Uncertainties in Extreme Value Analysis and Their Effect on Load Factors

A method for deriving site-specific load factors for fixed offshore structures that include uncertainties in extreme values is described. The method consists of an extreme value analysis, to derive the extreme value behaviour and the uncertainties involved, and a model, based on the Load Resistance Factor Design fo the API rules, to derive site-specific load factors with or without the inclusion of uncertainties in the long term load distribution. The application of site-specific load factors could lead to a harmonised calculation of the probability of failure for platforms around the world. The extreme value analysis that is applied is a threshold based Generalized Pareto Distribution in combination with Maximum Likelihood Estimation. This is applied to a Peaks-over-Threshold data selection from hindcast studies. The Extreme Value Analysis is used to derive four extreme value characteristics (shape and scale parameter, the threshold and the number of data points,) that are used to quantify the extreme value behaviour. The North Sea, West African Coast and Gulf of Mexico are analysed. These are three ocean basins that capture the main characteristic climates around the world: extra-tropical storms, swell dominated extremes and tropical storms, respectively. The uncertainties in the extreme value analysis are dependent on the four characteristics. A simulation study is used to extract the confidence intervals of the parameters (shape, scale and significant wave height) as a function of the characteristics. The simulation of extreme value data sets gives a bias in the results, and therefore a practical method is proposed to correct for the bias and to extract confidence intervals. The bias correction is used to derive the shape and size of the confidence intervals for the extreme significant wave heights at the North Sea, West African Coast and Gulf of Mexico. The load factors are computed on the basis of the Reserve Strength Ratio requirement. Only wave loads are incorporated in the determination of the loading on fixed platform; the current and wind loading are not considered. There computation methods for load factors are investigated; a traditional approach without the implementation of uncertainties, and two new methods that include the uncertainties derived with simulation studies. The three methods show that there are regional differences in load factors and that the implementation of uncertainties leads to an increase in the factors.