新型海洋立管涡激振动抑振装置的敏感性试验

将月牙扰流器按"八"字布置于立管表面用以抑制立管涡激振动,为研究其抑振效果,分析其抑振敏感性,在大型波、浪、流水槽进行涡激振动抑振试验。试验中采用有机玻璃管模拟立管,立管模型长1.5m,外径18mm,壁厚2mm,两端铰接,外流速分别为0.6、0.7m/s。按照不同螺高、螺距及迎水面形式设计8根具有正八字、倒八字抑振装置的不同类型抑振管,同时设计一根与抑振管相同配重的裸管,在立管表面粘贴应变计以测得动态应变数据,经过数据处理得到立管振动位移值。通过各抑振管间以及抑振管与配重裸管之间横向振动位移幅值的对比,研究抑振装置的抑振敏感性。结果表明:在适当尺寸及布置形式条件下,表面按"八"字布置月牙扰流器能够有效抑制立管横向涡激振动;迎水面月牙布置形式不同,抑振效果略显不同,迎水面布置为倒八字其抑振效果优于正八字,但该种抑振装置仍可实现全向性;抑振装置的螺高、螺距对抑振效果有一定影响,螺高增大、螺距减小有利于提高抑振效果。     

海洋立管涡激振动的研究现状、热点与展望

随着深海油气资源的开采,越来越多的研究者开始关注海洋立管的涡激振动问题.在海洋环境下,洋流是海洋立管的涡激振动的主要原因.当洋流流经立管时会在立管的两侧产生交替的泄涡,导致立管受到横流向和顺流向的脉动流体力.这被认为是海洋立管涡激振动的主要诱因.海洋立管的涡激振动是一个异常复杂的工程问题,它涉及许多科学上悬而未决的难题,如紊流、流动分离、分离点的漂移等等.此外,事先无法确定的立管的位置和立管与洋流之间的相互作用又大大增加了解决这一问题的难度.尽管近几十年里科学界在此方面做了大量的研究工作,一个能够准确、高效、经济地预报海洋立管涡激振动的方法仍然没有得到.即便如此,最近的研究工作依然在许多方面作出了突出的成就.首先介绍了涡激振动的背景知识和基础理论.随后,回顾了近年来海洋立管涡激振动方面的研究成果.接着,重点介绍了当前海洋立管涡激振动领域内的两个热点研究问题,即:在多大程度上立管的顺流向振动能够影响立管的横流向振动,以及尾流的三维效应是如何影响立管的涡激振动响应的.最近的研究发现,当结构与流体的质量比小于6时,顺流向振动能显著增大横流向振动的振幅.最近的研究还发现,立管尾流的三维特性和立管受到流体力的轴向相关度有密切关系.随着流动的发展(海流折合速度从0增加到12),立管尾流的三维特性发生变化,在初期,立管尾流的三维特性不明显,流体力的轴向相关度基本等于1,也就是说,流体力和立管的位移响应是同步的,因此能量不断地由海流向立管转移,导致立管的振幅不断增大.当海流折合速度大于6时,流体力的轴向相关度由1锐减到负值,此时,立管尾流的三维效应显著.最后针对今后海洋立管的涡激振动的研究提出一些建议.     

海中输流立管涡激振动试验研究及疲劳寿命分析

为研究海洋立管涡激振动响应并预测其疲劳寿命,在中国海洋大学物理海洋实验室大型风-浪-流水槽进行海洋立管涡激振动模型试验.考虑管内流体的流动,运用相似理论将实际海洋立管缩放为试验模型,施加不同流速的外流,测得立管在涡旋脱落时顺流向及横向振动的应变时程曲线,根据实测结果,采用Miner理论对立管进行疲劳寿命分析.结果表明:立管横向振动比顺流向振动强烈,大约高一个数量级;随骀着外流流速的增加,管道横向及顺流向振动明显增加,立管的疲劳寿命降低;立管中部及端部振动比较强烈,疲劳寿命较其它位置处低,容易发生疲劳破坏.     

月牙肋抑振装置对柔性立管涡激振动抑振作用的试验研究

海洋立管的涡激振动会对立管结构的疲劳寿命产生严重的影响。提出1种月牙肋抑振装置,通过在室内水槽中进行物理模型试验,研究该装置在涡激振动情况下对立管的抑振作用。测得该装置在立管表面不同布置方式时顺流向及横流向振动的应变时程曲线,使用DASP软件对所测数据进行分析处理,得到立管的振动幅值和功率谱。试验结果表明:这种月牙肋抑振装置可明显降低立管模型的振动幅值,且对振动频率有一定的影响,同时也表明不同的布置方式对立管的抑振效果也不相同。     

三根附属控制杆对海洋立管涡激振动抑制作用实验研究

海洋立管的涡激振动会严重影响立管结构的使用寿命.通过室内水槽实验研究在立管模型周围等分布置三根附属控制杆来减小立管涡激振动响应的新型抑制措施.实验中观测了0.24 m/s、0.31 m/s、0.37 m/s以及0.44 m/s四种均匀流和两个极限来流方向下的涡激振动抑制效果.实验结果表明:三根附属控制杆抑制措施可明显降低立管模型的横向振动幅值,但对主管的振动频率改变不大;同时,这一抑制措施对来流方向有较强的适应性,避免了以往单根控制杆在流向发生改变时可能加剧立管涡激振动的弊端.     

