海洋内孤立波作为一种大尺度的海洋内波，是连接海洋洋流运动和小尺度旋涡运动的重要纽带。在内孤立波的传播演化过程中，其诱导的特殊流场对海洋生态系统、物质和能量输运等具有深刻的影响。此外，海洋内孤立波诱导的剪切流场贯穿整个水深，势必会对海洋工程中的大长径比柔性立管造成巨大的威胁。因此，研究内孤立波自身的传播演化规律及其对柔性立管的影响具有重大的科学意义和工程应用价值。本文采用理论分析、数值模拟，并结合现场观测数据分析，对内孤立波的分裂演化规律以及典型柔性立管在内孤立波作用下的动力响应开展了研究。

首先，利用弱非线性KdV模型的拓展形式，综合考虑了变浅效应和海底摩擦效应对海洋内孤立波传播演化的影响。通过理论分析，发现模型中摩擦效应项的Chezy系数与内孤立波初始波幅成反比，并得到我国南海实际观测数据的良好验证，同时给出了中国南海Chezy摩擦系数的取值方法。提出了一个表征变浅效应和海底摩擦效应相对重要性的无量纲参数，应用于中国南海，可简明揭示南海海盆、大陆坡和大陆架区域内孤立波波幅的演化规律。

其次，对典型的连续密度剖面模型进行了改进，使其在保持原模型所有优点的前提下，可以更好地模拟实际海洋密度剖面，并证明了该密度剖面模型在不同海域和不同时间密度分布的广泛适用性。利用变系数KdV模型，研究了内孤立波在变地形条件下的分裂规律，总结了分裂内孤立子的分布特征。据此，提出了通过内孤立子波包中前两个孤立子的波幅来预测所有孤立子个数和波幅的方法，并对这一方法进行了验证。针对内孤立波在陡变地形下的分裂现象展开研究，得到了分裂内孤立子及二次尾波的质量和能量分配关系。并推导了一般情形的eKdV方程到标准KdV方程的方法，从而实现了两种方程的统一。

第三，研究了内孤立波作用下顶张紧立管的动力响应。通过振型叠加法，获得了内孤立波作用下简支梁动力响应的理论解。利用ABAQUS有限元软件的二次开发功能，建立了在两层流体中的内孤立波作用下的顶张紧立管响应有限元模型。模拟结果表明，立管的最大水平位移和最大弯矩出现在上下层流体的分界面附近，且立管的最大位移和最大弯矩随着内孤立波波幅的增加近似线性增长。

最后，建立了内孤立波作用下钢悬链线立管响应的三维有限元模型，分别研究了深水（3000m）和浅水(300m)海域中立管在不同传播方向内孤立波作用下的动力响应。结果表明，两种水深下立管的应力分布呈现出截然不同的特点，且钢悬链线立管的奇偶模态的振型相差很大，奇偶频率呈现出独立的变化趋势，观察到了奇偶振动频率曲线的“杂交”现象。在相同波幅内孤立波作用下，浅水立管的运动位移小于深水立管，但立管应力的变化远大于深水立管。此外，当内孤立波传播方向与立管平面夹角为90°时，立管的位移最大，但应力变化最小；当内孤立波传播方向位于立管平面内时，立管的位移最小，但应力变化最大。

Oceanic internal solitary waves are the key factor linking large scale ocean currents and small scale eddy motions. The flow field induced by internal solitary waves is special in that water motion occurs in the whole water depth and the upper-layer flow is generally opposite to the lower-layer flow making strong shear near the pycnocline. Thus, internal solitary waves play a significant role in marine ecosystem, sediment transportation and energy transfer. They also act as a great potential threat to the typical marine structures, such as piles, risers, mooring chains, etc. Therefore, it is of great scientific significance and practical importance to investigate the evolution of internal solitary waves and their impact on flexible risers. In this thesis, we carry out research on the disintegration of internal solitary waves and the dynamic response of flexible risers to internal solitary waves via theoretical analysis and numerical simulations in combination with observation data.

Firstly, the extension form of weakly nonlinear KdV model is used to study shoaling and friction effects on the evolution of internal solitary waves. Based on theoretical analysis and in situ measurements, the Chezy friction coefficient is proved to be inversely proportional to the initial amplitude of an internal solitary wave. Moreover, a dimensionless parameter defining the relative importance of shoaling and friction is presented. This parameter is used to investigate the amplitude variation of internal solitary waves in the South China Sea. According to the variation of amplitude along the propagation path, the evolution law of internal solitary waves in the deep basin, slope shelf, shelf break and continental shelf is obtained.

Secondly, a revised density profile model is proposed, which is validated by the data of different seas at different time. It is proved more accurate and more widely used. Then, the disintegration law of internal solitary waves over a sharply varied bathymetry and the distribution of their disintegrated solitons are explored with the KdV equation, a method is proposed to forecast the number and amplitudes of disintegrated solitons just by the amplitudes of the leading two solions in a wave packet. And the distribution of mass and energy of each soliton and the secondary tail wave are obtained. Furthermore, a transformation is derived to turn the general eKdV equation into a standard KdV equation.

Thirdly, the dynamic response of a top tensioned riser under internal solitary waves is investigated. The analytical solution to the dynamic response of a simply supported beam under internal solitary waves is acquired with the mode superposition method. By using ABAQUS, a finite element model for analyzing the dynamic response of a top tensioned riser under internal solitary waves is established. It is demonstrated that the displacement and moment of a top tensioned riser reach their maxima near the pycnocline, and they approximately increase linearly with the amplitude of internal solitary waves. The stress at the bottom of the riser experiences a significant increase as internal solitary waves pass by.

Finally, a three dimensional finite element model is established for analyzing the dynamic response of a steel catenary riser to internal solitary waves. Two cases with water depth of 300m and 3000m are considered to represent shallow and deep water, respectively. It is demonstrated that the axial distribution of stress is distinctly different in shallow and deep water. The odd and even vibration modes behave differently. The odd and even frequencies vary independently, and the odd and even modes are found to be “hybrid” in some frequency range. Under the same amplitude internal solitary waves, the displacement of the riser in shallow water is larger than in deep water, but the variation of stress is smaller. Moreover, when the incident internal solitary wave is perpendicular to the plane of the catenary riser, the displacement of the riser reaches its maximum, but the variation of stress reached minimum. When the incident internal solitary wave is in the plane of the catenary riser, the displacement of the riser reaches its minimum, but the stress reaches its maximum.