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From an operation & maintenance (O&M) point of view, it is necessary to model the aero-hydro-servo-elastic (AHSE) dynamics of each wind turbine but, on the other side, wind farms generally include hundreds of wind turbines. Simply using and linking several advanced, single wind turbine models of dynamics to represent a wind farm can be computationally prohibitive. To this end, this paper developed a reduced-order model (ROM), able to capture the relevant dynamics of the system for a specific failure, having a lower computational cost and therefore more easily scalable up to a wind farm level.First, a nonlinear AHSE model is used to derive the time-domain response of the wind turbine degrees of freedom (DOFs). The failure mode, its relevant DOF, and the relevant operational conditions during which the failure is likely to occur are identified.A linearisation of the nonlinear aero-hydro-servo-elastic-drivetrain (AHSE-DT) model is then carried out. Subsequently, a number of linear ROMs are developed based on the linear full-order system but excluding high-frequency states using the modal truncation (MT) method. For the targeted DOF (rotor torque signal) and the load cases simulated, the results from the linear ROMs showed that the blade modes are important to capture not only the DOF of extreme values, but also the DOF of high-frequency responses (above 1.5 Hz). The results from nonlinear ROMs showed that the ROM eliminating all the tower modes (rigid tower) is acceptable to capture the DOF of low-frequency response (below 0.5 Hz), as it has almost the same spectral responses as the full-order nonlinear model.

The interest in floating offshore wind turbines (FOWT) has been growing substantially over the last decade and, after a number of prototypes deployed [1], the first offshore floating wind farms have been approved and are being developed. While a number of international research activities have been conducted on the dynamics of offshore floating HAWT systems (e.g. OC3-Phase IV2, OC4-Phase II3), relatively few studies have been conducted on floating VAWT systems, despite their potential advantages [2]. Due to the substantial differences between HAWT and VAWT aerodynamics, the analyses on floating HAWT cannot be extended to floating VAWT systems. The main aim of the present work is to compare the dynamic response of the FOWT system adopting two different mooring dynamics approaches. Two version of the in-house aero-hydro-mooring coupled model of dynamics for VAWT “FloVAWT” [3] are used: one which adopts a mooring quasi-static model, and solves the equations using an energetic approach [4], and a modified version of FloVAWT, which uses instead the lumped-mass mooring line model “MoorDyn” [5]. The floating VAWT system considered is based on a 5MW Darrieus type rotor supported by the OC4-Phase II3 semi-submersible. The results for the considered metocean conditions show that MoorDyn approach estimate larger translational displacements of the platform, compared to the quasi-static rigid approach previously implemented in FloVAWT. As expected, the magnitudes of the forces along the lines are lower, being part of the energy employed for the elastic deformation of the cables. A systematic comparison of the differences between the two approaches is presented.

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