Assessing shipping water vertical loads on a fixed structure by convolution model and wet dam-break tests

Estimation of loads derived from shipping water events in naval and offshore structures is of importance to improve their structural design or to predict changes in their dynamics. For the case of vertical loads on deck of a fixed structure, it is possible to estimate analytically their evolution in time by considering the distribution of shipping water elevations. However, the classical approach to estimate this distribution (i.e., dam-break method) tends to overestimate the amount of water on deck and does not follow the generated decay trends observed experimentally. In the present work, the time evolution of the vertical loads due to shipping water events was studied analytically and experimentally. The validation of the use of a convolution model to estimate the time evolution of vertical loads is presented, aiming to improve the results obtained with classical approaches. A systematic experimental study has been conducted using the wet dam-break method to generate isolated shipping water events, measuring the slow-varying vertical loads on a rectangular fixed structure. A force balance and a high-speed camera have been used at the same sampling rate to monitor the vertical loads and the shipping water evolution on the deck. Results demonstrated that the use of the convolution model improved the representation of the time series of loads compared with the traditional dam-break approach. With this new method, it was possible to capture the peaks and the decay tendencies observed in the experimental data in an approximated way.     

Numerical analysis of shipping water impact on a deck using a particle method

The objective of this study is to simulate three-dimensional behavior of shipping water and to predict the impact pressure on the deck using a particle method. An experiment carried out in a two-dimensional wave tank with a fixed deck model was numerically analyzed in three dimensions. The fluid motion, the surface elevation, and the impact pressure on the deck were compared between the experiment and the calculation and good agreement was obtained.     

基于Boussinesq水波模型的聚焦波模拟

基于最高导数为3阶的单层Boussinesq方程,建立了聚焦波的时域波浪计算模型。数值模型求解采用了预报−校正的有限差分法。对于时间差分格式,预报和校正分别采用3阶Adams-Bashforth格式和4阶Adams-Moulton格式。首先,针对不同水深条件下水槽中传播的强非线性波进行模拟,并将数值结果与流函数的数值解析解进行了比较,结果表明无论是波面位移、波面处的水平速度和垂向速度均与解析解符合较好,最大波峰面的速度分布伴随水深的增加与解析解吻合程度变差,非线性速度分布的适用范围与线性解析解适应范围kh<3.5基本一致。其次,对深水聚焦波演化进行了模拟研究,研究中聚焦波的生成采用在边界点累加不同频率线性规则波的方法。应用聚焦波物理模型实验结果验证模型,计算聚焦位置处的波面位移和沿水深的速度分布与实验结果的对比表明,波面位移吻合程度较好,垂向的水平速度分布基本吻合。最后,保持中心频率(周期)不变,数值模拟了周期范围变化下最大聚焦波峰面以及波峰面水平速度的变化趋势,结果表明波峰面值和波峰面水平速度随着周期范围缩小而增大。     

On the radiation and diffraction of water waves by a rectangular structure with a sidewall

A radiation and diffraction boundary value problem is investigated. It arises from the interaction of linear water waves with a freely floating rectangular structure in a semi-infinite fluid domain of finite water depth with the leeward boundary being a vertical wall. Analytical expressions for the radiated potentials and the diffracted potential are obtained by use of the method of separation of variables and the eigenfunction expansion method. The added masses and damping coefficients for the structure heaving, swaying and rolling in calm water are obtained by use of the corresponding radiated potentials and the wave excitation forces are calculated by use of the diffracted potential. To verify the correctness of the method, a boundary element method is used. A comparison of the analytical results with those obtained by the boundary element method is made and good agreement is achieved, which shows that the analytical expressions for the radiated and diffracted potentials are correct. By use of the present analytical solution, the added mass, damping coefficients, wave excitation force, together with the hydrodynamic effects of the draft, width of the structure and the clearance between the structure and the sidewall are also investigated.     

Numerical solution of a mathematical model for water wavesin large coastal areas

on the evolution equation for water waves,a mathematical model for wave propagation in large mild-slope areas is derived.The model is solved by the finite difference method with the staggered grid system.The computational results are in good agreement with experimental data and show that the model can obtain better results with relatively coarser grids.The model can be used to simulate water wave propagation in large coastal areas and can be efficiently solved without much programming effort.     

