最大熵原理应用于海浪波高分布的研究

利用最大熵原理从理论上推导出波高的最大熵分布,在此基础上研究了状态参量对波高分布和波高熵的影响。影响最大熵分布的因子是平均波高和状态参量,不同海况对应的状态参量是不同的。利用波高实测资料,得出3种不同海况下的最大熵分布,通过比较发现最大熵分布很好地符合实测数据。把最大熵分布与目前广泛应用的瑞利分布作了比较,结果表明,最大熵分布有2个优点：没有对波高作出任何限制性假定和能够描述不同海况下的波高分布。     

华南沿海深水区不同重现期设计波浪的估算方法

根据华南沿海近３１年热带风暴及台风的天气图,分析统计出该海区不同纬度处台风中心气压、台风移动速度的长期概率分布。从这些概率分布曲线可得到不同重现期的设计台风,再用Ｂｒｅｔｓｃｈｎｅｉｄｅｒ１９７２年的半经验公式即可快速估算出不同纬度处的不同重现期的最大设计波浪及最大设计风速。该法效果良好,可供海洋工程可行性研究阶段参考使用。     

On the measurement of directional wave spectra at the southern Brazilian coast

The detailed reconstruction of the directional spectrum of wind waves from measurements of the wave field is an essential requirement for several applications, including the numerical modeling of wave evolution. Three reconstruction techniques that provide estimates of the directional distribution function D(f,θ), given the one-dimensional frequency spectrum, are compared using data from a coastal locality at the southern Brazilian coast. The techniques are the maximum entropy method (MEM), the Fourier Expansion Method using a cos² type function (FEM_cos) and the Fourier Expansion Method using a sech type function (FEM_sech). The main patterns of the wave climate at the study site are qualitatively assessed. Three main sea states, including swell, transition between local sea and swell, and directionally bimodal wind sea, are identified. Time series from three events associated with the main sea states provide test cases for inter comparison of the three reconstruction techniques. Maximum entropy estimates of D(f,θ) provide results that are more consistent than those obtained from the two FEM techniques in all cases considered.     

Simple expressions for correcting wind speed data for elevation

Using the logarithmic wind profile and typical values of dimensionless sea roughness for different sea states, simple power expressions are derived for the wind profile over water. The power expressions are useful for correcting measured wind speed data at a given elevation (up to an elevation of 60 m) to the standard elevation of 10 m commonly used in coastal engineering studies. The analysis shows that the 1/7 power expression (suggested in [U.S. Army Corps of Engineers, 1984. Shore Protection Manual, Vol. 1. Coastal Engineering Research Center, Vicksburg, USA.] and commonly used today) is a good approximation (<3% error) of the logarithmic profile for various sea states up to an elevation of 20 m. Above this elevation, the 1/7 power expression underestimates the wind speed for moderate to fully developed sea states.     

BPF法在海图基准面计算中的应用

为了船只航行安全，在海图上所标的水深一般并非指平均海面到海底的距离(平均水深)，而是从低于平均海面的某一个面到海底的距离，这个面叫做海图基准面。基准面的确定并无统一的方法，国际上只有一个原则的规定，即实际海面应当可能但很少可能低于它。这样规定既避免了在海图上给出过分浅的水深数值，又可保证航行的安全。 但世界海洋国家在实际确定基准面深度时方法很不统一，甚至有些并不甚符合上述原则。我国历史上曾采用过国外的许多种方法，十分混乱。五十年代以来曾采用弗拉基米尔斯基方法，但这种方法计算手续相当麻烦，效果有时也不够理想。这里我们将提出一种以 BPF 法为基础的计算海图基准面方法，并将由此得出的面叫做“近最低潮面”。此法计算较简便，用实测资料检验表明，所得结果较为合理。     

