Modal matrix reduction for fully coupled integrated load analysis of floating structures

Structural elasticity of floating wind turbines in integrated load analysis (ILA) is typically addressed by modelling the substructure with simplified beam models. The main reason can be found in the computational cost of the structural solver when solving the fully coupled hydroelastic problems. In this work, a reduce order method based on modal matrix reduction is applied to reduce the computational cost of the structural solver. The main idea is to largely reduce the number of degrees of freedom of the structural system by retaining only those modes with significant energy.

The seakeeping hydrodynamics is solved using the finite element framework SeaFEM. The structural particulars are introduced into this framework to fully integrate the fluid-structure interaction. The hydroelastic model is also coupled with the wind turbine solver OpenFAST, resulting in a complete aero-hydro-servo-elastic tool for the ILA analysis of floating turbines.

A methodology is proposed to identify critical conditions and hotspots based on the structural energy. An application case of the present strategy is presented for a detailed structural design of the well-known OC4-DeepCwind. The consistency of the modal approximation and methodology is verified against the FE structural solution. The capabilities of the proposed ILA framework are demonstrated in a fully coupled and detailed structural analysis, instead of at component level, with a significant reduction of its computational time.