| 摘要 |
A steerable rotary drilling system includes a roll stabilised instrument assembly 12 having an output control shaft 85, the rotational orientation of which represents a desired direction of steering. The control shaft is coupled to a bottom hole assembly which includes a drill bit 50 and a synchronous modulated bias unit including means 59, 60, 61, 62 for applying a lateral displacement to the drill bit. Rotation of the bias unit relatively to the control shaft 85 modulates the lateral displacement in synchronism with rotation of the drill bit, and in a phase relation thereto determined by the rotational orientation of the control shaft, so that the maximum lateral displacement is applied to the bit at a rotational orientation thereof which is dependent on the rotational orientation of the control shaft, thereby causing the bit to become displaced laterally in the desired direction as drilling continues. Means 86 are provided for decoupling the control shaft 85 from the bias unit while maintaining the integrity of both the roll stabilised assembly and the bias unit. <IMAGE> |
