Freak Wave Events Within the Second Order Wave Model

2004 ◽  
Author(s):  
Elzbieta M. Bitner-Gregersen ◽  
O̸istein Hagen
Keyword(s):  
Wave Model ◽  
Wave Theory ◽  
Rogue Waves ◽  
Offshore Structures ◽  
Second Order ◽  
Short Term ◽  
Freak Waves ◽  
Freak Wave ◽  
Offshore Industry

Recently significant interest has been paid to abnormal waves, often called rogue waves or freak waves. These waves represent operational risks to ship and offshore structures, and are likely to be responsible for a number of accidents. As shown by several authors, in ‘the second order world’ the freak waves are pretty rare events. The present study focuses on statistical properties of freak waves. The analyses are based on second order time domain simulations, short term distributions for crest statistics obtained from the literature, and long term field data. Time series of wave elevations are generated using the Pierson-Moskowitz, JONSWAP and two-peak Torsethaugen frequency spectrum for long-crested seas and deep water. Effects of combined seas (swell and wind sea) on wave statistics are discussed. Assuming 2nd order wave theory, the short term and long term probability of occurrence of a freak wave is estimated. The difference between a “freak wave” and a “dangerous wave” is pointed out. Finally, 100 year and 10000 year crest events obtained by analysis procedures used in the offshore industry are discussed in relation to freak waves.

Reconstruction and Analysis of Freak Waves Generated From Unidirectional Random Waves

2020 ◽  
Vol 142 (4) ◽  
Author(s):  
Yuxiang Ma ◽  
Changfu Yuan ◽  
Congfang Ai ◽  
Guohai Dong
Keyword(s):  
Time Series ◽  
Extreme Event ◽  
Wave Model ◽  
Rogue Waves ◽  
Random Wave ◽  
Random Waves ◽  
Long Distance ◽  
Freak Waves ◽  
Freak Wave ◽  
Wave Flume

Abstract The generation of two freak waves in a broadband and a narrowband random series registered in the experiments of Li, J. X., Li, P. F., and Liu, S. X. (2013, “Observations of Freak Waves in Random Wave Field in 2D Experimental Wave Flume,” China Ocean Eng., 27(5), pp. 659–670) is precisely reconstructed using a fully non-hydrostatic water wave model. The simulation results indicate that even when the background spectral bandwidths are different, the evolution processes of the two freak waves are similar. Both freak waves emerge quickly during the transition from normal states to extreme events. The freak waves can persist over a long distance, i.e., approximately 5 peak wavelengths. The reconstructed time series in both the backward and forward locations at which the freak waves were recorded reveal that the largest freak wave crests were not captured in the experiment. The freak waves gradually emerged from an intense wave group. The waves developed quickly during the transition from a normal state to an extreme event. Very deep troughs were also formed in the evolution process. The two freak waves were actually generated via different spectral bandwidth processes, but the generation mechanisms of the rogue waves were similar. By analyzing the time series of the freak wave groups, the formation of the freak waves is found to result from the combined effect of the dispersive focusing, the third-order resonant wave interactions, and the higher harmonics.

scholarly journals Dynamic analysis of a tension leg platform under extreme waves

2013 ◽  
Vol 10 (1) ◽  
pp. 59-68 ◽  
Author(s):  
Srinivasan Chandrasekaran ◽  
Koshti Yuvraj
Keyword(s):  
Wave Structure ◽  
Wave Model ◽  
Offshore Structures ◽  
Extreme Waves ◽  
Sea State ◽  
Freak Waves ◽  
Freak Wave ◽  
Offshore Installations ◽  
Wave Directionality ◽  
Type Response

Recent observations of the sea state that result in the undesirable events confirm the presence of extreme waves like freak waves, which is capable of causing irreparable damages to offshore installations and (or) create inoperable conditions to the crew on board. Knowledge on the extreme wave environment and the related wave-structure interaction are required for safer design of deep-water offshore structures. In the current study, typical long crested extreme waves namely:  i) New Year wave at offshore Norway; and ii) Freak wave at North Sea are simulated using the combined wave model. Dynamic response of the Tension Leg Platforms (TLP) under these extreme waves is carried out for different wave approach angles. Based on the analytical studies cared out, it is seen that the TLPs are sensitive to the wave directionality when encountered by such extreme waves; ringing type response is developed in TLPs which could result in tether pull out.DOI: http://dx.doi.org/10.3329/jname.v10i1.14518

