Patent Publication Number: US-9410570-B2

Title: Clamp

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional application and claims benefit under 35 U.S.C. §120 to U.S. patent application Ser. No. 11/661,070, filed Nov. 22, 2007, issued as U.S. Pat. No. 8,388,256, issued on Mar. 5, 2013, which claims priority, pursuant to 35 U.S.C. §119(e), to PCT Application No. PCT/GB2005/003273 filed Aug. 23, 2005 and Great Britain Application No. 0418796.9 filed Aug. 24, 2004. All of these applications are incorporated by reference in their entirety. 
    
    
     The present invention relates to clamps and in particular, though not exclusively, to a c-ring clamp for engaging cylindrical portions of a sleeve located around a cylindrical body of a downhole tool. 
     BACKGROUND 
     In the drilling and production of oil and gas wells, downhole tools are located on work strings and run into well bores to perform various tasks within the well. Due to the cylindrical nature of the well bore together with the tubing used in the well, these tools typical comprise a cylindrical body located in the work string and annular or part annular components located on or around the body. When run in the well, these components provide for operation of the tool and a desired task is performed in the well. 
     In constructing the downhole tools, one example of a component is an annular sleeve. Relative movement between the sleeve and body operates the tool. The sleeve is typically formed as two semi-cylindrical portions held together along the two longitudinally arranged mating surfaces. The clamping of the portions is achieved by forming a hole through the portions at the mating surfaces. Typically the hole is perpendicular to the mating surface. A recess is formed at each end of the hole to allow a nut and bolt to be used as a fastener to clamp the portions together. 
     A disadvantage of this type of fixing is that the bolts are not properly constrained and consequently the nuts and/or bolts are easily lost and the assembled tool can be tampered with as the fastener is easily accessible. This can result in some tools being used with missing or loose fasteners which can result in failure of the tool downhole. 
     To reduce the number of fastenings, some sleeves are formed with the portions hinged so that fastenings are only required at a single mating surface. However, the single row of fasteners produce a sleeve with asymmetric loading and the fasteners can still be tampered with. 
     To prevent tampering with the fasteners it is known to use fasteners which can only be operated by a unique or specialist implement. Such fasteners have the disadvantage that the implement must be carried beside the tool for times when release of the fasteners is required. This is very inconvenient for tools which are assembled onshore and transported offshore for use. 
     A further example of an annular component is a scraper or brush. On a downhole cleaning tool there will be rows of brushes or scrapers generally biaised outwards to contact the wall of the liner or casing of the well. Movement of the brushes/scrapers over the wall clean debris and other detritus from the wall. On a typical tool, such as that illustrated in U.S. Pat. No. 4,648,447, the body is made up in sections which are threaded together. The cleaning elements, scrapers in this case, are slid over a section of the body to locate under lips or recesses and hold them in position. The second part of the body is then connected to the first and further lips retain an opposite end of the elements. 
     These tools have a major disadvantage in that the work string must be disconnected and reconnected everytime the cleaning elements need to be replaced. This is both time consuming and requires skilled engineers to ensure the biaised elements are correctly positioned when the tool is re-assembled. 
     Another method of retaining the cleaning elements without disconnecting the work string is to locate an annular sleeve over the ends of the elements. This is illustrated in U.S. Pat. No. 5,711,046. However, the annular sleeves have the same disadvantages as discussed hereinbefore. 
     BRIEF SUMMARY 
     It is an object of the present invention to provide a clamp which when in use, the fastener cannot be removed. The clamp is thus tamperproof. 
     It is a further object of at least one embodiment of the present invention to provide a downhole tool having a body on which is located a sleeve with resistance to backing out. 
     It is a further object of at least one embodiment of the present invention to provide a cleaning element for mounting on a downhole tool with resistance to backing out and does not require disconnection of the work string. 
     According to a first aspect of the present invention there is provided a clamp, the clamp comprising first and second connecting members and a fastener operable by a tool to engage the first and second connecting members, each member having an inner surface, an outer surface and a mating surface therebetween, the first connecting member including a fastener retainer located at the mating surface, the second connecting member having a recess at the inner surface proportioned to accept the fastener and a port at the outer surface proportioned to allow entry of the tool to operate the fastener, wherein an aperture between the recess and the port is sized to prevent release of the fastener through the port. 
