Reproduction of Freak Waves Using Variational Data Assimilation and Observation

2018 ◽  
Author(s):  
Wataru Fujimoto ◽  
Takuji Waseda
Keyword(s):  
Data Assimilation ◽  
Variational Method ◽  
Wave Field ◽  
Wave Spectrum ◽  
Offshore Structures ◽  
Estimation Accuracy ◽  
Wave Impact ◽  
Freak Waves ◽  
Freak Wave ◽  
Squared Error

This study proposes ideas to reproduce freak waves from observational data. The reproduced data will apply to investigations on freak wave impact to offshore structures. Four-dimensional variational method (4DVAR) was used for the freak wave reproduction. Under a dynamical constraint, 4DVAR minimizes the squared error between observation and model prediction by adjusting the initial condition iteratively. This study utilizes the Higher Order Spectral Method (HOSM) to predict the nonlinear wave evolution, which is essential for freak wave generation. Information on wave spectrum estimated beforehand by a wave model is also employed to stabilize the reproduction. To increase convergence speed with fewer efforts of coding, a type of ensemble-based variational method (a4dVar) was adopted. The a4dVar performs perturbed ensemble simulations to evaluate the gradient of the squared error and is easy to parallelize and implement. This paper conducted twin experiments of HOSM+a4dVar data assimilation. HOSM model generated the true state of the uni-directional wave field, and the spatiotemporal wave field was reconstructed from time series of one virtual wave gauge located in the model. It is assumed that the virtual wave gauge detected a freak wave. The estimation accuracy of linear estimation and HOSM estimation were compared.

scholarly journals 3-D HOS simulations of extreme waves in open seas

2007 ◽  
Vol 7 (1) ◽  
pp. 109-122 ◽  
Author(s):  
G. Ducrozet ◽  
F. Bonnefoy ◽  
D. Le Touzé ◽  
P. Ferrant
Keyword(s):  
Wave Spectrum ◽  
Original Study ◽  
Test Cases ◽  
Extreme Waves ◽  
Extreme Wave ◽  
Freak Waves ◽  
Freak Wave ◽  
Directional Spectrum ◽  
Long Time ◽  
Directional Wave

Abstract. In the present paper we propose a method for studying extreme-wave appearance based on the Higher-Order Spectral (HOS) technique proposed by West et al. (1987) and Dommermuth and Yue (1987). The enhanced HOS model we use is presented and validated on test cases. Investigations of freak-wave events appearing within long-time evolutions of 2-D and 3-D wavefields in open seas are then realized, and the results are discussed. Such events are obtained in our periodic-domain HOS model by using different kinds of configurations: either i) we impose an initial 3-D directional spectrum with the phases adjusted so as to form a focused forced event after a while, or ii) we let 2-D and 3-D wavefields defined by a directional wave spectrum evolve up to the natural appearance of freak waves. Finally, we investigate the influence of directionality on extreme wave events with an original study of the 3-D shape of the detected freak waves.

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.

The Relationship Between the Shape of Freak Waves and Nonlinear Wave Interactions

2016 ◽  
Author(s):  
Wataru Fujimoto ◽  
Takuji Waseda
Keyword(s):  
Limit State ◽  
Offshore Structures ◽  
Class I ◽  
Finite Width ◽  
Wave Interactions ◽  
Freak Waves ◽  
Freak Wave ◽  
Irregular Wave ◽  
The Relationship ◽  
Different Order

The local shapes of freak waves are essential to estimate responses of ships or offshore structures by freak waves for limit state design or maritime accident survey. It is known that freak waves deform like a crescent and their trough depth become asymmetric in directional and irregular wave fields. Meanwhile, Class I & II instabilities also affect wave shape. We discussed how those instabilities affect the geometry of freak waves, using Higher Order Spectrum Method (HOSM) which is a fast simulator of water wave. This paper investigated the relationship between Class I & II instabilities and the nonlinear order of HOSM to separate the effects of the different order nonlinear instabilities on freak waves. This investigation and freak wave simulations by HOSM clarified that four-wave Class I instability with finite width wave spectra affected both the crescent deformation and the asymmetry. The results showed that Class II instability effects to the freak wave shapes were not significant.

