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1.
A comprehensive study is performed on the dynamic behavior of offshore wind turbine (OWT) structure supported on monopile foundation in clay. The system is modeled using a beam on nonlinear Winkler foundation model. Soil resistance is modeled using American Petroleum Institute based cyclic p–y and t–z curves. Dynamic analysis is carried out in time domain using finite element method considering wind and wave loads. Several parameters, such as soil–monopile–tower interaction, rotor and wave frequencies, wind and wave loading parameters, and length, diameter and thickness of monopile affecting the dynamic characteristics of OWT system and the responses are investigated. The study shows soil–monopile–tower interaction increases response of tower and monopile. Soil nonlinearity increases the system response at higher wind speed. Rotor frequency is found to have dominant role than blade passing frequency and wave frequency. Magnitude of wave load is important for design rather than resonance from wave frequency.  相似文献   

2.
The observation of extreme waves at FINO 1 during storm Britta on the 1st November 2006 has initiated a series of research studies regarding the mechanisms behind. The roles of stability and the presence of the open cell structures have been previously investigated but not conclusive. To improve our understanding of these processes, which are essential for a good forecast of similarly important events offshore, this study revisits the development of storm Britta using an atmospheric and wave coupled modeling system, wind and wave measurements from ten stations across the North Sea, cloud images and Synthetic Aperture Radar (SAR) data. It is found here that a standard state-of-the-art model is capable of capturing the important characteristics of a major storm like Britta, including the storm path, storm peak wind speed, the open cells, and peak significant wave height (H s ) for open sea. It was also demonstrated that the impact of the open cells has negligible contribution to the development of extreme H s observed at FINO 1. At the same time, stability alone is not sufficient in explaining the development of extreme H s . The controlling conditions for the development of Britta extreme H s observed at FINO 1 are the persistent strong winds and a long and undisturbed fetch over a long period.  相似文献   

3.
系统自振频率限制是海上风机结构设计中的一个关键因素。运行状态下风机动力荷载会引起基础的水平侧移,较大的水平侧移会导致基础刚度的降低,进一步影响风机系统的自振频率。该文基于有限元软件ABAQUS平台,建立单桩式海上风机结构系统的自振频率数值模型,并讨论运行状态下基础水平侧移对大直径海上风机系统自振频率的影响。模型中考虑了塔筒的变截面特性;桩-土相互作用通过p-y曲线方法模拟;桩和塔采用梁单元模拟;通过Pushover分析汇总出水平侧移引起的桩顶水平刚度。研究结果表明:桩基侧向位移会降低风机结构体系的自振频率;桩基侧向位移对基频的影响较小,对高阶频率的影响显著;大直径海上风机的频率计算中可忽略风机运行状态对体系自振频率的影响。  相似文献   

4.
Monopile foundations of offshore wind turbines modify the hydrodynamics and sediment transport at local and regional scales. The aim of this work is to assess these modifications and to parameterize them in a regional model. In the present study, this is achieved through a regional circulation model, coupled with a sediment transport module, using two approaches. One approach is to explicitly model the monopiles in the mesh as dry cells, and the other is to parameterize them by adding a drag force term to the momentum and turbulence equations. Idealised cases are run using hydrodynamical conditions and sediment grain sizes typical from the area located off Courseulles-sur-Mer (Normandy, France), where an offshore windfarm is under planning, to assess the capacity of the model to reproduce the effect of the monopile on the environment. Then, the model is applied to a real configuration on an area including the future offshore windfarm of Courseulles-sur-Mer. Four monopiles are represented in the model using both approaches, and modifications of the hydrodynamics and sediment transport are assessed over a tidal cycle. In relation to local hydrodynamic effects, it is observed that currents increase at the side of the monopile and decrease in front of and downstream of the monopile. In relation to sediment transport effect, the results show that resuspension and erosion occur around the monopile in locations where the current speed increases due to the monopile presence, and sediments deposit downstream where the bed shear stress is lower. During the tidal cycle, wakes downstream of the monopile reach the following monopile and modify the velocity magnitude and suspended sediment concentration patterns around the second monopile.  相似文献   

