Application of 7%Ni-TMCP Steel Plate to Large Scale LNG Storage Tank

Abstract

Increase of natural gas demand has led the increase of constructions of Liquefied Natural Gas (LNG) storage tanks worldwide. The inner tank material for above ground LNG storage tanks has mostly been made of 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. Meanwhile, technologies of steel making, welding, design, construction and inspection of 9%Ni steel plate have been improved significantly and enable us to achieve enlarging volumetric capacity of the tank to 2–3 times. It is known that nickel is an expensive and valuable rare metal and recent tendency of increase in nickel price has influenced the price of 9%Ni steel plate. In such a circumstance lowering the Ni content would save cost and this has led to an extensive research and development resulting in the 7%Ni-TMCP steel plate. This 7%Ni-TMCP steel plate developed is characterized by adjustment of chemical compositions and application of suitable Thermo-Mechanical Controlled Process (TMCP). 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 from 6 to 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. As a result it was demonstrated that the 7%Ni-TMCP steel plate had a quite excellent resistance to brittle fracture of the inner tank steel and its welds exposed to the cryogenic temperature of LNG. Furthermore, other various issues such as extent of applicable heat input, repair weld, difference of physical constants and so on were investigated and found acceptable. In Japan Ministry of Economy, Trade and Industry (METI) approved to apply this steel to new LNG storage tanks of Osaka Gas. After that Osaka Gas decided to construct a new full containment LNG storage tank applying 7%Ni-TMCP steel in Senboku terminal 1. The capacity is 230,000m3, which is larger than present largest scale of 180,000m3 in Japan. The construction is planned to start in September 2012 and be completed by November 2015.