The Effect of Nickel on Fracture Toughness at Low Temperature for Hydrogen Pre-Charged Steel Samples

Production of oil and gas in the Brazilian pre-salt will face several technical challenges. One of them that is a major concern is the presence of CO₂ in high concentrations. Indeed, since the local regulatory agency requires increasingly stricter recommendations, it seems unlikely the possibility of simply release CO₂ in the atmosphere. Besides that, as it happens since the 1970’s in USA, enhanced oil recovery (EOR) using CO₂ might be a great opportunity and has to be considered. If that is the case, as soon as CO₂ is separated from the oil in the top side, it has to be pressurized and transported through pipelines into the reservoir. The material of choice for that pipeline would be API 5L X65 since it is widely used and most available. The working pressure can reach up to 500 bar or more and it is an important issue to consider the fact that a suddenly depressurization due to a crack in the pipeline or a failure in a fastener or flange can promote a local abrupt decrease in temperature down to −60°C or even lower. That concern in addition to the fact that cathodic protection has to be used in the pipeline for corrosion control has the potential to produce embrittlement due to a combination of low temperature and hydrogen charging and ultimately lead to a catastrophic failure. The use of nickel to improve steel toughness has been used extensively and some grades or classes of nickel containing steels have been created for special applications. The aim of this work is to evaluate whether a nickel containing steel would be a more suitable material to manufacture high pressure CO₂ transporting pipelines, taking in account a possible leak and the corresponding effect of a local and abrupt drop in temperature on the fracture toughness of the material. The effect of hydrogen due to cathodic protection on fracture toughness is also evaluated. The results indicated that under the experimental conditions and materials examined here the 9%wt nickel steel is not sensitive to low temperature and hydrogen, practically maintaining its original fracture toughness.