Dynamic Analysis of A Deepwater Drilling Riser with A New Hang-off System

The safety of risers in hang-off states is a vital challenge in offshore oil and gas engineering.A new hang-off system installed on top of risers is proposed for improving the security of risers.This approach leads to a challenging problem:coupling the dynamics of risers with a new hang-off system combined with multiple structures and complex constraints.To accurately analyze the dynamic responses of the coupled system,a coupled dynamic model is established based on the Euler-Bernoulli beam-column theory and penalty function method.A comprehensive analysis method is proposed for coupled dynamic analysis by combining the finite element method and the Newmarkβmethod.An analysis program is also developed in MATLAB for dynamic simulation.The simulation results show that the dynamic performances of the risers at the top part are significantly improved by the new hang-off system,especially the novel design,which includes the centralizer and articulation joint.The bending moment and lateral deformation of the risers at the top part decrease,while the hang-off joint experiences a great bending moment at the bottom of the lateral restraint area which requires particular attention in design and application.The platform navigation speed range under the safety limits of risers expands with the new hang-off system in use.

Simulation of isothermal precision extrusion of NiTi shape memory alloy pipe coupling by combining finite element method with cellular automaton

In order to present the microstructures of dynamic recrystallization(DRX) in different deformation zones of hot extruded NiTi shape memory alloy(SMA) pipe coupling,a simulation approach combining finite element method(FEM) with cellular automaton(CA) was developed and the relationship between the macroscopic field variables and the microscopic internal variables was established.The results show that there exists a great distinction among the microstructures in different zones of pipe coupling because deformation histories of these regions are diverse.Large plastic deformation may result in fine recrystallized grains,whereas the recrystallized grains may grow very substantially if there is a rigid translation during the deformation,even if the final plastic strain is very large.As a consequence,the deformation history has a significant influence on the evolution path of the DRX as well as the final microstructures of the DRX,including the morphology,the mean grain size and the recrystallization fraction.

Free vibration characteristics of sectioned unidirectional/bidirectional functionally graded material cantilever beams based on finite element analysis

Advancements in manufacturing technology,including the rapid development of additive manufacturing(AM),allow the fabrication of complex functionally graded material(FGM)sectioned beams.Portions of these beams may be made from different materials with possibly different gradients of material properties.The present work proposes models to investigate the free vibration of FGM sectioned beams based on onedimensional(1D)finite element analysis.For this purpose,a sample beam is divided into discrete elements,and the total energy stored in each element during vibration is computed by considering either the Timoshenko or Euler-Bernoulli beam theory.Then,Hamilton’s principle is used to derive the equations of motion for the beam.The effects of material properties and dimensions of FGM sections on the beam’s natural frequencies and their corresponding mode shapes are then investigated based on a dynamic Timoshenko model(TM).The presented model is validated by comparison with three-dimensional(3D)finite element simulations of the first three mode shapes of the beam.

Self-adaptive strategy for one-dimensional finite element method based on EEP method with optimal super-convergence order

Based on the newly-developed element energy projection （EEP） method with optimal super-convergence order for computation of super-convergent results, an improved self-adaptive strategy for one-dimensional finite element method （FEM） is proposed. In the strategy, a posteriori errors are estimated by comparing FEM solutions to EEP super-convergent solutions with optimal order of super-convergence, meshes are refined by using the error-averaging method. Quasi-FEM solutions are used to replace the true FEM solutions in the adaptive process. This strategy has been found to be simple, clear, efficient and reliable. For most problems, only one adaptive step is needed to produce the required FEM solutions which pointwise satisfy the user specified error tolerances in the max-norm. Taking the elliptical ordinary differential equation of the second order as the model problem, this paper describes the fundamental idea, implementation strategy and computational algorithm and representative numerical examples are given to show the effectiveness and reliability of the proposed approach.

Shape Optimal Design of Hydrodynamic Bearing Based on Dynamic Characteristics

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ON THE SELECTION OF SHAPE FUNCTION SPACES OF TRIANGULAR PLATE ELEMENTS

Using the method of undetermined coefficients, we construct a set of shape function spaces of nine-node triangular plate elements converging for any meshes, which generalize Spect's element and Veubeke's element.

