On the Average Shape of the Largest Waves in Finite Water Depths

2020 ◽  
Vol 50 (4) ◽  
pp. 1023-1043
Author(s):  
Ioannis Karmpadakis ◽  
Chris Swan
Keyword(s):  
Single Point ◽  
Random Wave ◽  
Wave Interactions ◽  
Frequency Spectra ◽  
Wave Measurements ◽  
Key Characteristics ◽  
Average Shape ◽  
Time Histories ◽  
Two Populations ◽  
Water Depths

AbstractThis paper investigates the average shape of the largest waves arising in finite water depths. Specifically, the largest waves recorded in time histories of the water surface elevation at a single point have been examined. These are compared to commonly applied theories in engineering and oceanographic practice. To achieve this both field observations and a new set of laboratory measurements are considered. The latter concern long random simulations of directionally spread sea states generated using realistic Joint North Sea Wave Project (JONSWAP) frequency spectra. It is shown that approximations related to the linear theory of quasi-determinism (QD) cannot describe some key characteristics of the largest waves. While second-order corrections to the QD predictions provide an improvement, key effects arising in very steep or shallow water sea states are not captured. While studies involving idealized wave groups have demonstrated significant changes arising as a result of higher-order nonlinear wave–wave interactions, these have not been observed in random sea states. The present paper addresses this discrepancy by decomposing random wave measurements into separate populations of breaking and nonbreaking waves. The characteristics of average wave shapes in the two populations are examined and their key differences discussed. These explain the mismatch between findings in earlier random and deterministic wave studies.

scholarly journals Quantifying wave measurement differences in historical and present wave buoy systems

2021 ◽  
Author(s):  
Robert Edward Jensen ◽  
Val Swail ◽  
Richard Harry Bouchard
Keyword(s):  
Complex Nature ◽  
Frequency Spectra ◽  
Measurement Systems ◽  
Wave Measurements ◽  
Directional Sensors ◽  
Wave Measurement ◽  
Western Us ◽  
Directional Wave ◽  
Wind Sea ◽  
Wave Properties

AbstractAn intra-measurement evaluation was undertaken, deploying a NOMAD buoy equipped with three National Data Buoy Center and two Environment and Climate Change Canada-AXYS sensor/payload packages off Monterey, California; a Datawell Directional Waverider buoy was deployed within 19 km of the NOMAD site. The six independent wave measurement systems reported hourly estimates of the frequency spectra, and when applicable, the four Fourier directional components. The integral wave parameters showed general agreement among the five sensors compared to the neighboring Datawell Directional Waverider, with the Inclinometer and the Watchman performing similarly to the more sophisticated 3DMG, HIPPY, and Triaxys sensor packages. As the Hm0 increased, all but the Inclinometer were biased low; however, even the Watchman reported reasonable wave measurements up to about 6–7 m, after which the Hm0 becomes negatively biased up to about a meter, comparable to previous studies. The parabolic fit peak spectral wave period, Tpp, results showed a large scatter, resulting from the complex nature of multiple swell wave systems compounded by local wind-sea development, exacerbated by a variable that can be considered as temporally unstable. The three directional sensors demonstrated that NOMAD buoys are capable of measuring directional wave properties along the western US coast, with biases of about 6 to 9 deg, and rms errors of approximately 30 deg. Frequency spectral evaluations found similarities in the shape, but a significant under estimation in the high frequency range. The results from slope analyses also revealed a positive bias in the rear face of the spectra, and a lack of invariance in frequency as suggested by theory.

scholarly journals Spectral shapes and parameters from three different wave sensors

2021 ◽  
Author(s):  
Anne Karin Magnusson ◽  
Robert Jensen ◽  
Val Swail
Keyword(s):  
Frequency Spectra ◽  
Measurement Systems ◽  
Wave Measurements ◽  
Climate Monitoring ◽  
Production Platform ◽  
Wave Sensors ◽  
Central North Sea ◽  
Wave Forecasting ◽  
Wave Properties

