【作者】 饶志标；

【导师】 杨建民；

【作者基本信息】 上海交通大学 ， 船舶与海洋结构物设计制造， 2010， 硕士

【摘要】 当海洋立管置于一定速度的来流中,流体由于立管的存在会在立管表面发生分离,从而产生漩涡,漩涡交替地产生与泻放就导致立管在横向和流向发生振动。立管振动的产生反过来又会改变尾流结构,这种流体-结构物相互作用的问题被称作“涡激振动”。涡激振动的产生会对系泊系统和立管产生疲劳损害,减小整个平台的疲劳寿命。因此以立管为代表的海洋结构物的涡激振动问题受到人们的高度关注。本文分析了频域中海洋立管的涡激振动响应,对频域理论中的一些基本问题进行了深入的研究。内容主要涉及以下几个方面:1,为了寻找垂直立管的模态振型非规则正弦曲线的原因,本文研究了简支垂直立管不考虑以及考虑弯曲刚度两种情况。前者分别应用了贝塞尔函数法和渐近法两种方法求解,后者则是采用SHEAR7理论解。2,详细介绍了SHEAR7和VIVANA所基于的频域理论。为了为钢悬链线立管的涡激振动预报提供初始数据,首先建立了悬链线方程,然后根据悬链线方程将钢悬链线立管等效为垂直立管。3、用SHEAR7研究了3048米水深下钢悬链线立管在不同顶部预张力下以及不同流剖面下的横向涡激振动响应,以及用VIVANA预报了1500米水深下躺底的钢悬链线立管在剪切流下的横向涡激振动响应。为了研究涡激振动三方面的特性,即预张力变化对模态振型的影响;模态振型对模态权重的影响;模态分析方法对低阶模态权重的影响,对MARINTEK的一个实验进行分析。研究结果发现,预张力的变化对模态振型影响几乎很小,模态振型对模态权重的影响也很小。因此这就表明了没有必要去求解复杂的模态振型,可以直接用正弦模态振型来代替求解模态权重。当低阶模态数偏离主控模态数时,低阶模态数的模态权重值不可靠,一部分是由于模态分析方法本身引起的,一部分是由于噪声对低阶模态权重的影响很大。4、为了研究柔性立管的柔软特性对附加质量系数的影响,本文基于有限元方法,提出一个直接由随时间变化的位移估算柔性立管各点处附加质量系数的方法。此公式不仅突破了涡激振动频域理论依赖附加质量实验数据的限制以及考虑柔性立管的柔软特性,而且克服了简单地将柔性立管的附加质量系数假定为常数1的缺陷。此公式还考虑了柔性立管各点的振幅以及立管在不同水深处流场不同特性的影响。更多还原

【Abstract】 When the flexible riser exposed to a certain current, Vortex-Induced Vibrations (VIVs) occur once shedding vortices exert periodical forces on the flexible riser in the cross flow (CF) and in line (IL) directions. The vibration of flexible riser, in turn, changes its wake structure; this fluid-structures interaction problem is called the "VIV". The presence of VIV would lead to fatigue damage of mooring system and riser, reduce the fatigue life of the entire platform. Therefore, more efforts have been put on the research activities on VIVs during the past ten years.Based on VIV theories in frequency domain, the present work explores some basic issues of VIV. The main contents and contributions of this thesis may be summarized as follows:1, In order to explain the reason why linear axial tension causes mode shapes unlike sine function, the present work considers two scenarios, namely vertical tensioned risers without and with the bending stiffness. The vibrations of former risers were solved with both Bessel and Asymptotic solutions and the latter ones were presented with SHEAR7 theory.2, It introduces theories of both SHEAR7 and VIVANA programs in detail. To provide the basic input parameters in VIV tools, the catenary equation was firstly established. According to the equation, the SCR was defined as an equivalent vertical straight riser with linearly varying tension.3, It predicts VIV responses of SCR under different water depths, various flow profiles and top tensions with both SHEAR7 and VIVANA. An experiment was studied to achieve three objectives: (1) the effect of changed axial tension on mode shapes; (2) the effect of mode shapes on modal weight; and (3) the effect of modal analysis method on low modal weights. It is found that the variation of axial tension exerts little impact on mode shapes for the weak self-weight and flexural stiffness reasons. The comparison results of two mode shapes, sinusoidal and non-sinusoidal, show also little effect of mode shapes on modal weights. Thus, sinusoidal mode shapes can take place of real mode shapes to obtain modal weights when the tension is dominating. When they are away from the dominating mode, low modal weights become unreliable with modal analysis method. The reasons come from two aspects, one is the modal analysis method itself and the other the presence of noise.4, In order to study the interaction of adjacent elements on the added mass coefficients, the present work proposes a formula, based on FEM, for estimating the added mass coefficients along the flexible riser under known displacements time series. This formula not only wasn’t limited by the experimental data of rigid circular cylinders and considered its flexible characteristics, but also overcame the flaw that added mass coefficients of flexible risers are assumed to be constant. In addition, it also takes into account the varying amplitudes of various points at the flexible risers and different characteristics of flow field at different depths.更多还原