Scour depth under pipelines placed on weakly cohesive soils

We here study the scouring processes that evolve around a submarine pipeline placed on a weakly cohesive seabed. We first analyze some laboratory tests carried out by Vijaya Kumar et al. [21], Xu et al. [25] and Zhou et al. [28] that focused on the scouring around a horizontal cylinder lying on a cohesive bed, subject to waves and currents. The specific purpose is that of finding a new formula for the prediction of the equilibrium scour depth under submarine pipelines. After a theoretical analysis of the main parameters, the sought formula has been found to be a function of: (i) the hydrodynamic forces acting on the cylinder (through the Keulegan–Carpenter parameter KC), (ii) the clay content of the soil C_c, and (iii) the burial depth e₀/D. In the presence of small amounts of clay (C_c< 5 %), the scour depth depends directly on KC (as confirmed by many literature works for pipelines lying on sandy soils, e.g. [18]) and inversely on C_c (as already seen for bridge abutments on cohesive soils, e.g. [1]), the best-fit law being characterized by a coefficient of determination R² = 0.62. If some burial depth is accounted for, this being a novelty of the present work, a more general formulation can be used, valid in the presence of weakly-cohesive soils and with burial depths of the pipe smaller than 0.5 (R² = 0.79). For large clay-content ranges (2% < C_c < 75 %), the scour depth depends directly on both KC and C_c, this giving R² = 0.79 (no burial depth) and 0.91 (some burial depth). However, this finding is at odds with the main literature, because, for large amounts of clay, it is fundamental to consider the liquidity index LI, which accounts for some important clay properties, like the plasticity. We argue that the absence of LI is balanced by the direct dependence of the scour depth on C_c. Notwithstanding the small number of available data, a formula for the prediction of the scour depth under pipelines lying on cohesive soils is fundamental for several engineering applications. The present contribution represents the first attempt to build such a formula, when the pipeline is subject to the wave-current forcing and the seabed is characterized by a relatively small clay content.     

水平定向钻进铺管孔／管直径比设计取值方法

孔／管直径比是水平定向钻进铺管工程的重要基础设计参数,其取值大小直接关系到工程的成败、效率、安全和铺设后的管道运行安全。采用力学分析方法建立的回拖阻力数值模型及开发的专用软件,可以方便快捷地对孔管直径比取值进行精确的量化评价,并能够较好地解决工程设计中的一些复杂的数值计算问题,具有良好的推广应用前景。     

石亭江水平定向钻穿越施工技术浅析

文章主要阐述了兰成渝输油管道石亭江定向穿越施工技术方法和过程中遇到的问题,其中取得的经验和教训对今后采用何种方法穿越大粒径、大范围、大厚度的卵石地层具有一定的借鉴意义。     

爆管法在北美地区的应用现状

随着市政部门不断寻求节约成本的创新技术来解决地下基础设施问题,非开挖管线更换技术诸如爆管法成为可行的选择。爆管法是一种用相等或更大口径的新管更换现有地下供水给水、污水排水和燃气输送管线的施工方法。它是唯一一种完全沿旧管线线路铺设的施工方法。这使得安装新管线施工时无须征求额外通行权。爆管法兴起于20纪80年代,现在已遍布北美地区。本文通过对来自2007—2010年加拿大和美国的886个项目的调查问卷结果的总结,讨论了目前非开挖更管技术的趋势,分析了爆管法施工中的因素：包括设备的使用、采购与支付的形式、管线扩径的限制和施工的风险。     

Experimental investigation of hydrate formation in water-dominated pipeline and its influential factors

Blockage in water-dominated flow pipelines due to hydrate reformation has been suggested as a potential safety issue during the hydrate production. In this work, flow velocity-dependent hydrate formation features are investigated in a fluid circulation system with a total length of 39 m. A 9-m section pipe is transparent consisted of two complete rectangular loops. By means of pressurization with gas-saturated water, the system can gradually reach the equilibrium conditions. The result shows that the hydrates are delayed to appear as floccules or thin films covering the methane bubbles. When the circulation velocity is below 750 rpm, hydrate is finally deposited as a “hydrate bed” at upmost of inner wall, narrowing the flow channel of the pipeline. Nevertheless, no plugging is observed during all the experimental runs. The five stages of hydrate deposition are proposed based on the experimental results. It is also revealed that a higher driving pressure is needed at a lower flow rate. The driving force of hydrate formation from gas and water obtained by melting hydrate is higher than that from fresh water with no previous hydrate history. The authors hope that this work will be beneficial for the flow assurance of the following oceanic field hydrate recovery trials.©2022 China Geology Editorial Office.     

