Identification Method of Damping for Ship Structural Vibration (2nd Report)

Abstract

In order to prevent harmful vibration in ships at the early stage of design, anti-vibration design is required on the basis of accurate estimation methods. To investigate vibration response of superstructure, very large scale of 3-dimensional finite element analysis including hull structure is sometimes conducted, however, significant improvement of accuracy is not yet achieved due to the difficulty in estimating damping and exciting forces.In the 1st report, a new identification method of damping factor is proposed by calculating dynamic response directly on the basis of Rayleigh's damping; [C] = α [M] β [K]. Then the capability of this method is verified by the measured data of frame structure model.In this 2nd report, this method is applied to the measured data of a 280, 000DWT VLCC's superstructure and its ability is confirmed in the following procedures : 1) Precise transfer functions of superstructure vibration were obtained by the sweep exciter test of the superstructure. This test was performed with the aid of the active mass damper controlled by electromagnetic force recently developed), which enabled to oscillate the superstructure with fine frequency increment.2) 3-dimensional FE model of superstructure including hull structure is analyzed to investigate the influence of FE modeling manner on the calculated natural frequency, in particular the influence of FE model realization of local structures such as radar mast, funnel and etc. Consequently the guideline of FE modeling of superstructure is proposed to estimate the accurate natural frequency of superstructure vibration.3) By means of the above accurate superstructure model with hull structure, the new identification method described in the 1st report was adapted to identify the measured response of the VLCC's superstructure. As a result of this identification, calculated response showed a good agreement with the measured data in both ballast condition and full load condition.4) Obtained damping ratio is ζ=0.00896 and 0.01025 in ballast condition, ζ=0.01297 and 0.01626 in full load condition. These values almost agree to the assumed lower limit of the published damping ratio.