【作者】 单铁兵；

【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， 2013， 博士

【摘要】 深海环境的不断恶化以及近年来超强台风等极具破坏力的极端海况频繁出现，对平台安全性提出了严峻挑战，诸多的安全性指标中，气隙性能一直是半潜平台设计过程中一个关键问题。气隙规定为海洋平台下层甲板至波面之间的垂直距离，是衡量平台甲板是否发生砰击和越浪的一个重要参数。传统的设计观念是下层甲板具有足够的高度以保证波浪打不到甲板。但增加平台初始气隙的花费相当的高，而且甲板的高度也受到自身重量和稳定性因素的制约，并不能无限制的增大。再者，随着海洋环境的不断恶化，仅仅通过增大干舷高度来避免越浪发生是很难实现的。大多学者都一致认为增加甲板结构的局部强度以抵抗平台负气隙引起的波浪砰击比初始设计时增加平台初始气隙来避免主甲板任何位置发生砰击更为经济和合理。准确预报平台各处的气隙响应，平台易发生越浪和非线性砰击的位置、砰击强度，以便在平台设计过程中合理布置甲板设备、加固局部位置的结构强度就显得至关重要。半潜平台波浪爬升和气隙响应的研究是目前海洋工程学科的前沿课题，也是我国南海油气开采战略中的重要课题。本论文采用理论分析、数值模拟和模型试验相结合的研究方法，对立柱绕射和波浪爬升效应的发生机理、不同的立柱横剖面形状和入射角度、波流非线性耦合、多柱结构的水动力干扰、半潜平台波浪爬升、气隙响应和非线性砰击效应等进行了较为深入的研究。采用连续性方程和动量方程为控制方程，VOF方法求解自由液面，运用摇板造波方式，建立了三维数值波浪水槽，为了防止水池末端的波浪发生反射而对工作区域造成影响，在动量方程中添加源项进行消波。对多种网格划分形式进行了详细研究，确定了最优的网格划分方案，并保证了数值方法的可行性。以单立柱为对象，采用模型试验和CFD方法对其周围的波浪爬升和波面扰动特性进行了详细分析。研究了入射波浪参数对立柱周围波浪爬升效应的影响；分析了边波的形成机理以及其沿立柱回荡过程中对波浪爬升的非线性谐频成分的影响，揭示了波浪爬升过程中能量的转化规律以及水质点运动分布特性；采用模型试验的方法验证了数值模型的准确性，并对立柱周围的流场分布以及自由面漩涡生成特性等进行了较为深入的机理性研究。早期的半潜式钻井平台由于作业水深和体型较小，立柱尺度不大，因此，横剖面形状多为圆形，随着钻井和采油不断向深海和超深海发展，平台立柱的形状也相应发生了变化，包括圆形、方形以及倒角率不等的方柱形，因此，有必要对不同横剖面形状的立柱周围波浪爬升效应展开预报。通过CFD计算和水池模型试验相结合的方法，得到了横剖面形状对波浪爬升效应的影响，获得了立柱周围波面扰动、表面漩涡形成、速度场分布、波浪爬升的幅度和非线性成分等随倒角率的变化规律，为今后平台立柱的选型提供了一定价值的参考。考虑到实际海况中波浪的多方向性，这将导致相对和垂直浪向的投影面形状发生改变，从而影响波浪的爬升效应。从波浪沿立柱周向的爬升幅值、曲线波动位置、立柱迎浪投影面形状、立柱等效直径等因素出发，探讨了入射角度影响波浪爬升效应和自由表面漩涡形成的内在机理。波浪传播至深层涡流和环流海域时，波浪和水流共存时的波流场并非纯波动场与纯水流场的简单叠加，而是一个较为复杂的组合过程，并影响平台周围波浪的非线性砰击、越浪、波浪爬升效应。因此，研究波流共同作用时海洋结构物所受的波浪载荷和波浪爬升显得尤为重要。建立了波流共同作用下的数值波浪水池，并在该水池内对立柱在波流共同作用下的波浪爬升和绕射问题进行了数值模拟，分析了海流的存在对立柱的波浪爬升幅度、非线性特征、波面扰动和水平拖曳力的影响，揭示了海流对立柱周围水质点运动轨迹和漩涡的干扰效应。半潜式平台通过多个立柱将平台下甲板与水下沉箱连接，立柱之间将存在较强的水动力干扰，导致波浪沿立柱表面爬升效应的增强以及自由面上局部波面的放大。针对不同组合方式的立柱结构，采用数值模拟和模型试验相结合的方法，详细分析了外部波浪参数、立柱间距以及入射角度的变化对柱体之间水动力扰动效应的影响；重点考察了四立柱结构下，内侧边波的相互干扰和叠加效应；揭示了波浪上涌现象的形成原因以及其对波浪爬升的影响；研究了不同入射角度下的波浪爬升、波浪叠加效应和各立柱的受力情况，深入探索了柱群内围区域发生近场干涉现象时波动场的分布特征以及波面迅速放大的内在机理。基于各立柱结构的研究工作，建立了用于求解波浪与半潜平台相互作用的CFD数值模拟方案。重点考察波陡参数和波浪散射参数对半潜平台周围波浪爬升、非线性砰击效应的影响；研究了影响砰击压强的各种因素，给出了波浪砰击平台下甲板时水舌的运动特征和形状的变化。研究了平台运动对立柱周围波浪爬升效应、平台下甲板处气隙响应、波浪非线性谐频成分和波浪时历的形状等的影响，并在水池展开了模型试验研究，验证了数值模拟的准确性。研究了水下沉箱的存在对平台的周围波浪爬升的影响，从波面的变化、水质点的运动特性如运动轨迹、方向以及速度大小等方面展开分析。研究表明水下沉箱将增大波浪的叠加和波面的扰动效应，并干扰水质点自由振荡的运动规律；另外，沉箱的上表面将托起部分水体，从而在一定程度上抬高了沉箱区域的波面。极限波浪是一种波高巨大、非线性极强的水波，对海洋平台的破坏性相当严重。采用波浪聚焦的方法研究了极限波作用下，半潜平台周围波浪爬升和非线性砰击效应与常规波浪的异同点。在实际的海洋环境条件下，半潜平台往往承受的是随机波浪的作用。采用水动力模型试验的方法，对随机波浪条件下，半潜平台的气隙响应，立柱附近相对波浪爬升的谱密度分布，流对平台气隙分布的影响等方面进行了详细分析；研究了极限波浪下，平台周围的波浪爬升，运动特征、气隙响应以及发生砰击现象时水体的非线性扰动效应；分析了平台运动同波浪之间的相位差对波浪爬升的影响，并研究了波浪之间的叠加作用对波面的放大率和波浪非线性特征所产生的影响。本文从入射波陡参数、波浪散射参数、立柱横剖面形状、入射角度、波流非线性耦合、多柱之间的水动力干扰、多柱结构的排列方式、立柱的间距、有无水下沉箱、平台的运动状态等方面着手，提出了合理的数值计算方法和模型试验步骤，深入研究了半潜平台周围波浪爬升、气隙响应和非线性砰击效应，得到了较为系统的、具有一定创新意义的结论，为我国深海立柱式平台的设计时，预报平台的波浪爬升和气隙响应提供了理论依据和技术支持。更多还原

【Abstract】 It poses a severe challenge to platform security, with the frequent worsening ofdeep-sea environment and the appearance of super typhoon. Air-gap performance has beena key issue in the design of semi-submersible platform. The air-gap is defined as thevertical distance between the lower deck of platform and the wave surface, which is aimportant parameter to judge whether some nonlinear phenomena such as slamming andgreen water happen or not.The traditional design concept is that the initial air-gap is large enough to ensure wavewould not impact the deck. However, the height of deck is constrained by its own weightand stability. In addition, with the continuous deterioration of ocean environment, it isdifficult to avoid wave impacts and green water by simply increasing