Parametric Stability Analysis of Marine Risers with Multiphase Internal Flows Considering Hydrate Phase Transitions

This study investigates the effects of multiphase internal flows that consider hydrate phase transitions on the parametric stability of marine risers.A numerical model of the multiphase internal flow that considers a hydrate phase transition is established.The model first solves the flow parameters and subsequently obtains the natural frequencies of risers with different gas intake ratios.The stability charts of marine risers with different gas intake ratios are plotted by applying Floquet theory,and the effects of the gas intake ratio on the instability and vibration response of the risers are identified.The natural frequency increases with an increase in the gas intake ratio;thus,instability zones move to higher frequency ranges in the stability charts.As the increasing gas intake ratio reduces the damping effect of the Coriolis force,the critical amplitude of the heave in the unstable region decreases,especially when hydrodynamic damping is not considered.As a result,higher-order unstable regions are excited.When in an unstable region,the vibration response curve of a riser with a high gas intake ratio excited by parametric resonance diverges quickly due to parametric resonance.     

对流边界层理论中不稳定非自治系统的数值解（英文）

为了求解对流边界层理论中一个非自治微分方程系统,作者采用伽略金有限元方法,此方法是通过将无限区间上的三阶非线性微分方程转化成有限区间上的二阶微分形式,并构造出相应的伽略金有限元方程来求得数值解,该数值解与先前一些作者的结果一致,并且计算效率显高于其它数值方法.     

成层土中轴横受荷桩水平响应的非线性解

对于承受轴向荷载的水平受荷桩，以往研究大多基于线弹性或弹塑性水平荷载传递模型。为提升轴横受荷桩的计算设计水平，采用轴向荷载传递法计算桩身轴力，考虑桩身轴力引起的P-Δ效应，基于双曲线型水平荷载传递模型考虑桩-土体系变形的非线性特征，对成层土中轴横受荷桩的水平响应进行分析求解，得到了轴横荷载作用下桩身变形和内力的非线性有限差分解，并采用MATLAB语言编制了计算程序。使用模型试验算例与基于现场试验的有限元算例对非线性解的准确性进行对比验证，结果表明:计算结果与算例数据吻合良好，可靠性较高；采用不同荷载传递模型的计算结果在不同荷载水平下有所差异，在较大荷载水平下桩-土变形的非线性特点不容忽视。      

The Coupling VIV Analysis of SCRs with Rigid Swing

With the development of deepwater oil and gas exploration, Steel Catenary Risers(SCRs) become preferred risers for resource production, import and export. Vortex induced vibration(VIV) is the key problem encountered in the design of SCRs. In this study, a new model, the rigid swing model, is proposed based on the consideration of large curvature of SCRs. The sag bend of SCRs is assumed as a rigid swing system around the axis from the hanging point to the touch down point(TDP) in the model. The torque, produced by the lift force and the swing vector, provides the driving torque for the swing system, and the weight of SCRs provides the restoring torque. The simulated response of rigid swing is coupled with bending vibration, and then the coupling VIV model of SCRs is studied in consideration of bending vibration and rigid motion. The calculated results indicate that the rigid swing has a magnitude equal to that of bending vibration, and the rigid motion affects the dynamic response of SCRs and can not be neglected in the VIV analysis.     

Numerical Simulation of Mechanical Characteristics of a Metal Net for Deep-Sea Aquaculture

The investigation on hydrodynamic characteristics of a cage is important for its application in the deep-sea aquaculture in our country. With finite element method, the beam element is used to simulate a three-dimensional metal chain net, and the connector element is introduced as the interaction between metal net lines. A mechanical model for the metal net is constructed to simulate the hydrodynamic characteristics of a metal net subjected to fluid current forces. The static simulation results show that the relative errors of the displacements are 2.13%, 4.19%, 6.64%, and 11.35% compared with static concentrated load tests under concentrated forces of 20, 40, 60, and 80 N, respectively. Both the transient hydrodynamic deformations and drag forces of the netting structures under different current velocities are obtained by solving the hydrodynamic equation of the netting structure. The average relative error of the current forces obtained by numerical simulations shows an 8.13% deviation from the drag tests of the metal nets in the tank under five current velocities. The effectiveness and precision of the simulation approach are verified by static and dynamic tests. The proposed simulation approach will provide a good foundation for the further investigation of the hydrodynamic characteristics of deep-sea aquaculture metal cages and the parameter design for the safety of such cage systems.     

