Experimental investigation of the keying process of OMNI-Max anchor     

Jun Liu  Congcong Han  Long Yu 《Marine Georesources & Geotechnology》2013,31(3):349-365

ABSTRACT

The OMNI-Max anchors are newly developed dynamically installed anchors for deep water mooring systems. After installation, the anchor is keyed to a new orientation and position by tensing the attached mooring chain, which is known as the “keying process”. This study conducted 1g model tests to study the trajectories and capacity developments of OMNI-Max anchors in homogeneous and lightly overconsolidated (LOC) clays. A testing arrangement was designed to simulate the anchor keying process with a constant pullout angle at the mudline. A half model anchor which could move against the box glass was used to determine the anchor trajectory in the soil. The effects of padeye offset angle, uplift angle at the mudline, anchor fluke thickness, anchor initial embedment depth, and soil strength on the anchor trajectory and capacity were systematically investigated. Moreover, the critical uplift angle at the padeye and the anchor critical initial embedment depth were discussed. The results indicate that the anchor can dive both in homogeneous and LOC clays under certain conditions. A padeye offset angle of 24–30° is recommended for the OMNI-Max anchor to maintain high capacity and diving trend simultaneously. Besides, the anchor diving trend can be improved with small uplift angles at the mudline and with thick anchor flukes. A critical initial embedment depth of 1.3 times the anchor length is recommended to preclude the anchor from being pulled out.  相似文献   

Reviewing evidence of marine ecosystem change off South Africa     

《African Journal of Marine Science》2013,35(3):427-448

Recent changes have been observed in South African marine ecosystems. The main pressures on these ecosystems are fishing, climate change, pollution, ocean acidification and mining. The best long-term datasets are for trends in fishing pressures but there are many gaps, especially for non-commercial species. Fishing pressures have varied over time, depending on the species being caught. Little information exists for trends in other anthropogenic pressures. Field observations of environmental variables are limited in time and space. Remotely sensed satellite data have improved spatial and temporal coverage but the time-series are still too short to distinguish long-term trends from interannual and decadal variability. There are indications of recent cooling on the West and South coasts and warming on the East Coast over a period of 20–30 years. Oxygen concentrations on the West Coast have decreased over this period. Observed changes in offshore marine communities include southward and eastward changes in species distributions, changes in abundance of species, and probable alterations in foodweb dynamics. Causes of observed changes are difficult to attribute. Full understanding of marine ecosystem change requires ongoing and effective data collection, management and archiving, and coordination in carrying out ecosystem research.  相似文献   

1211号“海葵”台风登陆后引发两段大暴雨过程的对比分析          下载免费PDF全文

吴海英  曾明剑  王卫芳  王磊  沈阳 《大气科学学报》2015,38(5):670-677

利用地面加密自动站观测资料以及NCEP再分析资料,对1211号“海葵”台风登陆后在江苏引发的两段降水对流特征差异明显的大暴雨天气进行对比分析。结果表明:第一段区域性大暴雨天气发生在台风环流中心及北侧偏东风急流附近,此时台风环流完整,中心维持正压结构,环流中心及其北侧偏东急流附近伴有较大范围的水汽辐合和强上升运动,有利于区域性大暴雨天气发生,但降水发生在近乎中性的层结下,降水分布较均匀,发展平缓,降水期间对流活动较弱;第二段大暴雨则发生在远离环流中心的台风倒槽顶部,降水期间暴雨区中高层伴有较明显的冷平流,有利于对流不稳定层结发展,降水发展过程中,地面风场出现中尺度扰动,增强了局地辐合和气旋性涡度,加之地面锋区发展,促进了中尺度对流系统的形成和发展,此段降水中尺度特征显著,发展迅速,雨强大,伴有明显的对流特征,导致出现局地特大暴雨天气。  相似文献   

