Abstract
Time–frequency extraction is a key issue to understand structural symmetry of dynamic responses of offshore oil platforms for early warning during drilling operations. Current popular methods for signal characteristics extraction can only obtain the attributes with a single dimension or poor precision. To solve this, a combined Hilbert–Huang transform (HHT) and variational mode decomposition (VMD) method is proposed to extract multidimensional dynamic response characteristics of time, frequency, and energy of offshore oil platforms. Based on the extracted time–frequency–energy information, the frequency-domain integration approach (FDIA) can be applied to calculate the displacement using accelerometer in the micro inertial measurement unit (MIMU). A complementary filtering algorithm was designed to measure the torsion angle of platforms using six degrees of freedom data from the MIMU to obtain the torsion angle information. The performance of the proposed method was validated using a series of simulation shaking-table tests and a field test conducted on an offshore oil platform at Dongying City, Shandong Province, China. During the field test, seven out of eight collisions were detected in the frequency range 5 Hz to 12 Hz. The intensity of the fifth collision was the highest, and the maximum displacement obtained by the accelerometer was 6 mm. In addition, the results show a correlation between the axes of the accelerometer and gyroscope, and their combination can measure a torsion angle up to 1.1°.
Highlights
Introduction published maps and institutional affilWith the rapid growth in world energy demand, the number of offshore oil platforms keeps increasing gradually with progressively upgrading potential security hazards
According to the frequency range obtained through the variational mode decomposition (VMD)–Hilbert–Huang transform (HHT) method, the minimum cutoff frequency was set as 1 Hz, and the maximum was set as 20 Hz
To obtain torsion angle responses caused by ship collisions, the gyroscope data of micro inertial measurement unit (MIMU) were analyzed by Mahony complementary filter
Summary
HHT can process nonstationary and nonlinear signals adaptively and is made up of EMD and Symmetry 2021, 13, 1443. EMD decomposes the original signal into several intrinsic mode functions (IMFs) adaptively. The instantaneous frequency is obtained by the Hilbert transform to each intrinsic mode function (IMF). The formula of Hilbert transform for each IMF is as follows: Z. The instantaneous frequency can be obtained by the differential processing of phase function as follows: dφi (t). Even as the core of Hilbert–Huang transform, EMD is limited in the application because of its drawbacks, such as mode mixing, the endpoint effect, uncertain center frequency, and bandwidth of intrinsic mode. Since the iterative calculation of EMD is very complicated, the computational efficiency of HHT is extremely low, failing to satisfy real-time project applications [12]
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