5.
A numerical study is conducted to investigate the impact of climate changes on ocean surface gravity waves over the eastern Canadian shelf (ECS). The “business-as-usual” climate scenario known as Representative Concentration Pathway RCP8.5 is considered in this study. Changes in the ocean surface gravity waves over the study region for the period 1979–2100 are examined based on 3 hourly ocean waves simulated by the third-generation ocean wave model known as WAVEWATCHIII. The wave model is driven by surface winds and ice conditions produced by the Canadian Regional Climate Model (CanRCM4). The whole study period is divided into the present (1979–2008), near future (2021–2050) and far future (2071–2100) periods to quantify possible future changes of ocean waves over the ECS. In comparison with the present ocean wave conditions, the time-mean significant wave heights (H s ) are expected to increase over most of the ECS in the near future and decrease over this region in the far future period. The time-means of the annual 5% largest H s are projected to increase over the ECS in both near and far future periods due mainly to the changes in surface winds. The future changes in the time-means of the annual 5% largest H s and 10-m wind speeds are projected to be twice as strong as the changes in annual means. An analysis of inverse wave ages suggests that the occurrence of wind seas is projected to increase over the southern Labrador and central Newfoundland Shelves in the near future period, and occurrence of swells is projected to increase over other areas of the ECS in both the near and far future periods.  相似文献   

6.
Preliminary design of offshore wind turbines requires high precision simplified methods for the analysis of the system fundamental frequency. Based on the Rayleigh method and Lagrange's Equation, this study establishes a simple formula for the analysis of system fundamental frequency in the preliminary design of an offshore wind turbine with a monopile foundation. This method takes into consideration the variation of cross-section geometry of the wind turbine tower along its length, with the inertia moment and distributed mass both changing with diameter. Also the rotational flexibility of the monopile foundation is mainly considered. The rigid pile and elastic middle long pile are calculated separately. The method is validated against both FEM analysis cases and field measurements, showing good agreement. The method is then used in a parametric study, showing that the tower length Lt, tower base diameter d0, tower wall thickness δt, pile diameter db and pile length Lb are the major factors influencing the fundamental frequency of the offshore wind turbine system. In the design of offshore wind turbine systems, these five parameters should be adjusted comprehensively. The seabed soil condition also needs to be carefully considered for soft clay and loose sand.  相似文献   

7.
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8.
An autonomous vessel, the Offshore Sensing Sailbuoy, was used for wave measurements near the Ekofisk oil platform complex in the North Sea (56.5º N, 3.2º E, operated by ConocoPhillips) from 6 to 20 November 2015. Being 100 % wind propelled, the Sailbuoy has two-way communication via the Iridium network and has the capability for missions of 6 months or more. It has previously been deployed in the Arctic, Norwegian Sea and the Gulf of Mexico, but the present study was the first test for wave measurements. During the campaign the Sailbuoy held position about 20 km northeast of Ekofisk (on the lee side) during rough conditions. Mean wind speed measured at Ekofisk during the campaign was 9.8 m/s, with a maximum of 20.4 m/s, with wind mostly from south and southwest. A Datawell MOSE G1000 GPS-based 2 Hz wave sensor was mounted on the Sailbuoy. Mean significant wave height (H s 1 min) measured was 3 m, whereas maximum H s was 6 m. Mean wave period was 7.7 s, while maximum wave height, H max, was 12.6 m. These measurements have been compared with non-directional Waverider observations at the Ekofisk complex. The agreement between the two data sets was very good, with a mean percent absolute error of 7 % and a linear correlation coefficient of 0.97. The wave frequency spectra measured by the two instruments compared very well, except for low H s (～1 m), where the motion of the vessel seemed to influence the measurements. Nevertheless, the Sailbuoy performed well during this campaign, and results suggest that it is a suitable platform for wave measurements in a broad range of sea conditions.  相似文献   

