The Direct Calculating Method Research Of Global And Local Strength Of Very Large Floating Structure

Very Large Floating Structure(VLFS)is a new type of offshore structure, which is made up of a number of connecting floating bodies that are similar with semi-submersible platform. Very Large Floating Structure could be used as airport on the sea, floating base and sea port, etc., and it is very important for marine engineering.At present, the domestic study of VLFS is still in its fundamental theory period. As a result, there is in urgent need of direct calculating method for its structural strength analysis,which would lay some relative theory foundation for its evaluation of structure safety.In this paper, single-module of VLFS is taken as study subject.First of all, based on ANSYS/ASAS-AQWA and design wave method, wave loads are calculated, and then the global strength is analyzed. As a result, the hotspots are determined. Secondly,the structure of hotspots are improved and strengthened, and the global strength analysis of VLFS are respectively calculated with ANSYS/AQWA and SESAM. At the same time, the results of the strength calculation are compared. Moreover, the local strength analysis are studied with ANSYS. At last a direct calculating method for VLFS’ structural strength analysis is obtained. The main works and conclusions are as follows:(1) Global strength analysis of VLFS with ANSYS/AQWA. First of all,based on design wave method and ABS specifications, hydrodynamic response and wave loads of VLFS are calculated with ANSYS/AQWA. Global strength of VLFS under typical dangerous wave loads is analyzed with direct calculating method. Analysis results show that the three connection parts of VLFS(connection parts between cross brace and floating body,connection parts between columns and upper box,connection parts between columns and floating body)have high stress concentration. Among these parts, the max Von Mises stress occurs at the connection parts between cross brace and floating body, second largest stress appears at the connection parts between columns and upper box.(2) The global strength analysis of VLFS after structural improvement with ANSYS/AQWA. According to the result of global strength analysis, the structure of the connection parts between cross brace and floating body are improved. Wave loads are predicted and the global strength analysis of VLFS after structural improvement are analyzed with ANSYS/AQWA. The results show that the Von Mises stresses of the improved structure are generally decreased, and the max stress value is fallen by 31%. So, it is very effective for reducing stress by improving and enhancing the structure.(3) The global strength analysis of VLFS after structural improvement with SESAM. First, the wave loads of VLFS are calculated, and then the global structure analysis is analyzed. The results show that the three connection parts of VLFS(connection parts between cross brace and floating body,connection parts between columns and upper box,connection parts between columns and floating body)still have high stress concentration. The max Von Mises stress still occurs at the connection parts between the cross brace and floating body.(4)The comparison of the global structure analysis of VLFS after structural improvement with ANSYS/AQWA and SESAM. It is shown that under the same sea condition, the hotspots of VLFS are similar with two kinds of software, that are the three connection parts of VLFS, namely connection parts between cross brace and floating body,connection parts between columns and upper box and connection parts between columns and floating body. The max Von Mises stress still occurs at the connection parts between cross brace and floating body.Moreover, the stresses of SESAM are generally decreased compared to that of ANSYS/AQWA.(5)The local strength analysis of key nodes of the VLFS with ANSYS. Firstly, local finite element models of key nodes are established. Secondly, the responses of global strength analysis are regarded as boundary conditions of local strength analysis. The results indicate the Von Mises stress of local strength analysis is generally increased compared to that of global strength analysis, and the largest stress of local strength still occurs at the connection parts between cross brace and floating body. Hence, the area should be thought as the hot spots for fatigue analysis, and it would be primarily researched in the future.