浮动海洋结构物诱发准陷波现象的模拟研究

Floating platforms are usually supported by several vertical columns. As the distance between the neighboring columns is typically small, the hydrodynamic interference effects are significant. At certain wave frequencies, an important phenomenon known as near-trapping could occur. When this phenomenon occurs, a large amount of trapped waves and a rather large amplification of free surface oscillation can be observed within the local vicinity of the columns, which brings extremely negative effects to structural safety and air-gap design. The motion response of offshore platforms has obvious nonlinear characteristics and the prediction of near-trapping phenomenon for moving bodies is a complicated task. Existed research theories and achievements about near-trapping phenomenon concerning fixed bodies cannot meet the requirements of practical engineering. In view of this, a numerical model is developed in this research based on the Cummins method and higher-order boundary element method (HOBEM), in which the nonlinear effects of the second-order incident waves and the motion response have been considered. As the research approaches based on the assumption of small-amplitude motion have limitations in investigating the hydrodynamics of bodies undergoing large-amplitude motions, the twice expansion analysis method has been employed in the research to overcome the limitations. In addition, the wave loading and wave elevation are considered as functions of incident wave amplitude only in the conventional Cummins method and the effects of the nonlinear motion response cannot be taken into account in a strict manner. To fix this problem, a modified Cummins method is developed based on the use of the quadratic transfer function (QTF) components in this research and the application of this modified method can ensure an accurate and strict prediction of the second-order wave loading and wave elevation. Then the present numerical model can be used as an effective tool for the investigation of the near-trapping phenomenon. Thereafter numerical simulation and experimental study are performed. The mechanism for the influence of the wave parameter, structural parameter and motion response on the near-trapping phenomenon and that for its occurrence have been investigated. Through the investigation in the present study more reasonable guidance can be provided for the air-gap design of offshore platforms.

海洋平台的支撑立柱通常布置在彼此相距较近的位置，特定条件的入射波会诱发准陷波现象，使大量入射波滞留在结构周围并引起大幅波面振荡，对结构安全带来不利影响。同时，海洋平台的运动响应具有明显的非线性特征，对浮动物体准陷波现象的预报是项复杂工作，现有关于固定物体准陷波问题的研究成果无法满足实际工程需要。为此，本项目将基于频时转换理论并应用高阶边界元方法建立计及物体运动非线性影响的二阶数值计算模型，通过应用两次展开来解决基于小位移假定的分析方法在求解大幅运动物体水动力问题上存在的不足，同时，通过引入平方传递函数分量来解决基于单参数摄动展开的频时转换过程无法根据实时物体运动计算结构荷载以及响应波高的问题，进而实现对浮动物体准陷波现象的有效分析。分别开展物理模型实验与数值模拟研究，探究波浪参数、结构几何特征以及物体运动特征对准陷波现象的影响规律，揭示准陷波现象的产生机理，为海洋结构物的气隙设计提供参考。

围绕波浪与漂浮以及固定多立柱式海洋结构相互作用问题开展系统研究工作，基于高阶边界元方法建立了单向、双向、单色以及双色海洋波浪与固壁以及开孔多立柱式结构相互作用的数值计算模型，通过应用自适应高斯积分以及建立辅助界分方程有效解决了准奇异以及奇异积分计算问题，通过分析散射波的远场演变特性，建立了无穷域上快速振荡积分的高效计算方法。基于上述分析方法，实现对入射波-入射波、入射波-物体运动以及物体运动-物体运动不同类别耦合作用问题的有效分析。将所建立数值模型已用于开敞、非开敞、漂浮以及座底柱群结构的分析研究中，对于布置于岸壁直墙前的柱群结果，通过应用镜像理论将非开敞水域下单向波作用问题转化为开敞水域下双向波作用问题。基于所建立数值模型开展系统的数值计算研究工作，并通过开展实验研究对数值工作进行补充和完善，对准陷波现象影响的产生机理与影响因素进行分析和研究。数值计算与实验结果表明，在非线性双色海洋波浪作用下，当不同入射波的合频以及差频频率与柱群结构的准陷波频率接近时，二阶准陷波现象将被激励起，因此，当波浪非线性效应较强时，准陷波现象可以在更为广泛的频率范围内被激励起。当采用开孔壁面结构时，由于水质点流经开孔介质时会产生能量的耗散，因此会有效降低准陷波现象发生时的响应波高。同时，由物体运动产生的辐射波会对陷波现象发生时的波面分布特性产生显著干扰，由于辐射波与局部波面运动间的相对相位关系，物体运动可以增强局部区域的陷波运动。