9.
The behaviour of offshore‐directed winds over coastal dune and beach morphology was examined using a combination of modelling (3‐D computational fluid dynamics (CFD)) and field measurement. Both model simulations and field measurements showed reversal of offshore flows at the back beach and creation of an onshore sediment transport potential. The influence of flow reversals on the beach‐dune transport system and foredune growth patterns has previously received little attention. Detailed wind flow measurements were made using an extensive array of mast‐mounted, 3‐D ultrasonic anemometers (50 Hz), arranged parallel to the dominant incident wind direction. Large eddy simulation (LES) of the offshore wind flow over the dune was conducted using the open‐source CFD tool openFOAM. The computational domain included a terrain model obtained by airborne LiDAR and detailed ground DGPS measurements. The computational grid (~22 million cells) included localized mesh refinement near the complex foredune terrain to capture finer details of the dune morphology that might affect wind flows on the adjacent beach. Measured and simulated wind flow are presented and discussed. The CFD simulations offer new insights into the flow mechanics associated with offshore winds and how the terrain steering of wind flow impacts on the geomorphological behaviour of the dune system. Simulation of 3‐D wind flows over complex terrain such as dune systems, presents a valuable new tool for geomorphological research, as it enables new insights into the relationship between the wind field and the underlying topography. The results show that offshore and obliquely offshore winds result in flow reversal and onshore directed winds at distances of up to 20 m from the embryo dune toe. The potential geomorphological significance of the findings are discussed and simple calculations show that incoming offshore and obliquely offshore winds with mean velocities over 13 m s?1 and 7 m s?1, respectively, have the potential to create onshore‐directed winds at the back beach with mean velocities above 3.3 m s?1. These are above the threshold of movement for dry sand and support previous conclusions about the significance of offshore winds in dune and beach budget calculations. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

10.
The effect of climate change on extreme waves in front of the Dutch coast   总被引：1，自引：1，他引：0  
Coastal safety may be influenced by climate change, as changes in wave conditions (height, period, direction) may increase the vulnerability of dunes and other coastal defences. Dune erosion depends on mean water level, storm surge height and wave conditions. In this paper, we investigate the change in wave conditions in the North Sea in a changing climate. Until now, the effect of climate change on annual maximum wave conditions has been investigated, while events with higher return periods are actually most damaging for the coast (e.g. severe dune erosion). Here, we use the 17-member Ensemble SimulationS of Extreme weather under Non-linear Climate changeE (ESSENCE) change of climate change simulations, to analyse A1b-induced changes in the mean wave climate, the annual maxima and wave conditions with return periods of up to 1:10,000?years in front of the Dutch coast. The mean wave climate is not projected to differ between 1961–1990 and 2071–2100, with both wave height (H s) and wave period (T m) remaining unaltered. In the annual maximum conditions, a decrease is projected; especially, the annual T m maximum decreases significantly by 0.3 to 0.6?s over the whole study area. Furthermore, we find that the direction of the annual maximum wave conditions shifts from north and north-west to west and south-west for both H s and T m. This is induced by a similar shift in the direction of the extreme wind speeds. Despite the decrease in annual maximum conditions, the return H s and T m are not projected to change significantly as a result of climate change in front of the Dutch coast for the period 2071–2100 relative to 1961–1990.  相似文献   

