In order to reduce their cost, offshore wind turbines (OWTs) must have a powerful generator and a minimum overall weight. This has the consequence of making the OWT structure sensitive to dynamic excitations even at low frequencies. Indeed, modern multi-megawatt OWTs are composed of slender flexible and lightly damped components. The excessive vibrations of the OWT structure can impact the wind energy conversion to electricity, decrease the fatigue lifetime and even result in a total collapse of the structure when exposed to harsh environmental conditions. It is therefore important to reduce the unwanted vibrations of an OWT by implementing an appropriate control device that enhances its structural safety.

Motivated by the potential of the structural control methods in suppressing OWTs vibration, this paper proposes the design of a controlled active tuned mass damper (ATMD) system to reduce the nacelle/tower out-of-plane vibration of a monopile-supported 10 MW DTU OWT subjected to combined wave and wind loads. Compared to previous works, the main originality of this paper is the inclusion of a state estimator, Linear Quadratic (LQ) observer, within an optimal control schema. The state observer aims to drastically reduce the number of required system states. Indeed, as some measurements are practically impossible, all system states cannot be obtained. In this study, a fully coupled multi-degree of freedom (MDOF) analytical model of a monopile-supported OWT developed in [4] is used for this purpose. The optimal control schema makes use of the robust LQR feedback controller to establish the ATMD actuator control force. The developed active control schema proved to efficiently reduce the nacelle/tower vibration.

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