考虑内流的海洋立管动力特性和动力响应的实验研究

本文考虑管内流体和管外流体的共同作用,对海洋立管进行试验研究。试验在中国海洋大学物理海洋试验室进行,通过试验得到了立管的一些有价值的规律:随着外流流速逐渐增大,立管的一阶激振频率也逐渐增大,在一定的外流速下,会激起立管的二阶激振频率甚至于三阶激振频率,由于立管的刚度较大,内流对立管的影响并不明显。但是当外流达到一定流速,这种影响就会显现出来,内流的存在会降低海洋立管的振动频率。     

海洋立管试验支座设计及其对涡激振动影响研究

针对大长细比顶张力立管内外流共同作用时涡激振动的试验研究,考虑端部铰接和固接两种边界条件,设计了可施加张力和内流的铰接支座和固接支座,并对两种支座进行数值模拟及受力分析,最后用于深水立管的涡激振动试验。立管模型材料采用6.2 m长铜管,外径20 mm,壁厚1.5 mm,分别支撑在铰接支座和固接支座上,施加不同的顶部张力、外流流速和内流流速。通过在立管表面粘贴光纤光栅应变计获得动态应变数据,分别从动态响应幅值、频率等方面对不同边界条件立管的试验数据进行对比分析,得出在顶张力、内流速相同的情况下,外流速相等时两端铰接立管的振动幅值大于两端固接立管,随着外流速的增加,两端铰接立管比两端固接立管率先发生锁振现象。     

深水海洋立管涡激振动试验流场设计及CFD数值模拟

针对阶段流作用下的大长细比海洋立管涡激振动试验所需的外流条件,根据实验室的基本情况,对试验流场进行了设计改造和数值模拟。根据数值模拟的结果,选定了试验流场的改造方案,并对改造后测得的试验数据和数值模拟的结果进行了对比分析。结果表明:在低流速时,模拟结果与试验流速吻合较好;在高流速时,数值模拟结果比试验流速略大。在误差允许的范围内,可以得到均匀分布的流场。     

深海顶张紧式立管涡激振动疲劳可靠性研究

以深海顶端张紧式立管为研究对象,基于圆柱体受迫振荡实验数据和能量平衡原理预报立管涡激振动响应及其诱发的疲劳损伤度,各主要参数随机化,应用响应面法建立了涡激振动疲劳安全系数与极限状态方程关系,研究了疲劳安全系数对结构可靠性的影响,并分析了各随机变量的灵敏度。研究结果表明:1)随着疲劳安全系数的增大,结构失效概率逐渐减小,且失效概率的减小幅度随疲劳安全系数的增加而趋缓。疲劳安全系数SF无需超过20。2)增加疲劳安全系数SF可降低疲劳载荷不确定量B、S-N曲线参数A、顶张力、流速、外径和壁厚等立管及环境随机变量对结构涡激振动疲劳损伤可靠性的影响。3)立管各参数中,流速、外径和壁厚均值越大,立管结构安全度越低,顶张力对结构影响相反;各参数的标准差越大,结构可靠度越低。均值灵敏度由大到小依次为顶张力、流速、外径和壁厚,标准差灵敏度由大到小依次为流速、张力、外径和壁厚。4)壁厚对深海顶张紧式立管涡激振动所致疲劳损伤影响很小,可忽略其影响。研究成果可为深海顶张紧式立管涡激振动疲劳安全系数的选取提供参考。     

指数剪切流作用下海洋立管振动特性研究

细长海洋立管在复杂来流作用下的涡激振动（VIV）是海洋工程领域备受关注的热点问题。采用双向流—固耦合方法,对立管在指数剪切来流作用下的涡激振动实施了数值仿真研究。基于均方根振幅包络图的显性模态分析发现,与线性剪切流工况相比,当流速较小时,立管模型在指数剪切流作用下振幅最大值相对较小;流速较大时,指数剪切流的非线性分布致使立管的振动响应增强。通过对立管模型均方根振幅包络峰、谷处的振动响应频率分析发现,在波峰位置处振动频率单一且较为稳定,而在相邻波谷位置处多频共存现象显著,两者有显著差异。沿管的轴向波形主要表现为驻波主导和驻波—行波混合模式,行波一般间歇发生,流速越大发生频次愈高,其在横流向的传播方向通常由高流速区段传递至低流速区段。指数剪切流作用下,沿管体轴向各截面位置处的旋涡脱落模式差异显著,高流速段尾流区的旋涡发放具有较强的周期性,涡管完整且与管轴存在一定偏离角度,低流速段蜂窝状离散涡旋较多。     

Experimental Investigation on Vortex-Induced Vibration of Steel Catenary Riser

Steel catenary riser (SCR) is the transmission device between the seabed and the floating production facilities. As developments move into deeper water, the fatigue life of the riser can become critical to the whole production system, especially due to the vortex-induced vibration (VIV), which is the key factor to operational longevity. As a result, experimental investigation about VIV of the riser was performed in a large plane pool which is 60 m long, 36 m wide and 6.5 m deep. Experiments were developed to study the influence of current speed and seabed on VIV of SCR. The results show that amplitudes of strain and response frequencies increase with the current speed both in cross-flow (CF) and in-line (IL). When the current speed is high, multi-mode response is observed in the VIV motion. The amplitudes of strain in IL direction are not much smaller than those in CF direction. The seabed has influence on the response frequencies of riser and the positions of damage for riser.     

