Patent ID: 9624762
Date: 2017-04-18
CPC Classifications: E21B

Claim:
1. A method for reducing or avoiding at least an axial or a torsional oscillation mode in a drillstring with a bit attached to its lower end and controlled by a hoisting and rotation mechanism attached to its top end, wherein available controllable variables are a vertical speed and a rotational speeds and available response variables are an axial tension force and a torque, wherein the available controllable variables and available response variables are referenced to the top end of the drillstring, the method comprising: choosing at least one oscillation mode to be controlled from among at least an axial or a torsional oscillation mode; choosing a selected controllable variable from among the available controllable variables and a selected response variable from among the available response variables; wherein these two selected variables are relevant for the oscillation mode monitoring the selected controllable variable and the selected response variable; determining oscillation period of the oscillation mode; evaluating from the selected response variable a dynamic bit speed of the oscillation mode; determining a speed pulse, defined as a time limited variation of the selected controllable variable, capable of generating a generated oscillation with an amplitude substantially equal to an amplitude of the dynamic bit speed; and starting an open-loop controlled speed variation by adding the speed pulse to an operator set speed command when the amplitude of the dynamic bit speed exceeds a certain threshold level and an anti-phase of the dynamic bit speed matches a phase of the pulse generated oscillation.