Kenji Sasa

Current Situation and Difficulty of Wave Forecast From Viewpoint of Ship Management

Numerical forecasts of weather and oceanography are increasingly common in the field of ship operations due to advances in computer science. However, in some situations, the accuracy of forecasts is too unreliable to ensure safe operations. In the current study, a nationwide questionnaire is used to determine the effectiveness of wave forecasts in enhancing navigation safety. Data analysis is also shown for two cases of failed forecasts in low-pressure weather systems near Japan in the winter. Finally, recommendations are made for improving wave forecasts from the viewpoint of ship operations.Copyright

Optimal Routing of Short-Distance Ferry From the Evaluation of Mooring Criteria

In optimal ship routing, the ship behavior in the ocean is the main concern. However, there is also different type of ship service, namely the short-distance ferry. It is evident that a novel optimal ship routing must be proposed. A ferry service in southern Japan is the focus of this study. This service takes into account the typhoons or atmospheric depressions that influence the ferry ports. The ferry service is sometimes suspended because of the considerable motion of the ships moored in harbors. A wave monitoring system has been operational since 2015; however, the methodology has not been established from the viewpoint of optimal operation. The current situation regarding the ferry service is summarized in detail in this study, and patterns of typhoons and atmospheric depressions are revealed. In particular, it is difficult for ferry users to understand the reason for suspension of the ferry service when typhoons pass several hundred kilometers away with offshore wave heights of less than 2 m. Firstly, the relation between long period waves and the actual operation is clarified. Secondly, moored ship motions are numerically simulated using measured wave data to evaluate mooring criteria. The estimation formula of the mooring criteria is modeled, and the novel concept of optimal routing for the short-distance ferry is proposed.Copyright

A Basic Study on Ship Speed Loss From the Viewpoint of Geographic Conditions and Ship Performance

Currently, for marine transportation, various factors have to be taken account of. Hence, it is necessary to understand the ship performance, especially under heavy sea conditions. In this study, measurement results for a 20,000 DWT class bulk carrier were analyzed, and eight events of remarkable speed loss in heavy seas were observed during the measurement period 20 months from July 2010. The variations in ship performance parameters were statistically analyzed, and a regression analysis was carried out. All cases of remarkable speed loss occurred in seas relatively close to lands (within 200 nautical miles). The magnitude of speed loss was larger in the Southern Hemisphere than in the Northern Hemisphere. This indicates that more accurate weather routing is necessary for the Southern Hemisphere, especially for evaluating ship performance from the viewpoint of intrinsic weather patterns.Copyright

Numerical Analysis of Failed Forecasts of Waves Under Low Pressures From Viewpoint of Ship Operation

Today, weather routing becomes more important under the current situation of international maritime transportation. However, the authors reveal that the current systems are not necessarily sufficiently reliable. It is also worthwhile to consider the background of the difficulty in numerically forecasting winds and waves. Here, two failed cases of wave forecasts are verified using WRF and SWAN, two numerical meso-meteorology models. In both failed cases, low pressures developed from the southern sea area of Japan with a northeast direction. First, wind direction can be reproduced correctly, even if the resolution of the simulation is changed. Second, if the resolution is insufficient, wind speed will be underestimated as smaller than the observed values. At the same time, wave height tends to be underestimated as compared to observed values, too. This is the feasibility study for improving the forecasting of wave growth pattern due to developing low pressures from the viewpoint of safe ship operation.Copyright

Analysis of Sea Shock Encountered by a Ship in North East Japan

In general, earthquake motion experienced by a navigable vessel is called a sea shock (or seaquake). Sea shock is assumed that the vertical ground motion of the sea floor propagates as a compressional (longitudinal) wave in water.Seaquakes are sudden, unexpected phenomena that are induced by undersea earthquakes. There are almost no examples of their systematic measurement and the phenomenon of seaquakes is said to not be thoroughly understood.The effect of seaquakes was studied theoretically during the 1990s, during research on the wave response of structures for the construction of VLFS. However, There are a few experimental studies on the effect of seaquakes on floating bodies because of the problem of reproducibility.We measured ship motion caused by sea shocks by examining seabed oscillation. Such measurements provided very valuable data. The conclusions obtained in the present study are that ship motion caused by sea shocks has high-frequency oscillation. In particular, the acceleration in the z direction is remarkable.Copyright