EEMD-HT transform for identifying modal parameters of fixed offshore jacket platforms using vibration response measureme
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ORIGINAL PAPER
EEMD‑HT transform for identifying modal parameters of fixed offshore jacket platforms using vibration response measurement Nguyen Thanh Trung1 Received: 22 April 2020 / Revised: 2 June 2020 / Accepted: 4 July 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Determination of modal parameters is one of the most important steps in a maintenance strategy and/or structure health monitoring of aging fixed offshore jacket platforms during service life. This paper presents a new Ensemble Empirical Mode Decomposition-Hilbert transform (EEMD-HT)-based analysis to identify natural frequencies of the offshore structures under an artificial and natural excitation. An EMD processing algorithm is used to decompose the vibration response signal into a set of intrinsic mode functions (IMFs) and the EEMD is developed from the EMD to solve the vibration mode mixing in identifying the natural frequencies of the structure. In this study, the experiment of vibration response measurement was conducted on the actual offshore structure at site for both the artificial and natural excitations. A numerical model in an Eigen value analysis was also simulated to evaluate the consistency with the experiment. The main results demonstrate that the instantaneous (time-dependent) frequencies of fixed offshore jacket platform were successfully identified by the EEMD-HT technique under both the artificial and ambient excitation. Keywords Fixed offshore jacket platform · Natural instantaneous frequency · EEMD-HT · Vibration response measurement
1 Introduction Nowadays, the dynamic response analysis in the structural health monitoring (SHM) is a new efficient approach based on the site measurement responses of a structure using a signal processing technique to evaluate the dynamic characteristics of a structure more realistically. The main signal processing techniques include a Fast Fourier transform (FFT), a wavelet transform (WT), and a Hilbert–Huang transform (HHT) to identify the modal parameters of the structure [1–6]. However, as commented by Huang and Shen [7], a Fourier spectral analysis is restricted to linear systems, and the signal data should be either periodic or stationary and only present the results in energy–frequency space. A wavelet transform (WT) allows energy–time–frequency resolution for non-stationary data; however, it is based only on a complete theory and is suitable for linear signal data. HHT is a powerful technique for analyzing data from nonlinear and non-stationary processes, and its theoretical base * Nguyen Thanh Trung [email protected] 1
University of Transport and Communications, Hanoi, Vietnam
is empirical. Its characteristics have been demonstrated to be more precise for decomposing a signal in the time–frequency domain than WT [8]. Thus, this technique is conformable with response signals of the offshore structure that are commonly nonlinear and non-stationary due to the effect of the marine environment. The HHT is based on two-steps, which are the EMD (empirical mo
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