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In the design of marine pipelines, like other thin-walled structures, structural stability plays major role. Generally, two kinds of instabilities, namely global buckling and local buckling (collapse) may occur in marine pipelines. However, for deep water pipelines in addition to occurrence of collapse, another concern is the potential occurrence of propagating of this collapse along the pipe due to high external pressure. In the present study, details of 2-D and 3-D finite element modeling for collapse propagation simulation are outlined. In order to verify the accuracy and validity of the finite element modeling, the numerical results, obtained from nonlinear finite element analyses for several pipe samples have been compared with the experimental results. These proposed 2-D and 3-D modeling methods are easily applicable. Also, the comparison shows that the results of these methods have very close agreement with the experimental behaviour. Using nominal geometric properties, finding minimum required imperfections to eliminate bifurcation points and using corrected Ramberg-Osgood material behaviour for steel pipe are the main characteristics of the present 3-D method. The study shows that this method gives more appropriate results than the previous proposed method by Toscano et al. (2002).

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In practice, steel tubulars lack a mathematically perfect cylindrical shape, due to non-uniformities introduced during the manufacturing process, construction methods, damages incurred in the transport. The imperfections might exist in the cross section and along the tubular member. Imperfections can greatly affect the behavior of tubular members. The work presented in this paper is perusing effect of geometrical imperfections on steel tubes under monotonic axial compressive loading. An experimental model testing along with a numerical simulation approach has been employed. The small scale experiments were conducted on X70 steel pipes with diameter 44 (mm) and D/t of 22. The results showed that under monotonic loading the presence of imperfection in the tubular member leads to local buckling of the tube and has a decreasing effect on the limit stress (σ_L) and on the strain corresponding to the ultimate load (ε_L). The geometry of the imperfections introduced to the numerical model is based on the scaled down buckling mode shapes of the tubular specimen. In general, a reasonable agreement has been noticed between the experimental and the numerical results. The numerical model has then been used to study the effect of geometry of imperfection and imperfection amplitude (A), D/t and (λ/L) on the response of steel tubes to monotonic axial compressive loading. The results showed that the monotonic response was more sensitive to higher mode shapes as compared to the lower modes and to symmetric modes as compare to non-symmetric modes. The limit stress and the strain corresponding to the ultimate load decrease as imperfection amplitude increases. The stress-strain path was also found to be affected by the geometry of imperfections, D/t and the imperfection wave length. The results of the current study also shows that the tube imperfection has a more profound effect on the limit strain (ε_L) in comparison to that on the limit stress (σ_L).