顶张力对深水刚性立管涡激振动及疲劳损伤的影响

分析讨论顶张力对深水刚性立管在剪切流中的涡激振动响应和疲劳损伤的影响。从理论上分析张力对刚性立管固有特性的影响,并利用OrcaFlex软件建立数值模型,模拟不同顶张力系数下立管于剪切流中的涡激振动响应,同时采用S-N曲线法计算立管疲劳损伤。结果表明,顶张力对立管的影响直接体现在刚度上,立管刚度随着张力的增大而增大,因此顶张力对降低立管疲劳损伤有一定积极作用。但是随着顶张力的增大,其对降低疲劳损伤的贡献越来越小,而对立管强度储备的危害却逐渐增大。     

Fatigue damage from time domain simulation of combined in-line and cross-flow vortex-induced vibrations

A semi-empirical method for time domain simulation of vortex-induced vibrations (VIV) is used to calculate the in-line and cross-flow fatigue damage of a tensioned riser in uniform and sheared flow. Simulations are run for flow velocities ranging from 0.3 m/s to 2.4 m/s, and a detailed comparison with experimental observations is performed. Results are reported in terms of dominating frequency, mode of vibration and mean of r.m.s. of displacement, as well as fatigue damage distribution along the length of the structure and maximum fatigue damage rates for each case. Fatigue damage is calculated by rainflow counting of the strain time series together with an idealized S–N curve with slope m = 3. The results show that the model reproduces the measured fatigue damage with a satisfactory level of realism, using a consistent set of parameters. This indicates that the model is usable for calculation of riser VIV fatigue damage in various current conditions, assuming the Reynolds number is in the subcritical range.     

平台运动下悬链线立管涡激振动响应特性分析

悬链线立管会在半潜平台运动带动下出现涡激振动现象,对立管疲劳寿命造成影响.借助海工结构物动力分析软件Orcaflex建立立管模型,使用Iwan-Blevins尾流振子模型进行数值模拟.通过与试验结果对比验证了该方法的可行性.同时建立实尺度悬链线立管模型,并考虑背景海流的影响,分析平台垂荡、纵荡运动下的立管涡激振动响应和疲劳损伤.研究表明,平台运动产生的非定常流场将引起悬链线立管发生涡激振动现象,并在背景洋流激励的基础上增大疲劳损伤,在立管结构实际工程设计时应予以关注.     

细长海洋结构物涡激振动研究综述

随着世界范围内深海石油开采的需要,近年来关于海洋结构物涡激振动的研究越来越受到重视.虽然此问题在数值模拟和实验方面都已取得了一定的进展,但是还有许多问题尚待解决.同时,新型海洋结构物的引入给涡激振动的预报方法和抑振手段提出了新的挑战.因此,细长海洋结构物的涡激振动仍将是未来几年里备受关注的研究课题.本文在介绍有关涡激振动基本概念和理论背景的基础上,总结了近年来关于以深水立管为代表的海洋结构物涡激振动的研究与进展,包括对现有涡激振动分析工具的分类与评估;对柱体及海洋结构物涡激振动的实验研究;对深水立管与涡激振动相关的疲劳评估准则的研究;海洋结构物的横向、流向及轴向涡激振动的耦合作用研究;关于海洋结构物涡激振动的抑振措施和设备的理论及实验研究.本文着重介绍了计算流体力学方法在海洋结构物涡激振动研究中的应用和进展.最后,对海洋结构物涡激振动相关的研究热点的现状进行了总结并对今后工作提出了展望.     

参激振动对顺应式垂直通路立管涡激振动的影响

顺应式垂直通路立管(CVAR)是目前处于研究阶段的一种新型的立管类型,在海流作用下产生涡激振动,在平台垂荡运动作用下产生参数激励振动。为了研究参数激励的影响,本文引入尾流振子模型模拟漩涡脱落对立管的作用,同时考虑浮式平台升沉运动产生的参数激励,建立了CVAR参激-涡激联合振动方程,获取联合作用下的动力响应,并与纯涡激振动响应进行对比。结果表明,在相同的流速下CVAR中部涡激振动幅值最大,流速的增大会导致涡激振动的频率增大,发生高阶锁振,高阶锁振振动幅值比低阶锁振振动幅值小。考虑参数激励之后,较纯涡激振动而言,立管的振动幅值增大;当参激频率与涡激振动频率接近时,立管的振动幅值最大。     

Fatigue damage prediction of ship rudders under vortex-induced vibration using orthonormal modal FSI analysis

