An actively controlled rotary steerable
drilling system for
directional drilling of wells having a tool collar rotated by a
drill string during
well drilling. A bit shaft has an upper portion within the tool collar and a lower end extending from the collar and supporting a
drill bit. The bit shaft is omni-directionally pivotally supported intermediate its upper and lower ends by a
universal joint within the collar and is rotatably driven by the collar. To achieve controlled steering of the rotating
drill bit, orientation of the bit shaft relative to the tool collar is sensed and the bit shaft is maintained geostationary and selectively axially inclined relative to the tool collar during
drill string rotation by rotating it about the
universal joint by an offsetting mandrel that is rotated counter to collar rotation and at the same frequency of rotation. An
electric motor provides rotation to the offsetting mandrel with respect to the tool collar and is
servo-controlled by
signal input from position sensing elements such as magnetometers, gyroscopic sensors, and accelerometers which provide real time position signals to the
motor control. In addition, when necessary, a
brake is used to maintain the offsetting mandrel and the bit shaft axis geostationary. Alternatively, a
turbine is connected to the offsetting mandrel to provide rotation to the offsetting mandrel with respect to the tool collar and a
brake is used to
servo-control the
turbine by
signal input from position sensors.