Rotary steerable system having multiple independent actuators

Abstract

A fully rotating bias unit for directional drilling rotates at bit speed while providing proportional control of directional response and steering force. The unit also provides added reliability by having increased redundancy (multiple independently controlled actuators). The unit disposed on a drillstring transfers rotation to a drill bit and has a bore communicating fluid from the drillstring to the drill bit. Directors are disposed on the unit to rotate with it. Each of the directors is independently movable between extended and retracted conditions relative to the unit's housing. Actuators of the unit are in fluid communication between the bore and the borehole or some other low pressure. Each actuator is independently operable to direct communicated fluid from the bore to extend a respective one of the directors toward the extended condition. Meanwhile, venting of the communicated fluid from the directors to the borehole or other low pressure dump allows the respective director to retract toward the retracted condition.