Optimal design of passive-adaptive pendulum tuned mass damper for the global vibration control of offshore wind turbines

This paper presents an optimal design procedure for a pendulum tuned mass damper (PTMD) to mitigate the global structural vibrations of offshore wind turbines (OWTs) in the fore-aft and side-side directions. The procedure is tested to the design of a PTMD to be applied to the 5-MW benchmark baseline monopile wind turbine proposed by the National Renewable Energy Lab (NREL). The computation of wind and wave spectra, as well as the evaluation of the hydrodynamic and aerodynamic loads, is conducted by using an in-house built MATLAB (R) routine working together with an ANSYS (R) 3-D finite element (FE) global model for evaluating the resultant peak displacement response at the OWT hub by a power spectral density (PSD) analysis. In order to validate the OWT FEM model, a result comparison is made with the NREL OpenFAST, finding good matches between the two codes. An in-house built genetic algorithm (GA) toolbox, coded in MATLAB (R), is then used to optimally design the parameters of a PTMD with a simplified 2-degrees-of-freedom (2DOF) model. The chosen GA fitness function targets the minimization of the peak response of the primary structure as evaluated by the 2DOF model. The design parameters of the PTMD are the flexural rigidity and damping, the mass ratio and pendulum length. After the 3-D FE model of the OWT without any control device has been validated, and the PTMD has been optimized by the simplified 2DOF model, the performances of the PTMD are examined on a 3-D global FE OWT + PTMD model in ANSYS (R).