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BACKGROUND 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a sealing system between a relatively rotating element and a stationary element, possibly a drill pipe and a wash pipe. 
         [0003]    2. Related Art 
         [0004]    Drilling is done by having a rotating drill pipe extending between the drill bit and the surface facility. The surface facility is either on land, on a floating vessel, a platform or other kind of installation. There will naturally be relative rotation between the drill pipe and the surface facility. At the same time there are fluid lines in the drill pipe which need to be connected to equipment on the surface facility for transfer of a fluid from a fluid path in the drill pipe to the surface facility. One possible solution for this is to provide swivel means in the connection between the drill pipe and the surface facility. Such swivel means may for instance be a so-called washpipe connected to the drill pipe. These swivel means should also prevent leakage of fluids to the environment and preferably be easy to use, assemble and repair. The swivel means also has to withstand high pressure and high speed drilling with the associated extensive abrasion and wear in the connection between the relatively rotating elements. 
         [0005]    There is known a washpipe assembly for a standard drill pipe where the system includes hydrodynamic seal lubrications where each seal has a dynamic sealing surface incorporating a wavy hydrodynamic inlet and a non-hydrodynamic exclusionary corner, pressure staging between the hydrodynamic seals where the drilling fluid pressure is divided among three pressure retaining seals, exposing each one to only a fraction of the pressure, where each sealed chamber is independently pressurized by a lubrication cylinder (lubricator energized by the drilling fluid pressure and pivoting articulation), as described in the paper IADC/SPE 59107 “A new hydrodynamic Washpipe Sealing system. Extends Performance Envelope and Provides Economic Benefit” by Morrow, Drury, Dietle and Kalsi. Such an assembly will enable it to withstand significantly higher pressures and surface speeds compared with conventional units. 
       SUMMARY 
       [0006]    According to one or more embodiments of the present invention, there is provided a sealing system between a relatively rotating element and a stationary element, comprising at least three sealing elements arranged between the two elements and arranged in series between a process fluid and an environment. There is a barrier fluid arrangement to provide a barrier fluid between the sealing elements, where the barrier fluid arrangement comprises at least two compensator devices where the pressure in the process fluid is acting on one side of a piston in the compensator device and the pressure in the barrier fluid is acting on the opposite side of the piston. 
         [0007]    According to one or more embodiments of the invention there is a difference in the cross sectional area of the two sides of the piston and this difference varies between the at least two compensator devices. This gives that with a given compensator device there is a given pressure difference between the pressure in the process fluid and the pressure in the barrier fluid. This pressure difference is set with the difference in cross section areas of the two sides of the piston for each compensator during assembly of the sealing system and thereby forming a passive sealing system. The sealing system is provided with filling means for adding barrier fluid to the system. These filling means may provide a possibility for providing barrier fluid to each one of the compensator devices at a given pressure before active use of the sealing system. The filling means may provide a possibility of providing barrier fluid from one fluid source at a given pressure to at least two compensator devices at the same time, which filing means when the sealing device is in active use are closed, or the barrier fluid source is removed from the filling means. The filling means may when the filling means provide the possibility of providing barrier fluid to two or more compensators at the same time be closed by a one-way valve in the connection to each of the compensator devices, preventing fluid from flowing out of the sealing system when the barrier fluid has been added through the filing means and also preventing barrier fluid to flow between the different compensator devices. The filing means may thereby be a one point contact between the sealing system and the source of barrier fluid, with valves in the connection to each of the compensator devices. Alternatively there may be more than one filing means connected to groups of compensator devices or possibly one filing means for each compensator device. The issue is that the barrier fluid is filled to the sealing system with a given pressure and then during active use the system as such will then provide a pressure in the barrier fluid in response to the pressure in the process fluid and by the construction of the different compensator devices the process fluid pressure is divided between the different compensator device and one thereby achieves a passive system which responds to the differences in the process fluid pressure. 
