基于GPU并行的锥体导管架平台结构冰激振动DEM-FEM耦合分析

该文为分析海冰与锥体海洋平台的相互作用,采用离散元（DEM）-有限元（FEM）耦合方法建立冰激海洋平台结构的耦合模型。通过具有粘结-破碎性能的球体离散单元对海冰的漂移及破碎现象进行计算,海洋平台锥体部分采用平板型壳单元构造,其整体构架及锥体内部的加劲肋采用梁单元构造,即建立壳单元与梁单元组合的锥体海洋平台有限元模型。为提高DEM-FEM耦合算法的计算规模和效率,发展了离散单元与平板型壳单元接触算法及GPU并行环境下参数传递算法。基于此耦合模型分别讨论了平台结构的冰载荷、冰激振动以及锥体应力分布,并与相关实测数据进行对比,为寒区锥体海洋平台的结构设计提供有益的参考。     

基于黏聚单元法的冰-柱碰撞流固耦合计算方法研究

在采用黏聚单元法模拟海洋结构物与海冰的相互作用过程中，大多未考虑水的作用，与实际应用场景不符。基于LS-DYNA有限元软件，构建两种不同厚度的黏聚单元冰模型，采用S-ALE流固耦合方法，对圆柱体直立结构与层冰碰撞进行了数值模拟，研究了碰撞过程中层冰的破坏模式及柱体所受冰载荷，数值模拟结果与实际观测现象相吻合。随后分析了流场网格大小对耦合效果及冰载荷的影响。结果表明：柱体所受冰载荷与流场网格呈正相关，当流场网格为冰体单元的2倍时，计算时间及耦合效果最佳。     

抗冰平台的冰振疲劳分析

基于现场原型结构的观测,对渤海抗冰导管架平台冰激疲劳寿命估计的必要性进行了分析。结果表明,由于渤海特殊环境条件与油藏分布决定了在该海域设计的导管架平台属于典型的柔性抗冰结构,冰激振动不仅能激起较大的加速度响应,而且引起的交变应力也比较明显,在结构设计中进行疲劳寿命估计是必要的。目前,精确估计冰激疲劳寿命的方法还不成熟,其中冰激励荷载与海冰疲劳环境模型是两个关键问题。结合现场观测,对建立锥体疲劳冰荷载与冰环境参数的研究方法进行了论述,为详细疲劳分析提供了基础。     

抗冰导管架平台疲劳可靠性优化

疲劳破坏是海洋工程结构的一种主要破坏形式。对于渤海有冰边际油田,需要在满足冰激疲劳要求的同时考虑平台成本。该文建立了抗冰导管架疲劳可靠性优化模型,利用虚拟激励法并结合兼顾整体和局部的混合有限元模型,实现了复杂管节点冰激疲劳的快速高效分析,并利用对数正态格式计算冰激节点疲劳可靠性。渤海JZ20-2NW平台算例分析表明,通过优化设计可以在满足节点疲劳可靠性要求的同时降低结构重量,可以为冰区海洋平台设计提供参考。     

海冰与锥体抗冰结构动力作用的数值模拟

结合渤海海冰的实际情况,应用显式动力分析软件LS-DYNA模拟海冰和海洋平台锥体抗冰结构相互作用的弯曲断裂过程。模拟结果展示出和现场观测一致的弯曲断裂过程,海冰断裂长度和冰力分布符合现场实测数据结果。平台结构所受最大冰力出现在环向裂纹出现时;通过分析不同冰速对断裂长度、冰力和平台响应的影响可以发现断裂长度对冰速变化并不敏感,对海冰破碎周期有明显的影响。     

海冰作用下平台结构自激振动的参数分析与响应的数值计算

本文依据Ｍａａｔｔａｎｅｎ的自激冰力模型,分析了冰和结构的各种参数对海冰作用下平台结构自激振动的影响;对不同动力特性的导管架平台结构简化模型,进行了海冰作用下结构自激振动响应的数值计算,发现了自激冰力和结构振动的一些特征,为进一步理解冰致平台结构自激振动现象,避免自激振动的产生及采取相应的控制措施提供了有益的结果。     

