scholarly journals Experimental Study of the Statistical Properties of Directionally Spread Ocean Waves Measured by Buoys

2020 ◽  
Vol 50 (2) ◽  
pp. 399-414 ◽  
Author(s):  
M. L. McAllister ◽  
T. S. van den Bremer
Keyword(s):  
Ocean Waves ◽  
Irregular Waves ◽  
Wave Crest ◽  
Free Surface Elevation ◽  
Spectral Parameters ◽  
Peak Period ◽  
Wave Statistics ◽  
Buoy Data ◽  
Crest Height

AbstractWave-following buoys are used to provide measurements of free surface elevation across the oceans. The measurements they produce are widely used to derive wave-averaged parameters such as significant wave height and peak period, alongside wave-by-wave statistics such as crest height distributions. Particularly concerning the measurement of extreme wave crests, these measurements are often perceived to be less accurate. We directly assess this through a side-by-side laboratory comparison of measurements made using Eulerian wave gauges and model wave-following buoys for randomly generated directionally spread irregular waves representative of extreme conditions on deep water. This study builds on the recent work of McAllister and van den Bremer (2019, https://doi.org/10.1175/JPO-D-19-0170.1), in which buoy measurements of steep directionally spread focused waves groups were considered. Our experiments confirm that the motion of a wave-following buoy should not significantly affect the measured wave crest statistics or spectral parameters and that the discrepancies observed for in situ buoy data are most likely a result of filtering. This filtering occurs when accelerations that are measured by the sensors within a buoy are converted to displacements. We present an approximate means of correcting the resulting measured crest height distributions, which is shown to be effective using our experimental data.

An Experimental Study of the Systematic Underestimation of Wave Crests Measured by Lagrangian Buoys, and a Retrospective Correction Method

2020 ◽  
Author(s):  
Mark McAllister ◽  
Ton van den Bremer
Keyword(s):  
Approximate Expression ◽  
Ocean Waves ◽  
Order Theory ◽  
Correction Method ◽  
Second Order ◽  
Spectral Parameters ◽  
Peak Period ◽  
Wave Statistics ◽  
Buoy Data ◽  
Crest Height

<p>Wave-following buoys are used to provide measurements of free surface elevation across the oceans. The measurements they produce are widely used to derive wave-averaged parameters such as significant wave height and peak period, alongside wave-by-wave statistics such as crest height distributions. Particularly concerning the measurement of extreme wave crests, these measurements are often perceived to be less accurate. We directly assess this through a side-by-side laboratory comparison of measurements made using Eulerian wave gauges and model wave-following buoys for directionally spread waves representative of extreme conditions on deep water. Our experimental measurements are compared to exact (Herbers and Janssen 2016, J. Phys. Oceanogr, 46, 1009-1021) and new approximate expression for Lagrangian second-order theory derived herein. We derive simple closed-form expressions for the second-order contribution to crest height representative of extreme ocean waves. Our experiments confirm that the motion a wave-following buoy should not significantly affect the measurements of wave crests or spectral parameters, and that discrepancies observed for in-situ buoy data are most likely a result of filtering. This filtering occurs when accelerations that are measured by the sensors within a buoy are converted to displacements. We present an approximate means of correcting the resulting measured crest height distributions, which is shown to be effective using our experimental data.</p>

Statistics of Wave Crests From Models vs. Measurements

2002 ◽  
Author(s):  
Marc Prevosto ◽  
Geoerge Z. Forristall
Keyword(s):  
Parametric Models ◽  
Wave Crest ◽  
Wave Statistics ◽  
Irregular Wave ◽  
Analysis Phase ◽  
Main Emphasis ◽  
Wave Models ◽  
Intercomparison Study ◽  
Crest Height

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.

Maximum Crest Heights Over an Area: Laboratory Measurements Compared to Theory

2015 ◽  
Author(s):  
George Z. Forristall
Keyword(s):  
Single Point ◽  
Ocean Waves ◽  
Wave Crest ◽  
Large Area ◽  
Array Measurements ◽  
Theoretical Predictions ◽  
Wave Heights ◽  
Significant Wave Heights ◽  
Model Platform ◽  
Crest Height

Platform decks cover a reasonably large area compared to the size of a wave crest. Ocean waves are dispersive and directionally spread. As they propagate, their crest heights change. A platform deck samples those waves at many different locations. The maximum crest height over the area of a deck during a storm will naturally be greater than the maximum at a single point. The principle is clear but measurements are needed to confirm quantitative theoretical predictions. Such measurements were made in Marin wave basins using an array of 100 wave probes. At prototype scale, they covered an area of 100 by 100 m. Random directionally spread waves with prototype significant wave heights from 12 to 15 m and peak periods from 12 to 15 sec were generated and run through the array. Measurements were also made with pressure gauges mounted underneath a model platform deck placed at 11.5 and 13.0 m above still water level. Numerical simulations are used to find the maximum linear crest height expected over these areas. The second order enhancement of crest is accounted for by factoring the Gaussian maximum. Empirical fits to the simulations were found that can be used for most practical problems.

