粘弹性阻尼材料减振性能试验评估方法

为在船体结构声学设计阶段或阻尼材料研制阶段预先评估粘弹性阻尼材料的减振效果,基于锤击法,建立平板振动试验模型,引入频响函数幅值平均衰减量参数,对3型粘弹性阻尼材料复合试样分别在不同频段内的减振效果进行对比分析。结果表明：基于损耗因数和频响函数幅值平均衰减量的粘弹性阻尼材料减振效果试验评估方法能迅速在不同频段对阻尼材料的减振性能进行排序;利用频响函数幅值平均衰减量,可对耗能机理较为复杂的约束阻尼减振效果进行评估,为结构设计或研发阻尼材料提供依据。     

Application of analytical model in the prediction of dynamic responses and fatigue damage of flexible risers: Part I – Improvement of analytical model considering shear deformation and varying tension effects

Flexible risers have been widely utilized for the transfer of oil and gas products from a well to production units. The components of flexible risers, unlike steel risers, experience complex contact phenomena during bending. The contact between helical wires and adjacent layers especially causes a significant level of bending nonlinearity, making it hard to estimate the structural responses. Accordingly, a large-scale dynamic analysis of flexible risers usually involves an analytical model that predicts the bending moment and axial stress of helical wires based on theoretical approaches. The analytical model consists of an axis-symmetrical model and a bending model. Among them, the bending model plays a critical role in the prediction of the bending responses of flexible risers. The conventional bending models usually neglect the shear deformation of internal layers and continuity of sliding force, which leads to a significant error of analysis. Furthermore, the previous bending models assume that the contact pressure on helical wires is constant during bending. In real operating conditions, however, most flexible risers experience a considerable change of tension that governs the slip of helical wires. Hence, the current study presents a new dynamic analysis method for flexible risers. The suggested analytical model improves the bending model based on an accurate estimation of the internal strain field considering the shear deformation and continuous sliding force. Also, this study proposes a stiffness update method to reflect the effect of varying tension in the dynamic analysis. The presented method updates the bending property of flexible risers considering the continuous change of the contact pressure from varying tension. For the validation of suggested method, the current study carries out numerical simulations with a pure bending and varying tension for the internal diameter 7 inches flexible risers. It is identified that the suggested analytical model provides accurate analysis results. Moreover, it is found that the effect of varying tension gives a significant impact on the bending behavior of flexible risers by changing the slip condition of helical wires. Part I of this series of papers describes the detailed formulation method for the analytical model and with some verification examples. The suggested analytical model is expanded to the large-scale dynamic analysis in Part II for the investigation of the effect of shear deformation and varying tension.     

非粘结柔性立管静态分析

海洋柔性立管因材料和结构上的复杂性在设计分析中存在许多技术难题。本文在变形能原理和能量守恒的基础上,推导出了柔性立管各层的刚度矩阵。将各层刚度矩阵进行叠加,得到柔性立管总体刚度矩阵,并用总体刚度矩阵求解静载荷作用下立管变形响应。同时基于ABAQUS软件建立八层非粘结柔性立管有限元模型,并将有限元计算结果和刚度矩阵计算结果进行比较。结果分析表明：运用推导得到的刚度矩阵求解静态载荷下立管的变形是一种简便且准确的方法。     

Bend stiffener linear viscoelastic thermo-mechanical analysis. Part I — Experimental characterization and mathematical formulation

Bend stiffeners are subjected to cyclic loading during offshore operation or when subjected to a controlled full-scale qualification test. Due to the viscoelastic nature of the polyurethane, energy is dissipated within the material volume and the structure may experience a temperature increase, a phenomenon known as self-heating. The top connection is a flexible riser critical region in terms of fatigue, being the bend stiffener the main responsible for curvature control. As the curvature distribution is highly affected by the nonlinear time–temperature bend stiffener response, a detailed thermo-mechanical assessment may become relevant for riser lifetime and polyurethane material failure assessment, specially during accelerated full-scale tests. In the present paper (Part I), the polyurethane experimental characterization and steady-state thermo-mechanical mathematical formulation are presented for the bend stiffener self-heating assessment. A steady-state formulation is derived for a temperature dependent linear viscoelastic large deflection beam model to estimate the heat generation during harmonic tip loading. The temperature field distribution is calculated through a three-dimensional steady-state thermal model considering the viscoelastic heat calculated from the mechanical model with an iterative scheme. Stress relaxation tests are performed at different temperatures to determine the viscoelastic properties followed by thermal properties characterization through differential scanning calorimetry and by the Flash method to determine the specific heat, thermal conductivity and diffusivity, respectively. In a companion paper (Part II) the iterative numerical scheme is detailed and a case study presented.     

