波致砂土海床剪切与液化破坏特征对比研究

波浪作用下海床的稳定性直接影响着海洋构筑物的安全。目前在波土相互作用的研究中,虽然较多地涉及到对海床液化或剪切破坏的分析,但缺乏不同海床计算厚度和饱和度等条件下二者破坏特征的对比研究。本文基于波浪作用下海床应力的解析解,对砂土海床的剪切破坏和瞬态液化破坏特征进行了详细研究和对比。结果分析表明,对于波浪作用下不同饱和度的砂土海床,其剪切破坏深度随海床计算厚度的增加表现为3种变化模式,而其液化深度随海床计算厚度的增加则只表现为1种变化模式。相比非饱和砂土海床,饱和砂土海床计算厚度较小时才可能发生液化,且其液化深度最小,但相同条件下对应的剪切破坏深度却最大。波浪作用下砂土海床存在一个最不稳定厚度,其数值约为(0.2~0.3)倍波长,此时海床最易发生破坏,且破坏深度较大。波浪作用下砂土海床的剪切破坏在波峰和波谷处均可能发生,而瞬态液化只发生在波谷位置,且其液化深度位于剪切破坏深度范围内。     

波流作用下悬浮隧道运动响应纵向截断模型试验研究

利用波流水槽构建悬浮隧道纵向截断模型试验,分析了水流及波浪作用对悬浮隧道运动响应的影响,获得了管体结构的垂向及横向运动响应特性。研究结果表明:水流流速较小时,模型振幅随约化流速Vr增大而增大,而在约化流速为5.8附近,管体振幅达到最大并在这一临界值之后形成较为稳定的振幅值,产生明显的结构共振和涡激振动“锁定”现象。振幅稳定后垂向幅值约为管径的2倍,横向幅值与管径相当。对于波浪单独作用,在波浪较弱时,横向幅值与垂向几乎相等,随着特征KC_n数的增大,响应振幅先增后减。在特征KC_n数小于0.51范围内,垂向振幅大于横向振幅,其余范围则相反。垂向振幅和横向振幅分别在特征KC_n数为0.42和0.55时达到峰值,且两个方向的最大振幅值均与管体的管径相当。相对响应频率随着特征KC_n数的增大而减少,且垂向大于横向。     

南海珠江口盆地贝壳砂土力学性质初步研究

通过对珠江口盆地广布的贝壳砂的化学组分、粒度组成、形成时代的研究以及对砂土进行直剪试验、高压固结试验、常规三轴剪切试验和动三轴试验，对贝壳砂的土力学性质及动载荷作用下的稳定性作了初步分析，获取了贝壳砂的基本物理力学参数并评价了地震作用下砂土液化的可能性，为海洋工程的设计和施工提供了基本的物理力学参数。     

南海珠江口盆地贝壳砂土力学性质初步研究

通过对珠江口盆地广布的贝壳砂的化学组分、粒度组分、形成时代的研究以及对砂土进行直剪试验、主高压结试验、常规三轴剪切试验和动三轴试验，对贝壳砂的土力学性质及动载荷作用下的稳定性作了初步分析，获取了贝壳砂的基本物理力学参数并评价了地震作用下砂土液化的可能性，为海洋工程的设计和施工提供了基本的物理力学参数。     

海底孔压对波浪响应试验研究及数值模拟

孔隙水压力在海底土尤其在砂性海床中扮演着一个非常重要的角色.波浪的周期性加载作用,在砂土及粉土中产生超静孔隙水压力,其幅值大小是海床发生液化破坏的控制因素.在海上工程的历次调查中发现孔压引起许多液化破坏现象,如粉土海床的塌陷、凹坑、坡度较大处的粉砂流及构筑物基础周围的过量冲刷造成石油管架下沉等等,因此研究海床中孔压对波浪的响应具有重要的理论意义和实践意义.     

垂荡板对浮式风机水动—气动耦合性能影响研究

随着风电产业向深远海发展,浮式风机已经成为海上风机未来的发展趋势.由于复杂的风浪联合环境载荷作用,浮式风机作业时通常会产生大幅度的运动响应,这一方面会使得浮式风机系统受到的水动力载荷更加复杂,另一方面会影响浮式风机的输出功率.因此,如何有效地抑制浮式风机系统的运动响应就成为了设计的关键.基于非稳态致动线模型和两相流求解器naoeFOAM-SJTU,进行了带垂荡板的浮式风机耦合性能研究.首先在OC3-Hywind Spar平台上附加垂荡板,并结合NREL-5 MW风力机建立带垂荡板的浮式风机模型.其次对比不同形状的垂荡板对Spar-5 MW型浮式风机气动—水动耦合结果,分析相同风浪联合作用条件下垂荡板形状对浮式风机耦合响应的影响.研究结果表明:垂荡板能够减小纵荡和垂荡等运动响应幅值,但是对纵摇运动响应影响较小;当垂荡板直径和吃水位置相同时,相同风浪条件下圆形垂荡板能使浮式风机的气动平均功率增大约0.844％,而正方形垂荡板却使平均功率减小1.492％,这说明圆形垂荡板对浮式风机系统的作用效果整体而言优于正方形.     

