Patent Publication Number: US-11661796-B2

Title: Sealing system for downhole tool

Description:
SUMMARY 
     The present invention is directed to a downhole tool. The downhole tool comprises a cylindrical outer tube, a cylindrical inner tube, a bearing assembly, a first piston, and a second piston. The bearing assembly is disposed between the inner tube and outer tube and configured to allow relative rotation of the inner tube relative to the outer tube. The first piston is disposed at a first end of the bearing assembly between the inner tube and outer tube. The second piston is disposed at a second end of the bearing assembly between the inner tube and the outer tube. The downhole tool is characterized by three regions, each having its own fluid pressure. The first region is bounded by the inner tube, outer tube, first piston and second piston. The second region is disposed partially within the inner tube and in fluid contact with the first piston and the second piston. The third region is disposed outside of the outer tube. 
     In another embodiment the invention is directed to a system. The system comprises a pair of concentric and independently rotatable shafts situated within an environment. An annular zone is situated therebetween. A sealed chamber of variable volume is within the annular zone. The chamber is bounded in part at each end by an independently movable piston. The pistons comprise a first piston having an external side exposed to the annular zone and an internal side exposed to the chamber. The pistons also comprise a second piston having an external side exposed to the environment and an internal side exposed to the chamber. One or more bearings are contained within the chamber and interposed between the shafts. A flow path is located between the annular zone and the environment, bounded in part by the external side of the second piston. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1 A  is a side view of a downhole tool including a drill bit, a beacon housing, and a bearing assembly. 
         FIG.  1 B  is a sectional side view of the downhole tool of  FIG.  1 A . 
         FIG.  2    is a cross-sectional side view of a bearing assembly for use with the downhole tool shown in  FIG.  1 B . 
         FIG.  3    is a cross-sectional side view of the bearing assembly with a zerk inserted into the bearing chamber. 
         FIG.  4 A  is a sectional side view of an external piston in a first position, in contact with a shoulder of the bearing assembly. 
         FIG.  4 B  is a sectional side view of the external piston in a second position, in which the piston is not in contact with the shoulder. 
         FIG.  5    is a sectional side view of the piston of  FIG.  4 B , in the second position, wherein a port is shown in the sectional view. 
         FIG.  6 A  is a cut-away side view of the external components of the bearing assembly, wherein the external piston is shown in a first position. An internal piston is shown in a first position. 
         FIG.  6 B  is a cut-away side view as in  FIG.  6 A , but with the external piston in a second position. The internal piston is shown in a second position. 
         FIG.  7 A  is a sectional side view of the internal piston in its second position within the downhole tool. 
         FIG.  7 B  is a sectional side view of the internal piston in its first position within the downhole tool. 
         FIG.  8    is a cross-sectional side view as shown in  FIG.  2   , but with an imaginary boundary line drawn between two sections of the downhole tool to demonstrate which portions of the tool rotate together. 
         FIG.  9    is a cross-sectional side view of the bearing assembly within a borehole annulus, with a first, second and third region, each having its own fluid pressure called out and marked. 
         FIG.  10    is an exploded view of the bearing assembly of  FIG.  2   , with the outer wall, external piston and internal piston offset to show components that would otherwise be hidden from view. 
         FIG.  11    is a diagrammatic representation of a horizontal directional drilling operation. 
     
    
    
     DETAILED DESCRIPTION 
     The current state of the art for utility-HDD rock drilling involves using a sealed bearing system to permit rotation of an inner shaft inside of an outer shaft to drive a drill bit. This system is assembled under atmospheric conditions, and as a result, the bearing chamber maintains an absolute pressure that is roughly equivalent to the absolute atmospheric pressure at the time of assembly. However, once the bearing assembly is inserted into the borehole for use, the sealing system is at times responsible for isolating internal pressures inside of the drill string from those of the borehole, which may reach pressure differentials close to 1500 psi. This differential pressure results in significant forces on the sealing components, namely the seals themselves, often resulting in accelerated wear when compared to other systems which are isolated from the internal drill string pressures. 