Recently, the fatigue failure of ship rudders owing to vortex-induced vibration has increased as commercial ships become faster and larger. However, previous methods are inappropriate for fatigue failure prevention owing to the lack of fluid–structure interaction considerations. This study aims to develop a fatigue damage prediction method that can be applied at the design stage to prevent fatigue failure of ship rudders under vortex-induced vibration. The developed prediction method employed the fluid–structure interaction (FSI) method to properly consider the fluid–structure interaction and implemented orthonormal mode shapes to reflect the complex geometry and boundary conditions of the ship rudders. For validation, vortex-induced vibration of the hydrofoil model was obtained using the developed method, and the prediction results matched well with the experimental results. Then, the fatigue damage of the ship rudder model under vortex-induced vibration was predicted using the developed method, and their characteristics are discussed. The stress distribution obtained using the developed method matched well with the geometrical characteristics of the ship rudders. The potential for fatigue failure due to the resonance of vortex-induced vibration was expected by comparing the stress distributions for various flow velocities to the S–N curves provided by the DNV classification.     

附属分离盘抑制圆柱涡激振动的数值模拟

基于动网格技术，编写UDF程序计算附属不同长度分离盘的圆柱双自由度涡激振动，并借助FLUENT软件模拟计算其周围流场。通过模拟计算不同长度附属分离盘的圆柱涡激振动，系统地对比分析其所受升阻力系数、振动响应、尾流涡形态、运动轨迹和频率特征等方面内容，并总结其一般规律。分析发现，添加合适长度的分离盘可以大大降低涡泄频率，有助于避开“锁定”区域，降低涡激振动的响应，同时还应该注意附属分离盘带来的多频和宽频振动特征。该数值模拟方法也为附属抑制装置的立管涡激振动数值模拟奠定了基础。降低，当约化速度Ur=5.5附近，分离盘长度越长，升阻力系数与振动响应越小。（2）添加附属分离盘后，裸圆柱所对应的双排尾流涡将变为单排尾流涡；随着分离盘长度增大，涡泄的位置往后推移，与此同时，分离盘的两侧逐渐出现一组次漩涡（分离盘上产生的漩涡）；分离盘长度L=0.5D时，因其未能完全阻隔上下两侧漩涡的相互作用，并将一侧漩涡切分为二，与另一侧漩涡在尾流形成2P形态的涡。（3）附属分离盘长度的增加使得圆柱振动范围不断缩小，但会造成多频的振动特征，而且还有效地改变来流向响应与横向响应的相位角。（4）添加附属分离盘后，一方面圆柱阻力的主频率明显降低，而且主频率所对应的功率谱密度也明显降低，说明分离盘能降低来流向的振动频率与振动强度，但会造成附属分离盘的圆柱阻力表现为多频、宽频的振动特征；另一方面添加附属分离盘的圆柱升力频率明显降低，但同样会造成多频和宽频的振动特征。总的来说，添加合适长度的分离盘可以大大降低涡泄频率，有助于避开“锁定”区域，降低涡激振动的响应，同时还应该注意附属分离盘带来的多频和宽频振动特征。本文的数值模拟方法也为附属抑制装置的立管涡激振动数值模拟奠定基础。     

Reduction of VIV using suppression devices—An empirical approach

Helical strakes are known to reduce and even eliminate the oscillation amplitude of vortex-induced vibrations (VIV). This reduction will increase the fatigue life. The optimum length and position of the helical strakes for a given riser will vary with the current profile.

The purpose of the present paper is to describe how data from VIV experiments with suppressing devices like fairings and strakes can be implemented into a theoretical VIV model. The computer program is based on an empirical model for calculation of VIV. Suppression devices can be accounted for by using user-defined data for hydrodynamic coefficients, i.e. lift and damping coefficients, for the selected segments.

The effect of strakes on fatigue damage due to cross flow VIV is illustrated for a vertical riser exposed to sheared and uniform current. Comparison of measured and calculated fatigue life is performed for a model riser equipped with helical strakes. A systematic study of length of a section with strakes for a set of current profiles is done and the results are also presented.     

Numerical analysis of vortex-induced vibration on a flexible cantilever riser for deep-sea mining system

The flexible cantilever riser, as a special form of the marine riser, can be encountered in a deep-sea mining system, where the bottom of the long vertical lifting pipeline is connected with the intermediate warehouse. The main objective of this paper is to investigate the effects of the bottom weight caused by the intermediate warehouse and the flow speed on the dynamic responses of the cantilever pipeline. A quasi-3D coupling algorithm based on the discrete vortex method and finite element method is employed to calculate the unsteady hydrodynamic forces and vortex-induced vibrations of this pipeline in the time domain, respectively. We first simulate the VIV of a long flexible riser with two fixed ends in a stepped flow to validate the feasibility of the present method. Then, systematic simulations of cross-flow VIV of the cantilever riser are carried out under a wide range of bottom weights and different current speeds. The number of the vibration mode shows the decreasing tendency with the increase of the bottom weight. In a certain range of the weight, the number of the dominant mode remains unchanged, while the vibration amplitude declines with increasing weight. An amplitude jump phenomenon can be observed when the transition of the dominant mode in two contiguous mode clusters occurs. Moreover, the higher-order modes are excited with the increase of the current speed.     

