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.

Study on Vortex-Induced Motions of A New Type of Deep Draft Multi-Columns FDPSO

A numerical simulation and an experimental study on vortex-induced motion（VIM） of a new type of deep draft multi-columns floating drilling production, storage and offloading（FDPSO） are presented in this paper. The main dimension, the special variable cross-section column and the cabin arrangement of the octagonal pontoon are introduced based on the result. The numerical simulation is adapted to study the effects of current incidence angles and reduced velocities on this platform’s sway motion response. The 300 m water depth equivalent truncated mooring system is adopted for the model tests. The model tests are carried out to check the reliability of numerical simulation. The results consist of surge, sway and yaw motions, as well as motion trajectories. The maximum sway amplitudes for different types of offshore platform is also studied. The main results show that the peak frequencies of sway motion under different current incidence angles and reduced velocities vary around the natural frequency. The analysis result of flow field indicates that the change of distribution of vortex in vertical presents significant influences on the VIM of platform. The trend of sway amplitude ratio curve of this new type FDPSO differs from the other types of platform. Under 45° current incidence angle, the sway amplitude of this new type of FDPSO is much smaller than those of other types of offshore platform at 4.4 ≤ V;≤ 8.9. The typical ‘8’ shape trajectory does not appear in the platform’s motion trajectories.

Modelling of the Vortex-Induced Motion of A Single-Column Platform with Non-Linear Mooring Stiffness Using the Coupled Wake Oscillators

A phenomenological model for predicting the vortex-induced motion (VIM) of a single-column platform with non- linear stiffness has been proposed. The VIM model is based on the couple of the Duffing-van der Pol oscillators and the motion equations with non-linear terms. The model with liner stiffness is presented for comparison and their results are compared with the experiments in order to calibrate the model. The computed results show that the predicted VIM amplitudes and periods of oscillation are in qualitative agreements with the experimental data. Compared with the results with linear stiffness, it is found that the application of non-linear stiffness causes the significant reductions in the in-line and transverse motion amplitudes. Under the non-linear stiffness constraint, the lock-in behavior is still identified at 8<Ur<15, and the trajectories of the VIM on the xy plane with eight-figure patterns are maintained. The results with different non-linear geometrically parameters show that both in-line and transverse non-linear characteristics can significantly affect the predict in-line and transverse motion amplitudes. Furthermore, the computed results for different aspect ratios indicate that the in-line and transverse motion amplitudes increase with the growth of aspect ratio, and the range of lock-in region is enlarged for the large aspect ratio.

naoe-FOAM-SJTU Solver for Numerical Study of Vortex-Induced Motions of a Buoyancy Can in Currents

This paper presents 3D （three-dimensional） CFD （computational fluid dynamic） simulation to analyse the FIR （flow-induced response） especially the yaw motion of a buoyancy can. The numerical cases are conducted with a buoyancy can under different reduced velocities utilizing our in-house code naoe-FOAM-SJTU, a solver based on the open source toolkit OpenFOAM. SST-DDES （shear-stress transport-delayed detached-eddy simulation） model is applied to handle the flowseparation and overset grid method is utilized to solve a large amplitude 6-DOF （6 degrees of freedom） motions. Free decay test and VIM （vortex-induced motion） test are built numerically. In VIM cases, the responses of trajectory, amplitude, frequency are calculated in a series of reduced velocities. With the increase of reduced velocity, yaw frequency is increased, which is similar to surge and sway frequency. And yaw frequency is equal to the sway frequency, which is consistent with experimental results. Furthermore, comparing two cases, one fixed in rotation and the other one free in rotation, it can be concluded that release in the degree of rotation can decrease the sway amplitude but make no difference in the surge amplitude.

Research on the Effects of in-Line Oscillatory Flow on the Vortex-Induced Motions of A Deep Draft Semi-Submersible in Currents

A Deep Draft Semi-submersible （DDS） under certain flow conditions could be subjected to Vortex-Induced Motions （VIM）, which significantly influences the loads on and life fatigue of the moorings and the risers. To investigate the VIM of a DDS with four rectangular section columns in waves coupled with a uniform current, a numerical study using the computational fluid dynamics （CFD） method was conducted. The issues of the VIM of multi-column floaters can be con＇,＇eniently converted to the issues of oscillating cylinders in fluid cross flows. This paper looks into the CFD numerical simulation of infinite cylinders having rectangular sections in a two-dimensional sinusoidal time- dependent flow field coupled with a uniform current. The resulted hydrodynamic forces and motion responses in different oscillatory flows plus currents both aligned in the same direction for the incidence of 135° of the DDS relative to the flow are compared with the ones in current only cases. The results show that the VIM response of this geometric arrangement of a DDS with four rectangular columns in a current combined with oscillatory flows is more evident than that in the current only case. The oscillatory flows and waves have the significant influence on the VIM response, forces and trajectory, in-plane motions of the DDS.

