随着石油、天然气开采行业向深海发展，用于输送石油、天然气等能源的立管结构长度会逐渐增加。在地面工程中，管道过长，可以通过沿结构轴向分布式地增加约束来提高管道的稳定性，然而由于海洋特殊环境的影响，在除了管道两端之外，其余地方施加额外约束较为困难，这就意味着，深海输流管道的稳定性问题变得更加严峻；另外，随着水深增加，竖直立管长度的增加会导致结构本身的重力不可忽略，使得管道承受了张力沿轴向变化的效应。这种张力的不均匀性，会使得管道流固耦合稳定性及动力学响应特征发生明显的变化，因此本文拟通过理论分析和数值模拟对该问题展开研究。
       首先针对输流管道的不同失稳模式，基于拉格朗日方程、元功沿管线积分法和受力分析，建立了管道失稳形式和边界条件之间的关联，并建立了考虑变张力的输流管道的流固耦合控制方程。发现由于张力延轴向的变化，悬臂管道存在多种失稳模式。然后，通过有限元数值模拟、Ritz-Galerkin方法计算了不同流速下变张力输流管道的动力学特征，并与一种简化的常张力管道进行了对比分析，得到了张力不均匀性对输流管道的稳定性及动力学特征的影响，结果表明，变张力输流管道的频率要低于常张力输流管道的频率；并且变张力输流管道的模态不再具有对称性或反对称性。
考虑到实际海洋石油开采过程中，深水浮式平台会在波流作用下运动，会使得与其相联的输流管道的上端点受到周期性激励。因此，在稳定性分析的基础上，本文通过有限元数值模拟分析简支输流管道动响应，给出的位移时空演化结果表明，在变张力简支输流管道中存在明显的波传播现象，随着张力沿轴向的减小，波长沿着轴向减小，振幅沿着轴向增大；并基于WKB理论，揭示了动响应中存在的波传播机理，即振幅和波长沿轴向的变化规律。最后通过Ritz-Galerkin方法和Bolotin方法研究了不稳定内流即脉动流作用下，两种输流管道的参数激励问题和各自对应的参数不稳定区域；进一步给出了无量纲流速和无量纲质量等无量纲参数对不稳定区域的影响，结果表明，流速和质量的增加，会扩大不稳定区域面积，重力效应会提高不稳定区域的频率。

      With the development of the oil and natural gas mining industry to the deep sea, the length of the riser structure used to transport oil, natural gas and other energy sources will gradually increase. In ground engineering, if the pipeline is too long, the stability of the pipeline can be improved by adding constraints distributed along the structural axis. However, it is difficult to impose additional constraints on other places except for the two ends of the pipeline under the influence of the special marine environment, which means that the stability of deep-sea pipelines has become more severe. In addition, as the water depth increases, the increase in the length of the vertical riser will make the gravity of the structure itself to be non-negligible, so that the pipeline bears the effect of the axial tension change. The inhomogeneity of tension will cause obvious changes in the pipe fluid-solid coupling stability and dynamic response characteristics. Therefore, this paper intends to study this problem through theoretical analysis and numerical simulation.

      First, for the different instability modes of the liquid pipeline, based on the Lagrangian equation, the element work along the pipeline integration method and the force analysis, the relationship between the pipeline instability form and the boundary conditions and the fluid-structure coupling governing equations of fluid-conveying pipelines with variable tension were established. It was found that there were multiple modes of instability in the cantilever pipe under the axial change of the tension. Through the finite element method and the Ritz-Galerkin method, the dynamic characteristics of the variable tension pipeline at different flow rates were calculated, and compared with a simplified constant tension pipeline, the influence of tension inhomogeneity on the stability and dynamic characteristics of the pipeline were obtained. The results showed that the frequency of the variable tension pipeline was lower than that of the constant tension pipeline, and the mode of the variable tension pipeline was no longer symmetrical or antisymmetric.

      In the actual offshore oil extraction process, the deep-water floating platform move under the action of waves and currents, which cause the upper end of the fluid pipeline connected to it to be periodically excited. Therefore, on the basis of stability analysis, the dynamic response of simple branch pipelines was analyzed by the finite element numerical simulation. The results of the displacement time and space evolution displayed that there were obvious wave propagation phenomena in simple branch pipes with variable tension. As the tension decreased along the axis, the wavelength decreased along the axis, and the amplitude increased along the axis. Based on the WKB method, it revealed the wave propagation mechanism in the dynamic response, that was, the changing law of amplitude and wavelength along the axis. Finally, the Ritz-Galerkin method and the Bolotin method were used to study the parameter excitation problems of the two kinds of fluid pipes and their corresponding parameter instability regions under the unstable internal flow (that is, the pulsating flow). The dimensionless flow velocity and the dimensionless flow rate were further given. The effect of non-dimensional parameters such as mass on the instability area. The results illustrated that the increase in flow velocity and mass enlarged the area of the instability area, and the gravity effects increased the frequency of instability area.