On the Effect of the UOE Manufacturing Process on the Collapse Pressure of Long Tubes

Abstract

ABSTRACT 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 of 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 diameter, 1.333 in 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 propertied 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. INTRODUCTION The tension-leg platform (TLP) is a technically and economically important concept for deep-water oil and gas production. The tensioned legs, i.e., tendons, which secure (or moor) the floating semisubmersible to the seafloor, are critical to this concept. Both steel tubulars (i.e., pipe) and parallel-strand wire rope are being considered for the tension-leg members. For water depths considered to date (approximately 3,000 ft (900m) - see Refs [1-3]), pipe has been selected. Tubular tendons are designed to withstand extreme loading during storm events such as hurricanes and are required to have fatigue life in excess of 109 wave-induced load cycles. In deeper waters, the tendons must also be designed to avoid collapse due to the high ambient external pressure combined with steady and time-varying axial tension. Resulting tendon designs are characterized by large diameters (20-40 in - 0.5- 1.0 m) and large thicknesses (l-2 in -25-50 mm). Due to the large sizes, stringent roundness requirements and the relatively limited current demand, pipe manufactured by standard manufacturing processes are currently the most cost effective. Of these, pipe manufactured by the U-O-E process is very attractive for this application. However, some concern exists regarding the collapse performance of such pipe. This concern is based on the observation that the collapse pressure of U-O-E pipe is typically lower, by as much as 20%, than that of otherwise identical pipe of the same grade produced by other manufacturing processes. Hence, a project was initiated to develop an understanding of the effect of this manufacturing process on the collapse performance of such pipe.