Computational Fluid Dynamics and Experimental Study of Lock-in Phenomenon in Vortex-Induced Motions of a Cell-Truss Spar

Spar platforms could be subject to vortex-induced-motions(VIM) in certain current conditions.Lockin is a phenomenon which occurs in a range of reduced velocities in VIM.In this paper,a new concept of spar platform called cell-truss spar is studied using both computational fluid dynamics(CFD) and model test to investigate the VIM of the spar under different reduced velocities.The unique configuration of the cell-truss spar is carefully considered,and the unsteady flow around the spar is calculated and visualized in CFD simulations.A physical model with a scale ratio of 1:100 of the cell-truss spar is fabricated,and model tests are carried out in the current-generating ocean engineering basin.Many important parameters in VIM of the cell-truss spar are obtained,the occurrence of lock-in phenomenon is successfully simulated,and the mechanism and rules of lock-in are analyzed.

基于正交设计和CFD模拟的Spar平台螺旋侧板水动力优化设计研究

螺旋侧板能够有效抑制Spar平台涡激运动,为选取较优的螺旋侧板参数,文章基于正交设计理论,采用数值计算方法求解RANS方程,研究侧板螺距比、侧板高度和来流速度等因素对Spar平台横向力系数的影响。采用极差分析法和方差分析法处理计算数据,得到各Spar平台螺旋侧板参数及其交互作用对减涡效果的影响程度。

浮力筒自由涡激运动的模型试验研究

对外径150 mm,长700 mm垂直放置的浮力筒模型开展了自由涡激运动试验研究,测量了浮力筒在不同试验工况下六个自由度的运动参数,并着重对浮力筒顺流向、横向和艏摇运动的运动特性进行了分析。试验中发现,当约化速度较小时浮力筒主要发生顺流向运动而横向和艏摇运动较小且不规则;当约化速度增大到一定程度后,顺流向运动频率是横向运动频率的2倍,此时艏摇运动出现了两个主要运动频率,其中较大的艏摇运动频率等于横向运动频率,因此认为横向运动和艏摇运动的激励力来源相同,即二者存在耦合作用,而且浮力筒的运动轨迹呈现明显的"8"字形。试验中还发现,浮力筒涡激运动的平均拖曳力系数比圆柱绕流的阻力系数增大了约100%。

Spar型浮式风电基础结构涡激运动实验研究

对Spar型浮式风电基础结构进行水槽模型实验研究,并借鉴海洋石油领域的减涡措施,为模型安装螺旋侧板作为对比。测试不同约化速度和固有频率下的运动响应及顺流向、横流向动水压力,从响应幅值、涡泄频率、受力分析和运动轨迹等多个角度出发,分析其涡激运动的关键特征。研究表明:涡激运动锁定区前后阶段,涡泄频率与固有频率比值随约化速度呈线性增加,且锁定区前St≈0.18,锁定区后St≈0.14;螺旋侧板对运动轨迹、频率改变十分明显,其减涡效果显著。

考虑阻尼比的Spar式浮式风力机基础结构涡激运动特性研究

利用数值模拟方法对不同阻尼比条件下Spar式浮式风力机基础结构进行涡激运动特性的研究。从涡激运动响应幅值、频率比、位移、升力及拖曳力等方面研究Spar式浮式风力基础结构涡激运动的关键特性。研究表明:涡激运动上端分支对应的频率比为f_(y)/f_(n)≈1,且该分支出现最大幅值。随着阻尼比的增大,上端分支约化速度范围有减小趋势。低阻尼比下,力与位移的曲线出现"拍"现象;下端分支对应的频率比f_(y)/f_(n)均大于1,不随阻尼的改变而发生变化。上端分支与下端分支间,存在一过渡区域,该区域对应的斯托哈尔数St≈0.17,不随阻尼比的改变而改变。

