管道热屈曲动力过程数值模拟

运输高温高压油气的海底管道会发生整体热屈曲现象。管道热屈曲过程中可能会产生平衡状态的跃迁(snapthrough),且这样的跃迁过程必然会伴随着动力响应。管道热屈曲动力过程中侧向弹出的速度以及轴向缩进的速度对管土相互作用参数的取值有很大影响,然而关于管道热屈曲动力过程的研究却很少。本文给出了数值模拟过程中管道系统阻尼值和升温速率的确定方法,研究了管道初始几何缺陷以及海床参数对管道热屈曲动力过程的影响。     

往复荷载作用下软粘土中浅埋管道的稳定性的振动台模型试验研究

软粘土在波浪荷载作用下,可能发生强度软化,使埋设在软粘土层中的海底管道的受力状态或支撑条件发生变化,从而出现上浮或下沉的趋势,对管道的稳定性发生影响。本文通过在振动台上进行的模型试验来模拟当土性因动荷作用而发生变化时管道的稳定情况,对这个问题进行了探索,得出了一些规律性的认识,可供海底管道的设计者参考。     

Model test based soil spring model and application in pipeline thermal buckling analysis

The buckling of submarine pipelines may occur due to the action of axial soil frictional force caused by relative movement of soil and pipeline, which is induced by the thermal and internal pressure. The likelihood of occurrence of this buckling phenomenon is largely determined by soil resistance. A series of large-scale model tests were carried out to facilitate the establishment of substantial data base for a variety of burial pipeline relationships. Based on the test data, nonlinear soil spring can be adopted to simulate the soil behavior during the pipeline movement. For uplift resistance, an ideal elasticity plasticity model is recommended in the case of H/D (depth-to-diameter ratio)>5 and an elasticity softened model is recommended in the case of H/D≤5. The soil resistance along the pipeline axial direction can be simulated by an ideal elasticity plasticity model. The numerical analyzing results show that the capacity of pipeline against thermal buckling decreases with its initial imperfection enlargement and increases with the burial depth enhancement.     

浮力装置触发深海管道水平向整体屈曲效果的数值模拟研究

海洋油气资源的运输主要通过海底管道进行,管道在工作时受到较大的温度荷载,会产生整体屈曲变形。深海管道设计中常采用人为装置触发一定程度的水平向整体屈曲变形,来释放轴向的温度应力,浮力装置是常用的触发方式之一。本文通过数值模拟研究,分析了不同浮力大小和不同浮力施加范围下,管道水平向整体屈曲的临界屈曲力,得出临界屈曲力随浮力大小和施加范围变化的情况;并研究了不同土体阻力下,浮力装置触发整体屈曲的效果。研究表明,水平向土体阻力较大时,浮力装置触发水平向整体屈曲的效果较好。浮力装置的触发效果对轴向土体阻力不敏感。     

Parametric study of the wave-induced residual liquefaction around an embedded pipeline

Wave-induced liquefaction in a porous seabed around submarine pipeline may cause catastrophic consequences such as large horizontal displacements of pipelines on the seabed, sinking or floatation of buried pipelines. Most previous studies in relation to the wave and seabed interactions with embedded pipeline dealt with the wave-induced instaneous seabed response and possible resulting momentary liquefaction (where the soil is liquefied instantaneously during the passage of a wave trough), using theory of poro-elasticity. Studies for the interactions between a buried pipeline and a soil undergoing build-up of pore pressure and residual liquefaction have been comparatively rare. In this paper, this complicated process was investigated by using a new developed integrated numerical model with RANS (Reynolds averaged Navier–Stokes) equations used for governing the incompressible flow in the wave field and Biot consolidation equations used for linking the solid–pore fluid interactions in a porous seabed with embedded pipeline. Regarding the wave-induced residual soil response, a two-dimensional poro-elastoplastic solution with the new definition of the source term was developed, where the pre-consolidation analysis of seabed foundation under gravitational forces including the body forces of a pipeline was incorporated. The proposed numerical model was verified with laboratory experiment to demonstrate its accuracy and effectiveness. The numerical results indicate that residual liquefaction is more likely to occur in the vicinity of the pipeline compared to that in the far-field. The inclusion of body forces of a pipeline in the pre-consolidation analysis of seabed foundation significantly affects the potential for residual liquefaction in the vicinity of the pipeline, especially for a shallow-embedded case. Parametric studies reveal that the gradients of maximum liquefaction depth with various wave and soil characteristics become steeper as pipeline burial depth decreases.     