AbstractThe quality of wave measurements is of primary importance for the validation of wave forecasting models, satellite wave calibration and validation, wave physics, offshore operations and design and climate monitoring. Validation of global wave forecasts revealed significant regional differences, which were linked to the different wave buoy systems used by different countries. To fully understand the differences between the wave measurement systems, it is necessary to go beyond investigations of the integral wave parameters height, period and direction, into the frequency spectra and the four directional Fourier parameters that are used to estimate the directional distribution. We here analyse wave data measured from three different sensors (non-directional Datawell Waverider buoy, WaveRadar Rex, Optech laser) operating at the Ekofisk oil production platform located in the central North Sea over a period of several months, with significant wave height ranging from 1 to 10 m. In general, all three sensors provide similar measurements of the integral wave properties and frequency spectra, although there are some significant differences which could impact design and operations, forecast verification and climate monitoring. For example, the radar underestimates energy in frequency bands higher than 8 s by 3–5%, swell (12.5–16 s) by 5–13%, while the laser has 1–2% more energy than the Waverider in the most energetic bands. Lee effects of structures are also estimated. Lower energy at the frequency tail with the radar has an effect on wave periods (they are higher); wave steepness is seen to be reduced by 10% in the wind seas. Goda peakedness and the unidirectional Benjamin-Feir index are also examined for the three sensors.

Seismic Analysis of a Double-Hinged Articulated Offshore Tower

2008 ◽  
Author(s):  
Syed Danish Hasan ◽  
Nazrul Islam ◽  
Khalid Moin
Keyword(s):  
Seismic Analysis ◽  
Offshore Structures ◽  
Random Wave ◽  
Wave Forces ◽  
Offshore Structure ◽  
Concentrated Mass ◽  
Energy Principle ◽  
Rotational Angle ◽  
Sea Bed ◽  
Time Histories

The response of offshore structures under seismic excitation in deep water conditions is an extremely complex phenomenon. Under such harsh environmental conditions, special offshore structures called articulated structures are feasible owing to reduced structural weight. Whereas, conventional offshore structure requires huge physical dimensions to meet the desired strength and stability criteria, therefore, are uneconomical. Articulated offshore towers are among the compliant offshore structures. These structures consist of a ballast chamber near the bottom hinge and a buoyancy chamber just below the mean sea level, imparting controlled movement against the environmental loads (wave, currents, and wind/earthquake). The present study deals with the seismic compliance of a double-hinged articulated offshore tower to three real earthquakes by solving the governing equations of motion in time domain using Newmark’s-β technique. For this purpose Elcentro 1940, Taft 1952 and Northridge 1994 earthquake time histories are considered. The tower is modeled as an upright flexible pendulum supported to the sea-bed by a mass-less rotational spring of zero stiffness while the top of it rigidly supports a deck in the air (a concentrated mass above water level). The computation of seismic and hydrodynamic loads are performed by dividing the tower into finite elements with masses lumped at the nodes. The earthquake response is carried out by random vibration analysis, in which, seismic excitations are assumed to be a broadband stationary process. Effects of horizontal ground motions are considered in the present study. Monte Carlo simulation technique is used to model long crested random wave forces. Effect of sea-bed shaking on hydrodynamic modeling is considered. The dynamic equation of motion is formulated using Lagrangian approach, which is based on energy principle. Nonlinearities due to variable submergence and buoyancy, added mass associated with the geometrical non-linearities of the system are considered. The results are expressed in the form of time-histories and PSDFs of deck displacement, rotational angle, base and hinge shear, and the bending moment. The outcome of the response establishes that seismic sea environment is an important design consideration for successful performance of hinges, particularly, if these structures are situated in seismically active zones of the world’s ocean.

