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.

scholarly journals ON THE PROBABILITY DISTRIBUTION OF FREAK WAVES IN FINITE WATER DEPTH

2012 ◽  
Vol 1 (33) ◽  
pp. 13
Author(s):  
Kyungmo Ahn ◽  
Sun-Kyung Kim ◽  
Se-Hyun Cheon
Keyword(s):  
Probability Distribution ◽  
Wave Height ◽  
Distribution Functions ◽  
Occurrence Probability ◽  
Extreme Waves ◽  
Wave Interactions ◽  
Freak Waves ◽  
Freak Wave ◽  
Wave Data ◽  
Finite Water Depth

This paper presents the occurrence probability of freak waves based on the analysis of extensive wave data collected during ARSLOE project. It is suggested to use the probability distribution of extreme waves heights as a possible means of defining the freak wave criteria instead of conventional definition which is the wave height greater than the twice of the significant wave height. Analysis of wave data provided such finding as 1) threshold tolerance of 0.2 m is recommended for the discrimination of the false wave height due to noise, 2) no supportive evidence on the linear relationship between the occurrence probability of freak waves and the kurtosis of surface elevation 3) nonlinear wave-wave interactions is not thh primary cause of the generation of freak waves 4) the occurrence of freak waves does not depend on the wave period 5) probability density function of extreme waves can be used to predict the occurrence probability of freak waves. Three different distribution functions of extreme wave height by Rayleigh, Ahn, and Mori were compared for the analysis of freak waves.

Extreme Waves and Stochastic Wave Groups

2006 ◽  
Author(s):  
Francesco Fedele ◽  
M. Aziz Tayfun
Keyword(s):  
Numerical Results ◽  
Resonance Interaction ◽  
Experimental Results ◽  
Wave Crest ◽  
Extreme Waves ◽  
Zakharov Equation ◽  
Wave Groups ◽  
Probability Of Exceedance ◽  
Random Seas ◽  
Crest Height

We introduce the concept of stochastic wave groups to explain the occurrence of extreme waves in nonlinear random seas, according to the dynamics imposed by the Zakharov equation (Zakharov, 1999). As a corollary, a new probability of exceedance of the crest-to-trough height which takes in to account the quasi-resonance interaction is derived. Furthermore, a generalization of the Tayfun distribution (Tayfun, 1986) for the wave crest height is also proposed. The new analytical distributions explain qualitatively well the experimental results of Onorato et al. (2004, 2005) and the numerical results of Juglard et al. (2005).

New Insights in Extreme Crest Height Distributions: A Summary of the ‘CresT’ JIP

2011 ◽  
Author(s):  
Bas Buchner ◽  
George Forristall ◽  
Kevin Ewans ◽  
Marios Christou ◽  
Janou Hennig
Keyword(s):  
Single Point ◽  
Field Measurements ◽  
Order Theory ◽  
Second Order ◽  
Wave Crest ◽  
Height Distribution ◽  
Extreme Waves ◽  
Wave Height Distribution ◽  
Floating Platforms ◽  
Crest Height

The objective of the CresT JIP was ‘to develop models for realistic extreme waves and a design methodology for the loading and response of floating platforms’. Within this objective the central question was: ‘What is the highest (most critical) wave crest that will be encountered by my platform in its lifetime?’ Based on the presented results for long and short-crested numerical, field and basin results in the paper, it can be concluded that the statistics of long-crested waves are different than those of short-crested waves. But also short-crested waves show a trend to reach crest heights above second order. This is in line with visual observations of the physics involved: crests are sharper than predicted by second order, waves are asymmetric (fronts are steeper) and waves are breaking. Although the development of extreme waves within short-crested sea states still needs further investigation (including the counteracting effect of breaking), at the end of the CresT project the following procedure for taking into account extreme waves in platform design is recommended: 1. For the wave height distribution, use the Forristall distribution (Forristall, 1978). 2. For the crest height distribution, use 2nd order distribution as basis. 3. Both the basin and field measurements show crest heights higher than predicted by second order theory for steeper sea states. It is therefore recommended to apply a correction to the second order distribution based on the basin results. 4. Account for the sampling variability at the tail of the distribution (and resulting remaining possibility of higher crests than given by the corrected second order distribution) in the reliability analysis. 5. Consider the fact that the maximum crest height under a complete platform deck can be considerably higher than the maximum crest at a single point.