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

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.

scholarly journals Numerical Analysis of Wave Characteristic In the Freak Wave-- “New Year Wave” Formation

2021 ◽  
Vol 290 ◽  
pp. 02013
Author(s):  
Yu Xiang-jun ◽  
Li Qing-hong ◽  
Li Mao-lin
Keyword(s):  
Wave Spectrum ◽  
Random Wave ◽  
Small Range ◽  
Essential Factor ◽  
Transient Time ◽  
Freak Waves ◽  
Large Wave ◽  
Freak Wave ◽  
The North ◽  
The North Sea

Freak waves are both extremely large waves and highly transient time. Such a wave may lead to damage of ships to deaths. In this paper, to describe the connection between freak wave and wave essential factor, we use WAVEWATCH III model simulating “New Year Wave” in the North Sea to explore freak wave, with the importing of ECMWF re-analysis wind field. By this way, we successfully simulate the formation of freak wave in the random wave. Analysis shows large wave steepness and small directional spread angle are necessary conditions for freak waves to easily occur. By analyzing the wave spectrum, it is found that the wave energy is distributed in a small range, and the propagation direction is relatively concentrated.

Qualification Criteria and the Verification of Numerical Waves: Part 2: CFD-Based Numerical Wave Tank

2021 ◽  
Author(s):  
Benjamin Bouscasse ◽  
Andrea Califano ◽  
Young Myung Choi ◽  
Xu Haihua ◽  
Jang Whan Kim ◽  
...  
Keyword(s):  
Wave Spectrum ◽  
Vertical Direction ◽  
Offshore Structures ◽  
Irregular Waves ◽  
Numerical Wave Tank ◽  
Regular Waves ◽  
Wave Impact ◽  
Wave Tank ◽  
Modeling Practices ◽  
Numerical Wave

Abstract There is increasing interest in numerical wave simulations as a tool to design offshore structures, especially for the prediction of stochastic nonlinear wave loads like those related to air-gap and wave impact. Though the simulations cannot replace all experiments, they are now competitive on some topics such as the computations of wind and current coefficients. To proceed further it is necessary to improve the procedure to account for another complex environmental factor, wave motion. This paper addresses an industrial collaboration to develop modeling practices and qualification criteria of CFD-based numerical wave tank for offshore applications. As a part of the effort to develop reliable numerical wave modeling practices in the framework of the “Reproducible Offshore CFD JIP”, qualification criteria are formulated for the wave solutions generated from either potential-flow based codes in Part 1 of this work. Part 2 presents first a set of solutions for forcing the qualified waves obtained with the potential codes in the CFD domain. Those solutions follow a set of coupling protocols previously proposed in the JIP framework. Two potential codes and two CFD solvers are combined, so that four possible methods of generating waves and modalities are described. Two different potential models are considered, one using the higher order spectral method for numerical wave tank (HOS-NWT), and another using the finite-element method in the horizontal direction and a modal expansion after a sigma transform in the vertical direction (solver is called TPNWT). Both are equipped with a breaking model to generate extreme sea states. The two CFD solvers tested are Simcenter STAR-CCM+ and OpenFOAM. Simulation setups are proposed for both software. Simulation results from eight academic or industrial partners are presented for two sets of 2D test cases in deep water, one with regular waves and one with irregular waves, both with one very steep condition (ratio of wave height over wavelength of 10% for regular waves and 1000 year return period for Gulf of Mexico for irregular waves). The irregular waves are simulated for 10 sets of 3 hours to apply a stochastic approach to verify the quality of the waves generated in the numerical domain. Attention is given to the wave spectrum and the ensemble probability of the crest distribution, both obtained from the wave elevation at the center of the domain.

