地震波斜入射条件下砂质海床响应特征与海底管道失稳机理研究

Submarine pipeline is the lifeline engineering of offshore oil and gas resources exploitation. Ensuring its safe operation under extreme load in service period is an important and emergent scientific task for our country. The seismic damage in offshore area of China is mainly caused by the offshore earthquakes and nearshore strong earthquakes, and most of them are shallow source earthquakes. Therefore, considering the oblique incidence of seismic waves, the characteristics of the seabed earthquake can be more truly reflected. A series of undrained cyclic loading tests are conducted on representative submarine sands. The stress paths under the condition of oblique incidence of seismic waves are simulated by controlling the cyclic loading mode. The dynamic constitutive relationships appropriately describing the influences of different stress paths on the deformation characteristics of the seabed soil are derived. The exact solution of the sea water pressure on the surface of the seabed induced by the earthquake is derived. The analysis model for nonlinear seismic response of sandy seabed at the oblique incidence of seismic waves is established. The characteristics of seismic response of sandy seabed site are analyzed, the mechanism of liquefaction instability is revealed and the criterion is suggested. The 3D numerical model of nonlinear dynamic response of submarine pipeline under condition of oblique incident wave is constructed. A series of large-scale 3D seismic numerical simulation for seawater-seabed-pipeline interaction system is carried out. The key factors causing pipeline instability and pipeline structural damage are summarized. The research results are expected to make a breakthrough in the key scientific problems of the safe operation of the subsea oil and gas transmission system and the control of the seismic risk.

海底管道是海洋油气资源开发的生命线工程，保证其在服役期内极端荷载作用下的安全运行已是一项紧迫的科学任务。我国近海海域震害主要来自海域地震和近岸陆地强震，多为浅源地震，故考虑地震波的斜入射能更真实地反映海底地震特点。本项目开展代表性海底砂土的系列不排水循环加载试验，通过控制循环加载模式模拟地震波斜入射条件下的土体应力路径，提出适宜描述不同应力路径对变形特征影响的海底砂土动本构关系。推导地震诱发的海底动水压力解析解，建立地震波斜入射时海底砂质场地的非线性地震反应分析模型，分析砂质海床场地地震反应特征，揭示其液化失稳机理并建议判别准则。构建海底管道在地震波斜入射条件下非线性动力响应的三维计算模型，开展一系列大尺度海水-海床-管道相互作用体系的三维地震反应精细化数值模拟，总结诱发海底管道失稳和管道结构损伤的关键因素。研究成果有望在海底油气输送系统安全运行与地震风险控制的关键科学问题上取得突破。

海底管道是海洋油气资源开发的生命线工程，保证其在服役期内极端荷载作用下的安全运行已是一项紧迫的科学任务。我国近海海域震害主要来自海域地震和近岸陆地强震，多为浅源地震，故考虑地震波的斜入射能更真实地反映海底地震特点。本项目开展了考虑不同粒径、不同细粒含量、不同密实度的饱和砂土的动力特性的试验和本构模型研究，通过与试验结果进行对比，证实了所改进的本构理论框架能够有效评估细粒含量等特征对砂土液化行为的影响。提出了地震波斜入射时海底砂质场地的地震反应分析模型，推导建立了地震作用下海床表面动水压力解析解，通过引入无量纲频率，提出了一个能考虑海水压缩性的海底界面地震动水压力修正公式。基于Biot的理论和均质孔隙流体的概念，根据海水深度、海床参数和基岩的地震特征，推导出了一组海床不同响应特征值的解析解，开展了地震作用下海底饱和海床的动力响应特征研究，并提出了两种常用海床动力分析模型，即FD（Partial Dynamic）和PD（Fully Dynamic）模型的适用条件。为研究考虑海水-海床耦合效应的海底管道地震响应规律，考虑海床土体和管道的动力非线性特性以及海水与海床之间的耦合效应，建立了海水-海床-管道全耦合的动力相互作用有限元模型，研究了在不同地震动输入、不同地震激励方向、不同上覆水深条件下海底管道的地震响应规律和损伤机理，总结诱发海底管道失稳和管道结构损伤的关键因素。从波浪和海流作用下不规则海床的动力响应和海底管线开挖边坡稳定性出发，建立了波浪在基槽开挖完成后的海床上传播的耦合模型，提出了获取最不利时刻海底管线开挖基础的稳定性动力指标，提出了复杂海洋动力环境下海底管道边坡稳定性定量分析的新方法，并将该方法已经成功应用在实际的大型海底管线基槽边坡稳定性分析与设计方案。该项目所取得的一系列研究成果可为海底油气输送系统安全运行与地震风险控制的关键科学问题提供有效解决途径。