New Impander Technology for Improved Collapse Resistance of Large Diameter Pipe for Deepwater Applications
Authors: Reichel Thilo , Pavlyk Vitaliy , Beissel Jochem , Kyriakides Stelios , Jang Wen-Yea 
Published: May 2011

Abstract
Current manufacturing technology for large diameter pipe, such as the UOE and JCO processes, is known to cause some reduction in the collapse pressure of low D/t pipes compared to seamless ones of the same steel grade and D/t. The main contributor to the degradation in collapse pressure is the cold expansion with which pipe is finished in both processes. This paper introduces a new manufacturing technology in which longitudinally welded pipe is finished by controlled compression. A recently developed cold sizing press, called Impander, is used to produce pipe that is rounder, has reduced residual stresses, and increased compressive yield strength. The combination of these factors leads to a significant increase in the collapse pressure of the pipe. The Impander technology is first introduced followed by experimental and analytical results that demonstrate the resultant improvements in pipe collapse pressure. The enhancement in collapse pressure is demonstrated using X65 grade, 20-inch pipe with one-inch wall. Pipes were compressed to different degrees, and their dimensional characteristics and compressive mechanical properties were measured. The measurements were then used in finite element models to calculate the collapse pressure demonstrating the improved performance. The evaluation of the process was completed in a full-scale collapse experiment on a pipe finished by Impansion of 1.1%. The test showed perfect agreement with the modelling. The collapse pressure was 37% higher than current design codes allow.

Introduction
Linepipe larger than 16 inches is commonly produced by cold forming 40 or 60 ft (12.2-18.3 m) long plates through the " UOE?? or the " JCO?? processes. UOE involves:crimping of the long edges of the plate;forming it into a " U?? shape essentially by bending over a semi circular punch;forming the " U?? into a circular shape, " O??;welding the seam, and final shaping and sizing using an internal expansion tool, " E?? (see Kyriakides et al. 2006; Herynk et al. 2007; Kyriakides and Corona 2007). In the JCO process the plate edges are again first shaped by crimping or by some other forming process. It is then bent into a circular shape by incrementally pressing part of the circumference with a shaped rod (Kyriakides and Corona 2007). After the seam is welded, the pipe is once more finished by expansion.
The manufacturing processes were originally developed for relatively high D/t land pipelines where the main design parameter is internal pressure. During the last two decades they have also been adapted to produce thicker linepipe used in offshore operations where the main design load is external pressure and the main limit state is collapse. The collapse pressure of such pipe is governed by the D/t, the material yield stress and by initial imperfections such as out-of roundness and specifically ovality (Kyriakides and Corona 2007; Murphey and Langner 1985; Yeh and Kyriakides 1986, 1988). Although modern pipe mills are capable of producing thick-walled pipe with ovality as low as 0.15%, it is well documented that the collapse pressure of UOE and JCO pipe is significantly lower than that of seamless pipe with the same D/t and steel grade. Extensive studies (Kyriakides et al. 1991; Kyriakides et al. 2006; Herynk et al. 2007; Kyriakides and Corona 2007) and full-scale tests (Stark and McKeehan 1995; DeGeers et al. 2005; Fryer et al. 2004) have shown that these processes can reduce the collapse pressure by as much as 30%. Simply put, expansion tends to increase the material yield stress in the circumferential direction in tension but decreases it in compression due to the Bauschinger effect.