Anordning og fremgangsmåte for bruk ved avviksboring

Appts. for use in drilling a directional borehole, comprises (a) tubular telescopic members connected in a rotary drilling string, one of the members carrying a piston having upper and lower faces that are subject respectively to the pressure of drilling fluids inside and outside the members, the one member being attached to an asymmetrical drill bit; and (b) a synchronously operable means for changing the pressure acting on the upper face of the piston during a portion of each revolution the members to cause the drill bit to drill a directional borehole. Also claimed are (i) appts. for use in controlling the downward force applied to a rotary drill bit during the drilling of a borehole; and (ii) a method of drilling a directional borehole.