考虑涡激运动下海洋柔性立管的疲劳损伤分析

基于ABAQUS/AQUA模块,当Spar平台发生涡激运动时,对与平台相连接的柔性立管进行时域动态响应分析,得到了不同折合速度下柔性立管的应力响应曲线。对立管触地区域和悬垂段进行了细化的分析,并利用S-N曲线和Miner线性累积损伤理论对平台涡激运动引起的柔性立管疲劳损伤进行了计算。研究结果表明:平台涡激运动对立管触地区域和悬垂段的影响较大,这两部分最容易在长时间的工作中产生疲劳损伤。特别是平台运动到近点时,立管触地区域受到的应力急剧增加,这个区域是立管工作的危险区域,应该特别注意。该研究结论可以作为立管结构设计和海底铺设时的参考,通过增加立管触地区域的结构强度和优化铺设方式,从而来提高立管的疲劳寿命和系统的安全性。

系泊刚度对半潜式平台涡激运动响应的影响研究

以四柱半潜式平台为研究对象,基于模型试验方法分析不同系泊刚度下的平台涡激运动(VIM)特征。由于涡激运动对系泊系统极限张力和疲劳寿命的重要贡献,因此重点研究系泊刚度对平台横向涡激响应的影响。结果显示:系泊系统刚度的变化对横向涡激运动的最大响应幅值影响较小,运动响应曲线随无量纲折合速度的分布趋势基本一致;但相同流速下平台具有完全不同的涡激运动响应特性,这表明若当前流速引起了平台大幅度的涡激运动,可通过调节系泊线预张力来改变系泊系统的刚度,使其固有频率避开平台旋涡脱落的频率范围,从而抑制平台的横向涡激运动;当平台受到来自不同方向的水流作用时,需采用不同的刚度调整策略。此外,文中还详细分析了平台在XY平面上的涡激运动轨迹、涡激锁定现象如共振和频率锁定、不同系泊刚度下涡激响应的谱。相关结论对实际平台作业具有重要的参考价值,为处于涡激共振状态下的平台运动抑制提供了一种新的思路。

深水半潜式平台的涡激运动Ⅱ.船型优化研究

基于CFD数值计算方法对深水半潜式平台的涡激运动开展优化研究,提出一种能够抑制涡激运动的半潜平台新形式,并应用模型试验进行对比验证.研究了新型平台和传统型平台的涡激运动特征,对纵向偏移、横荡响应的幅值和分布规律,锁定区间等关键因素进行对比分析;研究了各方向耦合作用下平台的轨迹特征;详细分析了新型平台的减涡机制,从平台泄涡结构和涡激响应的均匀性、稳定性以及响应幅值等方面展开研究,阐述立柱的横剖面形状,排列形式等对平台涡激运动性能的影响.研究表明,这种"立柱的横剖面形状为梯形,呈外八字对称排列"的新型平台形式对抑制平台涡激运动具有积极影响,有助于减少或延缓强非线性涡激运动对系泊、立管系统的破坏作用.

深水半潜式平台的涡激运动Ⅰ.关键特性研究

采用数值计算和模型试验相结合的方法对深水半潜式平台涡激运动特性进行了系统研究.模型试验在拖曳水池内开展,采用4点水平系泊方式;数值计算选用黏性流的计算流体力学方法,建立适用于研究平台涡激运动的分析方法,开发合理的网格划分模式,引入网格变形和重构技术解决非线性的流固耦合问题;研究半潜式平台在不同来流角度下的横荡运动、纵向偏移特征;深入研究涡激锁定现象,给出不同角度下的漩涡锁定区间,获得涡激锁定的相关特性;研究平台涡激响应随折合速度所经历的演化过程;揭示了平台涡激运动随时间变化的发展阶段,分析各阶段漩涡泄放的特点.

张力腿平台涡激运动特性数值模拟与模型试验研究

针对张力腿平台显著的涡激运动可能威胁平台安全的问题,以原计划用于开发南海流花16-2油田群的某张力腿平台为研究对象,通过数值模拟和模型试验方法,研究了典型流向角下张力腿平台的涡激运动特性。研究表明:0°流向角下张力腿平台的横向运动响应最显著,其最大幅值的无量纲标称值约为0.40,而45.0°流向角下其横向运动响应幅值最小,最大无量纲标称值约为0.23;0°和45.0°流向角下张力腿平台的首摇运动响应相当;数值计算中,采用多面体网格和六面体网格的模拟结果相差不大,二者与模型试验的结果均在较小的折合速度下吻合较好,但在大的折合速度下有较大的偏差,且数值计算的响应值相对模型试验结果偏小;整体而言,六面体网格的模拟结果与模型试验结果的吻合度更高。本文研究为张力腿平台的研究与设计提供了参考。