A nonlinear vortex induced vibration model of marine risers

With the exploitation of oil and gas in deep water, the traditional vortex induced vibration (VIV) theory is challenged by the unprecedented flexibility of risers. A nonlinear time-dependent VIV model is developed in this paper based on a VIV lift force model and the Morison equation. Both the inline vibration induced by the flow due to vortex shedding and the fluid-structure interaction in the transverse direction are included in the model. One of the characteristics of the model is the response-dependent lift force with nonlinear damping, which is different from other VIV models. The calculations show that the model can well describe the VIV of deepwater risers with the results agreeing with those calculated by other models.     

Adaptive finite element modeling of marine controlled-source electromagnetic fields in two-dimensional general anisotropic media

In this paper, we extend the scope of numerical simulations of marine controlled-source electromagnetic (CSEM) fields in a particular case of anisotropy (dipping anisotropy) to the general case of anisotropy by using an adaptive finite element approach. In comparison to a dipping anisotropy case, the first order spatial derivatives of the strike-parallel components arise in the partial differential equations for generally anisotropic media, which cause a non-symmetric linear system of equations for finite element modeling. The adaptive finite element method is employed to obtain numerical solutions on a sequence of refined unstructured triangular meshes, which allows for arbitrary model geometries including bathymetry and dipping layers. Numerical results of a 2D anisotropic model show both anisotropy strike and dipping angles have great influence on the marine CSEM responses.     

Vortex-induced vibration of two parallel risers: Experimental test and numerical simulation

The vortex-induced vibration of two identical rigidly mounted risers in a parallel arrangement was studied using Ansys-CFX and model tests.The vortex shedding and force were recorded to determine the effect of spacing on the two-degree-of-freedom oscillation of the risers.CFX was used to study the single riser and two parallel risers in 2–8D spacing considering the coupling effect.Because of the limited width of water channel,only three different riser spacings,2D,3D,and 4D,were tested to validate the characteristics of the two parallel risers by comparing to the numerical simulation.The results indicate that the lift force changes significantly with the increase in spacing,and in the case of 3D spacing,the lift force of the two parallel risers reaches the maximum.The vortex shedding of the risers in 3D spacing shows that a variable velocity field with the same frequency as the vortex shedding is generated in the overlapped area,thus equalizing the period of drag force to that of lift force.It can be concluded that the interaction between the two parallel risers is significant when the risers are brought to a small distance between them because the trajectory of riser changes from oval to curve 8 as the spacing is increased.The phase difference of lift force between the two risers is also different as the spacing changes.     

3D GIS下地下水流有限元数值模拟方法与应用

目前缺乏针对较强空间特性水文地质计算参数的高效采集方法与管理机制,而且现有孔隙地下水流有限元数值模拟在三维可视化与空间分析等方面仍存在很多不足。因此,本文以有限元数值计算方法和三维地理信息平台为基础,结合GIS空间分析算法和计算机图形学理论,针对孔隙承压地下水有限元分析过程的关键步骤（概念模型构建、空间离散、水文地质参数提取与赋值等）,提出了孔隙地下水有限元数值模拟过程在3D GIS下的实现方法和技术框架。基于水文地质钻孔数据和GTP（Generalized Tri-Prism）空间数据模型,以盐城市滨海平原水文地质区为研究区域,实现了地下水系统空间结构特征的三维表达,同时与地下水流有限元数值模型相耦合,提出了3D GIS模型与地下水非稳定流有限元数值模型耦合的核心处理算法,并在此基础上开发了3D GIS地下水流数值模拟系统。应用实例表明,该技术框架可有效简化有限元分析流程、优化模型计算效率、实现地下水有限元数值模拟过程及计算结果的三维可视化。     

Violent transient sloshing-wave interaction with a baffle in a three-dimensional numerical tank

