On Weibull Extreme Response of Offshore Structures

This paper investigates the so-called two-parameter and three-parameter Weibull distribution functions with regard to their technical formulation and application to prediction of extreme responses of offshore structures. The validity of predicted Weibull extreme responses is evaluated using time-history responses of two floating structures (a Spar and a Truss Semi) and a fixed-base compliant tower platform. All these structures were previously designed for harsh environments in deepwater beyond 1,700 ft. in the Gulf of Mexico. The evaluation results indicate that the maximum responses predicted by both two-parameter and three-parameter Weibull distribution functions with a single 3-hr simulation agree well with those obtained from a more lengthy and rigorous process with numerous 3-hr time-history responses. It also shows that the predicted maxima from the two-parameter Weibull distribution function are more reliable and have less statistical variability than those from the three-parameter Weibull distribution function, particularly for highly nonlinear wide-banded non-Gaussian responses such as minimum air gaps at the cellar deck of the topsides of the floating structures. The approach using the two-parameter Weibull function as presented in this paper is thus an appealing tool in predicting maximum responses and will greatly improve the process in the early design stage of a fast track project.