Long-Term Area Statistics for Maximum Crest Height Under a Fixed Platform Deck

2018 ◽  
Author(s):  
Øistein Hagen ◽  
Jørn Birknes-Berg ◽  
Ida Håøy Grue ◽  
Gunnar Lian ◽  
Kjersti Bruserud ◽  
...  
Keyword(s):  
Time Series ◽  
North Sea ◽  
Extreme Values ◽  
Second Order ◽  
Irregular Waves ◽  
Scatter Diagram ◽  
Short Term ◽  
Wave Impact ◽  
Crest Height

As offshore reservoirs are depleted, the seabed may subside. Furthermore, the extreme crests estimates are now commonly higher than obtained previously due to improved understanding of statistics of non-linear irregular waves. Consequently, bottom fixed installations which have previously had sufficient clearance between the deck and the sea surface may be in a situation where wave impact with the deck must be considered at relevant probability levels. In the present paper, we investigate the long-term area statistics for maximum crest height under a fixed platform deck for 2nd order short crested and long crested sea based on numerical simulations as a function of platform deck dimension for jackets. The results are for one location in the northern North Sea, but some key results are also reported and verified for a more benign southern North Sea location. Time domain simulations for long crested and short crested waves over a spatial domain with dimension of a platform deck are performed, and relevant statistics for airgap assessment determined. Second order waves are simulated for the different cells in the (Hs, Tp) scatter diagram for Torsethaugen two-peak wave spectrum for long-crested and short-crested sea. A total of 1000 3-hour sea states are generated per cell, and time series generated for 160 spatial points under a platform deck. Short-term and long-term statistics are established for the maximum crest height as function of platform dimension; inline and transverse to the wave direction, and over the area. Results are given for the linear sea and for the second order time series. The annual q-probability estimates for the maximum crest height over area as a function of platform dimension is determined for a location at the Norwegian Continental Shelf by weighting the short-term statistics for the individual cells in the scatter diagram with the long-term probability of occurrence of the sea state. To reduce the number of numerical second order simulations, the effect of excluding cells that have a negligible effect on the long term extreme crest estimate is discussed. The percentiles in the distribution of maximum crest (over area) in design sea states that corresponds to the extreme values obtained from the long-term analysis are determined for long crested and short crested sea. The increase in the extreme crest over an area compared to the point in space estimate is estimated for both linear and second order surface elevation.

Can swell increase the number of freak waves in a wind sea?

2010 ◽  
Vol 650 ◽  
pp. 57-79 ◽  
Author(s):  
ODIN GRAMSTAD ◽  
KARSTEN TRULSEN
Keyword(s):  
Nonlinear Evolution ◽  
Wind Waves ◽  
Short Wave ◽  
Freak Waves ◽  
Freak Wave ◽  
Short Waves ◽  
Term Evolution ◽  
Wind Sea ◽  
Modified Equation

The effect of a swell on the statistical distribution of a directional short-wave field is investigated. Starting from Zakharov's spectral formulation, we derive a new modified nonlinear Schrödinger equation appropriate for the nonlinear evolution of a narrow-banded spectrum of short waves influenced by a swell. The swell-modified equation is solved analytically to yield an extended version of the result of Longuet-Higgins & Stewart (J. Fluid Mech., vol. 8, no. 4, 1960, pp. 565–583) for the modulation of a short wave riding on a longer wave. Numerical Monte Carlo simulations of the long-term evolution of a spectrum of short waves in the presence of a monochromatic swell are employed to extract statistical distributions of freak waves among the short waves. We find evidence that a realistic short-crested wind sea can on average experience a small increase in freak wave probability because of a swell provided the swell is not orthogonal to the wind waves. For orthogonal swell and wind waves we find evidence that there is almost no significant change in the probability of freak waves in the wind sea. If the short waves are unrealistically long crested, such that the Benjamin–Feir index serves as indicator for freak waves (Gramstad & Trulsen, J. Fluid Mech., vol. 582, 2007, pp. 463–472), it appears that the swell has much smaller relative influence on the probability of freak waves than in the short-crested case.

