Long period waves with periods of 1 ~ 5 minutes sometimes terminate the cargo handling and cause the break of mooring ropes in several ports. They are composed of set-down waves bounded in the grouping waves and free long waves and the latter become remarkable in port area. The standard and simplified frequency spectra including the long and short period wave components DR proposed to prevent the terminal damages by long waves.

INTRODUCTION

In several Japanese ports, the termination of loading and unloading is often induced by the large surge motion of a moored vessel even when the wave heights in a harbor are small (Suganume et al.,1995). The large surge motion is mainly caused by the resonance of mooring system (vessel-mooring rope) to the long period waves (Hiraishi et al.,1996a). Long period waves observed offshore ports are considered to be induced mainly by the group-bounded waves(set-down waves) (Sand,1982). Such long period waves easily penetrate into the inner harbors with no reduction in case that the breakwater opening is relatively wide. The cargo handling is often terminated because the surge motion of moored vessels is remarkably amplified by the resonance to the long period waves. The extension of breakwater, the construction of long groins and the improvement of mooring facilities are presently proposed as countermeasures to long period waves in industrial and comercial ports. For the establishment of countermeasure planning, determination of long period wave conditions like amplitude and period is great important. The generalized characteristics of long period waves applicable to coastal and offshore structure design have not been proposed yet though the theoretical analysis for generation of long period wave components by the non-linear interaction of wind wave components has been carried out. Hiraishi(1996b) has suggested that the offshore long waves are composed not only of bounded waves in the grouping waves but also of free waves formated by the atomospheric low pressures and reflected long period waves in shallow water areas.

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