Development of 7%Ni-TMCP Steel Plate for LNG Storage Tanks

Demand of natural gas continues to increase in the recent years due to the rise of environmental issue and the drastic increase of crude oil price. These events led the increase of constructions of LNG storage tanks worldwide. The inner tank material for above ground LNG storage tanks have mostly been made of a 9% nickel steel plate over the last 50 years as it has excellent mechanical properties under the cryogenic temperature of −160deg-C. During this period, the LNG storage tanks made of 9%Ni steel plate have safely been operated at the many LNG export and import terminals in the world. Meanwhile, technologies of steel making, refinement, design, analysis, welding and inspection have been improved significantly and enable us to achieve enlarging volumetric capacity of the tank to 2–3 times. There was a tendency of increase in nickel price in the recent years and hence the preparation corresponding to the risk to a great rise on the price of 9%Ni steel plate becomes necessary. In such a circumstance lowering Ni content was focused attention on the 9%Ni steel as nickel is an expensive and valuable rare metal and the 7%Ni steel plate was eventually researched and developed. It was a steel making process so that a 7%Ni steel would be made by adjustment of chemical compositions and processed by suitable Thermo–Mechanical Controlled Process (TMCP). As a result, it was demonstrated that 7%Ni-TMCP steel plate had excellent physical and mechanical properties equivalent to those of 9%Ni steel plate. In order to evaluate fitness of the 7%Ni-TMCP steel plate and its weld for LNG storage tanks a series of testing was conducted. Several different kinds of plate thicknesses i.e. 6,10,25,40 and 50 mm were chosen to run large scale fracture toughness tests such as duplex ESSO tests, cruciform wide plate tests as well as small scale tests. In this study, a comprehensive assessment was performed to confirm applicability of the 7%Ni -TMCP steel plate and its weld to LNG storage tanks. It was concluded that the 7%Ni-TMCP steel plate warrants serious consideration for use in LNG storage tanks. This paper reports details of the research and development of the 7%Ni-TMCP steel plate.