南极中山站附近冰盖近地面层湍流参数的观测研究          下载免费PDF全文

林忠  卞林根  马永锋  逯昌贵 《极地研究》2009,21(3):221-233

利用2007/2008年中国第24次南极考察队在南极中山站附近冰盖上观测试验获得的湍流脉动及相关资料,对原始资料通过坐标旋转订正后,应用涡动相关法计算分析了冰盖近地面层的湍流强度(I)、稳定度参数(z/L)、摩擦速度(u*)、拖曳系数(Cd)、地表粗糙度(z0)及动量通量(τ)和感热通量(H),并与空气动力学方法的计算结果进行对比。结果表明,Louis方案能够较好地模拟近地面层湍流通量;在平均状态下,全天雪面以感热形式从大气获得净的能量;近中性层结下地表粗糙度z0为4.54×10-4m,拖曳系数Cd=1.7×10-3,在非中性条件下,稳定度越小Cd越大,反之,则稳定度越大Cd越小。  相似文献   

损伤和缺陷对海洋平台抗冰抗震性能的影响   总被引：1，自引：0，他引：1  

柳春光  金书成 《世界地震工程》2009,25(4)

现役导管架式海洋平台可能会存在裂纹、凹痕、腐蚀等缺陷和损伤,并可能受到海洋生物附着及地基土冲刷的影响,这些问题将减小构件强度或者放大荷载作用.取一座导管架海洋平台为例,应用动力时程及整体推进法进行分析,对比损伤前后动冰力作用下结构响应、结构抗冰能力曲线及剩余强度储备系数,并取3个地震波对损伤前后结构的地震响应、抗震能力曲线、抗震安全裕度系数进行对比.结果显示损伤和缺陷极大地降低了海洋平台抗冰抗震性能,对现役海洋平台的检测和维修也有一定借鉴意义.  相似文献   

The new Caribbean complexity: Mobility systems, tourism and spatial rescaling   总被引：1，自引：1，他引：0  

Mimi Sheller 《Singapore journal of tropical geography》2009,30(2):189-203

The Caribbean region is being respatialized, rescaled and reterritorialized in the face of contemporary processes of neoliberal development, shifting mobilities and spatial restructuring. Drawing on the field of mobilities research, this paper argues that new transregional approaches to spatial dynamics are needed to describe the complex, polymorphic, and multiscalar geographies in the neoliberalizing Caribbean. It first analyzes how new spatializations of Caribbean mobilities are part of larger transnational processes of urban restructuring. It then examines how neoliberalizing policies have promoted the 'opening' of once publicly owned infrastructures such as ports, airports, and telecommunications, contributing to a rescaling of Caribbean territoriality. Finally it considers how tourism mobilities associated with the cruise ship industry and private luxury property developments facilitate interlocality competition and outside access to the region while circumscribing local access and mobility rights. The paper proposes a postcolonializing island studies that recognizes the complexity of contemporary Caribbean rescaling and the subtle ways in which modernized built environments and infrastructures of mobility and connectivity contribute to the weakening of island-state sovereignty, territorial integrity and democratic citizenship.  相似文献   

高强高性能混凝土损伤的细观数值演化          下载免费PDF全文

郑捷  左河山  陈景恒  曹文平  郑山锁 《地震工程学报》2016,38(5):745-750,769

在细观层次上,混凝土可以被看作由水泥基、分散粒子和界面过渡层组成的三相复合材料。首先,基于骨料分布和形态的随机特性,将瓦拉文公式推广应用于确定二维混凝土试件截面凸多边形骨料分布,提出圆形骨料模型中以圆骨料的面积为控制参数,以圆内接多边形为基架的凸多边形随机骨料算法。进而以C80高强高性能混凝土为例,对数值试样进行单轴受压的数值模拟,得到相应的应力-应变曲线和损伤演化图。数值模拟结果与物理试验结果对比表明本文提出的数值模型合理可行。  相似文献   

Satellite-observed variability of phytoplankton size classes associated with a cold eddy in the South China Sea     

