海底多跨柔性管线涡激振动与冲刷耦合作用机理研究

The large free-span and vortex-induced vibration (VIV) of multi-span submarine flexible pipeline under the complex seabed condition and strong hydrodynamic load are the main factors causing damage of pipeline damage, but the coupling mechanism of the above processes still requires to be clarified. The project will establish a sediment transport model based on Lattice Boltzmann method to study the structure of three-dimensional flow field and pattern of seabed evolution during the scour propagation process of multi-span flexible pipeline, and to determine the critical conditions for clean water and living-bed scour, and to analyze the condition for the onset of large free-span; the project will put forward a numerical method for fluid-structure coupled simulation to investigate the influence of the free span morphology, the properties of pipeline structure and the pipeline-soil contacting on the pattern, lock-in frequency range and amplitude variation of VIV, and to analyze the dangerous area of pipeline due to stress concentration and the stability of the span shoulder, so that to find out the influencing factors for VIV interaction among multi-spans; the project will establish a flow-pipeline-soil coupled numerical model to simulate the coupling dynamic process of VIV and scour propagation under strong hydrodynamic load, to analyse the variation of VIV amplitude and frequency along with the scour propagation and the influence of VIV on the scouring process and balanced scour profile, so as to reveal the coupling mechanism between them. The proposed numerical method will be verified by physical model test. The research outputs will provide useful analytical tools and theoretical basis for the engineering design and scientific research of deep-sea flexible pipeline.

多跨柔性管线在复杂海底地貌和强水动力作用下出现的长距离悬跨和涡激振动，是引起管线破坏的主要因素，两者的耦合作用机理亟待明确。项目基于格子玻尔兹曼方法建立沉积物输运模型，分析多跨柔性管线冲刷扩展过程中的三维流场结构和海床演变规律，得出长距离悬跨的形成机理；提出多跨柔性管线流固耦合计算方法，研究悬跨形态、管土接触性质和管线结构参数对涡激振动模式、频率锁定范围和振幅变化等特性的影响，分析管线的应力危险区和跨肩土的稳定性，归纳柔性管线涡激振动多跨间发生相互作用的影响因素；建立流管土耦合数值模型，研究多跨柔性管线在强水动力作用下涡激振动与局部冲刷的耦合动力过程，分析涡激振动的振幅和频率随冲刷扩展的变化特性以及涡激振动对冲刷过程和平衡剖面的影响规律，揭示两者的耦合作用机理；提出的数值计算方法将通过物理模型试验进行验证。项目研究成果将为深海柔性管线的工程设计和科学研究提供分析工具和理论基础。

本项目以数值模拟为主要手段，深入探究多跨柔性管线的水动力特性，进行了光滑及粗糙海床在波浪作用下的边界层转捩机理研究，探究了在不同类型的有限振幅扰动下振荡边界层中不稳定性的非模态增长，分析了在转捩过程中两种不同拟序结构的出现机理、发展过程及其相互作用，揭示了波浪边界层的层流-湍流旁路转捩机理和周期性自我维持机制，明确了湍流结构的时空特性及统计学规律，为进一步研究波流环境荷载作用下海床发生冲刷的临界条件提供了理论基础。本项目还研究了不同流态下单自由度和双自由度圆柱振幅、振动频率、振动偏心距、升力系数及阻力系数等振动特性，分析了单自由度下圆柱前方水跃现象对圆柱垂向振动特性的影响规律、圆柱涡脱落与自由液面耦合作用规律、双自由度圆柱振动轨迹及其垂向振动在流向自由度作用下的变化规律，对于防止海洋环境下海床发生冲刷时管道疲劳破坏有着重要意义。本项目通过实验手段分析了不同管道布置，管道埋深、间距和流速对在复杂水动力环境下多跨柔性管线冲刷特性的影响，充分认识了海底管道的局部冲刷机制，对于深海柔性管线设计以及破坏的预测和防治具有重要的研究和科学意义。