Vortex-induced motions (VIM) of offshore floaters occur when the frequency of the vortex shedding is close to the natural frequency in the transverse direction of the unit subjected to the current, and this can severely affect the fatigue life of mooring lines and risers. Literatures in recent years have shown an increasing interest in the VIM behavior of semi-submersibles (SS), the geometry of which implies a more complex VIM phenomenon. In the case of multi-column semi-submersibles, the vortices shed around each column. Thus the shape of the columns, circular or square, and the wake interference different for each current heading, characterize the VIM of the unit.
Based on verification by the VIM towing tests for a semi-submersible with four rounded square columns, VIM simulations were conducted using CFD method in the model scale for four semi-submersibles with different column designs: a SS model with four rounded square columns (SRC-SS), a SS model with four circular columns (CC-SS), a SS model with two tandem rounded square columns and two tandem circular columns (SRCT-CCT-SS), and a SS model with two staggered rounded square columns and two staggered circular columns (SRCS-CCS-SS). The current headings ranged from 0° to 180° and reduced velocities of 4 up to 14 were considered.
Different semi-submersibles present different VIM performances owing to the influence of the column design. The most significant transverse motions of the CC-SS model occur at the 0° current heading with the largest nominal transverse amplitudes around 74% of the column diameter. On the other hand, the maximum amplitudes in the transverse direction of the SRC-SS model, approximately 63% of the column width, are observed at the 45° current heading. Additionally, the VIM responses could be mitigated when the semi-submersible consists of combined circular-section and square-section columns.