Seakeeping Prediction of a Survey Vessel Operating in the Caspian Sea

2018 ◽  
Author(s):  
Elisabeta Burlacu ◽  
Leonard Domnisoru ◽  
Dan Obreja
Keyword(s):  
Caspian Sea ◽  
Experimental Tests ◽  
Irregular Waves ◽  
The Caspian Sea ◽  
Scaled Model ◽  
Spectral Parameters ◽  
Peak Period ◽  
Fibre Glass ◽  
Strip Theory ◽  
Response Amplitude Operators

This paper presents the numerical and experimental analysis of the seakeeping performances of a survey vessel operating in the Caspian Sea. For the numerical analysis we have developed our own code, based on a linear hydro-dynamic strip theory formulation. The irregular waves are modelled by short-term power density spectra JONSWAP, for the spectral parameters significant wave height and peak period corresponding to the Caspian Sea scattering diagram. The experimental study is developed at the towing tank from Naval Architecture Faculty of Galati, using a semi-captive scaled model 1:16 of the survey vessel, made of fibre glass and wood, being recorded the heave, pitch, roll motions and wave elongation. The experimental tests are carried out for two speeds and several significant heading cases: head, follow and beam regular waves. This study delivers the prediction of the survey vessel seakeeping capabilities and the validation of the numerical response amplitude operators by experiment.

Experimental Study of Turbulent Flow Induced by Regular and Irregular Waves Above a Rock-Armored Slope

2014 ◽  
Author(s):  
Konstantina A. Galani ◽  
Giannis D. Dimou ◽  
Athanassios A. Dimas
Keyword(s):  
Experimental Study ◽  
Turbulent Flow ◽  
Velocity Profiles ◽  
Irregular Waves ◽  
Wave Crest ◽  
Regular Waves ◽  
Mean Velocity ◽  
Peak Period ◽  
Wave Trough ◽  
Wave Heights

The aim of the present work is the experimental study of the turbulent flow induced by waves above a physical model of a rock-armored slope of 1/3. The armor consisted of two layers of rocks with characteristic diameter D50 = 4.4cm. Measurements of the instantaneous velocity fields were conducted using an underwater planar PIV system. Four cases of incoming waves were tested, two cases of regular waves of 1st order Stokes theory with wave period of 1.134s and wave heights of 0.04m and 0.08m, respectively, and two cases of irregular waves, generated from a JONSWAP spectrum, with a peak period of 1.134s and significant wave heights of 0.04m and 0.08m, respectively. For the regular waves, the period-averaged velocity profiles show the existence of a strong undertow current heading towards deep water, while turbulence is not homogeneous with larger horizontal fluctuations. The phase-averaged horizontal velocity profiles present systematically larger values during wave trough passage than during wave crest passage. Furthermore, as the depth becomes smaller, the waveform loses its symmetry, with the wave trough becoming wider and the wave crest steeper. For the irregular waves, the mean velocity profiles show the existence of an undertow current weaker in magnitude than the one in the regular waves, while turbulence is still not homogeneous with larger horizontal fluctuations. For both wave cases, spanwise vorticity, which is generated at the rough surface of the rock-armored slope, is transported landward by the turbulent velocities.

Maximum Crest Heights Over an Area and the Air Gap Problem

2006 ◽  
Author(s):  
George Z. Forristall
Keyword(s):  
Numerical Simulations ◽  
Single Point ◽  
Ocean Waves ◽  
The Other ◽  
Wave Crest ◽  
Two Dimensional ◽  
Extreme Wave ◽  
Small Areas ◽  
Dimensional Version ◽  
Crest Height

Ocean waves are dispersive and directionally spread, changing size and shape as they propagate. Therefore the maximum crest height over an area in a given length of time will be larger than the maximum crest at a single point. Extreme crest heights are usually calculated from single point statistics, but the designer of a platform is really interested in the probability of a wave crest reaching any part of the deck area. Statistics for the maximum crest over an area have been developed using a combination of analytic theory and numerical simulations. The resulting crest heights are significantly higher than given by point statistics even for relatively small areas. On the other hand, only a small fraction of the deck may be inundated. That fraction can be estimated by a applying a two dimensional version of the NewWave method that finds the most probable shape of an extreme wave.

