Experimental And Theoretical Study On Modal Analysis Of Ship Structures Under Ambient Excitation

The vibration of ship structures has been one of the test indexes of ship structure design, the trial and the delivery of ships at present. It has great influence on the security of ship structures and the normal work of the sailors and equipments, so study on modal analysis of ship structures and vibration control methods is significant. But traditional experimental modal analysis can not achieve satisfactory effect because of its many limitations.Operational modal analysis-OMA is a procedure which allows extracting modal parameters from structures only based on response data under ambient excitation. It has become a research focus in many fields recently because it doesn't need excitation devices and also doesn't influence the normal operation of the equipments. But there are fewer researches on operating ship structures analysis now. This paper mainly studies the feasibility of operational modal analysis on ship structures, and proposes a method to identify the modal parameters of an operating ship structure. The effectiveness of this method is verified with two experiments, and this promotes the application of operational modal analysis to the dynamic analysis of ship structures. The following work is completed:First, some methods of operational modal analysis are summarized based on the study on related literatures. The feasibility of OMA on an operating ship is discussed. The excitations to an operating ship are assumed to be white noise including harmonic excitations, and this breaks the limits of white noise excitation of traditional OMA method.Then the theory of signal process, dynamic test and modal analysis is studied. Band stop digital filter is discussed to filtering the obvious harmonic energy in excitations to reduce the influence on the identification of the real mode. The filtering auto-cross power spectrum is proposed to identify the modal parameters of an operating ship.Finally, two experiments are performed to verify the effectiveness of the proposed method. A simply supported beam experiment with white noise and harmonic excitations at the same time and the vibration test and analysis of a mast during a ship trial are performed to verify the mentioned method. It is concluded that the proposed method is feasible and significant to identify modal parameters of operating ship structures under ambient excitation.