黄河水下三角洲海底管道沉降量确定方法研究

提出一种海底管道沉降计算方法——递推法,应用到胜利油田埕岛海域海底管道沉降计算中,并将计算结果与目前常用的日本规范法和极限法进行比较。研究发现在粉土与粉质粘土为主的强度较高的海床上,递推法与日本规范法计算结果比较接近,极限法的最小;在淤泥质土组成的强度较低的海床上,日本规范法的计算值最大,递推法的居中,极限法的最小。逆推法可计算裸置和埋设两种形式的管道沉降量,而另外两者只能计算裸置管道的沉降。文中进一步讨论了引起管道沉降的影响因素,土体强度和压缩性对管道沉降影响较大。且管道埋深越大,沉降也越大。最后分析了黄河水下三角洲埕岛海域4个区的管道沉降量,Ⅰ区争Ⅱ区管道沉降较小,可以忽略,Ⅳ区沉降较大,在淤泥质软土上的管道,可能会完全陷入土中。     

Long term prediction of non-linear vertical bending moments on a fast monohull

An assessment is made of the maximum wave induced bending moment expected to occur during the operational lifetime of a fast monohull, based on long term distribution calculations of the non-linear vertical bending moment at mid-ship. The ship is assumed to operate in the Northern North Sea, with an operational life of 25 years. A succession of short-term stationary ship responses represented by Rayleigh distributions of peaks results in the long-term distribution of the structural loads. Non-linear pseudo transfer functions are used in the procedure to calculate the variances of the short-term responses. The amplitudes of these transfer functions are calculated by a non-linear time domain seakeeping program. The results are compared with rules minimum required values and also with results from simpler calculation procedures such as adopting design sea states.     

A hybrid efficient method to downscale wave climate to coastal areas

Long-term time series of sea state parameters are required in different coastal engineering applications. In order to obtain wave data at shallow water and due to the scarcity of instrumental data, ocean wave reanalysis databases ought to be downscaled to increase the spatial resolution and simulate the wave transformation process. In this paper, a hybrid downscaling methodology to transfer wave climate to coastal areas has been developed combining a numerical wave model (dynamical downscaling) with mathematical tools (statistical downscaling). A maximum dissimilarity selection algorithm (MDA) is applied in order to obtain a representative subset of sea states in deep water areas. The reduced number of selected cases spans the marine climate variability, guaranteeing that all possible sea states are represented and capturing even the extreme events. These sea states are propagated using a state-of-the-art wave propagation model. The time series of the propagated sea state parameters at a particular location are reconstructed using a non-linear interpolation technique based on radial basis functions (RBFs), providing excellent results in a high dimensional space with scattered data as occurs in the cases selected with MDA. The numerical validation of the results confirms the ability of the developed methodology to reconstruct sea state time series in shallow water at a particular location and to estimate different spatial wave climate parameters with a considerable reduction in the computational effort.     

高海况下无人水下航行器的横摇运动分析

为明确高海况下海况等级对无人水下航行器横摇运动的影响,得出满足某型无人水下航行器安全回收的海况条件,对高海况下无人水下航行器的横摇运动进行分析。运用频率响应法,分别在4级、5级和6级海况下,根据海浪谱密度函数和该型航行器的横摇频率响应函数,求得其横摇运动谱密度函数。再运用公式推导,得到其横摇运动的时域函数。使用MATLAB仿真软件对该型航行器进行横摇运动仿真,得到3种海况条件下,其横摇运动的时域函数图像,并验证仿真结果的准确性。仿真结论:高海况下的海况等级对无人水下航行器的横摇幅度有较大影响,满足其安全回收的海况条件为5级海况。     

垂向一维悬浮物浓度模式

在只考虑悬浮物的垂向输运过程和海底处的沉降与再悬浮过程的前提下,建立了一个一维的垂向粒子追踪模式,并用该模式模拟了2000年黄河口海域的一组资料,得到了与实测较为符合的结果。同时,对该模式中涉及的参数值进行了调整,进而据此探讨了参数值的变化对垂向悬浮物输运的影响规律,故该模式可为三维悬浮物输运模式中参数值的选取提供依据。     

