The Analysis Of Ultimate Strength Of FPSO Structure With Initial Defects

FPSO compared with the ideal state, in the reality must exist defect damage, this kind of situation in the Ship and marine structures design and manufacture are named the initial defects. The fracture accidents of marine structure not only jeopardize the personal safety of staff, but also cause huge economic losses. So in order to reflect the reality of the FPSO structure, we must simulate the initial defects in the structure before checking the ultimate strength.The ultimate strength of FPSO structure with initial deformation, residual stress and dent by the nonlinear finite element software ANSYS. this paper mainly consider the three forms of initial defects effect on the ultimate strength of stiffened plate structure, then research the changing rule of the ultimate moment of FPSO’s compartment structure under this three initial defects. Around this goal, this paper carried out the following work:(1) In the initial deformation effect on the ultimate strength of stiffened plate structure, this article uses three forms initial deformations, they are plate type initial deformation, reinforced lateral initial deformation and reinforced beams initial deformation.The initial deformations are through APDL language change node coordinates. Research under axial compression loading, stiffened plate structure in different thickness, different amplitude ratio values(the average amplitude is unchanged) and slight, average and serious cases, the change tendency of the ultimate strength. The results show that the thickness is thicker, the greater the bearing capacity of stiffened plate; On the basis of the average amplitude is unchanged, the amplitude ratio on the ultimate strength of stiffened plate structure is small; The more serious of initial deformation, the greater impact on the bearing capacity of structure.(2) In this paper, change the residual stress into the form of stress at the beginning of the file, then loaded into the stiffened plate structure. The residual stress including hot rolling residual stress and welding residual stress two forms, main consider including residual stress stiffened plate under axial pressure change trend of ultimate strength, and through the thermal analysis under different thickness and weld material height distribution of residual stress. The results show that the residual stress has significantly affected on ultimate strength of stiffened plate, but little influence of grid size in it.(3) The ultimate strength of stiffened plate with dent imperfection under uniaxial compression loading by the nonlinear finite element software ANSYS. The dent is assumed to formed by the ball impacting on the stiffened plate. Three typical dent positions are selected as the middle of plate, the middle of flange and the plate strenghehed by stiffener. The deformation state of stiffened plate after been impacted was kept, then the stiffened plate was compressed gradually. The collapse process of the dented stiffened was studied, finally, the ultimate strength is obtained. The influence of speed and weight of the impactor on the ultimate strength of stiffened panels is analyzed. It is indicated that the dent would decrease the ultimate strength of stiffened panels, and the ultimate strength decreases with the increase the area and depth of depression. The influence of area of the dent to the ultimate strength of the stiffened plate is greater than that of the depth of dent. Compared with dent in the middle of flange and the middle of stiffener, the influence to the ultimate strength of dented stiffened panel in the middle of the plate is greatest.(4) On the basis of the above research, consider three kinds of initial defects role at the same time the changing trend on ultimate strength of stiffened plate structure, and compared with the ultimate strength when considered separately. The results show that the initial defects exist at the same time has a greater influence on the bearing capacity of stiffened plate.At the same time, the residual stress relative to initial deformation and dent has a greater influence on the ultimate strength of structure.(5) Establish FPSO compartment structure, train dent and the residual stress in the deck plate, in the bottom plate loading initial deformation and dent, research ultimate moment of FPSO structure with initial defects under sagging and hogging conditions. The finite element calculation results show that the ultimate moment value in hogging condition is greater than that in sagging condition.The existence of initial defects not only change the stress distribution of structure, but also reduce the ultimate bearing capacity of structure.