| 摘要 |
A drilling system for drilling a borehole into an earth formation is provided. The drilling system comprises a drill string (1) extending into the borehole whereby an annular space (38) is formed between the drill string and the borehole wall, the annular space containing a body of fluid (40), the drill string including a longitudinal fluid passage having an outlet opening at the lower end part of the drill string, pumping means (19, 30) for selectively pumping drilling fluid via said passage and outlet opening into the body of fluid, and a fluid discharge conduit (48) for discharging fluid from the body of fluid; wherein the drilling system further comprises pressure control means (50, 52, 60, 62) for controlling the fluid pressure in the body of fluid when the pumping means (19, 30) is inoperative to pump drilling fluid into the body of fluid.
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