深水半潜式钻井平台简化疲劳分析

研究适用于中国南海的深水半潜式钻井平台结构简化疲劳分析方法。依据中国南海环境条件,计算平台结构应力响应,得到平台结构应力范围长期Weibull分布形状参数。通过结构波浪应力的长期预报,计算得到平台寿命期一遇波浪载荷,依据寿命期一遇波浪载荷计算平台结构寿命期一遇最大热点应力范围。采用简化疲劳分析方法计算平台结构疲劳寿命,用谱疲劳分析方法验证简化疲劳分析结果的准确性,证实了简化疲劳分析方法相对保守,可应用于平台的设计。     

基于谱分析LNG船典型热点疲劳可靠性分析

研究LNG船在疲劳载荷作用下应力热点的疲劳可靠性。通过整船有限元分析得到不同浪向角规则波中各热点的应力频响函数,用谱分析法确定各热应力点在各短期海况下的应力范围模型,进而通过长期预报,用Weibull分布对各热点应力范围分布模型进行拟合并探讨热点型式和受载特性对参数变化的影响。基于线性累积损伤理论,利用S-N曲线计算各热点的疲劳累积损伤值并建立极限状态方程,评估LNG船舯部典型热点的疲劳可靠性和结构的疲劳安全性。针对不同热点讨论了应力范围长期分布函数—Weibull分布参数的取值对典型热点可靠性指标的影响,并给出典型热点目标可靠性指标和长期应力范围极值之间的关系以用于指导设计和安全性评价。     

波浪载荷及气动力引起的半潜浮式风机疲劳损伤分析

研究浅水半潜式大功率浮式风力机波浪载荷和气动力引起的基础结构疲劳损伤,揭示基础结构的疲劳损伤机理。采用谱疲劳损伤计算分析方法,以10 MW风力机为例,计算波浪载荷引起的热点应力及多种海况引起的疲劳损伤。采用叶素动量理论并基于所在海域的风速分布,计算叶轮转动引起的气动力及其引起的疲劳损伤。计算结果表明,对于半潜式三立柱浮式风力机,波浪载荷引起的基础结构应力远大于气动力引起的基础结构应力,基础结构损伤主要是由波浪载荷引起,气动力引起的浮式基础结构的损伤为10^-3量级,而波浪载荷引起的损伤为10^-1量级。     

基于谱分析法的深水半潜式平台疲劳强度分析

以某新型深水半潜式钻井平台为分析对象,依据三维绕射理论计算单位波幅下波浪诱导载荷,通过建立三维有限元模型计算完成了热点应力响应。运用热点应力的谱分析法计算得到危险节点的疲劳寿命,并进一步分析了各个短期海况和不同浪向对节点总的疲劳损伤度的贡献。研究结果表明,危险节点的疲劳寿命都满足设计寿命要求;在各种中低海况下,疲劳损伤主要来自于平均跨零周期在6 s附近,有义波高转大的短期海况;平台关键连接部位的疲劳损伤主要是在横浪和斜浪状态下产生的,并且对某单一方向的波浪非常敏感。此研究对该类型平台的连接处结构设计和疲劳分析方法有参考意义。     

宽带波浪载荷下海洋结构物内应力范围及疲劳损伤分析

研究短期海况中海洋结构物在宽带波浪载荷作用下交变应力过程中的应力范围的概率分布及结构疲劳损伤问题.首先从应力的功率谱密度出发,利用小波变换作为工具,对应力过程进行模拟.然后用雨流计数法识别出应力过程中对结构造成疲劳损伤的应力范围,并用Weibull分布对应力范围的分布进行了拟合;最后给出了应力范围服从Weibull分布时结构疲劳损伤的计算方法.     

基于S-N曲线的深水半潜平台疲劳寿命分析

深水半潜平台工作环境恶劣,造价成本高,为保证安全工作,有必要对其进行疲劳寿命分析。首先利用SESAM软件建立目标平台整体模型并分析总体结构响应。然后根据平台总体结构响应计算结果,选取立柱与横撑连接处、立柱外板与上甲板连接处两个关键节点,建立关键节点有限元模型,计算其热点应力。最后采用S-N曲线分析该平台在中国南海环境条件下的疲劳寿命。分析结果显示该平台疲劳寿命符合规范要求,但立柱与横撑连接处的疲劳寿命偏低,应加强日常检修和维护。     

基于实测应力的半潜平台疲劳损伤分析

疲劳破坏是海洋结构物发生破坏的一种主要形式,深水半潜平台在海上运输、作业、停滞时会受到交变的风、浪、流载荷作用,进而引起结构关键节点处的疲劳损伤,影响平台安全性。以"海洋石油981"半潜钻井平台为研究对象,利用光纤光栅应变传感器对结构高应力区进行应力监测,获取测点处应力时历数据。将监测位置划分为四个监测板块,采用四点雨流计数法对测点应力进行处理,得到以离散形式给出的应力循环幅度与相应循环周次。参照DNV《海上钢结构疲劳设计规范》推荐的S-N曲线,基于Miner线性疲劳累计损伤原则,得到各个测点处的总体疲劳损伤度。选取具有最大疲劳损伤度的危险测点,并结合海上实测总时长,预测半潜平台的总体疲劳寿命。     

半潜平台立柱与浮筒中部不同形式连接节点疲劳寿命分析

为了研究半潜式平台的立柱与浮筒中纵舱壁连接节点出现焊缝开裂这种疲劳失效现象,利用简化疲劳方法以及三维细化有限元模型,对目前主流平台中所采用两种不同的立柱与浮筒中纵舱壁连接节点形式进行了疲劳寿命评估与分析。首先从结构刚度分配角度,平均应力效应角度分析疲劳裂纹产生的原因;然后利用疲劳分析结果评估不同连接节点对整个平台结构可靠性的影响;最后,基于结构力学性能和刚度匹配角度分析两种节点形式的优劣,确定最优节点设计方案。     

半潜式平台结构整体可靠性分析方法

针对海洋环境中的半潜式平台多方面不确定性,基于半潜式平台结构整体强度分析方法,考虑多种波浪荷载作用的结构响应,采用非线性逐步垮塌分析方法,研究平台在不同控制工况下的极限承载力模型,建立相应的功能函数.根据结构体系可靠度理论和失效模式之间的关系,采用串联体系的可靠度计算方法.最后,对一座具体半潜式平台在100年重现期波浪荷载作用下进行了体系可靠性计算.计算结果表明平台整体可靠度水平由波浪引起的主要结构响应形式决定.通过计算,验证可靠度计算方法具有高效性和可行性,可以为大型深水浮式平台结构的设计、维护和评估提供理论依据.     

半潜式海洋平台结构的疲劳失效概率计算研究

本文根据Ｍｉｎｅｒ线性累积损伤原理，引入随机变量，客观地分析了半潜式平台结构因疲劳引起损伤的情况，提出了平台结构在设计寿命期间的疲劳可靠性分析方法，通过算例，验证了方法的可靠性，并得出有意义的结论。     

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.     

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.     

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.     

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.     

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。     

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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