You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
[0001]     This invention relates to a connector which establishes a fluid tight connection to a drill-string and preferably establishes a fluid tight connection between a drill-string and a top-drive.  
       BACKGROUND  
       [0002]     It is known in the oil and gas industry to use a top drive motor and a drill-string to drill wells. It is the top drive motor that provides the torque to rotate the drill-string, which in turn rotates the drill bit at the bottom of the well. The drill-string itself consists of a series of hollow pipes, typically 30 ft (9.14 m) in length, and these are attached to each other via a threaded connection. The top drive is also attached to the drill-string via a threaded connection.  
         [0003]     During the drilling process, drilling-mud is pumped through the connection between the top drive and the drill-string. This drilling-mud travels through the drill-string and ensures sufficient lubrication, cooling and the removal of cuttings. It is often necessary to remove the drill-string from the well (to replace the drill bit for example) and under such circumstances drilling-mud is pumped through the drill-string to displace and support the retreating drill-string and maintain hydraulic balance in the well bore. This ensures that a vacuum is not created and that the force required to remove the drill-string is minimised, allowing the removal to occur more quickly. In a conventional arrangement, the drilling-mud is pumped through the same connection, between the top drive and drill-string, as used when drilling.  
         [0004]     When removing a drill-string from a well (which in the industry is known as tripping-out), successive sections of the drill-string have to be disconnected from the remaining sections of the drill-string. Furthermore, the section being removed also has to be disconnected from the top drive. A new connection is then established between the top drive and the remaining sections of the drill-string. However, making and breaking these threaded connections is-very time consuming and slows down the process of removing a drill-string from a well. This has a serious impact on the productivity of the well.  
         [0005]     Previous attempts have been made at speeding up the process of tripping-out. GB2156402A discloses methods for controlling the rate of withdrawal and the drilling-mud pressure to maximise the tripping-out speed. However, the time taken to connect and disconnect each section of the drill-string to the top drive is not addressed. Other attempts include removing several sections at a time, as discussed in GB2156402A. However, this approach is limited by the height of the derrick holding the top drive.  
       STATEMENTS OF INVENTION  
       [0006]     According to the present invention, there is provided a connector which provides a fluid tight connection between a fluid supply and a drill-string, the connector comprising a body portion and an extendable seal portion, the seal portion having a seal which is adapted to sealingly engage the drill-string when the seal portion is at least partially extended from the body portion.  
         [0007]     The seal may comprise a tapered bung, which may be forced into the open end of the drill-string, when the seal portion is at least partially extended from the body portion.  
         [0008]     The seal portion may comprise a piston-rod having a cap and a shaft which are joined together, the shaft being slidably mounted within the cylinder. The cap and part of the shaft may be located inside the cylinder.  
         [0009]     The connector may further comprise a piston, the piston being slidably mounted on the shaft within the body portion. The body portion may comprise a cylinder. The piston and cap may divide the cylinder into two chambers: a first-chamber and a second-chamber. The first chamber may contain drilling-mud, whilst the second chamber may contain air.  
         [0010]     In one embodiment, the piston-rod may have a central flow passage which provides a flow communication path between the first-chamber and the drill-string. Furthermore, a flow communication path from the inside of the hollow shaft to the cylinder may be provided by a hole in the piston-rod. The inside of the hollow shaft is not in flow communication with the cylinder when the piston covers the hole in the piston-rod.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the following drawings, in which:  
         [0012]      FIG. 1  is a schematic of the connector and shows the connector in position between the top drive and the drill-string;  
         [0013]      FIG. 2  is a sectional side projection of the connector and shows the connector prior to engagement with the drill-string;  
         [0014]      FIG. 3  is a sectional side projection of the connector and shows the connector when engaged with the drill-string;  
         [0015]      FIG. 4  is a more detailed sectional view of the connector and shows the connector in position to transfer drilling-mud to the drill-string. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0016]     With reference to  FIG. 1 , a drill-string  4  is removed from a well by raising a top drive  2 . The drill-string  4  is connected to the top drive  2  in two ways. Firstly, elevators  6  clamp around the drill-string  4 , and these transmit the force required to raise (or lower) the drill-string  4 . Secondly, the top-most section  3  of the drill-string  4  is provided with a female thread which engages a male threaded connector  5  on the top drive  2  to provide a connection to allow drilling-mud to be pumped into the drill-string  4 . Once a section of the drill-string  4  is removed from the well it must then be disconnected from the rest of the drill-string  4  and the top drive  2  before it can be taken away (or racked into the derrick (not shown)). The remaining sections of the drill-string  4  are held in place by conventional slips on a rotary table (not shown). In conventional arrangements, the join between the top drive  2  and the drill-string  4  is a threaded connection. Making and breaking this connection is time consuming, particularly when removing an entire drill-string  4 . The present invention relates to an alternative means for establishing this connection.  
         [0017]     With reference to  FIG. 2 , a connector  10 , according to the present invention, comprises a cylinder  15  and a piston-rod  20 , the piston-rod  20  being slidably engaged in the cylinder  15 . The piston-rod  20  further comprises a hollow shaft  30 , on which is mounted a cap  40 , the shaft  30  being slidably engaged in the cylinder  15  such that a first end of the shaft  30  protrudes outside the cylinder  15  and a second end is within the cylinder  15 . The cap  40  is mounted on a second end of the shaft  30 , whilst on a first end of the shaft  30  there is located a bung  60  and seals  130 . The bung  60  is preferably made from nylon and is shaped to fit into the top end of a drill-string  4 .  
