1) a hole in the rig floor, 30 to 35 ft (9 to 11 m) deep, which is lined with casing that projects above the floor and into which the kelly and swivel are placed when hoisting operations are in progress.
2) a hole of a diam smaller than the main hole and drilled in the bottom of the main hole. v to reduce the size of the wellbore and drill ahead.
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