Dynamic Stability of Drillstrings Under Fluctuating Weight on Bit

Summary This paper describes the theory and the underlying formulation behind the development of a drillstring-dynamics simulator that predicts rapidly growing lateral vibrations triggered by axially induced bit excitations. The analyses center on calculations of stable rotary speed ranges for a given set of drillstring parameters and are presented in vibration "severity" vs. rotary speed plots. The critical rotary speeds, which correspond to the rapidly growing lateral vibrations, are pinpointed by spikes on the severity plots. Some applications of the drillstring-dynamics simulator are presented, and limitations and further model development are discussed.