Abstract:
A circulating sub apparatus including a substantially tubular outer body member having a throughbore formed therein and a substantially tubular inner body member. The outer body member having at least one hole formed therein and a displacement mechanism for producing movement of the inner body member relative to the outer body member such that the inner body member is moveable between an open configuration and an obturated configuration. The inner body member includes a seat member adapted to catch a dropped object, the seat member is located upstream of the hole(s) of the outer body member in both the open and obturated configurations, and wherein the seat member is adapted to permit at least a proportion of fluid to flow past the dropped object when it is seated thereon.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119 from Great Britain Patent Application No. 0921440.4 filed on Dec. 8, 2009, the disclosure of which is incorporated by reference herein. 
     RELATED ART 
     1. Field of the Invention 
     The present disclosure relates to an apparatus and method relating to a circulating sub and also to a drop ball, and more particularly to a multi-activation circulating sub for use in energy exploration and drilling that can be opened and closed with dropped objects and more particularly can be repeatedly operated without having to use objects that increase in size. 
     2. Brief Discussion of Related Art 
     Circulating subs are used to redirect circulation of downhole fluid to transport debris or cuttings produced from the cutting action and also to allow pumping of Lost Circulation Material (LCM). Generally, circulating subs can be operated in an open and in a closed position. Often, a conventional circulating sub can only be moved once from one to the other position. Other conventional circulating subs can be opened by dropping a first object such as a drop ball which can leave the circulating sub when the seat enlarges, for example when it is moved into a recess. Depending on the design of the circulating sub, it can only be operated again either when a second drop ball larger than the first is dropped into the circulating sub to land on the enlarged seat, or can only be operated a particular number of times because the drop balls will fill up a drop ball catching chamber. 
     INTRODUCTION TO THE INVENTION 
     According to a first aspect of the present disclosure, there is provided a circulating sub apparatus comprising: 
     a substantially tubular outer body member having a throughbore formed therein; 
     a substantially tubular inner body member; 
     wherein at least the outer body member further comprises one or more holes formed therein; and 
     a displacement mechanism for producing movement of the inner body member relative to the outer body member such that the inner body member may be moved between:
         an open configuration, in which the one or more holes on the outer body member are open such that fluid may pass between the throughbore and the outside of the circulating sub apparatus via the one or more holes; and   an obturated configuration, in which the one or more holes on the outer body member are obturated;       

     wherein the inner body member comprises a seat member adapted to catch a dropped object characterized in that the seat member is adapted to permit at least a proportion of fluid to flow past the dropped object when it is seated thereon. 
     In exemplary form, the seat member is located upstream of the one or more holes of the outer body member in both the open and closed configurations. 
     According to an alternative first aspect of the present disclosure, there is provided a circulating sub apparatus comprising: 
     a substantially tubular outer body member having a throughbore formed therein; 
     a substantially tubular inner body member; 
     wherein at least the outer body member further comprises one or more holes formed therein; and 
     a displacement mechanism for producing movement of the inner body member relative to the outer body member such that the inner body member may be moved between:
         an open configuration, in which the one or more holes on the outer body member are open such that fluid may pass between the throughbore and the outside of the circulating sub apparatus via the one or more holes; and   an obturated configuration, in which the one or more holes on the outer body member are obturated;       

     wherein the inner body member comprises a seat member adapted to catch a dropped object characterized in that the seat member is located upstream of the one or more holes of the outer body member. 
     In exemplary form, the seat member of the alternative first aspect is adapted to permit at least a proportion of fluid to flow past the dropped object when it is seated thereon. 
     Typically, when in the obturated configuration, the one or more holes on the outer body member are obturated by the inner body member. 
     In exemplary form, the object is a ball and the seat member is adapted to catch a ball which is dropped down the throughbore of the circulating sub apparatus from the surface of a borehole into which the circulation sub is run on a string of tubulars. 
     Typically, the dropped object substantially blocks the throughbore of the circulating sub when it lands on the seat member but, in exemplary form, the seat member comprises slots, apertures or other suitable forms of bypass channels which remain open or unblocked when the object is landed on the seat member and the slots or the like permit a certain proportion of fluid to flow past the dropped object when it is seated on the seat member. 
     When the object blocks the inner passage of the circulating sub the downhole fluid pressure and/or the force caused by the fluid flow acting on the inner body member is increased and displaces it in a downward or downstream direction. In both the open and the obturated configuration, downhole fluid flows past the seat member and thus past an object when seated in the seat member. 
     The displacement mechanism is controlled by downhole fluid flow and/or pressure that acts on at least a portion of the displacement mechanism and/or the inner body member. 
     In the open configuration of the circulating sub, downhole fluid can flow from the surface of the borehole, through an inner passage such as a throughbore of a tubular string, wherein the inner passage is typically substantially parallel to the longitudinal axis of the circulating sub and typically from the throughbore of the circulating sub and from the throughbore of the inner body member wherein at least a portion of the fluid will flow through the one or more holes in the outer body member to the borehole annulus located outside of the circulating sub. 
