Abstract:
Subsea controlled milling apparatus and methods. In a described embodiment, a method of controlling displacement of a cutting device conveyed on a tubular string in a subterranean well includes the steps of: interconnecting an apparatus in the tubular string, the apparatus including an axial advancement device and an anchoring device; actuating the anchoring device to anchor the apparatus in the well; applying a pressure differential to the advancement device, thereby displacing the cutting device relative to the apparatus; and operating the cutting device to cut a structure in the well.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates generally to drilling, milling and similar operations performed in conjunction with a subterranean well and, in an embodiment described herein, more particularly provides controlled milling in subsea wells. 
   It is frequently desirable to isolate a cutting device, such as a drill bit or a mill, from the motion of a tubular string on which the cutting device is carried. For example, where a cutting operation is being performed from a floating rig (sometimes referred to as a “floater”), the tubular string suspended from the floater may rise and fall due to a heaving motion of the rig. Some floaters may be equipped with devices known as heave motion compensators, but these devices are not typically capable of removing all rising and falling motion from a suspended tubular string. 
   In some circumstances, accurate axial advancement of the cutting device in the well may be required. This accurate advancement is compromised by the rising and falling of the tubular string. For example, the cutting device may be a mill which may be damaged if the mill suddenly impacts a structure downhole. Of course, many other circumstances also require accurate axial advancement of a cutting device, whether the operations are performed from a floater or a land-based rig. 
   From the foregoing, it can be seen that it would be quite desirable to provide an apparatus which permits accurate axial advancement of a cutting device. It is accordingly an object of the present invention to provide such an apparatus and associated methods of controlling displacement of a cutting device in a well. 
   SUMMARY OF THE INVENTION 
   In carrying out the principles of the present invention, in accordance with an embodiment thereof, an apparatus is provided which includes an anchoring device and an axial advancement device. The apparatus is specially configured to control a milling operation in a subsea well. Associated methods are also provided. 
   In one aspect of the present invention, method of controlling displacement of a cutting device conveyed on a tubular string in a subterranean well is provided. The method includes the steps of: interconnecting an apparatus in the tubular string, the apparatus including an axial advancement device and an anchoring device; actuating the anchoring device to anchor the apparatus in the well; applying a pressure differential to the advancement device, thereby displacing the cutting device relative to the apparatus; and operating the cutting device to cut a structure in the well. 
   In another aspect of the invention, a system for controlling displacement of a cutting device in a cutting operation in a subterranean well is provided. The system includes the cutting device interconnected at a lower end of a tubular string; and an apparatus interconnected in the tubular string above the cutting device. The apparatus includes an anchoring device operative to anchor the apparatus in the well, and an advancement device responsive to a pressure differential in the apparatus. The advancement device controls axial displacement of the cutting device relative to the apparatus. 
   In yet another aspect of the invention, an apparatus for controlling displacement of a cutting device in a subterranean well is provided. The apparatus includes an advancement device responsive to a pressure differential in the apparatus to axially displace the cutting device relative to the apparatus and an anchoring device configured to anchor the apparatus in the well. 
   These and other features, advantages, benefits and objects of the present invention will become apparent to one of ordinary skill in the art upon careful consideration of the detailed description of representative embodiments of the invention hereinbelow and the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic cross-sectional view of a method embodying principles of the present invention; 
       FIG. 2  is a schematic view of the method of  FIG. 1 , wherein further steps of the method are being performed; and 
       FIGS. 3-9  are schematic cross-sectional views of successive axial portions of a subsea milling apparatus embodying principles of the present invention. 
   

   DETAILED DESCRIPTION 
   Representatively illustrated in  FIG. 1  is a method  10  which embodies principles of the present invention. In the following description of the method  10  and other apparatus and methods described herein, directional terms, such as “above”, “below”, “upper”, “lower”, etc., are used only for convenience in referring to the accompanying drawings. Additionally, it is to be understood that the various embodiments of the present invention described herein may be utilized in various orientations, such as inclined, inverted, horizontal, vertical, etc., and in various configurations, without departing from the principles of the present invention. 
