Classification of breaking wave impact loads on a fixed surface-piercing square column with an overhanging deck

In extreme sea states, steep or breaking waves can produce destructive wave impact loads on surface-piercing structures, such as semi-submersible platforms primarily composed of decks, columns, and pontoons. To reveal the characteristics of breaking wave loads and gain a deeper insight into the wave impacts induced by different breakers, a series of physical tests on a truncated square column with an overhanging deck were conducted in a wave flume. Focused waves were generated to reproduce breaking waves, and a total of 60 test runs were performed. Based on high-speed video recordings of wave motion, four types of wave breaking in the presence of the column structure were identified, including the upward deflected breaker without entrapped air, spilling breaker with air cavity, plunging breaker with small air cushion, and well-developed plunging breaker with large air cushion. Identification parameters and classification criteria for breaker types were proposed. The wave motion features of each breaker type were analyzed in detail, and the spatio-temporal distribution of the resulting local impact pressure and total impact force were discussed. Furthermore, the spatial variation of the pressure impulse and the durations of breaking wave impacts were investigated. The results showed that the wave impact loads caused by different breakers have distinctive spatio-temporal characteristics, and the near- breaking waves usually produce greater impact loads. The location of the maximum local impact load is related to the breaker type, and the spilling breaker is found to cause the most severe wave impact on the column structure. Due to the air entrainment of a larger volume, the local impacts produced by the well-developed plunging breaker should also be attached importance. The research demonstrates that the breaker type could be identified from wave impact loads without using any records of the wave motion. The breaker type must be taken into account for the prediction of breaking wave impact loads on surface-piercing structures in the future.