Effects of the UOE/UOC pipe manufacturing processes on pipe collapse pressure

Large-diameter pipes used in offshore applications are commonly manufactured by cold-forming plates through the UOE process. The plate is crimped along its edges, formed into a U-shape and then pressed into an O-shape between two semicircular dies. The pipe is welded closed and then circumferentially expanded to obtain a highly circular shape. Collapse experiments have demonstrated that these steps, especially the final expansion, degrade the mechanical properties of the pipe and result in a reduction in its collapse pressure upwards of 30%. In this study the UOE forming process has been modeled numerically using a 2-D finite element model. The model can assess the effects of press parameters of each forming step on the final geometry and mechanical properties of the pipe. The final step involves simulation of pipe collapse under external pressure in order to quantify the effect of the forming variables on its performance. Examples of these variables are the radii of the forming dies, the chosen displacements of the dies, the compression strain in the O-step, the expansion strain, etc. An extensive parametric study of the problem has been conducted, through which ways of optimizing the process for improved collapse performance have been established. For example, it was found that optimum collapse pressure requires a tradeoff between pipe shape (ovality) and material degradation. Generally, increase in the O-strain and decrease in the expansion strain improve the collapse pressure. Substituting the expansion with compression can not only alleviate the UOE collapse pressure degradation but can result in significant increases in collapse performance.