   In the method  10  as depicted in  FIG. 1 , a whipstock  12  has been anchored in a parent or main wellbore  14  using an anchoring device  16 , such as a packer. A window  18  has been milled through casing  20  lining the wellbore  14  by deflecting one or more cutting devices, such as mills, (not shown) off of the whipstock  12 . A branch or lateral wellbore  24  has been formed extending outwardly from the window  18  by deflecting one or more other cutting devices, such as drill bits, (not shown) off of the whipstock  12 . A liner  22  has been positioned in the lateral wellbore  24  by deflecting it off of the whipstock  12 , and the liner is cemented within the lateral wellbore. 
   Note that a transition joint or upper end portion  26  of the liner  22  remains in the parent wellbore  14 , partially blocking the wellbore. Although not specifically illustrated in  FIG. 1 , the upper end  26  would preferably extend axially upward farther within the casing  20 . Additionally, the whipstock  12  and packer  16  should be removed if access to the parent wellbore  14  below its intersection with the lateral wellbore  24  is desired. Preferably, the upper end  26  of the liner  22  extending through the window  18  would be cut off and the whipstock  12  would be retrieved in a single trip into the well. However, this method generally requires the use of a cutting device known to those skilled in the art as a washover tool or burning shoe (not shown in  FIG. 1 ) having a relatively thin wall thickness, due to the small space radially between the whipstock  12  and the casing  20 . 
   The thin walled washover tool is used to cut off the upper end  26  of the liner  22 , to washover the whipstock  12 , and to release the whipstock from the packer  16 . Unfortunately, however, if the method  10  is performed from a floater, it may be very difficult to control the advancement of the washover tool in this operation. Thus, the washover tool may abruptly contact the upper end  26  of the liner  22 , thereby damaging the tool, or, after cutting has commenced, it may be very difficult to maintain relatively uniform advancement of the washover tool. Furthermore, if a mud motor is used to drive the washover tool, and the motor stalls during the cutting operation, it may be very difficult to accurately disengage the washover tool from the structure being cut, and then to begin the cutting operation again. This situation makes it hazardous and inefficient to perform such cutting operations from a floater. Of course, similar situations may arise with land-based rigs (i.e., the need for accurate advancement of a downhole cutting device), and so it is to be clearly understood that the principles of the present invention are not limited to use in operations performed from a floater. 
   Referring additionally now to  FIG. 2 , the method  10  is depicted in which additional steps have been performed. A milling apparatus  30  embodying principles of the present invention has been interconnected in a tubular string  32 , such as a drill string, above a cutting device  34 , such as a burning shoe or washover tool. A downhole motor  36 , such as a mud motor, which is operated by circulating fluid through the drill string  32 , may be interconnected between the milling apparatus  30  and the washover tool  34 . Alternatively, the washover tool  34  may be rotated by rotating the drill string  32 , as described below. It is to be clearly understood that cutting devices other than the washover tool  34  and driving means other than the motor  36  or drill string  32  may be utilized in methods and apparatus incorporating principles of the present invention. 
   The milling apparatus  30  functions to isolate the washover tool  34  from the upward and downward motion of the drill string  32  thereabove. Thus, if the drill string  32  at the surface is rising and falling, this rising and falling motion is not transmitted to the washover tool  34 . This result is accomplished by including an anchoring device  38  and an advancement device  40  in the milling apparatus  30 . 
   The anchoring device  38  secures the milling apparatus  30  in position in the wellbore  14 , isolating the washover tool  34  from the rising and falling motion of the drill string  32  above the milling apparatus, while the advancement device  40  displaces the washover tool  34  and motor  36  (and the remainder of the drill string  32  below the milling apparatus) toward the structure to be cut. The advancement device  40  also includes a recocking or restroking feature which permits the washover tool  34  to be repositioned lower in the casing  20  during the milling operation (e.g., to cut further through the structure being cut), or retracted out of engagement with the structure being cut (e.g., in the event that the motor  36  stalls), and then to be advanced again into contact with the structure. 
   Referring additionally now to  FIGS. 3-9 , a milling apparatus  50  embodying principles of the present invention is representatively illustrated. The milling apparatus  50  may be used for the milling apparatus  30  in the method  10 , or it may be used in other methods. In  FIGS. 3-9 , the milling apparatus  50  is depicted received within casing  52  and interconnected in a tubular string  54 . 
   The milling apparatus  50  includes an advancement device  56  and an anchoring device  58 . The advancement device  56  includes a piston  60  reciprocably and sealingly received within a bore  62  formed in a mandrel assembly  64 . The anchoring device  58  includes a latch assembly  66  having keys or collets  68  which engage a radially enlarged internal profile or recess  70  formed in the casing  52 . 