     The present invention provides a solution to the above problem by equalizing the pressure between the bearing chamber and the internal passage without fluid communication. The invention further provides a path for high pressure fluid to leak from the internal passage of a downhole tool without entering the internal bearing chamber within the bearing assembly. Finally, the system provides a reliable method of lubricating downhole parts which rotate relative to one another and the environment. 
     Turning now to the figures,  FIGS.  1 A,  1 B and  11    show a bearing assembly  52  as a part of a downhole tool  53 . The downhole tool  53  supports a drill bit  54  which rotates to open a borehole in an underground location. The downhole tool  53  is located at an end of a dual member drill string  150 . The drill string  150  is made up of individual segments  152 . Thrust and rotation is provided to the drill string  150  by a horizontal directional drill  154  disposed at an uphole location at an end of the drill string. 
     The downhole tool  53  comprises a beacon housing  56 . The beacon housing  56  supports a beacon for conveying information about the position and orientation of the downhole tool  53  to an above ground location. This beacon housing  56  also comprises a connection  58  to an outer member of a dual member drill string  150  which provides thrust and rotational force to the downhole tool  53 . 
     As best shown in  FIG.  1 B , the downhole tool  53  has an internally-disposed rotating shaft  60 . The shaft  60  is coupled to an inner drill rod of the dual-member drill string  150 . The shaft  60  is disposed in an internal passage  62  of the bearing assembly  52  of the downhole tool  53 . 
     The present disclosure is directed to the sealed bearing chamber  50  within the bearing assembly  52  which is pressure compensated by the drilling fluid. Specifically, as shown in  FIGS.  2 - 7 B , an internal piston  10  and an external piston  12  work in concert to provide a path for leakage of drilling fluid which avoids the bearing chamber  50 . The external piston  12  is exposed to the borehole which is being excavated by the drill bit  54 . The internal piston  10  is not exposed to the borehole. 
     With reference now to  FIG.  2   , the bearing chamber  50  is shown in more detail. It should be understood that the bearing chamber  50  is disposed between an internal wall  100  and an outer wall  102  and houses multiple thrust bearings  14 . Outer wall  102  is generally rotatable with the drill bit  54 , and therefore the inner shaft  60  of the drill string. Inner wall or tube  100  is connected to and rotatable with the outer pipe of a dual member drill string (not shown). 
     The bearings  14  carry thrust between a shoulder  101  of the internal wall  100  and a shoulder or shoulders  103  of the outer wall. This allows thrust provided to the outer drill string (and thus the internal wall  100 ) to provide force at the drill bit  54  ( FIGS.  1 A- 1 B ). At the same time, the bearings  14  allow relative rotation between the internal wall  100  and the outer wall  102 . 
     As shown, the bearings  14  are in face-to-face and coaxial relationship. For example, as best shown in  FIGS.  2  and  10   , a first annular thrust bearing  14 A transfers thrust from the shoulder  101  to a second annular thrust bearing  14 B which is similarly formed and co-axial about a center axis  61  of the assembly. 
     Each bearing  14  has an inner ring  130  and an outer ring  132  that rotate relative to one another due to a plurality of ball bearings  134  interposed therebetween. 
     The pistons  10 ,  12  are disposed between the internal wall  100  and external wall  102  and allow pressure to equalize between the bearing chamber  50  and internal passage  62 . The internal piston  10  and external piston  12  are capable of axial movement. This movement is parallel to the center axis  61 . 
     Rings  18  are disposed about the internal wall  100 . The rings  18  carry thrust from the thrust bearings  14 . The rings  18  seal against dynamic seals  15  disposed in pistons  10  and  12 . Static seals  16  are disposed against pistons  10  and  12  within the external wall  102 . Static seals  17  are disposed in the rings  18  and seal against the internal wall  100 . The seals  15 ,  16 ,  17  prevent fluid from within the internal passage  62  from infiltrating the bearing chamber  50 . The external seals  16 ,  17  may be elastomeric, as each surface contacting such seals does not rotate relative to the seal. Dynamic seals  15  may also be elastomeric, though other seal materials may be used. The dynamic seals  15  are seated in pistons  10 ,  12  but seal against rings  18 . As shown in  FIG.  8   , these features rotate relative to one another. 
