A Study on Wave Bending Moment of Ships from Results of Statistical Analysis of Records in Logbooks

Abstract

The long-term statistical distribution of maximum values of wave bending moment of a ship is affected by the sea state conditions during voyages throughout her life. Authors analyzed statistically the following records in logbooks of five ships, ship speed, ship course, wind velocity, wind direction, and wave height.From the analysis, it was found that the probability of changing course of ship to avoid heavy storms during her voyages had less effects on statistical distribution of wind velocity than that of wave height. And the relation between wind velocity and wave height observed on a ship was affected by the service condition, speed, and seakeeping quality of the ship. Generally the ratio of the wave height to the wind velocity derived from long-term observation on a ship which had more chance of being caught by heavy storms judging from her service condition etc., was greater than that was derived from the observation on another ship which has less chance.The long-term distributions of wave bending moment of a ship in several sea state conditions derived from our analysis were compared with that of the same ship in the North Atlantic Ocean. From the comparison, it was showed that the probability of escaping from the severe wave conditions of some ships had large affection on the distributions. Examining the relation between the extreme value of wave bending moment expected to be exceeded with the probability of 10-8 or near order and sea state condition, it was found that the extreme value was ruled by only the probability of encounter of one or at most several severe wave conditions in a given long-term sea state condition. An approximate method of estimating the extreme value in a given long-term. sea condition was suggested from the fact.