Disconnect and other devices for downhole assembly

A disconnect tool for incorporation in a drill string between a downhole assembly and a drill pipe to selectively disconnect the downhole assembly from the drill pipe when the downhole assembly is stuck in a wellbore, said disconnect tool comprises:

• a disengagement apparatus and first and second parts, the first and second parts each being coupled to the disengagement apparatus such that the first and second parts are releasably connected to one another by the disengagement apparatus, one of said first and second parts being adapted for connection to said drill pipe and the other of said first and second parts being adapted for connection to said downhole tool, wherein
• said disengagement apparatus comprises a first actuator and first and second coupling elements,
• the first coupling element comprising:
• a die retention sleeve, axially movable in the first part from an operational position towards a disconnect position of the disengagement apparatus;
• a clutch housing, disposed within said die retention sleeve, said clutch housing being axially and rotationally fixed in the first part;
• windows in said clutch housing circumferentially spaced around the clutch housing;
• and
• radially displaceable capture dies housed in said windows, and
• the second coupling element comprising:
• an interface of said second part adapted to be engaged by said capture dies, wherein,
• the first actuator moves the retention sleeve between the operational and disconnect positions, so that
• when the first and second parts are engaged with one another and the retention sleeve is in the operational position, the capture dies bear against both the die retention sleeve and said interface of the second part to lock said first and second coupling elements and parts together, and
• when the retention sleeve is moved to the disconnect position, the capture dies can move radially to disengage from said interface so that said coupling is unlocked and said parts can separate, wherein said first actuator comprises the sleeve being screw threaded on said first part and having a circumferential rack driven by a pinion of a first motor, whereby screwing of the sleeve on the first part moves the sleeve axially between said operational and disconnect positions,
• characterised in that said pinion is fixed to an output shaft so that relative axial movement between the pinion and the output shaft is prevented, the disconnect tool further comprising a second motor and a second actuator controlled by said second motor, said second actuator comprising a mechanical brake having an engaged position and a disengaged position, said mechanical brake being configured to prevent screwing of the sleeve when said mechanical brake is in said engaged position.