Study Of System Identification And Vibration Control For Offshore Platforms

System identification and structural control techniques have made great progress in recent years and been taken applications in civil engineering. Only limited studies, however, have been devoted to system identification and vibration control of offshore structures. The primary objective of the present dissertation focuses on the techniques of system identification and vibration control for offshore platforms subjected to wave loading. Three efforts are as follows:Research on system identification on offshore platforms includes two topics. One is modal parameter identification and the other is the structural parameter identification. The modal parameter identification using natural excitation technique and eigensystem realization algorithm are studied. Reasonable results are identified. By introducing statistics to dispose the modal frequency of the structure, the preciseness and precision of the identified system mode are improved. The recursive equation of structural physical parameters of shearing system are developed to identify the floor mass and stiffness, which only uses any two identified modes and the total mass of the structure.Field measurements are carried out to record the vibration data of a template offshore platform. Dynamical characteristics and responses are analyzed to search the cause of the excessive vibration of the platform under mildly sea state.Research on structural vibration control contains vibration mitigation using tuned liquid damper and MR damper, respectively. The sloshing characteristics of TLD are investigated using shallow water wave theory and the effectiveness and feasibility of the TLD on platform vibration suppression are investigated. Considering that the yielding stress of the MR liquid can be changed continuously by adjusting the magnetic intensity in a certain range, an optimal control theory based semi-active control strategy is proposed and the effectiveness of the MR damper on offshore platform is illustrated by simulation techniques.