【作者】 林海花；

【导师】 王言英；

【作者基本信息】 大连理工大学 ， 船舶与海洋结构物设计制造， 2008， 博士

【摘要】 隔水管是海洋浮式生产系统中的重要附属设备。为了进一步探讨在波浪、海流等外载荷作用下隔水管的运动特性,以便更好地应用于工程实际中,本文对隔水管的涡激动力响应及疲劳损伤可靠性等进行了深入的研究。采用Morison方程对属于小尺度结构物的隔水管进行受力计算,其中的关键是阻尼系数C_d和惯性力系数C_m的确定。本文基于人工神经网络技术,构造具有一层隐含层的BP网络来获得合适的水动力系数。在网络中运用附加动量法和自适应学习速率进行改造,其中的影响参数选定为雷诺数Re、Keulegan-Carpenter数KC和粗糙度参数κ.另外,在涡激动力计算中还考虑了横向涡激对C_d的影响。结果表明,采用这种方法所得到的水动力系数与试验结果符合得较好。通过变分法和最小势能原理推导了隔水管的运动控制方程,该方程在顺流向和横流向上具有类似的形式。在涡激振动计算中,以Matteoluca改进的Van der Pol尾流振子模型为基础计算横流向上隔水管与流体之间的相互作用。这里的隔水管涡激动力响应问题是一个存在多重非线性的问题,首先隔水管的受力计算中存在速度的平方项;其次水动力系数的确定受很多因素的影响,是一个严重非线性问题;再者,涡激问题本身也是一个非线性问题,在流体载荷的作用下隔水管产生运动,流体和隔水管之间存在相互作用,即流固耦合非线性;另外,隔水管的运动还呈现出大位移小变形的几何非线性等等。本文综合考虑了以上各种非线性的影响,通过Hermite插值函数对隔水管的动力微分方程进行有限元离散,利用UL法描述几何非线性的影响,并采用Newton-Raphson迭代法和Newmark方法相结合的过程建立了空间隔水管非线性涡激动力响应的迭代算法。频域内分析了隔水管固有频率特征,时域内考虑了不同海流分布情况、不同波浪参数、不同顶部预张力、浮体的水平和升沉运动等任意组合的影响,给出了以上不同参数组合下相应的位移分布包络线以及横向涡激振动响应曲线。通过大量的计算实例,验证了本文计算方法及程序设计的有效性和适用性。正因为在外力作用下隔水管在平衡位置附近来回振动,使得隔水管产生疲劳损伤,严重时甚至产生疲劳断裂,造成重大的经济损失和环境污染等。因此,如果能够很好地抑制隔水管的振动,则在很大程度上可以降低危险发生的概率。针对目前的抑制装置及其研究成果,本文选择螺旋边条作为抑制隔水管涡激振动的装置。为了寻找到螺旋边条的最优形式以便于最大程度地抑制隔水管的涡激振动,本文利用Fluent软件模拟了三维空间中隔水管周围的流场,给出了不同流态下安装不同螺旋边条的模拟结果,最后建立优化模型并对模型进行分析求解得到了螺旋边条的最优形式,为其实际工程应用提供了理论依据。很多学者都只考虑涡激振动对隔水管引起的疲劳损伤进行分析,事实上,隔水管的顺向振动同样重要,尤其是水深不大的情况下。本文通过Von Mises应力处理隔水管多轴应力问题,并考虑了平均应力的影响,计算得到隔水管关键点处的应力时间历程,再根据雨流计数法、S-N曲线和Miner线性累积损伤模型估算隔水管的疲劳寿命,并采用Wirching方法进行了可靠性分析。计算结果表明,所采用的方法正确、有效,可以为隔水管的疲劳可靠性分析提供依据。更多还原

【Abstract】 Marine riser is an important appertain equipment of ocean oil producing system. An investigation emphasizing on dynamic response including VIV of marine riser subjected to waves and ocean currents in 3-D space was presented in this paper in order to be used in the actual engineering.The key is to determine the hydrodynamic coefficients C_d and C_m in Morison equation for the force calculation of marine riser. The two coefficients were calculated by applying BP neural network which had one hidden layer, then used additional momentum method and auto-adjustive learning rate to modify the network. Results indicate that this method is reliable and it can calculate Morison equation’s hydrodynamic coefficients according to different Re、KC and roughness number k.Based on the minimum potential energy principle, the marine riser model was developed containing the strain energy due to bending and effective tension, the kinetic energy due to both the riser and the internal fluid motions, the work due to external forces. Then the governing equation of marine riser was derived through functional variation method, and the forms of the equation at inline and transverse direction are similar. And, VIV calculation was based on Matteoluca’s wake oscillator model. The dynamic response problem of marine riser is a highly nonlinear problem, including the square term of the velocity, the determination of the hydrodynamic coefficients, VIV, fluid-solid coupling between the riser and fluid and geometrical nonlinearities. All these nonlinear factors were considered in this paper, the governing equation is dispersed by using Hermite interpolation function. An iterative-incremental method based on the updated Lagrangian formulation of beam element, Newton-Raphson and Newmark method was used to solve the nonlinear problems. Finally, the natural frequency characteristics of the marine riser were given in the frequency domain. And in the time domain, the displacements in the direction of in-line and cross-flow were given under different currents, waves, pretensions, movement of the platform etc. The results demonstrate that the dynamic response of marine riser was described well by using these methods.The flow over a marine riser with and without helical strakes was modeled to explore the VIV phenomena through FLUENT software. The simulation model can be specified over a range of Res. The number of the strake, the strake width and the pitch were variable in order to find the better form to suppress VIV. Finally, the instantaneous vorticity and total pressure, drag and lift coefficients were displayed. The results indicate that the response characters of a bare riser can be quite distinct from that of a riser with helical strakes, and their performances were dependent on their geometry. Finally an optimization model was built and solved, thus the optimal form of the helical strake was obtained in this problem, and these results may provide a theoretical basis for the practical engineering application.Many fatigue damage of marine riser were only analyzed to VIV. Acturally, it is also important to in-line vibration especially for the small sea depth. The time history of marine riser’s key point’s stress can be got through Von Mises method including the effect of multiaxial stress and mean stress. Thus the fatigue life of the marine riser can be solved through rain-flow method, S-N curve and Miner linear cumulative law model and the reliability analysis can also be solved through Wirching method. The results demonstrate that these methods are valid and they can provide basises for the analysis of fatigue damage and reliability.更多还原