     As the fastener can not pass out through the port the fastener cannot be pulled from the clamp via the outer surface where a user would have access. In this way the clamp is tamperproof, provides a resistance to backing out so that the fasteners will not be lost. 
     Preferably the recess includes oppositely directed engaging lips at the mating surface. This provides an anchor for the fastener. The lips will also leave an opening for the fastener to access the fastener retainer. 
     Advantageously the aperture comprises a portion of the port. In this way a single bore which is sized to accept the tool but is smaller than the fastener, is all that is required for the port to access the recess. The port may be a cylindrical bore. 
     In a preferred embodiment the fastener is a threaded bolt, operable to engage a threaded bore being the fastener retainer. This arrangement does not require a nut. 
     Preferably the fastener is operable by an Allen key. Allen keys are advantageous in that they are widely available, but can only operate if the exact size is used. 
     Preferably the fastener includes a locking washer. Thus the bolt is held closed once in the fastener retainer. Advantageously the washer could be sprung such that the fastener is advantageously held in the recess until the tool engages. 
     In an embodiment the connecting members are part cylindrical. Thus the outer surface is accessible and the inner surface is inaccessible when the clamp is in use. 
     In a preferred embodiment, the connecting members are c-rings and each member includes the recess and the port, wherein the port is arranged on the mating surface and the recess is arranged on a further mating surface so that, when the mating surfaces are engaged to the further mating surfaces, respectively, a c-ring clamp is provided. 
     According to a second aspect of the present invention there is provided a downhole tool comprising a substantially cylindrical body upon which is located a sleeve, the sleeve comprising at least two part cylindrical portions and at least two fasteners operable by a tool to engage adjacent portions, each portion having an inner surface, an outer surface and first and second mating surfaces therebetween for engagement with opposite mating surfaces of adjacent portions, a fastener retainer located at the first mating surface, a recess at the inner surface proportioned to accept the fastener and a port at the outer surface proportioned to allow entry of the tool to operate the fastener, wherein an aperture between the recess and the port is sized to prevent release of the fastener through the port. 
     Preferably the recess includes oppositely directed engaging lips at the mating surface. This provides an anchor for the fastener. The lips will also leave an opening for the fastener to access the fastener retainer. 
     Advantageously the aperture comprises a portion of the port. In this way a single bore which is sized to accept the tool but is smaller than the fastener, is all that is required for the port to access the recess. The port may be a cylindrical bore. 
     In a preferred embodiment the fastener is a threaded bolt, operable to engage a threaded bore being the fastener retainer. This arrangement does not require a nut. 
     Preferably the fastener is operable by an Allen key. Allen keys are advantageous in that they are widely available, but can only operate if the exact size is used. 
     Preferably the fastener includes a locking washer. Thus the bolt is held closed once in the fastener retainer. Advantageously the washer could be sprung such that the fastener is advantageously held in the recess until the tool engages. 
     According to a third aspect of the present invention there is provided a downhole tool comprising a substantially cylindrical body upon which is located a plurality of cleaning elements, each element comprising a cleaning member located on a base portion and at least two fasteners operable by a tool to engage each portion to the body, each portion having an inner surface, an outer surface and a portion mating surface for engagement with a body mating surface of the body, a fastener retainer located at the portion mating surface, a recess at the inner surface proportioned to accept the fastener and, a port at the outer surface proportioned to allow entry of the tool to operate the fastener, wherein an aperture between the recess and the port is sized to prevent release of the fastener through the port. 
     Preferably the recess includes oppositely directed engaging lips at the mating surface. This provides an anchor for the fastener. The lips will also leave an opening for the fastener to access the fastener retainer. 
     Advantageously the aperture comprises a portion of the port. In this way a single bore which is sized to accept the tool but is smaller than the fastener, is all that is required for the port to access the recess. The port may be a cylindrical bore. 
     In a preferred embodiment the fastener is a threaded bolt, operable to engage a threaded bore being the fastener retainer. This arrangement does not require a nut. 
