Nonlinear hydrodynamic forces acting on an offshore structure n waves

在波浪中系泊的近海建筑物经常经历缓慢的漂移运动。由于系泊系统所提供的恢复力较弱，自然周期较长，足以匹配缓慢变化的非线性波激振，从而产生幅度较大的水平偏移共振。准确预测这些漂移运动对系泊装置的设计至关重要。众所周知，当谐振被触发时，慢漂移阻尼对确定漂移运动的振幅起着关键作用。以往的研究表明，相对于其他慢漂移阻尼，如粘性阻尼和系泊阻尼，冷波漂移阻尼起着重要的作用，有时甚至占主导地位。本课题首先考虑了单圆柱在波浪和慢流中自由振荡的问题。对波浪漂移阻尼进行了一系列计算，以说明圆柱吃水和水深对波浪漂移阻尼的影响。然后考虑了多个圆柱之间的相互作用。并将计算结果与实验数据进行了比较。在这些工作中，采用特征函数展开的方法来解决边值问题。它在应用于任意形状的物体上有一个限制。另一方面，边界元法也被用于解决波浪和慢流中漂浮结构的问题。该方法应用于一个圆柱体。将计算结果与半解析法进行了比较，验证了该方法的有效性。然后对不同宽度和吃水的椭球体进行了一系列的计算，考察了椭球体形状对波漂阻尼的影响。研究了高频非线性波激励。用特征函数展开法计算了规则波列中的双频和三频激励。它们可能引起张力腿平台的弹跳和响铃现象。少

Offshore structures moored in waves often undergo slow drift motions. Because the restoring force offered by the mooring system is weak, the natural period is long enough to match up with the slowly varying nonlinear wave excitaion, which will cause resonance with large amplitude of horizontal excursion. It is critical for the design of the mooring devices to predict these drift motions precisely. It is well konown that the slow drift damping plays a key role to determine the amplitude of the drift motions when the resonamce is triggered. Previous works show that the so-colled wave drift damping makes an important contribution sometimes even dominant compared with the other components of the slow drift damping such as viscous damping and mooring damping.First of all concerning this topic, the probelm of a single circular cylinder freely oscillating in both waves and slow current was considered. Series calculations of wave drift damping were made to show the effects of cylinder draft an … More d water depth. Then the ineraction among multiple circular cylinders was considered. The comparison of calculated results with the experimental data were presented. In these works, the method of eigenfunction expansion was used to solve the boundary value problem. It has a limitation to be applied to bodies with arbitrary shapes.On the other hand, the boundary element method is also used to solve the problem of floating structures in both waves and slow current. The method is applied to a circular cylinder. Calculated results are compared with those by the semi-analytical method to validate the method. Then a series calculation are carried out on ellipsoids with various breadth and draft to examine the effects of body shape on the wave drift damping.High-frequency nonlinear wave excitation is also studied. Double frequency and triple frequency excitations in regular wave trains are computed by an eigenfunction expansion method. They may cause springing and ringing phenomena of a tension leg pratform. Less