Reliability Analysis Of FPSO Based On Ultimate Strength During Lifetime

Catastrophic accidents of ships and marine structures which occur during its service not only seriously jeopardize the safety of the staff,but will also cause huge economic losses and serious environmental pollution.Therefore,it needs to be considered in the initial stage of ship design the ultimate strength of the whole life cycle of a ship.The traditional strength check are based on deterministic methods,since deterministic method does not take into consideration all the factors of uncertainty,so we need to adopt reliability analysis based on probility theory.In this paper,a nonlinear finite element method was used to analyze the remaining ultimate strength of crevice corrosion damaged stiffened panels with axial compressive load from pointviews of the distribution of the relative position of crevice corrosion, corrosion number,crack length,corrosion pits radius and other factors,which was also compared with the experiment results.On the basis of the above and by reliability analysis, we analyzed the remaining ultimate strength of FPSO cabin with crevice corrosion. Specific contents are followed :(1) This paper used non-linear finite element method to calculate remaining ultimate strength of the stiffened panels and analyze the influence of the mesh size,crevice corrosion's relative position,the number of corrosion pits,crack length,corrosion pits radius and other factors on the remaining ultimate strength of the stiffened panels.The results show that the crack length change has the most obvious impact while the influence of the mesh size and crevice corrosion's relative position are weak.(2) Based on the calculated results by the finite element crevice corrosion damaged stiffened panels specimen was designed to conduct axial compression test and measure the initial deformation of stiffened plate.By comparing the test results with the finite element calculated results,we found that the test results and finite element results are in good agreement,indicating the feasibility of the finite element method.(3) FPSO cabin was calculated by nonlinear finite element.We calculated the complete structure of a rib and a half boat model,a rib and a full boat model,four ribs and a full boat model,and transverse frame model joined among four ribs through nonlinear calculations, analyzed the impact of selection of model range on calculation results;we got the stress distribution and ultimate bending moment changing rule respectively of a complete cabin structure,cabin only with crack,cabin only with corrosion,and crevice corrosion damaged cabin under middle-hogging and middle-sagging state;considering corrosion changes in service life,we analyzed the impact of 10-year-corrosive pitting,15-year-corrosive pitting and 20-year-corrosive pitting on the residual ultimate strength of the cabin,and made a comparative analysis with uniform corrosion of 20 years,then we found that pitting has little impact on the ultimate strength of the whole structure while uniform corrosion has strong effect.(4) Analyze the statistical characteristics of the ultimate strength of the ship body,still water bending moment,and wave bending moment.In order to get statistical characteristics of the hull's ultimate strength,we combined Monte Carlo method with ANSYS software. Random variables,such as yield limit, elastic modulus,thickness were used and 100 sets of data were extracted.Then based on the 100 sets of data and the calculated results and by the response surface method to fit out the function formula of the first item,the quadratic item, and cross-iterm of the ship ultimate strength,and by using FOSM method to figure out reliability index of the ship structure and the failure probability.From the above analysis, we found that the reliability degree under hogging state is always greater than the reliability under sagging state and the response surface method to calculate the reliability is more conservative.