     In the obturated or closed configuration of the circulating sub, downhole fluid typically can flow from the surface of the borehole, through an inner passage such as a throughbore of a tubular string, wherein the inner passage is typically substantially parallel to the longitudinal axis of the circulating sub and typically from the throughbore of the circulating sub and from the throughbore of the inner body member and flow out of a bottom end of the circulating sub, for example to the throughbore of equipment located in the tubular string below the circulating sub. 
     The circulation sub apparatus is, in exemplary form, used with an object that is adapted to erode or dissolve over time when it is landed on the seat member. The object and, in exemplary form, the ball is typically eroded over a certain time period by the action of the downhole fluid that is passing the ball while flowing through the slots of the seat member. The ball in exemplary form consists of a material that will not be eroded to an extent which would make it impossible to complete the opening operation of the circulating sub until the operation is completed. 
     The inner body member and/or the displacement mechanism in exemplary form comprise a piston. 
     Typically, the inner body member further comprises one or more holes therein. 
     Typically, the one or more holes of the inner and/or outer body member are substantially transverse to the longitudinal axis of the inner and/or outer body member. 
     Typically, movement of the inner body member into the open configuration moves the one or more holes of the inner body member into fluid communication with the one or more holes of the outer body member. 
     In exemplary form, the displacement mechanism is adapted to permit the inner body member to be repeatedly moved between the open position and the obturated position. 
     In exemplary form, the seat member is provided on or towards the upper end of the inner body member and typically, the seat member is located above the one or more holes of the inner body member as well as above the one or more holes of the outer body member. 
     The inner body member in exemplary form comprises a lower portion and an upper portion. Typically, the upper portion comprises the seat member and the one or more holes. The upper portion can further comprise a blocking portion which is provided such that it obturates the holes of the outer body member from inside the outer body member when the circulating sub is in the closed configuration. The lower portion typically engages at least a portion of the displacement mechanism when the inner body member is moved due to the force fluid flow and/or pressure. 
     Typically, the displacement mechanism further comprises a locking mechanism for locking the inner body member in at least two (or three) positions relative to the outer body member. 
     In exemplary form, the displacement mechanism further comprises a cam member comprising one of a lock device and which may comprise a key device and a guide mechanism which may comprise a slot arrangement for engagement with the lock device. 
     The displacement mechanism can further comprise a biasing mechanism which can, in exemplary form, comprise a spring member for biasing the inner body member towards or into one of the open and closed configuration. The biasing mechanism is in exemplary form arranged such that it resists and/or stores energy when the inner body member is moved downwards or downstream and/or is positioned in the open configuration due to pressure or force exerted on the inner body member by fluid flow and/or pressure. Typically, the biasing mechanism is adapted to release the stored energy and thereby expand when the said force is released. 
     In exemplary form, the displacement mechanism further comprises a biasing mechanism retaining member which may comprise a substantially tubular hollow member positioned below the inner body member to engage the inner body member and the spring member and the substantially tubular hollow member comprises a shoulder to separate and thereby prevent the spring member from engaging the cam member. 
     The cam member in exemplary form provides at least three locking positions for locking the inner body member in at least three positions relative to the outer body member by means of the locking member. The locking positions can be provided such that the ports of the inner and outer body member are in fluid communication and more in exemplary form are in a substantially aligned relationship in at least two of the at least three locking positions and in an obturated configuration such that the fluid is not able to communicate between the holes of the inner and outer body members in the at least one other locking position. In one of the said two fluid communication locking positions, the circulating sub can be in a fully open port configuration. This provides the advantage that downhole fluid can flow through the circulating sub and the said holes without a dropped object partially blocking the seat. The fully open configuration is in exemplary form provided when the object dropped into the circulating sub is no longer caught in the seat member and has been eroded and flushed out of the lower end of the circulating sub. 
     The locking positions are more in exemplary form provided such that when the locking member is positioned in a first locking position, in which the holes are in an obturated position, and the cam member is rotated, the following two locking positions provide the open hole configuration of the holes of the inner and outer body member. 
     Typically, the holes of the inner body member are elongated along the longitudinal axis of the inner body member such that an aligned position of the holes of the inner and outer body member can be established over a certain section or length of the inner body member. The length of the said certain section may be in the region of a length equivalent to the longitudinal length of the elongated holes of the inner body member. 
     Typically, the holes of the outer body member are provided as nozzles or ports formed through a side wall thereof. 
     In exemplary form, the inner body member comprises one or more grooves for a retaining seal on an outer surface thereof transverse to its longitudinal axis. Typically, the one or more grooves can be provided on an outer surface of the seat member and/or on an outer surface of the blocking portion. The grooves and the seal are adapted to prevent downhole fluid from flowing past the outer surface of each of the seat member, the blocking portion and/or the lower portion of the inner body member. 
     According to a second aspect of the disclosure, there is an object for dropping into a fluid flow pumped down a borehole in a downhole well, the object comprising one or more chambers therein. 
     In exemplary form, the object is a ball and more in exemplary form the object is hollow. The chamber may be a void comprising a vacuum but may in exemplary form comprise a chamber that is filled with a material that differs in physical properties such as burst or collapse strength compared to the rest of the object. Typically, the chamber may be filled with a gas at a pre-determined pressure and in further exemplary embodiments may be filled with air at atmospheric pressure. In exemplary form, the chamber is sealed from the environment outside of the ball and is in exemplary form sealed by the rest of the material that forms the sidewall or body of the ball. 
