Downhole disconnect tool

A hydraulic disconnect is disclosed which allows flow therethrough for normal downhole operations. A shiftable sleeve is provided which closes off internal flow passages to permit pressure build-up which is in turn used to move a piston to undermine a dog to facilitate release. Shifting of a sleeve is accomplished by either creating backpressure through flow or by differential pressure between the inside of the joint and the annulus which is used to shift a piston which in turn acts on the release sleeve which in turn blocks a passage so that subsequent pressure build-up can result in shifting of another piston to undermine dogs which allows the joint to separate. The tool transmits torque through its body without stressing the locking dogs. Tensile and compressive loads are handled through the body giving the tool the connection strength of a tool joint.