Modal identification of a jacket-type offshore structure using dynamic tilt responses and investigation of tidal effects on modal properties

Dynamic responses including acceleration, dynamic tilt, and strain were measured using a long-term monitoring system for the Uldolmok tidal current power plant, which is a jacket-type offshore structure. Modal properties such as natural frequencies and modal damping ratios for the lowest three vibration modes were identified using the proposed least square-based frequency domain decomposition method based on measured dynamic tilt responses. Long-term trends of the identified modal properties were investigated in the time and frequency domains, and it was found that the identified modal properties of the Uldolmok tidal current power plant periodically fluctuated approximately twice a day and also twice a month, and also the identified modal properties were highly correlated with the tidal components M2 and M4. Finally, nonparametric regression models were considered as a means of modeling the relationship between tidal changes and modal properties and the model with tidal height, tidal current speed and the multiplication of tidal height and speed was found to be the most appropriate one.

► Least square-based frequency domain decomposition method (LS-FDD) was proposed. ► Modal parameters for uldolmok tidal current power plant were extracted using LS-FDD. ► They were significantly fluctuated twice per day and also twice per month. ► They were strongly related to the changes of tidal height and tidal current speed. ► Optimal regression model was investigated to correlate them with tidal changes.