海上风机斜壁桶形基础承载特性研究

基于验证的三维有限元方法,考察了斜壁桶形基础的承载特性,得到了变形网格、位移增量分布、位移等值面分布等结果,探讨了斜壁倾角与各极限承载力之间的定量关系。计算表明,桶形基础发生竖向位移时,主要是桶体内部和桶基正下方的土体发生沉降,而桶侧的土体基本不发生沉降。桶形基础受到水平荷载发生转动时,转动中心轴大致位于桶基底面内,桶基水平承载力主要由桶内土体和桶基外侧中上部受压侧土体产生的抵抗反力构成。桶基因受到较大竖直向上荷载而失效时,桶内土体和桶基外侧靠近海床面附近土体产生了较大的向上位移。桶壁倾角β每增加1°,竖向抗压极限承载力、竖向抗拔极限承载力、水平极限承载力分别提高12%、17.4%及3.8%。     

长江口北槽近底高质量浓度泥沙形成关键机制分析

通过对长江口北槽水文泥沙观测数据的分析,并结合前人的研究成果,定性地确定了制约沉降及紊动抑制作用在长江口近底高质量浓度悬浮泥沙的形成中有重要的作用;通过一个基于FVCOM的一维垂向泥沙模型定量地确定了制约沉降对水体含沙量浓度的增加作用,但紊动抑制显著地改变了泥沙浓度的垂向结构,增强了近底层的水体层化,从而形成了较为明显的近底高质量浓度泥沙悬浮;与实测数据的对比结果显示,制约沉降与紊动抑制作用的联合作用对近底高质量浓度悬浮泥沙模拟结果有显著的改善作用,模型能够较好地模拟大潮期间的长江口近底高质量浓度泥沙垂向剖面过程。     

在平台振荡条件下TLP张力腿的涡激非线性响应

给出了预测张力腿涡激横向振动的时域分析,考虑了波浪、海流、张力腿平台的横漂与垂荡诸因素.张力腿平台的垂荡引起张力周期性变化,对张力腿的涡激横向振动起参数激励的作用,使其动力响应更为复杂.讨论了参数激励下涡激响应的共振条件;以尾流振子模型为基础,发展了分析张力腿在动张力作用下的涡激振动的工程方法,并分析了一座TLP实例,给出了在横漂周期内张力腿上质点的相对位移、弯矩、剪力幅值的时间历程.结果表明,平台垂荡使张力腿的涡激响应幅值升高、高频成分增加,因此它对张力腿的疲劳设计是重要的.     

基于2010—2020年东海风浪统计数据的海上风电吸力桶基础动荷载承载特性

为了得到桶基础承受风、浪动荷载的承载性能,结合我国东海地质条件,统计中国东海2010—2020年浅、深海风速和最大海浪数据,通过计算得到风浪荷载并制成ABAQUS软件幅值曲线施加到海上风电吸力桶基础上,分析了吸力桶基础在动荷载作用下的承载特性。结果表明：吸力桶基础受风、浪荷载影响明显,桶基础迎力面受到风浪动荷载产生拉拔现象。在浅海区,桶基础在风浪荷载作用下桶身自上而下变形不协调,风浪荷载最大的10月份桶基础迎力面最大位移超出了规范0.02D的限制,基础失稳;桶基础背力面受压变形较小。在深海区,桶基础在风浪荷载作用下桶身自上而下变形相对协调,施加风浪荷载后桶体迎力面最大位移为14.9 mm,整体上迎力面位移比背力面位移大4 mm,桶体处于稳定状态。相较浅海区的吸力桶单桶结构,深海区吸力桶结构由于尺寸增加,桶体的稳定性得到提高,说明吸力桶结构的尺寸对稳定性起决定性作用。     

Microbial Biomass and Organic Nutrients in the Deep-Sea Sediments of the Central Indian Ocean Basin

In order to assess the impact of deep-sea mining on the in situ benthic life, we measured the microbial standing stock and concentration of organic nutrients in the deep-sea sediments of the Central Indian Ocean Basin in the Indian pioneer area. Sediments were collected using box core and grab samples during September 1996. The total bacterial numbers ranged from 10 10 -10 11 cells per g -1 dry weight sediment. There was a marginal decrease in the number of bacteria from surface to 30 cm depth, though the subsurface section registered a higher number than did the surface. The highest numbers were encountered at depths of 4-8 cm. The retrievable number of bacteria were two orders less in comparison with the direct total counts of bacteria. An almost homogeneous distribution of bacteria, total organic carbon, living biomass, and lipids throughout the depth of cores indicates active microbial and benthic processes in the deep sea sediments. On the other hand, a uniform distribution of total counts of bacteria, carbohydrates, and total organic carbon in all the cores indicates their stable nature and suggests that they can serve as useful parameters for long-term monitoring of the area after the benthic disturbance. Further studies on temporal variability in this region would not only verify the observed norms of distribution of these variables but would also help to understand restabilization processes after the simulated benthic disturbance.     

