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Research Papers: Piper and Riser Technology

An Empirical Procedure for Fatigue Damage Estimation in Instrumented Risers

[+] Author and Article Information
C. Shi

School of Petroleum Engineering,
China University of Petroleum (East China),
Qingdao, Shangdong 266580, China

L. Manuel

Department of Civil, Architectural,
and Environment Engineering,
University of Texas,
Austin, TX 78712

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 1, 2016; final manuscript received November 10, 2016; published online February 17, 2017. Assoc. Editor: Marcelo R. Martins.

J. Offshore Mech. Arct. Eng 139(3), 031701 (Feb 17, 2017) (8 pages) Paper No: OMAE-16-1076; doi: 10.1115/1.4035303 History: Received July 01, 2016; Revised November 10, 2016

In order to assess the effects of vortex-induced vibration (VIV) and to ensure riser integrity, field monitoring campaigns are often conducted wherein the riser response is recorded by a few data sensors distributed along the length of the riser. In this study, two empirical techniques–proper orthogonal decomposition (POD) and weighted waveform analysis (WWA)–are sequentially applied to the data; together, they offer a novel empirical procedure for fatigue damage estimation in an instrumented riser. The procedures are briefly described as follows: first, POD is used to extract the most energetic spatial modes of the riser response from the measurements, which are defined only at the available sensor locations. Accordingly, a second step uses WWA to express each dominant POD mode as a series of riser natural modes that are continuous spatial functions defined over the entire riser length. Based on the above empirically identified modal information, the riser response over the entire length is reconstructed in reverse–i.e., compose identified natural modes into the POD modes and, then, assemble all these dominant POD modal response components into the derived riser response. The POD procedure empirically extracts the energetic dynamic response characteristics without any assumptions and effectively cleans the data of noisy or less important features; this fundamental application of WWA is used to identify dominant riser natural modes–all this is possible using the limited number of available measurements from sensor locations. Application of the procedure is demonstrated using experimental data from the Norwegian Deepwater Programme (NDP) model riser.

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Figures

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Fig. 1

Locations of the 23 input sensors and one target sensor (sensor no. 4) used to illustrate the empirical procedure. Note pinned-pinned boundary conditions are in effect at x/L = 0 and x/L = 1.

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Fig. 2

Normalized PSD functions of the 23 input (measured) strain time series

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Fig. 3

Normalized PSD functions for the first 13 POD subprocesses

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Fig. 4

Normal and Hilbert shape components of the fifth natural mode of the NDP model riser (with pinned-pinned boundary conditions)

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Fig. 5

Normalized PSDs and mode shapes of the first three dominant POD modes that account for 45.6%, 33.2%, and 12.2% of the total energy, respectively

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Fig. 6

Normalized PSDs and modal shapes of the fourth and seventh POD modes that account for 1.8% and 0.9% of the total energy, respectively

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Fig. 7

Strain time series at a selected sensor location (z/L = 0.11): direct measurements versus reconstruction

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Fig. 8

RMS values of CF strains over the entire riser span

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Fig. 9

Fatigue damage ratio estimated using the proposed empirical method (POD combined with WWA) with 23 strains as input. The range of damage ratios estimated with each data set is indicated in the legend.

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Fig. 10

Fatigue damage ratio estimated using the POD method (with interpolation using third-order polynomials) with 23 strains as input (an earlier study by Shi et al. [7]). The range of damage ratios estimated with each data set is indicated in the legend.

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