Effect of Spectrum Tail Length on Modulational Instability and Freak Wave Occurrence in JONSWAP Sea States

2019 ◽  
Author(s):  
Cagil Kirezci ◽  
Alexander V. Babanin
Keyword(s):  
High Frequency ◽  
Modulational Instability ◽  
Wave Spectrum ◽  
Tail Length ◽  
Shape Parameters ◽  
Extreme Wave ◽  
Freak Wave ◽  
Nonlinear Phase ◽  
Wave Forecasting ◽  
Quantitative Indicators

Abstract In this study, probability of freak wave occurrence due to modulational instability in JONSWAP sea states are investigated. This investigation has been conducted based on the quantitative indicators of instability in wave spectrum, which are two Benjamin-Feir index (BFI) [1,2] with different spectral bandwidth definitions and Π number [3]. Evolution of wave field are simulated using fully nonlinear phase-resolving Chalikov-Sheinin (CS) numerical model [4,5]. Initial sea surface is controlled with JONSWAP shape parameters (α and γ) and random initial phases. Effect of high frequency end of spectrum on modulational instability and freak wave evolution are discussed by considering 4 different tail lengths. According to simulation results, all parameters that are considered here perform as an indicator for the occurrence of extreme events which makes it possible to define a certain interval for indicators, where freak wave occurrence probability is the highest and potentially dangerous, to be possibly used in extreme wave forecasting. Another key finding is that, modulational instability increases when high frequency part of spectrum is present (longer tail) as expected. Nevertheless, after certain nonlinearity, modulational instability is more prone to result in breaking which significantly decreases the probability of occurrence of freak events. Therefore, spectra with shorter tail length result in more dangerous sea states.

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.

Wave spectrum analysis for extreme wave oscillation inside Paradip port

2019 ◽  
Author(s):  
Gulshan ◽  
Prashant Kumar ◽  
Prashant Patel ◽  
Rupali ◽  
Sukhwinder Kaur
Keyword(s):  
Spectrum Analysis ◽  
Wave Spectrum ◽  
Extreme Wave ◽  
Wave Oscillation

Flexible Riser Response Induced by Springing of an FPSO Hull

2007 ◽  
Author(s):  
Jelena Vidic-Perunovic ◽  
Niels J. Risho̸j Nielsen ◽  
Haiwen Zhang
Keyword(s):  
High Frequency ◽  
Wave Spectrum ◽  
Analytical Form ◽  
Second Order ◽  
Wave Frequency ◽  
Natural Vibration Frequency ◽  
Linear Wave ◽  
Flexible Riser ◽  
Excitation Wave ◽  
Non Linear

The hydrodynamic analysis of the flexible riser for offshore application is usually limited to the first order wave frequency motions of the floating vessel that holds the riser top end. In this paper effort is made to investigate the influence of non-linear second order springing deflection of the production vessel hull on flexible riser response. The system selected in this study consists of a free-hanging flexible riser configuration attached to an FPSO. Due to resonance between the excitation wave frequency and the natural vibration frequency of the hull, second order flexible vertical motions of the FPSO increase. This may influence the riser loads, presumably the tension force. Vertical motions including the second order high frequency contribution are assigned to the flexible riser at a point of attachment to the vessel. To account for the environmental loading, irregular sea is applied, characterized by modified linear wave spectrum. Second order excitation wave spectrum is truncated by use of WAFO routines for random second order wave simulation and an analytical form of the spectrum that accounts for the non-linear wave effects is proposed. Several environmental conditions are examined in order to consolidate the tendency in riser behaviour. The significance of the high-frequency quadratic terms in the loads along the flexible riser is discussed.

scholarly journals Wave spectral shapes in the coastal waters based on measured data off Karwar, west coast of India

2017 ◽  
Author(s):  
M. Anjali Nair ◽  
V. Sanil Kumar
Keyword(s):  
Wave Height ◽  
Coastal Waters ◽  
High Frequency ◽  
Significant Wave Height ◽  
Wave Spectrum ◽  
Temporal Variations ◽  
Wave Spectra ◽  
Monsoon Period ◽  
Significant Wave ◽  
Spectral Shapes

Abstract. Understanding of the wave spectral shapes is of primary importance for the design of marine facilities. In this paper, the wave spectra collected from January 2011 to December 2015 in the coastal waters are examined to know the temporal variations in the wave spectral shape. For 31.15 % of the time, peak frequency is between 0.08 and 0.10 Hz and the significant wave height is also relatively high (~ 1.55 m) for waves in this class. The slope of the high-frequency tail of the monthly average wave spectra is high during the Indian summer monsoon period (June–September) compared to other months and it increases with increase in significant wave height. There is no much interannual variation in slope for swell dominated spectra during the monsoon, while in the non-monsoon period when wind-seas have much influence, the slope varies significantly. Since the high-frequency slope of the wave spectrum is within the range 3–4 during the monsoon period, Donelan spectrum shows better fit for the wave spectra in monsoon months compared to other months.