     Preferably the fastener is operable by an Allen key. Allen keys are advantageous in that they are widely available, but can only operate if the exact size is used. 
     Preferably the fastener includes a locking washer. Thus the bolt is held closed once in the fastener retainer. Advantageously the washer could be sprung such that the fastener is advantageously held in the recess until the tool engages. 
     Preferably a pair of fasteners are located at each end of the cleaning element. More preferably a common recess is provided for each pair of fasteners. 
     Preferably, the cleaning member comprises a brush. Alternatively the cleaning member comprises a scraper. Advantageously the cleaning member is radially biaised from the body in use. 
     According to a fourth aspect of the present invention there is provided a method of fixing between first and second members, the method comprising the steps:
         (a) inserting a fastener in a fastener-sized recess on an inner surface of the first member;   (b) bringing the members together so as to locate the fastener at a fastener retainer of the second member;   (c) inserting a tool through a port smaller than the recess on an outer surface of the first member;   (d) operating the fastening using the tool to engage the first member to the second member.       

     Preferably the fastener is a threaded bolt and the tool is an Allen key. 
     According to a fifth aspect of the present invention there is provided a method of retaining a substantially annular sleeve around a cylindrical body in a downhole tool, the method comprising the steps:
         (a) locating a fastener into a substantially fastener-sized recess in an inner surface of at least two part cylindrical portions;   (b) arranging the part cylindrical portions around the body by locating adjacent mating surfaces of the portions together to provide the sleeve, while aligning each fastener with a fastener retainer at the mating surface of an adjacent portion;   (c) inserting a tool through a port smaller than the recess in an outer surface of a portion to engage a fastener;   (d) operating the fastener to partially engage the adjacent portions; and   (e) repeating steps (c) and (d) on adjacent portions until the portions are fully engaged to each other.       

     Preferably the fastener is a threaded bolt operated by an Allen key. In this way the fasteners need to be tightened sequentially to bring the portions together. This provides an additional anti-tamper and safety feature as the portions can only be released by sequentially slackening off each fastener. 
     In a preferred embodiment there are two part cylindrical portions forming the sleeve. This provides the simplest construction and quickest assembly. 
     According to a sixth aspect of the present invention there is provided a method of retaining cleaning members to a cylindrical body in a downhole cleaning tool, the method comprising the steps:
         (a) locating a pair of fasteners into substantially fastener-sized recesses in an inner surface at each end, respectively of the cleaning member;   (b) arranging the cleaning member on the body and aligning the fasteners with a fastener retainer on the body;   (c) inserting a tool through a port, at an end of the member, smaller that the recess in an outer surface of the member to engage a fastener;   (d) operating the fastener to partially engage the end of the member and body;   (e) inserting the tool through a port, at an opposite end of the member to engage the respective fastener;   (f) operating the fastener to partially engage the respective end of the member and the body; and   (g) repeating steps (c) through (f) until the member is fully engaged to the body.       

     Preferably the fasteners are threaded bolts. More preferably there is a pair of fasteners at each end of the member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will now be described, by way of example only, with reference to the following drawings of which: 
         FIG. 1  is a cross-sectional view of a clamp according to an embodiment of the present invention; 
         FIG. 2  is a cross-sectional view through the section A-A of  FIG. 1 ; 
         FIG. 3  is an illustration of a c-ring clamp according to an embodiment of the present invention; 
         FIG. 4  is an illustration of the port of the c-ring clamp of  FIG. 3 ; 
         FIG. 5  is an illustration of a downhole tool according to an embodiment of the present invention; 
         FIG. 6  is an illustration of the sleeve of  FIG. 5 ; 
         FIGS. 7( a ) and 7( b )  are illustrations of a downhole cleaning tool according to an embodiment of the present invention; 
       and  FIGS. 8( a )-( f )  are illustrations of a cleaning member of the tool of  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION 
     Reference is initially made to  FIGS. 1 and 2  of the drawings which illustrate a clamp, generally indicated by reference numeral  10 , in accordance with an embodiment of the present invention. Numeral  10  comprises first  12  and second  14  mating portions which engage each other. It will be appreciated that mating portions  12 , 14  may be part of any system to be held together. In particular, portions  12 , 14  can be arranged around an object to provide a clamp. 
