Sparse Modal Decomposition Method Addressing Underdetermined Vortex-Induced Vibration Reconstruction Problem for Marine Risers

When investigating the vortex-induced vibration(VIV)of marine risers,extrapolating the dynamic response on the entire length based on limited sensor measurements is a crucial step in both laboratory experiments and fatigue monitoring of real risers.The problem is conventionally solved using the modal decomposition method,based on the principle that the response can be approximated by a weighted sum of limited vibration modes.However,the method is not valid when the problem is underdetermined,i.e.,the number of unknown mode weights is more than the number of known measurements.This study proposed a sparse modal decomposition method based on the compressed sensing theory and the Compressive Sampling Matching Pursuit(Co Sa MP)algorithm,exploiting the sparsity of VIV in the modal space.In the validation study based on high-order VIV experiment data,the proposed method successfully reconstructed the response using only seven acceleration measurements when the conventional methods failed.A primary advantage of the proposed method is that it offers a completely data-driven approach for the underdetermined VIV reconstruction problem,which is more favorable than existing model-dependent solutions for many practical applications such as riser structural health monitoring.

Experimental Study on Vortex-Induced Vibration of Rough Risers with Coupling Interference Effect Under Side-by-Side Arrangement

A vortex-induced vibration(VIV)experiment of rough risers with coupling interference effect under a side-by-side arrangement was carried out in a wave-current combined flume.The roughness of the riser was characterized by arranging different specifications of surface attachments on the surface of the riser.Rough risers with three different roughnesses were arranged side by side with smooth risers to explore the VIV response of the riser under the combined action of roughness and interference effect,and to reveal the coupling mechanism between roughness and interference effect.The experimental results show that,compared with that of a smooth riser,the VIV of a rough riser under the coupling interference effect has a wider"lock-in"region,and the displacement decreases more significantly at a high reduced velocity,which is more likely to excite higher-order modes and frequency responses.In addition,the displacement response and frequency response of the smooth riser are not significantly affected by wake interference from the rough riser,which is caused by the decrease of the wake region due to the delay of the boundary layer separation point of the rough riser.

Experimental Study on Vortex-Induced Vibration of Rough Risers Coupling with Interference Effect in Tandem Arrangement

In order to study the response law of vortex-induced vibration(VIV)of marine risers under the combined action of roughness and interference effects,and to reveal the coupling mechanism of roughness and interference effects on the riser,a VIV experiment of rough risers in tandem arrangement was conducted in a wave−current combined flume.The experiment characterized the risers’roughness by arranging different specifications of attachments on the surface of the risers.Three rough risers with different roughness and smooth risers were arranged in tandem arrangement,with the rough risers arranged downstream.The experimental results indicate that the suppression of the attachments on the downstream risers’vibration are more significant both in the CF and IL directions as the reduced velocity increases.For the downstream riser,the amplitude response of rough riser is more significantly weakened compared with the smooth one at high reduced velocity.For the upstream risers,changes in the roughness and spacing ratio have an impact on their‘lock-in’region.When the roughness of downstream risers is relatively large(0.1300)and the spacing between risers is small(S/D=4.0),the reduced velocity range of‘lock-in’region in the CF direction of upstream risers is obviously expanded,and the displacement in the‘lock-in’region is severer.

Experimental Study on Vortex-Induced Vibration of Marine Riser Model with Coupling Interference Effect Under Combined Internal and External Flow

A partition model of interference efficiency was constructed to study the coupling interference effect under combined internal and external flow.The concept of“internal flow efficiency”,“velocity ratio”and“interference efficiency”were introduced to quantify the effect of internal flow and interference,and reveal the coupling mechanism among internal flow,external flow and interference effect.The results showed that the dynamic response of risers under variable angles was significantly different after considering the effect of internal flow.When the external flow velocity was smaller than 0.25 m/s,the vibration of risers was promoted by the internal flow.With the increase of external flow velocity,the effect of internal flow was weakened and the dynamic response of riser mainly depended on the external flow and interference effect.Under the effect of different internal flow,the interference efficiency had similar change trend.The interference effect amplified the complex secondary flow effect inside the riser,making the dynamic response of riser complex and random.In this paper,the overlap area and subdivision criterion of interference effect were constructed within the range of experimental velocity ratio,and the change curve of interference efficiency was obtained with an average meaning,which may have important practical meaning.