     In exemplary form the ball is formed from a material around the chamber that is erodible in the fluid flow and more particularly is adapted to be eroded to a certain extent and then collapse or implode due to the pressure of the external fluid being far higher than the internal pressure of the ball. 
     In exemplary form, the ball is particularly for use with the circulating sub according to the first aspect of the disclosure such that the erodible hollow ball is adapted to be landed on the seat member of the circulating sub. The exemplary features of the second aspect of the disclosure can be incorporated into the first aspect of the disclosure as appropriate. 
     Embodiments in accordance with the first aspect of the disclosure have the advantage that they can effectively be used with embodiments of an erodible ball in accordance with the second aspect of the present disclosure. A fully open configuration of the circulating sub, which is the configuration in which downhole fluid can flow through the circulating sub and the holes or ports without a ball in the seat, can be established in a rather short period of time. From this open port configuration, the circulating sub can easily be returned to a closed port configuration by dropping another erodible ball, which is similar to the first one, into the downhole string. The circulating sub will then be closed in about the same time that was needed to establish the fully open configuration because the ball is exposed to the same conditions as the first ball, i.e. a pressure affecting the ball and/or the amount of fluid flowing past the ball inside the circulating sub creating friction on the ball which erodes or for certain materials of ball will dissolve the ball. This advantage results from a number of aspects including the seat member being located upstream of the ports in both the open and the closed configuration of the circulating sub. Furthermore, when the ball is hollow, it does not need to be eroded completely but rather to an extent in which the outside pressure is sufficient to crush the ball due to the differential pressure inside the ball. Furthermore, erodible balls may be used instead of dissolvable balls because the erodible ball will not experience much erosion on the path from the surface of the borehole to the seat because there is much less friction acting on the ball during that time because the ball is being carried along by the fluid through the string as opposed to being eroded away when it is caught by the seat member due to the friction acting on it from the relatively high velocity downhole fluid travelling past the ball. To the contrary, a dissolvable ball may suffer from the disadvantage that it could dissolve before it reaches the seat because it will dissolve in static fluid as well as fluid moving past the ball and therefore erodible balls may be preferred to dissolvable balls. 
     Furthermore, an erodible ball provides the advantage that the material can for example be rather slowly erodible such that the ball will not be substantially eroded on its way through the downhole string (even though it is in contact with the downhole fluid) and would thus not be substantially eroded and therefore be relatively useless before an opening or closing operation has been started or is completed. The ball used according to the disclosure will then, in combination with the downhole fluid pressure on the ball, only be sufficiently small to be flushed through the seat member down the circulating sub, e.g. by being eroded or collapsing/imploding on itself when having been eroded to a certain extent, after it has served its purpose. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present disclosure will now be described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  is a part cross-sectioned perspective side view of a circulating sub apparatus according to the first aspect of the present disclosure in a closed configuration; 
         FIG. 2   a  is a cross-sectional side view of the  FIG. 1  apparatus in the same closed configuration as that of  FIG. 1 ; 
         FIG. 2   b  is a cross-sectional side view of the  FIG. 1  apparatus in a first open configuration; 
         FIG. 2   c  is a cross-sectional side view of the  FIG. 1  apparatus in a second open configuration; 
         FIG. 3  is a perspective side view of a cylindrical cam sleeve incorporated in the circulating sub apparatus of  FIG. 1 ; 
         FIG. 4  is a diagram of the path steps of the cam sleeve of  FIG. 3 ; 
         FIG. 5  is a perspective side view of an inner body member incorporated in the circulating sub apparatus of  FIG. 1 ; 
         FIG. 6  is another part cross-sectioned view of the  FIG. 1  apparatus in the same closed configuration as  FIGS. 1 and 2   a  and just prior to a drop ball in accordance with the second aspect of the present disclosure landing on a seat of the circulating sub apparatus; 
         FIG. 7  is another part cross-sectioned perspective side view of the  FIG. 1  apparatus in the same closed configuration as  FIGS. 1 ,  2   a  and  6  but immediately as the drop ball of  FIG. 6  has landed on the seat, but before the effect of the drop ball landing on the seat is experienced by the circulating sub; 
         FIG. 8  is another part cross-sectional perspective side view of the  FIG. 1  apparatus in the same first open configuration as that of  FIG. 2   b  and with the drop ball of  FIG. 6  still in position on the seat; 
         FIG. 9  is another part cross-sectional perspective side view of the  FIG. 1  apparatus in the same first open configuration as that of  FIG. 2   b  immediately prior to the ball having been eroded, but before that erosion is experienced by the circulating sub; 
         FIG. 10  is another part cross-sectional perspective side view of the  FIG. 1  apparatus in the same second open configuration as that of  FIG. 2   c;    
         FIG. 11  is another part cross-sectional perspective side view of the  FIG. 1  apparatus in the same second open configuration as that of  FIG. 2   c  and just prior to another drop ball in accordance with the second aspect of the present disclosure landing on the seat; 
         FIG. 12  is another part cross-sectional perspective side view of the  FIG. 1  apparatus in the same second open configuration as that of  FIG. 2   c  and immediately after the other drop ball of  FIG. 11  has landed on the seat but before the effect of the drop ball landing on the seat is experienced by the circulating sub; 
         FIG. 13  is another part cross-sectional perspective side view of the  FIG. 1  apparatus in the same first open configuration as that of  FIG. 2   b  but with the second drop ball on the seat; 
         FIG. 14  is another part cross-sectional perspective side view of the  FIG. 1  apparatus in the same first open configuration as that of  FIG. 2   b  but immediately after the second drop ball has eroded away, but before the effect of that erosion is experienced by the circulating sub; 
         FIG. 15  is another part cross-sectional perspective side view of the  FIG. 1  apparatus in the same closed configuration as that of  FIG. 2   a ; and 
         FIG. 16  is a cross-sectional side view of a drop ball in accordance with the second aspect of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The exemplary embodiments of the present invention are described and illustrated below to encompass apparatus and method relating to a circulating sub and also to a drop ball, and more particularly to a multi-activation circulating sub for use in energy exploration and drilling that can be opened and closed with dropped objects and more particularly can be repeatedly operated without having to use objects that increase in size. Of course, it will be apparent to those of ordinary skill in the art that the embodiments discussed below are exemplary in nature and may be reconfigured without departing from the scope and spirit of the present invention. However, for clarity and precision, the exemplary embodiments as discussed below may include optional steps, methods, and features that one of ordinary skill should recognize as not being a requisite to fall within the scope of the present invention. 
