【作者】 李小超；

【导师】 王永学；

【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】 平铺在海床上的海底管线由于海底地形凹凸不平、海床冲刷等因素容易出现悬跨段,悬跨段往往在海流、波浪等海洋环境载荷的作用下容易发生涡激振动,引起管线的疲劳破坏。本文通过物理模型实验和数值模拟相结合的方法对海底管线悬跨段涡激振动问题进行深入研究,诣在加深对涡激振动机理的理解,预报管跨在复杂海洋环境条件下的动力响应,为海底管线的失稳、强度破坏、疲劳损伤安全评估提供理论基础和科学依据。在波流水槽内开展了固定海床上海底管跨涡激振动模型实验,实验的目的在于考察海床近壁对涡激振动的影响、涡激共振由一阶共振向二阶共振过渡的模态过渡以及过渡范围内管线的振动特征。实验制作了两个模型,模型具有不同的质量比(单位长度管线质量与其排开流体质量之比),分别为2.62和4.30,模型均设计成受抗弯刚度支配的柔性梁模型。模型两端采用万向节模拟简支边界,由于重力等静载荷的影响模型跨中在水下分别存在约14mm和50mm的初始下垂。考虑到跨中初始下垂的限制,实验时质量比为2.62的模型端部间隙比选为2.0、4.0、6.0和8.0,质量比为4.30的模型端部间隙比选为4.0、6.0和8.0。基于一阶自振频率的约化速度在0～16.7范围内变化。采用光纤光栅应变传感器测量模型的动力响应,利用模态分析方法对应变数据进行处理得到模型的振动位移响应,进而分析得到振动频率、响应幅值、模态特征等数据。对海床间隙对涡激振动的影响、顺向和横向两个方向振动的耦合等进行了探讨。实验结果表明,随着间隙比的减小,主模态的转移发生在较大的约化速度下,当主模态从一阶模态转移到二阶模态时,响应频率出现了大幅度的增加；对于e/D>2.0,管线横截面运动曲线表现为常见的8字形,但是对于e/D=2.0,管线横截面做泪滴形轨迹运动。重点分析了随着流速的增加涡激振动由一阶模态共振向二阶模态共振的过渡,揭示出各海床间隙比条件下涡激振动由一阶模态向二阶模态的过渡特征。针对管跨两端管土作用复杂边界对涡激振动的影响在波流水槽内开展了第二组涡激振动模型实验。模型采用第一组实验中质量比为4.30的模型,模型两端平放在土壤上,中部具有2.138m的悬空长度,模型端部间隙比为6.0。针对沙土和粘土两种不同特性的海床土壤进行了实验。实验首先在空气中和静水中分别针对不同土壤进行自由衰减试验,测量模型的自振频率,之后在水流作用下进行涡激振动测试。水流流速在0-0.6m/s的范围内变化。实验对横向振动频率和应变时程进行了分析,结果表明,模型的自振频率具有模糊特性,但是水流作用下模型的振动频率是确定的,并且随着流速的增加而线性增加；土壤支撑的引入没有改变涡激振动的简谐特性。针对海底管跨涡激振动问题编制了有限元分析程序,程序采用柔性梁模拟管跨结构,水动力系数由VIVANA模型得到,模型控制方程采用Newton-Raphson迭代算法求解。采用Larsen et al. (2002,2004)给出的两组不同的用于确定升力曲线的参数曲线进行计算,将计算结果与前人及本文模型实验结果进行了比较。结果表明,Curve1 (Larsen et al.2002)参数曲线较Curve2 (Larsen et al.2004)参数曲线更适合于海底管跨的涡激振动分析；本文的涡激振动数值模型能够较好地计算无海床间隙影响时海底管跨一阶共振区内的涡激振动响应,以及较好地识别出主模态转移。最后,为了考察海床近壁对水动力系数的影响及预报近壁影响下管跨的涡激振动响应,利用简单支撑柔性梁实验获取的响应数据进行有限元反力分析,求解作用在柔性梁上的涡激振动载荷,计算相关的水动力系数。利用反力计算得到的水动力系数进行响应计算,将计算结果与实验值进行比较。更多还原

