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RELATED APPLICATION DATA 
     This is a continuation in part of application Ser. No. 12/361,352 filed Jan. 28, 2009 entitled “Retractable Downhole Backup Assembly for Circumferential Seal Support.” 
    
    
     FIELD OF THE INVENTION 
     The field of the invention is downhole backup devices for seals and more particularly devices that are retractable and positioned between seals for protection from well fluids and protection of the surrounding tubular from incremental stress from applied pressure differentials and most particularly to segmented slip segments that form a support ring and end treatment for such ring adjacent a seal to minimize seal damage from relative axial slip segment movements. 
     BACKGROUND OF THE INVENTION 
     Packers are used downhole to isolate zones in a wellbore. Many styles of packers are in use depending on the application and well conditions. A common design uses an annularly shaped sealing element that is axially compressed by setting down weight, or a setting tool that holds a mandrel and pushes down on a setting sleeve or a hydraulic mechanism that involves blocking a path through the packer and building pressure on a piston assembly to compress the sealing element. When the sealing element is compressed axially it extends radially into a sealing relationship with the surrounding tubular. To enhance the grip of the extended element there is also an upper and a lower set of slips disposed on opposed sides of the sealing element. The slips generally comprise tapered segments with exterior wickers that bite into the surrounding tubular when ramped out on tapered surfaces during the process of axially compressing the sealing element. 
     One issue with the compression set sealing elements is extrusion in the uphole or the downhole directions. Frequently, anti-extrusion rings are placed at the opposed ends of the sealing element. They plastically deform when the sealing element is axially compressed and engage the surrounding tubular to create a barrier at opposed ends. The problem with anti-extrusion rings is when the packer is retrieved. The plastically deformed rings retain their deformed shape despite extension of the packer mandrel assembly that allows the sealing element to extend axially and radially retract. In essence, the backup rings can still be in contact with the surrounding tubular after the sealing element has retracted away from the backup rings in a radial and an axial direction. When the packer is pulled out in this condition, the backup rings can swab the well as the packer is removed. Swabbing is the act of reducing pressure by removal of a tool that seals as it is being retrieved. This swabbing can cause formation damage or lead to the well coming in and a potential loss of well control. Also, well fluid above the packer is displaced upward or through a small bypass in the tool. This condition severely limits retrieval speed. Another problem is that the backup rings can get mangled on the trip out of the hole and cause the packer to hang up and in severe cases the packer may have to be milled to remove it. 
     Traditional designs have slips above and below the sealing element. A problem with this design is that when in service, and exposed to pressure differentials acting on the mandrel with the packer set there is a transfer of the applied pressure differential to the wickers of the uphole slips if the differential pressure is in the uphole direction and on the downhole slips if the pressure differential is in the downhole direction. This arrangement creates added stress on the surrounding tubular from the force increment on the slips created by the applied pressure differential. 
     There is yet another issue with debris in the well such as sand or gravel settling on top of the anti-extrusion rings, thus making it difficult to extract the packer after release. 
     Extrusion barriers different from continuous pliable rings that plastically deform have been tried. The idea behind a segmented ring design is the ability to maintain an overlapping relationship of the segments as they are ramped out on a tapered surface. This design is illustrated in U.S. Pat. No. 7,290,603. The problem with this design that used long return springs in the hope of biasing the segments to retract is twofold. The long spring members are exposed and can get damaged during run in. The debris in the well can get on the ramp surface or under the long spring elements and prevent the segments from retracting. This design also transfers load from differential pressure into the slips to increase stress in the surrounding tubing wall. 
     What is needed is an anti-extrusion system that is protected from well fluid debris after it is set while also minimizing the forces created from pressure differentials while in service from further stressing the surrounding tubular. An improved retraction system for a fully circumferential extrusion barrier is also provided to a barrier shielded from well fluids between seals. The barrier elements can have external wickers and function as slips as well as a barrier. The elements can also have a ring segment mounted to their wide dimension where the ring segments span over the region where the elements move relatively in the axial direction to change diameter. In the gripping position the seal is further isolated from exposure to relatively moving segments that can damage the seal. These and other features of the present invention will become more readily apparent to those skilled in the art from a review of the description of the preferred embodiment below along with the associated drawings, while recognizing that the full scope of the invention is to be found in the literal and equivalent scope of the appended claims. 
