On the influence of environmental and operational variability on modal parameters of offshore wind support structures

The present paper explores the effect of environmental and operational variability (EOV) on the modal parameters of offshore wind structures. A non-linear numerical model of a representative offshore wind turbine supported by a jacket substructure is established and analyzed under the exposure of EOV. In particular, the study investigates time-periodic effects, non-classical aerodynamic damping, and operational variability imposed by the turbine controller. The modal parameters of different operational states are computed using linearized, time-periodic system formulations, and the variability in the modal parameters is discussed. The results illustrate the variation of the extracted modal parameters, which is found to be governed by two main sources; namely, 1) controller variability and 2) interaction of system modes. The former explains the variation in modes that are highly affected by modifications of the controller parameters. The latter explains the variation in the modes that become closely spaced and hence prone to mutual interaction.