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Bent Sub

In this system, the drilling mud is pumped to the drill head under pressure where it is used to rotate the drill bit via a mud motor. In addition, the section of drill pipe behind the drill bit is deviated by a few degrees (termed a "bent sub") which will cause the borehole to progressively deviate if the drill bit is advanced without rotation of the drill string. If the drill string is rotated while the drill bit is advanced, the borehole will advance in an approximately straight direction. Combining straight drilling with curved sections allows a preplanned borehole with curved sections to be followed.

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