Dynamic Response of Articulated Tower Platforms to Random Sea Environment

2009 ◽  
Author(s):  
Mohd Moonis Zaheer ◽  
Nazrul Islam
Keyword(s):  
Dynamic Response ◽  
Excitation Frequency ◽  
Response Analysis ◽  
Wave Forces ◽  
Wind Gust ◽  
Wave Age ◽  
Wind Speeds ◽  
Positive Side ◽  
The Mean ◽  
Low Wind Speeds

Articulated tower motions have been characterized by rigid body mode of vibrations falling in the wind excitation frequency range due to its compliant nature. Dynamic response analysis of a multi hinged articulated tower platform to random wind and wave forces are presented in this paper. The wave forces on the submerged elements of the tower are calculated by using Morison’s Equation. The fluctuating wind is modeled with Ochi and Shin wind gust spectrum. The effect of wave age (young, intermediate and fully developed waves) on the wind gust spectrum is incorporated by adopting the wave age dependent Volkov, and wave age independent Charnock sea surface roughness models. The response of the tower is determined by a time domain iterative method. An example of results demonstrates the clear effect of wave age on the nonlinear dynamic response on the system. The mean wind modifies the mean position of the surge response to the positive side, causing an offset. Moreover, for high mean wind speeds the total wind response is much smaller than the wave response, but for low wind speeds the wind appears to be more important.

Dynamic Response Analysis of a Spar Platform Subjected to Wind and Wave Forces

2008 ◽  
Author(s):  
Bernt J. Leira ◽  
Dag Myrhaug ◽  
Jarle Voll
Keyword(s):  
Dynamic Response ◽  
Roughness Parameter ◽  
Response Analysis ◽  
Wave Forces ◽  
Wind Gust ◽  
Wave Age ◽  
Spar Platform ◽  
Dynamic Response Analysis ◽  
Wind Speeds ◽  
Wind Response

Results from a study on dynamic response analysis of a floating production unit (FPSO) excited by wave and wind forces are presented. The FPSO is examplified by a Spar platform considering the motion in surge and pitch. The wind gust is modelled with the Harris [4] and Ochi and Shin [7] wind gust spectra. The effect of the wave age on the wind gust spectrum is included by adopting the Volkov wave age dependent sea surface roughness parameter [10]; the wave age independent Charnock roughness parameter [2] is also used as a reference. Examples of results demonstrate clear effects of wave age on the dynamic response. Moreover, for high mean wind speeds the total wind response is much smaller than the wave response, but for low mean wind speeds the wind appears to be more important.

scholarly journals Rapid coral mortality following unusually calm and hot conditions on Iriomote, Japan

F1000Research ◽  
2018 ◽  
Vol 6 ◽  
pp. 1728 ◽  
Author(s):  
Andrew H Baird ◽  
Sally A. Keith ◽  
Erika Woolsey ◽  
Ryuta Yoshida ◽  
Tohru Naruse
Keyword(s):  
Water Movement ◽  
Sea Surface ◽  
Bleaching Event ◽  
Coral Mortality ◽  
Average Wind Speed ◽  
Common Cause ◽  
Wind Speeds ◽  
Average Wind ◽  
The Mean ◽  
Low Wind Speeds

Coral bleaching can be induced by many different stressors, however, the most common cause of mass bleaching in the field is higher than average sea surface temperatures (SST). Here, we describe an unusual bleaching event that followed very calm sea conditions combined with higher than average SST. Patterns of mortality differed from typical bleaching in four ways: 1) mortality was very rapid; 2) a different suite of species were most affected; 3) tissue mortality in Acropora spp. was often restricted to the center of the colony; 4) the event occurred early in summer. The two weeks prior to the event included 8 days where the average wind speed was less than 3 ms-1. In addition, SSTs in the weeks preceding and during the event were 1.0-1.5°C higher than the mean for the last 30 years. We hypothesize that this unusual bleaching event was caused by anoxia resulting from a lack of water movement induced by low wind speeds combined with high SST.

scholarly journals On the influence of wind and waves on the underwater light field

2012 ◽  
Vol 9 (2) ◽  
pp. 1231-1272
Author(s):  
M. Hieronymi ◽  
A. Macke
Keyword(s):  
Gravity Waves ◽  
Light Field ◽  
High Spatial Resolution ◽  
Radiative Transfer Model ◽  
Transfer Model ◽  
Ocean Mixed Layer ◽  
Underwater Light Field ◽  
Wind Speeds ◽  
The Mean ◽  
Low Wind Speeds

