初始几何缺陷对管道极限承载力影响研究

初始几何缺陷被认为是影响管道极限承载力和稳定性的重要因素,但是大部分的管道力学特性研究都没有考虑初始缺陷的影响。基于管道几何尺寸测量机,获得管道的壁厚和直径沿轴向以及环向的分布规律。据此建立了四个三维实体有限元模型,分别为完好管道模型、只考虑直径缺陷的管道模型、只考虑壁厚缺陷的管道模型以及考虑所有缺陷的管道模型。分析了初始缺陷对管道的极限内压承载力、极限轴力承载力和极限弯矩承载力的影响。结果表明,直径缺陷对管道的极限内压承载力影响较大;壁厚缺陷对管道在复杂荷载作用下的极限弯矩承载力影响较大。     

考虑椭圆化和材料各向异性的管道极限弯矩承载力解析解研究

已有的管道极限弯矩承载力解析方法忽略了截面的椭圆化变形,且假设管道截面达到全塑性抵抗力,这对于薄壁管道是不合理的。针对这些不足,在已有解析解的基础上,考虑管道截面塑性区的椭圆化变形以及管道轴向与环向材料的各向异性,推导了管道在内压、轴向力和弯矩联合荷载作用下的极限弯矩承载力解析解。并通过定义材料屈服后不同的广义模量,提出了管道极限弯矩承载力的上限解析解和下限解析解。在此基础上,研究了材料各向异性系数、径厚比以及初始荷载等参数对极限弯矩承载力上限和下限解析解的影响,并得到了一些有益的结论。     

含腐蚀缺陷高强钢厚壁管道压溃压力预测方法

腐蚀是管道常见的缺陷形式之一,会极大降低管道的压溃压力,同时深海环境下高静水外压易引发管道压溃失效,威胁管道的安全运行,因此准确预测管道压溃压力显得尤为重要。采用数值模拟方法研究了含腐蚀缺陷的高强钢厚壁管道压溃失效模式,分析了管材、管道径厚比、腐蚀深度、长度和宽度等参数对高强钢厚壁管道压溃压力的影响规律。分析结果表明:管道径厚比愈大,对厚壁管道的承压能力提升愈显著;腐蚀缺陷的存在对管道压溃压力具有减小作用;随着管道材料等级的提升,压溃压力有明显提高。提出了相应的压溃压力预测公式,为厚壁管道完整性评价提供了参考依据。     

考虑轴向荷载和外压影响的非黏结柔性管道骨架层失效分析

非黏结柔性管道骨架层由不锈钢材料互锁缠绕而成,主要用于抵抗外压防止软管发生压溃失效.骨架层失效是柔性软管故障的最大风险,骨架层失效模式主要分为压溃失效和拉伸失效.采用非线性有限元方法并且考虑几何大变形、材料非线性、接触和摩擦等非线性效应的影响,对"S"型骨架层结构建立三维有限元模型,进行了单外压作用下的压溃失效分析和单一轴向拉力作用下的拉伸失效分析,而后开展轴向拉力和外压载荷组合作用下骨架层压溃失效分析.结果表明轴向拉力会一定程度上降低骨架层压溃承载力,相关结论可以为柔性软管结构设计和完整性评价提供参考.     

非对称局部壁厚减薄海底管道的屈曲分析

在深海环境中,海底管线不仅承受较高外压,还会因为海水及运输介质的常年侵蚀而形成腐蚀缺陷,而腐蚀缺陷往往会导致管道的外压承载力下降。基于壳体稳定性理论,建立了含有非对称局部壁厚减薄管道在外压作用下的屈曲压力理论公式。公式具有广泛的适用性,当内、外局部壁厚减薄深度相等时,可用于计算含有对称局部壁厚减薄管道屈曲压力,而当内部或外部缺陷深度为零时,便可用于计算只含外部或者内部腐蚀缺陷的管道屈曲压力。通过有限元分析验证了该公式的正确性,结果表明公式可以准确预测不同缺陷位置及尺寸时管道的屈曲压力。详细研究了局部壁厚减薄缺陷位置、长度和深度等参数对屈曲压力的影响。研究表明,局部腐蚀对管道的屈曲压力产生重要影响,尤其当腐蚀角度和深度较大时,在腐蚀形成初期就会造成管道的承载力急剧下降,并且管道的屈曲压力与缺陷的径向位置有关,腐蚀缺陷位于管道外侧时的屈曲压力明显大于其位于管道内侧时的屈曲压力。     