Statistics of Wave Crests From Models vs. Measurements

2004 ◽  
Vol 126 (1) ◽  
pp. 43-50 ◽  
Author(s):  
Marc Prevosto ◽  
George Z. Forristall
Keyword(s):  
Parametric Models ◽  
Wave Crest ◽  
Wave Statistics ◽  
Irregular Wave ◽  
Analysis Phase ◽  
Main Emphasis ◽  
Wave Models ◽  
Intercomparison Study ◽  
Crest Height

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 3-D 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.

A Methodology for Calculating Wave Crest Enhancement in Extreme Seas

2013 ◽  
Author(s):  
Bruce Martin ◽  
Oriol Rijken
Keyword(s):  
Enhancement Factor ◽  
Wave Train ◽  
Irregular Waves ◽  
Wave Crest ◽  
Local Enhancement ◽  
Sea State ◽  
Incoming Wave ◽  
Numerical Predictions ◽  
Very High ◽  
Crest Height

The deck height of a tension leg platform or semi-submersible depends in large part on the expected crest height. This expected crest height is the result of the sea state, i.e. the incoming wave train, and local enhancement due to the vessels diffraction of the wave train. These local enhancements are usually determined by a combination of numerical computations and model tests. Quite often a crest enhancement factor is defined which takes into consideration these local amplification effects. Extrapolating the enhancement factor from extreme conditions to survival conditions may lead to significantly large crests and result in a very high deck elevation. Many studies, including the CresT JIP address the characteristics of the crests within a given sea state and in the absence of a vessel. This paper addresses the effect of the presence of a vessel on the crest heights, and in particular the high crests which will ultimately determine deck height. The paper is based on experimental measurements of wave elevations underneath and around various tension leg platforms and semi submersibles. The investigated sea states comprise of a series of long crested irregular waves, generated in a model basin, which describe extreme and survival conditions in the Gulf of Mexico. The crest heights underneath the vessel are measured and compared with crests which occur without the presence of the vessel. Numerical predictions of the local amplification are also made, based on 1st order diffraction analysis and the as-measured incident wave train. A narrative is provided on the differences in crest height and observed phenomena.

scholarly journals Time Scale for Scour Beneath Pipelines Due to Long-Crested and Short-Crested Nonlinear Random Waves Plus Current

2021 ◽  
Vol 9 (2) ◽  
pp. 114
Author(s):  
Dag Myrhaug ◽  
Muk Chen Ong
Keyword(s):  
Time Scale ◽  
Current Flow ◽  
Irregular Waves ◽  
Wave Crest ◽  
Random Wave ◽  
Random Waves ◽  
Flow Conditions ◽  
Regular Waves ◽  
Nonlinear Random Waves ◽  
2D And 3D

This article derives the time scale of pipeline scour caused by 2D (long-crested) and 3D (short-crested) nonlinear irregular waves and current for wave-dominant flow. The motivation is to provide a simple engineering tool suitable to use when assessing the time scale of equilibrium pipeline scour for these flow conditions. The method assumes the random wave process to be stationary and narrow banded adopting a distribution of the wave crest height representing 2D and 3D nonlinear irregular waves and a time scale formula for regular waves plus current. The presented results cover a range of random waves plus current flow conditions for which the method is valid. Results for typical field conditions are also presented. A possible application of the outcome of this study is that, e.g., consulting engineers can use it as part of assessing the on-bottom stability of seabed pipelines.

scholarly journals Comprehensive Wind and Wave Statistics and Extreme Values for Design and Analysis of Marine Structures in the Adriatic Sea

2021 ◽  
Vol 9 (5) ◽  
pp. 522
Author(s):  
Marko Katalinić ◽  
Joško Parunov
Keyword(s):  
Adriatic Sea ◽  
Extreme Values ◽  
Wave Climate ◽  
Offshore Structures ◽  
Model Parameters ◽  
Peak Period ◽  
Significant Wave ◽  
Wave Statistics ◽  
Wave Heights ◽  
Almost All

Wind and waves present the main causes of environmental loading on seagoing ships and offshore structures. Thus, its detailed understanding can improve the design and maintenance of these structures. Wind and wave statistical models are developed based on the WorldWaves database for the Adriatic Sea: for the entire Adriatic Sea as a whole, divided into three regions and for 39 uniformly spaced locations across the offshore Adriatic. Model parameters are fitted and presented for each case, following the conditional modelling approach, i.e., the marginal distribution of significant wave height and conditional distribution of peak period and wind speed. Extreme significant wave heights were evaluated for 20-, 50- and 100-year return periods. The presented data provide a consistent and comprehensive description of metocean (wind and wave) climate in the Adriatic Sea that can serve as input for almost all kind of analyses of ships and offshore structures.

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