【作者】 王颖；

【导师】 杨建民；

【作者基本信息】 上海交通大学 ， 船舶与海洋结构物设计制造， 2010， 博士

【摘要】 随着离岸深水工程技术的不断发展,Spar平台在深海油气开采中得到了越来越广泛的应用。我国油气开采进军深海领域,亟待先进的海洋开发技术来支持,Spar平台这种深水和超深水海域中极具竞争力的平台类型,必将在不远的将来成为我国深海开发的重要工具。相对于其排水量来说,Spar平台水线面小,重心低,这使得Spar平台拥有极好的稳性,并且对波浪激励具有较小的响应,这些优良性能使得Spar平台很快得到了业界的认可并进入繁荣发展的时代。但是与其它类型的海洋平台相比,其特殊的深吃水立柱式结构容易引起来流作用下结构后方的旋涡脱落,从而导致涡激运动。所有的Spar平台,包括Classic Spar,Truss Spar,Cell Spar等,在一定的海洋环境条件下都可能发生涡激运动。在Spar上工作的人员如果长期受到涡激运动的影响,很容易疲劳和影响健康;长时间持续的涡激运动可能引起立管损坏、锚泊系统疲劳、甚至平台停止作业,因此在立管及锚泊系统的强度和疲劳特性设计过程中,以及在平台安全性方面,都必须充分考虑到涡激运动的影响。Spar平台涡激运动研究是目前海洋工程学科的前沿课题,是我国南海油气开采战略中的重要课题,也是国际上方兴未艾的研究热点。本论文在国家863计划的支持下,以Spar平台涡激运动的关键特性为主要研究内容,通过理论分析、CFD计算和模型试验相结合的方法,从涡激运动发生机理、关键特性、抑制方法等多方面寻找突破口,探索适用于Spar平台涡激运动预报和抑制的数值和试验方法,对Spar平台涡激运动现象进行系统深入的研究,在锁定现象、高Re数下涡脱落特征、系泊系统的非线性特征、多柱式主体减涡机制、波流联合作用下的涡激运动研究等方面进行了新的探索。论文第一章介绍了Spar平台的诞生和发展过程、目前在国际范围内的应用及研究情况,并对三代主要Spar平台的总体结构、水动力特征进行了系统的总结和阐述,进而介绍了Spar平台涡激运动问题的概况,并分别从容易引起涡激运动的海洋环境条件、涡激运动的原始激励及主导运动、影响因素和特征参数等方面对涡激运动现象的基本情况进行了阐述,总结归纳了目前国际上涡激运动研究相关进展、涡激运动研究及预报的主要方法,以及涡激运动研究的热点、难点、目前研究中所存在的不足、面临的问题和目前研究发展的方向,最后介绍了本文研究工作的意义、主要研究内容和研究方法,并提出了论文的创新性。涡激运动现象涉及到流体力学中实际流体绕流物型时所产生的很多问题,如速度和压强分布、边界层分离现象、绕流阻力与升力等等。深入研究涡激运动现象的机理及本质特征,必须建立在流体力学基础理论之上。本文第二章就涡激运动问题所涉及的流体物理性质、流体力学理论和数值计算方法进行了讨论,并总结分析了圆柱绕流问题的一些经典结论,为以后章节的数值计算和物理试验研究奠定了理论基础。同时,对国际上研究圆柱涡激振动的半经验模型法进行了一些介绍,并对其优缺点进行了分析。涡激运动之所以成为一种十分复杂的现象,首先归因于引起涡激运动的物理机制背后隐藏着复杂的流体力学机理。因此在进行实际Spar平台的涡激运动研究之前,对低质量比浮式柱体在来流中的泻涡情况、受力情况等重要特征进行相应的基础性研究是十分必要的,这有助于进一步深层次理解和掌握涡激运动现象背后的机理和本质。本文第三章主要从机理方面对研究涡激运动的流体激励、尾流流场、响应特征等进行了较为深入的探索,采用直接模拟流体力的CFD方法,结合模型试验,去除了海洋环境、平台具体结构、平台深水锚泊系统等外部复杂因素的影响,对低质量比、低长径比的较大尺度圆柱在均匀流中的绕流情况及涡激运动响应特征进行了模拟和研究。圆柱特征参数的确定以典型Truss Spar主尺度为参考,可视为Truss Spar主体硬舱部分的1:100缩尺比模型。研究了固定圆柱绕流的边界层分离情况、涡的形成和脱落情况、尾流区流场结构情况,对作用在圆柱上的包括升力和拖曳力在内的流体力进行了测量、计算和分析,从而对旋涡脱落所引起的流体激励特征进行研究;研究了强迫运动幅值及频率对圆柱受力的影响,研究流固耦合系统中结构物对流体的反作用机制,从根本性的涡脱激励入手,掌握引起涡激运动的流体动力特征;考虑涡激运动中的流固耦合效应,对流场和结构物相互作用的整个系统进行分析,研究了不同来流条件、不同系泊情况下圆柱的涡激运动情况,分析了锁定现象的发生机理和规律;研究了螺旋式侧板的减涡机理;特别地,在试验过程中通过一组简单有效的流场可视化系统,对不同情况下圆柱周围的流场进行了观测,通过更直观、更有说服力的试验观测结果对数值模拟所得到的流场进行了验证,进一步加深了对涡激运动现象的认识和理解。第四章在通过简单浮式圆柱进行涡激运动机理性研究的基础上,对Spar平台的涡激运动进行了研究。与上一章研究的简单圆柱不同,实际作业条件下的Spar平台在洋流中的涡激运动受到很多因素的影响,其中包括海洋环境条件因素、平台自身结构及附属物因素、深水锚泊系统特征等相关因素。本章以一种新形式的Spar平台作为研究对象,对该平台在没有安装减涡装置的情况下进行了涡激运动的预报,对其涡激运动响应特征进行了研究和分析。多柱桁架式Spar平台(Cell-Truss Spar)是一种新的Spar平台概念,该平台融合Classic Spar、Truss Spar以及Cell Spar平台的设计特点,并且应用了优化的垂荡板设计,目标是使其在具有前面三代Spar平台良好的运动性能的基础上,进一步改善水动力性能、降低建造难度和成本。相对于Classic Spar、Truss Spar、Cell Spar等平台形式,Cell-Truss Spar在结构形式上有其自身的特征,研究中充分考虑了这一特征,对其建立适当的CFD计算模型,并制作了精细的物理模型,对平台的主体结构、惯性特征进行了模拟。对于平台的系泊系统,在数值模拟过程中,对系泊系统横荡、纵荡两个水平方向的刚度曲线进行了拟合,并将拟合得到的刚度表达式代入涡激运动微分方程求解程序,采用龙格-库塔方法对涡激运动进行求解;在模型试验过程中,采用水深截断系泊系统,模拟了平台系泊的实际分布及刚度特征。探索了飓风惯性流、环流等具有不同流速剖面特征的来流条件的计算及模拟方法。研究得到了该平台涡激运动的响应特征和关键性参数,为进一步的涡激运动抑制研究奠定了基础。涡激运动的抑制是涡激运动研究中的重要方面。Spar平台一般在硬舱部分安装螺旋状的减涡侧板,用于抑制平台在来流中的涡激运动。本文第三章中对螺旋式减涡侧板的作用原理进行了研究。试验和实测数据表明,这种螺旋式减涡侧板对于抑制平台的涡激运动效果比较明显,是目前工程界普遍采用并认可、实用性最强的减涡方式。对于目前处于概念设计阶段的新型的Cell-Truss Spar平台来说,减涡侧板的设计工作也是抑制平台涡激运动、优化其水动力性能的重要方面。第五章在研究掌握了该Spar平台涡激运动响应特性的基础上,进一步探讨了涡激运动的抑制方法,对该Spar平台的螺旋式减涡侧板进行了设计和优化研究。