Abstract:
Methods and apparatus for providing enhanced seal life of a swivel seal assembly by decreasing the effective differential pressure across a given rotating seal. The differential pressure is decreased by providing a pressurized fluid on the low-pressure side of the seal at a predetermined fraction of the operating pressure of the swivel seal assembly. A preferred swivel seal assembly utilizes a washpipe and packing box that are rigidly affixed to their respective conduits such that misalignment and dynamic run-out is compensated for in the gap between the washpipe and the packing box. A control system monitors and regulates the pressurized fluid supplied to the assembly.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    Not applicable  
         STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
         [0002]    Not applicable.  
         BACKGROUND OF THE INVENTION  
         [0003]    The present invention relates generally to methods and apparatus for providing high pressure fluid communication between relatively rotatable, generally coaxial conduits, and in particular to washpipe assemblies such as those used in rotary drilling swivels.  
           [0004]    Rotary drilling swivels generally include a washpipe that sealingly engages a set of circumferential seals contained within a seal housing. Frequently, the washpipe remains stationary while the seals and the seal housing rotate. Swivel seal assemblies have conventionally included a series of reinforced, elastomeric, chevron-type seals interspersed with a series of reinforcing back-up rings. Generally, one seal is exposed to full hydraulic pressure on one side, and atmospheric pressure on the opposite side. These seals tend to function in a redundant manner, wherein the full differential pressure of the mud acts on one seal until that seal fails and the next seal in the assembly acts as the primary seal.  
           [0005]    The rotating components of the swivel are often subject to some amount of radial misalignment, or dynamic run-out, due to tolerances and inherent operating conditions. Dynamic run-out between the washpipe and the seal housing has normally been compensated for by means of the seal extrusion gap between the two rotating components since most conventional swivel designs have both the washpipe and the seal housing radially fixed relative to their respective mountings. Some prior art swivel designs have sought to compensate for potential run-out and offset problems by allowing the washpipe and the seal housing to articulate.  
           [0006]    Current drilling technology has increased the desired operating pressure of rotary drilling swivel seal assemblies to very high pressures. Many conventional swivel seal designs have yielded less than desirable seal life or displayed other unsatisfactory performance in these high-pressure environments. Thus, there remains a need in the art for methods and apparatus for swivel seal assemblies that can withstand high pressures for extended periods of time. Therefore, the embodiments of the present invention are directed to methods and apparatus for swivel seal assemblies that seek to overcome the limitations of the prior art.  
         SUMMARY OF THE PREFERRED EMBODIMENTS  
         [0007]    Accordingly, there are provided herein methods and apparatus for increasing the sealing life of a swivel seal assembly by decreasing the effective differential pressure across a given rotating seal. The effective differential pressure is decreased by providing a pressurized fluid on the low-pressure side of the seal at a predetermined fraction of the operating pressure of the swivel seal assembly. A preferred swivel seal assembly utilizes a washpipe and packing box that are rigidly affixed to their respective conduits. The radial misalignment and dynamic run-out is compensated for by the seal extrusion gap between the washpipe and the metal backing surfaces.  
           [0008]    One preferred embodiment includes a swivel sealing assembly having a washpipe rigidly mounted to a rotating conduit and a packing box rigidly mounted to a stationary conduit. The packing box engages the washpipe such that a circulating fluid flows through the washpipe and packing box at an elevated pressure. A plurality of seals are contained within the packing box and adapted to seal against the washpipe and the packing box forming a plurality of annular volumes defined by the sealing interfaces between the washpipe, the packing box, and two adjacent seals. One or more pressure ports provide fluid access to one or more of the plurality of annular volumes. A pressure control system is adapted to provide pressurized fluid to the pressure ports at a predetermined fraction of the elevated pressure of the circulating fluid.  
