Marc Prevosto

Probability distributions for maximum wave and crest heights

The paper discusses short- and long-term probability models of ocean waves. The Gaussian theory is reviewed, and nonlinear short-term probability distributions are derived from a narrow band second-order model. The nonlinearity has different impact on different measurement techniques, and this is further demonstrated for wave data from the WAVEMOD Crete measurement campaign and laser data from the North Sea. Finally, we give some examples on how the short-term statistics may be used to estimate the probability distributions for the maximum waves during individual storms as well as in a wave climate described by long-term distributions.

Bivariate simulation of non stationary and non Gaussian observed processes: Application to sea state parameters

A method for artificially generating operational sea state histories has been developed. This is a distribution free method to simulate bivariate non stationary and non Gaussian random processes. This method is applied to the simulation of the bivariate process (Hs, Tp) of sea state parameters. The time series respects the physical constraints existing between the significant wave height and the peak period. Furthermore, we show that the persistence properties of the simulations match to those of the observations.

Wave Crest Sensor Intercomparison Study: An Overview of WACSIS

The Wave Crest Sensor Intercomparison Study (WACSIS) was designed as a thorough investigation of the statistical distribution of crest heights. Measurements were made in the southern North Sea during the winter of 1997-1998 from the Meetpost Noordwijk in 18 m water depth. The platform was outfitted with several popular wave sensors, including Saab and Marex radars, an EMI laser, a Baylor wave staff and a Vlissingen step gauge. Buoys were moored nearby to obtain directional spectra. Two video cameras viewed the ocean under the wave sensors and their signals were recorded digitally. The data analysis focused on comparisons of the crest height measurements from the various sensors and comparisons of the crest height distributions derived from the sensors and from theories. Some of the sensors had greater than expected energy at high frequencies. Once the measurements were filtered at 0.64 Hz, the Baylor, EMI and Vlissingen crest height distributions match quite closely, while those from the other sensors were a few percent higher. The Baylor and EMI crest distributions agreed very well with the statistics from second order simulations, while previous parameterizations of the crest height distribution were generally too high. We conclude that crest height distributions derived from second order simulations can be used with confidence in engineering calculations. The data were also used in investigations of crest and trough shapes and the joint height/period distribution INTRODUCTION Knowledge of the statistical distribution of crest heights given the wave spectrum is central to the problem of setting deck heights. Unfortunately, there is still considerable uncertainty about the form of this distribution. The empirical evidence is confusing, since different types of instruments have tended to give different results. The theoretical problem is difficult since it is essential to account for the non-linearity of the waves. The participants at the E&P Forum (1995) Workshop on Uncertainties in the Design Process felt that there was a need to

Influence of an Improved Sea-State Description on a Wave Energy Converter Production

Sea-states are usually described by a single set of 5 parameters, no matter the actual number of wave systems they contain. We present an original numerical method to extract from directional spectra the significant systems constituting of a complex sea-state. An accurate description of the energy distribution is then given by multiple sets of parameters. We use these results to assess the wave climatology in the Bay of Biscay and to estimate the power harnessable in this area by a particular Wave Energy Converter, the SEAREV. Results show that the fine description of sea-states yields a better assessment of the instantaneous device response. The discrepancy between the classical and multi-sets descriptions show that the new one is preferable for the assessment of harnessable power and for device design.© 2007 ASME

Stochastic Doppler shift and encountered wave period distributions in Gaussian waves

We show how to calculate the encountered wave period distribution for a ship traveling with constant speed on a Gaussian random sea with a directionally distributed frequency spectrum.

West Africa Swell Spectral Shapes

Wave spectra measured at sites off West Africa are dominated by the constant presence of one or several swell wave systems. The West Africa Swell Project (WASP JIP) was carried out to propose and assess parametric models for the shap es of the swell components. Bias, variability, and dispersionof estimates are affected by the length/stationarity comprom ise of the record lengths and the window-tapering used to reduce th eir variability. In particular, shapes with sharp angles are st rongly smoothed, for instance a triangular peak would appear round and reduced by 15 to 25% with rectangular or Tuckey windowing. Models that consider each wave system individually, an d an arbitrary number of those, were preferred to global ones. Pa rtitioning of directional spectra is thus a prerequisite, an d needs to be tuned taking account some prior knowledge of the swell characteristics. Triangular, log-normal, Gaussian and Gl ennJonswap shapes were considered. Sampling variability make s it difficult to distinguish between those shapes as far as swell s are concerned. The models also indicate that the width of the spe ctrum in frequency should be inversely proportional to the pe ak frequency. Directional spreading width shows a similar tre nd.

Observations of the High-Frequency Range of the Wave Spectrum

This paper takes a new look at the high frequency range of the wave spectrum. The analysis is based on data sets from two recent field campaigns offshore Portugal and Crete carried out in the MAST II WAVEMOD project, data from the WADIC experiment in the North Sea, and deep sea data from Haltenbanken and Voeringplataaet offshore Norway. In addition, the authors also show spectra obtained by spectral inversion of ERS-1 SAR imagery. The influence and calibration of wave measuring instrumentation and the use of wavenumber spectra when comparing spectra from shallow water is emphasized.

THE WEST AFRICA SWELL PROJECT (WASP)

The responses of floating systems are sensitive to the detailed shape of the swell portion of the wave spectrum. Knowledge of swell is particularly important for sites off West Africa. The West Africa Swell Project (WASP JIP) was formed to analyze the available data on West African swell. Measurements were obtained from Shell, Ifremer, Chevron and Marathon. Hindcast data came from Oceanweather and the NOAA Wavewatch model. Sites ranging from Nigeria to Namibia were considered.Modeling the dispersion of swell over long distances indicates that the resulting power spectrum will have a triangular or lognormal shape. Sampling variability makes it difficult to distinguish between those shapes or Jonswap or Gaussian forms, but lognormal spectra generally provided good fits. The models also indicate that the width of the spectrum in both frequency and direction should be inversely related to the peak frequency. Fits to the measurements established detailed relationships for each location.Calculating the response of single degree of freedom oscillators to the measured and hindcast spectra produces response spectra which give the maximum response as a function of natural period and damping. Extreme values of system response can be calculated from the response spectra. The largest responses come from uni-modal spectra, and design spectra can be estimated by inflating them.Copyright

Statistics of Wave Crests From Models vs. Measurements

The analysis phase of the Wave Crest Sensor Intercomparison Study (WACSIS) focussed on the interpretation of the wave data collected by the project during the winter of 1997–98. Many aspects of wave statistics have been studied, but the main emphasis has been on crest height distributions, and recommendations for crest heights to be used in air gap calculations. In this paper we first describe comparisons of the crest height distributions derived from the sensors (radars, wave staffs, laser) and from simulations based on 3D second order irregular wave models. These comparisons permit us to make conclusions on the quality of these models and to qualify the ability of some sensors to measure the crest heights accurately. In the second part two new parametric models of the crest height distributions are discussed and their superiority to standard parametric models is demonstrated.Copyright

Modeling process asymmetries with Laplace moving average

Many records in environmental science exhibit asymmetries: for example in shallow water and with variable bathymetry, the sea wave time series shows front–back asymmetries and different shapes for crests and troughs. In such situation, numerical models are available but their computational cost and complexity are high. A stochastic process aimed at modeling such asymmetries has recently been proposed, the Laplace moving average process, which consists in applying a linear filter on a non-Gaussian noise built using the generalized Laplace distribution. The objective is to propose a new non-parametric estimator for the kernel involved in the definition of this process. Results based on a comprehensive numerical study will be shown in order to evaluate the performances of the proposed method.