F(P)SO Global Responses in the West of Africa Squall Environment

2005 ◽  
Author(s):  
Zhibin Zhong ◽  
Yong Luo ◽  
Dusan Curic
Keyword(s):  
Time Series ◽  
Wind Speed ◽  
Wind Direction ◽  
Time History ◽  
The Past ◽  
Transient Nature ◽  
Time Histories ◽  
The Time Domain ◽  
Water Depths ◽  
Tandem Offloading

Mooring design for F(P)SOs in West of Africa offshore environment is in many cases governed by the squall driven condition. In the past, the squall condition was typically analyzed by using the peak wind speed with associated wind direction. However, due to its inherent transient nature, the squall event formulated in the time history with varying wind speed and direction is more appropriate and could be potentially more critical for the mooring system design. This approach has been adopted in the design and analysis of recent F(P)SO mooring systems. The F(P)SOs are turret-moored in various water depths in offshore West of Africa. A series of squall time histories have been applied to predict the global responses of the F(P)SO in the time domain. Each squall time history, which provides a unique combination of wind speed and direction variations, is analyzed in five nominal directions covering a sector of 90 degrees from East to West. Squall time histories are also applied to analyze the tandem offloading operation. The results are compared with those of the conventional constant wind speed approach and a few interesting observations are made. The paper also provides some insights into the F(P)SO yaw motions, as well as their relations to the changing wind direction. Analysis results demonstrate that using the squall time series with changing wind speed and direction is more critical than the conventional constant wind speed approach in the tandem offloading scenario. It is therefore recommended that mooring analysis using squall time series should at least be used for the tandem offloading simulations.

scholarly journals Space–Time Wave Extremes: The Role of Metocean Forcings

2015 ◽  
Vol 45 (7) ◽  
pp. 1897-1916 ◽  
Author(s):  
Francesco Barbariol ◽  
Alvise Benetazzo ◽  
Sandro Carniel ◽  
Mauro Sclavo
Keyword(s):  
Wind Speed ◽  
Wave Spectrum ◽  
Domain Size ◽  
Space Time ◽  
Statistical Prediction ◽  
Random Wave ◽  
Spectral Parameters ◽  
Frequency Spectra ◽  
Space Domain ◽  
Time Dynamics

AbstractWave observations and modeling have recently demonstrated that wave extremes of short-crested seas are poorly predicted by statistics of time records. Indeed, the highest waves pertain to wave groups at focusing that have space–time dynamics. Therefore, the statistical prediction of extremes of short-crested sea states should rely on the multidimensional random wave fields’ assumption. To adapt wave extreme statistics to the space–time domain, theoretical models using parameters of the directional wave spectrum have been recently developed. In this paper, the influence of metocean forcings (wind conditions, ambient current, and bottom depth) on these parameters and hence on wave extremes is studied with a twofold strategy. First, parametric spectral formulations [Pierson–Moskowitz and Joint North Sea Wave Project (JONSWAP) frequency spectra with cos2 directional distribution function] are considered to represent the dependence of wave extremes upon wind speed, fetch, and space domain size. Afterward, arbitrary conditions are simulated by using the SWAN numerical model adapted to store the spectral parameters, and the effects on extremes of current- and depth-induced shoaling are investigated. Preliminarily, the space–time extremes prediction model adopted is assessed by means of numerical simulations of Gaussian random seas. Compared to the significant wave height of the sea state and for a given space domain size, results show that space–time extremes are enhanced by opposite currents, whereas they are weakened by increasing wind conditions (wind speed and fetch) and by depth-induced shoaling. In this respect, the remarkable contribution to wave extremes of the size of the space domain is substantiated.

Detection of Cracks in Rotating Timoshenko Shafts Using Axial Impulses

1991 ◽  
Vol 113 (1) ◽  
pp. 74-78 ◽  
Author(s):  
K. R. Collins ◽  
R. H. Plaut ◽  
J. Wauer
Keyword(s):  
Piecewise Linear ◽  
Equations Of Motion ◽  
Crack Depth ◽  
Effective Means ◽  
Linear Equations ◽  
Free Vibrations ◽  
Frequency Spectra ◽  
Time Histories ◽  
Rotating Shafts ◽  
Linear Equations Of Motion

A rotating Timoshenko shaft with a single transverse crack is considered. The crack opens and closes during motion and is represented by generalized forces and moments. The shaft has simply supported ends, and the six coupled, piecewise-linear equations of motion (including longitudinal, transverse, and torsional displacements) are integrated numerically after application of Galerkin’s method with two-term approximations for each of the six displacements. Time histories and frequency spectra are compared for shafts with no crack and with a crack for which the crack depth is one-fifth of the shaft diameter. Free vibrations and the responses to a single axial impulse and periodic axial impulses are analyzed. The last case appears to provide an effective means for detecting cracks in rotating shafts.

