Comparative study on probability distributions of riser responses

The responses of a flexible riser caused by motions of a weather-vaning FPSO subjected to two cyclonic storm events are investigated numerically. The objective was to comparatively study probability distributions of the riser responses to determine their statistical properties and the distribution types appropriate for particular responses. This will contribute to a method for the prediction of the extreme responses of the flexible riser during cyclonic events. Simulations of the riser responses in two cyclonic storm events were conducted including effects of wind, waves and current. Several types of probability distributions were fitted to the generated datasets using maximum likelihood estimates. Distributions tested included Rayleigh, Normal, Lognormal, Weibull, Generalized Extreme Value, Gamma, Burr, Birnbaum-Saunders and Beta. In addition, Hermite moment models were fitted in an attempt to use a single probabilistic model for all responses. A comparison was conducted using the goodness of fit tests to determine the distribution that best represented the generated data. Hermite moment models based on four moments produced good fits across both storm cases and for the majority of responses. The findings of this study provide foundations for development of more bespoke distributions for riser responses. The implication of using appropriate parent distributions for short and long term response analysis and the association between parent and extreme value distributions in both time and frequency domain contexts are discussed.