Uncertainty of wave-induced response of marine structures due to long-term variation of extratropical wave conditions

Wave conditions in extratropical regions have seasonal variations. The wave condition in a period of one or more years exhibits variation, as displayed by samples of data for such a period. It is important to have a measure of the variability in the wave-induced extreme response and fatigue damage for marine structures due to this statistical uncertainty for sea states in various service periods. For instance, this variability needs to be considered when assessing the safety of a structure until next inspection or repair after damage. Moreover, this variability illustrates the uncertainty due to limited data, say in connection with measuring service behaviour in periods of limited duration. Scatter diagrams for the northern North Sea for 1-, 2- and 4-year periods as well as a 29-year period are applied in this study to determine the typical wave-induced response of an FPSO and a semi-submersible. The wave conditions are considered representative for extratropical climate. The marginal distribution of the significant wave height is modelled with a hybrid lognormal-Weibull model and the conditional distribution of peak period is fitted by a lognormal distribution. The long-term response of fatigue stress ranges is fitted by a generalized gamma distribution, which includes the frequently used two-parameter Weibull distribution as a special case. The extreme values are described by a two-parameter Weibull tail model. Compared with the 29-year distribution, the variation in the annual distributions is quite considerable as measured either by the distribution parameters, or the predicted extreme value of significant wave height, or the long-term response for extreme and fatigue loading. The correlation between the extreme and fatigue response and various measures of the significant wave height, HS, is also studied. High correlation is for instance found between the extreme HS and extreme response, as well as between the HS which is exceeded by a probability of 10% and fatigue damage.