大直径单桩基础是海上风机最常用的基础型式。在桩基安装完成后，由于波浪和海流等环境的影响，会使桩基周围产生局部冲刷，进而大幅度削弱桩基的承载性能。在复杂的水流环境下，桩基础周围的冲刷深度呈现随时间变化的发展过程。准确预测桩基周围冲刷深度随时间的发展，对合理评价桩基的稳定性至关重要，同时也对铺设合适冲刷防护措施的施工周期选取给出参考性建议。本文结合理论分析和物理模型试验等手段研究桩基局部冲刷的发展，对单向恒定流作用下桩基局部冲刷发展预测模型进行理论推导，并对潮流作用下的桩基冲刷特性和简化方法开展相应研究，以此来揭示桩基局部冲刷深度随时间的演变规律，为现场海上风机单桩基础周围冲刷深度的预测提供了理论依据和合理指导。

结合湍流唯象理论和能量学原理，得到作用在泥沙颗粒上的剪切应力比例表达式。引入平床输沙率公式和沉积物质量守恒定律，可以推导出水流作用下桩基局部冲刷发展的时变预测模型。该模型是一个具有物理机制的常微分方程，包含控制冲刷过程的所有参数，不存在尺度效应，可以用于现场原位桩基的冲刷预测中。忽略来流输沙项的影响，将时变模型退化到清水冲刷条件，进一步得到模型解析解，并结合72组试验数据对模型的关键系数进行率定。通过与以往的预测模型对比，发现所提出的模型预测精度和泛化能力更好，误差的概率密度函数分布集中在±10%范围内。将模型应用到现场原型单桩基础的清水冲刷发展预测中，发现预测的平衡冲刷时间随桩径、流速和沉积物粒度的变化与之前的试验观测结果一致。基于常微分方程，推导出可以用于刻画冲刷发展的时间尺度理论表达式，其预测结果和试验数据十分接近。

该模型的解析解也可作为拟合外推公式使用，该公式受到平衡冲刷深度、特征时间尺度、冲刷深度系数与主马蹄涡系数之比控制。可以采用冲刷过程数据去拟合这三个控制参数，进而确定整个冲刷过程。本文的拟合外推公式能解决现有公式存在的两个主要问题：（a）传统经验公式对冲刷发展的预测精度较差，会低估平衡冲刷深度，且提前达到平衡阶段；（b）一些公式在外推过程中存在收敛异常现象，导致平衡冲刷深度过大，与现实相悖。对不同拟合数据时长进行分析，发现本文的拟合外推公式在保证精度的同时也表现出较好的预测稳定性。进一步探索了该公式在非零初始条件、非恒定流情况下的应用前景。

实际海洋中流速的大小和方向是时变的，在流固耦合水槽中对两种典型潮流（正弦和方形潮流）下桩基周围的局部冲刷过程进行了一系列试验研究。结果表明，往复潮流导致桩周围的沉积物处于不断冲刷和回填过程，尽管正弦和方形潮流的冲刷坑形状相似，但冲刷深度的演变过程存在显著差异，分别呈现“短平台”和“锯齿状”特征。冲刷发展也可以看成水流对泥沙颗粒的做功过程，分析发现无量纲冲刷深度与无量纲有效流功之间存在一致关系，可以用三参数指数函数来表征。随无量纲有效功的增大，无量纲时间尺度呈下降趋势。基于无量纲有效流功，进一步提出了正弦和方形潮流的等效速度表达式，用来实现不同潮流之间的等效替换，并利用现有的试验数据进行了验证。考虑潮流双向输沙影响，提出了潮流和单向流作用下桩基冲刷深度之比的经验表达式，用来预测潮流下桩基局部冲刷深度。