近岸大区域水波数学模型及其数值求解

从水波发展方程出发,导出了大区域缓坡水波数学模型,并采用交错网格系统下的有限差分法对该数学模型进行了数值求解,计算结果表明该数学模型在粗网格下也能得到与实验结果比较一致的结果,从而表明该模型可用于较大区域的水波问题.该模型具有编程简单、求解比较快速、经济的优点.     

Effect of a submerged plate on the near-bed dynamics underincoming waves in deep water conditions

It is well known that wave induced bottom oscillations become more and more negligible when the water depth exceeds half the wavelength of the surface gravity wave. However, it was experimentally demonstrated for regular waves that the bottom pressure oscillations at both first and second wave harmonic frequencies could be significant even for incoming waves propagating in deep water condition in the presence of a submerged plate [16]. For a water depth h of about the wavelength of the wave, measurements under the plate (depth immersion of top of plate h/6, length h/2) have shown bottom pressure variations at the wave frequency, up to thirty times larger than the pressure expected in the absence of the plate. In this paper, not only regular but also irregular wave are studied together with wave following current conditions. This behavior is numerically verified by use of a classical linear theory of waves. The wave bottom effect is explained through the role of evanescent modes and horizontally oscillating water column under the plate which still exist whatever the water depth. Such a model, which allows the calculation of the velocity fields, has shown that not only the bottom pressure but also the near bed fluid velocity are enhanced. Two maxima are observed on both sides of the location of the plate, at a distance of the plate increasing with the water depth. The possible impact of such near bed dynamics is then discussed for field conditions thanks to a scaling based on a Froude similarity. It is demonstrated that these structures may have a significant impact at the sea bed even in very deep water conditions, possibly enhanced in the presence of current.     

Green water overtopping analyzed with a SPH model

Wave overtopping on the decks of offshore platforms and ships can cause severe damage due to the high forces generated by the water. This phenomenon is analyzed within the framework of the Smoothed Particle Hydrodynamics (SPH) method. The presence of a fixed horizontal deck above the mean water level modifies strongly the wave kinematics. In particular, the flow in the wave crest is split into two, showing a different behavior above and below the deck. Numerical results generated by the SPH method are compared to laboratory experiments. The formation of a jet in the rear of the deck after overtopping is observed under extreme conditions.     

The diel vertical migration of the sound-scattering layer in the Yellow Sea Bottom Cold Water of the southeastern Yellow Sea：focus on its relationship with a temperature structure

Using the hydroacoustic method with a 200 kHz scientific echo sounding system,the diel vertical migration(DVM) of the sound-scattering layer(SSL) in the Yellow Sea Bottom Cold Water(YSBCW) of the southeastern Yellow Sea was studied in April(spring) and August(summer) of 2010 and 2011.For each survey,13–27 hours of acoustic data were continuously collected at a stationary station.The acoustic volume scattering strength(Sv) data were analyzed with temperature profile data.In the spring of both 2010 and 2011,the SSL clearly showed the vertical migration throughout the entire water column,moving from the surface layer at night to near the bottom during the day.Conductivity,temperature,and depth data indicated that the entire water column was well mixed with low temperature of about 8 C.However,the SSL showed different patterns in the summers of 2010 and 2011.In the summer of 2010(28 C at the surface),the SSL migrated to near the bottom during the day,but there were two SSLs above and below the thermocline at depth of 10–30 m at night.In the summer of 2011(20 C at the surface),the SSL extended throughout the entire water column at night,possibly owing to an abrupt change in sea weather conditions caused by the passage of a Typhoon Muifa over the study area.It was concluded that the DVM patterns in summer in the YSBCW area may be greatly influenced by a strengthened or weakened thermocline.     

Radiation and diffraction of linear water waves by an infinitely long submerged rectangular structure parallel to a vertical wall

The radiation and the diffraction of linear water waves by an infinitely long floating rectangular structure submerged in water of finite depth with leeward boundary being a vertical wall are analyzed in this paper by using the method of separation of variables. Analytical expressions for the radiated and diffracted potentials are derived as infinite series with unknown coefficients determined by the eigenfunction expansion matching method. The expressions for wave forces and hydrodynamic coefficients are given. A comparison is made between the results obtained by the present analytical solution and those obtained by the boundary element method. By using the present analytical solution, the hydrodynamic influences of the submergence, the width, the thickness of the structure, and the distance between the structure and the wall on the wave forces and hydrodynamic coefficients are discussed in detail.     