计及海底井口影响的隔水管涡激振动响应特性试验

在靠近海底井口位置,隔水管周围流场受井口系统的影响发生变化,流固耦合下涡激振动可能诱发隔水管在横流向(CF方向)更为剧烈的振动。为了研究受管土装置影响的隔水管涡激振动响应特性,使用自主设计的管土装置模拟海底井口,采用8 m柔性立管,进行了均匀流下单管和受管土装置影响的涡激振动对比试验。试验通过FBG光纤应变片测得应变信息,使用模态叠加法、FFT变换处理分析试验数据,对比分析2组隔水管的主导频率、应变时历与幅值谱、无因次振幅以及激励力系数等参数沿管径的分布情况。结果发现管土装置影响下,隔水管主导频率减小;CF方向涡激振动增大,振幅沿径向呈非对称特征;涡激振动振幅更大,隔水管受流场的激励与阻尼更为剧烈。     

低质量-阻尼因子圆柱体的涡激振动预报模型

本文考查了在均匀来流中作横向振荡的圆柱体与周围流体之间的能量转移,由此建立了基于受迫振荡实验数据的弹性支撑圆柱体在均匀流中的涡激振动响应预报模型.根据此模型,分析了低质量-阻尼因子圆柱体的涡激振动响应特性.就水中圆柱体涡激振动响应特性相关的几个关键性问题进行了深入的讨论,包括响应振幅的决定因素、附加质量对锁定范围及响应频率的影响.正确理解这些问题对于深水立管涡激振动响应的有效预报至关重要.     

柔性立管涡激振动响应轨迹特性研究

为了深入研究细长柔性立管的涡激振动响应特性,进行了柔性立管的拖曳水池试验。由拖车拖动立管产生相对来流,根据应变测试得到的应变数据,基于模态叠加法得到位移响应。试验分析前,通过数值方法先针对刚性立管的涡激振动响应轨迹特性进行了分析。紧接着,通过试验方法对柔性立管的单模态以及多模态涡激振动响应轨迹特性进行了深入的分析和讨论。通过分析发现:柔性立管在低速下具有与刚性立管类似的轨迹响应特性,均呈现经典的8字形状;柔性立管在高速下,其轨迹开始变得混乱,这主要是由位移的多模态响应特性所产生。     

表面粗糙度对立管涡激振动响应影响的试验研究

应用模型试验的方法,研究了表面粗糙度对立管涡激振动响应特性的影响规律,对不同粗糙度条件下立管所受拖曳力、升力、端部张力、漩涡泄放频率、结构振动响应频率、位移响应等参数的变化规律进行了对比分析。结果表明：与立管横向振动相比,立管流向振动更早出现锁定现象,因此当折合速度较低时,立管流向振动的涡激振动响应要大于横向振动。立管张力均存在两个峰值频率,其中一个峰值频率为主导频率,与拖曳力主导频率吻合,由流向涡激振动所产生;另一个峰值频率为主导频率的一半,与升力主导频率吻合,由横向涡激振动所产生。因此可以看出：横向涡激振动与流向涡激振动通过张力作用而相互影响。与光滑立管相比,表面粗糙度降低了立管的涡激振动位移响应,减小了涡激振动的锁定区域,但提高了漩涡泄放频率。对于不同粗糙度下的粗糙立管,随着粗糙度的增加,立管的锁定区域开始点逐渐提前,锁定结束点逐渐推迟,锁定区域逐渐变宽。     

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.     

Time domain approach for coupled cross-flow and in-line VIV induced fatigue damage of steel catenary riser at touchdown zone

Existing VIV prediction approaches for steel catenary riser (SCR) typically employ truncation model without considering the interaction between the SCR and soil, and only allow for cross-flow (CF) VIV. In this study, a time domain approach accounting for the SCR-soil interaction is proposed to predict the CF and in-line (IL) VIV induced fatigue damage of a SCR at touchdown zone (TDZ). The hydrodynamic force resulting from the vortex shedding is modeled using the forced oscillation test data of a rigid cylinder and an empirical damping model, which are defined as functions of the non-dimensional dominant frequency and amplitude of the SCR response. Due to the coupling effect, the IL VIV force is magnified based on the CF VIV amplitude. By combining a linear hysteretic interaction model with a trench shape model, some particular phenomena during the vertical SCR-soil interaction are captured and qualitatively discussed, while for the horizontal direction, the seabed is simplified as nonlinear spring model. Based on these models, parametric studies are conducted to broaden the understanding of the sensitivity of VIV induced fatigue damage to the seabed characteristic. The results indicate trench depth, vertical and lateral stiffness, and clay suction are significantly affect the VIV induced maximum fatigue damage at TDZ.     

