Complexity of oilwell drillstrings makes unfeasible the direct application of control methodologies to suppress nondesired oscillations existing in these systems. A reasonable alternative is to develop operation recommendations and parameter selection methods to guide the driller to avoid such oscillations. In this paper, by using dynamical analysis tools, operation recommendations and the detection of safe drilling parameters in a conventional oilwell drillstring are presented. To this end, a more generic lumped-parameter model than those considered until now is proposed. Particularly, this model takes into account the fact that the drillstring length increases as drilling operation makes progress. The analysis of a sliding motion giving rise to self-excited bit stick-slip oscillations and bit sticking phenomena at the bottom-hole assembly is performed. Finally, the identification of key drilling parameters ranges for which non-desired torsional oscillations are present is carried out by studying Hopf bifurcations in the vicinity of the system equilibrium point when rotary velocities are greater than zero.

Abstract-A dynamical sliding-mode control is used to avoid different bit sticking problems present in a conventional vertical oilwell drillstring. The control goal of driving the rotary velocities of drillstring components to a constant positive value is achieved by means of this control. A discontinuous lumpedparameter torsional model of four degrees of freedom is considered. This model allows to describe drill pipes and drill collars behavior. The closed-loop system has two discontinuity surfaces. One of them gives rise to self-excited bit stick-slip oscillations and bit sticking phenomena. The other surface is introduced to accomplish the control goal despite variations in the weight on the bit (key to the dynamics) and other system parameters.

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Induction motors (IM) have been a fundamental part of industrial applications for over a century and the number of their applications continues to expand. A significant amount of the world’s total energy expense is consumed by this kind of motor. Hence, it is very important to increase the energy efficiency of these machines. Due to its good performance, field-oriented control (FOC) is the most common strategy to control IM. FOC requires references for stator current and rotor magnetic fluxes. For velocity regulation, a velocity reference is used instead of a stator current reference. However, at motor start-up or when a change of torque is required, it would be convenient for these references to be variable in order to reduce energy consumption. In this work, it is shown that this is indeed the case, and a technique to find optimal time-variable references for stator currents and magnetic rotor fluxes to reduce energy consumption is proposed. It is shown that, depending on the mechanical load, an energy reduction of 20–45% can be achieved.