【作者】 唐国强；

【导师】 滕斌； 吕林；

【作者基本信息】 大连理工大学 ， 港口、海岸及近海工程， 2011， 博士

【摘要】 立管是海洋工程中连接海底和上部平台的重要设备。通过立管,可以将石油、天然气等资源从海底输送至海面的平台。立管通常都是细长的结构,并且始终受到海洋环境荷载的作用。当流体经过立管结构物时,会产生周期性的旋涡脱落和流动作用力,引起立管在横流向和顺流向发生振动。这种现象被称之为“涡激振动”。当立管结构的固有频率和涡脱落频率相接近时,会导致横流向及顺流向发生较大振幅的振动。该现象被称之为“锁定”。涡激振动是导致海洋立管发生疲劳破坏的重要因素。因此,随着海洋油气资源的勘探和生产不断向深海发展,具有明显柔性特征的大长细比深水立管的动力响应问题受到了越来越多的关注。为了研究涡激振动的物理机制,首先针对低雷诺数下圆柱受迫振动问题,在任意拉格朗日-欧拉参考系下建立了基于Navier-Stokes方程的计算流体动力学模型。研究了圆柱在不同振幅和频率下的受力问题。其中,圆柱振动的无量纲振幅为0.2-0.6,频率比(圆柱振动频率与固定圆柱涡脱落频率的比值)为0.4-1.6。数值计算的结果可以看出,当圆柱的振幅为0.6 D时,圆柱后方的尾涡脱落形式为2P,而当圆柱以相对较小的振幅振动时,圆柱后方的尾涡脱落以2S为主。同时,根据数值模拟结果给出了与加速度同相位的流体力和与速度同相位的流体力同约化速度之间的关系。在上述圆柱受迫振动问题的基础上,进一步建立了圆柱自激振动的计算流体力学模型。此时,允许圆柱在横流向及纵流向发生自由振动。对于质量比(m*=10)情况,数值模型正确模拟了横流向位移的初始分支、上分支及下分支,以及各分支与涡流场模式之间的依赖关系。利用模型,进一步研究了质量比和阻尼比对圆柱自激振动的影响。计算结果表明：低质量比情况下,自激振动发生锁定所对应的约化速度带宽明显增大,但阻尼比的变化不会引起振动系统发生本质上的改变。虽然计算流体力学模型在认识涡激振动物理机制方面有重要作用,但是由于计算量巨大,目前还远不能满足实际工程的需要。为此,在SHEAR7模型的理论基础上,建立了可用于深水大长细比立管涡激振动动力响应及疲劳损伤预报的经验模型,并开发了相应的分析程序,通过与SHEAR7结果的比较,验证了该经验模型的有效性。为了研究细长柔性立管在高模态下的涡激振动动力响应问题,并为经验模型提供验证的数据,开展了室内水池拖曳实验,研究均匀流下大长细比立管模型的涡激振动特性,其中立管模型的长度为28.04 m,直径为1.6 cm,长细比为1750。通过光纤光栅传感器测量立管模型在横流向和顺流向的应变,进而通过模态分解的方法,获得振动响应位移。文中研究了位移的频谱特征及位移标准差的空间分布等问题。实验结果发现,顺流向对立管疲劳的贡献基本和横流向是相等的,而在以往的研究工作中,人们往往只关注了横流向振动的作用。当前广泛使用的涡激振动经验模型如SHEAR7和VIVANA等,其水动力系数主要来源于受迫振动圆柱的实验结果,难以充分考虑大长细比立管的高模态及多模型共同作用的现象。本文以涡激振动试验的实测位移作为输入的数据,结合有限元结构分析,通过数值计算得到了立管模型所受到的流体力沿立管管长的分布。然后,将流体力分解为与速度同相位的项(流体激振力系数)和与加速度同相位的项(附加质量系数)。最终获得了大长细比立管模型发生涡激振动时的水动力学系数。给出了单模态和多模态响应下的水动力学系数并计算了高频响应下对应的水动力学系数。应用本文建立的经验模型,分析了立管顶部张力、水流速度的空间分布、立管的外径、内径及壁厚对涡激振动的影响。对于工作水深超过1000 m配备浮力块的隔水管涡激振动问题,文中利用所建立的数值模型,研究了不同的浮力块配置方案对涡激振动的影响。数值表明：不合理的浮力块配置方案会给钻井隔水管带来严重的疲劳损伤。当采用连续铺设浮力块的方案时,可以选择40%-60%覆盖的方案；当采用浮力块和裸管交替布置的方案时,裸管和浮力块的空间比值可以选为1：2。更多还原