STIFFNESS EQUATION OF FINITE SEGMENT FOR FLEXIBLE BEAM-FORMED STRUCTURAL ELEMENTS

The finite segment modelling for the flexible beam-formed structural elements is presented, in which the discretization views of the finite segment method and the difference from the finite element method are introduced. In terms of the nodal model, the joint properties are described easily by the model of the finite segment method, and according to the element properties, the assumption of the small strain is only met in the finite segment method, i. e., the geometric nonlinear deformation of the flexible bodies is allowable. Consequently,the finite segment method is very suited to the flexible multibody structure. The finite segment model is used and the are differentiation is adopted for the differential beam segments. The stiffness equation is derived by the use of the principle of virtual work. The new modelling method shows its normalization, clear physical and geometric meanings and simple computational process.

Study on the Dynamic Characteristic of Lathe Headstock

In order to provide reliable data for the dynamic design or modification of a tool machine,the dynamic character- istics of the headstock,which is the main component to bear moment,must be obtained precisely.In the paper,the method based on the combination of calculation mode and experiment mode is proposed to analyze the dynamic characteristics of the headstock.The modal parameters and the mode shapes are calculated by ANSYS7.1 software.According to the FEM calculating results,the ex- periment parameters can be selected correctly.The modal parameters of the headstock have to be calculated and identified precisely. On the basis of these modal parameters,the faults of the headstock are shown and its weak points of design are illustrated.A con- clusion is drawn that some reasonable reinforce positions could greatly improve the dynamic characteristics of the system and this ap- proach is proved to be precise and reliable.

基于分层法的石墨烯增强功能梯度Timoshenko梁动力特性有限元分析

基于分层法对石墨烯增强功能梯度Timoshenko梁的动力特性问题进行有限元分析。在有限元建模过程中,首先将Timoshenko梁沿厚度方向分成若干层,然后利用改进的Halpin-Tsai细观力学模型计算各层的弹性模量,相应的泊松比和密度则根据混合率法则进行计算,最后对各层均采用4节点四边形板单元离散。通过数值算例分析分层数和单元尺寸比例的合理性,探究石墨烯片的分布形式、质量含量、几何形状和尺寸等因素对Timoshenko梁动力特性的影响。结果表明,添加少量的石墨烯能显著提高Timoshenko梁自由振动的频率,尤其在梁的上下部位分布较多正方形石墨烯片时效果最佳。随着石墨烯片长厚比的增加,Timoshenko梁的自由振动基频不断增大。当石墨烯片长厚比超过1 000之后,梁的基频变化不明显。

Mass-Stiffness Templates for Cubic Structural Elements Dedicated to Professor Karl Stark Pister for his 95th birthday

1 This paper considers Lagrangian finite elements for structural dynamics constructed with cubic displacement shape functions.The method of templates is used to investigate the construction of accurate mass-stiffness pairs.This method introduces free parameters that can be adjusted to customize elements according to accuracy and rank-sufficiency criteria.One-and two-dimensional Lagrangian cubic elements with only translational degrees of freedom(DOF)carry two additional nodes on each side,herein called side nodes or SN.Although usually placed at the third-points,the SN location may be adjusted within geometric limits.The adjustment effect is studied in detail using symbolic computations for a bar element.The best SN location is taken to be that producing accurate approximation to the lowest natural frequencies of the continuum model.Optimality is investigated through Fourier analysis of the propagation of plane waves over a regular infinite lattice of bar elements.Focus is placed on the acoustic branch of the frequency-vs.-wavenumber dispersion diagram.It is found that dispersion results using the fully integrated consistent mass matrix(CMM)are independent of the SN location whereas its lowfrequency accuracy order is O(κ8),whereκis the dimensionless wave number.For the diagonally lumped mass matrix(DLMM)constructed through the HRZ scheme,two optimal SN locations are identified,both away from third-points and of accuracy order O(κ8).That with the smallest error coefficient corresponds to the Lobatto 4-point integration rule.A special linear combination of CMM and DLMM with nodes at the Lobatto points yields an accuracy of O(κ10)without any increase in the computational effort over CMM.The effect of reduced integration(RI)on both mass and stiffness matrices is also studied.It is shown that singular mass matrices can be constructed with 2-and 3-point RI rules that display the same optimal accuracy of the exactly integrated case,at the cost of introducing spurious modes.The optimal SN location in two-dimensional,bicubic,isoparametric plane stress quadrilateral elements is briefly investigated by numerical experiments.The frequency accuracy of flexural modes is found to be fairly insensitive to that position,whereas for bar-like modes it agrees with the one-dimensional results.