Large-diameter monopile is the most commonly used foundation type for offshore wind turbines. Local scour will occur around the pile foundation after the installation due to environmental factors such as waves and currents, which will weaken the bearing performance of the offshore wind turbine structure. In complex water flow environments, the scour depth around pile foundations exhibits a dynamic development process that varies with time. Accurately predicting the scour development around pile foundations over time is crucial for evaluating the stability of pile foundations in a reasonable manner, and also provides reference suggestions for selecting the construction period for laying appropriate scour countermeasures. In the present study, the scour development around pile foundations is studied by combining theoretical analysis and physical model experiments. The prediction model of scour development around pile foundations under the action of unidirectional current is deduced theoretically. The scour characteristics and the simplified method of pile foundation under the action of tidal currents are studied. In this way, the evolution law of scour depth around pile foundation with time is revealed. It provides theoretical basis and reasonable guidance for the prediction of scour depth around pile foundations of offshore wind turbines on site.

A proportion expression of shear stress acting on sediment particles can be obtained based on the phenomenological theory of turbulent and energy principles. By introducing the sediment transport rate and the mass conservation law of sediment, a time-development prediction model for the scour around pile foundations under the steady current can be derived. This model is an ordinary differential equation with physical mechanisms, which includes all parameters controlling the scour process and does not have scale issues. It can be used for predicting the local scour of in-situ pile foundations. Neglecting the influence of incoming sediment transport, the prediction model can be degraded to the clear-water condition, and the analytical solution is further obtained. The key coefficients of the prediction model are calibrated based on 72 sets of experimental data. Compared with previous models, it is found that the proposed model has better prediction accuracy and generalization ability, with the probability density function distribution of errors concentrated within the range of ±10%. The model was applied to evaluate the scour development on the prototype pile foundation under clear-water condition, and it is found that the predicted equilibrium scour time changes with the pile diameter, flow velocity, and sediment particle size, which are consistent with previous experimental observations. Based on ordinary differential equations, a time scale theoretical expression is derived, which can be used to describe the scour development and its predicted results are very close to experimental data.

The analytical solution of this model can also be used as a fitting and extrapolation formula, which is controlled by the equilibrium scour depth, the characteristic time scale, and the ratio of the scour depth coefficient to the primary horseshoe vortex coefficient. The scour data can be used to fit these three control parameters and determine the entire scour process. The fitting and extrapolation formula proposed in this study can solve two main problems of existing formulas: (a) Traditional empirical formulas have poor prediction accuracy for the scour development, and there is a problem of underestimating the equilibrium scour depth and reaching the equilibrium stage ahead of time. (b) Some formulas exhibit a convergence anomalies phenomenon during the extrapolation process, resulting in excessive equilibrium scour depth, which contradicts reality. Through the analysis of different fitting data durations, it is found that the proposed formula not only guarantees the accuracy but also shows great prediction stability. The application prospect of the proposed formula in non-zero initial and unsteady current conditions are further explored.

The magnitude and direction of flow velocity in the actual ocean are time-varying. A series of experiments were conducted to investigate the local scour process around a pile foundation under two typical tidal velocity hydrographs (i.e., sinusoidal and square tidal currents) in fluid-structure-soil coupling flume. The results demonstrate that reciprocating tidal currents lead to a continuously evolving process of sediment erosion and backfilling around the pile. Although the shapes of the scour holes are similar between sinusoidal and square tidal currents, there are significant differences in the evolving process of the scour depth, presenting "short platform" and "serrated" characteristics, respectively. The scour development can be considered as a work process of approach flow moving sediment particles. Analysis shows that there is a consistent relationship between dimensionless scour depth and dimensionless effective flow work, which can be characterized by a three parameter exponential function. As the dimensionless effective flow work increases, the dimensionless time scale shows a decreasing trend. Based on the dimensionless effective flow work, an equivalent velocity expression for sinusoidal and square tidal currents is further proposed to achieve equivalent substitution between different tidal currents and verified using existing experimental data. Taking into account the influence of bidirectional sediment transport by tidal currents, an empirical expression for the ratio of scour depth under the tidal currents and unidirectional currents is proposed to predict local scour depth around pile foundation under tidal currents.