Applying POT methods to the Revised Joint Probability Method for determining extreme sea levels

Newly exposed concepts of POT declustering (Bernardara et al., 2014) within the GPD-Poisson model are applied to the joint probability of tide and surge for determining extreme sea levels, as a variation of the Revised Joint Probability Method (RJPM, Tawn and Vassie, 1989). A mixture model is proposed for the meteorological residual (surge) component with a non-parametric (empirical) density for the bulk values and parametric models for both the lower and upper tails. In particular, a distinction is made between values observed at regular time steps, called sequential values, and the clusters of extreme values, or events, on which the statistical extrapolations are performed. The sea level distribution is obtained by convolution of the tide and surge density functions. Confidence intervals are also proposed. This model is applied to the case study of Brest, France using both hourly and high water values. Two methods for handling tide–surge interaction are presented and discussed and a comparison with a direct approach is made.     

TOPEX卫星高度计有效波高数据分析与极值统计预报

利用 TOPEX卫星高度计测量的有效波高资料 ,对中国近海 9个海域进行卫星高度计的 C,Ku两波段测量值的比较分析与处理 ,得出合理的有效波高数值。利用三参数 Weibull分布 ,以渤海海域、上海附近海域和南海东部海域作为特定区进行有效波高的极值统计预报 ,并进行了统计分析     

Quality evaluation and calibration of the SWIM significant wave height product with buoy data

The significant wave height (SWH) is one of the main parameters that describe wave characteristics and is widely used in wave research fields. Wave parameters measured by radar are influenced by the offshore distance and sea state. Validation and calibration are of great significance for radar data applications. The nadir beam of surface wave investigation and monitoring (SWIM) detects the global-ocean-surface SWH. To determine the product quality of SWIM SWH, this paper carried out time-space matching between SWIM and buoy data. The data qualities were evaluated under different offshore distances and sea states. An improved calibration method was proposed based on sea state segmentation, which considered the distribution of the point collocation numbers in various sea states. The results indicate that (1) the SWIM SWH accuracy at offshore distances greater than 50 km is higher than that at distances less than 50 km, with an root mean squared error (RMSE) of 0.244 4 m, scatter index (SI) of 0.115 6 and relative error (RE) of 9.97% at distances greater than 50 km and those of 0.446 0 m, 0.223 0 and 18.66% at distances less than 50 km. (2) SWIM SWH qualities are better in moderate and rough sea states with RMSEs of 0.284 8 m and 0.316 9 m but are worse in slight and very rough sea states. (3) The effect of the improved calibration method is superior to the traditional method in each sea state and overall data, and the RMSE of SWIM SWH is reduced from the raw 0.313 5 m to 0.285 9 m by the traditional method and 0.198 2 m by the improved method. The influence of spatiotemporal window selection on data quality evaluation was analyzed in this paper. This paper provides references for SWIM SWH product applications.     

Wave height statistics for seakeeping assessment of ships in the Adriatic Sea

The paper presents methodology and results of the development of sea states statistics for the Adriatic Sea. Such statistics is still lacking despite a need of the shipping industry. The presented study is based on the Atlas of Climatology containing statistics of sea states observations in the Adriatic Sea made by merchant ships during the period of 15 years. The results, presented in the Atlas in the form of “wave roses”, are digitalized and empirical frequencies of sea state occurrences are obtained. The 3-parametric Weibull distribution is then fitted through empirical data points enabling the “smoothening” of the histogram. The resulting histogram is compared with other studies for the long-term prediction of the sea states in the Adriatic Sea. The paper concludes with the discussion on the accuracy and applicability of the results.     

Maximum entropy estimation of directional wave spectra from an array of wave probes

A procedure for estimating directional wave spectra from an array of wave probes based on the Maximum Entropy Method (MEM) is developed in the present paper. The MEM approach yields an angular spreading function at each frequency band consistent with the input cross-spectral density matrix. The method is evaluated using numerical simulations of directional sea states. The MEM is also used to analyze data obtained from the three-dimensional wave basin of the Hydraulics Laboratory, National Research Council of Canada. Finally, the MEM is compared with the Maximum Likelihood Method (MLM) and is shown to be a powerful tool for directional wave analysis.     