由于Cell-Truss Spar平台在平台型式上与以前三代Spar平台相比,有着鲜明的自身特征,特别是由于上部硬舱的特殊结构形式,使得平台主体并不像Classic Spar和Truss Spar那样构成规则的圆柱体,又不完全同于Cell Spar平台的主体,因此针对Cell-Truss Spar平台的具体结构特征进行了具体的研究,提出适合于该平台的减涡侧板结构形式,通过数值计算,模拟了侧板附近的流场变化情况,以及平台在安装侧板后的受力情况和旋涡破碎情况,最后通过数值计算与模型试验相互结合,对侧板的减涡效率、安装侧板后平台的涡激运动性能等进行了研究。在实际的海洋环境条件下,海洋平台所承受的往往是风浪流的共同作用。Spar平台的涡激运动是由流的作用引起并决定的,但是在实际海况下,涡激运动往往是和风浪流作用所引起的其他水动力响应共同发生并相互影响的,因此讨论平台在实际海况中的涡激运动,特别是波浪的作用对涡激运动的影响,对进一步了解和掌握涡激运动的响应特征,从而在实践中对Spar平台在实际海况下的涡激运动进行研究和预报具有重要的意义,对平台的设计安装、现场作业等都具有指导性的意义。波浪在涡激运动中的影响尤为重要。在波浪和流的共同作用下,波浪运动中水质点的轨圆运动,导致平台前后都有旋涡脱落,使涡激运动更为复杂。第六章通过水动力模型试验的方法对波流联合作用条件下波浪对平台涡激运动响应的影响、平台在实际海洋环境下的涡激运动响应特征等方面进行了研究。分别从规则波和不规则波两个方面对波流交互作用入手进行研究,在规则波方面,考虑三种不同波高、不同周期的规则波,讨论了规则波参数对涡激运动响应的影响;在不规则波方面,分别以墨西哥湾和中国南海为预计作业地点,考虑了两种典型的波流联合作用极限海况,对平台的涡激运动进行了研究和分析,并对相应的系泊系统受力进行了研究。最后,文章在第七章对论文研究工作的主要内容及得到的相关结论进行了总结,对涡激运动研究未来的发展方向进行了展望,并在实际工程应用方面提出了相关建议。综上所述,本文研究中,从来流条件、Re数、平台主体形式、平台尺度及惯性特征、系泊系统、减涡侧板特征等影响Spar平台涡激运动特征的不同因素着手,提出了合理的计算方法及试验步骤,对Spar平台进行涡激运动预报及抑制研究,从而深入理解涡激运动的发生机理,得到平台周围流场的特征及变化规律,了解涡激运动的规律,掌握涡激运动关键特征、环境条件、影响因素、决定性参数及抑制方法,对复杂的多柱式主体的减涡装置提出设计方案及优化结果,得到了较为系统的、具有一定创新意义的结论,为我国深海油气开采提供了Spar平台涡激运动预报及抑制方面的参考依据和技术支持。更多还原

【Abstract】 Along with the further development of ocean engineering technology, more and more Spar platforms have been fabricated and applied in different regions of the world, and they have become efficient facilities in deepwater oil and gas exploitation. The step to deepwater exploitation of our country is on the horizon, and the most advanced techniques and equipments are urgently needed. As a competitive configuration in deepwater and ultra-deepwater region, the Spar platform will become one of the most important facilities in our deepwater exploitation career in the near future.The Spar platform has become an important design type for offshore deepwater operations due to many advantages compared to other type of platforms such as small waterline area, low center of gravity, excellent stability and small responses to waves. These excellent performances make the Spar platform accepted by the world soon after it appears, and now this design type is moving towards a period of flourish. However, due to the deep draft cylinder hull piercing into the water, all types of Spars, including Classic Spar, Truss Spar, Cell Spar, etc, could be subjected to Vortex- Induced Motions (VIM) in certain flow conditions such as strong currents like the hurricane and loop currents in the Gulf of Mexico. Since long time standing VIM can cause riser damage, mooring line fatigue, and even work stoppage, it’s a major consideration both in the riser and mooring strength/fatigue design and the safeness assessment of the platform. VIM of the Spar platform, as an important phenomenon which affects the motion performances of a Spar in certain current conditions, has become a new subject in the ocean engineering field.VIM study of Spar platform is a hot frontier topic in worldwide ocean engineering field, and it is also an important subject in the deepwater oil and gas exploitation in South China Sea. This dissertation, which is financially supported by the National High Technology Research and Development Program (863 program) of China, focuses mainly on the key characteristics of Spar VIM performance. The vortex