The Effect of Internal Fluid on the Response of Vortex-Induced Vibration of Marine Risers

Based on Iwan′s wake oscillator model developed with the classical van der Pol equation, the differential equation for the response of the vortex-induced vibration (VIV) of the riser considering the effect of the internal flowing fluid and the external marine environmental condition is derived. The effect of the internal flowing fluid on the response of VIV of the riser is studied by means of the Finite Element Method. The results show that the effect of the internal fluid velocity on the VIV of the riser is strong when the natural frequency of the riser is close to the vortex shedding frequency. In addition, the increase of the top tension can decrease the sensitivity of the riser to the internal fluid velocity.

Simplified Prediction Model for Vortex-Induced Vibrations of Top Tensioned Risers

According to the characteristics of deepwater top tensioned risers, a simplified model is presented to predict the multi-modal response of vortex-induced vibration （VIV） in non-uniform flow based on energy equilibrium theory and the exporimental data from VIV self-excited and forced oscillations of rigid cylinders. The response amplitude of each mode is determined by a balance between the energy fed into the riser over the lock-in regions and the energy dissipated by the fluid damping over the remainders. Compared with the previous prediction models, this method can take fully account of the intrinsic nature of VIV for low mass ratio structures on lock-in regions, added mass and nonlinear fluid damping effect, etc. Moreover, it is the first time to propose the accurate calculating procedure for VIV amplitude correction factor by solving energy equilibrium equation and a closed form solution is presented for the case of a riser of uniform mass and cross-section oscillating in a uniform flow. The predicted values show a reasonable agreement with VIV experiments of riser models in stepped and sheared currents.

Nonlinear Analysis of Bidirectional Vortex-Induced Vibration of A Deepwater Steep Wave Riser Subjected to Oblique Currents

An improved three-dimensional(3D)time-domain couple model is established in this paper to simulate the bidirectional vortex-induced vibration(VIV)of a deepwater steep wave riser(SWR)subjected to oblique currents.In this model,the nonlinear motion equations of the riser are established in the global coordinate system based on the slender rod theory with the finite element method.Van der Pol equations are used to describe the lift forces induced by the x-and y-direction current components,respectively.The coupled equations at each time step are solved by a Newmark-βiterative scheme for the SWR VIV.The present model is verified by comparison with the published experimental results for a top-tension riser.Then,a series of simulations are executed to determine the influences of the oblique angle/velocity of the current,different top-end positions and the length of the buoyancy segment on the VIV displacement,oscillating frequency as well as hydrodynamic coefficients of the SWR.The results demonstrate that there exists a coupled resonant VIV corresponding to x-direction and y-direction,respectively.However,the effective frequency is almost identical between the vibrations at the hang-off segment along x and y directions.The addition of the buoyancy modules in the middle of the SWR has a beneficial impact on the lift force of three segments and simultaneously limits the VIV response,especially at the decline segment and the hang-off segments.Additionally,the incident current direction significantly affects the motion trajectory of the SWR which mainly includes the fusiform and rectangle shapes.

Laboratory Measurements of Vortex- and Wake-Induced Vibrations of A Tandem Arrangement of Two Flexible Risers

The dynamic response of two flexible model risers in tandem arrangement immersed in a stepped current was analyzed. The risers, with an external diameter of 20 mm and a total length of 6200 mm, had an aspect ratio of 310. They were hinged to the support structure at the center-to-center distances away 3-12 times the external diameter. The top 1200 mm was exposed to a uniform current at a speed which was up to 0.9 m/s（the Reynolds number was 18000） and the rest in still water. The dynamic responses, which were obtained through the Fiber Bragg Grating strain gauges mounted on the surface, were analyzed by studying the cross-flow amplitudes and modal weights. The cross-flow vibration were observed up to the third mode, and the modal transformation from the second mode to the third mode was clearly observed. The experiment confirmed that the typical vortex-induced vibration（VIV） had occurred on the up-stream riser. But for the down-stream riser, the main excitation mechanism was wake-induced vibration（WIV）. The modal transformation of WIV was more complex than that of VIV, which might be helpful for other researchers to study the interference effect.