       FIG. 1  shows an example of a circulating sub apparatus  10 , also referred to as circulating sub  10  below, according to the first aspect of the present disclosure with an outer body member  12  and an inner body member  14 . The outer body member  12  comprises a pin connection  16  on a lower end of the outer body member  12  and a box connection  18  on an upper end of the outer body member  12 . The pin connection  16  comprises a screw threaded OCTG connection that allows the circulating sub  10  to be coupled to another downhole tubular such as a drill pipe or the like with a corresponding box connection. The box connection  18  also comprises a screw threaded OCTG connection that also allows another piece of Bottom Hole Assembly (BHA) or drill pipe or the like with a corresponding pin connection to be coupled to the circulating sub  10 . In this view, two ports  20  are provided as apertures or holes through the sidewall of the outer body member  12 . Further ports or holes through the sidewall of the outer body member  12  may be positioned on a back side of the outer body member  12  and/or on the part of the outer body member  12  which is cut away in  FIG. 1  to provide additional potential fluid pathways through the sidewall of the outer body member  12 . The inner body member  14  has elongated holes or slots  22  formed in an upper portion  24  thereof. A seat member  26  for catching or arresting movement of a drop ball (not shown) is provided on the top of the upper portion  24 , above the holes  22  of the inner body member  14 . The seat member  26  comprises slots  28  such that some downhole fluid can bypass the seat member  26  through these slots  28  when a drop ball is landed on the seat member  26 . 
     At about its longitudinal midpoint, the inner body member  14  has a shoulder  72  which forms an upper end of a blocking or obturating portion  30  of the inner body member  14  and is described in further detail below. 
     The circulating sub  10  also comprises a displacement mechanism which is primarily adapted to control movement of the inner body member  14  relative to the outer body member  12 . The displacement mechanism comprises a locking member in the form of a key  32 , a cam sleeve  34 , a tubular spring retainer  36  and a biasing mechanism, which is in exemplary form in the form of a coil spring  38 . 
     The cam sleeve  34  is in exemplary form arranged such that it can freely rotate with respect to the inner body member  14  but in an alternative embodiment, the cam sleeve  34  can be rotationally locked to a lower portion  40  ( FIG. 2   a ) of the inner body member  14  by, for instance, a spline arrangement (not shown). Thereby, it covers a least the length of the lower portion  40  of the inner body member  14 , which is thus not shown in  FIG. 1 . A locking member in the form of a key  32  which is fixedly mounted on the outer body member  12  is engaged in a channel or slot  42  on an outer surface of the cam sleeve  34  such that the inner body member  14  can be selectively axially (longitudinally) locked with respect to the outer body member  12  according to locking positions  44 ,  46 ,  48  provided on the slot  42 . The cam sleeve  34  is shown in more detail in  FIGS. 3 and 4 . 
     The tubular spring retainer  36  is secured to the lower end of the inner body member  14  and traps the cam sleeve  34  in position around the lower portion  40 . The coil spring  38  is positioned in a lower part of the outer body member  12  immediately above the pin connection  16 . At least some coils of the coil spring  38  are positioned around the tubular spring retainer  36 . The tubular spring retainer  36  comprises a flange at its uppermost end which provides a shoulder  50  to prevent the coil spring  38  from contacting with the cam sleeve  34 . 
     The significant parts having been described above, the operation of the circulating sub  10  will now be explained. 
