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Petya G. Petrova 《Ocean Engineering》2011,38(4):658-672
The paper presents results for the distribution of wave heights from laboratory generated bimodal sea states. Data collected at the DHI offshore basin are analyzed and compared with results based on wave records from the MARINTEK offshore basin. The comparisons are done for three groups of mixed sea states: wind-sea dominated, swell-dominated and energy-equivalent, determined on the basis of the parameter sea-swell energy ratio (SSER), which have been generated according to the model of Guedes Soares (1984). In some sea states abnormal or freak waves have been observed.The quasi-determinism theory of Boccotti is used to expand some linear narrowband models to second order, thus providing validation of the adequacy of the equations to represent the linear components of the wave heights. Also, the data are compared with the predictions of a third order model using as a nonlinear correction the coefficient of kurtosis. Due to the coexistence of wind-sea and swell, the core of the autocovariance function in some cases demonstrates a global minimum which is the second local minimum in the sequence. This can affect the fitting ability of distributions whose parameters depend on the form of the autocorrelation function or its envelope.The results for MARINTEK and DHI data show similar patterns of fit between predicted and observed exceedance probabilities for the considered classes of bimodal spectra. 相似文献
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The temporal behaviour of swell is examined using the wavelet transform, and a test of stationarity using the wavelet transform is described. The method is demonstrated on time series data from a Directional Waverider buoy deployed at Duck, North Carolina, and it is shown that non-stationary sea states are usually associated with local wind-sea growth rather than significant changes in the swell component, which can be considered stationary for at least as long as the 160-min records. This is close to the 3-h duration that is typically assumed for a stationary sea state. The stationary character of swell is an important result for offloading from an LNG barge to carrier. 相似文献
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基于理论的气旋条件下双极化C波段合成孔径雷达波参数检测 总被引：2，自引：1，他引：1
Theoretical-based ocean wave retrieval algorithms are applied by inverting a synthetic aperture radar(SAR)intensity spectrum into a wave spectrum, that has been developed based on a SAR wave mapping mechanism. In our previous studies, it was shown that the wave retrieval algorithm, named the parameterized first-guess spectrum method(PFSM), works for C-band and X-band SAR at low to moderate sea states. In this work, we investigate the performance of the PFSM algorithm when it is applied for dual-polarization c-band sentinel-1(S-1) SAR acquired in extra wide-swath(EW) and interferometric wide-swath(IW) mode under cyclonic conditions.Strong winds are retrieved from six vertical-horizontal(VH) polarization S-1 SAR images using the c-band crosspolarization coupled-parameters ocean(C-3 PO) model and then wave parameters are obtained from the image at the vertical-vertical(VV) polarization channel. significant wave height(SWH) and mean wave period(MWP) are compared with simulations from the WAVEWATCH-III(WW3) model. The validation shows a 0.69 m root mean square error(RMSE) of SWH with a –0.01 m bias and a 0.62 s RMSE of MWP with a –0.17 s bias. Although the PFSM algorithm relies on a good quality SAR spectrum, this study confirms the applicability for wave retrieval from an S-1 SAR image. Moreover, it is found that the retrieved results have less accuracy on the right sector of cyclone eyes where swell directly affects strong wind-sea, while the PFSM algorithm works well on the left and rear sectors of cyclone eyes where the interaction of wind-sea and swell is relatively poor. 相似文献
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The Louis scheme and the COARE algorithm （version 3.0） are tested against eddy covariance and inertial dissipation methods for friction velocity estimates in different wind-sea/swell regimes. Atmospheric forcing data, tabulated by Donelan et al. （1997.J Phys Oceanog, 27：2087-2099）, were collected from a mast on the foredeck of a SWATH （small water-plane area, twin hull） ship in deep sea off the State of Virginia during the surface wave dynamics experiment. These data are representative of low to moderate wind regimes. The aerodynamic roughness length is determined by using the Charnock relationship. The intercomparison shows that the Louis scheme and the COARE algorithm underestimate the friction velocity by 6% and 3% respectively under pure wind sea conditions, 15% and 13% respectively under cross swell conditions, and 21% and 17% respectively under counter swell conditions. The analysis shows that these underestimations were caused by the method chosen to determine the aerodynamic roughness length because it significantly underestimates the aerodynamic roughness length. It is especially true under the cross swell and counter swell conditions. 相似文献
