Residual Ultimate Strength Analysis Of Cracked FPSO Structure

The fracture accidents of marine structure jeopardize the personal safety of staff, and cause huge economic losses. Thus naval architects try to find a way to examine the ultimate strength of ship structure, and the residual ultimate strength assessment of cracked ship structure has captured naval architects’ attention.Therefore, this thesis aims at thoroughly analysis the ultimate strength, buckling mode and stress intensity factor of cracked stiffened panels based on comprehensively understanding the current methods of study cracked structures, investigating the effects of various crack parameters, different load conditions and initial deformation on the ultimate strength of typical stiffened panels, the results providing in this thesis have significant value in engineering practice. To achieve this purpose, the following efforts and contributions have been made in this thesis:(1)The first part of this paper using test method studies the residual ultimate strength of cracked stiffened panels. Six specimens with through-thickness crack are tested under axial compression. These specimens have different crack characteristics such as crack size,location and angle, the effects of crack damage on buckling mode and residual ultimate strength of stiffened panel were validated by the test results. The test results show that crack length and location altered the distribution of stress which will eventually change the collapse mode of stiffened panel; Furthermore, the initial imperfection has great influence on the residual ultimate strength of cracked stiffened panels that we cannot ignore.(2) The residual ultimate strength of the cracked stiffened plate was analyzed through nonlinear FEA method; this part of the work takes into consideration the factor of initial imperfection, different crack locations and sizes. The results of stress distribution characteristics and buckling mode of cracked stiffened panels under axial compression action with were given. Calculate the stress intensity factor of model with edge and center through-thickness crack, and using K-criterion estimate whether the crack propagate or not,then analysis the stress at the location of crack tip.(3) This part of paper is using nonlinear finite method to examine the effect of different boundary condition on residual ultimate strength of two bay single stiffener or three stiffeners panels. A series of stiffened panels with varying crack sizes and different crack location under axial compressive actions were analyzed, and compare its results to stiffened panel structures under combined axial compression and lateral pressure actions, in both load conditions considered the influence of initial deformation. The FEA results show that: the ultimate strength of three stiffeners model is stronger than single stiffener model, which is closer to the real ultimate strength of steel stiffened panel. All the data shows that boundary condition has a significant influence on stiffened panel’s ultimate limit state analysis.Lateral pressure is equivalent to add constraint to main board, thus alter the stress response of stiffened panels, and its existence reduced the strength of stiffened panel.(4)Using nonlinear finite method analysis the ultimate strength of FPSO hull girder in sagging and hogging conditions, then give the stress distribution and the ultimate moment of the model with two type crack locations, i.e. crack in the deck and bottom of the FPSO model.