Due to the high flexibility of new offshore wind turbines (OWT) and dynamic content of the loading from wind, wave and earthquake applied to OWT systems, they are prone to excessive vibrations and this may result in serviceability issues as well as low fatigue lives of structural members. Therefore, control systems capable of mitigating undesired vibrations can help to reduce serviceability issues and increase fatigue lives. In this research, numerical models of jacket and monopile OWTs are developed and the effect of different types of vibration control devices such as Tuned Mass Dampers (TMDs) and Tuned Liquid Columns Dampers (TLCDs) on the dynamic responses of OWTs under various loadings is investigated. The dynamic responses are compared with those of the baseline system in which no vibration control device is used. Then, fragility analysis based on Multiple Stripe Analysis is performed to estimate the fragility reduction due to the implementation of vibration control devices. Different limit states for different equipment inside the nacelle are considered in the fragility analysis. The results show that the fragility reduction offered by the tuned liquid column dampers is higher for low-intensity earthquakes. In addition, the corresponding values for the parked condition are greater due to the lack of aerodynamic damping in this condition.Therefore, the implementation of an optimal TLCD can increase the overall reliability of the system, especially for parked conditions under low-intensity earthquake motions. Then a model for offshore wind turbine systems equipped with a semi-active time-variant tuned mass damper is developed considering nonlinear soilâpile interaction phenomenon and time-variant damage conditions. The adaptive concept of this tuned mass damper assumes a slow change in its structural properties. Stochastic wind and wave loadings in conjunction with ground motions are applied to the system. Damages to soil and tower caused by earthquake strokes are considered and the semi-active control device is retuned to the instantaneous frequency of the system using short-time Fourier transformation (STFT). The performance of semi-active time-variant vibration control is compared with its passive counterpart in operational and parked conditions. The results show that a semi-active mass damper with a mass ratio of 1% performs significantly better than a passive tuned mass damper with a mass ratio of 4%.
|Date of Award||1 Mar 2019|
- University Of Strathclyde
|Sponsors||University of Strathclyde|
|Supervisor||Erkan Oterkus (Supervisor) & Nigel Barltrop (Supervisor)|