Research On Modal Parameter Identification Based On Ambient Excitation For Offshore Platforms

Offshore platforms continually serve in atrocious ocean environment, their health monitoring and safety assessment are hot issues in offshore engineering. And the accuracy of modal parameter identification is directly related to the accuracy and reliability of Structure Health Monitoring (SHM) and safety assessment for large-scale structures, and it is considered as one of the core technologies in SHM and safety assessment, it is therefore necessary to research modal parameter identification.For exsiting offshore platforms, artificial exciting method for identifying modal parameter has many shortcomings, it requires expensive and specialized equipments and technical staffs, and it may affect the normal operation of the platform and even induce damages to the structure. The above shortcomings can be overcome while using modal parameter identification based on ambient excitation; and it is basis to develop the online SHM system under ambient excitation. Thus the development of modal parameter identification technology based on ambient excitation has good prospects for engineering applications.This thesis focuses on two output-only modal parameter identification technologies under ambient excitation: ITD (Ibrahim Time Domain) method and ARMA (Auto-Regressive and Moving Average Model) method. A three-DOF (Degree of Freedom) mass-spring-damper model and an offshore jacket platform structural model are established, and these two models are researched on the selection of several key parameters in order to reduce the noise impact to modal parameter identification by using ARMA and ITD methods. Studies demonstrate that increasing the number of virtual testing points can significantly reduce its sensitivity to noise while using ITD method. And then in order to validate the two methods, modal parameter identification of a jacket platform physical model is performed. The physical model used is a reduced scale model of an in-situ offshore platform. The model testing proves that the ITD method and ARMA method are capable of identifying a set of acceptable modal parameters by employing the measured signals. This model testing also shows that vibration testing under ambient excitation for offshore platforms is practicable; and the experimental scheme have referential significance on the field modal testing for offshore platforms.