近岸水质遥感评价参数的选择及评价模型的研究

利用遥感技术的水质评价是海洋环境监测的必然趋势,但目前可通过遥感技术反演的水质参数种类远少于《海水水质标准》中的35项,因此遥感监测时必须反演的水质参数有哪些?利用有限的参数该建立怎样的水质评价模型?成为我们关心的问题。本文以雷州半岛海域为研究对象,通过对实测数据的分析得出13种水质参数中的主导参数为总氮(N)、总磷(P)、化学需氧量(COD)、酸度(pH)、溶解氧(DO)这5种;再通过数理统计理论得出这5种参数对水质分类判别能力的大小关系为:COD> DO> P> N> pH,分别建立对应的五参数、四参数、三参数水质评价模型,并确定最优评价模型。即,雷州半岛海域水质遥感评价时必须反演的参数为COD、DO、P和N,水质综合评价模型为四参数模型,为后续其他海域水质的遥感监测提供可借鉴的方法。     

Wave-induced motions of an air cushion structure in shallow water

Model tests were conducted on two 1:100 scaled models of a typical concrete gravity substructure at the University of Western Australia. The two models had dimensions 0.5 m length×0.5 m width with the first model being a sealed closed bottom box of height 0.1 m and the second model being an open bottom box with skirt length of 0.1 m. The mass of the air cushion model was changed to accommodate various water plug heights within the skirt chamber. Each model was floated at a constant draft of 0.1 m and tested in water depths ranging between 0.03 m (shallow) and 0.8 m (deep). The environment comprised regular waves with periods ranging between 0.6 and 3.5 s and amplitude of 0.08–0.02 m. To quantify the dynamic response the heave and pitch motions of each model were measured.A simplified theoretical solution based on long wavelength, linear wave assumptions was developed and applied to the geometries in consideration. Improvements to the theory are sought using the forcing function from a boundary element code, as well as utilizing added mass coefficients from free decay experiments. Results show that experimental trends compare reasonably well with analytical solution in particular for periods longer than the natural period. The results show that introducing air cushion support into a CGS increases the pitch response, while having little effect of the heave motion.     

夏季长江冲淡水扩展的数值模拟

建立一个σ坐标系下三维非线性斜压陆架模式,研究夏季径流量、台湾暖流、黄海冷水团、风场对长江冲淡水扩展的影响。数值试验基本再现了夏季长江冲淡水低盐水舌伸向东北的现象和渤、黄、东海的环流结构。长江径流量只影响近口门附近冲淡木朝东南方向扩展势力和整个冲淡水扩展范围的大小。台湾暖流深受底形的影响,流动路径稳定,且不受自身强度的影响,又主流远离长江口,对长江冲淡水扩展的影响不大。黄海冷水团产生的余流在长江口海区阻碍着冲淡水沿岸向南扩展,在远离长江口海区诱导冲淡水向东南运动。总的黄海冷水团的作用是使长江冲淡水低盐水舌伸向东北。黄海冷水团越强,这种作用就越明显。夏季风场在冲淡水转向东北的过程中作用显着。     

浅水半潜型浮式风机基础概念设计与模型试验研究

目前我国海上风电开发主要集中在近海海域。我国近海海域水深相对较浅,在浅水非线性波浪载荷作用下,浮式基础动力响应呈现显著非线性特性,导致浮式风机基础及其系泊系统设计极具挑战。针对我国近海浅水海域环境条件,基于半潜型浮式风机基础概念SPIC,采用参数化建模与优化方法,开展了42 m水深条件下半潜型浮式风机基础概念设计,并在海洋工程水池开展1∶50缩尺模型试验。模型试验的结果表明：半潜式浮式风机基础概念设计具有稳定性好、动力性能优良等优点,浮式基础纵摇运动、机舱加速度等指标满足规范要求。模型试验结果验证了该概念的可行性。     