NUMERICAL PREDICTION OF FATIGUE DAMAGE IN STEEL CATENARY RISER DUE TO VORTEX-INDUCED VIBRATION

For studying the characteristics of Steel Catenary Riser （SCR）, a simplified pinned-pinned cable model of vibration is established. The natural frequencies, the normalized mode shapes and mode curvatures of the SCR are calculated. The fatigue damage of the SCR can be obtained by applying the modal superposition method combined with the parameters of S -N curve. For analyzing the relation between the current velocity and the SCR＇s fatigue damage induced by the vortex-induced vibration, ten different current states are evaluated. Then, some useful conclusions are drawn, especially an important phenomenon is revealed that the maximum fatigue damage in the riser usually occurs near the area of the boundary ends.

Identification of hydrodynamic coefficients from experiment of vortex-induced vibration of slender riser model

One of the challenges in predicting the dynamic response of deepwater risers under vortex-induced vibration （VIV） is that it runs short of believable fluid loading model. Moreover, the hydrodynamic loading is also difficult to be measured directly in the VIV experiments without disturbing the fluid field. In the present work, by means of a finite element analysis method based on the experimental data of the response displacements, the total instantaneous distributions of hydrodynamic forces together with the hydrodynamic coefficients on the riser model with large aspect ratio （length/ddiameter） of 1750 are achieved. The steady current speeds considered in the experiments of this work are ranging from 0.15 rn/s to 0.60 m/s, giving the Reynolds Number between 2400 and 9600. The hydrodynamic coefficients are evaluated at the fundamental frequency and in the higher order frequency components for both in-line and cross-flow directions. It is found that the Root-Mean Squared hydrodynamic forces of the higher order response frequency are larger than those of the fundamental response frequency. Negative lift or drag coefficients are found in the numerical results which is equivalent to the effect of fluid damping.

钻井立管下放安装过程中涡激振动响应实验研究

为研究海流作用下钻井立管在下放安装过程中的涡激振动(Vortex-induced vibration,VIV)响应特性,本文基于光纤光栅应变传感器技术,考虑立管下部组件(Lower marine risers package,LMRP)设计制作了多种钻井模型立管,对其下放安装过程中自由悬挂状态下的动力响应进行了水槽实验研究。实验制作了具有移动滑轨的下放装置,测试得到了0.4 m/s的均匀水流下模型立管动态应变响应时程曲线,并基于有限元法求得立管模型的固有频率和振型函数,再利用模态叠加法重构了立管模型位移响应。分析结果表明:立管在横流向(Cross-line,CF)的应变大于顺流向(In-line,IL)的应变,但两个方向的频率相同;自由端顺流向位移随LMRP质量增大而减小,但当质量超过管重时位移变化趋于稳定;自由端最大振幅受底部LMRP质量的影响较小,约为立管最大振幅的十分之一。

海洋立管的涡激振动模型预测方法

本文给出了海洋立管涡激振动模型预测方法，评述了近年来国内外研究进展，并对未来的发展趋势进行展望，为海洋工程领域的立管研究提出建议。

波流共同作用下隔水管动力响应非线性分析

为了探讨在波浪海流共同作用下隔水管的动力响应及涡激振动力分析,根据三维空间中隔水管运动的微分控制方程,以Matteoluca改进的Vanderpol尾流振子模型为基础计算涡激振动时隔水管与流体之间的相互作用。通过Hermite插值函数对隔水管的运动微分方程进行有限元离散,基于非线性分析理论利用当前拉氏描述(UL)建立了考虑几何非线性、预应力、涡激流固耦合等复杂因素影响的综合非线性增量平衡方程,并采用New-ton-Raphson迭代法和Newmark方法相结合的方法建立了空间隔水管非线性涡激动力响应的增量迭代算法。最后给出了考虑几何非线性与对外界荷载作用简化的位移分布包络线,以及横向涡激振动响应曲线。计算结果表明,所采用的方法正确、有效,可以为隔水管的生产设计及理论分析提供依据。

