Met-Ocean Contour Lines for Design; Correction for Omitted Variability in the Response Process

摘要

The concept of met-ocean contour lines was introduced as a rational way of defining met-ocean input for estimating characteristic load- and responses for offshore structures. The aim is to represent a tool for obtaining proper estimates of long term extremes (i.e. extremes corresponding to a given annual probability of exceedance) utilizing short-term methods. If the short- term variability can be neglected, the median sea state maximum is a proper short-term characteristic. Generally, the short-term variability needs to be accounted for and a short-term characteristic somewhat larger than the median should be selected. The selection of short-term characteristic has to be such that the annual probability of exceeding the maximum load (maximum of the selected short term characteristics) estimated along the contour line corresponds to the annual probability of the contour line. Several corrective actions are proposed in order to correct for the effect of the variability that is omitted by use of met-ocean contour lines. These are inflation of met-ocean contours, application of a correction factor on the extreme estimate and application of a percentile value in the short-term extreme load- or response distribution different from the median The alternative corrective actions are discussed and results from longterm analysis of systems with different characteristics are compared with results from use of contour lines.