Numerical analysis of cross-section ovalization in the deep-sea pipeline lateral buckling process

Abstract

The deep-water pipeline is the main means of transportation in offshore oil and gas development engineering. The deep-water pipeline may incur lateral global buckling due to the high temperature and pressure that are applied on the pipeline to ensure the contents’ liquidity. With the increasing operating water depth, a higher temperature and pressure are applied to the pipeline, causing large lateral deformation and a large bending moment. Due to the inhomogeneous distribution of the bending moment on the cross-section, different points on the cross-section will deform differently. This kind of deformation causes the cross-section to turn into an oval ring. The cross-section ovalization caused by global buckling was rarely analyzed in former engineering practice since the load is relatively low. With the increase in operation water depth and operation load, the ovality caused by global buckling is noticeable. This article analyzed cross-section ovalization caused by pipeline lateral global buckling with a numerical simulation method. The pipelines with different initial cross-section shapes were simulated, and the influence of several impact factors, including load, pipeline and soil factors on the ovality of the cross-section, were analyzed. The results show that the initial cross-section shape type has little effect on the pipeline ovalization pattern. The initial ovality of the pipeline with an oval ring cross-section shape has little influence on the residual ovality. Among all the factors analyzed in this paper, the pressure difference is the primary factor that should be considered in a pipeline ovalization check.     

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.     

海底管道整体屈曲的枕木-浮力耦合法研究

不埋海底管道在高温高压作用下,易发生水平向整体屈曲。实际工程中,常通过在管道路由上设置整体屈曲触发装置,实现对水平向整体屈曲的有效控制,其中以枕木法的成功应用最为多见。本文分析了枕木法的主要影响因素并验证了采用枕木法会出现管道屈曲段应力集中的现象,对比了枕木法、分布浮力法和枕木-浮力耦合法对管道整体屈曲变形规律的影响,采用数值模拟方法系统研究了枕木及浮力参数对管道水平向屈曲和后屈曲的影响规律。研究表明,在枕木两侧设置浮力段的人工触发装置可有效触发管道整体屈曲,同时促使管道虚拟锚固点间轴力的释放,降低了管道中屈曲段的应力,相较枕木法,枕木-浮力耦合法可将管道中的最大应力降低23%。     

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。     

波流共同作用下海底子母管线水动力的物理模型试验研究

通过物理模型试验研究海底子母管线分别在规则波加流和不规则波加流作用下的水动力特性。基于Morison方程,采用"等效直径法"分析得到子母管线拖曳力系数C_D,惯性力系数C_M和升力系数C_L(C_D+,C_L-)。试验分别考察了流速比U_c/U_w,母管与海床间隙比e/D及子母管间的相对缝隙G/D对海底子母管线水动力系数的影响。结果表明水动力系数随U_c/U_w的增大而减小;当e/D<0.5时,海床对子母管线受力的影响比较明显,C_D,C_M及C_L+均随e/D增大而减小,|C_L-|随e/D增大而增大;对子母管间的相互影响也不可忽略,C_D,C_M和|C_L-|均随G/D增大而减小,C_L+值随G/D增大而增大。     

波流共同作用下海底子母管线水动力的物理模型试验研究

通过物理模型试验研究海底子母管线分别在规则波加流和不规则波加流作用下的水动力特性。基于Morison方程,采用"等效直径法"分析得到子母管线拖曳力系数C_D,惯性力系数C_M和升力系数C_L(C_D⁺,C_L^-)。试验分别考察了流速比U_c/U_w,母管与海床间隙比e/D及子母管间的相对缝隙G/D对海底子母管线水动力系数的影响。结果表明水动力系数随U_c/U_w的增大而减小;当e/D<0.5时,海床对子母管线受力的影响比较明显,C_D,C_M及C_L⁺均随e/D增大而减小,|C_L^-|随e/D增大而增大;对子母管间的相互影响也不可忽略,C_D,C_M和|C_L^-|均随G/D增大而减小,C_L⁺值随G/D增大而增大。     

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。     

双拱初始缺陷海底管线水平向整体屈曲数值模拟分析

为了研究具有双拱反对称初始缺陷海底管线的整体屈曲特性,采用模态分析法将最可能出现的缺陷形态引入数值分析模型中。针对管线在高温高压作用下发生整体屈曲的动态变形特征,运用显式动力数值模拟方法研究了管线整体屈曲过程中水平向变形与轴向变形随温度和内压的变化规律,建立了在整体屈曲过程中屈曲管段与滑动管段轴力的变化过程与初始缺陷形态的关系。将数值模拟结果同经典解析解和室内模型实验结果进行对比,验证了本方法的可靠性。工程算例的分析结果表明,管线整体屈曲的发生是一个由低阶向高阶发展的过程,具有双拱缺陷的管线首先发生二阶模态的整体屈曲,而后过渡到四阶模态;管线整体屈曲的变形包括屈曲段的水平向变形和滑动管段的轴向缩进,其中水平变形释放了管壁内的轴力,轴力的释放量随初始缺陷尖锐程度的降低而增大;轴向缩进变形由于受到地基土的摩阻力使滑动管段内的轴力发生累积,轴力的累积量随初始缺陷的尖锐程度的降低而增加。以上研究成果对指导实际工程具有现实意义。     

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.     

