Underreamer.

摘要

This underreamer has a lower body and a tubular upper body connected above it. A pair of extendible cutter arms are scissored in the lower body for pivoting between retracted and extended positions. A sleeve fixed to the lower body extends upwardly within the upper body with an outside diameter smaller than the inside diameter of the upper body to leave a fluid flow annulus between the sleeve and upper body. A piston in the sleeve is axially movable from an upward position toward a lower position in response to application of fluid pressure through a drill string for pivoting the cutter arms from the retracted position toward the extended position. Passages through the lower body convey drilling fluid through the lower body from the annulus between the upper body and the sleeve. Such fluid is discharged outside the lower body adjacent to cutters on the cutter arms, into a pocket in the lower body occupied by the cutters in their retracted position, and through the lower end of the lower body.