           [0009]    An alternative embodiment includes a method for increasing the life of a swivel seal assembly having a plurality of seals contained with a packing box and adapted to seal against a washpipe by monitoring the internal pressure of the swivel seal assembly, wherein the packing box is rigidly mounted to a stationary conduit and the washpipe is rigidly mounted to a rotating conduit, providing a pressurized fluid at a first fraction of the swivel seal internal pressure to a first annular volume between adjacent seals, and providing a pressurized fluid at a second fraction of the swivel seal internal pressure to a second annular volume between adjacent seals.  
           [0010]    Thus, the present invention comprises a combination of features and advantages that enable it to extend the useful life of a swivel seal assembly by regulating the differential pressure across the rotating seals. These and various other characteristics and advantages of the present invention will be readily apparent to those skilled in the art upon reading the following detailed description of the preferred embodiments of the invention and by referring to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    For a more detailed understanding of the preferred embodiments, reference is made to the accompanying Figures, wherein:  
         [0012]    [0012]FIG. 1 is a partial cross-sectional view of a swivel seal assembly;  
         [0013]    [0013]FIG. 2 is a partial cross-sectional view of a swivel seal assembly with a pressure control system;  
         [0014]    [0014]FIG. 3 is a schematic view of one embodiment of a pressure control system; and  
         [0015]    [0015]FIG. 4 is a schematic view of an alternate embodiment of a pressure control system. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]    In the description that follows, like parts are marked throughout the specification and drawings with the same reference numerals, respectively. The drawing figures are not necessarily to scale. Certain features of the invention may be shown exaggerated in scale or in somewhat schematic form and some details of conventional elements may not be shown in the interest of clarity and conciseness. The present invention is susceptible to embodiments of different forms. There are shown in the drawings, and herein will be described in detail, specific embodiments of the present invention with the understanding that the present disclosure is to be considered an exemplification of the principles of the invention, and is not intended to limit the invention to that illustrated and described herein. It is to be fully recognized that the different teachings of the embodiments discussed below may be employed separately or in any suitable combination to produce the desired results.  
         [0017]    In particular, various embodiments of the present invention provide a number of different methods and apparatus for sealing between a rotating conduit and a non-rotating conduit. The concepts of the invention are discussed in the context of a rotary swivel seal assembly but the use of the concepts of the present invention is not limited to swivel seal assemblies and may find application in other rotating applications, both within oilfield technology and other high pressure, heavy duty applications to which the concepts of the current invention may be applied.  
         [0018]    In the context of the following description, the terms “rigid” or “substantially stationary” should be taken to mean a relationship in which two connected components are generally fixed relative to each other. It is understood that due to manufacturing, assembly, or other design details the two components may have some relative movement. The pressures referred to herein are pressures relative to ambient pressure, so 0 psi should be taken to mean ambient pressure.  
         [0019]    [0019]FIG. 1 shows a partial sectional view of one embodiment of a washpipe assembly  10  including rotating conduit  11  and stationary conduit  12 . Nut  28  is connected to rotating conduit  11  and holds washpipe  14  in rigid engagement with the rotating conduit. Washpipe  14  extends from rotating conduit  11  into a sealing engagement with rotary swivel seals  16 A-D that are contained within packing box  18  attached to stationary conduit  12 . Packing box  18  is rigidly attached to stationary conduit  12  by threaded connection  22 .  
         [0020]    Rotary swivel seals  16 A-D and spacers  24  are disposed within box  18 . Rotary swivel seals  16 A-D are preferably constructed from a resilient, compliant seal material such as fiber backed nitrile. Spacers  24  are preferably constructed from substantially rigid material such as alloy steel, and act as a rigid backup to the seal. Rotary swivel seals  16 A-D are generally unidirectional seals designed to hold pressure in only one direction, such as chevron-type seals. Other seal designs may be used as long as the seals do not require an elevated hydraulic pressure above the working pressure of the washpipe assembly or posses a special configuration that pumps fluid across the sealing face that acts as a lubricant.  