【Abstract】 Risers are widely used in the offshore industry to convey fluids, such as oil and gas, from the seabed to the platform. They are slender structures exposed to complex ocean conditions. As the fluid passes risers, the well-known vortex shedding is observed, resulting in the fluctuating forces on the structures and finally inducing the vibrations of the structures in both cross-flow and in-line directions. This phenomenon is commonly called as vortex-induced vibration (VIV). When the natural frequency of the riser is close to the vortex shedding frequency, large amplitude oscillations is observed in both directions. This phenomenon is usually called lock-in. Vortex-induced vibration is one of the most important factors accounted for the fatigue damage of risers in deep water. The dynamic effects of long flexible risers under vortex-induced vibration become of increasing concern.In order to investigate the physical mechanism of vortex-induced vibration, a Computational Fluid Dynamics (CFD) model based on Navier-Stokes equations under the Arbitrary Lagrangian-Eulerian (ALE) reference coordinate system was established to simulate the forced vibration circular cylinder. In this study, the ratio of displacement amplitude to cylinder diameter is from 0.2 to 0.6, and the frequency ratio (the oscillating frequency of the cylinder and vortex shedding frequency) ranges from 0.4 to 1.6. The numerical results show that when the circular cylinder vibrates at the amplitude of 0.6 D, the vortex structure in the wake of the circular cylinder is 2P mode. Otherwise, the 2S mode can be observed when the circular cylinder oscillates at rather smaller amplitude. According to the numerical results, the fluid forces in phase with acceleration and velocity of the circular cylinder versus reduced velocity are also calculated in this study.Based on the previous forced oscillating circular cylinder model, a self-excited model is developed. The circular cylinder is free to vibrate in both the cross-flow and in-line directions. For the large mass ratio m*=10.0, the numerical results present the initial branch, upper branch and lower branch for the cross-flow displacement. The relationship between the different branches and the vortex shedding mode is examined. Employing the self-excited model, the influence of mass ratio and damping ratio on the vortex-induced vibration was studied. The numerical results indicate that the decreasing in mass ratio results in the increase of the lock-in range. However, the damping ratio cannot change the essence of the dynamic system.Though many efforts have been made to research the physical mechanism of vortex-induced vibration with CFD method, it is far from the practical applications due to its huge time consuming. According to the theory of SHEAR7, a mathematical model used to evaluate the dynamic response and fatigue damage of deepwater risers under vortex-induced vibration was developed.In order to examine the VIV response of a long flexible riser oscillating at rather higher mode and providing the validation data for the empirical model, Laboratory tests were conducted to investigate the multi-mode dynamic responses of riser model subjected to steady uniform flow. The widely used semi-empirical methods in predicting VIV of deepwater risers, such as SHEAR7 and VIVANA, mainly rely on the observations and experimental results of a forced oscillating rigid cylinder under steady currents. Therefore, these empirical models are not able give exact prediction to the dynamic response of the long flexible risers involving higher mode and multi-mode. In this work, by using the available displacements data of VIV experiment as the input data, the time history of total hydrodynamic forces exerted along the axis of the riser model is obtained by the finite element analysis method. The total hydrodynamic forces are further decomposed into a component in phase with velocity (fluid exciting fluid coefficients) and a component in phase with acceleration (added mass coefficients). The results associated with both single mode and multi-mode responses are presented in this work. The hydrodynamic coefficients considering the higher order modes are addressed in this work.Employing the empirical model developed in this work, the influences of the top tension, the distribution of incident current velocity along the axis of the riser, outer diameter, inner diameter and wall thickness of risers on the dynamic response were investigated. For the problems of deepwater risers equipped with buoyancy modules, the influence of the buoyancy modules on the dynamic VIV response of the riser is studied. The numerical predictions indicate that the unreasonable arrangement of the buoyancy modules may lead to serious fatigue damage of the riser. As for the continuous paving buoyancy modules, the 40%-60% coverage rates are suggested. Otherwise, the space ratio of the buoyancy modules to bare riser should be 1:2 for the alternant paving buoyancy modules used according to the numerical results of this work.更多还原