     As shown, the rings  18  may be formed in two parts, though solid rings may also be used. As best shown in  FIGS.  7 A- 7 B , ring  18  is formed of a first section  32  and a second section  34 . The first section  32  is internally threaded and attached to externally-formed threads on the internal wall  100 . The second section  34  provides a sealing surface for dynamic seals  15  within the internal piston  10 . The sections  32 ,  34  may be connected by one or more bolts  36 . A washer  38  is disposed between the first section  32  (or the ring  18  if unitary) and the bearings  14 . The washer  38  applies substantially constant pressure to the thrust bearings  14  to keep them in place during operation. 
     Pressures in the bore annulus  64  are typically less than 30 psi absolute. Conversely, internal pressures found inside the internal passage  62  of the drill string will typically be from 50 psi to 1200 psi more than annular borehole pressures. In prior art bearing assemblies, the bearing chamber is subject to the pressure differential between the annular borehole pressure and the internal drill string pressure. Such pressure differential tends to cause fluid to escape from the internal drill string along a path which includes the bearing chamber, causing damage to the seals and infiltrating the chamber with abrasive drilling fluid. 
     For the purposes of this specification, it is instructive to define three pressure regions within and about the bearing assembly  52 . The bearing chamber  50 , including the area housing bearing  14  within the chamber between the sets of static seals  16 ,  17  and dynamic seals  15  is referred to herein as a first region. The internal passage  62  of the drill string and areas in direct fluid communication with the internal passage, is referred to herein as a second region. The region outside of the outer wall  102  and within the bore annulus  62  is referred to herein as a third region. 
     Each region has its own pressure profile which may change during operations. Because the internal piston  10  and external piston  12  are axially movable and each is bounded by the first and second regions, these regions tend to equalize pressure due to forces applied by the pistons and any other axially-movable components. 
     While drilling using the drill string and drill bit  54 , internal pressures from the second region act upon the internal piston  10 . The internal piston  10  and seals  15 ,  16 ,  17  thus tend to apply a pressure to fluid within the bearing chamber  50 . High pressure within the bearing chamber  50  tends to lower its volume, moving the internal piston  10  towards the bearing chamber  50  as the force is applied. 
       FIG.  7 A  shows the internal piston when it has been moved towards the bearing chamber  50  due to high pressure.  FIG.  7 B  shows the internal piston  10  at its furthest axial extent from the bearing chamber, such as when pressures in the first and second regions are low. It should be understood that distances travelled by the internal piston  10  are exaggerated for clarity. 
     Simultaneously, a port  90  formed in the inner wall between the internal passage  62  and a cavity  84  ( FIGS.  4 A,  4 B,  5   ) allows pressure from the second region to act on the external piston  12 . The absolute pressure in the cavity may be lower than the pressure of the second region due to the interposed port  90 . Such pressure results in application of a force on the external piston  12  which is opposite but parallel to the force on the internal piston  10 . 
     The movement of pistons  10 ,  12  towards one another pressurizes the first region within the bearing chamber  50 . While the pressure differential between the first and second region is non-zero, the relative equalization keeps wear on seals  16 ,  17  to a minimum. Because lubricating fluid within the bearing chamber  50  is highly incompressible, very little movement of the pistons  10 ,  12  results in a much higher pressure within the bearing chamber  50 . 
     Ideal lubricants are grease or oil, but the lubricant could be any non-compressible fluid with or without lubricating properties. The use of compressible fluids would require pressurization of the bearing chamber  50  but could accomplish the same goal of downhole pressure equalization and wear mitigation. 
