Koji Kagawa

Study on Vibratory Response of Deck in Superstructure

In general the design of deck panels is performed to be tuned of the lowest natural frequency of panels over frequencies in exciting forces. Recent passenger ships or some types of research ships with wide area for more accomodations, however, result in the occurence of lowered frequencies of panels. This means that it is very difficult to keep the lowest frequency of panels over exciting frequencies, and in such a case, it is necessary to verify that the higher modes of panels do not resonate with the exciting forces and/or those resonant amplitudes are small allowably.This paper presents a simplified method to calculate the natural frequencies and the responses of deck panels, and also investigates the vibration of main hull girder as an excitation source, considering the effect of long superstructure. Through some vibration tests on actual ships to obtain several data required for correlating the calculation with the design, it is found that the deckcovering materials have very important roles on estimating the damping of panels.Finally a design-based simplified technique is proposed to predict the characteristics for the vibration of deck panels, together with several recommendations related to the anti-vibration design of deck panels.It is concluded that it is very important and effective to sustain higher mode frequencies of panels apart from the lowest one, for example, by the panels stiffened in two perpendicular directions and to avoid the resonance in the lowest vibration mode, from the fact that response amplitudes of higher modes are much smaller than that of the lowest frequency.