海洋工程中双柔性圆柱流激振动响应特性

海洋立管、海底管线和浮式平台系泊缆等柔性圆柱结构的流激振动会造成严重的疲劳损伤。目前,关于单根柔性圆柱流激振动的研究较多。为研究多根柔性圆柱系统的流激振动特性,开展了双柔性圆柱流激振动的模型试验。采用模态分析法将试验测量应变转化为位移。通过分析双柔性圆柱在并列、串列和交错排布方式下的响应位移、控制频率,揭示了双柔性圆柱的流激振动特性。研究结果表明:柔性圆柱并列排布时,两圆柱的相互作用使结构顺流向位移增大;柔性圆柱串列排布时,受遮蔽效应的影响,下游圆柱控制模态较低。由于柔性圆柱的多模态响应特性,"锁频"区后下游圆柱的位移并未显著增大;柔性圆柱交错排布时,下游圆柱发生"Wake-flutter",横流向和顺流向的控制频率接近,顺流向的位移显著增大。     

基于能量竞争载荷模型的海洋立管双向涡激振动时域预报方法研究

海洋立管是深海油气开发中用于连接海底井口和水面浮体的唯一通道。立管在洋流作用下极易发生涡激振动（vortex-induced vibration,简称VIV）,发展快速经验性涡激振动时域预报方法对立管的安全设计具有重要意义。通过柔性立管模型试验,结合载荷重构方法和最小二乘法,识别建立了能量竞争载荷模型下的经验水动力载荷系数模型。应用识别建立的经验水动力载荷系数模型,发展形成了海洋立管顺流向及横流向双向涡激振动时域预报方法。将预报结果与试验结果对比,结果表明：基于能量竞争载荷模型的海洋立管双向涡激振动预报方法能够有效预报海洋立管涡激振动主导模态、主导频率、流向平均位移响应和涡激振动位移响应等力学行为特性。研究成果对发展更为有效的涡激振动预报手段具有有益参考。     

海洋立管涡激振动的研究现状、热点与展望

随着深海油气资源的开采,越来越多的研究者开始关注海洋立管的涡激振动问题.在海洋环境下,洋流是海洋立管的涡激振动的主要原因.当洋流流经立管时会在立管的两侧产生交替的泄涡,导致立管受到横流向和顺流向的脉动流体力.这被认为是海洋立管涡激振动的主要诱因.海洋立管的涡激振动是一个异常复杂的工程问题,它涉及许多科学上悬而未决的难题,如紊流、流动分离、分离点的漂移等等.此外,事先无法确定的立管的位置和立管与洋流之间的相互作用又大大增加了解决这一问题的难度.尽管近几十年里科学界在此方面做了大量的研究工作,一个能够准确、高效、经济地预报海洋立管涡激振动的方法仍然没有得到.即便如此,最近的研究工作依然在许多方面作出了突出的成就.首先介绍了涡激振动的背景知识和基础理论.随后,回顾了近年来海洋立管涡激振动方面的研究成果.接着,重点介绍了当前海洋立管涡激振动领域内的两个热点研究问题,即:在多大程度上立管的顺流向振动能够影响立管的横流向振动,以及尾流的三维效应是如何影响立管的涡激振动响应的.最近的研究发现,当结构与流体的质量比小于6时,顺流向振动能显著增大横流向振动的振幅.最近的研究还发现,立管尾流的三维特性和立管受到流体力的轴向相关度有密切关系.随着流动的发展(海流折合速度从0增加到12),立管尾流的三维特性发生变化,在初期,立管尾流的三维特性不明显,流体力的轴向相关度基本等于1,也就是说,流体力和立管的位移响应是同步的,因此能量不断地由海流向立管转移,导致立管的振幅不断增大.当海流折合速度大于6时,流体力的轴向相关度由1锐减到负值,此时,立管尾流的三维效应显著.最后针对今后海洋立管的涡激振动的研究提出一些建议.     

