| 摘要 |
A drilling unit for rotary drilling of wells using casing as the drill string. The drilling unit is a self-contained assembly designed to be bodily inserted and withdrawn through the bore of a well casing string and comprises releasable latch means for non-rotatively locking the structure to the casing; a rotary power unit supported by the latch means and rotatively connected to a bit assembly. The latter comprises a main or pilot bit and a plurality of radially extendible and retractable underreamers actuated by fluid pressure which may first be employed for driving the power unit.
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