Action Of Internal Solitary Wave On Offshore Structure And Its Response

Internal solitary waves can be observed in many sea areas around the world, especially in domains with rapidly changing topography, such as near continental shelves and straits. It is estimated that there are more internal solitons whose amplitude is often larger in the northern South China Sea, the sea of northeastern Taiwan and Bashi Channel than other sea areas. Compared with surface wave, the resilience of internal soliton is smaller, while the amplitude is larger and the frequent is lower. In this paper, the action of internal solitons on offshore structures and motion respondences of these structures are both computed; and the numerical results are compared with effects of surface wave, respectively.First of all, the internal solitary wave surface is simulated through the Kdv equation, and the velocity of water particle and the pressure of any point in the flow field of internal soliton is calculated and analyzed.Then, a numerical model to compute the effect on moving structure of internal solitary wave is established. In order to increase the adaptability of this model, marine structures are decomposed into pole elements and face elements. The forces of internal soliton on the pole elements are computed by Morison equation, on the face elements by the integral of dynamic pressure. The solitary wave force and buoyancy are solved through Kdv internal soliton theory. Based on the catenary theory, this paper calculates the force of the sectional anchor mooring system and the nonlinear relationship between anchor force and displacement. The motion respondences of the Spar platform under influence of internal solitary wave are also simulated by making use of the fourth order Runge-kutta method. With these results, the paper detailedly presents the actions of internal solitary wave on the Spar platform, its responses and changes of the anchor force.The boundary element method in frequency domain is applied to compute the action of internal solitary wave on the Spar platform and its response. It is proved that the action of internal soliton on offshore structure can not be ignored by comparing the action of internal solitary wave and surface wave on the Spar platform. Under the force of the internal soliton, the Spar platform produces a great horizontal displacement. Therefore, the low-frequency effect of internal solitary wave plays a significant role in the horizontal displacement of the Spar platform. It is simulated that when the maximum horizontal force on the Spar platform of the internal soliton is about 5%of that of the surface wave, the maximum horizontal displacement of Spar platform under internal solitary wave is about 3 times of that under surface wave. It can be concluded that the influence of internal soliton on marine structure is represented by great forces as well as the low-frequency effect.