Steerable rotary drilling systems

A method of maintaining a downhole instrumentation package in a roll stabilised orientation with respect to a drill string, comprising the steps of:

• mounting the instrumentation package in an instrument carrier which is rotatable about a longitudinal axis relatively to the drill string;
• rotating the instrument carrier about its longitudinal axis by means of an impeller disposed in a flow of drilling fluid passing along the drill string;
• and
• controlling the torque applied to the instrument carrier, in response to signals indicative of the rotational orientation of the instrument carrier, to vary the rate of rotation of the instrument carrier relatively to the drill string to provide roll stabilisation of the instrument carrier with respect to the drill string.

A steerable rotary drilling system comprising:

• a support connectable to a drill string;
• an instrument carrier carried by the support;
• means carried by the support for permitting the instrument carrier to rotate about the instrument carrier's longitudinal axis;
• means for transmitting a torque to the instrument carrier to cause it to rotate about its longitudinal axis relatively to the support in a direction opposite to the direction of rotation of the support and drill string;
• sensors carried by the instrument carrier for sensing the rotational orientation of the instrument carrier about its longitudinal axis and producing a signal indicative of said rotational orientation;
• control means for controlling, in response to said signal, the torque applied to the instrument carrier to vary the rate of rotation of the instrument carrier relatively to the support to provide roll stabilization of the instrument carrier;
• a bottom hole assembly including a drill bit and a synchronous modulated bias unit including means for applying to the drill bit a displacement having a lateral component at right angles to the axis of rotation of the drill bit;
• an output control shaft coupled between the instrument carrier and the bias unit, the rotational orientation of the shaft represents a desired direction of steering;
• means operated by rotation of the bias unit relatively to said output control shaft for modulating the lateral displacement component in synchronism with rotation of the bias unit and in a phase relation thereto determined by the rotation orientation of the control shaft, whereby the maximum value of the lateral displacement component is applied to the drill bit at a rotational orientation of the bias unit dependent on the rotational orientation of the control shaft, thereby to cause the drill bit to become displaced laterally in the desired direction as drilling continues;
• and
• means for readily decoupling the control shaft from the instrument carrier and the bias unit.