Vortex-Induced Yaw Motion (VIY) was studied experimentally on a large-volume semi-submersible platform considering four different current incidence angles, 0, 15, 30 and 45 degrees. The results showed a synchronization range of reduced velocities from 6 up to 8, this one defined by means of the yaw natural motion period in still water. The maximum angular amplitude of yaw was approximately 5 degrees, verified for the 0 degree incidence. This periodic yaw motion of large amplitudes, together with the well known translational motions, named Vortex-Induced Motion (VIM), may contribute negatively to the fatigue life of mooring lines and risers; therefore, this new modality of yaw synchronization should be taken into account during the design of semi-submersible platforms. Furthermore, the effects of the lower draft condition and damping on this new type of synchronization were addressed, showing they are important for the model tests because they contribute to decreasing the yaw amplitudes.
The vortex shedding around multiple bluff bodies is a complex phenomenon. Mostly, the columns of semi-submersible platforms act as an array of four bluff bodies in a square configuration under an incident flow. In fact, the vortex shedding of each cylinder can be different due to the interference between columns for distinct current incidence angles, causing differences in the drag and lift forces, as well as in the frequency of shedding vortices. Moreover, the moment caused by the different forces acting on the columns and the periodic nature of the vortex shedding can also induce a yaw motion in such platforms. The semi-submersible studied herein has four square columns, see Figure 1, which increase the differences as compared with circular cylindersNorberg (1993) experimentally investigated the flow and pressure forces on fixed rectangular cylinders at angles of incidence from 0 up to 90 degrees.