     In certain operations, the circulating sub  10  is required to be run into a borehole in a closed or obturated position such as that shown in  FIG. 1 . The ports or holes  20 ,  22  of the outer and the inner body member  12 ,  14  are not aligned, so downhole fluid that is pumped from the surface down the throughbore of the drilling string to the drill bit at the very bottom of the drill string is forced to flow through the throughbore (i.e. inner passage  52 ) of the circulating sub  10  and subsequently downwards to equipment located below the circulating sub  10  such as a motor for drilling (not shown). In the closed or obturated configuration of the circulating sub  10 , the holes or ports  20  of the outer body member  12  are additionally sealed with respect to the holes  22  of the inner body member  14  by suitable seals such as ‘O’ ring seals  31 ,  33  which are provided in corresponding grooves  74 ,  76  ( FIG. 5 ), one of which is located above and one being located below the holes  22  on the obturating portion  30 . At this point, (i.e. in the closed configuration) the key  32  is positioned in a closed locking position  44  which is the locking position on a lower part of the cam sleeve  34 . The circulating sub  10  is also shown in the obturated configuration in  FIG. 2   a.    
     If the operator wishes to open the ports  20 ,  22  (e.g. to pump LCM to plug the borehole when losses are experienced or to assist lifting drill cuttings back up to the surface from a particular location of the borehole), the operator drops a ball  54  into the fluid pumped down the throughbore at the surface. The ball  54  is of such a diameter that it is pumped down the throughbore of the drill string until it lands on the seat member  26  as shown in  FIG. 2   b . Downhole fluid flowing into the circulating sub  10  has therefore carried the ball  54  and landed it on the seat member  26  on the inner body member  14  because the diameter of the ball  54  is greater than the throat diameter of the seat  26 . Although some downhole fluid may flow past the seat member  26  through the slots  28 , the pressure in the downhole fluid located above the seat member  26  will increase high enough to overcome the biasing force of the coil spring  38  such that the inner body member  14  will move downwards. The cam sleeve  34  and the spring retainer  36  have also moved down consequentially. By means of vertically or longitudinally moving the cam sleeve  34 , it is forced to rotate due to the fixedly mounted interaction of the key  32  in the slot  42 . This way, the key  32  arrives at a first open locking position  46 A (shown in  FIGS. 3 and 4 ) on an upper end of the cam sleeve  34 . The circulating sub  10  is now in an open configuration, in which the inner body member  14  is in its furthest position of travelling downwards with respect to the outer body member  12  within the circulating sub  10 . The coil spring  38  is now in a compressed state and the lower end of the spring retainer  36  is in contact with a shoulder  51  at the lower end of the circulating sub  10  immediately above the pin connection  16 . The elongated holes  22  of the inner body member  14  are positioned such that an upper part of them is aligned with the ports  20  of the outer body member  12 . Indeed, downhole fluid is thus able to flow out of the circulating sub  10  through the ports  20  when the inner body member has moved a certain distance in the downward direction such that any part of the elongated holes  22  overlap the ports  20 . 
     As shown in  FIGS. 2   b  and  16 , the hall  54  in accordance with the second aspect of the present disclosure and seated in the seat member  26  is hollow to a certain extent, for example 50 percent, of its diameter such that it contains a sealed chamber  55  at its centre. The chamber  55  may be filled with air or any other suitable gas or it could be void such that it contains a vacuum at its centre  55 . Since the seat member  26  is positioned upstream of the elongated holes  22  of the inner body member  14 , the downhole fluid which flows into the circulating sub  10  always has to flow past the ball  54  and through the slots  28  of the seat member  26  to flow out of the circulating sub  10  (whether through the ports  20  in the open configuration or through the bottom end  16 ). 
     The ball  54  is formed from a material which will erode due to the passing downhole fluid and examples of suitable erodible materials may be cement, or a mixture of sand and resin. Alternatively, the ball  54  could be formed from a soluble material such that the ball  54  dissolves rather than erodes, and an example of a suitable soluble material for such a dissolvable ball  54  is that used by Santrol (www.santrol.com) in their BIOBALLS MR®, but other erodible or soluble materials could also be used. 
     If an erodible material is used, the ball  54  will be eroded when it is exposed to downhole fluid for a certain period of time. When the erosion has proceeded to an extent at which the differential pressure between the internal atmospheric pressure of the ball  54  and the external downhole fluid pressure is sufficiently great, the ball  54  will collapse or implode on itself. Once the ball  54  has collapsed or imploded, the small debris is flushed through the seat member  26  down the circulating sub  10  with the downhole fluid. The pressure and thus the force exerted on the inner body member  14  is released as the inner passage  52  of the circulating sub  10  is no longer partially blocked by the ball  54 . Due to the decreasing force on the inner body member  14 , the biased coil spring  38  expands again, thereby moving the inner body member  14  and the cam sleeve  34  upwards. When moving upwards, the cam sleeve  34  is rotated due to the key  32  engaging the slot  42  ( FIG. 1 ). The upward movement of the inner body member  14  is stopped in an intermediate position when the key  32  latches into an intermediate locking position  48  (see  FIGS. 3 and 4  for details). This state of the circulating sub  10  is shown in  FIG. 2   c.    
     In the state shown in  FIG. 2   c , the circulating sub  10  is still in an open configuration and can be considered an intermediate open configuration in which a lower part of the elongated holes  22  of the inner body member  14  is aligned with the ports  20  of the outer body member  12 , so the downhole fluid can still flow out through the ports  20 . The coil spring  38  is still compressed to a certain extent. The circulating sub  10  will remain in the intermediate state shown in  FIG. 2   c  (intermediate locking position  48  of  FIG. 4 ) even when no downhole fluid is pumped through the circulating sub  10 . 
