| 摘要 |
A knee-shaped underreamer (3) is provided with external cutting inserts and is adapted to be fitted within a drill string (2) above a bit (4). Weight from the drill string (2) urges the bit (4) against the bottom of a bore hole (1). Due to the knee-shape, the underreamer (3) is pressed out toward the wall of a bore hole (1). When the drill string (2) rotates, the underreamer (3) describes a circle, and the external cutting inserts of the underreamer (3) scrape off material from the wall of the bore hole (1) at the same time as the bit (4) lengthens the bore hole (1). A motor (6) drives the bit (4), independently on the rotation of the drill string (2). A stabilizer (5) having variable diameter is fitted within the drill string (2) above the underreamer (3). The stabilizer (5) assists in increasing the bearing force of the underreamer (3) against the wall of the bore hole (1), simultaneously reducing the risk of having the drill string (2) hurled out toward the wall of the bore hole (1). |
