It is very important to know the properties about long period waves from the point of view of moored ship motions in a harbour facing to the open sea. A lot of studies related long period waves are carried out, and some relations among long period waves and moored ship motions are obtained. However, there are very few studies concerning wave periods and the correlation of long period waves, as well as wave heights. In this study, observed wave data at a harbour facing to the Pacific Ocean in Japan when it becomes stormy weathers due to typhoons or atmospheric depressions are statistically analyzed. Therefore, the correlation between significant waves and long period waves are calculated, then some estimation methods and the new type of spectrum of long period waves are proposed.
Recently, the importance of moored ship motions in a phase of harbour planning is recognized as one of the main factors of the field of coastal engineering as well as the port operation(Shiraishi, et al., 1999). Results of some studies show that these phenomena are strongly correlated with the existence of long period waves around 1–3 minutes(Shiraishi, 'et al., 1996). Many studies that refers to long period waves are carried out(Hiraishi, et al., 1997), and some properties about long period waves are obtained by observed wave data. Nevertheless, most of them are mainly focused on wave heights, and they don't discuss about the property of wave periods so much. When we consider the moored ship motions, wave periods are very important factor as well as wave heights, especially in case of long period waves. There are very few studies considering the frequency property of long period waves that are divided into some frequency bands.