A Generic Method to Model Frequency-Direction Wave Spectra for FPSO Motions

2008 ◽  
Author(s):  
Hermione J. van Zutphen ◽  
Philip Jonathan ◽  
Kevin C. Ewans
Keyword(s):  
Frequency Spectrum ◽  
Two Dimensions ◽  
Wind Forcing ◽  
Wave Spectra ◽  
Spectral Form ◽  
New Approach ◽  
Spreading Function ◽  
Wind Sea ◽  
Model Frequency ◽  
Sea Wave

We report a new approach to model the frequency-direction spectrum, in which the frequency-direction spectra from measurements or hindcast studies are fitted simultaneously in two dimensions, frequency and direction. Depending on the amount of wind forcing on the partition, either a unimodal (swell) or bimodal (wind-sea) wave spreading function is adopted together with the spectral form which best fits the frequency spectrum. This paper describes the new method and presents the results on a measured dataset.

Realistic simulation of mixed sea using multiple spectrum-based wave systems

SIMULATION ◽  
2019 ◽  
Vol 96 (3) ◽  
pp. 281-296
Author(s):  
Sekil Park ◽  
Jinah Park
Keyword(s):  
Real Time ◽  
Smooth Transition ◽  
Statistical Characteristics ◽  
Fourier Domain ◽  
Wave System ◽  
Wave Spectra ◽  
Spectral Parameters ◽  
Realistic Simulation ◽  
Wind Sea ◽  
Sea Wave

The sea has a very wide, irregular, and continuously changing surface and is usually a mixed sea composed of several wave systems. Each wave system is generated from different locations and conditions and has its own characteristics. The Fourier domain approach using sea wave spectra is an effective technique for the realistic simulation of sea surfaces in real time, but the conventional Fourier domain approach cannot independently simulate the characteristics of each wave system. In this paper, we propose a realistic and real-time simulation method of the mixed sea using multiple spectrum-based wave systems for maritime simulators. We recognize the importance of the visual and physical contributions of each wave system and faithfully reproduce all wave systems in the mixed sea. In order to simulate the mixed sea, our method generates and combines multiple spectrum-based wave systems using adaptive spectral sampling of the separated spectrum of the multi-peaked spectrum. The unique characteristics of each wave system can be set independently through spectral parameters, sampling number and range, wave direction and spread, and the shape factor of waves. The proposed method also supports the smooth transition between sea states, such as wind sea, swell, and mixed sea. Through the experiments, we verify that the proposed method effectively reflects sea wave spectra and the reproduced sea has very similar statistical characteristics to the actual sea. Experimental results also show that our approach can simulate the mixed sea, which has high-frequency wind sea and low-frequency swell.

Alternative Environmental Contours for Marine Structural Design: A Comparison Study

2014 ◽  
Author(s):  
Erik Vanem ◽  
Elzbieta M. Bitner-Gregersen
Keyword(s):  
Structural Design ◽  
Joint Distribution ◽  
Traditional Approach ◽  
Physical Space ◽  
Probability Density Functions ◽  
Density Functions ◽  
Comparison Study ◽  
New Approach ◽  
Contour Lines ◽  
Wind Sea

A new approach to estimating environmental contours has recently been proposed, where the contours are estimated in the original physical space by Monte Carlo simulations from the joint distribution directly rather than applying the Rosenblatt transformation. In this paper, the new and the traditional approach to estimating the contours are presented and the assumptions on which they are based are discussed. The different results given by these two methods are then compared in a number of case studies. Simultaneous probability density functions are fitted to the joint distribution of significant wave height and wave period for selected ocean locations and, for each area, environmental contours are estimated for both methods. The chosen locations are characterised by different wave climates. Thus, the practical consequences of the choice of approach are assessed. Particular attention is given to mixed sea systems, i.e. a combination of wind sea and swell. In these situations, the new approach for environmental contours may fail to identify realistic conditions along some parts of the contours while for other wave conditions the contours are quite similar. The paper also briefly discusses possible ways of amending the new approach to estimating the contours to obtain more realistic conditions all along the contour lines.

scholarly journals On the Doppler distortion of the sea-wave spectra

2008 ◽  
Vol 237 (21) ◽  
pp. 2767-2776 ◽  
Author(s):  
A.O. Korotkevich
Keyword(s):  
Wave Spectra ◽  
Sea Wave