11.
Surf zone drifters and a current meter were used to study the nearshore circulation patterns in the lee of groynes at Cottesloe Beach and City Beach in Western Australia. The circulation patterns revealed that a persistent re-circulation cell was present in the lee of the groyne which was driven by changes in wave set-up resulting from lower wave heights in the lee of the groyne. The re-circulation consisted of a longshore current directed towards the groyne which was deflected offshore due to groyne resulting in a rip current along the groyne face. The offshore-flowing rip current and the incoming waves converged at the offshore extent of this circulation cell, with the deflection of the rip current parallel to the shoreline and then completing the recirculation through an onshore component. The Eulerian measurements revealed that 55% of the currents on the lee side of the groyne were directed offshore and that these currents had a maximum speed of 2 m s?1. Spectral analysis of the wave heights and the currents revealed several corresponding peaks in the measured spectral densities with timescales between 12 s and 50 min. Numerical simulations of an idealised beach with a shore-normal groyne were conducted using a circulation model driven by waves, and confirmed the formation of a persistent eddy in the lee of the groyne. Sensitivity studies indicated that the incident wave angle, wave period, and especially the wave height controlled the circulation. The eddy vorticity, a measure of an eddy's strength, increased roughly proportional to an increase in the incident wave energy flux.  相似文献   

12.
Severe sea states in the North Sea present a challenge to wave forecasting systems and a threat to offshore installations such as oil and gas platforms and offshore wind farms. Here, we study the ability of a third-generation spectral wave model to reproduce winter sea states in the North Sea. Measured and modeled time series of integral wave parameters and directional wave spectra are compared for a 12-day period in the winter of 2013–2014 when successive severe storms moved across the North Atlantic and the North Sea. Records were obtained from a Doppler radar and wave buoys. The hindcast was performed with the WAVEWATCH III model (Tolman 2014) with high spectral resolution both in frequency and direction. A good general agreement was obtained for integrated parameters, but discrepancies were found to occur in spectral shapes.  相似文献   

13.
Simulations from a numerical model address the impact of nearshore morphology on surf zone retention on, open coast, rip-channelled beaches exposed to shore-normal waves. In the model, rip channels are regularly spaced alongshore with a given spacing λ. For a given reference case bathymetry (λ= 200 m), rip current circulations retain floating material at a hourly rate R of about 80 % which is in line with most existing field and laboratory studies in similar settings. The influence of a surf zone rip-channel morphology on surf zone retention is evaluated by a number of morphologic parameters. Results show that rip spacing is important. The ratio of the surf zone width X s to rip spacing λ controls surf zone retention with R rapidly increasing with increasing X s /λ up to a threshold of about 1 above which R levels off to become asymptotic to 100 %. The impact of the presence of a rip head bar is profound but nonlinear. The onset of wave breaking across the rip head bar drives a weak seaward located circulation providing major pathways for surface water exiting the surf zone compartment. Additional simulations suggest that alongshore variations in the offshore bathymetry are important. Patterns in the wave field enforced by wave refraction and potentially wave breaking across offshore bathymetric anomalies can provide a conduit for transporting floating material out of the surf zone and into the inner shelf region. This has major implications for surf zone flushing by inner-bar rips on multiple-barred beaches and on beaches facing bathymetric anomalies on the inner shelf.  相似文献   

14.
Wave climate simulation for southern region of the South China Sea   总被引：2，自引：0，他引：2  
This study investigates long-term variability and wave characteristic trends in the southern region of the South China Sea (SCS). We implemented the state-of-the art WAVEWATCH III spectral wave model to simulate a 31-year wave hindcast. The simulation results were used to assess the inter-annual variability and long-term changes in the SCS wave climate for the period 1979 to 2009. The model was forced with Climate Forecast System Reanalysis winds and validated against altimeter data and limited available measurements from an Acoustic Wave and Current recorder located offshore of Terengganu, Malaysia. The mean annual significant wave height and peak wave period indicate the occurrence of higher wave heights and wave periods in the central SCS and lower in the Sunda shelf region. Consistent with wind patterns, the wave direction also shows southeasterly (northwesterly) waves during the summer (winter) monsoon. This detailed hindcast demonstrates strong inter-annual variability of wave heights, especially during the winter months in the SCS. Significant wave height correlated negatively with Niño3.4 index during winter, spring and autumn seasons but became positive in the summer monsoon. Such correlations correspond well with surface wind anomalies over the SCS during El Nino events. During El Niño Modoki, the summer time positive correlation extends northeastwards to cover the entire domain. Although significant positive trends were found at 95 % confidence levels during May, July and September, there is significant negative trend in December covering the Sunda shelf region. However, the trend appears to be largely influenced by large El Niño signals.  相似文献   

