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Research Papers

Systematically Varied Rogue Wave Sequences for the Experimental Investigation of Extreme Structure Behavior

[+] Author and Article Information
Günther F. Clauss

Ocean Engineering Section, Technical University Berlin, 10587 Berlin, Germanyclauss@naoe.tu-berlin.de

Christian E. Schmittner

 MARIN, Wageningen 6700 AA, The Netherlandsc.schmittner@marin.nl

Janou Hennig

 MARIN, Wageningen 6700 AA, The Netherlandsj.hennig@marin.nl

J. Offshore Mech. Arct. Eng 130(2), 021009 (Jun 09, 2008) (11 pages) doi:10.1115/1.2913598 History: Received September 29, 2006; Revised March 01, 2008; Published June 09, 2008

For an improved design of ships and offshore structures with regard to their behavior under severe weather conditions, wave height and steepness as well as the shape of the wave profile have to be considered. In this paper, the extreme new year wave as documented in numerous publications is varied with respect to wave height and period. These varied wave sequences are realized and measured in a model tank and applied to the investigation of motions and bending moments of a floating production storage and offloading ship. The results are compared to the responses in the original wave train. An investigation of the riskiness of extreme wave sequences in comparison with existing rules concludes this paper.

Copyright © 2008 by American Society of Mechanical Engineers
Topics: Waves , Wave packets
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References

Figures

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Figure 1

Record of a rogue wave, the so-called new year wave, measured in the North Sea at the Draupner jacket platform on Jan. 1, 1995 (6), Hs=11.92m, Hmax=25.63m=2.15Hs, ζc=18.5m=0.72Hmax, and water depth d=70m.

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Figure 2

Record of rogue wave measured at north alwyn on Nov. 19, 1997, Hs=8.64m, Hmax=22.03m=2.55Hs, Tp=13.11s, and d=126m. From statistics, such an extreme wave with Hmax=2.55Hs would be expected in a storm lasting 51days.

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Figure 3

Analysis of a storm recorded at North Alwyn on Nov. 16–21st, 1997, maximum Hs=10.67m, maximum Hmax=22.03m, maximum Hmax∕Hs=2.55 (20minute records). Within 5days, 21 waves with Hmax∕Hs>2 are observed, confirming that rogue waves are rather normal events.

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Figure 4

Process of wave generation: Calculation starts from the desired target wave train, defined by particular parameters (1). Proper modeling wave propagation, the wave train at the position of the wave maker (2), as well as the corresponding wave maker control signals (3) are calculated. The resultant wave train can be measured at the target position (4) and compared to the given target wave (5).

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Figure 5

Linear wave elevation with envelope (above) and corresponding Stokes third order wave packet (below) as well as related Fourier spectra (smooth curve: linear wave train)

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Figure 6

Transient wave packet measured close to the wave board at x=8.82m and corresponding envelope calculated by Hilbert transform: Linear wave theory is still applicable for its description

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Figure 7

Transient wave packet at x=85.03m: Comparison of registration with linear calculation (linear transformation from x=8.82m—see Fig. 6) illustrates that linear wave theory gives inaccurate results

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Figure 8

Calculation of wave numbers kij(ωj,a(ti)) as function of the instantaneous wave envelope ai at time step i for one spatial step xl. Propagation velocity cij=ωj∕kij increases with ‘wave amplitude’ ai (see Eqs. 10,11).

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Figure 9

Transformation of wave packet in Fig. 6(x=8.82m) to position x=80.03m using the described nonlinear calculation procedure (Iteration Step 105). Although the transformation comprises more than 76m, the result agrees very well with the registration.

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Figure 10

Schematic view of flap and piston type wave makers

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Figure 11

Coordinate system for modeling the wave maker

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Figure 12

Biésel transfer function for wave generation with a flap type wave maker (lower (blue) graph, b=3.44m, d=5.6m) and a piston type wave maker (upper (red) line, d=1.5m)

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Figure 13

Comparison of model wave (measured at scale 1:81) with the registered new year wave, presented as full scale data

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Figure 14

Wave tank evolution of the new year wave, target position at xtarget=28.95m

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Figure 15

Segmented model of the FPSO during tests in the wave tank (scale 1:81)

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Figure 16

New year wave (black), new year wave with elongated local period (blue), and new year wave with elongated local period and increased local wave height (red) as well as resultant heave and pitch motion and vertical bending moments at midship and at quarter length from the bow. The wave history is kept unchanged, thus enabling the relation of changes in response to the modifications introduced into the new year wave.

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