a bent sub
This is a feature of directional drilling equipment, the last section of the drill string consists of the turbine, and maybe some 20 - 30 feet in length, but has a slight bend midway of say 1-2deg, this makes it slightly like a banana. With the bit on the bottom of the hole, and the pipe not rotating, (the bit rotation is achieved by the turbine), but the bend will cause the hole to deviate. Downhole survey equipment can be used to orientate the pipe so the direction can be chosen. The radius of the bend is determined by the bend in the sub. In order to drill straight, the pipe is rotated, so the effect of the slight bend is now eliminated, this bent section wobbles as it rotates but is not sufficient to be a problem. To drill straight, rotate the drill pipe, to drill in a curve, keep the pipe still, relying on turbine rotation only, and allow the bit to deviate in the direction of the bend.
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