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A study of the seakeeping performance of a set of fishing vessels is carried out aiming to identify the seakeeping criteria, and vessel conditions that limit the operability of the fishing vessels in certain sea states. Ship motions and derived responses are obtained in fully developed sea states using the transfer functions of the hull forms. Those responses are assessed against the prescribed values, for the chosen criteria, to determine the vessels operational conditions that might result in hazards or seasickness. For the purpose of this study, each fishing vessel is considered operating in sea states 5 and 6, with different Froude numbers and heading angles, and their short term responses are assessed against the most relevant criteria related with the absolute and relative motions, accelerations, slamming and green water on deck. The results obtained show that roll and pitch criteria are most critical for seakeeping performance, and there is a significant influence of the transverse metacentric height, GMt, and the location of the reference checking points in the seakeeping performance of these fishing vessels. 相似文献
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A reliability based approach for analysis of offloading operations with two identical LNG carriers moored in a side-by-side configuration is suggested. The approach, although adopted for two tankers, is generally applicable to different types of LNG terminals. Different heading controls are conceivable for the discharging vessel in the case that the heading is not fixed: heading toward wind-sea, heading toward swell, heading toward wind and free weather vaning. The analyses show that heading toward the governing sea state is the most beneficial option for two identical LNG carriers moored in a side-by-side configuration. Strategies are discussed for estimation of limiting weather criteria that are consistent with prescribed target failure probabilities. 相似文献
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The degree of dependence between successive wave heights and periods is examined for sea states resulting from the combination of a remotely generated wave field and a locally generated wave system, based on simulated wave records. The sea states analysed represent situations that are swell dominated, wind–sea dominated or they have equivalent energy in the wind–sea and swell components. Results of the analysis of the simulated data have been compared with those expected from the theories for the joint distributions of consecutive wave heights and periods and with the results from a Pierson–Moskowitz target spectrum. 相似文献
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This paper concerns the calculation of the wave crest height exceedance probabilities in fully nonlinear mixed sea states. The exceedance probabilities have been calculated by incorporating a fully nonlinear wave model into a Transformed Rayleigh method. This is an efficient approach to the calculation of wave crest exceedance probabilities and, as all of the calculations are performed in the probability domain, avoids the need for long time-domain simulations. The nonlinear mixed sea states studied include a swell dominated sea state, two wind sea dominated sea states, and two states of mixed wind sea and swell with comparable energy. The wave steepness influence and the finite water depth effects are also considered in the study. The accuracy and efficiency of the Transformed Rayleigh method are validated by comparing the results predicted using the method with those predicted by using the Monte Carlo simulation method, the theoretical Rayleigh method and some empirical formulas. 相似文献
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When long, fast swell waves travel in approximately the same direction as the wind, the surface stress is reduced compared with under wind-sea conditions. Using measurements from the Östergarnsholm site in the Baltic Sea, new expressions of the roughness length were developed for wind sea and swell. These new expressions were implemented in the RCA3 regional climate model covering Europe. A 3-year simulation and two case studies using the wavefield from the ECMWF reanalysis (ERA-40) were analysed using the improved formulations. Wind-following swell led to a significant reduction of mean wind stress and heat fluxes. The mean surface layer wind speed was redistributed horizontally and the marine boundary layer cooled and dried slightly. This cooling was most pronounced over North Sea and the Norwegian Sea (almost 0.2 °C annually on average) whereas the drying was most pronounced over the Mediterranean Sea (almost 0.4 g kg−1 ). Somewhat less convective precipitation and low-level cloudiness over the sea areas were also indicated, in particular over the Mediterranean Sea. The impact on the atmosphere, however, is significantly locally greater in time and space. 相似文献
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基于星载波谱仪海浪方向谱探测原理，仿真了不同海况、风速下的海浪波谱仪接收信号，并采用周期图法、Welch法、AR模型法以及最小方差法共4种不同的调制谱估计方法反演出海浪谱，比较各种调制谱估计方法的海浪方向谱反演性能。仿真结果表明:对于一定方位向下的一维海浪谱反演，不同调制谱估计方法反演海浪谱性能优劣没有绝对的顺序。对于二维海浪谱反演，在成长中海浪条件下周期图法反演性能最差，其他3种方法的反演性能没有绝对优劣顺序；对于成熟风浪，最小方差法在积分能量误差、有效波高误差两个指标上的反演性能最好，在主波波向、主波波长误差上，周期图法反演性能最差，其他3种方法没有绝对优劣顺序。在涌浪条件下，AR模型法反演性能优于其他3种方法。在不同海况下，随着波谱仪中心入射角的下降，反演性能会下降。基于这些仿真结果，本文推荐最小方差法作为充分成长海浪的海浪方向谱反演的调制谱估计方法，AR模型法作为涌浪海浪方向谱反演的调制谱估计方法。 相似文献
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An extensive database of one-dimensional spectra observed during the passage of hurricanes is examined. Spectra recorded within eight times the radius to maximum winds of the centers of hurricanes are unimodal and remarkably similar to previously recorded fetch limited spectra. Parametric spectral forms such as JONSWAP and that of Donelan et al. approximate the data well. In addition, the functional dependence of the spectral parameters α and γ on the inverse wave age is consistent with that observed under fetch limited conditions. It is concluded that the similarity of the hurricane wave spectra to those observed for simpler fetch limited spectra is largely due to the effects of nonlinear interactions. The shape stabilizing effects of the nonlinear interactions reshape spectra consisting of a mix of swell and wind-sea to conform with typical one-dimensional fetch limited wind-sea forms. 相似文献