         [0021]    The threaded connection  22  between stationary conduit  12  and packing box  18  rigidly connects the box  18  to the conduit  12  and compresses seals  16 A-D and spacers  24 . The compression of seals  16 A-D energizes and expands the seals to form a sealing engagement with washpipe  14 . Seal  26 , preferably an o-ring type seal, is compressed between the uppermost spacer  24  and the stationary conduit  12  and seals against the internal pressure of stationary conduit  12 . Thus, once packing box  18  is installed, the fluid pressure inside the stationary conduit  12  is contained by static seal  26  and rotating seals  16 A-D.  
         [0022]    Washpipe  14  is held in place by holding nut  28  that attaches to conduit  11  by threaded connection  30 . Holding nut  28  also fixes in place holding ring  32  that carries seals  34  and  36  as well as snap ring  38 . Holding nut  28  compresses seal  34  to energize and expand the seal against washpipe  14 . Holding ring  32  compresses seal  36  against the rotating conduit  11  forming a static seal. Seals  34  and  36  act to isolate the elevated internal pressure of the swivel from external, atmospheric pressure. Seals  34  and  36  are respectively similar to seals  16 A-D and  26  described above. Snap ring  38  maintains the relative axial positions of washpipe  14  and holding ring  32  during the installation process. The radial clearance between washpipe  14  and nut  28  is limited so as to hold the washpipe substantially stationary relative to conduit  11 . The preferred radial clearance between washpipe  14  and nut  28  typically is between 0.020″ and 0.025″ on diameter.  
         [0023]    Because the washpipe  14  and the rotary swivel seals  16 A-D in packing box  18  are both substantially stationary relative to their respective conduits, any potential dynamic run-out between the rotating and non-rotating components is compensated for in the radial gap between the washpipe and the spacers. This radial gap defines the extrusion gap across which seals  16 A-D act to contain the internal pressure in the assembly. Therefore, the inside diameter of box  18  and spacers  24  are preferably sized so as to permit some amount of radial offset or run-out between the conduits while maintaining an extrusion gap across which seals  16 A-D can operate. In the preferred embodiments, this extrusion gap is between 0.01″ and 0.015″, and preferably approximately 0.013″.  
         [0024]    The performance of rotating seals  16 A-D is also greatly effected by the differential pressure across each seal. Pressure ports  20 A-C are provided so that the pressure differential across seals  16 A-D can be regulated. Pressure ports  20 A-C provide hydraulic access to the annular pressure volumes between adjacent seals  16 A-D. The annular pressure volumes are defined by the sealing interfaces between two adjacent seals, for example  16 A and  16 B, and the washpipe  14  and the packing box  18 .  
         [0025]    In the preferred embodiments, pressure ports  20 A-C are used to inject a pressurized fluid into the annular volumes between the seals  16 A-D. The pressurized fluid is preferably injected at a pressure above ambient pressure but below the internal pressure of swivel  10 . In other embodiments, the pressure between each seal  16 A-D is regulated such that each of the seals see substantially the same differential pressure. The pressurized fluid is not injected at a pressure higher than the swivel internal pressure, which would cause injected fluid to be forced across the seal face. Such an over-pressurization of fluid in order to lubricate and energize the seal face is an important feature of certain prior art applications that sought to use hydrodynamic sealing elements.  
         [0026]    [0026]FIG. 2 is a general schematic drawing illustrating a preferred swivel seal assembly  10  coupled with a pressure control system  40 . Pressure control system  40  monitors the internal pressure in swivel assembly  10  via line  42 , and, according to predetermined operating parameters, supplies pressurized hydraulic fluid through control lines  44 ,  46 , and  48  to pressure ports  20 A-C. In some embodiments, control system  40  may monitor the resulting pressure differentials across the seals  16 A-D, in order to provide early detection of leakage and potential seal failure. Control lines  44 ,  46 ,  48  may also include check valves  49  to prevent fluid from flowing from swivel assembly  10  to control system  40 . Check valves  49  maintain a volume of hydraulic fluid in assembly  10  to serve as seal lubrication in case of loss of pressure from control system  40  and prevent elevated pressure from traveling up the control lines to the control system.  
