Classification and Comparison of Wave Impact Modes on Semi-Submersibles

Semi-submersibles for offshore oil exploration and exploitation often suffer from severe wave impacts in extreme ocean environments. Owing to the complex wave interactions among structural components of semi-submersibles, in-depth analyses on the characteristics of wave impact events are of significance for both industry and academia. An experimental study was carried out to investigate the local wave impact loads on a semi-submersible, with focus on understanding the wave impacts by identifying typical impact modes. Quantitative criteria are proposed to classify major wave impacts on the semi-submersible into six modes and two types. The results show that the classification is reasonable and provides valuable information for studying wave impacts on semi-submersibles. The incident wave characteristics at the fore column of the semi-submersible have important influence on the wave impact mode. The fore-column dominating wave impacts exert the most intense loads on the fore column and feature well-developed breaking waves or slightly breaking waves at the fore column. However, the aft-column dominating wave impacts exert the most intense loads on the aft column or the deck bottom and feature non-breaking waves at the fore column. Energy loss during the fore-column impact weakens the impact severity on the aft column in the fore-column dominating wave impacts. The shoaling effect of the submerged pontoon and different motion configurations of the platform result in higher occurrence rate of the aft-column dominating wave impacts. Different impact modes are also distinguished by different spatial distributions of wave impact loads.