A new method dealing with the dynamic of mooring line is present in this paper. The perturbation method was used. First we calculated the frequency domain result which is called frequency response function. The impulse response function was then obtained by the inverse Fourier transform of such frequency domain solution. The time history of the tension induced by the oscillation of the upper end of the mooring line then can be calculated by a convolution integral alone the time axis with the corresponding impulse function and the oscillation velocity. In order to verify the validity of the proposed method some experiments have been conducted on a model mooring line by an oscillating apparatus which performing both regular and irregular oscillations. The measured horizontal forces acted on the upper end of the mooring line of the regular oscillations were used to compare with the calculated results in time domain, while the frequency response function which was analyzed by the B-T method from the irregular oscillation results was used to compare with the frequency domain result. The coincidence of these two results is satisfactory. We also discussed the damping of the mooring line in this paper.
The mooring line dynamic problem has been studied by several researchers in the past decades. But most of them are concerning with the problem of the load on the mooring line itself. For mooring system of the floating structure, it is recognized that the most important factor for the determination of the load of the mooring line may be the amplitude of slowly oscillation which always happen when the structure meets with the random wave tram. Because of the long oscillation period, the dynamic effect of the oscillation is very slight compared with the static component of the tension acted on the mooring line.
