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CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims benefit of Norwegian patent application serial number NO 20031305, filed Mar. 21, 2003, which is herein incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention concerns a device and a method to enable disconnection of a tool and a pipe string. More particularly, it concerns a disconnection device to be used especially in connection with coiled tubing operations, in which the connection part attached to the pipe string is of an external transverse dimension that may be equal or smaller than that of the pipe string. The invention also comprises a method of effecting the disconnection. 
   During coiled tubing operations, and especially when using a coiled tubing in for example a borehole, a tool attached to the pipe string may become solidly stuck in the borehole to a degree rendering the pipe string useless for pulling it loose. 
   Pipe strings are commonly provided with a disconnection device enabling disconnection of the tool and the pipe string, after which the pipe string and the disconnected part of the tool may be retrieved from the borehole. The tool may subsequently be pulled up using fishing tools. 
   Known disconnection devices are generally formed with a transverse dimensions larger than that of the pipe strings onto which they are attached. Due to this situation, there can be a problem pulling the part of the connection device connected to the pipe string through restrictions located close to the surface. In the past, this problem as been solved for instance through the use of an explosive charge that is introduced into the pipe string immediately above the connection, after which the connection device part is disengaged through blasting from the pipe string. The pipe string then may be pulled up to the surface. 
   Prior to the positioning of the charge, any hydraulic lines and cables present in the pipe string have to be disconnected and retracted to the surface. 
   2. Description of the Related Art 
   The object of the invention is to remedy the disadvantages associated with the prior art. 
   The object of the invention is achieved through features disclosed in the specification below and in the subsequent claims. 
   In accordance with one aspect of the present invention there is provided a disconnection device for disconnecting a tool and a pipe string, the device comprising a first connection part releasably connected to a second connection part by means of a locking device and a release object, wherein at least a section of the release object is soluble. 
   At least in preferred embodiments, a first connection part having an outer transverse dimension equal to or less that that of the pipe string is connected, possible via intermediate parts, to the lower end section of the pipe string. By means of an axially split connector ring of known type, the first connection part is releasably connected to a second connection part. The second connection part is connected, possible via intermediate parts, to a tool. 
   The split connector ring is maintained in its locking position by means of a pre-stressed locking object aimed in the direction of opening, a release object preventing the locking object from shifting away form its locking position. 
   At least in preferred embodiments, a first connection part having an outer transverse dimension equal to or less than that of the pipe string is connected, possible via intermediate parts, to the lower end section of the pipe string. By means of an axially split connector ring of known type, the first connection part is releasably connected to a second connection part. The second connection part is connected, possible via intermediate parts, to a tool. 
   During pipe string operations of the stated type, at least one hydraulic pipe is commonly introduced down to the tool inside the pipe string. By connecting this hydraulic pipe to the release object, the object may be dissolved for instance upon pumping acid down through the hydraulic pipe. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred embodiment of the invention will now be described by way of example only and with reference to the accompanying drawings, in which: 
       FIG. 1  shows a connector placed between a pipe string and a tool, the assembly being located in a borehole; 
       FIG. 2  shows an enlarged view of the connector of  FIG. 1 , the connector being in its locking position; 
       FIG. 3  shows the connector of  FIG. 2  when the release object is weakened, thereby causing a spring to displace the locking object away from its locking position; 
       FIG. 4  shows the connector of  FIG. 3  when the split connector ring is displaced away from its locking position, and the first connection part of the connector is being displaced away from the second connection part of the connector; and 
       FIG. 5  shows a smaller scale view of the connector in a released position. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   In the figures reference numeral  1  denotes a connector that connects a pipe string  2  with a tool  4 , all of which are placed in a borehole  5 . 
   The connector  1  consists of a first connection part  6  which is fixedly attached to the pipe string  2 , and which is of a design enabling it to be displaced into a second connection part  8  in a releasable and locking manner. The second connection part  8  is fixedly attached to the tool  4 . 
   The first connection part  6  is provided with a through-going bore  10 , as shown in  FIG. 2 . The bore  10  is of a relatively large diameter extending from the lower end section of the first connection part  6  and onwards to a shoulder  14  located between the two end sections of the first connection part  6 . The exterior of the first connection part  6  is provided with three encircling grooves  16  matching in a complementary manner an axially split locking ring  18 . 
   Axially the locking ring  18  is positioned against a ledge  20  within the second connection part  8  and is held radially in its locking position by means of a locking sleeve  22  displaced inward and over the locking ring  18 . The locking sleeve  22  is displaceably located within a bore  24  of the second connection part  8 . 
   A spring  26  for displacing the locking sleeve  22  out of its locking position is placed between the locking sleeve  22  and the first connection part  6 . 
   A first hydraulic pipe  28  running through the pipe string  2  is connected to a first coupling nipple  30 . The first coupling nipple  30  is positioned against the shoulder  14  and is connected to a tubular release object  34  via threads  32 . The releasable object  34  forms a portion of a hydraulic circuit. 
   A second hydraulic pipe  36  attached to the tool  4  is connected to a second coupling nipple  38 . The second coupling nipple  38  is positioned against the end portion  40  of the locking sleeve  22  and is connected to the tubular release object  34  via threads  42 . 
   The release object  34  together with the first coupling nipple  30  and the second coupling nipple  38  thereby prevent the locking sleeve  22  from being displaced out of its locking position. 
   Upon disengaging the tool  4  from the pipe string  2 , acid is pumped down through the first hydraulic pipe  28  and down to the release object  34 . Being of a material that is relatively easy to dissolve, for example an aluminium alloy, the release object  34  is partially dissolved after being affected by acid for a period of time. Preferably the release object  34  is provided with a section  44  having reduced wall thickness. The coupling nipples  30 ,  38  are provided with seals  46  that prevent acid from flowing out of the bore  10  as the acid is pervasively corroding the release object  34 . 
   Having consumed a transverse section of the release object  34 , thereby dividing it into at least two parts, the spring  26  displaces the locking sleeve  22  axially and out of its locking position, as shown in  FIG. 3 . 
   Then the axially split locking ring  18  is displaced radially out of the grooves  16  in the first connection part  6  and outwards into the bore  24 , as shown in  FIG. 4 . Thereby the first connection part  6  is released from the second connection part  8 , and the connection parts  6 ,  8  then may be displaced axially and away from one another, as shown in  FIG. 5 . 
   It will be appreciated that variations in the above described embodiments may still fall within the scope of the invention, which is set out in the accompanying claims.

Summary:
A disconnection device for disconnecting a tool and a pipe string comprises a first connection part releasably connected to a second connection part by means of a locking device and a release object. The release object is soluble by means of, for example, acid.