15.
There is a paucity of field data to describe the transition in nearshore circulation between alongshore, meandering and rip current systems. A combination of in‐situ current meters and surf zone drifters are used to characterize the nearshore circulation over a transverse bar and rip morphology at Pensacola Beach, Florida in the presence of relatively low energy oblique waves. Current speeds vary in response to the relative wave height ratio (Hs/h), which defines the degree and extent of breaking over the shoal. In the absence of wave breaking the nearshore circulation was dominated by an alongshore current driven by the oblique waves. As waves begin to break across the shoal (0.2<Hs/ h<0.5) the nearshore circulation is characterized by a meandering alongshore current. As conditions became more dissipative (Hs/h>0.5), the meandering current is replaced by an unsteady rip circulation that moves offshore between the shoals before turning alongshore in the direction of wave advance outside the surf zone. The increase in wave dissipation is associated with an increase in very low frequency (VLF) variations in the current speed across the shoal and in the rip channel that caused the circulation to oscillate between an offshore and an alongshore flow. The unsteady nature of the nearshore circulation is responsible for 55% of all surf zone exits under these more dissipative conditions. In contrast, only 29% of the drifters released from the shoal exited the surf zone and bypassed the adjacent shoal with the alongshore‐meandering current. While the currents had a low velocity (maximum of ~0.4 m s‐1) and would not pose a significant hazard to the average swimmer, the results of this study suggest that the transverse bar and rip morphology is sufficient to create an alongshore variation in wave dissipation that forces alongshore meandering and low‐energy rip circulation systems under oblique wave forcing. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

16.
The 1994 Shikotan earthquake tsunamis   总被引：1，自引：0，他引：1  
The 1994 Shikotan earthquake was one of the greatest earthquakes in recent years with a magnitude ofM s 8.0. A tsunami survey was conducted by Russian and U.S. geophysicists from October 16–30, 1994, less than two weeks after the earthquake. The survey results and a numerical hindcast simulation are reported. Tsunami focusing effect at locations supposedly sheltered by the island chain is discussed. Based on the obtained data, tsunamis which attacked Shikotan Island are characterized as long waves (the order of 10–20 min wave period) with a positive leading wave. Possible consequences of the positive leading wave form are discussed in relation to the observed minimal destruction of beach vegetation and relatively small transport of marine sediment onto the shore. The high-quality tide-gage record in Malokurilskaya Bay indicates the occurrence of a 53 cm subsidence at the site.  相似文献   

17.
Optimization of Surface Wave Identification and Measurement   总被引：1，自引：0，他引：1  
—?Accurate and reliable measurement of surface waves is important to Comprehensive Nuclear-Test-Ban Treaty (CTBT) monitoring because the M s :m b discriminant and its regional variants can in many cases unambiguously identify events as earthquakes or explosions. Surface wave processing at the International Data Center (IDC) is designed to be completely automated and is performed using the program Maxsurf. Maxsurf searches for surface wave characteristics in the expected surface wave arrival time window for all continuous long-period and broadband data in the IDC processing stream. The Prototype IDC GSETT3 Reviewed Event Bulletin (REB) now contains a very large and growing data set of surface wave measurements. Users of this data set need to be aware of processing changes and calibration errors in the GSETT3 experimental bulletin. The prototype International Monitoring System (IMS) surface wave detection threshold is approximately one magnitude unit lower than the detection threshold of other global networks that use visual identification of surface waves. Surface wave identification and measurement can be improved through development of regionalized earth models, phase-matched filtering and the use of path corrected spectral magnitudes in place of M s . Regionalized earth models are developed through tomographic inversion of a very large data set of phase and group velocity dispersion measurements. Discrimination capability can be improved through the use of maximum likelihood magnitudes and maximum likelihood upper bounds.  相似文献   