Theoretical and numerical analysis of bending behavior of unbonded flexible risers

This paper presents theoretical and numerical study on bending properties of unbonded flexible risers. To capture nonlinearities in layer's sliding, the stress component due to slip-stick behavior is considered and energy conservation principle considering sliding-caused heat consumption is employed in the analytical model. Besides, a finite element model estimating mechanics of unbonded flexible risers' bending is proposed. In the finite element model, couplings between bending moment–curvature and axial stress as well as contact interaction among layers and tendons have been considered. The theoretical and numerical results were validated against the corresponding experimental data in literature and mutually compared in analyzing nonlinear bending behavior of flexible risers. Moreover, the impacts of axisymmetric loads on riser's bending behavior have been further investigated.     

深水环境下粘弹性复合材料夹层结构蠕变特性研究

文章结合单轴压缩蠕变试验和粘弹性材料广义Maxwell建模理论得到吸声和浮体两种粘弹性填充材料的松弛模量Prony级数系数,开展了深水环境下粘弹性复合材料夹层结构蠕变特性试验,并将之与仿真研究结果进行对比,得到如下结论：复合材料夹层结构长时蠕变初期,芯材蠕变较表层复合材料占主导地位;随着时间增加,表层复合材料基体的粘弹性特性所表现出的蠕变和松弛现象交织出现,表层应变出现波动;芯材蠕变对复合材料夹层结构长时蠕变变形的贡献约为60%。     

基于波传播分析的水下粘弹性复合圆柱壳振动功率流研究

采用波传播分析方法,讨论了外部敷设粘弹性自由阻尼材料的无限长圆柱壳在流场中受径向简谐激励的振动功率流.用复模量形式计及粘弹性阻尼的损耗因子,研究了自由阻尼圆柱壳在外激励作用下的输入功率流和沿壳体传播的功率流.结果表明粘弹性阻尼层可以明显降低激振力输入壳体的功率流,加快振动波在结构内的衰减,为潜艇结构和水下各种管道的减振降噪提供了一定的理论参考.     

Numerical prediction of the modal response of flexible cylinders in cross-flow with a current dependent form of damping

A quadratic eigenvalue problem (QEP) was posed in order to study the dynamics of flexible cylinders in cross-flow, simulating slender offshore structures such as risers, catenaries or tendons. The Euler–Bernoulli equation was used to model the structure assuming a fluid loading model, and yielding a quadratic eigenvalue problem that included a form of damping dependent not only on the structural damping itself, but also on the free stream velocity and the fluid force coefficients. We solved the QEP using the finite element method. We also derived a simplified analytical solution in this work for comparison with the QEP, however this solution does not consider changes in tension along the length of the cylinder as the QEP does. In our study, the QEP solutions were first validated against the simplified analytical solution, and also against a well-known experimental dataset obtained in 2003, in which a flexible circular cylinder model was used to model the dynamics of a riser undergoing multi-mode vortex-induced vibrations.     

用粘弹性梁控制加筋板稳态简谐响应的研究

应用模态叠加法研究了以粘弹性梁为阻尼处理的加筋板的稳态简谐响应。在分析中，板和梁均采用每个结点三个自由度的有限元，考虑了梁的偏心作用。以结构幅频特性曲线的方式来研究粘弹性梁阻尼处理对结构响应的控制作用，分析了阻尼材料杨氏模量实部、损耗因子和粘弹性梁剖面尺寸对结构响应的影响。结果表明，粘弹性材料能够有效地控制结构的共振响应。粘弹性梁阻尼处理仅对某些结点和模态起的作用较大，将其与其他阻尼敷设形式合理配置，相信会起到良好的减振效果。     

海洋立管的模态及参数敏感性分析

为分析海洋立管受不同形式的轴向张力作用时的模态求解问题,用哈密尔顿原理建立理论模型,讨论模态的计算方法,并分析相关参数对结果的影响。结果表明,在相同的张力因子下,随着立管长度增大其固有频率主要受张力的控制;在深水中因表观重力引起的沿轴向张力的线性变化不可忽略,对频率和振型都有重要影响,张力因子的增大使频率提高,这种影响对结构长度变化敏感;频率随附加水质量系数的增大而降低。     