Current-induced scour beneath initially elevated subsea pipelines

When a subsea pipeline is laid on an uneven seabed, certain sections may have an initial elevation with respect to the far-field seabed, e_o, and thus potentially affecting the on-bottom stability of the pipeline. This paper focuses on quantifying the effects of the upstream dimensionless seabed shear stress, θ_∞, and Reynolds number, Re, on: (1) the maximum dimensionless seabed shear stress beneath the pipe, θ_max, to be compared to the critical shear stress in order to determine whether scour would occur and progress towards an equilibrium state; and, (2) the dimensionless equilibrium scour depth beneath the pipe, S_eq/D. Using a 2-D Reynolds averaged Navier-Stokes (RANS) approach along with the k-ω Shear Stress Transport (SST) turbulence model, a parametric study involving 243 computational fluid dynamics (CFD) simulations was conducted. The simulation results were used to develop a closed-form equation for the prediction of θ_max. Subsequently, experimental measurements of S_eq/D have been compiled from published literature, to develop a new closed-form equation for the prediction of S_eq/D with a high correlation to the experimental data. In summary, we present two closed-form equations for the prediction of θ_max and S_eq/D for pipelines with an initial e_o/D, which are applicable for both clear-water and live-bed conditions. The effects of θ_∞ and Re have been included, albeit Re having a small influence as compared to the other parameters.     

海底管线整体屈曲过程中塑性变形研究

海底管线是海洋油气工程中主要的输送手段。在工作状态下,受高温高压的影响,深海管线可能会发生水平向整体屈曲。随海洋油气作业水深的增大,施加于管线的温度和压强也逐渐增加,导致管线产生较大的屈曲位移和截面应力,使得截面产生塑性应变。本文采用数值模拟方法,对海底管线整体屈曲过程中塑性区的分布及其与整体屈曲影响因素的关联性、塑性变形对水平向变形的影响和塑性变形造成的截面椭圆度的变化规律进行分析,研究塑性变形对整体屈曲过程影响的规律。     

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.     

An experimental study of seabed responses around a marine pipeline under wave and current conditions

Experiments on three types of soil (d₅₀=0.287, 0.057 and 0.034 mm) with pipeline(D=4 cm) either half buried or resting on the seabed under regular wave or combined with current actions were conducted in a large wave flume to investigate characteristics of soil responses. The pore pressures were measured through the soil depth and across the pipeline. When pipeline is present the measured pore pressures in sandy soil nearby the pipeline deviate considerably from that predicted by the poro-elasticity theory. The buried pipeline seems to provide a degree of resistance to soil liquefaction in the two finer soil seabeds. In the silt bed, a negative power relationship was found between maximum values of excess pore pressure p_max and test intervals under the same wave conditions due to soil densification and dissipation of the pore pressure. In the case of wave combined with current, pore pressures in sandy soil show slightly decrease with time, whereas in silt soil, the current causes an increase in the excess pore pressure build-up, especially at the deeper depth. Comparing liquefaction depth with scour depth underneath the pipeline indicates that the occurrence of liquefaction is accompanied with larger scour depth under the same pipeline-bed configuration.     

Physical modeling and swirling strength analysis of vortex shedding from near-bed piggyback pipelines

The vortex shedding from near-bed piggyback pipelines in a steady flow has been investigated experimentally in a large water flume. A specially arranged PIV system with upward-illumination of pulsed laser arrays from the flume bottom was employed for the flow visualization and quantitative measurement of the lee-wake flow in a sub-critical regime around the piggyback pipelines in the proximity of a plane boundary. Based on dimensional analyses, a dimensionless maximum swirling strength (W_m) is used for analyzing the vortex shedding intensity and its frequency. Time-averaged swirling strength analyses indicate that the lee-wake patterns for the near-bed piggyback pipelines are dependent on the configuration factors, including the gap-to-diameter ratio (e/D), the spacing-to-diameter ratio (G/D), and the diameter ratio of two pipes (d/D), etc. The swirling strength in the lee-wake is obviously asymmetric for piggyback pipelines with bed proximity. For the fixed values of G/D and d/D, the maximum swirling strength decreases with the decrease of e/D. Moreover, for the examined G/D range (0 ≤ G/D ≤ 0.5), minimum values of W_m and corresponding VIV amplitude for the piggyback pipelines are evidently within the same range of spacing-to-diameter ratio G/D ≈ 0.05–0.20.     

