Pls help:Local buckling and Propagation buckling

Hello everyone,

 I am using Ansys 11 to analyse local buckling and propagation buckling on an offshore pipeline subjected to external hydrostatic pressure.Nonlinear Static analysis with Multi-linear material model with true stress and true strain data, is used. Shell 181 is used for the pipe. The Boundary conditions: all DOF for the the farther end of the pipe and the base line of the pipe constrained along Y axis(vertical displacement). Arc length method was used with the option of terminating the analysis at Uy limit. Though collapse(local buckling) was observed, I don't know how to obtain a buckle to propagate along the length of the pipe as shown in the attached file. 

Please help.....