Method and apparatus for determining drill string movement mode

A method for determining a mode of motion of a drill string, comprising:

• measuring a parameter related to acceleration of the drill string along at least one direction;
• spectrally analyzing the measurements of acceleration;
• and
• determining existence of a particular mode from the spectrally analyzed measurements, wherein the determining the particular mode comprises identifying a component frequency in the spectrally analyzed measurements which corresponds to at least one of a resonant frequency and a whirl frequency of a selected component of the drill string.

A method for estimating wear on a drill string, comprising:

• determining a mode of motion of the drill string;
• calculating side forces generated by contact between affected components of the drill string and a wall of a wellbore as a result of the mode of motion;
• and
• estimating a wear rate corresponding to the side forces and a rate of rotation of the drill string.

A method for estimating fatigue on a drill string, comprising:

• determining a mode of motion of the drill string;
• calculating flexural forces generated as a result of the mode of motion;
• and
• estimating a fatigue rate from the flexural forces.

An apparatus for determining mode of movement in a drill string, comprising:

• a sensor for measuring lateral acceleration of the drill string;
• means for determining a lateral position of the drill string with respect to time from the acceleration measurements;
• and
• means for determining the mode from the position with respect to time.

A apparatus for determining a mode of motion of a drill string, comprising:

• a sensor for measuring a parameter related to acceleration of the drill string along at least one direction;
• a spectral analyzer operatively coupled to the sensor;
• and
• means for determining existence of a particular mode from the spectrally analyzed measurements the means for determining the particular mode comprising means for identifying a component frequency operatively coupled to the spectral analyzer, the component frequency corresponds to at least one of whirl rate, torsional resonance, axial resonance and lateral resonance of the selected component of the drill string.