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.

Christchurch Women's Hospital: Analysis of Measured Earthquake Data during the 2011–2012 Christchurch Earthquakes

2014 ◽  
Vol 30 (1) ◽  
pp. 383-400 ◽  
Author(s):  
Akshay Sridhar ◽  
Adam Kuang ◽  
Joseph Garven ◽  
Stefanie Gutschmidt ◽  
J. Geoffrey Chase ◽  
...  
Keyword(s):  
New Zealand ◽  
Response Patterns ◽  
Torsional Motion ◽  
Frequency Spectra ◽  
Displacement Sensors ◽  
Richter Scale ◽  
Key Characteristics ◽  
Extensive Range ◽  
Earthquake Data

A network of acceleration and displacement sensors installed in the Christchurch Women's Hospital (CWH) in July 2011 captured an extensive range of earthquake signals, allowing for a unique opportunity to analyze the performance of the New Zealand South Island's only base-isolated structure. Key characteristics of a range of earthquake signals, including frequency spectra and response patterns, are identified, with particular focus on the swarm of earthquakes on 23 December 2011, including four earthquake events greater than magnitude 5.0 on the Richter scale. The findings indicate that the response of the isolators and the superstructure was essentially elastic for the events analyzed during this period. Accelerations measured above and below the isolators were similar, indicating that the behavior of the devices resembled that of rigid blocks. No significant rocking or torsional motion of the building was observed.

scholarly journals WAVES IN WETLANDS: HURRICANE GUSTAV

2011 ◽  
Vol 1 (32) ◽  
pp. 29 ◽  
Author(s):  
Jane McKee Smith ◽  
Robert E. Jensen ◽  
Andrew B. Kennedy ◽  
J. Casey Dietrich ◽  
Joannes J. Westerink
Keyword(s):  
Circulation Model ◽  
Wave Model ◽  
High Energy ◽  
Generation Model ◽  
Wave Modeling ◽  
Wave Measurements ◽  
The Waves ◽  
Variable Water ◽  
Water Depths ◽  
Made In

Few wave measurements have been made in wetlands during high-energy, surge events, such as hurricanes. During Hurricane Gustav in 2008, many nearshore wave measurements were made in Southeastern Louisiana. These data are used to verify a nearshore wave modeling system and to explore the characteristics of hurricane waves in wetlands. The modeling system consists of the wave generation model WAM, the nearhsore wave model STWAVE, and the circulation model ADCIRC. The measurements confirm reasonable success in modeling the waves. The measurements and modeling also expose some of the problems of measuring waves in highly-variable water depths under hurricane forcing and modeling waves in rapidly degrading wetlands.

scholarly journals In situ measurements of directional wave spectra from an unmanned aerial system

2021 ◽  
Author(s):  
Clarence Collins ◽  
Katherine Brodie
Keyword(s):  
Single Point ◽  
Field Research ◽  
Ocean Waves ◽  
Adaptive Method ◽  
Technical Note ◽  
Sea Surface ◽  
Unmanned Aerial System ◽  
Frequency Spectra ◽  
Directional Spectrum ◽  
Data Adaptive

This Coastal and Hydraulics Engineering Technical Note (CHETN) describes the ability to measure the directional-frequency spectrum of sea surface waves based on the motion of a floating unmanned aerial system (UAS). The UAS used in this effort was custom built and designed to land on and take off from the sea surface. It was deployed in the vicinity of an operational wave sensor, the 8 m* array, at the US Army Engineer Research and Development Center (ERDC), Field Research Facility (FRF) in Duck, NC. While on the sea surface, an inertial navigation system (INS) recorded the response of the UAS to the incoming ocean waves. Two different INS signals were used to calculate one-dimensional (1D) frequency spectra and compared against the 8 m array. Two-dimensional (2D) directional-frequency spectra were calculated from INS data using traditional single-point-triplet analysis and a data adaptive method. The directional spectrum compared favorably against the 8 m array.

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