SYSTEM FOR ROTARY-PERCUSSION DRILLING IN AN EARTH FORMATION

摘要

A drilling system for drilling a borehole in an earth formation, comprising a drill string having a central longitudinal (24) axis and being provided with a drill bit (30) having a bottom surface including a plurality of cutters (34) arranged so that said bottom surface has an envelope of a stepped profile in radial direction. First drive means is provided for rotating the drill bit in the borehole so as to induce a scraping movement of the cutters along the borehole bottom (15) and second drive means is provided fo r inducing a longitudinal reciprocal movement of the drill bit in the borehole so as to induce the cutters to exert a percussion force to the borehole bottom.