Impact Analysis of Air Gap Motion with Respect to Parameters of Mooring System for Floating Platform

In this paper, the impact analysis of air gap concerning the parameters of mooring system for the semi-submersible platform is conducted. It is challenging to simulate the wave, current and wind loads of a platform based on a model test simultaneously. Furthermore, the dynamic equivalence between the truncated and full-depth mooring system is still a tuff work. However, the wind and current loads can be tested accurately in wind tunnel model. Furthermore, the wave can be simulated accurately in wave tank test. The full-scale mooring system and the all environment loads can be simulated accurately by using the numerical model based on the model tests simultaneously. In this paper, the air gap response of a floating platform is calculated based on the results of tunnel test and wave tank. Meanwhile, full-scale mooring system, the wind, wave and current load can be considered simultaneously. In addition, a numerical model of the platform is tuned and validated by ANSYS AQWA according to the model test results. With the support of the tuned numerical model, seventeen simulation cases about the presented platform are considered to study the wave, wind, and current loads simultaneously. Then, the impact analysis studies of air gap motion regarding the length, elasticity, and type of the mooring line are performed in the time domain under the beam wave, head wave, and oblique wave conditions.     

Downwind sail aerodynamics: A CFD investigation with high grid resolution

A computational fluid dynamics (CFD) code was applied to an America's Cup Class Yacht to investigate sailing performance in a downwind configuration. Apparent wind angles at 45°, 105° and 120° are reported, sailed with mainsail and asymmetrical spinnakers. Numerical results are in good agreement with wind tunnel data. A large mesh investigation was performed, ranging from 60,000 elements up to 37 million elements, which shows a converging trend to the experimental values with differences smaller than 3% in both lift and drag. The most commonly used turbulence models in sail applications were tested and the results are presented here in two meshes with 1 million elements and 6.5 millions, respectively. All turbulence models over-estimate forces more than solving the Navier–Stokes system without any additional equations, hence turbulence models do not increase solution accuracy according to these results.     

Circular plumes in Lake Pontchartrain estuary under wind straining

Circular shaped density plumes of low turbidity, low fecal indicator (Escherichia coli and enterococci) concentrations, and high salinity have been observed near the Industrial Canal in Lake Pontchartrain, north of the City of New Orleans. A conceptual model in polar coordinates and a numerical model are developed, together with data analysis, to illustrate the dense plume. It is demonstrated that the northward expansion of the plume occurs under northerly winds. The northward expansion of the plume occurs under northerly winds that drive downwind flow at the surface and upwind radial flow at the bottom. Northerly wind-induced straining, similar to tidal straining, promotes vertical stratification. As a result, the water becomes stratified near a thin bottom layer (<1 m), within which density currents are facilitated. The stability of the stratified plume suppresses wind-induced turbulent mixing inside the plume. The bottom water outside of the plume is more effectively stirred by the wind, the result being that the suspended sediment concentration outside of the plume area is much higher than inside. This contrast in mixing makes the plume visible from the surface by satellites even though the stratification is at the bottom. Laterally, wind stress produces a torque (vorticity) in areas of non-uniform depth such that upwind flow is developed in deep water and downwind flow in shallow water. The continuity requirement produces an upwind flow along the axis of the Industrial Canal (IC). The upwind flow is balanced by the downwind flow over the shallower peripheral areas along the coast.     

Model analysis of the currents and wind waves in the Vistula Lagoon of the Baltic Sea

This paper considers the results of the numerical simulation of the steady currents and waves field structures in the Vistula Lagoon under constant wind forcing. The currents?? structure is essentially 3-dimensional. The direction of the near-shore drift is determined by the wind and shore orientation, but the currents involve two layers in the deeper part of the basin: the upper layer downwind current and the upwind compensative current in the underlying layer. The wind waves depend upon the wind speed, fetch, and depth until the wind speed is less than 6?C8 m/s. As the wind increases, the fetch??s dependence disappears. The wind height depends only on the wind??s magnitude, and, under stormy conditions (when the wind??s speed exceeds 15 m/s), the waves are limited by the water depth alone.     

Considerations on the squat of unevenly trimmed ships

In existing publications about empirical studies of ship squat trim is only dealt with implicitly by focussing the concern on the maximum squat. This restriction is only sufficient if a non-zero static trim is always increased dynamically, which, however, turns out to be wrong. Instead of maximum squat, we suggest a more suitable definition of the quantity relevant for under-keel clearance. In an example, it is shown that a non-zero static trim can actually be reduced dynamically, indicating a possible optimisation of trim. For a correct estimation of the squat effect on under-keel clearance, trim must be treated as an explicit parameter. Our arguments are based on full-scale experiments as well as results from numerical simulations.     