深水浮式平台监测系统研制及应用

监测系统是获得浮式平台真实情况的可靠手段,跟水池模型试验一样,对评估平台安全非常有价值。本文研究浮式平台监测系统研发要点,并在我国"南海挑战号"FPS平台上成功实施了1套包含环境条件、浮体运动和水下系泊系统的监测系统,实现了监测系统远程控制和"互联网+"数据共享,实现了较为宝贵的台风期间监测和数据传输的功能,获得了多个台风数据和超过3年的监测信息,可为今后该平台的安全评估、深水浮式平台设计反馈、现场作业支持或预警等提供数据和依据,为将来监测系统推广应用提供了典型示范。建议下一步通过海上现场监测数据来研究无法准确通过数值模拟但对立管安全影响非常大的浮式平台涡激运动(VIM)。

带螺旋侧板的Spar平台涡激运动数值模拟

[目的]长时间持续的涡激运动可能会引起Spar平台锚泊系统疲劳甚至是平台结构损坏,因此,必须充分考虑到涡激运动的不利影响。[方法]利用自主开发的CFD求解器naoe-FOAM-SJTU以及动网格方法,对Spar平台的涡激运动流场特性进行数值模拟。采用基于剪切输运应力方程的延时分离涡(SST-DDES)湍流模式,对带螺旋侧板的Spar平台的三维精细尾涡结构进行模拟,研究其在不同折合速度下的纵向、横向、艏摇运动响应以及运动轨迹。采用水平线性弹簧系统替代模拟平台系泊系统,并在研究中考虑平台不同自由度间的耦合作用对平台涡激运动的影响,然后将数值模拟结果与模型试验结果进行对比,分析带螺旋侧板的Spar平台横向和流向运动时间历程、频谱特性、锁定现象等,以揭示其涡激运动的内在机理。[结果]结果显示,加装螺旋侧板可有效减小Spar平台涡激运动响应幅值;采用自主开发的CFD求解器计算模拟海洋平台涡激运动问题具有良好的精度和可靠性。[结论]带螺旋侧板Spar平台涡激运动的数值模拟分析对Spar平台实际设计工作具有重要意义。

基于运动感知和眼动特征的视觉诱导晕动症检测研究

目的视觉诱导晕动症是造成用户VR体验舒适度降低的主要原因。如何有效检测并减少视觉诱导晕动症给用户带来的影响是VR产品普及过程中一大亟待解决的问题。旨在探讨视觉诱导晕动症检测方法。方法采用虚拟现实漫游实验收集场景变化参数和用户眼动数据,并从用户运动感知和眼动特征两方面进行数据分析。结果场景变化会引发用户的运动感知,用户本身对于视觉刺激的晕动症易感性会使这种运动感知诱发不同程度的视觉诱导晕动症症状,这种差异体现在眼动特征中。采用运动感知和眼动特征指标构建视觉诱导晕动症检测模型。结论建立的视觉诱导晕动症检测模型可以为实时视觉诱导晕动症检测提供理论支撑。

涡激运动的研究性实验设计

文章结合涡激运动的基本原理与传统拖曳水池的设计,提出了一种小型的研究涡激运动现象的模型实验设计,并描述了其实验装置的结构与原理。在分析了涡激运动理论成因的基础上,进行了涡激运动现象研究性实验:观测到了圆柱浮力筒涡激运动在水平面内运动的标志性“8”字形轨迹,并着重研究了涡激运动的运动响应随水流速度的变化关系。本实验可以使学生更加直观地感受流体力学中的复杂现象,提高学生的创新实践能力。

考虑艏摇的半潜式平台涡激运动试验研究

为研究半潜式平台的涡激运动特征,进行了水槽模型试验研究。测试了流向角为0°、15°、30°及45°时,在不同来流速度下半潜式平台的横荡、纵荡及艏摇运动响应;并以艏摇运动为重点,从响应幅值、运动频率等角度出发,探讨其涡激运动的关键特征。结果表明:在不同来流方向情况下,半潜式平台均未发生明显的频率锁定现象。当流向角为0°和15°时,平台三自由度响应随约化速度的增加近似呈线性增大的趋势;当流向角为30°和45°时,在约化速度4~10附近平台响应出现了明显的共振现象,并在此现象结束后响应大幅减小,之后,随着约化速度的增加再次大幅增大。除个别工况外,艏摇与横荡均高度耦合,其响应频率主峰值基本保持1∶1关系;艏摇固有周期对平台三自由度运动频率及幅值影响均不明显。流向角为45°时,平台的艏摇运动最为剧烈,在各个约化速度下响应幅值均数倍于其他流向角。另外,在0°流向角时观察到了“驰振”现象;15°流向角的多个约化速度下观察到了“自激”现象。