大长细比柔性杆件涡激振动实验

涡激振动(vortex-induced vibration,VIV)是导致深海细长柔性立管发生疲劳破坏的重要因素。采用实验观测手段研究了长细比为1 750的柔性立管多模态涡激振动特性。实验中,通过采用拖车拖拉立管模型在水池中匀速行进来模拟均匀流作用下的涡激振动响应。利用光纤光栅传感器测量立管模型在横流向(cross-flow,CF)和顺流向(in-line,IL)的应变,进而通过模态分解的方法,获得立管模型涡激振动的位移。在此基础上,研究了CF以及IL方向的响应频率、位移标准差的平均值和最大值等随流速的变化规律,并分析了立管模型上测点的运动轨迹及其影响因素。     

悬跨海底电缆涡激振动特性及疲劳损伤分析

悬跨海底电缆作为细长柔性结构,在静力平衡状态下具有一定的垂度,在水流作用下的涡激振动特性与海底管线和海洋立管等结构也有很大的不同,其振动模态受垂跨比影响很大。通过物理模型试验开展了不同垂跨比下悬跨海缆的涡激振动和疲劳损伤特性研究。试验模型按照水弹性相似准则设计,试验中测量了不同流速下海缆模型产生涡激振动时的应变历时数据,采用模态分析法获得了模型涡激振动时的振动模态和振幅。分析了不同流速下海缆模型的振动模态、应变和疲劳损伤的变化和分布特征。试验结果表明：垂跨比显著影响了海缆的涡激振动模态和应变幅值大小。在本试验流速范围内,对一定长度的悬跨海缆模型,当垂跨比较大时,随着流速的增大,模型涡激振动的主响应振动依次出现反对称1阶和对称1阶模态;当垂跨比较小时,模型涡激振动的主响应模态依次出现反对称1阶和对称2阶模态。当涡激振动主响应模态为反对称1阶时,疲劳损伤最大值达到0. 1~0.7。     

平台运动下悬垂立管动力响应及疲劳损伤特性试验研究

悬垂立管因其经济性被广泛应用在深海油气开发中。悬垂立管在顶端平台运动作用下,会不断产生横流向以及顺流向动力响应,循环累积造成疲劳损伤。为了研究悬垂立管在平台运动作用下的疲劳损伤特性,开展了大尺度悬垂立管模型试验。采用小波分析、模态分析、雨流计数法等方法处理分析试验数据。分析平台运动下无内流作用下大尺度悬垂立管动力响应及疲劳损伤特性,发现横流向弯曲应变结果较为复杂,局部呈现行波特性,主导频率具有时变性。相比之下,顺流向弯曲应变结果比较有规律性,整体呈现行波特性,主导频率不具有时变性,并始终保持与顶端平台运动频率相同。平台运动引发的悬垂立管涡激振动疲劳损伤和KC数有关。各点面内疲劳损伤值延管长分布趋势和面内应变RMS延管长分布趋势相似。横流向(CF方向)的疲劳损伤的数量级为顺流向(IL方向)的10-2倍左右。     

不同运动自由度组合串列双圆柱涡激振动数值模拟

基于自主研发的紧致插值曲线CIP (Constrained Interpolation Profile)方法数学模型,对均匀来流条件时不同运动自由度组合下的串列双圆柱涡激振动问题开展二维数值模拟。模型针对雷诺数Re=100,质量比m*=2的串列双圆柱涡激振动问题,选取上、下游圆柱不同运动自由度组合工况进行模拟。重点分析圆柱的升阻力系数、运动位移随折合速度Ur变化的响应。研究表明:当上游圆柱双自由度运动时,随着下游圆柱运动自由度的增加,下游圆柱对上游圆柱涡激振动响应的影响减弱;当下游圆柱双自由度运动时,随着上游圆柱运动自由度的增加,上游圆柱对下游圆柱涡激振动响应的影响变强。研究结果表明圆柱运动自由度组合形式对串列双圆柱涡激振动的影响不可忽略。     

质量比对柔性立管涡激振动影响实验研究

质量比是影响海洋立管涡激振动的一个重要因素.通过在室内物理实验中使立管模型内部分别充填空气、水和沙来改变立管的质量比,从而研究质量比对柔性细长立管涡激振动的影响.实验结果表明:在相同流速下,质量比大的立管模型所激起的模态更高.在低约化速度区域,空管和水管的涡激振动响应频率与涡脱落频率相同,沙管的响应频率则与自振频率更接近,三种质量比立管的响应位移较接近;在高约化速度区域,三种质量比的立管模型的响应频率处于自振频率和涡脱落频率之间,但空管的响应频率随约化速度的增大而不断增大,同一流速下,质量比大的立管模型响应位移小,其中空管的涡激振动响应一直处于大振幅的锁定状态下.共振区域对应约化速度的范围随着质量比增大而减小.     

