深远海大长径比立管系统内外流耦合机理研究

The continuous global demand for natural resources has driven offshore exploration into deep-waters characterized by long distance from the mainland, great depth of water, severe and capricious weather, and thus to ensure the safety and integrity of deep-water equipment is the most critical concern in offshore operations. A marine riser is inherently an extensible and flexible tubular structure, and as the increase of aspect ratio, the internal flow effect becomes much more remarkable inevitably owing to the increased flexibility. Although the research on the internal flow effect of a fluid-conveying flexible pipe in the air has been extensively undertaken, the coupling mechanism of a riser subjected to both internal and external flows has not been well understood. For example, one contradiction regarding the dynamic stability of a suspended and submerged pipe carrying fluid upwards exists, as the theory predicts instability for small internal flow velocities while experiments do not show such instability. This study is motivated to develop a time domain prediction algorithm of fluid-conveying marine risers based on Hamilton’s principle, and it tries to explore the intrinsic multi-field fluid-structure interaction nature by use of linear analytical method, numerical iteration schemes and scaled-model experimental tests. The dynamic characteristics of marine risers experiencing vortex-induced vibration and/or parametric excitation, and the recoil response of a deep-water drilling riser after an emergency disconnection are studied by examining the energy transfer and internal stress distribution along the riser, in order to figure out how the FSI (Fluid-Structure-Interaction) mechanism affects the riser stability, how the energy transfer sustains the oscillation mode, and how the coupling force triggers new natural modes, This study not only has academic significance for developing fundamental theory of FSI mechanics, but also beneficial for the design and safety operation of long, flexible fluid-conveying risers when operating in extreme water depths.

深远海的特点是离岸远、水深深、地质条件复杂、环境恶劣且多变，确保作业安全是研发深远海装备要解决的首要命题。深远海立管由于长径比的大幅增加，柔性增强，内流效应显著,但其内外流耦合机理尚未得到很好理解。本课题拟利用实验数据，基于汉密尔顿原理建立大长径比立管系统的内流-管结构-外流多元场液固耦合半经验分析模型，运用线性解析、数值计算和模型试验等手段，探究系统能量变化和内外流体力联合作用规律，进而揭示内外流耦合机理。本项目将对深远海立管在内外流耦合激励下（包括立管内流、涡激耦合效应,立管内流、参激耦合效应，海洋悬浮输流管内流、参激、涡激联合作用，钻井立管紧急脱离泄流反冲等）的响应特征进行研究，通过分析系统内外流能量传输机制、管轴向能量传递规律和内应力分布变化等来诠释系统响应特征及稳定性。本研究成果对发展内外流耦合基础理论有重要学术价值，亦可为深远海大长径比立管系统设计和安全作业提供理论依据和技术支撑。

内流效应主要取决于内流方向、单相流还是多相流、输流管形状和边界条件以及周围环境等。为了探究深远海大长径比立管系统内外流耦合机理，首先解决了内流效应在工程应用研究中遇到的两个基础悖论问题，包括简支输流管在内流速度超临界区发生模态耦合颤振悖论和自由端含有集中质量悬臂输流管固有频率计算悖论，发展了内流效应基础理论。然后针对深海立管系统典型边界条件进行精细化建模和分析，包括基于Stewart平台建立六自由度波浪补偿平台开展五级海浪环境和干扰下立管动态响应分析并设计波浪补偿控制系统；构建直接作用式张紧器非线性模型以分析张紧器结构非线性对参激稳定性的影响；建立硬悬挂海洋立管模型进行动态响应分析并完成再入控制系统设计。最后探究深远海立管在内外流联合激励下（包括保守内流效应与立管横流、顺流涡激振动耦合、钻井立管紧急脱离反冲过程中非保守泄流效应、平台运动、立管反冲运动耦合；段塞流与立管涡激振动耦合）的响应特征，通过分析系统内外流能量传输机制、立管轴向能量传递规律和内应力分布变化等诠释系统响应特征及稳定性。本研究成果对大长径比输流管系统内外流耦合基础理论具有重要学术价值，亦可为深远海立管系统设计和安全作业提供理论依据和技术支撑。