Development of API X70-X80 Grades Heavy-Wall Seamless Pipes With Low Temperature Toughness

Abstract

Abstract Based on the market demands regarding exploration in deep water fields, e.g. high pressures, low temperature and sour reservoirs, Vallourec & Mannesmann Tubes have developed new high strength steels up to grade X100 with low temperature toughness, sour service resistance and good weldability. This work is part of a development program for high performance heavy wall seamless pipes for special applications such as J-lay collars, buckle arrestors and risers. The paper presents the latest results on industrial studies on high strength heavy-wall pipes in grades X70 and X80 with wall thickness up to 80mm, which are manufactured by seamless hot-rolling and subsequent quench and temper treated. Vallourec & Mannesmann Tubes's alloying concept based on carbon content below 0.10% and micro-alloying of vanadium and niobium used for grades X60/X65/X70 was extended to heavy wall seamless pipes in grades X70/X80. Mechanical properties, advanced metallographic examinations and results of sour service resistance are reported. To demonstrate weldability, test welds were performed and examined. Introduction The market demands regarding exploration in deep water for oil and gas reservoirs lead to technological trends in using high strength steels (grade X65 to X80 and higher) with special requirements regarding e.g. corrosion, toughness. Increased yield strength and therefore thinner wall thickness enables the industry to reduce the costs of all components through the limitation of the string weight. On the same time the pipeline design includes special parts like J-lay collars for the pipe laying operation and buckle arrestors at regular intervals along the line for protection against an initiation and propagation of collapse and buckling [1]. All parts have to follow the demands for strength, ductility, toughness and sour service resistance combined with adequate welding performance [2, 3, 4, 5]. Over the last years Vallourec & Mannesmann Tubes have developed new high strength steels up to grade X100 [6]. Not only material development but also new technologies in the field of heat treatment capabilities allow the manufacturing of heavy wall seamless pipes with high strength and excellent corrosion properties. In this paper results of studies on X70 and X80 pipes with heavy wall (wall thickness between 40mm and 80mm) produced by the pilger mill are discussed. The pipes are quenched and tempered to create the suitable microstructure which provides good toughness and sour service properties. This work is a part of R&D program for high performance Q&T seamless pipes for deep water applications.