It is widely said that the decrease in cargo handling efficiency due to long period waves is mainly caused by low-frequency ship motions in the longitudinal direction of a moored ship. It is generally considered that the low-frequency ship motions are generated by the resonance phenomenon of the mooring system and the low-frequency fluctuation of water level inside a harbor. The mooring system consists of the moored ship and mooring ropes.
　In this study, the authors developed a reduction system of low-frequency ship motions by using mooring ropes. This system is capable of effectively reducing the low-frequency motions of the moored ship. The authors also proposed the system as one construction method for countermeasures against long period waves. The reduction system of low-frequency ship motions can reduce the surge motions of the moored ship by preventing the resonance with the long period waves. This is implemented by forcibly changing the natural period of the mooring system with the mooring winches automatically controlled by a personal computer. In this study, the authors verified the effectiveness of the reduction system of low-frequency ship motions by the fundamental model experiments, and carried out the trial designs of an actual system in consideration for on-site applications.