Browsing by Subject "Bit/rock interaction"

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    Influence of Bit Design on the Stability of the Rotary Drilling System
    (2020-06) Tian, Kaixiao
    Rotary drilling systems used to drill deep boreholes for hydrocarbon exploration and production experience self-excited vibrations, which can lead to drilling dysfunctions such as bit bounce, stick-slip and whirling. These dysfunctions cause premature failure of drill bits and other components within the system, ultimately leading to increased cost of the drilling operation. It is of interest, therefore, to understand the mechanism of self-excited vibration, with the objective of mitigating the drilling dysfunctions. This thesis focuses on establishing the link between the stability of the system in regard to coupled axial-torsional vibrations and features of PDC bits, which consist of multiple fixed blades or cutters mounted on a bit body. It has been established that a state-dependent delay system can depict the mechanism of the stick-slip drillstring vibrations. The complex arrangement of cutters of a realistic PDC bit, however, are responsible not only for multiple delays but also for a delay distribution that depends on the bit motion history. Since the delay distribution controls the stability of the bit, it is imperative to develop a model capable of capturing the exact design of PDC bits. This additional consideration, although greatly complicating the analysis, is expected to give more realistic insights into the mechanism of the stick-slip vibrations of a rotary drilling system.
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    Integrated dynamical models of down-the-hole percussive drilling
    (2014-07) Depouhon, Alexandre F.B.E.
    Due to the overall process complexity, studies about percussive drilling usually focus on a limited set of the (sub)processes underlying it, e.g., the hammer thermodynamics or the interaction between the bit and the rock. Following this paradigm, the assessment of the process performance is typically performed by considering a single percussive activation and a single interaction cycle between the bit and the rock, from arbitrary initial conditions.The need for an integrated approach to evaluate drilling performance, based on the dynamical interaction of the (sub)processes underlying drilling, is evident. Such an approach requires simplified models, however, as the computational cost associated with full scale models is simply unbearable. In this thesis, three dynamical integrated models are proposed and a preliminary analysis is conducted for a reference configuration and around it. The models couple three modules that represent: (i) the dynamics of the mechanical system, (ii) the interaction between the bit and the rock, and (iii) the activation of the mechanical system. For each module, simple representations are considered; of particular importance is the bit/rock interaction model which is a generalization to repeated interactions of experimental evidence observed for a single interaction.In the first model, the dynamics of a rigid bit is cast into a drifting oscillator and the activation modeled as a periodic impulsive force. The second and third models account for the dynamics of the piston and the activation results from the impact of the piston on the bit. They are respectively based on elastic and rigid representations of the two bodies. In the rigid model, analytical results of wave propagation in thin rods are used to represent the contact interaction between the piston and the bit. In the elastic model, wave propagation is resolved.Their preliminary analysis has revealed the occurrence of complex dynamical responses in the space of parameters. Expected trends are recovered around a reference configuration corresponding to a low-size hammer, with an increase of the rate of penetration with the feed force and the percussive frequency. An important sensitivity of the rate of penetration to the latter parameter is uncovered. Interestingly, our analyses show that when the activation period has the same order of magnitude as the timescale associated with the bit/rock interaction, a lower power consumption is observed, indicating a possible resonance phenomenon in the drilling system. Also, the predictions of the rigid model are shown to be in good agreement with the ones of the elastic model, in the explored range of parameters.Given the piecewise linear nature of the proposed models, dedicated numerical tools have been developed to conduct their analysis. As such, the thesis proposes a high-order time integration scheme for linear structural dynamics as well as a novel framework to evaluate the accuracy of such schemes, and a root-solving module to perform event-detection, for coupling with event-driven integration strategies. Specific to the framework is the account for both structural damping and external forcing in the evaluation of the scheme order of accuracy. Specific to the root-solving module is the forcing of event occurrence in the localization procedure.