Research Papers: Structures and Safety Reliability

Signal Processing for Operational Modal Analysis of a Jacket-Type Offshore Platform: Sea Test Study

[+] Author and Article Information
Xingxian Bao

Department of Naval Architecture
and Ocean Engineering,
China University of Petroleum (East China),
Qingdao 266580, Shandong, China
e-mail: baoxingxian@upc.edu.cn

Zhihui Liu

Department of Naval Architecture
and Ocean Engineering,
China University of Petroleum (East China),
Qingdao 266580, Shandong, China
e-mail: liuzhihui1984_2000@126.com

Chen Shi

Department of Naval Architecture
and Ocean Engineering,
China University of Petroleum (East China),
Qingdao 266580, Shandong, China
e-mail: shichen2004@hotmail.com

1Corresponding author.

Contributed by the Ocean, Offshore, and Arctic Engineering Division of ASME for publication in the JOURNAL OF OFFSHORE MECHANICS AND ARCTIC ENGINEERING. Manuscript received July 25, 2018; final manuscript received September 10, 2018; published online October 12, 2018. Assoc. Editor: Jonas W. Ringsberg.

J. Offshore Mech. Arct. Eng 141(2), 021605 (Oct 12, 2018) (10 pages) Paper No: OMAE-18-1108; doi: 10.1115/1.4041510 History: Received July 25, 2018; Revised September 10, 2018

Operational modal analysis (OMA) has been widely used for large structures. However, measured signals are inevitably contaminated with noise and may not be clean enough for identifying the modal parameters with proper accuracy. The traditional methods to estimate modal parameters in noisy situation are usually absorbing the “noise modes” first, and then using the stability diagrams to distinguish the true modes from the “noise modes.” However, it is still difficult to sort out true modes because the “noise modes” will also tend to be stable as the model order increases. This study develops a noise reduction procedure for polyreference complex exponential (PRCE) modal analysis based on ambient vibration responses. In the procedure, natural excitation technique (NExT) is first applied to get free decay responses from measured (noisy) ambient vibration data, and then the noise reduction method based on solving the partially described inverse singular value problem (PDISVP) is implemented to reconstruct a filtered data matrix from the measured data matrix. In our case, the measured data matrix is block Hankel structured, which is constructed based on the free decay responses. The filtered data matrix should maintain the block Hankel structure and be lowered in rank. When the filtered data matrix is obtained, the PRCE method is applied to estimate the modal parameters. The proposed NExT-PDISVP-PRCE scheme is applied to field test of a jacket type offshore platform. Results indicate that the proposed method can improve the accuracy of OMA.

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Fig. 1

The flowchart of the OMA procedure NExT-PDISVP-PRCE

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Fig. 2

The tested jacket-type offshore platform located in Bohai Bay, China

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Fig. 9

Stability diagrams corresponding to: (a) original, (b) filtered (with rank = 6) and (c) filtered (with rank = 4) correlation functions with reference channel x3 (“o”, instability; “*”, stability)

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Fig. 10

Mode shapes corresponding to filtered correlation functions with reference channel x3

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Fig. 3

The simplified finite element (FE) model of the tested jacket-type offshore platform

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Fig. 4

The mode shapes of the FE model: (a) the first-order, (b) the second-order, and (c) the third-order

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Fig. 5

Acceleration responses from eight accelerometers mounted on four legs in x and y directions

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Fig. 6

Cross-correlation functions x3x4 and y3x4

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Fig. 7

Magnitude-frequency curve of cross-correlation functions x3x4 and y3x4

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Fig. 11

Stability diagrams corresponding to: (a) original and (b) filtered correlation functions with reference channel y3 (“o,” instability; “*,” stability)

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Fig. 12

Mode shapes corresponding to filtered correlation functions with reference channel y3

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Fig. 8

Normalized singular values of the block Hankel matrix associated with correlation functions with reference channels x3 and y3, respectively



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