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The overall performance of ships depends on the seakeeping performance in specified sea areas where the vessel is designed to operate. The seakeeping performance procedure is based upon the probability of exceeding specified ship motions in a sea environment particular to the vessel's mission. Given the operational area of the vessel, the percentage of time the vessel operates in a particular sea state can be determined from an oceanographic database through application of the response amplitude operators. The predicted motions are compared to the motion limiting criteria to obtain the operability indices. However, the operability indices are strongly affected by the chosen limiting criteria. This is particularly the case for passenger vessels where many conflicting criteria are used to assess the effect of motions and accelerations on comfort and well-being of passengers. This paper investigates the effect of seakeeping criteria on seakeeping performance assessment for passenger vessels. Conventional seakeeping performance measures are evaluated for various levels of vertical accelerations defined by the ISO 2631 standard. It is shown that the estimated seakeeping performance of a passenger vessel greatly depends on the level of limiting value selected as the seakeeping criteria. 相似文献
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Abstract. Africa's southernmost coral reefs are situated in Natal Province, South Africa. The Natal coast is exposed to open Ocean swells and episodic storm swell conditions. Benthic communities on these reefs differentiated into three community types: shallow reefs (8–18 m) were dominated by alcyonacean corals and low-growing, massive Scleractinia; intermediate reefs (18–25 m) were dominated chiefly by branching and tabular Scleractinia of the genus Acropora (A. austera, A. clathrata); deep reefs were not dominated by corals but by sponges. Breakage and recovery experiments indicated that the difference in Acropora dominance between shallow and intermediate sites was caused by breakage in high swell conditions. Survival of experimentally produced A. austera fragments was significantly higher in intermediate than in shallow sites, where higher surge made re-attachment and regeneration unlikely. Also, colony morphology was adapted to differential surge conditions: colonies on the shallow reefs were smaller with shorter branches, while on intermediate reefs they were much bigger with long, widely spreading branches. Episodic breakage and low fragment survival due to high water-motion thus excluded branching corals from shallow reef sites. 相似文献
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This study aims at assessing the adequacy for describing bimodal sea states of different non-linear probability distributions that have been developed for single sea states. It is based on data collected at an offshore test basin. The measurements represent three bimodal sea states with individual unidirectional wave systems propagating at 60, 90 and 120 from each other. The wave spectra are separated into swell and wind sea components and the relative energy ratio between the areas under the associated spectral curves is estimated and is related with the statistics of the time series considered. Dependence is found between the normalized high order cumulants, which describe the non-Gaussian surface, and the predominant contribution of the wind sea energy. Furthermore, the probabilities of exceedance of the individual wave heights are estimated and compared with the Rayleigh model and with other models that take into account either the effect of spectral bandwidth or the effect of wave nonlinearities. The results are discussed with respect to three classes of sea states that reflect the relative contribution of swell and wind sea energy. 相似文献
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We consider the influence of the sea surface state on the backscattered radar cross section and the accuracy of the wind speed retrieval from the scatterometer data. We used a joint set of radars and buoys to determine the type of sea waves. Three types of sea waves were distinguished: developing wind waves, fully developed wind waves, and mixed sea. It is shown that the retrieval error of the near surface wind speed using a one-parameter algorithm is minimal in the case of fully developed wind waves. We compared these data with the results of radio-altimeter data analysis and showed that in both cases underestimation of the retrieval wind speed exists for developing wind waves and overestimation occurs for mixed sea. A variety of swell parameters (length of the dominating wave, swell height, swell age) significantly influence the backscattered radar cross section, leading to a growth in the mean square error of the retrieved wind speed during vertical sounding (radio-altimeter data), and only slightly influence the mean square error of the scatterometer data (medium incidence angles). It is necessary to include the information about the parameters of sea waves in the algorithms and take into account the regional wave properties to increase the accuracy of wind speed retrieval. 相似文献
