STATISTICAL PRESENTATION OF SEA SEVERITY

Probability distribution of significant wave height

Statistical presentation of sea severity provides information vital for the design and operation of marine systems. For the design of marine systems, information is necessary not only on the severest sea condition expected to occur during the system's lifetime (50 years for example), but also on the frequency of occurrence of all sea conditions, the latter being especially necessary for evaluating fatigue loadings.

The most commonly available information on sea severity is the statistical tabulation of significant wave height constructed from data accumulated over several years. Of course, the greater the number of accumulations, the more reliable the data. As to the time interval between data sampling, it is highly desirable that data be obtained at least at 3-hour intervals so that a relatively fast change in sea condition will not be missed. During a storm, sampling at no more than one hour intervals is strongly recommended (see Section 5.2.1).

Table 5.1 is a tabulation of 5412 significant wave heights obtained over a 3-year period in the North Sea (Bouws 1978). The data indicate that the measurements were made, on average, five times per day throughout the 3 years.