Improvement in UOE Pipe Collapse Resistance by Thermal Aging

Abstract

Abstract A recent investigation has shown that the UOE pipe manufacturing process significantly degrades the collapse resistance of high strength line pipe. This paper assesses the degree of strength recovery that can be achieved by aging the pipe. An experimental investigation was performed in order to develop a quantitative relationship between time andtemperature of aging, which produces an increase in compressive yield strength with consequent increase in collapse resistance to external pressure loading. Introduction The mechanical properties of high strength large diameter, submerged-arc welded line pipe depend on its chemical composition and thermo-mechanical history, with the latter including the plate mill rolling practice and the cold deformation that occurs during the pipe forming and expansion operations. Pipe forming and expansion tend to induce material anisotropy which is a measure of the same property in different directions. Anisotropy is usually more pronounced in the plastic. The anisotropy is evident from the reduction in compressive yield strength in the hoop direction when the axial strength which is less affected by the pipe forming process. As can be seen Figure 1, the degree of anisotropy varies across the thickness, being more pronounced at the outside of the pipe wall. The differences shape between hoop and axial stress curves is also significant. For pipelines which buckle in the plastic range, such an anisotropy directly affects the collapse pressure. A fill scale test program was completed in 1994 which investigated the collapse strength of UOE pipe. This test program was necessary because of the absence of previous experimental results for pipe of the size required for the Oman India Pipeline which will cross the Arabian Sea at depths if up to 3500 meters. This test program successfully evaluated the reduction in collapse strength manufacturing process and showed resulting the UOE that the loss of material compressive strength can be substantially after stress relieving the pipe by furnace heat treatment (typically 560°CA 10° C for one hour per inch thickness). However, this process is highly energy and time demanding and can produce detrimental effects on weld fracture toughness properties. A thermal aging process was suggested by a leading pipe manufacturer as a possible alternative to full stress relief. This process requires far less time and energy than stress relief and offers the possibility of application into a continuous pipe manufacturing production line. The Oman India Pipeline project team decided to investigate the effective of thermal aging and therefore performed a study of the phenomenon which included a series of scale tests and small scale experiments. A literature search revealed a good number of previous research efforts directed at material aging. However, generally these investigations into aging behavior have mainly been concerned with the negative impact that such aging has on the material mechanical properties, especially fracture toughness.