A MODEL TO ASSESS THE SUSCEPTIBILITY OF A MULTICOLUMN FOWT PLATFORM TO VORTEX-INDUCED MOTIONS IN EARLY DESIGN STAGES

As well known, VIM is a complex phenomenon that requires nonlinear models to be constructed and simulated, either through computationally demanding CFD approaches or by making use of reduced-order models (ROM), which are based on phenomenological schemes to emulate the vortex wake dynamics. However, even ROM approaches might be relatively time demanding to be used efficiently during initial stages of design, particularly within optimization schemes. A simple model is herein proposed to treat this rather complex problem in the design context. For a range of current intensities and for a complete turn of current directions, offsets and headings are determined, for which maps of natural periods and corresponding modes of oscillations are constructed. A criterion to inspect modes that are prone to resonance is proposed and polar maps of susceptibility to VIM are plotted. The criterion considers ratios between exciting and natural frequencies. Dominances of translational or yaw motions on resonant modes are quantified and discussed. To assess the proposed strategy, VIM susceptibility polar maps are confronted with results of time domain simulations, using a nonlinear ROM. The OC4-DEEPCWIND Floating Wind Turbine is taken as case-study.