Numerical Analysis of Seismic Dynamic Response of Saturated Porous Seabed Around a Buried Pipeline

The stability of submarine pipelines has been extensively studied by coastal engineers in recent years. Seismic-induced pore pressure and effective stresses in the saturated porous seabed and pipeline are the main important factors in the analysis of foundation stability around submarine pipelines. The majority research of the seismic-induced dynamic response around an offshore pipeline has been limited to two-dimension cases. In this paper, a three-dimensional finite element model including buried pipeline is established by extending DYNE3WAC. Based on the proposed numerical model, a parametric study is conducted to examine the effects of soil characteristics and pipeline configurations on the seismic-induced soil response around offshore pipelines.     

Lateral Global Buckling of Submarine Pipelines Based on the Model of Nonlinear Pipe–Soil Interaction

With the increasing development and utilization of offshore oil and gas resources, global buckling failures of pipelines subjected to high temperature and high pressure are becoming increasingly important. For unburied or semi-buried submarine pipelines, lateral global buckling represents the main form of global buckling. The pipe–soil interaction determines the deformation and stress distribution of buckling pipelines. In this paper, the nonlinear pipe–soil interaction model is introduced into the analysis of pipeline lateral global buckling, a coupling method of PSI elements and the modified RIKS algorithm is proposed to study the lateral global buckling of a pipeline, and the buckling characteristics of submarine pipeline with a single arch symmetric initial imperfection under different pipe–soil interaction models are studied. Research shows that, compared with the ideal elastic–plastic pipe–soil interaction model, when the DNV-RP-F109 model is adopted to simulate the lateral pipe–soil interactions in the lateral global buckling of a pipeline, the buckling amplitude increases, however, the critical buckling force and the initial buckling temperature difference decreases. In the DNV-RP-F109 pipe–soil interaction model, the maximum soil resistance, the residual soil resistance, and the displacement to reach the maximum soil resistance have significant effects on the analysis results of pipeline global buckling.     

海底滑坡作用下滩海管道结构安全分析

海底滑坡作为常见的海洋地质灾害,对海洋油气工程安全产生巨大威胁。海床土体失稳引起滑坡体滑动,会对海底管道产生拖曳作用。基于计算流体动力学方法(CFD)建立海底滑坡体对管道作用的评估模型,采用H-B模型描述块状滑坡体并与试验比较验证,分析不同海床倾斜度滑坡对管道的作用并拟合表达式;研究了海底管道在滑坡作用下的力学响应,并采用极限状态方法开展海底滑坡作用下管道结构极限安全分析,探讨了管道埋地状态时的极限安全界限,建立滑坡作用下管道结构安全分析方法。研究表明:滑坡对管道作用力与海床倾角呈现正相关,而覆土层厚度对作用力影响较小;随着不排水抗剪强度的减小,允许的滑坡宽度和速度均增加,表明土体不排水抗剪强度与引起的拖曳力呈正相关;滑坡土体宽度对极限安全速度影响较大。     

