Instantaneous Center of Rotation of a Vessel Submitted to Oblique Waves

When under influence of an incident wave system, any freely floating body presents a general motion with all six degrees of freedom. The Instantaneous Center of Rotation, as defined in classical mechanics, is a concept that allows the description of a general motion in 6 degrees of freedom as a pure rotation around such point. This approach, although not widely used in ocean engineering, might be an alternative tool that allows fast and precise analysis in many cases. Recent studies have shown that under specific conditions, such as a heading wave condition, the ICR varies in time but it is always located along a line for one wave frequency. Similar results were presented regarding beam waves as well. The present work continues with the investigation regarding the behavior of ICR under more generic conditions, assuming oblique waves exciting a vessel with typical geometry of a FPSO platform. The study extends the knowledge derived based on 2D approaches from previous works, comparing the results obtained from the different methods. An analytical model is presented, assuming only harmonic motion to all 6 degrees of freedom and showing that, similar to what was observed in the simplified 2D cases, the ICR tends to present dependence on the frequency of motion. Numerical data acquired from commercial codes based on potential theory is also presented.