Reliability based approach for offloading operation related to motion of two side-by-side moored LNG carriers

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.     

The Behaviour of LNG Carrier Moored to a Jetty Exposed to Waves,Swell,Unsteady Wind and Current

—Most terminals for tankers are piers and sea islands,while other types include single pointmoorings and multiple-buoy moorings.The LNG and LPG carrier moored to the jetty is a very commonterminal for transfer of gas in open seas.It is important to estimate the motions and line tensions of theLNG carrier when it moors to a jetty in metocean environment.Normally,the motions of the LNG carrierwould be restricted by the loading arm,which is connected to LNG carrier＇s manifold.An example of125,000m~3 LNG carrier moored to a jetty exposed to a set of environment conditions is given.Amathematical model which is based on the equations of motion in the time domain is used to the analysisof LNG moored to an offshore jetty exposed to waves,swell,wind and current.By means of a time do-main computer program TERMSIM computations are carried out to determine and optimize the lay-outand/or orientation of the jetty and mooring gear in terms of forces in mooring lines and fenders and theenvelope of motions of the loadi     

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.     

拉伸与扭转载荷作用下LNG低温复合软管的力学性能研究

LNG低温复合软管结构复杂,且在运行时受风、浪、流等多种载荷联合作用,其在超低温环境下的力学性能直接关系到其能否安全运行。首先参考江苏某软管公司生产的4英寸LNG低温复合软管,建立软管精细化有限元模型,接着对软管在拉伸、扭转两种轴对称载荷作用下的力学性能进行研究,最后就软管结构尺寸对其力学性能的影响展开分析。结果表明：在拉伸及扭转载荷作用下,不同层芳纶布以及同层芳纶布不同材料主方向在对应波峰、波谷处的受力情况不同。芳纶布铺设角度、芳纶布铺设层数、钢丝螺距以及钢丝直径对复合软管力学性能影响显著,分析相应的灵敏度规律从而对结构尺寸做出调整。研究成果可以为LNG低温复合软管的结构设计提供参考。     

A probabilistic assessment of design sloshing pressure time histories in LNG tanks

The violent motion (sloshing) of liquefied natural gas (LNG) in cargo tanks has attracted significant attention. Transformations of the LNG market have led to the increased transport of LNG in partially filled tanks, but established technology is mainly based on engineering experience with completely filled containers. This paper investigates a large sample of sloshing pressure measurements. It focuses on the magnitude of individual sloshing impact events, and their associated temporal and spatial patterns. The durations of these impacts are comparable to the natural frequency of an LNG container wall, so the details of their time histories are important in determining the structural response. Experiments are performed on tanks with high (92.5%) and low (30%) filling levels, for various wave headings. The common post-processing approach of representing impact pressure histories by a triangular profile is studied, and an alternative approach is presented. Two statistical models are used to describe the distribution of maximal pressures in sloshing impacts: a three-parameter Weibull model and a generalized Pareto model. The latter is found to be of questionable utility due to small sample sizes. It is observed that for low filling levels the sloshing impacts are of greater magnitude, having longer durations, smaller ratios of rise time to duration, and larger spatial extents. All these factors should in principle increase the structural response.     

Experimental Study on Aerodynamic Characteristics of the Model Wind Rotor System and on Characterization of A Wind Generation System

In order to investigate the aerodynamic characteristics of 6-MW wind turbine, experimental study on the aerodynamic characteristics of the model rotor system and on characterization of a wind generation system is carried out. In the test, a thrust-matched rotor system and a geometry-matched rotor system, which utilize redesigned thrustmatched and original geometry-matched blades, respectively, are applied. The 6-MW wind turbine system is introduced briefly. The proper scaling laws for model tests are established in the paper, which are then implemented in the construction of a model wind turbine with optimally designed blades. And the parameters of the model are provided. The aerodynamic characteristics of the proposed 6-MW wind rotor system are explored by testing a 1:65.3 scale model at the State Key Laboratory of Ocean Engineering at Shanghai Jiao Tong University. Before carrying out the wind rotor system test, the turbulence intensity and spatial uniformity of the wind generation system are tested and results demonstrate that the characterization of the wind generation system is satisfied and the average turbulence intensity of less than 10% within the wind rotor plane is proved in the test. And then, the aerodynamic characteristics of 6-MW wind rotor system are investigated. The response characteristic differences between the thrust-matched rotor system and the geometry-matched rotor system are presented. Results indicate that the aerodynamic characteristics of 6-MW wind rotor with the thrust-matched rotor system are satisfied. The conclusion is that the thrust-matched rotor system can better reflect the characteristics of the prototype wind turbine. A set of model test method is proposed in the work and preparations for further model basin test of the 6-MW SPAR-type floating offshore wind turbine system are made.     

