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.

During downhole drilling, severe vibration loads can occur that affect the reliability and durability of tools in bottom-hole assemblies (BHA). These high-frequency torsional oscillations (HFTO) can cause premature damage to tools or their subcomponents. This paper presents dynamic simulations, prototype testing, and field test results of a BHA component that isolates the upper part of the BHA from HFTO. This isolation increases the performance and reliability of BHA components through reduction of the vibration load. The paper provides a brief summary of the theoretical background for predicting the critical torsional eigenfrequencies, mode shapes, and susceptibility of BHA design to HFTO. Finite-element (FE) models were used to simulate the effect and efficiency of the isolation. The isolator tool was tested in a full-scale laboratory test setup that emulated the critical mode shapes to analyze the torsional dynamics and the expected isolation effect. A BHA with the isolator tool positioned between a rotary steerable system (RSS) and measurement while drilling (MWD) tools was excited in the critical HFTO mode shapes with an electrodynamic shaker. Vibration response was measured using triaxial accelerometers. Later, the tool was tested in the field in a BHA configured to log the vibration below and above the tool at sufficiently high sample rates to validate the isolation effect of the tool. The laboratory results showed the isolating effect between BHA components above and below the isolator tool, in particular for the critical HFTO-frequencies and mode shapes as predicted by the finite-element simulation. The torsional deflection shapes from testing using distributed triaxial accelerometers showed a high correlation with the predicted mode shapes from the FE model. The measurements from the field test also identified the isolation effect between the BHA sections. The measurement data showed that as predicted only the lower part of the BHA below the isolator tool oscillated at non-critical torsional eigenfrequency. The isolator tool reduced torsional vibrations and improved tool reliability, tool lifetime and service delivery, especially while drilling in formations that are susceptible to excite HFTO.

In downhole drilling systems, self-excited torsional vibrations caused by the bit-rock interactions can affect the drilling process and lead to the premature failure of components. Especially self-excited oscillations of higher-order modes lead to critical dynamic loads. The slim drill string design and the naturally limited drilled borehole diameter limit the installation space, power supply and lead to numerous potentially critical self-excited torsional modes. Consequently, small and robust passive damping concepts are required. The variety of possible downhole boundary conditions and potential damper designs necessitates analytical solutions for effective damper design and optimization. In this paper, two nonlinear passive damper concepts are investigated regarding design and effectiveness to reduce self-excited high-frequency torsional oscillations in drill string dynamics. Based on a finite element model of a drill string, a suitable minimal model based on the identified critical mode is generated and solved analytically using the Multiple Scales Lindstedt-Poincaré (MSLP) method. The advantages of MSLP compared to conventional MS methods are shown for this example. On the basis of the analytical solution, parameter influences are determined, and design equations are derived. The analytical results are transferred to self-excited drill string vibrations and discussed using time domain simulations of the drill string model.

The drilling process is impacted by vibrations through limited drilling efficiency and rate of penetration, reduced reliability and increased non-productive time. The understanding of the mechanisms and physics that lead to high levels of vibrations is extremely important to elaborate vibration mitigation strategies. A typical vibration excitation mechanism is forced response excitation, e.g., caused by the imbalance of the mud motor that can lead to lateral resonance with severe impact on tool life. Self-excitation is prominently caused by the bit-rock interaction and mainly excites torsional oscillations if PDC bits are used. Representations are stick/slip with low frequencies (<1 Hz) and high-frequency torsional oscillations (HFTO) with frequencies up to 500 Hz. The large frequency gap between stick/slip and HFTO allows for different effects and excessively increasing loads. The nature of this interaction is diverse and requires different strategies to reduce the loads associated to HFTO and stick/slip to a minimum. The interaction between stick/slip and HFTO is analyzed and appropriate drilling optimization strategies are proposed. Several scenarios are discussed by examination of high-frequency downhole data (1000 Hz) measured in different field applications and physical modeling. It is shown that averaged statistical data or diagnostic data that are typically available can lead to misinterpretation of the drilling conditions. The first scenario is pure HFTO. The second scenario is stick-slip with superimposed HFTO that can lead to an amplification (up to factor two) or reduction of HFTO loads compared to the first scenario. Influencing parameters are discussed that determine either an amplification or a reduction of the loads in the second scenario. The third scenario shows the interaction in the context of stability measures that are determined by the operational parameters. The increased rotary speed in the slip phase of stick/slip can lead to a stabilization of HFTO and actually decreasing amplitudes. The observations in the field are further validated using theoretical drilling scenarios. For each scenario different strategies are presented to reduce the field loads associated to HFTO and the compromise of the strategies to the drilling efficiency and rate of penetration is discussed. The nature of the interaction between stick/slip and HFTO is analyzed and unveiled. Clearly, the necessary depth of understanding can only be achieved by analysis of high-frequency downhole data. The physics-based interpretation of the problem allows the development of very specific drilling optimization strategies. Depending on the scenario a complete mitigation of HFTO or at least a significant reduction of loads can be achieved. Ultimately, the drilling process can be optimized leading to a reduced cost of the well delivery since HFTO can be a major cause for non-productive time if not handled properly.

Vibrations impact the drilling process by reducing reliability, increasing maintenance costs and reducing rate of penetration and drilling efficiency. Herein, torsional vibrations are typically distinguished into low-frequency torsional oscillations and stick/slip with frequencies below 1 Hz and high-frequency torsional oscillations (HFTO) with frequencies up to 450 Hz. HFTO is associated to high accelerations with critical values above 150 g and dynamic torsional torque values above the make-up torque. A HFTO mitigation strategy is mandatory for prone applications to guarantee high reliability and drilling efficiency. In this work different best practices for mitigation of HFTO and their interdependency are discussed along with their operational efficiency. The discussed scenarios include coupling between stick/slip and HFTO, different formation properties, and tools that are used for vibration mitigation such as isolators and mud motors. The analysis includes the review of time-based acceleration and load data with a sampling frequency of 1000 Hz and 2500 Hz and numerical modeling to determine the application and environment specific critical range of operational parameters in a holistic approach. The extracted critical range of operational parameters enables operations to optimally adjust parameters with a minimum of torsional loads and optimal drilling efficiency. This analysis again unveils that the general effect of HFTO is triggered by the cutting forces between PDC bits and hard and dense formation. It is shown qualitatively and quantitatively that high WOB values and low rotary speed values correspond to HFTO. The case study shows opportunities of a reduction of HFTO related loads by increase of the rotary speed without compromising the ROP in specific applications and environments. It is shown that special tools for vibration mitigation influence the stable operational window and need to be considered. Depending on the scenario a complete mitigation of HFTO or at least a significant reduction of loads can be achieved by targeted adjustment of operational parameters and use of tools for vibration mitigation. The drilling process can be optimized leading to a reduced cost of the well delivery since HFTO can be a major cause for non-productive time if not handled properly.