     SUMMARY OF THE INVENTION 
     A packer features spaced apart sealing elements with an extrusion barrier between them. When the packer is set the extrusion barrier is protected from debris in the well. The barrier provides full circumferential extrusion protection using one or more rings made of wedge shaped segments that have a keyway at their edges and are assembled in an alternating manner so as to be able to increase or decrease in diameter when mandrel components are moved toward or away from each other. The segments have an opening through which a mandrel projection extends so as to force the segments into the smaller diameter for removal. Travel stops for the segments in the form of machined flats are provided on the relatively movable mandrel components. 
     In a variation, the wedge shaped elements form a ring structure that can increase in diameter for a grip using relative axial motion of adjacent segments. The adjacent seal is further separated from access to the edges of the adjacent segments that move relatively by ring segments attached to the wide dimension of the segments that face the seal. The ring segments move out with the wedge elements to which they are attached so that in the set position of the seal there is an enhanced barrier against the surrounding tubular with the ring segments. The ring segments further block access of the seal under compressive loading to the interface locations between the wedge shaped elements so that their relative axial movement does not trap a portion of the seal and initiate cracks in the seal that can lead to leakage past the seal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of mandrel components that move relatively to actuate the segments of the backup system between retracted and extended positions; 
         FIG. 2  is a part cutaway view of an application of the backup system of claim  1  to a packer with multiple seals where the backup system is between the seals; 
         FIG. 3  is an alternative embodiment using two segmented ring backup systems that double as slips shown between seals and in the run in position; 
         FIG. 4  is the view of  FIG. 3  shown in the set position; 
         FIG. 5  shows the edge interface between adjacent segments of opposed orientation; 
         FIG. 6  is an alternative embodiment using the segmented ring for an extrusion barrier between the slip housing and the slip wedge ring shown in the run in position (without showing the slip housing); 
         FIG. 7  is the view of  FIG. 6  with the backup ring segments against the slip housing in the set position of the wedge slip ring; 
         FIG. 8  is the view of  FIG. 7  but in plan in the set position looking through the slip housing and showing how the wedge segments rotate the backup ring segments for the set position; 
         FIG. 9  is an alternative to the view in  FIG. 6  and shown in the run in position where the backup ring segments cannot pivot with respect to the wedge segment to which they are attached with spaced fasteners; 
         FIG. 10  is the view of  FIG. 9  but in the set position showing the backup ring segments moved out with the wedge slip segments; 
         FIG. 11  is a view along lines  11 - 11  of  FIG. 9 ; and 
         FIG. 12  is a view along lines  12 - 12  of  FIG. 10 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  illustrates the elements of the backup system that can be used downhole in a variety of applications and configurations, as will be explained below. While a given downhole tool will have many other components to accomplish its intended purpose, the basic components of operation of the backup system of the present invention are relatively movable components  10  and  12  that are part of a mandrel assembly  14  with a through passage  16 . Component  10  has a fully circumferential exterior ring  18  with a radial pushing segmented surface  20  interrupted by tapered flats  22 . A lower hub  24  extends beyond ring  18  and has a plurality of radial projections  26  that are preferably rectangular in cross-section, although other shapes can be used. The spacing on the projections is such that they line up with openings  28  on tapered segments  30  that have their noses  32  pointing in the same direction. Between segments  30  are tapered segments  34  that have their noses  36  pointing in the opposite direction from noses  32 . Preferably noses  32  and  36  have a rounded profile so that when the set position is obtained in a packer application seen in  FIG. 2  there will not be damage to the sealing elements  38  and  40  that preferably are disposed on opposed sides of the circumferential ring  42  a part of which is shown on an end view in  FIG. 5  to show how segments  30  and  34  can be secured on their edges as they slide axially with respect to each other which results in the diameter changing in opposed directions when components  10  and  12  are moved axially with respect to each other. A ball  44  extends into a socket  46  of an adjacent segment edge. Other edge retention devices such as dovetailed L-shapes that permit relative axial sliding on abutting edges while holding the overall ring shape  42  are contemplated to be within the scope of the invention. 