     Mating portions  12 , 14  are arranged to provide mating surfaces  16 , 18  respectively. Mating surfaces  16 , 18  are abutted and are located between inner surfaces  20 , 22  respectively and outer surfaces  24 ,  26  respectively. 
     Located in the first mating portion  12  is a fastener retainer  28 , this being a threaded hole to receive a threaded bolt. The fastener retainer  28  is located upon the mating surface  16 . To access retainer  28 , fasteners must be located through the mating surfaces  16 , 18  from the second portion  14  to the first portion  12 . It will be appreciated that although the diagram illustrates that the mating surfaces  16 , 18  are of an identical size and shape this may not necessarily be the case and as long as there is a portion of a mating surface where the two can come together and the retainer  28  can be accessed the invention will be met. 
     Located in the second portion  14  is a recess  30 . Recess  30  is a hollowed out section of portion  14 . It is hollowed out from the inner surface  22 . The recess  30  is bounded at a base  32  by a pair of overhanging members or lips  34   a,b . Lips  34   a,b  provide the mating surface  18 . The lips  34   a,b  are separated by a passageway or slot  36 . 
     A fastener in the form of a bolt  38  is located in the recess  30  through the slot  36  and in the figures is shown as screwed into the retainer  28  by passing across the mating surfaces  16 , 18 . The bolt  38  provides a lower circular edge  40  which is arranged to rest upon the lips  34   a,b  when located in the retainer  28  and this provides a bias or downward force to hold the portion  14  against the portion  12  to operate the clamp  10 . 
     Portion  14  further includes a cylindrical bore  42 . Bore  42  originates in the recess  30  and ends on the outer surface  26 . This bore  42  allows access for a tool  44 , such as an Allen key, to be inserted into the recess  30  from the outer surface  26 . 
     In use, the bolt  38  is pushed into the recess  30  from the inner surface  22 . When inserted, the bolt  28  is slid through slot  36  into the recess  30  and is located in a position above the position of the retainer  28 . From the drawings it is apparent that the bolt  38  is sized to fit neatly within the recess  30  and thus will be held within the recess  30  until the portions  12 , 14  are brought together. Access to insert the bolt  38  into the recess  30  is transverse to the direction of engagement of the bolt  38  and thus is parallel to the mating surfaces  16 , 18 . 
     Once retainer  38  is located in recess  30 , portions  12 , 14  are brought together to mate. Allen key  44  is then introduced through the bore  42  from the outer surface  26 . It is noticed that no access is required from the inner surface from this point onwards. Bore  42  is sized so that it is smaller than the head size of the bolt  38  thus the bolt  38  can not be released through the bore  42  to the outer surface  26 . In fact, there is no available passage for the fastener to the outer surface. 
     It will be recognised by those skilled in the art that Allen Key  44  may be referred to as an Allen wrench, hex key or hex head wrench and is a well known tool used to drive screws and bolts with a hexagonal socket in the head. Allen keys are used as they provide the known advantages of six contact surfaces between bolt and driver; contact surfaces of the bolt are protected from external damage; the bolt can be inserted into its socket using the key; torque applied constrained by the length and thickness of the key; very small bolt heads are accommodated; and can be used with a headless screw. 
     The Allen key  44  is located within the bolt  38  via a hex recess located in the top of the bolt  38 . Turning the Allen key  44  will engage the bolt  38  into the fastener retainer  28  by the mating of the screw threads in each. The bolt  38  can be tightened until the edge  40  applies sufficient pressure on the lips  34   a,b  to secure engage the mating surfaces  16 , 18  to each other. Thus portions  12 , 14  are now clamped together. 
     The Allen key  44  can then be removed. With the clamp  10  now secured it is impossible to access the fastener from the outer surface in order to release it without having the Allen key. Further in the event that, for any reason, the fastener  38  loosens and releases, the fastener  38  will not be lost, it will be held within the recess  30  until such time as a further Allen key  44  can be found and used to tighten the clamp  10  again. 