On the Estimation of the Kurtosis in Directional Sea States for Freak Wave Forecasting

2011 ◽  
Vol 41 (8) ◽  
pp. 1484-1497 ◽  
Author(s):  
Nobuhito Mori ◽  
Miguel Onorato ◽  
Peter A. E. M. Janssen
Keyword(s):  
Monte Carlo ◽  
Energy Distribution ◽  
Monte Carlo Simulations ◽  
Initial Conditions ◽  
Laboratory Data ◽  
Dispersion Effect ◽  
Freak Wave ◽  
Wave Forecasting ◽  
Directional Sea States ◽  
Good Agreement

Abstract Based on Monte Carlo simulations of the nonlinear Schrödinger equation in two horizontal dimensions, the dependence of the kurtosis on the directional energy distribution of the initial conditions is examined. The parametric survey is carried out to obtain the behavior of the kurtosis as function of the Benjamin–Feir index and directional spread in directional sea states. As directional dispersion effect becomes significant, the kurtosis monotonically decreases in comparison with the unidirectional waves. A parameterization of the kurtosis estimated from directional spectra is proposed here; the error of the parameterization is at most 10%. The parameterization is verified against laboratory data, and good agreement is obtained.

scholarly journals Evolution of a Random Directional Wave and Freak Wave Occurrence

2009 ◽  
Vol 39 (3) ◽  
pp. 621-639 ◽  
Author(s):  
Takuji Waseda ◽  
Takeshi Kinoshita ◽  
Hitoshi Tamura
Keyword(s):  
Wave Height ◽  
Wave Interaction ◽  
Northwest Pacific ◽  
Frequency Bandwidth ◽  
Extreme Wave ◽  
Spectral Parameters ◽  
Freak Wave ◽  
Resonant Wave ◽  
Directional Wave ◽  
Wind Sea

Abstract The evolution of a random directional wave in deep water was studied in a laboratory wave tank (50 m long, 10 m wide, 5 m deep) utilizing a directional wave generator. A number of experiments were conducted, changing the various spectral parameters (wave steepness 0.05 < ɛ < 0.11, with directional spreading up to 36° and frequency bandwidth 0.2 < δk/k < 0.6). The wave evolution was studied by an array of wave wires distributed down the tank. As the spectral parameters were altered, the wave height statistics change. Without any wave directionality, the occurrence of waves exceeding twice the significant wave height (the freak wave) increases as the frequency bandwidth narrows and steepness increases, due to quasi-resonant wave–wave interaction. However, the probability of an extreme wave rapidly reduces as the directional bandwidth broadens. The effective Benjamin–Feir index (BFIeff) is introduced, extending the BFI (the relative magnitude of nonlinearity and dispersion) to incorporate the effect of directionality, and successfully parameterizes the observed occurrence of freak waves in the tank. Analysis of the high-resolution hindcast wave field of the northwest Pacific reveals that such a directionally confined wind sea with high extreme wave probability is rare and corresponds mostly to a swell–wind sea mixed condition. Therefore, extreme wave occurrence in the sea as a result of quasi-resonant wave–wave interaction is a rare event that occurs only when the wind sea directionality is extremely narrow.

scholarly journals A Multisensor Comparison of Ocean Wave Frequency Spectra from a Research Vessel during the Southern Ocean Gas Exchange Experiment

2013 ◽  
Vol 30 (12) ◽  
pp. 2907-2925 ◽  
Author(s):  
Alejandro Cifuentes-Lorenzen ◽  
James B. Edson ◽  
Christopher J. Zappa ◽  
Ludovic Bariteau
Keyword(s):  
High Frequency ◽  
Doppler Radar ◽  
Inertial Sensors ◽  
Wave Spectrum ◽  
Ocean Wave ◽  
Modulation Transfer ◽  
Laser Altimeter ◽  
Frequency Spectra ◽  
Wave Statistics ◽  
Wave Measurements

