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You are an expert at summarizing long articles. Proceed to summarize the following text: 
CROSS-REFERENCE TO RELATED APPLICATIONS 
   Not Applicable. 
   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not Applicable. 
   NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT 
   Not Applicable. 
   BACKGROUND OF THE INVENTION 
   The present invention relates to apparatus and method for use in oil and gas exploration and production, in particular, but not exclusively, to a burr mill for selectively performing milling and/or burr removal within a well. 
   When an oil or gas well is drilled it is common to insert a liner or casing into the well in order to support the walls as the depth of the well is increased. In order to access oil or gas containing formation outside the casing, the casing is commonly perforated by means of explosives. As the casing is made of a hardwearing material such as steel, when perforation takes place the steel casing is deliberately damaged to provide access from the wellbore through to the formation and as a result, sections of the casing will be left with exposed metal shards or burrs directed into the wellbore. 
   Consequently, the insertion of any other tools into the wellbore are susceptible to damage due to collisions with or scraping against the burrs formed during perforation. In particular, delicate screens used for the filtering of fluids downhole can easily be ruptured on contact with the burrs. It would therefore be advantageous to find a method of removing these burrs to avoid damaging tools downhole. 
   It is already known to attach a mill to a drill string and by rotation of the drill string through the wellbore, burrs may be removed. These tools have the disadvantage that once they have successfully milled off the burrs they become redundant within the well and if left in place they can both cause unwanted wear on the casing and be exerted to unwanted wear on the milling surfaces of the tool as they are subjected to continuous buffering on the inside diameter of the casing. 
   It is an object of at least one embodiment of the present invention to provide a downhole tool for the removal of burrs or other unwanted debris from inside a wellbore which obviates or mitigates disadvantages in the prior art. 
   It is an object of at least one embodiment of the present invention to provide a downhole tool in the form of a burr mill which is disengagable so that the milling elements can be removed from the surfaces on which the burrs occur. 
   BRIEF SUMMARY OF THE INVENTION 
   According to the first aspect of the present invention, there is provided a downhole tool for the removal of burrs and other unwanted material from an inside surface of a pipeline, well casing or other tubular, the tool including a tool body mountable on a work string, the body supporting a plurality of milling elements which mill the surface and retraction means for disengaging the milling elements from the surface when milling is no longer required. 
   Thus, the tool is capable of providing a milling action to remove burrs when the tool body is rotated on a drill string as it enters the well and at any location where the string requires to be circulated but no milling is required, the milling elements can be disengaged and retracted back into the tool to stop their contact with the inside surface of the casing or liner. 
   Preferably the tool includes biasing means to bias the one or more milling elements in an outward radial direction. Preferably the milling elements are biased into engagement with the inside surface. Advantageously the biasing means comprise springs held under compression. Preferably also the tool includes an outer sleeve, the outer sleeve including one or more apertures through which the milling elements protrude. More preferably the apertures include overhanging portions which engage a part of the milling element and limit the radial movement of the milling element. By limiting the radial movement of the milling elements the springs are held in compression. 
   Preferably the retraction means comprises release means to remove the compression on the springs. Advantageously the release means operates by re-positioning the springs relative to the tool body. The release means may comprise an inner sleeve mounted in a central bore of the tool body into which are located ends of the springs. The springs are re-positioned by virtue of movement of the inner sleeve from a first position in which the milling elements are engaged to the inside surface and a second position where the milling elements are disengaged. 
   Preferably the inner sleeve is held in the first position by at least one shear pin. More preferably the inner sleeve includes a ball seat into which a drop ball can locate. Once located a pressure build up behind the ball will force the ball against the drop inner sleeve until the shear pin shears and the inner sleeve falls into the second position. 
   Preferably the retraction means further includes one or more magnets. Preferably the magnets hold the milling elements against the tool body when disengaged. 
   Preferably also the tool includes a by-pass means which maintains fluid flow through the central bore by allowing fluid to by-pass the drop ball when the tool is disengaged. Advantageously the by-pass means comprises one or more radial ports in the inner sleeve and one or more recesses in the tool body. When the inner sleeve is in the second position, the one or more recesses are located adjacent the drop ball and one or more flow paths are created as the one or more ports align with the one or more recesses thereby directing fluid around the drop ball. 
