On the Effect of the U-O-E Manufacturing Process on the Collapse Pressure of Long Tubes

A commonly used process for manufacturing large-diameter tubes for offshore pipeline, riser and tension-leg platform tether applications involves the cold forming of long plates. The plates are bent into a circular shape and then welded. The circumference of the pipe is then plastically expanded to develop a high tolerance circular shape. Collectively, these steps comprise the U-O-E manufacturing process. These mechanical steps cause changes in the material properties and introduce residual stresses in the finished pipe. This paper presents the results of a combined experimental and analytical study of the effect on the U-O-E process on the capacity of the tube to resist collapse under external pressure loading. The U-O-E manufacturing process for a 26 in. (660 mm) diameter, 1.333 in. (33.86 mm) wall thickness pipe was simulated numerically. The numerical process was validated by comparing the predicted stress-strain behavior of the material at two stages in the process with properties measured from actual pipe specimens obtained from the mill. Following the simulation of the U-O-E process the collapse pressure was calculated numerically. The manufacturing process was found to significantly reduce the collapse pressure. A similar pipe for which the final sizing was conducted (simulated) with circumferential contraction (instead of expansion) was found not to have this degradation in collapse pressure.