Abstract. We present a detailed study of the influence of various wind and wave conditions on the distribution of downwelling irradiance within the upper ocean mixed layer down to 100 m water depth. The work is based on a two-dimensional Monte Carlo radiative transfer model with high spatial resolution. We treat conditions that are favorable for the development of extreme light fluctuations, e.g. light at 490 nm and very clear oceanic water. Local wind determines the steepness of capillary-gravity waves which in turn dominate the irradiance variability near the surface. Maximum irradiance peaks that exceed the mean irradiance by a factor of more than 10 can be observed at low wind speeds up to 5 m s−1. Sea states influence the light field much deeper; gravity waves can cause considerable irradiance variability even at 100 m depth. The simulation results show that under realistic conditions 50 % radiative enhancements compared to the mean can still occur at 30 m depth.

scholarly journals Rapid coral mortality following doldrums-like conditions on Iriomote, Japan

F1000Research ◽  
2017 ◽  
Vol 6 ◽  
pp. 1728 ◽  
Author(s):  
Andrew H Baird ◽  
Sally A. Keith ◽  
Erika Woolsey ◽  
Ryuta Yoshida ◽  
Tohru Naruse
Keyword(s):  
Water Movement ◽  
Sea Surface ◽  
Bleaching Event ◽  
Coral Mortality ◽  
Average Wind Speed ◽  
Wind Speeds ◽  
Average Wind ◽  
Thermal Bleaching ◽  
The Mean ◽  
Low Wind Speeds

Coral bleaching can be induced by many different stressors, however, the most common cause of mass bleaching in the field is high sea temperatures (SST). Here, we describe an unusual bleaching event that followed very calm sea conditions combined with higher than average sea surface temperatures (SST). Patterns of mortality differed from typical thermal bleaching in four ways: 1) mortality was very rapid; 2) the suite of species most affected was different; 3) tissue mortality in Acropora spp. was often restricted to the center of the colony; 4) the event occurred early in the summer. The two weeks prior to the event included 8 days where the average wind speed was less than 3 ms-1. In addition, SSTs in the weeks preceding this event were 1.0-1.5°C higher than the mean for the last 30 years. We hypothesize that the lack of water movement induced by low wind speeds combined with high SST to cause colonies anoxic stress resulting in this unusual bleaching event.

scholarly journals Automated Wind Turbine Wake Characterization in Complex Terrain

2019 ◽  
Author(s):  
Rebecca J. Barthelmie ◽  
Sara C. Pryor
Keyword(s):  
Wind Turbine ◽  
Complex Terrain ◽  
Atmospheric Stability ◽  
Average Height ◽  
Wind Speeds ◽  
Stable Conditions ◽  
The Mean ◽  
Low Wind Speeds ◽  
Turbine Wake ◽  
Wind Turbine Wake

Abstract. An automated wind turbine wake characterization algorithm has been developed and applied to a dataset of over 19,000 scans measured by scanning Doppler lidar at Perdigão over the period January to June 2017. The algorithm correctly identifies the wake centre position in 62 % of possible wake cases, 46 % having a clear and well-defined wake centre while 16 % have split centres or multiple lobes. Only 5 % of cases are not detected, the remaining 33 % could not be categorized either by the algorithm or subjectively, mainly due to the complexity of the background flow. Average wake centre heights categorized by inflow wind speeds are shown to be initially lofted (to 2 rotor diameters, D) except when the inflow wind speeds exceed 12 ms−1. Even under low wind speeds, by 3.5 D downstream of the wind turbine, the mean wake centre position is below the initial wind turbine hub-height and descends broadly following the terrain slope. However, this behaviour is strongly linked to hour of the day and atmospheric stability. Overnight and in stable conditions the average height of the wake centre is 10 m higher than in unstable conditions at 2 D and 17 m higher at 4.5 D downstream of the wind turbine.

scholarly journals On the influence of wind and waves on underwater irradiance fluctuations

Ocean Science ◽  
2012 ◽  
Vol 8 (4) ◽  
pp. 455-471 ◽  
Author(s):  
M. Hieronymi ◽  
A. Macke
Keyword(s):  
Gravity Waves ◽  
Radiative Transfer Model ◽  
Transfer Model ◽  
Sea State ◽  
Wind Speeds ◽  
Light Fluctuations ◽  
Underwater Irradiance ◽  
The Mean ◽  
Simulation Results ◽  
Low Wind Speeds