深水浮式平台垂荡运动与水下柔性立管涡激振动的动力耦合

基于有限元数值模拟,进行了"平台垂荡-顶张力立管涡激振动"整体系统的动响应数值模拟。动响应模型考虑了立管尾迹流场的水动力与结构动力的耦合和垂荡引起的立管结构刚度的时变特性;分析了平台垂荡运动的频率、模态阶数等因素对水下顶张力立管涡激振动的影响。数值结果表明:与不考虑平台运动相比,立管的动响应位移会增大;立管响应幅值随着模态阶数的降低而增大;在响应过程中,尤其对于低阶模态,会出现响应的模态转换现象。鉴于在平台垂荡和涡激振动的共同作用下,立管的动响应会大于涡激振动、参数激励分别单独作用的响应,建议在立管实际工程设计中应该考虑平台运动和涡激振动耦合激励作用下的结构动响应。     

内波流场中悬链线立管涡激振动数值研究

基于内波流场的eKdv理论并结合尾流振子模型和挠性立管的非线性振动方程建立内波流场中悬链线立管计算模型,研究了内波流场中悬链线立管升力方向运动的响应问题,并分析了内孤立波波高和密度跃层位置对悬链线立管升力方向响应的影响。结果表明:内孤立波经过悬链线立管时会引起突然增大的涡激振动,其中高阶模态占主导;涡激振动最大位移发生于流体密度跃层位置附近;内孤立波波高增大以及密度跃层位置加深会使立管转变为更高阶模态振动,并且密度跃层位置加深会使立管涡激振动位移明显减小。     

Assessing the Validity of Seismo-Acoustic Predictor Equations for Obtaining Seabed and Sub-Surface Sediment Physical Properties

The interdependence between the seismo-acoustic properties of a marine sediment and its geotechnical/physical parameters has been known for many years, and it has been postulated that this should allow the extraction of geotechnical information from seismic data. Though in the literature many correlations have been published for the surficial layer, there is a lack of information for greater sediment depths. In this article, a desktop study on a synthetic seafloor model illustrates how the application of published near-surface prediction equations to subsurface sediments (up to several tens of meters burial depth) can lead to spurious predictions. To test this further, acoustic and geotechnical properties were measured on a number of sediment core samples, some of which were subjected to loading in acoustically-equipped consolidation cells (oedometers) to simulate greater burial depth conditions. For low effective pressures (representing small burial depths extending to around 10 meters subsurface), the general applicability of established relationships was confirmed: the prediction of porosity, bulk density, and mean grain size from acoustic velocity and impedance appears generally possible for the investigated sedimentary environments. As effective pressure increases through, the observed relationships deviate more and more from the established ones for the near-surface area. For the samples tested in this study, in some instances increasing pressure even resulted in decreasing velocities. There are several possible explanations for this abnormal behavior, including the presence of gas, overconsolidation, or bimodal grain size distribution. The results indicate that an appropriate depth correction must be introduced into the published prediction equations in order to obtain reliable estimates of physical sediment properties for greater subsurface depths.     

海上大直径钢管桩打桩过程中桩周土强度弱化研究

海上大直径钢管桩打桩过程中,桩周土体受到强烈扰动而发生强度弱化,掌握桩周土体强度弱化规律对于准确预测打桩过程、保证工程安全具有重要意义。为研究土体强度弱化规律,开展了环剪试验模拟打桩对桩周土体的扰动,测试土体强度随剪切速率的变化规律,建立了描述土体强度弱化规律的拟合公式,引入到打桩分析软件中。研究结果表明：土体的强度折减程度不仅与土体本身的性质有关还受到土体的埋深和剪切速率的影响,埋深越深土体强度折减程度越低,剪切速率越高土体强度折减越高,在打桩分析中可采用这里推荐的线性折减方法来模拟不同深度处土体强度的折减规律。     

Interannual Sea Level Variations and Annual Tides in the Northwestern Pacific

Specific properties of the interannual sea level variations and annual tides in the Northwestern Pacific were studied. Several tide stations were monitored. The monthly mean sea level for the year of 1995 was analyzed at each tide station. A seismic event in 1995, some tectonic activity around the subject area, and the Kuroshio (the oceanic western boundary current) may possibly contaminate results which would have occurred from the astronomical annual tide alone.     

