Operational Modal Parameters Identification Of Ship Structure Based On HHT Method

The ship is a large and dynamic structure which is sailing on water. The excitation from the propeller, host, wave and others will definitely cause the vibration of ship. It will not only affect the comfort life of crew but also the normal operation of equipments. Moreover, the vibration problems may lead to damage of ship structure. In that condition, vibration of ship structure attracts people’s attention. How to reduce the level of ship vibration is becoming an issue. Modal parameter plays a vital role on the evaluation and control of ship vibration. It also has guiding significance for ship design, modification and optimization. The performance of ship structure can be assessed according to modal results, refer to natural frequencies, modal shapes and modal damping ratios.During a traditional modal test, both the input force and the output response are measured. For a ship, it is difficult to find an applicable excitation. Operating modal analysis is a produce which can extract modal parameters from measured response without knowing the input force. It is a new method for modal parameter identification of ship structure.In this paper, research is mainly focused on the application of operational modal parameter identification on ship structure. And a brief discussion of the signal reduction on parameter identification is presented. Firstly, a review of modal parameter identification is summarized. The advantage and disadvantage of these methods on frequency domain, time domain and frequency-time domain are discussed. And some popular software for operational modal parameter identification are introduced. The basic theory of Hilbert-Huang method is introduced in detail. The Hilbert-Huang transform algorithm is realized in the Matlab program language. Characteristics of this method are discussed by analyzing some typical signals. And the paper introduces the instantaneous frequency and the empirical mode decomposition. Then, the experiments of a simply supported beam, a submerged plate and a ship model are done to verify the feasibility of the program. Finally, the signal noise reduction of the empirical mode decomposition is analyzed.