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CROSS REFERENCE TO EARLIER APPICATION  
       [0001]     The present application is a continuation application of U.S. application Ser. No. 10/520,606 filed 6 Jan. 2005, which is a national stage application of International application No. PCT/EP2003/006750, which claims priority on European Patent Application 02254746.7 filed 8 Jul. 2002. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention is related to a drilling system, more particularly, a system for controlling formation pressure while drilling.  
         [0003]     According to prior art systems, the pressure is controlled by pumping fluid into the bore hole, via the annulus. Such a drilling fluid outlet system could further comprise a safety choke, in case the pump fails.  
         [0004]     However due to large rock debris or contaminated mud, the safety choke could get clogged or could be damaged, which jeopardises correct control over the pressure in the well bore.  
       SUMMARY OF THE INVENTION  
       [0005]     The invention relates to a drilling system for drilling a well bore into an earth formation, comprising: 
        drilling means for drilling a well bore;     pumping means for pumping drilling fluid into the well bore during drilling; and     a drilling fluid outlet system for retrieving drilling fluid from the well bore.        
 
         [0009]     It is an object of the invention to alleviate the disadvantages of the prior art and to provide a system that is capable of regulating the pressure while the flow may contain large rock debris or contaminated mud.  
         [0010]     This object is achieved by a drilling fluid outlet system comprising choke means for choking the returned flow of retrieved drilling fluid and alternating means for alternating the flow direction through the choke means.  
         [0011]     The invention relates also to a drilling fluid outlet system.  
         [0012]     In another aspect the invention relates to a method of drilling a well bore into an earth formation.  
         [0013]     In accordance with this aspect of the invention, the method comprises: 
        drilling the well bore by operating drilling means;     pumping drilling fluid into the well bore during said drilling; and     retrieving drilling fluid from the well bore in a drilling fluid outlet system, said drilling fluid outlet system comprising choke means for choking the return flow of retrieved drilling fluid, whereby the flow direction of retrieved drilling fluid through the choking means is alternated for flushing away any debris from the choke means.        
 