平台运动影响下张力腿涡激振动特性研究

张力腿涡激振动易引起系泊系统的疲劳损伤,危害张力腿平台的安全性和可靠性。该文借鉴细长立管涡激振动的相关研究,同时考虑张力腿与立管在截面尺度、预张力大小以及边界条件存在的巨大差距,采用切片理论结合CFD数值模拟方法,对平台运动影响下张力腿涡激振动开展了研究。考虑定常张力与时变张力对涡激振动的影响,采用GAMBIT软件分区建立了多切片计算网格,将编制求解涡激振动的UDF程序嵌入Fluent软件中,采用动网格技术实现流场的更新并计算作用于张力腿上的瞬时升力和拖曳力。文中还比较了仅考虑流和考虑浪、流联合作用张力腿的时变张力影响,研究了两种典型波高共计六种工况,计算结果表明：随着流速的增加,各模态权重比例将发生跳转现象;尽管由于平台运动导致张力腿受力的随机性,但平台运动对张力腿涡激振动影响较小。     

深海立管涡激振动疲劳损伤影响因素分析

根据我国南海、墨西哥湾和西非海域的流速沿深度方向的分布规律,利用涡激振动预报程序计算了立管在不同参数下的动力响应.在此基础上计算了立管沿长度方向涡激振动引起的疲劳损伤.分别讨论了立管顶部预张力、管内流体密度、管外流速分布、立管外径、立管壁厚等参数的变化对疲劳损伤的影响.结果显示立管顶部预张力、管外流速分布、立管外径对疲劳损伤的影响非常明显,而管内流体密度和立管壁厚因其变化范围有限,对疲劳损伤的影响没有上述三个参数显著.     

海底管线悬空振动的研究现状

海底管线管跨段的涡激振动,是引发海底管线疲劳失效的主要因素之一。介绍了管跨段的形成原因,总结国内外关于管跨段临界长度的计算方法,探讨了近年来对涡激振动问题理论及实验研究的研究现状,提出了在今后的管跨段研究中有待解决的问题。     

顶张力立管涡激振动抑振装置性能比较

为降低甚至消除漩涡脱落引起的顶张力立管振动响应从而提高其疲劳寿命,设计了五种型式的涡激振动抑振装置,并在中国海洋大学物理海洋实验室对装有抑振装置的模型立管进行了实验研究.得到了模型立管在几种不同流速下顺流向和横向涡激振动应变时程曲线,同时为了对比分析也测量了没有安装抑振装置的裸管的响应曲线.对比分析了三种外流流速下抑振装置对立管涡激振动响应的影响,结果表明各种抑振装置对立管顺流向和横向涡激振动幅值和频率都有不同程度的降低,但其影响和有效性各不相同.     

Fatigue damage induced by vortex-induced vibrations in oscillatory flow

Vortex-induced vibration (VIV) of a flexible cylinder in oscillatory flow was experimentally investigated in an ocean basin. An intermittent VIV was confirmed to have occurred during the tests. The fatigue damage caused by VIV was calculated based on rainflow counting and a standard S–N curve. There are 3 main observations for fatigue damage from VIV in oscillatory flow: 1) the damage varied significantly with the KC number, which is a unique feature for VIV in oscillatory flow. 2) Fatigue damage at small KC number cases was found to be larger compared to those at large KC numbers owing to the fact that number of vortex shedding cycles per half of the motion cycle is low, and damping within half of the motion cycle will hence become low. The fact that vortices from the previous cycle still are active during the next may also contribute to the large response at small KC numbers. 3) ‘Amplitude modulation’ and ‘mode transition’, two specific features for VIV in oscillatory flow, were found to have a strong influence on fatigue. Fatigue damage has also been calculated by an empirical VIV prediction model assuming that all cases have steady flow at an equivalent velocity. Finally, a simplified method for calculating fatigue damage from VIV in oscillatory flow based on steady flow conditions is proposed. A modification factor diagram is presented, but the scope of the present study is too limited to provide a good basis for a general model for this factor. A general model for how to apply results from constant current analysis to predict fatigue in oscillatory flow will therefore need further research.