Based on global sensitivity analysis (GSA), this paper proposes a new risk assessment method for an offshore structure design. This method quantifies all the significances among random variables and their parameters at first. And by comparing the degree of importance, all minor factors would be negligible. Then, the global uncertainty analysis work would be simplified. Global uncertainty analysis (GUA) is an effective way to study the complexity and randomness of natural events. Since field measured data and statistical results often have inevitable errors and uncertainties which lead to inaccurate prediction and analysis, the risk in the design stage of offshore structures caused by uncertainties in environmental loads, sea level, and marine corrosion must be taken into account. In this paper, the multivariate compound extreme value distribution model (MCEVD) is applied to predict the extreme sea state of wave, current, and wind. The maximum structural stress and deformation of a Jacket platform are analyzed and compared with different design standards. The calculation result sufficiently demonstrates the new risk assessment method’s rationality and security. 1. Introduction In August 2005, Hurricane Katrina and Rita attacked the coastal areas of the USA and the Gulf of Mexico. Including causing serious damage in New Orleans, it also caused a great loss to petroleum industry in the Gulf of Mexico. According to the incomplete statistic, Katrina and Rita resulted in 116 destroyed and damaged platforms and more than eight oil refineries were forced to cease production. And just in March 11, 2011, a huge earthquake of magnitude 9.0 occurred besides the northeast coast of Japan. The powerful quake sparked the tsunami wave of ten meters in Japan which led to an unexpected nuclear leakage accident. This accident became a death blow to the Japanese economy. Previous studies concluded that a tendency toward the growth of intensity and losses of each disaster is also appearing [1–3]. Most of these disasters resulted from the inaccurate risk assessment of natural hazards. Although natural disasters are dangerous to human, the great loss by each disaster is mainly caused by human’s ignorance of disaster prediction. Due to the complexity of the natural events, it is very hard to forecast the occurrence of natural hazards accurately without the global uncertainty analysis (GUA). In addition, as the law of nature is not clear enough, the design process for offshore structure usually could not predict the real extreme sea state accurately as a result of many kinds of
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