Study On Effects Of Bimodal Spectral And Long-Period Waves Upon Mooring Ships

The motion of mooring ships has always been regarded as an essential issue in design and use of ports, which is closely related to mooring duration, efficiency and security of uploading and downloading operations, and self-movements of mooring ships. Such motion of mooring ships is caused not only by the coupling effects of a variety of environmental dynamic factors, but also the characteristics of mooring ships own motion responses. At large open piers, such motion responses of mooring ships caused by factors like mixed waves, long-period waves, and fluctuating winds, etc. have become a burning problem that should be solved urgently.Mainly based on a series of physical model experiments and combined with the method of numerical simulation, the paper has been engaged in a systematic study on the burning problems mentioned above.The results of the physical experiment which was fulfilled wish mooring ship in long-period waves (with testing wave periods ranging from10s to50s) have manifested that, dynamic magnifications of a specific movement of a mooring ship are closely related to the natrual period of the movement (which is inevitably relevant to the loading conditions of the mooring ship). However, such kind of dynamic magnifications have little to do with wave heights.Within the experimental range of wave periods,●the movement of surge increases along with the wave period; when a dynamic magnification of surge appears, its corresponding spectral peak period of waves and the natural period of surge are approximate in value.●the movement of sway increases along with the wavec period; meanwhile, the movement of sway has dynamic magnifications for a couple of times (the peak value of the movement of sway appears for several times).●the movement of heave goes upward with the progress of wave period; when the spectral peak period of waves is greater than30s, the value of the movement of heave is basically maintained at a certain level (no more growth), and its motion amplitudes and acting wave height are roughly the same in value.●under the action of transverse waves, the movement of pitch basically does not change with the growth of spectral peak period. ●the movement of roll also has dynamic magnifications for many times, but its motion amplitude each time is roughly the same with each other.●the movement of yaw gradually increases with the progress of wave period, when the movement of yaw has a dynamic magnification, the wave period and its corresponding natural period of yaw are basically equal to each other.The results of the physical model experiment which was fulfilled with a mooring ship in bimodal spectral waves (the spectral peak period employed in the experiment:high-frequency (wind wave) period at6s; low-frequency (swells) period ranging from12s to24s) have shown that:●with the increase of low-frequency spectral peak period, the laws of a mooring ship’s motion changes under the action of three different kinds of bimodal waves are the same. These laws are also the same with the ones of the mooring ship’s motion changes with the variation of spectral peak period under the action of long-period waves.●when the total energy of bimodal spectral waves is certain, both of the movements and line tensions of the mooring ship will go up with the increasing contributions of low-frequency energy of the total.●within the experimental range of wave periods (high-frequency spectral peak period of bimodal spectral waves:6s; low-frequency spectral peak period:12-24s; contribution of low-frequency energy:20%～80%; effects of transverse waves), the motion responses of a mooring ship in bimodal spectral waves are far greater than the ones in unimodal spectral waves(wind waves), when they are identical in the total energy of waves.●within the experimental range of wave periods (high-frequency spectral peak period of bimodal spectral waves:6s; low-frequency spectral peak period:12-24s; contribution of low-frequency energy:20%～80%; effects of transverse waves), the ratio of the movements of the mooring ship in bimodal spectral waves and in long-period waves will grow with the increase of low-frequency energy, when the two types of waves are identical in total energy.●when the characteristic parameters of high-frequency part of mixed waves are constant, the mooring ship’s movements (except the movement of pitch) and the line tensions will increase with the growth of low-frequency energy of bimodal spectral waves.●when the characteristic parameters of low-frequency part of mixed waves are constant, the mooring ship’s movements (except the movement of pitch) and the line tensions will increase slightly with the growth of high-frequency energy of bimodal spectral waves.●when the characteristic parameters of low-frequency part of mixed waves are constant, the mooring ship’s movements (except the movement of pitch) and the line tensions will increase slightly with the growth of high-frequency energy of bimodal spectral waves.The results of the physical model test which was accomplished upon mooring ships under the effects of fluctuate wind (wind velocity in the test:8.5～21.4m/s) have indicated that:●there is no significant effect wind has on the mooring ships when the wind is on less than6th wind scale (14m/s). Therefore, the6th wind scale has been regarded as the start-up wind speed of a ship.●the effects of transversal offshore wind on the8th scale (17～20m/s) have upon the movement of sway is roughly equivalent to the ones the transversal wind waves with7s of period,1.5～2m of wave height.●when the mooring ship is ballasted, the wind velocity is greater than6th wind scale, as the wind speed increases, line tension increases rapidly.The numerical simulation results have shown that:●a numerical simulation model has been established for research in the paper, whose calculation results of the model mooring ships’motions and dynamic responses agree well with the experimental results, which can be used for a real situation.●when the low-frequency energy of bimodal spectral waves accounts for20%of the total, the periodic changes of high-frequency spectral peak have tremendous impact on the motion responses (the movement of pitch excluded) of mooring ships under the action of bimodal spectral waves; with the increasing contributions of low-frequency energy, the periodic changes of high-frequency spectral peak have begun to have smaller impact on the motion responses and line tensions of mooring ships.