State of Art

The interest in Floating Offshore Wind Turbines (FOWTs) has rapidly grown as a way to access a large worldwide deep-water wind energy resource [1]–[3], as already confirmed by various public institutions [4]–[9]. The total capacity of offshore wind energy has increased considerably in the last decade, with predictions of up to 120 GW to be installed by 2030. With the 30 MW Hywind project in Scotland, the 24 MW Windfloat Atlantic project in Portugal and the promising 250 MW 7Seas Med project offshore Sicily (construction is expected to start in 2023), Europe and, especially, the Mediterranean countries, will become the global technology leaders for floating wind installations [10]–[14].

Structural modelling

FOWTs account for the combination of aerodynamics and hydrodynamics loads, flexible structural components, mooring and control system. Because of their complexity, a comprehensive theoretical, numerical and experimental analysis is required for the characterization of their overall behavior [15], [16].

While, in view of the ease of interpretation and developed expertise, the preliminary FOWTs design can rely on linearized models, for a deeper understanding of FOWTs dynamics requires account of nonlinear phenomena. These not only lead to negative impacts on the system, but could also be exploited to improve the modelling, design, and control [17]–[20]. At present, FOWTs have been studied, from a structural point of view, using a rigid-flexible multi-body model [21]–[23], including nonlinear beam models for the rotor blades [24], nonlinear interactions of the mooring lines (taut or slack cables) [25], [26] and power cables with the floating platform [27]–[30]. Nevertheless, previous approaches lack of comprehensive analysis on the role of nonlinear phenomena (internal, sub- and super-harmonic resonances; parametric excitation; multistability; chaos; etc.), including their possible exploitation to optimize the performances. Only single structural parts, and single (nonlinear) dynamic aspects have been studied, while a general, holistic, view of the FOWTs as a unique complex dynamic system is missing, also because of the significant computational difficulties.

Fluid-structure interaction

The use of highly accurate computational models able to simulate the Fluid-Structure Interaction (FSI) problem involving (semi-) submersed bodies is still tied down by an unaffordable computational cost. A trade-off between accuracy and efficiency has been somehow found by developing mid-fidelity time-domain solvers. Most of the numerical models adopt standard engineering tools, such as FAST [31], for the analysis of the entire coupled system. These approaches, in particular for the platform-waves interaction, are commonly based on first or second order models [32]–[35] and on the assumption of small displacements and rotations of the structure. These hypotheses are no longer acceptable for structures like Tension Leg Platforms (TLP) and semi-submersible, especially under extreme load conditions. Moreover, despite incorporating second order wave forces, the surge motions is still significantly underpredicted if compared with experimental results [36], [37].

To improve numerical results, high-fidelity CFD modeling is typically used. For example, the fully nonlinear interaction of the FOWT platform and mooring system with the wave forcing can be properly simulated with CFD [38]. In recent years, CFD simulations has increased, in spite of the large computational cost [39]–[42], revealing an overall good agreement with potential flow theory and weakly-nonlinear numerical models [43], [44], and permitting to extend them. However, the modeling of the mooring cable nonlinearities within a CFD framework still poses a challenge to computationally efficient modeling [45]. The OC3 projects [46], [47] reports a series of performed code-to-code comparisons, but detailed code-to-experiment comparisons are still relatively few because of the scarcity of available FOWT test data.

Experimental studies

FOWTs have been studied with scale experiments (and numerical approaches) [48]–[52], to reproduce the effective energy production, the dynamic response of the floating support system and the stresses on the different components of the structure. Experiments on HYWIND floating wind turbines were performed at MARINTEK in Trondheim, by means of a 1:47 Froude-scaled model subjected to coupled wind and wave loads [53]. A comparison between experimental tests and the OrcaFlex numerical model was performed on a 1:100 scale wind turbine mounted on a stepped spar with four mooring lines [48]. More recently, [54]–[58] tested scaled OC3-Spar FOWTs under wind and wave loads. Further investigations are needed (i) to achieve a complete understanding of the nonlinear behavior of FOWT under contemporary acting aerodynamic and hydrodynamic forcing, and (ii) to have up-to-date tests. It is underlined that none of the mentioned numerical and physical modelling has been tailored for the Mediterranean Sea metocean conditions.

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NEPTUNE

NonlinEar Phenomena in floaTing offshore wind tUrbiNEs

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