     As shown in  FIG. 4 , the next state the circulating sub  10  may assume in this example is the second open configuration as shown in  FIG. 2   b  in which the inner body member  14  is in its furthest position of travelling downwards the circulating sub  10  and where the key  32  arrests in the second open locking position  46 B. This can be established when another (second) ball  54  in exemplary form in accordance with the second aspect of the present disclosure, which may be similar to the ball  54  shown in  FIG. 2   b , is dropped into the circulating sub  10  and lands on the seat member  26  and downhole fluid is pumped into the throughbore of the circulating sub  10 . 
     The cam sleeve  34  can be provided such that the circulating sub  10  will return to a closed configuration. This operation will occur when the other (second) ball  54  leaves its place on the seat member  26 , e.g. when it is eroded and collapsed/imploded on itself, so that the inner body  14  and consequently the cam sleeve  34  move upward again, and the key  32  finally latches into the closed locking position  44  ( FIG. 1 ) again. 
       FIG. 3  shows an example of a cam sleeve  34  to be utilized in the circulation sub  10  as shown in  FIGS. 1 ,  2   a ,  2   b  and  2   c . The cam sleeve  34  has a generally tubular body. On its outer cylindrical surface  56 , the cam sleeve  34  is provided with a “W” shaped channel or slot  42  in which a locking member in the form of a key  32  ( FIG. 1 ) can engage. The slot  42  is not as deep as the tubular body itself and is jagged in an unsymmetrical way around the outer side  56  of the cam sleeve. Four “V”-like shaped locking positions  44 ,  46 A,  46 B,  48  are shown, with two locking positions  44 ,  48  pointing with the vertex of the “V” towards a lower end  58  of the cam sleeve  34 . These locking positions  44 ,  48  are modeled in a lower side  60  of the slot  42 . One of these two locking positions is closer to the lower end of the cam sleeve  34  and is also referred to as the closed locking position  44 , whereas the other is closer to a middle portion of the cam sleeve  34  and is also referred to as the intermediate open locking position  48 . The two locking positions  46 A,  46 B pointing with the vertex of the “V” towards an upper end  62  of the cam sleeve  34  are also referred to as the first  46 A and second  46 B open positions. They are modeled in an upper side  64  of the slot  42 . In cooperation with the key  32 , the cam sleeve  34  is responsible for stopping the inner body member  14  ( FIG. 1 ) in different positions, as shown for example in  FIGS. 1 ,  2   a ,  2   b  and  2   c . When the key  32  is positioned in the closed locking position  44 , the inner body member  14  is in a position in which it obturates the ports  20  ( FIG. 1 ) of the outer body member  12  as shown in  FIG. 1  and  FIG. 2   a.    
       FIG. 4  is a diagram of the path steps of the cam sleeve of  FIG. 3  and operation of the displacement mechanism will now be described in more detail. In the diagram, the slot  42  of the cam sleeve  34  is shown in a planar view. A path  66  is shown to illustrate the path of the key  32  ( FIG. 1 ) when the circulating sub  10  ( FIG. 1 ) is activated through one cycle of the various configurations. On the left side of the diagram of  FIG. 4 , the status of the circulating sub  10  is indicated, i.e. closed or opened (in a first and a second configuration). Above the diagram, the status of a pump (for pumping downhole fluid into the downhole string) and whether there is a ball in the seat member is indicated. The path  66  of the locking member  32  starts at the closed locking position  44  at a closed status or closed configuration of the circulating sub  10 . In this state, downhole fluid can be pumped into the circulating sub  10  or not without affecting movement of the inner body member  14  relative to the outer body member  12 . There is no ball  54  ( FIG. 2   b ) in the seat member  26  ( FIG. 1 ). 
     When the ball  54  is dropped and the inner body member  14  ( FIG. 1 ) is moved downwards, the cam sleeve  34  also moves straight vertically downwards (i.e. without rotation) until the key  32  ( FIG. 1 ), which is fixed to the outer body member  12  ( FIG. 1 ), engages an upper side  64  of the slot  42 . Further downward moving of the inner body member  14  will then force the cam sleeve  34  to rotate clockwise (when viewed from above) (either with or around the lower portion  40  ( FIGS. 2   a ,  2   b ,  2   c ) of the inner body member  14  depending on if the sleeve  34  is respectively splined to the lower portion  40  or not) and the key  32  is guided through the narrow part of the slot  42 . The elongated holes  22  ( FIG. 1 ) of the inner body member  14  and the ports  20  ( FIG. 1 ) will then start to overlap such that the closed status of the circulating sub  10  changes to an open status. The inner body member  14  is moved further downwards until the key  32  latches into the first open locking position  46 A. This state of the circulating sub  10  ( FIG. 1 ), the inner body member  14  and the cam sleeve  34  is shown in  FIG. 2   b . An open status or configuration of the circulating sub  10  is provided in which downhole fluid can flow out through the ports  20 . 