隔水管悬挂模式下的纵向振动特性仿真研究

考虑平台升沉运动及张紧器影响,基于动力学基本原理建立隔水管悬挂模式下的纵向振动力学仿真模型,利用有限差分法进行离散并求解,分析悬挂方式、浮力块、水深、壁厚和LMRP(Lower MarineRiserPackage)等因素对隔水管纵向振动的影响。仿真结果表明:软悬挂模式优于硬悬挂模式;配置浮力块会加剧隔水管纵向振动;隔水管纵向位移和应力从顶部到底部先减小后增大,且底部增大的趋势随着水深的增加而逐渐减弱;隔水管的应力和纵向位移随着壁厚的增加逐渐减小;应力随着LMRP质量增加而增大。

浪流共同作用下隔水管涡激动力响应分析

隔水管是海洋浮式生产系统中的重要附属设备.为了探讨在波浪海流共同作用下隔水管的动力响应及进行涡激振动分析,文中给出了三维空间中隔水管运动的控制方程,其中以Matteoluca改进的Vanderpol尾流振子模型为基础计算涡激振动时隔水管与流体之间的相互作用.利用Hermite插值函数对隔水管的运动方程进行有限元离散,采用Newmark方法求解隔水管的动力响应.同时绘出了不同外界条件下相应的位移分布的包络线以及横向涡激振动响应曲线.计算结果表明,所采用的方法正确、有效,可以为隔水管的生产设计及理论分析提供依据.

应用BP神经网络的柔性圆柱涡激振动预报

涡激振动是导致海洋立管等柔性圆柱结构疲劳破坏的重要诱因。柔性圆柱的涡激振动现象复杂多变,传统的计算流体力学方法和经验模型很难对其做出准确有效的分析。本文采用BP神经网络,在实验数据样本的基础上,建立了4个涡激振动响应预报模型,分别可在一定的轴向力和流速范围内预测柔性圆柱涡激振动的横流向及顺流向位移和频率响应。将模型预测结果与实验数据进行对比,发现两者吻合良好,证明此预报模型精准可靠。本文的研究结果可为涡激振动问题的预报和分析提供思路。

海洋立管涡激损伤分析的虚拟激励法概述

立管是连接海底钻井和海上平台的重要系统.涡激振动是造成立管疲劳损伤的主要因素.受海流的随机作用,立管涡激振动表现出不确定性,对这种随机振动进行快速有效地分析,对于立管的工程设计具有非常重要的意义.考虑随机激励作用,要比确定性激励困难得多,虚拟激励法对于克服这种困难具有很好的效果.近年来,应用虚拟激励法分析立管的涡激损伤问题,取得一些成果,该文对此作一概略的叙述.

海洋柔性立管二维振动的输出反馈控制

考虑了海洋柔性立管在时变内流和海流作用下二维振动控制问题。柔性立管系统动力学行为由两个偏微分方程和六个常微分方程描述。为提高其振动控制效果并避免控制溢出,直接基于其无限维分布参数模型,利用Lyapunov直接法和高增益观测器技术研发了输出反馈边界控制来控制立管的二维振动。随后证明了海流扰动作用下控制系统的一致最终有界稳定性和闭环状态的一致有界性。最后,仿真结果验证了文中所设计二维输出反馈边界控制算法的有效性。

计及内输流作用的立管涡激振动响应特性研究

为研究内流对输液立管涡激振动响应的影响,基于计及内流作用的海洋立管运动微分方程,结合涡激振动状态下的立管水动力载荷表征方法,引入相位追赶和频率锁定模型,建立计及内流作用的柔性立管涡激振动时域预报方法。结合有限元法和Newmark-β法,实现对内流作用下的立管涡激振动响应的预报。对比均匀流场下的无内流立管预报结果与试验结果,验证该预报方法的可行性和准确性;同时,对不同外流和内流作用下的输液立管涡激振动响应进行预报。着重观测不同工况下的输液立管时空分布图、RMS幅值、主导响应频率和主导模态。结果表明:内输流的存在会使立管涡激振动呈现出更显著的行波现象;涡激振动响应幅值和主导模态随着内流流速的增大而增大。

海洋立管非线性耦合振动系统的分段同伦分析

海洋立管是连接海上平台和海底井口的主要传输管道,涡激振动是导致海洋立管产生疲劳损伤的重要因素。通过研究立管涡激振动的非线性耦合振动系统,应用分段同伦分析方法,给出了该非线性振动系统的近似解析解,检验了分段同伦分析方法应用于海洋立管涡激振动的有效性。