     Reference is now made to  FIGS. 3 and 4  of the figures which illustrate a c-ring clamp, generally indicated by reference numeral  110 , according to a further embodiment of the present invention. C-ring clamp  110  is similar to the clamp  10  shown in  FIGS. 1 and 2  and thus for clarity like parts will be given the same reference numeral with the addition of  100 . Each mating portion  112 , 114  comprises a semi-cylindrical portion having an outer surface  124 , 126  and an inner surface  120 , 122 . The portions  112 , 114  have abutting mating surfaces  116 , 118 . 
     The portions  112 , 114  provide a hollow cylindrical section having a bore  150  formed from the inner surfaces  120 , 122 . In this embodiment, there are now four mating surfaces  116   a,b  and  118   a,b  as there are two points where the portions  112 , 114  now abut to form the c-ring clamp  110 . Each mating surface  116   a,b , 118   a,b  is located against an opposite mating surface  118   a,b , 116   a,b.    
     At the mating surfaces  116   a , 118   b  there is located recesses  130   a,b  ( 130   b  is not shown in the diagram as it is hidden from view). The arrangement of the fasteners  138   a,b  is identical to that of the bolt  38  and thus it will be appreciated there are now two fasteners  138   a,b  to hold the c-ring clamp  110  together. 
     In this embodiment, the fasteners  138   a,b  are threaded bolts which each include a lock washer  152   a,b . The lock washers are preferably Nord-Lock® washers which positively lock the fastener in place and further prevent backing off. The lock washers  158   a,b , when the bolts  138   a,b  are tightened by the Allen key (not shown in this figure), will hold the retainer  138   a,b  in the engaged position against the lips  134   a,b.    
     A Nord-Lock® washer comprises two interlocking rings. The mating surfaces are cams having an angle of movement or rise, “A”. The outer surfaces are ridged to help grip between the fastener head and the workpiece. A Nord-Lock® washer is selected such that the angle “A” is greater than the pitch angle “B” of the threads of the fastener. The key is the difference in angles. Since angle “A” is larger than the angle “B”, the pair of washers expand more than the corresponding pitch of the thread. This tension makes the bolt self-locking. 
     In an alternative embodiment lock washers  152   a,b  are sprung locking rings. Each sprung locking ring provides the abutment edge  140   a,b  to the lips  134   a,b . Each sprung locking ring also insures that the fastener  138   a,b  will remain in the recess  130   a,b  until the pieces  112 , 114  are brought together. This is because the sprung action biases the head  154   a,b  of the fastener  138   a,b  to the roof  156   a,b  of the recess  130  opposite the mating surface  116   a,b . Further the locking ring  152   a,b , when tightened by the Allen key (not shown in this figure), will hold the retainer  138   a,b  in the engaged position against the lips  134   a,b.    
     Although they are not shown in the figures, it will be appreciated that the mating surfaces  116   b , 118   a  include fastener retainers  128   a,b . The fastener retainers  128   a,b  will be located below the fasteners  138   a,b  in an identical manner to that shown in clamp  10 . 
     In use, retainers  138   a,b  are pushed into the recesses  130   a,b  and will be held in place by the locking rings  152   a,b  as they bias the heads of the fasteners  138   a,b  to the roof of the recesses  130   a,b . The two portions  112 ,  114  are then brought together to mate the surfaces  116   a , 118   a  and  116   b , 118   b . The fasteners  138   a,b  in the form of bolts will be located at fastener retainers  128   a,b.    
     The portions  112 , 114  will be used to clamp around a cylindrical or similar shaped body. Alignment means  160  can be located on the inner surface  120  to engage further with the body to be clamped. Similarly, although not shown, the mating surfaces  116 , 118  could be keyed to improve the accuracy of the mating of the two portions  112 , 114 . Once located, an Allen key (not shown) is inserted through cylindrical bore  142   b  to engage a hex fitting  162   b  in the fastener  138   b  as described hereinbefore. This is as illustrated in  FIG. 4 , where the head of the fastener  138   b  can be seen through the bore  142   b  and is accessible to an Allen key, but the head is too broad to pass through the bore  142   b.    
