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Journal of Offshore Mechanics and Arctic Engineering | Volume 131 | Issue 4 | Ocean Engineering
Ocean Engineering

Statistics of Extreme Events in Airgap Measurements

[+] Author and Article Information
A. Naess

Department of Mathematical Sciences, Centre for Ships and Ocean Structures, Norwegian University of Science and Technology, NO-7491 Trondheim, Norwayarvidn@math.ntnu.no

C. T. Stansberg

 MARINTEK, NO-7450 Trondheim, Norwaycarl.t.stansberg@marintek.sintef.no

O. Gaidai

 MARINTEK, NO-7450 Trondheim, Norwayoleg.gaidai@marintek.sintef.no

R. J. Baarholm

 MARINTEK, NO-7450 Trondheim, Norwayrolf.j.baarholm@marintek.sintef.no

J. Offshore Mech. Arct. Eng 131(4), 041107 (Oct 01, 2009) (8 pages) doi:10.1115/1.3160652 History: Received March 18, 2008; Revised February 25, 2009; Published October 01, 2009
Article
References
Figures
Tables
Citing Articles

The paper presents a study of the extreme value statistics related to airgap measurements on a scale model of a semisubmersible platform subjected to random waves in a wave basin. Relative wave elevation records corresponding to totally 24 h storm duration are considered, made up by 8×3 h realizations. The focus is on a comparison of two alternative methods for the prediction of extreme values from finite recordings at two different locations at the platform. One is a standard method used in the wave basin, making use of a Weibull-tail fitting procedure. The other is a novel method based on the level upcrossing function combined with an optimization procedure that allows prediction at extreme response levels. Similar results are obtained in the mean values by the two methods, while the latter shows less variability in the predictions from single 3 h records.
Copyright © 2009 by American Society of Mechanical Engineers
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References

1
Sweetman, B., 2004, “Practical Airgap Prediction for Offshore Structures,” ASME J. Offshore Mech. Arct. Eng., 126 (2), pp. 147–155.  [CrossRef]
2
Coles, S., 2001, "An Introduction to Statistical Modeling of Extreme Values", Springer-Verlag, London.
3
Stansberg, C. T., Baarholm, R., Kristiansen, T., Hansen, E., and Rortveit, G., 2005, “Extreme Wave Amplification and Impact Loads on Offshore Structures,” "Proceedings of the 20th Offshore Technology Conference (OTC 2005)", ASME, IPTI, May, Paper No. OTC 17487.
4
Nielsen, F. G., 2003, “Comparative Study on Airgap Under Floating Platforms and Run-Up Along Platform Columns,” Mar. Struct., 16 , pp. 97–134.  [CrossRef]
5
Stansberg, C. T., and Kristiansen, T., 2005, “Non-Linear Scattering of Steep Waves Around Vertical Columns,” Appl. Ocean Res., 27 , pp. 65–80.  [CrossRef]
6
NORSOK N-003,1999, “Actions and Action Effects,” Norwegian Technology Standards Institution, Oslo, Norway, n-003, 1st revised ed.
7
DNV-RP-C205, 2007, “Environmental Conditions and Enviromental Loads,” DNV Recommended Practice, Oslo, Norway, .
8
Naess, A., Gaidai, O., and Haver, S., 2007, “Efficient Estimation of Extreme Response of Drag Dominated Offshore Structures by Monte Carlo Simulation,” Ocean Eng., 34 (16), pp. 2188–2197.  [CrossRef]
9
Naess, A., 1984, “On a Rational Approach to Extreme Value Analysis,” Appl. Ocean Res., 6 (3), pp. 173–174.  [CrossRef]
10
Naess, A., and Gaidai, O., 2008, “Estimation of Extreme Values From Sampled Time Series,” Struct. Safety, 31 , pp. 325–334.  [CrossRef]
11
Naess, A., and Gaidai, O., 2008, “Monte Carlo Methods for Estimating the Extreme Response of Dynamical Systems,” J. Eng. Mech., 134 (8), pp. 628–636.  [CrossRef]
12
Gill, P., Murray, W., and Wright, M. H., 1981, "Practical Optimization", Academic, London.
13
Leadbetter, M. R., Lindgren, G., and Rootzen, H., 1983, "Extremes and Related Properties of Random Sequences and Processes", Springer-Verlag, New York.