基于mbS模式的有限单元法

mbS模式及其有限元法是在固体和结构分析模型中引入薄膜、弯曲和剪切理论,且采用纯拉压、纯弯和纯剪单元进行分析的数值方法.在时空系中剖分物质单元和时间单元上构造以指数函数和贝塞尔函数为插入函数且按Lagrange插值条件的薄膜、弯曲和剪切等基本位移函数,由此得到更加完备和耦合的固体和结构实体单元的变形模式,根据能量泛函变分原理得到静动力有限元基本方程的一致格式.研究表明,mbS模式及其有限元法可用于梁柱和板壳等结构的静动力分析及屈曲分析.

非均质压电结构动力学问题的多尺度有限元法研究

基于多尺度有限元法,通过构造位移多尺度基函数和电势多尺度基函数将细观非均质信息引入到宏观响应中,同时引入耦合附加项考虑各坐标方向位移场间的耦合作用。所构造的多尺度基函数可以直接高效地双向传递粗尺度与细尺度之间的信息,使问题在宏观尺度上求解,降低计算消耗。数值模拟结果表明,与传统有限元法相比,采用多尺度有限元法求解非均质压电结构的动力学问题,在保证计算精度的同时,还具有较高的计算效率,为非均质压电结构动力学问题的数值模拟提供了一种有效手段。

压电材料平面Ⅰ型裂纹模拟仿真分析

文中利用复变函数知识,基于压电材料第二类本构方程推导了压电材料平面Ⅰ型裂纹尖端的应力场和电位移场及机械应变能释放率的解析解,采用虚拟闭合裂纹法和Abaqus软件对压电材料平面型裂纹进行模拟仿真分析,根据仿真结果得到的机械应变释放数值解,讨论了裂纹参数与施加力电载荷对裂纹扩展的影响。将仿真模拟的数值解与解析解相对比,数值解结果与解析解符合程度较好,验证了虚拟裂纹闭合法与有限元软件结合方法的有效性。

基于MLS形函数处理边界的电阻率法有限单元正演模拟

电阻率法有限单元正演模拟中,采用第三类边界条件时为保证精度仍要求较大范围的计算域.无单元法为地球物理领域的新兴正演模拟方法,其计算效率低,但其中采用的移动最小二乘(MLS)形函数相比于有限单元法形函数具有良好的连续性,模拟精度高.本文将MLS形函数应用于电阻率法有限单元2.5维正演的第三类边界条件处理,提出电阻率法有限单元-移动最小二乘(FEM-MLS)耦合正演方法.通过不同正演方法的模型算例模拟结果对比,验证了本文算法的有效性,并讨论了各个参数选择对模拟结果的影响.本文数值模拟结果表明采用第三类边界条件时,在同等计算精度前提下,FEM-MLS耦合法相比于有限单元法可进一步缩小计算域并提高了计算效率,相比于采用较大计算域满足边界条件的有限单元法计算效率提高了约一倍,相比于采用相同小范围计算域的有限单元法平均精度提高了约一倍.

涡轮增压器超高速轴承-转子动力学建模及模态参数识别

针对传统涡轮增压器超高速轴承-转子系统动力学建模精度不高、模态参数不易识别等问题,基于Timoshenko梁理论建立了涡轮增压器动力学模型及运动微分方程,利用锤击法试验对增压器不同激励位置、有无预紧力等工况下的模态参数进行了识别,并将理论与试验分析结果进行了对比验证。研究结果表明:激励涡轮增压器不同位置时,系统的激发模态不尽相同;预紧力对转子动力学分析结果具有重要影响,增加预紧力可显著提高转子刚度;建立的动力学模型和模态识别方法具有一定的准确性和适用性,可以为后续涡轮增压器结构优化设计、模态参数识别提供理论与试验基础。

