【作者】 吴学敏；

【导师】 黄维平；

【作者基本信息】 中国海洋大学 ， 港口、海岸及近海工程， 2013， 博士

【摘要】 深海环境复杂，这必然对深水油气开发设备的要求有了质的提高，在这些必不可少的油气开发设备中，深水立管是连接海底与海面作业平台的关键设备。在复杂的海洋环境中，造成立管疲劳破坏的主要因素是海流引起的涡激振动（VIV），如果立管发生疲劳破坏，不但会影响巨大的经济利益，还会使油气泄露，导致爆炸火灾甚至造成海洋环境的污染。所以准确地预测深水立管的涡激振动响应具有极其重要的意义。本文在总结国内外学者对深水立管涡激振动研究现状的基础上，采用立管模型试验、理论推导和数值分析相结合的方法，提出了深水立管涡激振动非线性分析模型，并对深水顶张力立管涡激振动的响应及强度进行研究。首先，基于课题组模型试验研究发现：当理论涡脱频率与管道的固有频率不一致时，作用在振荡管道上的涡激力并非简谐扰力，而是具有一定带宽的窄带随机扰力，所以管道的涡激振动响应是一个随机过程，由此考虑到立管涡激振动过程中流体与结构之间的耦合作用，将熟知的涡激力模型进行修正，并将该修正的涡激力模型应用于深水立管的涡激振动响应分析，提出考虑流固耦合的深水立管非线性涡激振动时域分析模型，编制了相应的深水立管动力响应分析程序TTRTD1.0，研究了服役于1500m水深的顶张力立管涡激振动的响应和强度，计算结果表明：提出的分析模型能够较好地预测出深水立管的涡激振动响应特征，包括高阶模态响应、多阶模态响应和非对称涡激响应大变形的特征。其次，深水柔性立管涡激振动响应具有大变形的现象，而由于深水立管大长细比的结构特点，传统的分析通常是基于小变形理论，本文通过理论推导将大变形考虑进深水立管涡激振动力学模型，对前一部分提出的非线性涡激振动时域分析模型的力学模型进行改进，提出了考虑大变形的深水立管涡激振动非线性时域分析模型，编制了相应的深水立管动力响应及强度分析程序TTRLD1.0，通过理论分析该模型立管的动力特性发现：大变形使得立管的固有频率降低，且低阶频率降低的幅度较大，而大变形对简支立管的振型并没有影响。同时，通过与不考虑大变形效应的深水顶张力立管非线性涡激振动时域分析模型响应的计算结果对比发现：随着流速的增大，立管响应的变形增大，大变形的影响使得结构振幅随之增大。但增大的幅度并不是呈线性增长。并且大变形效应增大了立管发生最大位移处的等效应力幅值，影响立管的疲劳寿命，因此，在深水立管设计分析时，应该充分考虑大变形效应对立管涡激振动响应的影响。最后，鉴于深水立管顶端与深水平台相连，深水平台在海洋环境中易发生较大幅度的垂荡运动，平台的垂荡运动引起的参数激扰将影响立管的涡激振动响应，所以，这一部分对前一部分提出的考虑大变形的深水立管涡激振动模型进一步改进，将浮式平台垂荡运动引起的参数激扰对立管涡激振动的影响考虑进分析模型中，以有限元理论为基础编制了深水立管动力响应分析程序TTRPD1.0，并研究了深水浮式平台垂荡运动对立管响应和应力强度的影响，研究发现：海洋平台的垂荡运动引起的参数激扰影响了立管的涡激振动响应和强度，并且参数激扰对立管响应的影响是环境荷载、立管参数、平台垂荡幅值和垂荡频率的函数。本文的创新主要体现在：对熟知的涡激力进行了修正，提出了能够预测深水柔性立管区别于传统刚性立管涡激振动特征的非线性时域分析模型；将深水柔性立管大变形效应和海洋平台的垂荡运动对立管涡激振动响应的影响考虑进立管结构的涡激振动力学模型中，研究了大变形和深海平台垂荡运动对立管涡激振动响应和强度的影响；最终提出了更加符合工程实际的适用于深水立管涡激振动响应预测的非线性时域分析模型。更多还原