     Each fastener  138  can only be tightened a short distance due to the c-ring nature of the portions  112 , 114 . Thus the fasteners  138  must be tightened gradually in a cyclic pattern until all are fully engaged. Once fully tightened the fasteners  138  clamp the mating surfaces  116 , 118  together to provide a c-ring clamp  110 . 
     By forming two part cylindrical portions which mate together a further safety feature is provided in the clamp  110 . In particular, even with a tool available to release the fasteners  138  from the outer surface  124 , a user will need to be aware that each fastener has to be released consecutively and separately by a small amount in order to open the clamp  110 . Such a procedure to release the clamp  110  prevents a user successfully tampering with a single fastener  138  on the clamp  110 . 
     As with the clamp  10  of  FIGS. 1 and 2 , the clamp  110   FIGS. 3 and 4  does not allow the fasteners  138  to be removed through the bore  142  and thus the fasteners cannot be removed during use. 
     Reference is now made to  FIGS. 5 and 6  of the drawings.  FIG. 5  shows a well bore  70  in which is located a work string or drill string  72  having mounted thereon a downhole tool, generally indicated by reference numeral  74 . Tool  74  is located on the work string by virtue of pin and box sections  76 ,  78  as is known in the art. 
     The tool  74  comprises a substantially cylindrical body  80  having a sleeve  82  located thereupon. Tool  74  operates by movement of the body  80  relative to the sleeve  82 . Sleeve  82  is formed by two semi-cylindrical portions  212  and  214 . Portions  212  and  214  are joined as described hereinbefore with reference to  FIGS. 1 to 4 . Bores  242   a,b  are seen on the portion  214  and are tool access points to attach the portions  212 , 214  together around the body  80 , to form the sleeve  82 . 
     Reference is now made to  FIG. 6  of the drawings which illustrates the sleeve  82  in greater detail. Like parts to those of the earlier figures will be given the same reference numerals now with the addition of  200 . Sleeve  82  is constructed in an identical manner to the c-ring clamp in  FIGS. 3 and 4 . Fasteners  238  are bolts having locking rings  252 , and are operated by an Allen key located through the bores  242 . In the sleeve  82 , there are four cylindrical bores  242 . There are three bores positioned equidistantly along each of two longitudinal axes of the sleeve  82  where the portions  212 , 214  join. 
     In use, the fasteners  238  will be located in each of the four recesses  230  and the portions  212 ,  214  located around the body  80 . A single Allen key is required to tighten the sleeve  82  as only a single fastener can be tightened at one time and the fasteners must be tightened sequentially by graded amounts in order that the mating surfaces  216 , 218  join successfully to provide a perfect sleeve  82  with a continuous outer surface  224 , 226 . Once constructed and the Allen key is removed, the sleeve cannot be tampered with as access to the fasteners is only available through the cylindrical bores and the fasteners cannot be removed through these bores. The fasteners are permanently located in the recesses  230  and thus are securely located within the tool and cannot be released to catch anywhere in the well bore  70 . Further if someone was to tamper with the tool or sleeve in any way they could only gain access to the fasteners to release them if they had a suitable sized Allen key. Even when they had such an Allen key, unfastening a single fastener would not release the portions  212 , 214  as each fastener needs to be loosened sequentially. This helps prevent tampering of the tool or accidental release of the sleeve  82 . 
     Reference is now made to  FIG. 7  of the drawings which illustrates a downhole cleaning tool at  FIG. 7( a ) , with the cross section view of the tool at  FIG. 7( b ) . The cleaning tool  360  comprises a cylindrical body  362  having a first end  364  including the box section  366  for connection to a workstring (not shown). At the opposing end  368  there is located a box section  370  for connection to a lower workstring. This tool  360  can be inserted into a workstring and run into a wellbore. The body  362  includes a longitudinal bore  370  axially there through to provide an internal passageway for fluids through the cleaning tool communicating with the workstring. 
