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 off shore pipeline, riser and tension-leg platform tether applications inv olves the cold forming of long plates. The plates are bent into a circ ular shape and then welded. The circumference of the pipe is then plas tically expanded to develop a high tolerance circular shape. Collectiv ely, these steps comprise the U-O-E manufacturing process. These mecha nical steps cause changes in the material properties and introduce res idual stresses in the finished pipe. This paper presents the results o f 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 exter nal pressure loading. The U-O-E manufacturing process for a 26 in. (66 0 mm) diameter, 1.333 in. (33.86 mm) wall thickness pipe was simulated numerically. The numerical process was validated by comparing the pre dicted stress-strain behavior of the material at two stages in the pro cess 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 fou nd to significantly reduce the collapse pressure. A similar pipe for w hich the final sizing was conducted (simulated) with circumferential c ontraction (instead of expansion) was found not to have this degradati on in collapse pressure.