Quantitative Evaluation of Ship Operational Effect in Actually Encountered Sea States

Since ships are being operated under consideration of the safety for lives and properties, economical reasons and so on, the sea states in natural phenomena and those actually encountered by ships are thought to be different, the latter has some effects of human operational factors (called as “ship operational effect”). Evaluating the ship operational effect in detail is important to consider rational wave design loads for hull structure strength. The purpose of this study is to evaluate the ship operational effect in actually encountered sea states quantitatively. As the first report, comparison was made between IACS Rec.34 (a kind of the observed sea states in natural phenomena) and forecasted sea states corresponding to AIS data of ships (a kind of the sea states data actually encountered by ships) on the North Atlantic. Comparisons among the encountered significant wave heights by merchant ships such as bulk carriers, oil tankers and container ships and those specified in IACS Rec. 34 were carried out. Furthermore, the wave headings regarding the encountered waves were investigated. Finally, the relationships between encountered significant wave heights and ship speeds were derived to confirm the ship operational effect. It was confirmed from the results that the actually encountered wave heights were smaller than those in IACS Rec. 34, through comparing the exceedance probability of the significant wave heights for each type of ships and IACS Rec. 34. The exceedance probability in the encountered beam seas is relatively lower compared with those in the encountered head and following seas. The results also show that ship speeds decrease when the encountered wave heights become larger.