Application of potential flow methods to fast displacement ships at transcritical speeds in shallow water

Computer codes implementing three different numerical methods for the prediction of ship squat at transcritical speeds in shallow open-water are tested. SlenderFlow is a potential flow code specifically for ships in very shallow water, based on partially dispersive slender body theory. Flotilla is a potential flow code based on fully dispersive thin-ship theory. Rapid is a general nonlinear free-surface panel code. Code predictions of transcritical sinkage, trim and resistance in laterally unrestricted water were compared to the experimental results of Graff (1964) for two Taylor series hulls in a finite-width towing tank. Once tank width effects were accounted for, each of the three codes was found to give good predictions within the valid range of the underlying theory. A simple method for estimating transcritical wave resistance from trim is presented.     

海浪微波散射理论模式

在假设海面白帽为球形气泡层的基础上,利用白帽海面的矢量辐射传输方程各随机粗糙面散射模型建立了海面的微波散射模型。辐射传输方程利用迭代法求解,随机粗糙面散射模型采用双尺度散射模型,利用白帽覆盖率的经验公式计算海面的微波散射特性。数值计算结果表明,随着气泡厚度的增加球形气泡散射系数越来越接近球形粒子散射系数;白帽对散射同的贡献随风速增大而增大;侧风情况比逆风和顺风情况影响均大;水平极化比垂直极化影响大     

A model on the turbulent wind field over wind-waves in curvilinear co-ordinates

The wave-induced fluctuations of wind velocity over wind-waves measured in the wind tunnel experiment (Ichikawa andImasato, 1976) are compared with the numerical results estimated by a linear model (Model II) on the turbulent wind field over a dominant component of wind-waves. In the Model II, the undulation of mean air flow is introduced by adopting the curvilinear co-ordinates, and the existence of viscous sublayer and the influence of underlying wind-waves to background atmospheric turbulence are taken into account. The numerical results estimated by the Model II are in good agreement with the experimental results. The good agreement, which was not obtained from the previous model (Model I) in the Cartesian co-ordinates, is shown to be attributed to the undulating mean flow introduced in the Model II.     

Sensitivity Analysis of Air Gap Motion with Respect to Wind Load and Mooring System for Semi-Submersible Platform Design

A design of semi-submersible platform is mainly based on the extreme response analysis due to the forces experienced by the components during lifetime. The external loads can induce the extreme air gap response and potential deck impact to the semi-submersible platform. It is important to predict air gap response of platforms accurately in order to check the strength of local structures which withstand the wave slamming due to negative air gap. The wind load cannot be simulated easily by model test in towing tank whereas it can be simulated accurately in wind tunnel test. Furthermore, full scale simulation of the mooring system in model test is still a tuff work especially the stiffness of the mooring system. Owing to the above mentioned problem, the model test results are not accurate enough for air gap evaluation. The aim of this paper is to present sensitivity analysis results of air gap motion with respect to the mooring system and wind load for the design of semi-submersible platform. Though the model test results are not suitable for the direct evaluation of air gap, they can be used as a good basis for tuning the radiation damping and viscous drag in numerical simulation. In the presented design example, a numerical model is tuned and validated by ANSYS AQWA based on the model test results with a simple 4 line symmetrical horizontal soft mooring system. According to the tuned numerical model, sensitivity analysis studies of air gap motion with respect to the mooring system and wind load are performed in time domain. Three mooring systems and five simulation cases about the presented platform are simulated based on the results of wind tunnel tests and sea-keeping tests. The sensitivity analysis results are valuable for the floating platform design.     

有多个出口的北极地区微潮咸水湖的天气导致的次潮流

Estuarine processes in the arctic lagoons are among the least studied but important subjects, especially considering the rapid warming of arctic water which may change the length of ice-free period in the summer. In this paper, wind-driven exchange flows in the micro-tidal Elson Lagoon of northern Alaska with multiple inlets of contrasting widths and depths are studied with in situ observations, statistical analysis, numerical experiments, a regression model on the basis of dynamics, and remote sensing data. Water velocity profiles were obtained from a bottom deployed acoustic Doppler current profiler(ADCP) in the northwestern Eluitkak Pass connecting the Beaufort Sea to the Elson Lagoon during a 4.9 day ice-free period in the summer of 2013. The subtidal flow is found correlated with wind(R~2 value ～96%). Frequently occurring east, northeast and north winds from the arctic atmospheric high-and low-pressure systems push water from the Beaufort Sea into the lagoon through the wide inlets on the eastern side of the lagoon, resulting in an outward flow against the wind at the narrow northwestern inlet. The counter-wind flow is a result of an uneven wind forcing acting through the asymmetric inlets and depth,an effect of "torque" or vorticity. Under northwest wind, the exchange flow at the northwestern inlet reverses its direction, with inward flows through the upwind northwestern inlet and outward flows through the downwind eastern inlets. A regression model is established based on the momentum equations and Taylor series expansions. The model is used to predict flows in July and August of 2015 and July of 2017, supported by available Landsat satellite images. About 73%–80% of the time the flows at Eluitkak Pass are out of Elson Lagoon for the summer of 2015 and 2017. Numerical experiments are conducted to corroborate the findings and illustrate the effects under various wind conditions. A quasi-steady state balance between wind force and surface pressure gradient is confirmed.     

