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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

## Abstract

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.

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## Figures

Figure 1

Schematic of the problem (see 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.

Figure 3

Snapshot from the ocean basin tests of the Kristin semisubmersible

Figure 4

Power spectra, 24 h records

Figure 5

Amplitude distribution, relative wave 8, and 3 h realization

Figure 6

Amplitude distribution, relative wave 8, and 24 h realization

Figure 7

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

Figure 8

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

Figure 9

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

Figure 10

Optimal transformed plot, 24 h, and relative wave 1

Figure 11

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

Figure 12

Optimal transformed plot, 24 h, and relative wave 8

Figure 13

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

Figure 14

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

Figure 15

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

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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