Showing papers on "Rogue wave published in 2002"

New method for numerical simulation of a nonstationary potential flow of incompressible fluid with a free surface

Abstract: New method for numerical simulation of potential flows with a free surface of two-dimensional fluid, based on combination of the conformal mapping and Fourier Transform is proposed. The method is efficient for study of strongly nonlinear effects in gravity waves including wave breaking and formation of rogue waves.

Extreme wave events in directional, random oceanic sea states

Abstract: We discuss the effect of the directional spreading on the occurrence of extreme wave events. We numerically integrate the envelope equation recently proposed by Trulsen et al. [Phys. Fluids 12, 2432 (2000)] as a weakly nonlinear model for realistic oceanic gravity waves. Initial conditions for numerical simulations are characterized by the spatial JONSWAP power spectrum for several values of the significant wave height, steepness, and directional spreading. We show that by increasing the directionality of the initial spectrum the appearance of extreme events is reduced.

Technical Note Analysis of freak wave measurements in the Sea of Japan

Abstract: This paper presents an analysis of a set of available freak wave measurements gatheredfrom several periods of continuous wave recordings made in the Sea of Japan during 1986–1990 by the Ship Research Institute of Japan. The analysis provides an ideal opportunity tocatch a glimpse of the statistics of freak waves in the ocean. The results show that a well-defined freak wave may occur in the developed wind–wave condition: S(f) f 4 , with single-peak directional spectra. The crest and trough amplitude distributions of the observed seawaves including freak waves are different from the Rayleigh distribution, although the waveheight distribution tends to agree with the Rayleigh distribution. Freak waves can be readilyidentified from the wavelet spectrum where a strong energy density occurs in the spectrum,and is instantly surged and seemingly carried over to the high-frequency components at theinstant the freak wave occurs. 2002 Elsevier Science Ltd. All rights reserved. Keywords: Freak wave; Wave statistics; Wave height distribution; Wavelet analysis; Sea of Japan* Corresponding author. Tel.: +81-471-821-181; fax:+81-471-847-142.E-mail address:mori@criepi.denken.or.jp (N. Mori).

On the characteristics of observed coastal freak waves

Abstract: The purpose of this study is to describe the characteristics of occurrence of coastal freak waves and to investigate their statistical and spectral structures. According to Ochi's definition of fre...

Time-Domain Investigation of a Semisubmersible in Rogue Waves

Abstract: Heave, pitch and roll motions as well as airgap are key characteristics of semisubmersibles in extreme seas which are defined by Ultimate Limit State design conditions (ULS) with a specified 100-year design wave height Hs and peak period Tp . The increasing number of reported rogue waves with unexpected large wave heights (Hmax /Hs > 2), crest heights (ζmax/ /Hmax > 0.6), wave steepness and group patterns (e.g. Three Sisters) may suggest a reconsideration of design codes by implementing an Accidental Limit State (ALS) with a return period of 104 years. For investigating the consequences of specific extreme sea conditions this paper analyses the seakeeping behaviour of a semisubmersible in a reported rogue wave, the Draupner New Year Wave embedded in irregular sea states. The numerical time-domain invegstigation using a panel method and potential theory is compared to frequency-domain results. In particular, the characteristics of the embedded rogue wave is varied to analyse the dynamic response of the semisubmersible in extreme wave sequences For validation, the selected sea condition is generated in a physical wave tank, and the sea-keeping behaviour of the semisubmersible is evaluated at model scale. In conclusion, the results deomstrate the consequences of rogue wave impacts, with respect to the relevance of present design methods and safety standards.Copyright © 2002 by ASME

Detection of extreme waves using synthetic aperture radar images

Abstract: Within the last years a considerable number of large ships have been lost due to severe sea state conditions. The cause of accidents are in many cases believed to be rogue waves, which are individual waves of exceptional wave height or abnormal shape. In particular steep breaking waves can be fatal for smaller ships. Damage is sometimes can also be caused by unusual grouping of waves, which can lead to dangerous ship motion. In situ measurements of extreme waves are sparse with most observations reported by ship masters after the encounter. In this paper a global data set of 5 /spl times/ 10 km sized synthetic aperture radar (SAR) images acquired by the European Remote Sensing satellite ERS-2 every 200 km along the track is used to analyse extreme ocean wave events. As the European Space Agency (ESA) does not provide this dataset as a standard product wave mode raw data were reprocessed to complex SAR images using the processor BSAR developed at the German Aerospace Center (DLR). About 1000 globally distributed SAR wave mode images are available every day. Two dimensional ocean wave fields are derived from SAR images by inversion of the SAR imaging mechanism. Individual high waves are detected in the derived wave fields using a matched filter technique. The inhomogeneity of ocean wave fields is analysed using a parameter, which describes the shift invariance of the wave spectrum.

