This paper presents a numerical and field experimental modal analysis on a real offshore platform in Bohai Bay, China. This platform is a steel jacket offshore platform with vertical piles. The field testing under wave-induced force and wind force etc. was conducted, in order to obtain the dynamic parameters of the structure, including the frequencies of the jacket platform, as well as the corresponding damping ratios and mode shapes. The Natural Excitation technology (NExT) combined with Eigensystem Realization Algorithm (ERA) and the peak picking (PP) method in frequency domain are carried out for modal parameter identification under operational conditions. The threedimensional finite element model (FEM) is constructed by ANSYS and an analytical modal analysis is performed to generate modal parameters. The analytical results were compared with experimental results. A good correlation was achieved between the finite element and ambient vibration test results. It is further demonstrated that the numerical and experimental modal analysis provide a comprehensive study on the dynamic properties of the jacket platform. According to the analysis results, the modal parameters identification under ambient excitation can calibrate finite element model of the jacket platform structures, or can be used for the structural health monitoring system.
The technology of experimental modal analysis has been used extensively in mechanical engineering, civil engineering for the last decades. In the classical modal parameters identification approach, the modal parameters are found by fitting a model to the so-called Frequency Response Function (FRF), a function relating input loading and output response. In fact, large civil engineering structures such as offshore platform, bridges, high-rise buildings suffer various kinds of loading, in many cases, the loading of large structures are not easily measured under operational condition. On the other hand, these structures are difficult to excite artificially.