3D computational modelling of subsea pipeline-soil interaction - for application in the design of controlled lateral buckling

背景在海洋石油和天然气工业中，海底管道是开采、加工和运输碳氢化合物产品的基础设施的重要组成部分。这些管道经常需要在相对于周围海底环境的高压和高温(HPHT)下运行。由此产生的轴向膨胀趋势，再加上来自海底土壤的摩擦约束，导致巨大的压缩力累积。由于管道是相对细长的结构元件，它们容易发生杆件屈曲：对于埋地管道，要么是隆起屈曲，要么是底部管道(直接铺设在海床上)的横向屈曲。目前，只有后一种方法在经济上和技术上在深水可行。事实上，对于在120摄氏度以上高温高压下运行的管道来说，受控侧向屈曲是唯一实用的解决方案。对于底部管道，侧向屈曲几乎是不可避免的，设计目标是控制屈曲过程。这是通过使用扳机(如蛇形、枕木、浮力模块)在预定位置启动扣件来实现的，以确保预测的横向位移、弯曲应力和(通常)塑性应变不会过大。海底管道的运行涉及多次关闭和重新启动，因此侧向屈曲是管道在海床上来回移动的循环过程。这种运动主要垂直于管道轴线，由于涉及的幅度很大--通常至少有10或20个管道直径--近地表海底材料有严重的塑性变形和重塑，土壤被犁入管道前面的土丘或护堤(图1)。这种行为的直接数值模拟(例如使用有限元分析)仍然具有极大的挑战性，在实践中使用了大大简化的管-土相互作用模型。管道在横向移动时也倾向于垂直移动，这两个自由度之间存在着特别复杂的耦合。最后，循环轴向运动是一个重要组成部分，因为相邻长度的管道在重新启动时进入扣环，并在关闭时送出。工业合作伙伴Crondall Energy-Backback Crondall Energy是一家专家咨询公司，在浮式生产、海底技术、流量保证、管道工程和海底系统设计方面提供独立的技术服务。Crondall Energy在全球拥有约50名工程师，其中包括在阿伯丁和纽卡斯尔工作的12名海底和管道工程师。我们的客户主要是大型石油和天然气运营公司，如BP、雪佛龙、壳牌、道达尔和伍德赛德。

BackgroundIn the offshore oil and gas industry, subsea pipelines are vital components of the infrastructure for extracting, processing and transporting hydrocarbon products. These pipelines are frequently required to operate at high pressure and high temperature (HPHT) relative to the ambient subsea environment. The resulting tendency for axial expansion, combined with frictional restraint from the seabed soil, causes large compressive forces to build up. Since pipelines are relatively slender structural elements, they are susceptible to bar buckling: either upheaval buckling in the case of a buried pipe, or lateral buckling in the case of an on-bottom pipe (laid directly on the seabed). At present, only the latter approach is economically and technically feasible in deep water. Indeed controlled lateral buckling is the only practical solution for pipelines operating at HPHT temperatures, above 120 Celsius.For on-bottom pipelines, lateral buckling is virtually inevitable, and the design objective is to control the buckling process. This is done by initiating buckles in pre-determined locations using triggers (e.g. snake-lay, sleepers, buoyancy modules) to ensure that the predicted lateral displacements, bending stresses and (usually) plastic strains, are not excessive. Operation of a subsea pipeline involves numerous shutdowns and restarts, so lateral buckling is a cyclic process in which the pipe moves back and forth across the seabed. This motion is predominantly normal to the pipeline axis, and because the amplitudes involved are large - typically at least 10 or 20 pipe diameters - there is severe plastic distortion and remoulding of the near-surface seabed material, with soil being ploughed into mounds or 'berms' in front of the pipe (Fig. 1). Direct numerical simulation of this behaviour (e.g. using finite element analysis) remains extremely challenging, and in practice greatly simplified pipe-soil interaction models are used. The pipe also tends to move vertically while it moves laterally, and there is a particularly complex coupling between these two degrees of freedom. Finally, there is a significant component of cyclic axial movement as adjacent lengths of pipe feed into a buckle during restarts, and feed out during shutdowns.Industrial partnerCrondall Energy - backgroundCrondall Energy is an expert consultancy, providing independent technical services in floating production, subsea technology, flow assurance, pipeline engineering and subsea system design. Crondall Energy employ about fifty engineers worldwide, including twelve Subsea and Pipeline Engineers based in Aberdeen and Newcastle. Our clients are predominantly the major oil and gas operating companies such as BP, Chevron, Shell, Total, and Woodside.