Application of analytical model in the prediction of dynamic responses and fatigue damage of flexible risers: Part II – Dynamic analysis of flexible risers in large-scale domain using a direct moment correction method

In recent years, the dynamic responses of flexible risers have been the focus of many researchers. Most flexible risers undergo a substantial level of irregular motion from environmental loadings, which involves a continuous slip of helical wires. The slip of helical wires especially leads to a hysteretic effect by reducing the bending stiffness, making it hard to predict the dynamic responses of flexible risers. The current study, as an extension to Part I, presents a new large-scale dynamic analysis method for flexible risers. The suggested method creates a large-scale model for the dynamic analysis that considers a geometric and bending nonlinearity of flexible risers. The kinematics of each beam element is formulated based on a Green-Lagrangian strain and the interaction with the seabed, providing a realistic analysis of flexible risers. In particular, the current study introduces a direct moment correction method that modifies the internal force vector using an improved analytical model. The improved analytical model is assigned at each node of the large-scale model and estimates an accurate bending hysteresis curve considering the effect of shear deformation and varying tension. The suggested method corrects the bending moment and shear force of all beam elements based on the bending hysteresis curves obtained from the improved analytical model, by which a complex bending behavior of flexible risers is reflected in a large-scale domain. As a result, this study achieves a more accurate prediction of the dynamic responses and fatigue damage of flexible risers. A new dynamic analysis program, called OPFLEX, is developed herein based on the suggested analysis method. Using the developed program, the current study conducts several numerical investigations to identify the effect of the shear deformation and varying tension. Consequently, it is confirmed that the shear deformation of internal layers reduces the fatigue damage of helical wires by delaying the increase of internal stress. It is also identified that the effect of varying tension deteriorates the fatigue life of flexible risers through a continuous change of contact pressure during bending.     

基于粘弹性阻尼层随机性的自由阻尼层板的振动和阻尼分析

本文基于粘弹性阻尼层的随机性用Monte Carlo直接抽样法对自由阻尼层板的振动和阻尼进行了分析研究。分析中随机变量取正态分布。针对不同的阻尼层厚度分别考察了复弹性模量的实部、虚部、材料的损耗因子的影响。考察了整体性和局部单元性阻尼导厚度的影响。结果表明，粘弹性材料弹性模量的随机性对结构固有频率的影响不大，对模态损耗因子影响较大；不论是整体的还是局部单元的，阻尼层厚度的随机性对模态损耗因子的影响很显著。因此粘弹性阻尼层的随机性对抑制结构共振响应和声辐射的影响是较大的。阻尼层厚度局部单元随机性影响表明了对阻尼结构采用随机分析的必要性。     

LQR control on multimode vortex-induced vibration of flexible riser undergoing shear flow

Under the actions of ocean currents and/or waves, deep-sea flexible risers are often subject to vortex-induced vibration (VIV). The VIV can lead to severe fatigue and structural safety issues caused by oscillatory periodic stress and large-amplitude displacement. As flexible risers have natural modes with lower frequency and higher density, a multimode VIV is likely to occur in risers under the action of ocean currents, which is considered as shear flow. To decrease the response level of the VIV of the riser actively, a multimode control approach that uses a bending moment at the top end of the riser via an LQR optimal controller is developed in this study. The dynamic equations of a flexible riser including the control bending moment in shear flow are established both in the time and state-space domains. The LQR controllers are then designed to optimize the objective function, which indicates the minimum cost of the riser's VIV response and control input energy based on the Riccati equation of the closed-loop system under the assumption that the lift coefficient distribution is constant. Finally, the VIV responses of both the original and closed-loop systems under different flow velocities are examined through numerical simulations. The results demonstrate that the designed active control approaches can effectively reduce the riser displacement/angle by approximately 71%–89% compared with that of the original system. Further, for multimode control, the presented mode-weighted control is more effective than the mode-averaged control; the decrease in displacement is approximately 1.13 times than that of the mode-averaged control. Owing to the increase in flow velocity as more and higher-order modes are excited, the VIV response of the original system decreases slightly while the frequency response gradually increases. For the closed-loop system, the response becomes smaller and more complicated, and the efficiency of the controller becomes lower at a certain flow velocity.     

粘弹性材料声辐射计算的二次建模理论研究

本文采用二次建模方法,对具有微观结构的粘弹性材料的声辐射问题进行了研究.第一次建模时针对包含单个内部结构的周期细胞进行精确的建模,根据有限元计算结果获得粘弹性阻尼层等效成正交各向异性材料的等效物性参数;第二次建模时,该粘弹性阻尼层可简单地用具有该等效物性参数的均匀材料替代.二次建模计算所划分网格的尺度较大,可选择的单元类型也更多.在作该材料的低频受激振动声辐射预报时,上述特点能够在保证计算精度的前提下,大幅度地降低计算成本,缩短计算时间.     

