带有柔性阻流器的海底管道冲刷防护试验研究

从管道尾流区泥沙颗粒的受力平衡着手,探讨了管道冲刷的临界压力差。在此基础上,提出了一种防止或减少管道冲刷的新方法,即在管道下方安装柔性阻流器。通过物理模型试验,研究了单向恒定流及规则波作用下,柔性阻流器对管道两侧床面压力、冲刷深度的影响。试验结果表明:柔性阻流器的安装能明显减小管道两侧床面的压力差。在单向恒定流作用下,管道的冲刷深度随阻流器长度的增加而减小;当阻流器达到临界长度时,冲刷消失。规则波时,因阻流器的扰动作用,冲刷深度有所增加;但随阻流器长度的增加,其扰动作用减小,对床面的防护作用增加,因而冲刷深度减小。     

海底子母管作用下海床冲刷的试验研究

子母管作为海洋工程中的新型管线结构,越来越受到重视。本文运用物理试验的方法研究了稳定流中海底子母管对海床局部冲刷的影响。通过控制不同海底子母管间间距,管径及来流流速,分析出了两管间隙比对冲刷坑内冲刷深度的影响,同时,从泥沙起动原理出发,利用流体的连续性特征,推导了与子母管间间隙比、管径及来流流速相关的冲刷坑深度理论公式,该理论公式与试验结果吻合良好,可预测子母管下泥沙的冲刷。通过试验可知,雷诺数及子母管间间隙对冲刷坑深度有较大的影响。在一定雷诺数下,子母管间间隙越小,冲刷坑深度越大,泥沙输移越也越严重。在管线间间隙比不变时,雷诺数增加同样能加强管线下方冲刷。     

往复流作用下海底管道局部冲刷机制数值模拟

基于开源的计算流体力学模式REEF3D,建立了海底管道局部冲刷水槽数值模型,在验证单向流实验结果的基础上,进一步对往复流作用下的海底管道局部冲刷机制进行了研究,并作对比分析。研究表明,总体上,往复流对管道所在海床局部冲刷规模比单向流弱。当流向改变后,原先下游的堆积区转变成上游,优先受到冲刷,并填充到管道下方的冲刷坑,同时使水流在管道下方的作用减弱。这种回填过程,使短周期下的往复流作用需要更长的冲淤平衡时间。在这种回填与冲刷的共同作用下,上下游的冲刷坑坡度会因流向变化而变化。     

孤立波作用下沿海路基冲刷实验研究

基于波浪水槽实验,对孤立波作用下沿海公路局部冲刷开展实验研究。实验采用1/10与1/20组合坡模拟海岸地形,对岸滩进行逐个多次波浪作用,用地形仪对作用后地形进行测量。通过改变路基在冲刷槽的绝对位置和相对位置,探讨路基位置因素对局部冲刷影响。实验结果表明:孤立波作用下沿海路基向海侧出现明显冲坑,冲刷深度随波浪作用个数增加,最后达到平衡。路基位置是路基局部冲刷主要因素,随着路基位置远离海岸线,路基向海侧冲刷深度呈减小趋势。     

基于Flow-3D的抗拖网海床基冲刷数值模拟

基于Flow-3D软件,建立抗拖网海床基沙土基床冲刷的三维数学模型,控制水流流速和泥沙粒径,对不同条件下抗拖网海床基周围沙土基床的冲刷进行数值模拟,并分析了抗拖网海床基最大冲刷深度随这两种因素改变的趋势。结果表明,抗拖网海床基沙土基床冲刷坑深度随海流流速增大而增加,在设定其他条件不改变的情况下,冲刷坑的加深速度随着流速增大会逐渐减缓,最终达到冲淤平衡。同时,由于海底环境复杂多变,导致模型与实际情况有一定的差异,还需在后续的研究中进一步进行优化。     