A CFD Study of Focused Extreme Wave Impact on Decks of Offshore Structures

2014 ◽  
Author(s):  
Xin Lu ◽  
Pankaj Kumar ◽  
Anand Bahuguni ◽  
Yanling Wu
Keyword(s):  
Real World ◽  
Offshore Structures ◽  
Air Gap ◽  
Irregular Waves ◽  
Linear Wave ◽  
Extreme Wave ◽  
Wave Impact ◽  
Loading Test ◽  
Wide Range ◽  
Wave Maker

The design of offshore structures for extreme/abnormal waves assumes that there is sufficient air gap such that waves will not hit the platform deck. Due to inaccuracies in the predictions of extreme wave crests in addition to settlement or sea-level increases, the required air gap between the crest of the extreme wave and the deck is often inadequate in existing platforms and therefore wave-in-deck loads need to be considered when assessing the integrity of such platforms. The problem of wave-in-deck loading involves very complex physics and demands intensive study. In the Computational Fluid Mechanics (CFD) approach, two critical issues must be addressed, namely the efficient, realistic numerical wave maker and the accurate free surface capturing methodology. Most reported CFD research on wave-in-deck loads consider regular waves only, for instance the Stokes fifth-order waves. They are, however, recognized by designers as approximate approaches since “real world” sea states consist of random irregular waves. In our work, we report a recently developed focused extreme wave maker based on the NewWave theory. This model can better approximate the “real world” conditions, and is more efficient than conventional random wave makers. It is able to efficiently generate targeted waves at a prescribed time and location. The work is implemented and integrated with OpenFOAM, an open source platform that receives more and more attention in a wide range of industrial applications. We will describe the developed numerical method of predicting highly non-linear wave-in-deck loads in the time domain. The model’s capability is firstly demonstrated against 3D model testing experiments on a fixed block with various deck orientations under random waves. A detailed loading analysis is conducted and compared with available numerical and measurement data. It is then applied to an extreme wave loading test on a selected bridge with multiple under-deck girders. The waves are focused extreme irregular waves derived from NewWave theory and JONSWAP spectra.

scholarly journals EXTREME WAVE PRESSURES AND LOADS ON A PILE-SUPPORTED WHARF DECK - INFLUENCES OF AIR GAP AND WAVE DIRECTION

2018 ◽  
pp. 5
Author(s):  
Andrew Cornett
Keyword(s):  
Offshore Structures ◽  
Model Tests ◽  
Scale Model ◽  
Extreme Waves ◽  
Wave Direction ◽  
Coastal Structures ◽  
Extreme Wave ◽  
Wave Impact ◽  
The Past ◽  
Water Depths

Many deck-on-pile structures are located in shallow water depths at elevations low enough to be inundated by large waves during intense storms or tsunami. Many researchers have studied wave-in-deck loads over the past decade using a variety of theoretical, experimental, and numerical methods. Wave-in-deck loads on various pile supported coastal structures such as jetties, piers, wharves and bridges have been studied by Tirindelli et al. (2003), Cuomo et al. (2007, 2009), Murali et al. (2009), and Meng et al. (2010). All these authors analyzed data from scale model tests to investigate the pressures and loads on beam and deck elements subject to wave impact under various conditions. Wavein- deck loads on fixed offshore structures have been studied by Murray et al. (1997), Finnigan et al. (1997), Bea et al. (1999, 2001), Baarholm et al. (2004, 2009), and Raaij et al. (2007). These authors have studied both simplified and realistic deck structures using a mixture of theoretical analysis and model tests. Other researchers, including Kendon et al. (2010), Schellin et al. (2009), Lande et al. (2011) and Wemmenhove et al. (2011) have demonstrated that various CFD methods can be used to simulate the interaction of extreme waves with both simple and more realistic deck structures, and predict wave-in-deck pressures and loads.

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