     Segment  12  is preferably identical to segment  10  and oriented in a mirror image as shown in  FIG. 1 . Segment  12  has a radial pushing surface  48  to abut segments  34  to push them in the opposite direction as radial surface  20  pushes segments  30  that are oppositely oriented from segments  34 . Radial surface  48  is interrupted by tapered flats  50 . When components  10  and  12  are pushed together, noses  32  ride over flats  50 , as best seen in  FIG. 4  showing an alternative embodiment, with a minimal clearance such as about 0.015 inches. Similarly noses  36  ride over flats  22  with a similar clearance. The reason for the minimal clearance is to close off an extrusion route for the seal such as  40  in the set position. As best seen in  FIG. 2 , there is a series of axial gaps  52  between the tops  54  of segments  30  and the adjacent seal  38  interspersed with noses  36  and the same pattern exists at the opposite end between noses  32  and seal  40 . However, axially between noses and an adjacent seal there is no place for extrusion as the tops such as  54  of the opposite oriented segment that is between the noses closes off any extrusion gaps by abutting against ring  18  on one side or ring  56  on the other. The noses  32  or  36  overly the flats  50  and  22  respectively in the set position against a surrounding tubular (not shown) with minimal clearance so that extrusion gaps for seals  38  or  40  are also effectively non-existent being so small. As a result full 360 degree extrusion protection is obtainable in the set position of  FIG. 2  for the ends of the seals  38  and  40  that face each other. The outside ends  58  and  60  better seen in  FIG. 3  abut sleeves  62  and  64  that are brought closer to each other when acted on by a setting tool shown schematically as arrows  66  and  68 . Those skilled in the art will appreciate that other parts have been left out for clarity such as body lock rings to hold a set position after the setting tool  66 ,  68  sets and automatically releases. To prevent extrusion past ends  58  and  60  when setting, there is a limit to the amount of axial movement of sleeve  62  with respect to sleeve  64 . The embodiment shown in  FIGS. 3 and 4  illustrates the modular nature of the backup system and uses two rings with opposed segments  70  and  72 . It has three spaced mandrel components as opposed to the two components  10  and  12  shown in  FIG. 2  when only one backup ring is used. Instead, in  FIG. 3  there are mandrel components  74 ,  76  and  78  that are spaced apart and relatively movable with respect to each other in response to operation of the setting tool  66 ,  68  for setting and in the opposite direction for removal with a known removal tool that extends the components away from each other. Seal  80  sits on component  74  and seal  82  sits on component  78 . Ring  70  is between components  74  and  76  and ring  72  is between components  76  and  78 . One travel stop is affected when sleeve  84  contacts top sub  86  as seen by comparing  FIGS. 3 and 4 . At the other end sleeve  88  runs into an unseen component to act as a second travel stop. As in the  FIGS. 1 and 2  embodiment the operation of an individual ring  70  or  72  is the same. For example, for setting, shoulders  90  and  94  respectively push oppositely oriented segments  92  and  96  toward each other. Segments  92  and  96  can also optionally serve as slips if they have wickers  98  and  100  on their respective external faces. For release, components  76  and  78  are pulled apart by a release tool (not shown) which results in radially extending tabs  102  in openings  104  in segments  92  pulling on those segments to move segments  92  with respect to oppositely oriented segments  96  so that the diameter of the ring  72  is positively pulled down to a smaller dimension so that removal from a surrounding tubular (not shown) is made possible. Those skilled in the art will see that the rings  72  and  70  work on the same principle and that the system is modular and can accommodate as many rings as desired. Wickers on the exterior face of any ring are an option for doing double duty as slips. Even within a given ring some components can have wickers while others do not. Note that in the  FIG. 1  embodiment where a single ring of segments  30  and  34  are used, both segments  30  and  34  have openings for radially extending members  26  or  106  so that the segments can be pulled apart for release. In the modular design of  FIGS. 3 and 4  only segments  92  in ring  72  are shown with radially extending members through openings to exert a force for release but the invention contemplates that all wedge shaped segments that make up a ring can have the openings through which the oppositely oriented segments are pulled to the lower diameter for removal. 
