Abstract
In this paper, a passive damper based on energy dissipation through shock and dry friction (shock-friction damper) is investigated regarding its design and effectiveness for damping self-excited torsional vibrations similar to those occurring in deep drilling. The results are compared to the results of conventional friction dampers. The effectiveness of the damper for different operational drilling parameters that change during the drilling process, such as the weight on the bit and the rotary speed of the bit, is analyzed. Two linear reduced order models of a drill string that are based on a complex finite element model are set up. One is reduced using the component mode synthesis and one is reduced to the identified critical mode. A lumped mass represents the inertia of a forcedly connected nonlinear damper. A combined reduced order model of the complex system and the inertia damper is introduced to investigate its dynamic motion and stability. Particular focus is on the energy flow within the dynamic system and on the change of the dissipation energy in the contact. A semi-analytical solution is derived using the harmonic balance method that is used to investigate the damping effect for various designs and operational parameters. Herein, the modal properties as well as parameters of the damper are examined regarding the damping effect and the stability of the system. Finally, the capability of the mechanism to suppress the self-excitation due to the bit–rock interaction in a drilling system is discussed, and recommendations are made with respect to the design parameters and placement of the damper.
Highlights
In downhole drilling systems, various types of vibrations occur that can reduce drilling performance and reliability as well as increase premature failure of components and nonproductive time [1, 2]
When drilling in hard and dense formations, high-frequency torsional oscillations (HFTO) occur [3] in a system-dependent frequency range between 50 Hz and 500 Hz. ese oscillations are self-excited torsional vibrations of higher-order modes that are caused by the bit-rock interaction [4] and that can lead to critical torsional loads [1, 5]
Downhole measurement data show that in most cases one high-frequency torsional mode dominates the dynamic of the drilling system [6, 7]. e selfexcitation mechanism, which leads to the critical torsional loads, can be modeled by a torque characteristic at the bit that is nonlinear with respect to the rotary speed (Figure 1) [8]
Summary
Various types of vibrations occur that can reduce drilling performance and reliability as well as increase premature failure of components and nonproductive time [1, 2]. In [25], shocks are used to suppress self-excited vibrations through energy transfer from low- to high-frequency modes, which results in an increased damping ratio. Another possibility to use shocks to reduce vibrations are various types of impact dampers. E challenges in drill string dynamic and the possibility of influencing the energy balance and, if applicable, the damping effect through shocks necessitates further investigation Optimization of these highly nonlinear damping concepts to increase the provided damping for critical torsional modes, stabilize the self-excited modes, is important. E resulting shocks can increase the damping effect through energy transfer from the self-excited structure to the damper and increase the dissipated energy in the friction contact. A semi-analytical and an analytical solution for the design of the damper is derived by using the harmonic balance method. e semi-analytical solutions are compared to time domain simulations of an entire selfexcited drilling system
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