Exploration of the wind-induced drift characteristics of typical Chinese offshore fishing vessels

The drift motion of a maritime distress target is the collective result of the balance of forces that comes from wind, currents and waves. The drift properties vary from one type of object to another. The objective of this paper is to explore the leeway drift characteristics of typical Chinese offshore fishing vessels as well as evaluate the leeway drift model. First, a series of field experiments were conducted in South China Sea to provide data source. Next, nine leeway coefficients in AP98 leeway model were derived by the least square fitting based on the experimental data. Furthermore, another drifting dynamics model based on a balance of forces on the drifting vessel due to wind and currents, was also calibrated for comparison. Finally, two cases of drift trajectory and searching areas were simulated by the two different models through Lagrangian particle tracking and Monte Carlo techniques. Results indicate that the AP98 leeway model is in better accordance with the observation compared with the drifting dynamics model. Additionally, the simulation accuracy of AP98 leeway model can be improved to some extent especially when the probability of positive crosswind (POPC) is considered. Considering the large number of the typical offshore fishing vessels in South China Sea, and their high accident rate, the model coefficients for this type of vessels are expected to be implemented into more search and rescue (SAR) models by SAR organizations.     

Comparison of wind and wave measurements and models in the Western Mediterranean Sea

We have hindcast the wind and wave conditions in the Mediterranean Sea for two one month periods. Four different meteorological models and three different wave models have been used. The results have been compared with satellite and buoy wind and wave observations.Several conclusions concerning both the instruments and the models have been derived. The quality of both wind and wave results has been assessed. Close to the coasts high resolution, nested wave models are required for sufficient reliability.A wave threshold analysis suggests a sufficient reliability only off the coast, with a substantial decrease for low wave heights.     

An experimental study of the wave excitation in the gap between two closely spaced bodies,with implications for LNG offloading

The side-by-side offloading of liquid natural gas (LNG) at offshore terminals involves a fixed and a floating body in close proximity; the offshore terminal being the fixed body and the LNG tanker the floating body. The closeness of the two bodies leads to the formation of a long and relatively narrow gap, within which there is the potential for large amplifications of the water surface elevation. The present paper uses experimental results to characterise both the size and nature of the excitation within the gap. It also illustrates the effect of the vessel motion on this amplification by considering a 1:100 scaled model of an LNG tanker as well as its fixed approximation. It is found that the body's ability to move acts to increase the frequency at which resonant amplification within the gap occurs (the resonance frequency). The incident wave conditions considered include regular and irregular waves in both beam- and head-sea orientations; the latter leading to very different gap end conditions. The nature of the resonant amplification for the floating LNG tanker is shown to be similar for the two orientations, suggesting that the gap end conditions do not drive the resonant amplification. Consideration of the nonlinearity within the gap illustrates that resonant amplification occurs at the resonance frequency, irrespective of whether the fluid motion is first or second harmonic. The present paper provides data relevant to the safe offloading operations of an LNG tanker and demonstrates the importance of incorporating the vessel motion in numerical modelling procedures.     

Temporal significant wave height estimation from wind speed by perceptron Kalman filtering

Significant wave height estimates are necessary for many applications in coastal and offshore engineering and therefore various estimation models are proposed in the literature for this purpose. Unfortunately, most of these models provide simultaneous wave height estimations from wind speed measurements. However, in practical studies, the prediction of significant wave height is necessary from previous time interval measurements. This paper presents a dynamic significant wave height prediction procedure based on the perceptron Kalman filtering concepts. Past measurements of significant wave height and wind speed variables are used for training the adaptive model and it is then employed to predict the significant wave height amounts for future time intervals from the wind speed measurements only. The verification of the proposed model is achieved through the dynamic significant wave height and wind speed time series plots, observed versus predicted values scatter diagram and the classical linear significant wave height models. The application of the proposed model is presented for a station in USA.     