Figures

Grahic Jump Location
+Schematic of the problem (see Ref. 3)
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Schematic of the problem (see Ref. 3)
Figure 1

Schematic of the problem (see Ref. 3)

Grahic Jump Location
+Bird’s eye view of the case study geometry. Wave probe locations used in this presentation are highlighted in Ref. 3.
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Bird’s eye view of the case study geometry. Wave probe locations used in this presentation are highlighted in Ref. 3.
Figure 2

Bird’s eye view of the case study geometry. Wave probe locations used in this presentation are highlighted in Ref. 3.

Grahic Jump Location
+Snapshot from the ocean basin tests of the Kristin semisubmersible
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Snapshot from the ocean basin tests of the Kristin semisubmersible
Figure 3

Snapshot from the ocean basin tests of the Kristin semisubmersible

Grahic Jump Location
+Power spectra, 24 h records
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Power spectra, 24 h records
Figure 4

Power spectra, 24 h records

Grahic Jump Location
+Amplitude distribution, relative wave 8, and 3 h realization
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Amplitude distribution, relative wave 8, and 3 h realization
Figure 5

Amplitude distribution, relative wave 8, and 3 h realization

Grahic Jump Location
+Crossing rate log plot with extrapolation, 24 h, and relative wave 8
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Crossing rate log plot with extrapolation, 24 h, and relative wave 8
Figure 11

Crossing rate log plot with extrapolation, 24 h, and relative wave 8

Grahic Jump Location
+Optimal transformed plot, 24 h, and relative wave 8
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Optimal transformed plot, 24 h, and relative wave 8
Figure 12

Optimal transformed plot, 24 h, and relative wave 8

Grahic Jump Location
+Plot of the 3 h extreme value from measurements and the predicted MPM for relative wave 1
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Plot of the 3 h extreme value from measurements and the predicted MPM for relative wave 1
Figure 13

Plot of the 3 h extreme value from measurements and the predicted MPM for relative wave 1

Grahic Jump Location
+Amplitude distribution, relative wave 8, and 24 h realization
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Amplitude distribution, relative wave 8, and 24 h realization
Figure 6

Amplitude distribution, relative wave 8, and 24 h realization

Grahic Jump Location
+Plot of the 3 h extreme values from measurements and the Weibull fit for relative wave 1
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Plot of the 3 h extreme values from measurements and the Weibull fit for relative wave 1
Figure 7

Plot of the 3 h extreme values from measurements and the Weibull fit for relative wave 1

Grahic Jump Location
+Plot of the 3 h extreme values from measurements and the Weibull fit for relative wave 8
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Plot of the 3 h extreme values from measurements and the Weibull fit for relative wave 8
Figure 8

Plot of the 3 h extreme values from measurements and the Weibull fit for relative wave 8

Grahic Jump Location
+Crossing rate log plot with extrapolation, 24 h, and relative wave 1
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Crossing rate log plot with extrapolation, 24 h, and relative wave 1
Figure 9

Crossing rate log plot with extrapolation, 24 h, and relative wave 1

Grahic Jump Location
+Optimal transformed plot, 24 h, and relative wave 1
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Optimal transformed plot, 24 h, and relative wave 1
Figure 10

Optimal transformed plot, 24 h, and relative wave 1

Grahic Jump Location
+Plot of the 3 h extreme value from measurements and the predicted MPM for relative wave 8
View Large  |  Save Figure  |  Download Slide (.ppt)
Plot of the 3 h extreme value from measurements and the predicted MPM for relative wave 8
Figure 14

Plot of the 3 h extreme value from measurements and the predicted MPM for relative wave 8

Grahic Jump Location
+Plot of the 3 h extreme value from measurements and the predicted 90% fractile extreme for relative wave 1
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Plot of the 3 h extreme value from measurements and the predicted 90% fractile extreme for relative wave 1
Figure 15

Plot of the 3 h extreme value from measurements and the predicted 90% fractile extreme for relative wave 1

Grahic Jump Location
+Plot of the 3 h extreme value from measurements and the predicted 90% fractile extreme for relative wave 8
View Large  |  Save Figure  |  Download Slide (.ppt)
Plot of the 3 h extreme value from measurements and the predicted 90% fractile extreme for relative wave 8
Figure 16

Plot of the 3 h extreme value from measurements and the predicted 90% fractile extreme for relative wave 8

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