   The milling apparatus  50  is positioned and anchored in a well by engaging the keys  68  with the profile  70 . An appropriate latch assembly for use as the latch assembly  66 , and an appropriate latch coupling having an internal profile for use as the profile  70 , are described in U.S. Pat. No. 6,202,746, the entire disclosure of which is incorporated herein by this reference. The keys  68  of the latch assembly  66  engage the profile  70  as the apparatus  50  is lowered through the casing  52 . Engagement between the keys  68  and the profile  70  prevents further axially downward movement of the apparatus  50  relative to the casing  52 , and preferably also prevents rotation of the apparatus within the casing. 
   Note that other types of anchoring devices may be used instead of the latch assembly  66  and profile  70 . For example, a hanger or packer having outwardly extendable slips could be used to anchor the apparatus  50  in the casing  52 . As another example, the latch assembly and coupling described in U.S. Pat. No. 6,382,323, the entire disclosure of which is incorporated herein by this reference, may be used. 
   After the anchoring device  58  anchors the apparatus  50  in the casing  52 , at least a portion of the weight of the string  54  is placed on the milling apparatus  50  by, for example, slacking off on the string at the surface. The string  54  is, thus, placed at least partially in compression above the milling apparatus  50 , thereby preventing any rising and falling motion of the upper end of the string from being transmitted through the milling apparatus. As depicted in  FIGS. 3-9 , weight of the string  54  has been placed on the apparatus  50  after it has been anchored in position within the casing  52 . 
   If a hanger or packer is used as the anchoring device  58 , then weight of the string  54  may be placed on the apparatus  50  in order to engage slips of the hanger or packer with the casing  52 . If the latch assembly and coupling described in the above-referenced U.S. Pat. No. 6,382,323 is used, then tension instead of compression is applied to the milling apparatus  50  by the string  54  after the latch engages the coupling. 
   The mandrel  64  is attached to the tubular string  54 , so that rotation of the tubular string at the surface also rotates the mandrel in the apparatus  50 . A bearing assembly  72  is interconnected between the mandrel  64  and the latch assembly  66  to permit rotation of the mandrel relative to the latch assembly after the apparatus  50  has been anchored in the casing  52  and weight of the string  54  has been placed on the apparatus. Thus, the bearing assembly  72  supports the weight of the string  54  placed on the apparatus  50  after the anchoring device  58  secures the apparatus relative to the casing  52 . 
   If the latch assembly  66  is of the type described in the U.S. Pat. No. 6,202,746 referred to above, then full engagement of the keys  68  in the profile  70  may require that the latch assembly be rotated within the casing  52  to appropriately align the keys with the profile. This rotation of the latch assembly  66  is accomplished by providing a clutch assembly  74  between the mandrel  64  and the latch assembly. The clutch assembly  74  includes a piston  76  which is displaced upward when a pressure differential exists between an internal longitudinal passage  78  formed through the apparatus  50 , and an annulus  80  formed between the apparatus and the casing  52 . Specifically, the pressure differential is between a pressure in a portion of the passage  78  above the piston  60  and pressure in the annulus  80 . 
   The piston  76  is displaced upward against a biasing force exerted by a spring  82  when the pressure differential is sufficiently large to produce an upwardly directed force on the piston greater than a downwardly directed force exerted by the spring. Thus, when the pressure differential is sufficiently large, the piston  76  displaces upward and thereby disconnects the mandrel  64  from the latch assembly  66  (i.e. rotation of the mandrel relative to the latch assembly is permitted, and rotation of the mandrel will not produce rotation of the latch assembly), and when the pressure differential is not large enough to upwardly displace the piston, the mandrel is connected to the latch assembly (i.e., rotation of the mandrel relative to the latch assembly is not permitted, and rotation of the mandrel produces rotation of the latch assembly). 
   When the apparatus  50  is being positioned in the well and the keys  68  are being engaged in the profile  70 , the pressure differential from the passage  78  above the piston  60  to the annulus  80  is preferably not sufficiently large to upwardly displace the piston  76 . Thus, the mandrel  64  may be rotated (e.g., by rotating the string  54  at the surface) to produce rotation of the latch assembly  66  and thereby fully engage the keys  68  in the profile  70 . When the milling process is initiated, as described more fully below, the pressure differential is sufficiently large to upwardly displace the piston  76  and permit relative rotation between the mandrel  64  and the latch assembly  66 . 
