in the  44  configuration in some embodiments of the invention, since the truncated wickers  32  on the drive slips  12  will still substantially balance stresses in the receiving tubular structure. It will also be noted that pointed wickers  44  should be hardened such that they are sufficiently durable to penetrate the inside diameter of the receiving tubular structure. It is further noted that the wickers  44 ,  46  may be employed on the frame  102  of the slip  100  in lieu of wickers  124 . 
     Addressing now the upright  42  of the key structure  48 , and referring to both  FIGS. 4 and 5 , it is apparent that the length  40  of the upright section  42  is longer than that of the comparable portion of slip  12 . The reason for the length of this portion of slip  14  is to delay a tensile force being applied to this slip  14  when retraction of the slip system  10  is desired. Referring back to  FIGS. 2 and 3  and reiterating that the T-shaped slots  22  and  24  are distinct, a review of the drawing will make clear that T-shaped slots  24 , upon an axial tensile load on ring  18 , will cause an immediate transfer of the tensile load to the associated slip  14 . This is distinct from the T-shaped slots  22  wherein the same tensile load applied to ring  18 , is not immediately transferred to the associated slip  14  but rather the ring  18  must axially move relative to the associated slip  14  until surface  50  contacts surface  52 . Upon this contact, the tensile load will be transmitted to the associated slip  14 . In such configuration it will be appreciated that every other slip  14 , in the illustrated embodiment, will be pulled in a direct commensurate with retracting the slip system  10  prior to the other slips  14  being so pulled. This reduces the force necessary to retract the slip system  10 . In the illustrated embodiment, the force is roughly halved while in other embodiments with differing numbers of alternating T-shaped slots  22  and  24 , the reduction in tensile force required will be describable as a percentage of the whole proportional to the number of earlier pulled slips relative to the total number of slips associated with the subject ring. In view of the keyed slip  100 , the length  40  of the upright section  42  may alternatively be consistent with that of the drive slip  12 . 
     As noted above, ring  16  is illustrated to contain only T-shaped slot  20 . The reason that the staggered T-shaped slots are not employed on ring  16  is that all of the associated slips  12  substantially lack gripping wickers and therefore, the tensile force required to unseat them is substantially less than that of the slips  14 . Therefore, there is no need to stagger the T-shaped slots in ring  16 . This is by no means to say that it is inappropriate to stagger T-shaped slots  20 , as it certainly is not only possible and functional, but rather merely to state that it is unnecessary. 
     When the slip  100  is employed, as exemplarily demonstrated in  FIGS. 2, 3 , and  6 , the necessity of having slips  14  positioned within varying slots  22 ,  24  is reduced as the slip  100  is designed to allow the slip system  10  to relax, thus easing retrieval of a packer or bridge plug. Thus, while the system  10  is illustrated as including slots  22 ,  24 , because the slips  100  reduce the force necessary to retract the slip system  10 , the system  10  may alternatively include slots of equal size for the slips  14  since the slips  100  can effectively be employed to remove the built in tangential load from the slip system  10 . 
       FIG. 2  shows the slips  100 ,  12 ,  14  in a set condition of the slip system  10 . With additional reference to  FIG. 1 , the first end  132  of the key  104  of the slip  100  is pushed towards the first end  110  of the interior  108  of the frame  102 , by a respective one of the rings  16 ,  18 , and the first and second sides  136 ,  138  of the key  104  push radially outwardly towards the first and second sides  114 ,  116  of the frame  102  to assist in the tangential loading of the system  10 . When retrieval of the packer or bridge plug is necessary, unsetting of the slip system  10  will involve applying a tensile load on at least one of the rings  16 ,  18  to pull on the second portion  146  of the key  104  in direction B. Because the key  104  is movable within the frame  102 , the frame  102  does not immediately move in the longitudinal direction with the key  104 . Also, because the key  104  does not include wickers and is therefore not embedded in a receiving tubular structure, it is readily pulled in the axial direction without the frame  102 . As soon as the key  104  is longitudinally moved within the frame  102  from a set condition to an unset condition, the tangential load that exists between the slips  100 ,  12 ,  14  in the system  10  is relieved, significantly improving retrievability of both the system  10  and any associated tool structure. The second end  134  of the first portion  130  of the key  104  then abuts with the second end  112  of the interior  108  of the frame  102  such that continued tensile load on the ring  16  or  18  will move the slip  100 , including the frame  102 , to the unset condition as shown in  FIG. 3 . 
     Referring to  FIG. 6 , an alternate embodiment of slip system  200  is substantially the same as the slip system  10  except that the slip system  200  includes a ring  18  which allows for the T-shaped structures on each of the slips  14  to be identical. In this embodiment, the T-shaped structure  48  is not required to be long, as it is illustrated in the  FIG. 2  and  FIG. 3  embodiments. It will be appreciated that the reason that the elongated section  42  is not needed, is that surface  50  of slots  22  is positioned closer to an end  60  of ring  18  than it is in the  FIG. 2  embodiment. One will also note that the clearances between the T-shaped structure  48  and the slots  22  has also been increased to account for potential axial movement of the system  200 . This additional clearance alleviates unnecessary load on the structure  48  when the system is set. In this embodiment, two slips  100  are shown replacing slips  14  for exemplary purposes only. As noted above, any number of slips  100  could be employed in the system  10 ,  200  to replace either or both of slips  12 ,  14 , and therefore the particular arrangements of slips  100 ,  12 ,  14  shown in the figures is meant to be illustrative of exemplary applications rather than limiting embodiments. 
     While the figures in this application may suggest to one of ordinary skill in the art the existence of a clear uphole end and downhole end of slip system  10 , based upon conventional illustration methods, it is to be understood that slip system  10  is usable with either end uphole. Generally, it will be desirable to impart a compressive setting force against ring  16  and the drive slips  12  while maintaining ring  18  and gripping slips  14  stationary. This is, however, not a requirement and the slip system  10  is to be understood to be actuable and retractable from either end. It is also to be understood that the system is actuable and retractable from a position downhole of the system of a position uphole of the system. 
     While the invention has been described with reference to an exemplary embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the claims. Also, in the drawings and the description, there have been disclosed exemplary embodiments of the invention and, although specific terms may have been employed, they are unless otherwise stated used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention therefore not being so limited. Moreover, the use of the terms first, second, etc. do not denote any order or importance, but rather the terms first, second, etc. are used to distinguish one element from another. Furthermore, the use of the terms a, an, etc. do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item.