Numerical investigation of vortex-induced motions of a paired-column semi-submersible in currents

Vortex-induced motions (VIM) is becoming a noteworthy issue for column-stabilized floating platforms, mainly due to its substantial fatigue damage to risers and mooring system. The VIM of deep-draft semi-submersible is more complex than single column floaters because of the wake interference between columns, as well as the considerable yaw motions. In the present work, a numerical approach for simulating VIM of deep-draft semi-submersible is proposed. Specifically, detached-eddy simulation is used for turbulence modeling and dynamic overset grid technique is used for moving objects. Simulations for stationary drag and VIM of a model-scale paired-column semi-submersible are conducted with the proposed approach. The numerical results are compared with experimental data. Transverse, in-line and yaw motions are allowed during VIM simulations and are further analyzed in frequency domain by Fast Fourier Transform (FFT). Different VIM characteristics are observed at different current velocities. The work done by each component of the structure is also discussed. Flow visualizations are presented for better understanding of the wake interferences during VIM. The accuracy and reliability of the current numerical approach is assessed.