     Located on the body  362  are cleaning elements  372 ,  382 . The cleaning elements comprise brushes  374  and scrapers  384 . Brushes  374  include six rows of bristles which clean wellbore casing when held against the wellbore casing and lining. Each set of bristles is located on a pad or element  376 . There are three elements  376  circumferentially around the body  362  at locations longitudinally spaced along the body  362 . While three elements  376  are shown at each location there may be a number of portions  376  incorporated depending on the coverage required around the body  362 . In an alternative embodiment for instance four pads with brushes  374  will be arranged at each location. Further it will be noted that the brushes  374  at each location are not collinear. That is a rotational alignment has been made between each set of brushes. This is to ensure complete circumferential coverage of the body  362  by the brushes  374 . Similarly the scrapers  382  comprise blades  384  being arranged in triplicate around the body  362  at three longitudinally spaced locations. Again the blades  384  are misaligned in the longitudinal access of the tool to ensure complete circumferential coverage of the body  362  by the blades  384 . Three locations are illustrated for both the brushes  374  and the scrapers  382 . It will be appreciated that any number of those cleaning elements may be incorporated. Additionally the tool is supplied with two stabilisers  386   a, b  together with a fixed stabiliser including a cutting blade  388  toward the forward end  368  of the tool. Additionally also there is a further fixed stabiliser including a magnet holder  390  located toward the upper end  364  of the tool. The magnet holder  390  will attract ferrous materials dislodged by the scrapers and brushes and thereby remove such materials from the well fluid in the wellbore. 
     Reference is now made to  FIG. 8  of the drawings which illustrates a single cleaning element  372 . Although element  372  will be described with reference to brushes it will be appreciated that the brush portion  374  can be replaced by the scraper blades  384 . Element  372  comprises a brush portion  374  including a plate  392  into which is fixed six brushes  394 . Attached to the rear  396  of the plate  392  is a spring in the form of linear expander  398  arranged longitudinally on the plate  392  and fixed by a screw  400 . Such a spring  398  provides a bias to force the plate  392  against lips  402   a    402   b  of a bridge clamp  404 . 
     Bridge clamp  404  comprises a part cylindrical section (best seen in  FIG. 8 f   ) which includes an aperture  406  in which is held the plate  392  and brushes  374 . It is the bridge clamp  404  which includes the clamping system according to an embodiment of the present invention. At each end  408 ,  410 , respectively of the clamp  404  is located a clamping system generally indicated by reference numeral  412   a, b . Like parts of the clamping system  412  to the system illustrated in  FIG. 1  have been given the same reference numeral with the addition of 300 to aid clarity. The clamping system  412  comprises a first mating portion  314  designed to be held against a second mating portion being the body  362 . The second mating portion is illustrated in  FIG. 7( b )  as that section of the body behind the elements  372 . Mating portion  314  provides a mating surface  316  to abut a mating surface  318  on the body  362 . The mating surfaces  316 ,  318  are abutted and are located between inner surfaces  320  and  322  respectively. 
     Located in the body mating portion  312  are pairs of fastener retainers  328 . The retainers are arranged on the body  362  as a pair of threaded recesses and aligned for each receiving a threaded bolt  344   a, b . The fastener retainers  328  (shown in  FIG. 7( b ) ) are located upon the mating surface  318 . To access the retainers  328 , fasteners must be located through the mating surfaces  316 ,  318  from the bridge clamp  404  to the body  362  across the portions  314  and  312 . Located in the portion  314  is a recess  330 . The recess  330  (shown in  FIG. 8( a ) ) is a hollowed out section of portion  314 . It is hollowed out from the inner surface  322  and is more clearly seen in  FIG. 8( b ) . On inner surface  322  there is a common access point  323 . The access point  323  has a larger portion in the centre  325  and narrower portions at each end  327   a, b  as illustrated. These narrowed portions  327   a, b  define a pair of overhanging members or lips  334   a, b . On the base  332 , lips  334   a, b  provide the mating surface  318 . Lips  334   a, b  are separated by a passageway or slot  336 . 