某底部设置V型分叉柱超限高层建筑结构设计

介绍了上海某156m高超限高层塔楼结构设计,该塔楼南侧底部有两组V型分叉柱,节点交汇形式复杂,因其存在楼板开洞、大堂3层挑空、1~4层有V型分叉柱、五边形平面等特点,结构存在平面不规则、竖向不规则、局部不规则等4项不规则内容,属于超限高层结构。塔楼结构体系采用钢管混凝土柱+钢梁+混凝土核心筒混合结构,即达到装配预制率要求,也满足结构刚度和强度的需求。根据塔楼的不规则情况选取合适的抗震性能目标,并对V型分叉柱结构、底部剪力墙等关键构件进行中、大震的抗震性能验算。对V型分叉柱结构采用ABAQUS实体模型进行有限元分析验证确保安全性。整体结构采用SAUSAGE软件进行大震动力弹塑性时程分析,发现在核心筒中高区连梁跳层设置钢骨可明显减小大震层间位移角,提高结构抗倒塌性能。最后为确保关键构件的延性性能,采取有针对性加强措施。

铁路钢轨扣件e形弹条动态性能研究及结构改进

以铁路钢轨扣件e形弹条为研究对象,运用有限元仿真和锤击试验方法对组装状态下弹条动态性能进行研究,随后对弹条进行结构及材料优化形成空心阻尼弹条。通过对原弹条进行强度应力和动态特性分析,弹条组装工作状态下模态试验与模态仿真前3阶固有频率误差分别为2.2%、1.8%、4.9%,仿真模型的有效性得到验证;改进后的空心阻尼弹条对影响弹条共振疲劳损伤的第三阶固有频率从原来DI弹条的747 Hz提高到940 Hz,固有频率提高25.8%,且避开地铁钢轨波磨段750 Hz左右的激励频率;同时对应的阻尼弹条其阻尼损耗因子相对原弹条提高92%,大大地降低弹条共振峰值响应,能够有效地提高弹条疲劳寿命。

An Improved Model Reduction Method on AIMs for N-S Equations Using Multilevel Finite Element Method and Hierarchical Basis

A numerical method is proposed to approach the Approximate Inertial Man-ifolds(AIMs)in unsteady incompressible Navier-Stokes equations,using multilevel fi-nite element method with hierarchical basis functions.Following AIMS,the unknown variables,velocity and pressure in the governing equations,are divided into two com-ponents,namely low modes and high modes.Then,the couplings between low modes and high modes,which are not accounted by standard Galerkin method,are consid-ered by AIMs,to improve the accuracy of the numerical results.Further,the multilevel finite element method with hierarchical basis functions is introduced to approach low modes and high modes in an efficient way.As an example,the flow around airfoil NACA0012 at different angles of attack has been simulated by the method presented,and the comparisons show that there is a good agreement between the present method and experimental results.In particular,the proposed method takes less computing time than the traditional method.As a conclusion,the present method is efficient in numer-ical analysis of fluid dynamics,especially in computing time.

断裂问题的扩展有限元法研究

扩展有限元(extended finite element method,XFEM)是近年来发展起来的、在常规有限元框架内求解不连续问题的有效数值计算方法,其基于单位分解的思想,在常规有限元位移模式中加入能够反映裂纹面不连续性的跳跃函数及裂尖渐进位移场函数,避免了采用常规有限元计算断裂问题时需要对裂纹尖端重新加密网格造成的不便。在推导扩展有限元算法的基础上,分析了应力强度因子的J积分计算方法及积分区域的选取。采用XFEM对I型裂纹进行了计算,有限元网格独立于裂纹面,无需在裂纹尖端加密网格;分析了积分区域、网格密度对应力强度因子计算精度的影响,指出了计算应力强度因子的合适参数,验证了此方法的可靠性和准确性。

一种改进的利用频响函数进行有限元模型修正的方法

在对机械结构的动态特性进行准确而可靠的预测时 ,有限元模型的设计参数的修正是很重要的。利用试验测试和预测的有限元模型计算得到的频响函数 (FRF) ,在结构动力缩聚技术的基础上 ,推导出了一种改进的基于频响函数的灵敏度分析的修正方程。数值实例研究结果表明该方法利用不完备的测量数据 ,也可在很宽的频率范围内 ,同时对多个参数进行修正 ,有限元模型修正解与真实结构参数完全吻合。