Design of a floating structure is supposed to be based on the extreme responses experienced by the components of the structure during its lifetime. The airgap response and potential deck impact of ocean structures under sea waves is of considerable interest. Non-linear diffraction models are usually called for a more consistent evaluation of the wave field under the deck and the wave run-up upon the columns, but even second-order analysis is not free of uncertainties. Therefore, air gap evaluation still relies heavily on experimental analysis. This paper presents some deep-tank results performed for the evaluation of the dynamic airgap of a large-volume semisubmersible platform. A series of model tests were carried out for the scale model of a horizontal moored semi in regular and extreme irregular wave conditions. Airgap response combined with run-up close to columns at total 11 locations on the deck was evaluated under oblique wave status. Motions and elevation data are analyzed by statistical treatment. Weibull-tail fitting procedure is realized to determine the extreme response levels.

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