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The presented work aims at validating the generalization of the asymptotic distribution model of Boccotti for large wave heights recently proposed by Tayfun [1] to laboratory generated mixed sea states with two-peak spectra. The input wave spectra are modelled as the sum of two JONSWAP spectra describing unidirectional wave systems with different or identical directions of propagation (crossing or following mixed seas). In order to account for the effect of the energetically dominant wave system on the largest observed waves, the Boccotti's parameters were calculated at the absolute minimum of the autocorrelation function which can differ from the first minimum for some cases of mixed seas, such as those dominated by the swell or seas with comparable contribution of the two spectral components. So far the proposed model has been validated elsewhere against samples of large wave heights exceeding the significant wave height in wind seas and in mechanically generated long-crested seas, both characterized by unimodal spectra and strong third-order nonlinearities. The present study demonstrates that it can predict equally well the tail of the distributions for mixed seas, irrespective of the type of the mixed sea, particularly when the third-order statistics is relatively large. Typically, the mixed seas from the considered offshore basin experiment display such conditions as the propagation distance from the wavemaker increases, though this effect is less pronounced for mixed following than for mixed crossing wave conditions. Moreover, the generalized model remains valid irrespective of the sign of the fourth-order sum Λ which is a key parameter of the distribution. Its good predictive ability is quantified here by the root-mean-square errors between observations and theory. 相似文献
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Accurate wave climate characterization, which is vital to understand wave-driven coastal processes and to design coastal and offshore structures, requires the availability of long term data series. Where existing data are sparse, synthetically generated time series offer a practical alternative. The main purpose of this paper is to propose a methodology to simulate multivariate hourly sea state time series that preserve the statistical characteristics of the existing empirical data. This methodology combines different techniques such as univariate ARMAs, autoregressive logistic regression and K-means clusterization algorithms, and is able to take into account different time and space scales. The proposed methodology can be broken down into three interrelated steps: i) simulation of sea level pressure fields, ii) simulation of daily mean sea conditions time series and iii) simulation of hourly sea state time series. Its effectiveness is demonstrated by synthetically generating multivariate hourly sea states from a specific location near the Spanish Coast. The direct comparison between simulated and empirical time series confirms the ability of the developed methodology to generate multivariate hourly time series of sea states. Finally, the potential of the proposed methodology to simulate multivariate time series at multiple locations and incorporate climate change issues is discussed. 相似文献
19.
C. Graham 《Coastal Engineering》1982,6(4)
This study sets out to define the basic forms in which wind speed and wave height persistence statistics may be defined for offshore engineering applications, and describes the development of a mathematical persistence model.The model incorporates some of the principles laid down by other workers, but it is fundamentally based on a new concept for parameterising persistence statistics, linking wind speed and sea state percentage probability of exceedance with the persistence average duration.North Sea measured wind and wave data have been used to calibrate and test the model. During the course of these test runs it proved necessary to fine tune the basic Weibull equation of the model, but following these adjustments the model runs were found to correlate well with the measured data.It is concluded that the model may be used to predict wind speed and wave height persistence statistics with acceptable accuracy for preliminary stage oil industry planning purposes and that the calibrated model has particular application for those areas where little measured data are currently available. 相似文献
20.
High-resolution acoustic measurements of low-frequency near-surface backscattering at low grazing angles have been made in the open ocean using vertical arrays of coherent sources. Over the range of wind speeds (4-18 m/s) encountered, the normalized data amplitudes exhibited variable non-Rayleigh behavior, from near Rayleigh in the highest sea states to near lognormal in low-to-moderate sea states. Seven probability density function (pdf) models were fit to the data, with the three-component Rayleigh mixture providing the most consistent fits and the least errors. One pdf model, the Poisson-Rayleigh, provided not only good fits to many data sets, but also physical insights into the scattering process. This model's estimates of the expected number of discrete scatterers ranged from 200/km/sup 2/ at low wind speeds to 2000/km/sup 2/ at high wind speeds, consistent with the expected densities of fish and subsurface bubble clouds, respectively. These results are encouraging with regard to developing physical models capable of using local results (such as these) to accurately predict long-range reverberation and clutter statistics. 相似文献