An empirical microwave emissivity model for a foam-covered sea

A model for computing microwave emissivity of a wind-driven foam-covered sea is presented. The effect of roughness and foam is modeled by combining early measurement results and theoretical analysis. Recent Seasat-SMMR measurements are used to fine tune the model and derive an "effective" fractional foam coverage expression in terms of frequency and wind speed. The model incorporates polarization characterization and view angle dependence of the foam-cover emissivity. For48.8degincidence angle and wind speed less than 15 m/s, the emissivity values calculated from this model differs by <8 percent from those calculated from Wilheit's model. At nadir and at 25 m/s wind speed, the emissivity calculated from the two models differ bysim15percent.     

一种计算台风风场的方法

揭示Ｒａｎｋｉｎｅ涡风场模式和Ｊｅｌｅｓｎｉａｎｓｋｉ风场模式之间的联系，并设计了一种台风风场分布模式，它的风速分布曲线落在Ｊｅｌｅｓｎｉａｎｓｋｉ和Ｒａｎｋｉｎｅ涡两个风场模式的风速分布曲线之间，具有一个既优于Ｊｅｌｅｓｎｉａｎｓｋｉ又优于Ｒａｎｋｉｎｅ涡的风速衰减速率，因此它同时克服了Ｒａｎｋｉｎｅ涡模式计算风速偏小和Ｊｅｌｅｓｎｉａｎｓｋｉ模式计算风速偏大的缺点，以一种比较合理的变化趋势向远方衰减，成为一个比较切合实际的台风风场分布模式。同时，文中提出的移行台风风场计算方法对宫崎正卫、上野武夫和Ｊｅｌｅｓｎｉａｎｓｋｉ模式都有一定的改进。     

Natural air–sea flux of CO2 in simulations of the NASA-GISS climate model: Sensitivity to the physical ocean model formulation

Results from twin control simulations of the preindustrial CO₂ gas exchange (natural flux of CO₂) between the ocean and the atmosphere are presented here using the NASA-GISS climate model, in which the same atmospheric component (modelE2) is coupled to two different ocean models, the Russell ocean model and HYCOM. Both incarnations of the GISS climate model are also coupled to the same ocean biogeochemistry module (NOBM) which estimates prognostic distributions for biotic and abiotic fields that influence the air–sea flux of CO₂. Model intercomparison is carried out at equilibrium conditions and model differences are contrasted with biases from present day climatologies. Although the models agree on the spatial patterns of the air–sea flux of CO₂, they disagree on the strength of the North Atlantic and Southern Ocean sinks mainly because of kinematic (winds) and chemistry (pCO₂) differences rather than thermodynamic (SST) ones. Biology/chemistry dissimilarities in the models stem from the different parameterizations of advective and diffusive processes, such as overturning, mixing and horizontal tracer advection and to a lesser degree from parameterizations of biogeochemical processes such as gravitational settling and sinking. The global meridional overturning circulation illustrates much of the different behavior of the biological pump in the two models, together with differences in mixed layer depth which are responsible for different SST, DIC and nutrient distributions in the two models and consequently different atmospheric feedbacks (in the wind, net heat and freshwater fluxes into the ocean).     

Pressure resistance of the corrugated stainless steel membranes of LNG carriers

Corrugated stainless steel membranes have been used as the primary barriers for LNG carriers to reduce thermal stresses at cryogenic temperatures. As the capacity of the LNG cargo is increased, however, the corrugated stainless steel membranes suffer from buckling and collapse due to increased sloshing loads in large-capacity LNG cargo carriers. In this study, the pressure resistance of the corrugated part of the stainless steel membrane was evaluated using a finite element method for better design of the LNG containment system, and an experimental equipment was developed and the pressure resistance of the stainless steel corrugations was measured and compared with the calculated results. In addition, a new membrane with high-pressure resistance and its reinforcing method were developed and evaluated experimentally.     