A study of sea-wave spectra in a wide wavelength range from satellite and in-situ data

2016 ◽  
Vol 52 (9) ◽  
pp. 888-903 ◽  
Author(s):  
V. G. Bondur ◽  
V. A. Dulov ◽  
A. B. Murynin ◽  
Yu. Yu. Yurovsky
Keyword(s):  
Wavelength Range ◽  
Wave Spectra ◽  
In Situ Data ◽  
Wide Wavelength Range ◽  
Sea Wave

Set-Up due to Random Waves: Influence of the Directional Spectrum

2013 ◽  
Vol 155 (A3) ◽  
Keyword(s):  
Frequency Spectrum ◽  
Mathematical Expression ◽  
The United States ◽  
Random Waves ◽  
Directional Spectrum ◽  
Flow Parameters ◽  
Spreading Function ◽  
Set Up ◽  
Correct Estimation ◽  
Buoy Data

The correct estimation of set-up is very important to evaluate coastal hazard and to design coastal structures. In this paper, we derived a mathematical expression for wave set-up in the context of random waves. The solution to this expression assumes straight, parallel depth contours and constant average flow parameters in the longshore direction. We then investigated the effect of different types of sea state taking account of different frequency spectrum and spreading function assumed in the expression on estimates of wave set-up. We found the set-up was highly influenced by the frequency spectrum used. Finally, we applied this expression to estimate set-up values at locations in Italy and in the United States using buoy data provided by ISPRA (Istituto Superiore per la Protezione e la Ricerca Ambientale) and NDBC (National Data Buoy Centre).

Directional Spreading in Two-Peak Spectrum at the Norwegian Continental Shelf

2002 ◽  
Author(s):  
Elzbieta M. Bitner-Gregersen ◽  
O̸istein Hagen
Keyword(s):  
Continental Shelf ◽  
Frequency Spectrum ◽  
Wave Spectrum ◽  
Single Mode ◽  
Reasonable Accuracy ◽  
Double Peak ◽  
Directional Distribution ◽  
Practical Applications ◽  
Wind Sea ◽  
Norwegian Continental Shelf

In practical applications, it is usually assumed that the wave spectrum is of a single mode form, and well modeled by a JONSWAP or Pierson-Moskowitz spectrum. This assumption is of a reasonable accuracy for severe sea states. However, moderated and low sea states are often of a combined nature, consisting of both wind-sea and swell and should be characterized by a double peak spectrum. The present paper discusses two-peak spectra observed in Norwegian waters, which are particularly affected by swell. The analysis is based on 5-year of directional data from Haltenbanken. A directional distribution for swell is suggested. Further, a procedure for including directional spreading in two-peak spectra is proposed. The method is illustrated for the two-peak Torsethaugen frequency spectrum (Torsethaugen, 1996), which is currently used by the Norwegian industry.

Estimation of Wind-Sea and Swell Components in a Bimodal Sea State

2004 ◽  
Vol 128 (4) ◽  
pp. 265-270 ◽  
Author(s):  
K. C. Ewans ◽  
E. M. Bitner-Gregersen ◽  
C. Guedes Soares
Keyword(s):  
Wave Spectrum ◽  
Low Frequency ◽  
Wave Spectra ◽  
Spectral Parameters ◽  
High Frequency Peak ◽  
Wave Measurements ◽  
Frequency Peak ◽  
Directional Wave ◽  
Wind Sea ◽  
Complete Frequency

Methods for separating the spectral components and describing bimodal wave spectra are evaluated with reference to wave spectra from directional wave measurements made at the Maui location off the west coast of New Zealand. Two methods involve partitioning bimodal wave spectra into wind-sea and swell components and then fitting a spectral function to each component, while the third assigns an average spectral shape based on the integrated spectral parameters. The partitioning methods involve separating the wave spectrum into two frequency bands: a low-frequency peak, the swell component, and a high-frequency peak, the wind-sea. One partitioning method uses only the frequency spectrum while the other analyzes the complete frequency-direction spectrum. Comparison of the spectral descriptions and derived parameters against the measured counterparts provides insight into the accuracy of the different approaches to describing actual bimodal sea states.

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