海湾水环境影响的约瑟夫-新德那模式预测

对应用约瑟夫-新德那模式进行海湾水环境影响预测中存在的问题作了初步分析,海域水流、混合深度值的选取及特殊污染物的存在等是影响预测结果准确性的主要因素。潮流憩流状态时,污染物的迁移扩散主要取决于污染物的混合速度,污染物基本上以近似于半圆形呈π弧度的角度向水体混合扩散,预测结果与污染物实际迁移扩散状况大致吻合;潮流非憩流或大部分时间为非憩流状态时,海域水流对污染物的迁移扩散影响较大,混合角度与海流流速成反比,污染物向水体扩散的实际混合角度要小于π弧度,预测的结果与污染物实际迁移扩散状况存在较大差距。海域水流对污染物迁移距离产生很大影响,水流愈大,污染物迁移的距离就愈远。混合深度的取值也直接关系到预测结果的准确性,若按可能受影响范围海域的平均水深值作为混合深度取值,其预测结果要比水深取2m时的预测结果更接近真实状况。对已达标排放的废水进行影响预测时,结果与实际情况接近,而对高浓度石油类的废水含量的影响预测却与实际情况差异极大。     

Validation of the three-wave quasi-kinetic approximation for the spectral evolution in shallow water

This paper aims at validating the three-wave quasi-kinetic approximation for the spectral evolution of weakly nonlinear gravity waves in shallow water. The problem is investigated using a one-dimensional numerical wave propagation model, formulated in the spectral representation. This model includes both a nonlinear triad interactions term and a wave breaking dissipation term. Some numerical tests were carried out in order to show the importance of using the triad nonlinear term in wave propagation spectral models, particularly to describe both behavior of the spectral integral parameters and of the spectral shape evolution in shallow water depth. Furthermore; a comparison against different set of experimental observations was carried out. Comparing the numerical results with the experimental observations made it possible to show the modeling efficiency of the three-wave quasi-kinetic approximation.     

Applications of sediment sudden deposition model based on the third-generation numerical model for shallow water wave

The existing numerical models for nearshore waves are briefly introduced, and the third-generation numerical model for shallow water wave, which makes use of the most advanced productions of wave research and has been adapted well to be used in the environment of seacoast, lake and estuary area, is particularly discussed. The applied model realizes the significant wave height distribution at different wind directions. To integrate the model into the coastal area sediment, sudden deposition mechanism, the distribution of average silt content and the change of sediment sudden deposition thickness over time in the nearshore area are simulated. The academic productions can give some theoretical guidance to the applications of sediment sudden deposition mechanism for stormy waves in the coastal area. And the advancing directions of sediment sudden deposition model are prospected.     

海底沉积物的基本地声结构与地声模型

为准确建立海底地声模型,本文探讨地声模型的基本组成和基本结构。通过样品实验室测量,分析南海海底表层沉积物的密度、孔隙度与声速随着埋深变化的关系,得出海底实际存在的低声速表面–声速缓慢变化类型、低声速表面–声速增大类型、高声速表面–声速缓慢变化类型和高声速表面–声速增大类型4种典型地声结构;对比钻探测量,分析黄海海底沉积物的密度、孔隙度与声速随埋深变化关系,得出海底地声模型分层特征与地声结构组合特征。研究表明,地声模型可以归结为4种基本地声结构的组合,通过与底层海水声速、同层内声速剖面以及与上层海底沉积物下表面声速的比较,可以建立各种海底地声模型;基于实验室测量法建立的地声模型可以作为参考地声模型,但需要考虑实际海底温度和压力梯度以及海底沉积物的频散特性等,借助于声速比校正法和频散性理论模型进行计算及修正。     

A 3-D model based on Navier–Stokes equations for regular and irregular water wave propagation

A spatial fixed σ-coordinate is used to transform the Navier–Stokes equations from the sea bed to the still water level. In the fixed σ-coordinate system only a very small number of vertical grid points are required for the numerical model. The time step for using the spatial fixed σ-coordinate is efficiently larger than that of using a time dependent σ-coordinate, as there is substantial truncation error involved in the time dependent σ-coordinate transformation. There is no need to carry out the σ-coordinate transformation at each time step, which can reduce computational times. It is important that wave breaking can be potentially modeled in the fixed σ-coordinate system, but in a time-dependent σ-coordinate system the wave breaking cannot be modeled. A projection method is used to separate advection and diffusion terms from the pressure terms in Navier–Stokes equations. The pressure variable is further separated into hydrostatic and hydrodynamic pressures so that the computer rounding errors can be largely avoided. In order to reduce computational time of solving the hydrodynamic pressure equation, at every time step the initial pressure is extrapolated in time domain using computed pressures from previous time steps, and then corrected in spatial domain using a multigrid method. For each time step, only a few of iterations (typically six iterations) are required for solving the pressure equation. The model is tested against available experimental data for regular and irregular waves and good agreement between calculation results and the measured data has been achieved.