This paper describes two principal sources of bottomhole-assembly (BHA) bending vibration: (1) drill-collar whirling and (2) linear coupling between weight-on-bit (WOB) fluctuations and bending vibration of an initially curved BHA. It also evaluates the consequences of bending vibration in terms of drill-collar wear and connection fatigue. Equations are given for forward and backward whirl rate and for tangential velocity at the borehole wall. Downhole measurements of bending moment are used to detect and to identify bending-vibration events, and data taken with a downhole vibration-measurement system are used to illustrate cases of linearly coupled bending vibration, forward and backward whirl, and bit bounce.

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