To accurately predict the air gap performance of an offshore platform and improve the safety of the platform, the hydrodynamic characteristics of the structure in the open sea area and in the flume were investigated with the help of HydroStar. Based on the linear potential flow theory and the boundary element method, a comparative study of the near-trapping phenomena of the four-cylinder structure was carried out under wave action with cylinders of various radii. The results show that natural frequencies of the near-trapped mode move to a high frequency with an increase in the radius of the cylinder, and it has a significant impact on the higher-order near-trapped mode and causes the change in wave surface distribution characteristics; the spacing between the sidewalls of the cylinders is a key factor to determine the frequency of a near-trapping phenomenon; there are coupling effects between the frequencies of near-trapping and horizontal resonance, and they are excited at the same time, resulting in a significant increase in wave elevation near the corresponding frequency. Copyright ©2021 Journal of Harbin Engineering University.
