부산대학교 도서관

This paper delves into the dynamics of the pipe-in-pipe system, crucial in offshore oil and gas exploration, featuring an inner drill string nestled within an outer riser. A reduced-order model with five degrees of freedom is proposed to capture the coupled dynamics of this system. Here, the inner drill string is depicted as a torsional-lateral coupled rotor constrained by the outer riser. In contrast, the outer riser is modeled as a suspended stator subjected to vortex-induced excitations and contact forces. The model incorporates nonlinearities, including dry frictions, collisions, loss of contact, and nonlinear damping through the van der Pol wake oscillator. This comprehensive model allows an in-depth exploration of the dynamics of the coupled pipe-in-pipe system. The research revealed that the overall coupling frequency of the PIP system is slightly lower than the natural frequency of a single riser. Additionally, vortex-induced vibration of the pipe-in-pipe system predominantly occurs near the coupling frequency (cross-flow vibration) and its second frequency (in-line vibration). Furthermore, the coupling frequency and its higher harmonics are significant frequency components of drill string whirl. Parametric analysis is conducted based on the non-dimensional numerical model, revealing a novel and stable whirling phenomenon termed “following whirling” at low rotary speeds of the drill string. This phenomenon arises from a combination of vortex-induced vibration and dry friction. During this stable following whirling, the riser’s vortex-induced vibration traces an “8”-shaped trajectory, while the drill string follows suit, oscillating between forward- and backward-whirling, forming a “C”-shaped trajectory relative to the riser. Additionally, within the frequency lock-in region of the vortex-induced vibration, frequency analysis indicates induced nonlinear resonances through the continuous contact whirling of the drill string, showcasing a strong coupling between the drill string and the riser. In contrast, within the non-lock-in region, the system dynamics are primarily dominated by the backward whirling of the drill string. Furthermore, the parametric study reveals that the backward whirling of the drill string can significantly reduce both the cross-flow and the in-line vortex-induced vibration of the riser. However, as a side-effect, the backward whirling of the drill string can lead to higher-frequency vibrations, potentially causing fatigue damage to both the outer riser and the inner drill string. This research lays a foundation for comprehending the coupled dynamics of pipe-in-pipe systems in offshore drilling applications.