         [0008]    According to an aspect of the invention the rotating element may be a drill pipe and the stationary element may be a washpipe and there may be a radial opening from the drill pipe through the wash pipe. This radial opening may be in addition to an axial opening. Such a configuration with a radial opening and an axial opening may be found in a dual drill pipe. A dual drill pipe may comprise a normal drill pipe with an inner pipe arranged within the drill pipe forming an annular space between the drill pipe and the inner pipe, in addition to the space within the inner pipe. This annular space may be connected to surface equipment through a radial opening. The inner space of the inner pipe may in a conventional manner have an axial opening at the top of the drill pipe. The annular space with the radial opening may be used to provide drilling fluid down to the drill bit, and the drill fluid with cuttings may be transported back to the surface through the inner space of the inner pipe. It is also possible to envisage an opposite transportation of fluids. 
         [0009]    According to an aspect the piston in the compensator device may be arranged with a piston rod on one side. The cross sectional area of the piston rod may then be used to adapt the difference in the cross sectional area of the two sides of the piston. 
         [0010]    According to another aspect the compensator devices may comprise cylinders for positioning of the piston, which cylinders have a similar inner diameter for at least two of the compensator devices in the sealing system. This will give similar pistons in several compensators, possibly all the compensators in the sealing system. The cross sectional difference may then be achieved by attaching piston rods with different cross sectional areas to the different pistons. 
         [0011]    According to another aspect at least one of the pistons may be an annular piston positioned around an inner piston. There may be several annular pistons arranged outside each other with a common centre axis. Another possibility is to have several sets of annular pistons or alternatively some annular pistons and some other pistons. 
         [0012]    According to another aspect there may be one fluid supply of barrier fluid to the at least two compensator devices. The one fluid supply may be to all the compensators or there is one barrier fluid supply to some compensators and another fluid supply to some other compensators. 
         [0013]    According to another aspect a first compensator device may be connected to a first space between a first sealing element, exposed to the process fluid, and a second sealing element, and it may be arranged to have the barrier fluid acting on the side of the piston with a smaller exposed area of the piston than the side of the piston acting on the process fluid. This will give a somewhat higher pressure in the barrier fluid than in the process fluid, limiting the exposure of the seal to the process fluid. During normal operations barrier fluid will leak towards the process fluid and not the other way. The compensator devices connected to other spaces between sealing elements may be arranged to have the barrier fluid acting on the side of the piston with a larger exposed area of the piston than the side of the piston exposed to the pressure within the process fluid. This gives a predefined pressure drop from the process fluid to the barrier fluid. 
         [0014]    According to another aspect the piston rods may be extending out of the compensator as a visual indicator. This may also indicate which piston is connected to which seals in the sealing system, as these piston rods may have different cross section area. 
         [0015]    According to another aspect the at least two compensator devices may be arranged at least partly within the outer relative stationary element. They may be arranged in a line or divided around the circumference of the outer element or as a combination. The barrier fluid supply may also be arranged at least partly within this element, or alternatively the at least one filling means are arranged easily accessible in the outer surface of the outer relative stationary element. Such a configuration will avoid external lines for supply of barrier fluid to the outer element. 
         [0016]    According to another aspect one compensator device may be providing a barrier fluid to two spaces between two sealing elements, one space on either side of the opening. With a radial opening such a configuration is a good solution. Such a configuration will give the need for half the amount of compensators compared with a solution with one compensator for each space. With a radial opening there will also be symmetry around the opening. The sealing elements and the spaces will also extend all the way around the drill pipe, forming ring shaped sealing elements and spaces for the barrier fluid. 
         [0017]    According to another aspect the cross sectional area on one side of the pistons in the compensator devices, exposed to the pressure in the barrier fluid may be mainly equal for almost all the compensator devices. This give similar pistons in all the compensators, and easy production. The cylinder for the movement of the pistons may be formed by a separate element or at least partly by the outer relative stationary element. 
         [0018]    According to one or more embodiments of the present invention, a method for operating a sealing system between a relatively rotating element and a stationary element comprises arranging at least three sealing elements in series between the two elements and between a process fluid and an environment, arranging a barrier fluid arrangement to provide a barrier fluid in spaces between the sealing elements, providing at least two compensator devices in the barrier fluid arrangement and arranging them such that the pressure in the process fluid is acting on one side of a piston in the compensator device and the pressure in the barrier fluid is acting on the opposite side of the piston, providing a difference in the cross sectional areas of the two sides of the piston and adapting the difference in the cross sectional areas of the pistons of the different compensator devices such that the pressure difference between the process fluid and the environment is divided between the different compensator devices. 
