Dynamic analysis and stability evaluation of floating crane under heaving motion

Abstract

Floating cranes are essential lifting equipment for engineering vessels. When lifting and transferring cargoes, the heaving motion of the vessel cause the lifted cargoes to swing, posing a safety hazard. In this study, the dynamic model of the floating crane is developed and simplified into a typical Mathieu equation, and the Floquet theory is used to analyze the Floquet multipliers, so as to distinguish the stable and unstable regions of floating crane operation. In different instability zones (corresponding to different rope length ranges), numerical simulations and experimental studies were conducted to investigate the dynamic response and stability of a floating crane during the lifting and lowering of cargo under heave motion. Comparing the experimental and simulation results, the dynamic response of the payload shows consistency. The results indicate that different rope lengths correspond to different instability zones due to varying natural frequencies. When the Floquet multiplier exceeds 1, the excitation frequency satisfies the parametric resonance condition of the corresponding instability zone, leading to unstable motion of the payload. The results also show that the lifting/lowering velocity has a weak impact on the stability, while the rope length is the key factor affecting the stability of the floating crane system. The findings of this study could provide guidance for offshore floating crane operations.