     While the term “incompressible” is used herein to describe lubricants within the bearing chamber  50 , one of skill in the art will understand that some volumetric change of the space between the pistons  10 ,  12  will occur at high pressure. This is because lubricant within the chamber will necessarily include entrained air, air pockets, or the like, which will compress at high pressures. Thus, enough compression occurs within bearing chamber  50  to allow external piston  12  to move away from the shoulder  86 . 
     With reference to  FIGS.  4 A- 4 B and  5   , the external piston  12  comprises a surface feature  80 . The surface feature  80  limits the contact between the external piston  12  and the shoulder  86 . As shown, the surface feature  80  is an annular notch. The contact point  82  between the external piston  12  and the shoulder  86  may be steel on steel, steel on polymer, ceramic on ceramic, ceramic on polymer, or steel on ceramic. 
     The cavity  84  is isolated from the bearing chamber  50  by dynamic seals  15 . When the pressure within the cavity  84  at surface feature  80  is low, pressure within the first region is also low. Because low pressure conditions are maximum volume conditions, the external piston abuts the contact point  82 , sealing the cavity  84  from the third region. This orientation is shown in  FIGS.  4 A and  6 A . 
     When pressure within the cavity  84  is increased due to high pressures within the second region, a differential pressure will be created between the first region and the second region and pressurization of the first region results. The pressure of the first region increases with the pressure of the second region, and the volume of the first region likewise tends to decrease. When the pressure within the first region exceeds a predetermined threshold, the force on the external piston  12  overcomes the static friction applied by seals  16 ,  15 . As a result, the external piston  12  moves away, slightly, from the contact point  82  as shown in  FIGS.  4 B,  5  and  6 B . 
     The external piston  12  therefore forms an intentionally unreliable seal, and opens a flow path  85  which allows movement of fluid from the cavity  84  to the third region outside of the outer wall  102  within the borehole annulus  64 . The pressure differential between the third region and second region would otherwise tend to force fluid through the first region, across seals  15 ,  16 ,  17 . 
     The flow of drilling fluid along flow path  85  further lubricates the outer surface of the bearing assembly  52  and outer wall  102 , as well as the interface between shoulder  82  and external piston  12 , where relative rotation occurs. Preferably, enough fluid flow occurs along flow path  85  during operation to maintain appropriate levels of lubrication. 
     The surface feature  80  on external piston  12  can be customized to particular pressure conditions. For example, the piston  12  may be sized so that it only partially reacts to the full force applied from the first region. This creates a less significant contact force at contact point  82  which is more easily overcome by pressure within the second region generally and the cavity  84  specifically. Alternatively, contact forces at contact point  82  may be externally increased or decreased by installation of a spring or other force carrying component (not shown). 
     The use of different wear materials at this location are also possible, each offering different sealing capacities or capabilities. The geometry of the contact point  82  may be formed to intentionally increase the length or restrictive properties of flow path  85 . For example, the flow path could be zig-zag or circuitous to lengthen the path  85 , or radial grooves may be cut into surfaces to add flow. 
     In any case, the intent for the device is to allow intentional, controlled leakage along the flow path  85  so that pressure differential between the second and third regions do not adversely affect the first region. Specifically, high pressure differentials between the internal passage  62  and annulus  64  might tend to damage internal seals  15 ,  16 . These are avoided by maintaining adequate fluid pressure within cavity  84  by allowing a restricted release of fluid from the cavity  84  into the bore annulus  64 . If the flow rate is such that fluid flows out of cavity  84  into annulus  64  faster than fluid flows into cavity  84  from internal passage  62 , significant pressure loss would occur within cavity  84 . This pressure loss would cause an unwanted pressure differential between the bearing chamber  50  and cavity  84 . 
     A diagrammatic representation of flow from passage  62 , through port  90 , and around external piston  12  is best shown in  FIG.  5   . It should be understood that the width of the flow passage  85  may be exaggerated for clarity. 