Onset of scour below pipelines and self-burial

This paper summarizes the results of an experimental study on the onset of scour below and self-burial of pipelines in currents/waves. Pressure was measured on the surface of a slightly buried pipe at two points, one at the upstream side and the other at the downstream side of the pipe, both in the sand bed. The latter enabled the pressure gradient (which drives a seepage flow underneath the pipe) to be calculated. The results indicated that the excessive seepage flow and the resulting piping are the major factor to cause the onset of scour below the pipeline. The onset of scour occurred always locally (but not along the length of the pipeline as a two-dimensional process). The critical condition corresponding to the onset of scour was determined both in the case of currents and in the case of waves. Once the scour breaks out, it will propagate along the length of the pipeline, scour holes being interrupted with stretches of soil (span shoulders) supporting the pipeline. As the span shoulder gets shorter and shorter, more and more weight of the pipeline is exerted on the soil. In this process, a critical point is reached where the bearing capacity of the soil is exceeded (general shear failure). At this point, the pipe begins to sink at the span shoulder (self-burial). It was found that the self-burial depth is governed mainly by the Keulegan–Carpenter number. The time scale of the self-burial process, on the other hand, is governed by the Keulegan–Carpenter number and the Shields parameter. Diagrams are given for the self-burial depth and the time scale of the self-burial process.     

非开挖工地的地下管线调查与探测

本文概述了探测地下管线的基本方法、地下管线探测仪的选择及地球物理探测方法的选择,介绍了在非开挖工地调查探测地下管线的步骤和方法。     

地下管网数据的动态更新技术研究

结合城市地下管网数据的现状,建立地下管网数据的时空数据结构;针对较少数据量的竣工测量数据和较大数据量的普查数据分别采用了基于要素和基于范围的更新方法,同时建立起数据的历史回溯机制,实现任意时间点数据的查询展示。实实在在解决了地下管网数据的现势性以及历史数据维护问题。     

Pipe–soil interaction and pipeline performance under strike–slip fault movements

The performance of pipelines subjected to permanent strike–slip fault movement is investigated by combining detailed numerical simulations and closed-form solutions. First a closed-form solution for the force–displacement relationship of a buried pipeline subjected to tension is presented for pipelines of finite and infinite lengths. Subsequently the solution is used in the form of nonlinear springs at the two ends of the pipeline in a refined finite element model, allowing an efficient nonlinear analysis of the pipe–soil system at large strike–slip fault movements. The analysis accounts for large strains, inelastic material behavior of the pipeline and the surrounding soil, as well as contact and friction conditions on the soil–pipe interface. The numerical models consider infinite and finite length of the pipeline corresponding to various angles β between the pipeline axis and the normal to the fault plane. Using the proposed closed-form nonlinear force–displacement relationship for buried pipelines of finite and infinite length, axial strains are in excellent agreement with results obtained from detailed finite element models that employ beam elements and distributed springs along the pipeline length. Appropriate performance criteria of the steel pipeline are adopted and monitored throughout the analysis. It is shown that the end conditions of the pipeline have a significant influence on pipeline performance. For a strike–slip fault normal to the pipeline axis, local buckling occurs at relatively small fault displacements. As the angle between the fault normal and the pipeline axis increases, local buckling can be avoided due to longitudinal stretching, but the pipeline may fail due to excessive axial tensile strains or cross sectional flattening. Finally a simplified analytical model introduced elsewhere, is enhanced to account for end effects and illustrates the formation of local buckling for relative small values of crossing angle.     

Analysis of buried pipelines subjected to reverse fault motion

Presently available simplified analytical methods and semi-empirical methods for the analysis of buried pipelines subjected to fault motion are suitable only for the strike-slip and the normal-slip type fault motions, and cannot be used for the reverse fault crossing case. A simple finite element model, which uses beam elements for the pipeline and discrete nonlinear springs for the soil, has been proposed to analyse buried pipeline subjected to reverse fault motion. The material nonlinearities associated with pipe-material and soil, and geometric nonlinearity associated with large deformations were incorporated in the analysis. Complex reverse fault motion was simulated using suitable constraints between pipe-nodes and ground ends of the soil spring. Results of the parametric study suggest that the pipeline's capacity to accommodate reverse fault offset can be increased significantly by choosing a near-parallel orientation in plan with respect to the fault line. Further improvement in the response of the pipeline is possible by adopting loose backfill, smooth and hard surface coating, and shallow burial depth in the fault crossing region. For normal or near normal orientations, pipeline is expected to fail due to beam buckling at very small fault offsets.