| 摘要 |
The drilling apparatus includes a safety sub mountable to the drill string outer end to retain the combination of an overshot assembly and an overshot adaptor extending within the drill string outer end during a drilling operation and preventing the drilling tool moving out of the drill string until the sub is removed. The overshot assembly has a fluid seal between the overshot tube and the drill string and a fluid bypass channel with valve means for blocking axial outward flow through the channel when the overshot adaptor is separated from the overshot assembly and when connected thereto, retaining the valve means in an open condition. When the overshot adaptor is removed from the overshot assembly, the overshot assembly is fluidly propellable to the bit end. A disconnect tool facilitates the removal of the adaptor from the sub.
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