Loads on Ships and Offshore Structures

2007 ◽  
Author(s):  
P. Temarel
Keyword(s):  
State Of The Art ◽  
Fatigue Loading ◽  
Rogue Waves ◽  
Offshore Structures ◽  
Green Water ◽  
Vortex Induced Vibrations ◽  
Ice Loads ◽  
Offshore Industry ◽  
Multi Body ◽  
Wave Induced

The Loads Committee of the International Ship and Offshore Structures Congress (ISSC) critically reviews the state of the art of environmental and operational loads. Amongst these, elements more relevant to the offshore industry will be presented in this paper. These comprise wave-induced loads, including linear and nonlinear methods, multi-body interactions, slamming, green water, sloshing and rogue waves, cables and risers, vortex-induced vibrations, ice loads, fatigue loading and, verification and validation.

Characteristic Levels of Strongly Nonlinear Extreme Wave Load Effects

2016 ◽  
Author(s):  
Thomas B. Johannessen ◽  
Øistein Hagen
Keyword(s):  
Wave Breaking ◽  
Nonlinear Problems ◽  
Offshore Structures ◽  
Short Term ◽  
Nonlinear Load ◽  
Load Effect ◽  
Strongly Nonlinear ◽  
Load Effects ◽  
Term Analysis

Offshore structures are typically required to withstand extreme and abnormal load effects with annual probabilities of occurrence of 10−2 and 10−4 respectively. For linear or weakly nonlinear problems, the load effects with the prescribed annual probabilities of occurrence are typically estimated as a relatively rare occurrence in the short term distribution of 100 year and 10 000 year seastates. For strongly nonlinear load effects, it is not given that an extreme seastate can be used reliably to estimate the characteristic load effect. The governing load may occur as an extremely rare event in a much lower seastate. In attempting to model the load effect in an extreme seastate, the short term probability level is not known nor is it known whether the physics of the wave loading is captured correctly in an extreme seastate. Examples of such strongly nonlinear load effects are slamming loads on large volume offshore structures or wave in deck loads on jacket structures subject to seabed subsidence. Similarly, for structures which are unmanned in extreme weather, the governing load effects for the manned structure will occur as extremely rare events in a relatively frequent seastate. The present paper is concerned with the long term distribution of strongly nonlinear load effects. Using a simple point estimate of the wave elevation correct to second order and a crest kinematics model which takes into account the possibility of wave breaking, the long term distribution of drag load on a column above the still water level is studied and compared with a similar loading model based on second order kinematics which does not include the effect of wave breaking. The findings illustrate the challenges listed above. Model tests are useful in quantifying strongly nonlinear load effects which cannot be calculated accurately. But only a relatively small number of seastates can be run in a model test campaign and it is not feasible to estimate short term responses far beyond the three hour 90% fractile level. Similarly, Computational Fluid Dynamics (CFD) is increasingly useful in investigating complex wave induced load effects. But only a relatively small number of wave events can be run using CFD, a long term analysis of load effects cannot in general be carried out. It appears that there is a class of nonlinear problems which require a long term analysis of the load effect in order for the annual probability of occurrence to be estimated accurately. For problems which cannot be estimated by simple analytical means, the governing wave events can be identified by long term analysis of a simple model which capture the essential physics of the problem and then analysed in detail by use of CFD or model tests.

scholarly journals Freak waves in 2005

2006 ◽  
Vol 6 (6) ◽  
pp. 1007-1015 ◽  
Author(s):  
I. I. Didenkulova ◽  
A. V. Slunyaev ◽  
E. N. Pelinovsky ◽  
C. Kharif
Keyword(s):  
Mass Media ◽  
Rogue Waves ◽  
Wave Impact ◽  
Freak Waves ◽  
Freak Wave ◽  
Open Sea

Abstract. Information about freak wave events in the ocean reported by mass media and derived from personal observations in 2005 is collected and analysed. Nine cases are selected as true freak wave events from a total number of 27 mentioned. Besides rogue waves in the open sea, the problem of freak wave events on the shore is emphasized. These accidents are related to unexpected wave impact upon the coast and shore constructions or to sudden intensive flooding of the coast. Of the nine events considered reliable here, three events correspond to open-sea cases, while the six others occurred nearshore.

