Second-order resonant motions of a deep-draft semi-submersible under extreme irregular wave excitation

Abstract Deep-draft semi-submersible platforms may present significant second-order resonant motions in severe sea states. Evaluating the slow-drift motions of such floating systems is important from the initial stages of their designs considering not only the offset requirement and mooring line load but also the air-gap responses beneath the lower deck. Theories and corresponding applications for these second-order resonant motions have been investigated for decades, but the problem has not been solved completely considering the requirements of a model that is sufficiently accurate and expedient. In this study, high-quality numerical simulations and model tests were conducted. Numerical calculations were carried out with the applications of full quadratic transfer function (QTF) matrices by means of commercial boundary element method (BEM) code. Comparisons show perfect agreement between the wave-frequency components obtained from both the model tests and the numerical calculations. Furthermore, the results also indicate that the second-order resonant motions can be to some extent estimated numerically by the second-order potential theory and that deviations can still be observed compared with those motions obtained by the model tests.