Vortex-induced dynamics of a pipe-in-pipe system with backward whirling: Nonlinear model predictive control with tension-based detuning

To advance the understanding of dynamics in offshore drilling systems, this study investigates the vortex-induced vibrations of a pipe-in-pipe structure under the combined effects of tension modulation of the riser and backward whirling of the drill string. A reduced-order dynamic model integrating gravity, variation of the top tension, and nonlinear fluid-structure interactions is developed. Parametric studies reveal that the backward whirling of the drill string enhances the system’s energy dissipation rate, reducing the vortex-induced vibrations by 15-40%, while direct tension modulation can break the lock-in vibration with slow convergence. An equivalent single-riser model is then derived, revealing that whirling introduces additional mass, stiffness, damping, and multi-frequency excitations. The equivalent model can capture key nonlinear dynamic characteristics of risers induced by the vortex shedding and the whirling, including response amplitudes and frequency components. Based on this equivalent model, a nonlinear model predictive control strategy is proposed, leveraging real-time tension modulation to detune structural frequencies from vortex shedding frequencies. Comparative analyses demonstrate rapid suppression within 20 s with a nonlinear model predictive control under noise interference, achieving 97% cross-flow and 83% in-line vibration reduction, outperforming the control-Lyapunov function approaches. Lift coefficients and phase trajectory analyses elucidate energy dissipation induced by backward whirling and phase-jump effects from active control. Besides, robustness tests under varying flow velocities confirm the adaptability of the proposed controller. This study demonstrates the feasibility of active tension control for VIV suppression to enhance deepwater drilling safety.