   According to a second aspect of the present invention, there is provided a method of removing burrs or other unwanted debris from an inside surface of a pipeline, well casing or other tubular, the method comprising the steps:
         a) inserting into the tubular one or more milling elements;   b) biasing the one or more milling elements against the surface to provide a milling action when the elements are moved in relation to the surface;   c) disengaging the one or more milling elements from the surface to prevent further milling.       

   Preferably the method further includes the step of actively retaining the milling elements in a retracted position away from the surface of the tubular. 
   Preferably step (c) includes the step of dropping a ball into the tool to cause parts thereof to move in relation to each other and thereby re-position springs within the tool. 
   More preferably the method includes the step of magnetically retaining the one or more milling elements against the tool body when disengaged. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     An embodiment of the present invention will now be described by way of example only with reference to the accompanying Figures in which: 
       FIGS. 1  ( a ) and  1  ( b ) are a schematic cross-sectional view of a downhole tool in both an engaged (LHS) and disengaged (RHS) position in accordance with an embodiment of the present invention; 
       FIG. 2  is a [top] view of a milling element of the apparatus of  FIG. 1 ; and 
       FIG. 3  is a sectional view through the line X—X of  FIG. 1  ( b ). 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Reference is initially made to  FIG. 1  of the drawings which depicts a downhole tool generally indicated by reference numeral  10  according to an embodiment of the present invention. Tool  10  includes a tool body  12  through which is axially located a central bore  14  for the passage of fluid through the tool  10 . At an upper end of tool body  12  is located a box section  16  and at a lower end of tool body  12  there is located a threaded pin  18 . Box section  16  and threaded pin  18  allow the tool  10  to be connected in a drill string (not shown). 
   Within the central bore  14  there is an inner sleeve  20 . Inner sleeve  20  includes four ports  22  which when the sleeve is moved can locate across a recess  24  in the tool body  12  and provide an alternative flow path. This is illustrated in  FIG. 1  at the upper end of the tool where the inner sleeve  20  has been moved by the action of a drop ball  26  being placed in the central bore  14 . Inner sleeve  20  is kept initially in place by the use of shear screws  28 . When ball  26  is dropped through the central bore  14 , it lands on the ball seat at the upper end of inner sleeve  20 . A fluid pressure build up behind ball  26  forces the ball  26  downwards with the result that the screws  28  shear under the force. Sleeve  20  then falls until it is prevented from exiting the lower end of the tool  10  by virtue of the lip  30 . 
   Milling elements  32  are arranged around the tool body  12 . In the embodiment shown there are three milling elements arranged equidistantly around the tool body as shown more clearly with the aid of  FIG. 3 . 
   Referring to  FIG. 2 , it is seen that each milling element has a milling surface which is arranged with projections to aid the milling action for the removal of burrs and other unwanted debris from the inside walls of the pipeline, liner or casing in use. Consequently, each milling surface  34  has a radial profile to provide a match to the pipeline wall (not shown). The milling elements  32  are not fixed to the tool body  12 . The milling elements  32  are free-floating and are held in the extended position against the pipeline walls by virtue of springs  36 ,  38  and  40  located between the milling elements  32  and the inner sleeve  20 . To aid the insertion of these springs  36 ,  38  and  40  when the tool is assembled, magnets  42  are located in recesses on a back surface  44  of the milling element wherein each spring  36 ,  38  and  40  locates in the recess and is held in place by the magnet  42 . The opposing end of each spring  36 ,  38  and  40  is held in a narrow recess  46  on the inner sleeve  20 . Also located on the back surface  44  of the milling element  32  are additional retraction magnets  48  and  50 . Magnets  48  and  50  are located adjacent elongate ports  52  and  54  into which are located socket head cap screws  56  and  58  whose purpose will be described hereinafter. 
