【作者】 宋吉宁；

【导师】 滕斌； 吕林； 陈兵；

【作者基本信息】 大连理工大学 ， 港口、海岸及近海工程， 2012， 博士

【摘要】 随着世界范围内海洋油气开采走向深海,出现了多种形式的海洋平台。立管系统是海洋平台进行海底油气输送的重要设施和关键部分,随着作业水深的发展,立管长度可达几千米。深海立管受海流作用出现的涡激振动问题是造成立管发生疲劳破坏的主要原因。而一旦立管破坏,除了造成严重的经济损失,也将引发漏油事故,可能导致严重的海洋环境污染和生态安全问题。由于对海流作用下立管涡激振动认识的不足,为了保证安全,在立管设计中只好采用非常高的安全系数,但这也导致海洋平台的造价巨额上升。如果能够对立管涡激振动进行准确的预报或者采取适当的涡激振动抑制措施,则可以大大降低海洋平台的造价,因此,这方面的研究也成为海洋工程领域近年来的热点问题。本论文工作依托于国家高技术研究发展计划(863计划)课题“3000米水深半潜式钻井平台关键技术研究”(No.2006AA09A103),旨在通过物理模型实验提高对海流作用下立管涡激振动特性和机理的认识,研究和探索涡激振动的抑制措施与方法,并基于离散涡理论建立模拟涡激振动的CFD计算模型,发展立管涡激振动的数值预报技术。主要研究内容归纳如下：1.在室内波浪水槽中开展了小尺寸的垂向管涡激振动物理模型实验。立管模型长度约2m,直径16mm,管内部分别填充空气、水和细沙,来改变立管模型的质量比。将新式的光纤光栅(FBG)测量技术应用于水下立管涡激振动实验。通过这个实验,研究了不同质量比的立管模型在水流作用下的涡激振动。应用模态分析法,计算了不同流速作用下涡激振动的响应模态。分别对三种不同质量比立管模型的涡激振动无因次振动频率和振幅与约化速度的关系进行了分析。通过数据分析,给出了不同流速下响应位移在沿立管长度方向上的分布情况。2.在立管涡激振动抑制措施方面,提出了在立管周围等角度间隔均匀布置多根附属控制杆的涡激振动抑制措施,并在室内水槽开展了这种抑制措施的实验研究。实验中观测了四种流速和两个极限来流方向下的涡激振动抑制效果。实验结果表明：三根附属控制杆抑制措施可明显降低立管的横向振动幅值,对主管的振动频率改变不大。并且,这一抑制措施对来流方向有较强的适应性,避免了以往单根控制杆在流向发生改变时可能加剧立管涡激振动的问题。3.为了研究高阶模态的立管涡激振动,在拖曳水池中开展了大长细比的柔性水平管涡激振动模型实验。立管模型长度约28m,长细比达到1750,雷诺数范围在3000到10000之间。在立管模型的横流向和顺流向分别安装了光纤光栅应变传感器,对两个方向的涡激振动都进行了观测。在本文的实验条件下,横向方向上的涡激振动模态可达到6阶,而纵向方向上的响应可以达到12阶模态。通过模态分析法,研究了单模态响应和多模态响应形式,并且本文实验证实,即使在均匀流作用下,立管也会出现多模态响应。而且,通过频谱分析发现,多模态响应中的这些参与模态具有相同的响应频率。即使在均匀流作用下,涡激振动响应在立管方向上分布也不对称,实验中出现许多这样的结果,进一步的分析表明,这种不对称性主要是由多个模态共同参与涡激振动响应引起的。实验中观测到了三倍频、四倍频和五倍频等高倍谐振,并对这种高倍谐振现象的形成机理进行了分析。4.建立了无网格的二维离散涡法时域模型,在离散涡诱导速度的计算中采用快速多极子方法,提高了计算效率,并对固定圆柱绕流和运动圆柱绕流都进行了数值模拟计算。其中,运动圆柱绕流问题包括两个方面,强迫圆柱绕流和弹性支撑刚性圆柱在水流作用下涡激振动。通过弹性支撑刚性圆柱的涡激振动模拟计算,进一步分析了涡激振动中位移响应振幅随约化速度的变化关系。5.采用“切片理论”方法,将二维离散涡法水动力模型与立管结构有限元模型结合,建立了计算长柔性立管涡激振动问题的“准三维”离散涡法时域模型。并应用这个时域模型,对两个长柔性立管涡激振动实验工况进行了模拟计算,与相应的实验结果比较接近,验证了算法的合理性和结果的准确性,表明本文建立的准三维离散涡法涡激振动时域模型可以较好地应用于长柔性立管涡激振动问题的预报计算。更多还原