   If, however, rotation of the string  54  is not used to rotate a cutting device  106  below the apparatus  50  (see FIG.  9 ), then the clutch assembly  74  may be eliminated from the apparatus. This would be the case if the mud motor  36  is used instead to rotate the cutting device  106 . 
   The piston  60  displaces in response to a pressure differential in the passage  78 . Specifically, the piston  60  is displaced downward by a differential between pressure in the passage  78  above the piston and pressure in the passage below the piston. For this purpose, the piston  60  includes a flow restricting orifice  84 . When fluid is circulated down the passage  78 , the orifice  84  creates a pressure drop from above to below the piston  60 . This pressure drop or pressure differential biases the piston  60  downwardly. 
   The passage  78  extends through a tube  86  attached to the piston  60  and extending downwardly therefrom. An annulus  88  is formed between the tube  86  and the mandrel  64 . A fluid, such as silicone oil or another hydraulic fluid, is contained in the annulus  88 . As the piston  60  displaces downward, the fluid is displaced downward with the piston. 
   One or more flow restricting orifices go are formed through a bulkhead  92  at a lower end of the annulus  88 . These orifices go meter the fluid flowing downward from the annulus  88  into another annulus  94  therebelow. This metering of the fluid flowing through the orifices go is used to control the rate of downward displacement of the piston  60  and tube  86 . 
   The orifices go may be enlarged to produce an increased rate of displacement, or the orifices may be made smaller to produce a slower displacement of the piston  60  and tube  86 . A floating piston  96  is used to separate the clean hydraulic fluid in the annulus  94  from well fluid therebelow. 
   The tube  86  is attached to a lower tubular extension  98 . The extension  98  is reciprocably received within the mandrel assembly  64  and extends downwardly therefrom through a bushing  100  at a lower end of the mandrel assembly. 
   The bushing  100  is of the type well known to those skilled in the art as a “kelly” bushing. The bushing  100  transmits torque from the mandrel assembly  64  to the extension  98  by preventing relative rotation therebetween. However, the bushing  100  does permit the extension  98  to displace axially therethrough. 
   For this purpose, the extension  98  preferably has a square-shaped outer side surface which is reciprocably received within a complementarily shaped inner side surface of the bushing  100  (indicated by dashed lines in FIG.  8 ). It should be understood that other shapes of the extension  98  and bushing  100  surfaces may be used in keeping with the principles of the invention, such as hexagonal, octagonal, etc. Furthermore, other means may be utilized for permitting relative axial displacement while preventing relative rotation between the extension  98  and the bushing  100 , such as a splined connection, a pin received in an axial slot, etc. 
   If, however, rotation of the string  54  is not used to rotate the cutting device  106  below the apparatus  50 , then the extension  98  and the bushing  100  may be eliminated from the apparatus. This would be the case if the mud motor  36  is used instead to rotate the cutting device  106 . 
   The extension  98  is connected at its lower end to a tubular sub  102  having a check valve  104  therein. The check valve  104  permits downward flow through the passage  78 , but prevents flow in the opposite direction. The check valve  104  could be, for example, a conventional float valve. 
   The sub  102  is connected at its lower end to the cutting device  106 , such as the burning shoe or washover tool  34  in the method  10  described above. Of course, there may in actual practice be other equipment connected between the sub  102  and the cutting device  106 , for example, to appropriately position the cutting device and apparatus  50  relative to each other and relative to the structure being cut in the well. 
   Operation of the apparatus  50  is described below as if the apparatus is used in the method  10 , it being understood that this is merely an example of a wide variety of methods in which the apparatus may be used. 
   The profile  70  is preferably interconnected in the casing  20  a known distance from the structure to be cut in the well (in this case the upper end  26  of the liner  22 ) when the casing is installed in the well. Of course, at this time the liner  22  has not yet been installed, so the profile  70  is positioned a known distance from the intended location of the upper end  26  of the liner  22 . Alternatively, the profile  70  may be formed in the casing  20  after it is installed in the well, for example, as described in U.S. patent application Ser. No. 10/147,567, filed May 16, 2002, the entire disclosure of which is incorporated herein by this reference. As another alternative, the apparatus  50  may be provided with another type of anchoring device, such as the anchoring device described in U.S. Pat. No. 6,286,614, the entire disclosure of which is incorporated herein by this reference. 