18.
A hybrid foundation for offshore wind turbines (OWT) is studied, combining a monopile of diameter d and length L with a lightweight circular footing of diameter D. The footing is composed of steel plates and stiffeners forming compartments, backfilled to increase the vertical load. A special pile–footing connection is outlined, allowing transfer of lateral loads and moments, but not of vertical loads. The efficiency of the hybrid foundation is explored through 3D finite element modelling. Hybrid foundations of L=15 m are comparatively assessed to an L=30 m reference monopile. A detailed comparison is performed focusing on a 3.5 MW OWT. While the moment capacity of the monopile is larger, the hybrid foundation exhibits stiffer response, outperforming the monopile in the operational loading range. Under cyclic loading, the hybrid foundation experiences less stiffness degradation and rotation accumulation. Besides installation, the cost savings depend on the design of the footing and buckling can be crucial. The rubble fill is shown to provide lateral restraint to the stiffeners, being beneficial for buckling prevention. Although seismic shaking is not critical in terms of capacity, it may lead to substantial accumulation of rotation and settlement. Combined with cyclic environmental loading, the latter may challenge the serviceability of the OWT, potentially leading to a reduction of its service life. To derive insights on the effect of seismic loading, two scenarios are investigated: (a) seismic loading; and (b) combined environmental and seismic loading. In the first case, even a D=15 m hybrid foundation may outperform the reference monopile. This is not the case for combined environmental and seismic loading, where a D=20 m hybrid system would be required to outperform the reference monopile.  相似文献   

19.
Measurements of midlatitude E region coherent backscatter obtained during four summers with SESCAT, a 50 MHz Doppler system operating in Crete, Greece, and concurrent ionosonde recordings from the same ionospheric volume obtained with a CADI for one of these summers, are used to analyse the long-term variability in echo and Es occurrence. Echo and Es layer occurrences, computed in percent of time over a 12-h nighttime interval, take the form of time sequences. Linear power spectrum analysis shows that there are dominant spectral peaks in the range of 2–9 days, the most commonly observed periods appearing in two preferential bands, of 2–3 days and 4–7 days. No connection with geomagnetic activity was found. The characteristics of these periodicities compare well with similar properties of planetary waves, which suggests the possibility that planetary waves are responsible for the observed long-term periodicities. These findings indicate also a likely close relation between planetary wave (PW) activity and the well known but not well understood seasonal Es dependence. To test the PW postulation, we used simultaneous neutral wind data from the mesopause region around 95 km, measured from Collm, Germany. Direct comparison of the long-term periodicities in echo and Es layer occurrence with those in the neutral wind show some reasonable agreement. This new evidence, although not fully conclusive, is the first direct indication in favour of a planetary wave role on the unstable midlatitude E region ionosphere. Our results suggest that planetary waves observation is a viable option and a new element into the physics of midlatitude Es layers that needs to be considered and investigated.  相似文献   

20.
The amplitude and time of maximum of the semidiurnal tide in the mesosphere and lower thermosphere above Davis varies through the summer months. In particular, the time of maximum can oscillate around a fixed value or, at some heights, shift by a whole cycle of local time. Recent studies of the semi-diurnal tide at the South Pole have suggested that an s=1 mode is common at high-southern latitudes. At mid-latitudes, the s=2 mode is thought to dominate suggesting that a region where these modes overlap is a possibility. Although it is not possible to discern which of these modes are present using single station data, the concept that more than one 12-h wave is present allows a new interpretation of the Davis semi-diurnal amplitudes and times of maximum. In this study, wind data obtained at Davis, Antarctica, during the summer of 1996/97 are used to show that the complex variations in time of maximum and amplitude can be explained by a sum of two waves. The simplest of models combines an invariant and a varying 12-h wave and yields characteristics for each component that are less complex than the original observation. There is also potential for changes to the interpretation of time of maximum vs. height profiles.  相似文献   

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