基于SAR图像雨团足印的海面风向提取方法

利用地球物理模式函数进行SAR海面风速反演时,需以风向作为地球物理模式函数的输入。本文应用了一种利用SAR图像上雨团足印顺风一侧比逆风一侧明亮的图像特征的海面风向提取方法,以进行海面风速反演。4景RADARSAT-2卫星SAR示例数据风向提取结果相对于ASCAT散射计的风向均方根误差满足不大于16°。分别以本文方法提取的风向和ASCAT散射计风向作为输入,利用地球物理模式函数CMOD5进行海面风速的SAR反演,两者的风速反演结果基本一致,其均方根误差差值不超过0.3 m/s。本文利用SAR图像雨团足印信息的风向提取方法准确可靠,可应用于SAR海面风速反演。     

Investigation on the hydrodynamic performance of an ultra deep turret-moored FLNG system

Hydrodynamic performance of an ultra deep turret-moored Floating Liquefied Natural Gas(FLNG) system is investigated.Hydrodynamic modeling of a turret-moored FLNG system,in consideration of the coupling effects of the vessel and its mooring lines,has been addressed in details.Based on the boundary element method,a 3-D panel model of the FLNG vessel and the related free water surface model are established,and the first-order and second-order mean-drift wave loads and other hydrodynamic coefficients are calculated.A systematic model test program consisting of the white noise wave test,offset test and irregular wave test combined with current and wind,etc.is performed to verify the numerical model.Owing to the depth limit of the water basin,the model test is carried out for the hydrodynamics of the FLNG coupled with only the truncated mooring system.The numerical simulation model features well the hydrodynamic performance of the FLNG system obtained from the model tests.The hydrodynamic characteristics presented in both the numerical simulations and the physical model tests would serve as the guidance for the ongoing project of FLNG system.     

High-resolution central difference scheme for the shallow water equations

1 IntroductionThe shallow water equations (SWE) are frequent-ly used as a mathematical model for water flows incoastal areas, lakes, estuaries, etc. Thus, they are animportant tool to simulate a variety of problems relat-ed to coastal engineering, environment, ecology, etc.(Bermúdez et al., 1998). On the basis of solving theone-dimensional (1D) SWE, Hu et al. (2000) have de-veloped a model capable of simulating storm wavespropagating in the coastal surf zone and overtopping asea wall. Ano…     

Computational study of sail performance in upwind condition

Airflow around yacht sails with imposed final geometry is simulated using a CFD code, reproducing experimental tests carried in a wind tunnel. Two configurations are considered: one, mast–main sail and the other, mast–jib–main sail. Both configurations were studied in the same flow conditions of air velocity and incidence angle. The grid is structured-like next to the sails and unstructured in the rest of the domain. The turbulence model used is Shear Stress Transport. The results are compared against experimental and numerical results.     

High-resolution mapping of oceanic wind fields with skywave radar

The direction of the mean surface wind field in the North Pacific Ocean was mapped on September 25 and 26, 1973, over an area of3 times 10^{6}(km)²by OTH-B HF radar. A spatial resolution of 60 km in range and 15 km in cross range was used at points spaced by 150 km in range and 80 km in cross range. Wind directions were inferred from the upwind/downwind first-order Bragg ratio and the measure of the maximum ratio occuring for radial winds at points near each observation. Over 90 percent of the recorded data were usable for this purpose.High spatial resolution is essential to make detailed measurements of the wind speed and direction across and along an atmospheric cold front. The location of the atmospheric cold front derived from the wind field agreed well with the ESSA VIII satellite frontal location.     

