【作者】 黄海洋；

【导师】 张新曙；

【作者基本信息】 上海交通大学 ， 船舶与海洋工程， 2019， 硕士

【摘要】 月池是船体或者油气勘探平台上的垂直开口,可用于管道铺设、立管升降、潜水员出入之类的海上工程操作。在入射波或者船体做简谐运动的激励下,月池内的流体可能会出现巨大的运动响应。这种运动通常发生在月池的固有频率附近,称为月池共振。月池共振包括两种形式,第一种形式是月池内流体做整体式的上下往复运动,称为piston mode;第二种是月池内流体在壁面之间做前后间的水平晃荡,称为sloshing mode。这种共振可能会对船体的结构和海上工程操作造成不利的影响,因此在船舶的概念设计阶段就要提前预报出其共振频率。本文通过势流理论中的两类方法对月池共振问题做了三个主题的研究。通过半解析方法研究二维月池、三维圆形月池的共振频率,通过边界元程序WAMIT研究固定和自由浮动状态下矩形月池的共振频率偏移问题。针对二维形态的月池,本文建立了对称和非对称两种形式的月池模型分别进行研究。本文扩展了Molin提出的针对二维月池和矩形月池的无限水深模型,建立了新的有限水深模型。通过区域分解法对拉普拉斯方程的边界值问题进行求解。在船体下方区域的外垂直边界上,假设速度势为零。通过不同区域间的速度势匹配和流体速度匹配,把边界值问题转化为矩阵特征值问题。通过求解矩阵方程的特征值和特征向量,得到了月池共振的固有频率和相应的模态形状。在对称模型中,本文进一步推导了出其单模态估算公式(SMA)。通过公式可以快速预报出月池共振的各阶固有频率。文章对比了当前方法计算出的结果和其他文献中得到计算出的结果,发现该方法比无限水深模型的计算结果准确性大大提高,同时该方法具有计算快速的特性。特对圆形的月池,本文继承了二维月池的理论基础,建立了多种形式的圆形月池模型,包括单层流中的圆形月池无限水深模型、单层流中的有限水深模型,两层流中的无限水深模型、两层流中的有限水深模型。针对各种模型,文章使用与二维月池类似的方法,通过详细的理论推导,得到了计算其固有频率的特征值方程。同时给出了每个模型的各阶共振频率的单模态估算公式。船体固定状态和自由浮动的状态下,月池的共振频率是不一样的,上述半解析方法计算出的固有频率是船体固定状态下的固有频率。针对这个问题,本文使用了边界元程序WAMIT对固定状态下和自由浮动状态下矩形月池的共振频率进行系统性的比较与分析。文中建立了十个不同尺寸的带有月池的船体模型,一部分船体中的月池带有台阶,一部分船体中的月池不带有台阶。针对每个模型,都在0?,45?,90?的规则入射波中,计算了固定状态和自由浮动状态下的月池内波面升高、垂荡和纵摇响应。选取波高幅频响应曲线上的局部最大极值点作为共振的发生点。我们发现,固定状态下的共振频率或多或少的都比自由浮动状态下的共振频率小。对于吃水浅和月池-船体面积之比较大的情况,piston mode的频率偏移非常明显,而sloshing mode的频率偏移不明显。月池台阶的长度的增加可以显著减少piston mode的频率偏移情况,但是会增大sloshing mode的频率偏移。此外,还发现月池共振会引起垂荡或者纵摇运动幅频响应曲线的上的局部最大值。更多还原

【Abstract】 ‘Moonpools’are referred to the vertical openings,through hull of ship or offshore oil and gas exploration platform,used for marine and offshore operations such as pipe laying,riser hang off and diver recovery.Being exposed to incident waves or harmonic ship motions,the fluid inside the moonpools may perform significant resonant motions.This water motion mostly occurs at the natural modes of the moonpool,including piston mode,where the water inside the moonpool heaves up and down,and sloshing modes,where the water insider the moonpool moves back and forth between the vertical walls.It is essential to predict the resonance frequencies for assisting the concept design and guiding the marine operation,because the resonance fluid motion may lead to negative impacts to the hull or deck structure.Focusing on the moonpool resonance,there are three research topics in this arti-cle through two methods of potential flow theory.The resonance frequencies of two-dimensional moonpool and cylinder moonpool are investigated by semi-analytical meth-ods.The difference in rectangular moonpool resonance frequencies between the fixed and floating conditions is studied using panel code WAMIT.Theoretical models are proposed for computing the natural frequencies and modal shapes of two-dimensional asymmetric and symmetric moonpools in the finite water depth.The boundary value problem is solved by using a domain decomposition approach.On the outer vertical boundary bounded by the beam of the two bodies,linearized velocity potential is assumed to be nil.Eigenvalue problem is formulated by matching the velocity potential and fluid flux on the common boundaries to obtain the natural frequencies and modal shapes of the free surface elevation.In the symmetric moonpool cases,so-called single mode approximations(SMA)have been derived and can be adopted for rapid esti-mation of the natural frequencies for both piston and sloshing modes.The present results have been extensively compared with the solutions using the two-dimensional infinite water depth model developed by Molin,the numerical solutions and experimental data by Faltinsen et al..It is found that the solutions have been improved from the infinite water depth model.It is demonstrated that the proposed models can well predict the reso-nance frequencies and modal shapes for the two-dimensional asymmetric and symmetric moonpools.For cylinder moonpools,the theoretical method of two-dimensional moonpools is extended.Variety of cylinder models are built,including the model in infinite water depth,the model in finite water depth,the model in infinite water depth in two-layer fluid,and the model in finite water depth in two-layer fluid.Detailed theoretical derivaiton is presented in the article,and the eigenvalue functions are obtained to computed the resonance frequencies.Meanwhile the single-mode approximation(SMA)formulas are provided for each model.The difference in three-dimensional moonpool resonance frequencies between the fixed and free-floating conditions is studied using the panel code WAMIT.Ten vessel models with moonpools and different geometry parameters are analyzed to examine how the vessel and moonpool configuration affects the shift of the resonance frequencies.For fixed and free-floating conditions,incident regular wave with different frequencies with wave headings being 0?,45?and 90?are selected for the present computations.The free surface elevation inside moonpool,heave and pitch motions with respect to incident wave frequencies are computed for both fixed and free-floating conditions.The resonance frequencies are obtained by searching for the local maximum in the elevation-frequency curves.The resonance frequencies in fixed and free-floating conditions are compared and analyzed systematically.The motion RAOs and the free surface wave elevation inside moonpool are also examined together.It is observed that the resonance frequencies in fixed condition are more or less smaller than that in free-floating condition.The difference is more obvious in piston mode resonance for moonpool with small draft or moonpool with large area.In contrast,the shift of the sloshing mode resonance frequency is less obvious.The increase of recess length can reduces the difference in piston mode resonance distinctly,though it can enhance the shift in sloshing mode resonance frequency.Moreover,it is revealed that the moonpool resonances can induce local maximum in heave or pitch motion RAO.更多还原