Nonlinear Effects on Local Mechanics of Freak Waves

2015 ◽  
Author(s):  
Wataru Fujimoto ◽  
Takuji Waseda
Keyword(s):  
Limit State ◽  
Nonlinear Effects ◽  
Offshore Structures ◽  
Freak Waves ◽  
Freak Wave ◽  
Wave Kinematics ◽  
Longitudinal Asymmetry ◽  
Two Factors ◽  
Structural Impacts ◽  
Particle Velocities

The local properties of freak waves, such as geometry and particle velocities, are still to be investigated and are essential in the limit state design of ships or offshore structures. We have focused on two factors for this research. The first is nonlinearity higher than 3rd order since local steepness around freak waves will be large. The 4th order nonlinearity deforms a perturbed regular wave like a crescent and also causes a longitudinal asymmetry that means that the shape of the wave is asymmetrical in the propagation direction. The second is higher wavenumber components that will increase particle velocities. We tried to research the effects of these two factors on freak waves with Higher Order Spectral Method. Consequently, a crescent shape and longitudinal asymmetry in freak wave shape were found. In addition, higher wavenumber components increased the maximum horizontal velocity of freak waves significantly. These results show that the 4th order nonlinearity and higher wavenumber components are important for local freak wave kinematics, as well as for determining structural impacts, the motion of floating objects, and wave breaking.

Long Term Analysis of Steep and Breaking Wave Properties by Event Matching

2018 ◽  
Author(s):  
Thomas B. Johannessen ◽  
Øystein Lande
Keyword(s):  
Wave Breaking ◽  
Offshore Structures ◽  
Return Periods ◽  
Short Term ◽  
Nonlinear Load ◽  
Load Effect ◽  
Strongly Nonlinear ◽  
Load Effects ◽  
Term Analysis

Offshore structures are typically required to withstand extreme and abnormal load effects with annual probabilities of occurrence of 10−2 and 10−4 respectively. For linear or weakly nonlinear problems, the load effects with the prescribed annual probabilities of occurrence are typically estimated as a relatively rare occurrence in the short term distribution of 100 year and 10 000 year seastates. For strongly nonlinear load effects, it is not given that an extreme seastate can be used reliably to estimate the characteristic load effect. The governing load may occur as an extremely rare event in a much lower seastate. In attempting to model the load effect in an extreme seastate, the relevant short term probability level is not known nor is it known whether the physics of the wave loading is captured correctly in an extreme seastate. Examples of such strongly nonlinear load effects are slamming loads on large volume offshore structures or wave in deck loads on jacket structures subject to seabed subsidence. The present paper is concerned with the long term distribution of strongly nonlinear load effects and a methodology is proposed which incorporates CFD analysis in a long term Monte Carlo analysis of crest elevations and wave kinematics. Based on a long term time domain simulation of a linear surface elevation, a selection of events is run in CFD in order to obtain a database of linear and corresponding fully nonlinear wave fields with the possibility of wave breaking included. In the subsequent long term analysis, a large linear event is then replaced by the closest matching event in the database. A technique is developed to Froude scale the database results and shift the origin in time and plane so that the database of typically only 100 events give a close match to all the events in the simulation. The method is applied to the simple case of drag loading on a cylinder which is truncated above the still water level such that only the largest waves impact with the structure. It is observed that whereas the Event Matching method agree well with a second order model for return periods lower than 100 years, the loading on the cylinder is significantly larger for longer return periods. The deviation is caused by the increasing dominance of wave braking in the largest crest and illustrates the importance of incorporating wave breaking in the analysis of wave in deck loading problems.

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