During the initial design stages of a Floating Offshore Wind Turbine, optimization techniques are usually applied. Being the floating unit a multicolumn semisubmersible platform, vortex-induced motions (VIM) due to current action may be of some concern in the engineering process, regarding the mooring system design or even to address possible effects on the dynamics of the whole system. As well known, VIM is a complex phenomenon that requires non-linear dynamic models to be constructed and simulated, either through computationally demanding CFD approaches or by making use of phenomenological models to emulate the vortex wake dynamics. Indeed, in the case of multicolumn semisubmersible platforms, a strategy based on wake-oscillators has been developed to construct a nonlinear reduced order model to cope with the VIM analysis task [1]. However, even such a reduced order model might be relatively time demanding to be used efficiently during initial stages of design. This is even more true if an optimization scheme is used, in which hundreds of alternatives are scrutinized, within a search towards the minimization of a stablished objective function. In that particular design context, a simple model could be though as an alternative to assess the susceptibility to VIM. Essentially, VIM is a nonlinear resonant phenomenon governed by the synchronization of the frequencies of the vortex-shedding hydrodynamic forces (as well as yawing moment, if a multicolumn platform is considered) with the natural frequencies related to oscillation modes of the moored unit on the horizontal plane. Those eigenfrequencies and corresponding (coupled) modes depend on the stiffness of the mooring system, which, in turn, depend nonlinearly on the offset and heading of the platform, taken at each environmental condition, [2]. A simple model is herein proposed to treat this rather complex problem. For a range of current intensities and for a complete turn of current directions, offsets and headings are determined, from 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 maps of susceptibility to VIM are plotted. Verifying the proposed strategy, such maps are confronted with results of time domain simulations, using the nonlinear ROM presented in [1]. The FOWT-OC4 four-column semisubmersible platform is taken as case study. Firstly, in 1:73 small-scale, for which experimental results exist. Then in full-scale. Discussions on the applicability of the susceptibility to VIM model for design are done and some conclusions are drawn.[1] &nbsp;&nbsp; Oliveira, E.L, Pesce, C.P., Mendes, B., Orsino. R.M.M., Franzini, G.R. A Reduced-Order Mathematical Model for the Current-Induced Motion of a Floating Offshore Wind Turbine. Proceedings of ASME 2020 the 3rd International Offshore Wind Technical Conference, IOWTC2020-3503.[2] &nbsp;&nbsp; Amaral, G.A., Pesce, C.P., Franzini, C.P. Mooring system stiffness: a general analytical formulation with an application to floating offshore wind turbines. Proceedings of ASME 2018 the 1st International Offshore Wind Technical Conference, IOWTC2018-1040.

A Model to Assess the Susceptibility of a Multicolumn FOWT Platform to Vortex-Induced Motions in Early Design Stages

Paper Proceedings: https://asmedigitalcollection.asme.org/OMAE/OMAE2021/volume/85116

OMAE 2021

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