Junfang Lin  Wenxi Cao  Guifen Wang  Shuibo Hu 《Marine pollution bulletin》2014

Ocean-color remote sensing has been used as a tool to detect phytoplankton size classes (PSCs). In this study, a three-component model of PSC was reparameterized using seven years of pigment measurements acquired in the South China Sea (SCS). The model was then used to infer PSC in a cyclonic eddy which was observed west of Luzon Island from SeaWiFS chlorophyll-a (chla) and sea-surface height anomaly (SSHA) products. Enhanced productivity and a shift in the PSC were observed, which were likely due to upwelling of nutrient-rich water into the euphotic zone. The supply of nutrients promoted the growth of larger cells (micro- and nanoplankton), and the PSC shifted to greater sizes. However, the picoplankton were still important and contributed ∼48% to total chla concentration. In addition, PSC time series revealed a lag period of about three weeks between maximum eddy intensity and maximum chlorophyll, which may have been related to phytoplankton growth rate and duration of eddy intensity.  相似文献   

Effect of seawater on incident plane P and SV waves at ocean bottom and engineering characteristics of offshore ground motion records off the coast of southern California,USA     

Hongqi Diao  Jinjun Hu  Lili Xie 《地震工程与工程振动(英文版)》2014,13(2):181-194

The effect of seawater on vertical ground motions is studied via a theoretical method and then actual offshore ground motion records are analyzed using a statistical method. A theoretical analysis of the effect of seawater on incident plane P and SV waves at ocean bottom indicate that on one hand, the affected frequency range of vertical ground motions is prominent due to P wave resonance in the water layer if the impedance ratio between the seawater and the underlying medium is large, but it is greatly suppressed if the impedance ratio is small; on the other hand, for the ocean bottom interface model selected herein, vertical ground motions consisting of mostly P waves are more easily affected by seawater than those dominated by SV waves. The statistical analysis of engineering parameters of offshore ground motion records indicate that:(1) Under the infl uence of softer surface soil at the seafl oor, both horizontal and vertical spectral accelerations of offshore motions are exaggerated at long period components, which leads to the peak spectral values moving to a longer period.(2) The spectral ratios(V/H) of offshore ground motions are much smaller than onshore ground motions near the P wave resonant frequencies in the water layer; and as the period becomes larger, the effect of seawater becomes smaller, which leads to a similar V/H at intermediate periods(near 2 s). These results are consistent with the conclusions of Boore and Smith(1999), but the V/H of offshore motion may be smaller than the onshore ground motions at longer periods(more than 5 s).  相似文献   

珠江三角洲经济区生态地球化学评价   总被引：2，自引：0，他引：2  

窦磊  杜海燕  游远航  赖启宏 《现代地质》2014,28(5):915-927

土壤重金属污染是珠江三角洲地区主要的生态环境问题之一,直接影响到区域生态系统的稳定和食品安全。通过开展多目标区域地球化学调查、区域生态地球化学评价、局部生态地球化学评价和总体综合评价,系统完成了珠江三角洲经济区47954 km²(包括10 m水深以浅的近岸海域)的生态地球化学调查与评价。按照双层网格化方法系统采集了陆域土壤、近岸海域和珠江水系的主要河流沉积物表层和深层样品,测试了71项元素和指标,建立了珠江三角洲经济区土壤、沉积物地球化学背景值和基准值,首次对珠江三角洲地区区域地球化学特征进行了系统全面的调查,全面查明了珠江三角洲经济区土壤环境质量状况,区内一级和二级土壤分别占总面积的19.9%和57.3%,三级和三级以上土壤占22.8%。定量计算了镉、铅、汞等主要异常元素经河流、成土过程等自然来源和大气干湿沉降、施肥、灌溉、使用农药等人为来源贡献量,阐明了其迁移转化途径及生态安全性。结合研究区特点,系统开展了三角洲形成演化过程的地球化学研究,并在国内首次制定了符合地区特点的珠江三角洲经济区土壤环境质量地方标准。  相似文献