Analyzing vibration responses to predict water-added mass and hydrodynamic damping in offshore jacket structures

Determining the natural frequencies and damping ratio is the most important step in the dynamic behaviour analysis of fixed jacket structures, such as offshore wind turbines and drilling rigs. The rise of added-water mass could result in the degradation of fundamental natural frequencies and increase the hydrodynamic damping of the structures. The jacket structures consist of many tubular components; the effect of water-added mass and damping is complex and different from the proposed equations. Therefore, this paper studied the application of vibration response measurement analysis using the Fast Fourier Transform (FFT) algorithm to identify the water-added mass and hydrodynamic damping ratio for the structures. The vibration test for three cases, including in the air, water, wave condition and numerical method, was conducted. The research proved that the effect of the water-added mass and hydrodynamic damping under the wave excitation on the structure was most significant in three cases.