Sheet flow and suspension of sand in oscillatory boundary layers

after revisionTime-dependent measurements of flow velocities and sediment concentrations were conducted in a large oscillating water tunnel. The measurements were aimed at the flow and sediment dynamics in and above an oscillatory boundary layer in plane bed and sheet-flow conditions. Two asymmetric waves and one sinusoidal wave were imposed using quartz sand with D₅₀ = 0.21 mm. A new electro-resistance probe with a large resolving power was developed for the measurement of the large sediment concentrations in the sheet-flow layer. The measurements revealed a three layer transport system consisting of a pick-up/deposition layer, an upper sheet flow layer and a suspension layer.In the asymmetric wave cases the total net transport was directed “onshore” and was mainly concentrated in the thin sheet flow layer (< 0.5 cm) at the bed. A small net sediment flux was directed “offhore” in the upper suspension layer. The measured flow velocities, sediment concentrations and sedimenl fluxes showed a good qualitative agreement with the results of a (numerical) 1DV boundary-layer flow and transport model. Although the model did not describe all the observed processes in the sheet-flow and suspension layer, the computational results showed a reasonable agreement with measured net transport rates in a wide range of asymmetric wave conditions.     

Aerodynamic Simulation of A Containership to Evaluate Cargo Configuration Effect on Frontal Wind Loads

Fuel consumption has always been a matter of concern for ships propulsion. In this research we aim to develop computer models of several containership cargo stacking configurations and discuss an optimal configuration at a constant front wind speed. The paper presents the simulation results by using ANSYS CFX for a 1:4 scale Post-Panamax 9000 TEU containership. The ship is modelled in a cubic domain that contains unstructured mesh with details, in such a way that can demonstrate the influence of the container configuration on wind force. Also the numerical results are verified versus wind tunnel test data. An optimal stack configuration led to about 25% reduction in air resistance. It is proposed that in order to reduce the wind drag force and consequently reduce the fuel consumption and pollutant emissions, empty spaces between the cargo containers and unbalanced cargo distribution over the deck should be inhibited. Also, it is advised to make the cargo distribution on the most forward and aftward deck areas more streamlined.     

Tower-based backscatter measurements of the sea

In September 1979, the radar scattering coefficient (sigmadeg) was measured at platform Noordwijk in the North Sea 10 km off the Dutch coast. This was done in conjunction with similar measurements by Dutch and French investigators as part of Project MARSEN (Marine Remote Sensing). Our measurements were made with vertical and horizontal polarizations, in the frequency baud 9-17 GHz, at incidence angles0deg - 70deg, with wind speeds from 2-22 m/s, and look directions upwind, downwind, and crosswind. This paper presents the scattering-coefficient variation with these radar and ocean parameters. In particular, the exponents for the windspeed response are compared with those from other investigators. Some of the exponents reported here are higher than reported previously, possibly because orthogonal regression was used rather than regression ofsigmadegversus windspeed.     

非对称风场对台风浪模拟效果的比较研究

分析四象限非对称风场模型与叠加风场模型的优缺点,将模型结果与实测风速进行对比验证;利用上述两种风场模型分别驱动第三代海浪模式SWAN,对发生在南海海域的三场台风浪进行了数值模拟计算。结果显示:四象限非对称模型关于风速的计算值与实测值吻合度更高,尤其是当台风中心距离测站较近时;四象限非对称模型驱动SWAN模拟的台风浪精度优于叠加风场模型,适用于南海台风浪的数值模拟。     

Roughness correction method for salinity remote sensing using combined active/passive observations

Roughness-induced emission from ocean surfaces is one of the main issues that affects the retrieval accuracy of sea surface salinity remote sensing. In previous studies, the correction of roughness effect mainly depended on wind speeds retrieved from scatterometers or those provided by other means, which necessitates a high requirement for accuracy and synchronicity of wind-speed measurements. The aim of this study is to develop a novel roughness correction model of ocean emissivity for the salinity retrieval application. The combined active/passive observations of normalized radar cross-sections (NRCSs) and emissivities from ocean surfaces given by the L-band Aquarius/SAC-D mission, and the auxiliary wind directions collocated from the National Centers for Environmental Prediction (NCEP) dataset are used for model development. The model is validated against the observations and the Aquarius standard algorithms of roughness-induced emissivity correction. Comparisons between model computations and measurements indicate that the model has better accuracy in computing wind-induced brightness temperature in the upwind/downwind directions or for the surfaces with smaller NRCSs, which can be better than 0.3 K. However, for crosswind directions and larger NRCSs, the model accuracy is relatively low. A model using HH-polarized NRCSs yields better accuracy than that using VV-polarized ones. For a fair comparison to the Aquarius standard algorithms using wind speeds retrieved from multi-source data, the maximum likelihood estimation is employed to produce results combining our model calculations and those using other sources. Numerical simulations show that combined results basically have higher accuracy than the standard algorithms.