The nonlinear dynamic responses of a payload hanging from an offshore large crane vessel are investigated numerically. A three dimensional fully nonlinear time domain multi-body model of crane and vessel based on pulley and cable drive is built to perform the analysis. The dynamic responses of payload under the hull motions of roll, pitch, heave and coupling motions of heave and roll, heave and pitch are investigated. The motion of the payload is proved to exhibit various nonlinear phenomena (for example, sub-harmonic motion, period doubling behavior) due to certain periodic chord motion presence of crane vessels. Conclusion drawn from this study could be used for payload pendulation forecast and devising techniques to control or damp the enormous motions.
Floating cranes are applied for a variety of tasks in offshore areas including transportation, assembling of costly structures and salvage operations. Efficient and safe operations of crane vessels at offshore are thus becoming increasingly important due to the increase in activities in deep water and particularly with a demand for higher lift capacity. Practical problems arise due to the difficulties in positioning accurately when the payload is handling with crane vessel operations. Even small disturbances in the state of the system could produce shaking of lifting weight and may entail the danger of collisions, which not only reduces the efficiency of the crane, but also threatens the safety of the staff, so the characteristic of the motion of the hull needs to be checked in advance in order to avoid potential risk by chaotic coupling effect.
Aiming at these issues, interdisciplinary research based on multi-body mechanics and virtual simulation technology has been carried out gradually by the scholars at home and abroad. Thomas Erling Schellin et al. (1991) developed a mathematical model, which treated hull and crane as one rigid body and considered elastic stretch of the hoisting rope assembly to predict the response of a shear-leg crane vessel in waves. Nonlinear large-angle swing of the suspended load coupled with surge, sway, heave, roll, pitch and yaw motions of the hull. Results showed the payload pendulation had little effect on motion of the hull. J. A. Witz (1995) used time domain numerical solution of the equations of motion to investigate the parametric excitation of loads suspended from crane vessels in random seas. Results showed large swinging motions would be generated when the dominant period of the incident waves was approximately equal to the natural period of the load or approximately equal to one half times the natural period. POSIADALA et al. (1996) established spherical pendulum model of the payload system, where the influence of vibrations in the hoist system on the crane and the load was considered. The highly nonlinear equations obtained had been numerically integrated using a fourth order Runge-Kutta algorithm. Large swinging motions were generated when the frequency of excitation applied on the base of crane was equal to the natural frequency of the payload system. C.CHIN (2001) presented the frequency content of the motion of crane vessel might contain significant energy near the natural frequency and/or twice the natural frequency of the free-swinging load. This situation could initiate an external and/or a parametric resonance. Moumen Idres et al. (2003) developed a nonlinear crane-vessel dynamic model that the vessel equations of motion were integrated simultaneously with the crane cargo equations. Simulations showed that large-amplitude response occurred at wave periods near the natural period of the hook load. Z. N. MASOUD et al. (2004) presented conditions that the frequencies of roll and pitch motions of the vessel were equal to the natural frequency of the payload pendulation and the frequency of heave motion was equal to twice the natural frequency of the payload pendulation were the worst-case excitation. B.W. Nam et al. (2017) applied both experiments and numerical calculations to investigate coupled motion responses of a floating crane vessel and a lifted subsea manifold during deep-water installation operations. For short wave periods, the efficiency of the passive heave compensator was maximized in reducing dynamic tension of hoisting wire. Y.J. Ha et al. (2018) investigated the mating operations of a topside module onto a FLNG by using a floating crane in waves. The lifted topside module showed two peak responses in sway and roll motions due to the interaction effect with the crane vessel. The higher peak happened at the roll resonant period of the floating crane vessel, while the other peak was caused by the pendulum resonance of the lifted topside module.