induced mechanism, the substaintial performances and the suppression methods of VIM response, are studied through theoretical analysis, CFD simulation and model test method, and many new explorations are carried out in lock-in phenomenon, vortex shedding characteristics with high Reynold number, nonlinear mooring system, strake design of the multi-surface hull, and so on.In Chapter 1, the development of Spar platforms are presented, and the configuration and hydrodynamic characteristics of the three generations of Spars are summarized. The phenomenon of Vortex-Induced Motions is introduced, and the general development and progress of the investigation on VIM is concluded. The main content and the research method of this thesis are generalized in the end.The phenomenon of VIM is related with several important issues in hydromechanics, such as the velocity and pressure distribution in the fluid field, the flow separation in the boundary layer, the vortex shedding, the lift and drag forces of viscous flow passing objects, and so on. The study of the mechanism and characteristics of VIM should be carried out on the base of hydromechanic theories. Chapter 2 is mainly about the physical property of the fluid, the hydromechanic theories, and the numerical calculation method involved in VIM study. Some classical conclusions on viscous flow around a circular cylinder are presented and discussed. This chapter sets up the theoretical basement of the successive investigations in the following chapters.VIM is a complex phenomenon essentially due to the complex physical mechanism behind the vortex shedding. Therefore, it is necessary to carry out a foundational investigation on the fluid field and vortex shedding characteristics around a low mass ratio cylinder, without the influence of the exterior factors such as the complex vertical profile of the current, the appurtenance on the hull surface and the deepwater mooring system, etc, before the study of VIM of Spars. This could be helpful to reveal the inherent characteristics of the vortex-induced motion mechanism. Chapter 3 studies the wake flow and the vortex-induced motions of a floating circular cylinder under uniform current conditions. The cylinder could be considered as a 1:100 scale model of the hard tank of a typical truss spar. In order to understand the underlying fundamental principles contributing to the complex VIM phenomenon, both experimental and Computational Fluid Dynamics (CFD) methods are adopted to study the wake field feature, the fluid exciting force and VIM performance of the cylinder. The flow field around the circular cylinder, the flow sepration, the vortex-shedding characteristics and the resulting vortex-induced motion performances under different current conditions are investigated. Fixed, forced-motion and elastically moored conditions of the circular cylinder are studied, and many important parameters and conclusions related to the vortex-induced motions of the cylinder are obtained, including the lift and drag exciting forces, the influence of the forced-motion to the fluid field and the forces