Abstract. The influence of various wind and wave conditions on the variability of downwelling irradiance Ed (490 nm) in water is subject of this study. The work is based on a two-dimensional Monte Carlo radiative transfer model with high spatial resolution. The model assumes conditions that are ideal for wave focusing, thus simulation results reveal the upper limit for light fluctuations. Local wind primarily determines the steepness of capillary-gravity waves which in turn dominate the irradiance variability near the surface. Down to 3 m depth, maximum irradiance peaks that exceed the mean irradiance Ed by a factor of more than 7 can be observed at low wind speeds up to 5 m s−1. The strength of irradiance fluctuations can be even amplified under the influence of higher ultra-gravity waves; thereby peaks can exceed 11 Ed. Sea states influence the light field much deeper; gravity waves can cause considerable irradiance variability even at 100 m depth. The simulation results show that under realistic conditions 50% radiative enhancements compared to the mean can still occur at 30 m depth. At greater depths, the underwater light variability depends on the wave steepness of the characteristic wave of a sea state; steeper waves cause stronger light fluctuations.

Dynamic Response of a Turbulent Cylinder Wake to Forced Excitation

2009 ◽  
Author(s):  
Efstathios Konstantinidis ◽  
Chunlei Liang
Keyword(s):  
Dynamic Response ◽  
Dimensionless Parameter ◽  
Excitation Frequency ◽  
Three Dimensional ◽  
Transverse Force ◽  
Cylinder Wake ◽  
Mean Velocity ◽  
Large Eddy ◽  
The Mean ◽  
Forced Excitation

Three-dimensional large-eddy simulations were carried out to determine the dynamic response of a turbulent cylinder wake to forced excitation at a subcritical Reynolds number of 2580. The excitation frequency was varied across the primary lock-on range while the amplitude of sinusoidal velocity perturbation and the mean velocity imposed at the inflow boundary were kept constant. The velocity fluctuations in the wake and the fluctuating forces on the cylinder are analyzed employing spectral, time-domain and phase-portrait methods. The results show that the dynamic response of the inline force is different than that of the transverse one on the border of the lock-on range; while the inline force exhibits a phase-locked response, the transverse force indicates an intermittent response. This behavior is linked to the wake dynamics which is similar to that of the transverse force. This result is explained on the basis of the Morison equation which shows that the inline force is biased by the inertial components associated with the added mass and pressure waves in unsteady flows. It is further shown that the existence of a mean velocity component alters radically the dynamics of the inline force and appropriate ranges of a dimensionless parameter are proposed to describe the response.

scholarly journals The Share of the Mean Turbulent Kinetic Energy in the Near-Neutral Surface Layer for High and Low Wind Speeds

2019 ◽  
Vol 172 (1) ◽  
pp. 81-106 ◽  
Author(s):  
M. Schiavon ◽  
F. Tampieri ◽  
F. C. Bosveld ◽  
M. Mazzola ◽  
S. Trini Castelli ◽  
...  
Keyword(s):  
Surface Layer ◽  
Kinetic Energy ◽  
Turbulent Kinetic Energy ◽  
Neutral Surface ◽  
Wind Speeds ◽  
The Mean ◽  
Low Wind Speeds ◽  
Neutral Surface Layer

Analysis of Offshore Guyed Tower Platforms to Wind and Wave Forces

1998 ◽  
Vol 120 (4) ◽  
pp. 256-262 ◽  
Author(s):  
R. S. Bisht ◽  
T. K. Datta ◽  
A. K. Jain
Keyword(s):  
Dynamic Response ◽  
Spectral Density ◽  
Power Spectral Density ◽  
Wind Velocity ◽  
Parametric Study ◽  
Response Analysis ◽  
Wave Forces ◽  
Wind Force ◽  
Displacement Response ◽  
Power Spectral

The fundamental frequency of guyed tower platforms is designed to be low, and, therefore, it is susceptible to low-frequency excitation caused by fluctuating components of the wind velocity. The dynamic response of a simplified model of the Lena guyed tower is obtained for the fluctuating component of the wind force and the wave force. Both wind velocity and water particle kinematics due to wave motion are modeled as stationary random processes and are simulated from their respective power spectral density functions. The response analysis is performed by an iterative frequency domain procedure which duly considers the nonlinearities produced due to the drag effect and the nonlinear guyline resistance. The responses are also obtained in the presence of current. A parametric study is conducted to evaluate the relative contribution to the responses made by the wind force. The parametric study also illustrates how the current modifies the overall dynamic response. It is shown that the wind force significantly influences the first mode displacement response of the tower; the bending moment and shear force are not much affected by it. The current velocity tends to modify the peaks of the power spectral density function of the displacement response.

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