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.

Polycrystalline diamond compact (PDC) bits with multiple fixed cutters drill through deep formations and are used in oil and gas, geothermal, and mining industry. The bit-rock interaction excites drill string vibrations, which need to be mitigated to improve drilling performance. A velocity-weakening characteristic of the bit torque can cause severe self-excited high-frequency torsional oscillations (HFTO) of the bottom hole assembly (BHA). The dependency on velocity can be attributed to rate-sensitive rock cutting forces at each cutter. The purpose of this paper is to model the rate effect of cutting forces and determine influencing factors in the single-PDC cutting process. In this paper, a modified bonded-particle model (discrete element method, DEM) for rock in deep drilling environments is applied to the pressurized cutting process. Potential modification strategies considering rate indicators at different scales are compared and discussed to reproduce rate dependency in the particle contact model. Two specific models, incorporating cutting-speed-dependent bond strength or grain elastoplasticity, are implemented and verified. The relationship between rate-sensitive cutting forces and the causal rock failure mechanisms is then clarified by interpreting the results from a continuum perspective. Moreover, the influences of axial force and cutter wear state on the rate dependency are investigated. In cutting simulations at a constant depth of cut and various cutting speeds, both models result in rate-dependent cutting forces in good qualitative agreement with rock experiments: Significant hardening is reflected in the axial force component. The modification of contact models combined with a macroscopic rate indicator (i.e. cutting speed) has proven to be a feasible and effective way to reproduce the rate effects in the cutting simulations. In subsequent simulations with constant axial force, the steady-state tangential force decreases as the cutting speed increases, matching the reduction of torque on the bit with increasing RPM. Rate dependency intensifies at higher axial forces or with dull cutters, which agrees with laboratory full-scale bit tests in the literature. This study proposes a practical approach to integrating observed rate effects of downhole rock cutting processes into numerical rock models. The results enhance our understanding of the rock cutting process and have positive implications for the improvement of the cutter design against undesirable rate effects.

Brake wear emissions can be reduced by altering the surface of brake disks. A parametric study using a gray cast iron and a laser-cladded brake disk was performed in a pin-on-disk experiment with integrated optical pin surface characterization and particle emission measurement. Significant differences in the friction, wear and emission behavior are present. The high wear-resistance of the laser-cladded disk led to a reduction of 70% of the particle number emission relative to the gray cast iron disk, but the coefficient of friction was unstable. The surface of the pin used with the gray cast iron showed an initial large debris extension and protruding patches that were removed at high braking energies, exposing white patches and creating holes. These observations correspond to known processes from the plateau theory. The surface of the pin used with the laser-cladded disk showed a topography dominated by holes with almost no protruding patches. The braking condition did not influence the pin surface, implying that the disk and not solely the pin surface might be governing the friction process, and therefore challenging the applicability of the plateau theory to laser-cladded disks. To further study this aspect, a segmentation method was developed for the pin surface images and topographical data to extract and quantify different features on the pin, such as debris, patches, holes and the tribolayer. The correlation of the surface coverage ratios of the feature classes with the braking conditions (speed and applied pressure), the coefficient of friction and the emissions confirmed the differences between the gray cast iron and laser-cladded brake disk.

This paper deals with the development of a novel mesoscopic model of polycrystalline diamond compact (PDC) drill bits that can be implemented in complex drill string models for simulations to analyse the influence of rock inhomogeneities or the impact of anti-whirl bits on drill string dynamics. In contrast to existing modelling approaches, the model is developed at a mesoscopic level, where the basic bit–rock interaction is taken from the macroscopic bit model and the cutting characteristics are summarised at a microscopic cutting level into a simplified configuration via cutting blades. This model can therefore effectively describe asymmetries and thus interactions between the torsional and lateral dynamics of the drill bit, and is particularly suitable for investigating the effects of drilling into rock inhomogeneities and fault zones on drilling dynamics. By integration into a complex drill string model, simulation studies of drilling through a sandwich formation were carried out. The simulation results allow detailed stability statements and show the influence of formation properties and bit design on torsional and lateral drill string dynamics.

In technical applications the jerk information will be used for i.a. motion control, comfort evaluation or prediction of acceleration. In the majority of cases the jerk will be determined indirectly by differentiation of measured acceleration signals and depends consequently on the acceleration signals' quality and the differentiation's algorithms. An accurate jerk determination can only be realized by using an appropriate filter with respect to the relevant frequency range. In order to avoid such limitation a method of direct jerk detection will be required. Based on a 2 DOF mass system with viscous coupling a possible method was presented in [1] to detect directly the jerk. Thereby, in certain conditions the jerk is proportional to the displacement of one mass. Another method of direct jerk detection is given implicitly in [2]. With this method the jerk information will be generated by an additional inner variable of a damped single mass system based on the fundamental idea of the Mesoscopic Particle Method [3]. Both presented methods will be investigated and evaluated regarding its limits of applicability as a result of arbitrary periodical and impact-like excitations. (© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)

The focus of this article is on the measurement of tire and road wear particles (TRWPs) during vehicle operation. The long-term objective is to determine the sources of particulate matter. Consequently, the development of sustainable tires can be supported in the future by identifying factors influencing the concentration of particulate matter in vehicle-based tire tests. In an initial campaign, a test vehicle was equipped with a total of seven low-cost sensors (LCSs) for measurement campaigns on an isolated outdoor test track. The purpose of this was to evaluate the particle measurements in combination with GNSS data and driving data such as acceleration and speed. The potential observed in the initial investigation led to further investigations with an advanced, interconnectable modular particle and environmental sensor system (iMPES), which was developed in-house. The iMPES records measurement data for PM10 via the PMS7003 and PM100 via the SDS198 at 1 Hz over a period of up to 6 h, using a mobile power supply. The findings of the study indicate a robust characterization of the particle concentrations over the temporal and local course of the campaign drives. The results demonstrate the potential of the method to be part of a methodology to differentiate the particle sources and to derive influencing factors on the particulate matter concentration. The paper proposes a methodology for the mapping and analysis of lap-based data on a normalized route. Consequently, an inquiry into the local and driving-dependent dynamics is conducted, alongside a comparison with driving data.