Patent attributes
A drillstring with a bit is characterized by controllable variables of vertical and rotational speeds and response variables of axial tension force and torque, referenced to the top of the drillstring. A method of reducing or avoiding at least an axial or a torsional oscillation mode in the drillstring includes:choosing at least one oscillation mode to be controlled;selecting and monitoring a relevant controllable variable and a relevant response variable;determining the oscillation period;estimating from the response variable a dynamic component of bit speed;determining a speed pulse capable of producing a generated oscillation with amplitude substantially equal to the amplitude of the dynamic component of bit speed; andUsing open-loop control to add the speed pulse to an operator set speed command when the dynamic component of bit speed has an amplitude exceeding a threshold level and an anti-phase matching a phase of the generated oscillation.