透空导流板对海底管线冲刷防护的试验研究

基于海底管线周围流场复杂,管线运行过程中易于出现冲刷悬空破坏等诸多特征。本文根据导流板作用下海底管线的冲刷防护机理,提出了一种透空导流板与海底管线结合的防冲刷措施。通过量纲分析得到了影响海底管线冲刷深度的无量纲公式,并通过物理模型试验,研究了单向流作用时,不同流速、不同管孔比、不同透水比的工况下透空导流板对海底管线的冲刷防护的影响。研究发现,由于透空导流板的存在,泥沙受到的推移力减小,导致泥沙淤积,海底管线的冲坑深度小于无导流板防护的冲坑深度,透空导流板防护的管线最大冲刷深度在管轴线前0.3 D处,且在管线下方冲刷坑深度明显减小。基于试验资料分析,拟合得到了平衡冲刷深度的计算公式,其公式与实测资料拟合较好,说明该公式具有较好的适用性。     

水流作用下斜桩局部冲刷及流场变化试验研究

为研究水流作用下斜桩周围局部冲刷特性及流场变化并掌握斜桩同垂直桩的差异,本研究在不同流速条件下开展了包含反斜桩、垂直桩、正斜桩等多种工况的水槽试验,测量分析了斜桩的冲刷历时曲线、床面形态、沿程流速分布、最大冲刷深度等数据.结果表明:清水条件下斜桩周围局部冲刷坑及桩后沙丘尺度明显减小;斜桩桩前最大冲刷深度随倾角的增加而逐...     

港珠澳大桥青州航道桥桥墩基础冲刷试验研究

桥墩基础冲刷是桥梁毁坏的重要因素,是桥梁基础设计的关键指标之一。目前国内外对于桥墩基础在复杂动力条件下冲刷深度的研究常采用物理模型试验方法,利用正态系列模型方法,在波流水槽中研究了水流、潮流和波流共同作用下青州航道桥索塔基础周围流态变化和局部冲刷特征。研究结果表明,桥墩最大冲刷深度和冲淤范围与水流流速、桥墩轴线与水流夹角和波浪等因素有关;在潮流最大流速和恒定流流速一致情况下,桥墩局部冲刷深度达到平衡后,将会达到与恒定流基本一致的最大冲深;波流共同作用下的最大冲刷深度比恒定流增加10%左右。设计桥墩形状在100年一遇水流和波浪共同作用下桥墩基础局部最大冲刷深度为13.7 m。     

后挖沟深度对深海海底管道屈曲影响数值分析

鉴于海底管道的服役水深越来越深,主要采用犁式挖沟机对预铺设于海床之上的海底管道采取后挖沟的方式将海底管道埋设于海床之下,以保护其免受不必要的损伤。针对后挖沟深度H是海底挖沟机的重要设计参数,也是影响管道悬跨的重要因素的问题,对SMD(UK)犁式挖沟机展开参数优化,确保作业过程中悬跨段管道在外部静水压力作用下,海底管道不会发生屈曲破坏。采用ABAQUS软件,分别建立了作业前和作业中两种工况下的悬跨模型,分析机械手对接触部分管道的损伤,结果显示,作业中的机械手对悬跨管道的损伤更大;同时,建立了作业中不同管径下,后挖沟深度对管道损伤的安全裕量关系曲线。进一步,结合作业中不同挖沟深度下的管跨段屈曲数值模型,对处于外部静水压力作用下的悬跨管的屈曲失效展开分析,结果显示,随着后挖沟深度的加大,不同管径下的悬跨段管道局部出现塑性压溃的临界压力值不断降低;管道外径的增大,降低了同一后挖沟深度下发生屈曲失效的压力值。最后,在后挖沟深度与外部静水压力组成的区域内,建立屈曲失效临界关系曲线,并划分出工作区和压溃区,为深海管道后挖沟埋管的施工提供工程参考。     