Freak Wave Generation in a Wave Basin With HOSM-WG Method

2015 ◽  
Author(s):  
Hidetaka Houtani ◽  
Takuji Waseda ◽  
Wataru Fujimoto ◽  
Keiji Kiyomatsu ◽  
Katsuji Tanizawa
Keyword(s):  
Control Method ◽  
Irregular Waves ◽  
Frequency Bandwidth ◽  
Freak Waves ◽  
Freak Wave ◽  
Wave Basin ◽  
Wave Trains ◽  
Directional Wave ◽  
Wave Maker ◽  
The Difference

A method to produce freak waves with arbitrary spectrum in a fully directional wave basin is presented here. This is an extension of Waseda, Houtani and Tanizawa at OMAE 2013[1], which used “HOSM-WG” based on the higher-order spectral method (HOSM). We used the following three methods to improve the HOSM-WG in [1]: “separation of free waves from bound waves,” “using Biesel’s transfer function in wavenumber space” and “using Schaffer’s 2nd-order wave maker control method.” Modulational wave trains, freak waves in unidirectional irregular waves and freak waves in short-crested irregular waves were generated in a wave basin. The experimental results using the improved HOSM-WG were compared to the HOSM simulation, and good agreements were found. The effectiveness of the improved HOSM-WG was ascertained. We showed that the difference between HOSM-WG and HOSM simulations became larger as wave steepness, frequency bandwidth of the spectrum or directional spreading became larger.

Freak wave statistics on collinear currents

2009 ◽  
Vol 637 ◽  
pp. 267-284 ◽  
Author(s):  
KARINA B. HJELMERVIK ◽  
KARSTEN TRULSEN
Keyword(s):  
Wave Height ◽  
Significant Wave Height ◽  
Constant Current ◽  
Local Criterion ◽  
Freak Waves ◽  
Freak Wave ◽  
Significant Wave ◽  
Wave Statistics ◽  
Global Criterion ◽  
Wave Distribution

Linear refraction of waves on inhomogeneous current is known to provoke extreme waves. We investigate the effect of nonlinearity on this phenomenon, with respect to the variation of significant wave height, kurtosis and occurrence of freak waves. Monte Carlo simulations are performed employing a modified nonlinear Schrödinger equation that includes the effects of a prescribed non-potential current. We recommend that freak waves should be defined by a local criterion according to the wave distribution at each location of constant current, not by a global criterion that is either averaged over, or insensitive to, inhomogeneities of the current. Nonlinearity can reduce the modulation of significant wave height. Depending on the configuration of current and waves, the kurtosis and probability of freak waves can either grow or decrease when the wave height increases due to linear refraction. At the centre of an opposing current jet where waves are known to become large, we find that freak waves should be more rare than in the open ocean away from currents. The largest amount of freak waves on an opposing current jet is found at the jet sides where the significant wave height is small.

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-in-Deck Impact Load Measurements on a Fixed Platform Deck

2014 ◽  
Author(s):  
Jule Scharnke ◽  
Tone Vestbøstad ◽  
Jaap de Wilde ◽  
Sverre Haver
Keyword(s):  
North Sea ◽  
Impact Load ◽  
Wave Theory ◽  
Model Tests ◽  
Wave Crest ◽  
Impact Loads ◽  
Load Prediction ◽  
Large Wave ◽  
Efficient Test ◽  
Crest Height

