Experimental study on vortex-induced motions of a semi-submersible platform with four square columns, Part I: Effects of current incidence angle and hull appendages

An experimental study on Vortex-Induced Motion (VIM) of the semi-submersible platform concept with four square columns is presented. Model tests were carried out to check the influence of different headings and hull appendages (riser supports located at the pontoons; fairleads and the mooring stretches located vertically at the external column faces; and hard pipes located vertically at the internal column faces). The results comprise in-line, transverse and yaw motions, as well as combined motions in the XY plane, drag and lift forces and spectral analysis. The main results showed that VIM in the transverse direction occurred in a range of reduced velocity 4.0 up to 14.0 with amplitude peaks around reduced velocities around 7.0 and 8.0. The largest transverse amplitudes obtained were around 40% of the column width for 30° and 45° incidences. Another important result observed was a considerable yaw motion oscillation, in which a synchronization region could be identified as a resonance phenomenon. The largest yaw motions were verified for the 0° incidence and the maxima amplitudes around 4.5°. The hull appendages located at columns had the greatest influence on the VIM response of the semi-submersible.

► Amplitudes of motion in the free surface plan, spectral analysis and results of force are presented.
► Eight current incidence angles tested for at least 18 reduced velocities up to 20.
► Largest transverse VIM for 30° and 45° with maximum amplitude around 40% column length.
► Maximum yaw motions for 0° with amplitude around 4.5°.
► Synchronization range for transverse and yaw motions confirms a resonant behavior.