Studies On Dynamic Response And Vibration Control For Offshore Platforms

Offshore platforms are generally used on-site, inevitably undergoing the environmental loads such as wind, waves, current, ice and earthquake etc., which will induce continuous vibration of the offshore platforms. The vibration, on one hand, will cause fatigue damage, decreasing the platform's reliability; on the other hand, the excessive vibration can't satisfy the basic psychological requirements of the personnel. Hence using smart control technology to enhance the structures' safety and reliability, prolong the service life and ensure the comfort of the personnel has become a very important research subject on ocean engineering and academic fields.This thesis is combined with the project "Identification of Nonlinear Structural Dynamic Systems and the Study on Structural Vibration Control for Offshore Platforms", which is supported by National Science Foundation of China. Based on the present situation on the vibration control for offshore platforms, this thesis studies the dynamic properties of offshore platforms under random wave loading, furthermore, proposes feedforward-feedback control strategy and H2 control method to suppress the vibration of platforms. The major research is shown as following: The dynamic properties of the offshore platform are analyzed by numerical simulation from time domain and frequency domain using finite element (FE) modeling. Feedforward-feedback control method is presented to suppress the vibration of platforms. And the controller is designed accounting for noise (model and measurement), the limits of AMD stroke and control input force. On the basis of setting up the uncertain model of platform-AMD, this thesis investigates the feasibility and effectiveness of H2 control strategy with robustness. The method of designing controller by Hj method is also presented accounting for the AMD constraints. The motion equation with time delay is presented. The time delay effects on the performance of active control system for platforms are investigated. Based on the Taylor series expanded and predict control method, two methods for time delay compensation are put forward, which can be used not only in feedback control system, but also in feedforward-feedback control system. On the basis of MATLAB6.1, the simulation software of vibration control is developed.