海底管道掏空与波浪力变化关系试验

鉴于海底油气管道在海洋能源开发中的特殊意义,前人开展了大量管道安全试验研究,特别注重不同底床、不同波浪条件、不同埋置深度等方面的组合试验,本次试验针对危害管道安全的掏空和悬跨开展了波浪水槽试验,探讨黄河三角洲为主的粉土海床上掏空与管道受力关系,弥补这方面研究的不足,结果表明,海底管道受力变化与掏空过程密切相关。连续试验过程表明,海底管道与海床土、波浪水体之间存在密切关系。海底管道在波浪作用下冲刷掏空经历了3个演变阶段:泥沙起动阶段、水流隧道发育阶段、快速掏空阶段和冲刷平衡阶段。水流隧道伴随的管涌对冲刷掏空起重要作用。伴随冲刷掏空,管道波浪力出现规律性转化,从正向作用逐渐过渡到负向作用,也可以分出4个演化阶段:冲刷初始阶段,管道波浪力以正向为主,管道表现为正向力作用下的振动;隧道发育阶段,管道受到的作用力主要是波峰上举力和波谷下拉力,表现为上下振动;管道悬空阶段,波峰负向作用力和波谷正向力交替出现,管道在波浪作用下表现为前后振动;冲刷平衡阶段,管道主要在负向作用力下振动。     

Numerical investigation of local scour below a vibrating pipeline under steady currents

Local scour below a vibrating pipeline under steady current is investigated by a finite element numerical model. The flow, sediment transport and pipeline response are coupled in the numerical model. The numerical results of scour depths and pipeline vibration amplitudes are compared with measured data available in literature. Good agreement is obtained. It is found that pipeline vibrations cause increases of scour depth below the pipeline. The scour pit underneath a two-degree-of-freedom vibrating pipeline is deeper than that under a pipeline vibrating only in the transverse flow direction. The effects of water depth are also investigated. The present numerical result shows that water depth has weak effect on the scour depth. However it does affect the time scale of the scour. The shallower the water depth is, the less time it requires to reaches the equilibrium state of scour. It is found that the vibration forces vortices to be shed from the bottom side of the pipeline. Then vortex shedding around a vibrating pipeline is closer to the seabed than vortex shedding around a fixed pipeline. This contributes to the increase of the scour depth.     

海底管线局部冲刷机理研究综述

局部冲刷造成的悬空是海底管线失效的最大诱发因素。对均流及波浪作用下铺设在海床表面管线的局部冲刷起动机理、临界条件以及2D局部冲刷机理、流—管—土耦合机理和3D局部冲刷机理等做了分析和总结。     

泥沙突变理论在海底管线冲刷中应用

基于泥沙突变理论,针对海底管线冲刷下泥沙的运动特征,建立恒定流作用下的泥沙起动模式,确定希尔兹数、无量纲参数、冲刷坑深度之间相互作用的非线性方程,推导了恒定流作用下海底管线冲刷坑深度的预测公式。将相同条件下该公式的计算结果与前人的试验资料进行了对比,可发现尽管计算结果存在一定的误差,但也基本能满足对冲刷坑深度的预测和判断,从而证明了泥沙突变模型在预测海底管线冲刷坑深度中的适用性。     

应用切割单元法对海底管道局部冲刷数值模拟

采用SIMPLEC算法的有限体积法,求解非定常流动的N-S方程,采用k-ω紊流模型,通过模拟均匀无粘性推移质的冲刷和淤积,得到海底管道由搁置在海床上的状态变为悬跨状态这一过程的管道周围局部海床冲刷情况,建立了海底管道局部冲刷的二维数值模型。海底管道从搁置在底床上到冲刷悬空的过程中,管道周围的空间产生了拓扑变化,这给采用贴体网格并在计算过程中进行网格重构的传统方法带来了很大困难,而采用切割单元法,把物体轮廓从静止的背景直角坐标结构化网格中切割出去,计算过程中不需要传统意义的网格重构过程。数值模型预测的海底管道周围局部冲刷结果与Mao的物理模型实验实测结果及Liang和Cheng等的数值模拟结果符合较好,验证了模型的准确性。     

