Introduction

The European Green New Deal indicates the increment of production of green and sustainable energy as one of the objectives for the future [1], and indicates an “Offshore Renewable Energy Strategy” targeting the production of 300GW wind energy by 2050, with the Floating Offshore Wind Turbines (FOWTs) as a key technology [2].

NEPTUNE is motivated by the growing interest in FOWTs, driven by the rapidly increasing need for renewable energies, along with the already planned and envisaged installations of offshore wind farms in the Mediterranean Sea. As onshore wind technology is mature, offshore wind has been embraced as the next game-changer in the energy transition [3]–[8]. FOWTs present modelling, design, and technological challenges compared with existing fixed-bottom turbines, mainly due to:

the coexistence of different environmental loads (wind, waves, and current), also in extreme conditions;
the hydrodynamic fluid-structure interactions;
the control system and the structural dynamics (blades and tower);
the effect of the mooring systems and dynamic power cables.

The aforementioned elements can induce significant coupled nonlinear phenomena in FOWTs mechanical behavior and can drastically change the prevailing dynamic response.

In order to be effective and to refer to specific applications, the project is focused on Tension Leg Platform (TLP), whose mooring system consists of a number of pre-tensioned vertical tethers that are anchored to the seabed.

Project overview

The Project is composed by two main activities:

theoretical and numerical modeling of nonlinear phenomena;
laboratory investigation of scaled-down FOWT, to validate/calibrate the analytical/numerical models, and possibly to highlight oversighted phenomena to be investigated.

The activities will consider environmental loads of the Mediterranean Sea, where the new FOWTs are expected to be realized in the nearest future, with a focus on nonlinear structural dynamic phenomena and fluid-structure interactions of mooring systems through physical and computational models, since it is felt that they have been less investigated for this kind of structures, while aerodynamics and aeroelastic aspects of the turbine will be treated considering consolidated methods. The interaction between laboratory experiments and theoretical modelling will be fundamental for the success of the project.

NEPTUNE will provide a link between modelling and technical applications addressing the topic in a systematic and comprehensive way. Recent trends in designing physical model tests include the concept of composite modelling [9], [10], integrating the benefits of both physical (experience) and analytical/numerical models (predictable capability), combined in a balanced approach. To outperform the classical existing methods, we take advantage of interpretable sparse regression data-driven techniques suited to leverage on experimental observation to boost the physical model, allowing also to discover terms that are not present in the envisaged theoretical model. The flow chart of the composite modelling approach is shown in figure.

The experimental results can promote the development of the theory, possibly highlighting unexpected mechanical behaviors (e.g., nonlinear resonances) and the theoretical and computational results need to be compared with the experiments to determine their accuracy. On the other hand, the theoretical guidance can improve the methodology of conduction of both experiments and numerical simulations.

Because of the size of the project and because it is felt that it has been less investigated for this type of structure, physical experiments will involve the structural and hydrodynamic aspects, and their interaction.

On the contrary, numerical simulations and theoretical models will consider all features, including also aerodynamic and aeroelastic aspects, by using trustable and available models and methods [11], taken from the literature and for which there already exists a consolidated expertise by members of the NEPTUNE Team [12], [13].

The dynamic response will be analyzed proposing semi-analytical solutions by virtue of perturbation methods and sophisticated analytical, geometrical and computational techniques employing powerful concepts of bifurcation and chaos theory [14]–[17]. At the same time, computational simulations, employing e.g. Isogeometric Analysis (IGA), Finite Element Method (FEM) and Boundary Element Method (BEM)-based approaches, and high-fidelity numerical models, including CFD, will be developed.

The results of this project will allow to highlight future possible exploitation of nonlinear phenomena, and from a practical point of view, the realization of the “NEPTUNE Italian Design Guidelines (NEPTUNE-IDG)” for FOWTs in the Mediterranean.

Flow chart of the "composite modelling" approach

References

[1] “A European Green Deal https://ec.europa.eu/info/strategy/priorities-2019-2024/european-green-deal_en.”

[2] “Offshore Renewable Energy Strategy https://ec.europa.eu/commission/presscorner/detail/en/fs_20_2099.”

[3] J. Lee and F. Zhao, “Global Wind Report 2021,” Glob. Wind Energy Counc., pp. 1–80, 2021.

[4] Joint Research Center European Commission and E. 29922 EN, “Low Energy Carbon Observatory - WIND ENERGY Technology market report,” 2019.

[5] DG Mare and Joint Research Centre European Commission, “EU Blue Economy Report,” 2020.

[6] International Energy Agency (IEA), “Offshore Wind Outlook – Special report,” 2019.

[7] International Renewable Energy Agency (IRENA), “Future of wind: Deployment, investment, technology, grid integration and socio-economic aspects (A Global Energy Transformation paper),” Abu Dhabi, 2019.

[8] Chinese Wind Energy Association (CWEA), “Statistics of Chinese installed wind power generation capacity 2018,” Beijing, 2019.

[9] G. R. Tomasicchio, F. D’Alessandro, and G. Barbaro, “Composite modelling for large-scale experiments on wave-dune interaction,” J. Hydraul. Res., 2011

[10] J. W. Kamphuis, “Coastal modeling: Indispensable design tool, but how?.,” in Proc. coastlab ’10, 2010, pp. 29–30.

[11] J. Jonkman and M. L. Buhl, “FAST User’s Guide,” vol. 123, no. 6, pp. 407–8, 2007.

[12] N. Bruschi, G. Ferri, E. Marino, and C. Borri, “Influence of Clumps-Weighted Moorings on a Spar Buoy Offshore Wind Turbine,” Energies, vol. 13, no. 23, p. 6407, Dec. 2020.

[13] S. Lenci, F. Clementi, and C. E. N. Mazzilli, “Simple formulas for the natural frequencies of non-uniform cables and beams,” Int. J. Mech. Sci., vol. 77, pp. 155–163, Dec. 2013.

[14] G. J. Vernizzi, G. R. Franzini, and S. Lenci, “Reduced-order models for the analysis of a vertical rod under parametric excitation,” Int. J. Mech. Sci., vol. 163, p. 105122, Nov. 2019.

[15] G. J. Vernizzi, S. Lenci, and G. Rosa Franzini, “A detailed study of the parametric excitation of a vertical heavy rod using the method of multiple scales,” Meccanica, vol. 55, no. 12, pp. 2423–2437, Dec. 2020.

[16] J. Warminski, L. Kloda, and S. Lenci, “Nonlinear vibrations of an extensional beam with tip mass in slewing motion,” Meccanica, vol. 55, no. 12, pp. 2311–2335, Dec. 2020.

[17] F. Clementi, S. Lenci, and G. Rega, “1:1 internal resonance in a two d.o.f. complete system: a comprehensive analysis and its possible exploitation for design,” Meccanica, vol. 55, no. 6, pp. 1309–1332, Jun. 2020.

NEPTUNE

NonlinEar Phenomena in floaTing offshore wind tUrbiNEs

MIUR Grant: Prot. 2022W7SKTL

e-mail: neptune2023.social@gmail.com