Nonlinear hydrodynamic forces acting on an offshore structure n waves

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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