D1.2 Higher Order Hydroelastic Module

This document is a deliverable of the FLOATECH project, funded under the European Union’s Horizon 2020 research and innovation programme under grant agreement No 101007142.

Global dynamic analysis of a single offshore (potentially floating) wind turbine is in itself a challenge as it involves strong interactions between the elastic turbine, its control system, its support and mooring system. This requires multiphysics modeling including aerodynamics, hydrodynamics, and structural dynamics. Over the last decade, numerous numerical simulation tools have been developed, mostly coupling offshore design tools including various hydrodynamic solvers to aero-elastic solvers used to design wind turbines.

In general, these simulators have an excellent accuracy and CPU time ratio which allows them to be used regularly in design offices for the analysis of the dynamic behavior of floating wind turbines. These simulators are sufficient to assess the global response of the system under combined wind and wave conditions but have certain limitations. In particular, they do not allow the structural dimensioning of the floating support since they often model a flexible wind turbine (tower and rotor) supported by a rigid platform. This form of "rigid-flexible" coupling ignores the flexible modes of the platforms. This design step is then performed in a decoupled way using a structural solver modeling the platform by means of a finite element formulation and considering only static loadings or linear wave loadings. While this approach can be sufficient for medium power rotors, the development of 10+MW rotors will require foundations of large dimensions for which hydroelastic effects will become significant.

To overcome this problem, recent works propose coupling approaches between hydrodynamic solvers and structural solvers [1, 2, 3, 4]. The hydrodynamic loads are then no longer represented in the form of a single force vector applied at a single point, but rather are applied in a distributed manner over the whole structure. Depending on the approach, the hydrodynamic loads are calculated either using a Morison formulation or a linear potential flow model. The structural deflections are modeled either directly through a finite element model or on the rigid and flexible structural modes of the platforms. The results of these works show that taking into account the flexibility of the structure can have a significant influence on the distribution of internal loads, on the structural resonance periods and consequently on the dimensioning.

QBlade Ocean (hereafter QB) follows the same trend aiming to be a tool used for the design of 10+MW floating wind turbines and so being able to calculate the structural response of floating platforms. This document presents an overview of the different numerical models that have been implemented into the hydrodynamic module of QB as well the coupling that has been made with the structural solver. The results of a step-by-step verification and validation process are successively presented. The results show that QB is able to simulate the wave-induced motions of various types of floating platforms under regular or irregular wave conditions by means of different modelling approaches. It shows that QB has results comparable to OpenFAST solver. Finally, preliminary results on the hydroelastic response of a floating wind turbine platform are presented.