Research On Global Strength And Structural Improvement Of Different Types Of Very Large Floating Structure

The Very Large Floating Structure(VLFS)is a new type of floating platform with various forms,which could be assembled by several semi-submersible or barge basic modules of different specifications through connectors according to functional requirements.As a new type of floating platform with the size which is much larger than that of conventional floating structures,VLFS has important research significance for the development of the ocean resource,the protection of marine rights and the improvement of the comprehensive support ability of the deep sea.The main contents are as follows:Based on the potential flow theory,long-term forecasting method and direct calculation theory,hydrodynamic analysis and global strength analysis was carried out on the VLFS with longitudinal pontoon to determine the high stress area of the structure.Then,an improved scheme was proposed for the columns,the cross braces and the lower pontoons to explore the influence of different improvement schemes on the structural stress.Finally,based on the strength analysis results of improved structure,an improved scheme for the VLFS with transverse pontoon is proposed and the strength analysis is carried out.(1)The hydrodynamic analysis of the VLFS with two different configurations of longitudinal pontoon and transverse pontoon is carried out to obtain the motion response and the wave load transfer function in the frequency domain;then the forcast of long-term wave load is carried out to determine the design wave parameters;the global strength analysis is carried out under the the most dangerous working condition(the condition of longitudinal torsion)to obtain the high stress area of the structure.The high stress areas of the two structures of the VLFS structure are similar,which are the connection parts between columns and the upper shell,the bell-mouth position,and the connection parts between columns and the floating body.(2)According to the strength calculation results of the VLFS with longitudinal pontoons,four kinds of structural improvement schemes for the columns,cross braces and floating body are proposed.The upper body and the columns should be connected with rounded edge,and the connection position of the floating body and the columns should be tranversed to rectangular;A-shaped diagonal braces was added and corss section of the braces should be changed to the drum-shape;the frames should be added and the lower floating decks and bottom boards should be changed to curved panels;the ends of the column should be transitioned into the rectangle shape,the shape of the floating body and the cross braces should be changed and diagonal braces should be added.(3)The hydrodynamic analysis and strength analysis of the improved structure of the VLFS with longitudinal pontoons is carried out.The results of the comparative strength analysis show that the optimization effect is poor when the two ends of the column are changed to a rectangular shape,especially for the stress redution of the transverse braces.Rounded edge of the column,addition of the inclined braces,the change of the lower floating decks and bottom boards could reduce the stress of the high stress area effectively.The change of the cross section of the tranverse braces could reduce the stress of the tranverse braces effectively,but it could not bring obvious influence on other high stress area.Campared with other schemes,addition of the inclined braces,the change of the lower floating decks and bottom boards could reduce the stress the most effectively,which is the most reasonable scheme.(4)According to the calculation results of the global strength analysis of the VFLS with tranverse pontoons,with reference to the improvement scheme of the VFLS with longitudinal pontoons,the scheme of change the cross section of the column and the tranverse braces,addition of the inclined braces could.Through the strength analysis,the improvement schem could reduce the stress of the high stress area,but it could add new high stress area of the connection position of the upper body and the inclined braces.