Statistics for the Draupner January 1995 Freak Wave Event

Abstract: The wave time series recorded at “Draupner”, January 1, 1995 at 15:20 exhibits an unsymmetrical, freak wave event. The statistics of the peak event is studied within the framework of traditional non-Gaussian process models. Assuming that the sea surface is adequately described by (one of) commonly applied non-Gaussian wave models, the paper shows that the probability of occurrence of a freak event like the one observed at Draupner January 1995 is very low.Copyright © 2002 by ASME

Extreme Waves Detected by Satellite Borne Synthetic Aperture Radar

Abstract: Within the last 20 years at least 200 supercarriers have been lost, due to severe weather conditions. In many cases the cause of accidents is believed to be ‘rouge waves’, which are individual waves of exceptional wave height or abnormal shape. I situ measurements of extreme waves are scarce and most observations are reported by ship masters after the encounter. In this paper a global set of synthetic aperture radar (SAR) images is used to detect extreme ocean wave events. The data were acquired aboard the European remote sensing satellite ERS-2 every 200 km along the track. As the data are not available as a standard product of the Europea Space Agency (ESA), the radar raw data were focused to complex SAR images using the processor BSAR developed by the German Aerospace Center. The entire SAR data set covers 27 days representing 34000 SAR imagettes with a size of 5km×10km. Complex SAR data contain information on ocean wave height, propagation direction and grouping as well as on ocean surface winds. Combining all of this information allows to extract and locate extreme waves from complex SAR images on a global basis. Special algorithms have been developed to retrieve the following parameters from the SAR data: Wind speed and direction, significant wave height, wave direction, wave groups and their individual heights. The satellite ENVISAT launched in March 2002 acquires SAR data with an even higher sampling rate (every 100 km). It is expected that a long-term analysis of ERS and ENVISAT data will give new insight into the physical processes responsible for rogue wave generation. Furthermore, the identification of hot spots will contribute to the optimization of ship routes.© 2002 ASME

Detection of extreme waves using radar-image sequences

Abstract: A method is presented to localize wave groups spatially and spatio-temporally utilizing synthetic aperture radar (SAR) images and nautical radar-image sequences of the ocean surface. Extreme waves can grow in space and time as a result of wave group evolution. These wave groups have to be taken into account for instance for the design of offshore platforms, breakwaters or ships, because they can cause severe damage on those structures. To detect extreme waves, dominant wave groups are selected from SAR images and radar-image sequences by considering the wave envelope. A radar-image sequence is transformed into the wave-number frequency domain using a 3D Fourier transform where the signal of the ocean gravity waves is filtered using a band pass filter based on the dispersion relation for linear surface gravity waves. Thereafter, a 3D Hilbert transform is applied to the filtered complex Fourier coefficients, which are then transformed back into the spatio-temporal domain applying an inverse 3D Fourier transform. The resulting spatio-temporal complex envelope of the wave field is investigated for the dominant wave groups, by considering the amplitude of the complex envelope. With slight changes the algorithm can also be applied to single radar images. To test and verify the algorithm, several radar image sequences were acquired with the wave monitoring system WaMoS-II, which is based on a nautical radar operating in the X-band (9.5 GHz) near grazing incidence. The instrument was operated on towers in the North Sea. All these data sets are exploited with respect to the localization of extreme waves and wave groups.