冰与柔性结构作用挤压破坏形成的动荷载

基于现场原型结构测量,研究了冰与柔性直立结构作用产生挤压破坏过程形成的交变冰力。结果发现:随着冰速的增加,可以出现三种不同冰力形式并分别引起结构准静态振动、稳态振动和随机振动响应。进一步对形成不同冰力模式的机理进行了分析,提出了随着冰速变化,冰与结构相对速率变化导致结构近场冰可以出现塑性破坏、裂纹损伤破坏与纯脆性破坏三种破坏模式,并解释形成交变荷载过程。     

钢筋混凝土框架结构破坏性能的离散单元法模拟

离散单元法是模拟结构破坏的一种有效的分析方法。通过引入节点单元和考虑混凝土的非线性对矩形离散单元模型进行了改进,并给出了改进后离散单元模型的破坏准则和基本方程以及弹簧系数等计算参数的确定;改进后的模型采用了双链表技术,提高了模型的计算效率。对在爆炸荷载作用下的钢筋混凝土框架结构的倒塌破坏过程进行了模拟,结果表明:采用改进后的离散单元法可以有效地模拟钢筋混凝土框架结构的倒塌破坏过程。     

海冰力测量的标定方法

海冰力是渤海海洋平台设计的主要控制荷载之一。为了测量渤海海冰对平台的作用力，我们采用三种不同参数的测量方法；即力、应变和加速度测量。为了检验测量系统的可靠性，取得良好的测量结果。在海冰到来之前用拖船给平台导管架施加一系列确定的外力，从而对各测量系统进行了现场标定。本文介绍了标定方法及其结果。     

海洋平台油气管道疲劳裂纹AE信号特征提取及识别研究

为了将声发射(AE)技术实际应用到监测海洋平台油气管道疲劳裂纹中,需要解决管道振动干扰以及疲劳裂纹AE信号有效特征提取的问题,而问题的关键在于对管道结构疲劳裂纹AE信号特征提取及识别算法的研究。在已有研究的基础上,提出了一种基于经验模态分解(EMD)为特征提取的疲劳裂纹识别方法,将管道振动干扰问题和疲劳裂纹AE信号有效特征提取问题联系在一起,对特征元素进行优化并剔除无效噪声干扰信息,通过概率神经网络(PNN)对疲劳裂纹信号进行识别。试验结果表明,PNN结合基于EMD为特征提取的疲劳裂纹识别法能够取得良好的效果,为声发射技术监测海洋平台油气管道疲劳裂纹提供了试验和理论依据。     

Validation of microparameters in discrete element modeling of sea ice failure process

A GPU-based discrete element method (DEM) with bonded particles is investigated to simulate the mechanical properties of sea ice in uniaxial compressive and three-point bending tests. Both the uniaxial compressive strength and flexural strength of sea ice are related to the microparameters in DEM simulation including particle size, sample size, bonding strength, and interparticle friction coefficient. These parameters are analyzed to build the relationship between the material macrostrengths of sea ice and the microparameters of the numerical model in DEM simulations. Based on this relationship, the reasonable microparameters can be calculated by given macrostrengths in the applications of simulating the failure processes of sea ice. In this simulation, both uniaxial compressive strength and flexural strength of ice increase with the increasing ratio of sample size and particle size. The interparticle friction coefficient is directly related to the compressive strength but has little effect on the flexural strength. In addition, numerical simulations are compared with experimental data to show the performance of the proposed model, and a satisfactory agreement is achieved. Therefore, this microparameter validation approach based on macrostrengths can be applied to simulate the complicated failure process of sea ice interacting with offshore platform structures.     

