Modal parameters identification of offshore structures is important for many engineering applications, such as damage detection, structural health monitoring, etc. Operational modal analysis has been widely used for large structures. However, measured signals are inevitably contaminated with noise and may not be clean enough for identifying the modal parameters with proper accuracy. The traditional methods to estimate modal parameters in noisy situation are based on over-determined system to absorb the “noise modes” firstly, and then using the stability diagrams to distinguish the true modes from the “noise modes”. However, it is difficult to sort out true modes when the signal noise ratio is low, especially, the “noise modes” will also tend to be stable as the model order increases. This study develops a noise reduction procedure for polyreference complex exponential (PRCE) modal analysis based on ambient vibration responses. In the procedure, natural excitation technique (NExT) is firstly applied to get free decay responses (auto- and cross-correlation functions) from measured (noisy) ambient vibration data, and then the noise reduction method based on solving the partially described inverse singular value problem (PDISVP) is implemented to reconstruct a filtered data matrix from the measured data matrix. In our case, the measured data matrix is block Hankel structured, which is constructed based on the free decay responses. The filtered data matrix should maintain the block Hankel structure and be lowered in rank. When the filtered data matrix is obtained, the PRCE method is applied to estimate the modal parameters. The proposed NExT-PDISVP-PRCE scheme is applied to field test of a jacket type offshore platform. Results indicate that the proposed method can improve the accuracy of operational modal analysis.
Damping Identification of Offshore Wind Turbines using Operational Modal Analysis: A Review