New Steel Plate for Liquefied-Natural-Gas Storage Tank

Abstract

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 183286, “New Steel Plate for LNG Storage Tank,” by T. Kagaya, H. Furuya, T. Kamo, Y. Takahashi, H. Wakamatsu, and T. Nagao, Nippon Steel and Sumitomo Metal; H. Nishigami and S. Tomita, Osaka Gas; and S. Hirai and M. Mitsumoto, Toyo Kanetsu, prepared for the 2016 Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, 7–10 November. The paper has not been peer reviewed. For more than 50 years, 9% Ni steel with excellent mechanical properties at a cryogenic temperature of less than –162°C has been used as inner-tank material for liquefied-natural-gas (LNG) storage tanks. However, a reduction of the nickel content in steel can save construction costs of LNG storage tanks. A new steel plate for LNG storage tanks has been developed by optimizing chemical composition and applying recent thermomechanical-control-process (TMCP) technology. Introduction With demand for LNG rising, construction of above-ground LNG storage tanks is expected to increase. 9% Ni steel plate has excellent strength and cryogenic toughness. For LNG storage tanks, a double-integrity structure has been proposed to prevent peremptory destruction (Fig. 1). Although a high safety standard is demanded for such steel plates, in terms of saving construction costs of LNG tanks, a reduction in the amount of nickel used was desired. The new steel plate, equivalent to conventional 9% Ni steel, has been developed by adopting a TMCP to obtain the refined microstructure and a large amount of retained austenite. Development of New Steel Concept. In the development of the new steel, key technologies are application of the TMCP and optimization of chemical compositions. The properties of the base plate and welded joint of the new steel are equivalent to those of 9% Ni steel, with excellent brittle-crack-initiation resistance and brittle-crack-propagation-arresting capability. A TMCP is a production process wherein the rolling temperature and cooling rate after rolling are controlled. TMCP technology, which improves strength, toughness, and weldability, was developed for use in shipbuilding steel or line-pipe steel. TMCP technology has been applied to plates for offshore structures, high-rise buildings, bridges, and several other structural applications. Microstructures obtained with TCMP technology are finer than those obtained with conventional processes. The production process of the new steel is a combination of controlled rolling, accelerated cooling, and appropriate heat treatment (intermediate heat treatment, known as lamellarizing). A very fine martensitic microstructure is formed by controlling the previous austenite grain size in the heating process and rolling conditions in the uncrystallized zone and quenching in the accelerated cooling process after rolling. Retained austenite, which improves toughness, is also formed by lamellarizing and tempering after direct quenching. The amount of retained austenite of the new steel is greater than that of 9% Ni steel.