【Abstract】 As the worldwhile activities of oil and gas exploration and production continue apace to deeper water, many kinds of platforms have been built. A key element of the platform is the marine riser system, which plays an important part in transporting the oil and gas from the seabed to the platform. Marine risers can be thousands of meters long as the prodution depth goes. The vortex-induced vibration (VIV) response of marine risers subjected to the ocean current accounts for the greatest contribution to overall riser fatigue damage. Since their failure may cause great economic damage and environmental problems, marine risers are designed with a great safety factor to guarantee the safe, but which leads to huge cost. Accurate prediction and/or suppression of the VIV response of the marine risers, which can greatly reduce the cost of the platform, has led to an intensification of research activity in ocean engineering in recent years.The present research presented in this dissertation is supported by the "National High-Tech Research and Development Program (863Program)"of China with Grant no.2006AA09A103. Experimental investigation has been conducted to improve the understanding of the VIV of marine riser and develop new VIV suppression method. A CFD model has been built based on a discrete vortex method to simulate the VIV response of marine risers. The main content is as follows:First, experimental tests of VIV of a small vertical riser pipe have been conducted in a wave flume. The length of the riser model is about2m with a diameter of16mm. In order to vary the mass ratio, the riser pipe is filled with air, water and sand, respectively. Fiber brag grating (FBG), a new technology, is used to measure the strain response in the present experiment. By modal analysis method, the vibrating modes of VIV response are analysed at different flow velocities. The dimensionless vibrating frequency and amplitude versus reduced velocity are also presented in this work. Moreover, the distributions of the displacement of VIV response along the length of the riser are analysed at different flow velocities.Second, a passive flow control method to suppress VIV has been tested, which is accomplished by arranging three identical small circular cylinder closed to the riser model with120°angle. The riser model with control rods is placed in a water flume subject to four different current velocities and two extreme attack angles. The experimental results show that the control rods can remarkably reduce the transverse responses of riser model while the vibration frequency of riser model is almost as same as that of a bare riser. The experiments also demonstrate that present flow control measure is not sensitive to the incoming flow direction which is a very important advantage for practical engineering applications.Third, laboratory tests have been conducted on vortex-induced vibration (VIV) of a long flexible riser towed horizontally in a wave basin in order to study high mode VIV responses. The riser model has a total length of28.0m with an aspect ratio of about1750. Reynolds numbers ranged from about3000to10000. Fiber optic grating strain gauges are adopted to measure the dynamic response in both cross flow and in-line directions. The cross-flow vibrations were observed to vibrate at modes up to6and the in-line reached up to12. The fundamental response frequencies can be predicted by a Strouhal number of about0.18. Multi-mode responses and the asymmetry of the bare pipe response in uniform flow were observed and analyzed. The experimental results confirmed that the riser pipe vibrated multi-modally despite it being subject to a uniform current profile and all of the excited modes vibrated at the Strouhal frequency. The asymmetrical distribution of displacement was mainly due to more than one mode participating in the resposne. Higher harmonics of the VIV response such as the third, fourth and fifth harmonics frequencies were found to be steady over the entire duration of the test even if they varied along the length of the riser pipe.Fourth, a2D grid-free discrete vortex model is established in order to predict VIV responses. Fast multipole method is used in the model in order to improve the efficiency of the calculation of the vortex-induced velocity. Not only flow past a fixed cylinder,but also flow past an oscillating cylinder has been simulated in present work. For flow past an oscillating cylinder, both VIV responses of an elastically mounted rigid cylinder and fluid forces of a forced rigid cylinder are concerned. Drag force, lift force and displacement responses are presented. The relationship between the response amplitude and reduced velocity is analysed in the present work.Fifth, coupled with the strip method, a quasi-3D DVM model is established by combining the2D DVM model with the FEM structure model in order to predict the VIV of long flexible risers. Then, numerical simulations of VIV of long flexible risers are conducted. The simulation results are compared with the experimental results, and the reasonable agreements are obtained,which show that the present DVM model can be used to predict the VIV of long flexible risers.更多还原