on the cylinder, the reaction of the moving cylinder to the fluid field, the property of the fluid-structure coupled system, and the vortex induced responses of the cylinder under different reduced velocities, lock-in mechanism, the efficiency of the helical strakes, and so on. The effects of mooring stiffness and distributions, current velocities and directions, and the function of helical strakes are all taken into consideration. Particularly, by using a flow visualization system, the unsteady flow around the cylinder and the vortices in the wake are captured and recorded during the model test procedure, and then compared with the CFD simulation. This study sets up a basis for the successive researches on VIM of real offshore platforms such as spars and semi-submersibles.Chapter 4 mainly concentrates on the VIM performance of Spar platforms. VIM of Spar platforms, which operate in deep sea, is influenced by many environment conditions such as sea state and current profile, the configuration and appurtenance of the hull, the mooring system distribution, and so on. In this chapter, a new concept of Spar platform called Cell-Truss Spar, which has been put forward recently, is studied using both CFD and model test method to research on its VIM performances under different reduced velocities in shear current conditions. The Cell-Truss Spar combines some good qualities of the Cell Spar and Truss Spar designs, aiming to bring in the lighter truss section and heave plate damping feature of the Truss Spar to obtain satisfactory heave motion performances, while reduce manufacture and installation difficulties by means of cell concept. In this study, the unique character of the Cell-Truss Spar is carefully considered in CFD simulations and the model tests, and the inertial parameters are modeled accurately. The actual distribution and the nonlinear stiffness of the mooring system are carefully considered in the study. The simulation method of different current profiles such as hurricane inertial current and the loop current are considered and suggested. Many important parameters in the vortex-induced motions of the Cell-Truss Spar are obtained, and the characteristics of vortex-induced-motion responses of the Cell-Truss Spar are analyzed and concluded. This is very important for the successive researches on VIM suppression of the Cell-Truss Spar. The suppression of vortex-induced motions is an important aspect of VIM study. To mitigate VIM, helical strakes are used on Spar hulls, and they have been proved to be effective and utility. For the Cell-Truss Spar, which is still on concept design stage at the present time, the design and optimization of the helical strakes is very important to control the VIM response and improve the hydrodynamic performance. In Chapter 5, the strake of the Cell-Truss Spar is designed and optimized, and the suppression of VIM is studied. As a result of the unique characters on the hull, the outer surface of the Cell-Truss Spar does not form a regular cylinder, and the strakes used on the existing Spars might not be suitable for the Cell-Truss Spar. In