     Those skilled in the art will appreciate that the preferred location of the backup assembly that can also function as a slip assembly is between sealing elements. When done in that manner, any added force from well pressures does not add to the stress on the surrounding tubular at the location where it is gripped by the wickers on the ring components. The preferred design provides a positive applied force to the opposed segments through an opening in the segments to move them relatively to each other to the smaller diameter position. The use of angled flats toward which the segment noses move creates a very small clearance adjacent a sealing element that is located between the flat ends of the oppositely oriented segments that sit against a radial surface. As a result, going around for 360 degrees, there is either no place for the seal material to be extruded or there is an array of segment noses with undercuts that run parallel to a tapered flat on the mandrel portion to present a very small clearance that has the effect of retaining the seal material against extrusion. The nose are made or machined to a rounded shape so that even if they abut the end of a sealing element, there will not be damage or any tearing of the sealing element. 
     While the preferred placement of the backup assembly is between sealing elements, other arrangements can be used such as putting the backup assembly on one or both ends of a sealing element and in a position of exposure to well pressures and fluids. The segments in the ring or rings that make up the backup assembly used in these locations can also be equipped with wickers and perform a double duty as a backup assembly providing circumferential anti-extrusion protection for an adjacent sealing element as well as an anchor for that tool. Other tools that need a backup or protection from extrusion of components when subjected to well pressure when set are also contemplated to be within the scope of the invention. 
     In an alternative embodiment that has several variations, an objective is to isolate a seal such as  38  in  FIG. 2  from the pockets such as  52  that open up in the set position when surface  54  moves away from the seal  38 . The same condition appears near seal  40  as segments  34  move away from seal  40  except that the gap near seal  40  is circumferentially offset from the gaps  52  adjacent seal  38 .  FIGS. 6-8  interpose a segmented barrier ring  200  that has individual components such as  202  and  204  at a location adjacent the pushing surfaces  20  and  48  shown in  FIG. 1 . Each segment  202  and  204  is, at the end shown in  FIG. 6 , attached to a wedge slip segment such as  30  in  FIG. 2  by a fastener  206  in a countersunk hole  208 . Each wedge segment  30  has a top surface  210  and an adjacent lower surface  212 . Each ring segment  202  and  204  is secured by fastener  206  to the surface  212 . The top surfaces  214  and  216  of the ring segments  202  and  204  are preferably flush with the top surfaces  210  of the slip wedge segments  30 . Each segment  202  and  204  can preferably pivot about the fastener  206 . The pivoting action can come about as the wedge segments  30  and  36  move axially relative to each other along edge dovetails such as  220 . As the diameter of the ring made up of wedge segments  30  and  36  grows, an inside surface  218  on ring segments  202  and  204  comes up against surface  222  on an adjacent wedge segment  30 . The fastener  206  provides some rotational moment and the contact point between inside surface  218  and surface  222  slides relative to the diameter change of wedge components  30  and  36 . 
     The assembly of the components that make up the barrier ring  200  have gaps between the segments  202  and  204  that allow the diameter of the ring  200  to increase or decrease. These gaps or breaks occur over surfaces  212  to avoid the edge dovetails  220  that exit at the edges of the segments  30  where the narrow end of segments  36  is disposed. The idea is to use the surface  212  to close off an extrusion path for the adjacent seal such as  38 . Adjacent ends of ring segments  202  and  204  have offset narrow projections  224  and  226  to maintain the continuity of the barrier ring  200  in the run in and the set positions. These projections continue to circumferentially overlap in the set position of  FIG. 7  or  8 . There are leading tapers  228  and  230  on the projections  224  and  226  respectively. These tapers are used to move any rubber that has advanced against surface  212  out of the way when it is time to move the segments  202  and  204  closer to each other. The surface  218  that induces the pivoting motion of the segments  202  and  204  about their respective fastener connection keeps the gap  232  between the tapers  228  and  230  to a minimum. 
