Wet damping estimation of the scaled segmented hull model using the random decrement technique

• Free decay signal of segmented hull model towed in model basin was obtained.
• Free decay signal obtained from autocorrelation and random decrement technique was identical.
• The derived wet damping ratio was larger than that obtained from wet hammering test in calm water.

Damping is one of the most uncertain factors in structural dynamic problems, and plays a very important role in resonance phenomena such as springing. This paper presents the wet damping estimation of a segmented hull model using a random decrement technique together with continuous wavelet transform. The 16 sea states were grouped together based on the speed of the ship to determine the possible influence of the ship speed on the damping ratio. The measured time histories of the vertical bending moment for each tested sea state were processed using the random decrement technique to derive the free decay signal, from which the damping ratios were estimated. In addition, the autocorrelation functions of the filtered signal were calculated and a comparison was made with the free decay signal obtained from the random decrement technique. The wet damping ratios for each sea state group, as well as precise wet natural frequencies, were estimated using a continuous wavelet transform. The wet natural frequencies derived from the measured signal did not show any significant discrepancy compared to those obtained using the wet hammering test, whereas a significant discrepancy was observed with the damping ratio. The discrepancy of the damping ratio between in calm and moving water might be due to the viscous effects caused by the dramatically different flow patterns and relative velocity between the vibrating structure and surrounding fluid particles.