         [0018]     The shaft  30 , cylinder  15 , bung  60  and cap  40  shown in  FIG. 2  are arranged such that their longitudinal axes are coincident. At the end of the cylinder  15 , beyond which the shaft  30  protrudes, there is mounted an end-cap  110 . The end-cap  110  seals the inside of the cylinder  15  from the outside, whilst also allowing the shaft  30  to slide in or out of the cylinder  15 . Seals, such as O ring seals  25  are used to seal between the end-cap  110  and shaft  30 .  
         [0019]     The connector  10  further comprises a piston  50 . The piston  50  is slidably mounted on the shaft  30  inside the cylinder  15  and is free to move between the cap  40  and the end-cap  110 . The whole assembly  20 ,  40 ,  50  and  60  is also able to slide in the cylinder  15 . The inside of the cylinder  15  is divided by the piston  50  to form a first chamber  80  and a second chamber  70 . The first and second chambers  80  and  70  preferably hold air and drilling-mud respectively. The piston  50  is sealed against the shaft  30  and cylinder  15 , for example by means of O ring seals  52  and  54 , to ensure no flow communication between the two chambers  70  and  80 . The first chamber  80  is in flow communication with an air supply via a port  100  and the second chamber  70  is provided with drilling-mud via a socket  90 . The top drive  2  is connected to the connector  10  via a conventional thread in the socket  90 .  
         [0020]     In the disposition of components shown in  FIG. 2 , the piston  50  and cap  40  are touching, so that drilling-mud cannot flow from the second chamber  70  to the drill-string  4 .  FIG. 3 , shown an alternative disposition of the cap  40  and piston  50 . With the cap  40  and piston  50  apart, holes  120  are exposed in the side of the cap  40 . These holes  120  provide a flow communication path between the second chamber  70  and the interior of the hollow shaft  30 . Thus drilling-mud can flow from the second chamber  70  to the drill-string  4 , via the holes  120  in the cap  40  and the hollow shaft  30 .  
         [0021]      FIG. 4  shows further detail of the structure of the cap  40  and piston  50 . In particular, the flow communication path between the second chamber  70  and the hollow shaft  30 , via the holes  120 , is further highlighted.  
         [0022]     In operation of the connector  10 , the pressure of the air in the first chamber  80  is kept at a constant value of approximately 100 psi. By contrast, the pressure of the drilling-mud in the second chamber  70  is varied and it is this pressure that controls the operation of the connector  10 .  
         [0023]     When the pressure of the drilling-mud pressure is sufficiently low, so that (accounting for the differences in the projected areas of the two sides of the piston  50 ), the force exerted on the piston  50  by the drilling-mud is less than the force exerted on the piston  50  by the compressed air, the piston  50  is biased towards the cap  40  and socket  90 . The piston  50  forces the retraction of the piston-rod  20  into the cylinder  15 . The piston  50  also abuts the cap  40 , thereby closing the holes  120  and ensuring no drilling-mud flows out of the connector  10 . When the piston-rod  20  is retracted, the bung  60  and the seals  130  are disengaged from the drill-string  4  and the top most section of the drill-string  4  can be removed.  
         [0024]     To extend the piston rod  20 , so that the bug  60  and seal  130  engage the drill-string  4 , the pressure of the drilling-mud is increased. Once this pressure exceeds a certain threshold, the force exerted by the drilling mud on the piston  50  exceeds the force exerted by the compressed air on the piston  50 , so that the cap  40  is forced toward the end-cap  110  and the piston-rod  20  extends. As the projected area of the cap  40  is greater than the projected area of the piston  50  and the air pressure is only exposed to the piston  50 , the piston  50  remains abutted to the cap  40 . Thus, whilst the piston-rod  20  is extending, the holes  120  are not exposed and drilling-mud cannot flow.  
         [0025]     Once the bung  60  and seals  130  are forced into the open threaded end of the drill-string  4 , thereby forming a fluid tight seal between the piston-rod  20  and the open end of the drill string  4 , the piston-rod  20 , and hence cap  40 , are no longer able to extend. By contrast, as the piston  50  is free to move on the shaft  30 , the piston  50  is forced further along by the pressure of the drilling-mud. The holes  120  are thus exposed and drilling-mud is allowed to flow from the second chamber  70 , through the piston-rod  20  and into the drill-string  4 . The drill-string  4  can then be lifted by clamping the elevators  6  to the drill-string  4  and raising them.  
         [0026]     As described above, the connector  10  replaces the traditional threaded connection between a top drive  2  and drill-string  4  during the removal of a drill-string  4  from a well. With this connector, the connection between the top drive  2  and drill-string  4  can therefore be established in a much shorter time and great savings can be achieved.

Summary:
A connector ( 10 ) which provides a fluid tight connection between a fluid supply and a drill-string ( 4 ), the connector ( 10 ) comprising a piston-rod ( 20 ) and a cylinder ( 15 ), the piston-rod ( 20 ) having a seal at or towards its free end which is adapted to sealingly engage the drill-string ( 4 ) when the piston-rod ( 20 ) is at least partially extended from the cylinder ( 15 ).