In this article, friction-based damping principles and their suitability for dampening self-excited torsional oscillations in drill strings are investigated. Analytical and semi-analytical solutions based on a modally reduced model are derived by approximating the dynamic behavior with the Harmonic Balance method. The results are validated through time-domain simulations, and limitations of the method are shown. The method enables consideration of the damper position that determines the local amplitudes experienced by the damper. In analytical solutions, the damper location is represented by the local value of the mass-normalized mode shape. The analytical approach can be used to calculate and optimize an equivalent damping ratio for every torsional mode based on the parameters and placement of the friction damper. The provided damping can then be used to estimate the stability of critical torsional modes.

No abstract

Drilling vibration can be harmful to the bit and the bottomhole assembly (BHA), resulting in damage and tool failure and subsequent non-productive time (NPT). Bit damage while drilling offshore is costly, and any improvement in bit life can save multiple unplanned trips and lead to savings for operators. Consequently, dynamic dysfunctions have been the focus of industry research to understand and mitigate their effects. In this paper, the authors present a field case study from an offshore application. A high-frequency, in-bit sensing device (1400-Hz sampling frequency) was installed into the bit shank in conjunction with a well-established measurement while drilling (MWD) tool in the BHA. The intent was to capture the dynamics behavior of the BHA and the dynamics directly at the bit. Multiple measurements along the BHA gave a better understanding of the behavior of the entire drilling system. The measurements were then used, along with dynamics modeling and simulation, to correlate bit and cutter damages with instances of backward whirl and stick/slip. The developed kinematic model of whirl corroborated well with measurements and showed how polycrystalline diamond compact (PDC) cutters can exhibit relative backward motion that potentially leads to severe cutter damage. The extended frequency range of the measurement module also enabled the capture of new dynamics phenomena at much higher frequencies than are usually reported in literature. The insights gained in bit dynamics and cutter damage helped to improve bit design by building dynamically stable PDC bits with increased rate of penetration (ROP) and reduced NPT. The need for high-frequency measurements is also discussed and the benefits are presented, i.e., avoiding bit design iterations around misunderstood vibration issues. In today's challenging drilling applications, the measurements are important in characterizing the harsh drilling environment and understanding high-frequency dynamics phenomena that are rarely measured or discussed.

In this paper the damping capability of piezoelectric shunting is analysed for bladings. Beside the broadly used inductance-resistance networks, negative capacitance techniques are considered. For the validation of the theoretic results, a test rig with a model of a bladed disk with eight blades has been manufactured and equipped with two collocated piezoceramics at each blade. One of the piezoceramics is used as an actuator for an engine order excitation. The second piezoceramics is used for shunt damping. The experimental results of the test rig are compared with numerical results. Therefore, the structure and the piezoceramics are modeled in a finite element program. The modal excitation forces of the piezoelectric actuators are derived for all modes of the structure by a static analysis with a specific voltage applied to the piezoceramics. In addition, using the modal displacement field of the static analysis the modal excitation forces can be calculated. Furthermore, the number of degrees of freedom of the system is reduced by a modal reduction technique. The electrical behavior of the piezoceramics connected to each blade is modeled by one degree of freedom and coupled with the mechanical system described above. The different damping concepts are compared with respect of their effectiveness.

Recent advances in downhole vibration monitoring have enabled detection of high-frequency torsional oscillations (HFTO) of drilling systems in the field. The general understanding of torsional behavior of drilling systems derived from elaborate efforts in the literature does not explain the mechanism responsible for HFTO. Even for the well-known and extensively studied stick/slip vibrations, the mechanism perpetuating the vibrations is not agreed upon by researchers. The paper discusses findings from experimental and analytical studies regarding the HFTO of bottomhole assemblies (BHAs). The phenomenon is investigated and reproduced in field tests, full-scale laboratory tests, and computer modeling. Drilling conditions are identified in which the drilling system is prone to exhibit the vibrations. Interaction of the bit and the drilling system, which can generate self-excited vibrations, is investigated using analytical methods. Cutter-rock interaction as a possible excitation source is studied using laboratory experiments to elucidate its interaction with the system. The integrated analysis investigating rock, cutter, bit, and BHA interaction identifies HFTO as a bit-induced BHA torsional resonance that can occur at vibration modes higher than the fundamental mode of the BHA. It is demonstrated that modes most susceptible to exhibiting HFTO can be identified and the dominant mode predicted. The understanding gained can be employed to mitigate torsional resonance and enhance drilling performance.