Drilling systems are subject to torsional vibrations that are excited by bit-rock interaction. These torsional oscillations can be distinguished by mode shape and natural frequency. Well-known stick/slip oscillations are characterized by relatively low frequencies and affect the entire drillstring. High-frequency torsional oscillations (HFTO), in contrast, denote the excitation of a high-order eigenmode [I, 2J. In the case of HFTO, high dynamic loads are mainly experienced by the bottom-hole assembly (BHA). All torsional oscillations can compromise drilling efficiency and tool reliability. It is important to estimate expected dynamic loads during the design process of new tools and for optimal design of bottom-hole assemblies. Therefore, models are necessary that can predict the expected dynamic loads.
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