Mooring line damping due to low-frequency superimposed with wave-frequency random line top end motion

The slow drift motions would lead to a serious influence on moored floating structures and cause the failure of mooring and riser systems. Mooring line damping which represents the transfer of energy is important for moored floating structures. In this paper, time domain finite element method was applied by using OrcaFlex. A series of mooring line top end motions was simulated to investigate the relationship between mooring line damping and low-frequency superimposed with wave-frequency random motion. A transformation method was introduced that wave-frequency random motion was transferred to an equivalent sinusoidal motion based on the spectral density of vessel motion. Then, the influence of equivalent sinusoidal motion and random motion on mooring line damping was compared. It can be found that mooring line damping could be reduced slightly if considering random motion. Finally, the influence of individual parameter which includes current speed, drag coefficient, added mass coefficient and pre-tension on mooring line damping was studied. The results showed that the significant status of drag coefficient and pre-tension on the predication of mooring line damping. But for current speed, the effect on mooring line damping cannot be overstated for considering random motion but the reverse is true for considering sinusoidal motion.