黄河水下三角洲海底管道沉降量确定方法研究

提出一种海底管道沉降计算方法——递推法，应用到胜利油田埕岛海域海底管道沉降计算中，并将计算结果与目前常用的日本规范法和极限法进行比较。研究发现在粉土与粉质粘土为主的强度较高的海床上，递推法与日本规范法计算结果比较接近，极限法的最小；在淤泥质土组成的强度较低的海床上，日本规范法的计算值最大，递推法的居中，极限法的最小。逆推法可计算裸置和埋设两种形式的管道沉降量，而另外两者只能计算裸置管道的沉降。文中进一步讨论了引起管道沉降的影响因素，土体强度和压缩性对管道沉降影响较大。且管道埋深越大，沉降也越大。最后分析了黄河水下三角洲埕岛海域4个区的管道沉降量，Ⅰ区争Ⅱ区管道沉降较小，可以忽略，Ⅳ区沉降较大，在淤泥质软土上的管道，可能会完全陷入土中。     

Upheaval buckling of surface-laid offshore pipeline

Offshore pipelines operating under high pressure and temperature are subjected to upheaval buckling. Pipeline behaviour in upheaval buckling depends on a number of factors including the shape of pipeline imperfection, installation stresses, loading types, seabed sediment behaviour and the flexural stiffness of the pipe. Current method of predicting upheaval buckling is based on simplified shapes of pipeline imperfection developed for idealized seabed conditions. To account for the effect of internal pressure, the pressure load is represented using an equivalent temperature. However, the applicability of these idealizations on the prediction of upheaval buckling has not been well-investigated. In this paper, the three-dimensional finite element modelling technique is used to investigate the applicability of idealized shapes and their effects on the upheaval buckling of pipeline for a seabed condition at offshore Newfoundland in Canada. The finite element model is then used to conduct a parametric study to investigate the effects of installation stress, loading types, seabed parameters and the flexural stiffness of the pipe. Finally, a design chart is developed to determine the optimum height of seabed features to manage pipeline stability against upheaval buckling under different temperature and pressure loadings.     

海底双层管单层连接管道结构受力分析

粘性高的海洋石油通常需要通过海底保温管道加温输送.温度变化会引起管道变形,并在管壁内产生较大的温度应力.同时,管道正常运营期间还受到管道内压、外压、管内流体粘滞力和土体摩擦力等环境荷载的作用.复杂的环境可能导致海底管道轴向应力过大发生破坏.为了提高铺管效率,提出了双层管单层连接管道这一特殊管道形式,并从理论上分析温度变化和环境荷载对该管道的影响,计算正常运行时管道不同位置处横截面内最大Von-Mises应力.最后得到了Von-Mises应力沿管道轴线分布情况,发现内管和单层连接管的应力一般比外管大,变径管和内管的焊缝处是Von-Mises应力最大的地方.     

Experimental and finite element study on lateral global buckling of pipe-in-pipe structure by active control method

Offshore oil and gas exploration are gradually heading toward the deep sea and even the ultra-deep sea. According, the working temperature and pressure intensity of subsea oil and gas pipelines have increased by a considerable degree. This situation is accompanied by the global buckling problem in deep sea pipelines, which has become increasingly common. Meanwhile, ordinary single-layer pipelines cannot last for a long time under harsh deep-sea working conditions. Thus, multilayer pipelines, such as the pipe-in-pipe (PIP) structure and bundled pipelines, have gradually become top choices. However, the global buckling mechanisms of these multilayer pipelines are more complicated than those of single-layer pipelines. The sleeper–snake lay pipeline, which is an active control method for global buckling, was used in this study. The change and development laws of global buckling in a PIP structure at different wavelengths and amplitudes were determined through an experimental study. A dynamic solution method that considers artificial damping was adopted to establish finite element global buckling models of a PIP structure with initial imperfections. The effects of various factors, such as pipeline laying shape, sleeper–pipe function, and seabed–pipe function, on global buckling were analyzed. By the result of finite element method analysis, the initial imperfection, and sleeper–pipeline friction were determined to be the key factors that influenced critical pipeline buckling force. Accordingly, a reference for the design of PIP structures is presented.     