钢悬链线立管整体干涉分析研究

深水海洋立管具有柔性,特别是极端海况条件下,相邻立管之间可能发生互相干涉而增大立管的应力,从而影响其疲劳寿命.钢悬链线立管相比顶张力立管,有效张力较小,对环境载荷作用更加敏感,更容易发生碰撞.基于动力学分析软件OrcaFlex建立有限元模型,从允许碰撞的角度出发,对串列布置于张力腿平台上的两根钢悬链线立管进行整体碰撞分析,研究立管间距、尾流模型、拖曳力系数、海流流速和柔性接头刚度对立管碰撞的影响,阐述对碰撞范围、上下游立管相对运动速度、最大碰撞速度和最大碰撞能量的影响规律,为实际工程中立管的空间布置和结构优化设计提供参考.     

环肋圆锥壳基座振动传递的粘弹性阻尼控制技术研究

敷设粘弹性阻尼材料是船舶结构进行减振降噪的一种有效方式。该文利用有限元迭代法对具有频变特性的粘弹性材料的振动响应进行计算,并验证了计算方法的正确性。同时对粘弹性阻尼材料在带有环肋圆锥壳基座结构的敷设方法进行了研究。研究结果表明对基座的腹板敷设约束阻尼层结构可有效地降低整个结构的振动响应。     

含损伤加筋板结构声辐射阻尼变异研究

从能量耗散角度提出了结构声辐射阻尼的有关概念,考虑到辐射阻尼的特性,文章采用结构振动时耗散能量与振动总能量的比来建立辐射阻尼的数学模型,并用数值计算的方法研究简支板结构的声辐射阻尼特性.在研究结构损伤对声辐射阻尼影响的时候,以加筋板结构为例,计算结构不同损伤情况下的声辐射阻尼.基于Mindlin理论,建立描述健康和损伤的四结点有限元板壳单元模型,采用有限元方法计算结构表面动力响应.各向同性损伤单元,采取刚度各向整体弱化的方法分析;对于各向异性损伤单元,采用Kachanov理论,引入了x和y两个方向的弹性损伤折减系数.考虑到不同损伤存在形式,计算分析了损伤对声辐射阻尼的影响.文章建立了一种含损结构的分析方法,通过对一些典型算例分析,在评价损伤对船舶与海洋结构物常用的加筋结构声辐射特性影响方面做出了一定的探索.     

舰船舱室减震降噪用泡沫金属铝的室温内耗性能研究

泡沫金属铝是一种新型船用减震降噪材料,在船舶舱室降噪中具有很好的工程实用价值.本文采用声频内耗仪测定了用熔体发泡法制备的工业用大规格泡沫金属铝在室温下的声频内耗,研究了泡沫金属铝在不同振幅和不同频率下的内耗特征,讨论了其内耗产生的机理.结果表明:泡沫金属铝具有较好的阻尼性能.在室温下的内耗随振幅的增加而增大,随频率的增加而减小.泡沫金属铝的内耗是因为其内部存在大量的孔洞,在交变载荷的作用下发生振动时,造成其内部应力应变分布不均匀,产生膨胀能和畸变能,使能量损耗.     

Fatigue analysis of water intake risers: Hydrodynamic damping effect and a hybrid frequency-time domain method

The fatigue performance is key to the design of water intake risers (WIRs), which is a novel concept used to convey cooling water for liquefaction of natural gas at sea. To estimate the fatigue life, it is crucial to accurately predict the response amplitude of the WIRs, which is dominated by hydrodynamic damping. In operational conditions, the motion amplitudes of WIRs are usually smaller than their diameter, and thus leading to a flow regime of KC < 5. It is found in this flow regime; the hydrodynamic damping largely depends on the motion magnitude of the risers. To consider this coupling effect, a hybrid frequency-time domain fatigue analysis method is proposed, where a nonlinear stress transfer function is adopted. The hybrid method accounts for the coupling effect between the hydrodynamic damping and the structural motion. Significantly reducing the computational cost, this method provides results as accurate as that from a time domain analysis based on the relative velocity model with a constant drag coefficient. Furthermore, recommendations for further simplification of the fatigue analysis recognizing the coupling effect are given.