Fully Nonlinear Numerical Wave Model In Open Water And Its Application In Ringing Phenomenon Of Platform

With the development of ocean exploration toward the deep water, marine environment is becoming worse, and the nonlinear effect of waves is more significant. Springing and ringing phenomena are easily induced by high-frequency wave components for marine structures, which is aggravating with the increase of water depth. Springing and ringing phenomena has a great influence on the safety of the structure. So the study on the nonlinear waves interaction with structures is of great importance. In this paper, a higher-order boundary element method (HOBEM) is developed to simulate water waves and3-D bodies interactions on the basis of the fully nonlinear time-domain solution in the potential theory. In the fully nonlinear time-domain model, the mixed Eulerian-Lagrangian (MEL) technique is utilized to track the free surface, and the free surface conditions and the rigid motion equation are integrated by the fourth-order Runga-Kutta method. For the calculation of nonlinear wave loads, some auxiliary functions are introduced, instead of directly predicting the time derivative of the velocity potential. In addition, a dynamic grid technique based on the spring analysis is introduced to remesh the instantaneous free surface and wetted body surface. The intepolation is applied to derive the physical and geometrical quantities of the new nodes on the free surface.A fully nonlinear numerical wave tank model is developed, where the push-pedal wave-generating method and source wave-generating method are used to generate incident waves. Source wave-generating method has the advantage of allowing transmission of the waves reflected from objects back to the input boundary. However, the reflection of the lateral surfaces and the resonance of transverse standing waves are still exsit. Consequently, it is hard to accurate simulate water wave problems in open water by means of the numerical wave tank. With regards to the limitation of the numerical wave tank, a fully nonlinear decomposition is adopted to develop a new nonlinear numerical wave model in open water. The whole problem is separated into an incident part and a scattered wave component. The incident flow is evaluated exactly from the fifth-order Stokes wave theory. Fully nonlinear free surface boundary conditions for the scattered velocity potential must be satisfied on the instantaneous free surface. Compared to the numerical wave tank, one advantage of this method is that the problem is calculated in the circular domain, so that the treatment of open boundary conditions is easy solved, avoiding the reflection of the lateral surfaces and the resonance of transverse standing waves. Another advantage is that it needs less elements with the same precision, therefore this method is more effient.Firstly, the fully nonlinear numerical wave tank is applied to study the wave propogation and the wave diffraction around a fixed body. Meanwhile, the limitation of the numerical wave tank is analyzed. Secondly, the open-water model is adopted to study the diffraction and the radiation problems. Numerical examples presented relate to the wave diffraction around a fixed body, the forced oscillation of a floating body, interactions between waves and a floating body. The wave forces, wave elevations and motion responds are presented and verified by comparing with the other published results. Simultaneously, the numerical results from the open-water model are compared with those from the second-order frequency theory, indicating the limitation of the second-order wave theory and the advantage of the present method. In addition, the third-order diffraction forces agree well with the results from the third-order frequency theory. Finally, the open-water model is applied to the simulation of the ringing phenomenon of TLP. The effects of wave amplitude and wave frequency on high-fequency resonance in the vertical and pitching direction and the tension of the tendon are systematically analyzed. It is found that, compared to that in the vertical direction, the high-frequency resonance in the pitching direciton is more pronounced, the influence frequency range is more broader, and the impact on the tendon is greater.