Research On The Ultimate Strength Of FPSO Hull Girder Subjected To Random Corrosion Wastage

Maritime accidents caused by marine structures' corrosion wastage in their life cycle can not only bring life-threatening for the staff on board, but also can directly cause serious economic losses and even may pollute the marine environment. Therefore, to improve the safety of ships is the common desire for many scholars and designers, which involves the correct assessment of the ultimate strength of the hull structures, particularly the study on residual ultimate strength of ship structures subjected to corrosion wastage is significant.In this paper, experimental research on residual ultimate strength of stiffened panel damaged by pitting corrosion subjected to axial compression load has been made, using 6different stiffened panels with pitting corrosion to do experiments. The influence of pitting corrosion factors on buckling failure and the residual ultimate strength of stiffened panel will be analyzed by changing the location, diameter, depth and number of pitting corrosion in specimen. Numerical simulation calculation is made respectively for specimen with the initial deformation and without initial deformation, comparing with the experimental results.Basing on progressive destruction theory, numerical simulation calculation on the ultimate strength of stiffened panel with pitting corrosion has been made using the nonlinear finite element method(fem) with the help of ANSYS platform, respectively discussing the influence of pits' location, diameter, number, depth and corroded volume loss on ultimate strength of stiffened panel, buckling failure and stress distribution. The residual ultimate strength of FPSO structures subjected to corrosion wastage has been researched with nonlinear finite element method(fem). FPSO structures' ultimate bending moment respectively in hogging and sagging state subjected to uniform corrosion and pitting corrosion is calculated, which the corrosion wastage location covers deck, broadside plate and bottom plate of ship.The varying regular pattern of ultimate bending moment and the mutual influence of the structure under external loads have been discussed. Considering the randomness of stiffened panel's material parameter, components' size and the size of pitting corrosion, the residual ultimate strength of stiffened panel and the ultimate bending moment of FPSO structures have been calculated with nonlinear Finite Element Method and Monte Carlo Method. Probability density scatter plot has been obtained basing on statistical knowledge. The fitting curve of scatter plot has been made and sensitivity of random variables is analysed.Results of the research in this paper show that pitting corrosion can weaken theultimate carrying capacity. Some stiffeners of stiffened panels are not failed in the same time,and there is time interval during the period of buckling. Ecperimental results and finite element simulation results are anastomotic. Initial deformation has great influence on the ultimate strength of stiffened panel. Pitting corrosion with different parameters affect the stress distribution, weaken the ultimate strength of structures and change specimen's buckling. The ultimate strength of stiffened panel when pits locate in sides of shell plate is greater than the ultimate strength of stiffened panel when pits locate in middle of shell plate.The diameter, number, depth and volume of pitting corrosion has significant influence on the ultimate strength of stiffened panel, which the relationship can approximately be dealed as monotonic function. The residual ultimate strength of stiffened panel is small when the value of influencing factors is great. In sagging or hogging limit state, partial components of FPSO structures reach the yield limit of the material. The variety of bending moment of FPSO structures with corrosion wastage has obvious regular pattern. When the volume loss caused by uniform corrosion equals to the volume loss caused by pitting corrosion, the ultimate bending moment of FPSO structures subjected to uniform corrosion is greater than the ultimate bending moment of FPSO structures subjected to pitting corrosion. The probability density function of ultimate strength of stiffened panel obeys the normal distribution, in addition, whether in sagging or in hogging, the probability density function of ultimate bending moment of FPSO structures obeys the normal distribution. The ultimate bending moment reduces when pit's depth inceases. Compared with hogging, pitting corrosion wastage has greater impact on ultimate bending moment when structures are in sagging. The ultimate bending moment of structures can be improved by reducing pitting corrosion damage.