【Abstract】 In recent years, owing to the large demand of crude oil and gas in the world,explorations have been moved to deeper water regions. Ocean structures such aslong flexible risers exposing to current flows at deep water are plagued with vortex-induced vibration problems. If failure occurs, it will not only cause economiclosses,but also cause environment pollution. Correctly predicting dynamiccharacters of deepwater risers become more and more important. This paperproposes a non-linear time-domain analysis model for VIV response of deepwaterrisers by model test, theoretic deduction and numerical analysis, and studies thedynamic responses and stress strength of deepwater TTRFirstly, on the base of the results of model test, the paper finds that thevortex-excited force on oscillating riser is not a harmonic force, but a random forcewith narrow bandwidth character. So the VIV responses of risers is random. Andthe paper holds that when the risers oscillate in fluid, the oscillation of riser acts onthe the fluid which is around structure, the fluid conversely acts on structure. Thuscoupling effect exists between structure and fluid which is around risers.Considering the coupling effect, the paper modifies Morrison vortex-excited force,and applies the modified vortex-excited force to analysis on VIV response ofdeepwater risers. A non-linear time-domain analysis model for VIV response ofdeepwater TTR is proposed, and corresponding program TTRTD1.0is designedwith MATLAB. The paper also studies on VIV response and stress strength ofdeepwater TTR operated in a water depth of1500m. The calculated results indicatethat the program TTRTD1.0can effectively predict the VIV responses characters ofdeepwater TTR on base of the proposed analysis model, including multi-modeexcited, high order mode excited, asymmetrical distribution of displacement andlarge displacement. Secondly, the paper modifies the non-linear time-domain analysis model forVIV response of deepwater risers by theoretic deduction considering the largedeformation character of deepwater risers when vortex-induced vibration occurs. Anew analysis model is proposed considering large deformations, Accordingly theprogram TTRLD1.0is designed on the base of the new model. And the dynamiccharacteristic of the new model is analysis ed theoretically. The theoretical analysisresults indicate that large deformation decreases the natural frequency of structure,especially low-order natural frequencies. But large deformation had no effect onvibration mode of simply supported risers. At the same time, The paper also studiesthe large deformation effect on the response and stress strength of deepwater risers.The calculated results compared with the results of no considering large deformationindicate that large deformation increase the amplitude of risers with the increase ofvelocity of current, but it is non-linear relationship between them. And t hecalculated results of stress strength indicate that large deformation increase the stressstrength amplitude of weak location of risers. All the results confirm that largedeformation go against fatigue life, so large deformation effect can’t be neglected.Lastly, deepwater risers are installed between wellhead at the sea bed andfloating platform.The heave of a floating platform induces a fluctuation in time ofthe axial tension of the riser, especially semi-submersible platform. A possible andundesirable phenomenon is the excitation of a transverse riser vibration caused bythis fluctuation. According to the above, the paper further improve the VIVpredicting model considering the heave of a floating platform. Accordingly theprogram TTRPD1.0is designed. The paper studies VIV response and stress strengthof deepwater TTR operated in a water depth of1500m. The calculated resultsindicate that the heave of a floating platform affect the amplitude of VIV responseand stress strength. And the effect of the heave of a floating platform ion VIVresponse of deepwater risers is the function of environmental load, riser parameter,the amplitude of heave and frequency of heave.The new ideas and innovative points lie in the following:1. This paperimproves the Morison vortex-induced force, and proposes a non-linear time-domain analysis model for VIV response of deepwater risers.2. This paper further improvesthe VIV predicting model considering large deformation, and studies the effect onVIV response of deepwater risers.3.This paper further improves the VIV predictingmodel considering the heave of a floating platform, and studies large its effect onVIV response and stress strength of deepwater risers.4. A more corresponded toengineering practice model for predicting VIV response of deepwater risers isproposed finally in this paper.更多还原