     Preferably the wickers on the segments  30  or  36  engage the surrounding tubular in a way that lets the barrier ring  200  come close or engage the surrounding tubular in the set position of  FIG. 7  or  8 . When a mandrel component such as  10  in  FIG. 7  pushes against the top surfaces  210  seen in  FIG. 6  and the barrier ring  200  grows in diameter to come close to or contact the surrounding tubular there is little to no gap at the tubular wall for extrusion of the seal such as  38 . Importantly, the access of the seal  38  to relatively moving edges of the wedge segments  30  and  36  is blocked as the ring segments  202  and  204  overlie that transition zone between adjacent wedge segments  30  and  36  at the periphery near the surrounding tubular wall and the pushing surface such as  20  shown in  FIG. 1  overlays the ends of the wedge segments  30  and  36  further radially inward of the barrier ring  200 . 
     It should be noted that in the design of  FIGS. 6-8  the bevels  22  and  50  shown in  FIG. 1  are optional and can be omitted. While this design embodiment has been discussed with respect to one side of a ring of wedge segments  30  and  36 , those skilled in the art will appreciate that the opposite side with respect to a seal  40  can also be used if oriented in minor image. The difference will be that the fixation with a fastener will be into the wide portion of segments  36  instead of segments  30  as described for the opposite end and shown in  FIGS. 6-8 . 
     A ramp  234  can be located on ring segment  202  opposite ramp  228  to push out rubber of seal  38  that had advanced into a space  236  defined between ramps  228  and  236  and above the surface  212  on the wedge segments  30 . 
       FIGS. 9-12  show a slightly different design. There is a segmented barrier ring  300  made of segments  302  and  304 . There are spaced apart fasteners  306  and  308  that go into top surface  310  of the wedge segments  30 . As a result there is no relative rotation as between the segments  302  and  304  and the wedge slip  30  to which each is secured. The segment  302  has an undercut  312  and an adjacent end segment  314  that has a square or rectangular cross-section. Segment  304  has an 1-shaped cutout  316  to accept the segment  314  as the diameter of the ring  300  changes. Gap  318  between surfaces  320  and  322  opens in the set position but that gap has a bottom at surface  324  on segment  304 . In the set position, the ring outer dimension  326  comes close to or into contact with the surrounding tubular  328  as shown in  FIG. 12 . Despite some small gaps  324  in the outer dimension  326 , those gaps are of minimal volume due to the overlapping nature of the segments  302  and  304  at the gap locations. This feature allows the location of the transition between segments  302  and  304  to be over the wedge segments  36  and the edge dovetails  330  since the outer dimension  326  goes to the tubular wall  328  results in isolation of the dovetail regions  330  from rubber or other material of seal  38  that is trying to extrude in that direction. Preferably the ends of the segments  302  and  304  stay in contact adjacent segments  314  as the diameter of the barrier ring  300  increases or decreases. 
     As an alternative the barrier rings  200  or  300  can be made of a single piece split ring where the opposed ends have details as described above. Using a split ring will eliminate the pivoting feature described with respect to barrier ring  200  but the one piece design would in other respects function the same way. 
     The above description is illustrative of the preferred embodiment and many modifications may be made by those skilled in the art without departing from the invention whose scope is to be determined from the literal and equivalent scope of the claims below:

Summary:
Wedge shaped elements form a ring structure that can increase in diameter for a grip using relative axial motion of adjacent segments. The adjacent seal is further separated from access to the edges of the adjacent segments that move relatively by ring segments attached to the wide dimension of the segments that face the seal. The ring segments move out with the wedge elements to which they are attached so that in the set position of the seal there is an enhanced barrier against the surrounding tubular with the ring segments. The ring segments further block access of the seal under compressive loading to the interface locations between the wedge shaped elements so that their relative axial movement does not trap a portion of the seal and initiate cracks in the seal that can lead to leakage past the seal.