Numerical analysis of the impact forces exerted by submarine landslides on pipelines

Submarine pipelines that transport crude oil and natural gas are often in a complex marine geological environment and may become unstable and fail upon impact by submarine landslides. Previous research has mostly focused on the impact forces exerted by submarine landslides on suspended pipelines, but the impact of submarine landslides on pipelines laid on the seafloor at various impact angles, θ, have been relatively infrequently discussed, and the effects of suspended height, H, on the impact forces exerted by submarine landslides on pipelines have not been thoroughly investigated. In this study, based on the Herschel–Bulkley model, the impact forces exerted by a submarine landslide on laid-on or suspended pipelines at various impact angles θ were simulated using the computational fluid dynamics (CFD) approach. Equations for calculating the axial and normal drag coefficients of a submarine pipeline were proposed. The CFD numerical simulation results were rearranged based on the soil mechanics approach. By comparing the parameters, an essentially corresponding relationship was found between the soil mechanics and CFD approaches when the equations were used to calculate the impact forces exerted by a submarine landslide on a pipeline. In addition, a semi-analytical expression for the failure envelope was provided. Furthermore, the effects of H on the forces on a pipeline were discussed, and an equation for calculating the acting forces on a pipeline along the flow direction of a submarine landslide that comprehensively accounts for the effects of θ and H was proposed. The lift force was discussed preliminarily and the results provide a basis for further investigation. The achievement of this study is applicable for selecting locations of submarine pipeline routes and for designing submarine pipelines.     

Numerical simulation of local scour around two pipelines in tandem using CFD–DEM method

In the field of offshore oil and gas engineering, the arrangement of multiple pipelines are becoming more common, the spacing between the pipelines and the incoming stream velocity will significantly affect the scouring process around the pipelines. In this study, the effect of space ratio (G/D) and the stream velocity on the scouring process around two pipelines in tandem are investigated using the coupled approach of computational fluid dynamics (CFD) and discrete element method (DEM). Here G is the spacing between the pipelines and D is the diameter of the pipeline. Specifically, the effect of space ratio and the stream velocity are discussed by simulating the gap ratio (G/D) between two pipelines ranging from 1 to 3 with an interval of 1, under the stream velocity U = 0.5,1 and 2 m/s, The results indicate that when G/D ≤ 2, the equilibrium scour depth below the upstream pipeline (S1) is slightly larger than that under the downstream pipeline (S2), S1 and S2 slightly increase as the gap ratio increases. Whereas for G/D > 2, the equilibrium scour depth beneath the upstream pipeline is slightly smaller than that under the downstream pipeline, S1 and S2 slightly decrease as the gap ratio increases. Furthermore, the scour depths are highly dependent on and positively related to the incoming stream velocity, the equilibrium bed profiles are similar under the same incident stream velocity with different gap ratios.     

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。     

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.     

Parametric study for underwater blast-induced pipeline response embedded in marine sediments

Abstract

Blast response of submerged pipelines has been a research focus in recent years. In this article, a three-dimensional numerical model is established to investigate dynamic response of pipelines due to underwater explosion. The u–p approximation is integrated into finite element method (FEM) to simulate pore water effect in the seabed. Numerical continuity between hydraulic pressure in the flow field and pore pressure in the marine sediment is guaranteed to realize the blast response of submerged pipelines in ocean environment. Both fluid–structure interaction (FSI) and pipeline–seabed interaction (PSI) have been considered in the proposed model simultaneously. A comprehensive parametric study is carried out after validation of the present model with test data from underground explosion and underwater explosion, respectively. The effect of embedment depth, TNT equivalent, stand-off distance, pipeline diameter, and pipeline thickness to blast response of the submerged pipelines is investigated based on numerical results. Variation of deformation patterns and stress distribution of the pipeline with various installation and structure parameters has been illustrated and discussed to facilitate engineering practice.     

How storm size matters for surge height

The observed trend of peak storm surge η_max increasing with storm size R_max, roughly as η_max ∞ R_max^0.22, particularly on gently sloping coasts, is discussed in relation to the simple 1D analytical solutions for forced long waves due to respectively surface pressure p_s and wind stress τ_w. At constant depth h, the τ_w-driven surge is proportional to storm size while the p_s-driven part is not. This could perhaps be seen to explain why the size-dependence is stronger on flatter slopes where the τ_w-driven surge dominates. However, this direct size dependence disappears in the sloping beach scenario if the typical depth is assumed proportional to storm size. The observed size dependence is then more likely due to a combination of two 2D effects: Firstly, the sideways radiation from a travelling surge which exceeds the “stationary height” Δ_p/ρg is relatively weaker for a wider system. Secondly, the wind stress field is a dipole, and the mutual cancellation of the two poles is weaker for larger systems.