| 摘要 |
A drilling system for drilling a borehole into an earth formation is provide d. 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 th e 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 bod y of fluid; wherein the drilling system further comprises pressure control mea ns (50, 52, 60, 62) for controlling the fluid pressure in the body of fluid whe n the pumping means (19, 30) is inoperative to pump drilling fluid into the bo dy of fluid.
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