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 Coherence and predictability of extreme events in irregular waves

2012 ◽  
Vol 19 (2) ◽  
pp. 199-213 ◽  
Author(s):  
A. L. Latifah ◽  
E. van Groesen
Keyword(s):  
Nonlinear Effects ◽  
Exceptional Case ◽  
Irregular Waves ◽  
Time Signal ◽  
Freak Waves ◽  
Position Information ◽  
Large Wave ◽  
Freak Wave ◽  
First Case ◽  
Time Trace

Abstract. This paper concerns the description and the predictability of a freak event when at a certain position information in the form of a time signal is given. The prediction will use the phase information for an estimate of the position and time of the occurrence of a large wave, and to predict the measure of phase coherence at the estimated focussing position. The coherence and the spectrum will determine an estimate for the amplitude. After adjusting for second order nonlinear effects, together this then provides an estimate of the form of a possible freak wave in the time signal, which will be described by a pseudo-maximal signal. In the exceptional case of a fully coherent signal, it can be described well by a so-called maximal signal. We give four cases of freak waves for which we compare results of predictions with available measured (and simulated) results by nonlinear AB-equation (van Groesen and Andonowati, 2007; van Groesen et al., 2010). The first case deals with dispersive focussing, for which all phases are (designed to be) very coherent at position and time of focussing; this wave is nearly a maximal wave. The second case is the Draupner wave, for which the signal turns out to be recorded very close to its maximal wave height. It is less coherent but can be described in a good approximation as a pseudo-maximal wave. The last two cases are irregular waves which were measured at MARIN (Maritime Research Institute Netherlands); in a time trace of more than 1000 waves freak-like waves appeared "accidentally". Although the highest wave is less coherent than the other two cases, this maximal crest can still be approximated by a pseudo-maximal wave.

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.

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.

A Psychometric Examination of the Anagram Persistence Task: More Than Two Unsolvable Anagrams May Not Be Better

Assessment ◽  
2018 ◽  
Vol 27 (6) ◽  
pp. 1198-1212 ◽  
Author(s):  
Gilles E. Gignac ◽  
Ka Ki Wong
Keyword(s):  
Processing Speed ◽  
Test Performance ◽  
Convergent Validity ◽  
Response Times ◽  
Nonlinear Effects ◽  
Intelligence Test ◽  
Factorial Validity ◽  
Test Taking ◽  
Two Factors ◽  
Administration Time

The purpose of this investigation was to examine a single-anagram, a double-anagram, and multi-anagram versions of the Anagram Persistence Task (APT) for factorial validity, reliability, and convergent validity. Additionally, a battery of intelligence tests was administered to examine convergent validity. Based on an unrestricted factor analysis, two factors were uncovered from the 14 anagram (seven very difficult and seven very easy) response times: test-taking persistence and verbal processing speed. The internal consistency reliabilities for the single-anagram, double-anagram, and multi-anagram (seven difficult anagrams) measures were .42, .85, and .86, respectively. Furthermore, all three versions of the APT correlated positively with intelligence test performance ( r ≈ .22). However, the double-anagram and multi-anagram versions also evidenced negative, nonlinear effects with intelligence test performance ( r ≈ −.15), which suggested the possibility of testee adaptation. Taking psychometrics and administration time into consideration, simultaneously, the double-anagram version of the APT may be regarded as preferred.

Non-Gaussian Extreme Waves in the Central North Sea

1997 ◽  
Vol 119 (3) ◽  
pp. 146-150 ◽  
Author(s):  
J. Skourup ◽  
N.-E. O. Hansen ◽  
K. K. Andreasen
Keyword(s):  
Wave Height ◽  
North Sea ◽  
Upper Bound ◽  
Wave Crest ◽  
Extreme Waves ◽  
Freak Waves ◽  
Freak Wave ◽  
Wave Trains ◽  
Central North Sea ◽  
Crest Height

The area of the Central North Sea is notorious for the occurrence of very high waves in certain wave trains. The short-term distribution of these wave trains includes waves which are far steeper than predicted by the Rayleigh distribution. Such waves are often termed “extreme waves” or “freak waves.” An analysis of the extreme statistical properties of these waves has been made. The analysis is based on more than 12 yr of wave records from the Mærsk Olie og Gas AS operated Gorm Field which is located in the Danish sector of the Central North Sea. From the wave recordings more than 400 freak wave candidates were found. The ratio between the extreme crest height and the significant wave height (20-min value) has been found to be about 1.8, and the ratio between extreme crest height and extreme wave height has been found to be 0.69. The latter ratio is clearly outside the range of Gaussian waves, and it is higher than the maximum value for steep nonlinear long-crested waves, thus indicating that freak waves are not of a permanent form, and probably of short-crested nature. The extreme statistical distribution is represented by a Weibull distribution with an upper bound, where the upper bound is the value for a depth-limited breaking wave. Based on the measured data, a procedure for determining the freak wave crest height with a given return period is proposed. A sensitivity analysis of the extreme value of the crest height is also made.

Toward Limit State Design of Ships and Offshore Structures Under Impact Pressure Actions: A State-of-the-Art Review

2006 ◽  
Vol 43 (03) ◽  
pp. 135-145
Author(s):  
Jeom Kee Paik
Keyword(s):  
Limit State ◽  
Structural Response ◽  
Time History ◽  
Rate Sensitivity ◽  
Offshore Structures ◽  
Impact Pressure ◽  
Green Water ◽  
Limit State Design ◽  
Structural Capacity ◽  
Pressure Time

In design of ships and ship-shaped offshore units, issues related to impact pressure actions arising from sloshing, slamming, green water, or explosion are of particular concern. The structural response under impact pressure actions is quite different from that under static or quasistatic actions. It has been recognized that the limit state approach is a more rational basis for structural design and safety assessment where both "demand" (loads) and "capacity" (strength) must be accurately defined. For impact pressure action cases, the demand is associated with hydrodynamics areas, taking into account the characteristics of impact pressure-time history, and the structural capacity is associated with structural mechanics areas, considering geometric and material nonlinearities together with strain rate sensitivity. This paper reviews recent advances and trends toward future limit state design of ships and offshore structures under impact pressure actions.

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