Method and apparatus for damping/absorbing rotational vibrations/oscillations

A vibration damping device for use with a downhole tool, the downhole tool having a tool axis, the vibration damping device comprising:

• a device housing mechanically coupled to the downhole tool, wherein the device housing includes a receptacle having a volume and an inner surface;
• and
• an inertia element movably supported in the receptacle and having a volume, a mass, and a non-zero moment of inertia about the tool axis;
• wherein the volume of the inertia element is less than the volume of the receptacle so that an interstitial volume is defined between the inertia element and the receptacle, and wherein the interstitial volume is occupied by a fluid or an elastomer;
• and
• wherein the inertia element is supported within the receptacle in a manner that allows the inertia element to move relative to the device housing,
• further including positioned between the inertia element and the inner surface of the receptacle, at least one of a longitudinal biasing means and longitudinal friction pad combination or a radial biasing means and radial friction pad combination.

A method for providing a damping tool for use with a bottomhole assembly (BHA), the damping tool including a torsional damping device and the torsional damping device including an inertia element and a damping fluid, comprising the steps of:

• a) calculating a set of natural frequencies and mode shapes of the BHA based on the mechanical properties of the BHA;
• b) selecting at least one desired frequency from the calculated natural frequencies;
• c) calculating or measuring the frequency dependent damping response of a damping device and adjusting at least one property of the damping device so that the calculated or measured frequency dependent damping response corresponds to the at least one desired frequency;
• d) using the calculated mode shapes to determine where to couple the damping device to the BHA.