addition, it is more difficult to fabricate the conventional Spar strake members to fit on the multi-surfaced exterior of the Cell-Truss Spar. For the sake of suppression study on the VIM of the Cell-Truss Spar, four different strake configurations are put forward to be applied on the Cell-Truss Spar. The fluid field around the hull, the vortex disturbance near the strakes, and the forces acting on the Spar hull with different strakes are simulated by CFD method, and the strake efficiency is assessed through model test combining with CFD computation. The optimized strake configuration is finally chosen, and the VIM performance of the strake-equipped Spar is studied.In real ocean environment, the platforms are usually affected not only by current, but also wind and waves. The vortex-induced motions due to the current are always come forth together with other hydrodynamic responses, and they might be influenced by each other. The interaction of current and wave is especially important to VIM. The circular motion of the fluid particles in the wave makes the vortex shedding appear back and forth. This is a more complex problem. Field measurements of the classic Neptune Spar after the passage of hurricane Georges in 2001 suggest that VIM response in“hurricane inertial current”conditions in the presence of waves need to be considered in addition to VIM response in uniform loop/eddy current conditions. Hence, a comprehensive test program was undertaken to study the effect of waves on Spar VIM response in Chapter 6. The interactions of the current with regular and irregular waves are studied respectively. For the regular wave effect, three regular waves with different wave height and period are tested together with current, and the effect of different wave parameters to VIM is studied. While for the irregular wave effect, two classical sea state of current combining with irregular wave in Gulf of Mexico and South China Sea is considered to study the VIM responses. The mooring line forces are also measured and analyzed. The study of Spar VIM in wave and current is very important to understand VIM ulteriorly.Finally, in Chapter 7, the main content of the thesis and the corresponding results are concluded, and the developing trend for further studies in this field is put forward. Some suggestions for the related engineering practices are proposed in the end. To sum up, this thesis launches on several factors which could affect the Spar VIM response characteristics, including the current conditions, the Reynolds number, the hull configuration, the mooring system, the strake design, and so on. Numerical simulations and model tests are carried out to study the prediction and suppression of VIM. The vortex shedding exciting mechanism is studied, many important parameters of VIM are obtained, the mechanism and rules of lock-in phenomenon are analyzed, and the adoption of helical strakes are considered. The inherent characteristics of VIM are revealed, and several important conclusions are drawn. These will be very useful to the deepwater oil and gas exploitation in the future.更多还原