冰区航行船舶冰阻力及六自由度运动响应的离散元分析EI北大核心CSCD

冰阻力是冰区船舶航行过程中的重要影响因素,对船舶航行性能及航行安全带来严峻挑战。该文采用扩展多面体离散元方法建立冰船动力作用过程的三维离散元方法,基于船舶推进功率与推进力间近似关系,将螺旋桨推力、冰载荷、舵力及水动力等载荷分开考虑,开展恒功率破冰船六自由度非线性操纵直航冰阻力及运动响应计算。为验证该离散元方法的可靠性,对比了碎冰区DUBROVIN及平整冰区LINDQVIST冰阻力公式计算结果。基于船舶在不同主机推进功率及复杂冰况下的直航冰阻力计算结果,展开对船舶破冰航速的影响因素分析。在此基础上,给出了船舶垂荡、横摇、纵摇等冰区船舶运动响应模拟结果和极地船舶运动响应预报结果,最后对碎冰及平整冰区船舶冰阻力及运动响应展开了对比分析。该方法可有效计算船舶冰阻力,其模拟结果可为冰区船舶运动响应及航行安全预警提供重要参考。     

破冰船冰区操纵性能离散元分析

针对浮冰区与平整冰区中航行船舶冰阻力与操纵性的预报问题,采用冰-水-船相互作用数值模型对“雪龙”船在浮冰区与平整冰区中的回转运动特性进行了数值分析。在数值模型中,采用具有黏结破碎特征的球形离散元模型来模拟冰盖的破坏过程,采用球体-三角形接触模型来模拟船-冰之间的碰撞过程,建立了考虑船舶桨力、舵力及水动力的六自由度操纵运动方程。在此基础上对船舶在不同厚度（1.2 m~1.7 m）、不同密集度（40%,60%,80%）浮冰区与不同厚度（0.8 m~1.2 m）平整冰区行进时的破冰阻力及回转特性进行了数值模拟。模拟结果表明:在相同厚度的浮冰区航行时冰阻力小于平整冰区航行的阻力值;浮冰区中回转圈直径约为敞水中回转直径的1倍~2倍;平整冰中回转直径约为敞水中回转直径的2倍~6倍。     

Ice protection of offshore platforms

Climate change-induced reduction in the extent and duration of sea ice cover, as well as an increase in energy demands, has caused renewed interest in exploring and drilling for oil in Arctic waters. Superstructure icing from sea spray and atmospheric icing in the Arctic may impact offshore platform operations. Though icing has not caused the loss of an offshore platform, it can reduce safety, operational tempo, and productivity. Historically, many ice protection technologies were tested on offshore platforms with little success. However, new technologies and modern versions of old technologies used successfully in aviation, the electric power industry, and ground transportation systems, may be adapted to an offshore environment. This paper provides a framework for assessing the relative threat of ice accumulation types, such as superstructure ice, glaze, rime, frost, and snow, to the safety of platform functions. A review of ice protection strategies for functional platform areas is also provided.     

Granular flow in the marginal ice zone

The region of sea ice near the edge of the sea ice pack is known as the marginal ice zone (MIZ), and its dynamics are complicated by ocean wave interaction with the ice cover, strong gradients in the atmosphere and ocean and variations in sea ice rheology. This paper focuses on the role of sea ice rheology in determining the dynamics of the MIZ. Here, sea ice is treated as a granular material with a composite rheology describing collisional ice floe interaction and plastic interaction. The collisional component of sea ice rheology depends upon the granular temperature, a measure of the kinetic energy of flow fluctuations. A simplified model of the MIZ is introduced consisting of the along and across momentum balance of the sea ice and the balance equation of fluctuation kinetic energy. The steady solution of these equations is found to leading order using elementary methods. This reveals a concentrated region of rapid ice flow parallel to the ice edge, which is in accordance with field observations, and previously called the ice jet. Previous explanations of the ice jet relied upon the existence of ocean currents beneath the ice cover. We show that an ice jet results as a natural consequence of the granular nature of sea ice.     