Flow Cytometric Analysis of Coastal Lagoon Bacterioplankton and Picophytoplankton: Fixation and Storage Effects

The effect of different fixation and storage protocols on the flow cytometric (FCM) simultaneous analysis of bacterioplankton and phytoplankton in coastal seawater samples (Mediterranean coastal lagoons) was investigated. FCM measurements (cell number, fluorescence and scatter characteristics) were obtained through DAPI staining. Three fixatives [glutaraldehyde (GA), formaldehyde (FA) and paraformaldehyde (PFA)] and two storage (3 months duration) methods (5 °C and −196 °C) were tested. Two dominant populations were detected in studied samples: bacteria and eukaryotic picophytoplankton. Adding fixatives (2% final concentration) appears necessary to obtain FCM exhaustive counts of all the bacteria and phytoplanktonic cells. This was related to the permeation effect of fixatives which allowed a better DAPI staining of the cells. Maximum fluorescence, i.e. optimal staining of the cells was obtained with FA or PFA, and significant lower fluorescences with GA. Fixed samples stored at 5 °C induced rapid cell loss. Only storage in liquid nitrogen of samples fixed with FA or PFA, allows mid-term (≥4 months) preservation of bacteria or picophytoplankton cell numbers, and limited evolution of DAPI-induced fluorescence and scatter characteristics.     

Phylogenetic diversity and phenotypic characterization of cultivable bacterioplankton isolated from polar oceans

A set of 27 marine planktonic bacteria isolated from the polar regions was characterized by 16S rDNA sequencing and physiological and biochemical testing. More than half of these bacteria were positive for caseinase, gelatinase and 13-glucosidase, and could utilize glucose, maltose or malic acid as carbon source for cell growth. Twelve isolates expressed nitrate reduction activities. Except for one antarctic isolate BSwlO175 belonging to Actinobacteria phylum, these isolates were classified as γ-Proteobacteria, suggesting that γ-Proteobacteria dominated in cultivable marine bacterioplankton at both poles. Genus Pseudoalteromonas was the predominant group in the Chukchi Sea and the Bering Sea, and genus ShewaneUa dominated in cultivable bacterioplankton in the Prydz Bay. With sequence similarities above 97%, genus Psychrobacter was found at both poles. These 27 isolates were psychrotolerant, and significant 16S rDNA sequence similarities were found not only between arctic and antarctic marine bacteria （ 〉 99% ）, but also between polar marine bacteria and bacteria from other aquatic environments （ ≥ 98.8% ）, including temperate ocean, deep sea, pond and lake, suggesting that in the polar oceans less temperature-sensitive bacteria may be cosmopolitan and have a bipolar, even global, distribution at the species level.     

INFORMATION FOR AUTHORS

<正>Acta Oceanologica Sinica(AOS)is a comprehensive academic journal edited by the Editorial Committee of Acta Oceanologica Sinica and is designed to provide a forum for important research papers of the marine scientific community which reflect the information on a worldwide basis.The journal publishes scholarly papers on marine science and technology,including physics,chemistry,biology,     

Estimation of Sediment Properties during Benthic Impact Experiments

Sediment properties, such as water content and density, have been used to estimate the dry and wet weights, as well as the volume of sediment recovered and discharged, during benthic impact experiments conducted in the Pacific and Indian Oceans. The estimates show that the weights of dry (355-1332 t) and wet sediment (1651-4888 t) recovered during the experiments, are not only a function of the total duration and distance covered during the experiment, but also depend on water content and density of the sediment. Estimation of dry sediment and its volumetric ratio in the discharge, are key parameters for calculating the discharged volume. The estimated volume of wet sediment recovered (1427-4049 m 3 ) as well as discharged (2693-6951 m 3 ) during the experiments, provide important inputs to evaluate the sediment resettlement and migration. Using these estimates, the average depth of excavation on the seafloor, can also been calculated. However, the stages of sediment recovery and discharge are expressed differently in some of the experiments. In order to standardize these, different stages have been identified, and definitions of certain terms have been suggested, for use in the future. The methods of calculating different properties, as well as weight and volume of discharged sediment are described in the paper for use in other applications concerning deep-sea discharges.