         [0017]     The invention will be now illustrated by way of example and in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0018]     In the accompanying drawing:  
         [0019]      FIG. 1  shows a schematic view of a drilling system according to the invention;  
         [0020]      FIG. 2  shows a schematic view of a first embodiment of a drilling fluid outlet system according to the invention; and  
         [0021]      FIG. 3  shows a second embodiment of a drilling fluid outlet system according to the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0022]     Described is a drilling system comprising an drilling fluid outlet system that comprises choke means for choking the returned flow of retrieved drilling fluid and alternating means for alternating the flow direction through the choke means.  
         [0023]     So if the choke means would get clogged, the returned flow is alternated, such that the choke means are cleaned and the debris is discharged.  
         [0024]     According to a preferred embodiment, the drilling fluid outlet system comprises an inlet and an outlet, the choke means comprise a first inlet/outlet connection, a second outlet/inlet connection and the alternating means connect the inlet alternatingly to the first or second connection and the outlet alternatingly to the second or first connection. So with these alternating means it is possible to connect the choke means such that the inlet is used as an inlet or outlet and that at the same time the outlet is used as outlet or inlet.  
         [0025]     In another embodiment of the drilling system according to the invention, filter means are arranged in the first and second connection of the choke means.  
         [0026]     These filter means block large pieces, which could damage the choke means. Upon alternating the flow direction blocked pieces at the filter means are discharged.  
         [0027]     In again another embodiment of the drilling system according to the invention, the alternating means comprise a four way valve, having four connections and wherein the connections are connected two by two. With such a valve, which could be actuated automatically, alternating of the flow is performed quickly. This will provide a minimum interruption of the flow.  
         [0028]     Preferably, the drilling fluid outlet system comprises an accumulator. This accumulator secures a constant flow at the moment that the flow direction is alternated.  
         [0029]     In yet another embodiment of the drilling system according to the invention, the choking means comprises a bi-directional choke. Such a bi-directional choke provides a choking action in both flow directions. This provides for a compact design.  
         [0030]     In another embodiment the choking means comprise at least two uni-directional chokes. Such uni-directional chokes provide a choking action in only one flow direction. Therefore two chokes are necessary to provide a choking action in both flow directions.  
         [0031]      FIG. 1  shows a schematic view of a drilling system  1  according to the invention. With this drilling system  1  a well bore  2  is drilled into the earth formation  3 .  
         [0032]     The drilling system  1  comprises a frame  4  onto which a fluid chamber  5  is slidably arranged along a guide  6 . A drill string  7  with on its bottom end a drill bit  8  is rotated by a top drive  9  in order to drill the well bore  2 . The well bore  2  is partially lined with a casing  10 . The drill string  7  is composes out of a plurality of drill string joints  11 , which are interconnected by connectors  12   a  and  12   b . Drill mud contained in a reservoir  13  is pumped by pump  14  into the drill string  7  to the drill bit  8 . The drilling mud  25  flows into the annulus between the well bore wall and the drill string  7  upwards. The annulus is closed off by a rotating blow out preventer  15 . The drilling mud is returned via pipe  16  and drilling fluid outlet system  17  back to the reservoir  13 . When connecting another drill string joint  11  the feed of drilling mud is taken over by pump  18 , which pumps the drilling fluid into the chamber  5 , such that it can flow into the drill string  7 .  
         [0033]     The drilling fluid outlet system  17  ensures that a certain pressure is maintained in the well bore  2  by choking the flow in the pipe  16 .  
         [0034]     In  FIG. 2  the drilling fluid outlet system  17  is shown in more detail. The system  17  comprises a valve  20  and a bidirectional choke  21 . In the position of the valve  20  as shown in  FIG. 2  the inlet pipe  16  is connected to the first connection pipe  22  of the choke  21 . The outlet pipe  19  is connected to the second connection  23  of the choke  21 . When the choke  21  gets clogged, the valve  20  is rotated, such that the inlet pipe  16  is connected to the second connection  23  of the choke  21  and the outlet pipe  19  is connected to the first connection  22  of the choke  21 . In this way the flow direction is alternated and any debris, which is clogging the choke  21  is flushed away through outlet pipe  19 .  
         [0035]     In  FIG. 3 a  second embodiment  30  of a drilling fluid outlet system according to the invention is shown. The system  30  again comprises a valve  31  and two uni-directional chokes  32  and  33 . Such chokes  32 , 33  have a choking action in just one flow direction. Furthermore, the system  30  comprises filter means  34 , 35  that filters large debris in the mud, which could damage the chokes  32 , 33 . An accumulator  36  is connected to the inlet pipe  16 , which ensures that a constant flow is maintained while the flow direction of the system  30  is alternated by turning the valve  31 . Any debris present on the filter means  34 , 35  or in the chokes  32 , 33  is cleaned by alternating the flow direction of the system  30  and is flushed away through outlet pipe  19 .  
         [0036]     It is remarked that WO-00 79092, for example, discloses a drilling system for drilling a well bore into an earth formation, comprising: 
        drilling means for drilling a well bore;     pumping means for pumping drilling fluid into the well bore during drilling; and     a drilling fluid outlet system for retrieving drilling fluid from the well bore. The drilling fluid outlet system is used to maintain control over the fluid pressure at the well bore wall, especially when drilling is stopped or during tripping of the drill string out of the bore hole. Without maintaining control over the pressure, there is a potential danger that undesired fluid flows from the earth formation into the bore hole, or that the borehole wall collapses.        
 
         [0040]     While the illustrative embodiments of the invention have been described with particularity, it will be understood that various other modifications will be readily apparent to, and can easily be made by one skilled in the art without departing from the spirit of the invention. Accordingly, it is not intended that the scope of the following claims be limited to the examples and descriptions set forth herein but rather that the claims be constructed as encompassing all features which would be treated as equivalents thereof by those skilled in the art to which this invention pertains.

Summary:
A drilling system and method for drilling a well bore in to an earth formation is provided with drilling means for drilling a well bore and pumping means for pumping drilling fluid into the wellbore during drilling. Also provided is a drilling outlet system for retrieving drilling fluid from the well bore. The drilling fluid outlet system has choke means for choking the return flow of retrieved drilling fluid, and alternating means for alternating the flow direction through the choking means.