Abstract
The selection of 9% Ni steel for high pressure dried CO2 injection lines, adopted in a topside plant according to ASME B31.3, was based on the high strength and low temperature toughness properties of this material. The material specification for piping included seamless ASTM (1) A333 Grade 8 pipes; ASTM A522 Type I flanges and ASTM A420 Grade WPL8 fittings. The minimum specified Yield Strength of 517 MPa was a very important condition to reduce the thickness of the pipes and weight of the lines, but even adopting a high strength material, the calculated pipe wall thickness for 8in (100 mm) nominal diameter is 1.25 in (31 mm) and for 14 in (350 mm) pipes the thickness is 2 in (50 mm) , which are heavy wall pipes to be welded. The requirements for low temperature toughness for these high pressure gas lines come from a minimum design temperature of down to -70°C. This is not a problem as the 9%Ni alloy steel presents ductile behavior for temperatures down to -195°C. Also the weldability of this material is considered good since no preheating and no post welding heat treatment (for stress relief) are usually required. In this specific project, due to the presence of a small amount of H2S and eventually a CO2 wet condition, the lines were considered “H2S service” and the user developed, tested and approved the special materials to be adopted in the project. But the term “H2S resistant” cannot be widely applied to the 9Ni steel since this high alloy (non corrosion resistant) is not prequalified according to ANSI (2)/NACE(3) MR0175/ISO(4) 15156 and cannot be even classified in this standard. This paper presents the complexity of microstructures for different products and heat treatments, as well as the welding effects for these materials, showing that to be considered H2S resistant a more in-depth investigation is still required for this material.