地震作用下海冰与桥墩的间距对桥墩动力响应的影响研究

位于冰水海域的桥墩,在波浪力作用下,桥墩结构与周围海冰往往出现间隙,地震作用下海冰与结构的动力相互作用呈现复杂多样化。本文在弹性力学中的弹性波传播理论及结构动力学的基础上,考虑不同的海冰与桥墩间距,建立了地震作用下桥墩、冰与水体的流固耦合动力方程。并以一矩形桥墩为研究对象,研究了地震作用下,海冰与桥墩的间距对矩形桥墩结构的动力响应及动水压力的影响。研究表明:地震作用下,海冰与桥墩间距对桥墩顶部的最大位移和最大加速度响应的影响不显著,但是对桥墩侧面动水压力和墩底内力影响较大;当海冰与桥墩存在间距且小于1倍结构尺寸时,墩底的弯矩和剪力达到最大值。     

Past, present and impendent hydroelastic challenges in the polar and subpolar seas

Current and emergent advances are examined on the topic of hydroelasticity theory applied to natural sea ice responding to the action of ocean surface waves and swell, with attention focused on methods that portray sea ice more faithfully as opposed to those that oversimplify interactions with a poor imitation of reality. A succession of authors have confronted and solved by various means the demanding applied mathematics associated with ocean waves (i) entering a vast sea-ice plate, (ii) travelling between plates of different thickness, (iii) impinging on a pressure ridge, (iv) affecting a single ice floe with arbitrarily specified physical and material properties, and (v) many such features or mixtures thereof. The next step is to embed simplified versions of these developments in an oceanic general circulation model for forecasting purposes. While targeted on specific sea-ice situations, many of the reported results are equally applicable to the interaction of waves with very large floating structures, such as pontoons, floating airports and mobile offshore bases.     

Uncertainty modeling in the fatigue reliability calculation of offshore structures

This paper presents a procedure of modeling uncertainties in the spectral fatigue analysis of offshore structures with reference to the reliability assessment. Uncertainties of the fatigue damage are generally embedded in response characteristics of the stress process and the damage-model used. Besides commonly accepted uncertainties in offshore structural analysis, which are associated with the modeling of structures and the random wave environment, there are also uncertainties arising from joint flexibilities that occur during the response, the wave–current and water–structure interactions. Uncertainties in joint flexibilities are associated with degradation of member connectivities during a response process. Uncertainties introduced by the wave–current interaction are related to the modeling of a random sea state, applied wave loads and water–structure interaction effects in general. The water–structure interaction, which is an important phenomenon to be considered in the analysis of dynamic-sensitive structures, introduces some added hydrodynamic damping. The associated uncertainties are reflected in the response analysis via the damping term. Therefore, in a quasi-static response analysis, these uncertainties disappear. In the spectral fatigue damage, in addition to the uncertainties of stress statistical characteristics there are some other uncertainties associating with the damage-model used. These uncertainties are related to experimentally determined fatigue data and configurations of selected joints at which damages are likely to occur due to high stress concentrations. This paper presents these uncertainty issues with emphasis on the application of a reliability assessment. However, some other uncertainties arise from approximations inherent in the model. They are assumed to be either comparatively negligible or can be considered within the current uncertainty models so that they are not treated further in this paper. In the calculation of the fatigue damage, a non-narrow banded stress process is used.     

The effect of oscillatory loads on the bearing capacity of floating ice covers

Parked vehicles with running engines, or motor driven machinery, subject an ice cover to a static load and to a relatively small oscillatory force, that is caused by the moving parts. Since for the driving frequencies in question the dominant feature is fatigue of the ice cover, while it is undergoing nonelastic time-dependent deflections, an experimental program was initiated to study this phenomenon by running a series of tests in one of the cold rooms at CRREL. An electronically driven shaker placed on the ice cover was used to simulate the dynamic case. A loading device of the same weight and base shape was used as a static control in the tests. Each test consisted of placing these two objects on an ice cover and recording how their vertical displacements vary with time, for a fixed driving frequency of the shaker. A comparison of these two curves established the effect of the oscillating force component. Eight tests were conducted. It was found that for urea ice covers and driving frequencies of 1, 10 and 30 Hz (60, 600, and 1800 rpm) the vibrating shaker increased the vertical downward displacements and substantially decreased the time to breakthrough.