L51851 Characterization of Burst for Line-Grade Steel Pipelines at Elevated Temperatures

In many offshore pipeline applications the hydrocarbons transported are at an elevated temperature (200-400 °F). Often, it is desirable that the line be kept at elevated temperature in order to avoid deposition of paraffin and hydrates which can clog the line. Thus, in many cases the line is insulated. Long-term exposure of steels to elevated temperature changes their mechanical properties including their rate dependence. An issue of concern is how to efficiently design the line against burst. This report contains the results of a study of burst of seamless pipes at elevated temperatures in the range of 75-350 °F (24-177 °C). The main objective of the study was to understand how changes in mechanical properties induced to the steels by elevated temperature affect the burst performance of the pipes. The materials tested are SAF 2507 super duplex and an X-52 line grade steel. The study involved three main tasks. In the first task the stress-strain response of the two materials was measured in uniaxialtests carried out at various temperature levels in the range of 75-425 °F (24-218 °C). At each test temperature stress-strain responses were measured at several strain rates. The second task involved bursting of tubular specimens at different temperatures under controlled conditions. The tubes were machined to precision out of pipe stock to end up with specimens with diameter-to-thickness ratio of 34.7 for SAF 2507 and 34.9 for X-52. Because the emphasis of the study was on the changes caused by elevated temperature on the steel properties and their effect on burst, the D/t of the test specimens was not varied. The third task involved constitutive modeling of the mechanical behavior of the materials and using them in models of the process of burst to simulate the experiments.