   After the casing  20  is installed and cemented in the parent wellbore  14 , the whipstock  12  and packer  16  are installed in the casing below the intended location for the window  18 . Then the window  18  is milled and the lateral wellbore  24  is drilled through the window. The liner string  22  is positioned in the lateral wellbore  24 , with the upper end  26  of the liner extending into the casing  12 . 
   The apparatus  50  is interconnected in the drill string  32  above the cutting tool  34 . The drill string  32  is lowered in the parent wellbore  14  until the keys  68  engage the profile  70 . At this point, the pressure differential from the passage  78  to the annulus  80  is either not present, or is not sufficiently large to actuate the clutch assembly  74  and rotationally disconnect the string  32  from the latch assembly  66 . 
   Thus, the string  32  may be rotated to rotate the latch assembly  66  and fully engage the keys  68  in the profile  70 . This engagement between the keys  68  and the profile  70  both rotationally and axially anchors the apparatus  50  in the casing  20 , although it is not necessary for the apparatus to be rotationally anchored in the casing. 
   Once the apparatus  50  is anchored in the casing  20 , sufficient weight of the string  32  (e.g., 10,000 lb.) is placed on the apparatus to isolate the apparatus from the rising and falling motion of the upper end of the string. Fluid is then circulated down the string  32  and through the passage  78  to the annulus  80  for return to the surface. This fluid flow creates a pressure differential from the passage  78  above the piston  60  to the annulus  80  due to the flow restricting orifice  84  in the piston. 
   The pressure differential causes the piston  76  of the clutch assembly  74  to rise and rotationally disconnect the latch assembly  66  from the mandrel  64 . The string  32  may now be rotated to rotate the mandrel  64 , without also rotating the latch assembly  66 . The weight of the string  32  applied to the apparatus  50  is borne by the bearing assembly  72 , permitting relatively unhindered rotation of the mandrel  64  relative to the latch assembly  66 . 
   The pressure differential in the passage  78  from above to below the piston  60  causes the piston to displace downward. This downward displacement of the piston  60  is metered by the flow restricting orifices go in the bulkhead  92 . Thus, downward advancement of the washover tool  34  (which is connected to the piston  60  via the tube  86 , extension  98  and sub  102 ) is in a controlled manner, isolated from any rising and falling motion of the upper end of the string  32 . 
   Rotation of the mandrel  64  is transferred to the extension  98  via the kelly bushing  100 . Thus, the washover tool  34  is rotated by rotation of the string  32 . Alternatively, the mud motor  36  could be interconnected between the apparatus  50  and the washover tool  34 , so that the circulation of fluid through the passage  78  and thence through the mud motor would cause rotation of the washover tool. 
   In this manner, the washover tool  34  is rotated and axially advanced in a controlled manner, even though the upper end of the string  32  may be rising and falling. If it is desired to cut farther through a structure than is available in a single stroke of the apparatus  50 , then the apparatus may be recocked downhole. This recocking is accomplished by ceasing the circulation of fluid through the passage  78 , disengaging the latch assembly  66  from the profile  70 , for example, by picking up on the string  32 , and then slacking off on the string with the washover tool  34  remaining in contact with the structure being cut. This will apply an upwardly directed force to the sub  102 , extension  98  and tube  86 , thereby forcing the piston  60  to displace upwardly. The apparatus  50  may then be anchored in the casing  20  again, either in the same position as before, or in a more downwardly disposed position, and the cutting operation may be resumed by circulating fluid through the passage  78  and rotating the string  32 . 
   When it is desired to retrieve the apparatus  50  from the well, the string  32  is picked up. This raises the mandrel assembly  64  relative to the anchoring device  58 . A latch assembly  110  having outwardly extending keys  114  eventually engages an internal profile  112  formed in the anchoring device  58 . A sufficient axial force applied upwardly to the anchoring device  58  will release the keys  68  of the latch assembly  66  from the profile  70 , permitting the apparatus  50  to be retrieved from the well. 
   Of course, a person skilled in the art would, upon a careful consideration of the above description of representative embodiments of the invention, readily appreciate that many modifications, additions, substitutions, deletions, and other changes may be made to these specific embodiments, and such changes are contemplated by the principles of the present invention. Accordingly, the foregoing detailed description is to be clearly understood as being given by way of illustration and example only, the spirit and scope of the present invention being limited solely by the appended claims.