A finite difference model for solving Navier Stokes equations with turbulence taken into account is used to investigate viscous liquid sloshing-wave interaction with baffles in a tank. The volume-of-fluid and virtual boundary force methods are employed to simulate free surface flow interaction with structures. A liquid sloshing experimental apparatus was established to evaluate the accuracy of the proposed model, as well as to study nonlinear sloshing in a prismatic tank with the baffles. Damping effects of sloshing in a rectangular tank with bottom-mounted vertical baffles and vertical baffles touching the free surface are studied numerically and experimentally. Good agreement is obtained between the present numerical results and experimental data. The numerical results match well with the current experimental data for strong nonlinear sloshing with large free surface slopes. The reduction in sloshing-wave elevation and impact pressure induced by the bottom-mounted vertical baffle and the vertical baffle touching the free surface is estimated by varying the external excitation frequency and the location and height of the vertical baffle under horizontal excitation.     

Comparative Analysis of Seismic Response Characteristics of Pile-Soil-Stnicture Interaction System

The study on the earthquake-resistant performance of a pile-soil-structure interaction system is a relatively complicated and primarily important issue in civil engineering practice. In this paper, a computational model and computation procedures for pile-supported structures, which can duly consider the pile-soil interaction effect, are established by the finite element method. Numerical implementation is made in the time domain. A simplified approximation for the seismic response analysis of pile-soil-structure systems is briefly presented. Then a comparative study is performed for an engineering example with numerical results computed respectively by the finite element method and the simplified method. Through comparative analysis, it is shown that the results obtained by the simplified method well agree with those achieved by the finite element method. The numerical results and findings will offer instructive guidelines for earthquake-resistant analysis and design of pile-supported structures.     

地下水流的有限分析数值解

本文介绍了一种求解地下水流问题的有限分析法,该方法将解析解结合到数值方法中,从而提高了计算精度,改善了稳定性。算例表明,在网格尺寸相同的情况下,有限分析解比有限差分解高一个数量级,与解析解吻合很好。     

Dynamic performance of a semi-submersible platform subject to wind and waves

By applying experimental and numerical simulations, the motion performance of a semi-submersible platform with mooring positoning system under combined actions of wind and waves is studied. The numerical simulation is conducted by the method of nonlinear time domain coupled analysis, and the mooring forces are calculated by the piecewise extrapolating method. The scale in the model experiment is 1:100, and the mooring system of the model is designed with the method of equivalent water-depth truncation by comparing the numerical and the experimental results, the platform motion and mooring forces subject to wind and waves are investigated. The results indicate that the numerically simulated mooring forces agree well with the experimental results in static equivalent field, but show some difference in dynamic equivalent field; the numerically simulated platform motions coincide well with the experimental results. The maximum motion of the platform under operating conditions is 20.5 m. It means that the horizontal displacement is 2% less than the water depth, which satisfies the operating requirements.     

河西重力变化的小波分解与地震活动关系的研究

讨论了小波分析在地震重力测量数据分析中应用的可能性，并采用基于样条函数的小波分析及其相应的Β小波分别计算了河西地区的重力变化资料，其结果如下：①小波分解可有效分离重力场时间变化的不同空间波长成分，更清晰地看清重力场变化与地震活动的关系。②河西地区重力资料分解结果表明：反映该地区浅部物质变化的重力变化高频部分和反映该地区深部物质变化的重力变化低频部分对永登Ｍｓ５．８级地震的反应不明显，而反映该地区中层物质变化的重力变化二、三阶小波分解结果对该地震的反应较明显。重力小波分解计算结果反映的密度变化深度与地震资料统计的震源深度基本一致。     

Hydro-mechanical analysis of rainfall-induced fines migration process within unsaturated soils

Seepage-induced fines migration under rainfall infiltration is a main cause leading to shallow failures in loose colluvial slopes. To describe the full process of fines migration within unsaturated soils during rainfall infiltration and the associated hydro-mechanical behaviors, a seepage-erosion-deformation coupled formulation is proposed in this paper. The governing equations proposed are implemented into a finite element code and used to investigate the influences of skeleton deformation on the rainfall infiltration process through unsaturated soil columns. The numerical results were presented in detail for a better understanding of the rainfall-induced fines migration process within unsaturated soils. Further, the obtained results are integrated into an infinite slope model for slope stability analysis. The results show that, the skeleton deformation will affect the rainfall infiltration rate and hence the timing of slope failures; meanwhile their influences are more evident if the fines deposition process is taken into account. Moreover, the slope stability could be reduced gradually due to the soil strength loss along with loss of fine particles. Therefore, particular attentions should be paid to analyzing the stability of soil slopes susceptible to internal erosion.     