         [0019]    According to another aspect the method may comprise providing a barrier fluid with a given pressure in the system before active use of the sealing system, and then removing the barrier fluid source from the sealing system until the sealing system again should be filled or filled up with barrier fluid. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  is an elevated sketch a washpipe in connection with a drill pipe, 
           [0021]      FIG. 2  is a cross section of the system in  FIG. 1 , and 
           [0022]      FIG. 3  is a schematic sketch of the barrier fluid system in the sealing system. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    Hereafter, embodiments of the invention will be described. In embodiments of the invention, numerous specific details are set forth in order to provide a more thorough understanding of the invention. However, it will be apparent to one of ordinary skill in the art that the invention may be practiced without these specific details. In other instances, well-known features have not been described in detail to avoid obscuring the invention. 
         [0024]    In  FIG. 1  there is shown an elevated sketch of a drill pipe  1 , forming the rotating element, with a washpipe  3 , forming the stationary element attached to the drill pipe  1 . There is in the drill pipe  1  indicated another inner pipe  6 . Between the drill pipe  1  and the inner pipe  6  three is formed an annular space  7  and there is an inner space  8  within the inner pipe  6 . Normally the annular space  7  will be used for transporting fluid, a process fluid into the well which is added to the annular space through a opening  5  in the washpipe  3 , as indicated with the arrows and a return fluid is moved out of the well through the inner space  8  of the inner pipe  6  as also indicated with the arrows. There may to the opening  5  be attached a pipe from the stationary surface equipment, comprising for instance a valve means for regulating the flow into the annular space  7  through the opening  5 . 
         [0025]    As shown in  FIG. 2  which is a cross section of the element in  FIG. 1  the drill pipe  1  is formed with holes  2  through the wall of the drill pipe  1 . These holes  2  leads to an inner annular cavity  4  formed in the inner surface of the washpipe  3 . This inner annular cavity  4  is in connection with the opening  5 . Between the opposing surfaces of the washpipe  3  and the drill pipe  1  there are arranged several sealing elements  10 , in series, on both sides of the annular cavity  4 . The sealing elements  10  are annular sealing elements and are arranged within grooves in the washpipe  3 . It is possible to envisage that the sealing elements are arranged in grooves in the drill pipe. As these sealing elements  10  are arranged around the circumference of the drill pipe  1  and in abutment against the drill pipe  1  and the washpipe  3  there are formed annular spaces  11  between two neighboring sealing elements  10 . There are nine sealing elements  10  arranged in series on both sides of the annular cavity  4  in the shown example. The series of sealing elements  10  may comprise three or more sealing elements  10  forming at least two annular spaces  11 . There may be for instance five, six, seven or eight sealing elements forming four, five, six or seven annular spaces. According to one or more embodiments of the present invention, there are similar series of sealing elements  10  on both sides of the annular cavity  4 . 
         [0026]    The wash pipe  3  is formed between two pipe flanges  22  attached to the drill pipe  1  with bearing arrangements  9  between the washpipe  3  and the pipe flanges  22  allowing and supporting relative rotational movement between the drill pipe  1  and the washpipe  3 . Another configuration is possible for allowing such relative movement. There is partly within the washpipe  3  arranged several compensator devices  21 . The compensator devices  21  comprises a cylinder  20 , wherein there is arranged a movable piston  16 . The cylinders  20  and pistons  16  are similar for all the compensator devices  21 . There is a sealing connection between the pistons and cylinders. To the piston  16  there is attached a piston rod  17 . The cross sectional area of the piston rod  17  is varied from one compensator device  21  to the next compensator device  21 ′. As one can see from  FIG. 1  the piston rods  17  extend out of the compensator device and work as a visual aid. The compensator devices  21  are also positioned partly within the washpipe  3  and arranged around the washpipe  3 . There are as indicated with the process fluid line  14  in the washpipe  3  from the annular cavity  4  to the different compensators  21  provided internally bores to avoid external fluid lines for process fluid and barrier fluid to the different compensator devices  21 . Such a construction will give a compact device with minimal external fluid lines. 