Mapping wind by the first-order Bragg scattering of broad-beam high-frequency radar

Mapping wind with high-frequency(HF) radar is still a challenge. The existing second-order spectrum based wind speed extraction method has the problems of short detection distances and low angular resolution for broad-beam HF radar. To solve these problems, we turn to the first-order Bragg spectrum power and propose a space recursion method to map surface wind. One month of radar and buoy data are processed to build a wind spreading function model and a first-order spectrum power model describing the relationship between the maximum of first-order spectrum power and wind speed in different sea states. Based on the theoretical propagation attenuation model, the propagation attenuation is calculated approximately by the wind speed in the previous range cell to compensate for the first-order spectrum in the current range-azimuth cell. By using the compensated first-order spectrum, the final wind speed is extracted in each cell. The first-order spectrum and wind spreading function models are tested using one month of buoy data, which illustrates the applicability of the two models. The final wind vector map demonstrates the potential of the method.     

风浪流中半潜式风机系统动力响应特性研究

以三浮筒(Tri-floater)型半潜式风机系统为对象,采用时域方法,考虑垂荡板的黏性阻尼效应,研究系泊缆直径、长度、预张力和布置方式等参数对其在风浪流中动力响应特性的影响规律,提出了一种系泊参数优化方法。在此基础上,根据东海和南海三个不同水深区域百年一遇海洋环境条件,通过调整Tri-floater型浮式基础的吃水和垂荡板等参数,有效地提高了其运动响应周期,减小了其运动响应幅值。同时,根据风机浮式基础运动幅值的设计规范要求,针对三个不同水深区域特点,设计了Tri-floater型浮式基础的系泊系统。计算结果表明,改进后的Tri-floater型半潜式风机系统能满足百年一遇极端环境下的作业要求,适合于东海和南海海洋环境下海上风能资源的开发。     

Derivation of Parametric Tropical Cyclone Models for Storm Surge Modeling

In this paper, the parametric tropical cyclone models for storm surge modeling are further developed. Instead of tangential wind speed via cyclostrophic balance and radial wind speed using a simple formulation of defection angle, the analytical expressions of tangential and radial wind speed distribution are derived from the governing momentum equations based on the general symmetric pressure distribution of Holland and Fujita. The radius of the maximum wind is estimated by tropical cyclone wind structure which is characterized by the radial extent of special wind speed. The shape parameter in the pressure model is estimated by the data of several tropical cyclones that occurred in the East China Sea. Finally, the Fred cyclone (typhoon 199417) is calculated, and comparisons of the measured and calculated air pressures and wind speed are presented.     

南海海面风、浪场时空变化特征及其关系分析

利用长时间序列的卫星观测数据,对南海海域的风、浪场时空分布及其相互关系进行了分析。结果显示,海面风距平场VEOF分解后得到的第一模态具有明显的季节变化,即季风特征,说明季风是影响整个南海风速的主要因素;第二模态具有较强的区域变化特征,是季风转换时期的距平场特征;第三模态反映的是海面风距平场受陆地地形影响所表现的分布特征。有效波高距平场EOF分解后得到的第一模态、第二模态与风距平场的前2个模态的空间分布较为相似,并且,风、浪距平场第一模态间的相关系数达0.76,均说明南海作为边缘海其波浪场与风场变化有很好的相关性。有效波高第三模态的分布与风场的第三模态相关性较弱,反映的是受海底地形影响所表现的分布特征。     

Evaluation of long-term extreme response statistics of jack-up platforms

This paper is concerned with the models appropriate for the dynamic assessment of jack-ups, concentrating particularly on the long-term response due to random ocean waves and on work-hardening plasticity models used for spud-can response. A methodology for scaling of short-term statistics, calculated using a Constrained NewWave technique, is shown in a numerical experiment for an example jack-up and central North Sea location. The difference in long-term extreme response statistics due to various footing assumptions is emphasised. Results for two environmental load conditions are described (one excluding and one including wind and current effects) and the role of sea-state severity in the variation of short-term extreme response statistics is also highlighted.