Experimental study on vortex-induced motions of a semi-submersible platform with four square columns, Part II: Effects of surface waves, external damping and draft condition

Aiming to complete the results presented before by Gonçalves et al. (2012d. Ocean Eng. 54, 150–169) the present work brings new experimental results on VIM of a semi-submersible platform with four square columns, particularly concerning changes in three different aspects: simultaneous presence of current and surface waves in the same direction, external damping level, and draft conditions. The VIM tests were performed in the presence of regular and irregular waves, both conditions with simultaneous current presence, to understand the wave effects. Considerable differences between the presences of regular and irregular waves were observed. The motion amplitudes in the transverse direction, in the tests with regular waves, were markedly lower than those with irregular waves, and the VIM behavior was not observed. In the sea state tests, the amplitudes are lower than current-only ones, yet a periodic motion characterized by VIM was observed. Furthermore, the effects of the lower draft condition and damping level were addressed, showing they are important for model tests because they contribute to decreasing VIM amplitudes.

► Three conditions of regular waves and three sea conditions were performed concomitantly with current in same direction.
► VIM presence with current plus waves depends on the in-line motion characteristics due to wave incidence.
► Ratio between wave and current velocities and Keulegan–Carpenter number affects VIM.
► Amplitude decrease with the increase in the external damping level.
► Low draft condition (lower exposed portion of the columns) completely attenuated the VIM.