Operational modal analysis of a ship model in the presence of harmonic excitation

Abstract

A ship is operated under an extremely complex environment, and waves and winds are assumed to be the stochastic excitations. Moreover, the propeller, host and mechanical equipment can also induce the harmonic responses. In order to reduce structural vibration, it is important to obtain the modal parameters information of a ship. However, the traditional modal parameter identification methods are not suitable since the excitation information is difficult to obtain. Natural excitation technique-eigensystem realization algorithm (NExT-ERA) is an operational modal identification method which abstracts modal parameters only from the response signals, and it is based on the assumption that the input to the structure is pure white noise. Hence, it is necessary to study the influence of harmonic excitations while applying the NExT-ERA method to a ship structure. The results of this research paper indicate the practical experiences under ambient excitation, ship model experiments were successfully done in the modal parameters identification only when the harmonic frequencies were not too close to the modal frequencies.

Keywords

natural excitation technique (NExT) / eigensystem realization algorithm (ERA) / ship structure / harmonic excitation / signal processing / modal parameters identification / ship model / operational model analysis

References

[1] Brinker R, Andersen P, Moller N. An indicator for separation of structural and harmonic modes in output-only modal testing. Proceedings of the 18th IMAC, San Antonio, 2000, 1649-1654

[2] Brown DL, Allemang RJ, Zimmerman R, Mergeay M. Parameter estimation techniques for modal analysis. Seventh International Seminar on Modal Analysis, 1985, Belgium: Katholieke Universiteit Leuven

[3] De Roeck G, Peeters B, Ren WX. Benchmark Study on System Identification through Ambient Vibration Measurement. 18th IMAC, San Antonio, 2000, 1106-1112

[4] Ibrahim SR. Computation of normal modes from identified complex modes. AIAA Journal, 1982, 21(3): 446-451

[5] Ibrahim SR, Mikulcik EC. A method for direct identification of vibration parameters from the free response. The Shock and Vibration Bulletin, 1977, 47(4): 183-198

[6] James GH, Carne JPL. The natural excitation technique (NExT) for modal parameter extraction from operating structures. Journal of Analytical and Experimental Modal Analysis, 1995, 10(4): 260-277

[7] Jiang DZ. Experimental and theoretical study on modal analysis of ship structures under umbient excitation, 2010, Dalian: Dalian University of Technology, 1-4

[8] Jiang DZ, Hong M, Zhou L. Study on operational modal parameters identification of ship structures. Journal of Ship Mechanics, 2011, 15(3): 313-324

[9] Juang JN, Pappa RS. An eigensystem realization algorithm for modal parameter identification and model reduction. Journal of Guidance, Control, and Dynamics, 1985, 8(5): 620-627

[10] Liu S. Study on identification of structure modal parameters based on the excitation of ship operational condition, 2012, Dalian: Dalian University of Technology, 37-52

[11] Mohanty P, Rixen DJ. Modified ERA method for operational modal analysis in the presence of harmonic excitations. Mechanical Systems and Signal Processing, 2006, 20: 114-130

[12] Pappa RS, Elliott KB. Consistent-mode indicator for the eigensystem realization algorithm. Journal of Guidance, Control, and Dynamics, 1993, 16(5): 852-858

[13] Peeters B, Guid DR. Stochastic subspace technique applied to parameter identification of civil engineering structures. Proceedings of New Advances in Modal Synthesis of Large Structures: Nonlinear, Damped and Nondeterministic Cases, Lyon, 1995, 151-162

[14] Pintelon R, Peeters P, Guillaume P. Continuous-time operational modal analysis in the presence of harmonic disturbances. Mechanical System and Signal Processing, 2008, 22: 1017-1035

[15] Shen F, Zheng M, Feng SD, Xu F. Using the cross-correlation technique to extract modal parameters on response-only data. Journal of Sound and Vibration, 2003, 259(5): 1163-1179

[16] Wan L. The study on modal parameters identification for ship structures based on ambient excitation, 2011, Dalian: Dalian University of Technology, 21-24