【Abstract】 Submarine pipeline spans can arise due to uneven seabed or scouring for the pipelines being laid on the cohesionless soil. Vortex-induced vibrations (VIVs) may occur when the spans are subjective to the effects of currents and cause unacceptable fatigue damage in the pipeline. Model tests and numerical simulations were performed to improve the understanding of the VIV phenomenon and predict response under various conditions in order to provide theoretical foundation and scientific basis for disaster process, damage mechanism and safe assessment of submarine pipelines in an offshore environmental condition.Flexible pipe tests were performed to investigate the effect of the proximity to seabed on VIV and the mode transition in VIV as the flow velocity varies. Two pipe models,16mm in diameter,2.6m in length and with mass ratios (mass/displaced mass) of 2.62 and 4.30 respectively were tested in a wave current tank. The pipe models were designed as bending stiffness dominated beams which are generally chosed to represent scaled models of pipeline spans, and were installed with universal joints at each end. The middle of the models had initial sags of about 14mm and 50mm respectively in water. The reduced velocity based on the first natural frequency is in the range of 0-16.7. Due to the restriction of the initial sags, the gap ratios at the pipe ends for the pipe model with the mass ratio of 2.62 were 2.0,4.0,6.0 and 8.0, and those for the pipe model with the mass ratio of 4.30 were 4.0,6.0 and 8.0. The response of the models was measured using fiber optic strain gauges. Modal analysis was applied to analyze the strain data from the tests, and the final results were the response amplitudes, response frequencies and the dominant modes. The effect of the proximity to seabed on VIV and the coupling of in-line and cross flow VIV were discussed. The results show that, as the gap ratio decreased, the shift in the dominant mode took place at a higher reduced velocity, and the dramatic increase in response frequency appears with the shift in dominant mode from the first mode to the second one; the pipe undergoes a common 8-shape motion for e/D>2.0, but a teardrop shape motion for e/D=2.0. Furthermore, the mode transition as the flow velocity increases is analyzed. The characteristics of VIV mode transition from the first mode to the second one for different gap ratios are revealed.A second set of experiments were conducted to investigate the effect of the pipe-seabed-interaction at the span shoulders on the VIVs of submarine pipeline spans. The pipe model with the mass ratio of 4.30 was tested in the wave current tank. The pipe was laid horizontally on the soil, and had a free span in length of 2.138m. The gap ratio at the pipe ends was maintained at 6.0. The tests in both still water and a current were conducted. The flow velocity was in the range of 0-0.60m/s. The frequency responses and the time-domain tracing of cross-flow strain responses are presented and analyzed. The experimental results exhibit several valuable features:the natural frequency of the model has a fuzzy property, but the response frequency in a current has not and increases linearly with the increase of flow velocity; the existence of soil support does not affect the harmonic characteristic of VIV.Combining the VIVANA model with a flexible beam model, a computer analysis program is developed for cross-flow VIV. The Newton-Raphson method is used to solve the dynamic equilibrium equation. Calculations are carried out using two different groups of parameter curves from Larsen et al. (2002,2004). The calculated results are compared with experimental results from both previous and present laboratory model tests. It is shown that Curve 1 (Larsen et al.,2002) where the maximum response amplitude for zero lift (i.e. CL=0) is from free vibration tests is more suitable for the VIV analysis of submarine free span pipelines. The agreement is in general good for the vortex-induced resonance at the first mode, and the VIV response model is able to identify the shift in the dominant mode.Finally, inverse force analysis is performed using the response data from the flexible pipe tests to investigate the effect of the proximity to seabed on the hydrodynamic force coefficients and predict the response with wall interface. The force contour plots and coefficients are generated. Response calculations are carried out using force coefficients from the inverse force analysis and the calculated results are compared with experimental data.更多还原