the height offreeboard of platform. Most scholars agree that it is more economical and reasonable toincrease the local strength of certain areas on the deck, which is apt to suffer the waveslamming, than avoiding any wave slamming on the main deck through increase the initialair-gap of platform. Therefore, it is greatly important to predict air-gap distributionsaccurately on the platform deck in order to reinforce local strength, arrange deckequipments etc.It is a hot topic of the research on wave run-up and air-gap response in oceanengineering field, and becoming an important issue in oil and gas exploitation strategy ofthe South China Sea. The mechanism of wave diffraction and run-up effects, thecross-sectional shape of column, the directions of incident wave, nonlinear couplingbetween wave and current, hydrodynamic interference of multi-columns structure, waverun-up and air-gap response of semi-submersible are studied based on the combinations oftheoretical analysis, numerical simulation and model tests.3D numerical wave tank is also created based on continuity and momentumequations. VOF method is applied to capture free surface. Wave is generated by flap-typemethods. In addition, source terms are added in momentum equations in order to preventthe reflection wave at the end of numerical wave tank. Several kinds of mesh creationstrategies are investigated in detail to ensure the feasibility of the numerical methods.Wave run-up and wave disturbance around a fixed truncated column is investigatedbased on model test and CFD method. The wave run-up effect according to differentincident wave parameters is investigated. The edge wave effects and non-linear harmonicfrequency components during wave scatters around column is also discussed. Data frommodel test verifies the accuracy of the numerical simulation. The flow distributions andvertex shedding characteristics are analyzed deeply. The column scale for early semi-submersible drilling platform is relatively small.The shape of corresponding cross-section is mostly rounded. As the drilling andproduction technology develops toward deep and ultra-deep sea, the shape of columnchanges accordingly. Therefore, it is necessary to predict wave run-up distributions aroundthe columns in different cross-section geometry. The effects of cross-section geometrytoward wave run-up, wave disturbance around column, vortex on free surface, velocitydistributions, amplitude and non-linear harmonic components of wave run-up areinvestigated based on the combination of CFD calculation and model test, which providesan important reference to the selection of cross-section geometry for next platformcolumns.Considering the multi-direction of ocean wave in actual sea conditions, which willlead to a change of projection surface, wave run-up around column in different directionof incident wave is studied. Wave run-up effects and the mechanism of vortex shedding onfree surface are discussed from some factors such as wave run-up amplitude alongcircumferential direction, curve fluctuations, projection shape, equivalent diameter ofcolumn.As is known, when the wave propagates to eddy and circulation current, the flowfield as wave and current coexist is not a simple sum between wave and current, but amore complex interactions between wave and current, which directly affects non-linearimpacts, green water, wave run-up effects. Therefore, it is greatly important to study thewave loading