Super Stainless Steels: Review Of Norwegian Offshore Practice

Abstract

ABSTRACT In the 80's there has been a quiet revolution regarding steels offshore Norway. Duplex stainless steel (type 2205) was installed in hydrocarbon systems (together with AISI 3161 and carbon steel). Highly alloyed austenitic stainless steels (type 6 Mo) were applied primarily in seawater-containing fluids (together with cupronickel). A small amount of highly alloyed duplex steel (super duplex) was used e.g. in a process system. The reason for this development was expected lower total life-time costs for the oil/gas production, due to improved corrosion resistance of the new steels. In the 90's it is presumed that titanium, glass fibre reinforced plastics, and perhaps a new super austenitic grade with increased nitrogen content will take a bigger share of the market for sea water containing fluids. This paper summarises the Norwegian practice with respect to steel properties, requirements, engineering features, and use, the latter including some elucidation of needed quantities. Finally, a brief outlook to future use and competitive materials is presented. INTRODUCTION During the 80's there was a quiet revolution in the North Sea regarding materials for offshore production platforms. It related especially to corrosion resistant steels, i.e. stainless steels. Apart from the well known 18-8M0 steel, AISI 316 L, particularly duplex, super duplex, and super austenitic (6 Mo) were used in lieu of carbon steel and cupro-nickel. The first large-scale application was offshore Norway where significant parts of the process and utility systems - piping and vessels - were made of duplex and 6 Mo steels. Shortly thereafter many British platforms were equipped with equal materials. The main reason for this development that still goes on, is lower total life-time costs expected with these steels. This again depends on the relatively high maintenance costs on the particular type of platforms installed in the North Sea. One must take into account that carbon steel corrodes quickly and may have to be repaired or replaced several times during the lifetime of the platform. THE SUPER STAINLESS STEELS Background When the converter processes AOD (argon - oxygen - decarburisation) and VOD (vacuum - oxygen - decarburisation) were introduced in the 70's, a revolutionary development of stainless steels started These refining methods resulted in very pure steel, especially with respect to oxygen, sulfur, carbon and slag inclusions; moreover, the methods allowed narrow compositional ranges. In this way it became possible to improve e.g. hot workability, weldability, and corrosion resistance of the steels, and more highly alloyed grades could be produced. The improvements were obtained to reduced costs, e.g. caused by less chromium loss during refining of low-carbon melts. Production costs were further lowered by applying continuous casting techniques. Even foundries have today found benefits by using the methods at least for premelted raw materials, and the most advanced of them have invested in small AOD-converters.