   Milling element  32  is limited in radial movement by stand off sleeves  60  and  62 . Each stand off sleeve  60  and  62  has opposite handed threads thus in this embodiment stand off sleeve  60  has a left hand thread while stand off sleeve  62  has a right hand thread. Each sleeve  60  and  62  includes a lip  64 ,  66  which engages the corresponding lip  68 ,  70  on the milling element  32  to prevent the radial movement. Thus, milling element  32  is biased radially outwards by the use of the springs  36 ,  38  and  40 . As better shown in  FIG. 3 , it will be appreciated that the springs  36  and magnets  42  may be paired up. Although those skilled in the art will appreciate that any number of milling elements may be used and the size and arrangement of the springs may be adjusted, as long as the overall effect is to bias the milling elements and in particular, the milling surfaces  34  outwardly. 
   In use the milling elements  32  are arranged on the tool body  12  in the configuration shown to the left hand side of  FIG. 1 . The tool  10  is attached to the drill string and the drill string rotated into the casing or liner. On entering the casing or liner the milling elements are in the expanded position by virtue of the springs  36 ,  38  and  40  radially biasing the milling surface  34  against the inner surface of the casing. The milling element  32  may move in relation to the diameter of the casing so that casing inner diameters of various sizes can be used with the tool. As the tool is rotated, burrs present on the inside wall of the casing will be dressed off and removed as will any other debris on the surface of the casing walls. When it is necessary to stop a de-burring or milling process but the drill string still requires to rotate to operate other tools which may be mounted thereon, a drop ball  26  is released into the central bore  14  of the tool. The drop ball  26  will typically be released at the surface and travel through the central bore of the drill string to enter the tool  10  at its location in the wellbore. Drop ball  26  will close the central bore  14  as it impacts on the inner sleeve  20 . Fluid pressure will build up behind the ball  26  and the resulting force will cause the shear screws  28  to shear thereby allowing the inner sleeve  20  to fall towards the lower end of the tool. In falling the port  22  will locate over recess  24  in the tool body  12  so that flow is maintained through the central bore  14  of the tool  10 . At the location of the milling elements  32 , movement of the inner sleeve  20  will cause the springs  36 ,  38  and  40  to be re-positioned longitudinally with respect to the tool body  12 . Narrow recesses  36  will ensure that the end of the springs  36 ,  38  and  40  and located in the narrow recess  46  will be forced downwards which will release the opposing end of each spring  36 ,  38  and  40  from the magnet  42 . Once the springs  36 ,  38  and  40  have been re-positioned, the milling element  32  will be pulled radially inwards by the action of the magnets  42  against the re-positioned springs  36 ,  38  and  40  with the result that the milling element  32  will be pulled to a retracted position away from the walls of the casing. Milling element  32  will be held in the retracted position by virtue of the retraction magnets  48  and  50  remaining attached and attracting the socket head cap screws  56  and  58 . Thus, in the disengaged position the milling elements  32  are held against the tool body  12  and the milling operation is stopped. In order to prevent passage of fluid into the region where the springs  36 ,  38  and  40  and magnets  42 ,  48  and  50  are located, the inner sleeve  20  includes a series of ‘O’ rings  72  and  74 . 
   The principle advantage of the present invention is that it provides a milling tool where the milling elements can be disengaged to reduce wear on the elements and on the casing walls in use. 
   It is a further advantage of the present invention that the milling elements are held against the tool body when the tool is disengaged. 
   Various modifications may be made to the invention described hereinbefore without departing from the scope thereof. For instance, the number and arrangement of milling elements may be varied as long as they are mounted around the tool body and have a milling rib or profile to interact with a surface of the inner wall of the casing. Additionally, there may be more than one set of milling ribs located longitudinally which can be operated by a single ball drop. It will also be appreciated by those skilled in the art that a number of these tools may be mounted in relation to each other on a drill string each being operated separately by means of different sized drop balls. Thus, the lowest positioned tool would have a small inner sleeve so that the drop ball would be small enough to fall through the central bore and inner sleeve of the milling tools placed above it.

Summary:
A downhole tool for use in the removal of burrs or other unwanted material from an inner surface of a pipeline, well casing or other tubular. The tool has a plurality of milling elements, which may be biased against the surface or retracted from the surface to disengage the tool from the tubular. A drop ball mechanism with a fluid by-pass is described for disengaging the milling elements.