深海管道非线性屈曲理论计算方法

针对深海管道在侧向静水压条件下发生局部屈曲压溃的情况,研究了关于管道前屈曲阶段变形响应和压溃压力的计算方法。假定管道前屈曲阶段发生沿轴向均匀一致的椭圆度变形,建立基于圆环的力学模型,考虑几何大变形和材料的弹塑性本构关系,利用虚功原理建立平衡方程。通过选用合适的位移离散函数对方程组数值离散,并利用牛顿法进行数值求解,得到含椭圆度缺陷的管道截面的变形响应和压溃压力。通过试验、有限元分析和规范对理论计算结果进行了对比验证,证明该方法精度可靠,与事实吻合。最后,计算了初始椭圆度缺陷对不同径厚比管道的压溃压力的影响。     

含有腐蚀缺陷海底管道极限载荷分析

利用有限元弹塑性分析方法,对含有腐蚀缺陷的海底管道进行材料非线性和几何非线性分析,探讨了确定管道极限载荷的准则。在此基础上,给出含有腐蚀缺陷的海底管道的极限载荷,研究腐蚀长度、深度和宽度对海底管道极限载荷的影响,提出了含有腐蚀缺陷的受内压的海底管道的极限载荷计算公式,并与试验结果进行了比较,证明该方法是有效的。     

海底双层管单层连接管道结构受力分析

粘性高的海洋石油通常需要通过海底保温管道加温输送.温度变化会引起管道变形,并在管壁内产生较大的温度应力.同时,管道正常运营期间还受到管道内压、外压、管内流体粘滞力和土体摩擦力等环境荷载的作用.复杂的环境可能导致海底管道轴向应力过大发生破坏.为了提高铺管效率,提出了双层管单层连接管道这一特殊管道形式,并从理论上分析温度变化和环境荷载对该管道的影响,计算正常运行时管道不同位置处横截面内最大Von-Mises应力.最后得到了Von-Mises应力沿管道轴线分布情况,发现内管和单层连接管的应力一般比外管大,变径管和内管的焊缝处是Von-Mises应力最大的地方.     

基于ABAQUS的海底管道静水压溃压力的敏感性分析

局部屈曲压溃是海底管道发生稳定性破坏的一种形式,随着管道的刚度相对越来越柔,厚度相对越来越薄,管道发生屈曲压溃的问题也越来越突出。运用ABAQUS有限元分析软件进行管道的非线性屈曲分析,确定不同径厚比、初始椭圆度、轴向拉力和弯矩作用下的管道静水压溃压力,以分析静水压溃压力对这些因素的敏感性。     

坠物碰撞下海底管道凹陷及损伤区域数值模拟研究

基于ANSYS/LS-DYNA有限元软件显示动力分析,采用非线性动态有限元法,对海底管道受坠物碰撞的动态过程进行数值模拟,对比分析坠物质量、坠物形状、轴向预加荷载因素对海底管道在撞击作用下凹陷及损伤区域的影响。结果表明:海底管道受撞击部位的凹陷及损伤区域随着坠物能量的增加而增大;坠物形状因素由于碰撞发生时接触面不同对管道凹陷及损伤区域造成不同的影响;海底管道内压的存在一定程度上抵抗了碰撞造成的局部塑性损伤变形;轴向受拉对海底管道损伤方面的影响很小可忽略不计;适当增加轴向压力可提升海底管道抗碰撞冲击能力而过大的轴向压力会加剧管道破坏。     

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.     

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.     