Nonlinear Amplitude Dispersion Effects in Extreme Deep-Water Random Waves

Abstract: The connection between extreme waves and nonlinear group formation is described. A statistical relation is shown, and a physical explanation based on nonlinear dispersion is investigated. Theoretically it can be described by the modified nonlinear Schr6dinger equation. Here a simplified theoretical approach is made, in order to interpret laboratory experiments. The results show that the balance between the energy envelope and the instantaneous frequency is essential. Extremes from experiments with random waves show a behaviour similar to effects in bichromatic wave trains. INTRODUCTION During recent years, it has become reasonably well established that individual extreme ocean waves on deep water are underpredicted by linear theory. In particular, the largest crest heights in severe storm conditions may be typically up to 15 20% higher than predicted by Rayleigh theory. Results from full-scale observations (Vinje and Haver, 1994; Forristall, 1998) have shown that second-order random wave models are in most cases able to describe and predict this, and laboratory experiments show similar findings (Stansberg, 2000a). For the use of such wave models in practical engineering design, theoretical and numerical tools are already available from the literature today. However, there are still some rare observations, in field as well as in laboratory data, which appear to be clearly beyond these predictions. They are sometimes called "freak", "rogue" or "abnormal" waves, see e.g. ISSC (2000). Whether they are simply a natural result of statistical scatter, or whether they are generated by separate physical mechanisms, is not yet clearly understood, but there is a number of possible mechanisms that might explain these. One such mechanism is the nonlinear self-focusing of energetic wave groups in deep water random wave trains, as observed experimentally in Stansberg (2000b). These effects are related with the modulational wave instabilities predicted by use of the Modified I Dr. ing., Offshore Structures, Norwegian Marine Technology Research Institute A.S. (MARINTEK), N-7450 Trondheim, Norway. carltrygve.stansberg@marintek.sintef.no

Detection of Extreme Waves in SAR Images and Radar-Image Sequences

Abstract: Extreme waves are often enclosed by other waves which are also higher than the average. These wave groups have to be taken into account for instance for the design of offshore platforms, breakwaters or ships, because successive high waves can cause more damage on those structures than the same waves separated by smaller waves. Further they can excite the resonance frequencies of moored structures like platforms due to non-linear effects or cause capsize. They are therefore of interest for engineers and scientists (e.g. Goda 1983). A method is presented to localize wave groups spatial and spatio-temporal utilizing synthetic aperture radar (SAR) images and nautical radar-image sequences. The approach to detect wave groups is based on the detection of the wave envelope. It is assumed that the sea surface elevation can be treated as a Gaussian process. The method is applied to SAR images acquired by the European satellite ERS-1 and to radar-image sequences recorded by tower-based nautical radars. In contrast to 1D sensors like buoys the SAR records an image and gives therefore a 2D description of the sea surface by measuring the radar backscatter from the sea surface. The measurements taken by a nautical radar provide the possibility to record time series of images and therefore to get a 3D description of the sea surface. Radar-image sequences are acquired by recording the spatial and temporal evolution of the sea surface backscatter, which is modulated through the surface wave field. Nautical radar-image sequences allow to detect wave groups within a time span that makes it possible to start safety programs before the group reaches a platform. The existing data sets are exploited with respect to the recognition of extreme wave events.Copyright © 2002 by ASME

Fully Nonlinear Models of Wave Transformation in Coastal Areas

Abstract: Fully nonlinear two-dimensional (2D) and three-dimensional (3D) models of ocean wave generation and propagation, i.e., so-called "Numerical Wave Tanks" (NWT), have recently become powerful tools for engineering design and scientific research in coastal and littoral processes. NWTs are now used to carry out many investigations for which, until recently, only physical laboratory wave tanks could be used. Despite their necessary idealization of actual problems, NWTs are often advantageous because of their rapid set­ up time and use, and thus lower cost, as compared to laboratory experiments. This is particularly true if many configurations of the same problem are to be studied in order to identiry the optimal geometry or siting of, say, an offshore or a coastal structure. Also, in many cases, NWTs provide unperturbed access to hydrodynamic wave parameters, such as pressure, velocity and acceleration, which often can only be measured in the laboratory using invasive gages and probes. This keynote lecture will present an overview of the author's own and collaborative recent experience in developing and applying NWTs, mostly based on potential flow theory. The coupling of such NWTs with VOF-based Navier-Stokes solver will also be briefly discussed. Specific applications will include : (i) shoaling and breaking of long­ crested 2D swells over sandy beaches, with comparison to laboratory experiments (wave and partic1e velocity measurements); (ii) shoaling and plunging breaking of 3D waves with specifie analysis of velocity and acceleration in the plunging jet; (iii) generation and propagation of waves (tsunamis) caused by submarine mass failure, with comparison with field case studies and laboratory experiments; and (iv) 3D wave energy focusing leading to freak waves formation.