     The cam sleeve  34  will stay locked with a key  32  ( FIG. 1 ) locked in the first open locking position  46 A until it is moved upward again with the inner body member  14  ( FIG. 1 ). This will happen when the pressure on the inner body member  14  is released, for example when the ball  54  ( FIG. 2   b ) is no longer located in the seat member  26  due to its erosion and/or collapse/implosion. When this is the ease, the cam sleeve  34  will not start rotating clockwise until the key  32  engages a lower side  60  of the channel  42 . Thereby, the cam sleeve  34  is rotated towards an intermediate locking position  48  so that the inner body member  14  is in a position which is shown in  FIG. 2   c . With the key  32  positioned in this intermediate locking position  48 , the inner body member  14  is in a position which is also referred to as intermediate open position and an open configuration of the circulating sub  10  ( FIG. 2   c ) is still provided. 
     Only with a further downward movement of the cam sleeve  34 , i.e. when higher pressure is exerted on the inner body member, for example when another ball  54  ( FIG. 2   b ) is landed on the seat member  26  ( FIG. 1 ), the key  32  ( FIG. 1 ) will leave the open intermediate locking position  48 . Thus, when the cam sleeve  34  rotates with or around the lower portion  40  ( FIG. 2   a ) of the inner body member ( FIG. 1 ), the next locking position is a second open locking position  46 B and therefore provides an open port configuration as shown in  FIG. 2   b.    
     Once the second ball  54  erodes or dissolves away, the cam sleeve  34  will again move upwards such that the key  32  leaves the second open locking position  46 B and upon rotation of the cam sleeve  34 , the key  32  will arrive again back where it started in the closed locking position  44  and thus provides a closed port configuration of the circulating sub  10  as shown in  FIG. 1 . 
     It is important that downhole fluid is pumped through the downhole string to exert pressure on the ball  54  and the inner body member  14  ( FIG. 1 ) when the hall  54  is seated and the cam sleeve  34  is moved such that the key  32  is positioned into the second locking position  46 . 
     Accordingly, with the cam sleeve  34 , the circulating sub  10  can be repeatedly actuated from a closed configuration to an open configuration by dropping one ball  54  and then to a closed configuration again by dropping another ball  54  into the downhole string, and this provides the advantage that the cycle can be repeated as many times as desired by the operator, with no limit on the number of cycles. 
       FIG. 5  shows an example of the inner body member  14 , also referred to as piston, to be utilized in the circulation sub  10  as shown in  FIG. 1 . The inner body member  14  comprises an upper portion  24  and a lower portion  40 . A seat member portion comprising the seat member  26  is located at the uppermost and upstream end of the upper portion  24 . The seat member  26  is provided to catch a ball  54  ( FIG. 2   b ) which is dropped down a downhole string (not shown) and the circulating sub  10  to at least partially block the inner passage or throughbore  52  ( FIG. 1 ) of the circulating sub  10  thereby operating the circulating sub  10  to an open configuration as shown for example in  FIGS. 2   b  and  2   c , as will be discussed in detail subsequently. Not shown in the  FIG. 5  view are slots  28  (seen in  FIG. 1 ) of the seat member  26  which allow downhole fluid to partially flow past the seat member  26  through the inner body member  14  even when a ball  54  has landed on the seat member  26 . The seat member portion comprises a circumferential or transverse groove  68  around an outer surface of the inner body member  14  in which a seal such as an ‘O’ ring seal  35  can be mounted to prevent downhole fluid from flowing past the outer side of the seat member  26 . At a lower end of the seat member  26 , there is a first shoulder  70  and below this shoulder  40 , the outer diameter of the inner body member  14  is slightly less than the outer diameter at the seat member portion  26 . In this reduced diameter part, the inner body member  14  comprises one or more holes or slots  22  which are evenly distributed around the circumference of the inner body member  14  and are elongated along a longitudinal axis of the inner body member  14 . The elongated holes  22  are in exemplary form located such that they are aligned with holes or ports  20  ( FIG. 1 ) of the outer body member  12  ( FIG. 1 ) as shown in  FIGS. 2   b  and  2   c  because this aligned arrangement reduces any frictional losses experienced by the fluid to a minimum, but the holes  22  and ports  20  need not be aligned because the fluid can pass around the annulus  37  between the outer surface of the upper portion  24  and the inner surface of the outer body member  12 . When the inner body member  14  is moved relative to the outer body member  12 , the elongated holes  22  allow an alignment with the holes or ports  20  of the outer body member  12  along a longitudinal distance up to the length of the elongated holes  22 . Below the portion of the inner body member  14  comprising the elongated holes  22 , the outer diameter of the inner body member  14  increases again at a second shoulder  72 . This increased outer diameter is only retained for a certain distance along the longitudinal axis of the inner body member  14 , thereby forming a portion of the inner body member  14  which can be referred to as a lower part of the upper portion  24  of the inner body member  14  or as a middle or blocking or obturating portion  30 . This is because it is provided such that it obturates the ports  20  or holes of the outer body member  12  from inside the outer body member  12  when the circulating sub  10  is in a closed configuration as shown in  FIG. 1  or  FIG. 2   a . Two transverse grooves  74 ,  76  are provided on the obturating portion  30  for mounting seals such as ‘O’ rings  31 ,  33 . When the ‘O’ ring seals  31 ,  33  are provided, downhole fluid is prevented from flowing past the outer side of the obturating portion  30  further downwards. This will inhibit damage or other negative effects of the operating mode of the cam sleeve  34  for instance. After a third shoulder  78 , the outer diameter of the inner body member  14  reduces again and the inner body member  14  comprises a lower portion  40  which is designated for being at least partially guided into the cam sleeve  34  as shown in  FIG. 3 . The cam sleeve  34  can in exemplary form freely rotate around (or is less in exemplary form rotationally locked to) the lower portion  40  of the inner body member  14  hereinbefore as described relating to  FIG. 3 . 
