When drilling a wellbore, several types of vibration can occur, which can cause premature failure of drill string components and increase the non-productive time. Especially in hard and dense formation, self-excited torsional vibrations of higher order modes that are caused by the bit-rock interaction lead to critical dynamic loads. Due to the specific boundary conditions in drill string dynamics, such as small installation space, limited energy supply, and variable critical drill string modes, conventional tools to reduce the critical high frequency torsional oscillations, such as dampers and isolators are limited. Thus, enhancements of these principles are necessary to reduce vibration and increase the drilling performance.
In this work, the energy transfer, due to the change of an adaptive local stiffness, between stable and unstable drill string modes is investigated. Furthermore, it is shown how this energy transfer can be used to direct energy towards dampers to increase the stabilizing damping effect. The energy transfer and the damping effect due to local switching is characterized using simulations of nonlinear finite element models of entire drilling systems. In order to efficiently perform robustness analysis and optimizations with respect to arbitrary dampers, an analytical description of the dynamic behaviour of the drill string is generated by section-wise linearization of this highly nonlinear system. Finally, the effect of dampers with and without energy transfer by local switching is discussed and compared with respect to the stabilizing effect in self-excited drilling systems.
 

Self-excitation,Switching,Drill string,Vibration control,Modal analysis