Study On The Ultimate Strength Method Of Semi-submersible Platforms Under The Key Component Damaged

Semi-submersible offshore platform is one of the most widely used floating deepwateroffshore platforms currently. During the service cycle, the environmental of platformcondition is harsh. Except the influence of environmental loads, the brace damaged, thecolumn collision and material corrosion also will bring damage to the Semi-submersibleoffshore platform. Therefore, it is necessary to study ultimate strength of the wholesemi-submersible platform for evaluating the safety of platform. A typical semi-submersibleoffshore platform structure is taken on this paper, calculation method and analysis processof ultimate strength are proposed, and consider the influence of the brace damaged, thecolumn collision and corrosion on the ultimate strength of the platform. The major worksand conclusions are as follows:Firstly, wave load long-term forecast and the overall strength of the platform indangerous conditions are analyzed. Based on Morrison formula and3d potential flow theory,adopt SESAM to do hydrodynamic analysis, and design wave analysis parameters are goton the basis of design wave theory. Load transfer analysis is carried out using the designparameters got before and wave load in each load case is known. Through FEM analysisoverall structure responses of the platform under wave loads are got, and evaluate strengthof platform are made by comparison between allowable stress and the result.Secondly, calculate ultimate strength of semi-submersible platform in intact state. Thispaper adopts quasi-static nonlinear finite element method, and calculates the local ultimatestrength of the pontoon and the main deck, then the global ultimate strength of the totalsemi-submersible platform is studied. Study results show that the quasi-static method canbe used to calculate the ultimate strength of semi-submersible platforms. There is a big gapbetween the local ultimate strength of the component and the global ultimate strength ofthe total platform.Finally, analyze the redundancy ultimate strength of the semi-submersible platformconsidering the influence of brace damaged, column collision and corrosion. By comparingultimate strength of semi-submersible platform in intact and damaged condition, the changeregular of the ultimate strength of the platform is got. Study results show that collision condition has no effect on the global ultimate strength of the total semi-submersibleplatform. The brace damaged condition has the most serious influence when the platform isin beam sea, and the corrosion condition has the most serious influence on hogging.Structure safety of semi-submersible platform in beam sea under damaged and corrosionstate should be paid close attention.