       FIG. 6  shows the circulating sub  10  in a closed configuration, similar to the configuration shown in  FIG. 1  and  FIG. 2   a . A ball  54  has already been dropped into the downhole string but has not yet landed on the seat member  26 . 
       FIG. 7  shows the circulating sub  10  also in the closed configuration as that at  FIGS. 1 and 2   a  but when the ball  54  has landed on the seat member  26  but the inner body member  14  has not yet moved downwards, for example when downhole fluid has not yet started to build up enough force to result in movement of the inner body member  14 . 
       FIG. 8  is the circulating sub  10  in an open configuration, with the ball  54  still in the seat member  26 , the inner body member  14  located in its furthest position down in the circulating sub, and the locking member  32  in the first open locking position  46 A. This open port configuration is also shown in  FIG. 2   b.    
       FIG. 9  shows the circulating sub  10  in the open port configuration of  FIG. 8 . The ball  54  has just dissolved, for example eroded to a certain extent and then collapsed and flushed down the circulating sub  10 . At the moment in which the bail  54  has left the seat member  26 , the pressure/force of the downhole fluid acting upon the inner body member  14  will immediately be reduced and the coil spring  38  will now force the inner body member  14  to move upwards again. This is shown in  FIG. 10 , where the inner body member  14  has moved upwards to an intermediate position, which still provides an open port configuration of the circulating sub  10 . The inner body member  14  cannot move further upwards with the key  32  in the intermediate locking position  48 , which is described in more detail with relation to  FIGS. 3 and 4 . 
     Furthermore, the circulating sub  10  will remain in the (intermediate) open position  48  no matter what the flow rate of the downhole fluid is (i.e. zero, full or any rate therebetween). 
     When the operator wishes to close the ports  22  to redirect all the downhole fluid down through the pin end  16  and onto other equipment below the circulating sub  10 , the inner body member  14  has to move downwards again to be released from this position  48 . Therefore, another ball  54  is dropped into the downhole fluid being pumped down the downhole string by the operator at the surface, as shown in  FIG. 11 . 
       FIG. 12  shows the configuration of the circulating sub of  FIGS. 10 and 11  but with the ball  54  landed on the seat member  26 . The inner body member  14  has not yet moved downwards, but will do so due to the force created by the downhole fluid acting on the ball  54  and the inner body member  14 . 
     In  FIG. 13 , the inner body member  14  has moved downwards from the intermediate position of  FIGS. 11 and 12  to its furthest position downstream in the circulating sub  10 . The key  32  is, after further rotation of the cam sleeve  34 , locked in a second open locking position  46 B. The ball  54  is still in the seat member  26 . The circulating sub  10  is in an open configuration similar to the configuration of  FIG. 2   b  or  FIG. 8 , but the downhole fluid flowing past the ball  54  (the majority of which will then flow out through the open ports  20 ) will start to erode the ball  54 . 
     In  FIG. 14 , the ball has completely eroded/dissolved from the seat member  26 . This state of the circulating sub  10  can be compared with the one described with relation to  FIG. 9 . 
     In  FIG. 15 , the inner body member  14  has moved upwards again due to the released force on the inner body member  14  when the ball has left the seat member  26 . The next locking position on the cam sleeve  34 , in which the key  32  latches upon rotation of the cam sleeve  34  due to upwards movement of the inner body member  14 , provides a closed configuration of the circulating sub  10 , which is similar to the configuration shown for example in  FIGS. 1 and 2   a . Thus, the cam sleeve  34  has completed one complete (360°) rotation and is now back to the position it started at and is ready for one or more further cycles of drop ball  54  operations if further circulation of downhole fluid through the ports  20  is desired or required. 
       FIG. 16  shows an example of a ball  54  according to the second aspect of the present disclosure which is hollow at its centre  55 . The material of the ball  54  is erodible but it could also or alternatively be a soluble material and a suitable erodible material is cement and a suitable bonding material or sand and a suitable bonding material such as resin. At the centre  55  of the ball  54 , there can be a vacuum or it can be filled with a suitable gas such as air at atmospheric pressure. 
     Following from the above description and invention summaries, it should be apparent to those of ordinary skill in the art that, while the methods and apparatuses herein described constitute exemplary embodiments of the present invention, the invention contained herein is not limited to this precise embodiment and that changes may be made to such embodiments without departing from the scope of the invention as defined by the claims. Additionally, it is to be understood that the invention is defined by the claims and it is not intended that any limitations or elements describing the exemplary embodiments set forth herein are to be incorporated into the interpretation of any claim element unless such limitation or element is explicitly stated. Likewise, it is to be understood that it is not necessary to meet any or all of the identified advantages or objects of the invention disclosed herein in order to fall within the scope of any claims, since the invention is defined by the claims and since inherent and/or unforeseen advantages of the present invention may exist even though they may not have been explicitly discussed herein.