New methods for estimation of extreme wave crest heights have resulted in an increase of the estimated 10,000 year crest height. At the Norwegian Continental Shelf this increase is typically 2 to 4 m, resulting in a crest height of 22 m to 24 m in the Central & Northern North Sea and the Haltenbanken area. As a result several fixed platforms designed prior to 2000 may experience negative air gap if being hit by the 10,000 year wave crest height. Numerical methods have been used for assessing wave-in-deck impact loads. The model tests discussed in this paper were conducted to be used as verification of the numerical codes. For the model tests two sea states along the 10,000 year contour line were considered. Several 3-hour (full scale time) realizations were calibrated in order to capture the natural variability of the most extreme crest heights. For wave deck impact problems, one is merely interested in the few very large wave crests out of a 3-hour simulation. A more efficient test scope would, therefore, be to generate only the largest wave groups of the realizations. For this reason the most extreme crest(s) per sea state were identified and most wave-in-deck tests were conducted by generating only the part of the time series containing the large crest(s). The wave calibration results were discussed in a previous paper, see [1]. For the wave-in-deck model tests, an existing North Sea jacket was built at scale 1:60 and instrumented in order to measure the global loads on the platform deck independently from the loads on the jacket itself. In this paper the model test setup as well as the measured wave-in-deck impact loads are discussed and compared to a simplified load prediction model. The presented results show that the simplified loading model, with wave properties based on Stokes 5th order wave theory, underestimates the measured horizontal deck loads.

Reliability Assessment of a Generic Jacket: Effects of Airgap Choices and Current Modelling

2002 ◽  
Author(s):  
Sverre Haver ◽  
Kenneth Johannesen Eik ◽  
Einar Nygaard
Keyword(s):  
Wave Height ◽  
Reliability Assessment ◽  
Wave Crest ◽  
Base Shear ◽  
Current Profile ◽  
Annual Probability ◽  
Current Design ◽  
Wave Heights ◽  
Norwegian Continental Shelf ◽  
Crest Height

A simplified reliability assessment is carried out for a generic jacket at 200m water depth. The purpose is to indicate the sensitivity of the annual failure probability to the selected airgap and current design profile. Two example cases are considered. For one case the required airgap is defined by the 10−4 wave crest height, while for the other the required airgap is defined from the 10−2 wave crest height plus an uncertainty margin taken to be 10% of the crest height. For both cases, the required minimum design base shear capacities are determined both using the 10-year current profile (earlier practice at the Norwegian Continental Shelf) and the associated current profile (i.e. the current profile which when used in combination with the n-year wave height yields the n-year load). The investigation shown herein clearly demonstrates that the chosen airgap is a crucial parameter regarding the annual probability of structural failure. It is, furthermore, demonstrated that if a wave-deck impact is required in order to fail the structure (which will be the case for most jackets), the current modeling is not very important. However, if the structure is designed such that failure may occur for wave heights not reaching deck level (either due to a highly utilized design or a very generous initial airgap), the current modeling (both in terms of selected design profile and joint description of wave height and current speed) may be far more important.

scholarly journals Time-frequency analysis of the sea state with the "Andrea" freak wave

2014 ◽  
Vol 2 (2) ◽  
pp. 1481-1503
Author(s):  
Z. Cherneva ◽  
C. Guedes Soares
Keyword(s):  
Time Series ◽  
Energy Distribution ◽  
North Sea ◽  
Sea State ◽  
Freak Wave ◽  
High Wave ◽  
Time Frequency ◽  
Wigner Spectrum ◽  
Central North Sea ◽  
Field Wave

Abstract. The non-linear and non-stationary properties of a special field wave record are analyzed with the Wigner spectrum with the Choi–Williams kernel. The wave time series, which was recorded at the Ekofisk complex in the Central North Sea at 00:40 UTC on 9 November 2007, contains an abnormally high wave known as "Andrea" wave. The ability of the Wigner spectrum to reveal the wave energy distribution in frequency and time is demonstrated. The results are compared with previous investigations for different sea states and also the state with the abnormal Draupner's New Year 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

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