The Investigation On Load And Dynamic Response Characteristics Of Spar Platforms Under Ocean Internal Solitary Waves

There are abundant storage of oil and natural gas in the South China Sea,so it will have a big effect on the energy safety of our nation if we can explore the oil and natural gas in the South China Sea.The ocean platforms play an important role in deep-sea oil and natural gas exploration.But there are some difficulties in the exploration due to the complex natural environments,such as the internal solitary waves happen frequently and often spread across a large area.Offshore platforms will be greatly influenced if they are placed in these places where there are frequent internal solitary wave activities,hence when designing the platforms,the influence of ocean environments must be taken into consideration.After reviewing and summarizing some relevant developments in the relative areas,a combination of model experiment and numerical simulation is taken to study the load characteristics and dynamic responses of Spar platforms with and without spiral strakes under internal solitary waves.The specific contents are organized as follows:In chapter Ⅱ,a series of experiments are conducted in large-scale density stratified water tank and the load characteristics of the Spar platforms with and without spiral strakes under internal solitary waves are investigated.According to experimental results,the longitudinal forces can be calculated by Morison equation of these two kinds of Spar platforms,and the methods of calculating the inertia coefficient and drag coefficient are given based on the experimental results.The time history characteristics of theory and experimental results are in good agreement.In the meanwhile,the magnitude of lateral loads is far less than the magnitude of longitudinal loads,this means that the lateral forces can’t be neglected in designing.The lateral forces of the Spar platform with spiral strakes are reduced nearly almost 50% compared with the Spar platform without spiral strakes.Hence the spiral strakes can not only reduce vortex induced motion,but also the lateral motion induced by internal solitary waves.The experimental results also show that the spiral strakes have little impact on the vertical loads,namely the vertical friction can be neglected.For Spar platforms with and without spiral strakes,the vertical loads can be calculated by Froude-Krylov equation.In chapter Ⅲ,based on the loads calculation method gotten from the second chapter,the loads characteristics and dynamic responses of the two kinds of Spar platforms under internal solitary waves are investigated.Firstly,the lumped mass method is used to the modeling analysis of mooring lines and the static structure and tension are calculated when given the top tension and its angle with the level.Based on the gotten results,the dynamic responses of mooring lines under top point displacement excitation are studied.The results show that when increasing the excitation frequency while keeping the excitation amplitude,the dynamic responses of nodes will increase correspondingly.And finally,the variation characteristics of loads and dynamic responses of the two Spar platforms with the changes of wave amplitudes and depth of the upper fluid layer are investigated.In chapter Ⅳ,summarizing the work in this paper and giving some advices on further research.