         [0027]    The connection between the different compensators  21 , the different process fluid lines  14  and barrier fluid lines  15  and the different spaces  11  between the sealing elements  10  are schematically given in  FIG. 3 . The stationary element  3  with the grooves and the different sealing elements  10  are shown. Also in in one or more embodiments of the present invention, there are nine sealing elements  10  in series on both sides of the annular cavity  4  leading to the opening  5  for the process fluid. To the cavity  4  and or the opening there are connected a process fluid line  14 , guiding the pressure in the process fluid to the different compensators  21 . There are eight compensators  21  all with similar cylinders  20  wherein there are arranged pistons  16 . To the pistons  16  there are attached piston rods  17 . The process fluid lines  14  leads to a given side of the piston  16 . The pistons  16  have a first cross sectional area  18  and a second cross sectional area  19 . The process fluid lines  14  leads to the side of the piston  16  with the second cross sectional area  19 . There are in the system also a barrier fluid source  13 , connectable to the barrier fluid lines  15  leading to the spaces  11  between the different sealing elements  10  and to the compensators  21 . The barrier fluid lines  15  lead to the side of the piston  16  with the first cross sectional area  18 . The area differences between the first cross sectional area  18  and the second cross sectional area  19 , given by the cross sectional area  18  divided by the cross sectional area  19 , are different for all the compensators  21 . 
         [0028]    There is one high pressure compensator  21 . 0  where the barrier fluid line  15  is connected to the first cross sectional area  18  where there to this side is connected a piston rod  17  and the second cross sectional area  19  is the full area of the cylinder  20 . This high pressure compensator  21 . 0  is connected to the space  11  between the sealing element  10  closest to the process fluid and the neighboring sealing element  10 . The high pressure compensator  21 . 0  provides a pressure in the barrier fluid delivered to the space  11  which is somewhat larger than the pressure in the process fluid in the annular cavity  4 . This higher pressure in the barrier fluid will give a leakage of the barrier fluid towards the process fluid, thereby preventing unnecessary abrasion of the sealing element  10  closest to the process fluid. The first compensator  21 . 1  is formed with the piston rod  17  connected to the second cross sectional area  19  of the piston  16 . The process fluid lines  14  are connected to this second cross sectional area  19  and the barrier fluid lines  15  are connected to the first cross sectional area  18 . The first compensator  21 . 1  delivers a barrier fluid with a pressure somewhat lower than the process fluid and is connected to the space  11  neighboring the space  11  connected to the high pressure compensator  21 . 0 . 
         [0029]    The second compensator  21 . 2 , the third compensator  21 . 3  etc all deliver a barrier fluid pressure to different spaces  11 , reducing the pressure in the spaces  11  gradually the further from the annular cavity  4  the space  11  is positioned. Outside the space  11  connected to the seventh compensator  21 . 7  it is the pressure of the environment. All the compensators  21  are connected to two spaces  11 , one on each side of the annular cavity  4 , mirroring the sealing system on both sides of the annular cavity  4 . There is indicated a barrier fluid supply  13 . This may be used to fill the barrier fluid lines  14  to a given pressure before the sealing system is attached to the process fluid pressure. By such a system one is dividing the process fluid pressure between all the compensators, where the division of pressures on the different spaces  11  in sealing system is given by the difference in cross sectional area across the pistons  16 . 
         [0030]    While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.

Summary:
A sealing system between a relatively rotating element and a stationary element has at least three sealing elements arranged between the rotating element and the stationary element and arranged in series between a process fluid and an environment, and a barrier fluid arrangement to provide a barrier fluid to spaces formed between the sealing elements. The barrier fluid arrangement has at least two compensator devices where under use the pressure in the process fluid is acting on one side of a piston in the compensator device and the pressure in the barrier fluid is acting on the opposite side of the piston. There is a difference between cross sectional areas of the two sides of the piston and the difference varies between the at least two compensator devices.