Abstract Obtaining accurate measurements of wave statistics from research vessels remains a challenge due to the platform motion. One principal correction is the removal of ship heave and Doppler effects from point measurements. Here, open-ocean wave measurements were collected using a laser altimeter, a Doppler radar microwave sensor, a radar-based system, and inertial measurement units. Multiple instruments were deployed to capture the low- and high-frequency sea surface displacements. Doppler and motion correction algorithms were applied to obtain a full 1D (0.035–1.3 ± 0.2 Hz) wave spectrum. The radar-based system combined with the laser altimeter provided the optimal low- and high-frequency combination, producing a frequency spectrum in the range from 0.035 to 1.2 Hz for cruising speeds ≤3 m s−1 with a spectral rolloff of f−4 Hz and noise floor of −20/−30 dB. While on station, the significant wave height estimates were comparable within 10%–15% among instrumentation. Discrepancies in the total energy and in the spectral shape between instruments arise when the ship is in motion. These differences can be quantified using the spectral behavior of the measurements, accounting for aliasing and Doppler corrections. The inertial sensors provided information on the amplitude of the ship’s modulation transfer function, which was estimated to be ~1.3 ± 0.2 while on station and increased while underway [2.1 at ship-over-ground (SOG) speed; 4.3 m s−1]. The correction scheme presented here is adequate for measurements collected at cruising speeds of 3 m s−1 or less. At speeds greater than 5 m s−1, the motion and Doppler corrections are not sufficient to correct the observed spectral degradation.

Local Energy Spectra in the Vicinity of an Extreme Wave Event

2003 ◽  
Author(s):  
Richard Gibson ◽  
Chris Swan
Keyword(s):  
Wave Spectrum ◽  
Wave Model ◽  
Extreme Wave ◽  
Time Frequency ◽  
Recent Advances ◽  
Rapid Changes ◽  
Wave Event ◽  
Local Wave ◽  
Water Particle Kinematics ◽  
Extreme Wave Event

Previous work by Baldock, Swan and Taylor [1], Johannessen and Swan [2,3], and Bateman, Swan and Taylor [4,5] has demonstrated that in the vicinity of an extreme wave event there are significant and rapid changes in the local wave spectrum. The present paper combines two recent advances. The first is a new fully nonlinear directional wave model, the application of which is particularly suited to the description of extreme waves arising in realistic sea states. The second involves recent advances in time-frequency analysis techniques. Unlike traditional spectral analysis, based upon the Fourier transform, these allow local and rapid changes in a wave spectrum to be clearly identified. By combining these methods the proposed paper will first highlight the occurrence of these changes in realistic seas and will subsequently demonstrate their significance both in terms of estimating crest height elevations and in predicting the associated water particle kinematics.

Linearization of the Wave Spectrum: A Comparison of Methods

2020 ◽  
Author(s):  
Dylan Barratt ◽  
Harry B. Bingham ◽  
Paul H. Taylor ◽  
Ton S. van den Bremer ◽  
Thomas A. A. Adcock
Keyword(s):  
Fourier Transform ◽  
Phase Separation ◽  
Wave Field ◽  
Wave Spectrum ◽  
Three Dimensional ◽  
Extreme Wave ◽  
Offshore Engineering ◽  
Wave Event ◽  
The Fourier Transform ◽  
Spurious Modes

Abstract The relative contributions of free waves and bound waves to the formation of an extreme wave event remains a topic of interest in offshore engineering. A variety of methods have been proposed for identifying and removing the bound wave components. The method of “phase separation” or “phase manipulation” repeats simulations/experiments of a wave field with an offset in the initial phase of the wave components and relies upon summation of the resulting wave fields to isolate the bound harmonics, following from a Stokes expansion in steepness; the method has proven effective in isolating bound harmonics but requires that all cases be repeated. Alternatively, the bound harmonics can be removed using a three-dimensional fast Fourier transform (3D-FFT) of the wave field. However, the Fourier transform requires periodicity in the signal and assumes homogeneity in space and stationarity in time, producing spurious modes otherwise. We compare the phase separation and 3D-FFT approaches for a steep, focusing wave group in deep water using the numerical simulation tool, OceanWave3D, and discuss the effectiveness of both methods.

Sign in / Sign up

Export Citation Format

Share Document