This invention concerns a core drilling tool with direct drive.
Such tools can be used for core drilling jobs in all conventional deep well drilling installations. It is not necessary to dismantle the entire drilling shaft in order to remove the core.
U.S. Pat. No. 4,518,050 describes a core drilling tool of the type defined initially which makes it possible to pull the core drilling appliance while the outer pipe remains in the borehole and whereby the core drilling appliance can be moved axially in the outer pipe during coring. The motor of this tool remains axially secured in the outer tube during coring and is rotationally coupled to the movable core drilling appliance by way of movable coupling elements. The core drilling appliance presents reaction faces to the drilling fluid such that they impose an axial feed force on it as a result of the drilling fluid pressure applied to it in combination with the extent of the surfaces. The drilling mud pressure applied through the core drilling appliance is caused by the throttling effect of the annular spaces and gaps in the drilling mud flowing through the core drilling appliance. A first space through which the flow passes is formed by a core pipe and an inside pipe that carries a drill crown. A second space through which flow passes is located between the above-mentioned inside pipe and an outer core barrel surrounding it.
Although the throttling effect of the first space remains constant, the throttling effect of the second space decreases with an increase in exposure of the core drilling appliance out of the outer core barrel. The drilling mud pressure and thus the axial feed force are proportional to the sum of the throttling effects of the two spaces. In addition to the great dependence of the axial feed force on the exposure of the core drilling appliance, another disadvantage is that the maximum value of the axial feed force is relatively low and furthermore the size of this force cannot be influenced.