     Fasteners in the form of bolts  344   a, b  are each located in the recess  330  by insertion through the gap  325  in the slot  336 . Each bolt  344   a, b , when in position, can be screwed into the respective retainers  328  on the body  362 . Each bolt  344   a, b  provides a lower circular edge  440  which is arranged to rest upon the lips  334   a, b  when located in the retainer  328  and this provides a bias or downwards force to hold the portion  314  of cleaning element  372  (shown in  FIG. 8( a ) ) against the portion  312  located on cylindrical body  362  (shown in  FIG. 7( b ) ) and thus operate as a clamp. Portion  314  of cleaning element  372  further includes two cylindrical bores  342   a, b . Each bore  342  extends from the recess  330  to the outer surface  326 . These bores  342  each allow access for a tool, such as an Allen key, to be inserted into the recess  330  from the outer surface  326 . 
     In use plate  392  is located against lips  402  so that the brushes  394  protrude from the outer surface  326 . Bolts  344  are located into the recess  330  through the aperture  325  and slid across the slot  336  to respective ends thereof. This occurs at each end  408 ,  410  of the element  372 . Each bolt  344   a, b  is sized to fit neatly within the recess  330 . Each bolt  344  if further held in position by a locking washer  345  as described herein before. Once each bolt  344  is located in the recess  330  respectively the element  372  is brought against the body  362  so that the bolts  344  align with the fastener retainers  328 . The Allen key is then introduced through a first bore  342   a  and inserted into the respective fastener  344   a  and turned to engage the element  372  to the body  362 . It should be noted that only a partial number of turns of the bolt  344   a  are required, then the tool should be removed through the bore  342   a  and inserted respectively into the other bores  342   b  at either end of the plate to successively tighten all four bolts  344  in turn in a cyclic pattern until the plate  372  is held firmly against the body  362 . It will be recognised that the Allen key has a clearance fit in the bore  342  while the bolt  344  has a head larger than the bore  342  so that each bolt  344  cannot be released from the plate  372  once it has been fastened to the body  362 . 
     Once the bridge clamp  404  is secured it is impossible to access any of the bolts  344  from the outer surface  326  in order to release the clamp  404  without having the required tool i.e. the Allen key. But in the event that any fastener  344  begins to loosen and release the respective fastener  344  will not be lost, it will be held within recess  330  until such time as a further Allen key is used to tighten the clamp  404  back into position. 
     In the event that the brushes  394  wear to such an extent that they need to be replaced, the above described process can be reversed. In this respect the Allen key is inserted through each of the bores  342  in sequence to slacken the fasteners  344  and remove the clamp  404 , from the body  362 . The plate  392  including the brushes  394  can therefore be replaced easily and a new plate  392  be inserted behind the lips  402  and the plate  404  connected to the body  362  as described herein before. 
     It will be recognised that this arrangement allows any plate  392  and brush set  394  or scraper  382  to be replaced without requiring all the brushes or scrapers to also be replaced either on a row or across the entire body. Further this arrangement allows the brushes and/or scrapers to be replaced without disconnecting the body  362  from the workstring. 
     The principal advantage of the present invention is that it provides a clamping system from which the fastener used for the clamp cannot be easily removed or lost when the clamp is in use. This gives a tamperproof feature to the clamp. 
     A further advantage of at least one embodiment of the present invention is that by providing a c-ring clamp with fasteners over at least two surfaces, the load is spread during the clamping operation. 
     It is yet a further advantage of at least one embodiment of the present invention in providing a downhole tool including a sleeve which has the advantageous clamping means. Such a sleeve is tamperproof while providing a secure fixing wherein the fasteners cannot back off and be released into the well bore. Further this arrangement provides an additional safety feature in that no one single fastener can be removed. They must all be removed consecutively. 
     It is still a further advantage of at least one embodiment of the present invention in providing a downhole cleaning tool where the cleaning members can be replaced without disassembling the work string and will not release during use. 
     Various modifications may be made to the invention herein described without departing from the scope thereof. For example though bolts have been described as the fasteners it will be appreciated that other screws may be used as could other fastening means such as clips, or other types which retain two surfaces together and are operated by a tool. Additionally while a hex fitting operated by an Allen key is described, other fittings such as slots, posi-drives and Phillips@ screw heads could be used in conjunction with the relevant screw drivers as the tool.