         [0027]    [0027]FIG. 3 illustrates on embodiment of a control system  40  that uses a plurality of pressure reducing cylinders  54 ,  56 ,  58 , to regulate the pressure between the rotating seals  16 A-D. Internal fluid pressure from the swivel assembly  10  is distributed to the pressure reducing cylinders  54 ,  56 ,  58  by manifold  52 . Each cylinder  54 ,  56 ,  58  takes the hydraulic pressure input from the manifold  52  and produces an output hydraulic pressure at some predetermined percentage of the input pressure.  
         [0028]    For example, in one embodiment, swivel assembly  10  has an internal pressure of 4000 psi. Manifold  52  distributes 4000 psi fluid to each of the reducing cylinders  54 ,  56 , and  58 . Cylinder  54  is chosen to provide a fluid pressure that is 0.75 times the swivel internal pressure. Therefore, cylinder  54  provides a 3000 psi fluid through hose  44  and port  20 A to the annular volume between the seal  16 A and seal  16 B. Cylinder  56  provides a fluid pressure that is 0.5 times the swivel internal pressure, thus providing a 2000 psi fluid through hose  46  and port  20 B to the annular volume between the seal  16 B and seal  16 C. Cylinder  58  provides a fluid pressure that is 0.25 times the swivel internal pressure, thus providing a 1000 psi fluid through hose  48  and port  20 C to the annular volume between seals  16 C and seal  16 D.  
         [0029]    Thus, the swivel internal pressure is 4000 psi, the pressure in the annular volume between seals  16 A and  16 B is 3000 psi, the pressure in the annular volume between seals  16 B and  16 C is 2000 psi, the pressure in the annular volume between seals  16 C and  16 D is 1000 psi, with the external pressure being 0 psi. Therefore, in the particular example set forth, each rotary swivel seal  16 A-D sees a differential pressure of only 1000 psi. This can be compared to most of the prior art systems in which one seal sees the full swivel internal pressure on one side and atmospheric pressure on the other side, which in this case would be a 4000 psi differential.  
         [0030]    [0030]FIG. 4 illustrates an alternative embodiment of a control system  40  that utilizes an electronic control system  60  to regulate hydraulic pumps  64 ,  66 ,  68  that supply fluid to the pressure ports. Control system  60  receives data via line  42  from a pressure transducer, or other device, measuring the swivel internal pressure. Control system  60  then activates pumps  64 ,  66 , and  68  as desired. The pumps draw fluid from a reservoir  62  and supply the pressurized fluid into the annular volume between adjacent rotary seals. In one embodiment, control system  60  may supply pressure such that the pressure in each annular volume is equal and differential pressure across each seal, other than the seal exposed to ambient pressure, is balanced. Control system  60  may also be used to regulate the internal pressure as described above such that the differential pressure across each seal is equal.  
         [0031]    Control system  60  may also be adapted to receive, by way of signal lines  70 , pressure information from pressure transducers, or other devices, reading the pressure within each one of the annular pressure areas. These additional readings could be used to sense pressure fluctuations and warning of impending seal failures. In the event of a seal failure, control system  60  could activate the pumps to adjust the differential pressure across the still intact seals.  
         [0032]    Although the above described embodiments act to reduce the differential pressure across individual rotary swivel seals  16 A-D, it is preferred that the seals be chosen to be able to withstand full operating pressure in the case of failure or temporary inoperability of the pressure control system  40  or  60 . Because the preferred embodiments utilize standard seal configurations not requiring hydrodynamic seal energization, these embodiments will continue to perform, although with a reduced life cycle, without the reduced differential pressure provided by the control system  40  or  60 .  
         [0033]    The embodiments set forth herein are merely illustrative and do not limit the scope of the invention or the details therein. It will be appreciated that many other modifications and improvements to the disclosure herein may be made without departing from the scope of the invention or the inventive concepts herein disclosed. Because many varying and different embodiments may be made within the scope of the present inventive concept, including equivalent structures or materials hereafter thought of, and because many modifications may be made in the embodiments herein detailed in accordance with the descriptive requirements of the law, it is to be understood that the details herein are to be interpreted as illustrative and not in a limiting sense.