深海立管参激—涡激联合振动与疲劳特性研究

发布时间：2018-04-10 08:38

本文选题：深海立管　切入点：固有振动　出处：《天津大学》2014年博士论文

【摘要】：随着油气资源需求量日益增大，海洋油气资源的开采渐渐由浅海向深海领域发展。立管作为连接海面与海底的一种通道，进行勘探、钻井、输液等工作，是深海开发中必不可少的关键装备。立管长期在水面以下工作，除了时刻承受风、浪、流等载荷作用，内部还有高压的油或气通过，此外立管顶端上部浮体的运动以及结构与流体之间的相互耦合都会对立管产生重要的影响。因此，研究深海立管在复杂海洋环境下的动力特性具有重要的理论与工程意义。由于平台升沉运动引起立管轴向力随浮体运动而发生时变，从而作为一个参数激励对立管涡激振动（VIV）产生重要影响。本文以Spar平台顶张力立管（TTR）为研究对象，旨在通过深海立管参激-涡激联合振动与疲劳特性研究，为立管的动力设计和优化提供理论基础和技术支持。本文的主要工作和结论如下：（1）根据立管组成和结构特点，以及立管所受外载荷情况，建立了深海立管参激-涡激联合振动分析模型；分析深水立管长细比、自重及顶端受浮体垂荡运动等因素对立管固有振动的影响，针对不同的分析模型，提出固有振动特性计算方法。（2）把单自由度参激振动经典Mathieu方程用于多自由度立管参激振动稳定性分析。通过Garlkin法将立管运动偏微分方程转化为Mathieu方程，依据Mathieu不稳定图对立管每一阶模态进行独立分析后叠加得到立管的不稳定图，并与数值算法结果进行对比。结果表明，立管各模态不稳定区存在大量重叠区域，由于高阶不稳定区范围狭窄且在阻尼作用下迅速减小，实际应用中，可只考虑立管一阶不稳定区。（3）考虑立管自重，基于Floquet理论研究了轴向变张力下深海立管参激稳定性特点。与恒张力模型相比，主要有2点不同：一是立管固有频率显著减小，各阶频率更加接近，在一定的频率范围内，有更多的模态被激发，参激不稳定区域显著增大，立管更易发生参激失稳；二是立管模态振型不正交，带来模态耦合效应，立管发生多模态振动，立管参激共振除满足主共振条件外，，还发生组合共振，组合共振的对立管的危害更大。（4）基于Van der Pol尾流振子模型，采用有限差分法计算立管的振动响应，研究了流速、顶张力和参数激励对立管涡激振动的影响。结果表明，参数激励加剧了立管的涡激振动，特别对于低流速下深海立管涡激振动影响更大；当流速较低而平台升沉幅值和频率较高时，立管横向振动频率约是参激频率的0.5倍；参数激励作用下，立管底部振动位移和弯曲应力明显增大。（5）提出了参激-涡激联合振动试验方法，设计了参激加载方法和装置，制作了合理的试验模型，研究了流速、顶张力、参数激励对立管涡激振动的影响，并验证了数值模拟结果的可靠性。（6）基于参激-涡激联合振动模型和Miner疲劳累积损伤理论，研究了立管参激-涡激联合振动疲劳损伤，揭示了立管振动疲劳损伤机理，提出了参激-涡激联合振动疲劳损伤预报方法，分析了参数激励对立管涡激振动疲劳损伤的影响。结果表明，立管疲劳损伤随水深增加而增大，最大疲劳损伤在立管底部。参数激励对立管底部疲劳损伤有较大影响。
[Abstract]:With the increasing demand of oil and gas resources , the exploitation of offshore oil and gas resources is gradually developed from the shallow sea to the deep sea .

The axial force of riser due to the heave motion of the platform is time - varying with the movement of the floating body , which plays an important role as a parameter to excite the vortex - induced vibration ( VIV ) of the opposing tube . In this paper , the Spar platform top tension riser ( TTR ) is used as the research object , which aims to provide theoretical basis and technical support for the dynamic design and optimization of the riser . The main work and conclusion of this paper are as follows :

( 1 ) Based on the composition and structural characteristics of riser and the external load of riser , a combined vibration analysis model of deep sea riser - vortex - excitation is established .
In this paper , the influence of the slender ratio , the dead weight and the top end of the deep water riser on the inherent vibration of the counter tube are analyzed , and the inherent vibration characteristic calculation method is proposed for different analytical models .

( 2 ) A single - degree - of - freedom parametric oscillation classic is used for the analysis of the stability of a multi - degree - of - freedom vertical tube reference vibration . By means of the Garlkin method , the vertical tube motion partial differential equation is converted into a vertical tube instability map , and compared with the numerical algorithm results .

( 3 ) Considering the self - weight of riser , the stability of deep sea riser under axial variable tension is studied based on the Floquet theory . Compared with the constant tension model , there are two different points : firstly , the natural frequency of the vertical tube is obviously reduced , the frequency of each step is more close , and more modes are excited in a certain frequency range ;
secondly , the mode vibration mode of the vertical pipe is not orthogonal , the mode coupling effect is brought , the vertical pipe generates multi - mode vibration , the vertical pipe parametric excitation resonance does not meet the main resonance condition , the combination resonance is also generated , and the harm of the combined resonance is greater .

( 4 ) Using finite difference method to calculate the vibration response of riser , the influence of velocity , top tension and parameters on the vortex - induced vibration of riser is studied .
When the flow velocity is low and the platform heave amplitude and frequency are high , the vertical tube transverse vibration frequency is about 0.5 times of the reference frequency ;
Under the action of parameters , the vibration displacement and bending stress of the bottom of the vertical pipe are obviously increased .

( 5 ) In this paper , a method of parametric excitation - vortex combined vibration test is put forward , and a reasonable experimental model is designed . The influence of flow velocity , top tension and parameters on the vortex - induced vibration of opposing tube is studied , and the reliability of numerical simulation results is verified .

( 6 ) Based on the theory of parametric excitation - vortex combined vibration model and Miner fatigue cumulative damage theory , the mechanism of vertical tube vibration fatigue damage is studied . The influence of parameters on the fatigue damage of vortex - induced vibration is analyzed . The results show that the fatigue damage of riser increases with the increase of water depth , and the maximum fatigue damage is at the bottom of riser .

【学位授予单位】：天津大学
【学位级别】：博士
【学位授予年份】：2014
【分类号】：P756.2

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