Vortex Induced Motion (VIM) of a Deep Draft Semi® with four square columns has been observed in model tests and in a prototype configuration. The flow field around the columns causes the vessel to oscillate along one of the vessel’s main diagonals under particular conditions. The observations and conclusions of three series of VIM model programs are combined in this report. Each test program was executed with a unique objective in mind and each setup was unique. Many similarities in response characteristics have been identified. Some of the differences in observed response are identified and discussed. A discussion is provided regarding the scaling of VIM problems, and the importance of the Reynolds number under model test conditions is discussed. The VIM of a typical Gulf of Mexico Deep Draft Semi® may affect the fatigue life of the mooring system and risers. A formulation is developed that predicts the distribution of VIM amplitudes which then can be used to estimate VIM induced fatigue damage. Observations and conclusions are provided of several experiments where VIM response was evaluated in the presence of a fatigue sea state and for a system with increased sway damping.

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