     While  FIGS.  4 A and  4 B  tend to show a large difference in the position of the external piston  12 , it should be understood that very little movement is required to allow drilling fluid to travel along the flow path  85  in sufficient volume to lubricate the contact point  82  and outside of the outer wall  102 , and to keep drilling fluid from entering the bearing chamber  50  and first region. 
       FIG.  3    is representative of the bearing chamber  50  at the time of assembly, while being filled with lubricant. Internal piston  10  and external piston  12  are positioned such that the bearing chamber  50  volume is at its minimum (for example, see  FIGS.  4 B and  7 A ). The pistons  10 ,  12  are each contacting internal stops  20 , which may be a surface of a thrust bearing  14 . A lubricant filling apparatus, such as a zerk  22 , is partially inserted into the bearing chamber  50 , and lubricant is pumped or poured into the chamber at a first end. A port  24  is disposed at a second end of the bearing chamber  50 . This port  24  is left open to allow air to escape during filling of the bearing chamber  50  with lubricant. As shown, the port  24  is disposed through ring  18 , though other structures may be suitable for such a port. The port  24  may be a one-way flow pressure-relieving port. 
     Once the bearing chamber  50  is filled with lubricant, the port  24  is sealed with a plug  25  ( FIG.  3   ). The addition of further lubricant through the zerk  22  pressurizes the bearing chamber  50 . This pressurization should overcome the friction of the seals  15  and  16  such that the pistons  10 ,  12  traverse axially until the pistons  10 ,  12  contact external stops  30  as shown in  FIGS.  3 ,  4 A and  7 B . As shown, the external stop  30  for the external piston  12  is the shoulder  86 . 
     The zerk  22  is removed, and pressure inside of the bearing chamber  50  returns to atmospheric pressure. Simultaneously, the contact forces decrease and external stops  30  are reduced to coincidental contact, with no residual forces left from filling the bearing chamber  50 . The zerk  22  is replaced with a plug, sealing the bearing chamber  50  and first region at the maximum volume/atmospheric pressure condition. The bearing chamber  50  is now ready for operation, as described above. 
     Because of the partially balanced relationship of the pressures described above, the leakage rate of lubricant is decreased. Moreover, as this lubricant is slowly leaked, the bearing chamber  50  can be flushed and recharged with lubricant by removing the plugs described above and flushing and refilling the bearing chamber  50  with desired lubricant in the same way as the cavity was filled during assembly. The resulting lower pressure differential reduces wear on seals  15 ,  16 , improving the life of the bearing chamber  50  and its components. 
     Throughout, the bearing assembly  52  is shown in cross-section to aid in understanding of the orientation of its parts across its volume. However, it should be understood that many of the seals, pistons, bearings, and other features described herein are annular in nature.  FIGS.  6 A and  6 B  show the bearing assembly  52  with the outer wall  102  cut away so that pistons  10 ,  12 , bearings  14 , and static seals  16  may be clearly seen in their annular forms. Further,  FIG.  10    shows the apparatus in exploded view for the same purpose, with pistons  10 ,  12  offset from the bearing assembly so that inner rings  18  and seals may be viewed. 
     With reference to  FIG.  8   , a boundary line  300  is shown to illustrate relative rotation of the components of the bearing assembly  52 . Features on a first side of the boundary line  300  rotate together, while features on a second side of the boundary line  300  also rotate together. For example, the internal shaft  60 , outer wall  102 , and pistons  10 ,  12  are on a first side of the boundary line  300 . Internal wall  100 , rings  18  are on the second side of the boundary line  300 . Thrust bearings  14  are split, such that the outer ring  132  is on the first side and inner ring  130  is on the second side. 
     In  FIG.  9   , the first region  410 , second region  420  and third region  430  are shown. The cavity  84  is in fluid communication with the second region  420 , but may have a lower pressure due to flow through the port  90 , and because of its position along the flow path  85  ( FIG.  5   ). 
     Changes may be made in the construction, operation and arrangement of the various parts, elements, steps and procedures described herein without departing from the spirit and scope of the invention as described in the following claims.