强潮海湾Spoiler海底管道冲刷机理分析

本文基于历年检测资料统计分析了杭州湾海底输油管道裸露埋深状态变化规律,结合水文地形测量资料、辅以数学模型手段,从Spoiler自埋设计、杭州湾水流特性、管道路由海床演变特性以及人类活动等多因素探索了杭州湾海底管道裸露埋深状态变化的原因。研究结果表明:杭州湾管道80%以上处于埋深状态,局部管段长期裸露,裸露管段主要位于北岸深槽和庵东边滩滩坡区域;管道冲刷与管道路由海床演变特性、水流与管道夹角以及Spoiler自埋设计的发挥密不可分;杭州湾南岸围垦工程的实施与该区域管道裸露、掩埋状态密切相关。     

海底管线水平向整体屈曲及失效判断方法研究

海底管线是海洋石油的重要输运手段。为满足输送工艺的需要,正常工作条件下管线往往被施加较高的温度和压强,高温高压使管线内产生附加应力,当附加应力大于土体对管线的约束力时,管线就会发生整体屈曲。过度的水平向整体屈曲会导致截面产生较大的弯曲应力和压缩应变,对管线系统的安全运行造成威胁,因此需要对发生水平向整体屈曲后的管线进行验算。采用解析解法、规范法和有限元法对管线的整体屈曲进行分析,提出了应用临界屈曲荷载值域空间和值域下限来判断不同缺陷大小下管线是否发生水平向整体屈曲的方法。结合工程实例,分别采用内力控制标准和位移控制标准对管线水平向整体屈曲后是否失效进行了验算。研究指出,相较于位移标准,内力控制标准更为严格。     

Numerical modelling of pipe-soil interaction for marine pipelines in sandy seabed subjected to wave loadings

Accurate assessment of pipe-soil interaction under cyclic wave actions is of pronounced importance for the stability analysis of submarine pipelines in sandy seabed. This paper presents a plane-strain numerical study on such a problem using a finite element program DBLEAVES, which incorporates an elasto-plastic soil model that is capable of capturing the cyclic mobility behavior of sandy soils under cyclic loadings. A detailed validation against analytical solution and model test results was provided to demonstrate the robustness of the present numerical model to mimic both pre- and post-liquefaction behavior of sands, before an extensive parametric study was introduced. It was found that the accumulation of excess pore pressure in the vicinity and far field of a pipeline was strongly affected by the existence of it, with an influential range of about two pipe diameters. The influences of wave and seabed properties (e.g. relative densities) on the uplift response of pipelines were then investigated, based on which an explicit model was developed to quantify the degradation effect of waves on the uplift bearing capacity of pipelines against thermally-induced buckling.     

Allowable span length of submarine pipeline in shallow water

Because of the complex geological conditions of the seabed, submarine pipelines buried beneath the ocean floor become suspended over the seabed under the long-term scour of waves eroding the surrounding sediment. Further, most oil fields were built in offshore areas while the country was developing. This gives the waves seen in shallow water obvious nonlinear features, and the abnormal characteristics of these waves must be considered when calculating their hydrodynamic forces. Particularly under such conditions, these suspended spans of submarine pipelines are prone to damage caused by the action of the external environment load. Such damages and eventual failures may result not only in great property losses but also pollution of the marine environment. The span length of these areas is a key predictive factor in pipeline damages. Therefore, determining the allowable span length for these submarine pipelines will allow future projects to avoid or prevent damage from excessive suspended span lengths. Expressions of the hydrodynamic loads placed on suspended spans of pipeline were developed in this work based on the first-order approximate cnoidal wave theory and Morison equation. The formula for the allowable free span length was derived for the common forms of free spanning submarine pipeline based on the point where maximum bending stresses remain less than the material’s allowable stress. Finally, the allowable free span length of real-world pipelines was calculated for a subsea pipeline project in Bohai Bay. This research shows that, with consideration for the complicated marine environment, existing suspended spans are within allowable length limitations. However, continuing to limit the length of these submarine pipeline spans in the Nanpu oil field will require ongoing attention.     