3D scour below pipelines under waves and combined waves and currents

This paper presents the results of an experimental investigation on three-dimensional local scour below a rigid pipeline subjected to wave only and combined wave and current conditions. The tests were conducted in a conventional wave flume. The major emphasis of the investigation was on the scour propagation speed (free span expansion rate) along the pipeline after local scour was initiated at a controlled location. The effects of flow ratio (steady current velocity vs. combined waves/current velocity), flow incidence angle and pipeline initial embedment depth on free span expansion rate were investigated. It was observed that the scour along the pipeline propagated at a constant rate under wave only conditions. The scour propagation rate decreased with increasing embedment depth, however, increased with the increasing Keuglegan–Carpenter (KC) number. Under combined wave and current conditions, the effect of velocity ratio on scour propagation velocity along the pipeline was quantified. Empirical relationships between the scour propagation rate (V_h) and key parameters such as the KC number and embedment depth (e/D) were established based on the testing results.     

Investigation on the Local Scour Beneath Piggyback Pipelines Under Clear-Water Conditions

A piggyback pipeline consists of two pipes such that the secondary line rides on the main pipe with a fixed distance between two pipes in length. The novel strategy is utilized in offshore areas instead of a single flow line. In this regard, there are only a handful of experimental and numerical studies investigating the effect of scour below a piggyback pipeline under steady current. Hence, this study focuses on examining the influential factors on scouring due to steady current including the pipe diameter and the gap between pipes through numerical simulations and experimental tests. Accordingly, at the first phase of the research, a single pipe was established and tested in laboratory to compare the results with those of an empirical equation. After finishing experimental verifications,piggyback pipelines were also assembled to study the scouring under steady current conditions. It was concluded that by increasing the gap distance between the pipes, the maximum scour depth decreases; however, an increase in the small pipe's diameter results in a larger maximum scour depth. Secondly, numerical simulations were carried out using the FLOW-3D software which was found to be a suitable tool for the numerical investigation of this study.Finally, the numerical results have been compared with the corresponding experimental data and a relatively good agreement was achieved between them.     

Dynamic interaction between two parallel submarine pipelines considered vortex-induced vibration and local scour

Abstract

The mechanism of local scour under two vibrating pipelines is investigated numerically in this research. A sediment scour model is adopted to estimate the motion of sediment. The general moving objects model, which is dynamically coupled with fluid flow, is set up to simulate the vortex-induced vibration (VIV) of the pipeline. The sediment scour model and pipeline vibration model are verified with the previous experimental results and show good agreement. Then, the coupling effects between the pipeline vibration and the local scour are investigated numerically. The effects of G/D (the ratio of the distance between the two pipelines to the diameter of the pipelines) on the local scour and the VIV of the pipeline are examined. The results indicate that the maximum scour depth under the vibrating pipelines is much larger than the scour depth under the fixed pipelines. Due to the shadowing effect of the upstream pipeline, the maximum scour depth under the upstream pipeline is deeper than that under the downstream pipeline. The pipeline vibration magnitude is closely related to the strength of the vortex that sheds behind the pipeline. The effect of G/D on the shape and strength of the vortices that shed behind the pipelines is significant.     

Mechanism of local scour around submarine pipelines based on numerical simulation of turbulence model

1 IntroductionSubmarine pipelines are important ocean engi-neeringequipm ents, especiallyfortheoiland gasin-dustries. M anyresearchershavebeendedicatedtotheproblem of local scour around the submarinepipelines, butmostoftheirwork arelaboratory testsand foc…     

Wave pressures and uplift forces on and scour around submarine pipeline in clayey soil

Experimental investigations are carried out on wave-induced pressures and uplift forces on a submarine pipeline (exposed, half buried and fully buried) in clayey soil of different consistency index both in regular and random waves. A study on scour under the pipeline resting on the clay bed is also carried out. It is found that the uplift force can be reduced by about 70%, if the pipeline is just buried in clay soil. The equilibrium scour depth below the pipeline is estimated as 42% of the pipe diameter for consistency index of 0.17 and is 34% of the pipe diameter for consistency index of 0.23. The results of the present investigations are compared with the results on sandy soil by Cheng and Liu (Appl. Ocean Res., 8(1986) 22) to acknowledge the benefit of cohesive soil in reducing the high pore pressure on buried pipeline compared to cohesionless soil.