Non-smooth dynamics of Anti Stick–Slip Tool

Abstract

This paper investigates non-smooth dynamics of an Anti Stick–Slip Tool (ASST) and its effects on a simulated downhole drilling. An ASST is a mechanical tool mounted on a Bottom Hole Assembly and designed to suppress stick–slip phenomena arising in drill-strings. In this paper the ASST is considered as a non-smooth dynamic system, where the presented modelling accounts for switching between Activated and Non-Activated states and proposes quantitative conditions for observing each state. In this tool, a helical spline couples axial and rotational movements acting together with an internal preloaded spring. The helical spline governs a relative motion of coupled parts and the preloaded spring determines a threshold value for activation of the ASST. The internal friction between coupled components contributes to conversion of excessive loads to a relative motion of two coupled parts. The proposed model describes the effect of internal friction forces on the operation of the tool whilst the tool is subject to nonlinear cutting forces from bit–rock interactions. Prescribed kinematics imposed on the tool allows to simplify the model of entire system and to provide an insight into the complex behaviour of the tool. Nonlinear responses of the developed model for a range of system parameters were numerically simulated to explore the dynamic behaviour of the ASST.