Axial-Torsional Vibrations Interaction Analysis and Decoupling in Drill String Systems

Rotary drilling system is an important and crucial electromechanical system used in petroleum industry, it is used to drill holes and extract oil and gas from targeted reservoirs beneath the surface. The rate of penetration (ROP) is one of the quantitative metrics used to assess the performance of the drilling system. However, the appearance of unwanted vibrations generally leads to decrease of this performance and increase the nonproductive time (NPT), in addition to drill string damages and wears. These vibrations are classified according to their propagation direction into three types: Axial, Lateral and Torsional. Many researches have been dedicated to designing robust controller to mitigate such vibrations separately. Nevertheless, vibrations often occur simultaneously, with interactions between them. This interaction can have a direct influence on the robustness of the designed controllers. Thus, in order to design a robust controller that mitigate the most frequent vibrations (Axial and torsional), it is necessary to analyze the interaction between them and decouple them before application of any controller. The main objective of this study is to analyze the interaction between the axial and torsional vibrations in the Two-input two-output (TITO) drill string model and to design appropriate decouplers for the system. Based on the obtained results, we demonstrate a significant interaction between the torsional and axial vibrations, and proved that the introduced decouplers have omitted these interaction terms with a minimum influence on the whole dynamic of drill-string system. Therefore, applying this decoupling process is highly recommended to enhance the robustness of the controller in mitigating axial and torsional vibrations simultaneously.