MS2.9 - Advances in Control of Structural Vibrations

The dynamic performance evaluation of two adjacent single-degree-of-freedom structures (SDOF) connected with novel negative stiffness-inerter-based dampers (NSIDs) is studied under base excitation. The base excitation is modelled as a stationary white noise random process. The equations of motion are written in state space form, and optimal parameters for maximum possible response reduction are evaluated. The optimal parameters are based on minimising the mean square response of relative displacement under white noise base excitations. The performance of optimised dampers connecting adjacent SDOF systems is then assessed under real earthquake records. The seismic response reduction of adjacent structures with NSIDs is compared with viscous dampers with the same damping coefficient. Numerical results indicate comprehensive seismic response reduction of both the connected structures when NSIDs are used as connecting systems compared to viscous connections. The analytical study demonstrates that the performance of viscous dampers can be enhanced by adding the negative stiffness and inerter elements to the dashpot.

Floating Offshore Wind Turbines (FOWTs) are have been proposed in recent years for deep water deployment where the installation of traditional fixed bottom turbines would be impossible. This opens up vast areas of the marine environment for offshore wind development. FOWTs have been realised in recent years in offshore wind farms in Scotland (Hywind & Kincardine), Portugal (Windfloat), Spain (Flocan 5 Canary), and France. However, there remain significant technical challenges to be overcome to make floating offshore wind a commercially attractive prospect. In general the structural characteristics of a FOWT are much more dynamic than an onshore turbine or a traditional fixed base offshore wind turbine. Due to this fact, novel structures and controllers must be developed specifically for FOWTs. The stability of the FOWTs (pitching and rolling of the platform) and reduction of aerodynamic loads on FOWTs is now the topic of considerable research. Much work focuses on the design of new pitch and torque controllers. Most commercial wind turbines use proportional integral (PI) collective blade pitch control to regulate rotor speed in the above rated wind speed regime. A significant drawback of this type of controller is that it assumes that the blades have identical structural properties and are subject to similar aerodynamic loads, which is seldom the case. Also, these controllers are designed to regulate the rotor speed and are not designed for structural vibration/load reduction. However, it is well known that blade pitch control can reduce structural loads on wind turbines. This opens up the possibility of designing controllers that use existing actuators and sensors like the blade pitch actuators to reduce structural loads/vibrations while maintaining the required rotor speed. Recent studies have investigated individual blade pitch control (IPC) to address these shortcomings. However, the vast majority of studies published in the literature depend on the availability of state measurement. Although sensors are commonly placed on all wind turbines, and some information is readily available, the measurement required by the typical state feedback controllers is usually not available. Displacements and velocities of the blade, the tower and the floating platform are difficult to measure. This paper develops an observer based individual blade pitch controller for load mitigation and power regulation of floating offshore wind turbines. We propose to use a Kalman filter to estimate the state from the accelerometer and strain gauge measurement for use in the state feedback controller. The state feedback controller was proposed previously by the authors and showed excellent performance. This paper extends the capability of the state feedback controller by designing an observer (Kalman filter) to estimate the state from limited measurements. The proposed observer based controller is compared against a baseline proportional integral collective blade pitch controller and full state feedback controllers to evaluate its performance. Numerical results show that the proposed output feedback controller offers performance improvements over the baseline controller, similar to the full state-feedback controller.

Floating offshore wind turbines (FOWTs) are the largest rotating structures on the earth. FOWTs are very flexible and dynamically sensitive, they are also installed in very harsh environments and exposed to stochastic environmental loading from wind and ocean waves. In recent years dampers have been installed in the towers of offshore wind turbines to mitigate vibrations. In this paper structural dynamic models are used to demonstrate improvements in the fatigue life of FOWT towers when they are equipped with a new type of damper - the tuned mass damper inerter (TMDI). A multi-body dynamic approach is used to model the wind turbine and the TMDI installed in the tower. The model is subjected to stochastically generated wind and wave loads of varying magnitudes to develop wind-induced probabilistic demand models for towers of FOWTs under model and load uncertainties. Numerical simulations are carried out to determine the improvements in fatigue life of FOWT towers that can be achieved by installing TMDIs in comparison to traditional tuned mass dampers (TMDs). Results show that the TMDI outperforms the classical TMD when considering fatigue life.