Vibration Control of Offshore Wind Turbines through the Hourglass Lattice Transition Piece

The Hourglass Lattice Structure (HLS), or Hourglass Lattice Transition Piece (HLTP), is a novel transition piece designed to mitigate excessive vibrations in wind turbines. This innovative concept translates the Reduced Column Section principles into offshore applications, relying on (i) the shift of the fundamental period, (ii) the induced rocking behaviour, and (iii) the increased damping achieved through material yielding or supplementary dashpot devices. In this study, a design procedure is proposed to determine the optimal parameters of the HLS. The procedure is based on establishing an equivalence between the lattice structure and a tapered beam formulation. The latter has been developed through an analytical frequency-dependent model, which has been further extended to account for stochastic aerodynamic and seismic excitations. The resulting formulation enables the derivation of a simplified wind turbine model suitable for fast and reliable stochastic simulations in the optimisation process. A case study based on the IEA 15 MW Reference Wind Turbine is presented, incorporating dynamic soil–structure interaction (SSI) effects into the proposed formulation. Several HLS configurations are investigated, including variations in hourglass geometry, lattice arrangement, induced plastic behaviour, and additional damping mechanisms. The results demonstrate the beneficial effects of the HLS in mitigating excessive vibrations, leading to reduced peak bending moments in both the tower and monopile, as well as an improvement in fatigue reliability.