Experimental investigation on the wave-induced pore pressure around shallowly embedded pipelines

A series of regular wave experiments have been done in a large-scale wave flume to investigate the wave-induced pore pressure around the submarine shallowly embedded pipelines.The model pipelines are buried in three kinds of soils,including gravel,sand and silt with different burial depth.The input waves change with height and period.The results show that the amplitudes of wave-induced pore pressure increase as the wave period increase,and decay from the surface to the bottom of seabed.Higher pore pressures are recorded at the pipeline top and the lower pore pressures at the bottom,especially in the sand seabed.The normalized pressure around pipeline decreases as the relative water depth,burial depth or scattering parameters increase.For the silt seabed,the wavelet transform has been successfully used to analyze the signals of wave-induced pore pressure,and the oscillatory and residual pore pressure can be extracted by wavelet analysis.Higher oscillatory pressures are recorded at the bottom and the lower pressures at the top of the pipeline.However,higher residual pressures are recorded at the top and the lower pressures at the bottom of the pipeline.     

海底管道测量声学剖面特征分析

海底管道的位置与埋深测量通常采用浅地层剖面仪等声学手段来实现,由于海管与地层之间的声阻抗差异,会以绕射弧的形态出现在声学剖面中。但当海管埋设于管沟中时,管沟中断棱的绕射与海管绕射易于混淆,给声学剖面图的解译和识别带来困难。基于地震勘探原理,结合浅地层剖面仪的性能、挖沟作业对地层的扰动等,分析了管沟绕射弧的类型与特点、挖沟作业扰动的声学特征等,提出了管沟中海管绕射弧的识别方法。结果表明,正确认识不同类型管沟的绕射、施工扰动产生的绕射等声学特征,才能辨识出海管的绕射现象。实际工作中应根据测量海区的水深、土质、海管属性等多种要素,选用适宜的仪器类型及测量参数。     

Subsea pipeline walking with velocity dependent seabed friction

With the increase in demand and supply gap in the oil and gas industry, new developments of oil and gasinfrastructure are moving into deeper water. This requires design and construction of long high temperature and high pressure pipelines from deep sea to shore. These pipelines are subjected to cyclic expansion during operating cycles. Accumulated axial movement due to repeated thermal cycles may lead to global displacement referred to as ‘walking’. Walking rates depend on the restraint associated with seabed friction. In conventional analyses, seabed friction is independent of the rate of thermal loading and expansion but it has been recognised that the sliding resistance between a pipe and the seabed varies with velocity, partly due to drainage effects. In this paper a numerical model is used to explore the effect of velocity-dependent seabed friction. A velocity-dependent friction model is implemented in commercial software ABAQUS and validated via single element and simple (flat seabed) pipeline cases. This model features upper and lower friction limits, with a transition that occurs as an exponential function of velocity. A parametric study is performed using differing rates of heating and cool-down in walking situations driven by seabed slope, SCR end tension and the difference between heat up and cool down rates. The walking behaviour is compared to cases with constant friction and solutions are proposed to express the velocity-dependent response in terms of an equivalent constant friction. These equivalent friction values can then be applied in existing simple solutions or more complex numerical analyses, as a short cut method to account for velocity-dependent friction.     

波浪作用下孔隙海床-管线动力相互作用分析

波浪作用下海床中的孔隙水压力与有效应力是影响海底管线稳定性的主要因素。然而,在目前的海床响应分析中一般将管线假定为刚性,并不能合理地考虑海床与管线之间的相互作用效应,同时也没有考虑土体和管线加速度对海床动力响应的惯性影响,从而无法确定由此所引起的管线内应力。为此考虑管线的柔性,分别采用饱和孔隙介质的Biot动力固结理论和弹性动力学理论列出了海床与管线的控制方程,进而采用摩擦接触理论考虑海床与管线之间的相互作用效应,基于有限元方法建立了海床-管线相互作用的计算模型及其数值算法。通过变动参数对比计算讨论了管线几何尺寸、海床土性参数对波浪所引起的管线周围海床孔隙水压力和管线内应力的影响。