Experimental Study on Response Performance of VIV of A Flexible Riser with Helical Strakes

Laboratory tests were conducted on a flexible riser with and without helical strakes. The aim of the present work is to further understand the response performance of the vortex induced vibration (VIV) for a riser with helical strakes. The experiment was accomplished in the towing tank and the relative current was simulated by towing a flexible riser in one direction. Based on the modal analysis method, the displacement responses can be obtained by the measured strain. The strakes with different heights are analyzed here, and the response parameters like strain response and displacement response are studied. The experimental results show that the in-line (IL) response is as important as the cross-flow (CF) response, however, many industrial analysis methods usually ignore the IL response due to VIV. The results also indicate that the response characteristics of a bare riser can be quite distinct from that of a riser with helical strakes, and the response performance depends on the geometry on the helical strakes closely. The fatigue damage is further discussed and the results show that the fatigue damage in the CF direction is of the same order as that in the IL direction for the bare riser. However, for the riser with helical strakes, the fatigue damage in the CF direction is much smaller than that in the IL direction.     

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.     

大长细比柔性杆件涡激振动实验

涡激振动(vortex-induced vibration,VIV)是导致深海细长柔性立管发生疲劳破坏的重要因素。采用实验观测手段研究了长细比为1 750的柔性立管多模态涡激振动特性。实验中,通过采用拖车拖拉立管模型在水池中匀速行进来模拟均匀流作用下的涡激振动响应。利用光纤光栅传感器测量立管模型在横流向(cross-flow,CF)和顺流向(in-line,IL)的应变,进而通过模态分解的方法,获得立管模型涡激振动的位移。在此基础上,研究了CF以及IL方向的响应频率、位移标准差的平均值和最大值等随流速的变化规律,并分析了立管模型上测点的运动轨迹及其影响因素。     

Numerical and Physical Investigation on Vortex-Induced Vibrations of Marine Risers

1 .Introduction Risers ,in deep water and strong current environments ,are prone to vortex-induced vibrations(VIV) .Vortex-inducedforces may excite the risersintheir normal modes of transverse direction.Un-der the“lock-in”condition,large resonant oscill…     

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

Based on Iwan‘s wake oseillator model developed with the classical van der Pol equalion, the differential equation for the response of the vortex-induced vihration (VIV) of the riser considering the effect of the internal flowing fluid and the external marine environmental condition is derived. The effect of the intermal 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.     

Numerical Prediction of Vortex Induced Vibrations on Top Tensioned Riser in Consideration of Internal Flow

In consideration of the effect of the internal flowing fluid and the external marine environmental condition on the vortex-induced vibration （VIV） of top tensioned riser （Till）, the differential equation is derived based on work-energy principles and the riser near wake dynamics is modeled by Facchinetti＇ s wake oscillator model. Then Galerkin＇ s finite element approximation is implemented to derive the nonlinear matrix equation of the coupled equations and file corresponding numerical programs are compiled which solve the coupled equations directly in the time domain. The comparison of the predicted results with the recent experimental results and the prediction of SHEAR7 is performed. The results show the validity of the proposed method on the prediction of VIV of deep water risers. The effect of internal flow on the dynamic characteristics and dynmnic response of the riser is analyzed and several valuable conelusions are drawn.     

基于OrcaFlex的钢悬链立管疲劳损伤预报

涡激振动是钢悬链立管疲劳损伤的重要诱导因素之一,因此涡激振动分析在钢悬链立管设计中占据重要地位。OrcaFlex在海洋工程领域功能强大、应用广泛,能够提供两种涡激振动疲劳损伤预报方法,分别是基于模态叠加法的频域预报方法和基于Iwan-Blevins尾流振子模型的时域预报方法。OrcaFlex的涡激振动时域预报模型只考虑了横流向涡激振动造成的疲劳损伤,基于OrcaFlex的时域预报方法开发了双自由度涡激振动尾流振子模型的接口,考虑了浮式平台运动对钢悬链立管的涡激振动激励作用,实现了钢悬链立管双自由度涡激振动疲劳损伤预报。     

内流对立管顶张力影响规律的试验研究

通过大长细比海洋立管物理模型试验,研究内流对其顶张力的影响。在不同的外流流速下用电子流量计读出内流流速,用动态电阻应变片测试得到顶端张力;然后直接分析内流对顶张力的影响规律。试验研究表明:内流的增加会使顶张力减小;施加的初始顶张力较大时,内流的影响较小;施加的外流较小时,内流对顶张力的影响较大。