Abstract

This paper presents a methodology for the calculation of internal loads, induced by wind and waves, on a floating offshore wind turbine. The ring shaped hull is divided in separated compartments on which diffraction radiation is calculated and then used in a time-domain dynamic analysis of the floater, also including wind turbine and mooring system. In addition to reproducing the dynamic behavior of a rigid body, the multibody modelling provides information of loads transmission inside the hull.

Introduction

Several ways of modelling floating wind turbines and accessing loads exerted in the hull can be considered. The most suitable model is usually chosen on the basis of the hydrodynamic and aeroelastic properties of the wind turbine. Several techniques were presented whereby the floating foundation was modelled on the basis of simple hydrodynamic models like in Utsunomiya et al. (2013) for a spar platform or Le Cunff et al. (2013) for a three floater semi-submersible platform, or where the full 3D pressure field of the waves was directly calculated at each time-step in time domain simulations and then mapped to finite element models as proposed by Cermelli et al. (2010). The floating foundation considered in the present paper is a large volume structure for which it is necessary to account for diffraction and radiation, but for which part of the wave loads are caused by viscous vortex-shedding effects. Both techniques mentioned above have been tried, but the technique proposed by Utsunomiya et al. (2013) and Le Cunff et al. (2013) could not effectively be used due to the limited accuracy of Morison's equation for the ring floater, while the latter technique used by Cermelli et al. (2010) is more adapted when most of the loads come from perfect fluid phenomena. In addition, the computation time of the structural analysis using direct pressure field application made the latter method difficult to handle in the detailed design of a floating foundation where a large number of load cases need to be considered.

This made the authors search for an alternative method described here, which would provide accurate hydrodynamic modelling of the foundation and direct information on the loads governing the structural design of the hull: tower loads, accelerations, but also bending, shears and torque of the caissons constituting the hull.

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