Investigation On The Dynamic Response Characteristics For Tension Leg Platform In Internal Solitary Waves In A Two-fluid System

It has abundant resources of oil and natural gas in the South China Sea,but the marine environment is very complicated,and the offshore structures often encounter many atrocious ocean conditions when explore the oil and natural gas resources in deep sea area.Many Satellite Remote Sensing observation results and Moored Array's monitoring data show that internal solitary wave is one of the most dangerous ocean conditions when explore the oil and natural gas resources in the deep sea area of South China Sea.The internal solitary wave usually has a large amplitude and a great characteristic wave length,travels tens or even hundreds of meters below the free surface and it unusually causes strong underwater currents which would bring great damages to many offshore structures.On the basis of reviewing and summarizing the recent study about offshore structures and internal solitary waves,the dynamic response characteristics of “Liuahua” tension leg platform were considered.According to the observation data about internal solitary waves in “Liuhua” area and the motion equations in time domain for tension leg platform,a theoretical dynamic responses model was established.The main work carried out in this paper is summarized as follows:In chapter ?,a series of experiments were conducted in a density stratified gravity type internal wave flume to investigate loads characteristics exerted on “Liuhua” tension leg platform by internal solitary wave in a two-fluid system.Based on the experimental results,a theoretical model for internal solitary wave loads exerted on “Liuhua” tension leg platform was established.Further study shows that the horizontal forces exerted on “Liuhua” tension leg platform are mainly consist of two parts: the Morison forces exerted on columns and the Froude-Krylov forces exerted on the pontoons and columns.The vertical forces exerted on “Liuhua” tension leg platform are mainly consist of the pressures exerted on the bottom of columns and the pressure discrepancy of the upper and lower surfaces of pontoons,furthermore,both the pressures exerted on the bottom of columns and the pressure discrepancy of the upper and lower surfaces of pontoons could be calculated by Froude-Krylov force formula.Further study results show that the theoretical calculation results of internal solitary wave loads,calculated from the theoretical loads model,have a good agreement with the experimental results.In chapter ?,the tendons of the mooring system are simplified to nonlinear beams,and based on the theoretical model of internal solitary wave loads exerted on “Liuhua” tension leg platform,a dynamic response model,which has three degrees of freedom motion of the tension leg platform,is established by combining the finite time-domain motion equations of the platform.According to the observation data of internal solitary waves at Liuhua area in the South China Sea,the dynamic responses of “Liuhua” tension leg platform,mooring in Liuhua area,are calculated by numerical methods.The numerical calculation results show that the internal solitary waves would cause great impact loads on the platform and large horizontal movement.Because of dramatically increasing of the mooring forces,the platform also has a significant vertical movement,but the amplitude of rolling is very small compared to the horizontal movement or vertical movement of the platform.Further study shows that the depth ratio of the upper and lower water also has a great effect on the platform's dynamic responsesBased on the experimental results,a theoretical model of coupled dynamic responses for “Liuhua” tension leg platform is established in this paper,and it will be a useful tool for the future practical engineering application.