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,     

Net Sand Transport Direction in a Tidal Inlet, using Foraminiferal Tests as Natural Tracers

The relationship between the flux of exotic benthic foraminiferal tests (i.e. tests which are supplied from open-sea sources alone) in a tidal inlet and that of bulk sediment was analysed, which can be expressed as two first-order linear equations. According to this relationship, in order to determine net sediment transport directions in the entrance, the test concentration in surficial sediments of the tidal basin can be compared against a ‘ critical level ’. The critical level is determined for the conditions that no net transport of bulk sediment is present within the entrance. If the observed concentration (averaged over the tidal basin) is higher than the simulated critical level, then the net sediment transport is directed to landward. This method is applied to the analysis of net sand transport at Christchurch Harbour, a tidal inlet system located in southern England. In this investigation, concentrations of exotic foraminiferal tests in the surficial sediments of the tidal basin and ebb tidal delta area were obtained from the analysis of sea-bed sediment samples. A series of probable critical levels were calculated based upon the data sets with regard to: (1) sediment discharge from the rivers; (2) magnitude of sediment discharge within the entrance during the ebb; (3) the test concentration outside the harbour; (4) the thickness of the moving layer; and (5) two parameters associated with dispersive processes. The results show that the concentration in the tidal basin sediment is higher than a number of simulated critical concentrations for representative cases. Consequently, the high level of the concentration of exotic benthic foraminiferal tests within the harbour should be explained as a result of landward net transport of sands within the entrance.     

Dynamic behaviour of axially pre-loaded tubular steel members of offshore structures subjected to impact damage

The behaviour of an axially pre-loaded cylindrical member of an offshore structure, hit by a supply ship, has been investigated. The effects of axial pre-loading on the dynamic properties of members, the extent of damage and propagation of dynamic instability in the tubular members have also been investigated. Numerical models have been validated using available experimental data from the literature although these are mostly static with no pre-loading. Axial pre-loading was found to change the dynamic characteristics of some cylindrical members quite dramatically while it had no significant effect on others. The study examined whether there was any change in the behaviour between local and global modes when the loading was dynamic rather than static. The effect of damping on the dynamic instability of axially pre-loaded tubes under lateral dynamic loads was also studied. It was observed that for some geometries the quasi-static response can be used to define the boundary between bounded and unbounded dynamic responses. The main contribution of this study, compared with previous investigations reported in the literature, is that both pre-loading and dynamic effects have been included.     

Assessment of Impact on Seafloor Features in INDEX Area

Observations on seafloor features from subbottom profiling and seabed photography data show that the undisturbed and natural conditions before the benthic disturbance undergo a substantial change after operation of the disturber. The effects can be seen in the form of deep trenches and grooves, formed by the action of the sleds and the pumping of the sediment along its path, as well as sediment piles on either side of the track. Biological traces such as fecal coils, casts, tubes, burrows, and trails get obliterated in the areas of disturbance and resedimentation. The smooth, uniform nature of the seafloor observed in predisturbance phase shows microtopographic changes in and around the disturbed area as a result of sediment excavation and resettlement. Vertical mixing as well as lateral transport of sediment alter the geological, biological, physical, and chemical conditions on the seafloor, which need to be monitored over time to assess the process and the time taken for restoration of environmental conditions.     

Regional Commercial Sponge Extinctions in the West Indies: Are Recent Climatic Changes Responsible?

Abstract. Long-term changes in the distributional patterns of commercial sponges ( Spongia spp. and Hippospongia spp.) within the West Indian Region indicates that: 1) commercial sponges had a widespread distribution throughout the whole West Indian Region and were ubiquitous in very shallow water until about the first half of the present century; 2) they were fished commercially not only in the traditional northern Caribbean sites (Florida, Gulf of Mexico, Bahamas) but also in the Greater ( e. g. , Hispaniola, Jamaica) and Lesser Antilles; and 3) they became extinct throughout most of the Lesser Antilles ( e. g. , Puerto Rico, Vieques, St. Thomas) sometime during the first half of this century. Mortalities of spongiids within the Antilles were found to differ from other marine mortalities in that: 1) species disappeared from a large region; 2) species vanished from different habitats and depths; and 3) natural populations never recovered. Species richness patterns suggest that commercial sponge genera ( Spongia and Hippospongia ) evolved under slightly cooler elimatic conditions than those found at present, and that these extinctions occurred as a direct or indirect effect of positive thermal anomalies in sea surface and atmospheric temperatures between 1900–1950. The concept that species diversity is stable on a regional scale is questioned.