The effect of initial imperfections on the pipeline global buckling is the theoretical basis of the global buckling stability analysis and buckling control of the submarine pipeline. The pipeline length for the analysis has a significant effect on the global buckling. According to the global buckling regularity of pipelines with different length, pipelines are divided into two types: the "long pipeline" and the "short pipeline". The global buckling of pipelines with different initial imperfections is simulated by the 3D-Explicit method. The effect of an initial imperfection on the global buckling of a "long pipeline" and a "short pipeline" are proposed by analyzing the effect of initial imperfections on the expansion length of buckling. The global buckling of a long pipeline and a short pipeline with the same initial imperfection are compared. It shows that the post-buckling characteristic values, including moment, strain and lateral displacement, of the long pipeline are larger than those of the short pipeline. It shows that the long pipeline with an initial imperfection is more prone to destructive global buckling than the short pipeline with the same initial imperfection.
With the exploration and exploitation of offshore oil and gas resources, the increasing of pipeline length and installation depth greatly increases the risk during construction and operation periods. The global buckling of the bare pipeline due to the action of temperature and pressure is one important failure form. Boreas Consultants, TWI and Cambridge University performed Joint Industry Project (JIP) and proposed that Out-Of-Straightness (OOS) is the cause of pipeline global buckling and OOS has the significant influence on the critical buckling force, the buckling amplitude, the axial force, the axial displacement and axial strain of buckling pipes (Bruton, 2005).
Many researches have been performed in the influence of initial imperfections on the global buckling of pipelines through analytical solutions and numerical analysis methods. Taylor derived an analytical solution to the global buckling of pipes with single-arch and double-arch geometric initial imperfection based on Hobb's analytical solution (Taylor, 1986). The influence of initial imperfections on lateral global buckling was analyzed by analytical and numerical methods and the relationship between initial imperfection dimension and critical buckling force were researched (Sriskandarajah, 1999). The vertical global buckling deformation of buried pipelines with initial imperfections was analyzed using the numerical method (Villarraga, 2004). In 2008, the influence of initial imperfections on lateral global buckling was researched by numerical methods which showed that pipelines with initial imperfections can be regarded as a snake pipeline with small span (Rathbone, 2008). As the numerical method improved, the influence of initial imperfections on lateral and vertical global buckling was analyzed by nonlinear finite element software (Karampour, 2013). The global buckling failure envelope of pipelines with different initial imperfections was proposed by using numerical simulation analysis (Liu, 2015). An analytical model based on strain design rules has been developed for more economic pipeline designs by full scale test results and numerical simulations (Gresnigt, 2016).