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Damage localization of marine risers using time series of vibration signals

Based on dynamic response signals a damage detection algorithm is developed for marine risers. Damage detection methods based on numerous modal properties have encountered issues in the researches in offshore oil community. For example, significant increase in structure mass due to marine plant/animal growth and changes in modal properties by equipment noise are not the result of damage for riser structures. In an attempt to eliminate the need to determine modal parameters, a data-based method is developed. The implementation of the method requires that vibration data are first standardized to remove the influence of different loading conditions and the autoregressive moving average (ARMA) model is used to fit vibration response signals. In addition, a damage feature factor is introduced based on the autoregressive (AR) parameters. After that, the Euclidean distance between ARMA models is subtracted as a damage indicator for damage detection and localization and a top tensioned riser simulation model with different damage scenarios is analyzed using the proposed method with dynamic acceleration responses of a marine riser as sensor data. Finally, the influence of measured noise is analyzed. According to the damage localization results, the proposed method provides accurate damage locations of risers and is robust to overcome noise effect.     

Study on deformation law of circular foundation under combined loading

The foundations of some ocean engineering structures are built to withstand not only the vertical gravity load V, but also the horizontal load H induced by sea waves and current. The horizontal load includes the concentrated force load, the moment load M, and the torque load T termed also as combined loading. It is of academic and engineering significance to study the deformation law of submarine seabed due to combined loading. On the basis of the three-dimensional elastic mechanics solution of circular foundation, numerical methods are used to analyze the deformation law of submarine soil under circular foundation with six degrees of freedom. The finite element analysis results give the elastic deformation law of soil in three dimensional spaces, modify the theoretical elasticity solution, and presents nonlinear soil deformation mechanism under the circular foundation with six degrees of freedom.     

青藏高原东南缘构造应力场重构

采用有限元方法,针对青藏高原东南缘建立更细致、更精确的三维有限元弹性模型。选取9种不同的应力边界条件,分别进行优化分析后处理,将对应台站形变模拟值与GPS实测值进行误差分析,最终选取最佳方案作为古构造应力场。结果表明,青藏高原东南缘4 Ma BP的古应力场主要起源于中国大陆周围板块的相互作用,特别是印度板块NNE向强烈碰撞作用,成为中国大陆尤其是西南部青藏高原地区构造应力场最主要的动力来源,控制各个块体相互作用的方式和运动格局。青藏高原东南缘古应力场主要包括几个力源：西北部青藏高原侧向挤压造成的WE向应力约105 MPa;西南部直接来自于印度板块的NE-WS向应力约70 MPa;南部NS向作用力33 MPa;东南部扬子块体侧向NW-SE阻挡力56 MPa;北东部受扬子块体强烈EW向阻挡力90 MPa。这些力源共同作用于青藏高原东南缘,形成现今复杂应力场。
     

Analysis on velocity distribution and displacement along the profile of a slope using both empirical and analytical methods

Assessing the slope deformation is significant for landslide prediction. Many researchers have studied the slope displacement based on field data from the inclinometer in combination with complicated numerical analysis. They found that there was a shear zone above the slip surface, and they usually focused on the distribution of velocity and displacement within the shear zone. In this paper, two simple methods are proposed to analyze the distribution of displacement and velocity along the whole profile of a slope from the slip surface to the slope surface during slow movement. In the empirical method, the slope soil above the shear zone is assumed as a rigid body. Dual or triple piecewise fitting functions are empirically proposed for the distribution of velocity along the profile of a slope. In the analytical method, the slope soil is not assumed as a rigid body but as a deformable material. Continuous functions of the velocity and displacement along the profile of a slope are directly obtained by solving the Newton's equation of motion associated with the Bingham model. Using the two proposed methods respectively, the displacement and velocity along the slope profiles of three slopes are determined. A reasonable agreement between the measured data and the calculated results of the two proposed methods has been reached. In comparison with the empirical method, the analytical method would be more beneficial for slope deformation analysis in slope engineering, because the parameters are material constants in the analytical solution independent of time t, and the nonlinear viscosity of the soil can be considered.