and run-up around floating structures in wave-current coupling conditions.This chapter uses the same numerical methods to establish the numerical wave tank underthe wave-current coupling. The effects of current to wave run-up amplitude, non-linearcharacteristics, wave perturbations, drag force, trajectory of wave particles and vortexshedding are analyzed in detail.The hydrodynamics interaction among columns is pretty strong for semi-submersible,which will induce large wave run-up and local surface amplification. A detailed analysis ofwave non-linear effects in different kinds of column combinations according to incidentwave parameters, column spacing and the directions of incident wave is presented. Inaddition, the interactions of edge wave, wave run-up round columns, wave superpositioneffects and forces are also investigated. The free surface characteristics and the mechanismof amplification as the near trapping phenomenon occurs, are studied in detail.Based on the work of each column-type, numerical wave tank used for investigatingthe interaction between waves and semi-submersible platform is established. The waverun-up and non-linear slamming around the platform according to incident wavefrequency and steepness are investigated. Many factors which affect impact pressure, movement characteristics of wave tongue are also presented. In addition, relative waverun-up and air-gap distributions, the non-linear harmonic components and shape of wavetime series around platform in motion conditions are obtained. In order to verify theaccuracy of numerical simulation, model test is conducted.Wave run-up distributions, free surface disturbances, movement characteristics ofwave particles with and without pontoon are all taken into consideration. It indicates fromthe results that pontoon under water will increase the effect of wave superposition anddisturbance, and interfere with the free oscillations of water particle motions. In addition,the upper surface of the pontoon would hold up some part of water, which to a certainextent to raise wave height above pontoon.The extreme wave is always huge and highly nonlinear, which poses a huge threat toplatforms. Wave focused method is applied to investigate the similarities and differencesin wave run-up and slamming between extreme and normal wave conditions.Under the actual ocean condition, semi-submersible platform are often subjected torandom waves. Air-gap responses, spectral density distributions of relative wave run-upand the effect of current around semi-submersible platform in random wave are consideredbased on model tests. Subsequently, wave run-up, movement characteristics, air-gapresponse and slamming are studied. In addition, some effects such as wave phasedifference between platform motion and incident wave, wave superposition are alsoanalyzed.The thesis launches on several factors such as incident wave steepness, scatteringparameters, cross-section geometry, the direction of incident wave, wave-current couplingcondition, hydrodynamic interfere among multi-columns, the arrangement ofmulti-columns, column spacing, pontoon, platform motion etc, in order to investigatewave run-up, air-gap response and non-linear slamming effects. More systematic andinnovative conclusions are drawn, which provides a theoretical basis and technical supportfor prediction of wave run-up and air-gap response when multi-column types of platformsare designed for deep water.更多还原