Representation of double-peaked sea wave spectra in Jiaozhou Bay

The spectrum shape characteristics of double-peaked sea wave spectia actually measured in the Jiaozhou Bay were analysed and one representation of double-peaked sea wave spectra with six-parameters which can be determined from a given sea wave spectrum is proposed in this paper. After verifying by fitting process with the data sets of double-peaked sea wave spectra measured in the Jiaozhou Bay, it can be seen from the results that the representation mentioned above may be used to express the double-peaked sea wave spectra generated in the Jiaozhou Bay under various sea states, and that the absolute values of maximum deviation index (D.I.) do not exceed 30. 0.     

Availability of offloading from an LNG barge

Long-term data sets are discussed for three locations – a southern North Sea location, where sea states are typically dominated by wind-sea conditions, a Northwest Shelf of Australia location, where sea states are often a mix of wind-sea and swell conditions, and a Guinea Gulf location offshore Nigeria, where sea states are dominated by swell. Various types of sea state statistics that have application to availability studies are presented; but databases of these type that also include wind and current data can also be used to estimate response parameters, which can give a more direct assessment of availability, and some example statistics of these are also given. Finally, the availability of offloading an FLNG barge to carrier is evaluated for each of the data sets, for specific operability criteria. Two types of operability criteria are used in the availability evaluations, and the results compared.     

The retrieval of the nonlinear random waves from the non-Gaussian characteristics of the sea surface elevation distribution

In this paper, without recourse to the nonlinear dynamical equations of the waves, the nonlinear random waves are retrieved from the non-Gaussian characteristic of the sea surface elevation distribution. The question of coincidence of the nonlinear wave profile, spectrum and its distributions of maximum (or minimum) values of the sea surface elevation with results derived from some existing nonlinear theories is expounded under the narrow-band spectrum condition. Taking the shoaling sea wave as an example, the nonlinear random wave process and its spectrum in shallow water are retrieved from both the non-Gaussian characteristics of the sea surface elevation distribution in shallow water and the normal sea waves in deep water and compared with the values actually measured. Results show that they can coincide with the actually measured values quite well, thus, this can confirm that the method proposed in this paper is feasible.     

Obtaining the distribution of quiescent periods directly from the power spectral densities of Sea waves

There is a growing practical interest in the ability to increase the sea states at which marine operations can be safely undertaken by exploiting the quiescent periods that are well known to exist under a wide range of sea conditions. While the actual prediction of quiescent periods at sea for the control of operations is a deterministic process, the long term planning of future maritime tasks that rely on these quiescent periods is a statistical process involving the anticipated quiescence properties of the forecasted sea conditions in the geographical region of interest. It is in principle possible to obtain such data in tabular form either large scale simulation or from field data. However, such simulations are computationally intensive and libraries of appropriate field data are not common. Thus, it is clearly attractive to develop techniques that exploit standard wave spectral models for describing the quiescence statistics directly from such spectra. The present study focuses upon such techniques and is a first step towards the production of a computationally low-cost quiescence prediction tool and compares its efficacy against simulations. Two significant properties emerge for a large class of wave spectral models that encompasses the ubiquitous Neumann and Pierson Moskowitz or Bretschneider forms. Firstly, the auto-correlation function of the wave profile that are required to produce the quiescence property can be obtained analytically in terms of standard special functions. This considerably reduces the computational cost making desktop computer-based planning tools a reality. Secondly, for each class of these parametric spectra, the probability of a given number of consecutive wave heights (normalised to the significant wave heights) less than some critical value is in fact independent of absolute wave height. Thus, for a broad class of practically interesting wave spectra all that is required to obtain the statistical distribution of the quiescent periods is simple rescaling.