Automatic drilling system

Method of controlling weight on bit while drilling a borehole with a rotary drilling rig that includes a rotatable drill string having a drill bit connected at the lower end thereof, and a drawworks for lifting and lowering the string with a drilling line rove about a drawworks drum, a drawworks motor means for rotating the drum and moving the string uphole and downhole and thereby changing the elevation of the bit;

• and a drawworks brake means for rendering the drum non-rotatable;
• comprising the steps of:
• (1) connecting another motor means to be rotated by said drum as said drilling line unspools from said drawworks drum;
• connecting a power source to said another motor means for applying a rotational force by said another motor means in opposition to the rotation as the drawworks drum unspools drilling line;
• (2) weighing the drill string while said string is suspended off bottom and converting the resultant measurement to a signal that is proportional thereto;
• (3) selecting a desired value of the weight on bit, and providing a signal that is proportional thereto;
• (4) combining the signals of steps (2) and (3) to provide an operating signal that is representative of the tension required at the upper end of the drill string in order to continuously support the string with an uphole force that results in a weight on bit substantially equal to step (3) while drilling a borehole;
• (5) controlling the rotational force that said another motor means applies to the drawworks drum in proportion to the magnitude of the operating signal of step (4) while drilling a borehole whereby the weight of said drill string rotates the drum to unspool cable therefrom while the another motor means resists unspooling of the cable so that the string is lowered at a rate which maintains said weight on bit of step (4) substantially constant.

A drilling rig having a top drive unit comprised of an electric motor connected to rotate the upper end of a drill string;

• a bit at the lower end of the string;
• a swivel supports the motor;
• drilling fluid flows through said swivel, and continues through a rotating hollow shaft of said motor;
• said swivel is supported from a traveling block which in turn is supported by a plurality of cable strands from a crown block, one end of the cable being rove about a drawworks drum so that rotation of the drum changes the elevation of the string;
• whereby as the tension in the cable is reduced the string is lowered and the bit engages and penetrates a formation;
• first motor means for rotating said drum to thereby lift and lower said traveling block whereupon said swivel, electric motor, drill string and bit are lifted uphole and lowered downhole;
• a second motor means connected to be rotated by unspooling of said drawworks drum;
• said second motor means applies a rotational force to said drawworks drum that is opposed to unspooling of said drawworks drum;
• said second motor means resists unspooling of the drawworks drum in proportion to the power delivered to said second motor means;
• means weighing said drill string while the bit is off bottom and converting the weight measurement into a signal proportional thereto;
• means selecting a weight on bit, and converting the weight on bit selection into a signal proportional thereto;
• the weight on bit being a fraction of the drill string weight;
• means combining the drill string weight signal and the selected weight on bit signal to provide an operating signal representative of the tension that must be effected in the cable in order to achieve said weight on bit;
• and means controlling the power delivered to said second motor means in proportion to the magnitude of said operating signal to continually adjust the rotational force of the second motor means and thereby adjust the weight on bit to a value substantially equal to said selected value of the weight on bit.

In a rotary drilling unit having a drawworks drum which spools and unspools cable therefrom to raise and lower a drill string uphole and downhole in a borehole;

• a bit at the end of the string, a first motor connected to rotate said drum in either desired direction of rotation, brake means to prevent rotation of said drum;
• the improvement comprising:
• means for weighing said drill string with the bit off bottom to provide a first signal which is proportional to said drill string weight;
• a second motor connected to be rotated by said drum;
• said second motor means being connected to resist unspooling the cable when energized;
• means for selecting a desired weight on bit during a drilling operation and providing a second signal proportional to said desired weight on bit;
• means combining the first and second signals to provide an operating signal representative of the weight of the drill string required in order to achieve said desired weight on bit;
• a controller for controlling the rotational force said second motor imparts into resisting the rotation of the drawworks drum;
• means connecting said operating signal to said controller so that the rotational power of said second motor changes in proportion to said operating signal to thereby unspool the cable at a rate to maintain said desired weight on bit substantially constant.

A weight on bit control system for a drilling rig that has a drill string including a bit attached at the lower end thereof, and means for rotating the upper end thereof;

• and a drawworks having a cable drum that rotates to spool a cable on and off the drum, the cable being attached to vertical movement of the upper end of the string, thereby enabling the drill string to be lifted and lowered within the borehole in response to drum rotation;
• a first motor means for rotating said drum and lifting said drill string uphole and lowering said string downhole;
• brake means by which the drum can be rendered non-rotatable;
• means forming a bit weight set point for selecting the desired weight on bit while making hole;
• means for weighing said drill string while the bit is off bottom and while the bit is drilling and making hole;
• and means for converting the selected bit weight and the weight of the drill string into an operating signal wherein the magnitude of the operating signal varies with the change of weight of the drill string while the bit is drilling and making hole;
• a second motor means connected independently of said first motor means and said brake means to reduce the rate of unspooling of said drum, said second motor means is connected to be rotated in one direction by the unspooling of the drawworks drum and to exert a torque in the other direction in response to the power delivered to said second motor means;
• means controlling the power delivered to said second motor means in proportion to said operational signal;
• whereby:
• the rate of descent of the drill string is controlled and thereby maintains the weight on bit substantially equal to said set point.