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.     

A Review of Some Environmental Issues Affecting Marine Mining

This article reviews information recently available from existing marine and coastal mining for responses to environmental issues affecting marine mining at different depths. It is particularly but not exclusively concerned with those issues affecting seabed biodiversity impact and recovery. Much information has been gathered in the past 10 years from shallow mining operations for construction aggregate, diamonds, and gold, from coastal mines discharging tailings to shallow and deep water, and from experimental deep mining tests. The responses to issues identified are summarized in a series of eight tables intended to facilitate site-specific consideration. Since impacts can spread widely in the surface mixing layer SML, and can affect the biologically productive euphotic zone, the main issues considered arise from the depth of mining relative to the SML of the sea. Where mining is below the SML, the issue is whether it is environmentally better to bring the extraction products to the surface vessel for processing (and waste discharge), or to process the extraction products as much as possible on the seabed. Responses to the issues need to be sitespecific, and dependent on adequate preoperational environmental impact and recovery prediction. For deep tailings disposal from a surface vessel, there are four important environmental unknowns: (1) the possible growth of "marine snow" (bacterial flocs) utilizing the enormous quantities of fine tailings particles (hundreds or thousands of metric tons per day) as nuclei for growth, (2) the possibility that local keystone plankton and nekton species may migrate diurnally down to and beyond the depth of deep discharge and hence be subjected to tailings impact at depth, (3) the burrow-up capability of deep benthos and their ability to survive high rates of tailings deposition, and (4) the pattern and rate of dispersion of a tailings density current through the deep water column from discharge point to seabed. Actions to obtain relevant information in general and site-specifically are suggested.     

Increased Particle Fluxes at the INDEX Site Attributable to Simulated Benthic Disturbance

Particle fluxes were measured 7 m above the sea bottom during the predisturbance, disturbance, and postdisturbance periods by using time series sediment traps attached to seven deep-sea moorings deployed in the INDEX experiment site in the Central Indian Basin. The predisturbance particle fluxes varied between 22.3 to 55.1 mg m -2 day -1 . Increased and variable particle fluxes were recorded by the sediment traps during the disturbance period. The increase observed was 0.5 to 4 times more than the background predisturbance fluxes. The increases in particle fluxes (~4 times) recorded by the sediment trap located in the southwestern direction (DMS-1) were the greatest, which could be the result of preferential movement of resuspended particles generated during the deep-sea benthic disturbance along the general current direction prevailing in this area during the experimental period. Also, the traps located closer to the disturbance area recorded greater fluxes than did the traps far away, across the Deep Sea Sediment Resuspension System path. This variability in recorded particle fluxes by the traps around the disturbance area clearly indicates that physical characteristics such as grain size and density of the resuspended particles produced during the disturbance had an important effect on particle movement. The postdisturbance measurements during ~5 days showed a reduction in particle fluxes of ~50%, indicating rapid particle settlement.     

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。     

Geoacoustic Inversion via Genetic Algorithm and Its Application to Manganese Sediment Identification

An acoustic inversion method using a wide-band signal and two near field receivers is proposed and applied to multiple layered seabed models including a manganese sediment. The inversion problem can be formulated into a probabilistic model comprised of signals, a forward model, and additive noise. The forward model simulates wide-band signals, such as chirp signals, and is chosen to be the source-waveletconvolution plane wave modeling method. The wavelet matching technique, using weighted least-squares fitting, estimates the sediment sound-speed and thickness on which determination of the possible numerical ranges for a priori uniform distribution is based. The genetic algorithm is applied to a global optimization problem to find a maximum a posteriori solution for determined a priori search space. Here the object function is defined by an L 2 norm of the difference between measured and modeled signals. Not only the marginal pdf but also its statistics are calculated by numerical evaluation of integrals using the samples selected during importance sampling process of the genetic algorithm.     

Determination of In Situ Sediment Shear Strength from Advanced Piston Corer Pullout Forces

The advanced piston corer (APC) has been used by the Ocean Drilling Program since 1985 for recovering soft sediments from the ocean floor. The pullout force measured on extracting the core barrel from the sediment is shown to correlate with the average shear strength of the sediment core measured in the ship's laboratory. A simple rule of thumb is derived relating the shear strength of the sediment to the pullout force. Multiple APC holes at individual sites allow the consistency of the pullout measurements to be assessed. The effects of different operational procedures during APC coring are also explored. Although generally applicable, the correlation between pullout force and laboratory measurements of shear strength breaks down for some APC holes, possibly because of the disturbance of some sediment types during the APC coring process. A better understanding of the physical process of APC coring, and its effect on the properties of the sediment both inside and immediately outside the core barrel, would indicate what confidence can be put on the measurement of pullout force as a way of evaluating the in situ shear strength of deep sea sediments.     

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.     

《Acta Oceanologica Sinica》Vol. 33(2014) contents

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