6% Mo Austenitic Stainless Steel Selection for Offshore Applications
Authors: Agarwal D.C. , Jasner M.R. , Rockel M.B. 
Published: October 2008

6% Mo austenitic stainless steels are increasingly being used in the offshore industry as a construction material for handling seawater and hydrocarbon/seawater mixtures containing hydrogen sulfide.

The paper describes the specific investigations conducted on the various components relates to the Saga Snorre project for their corrosion and metallurgical characteristics. Over 1,000 tons of seamless/welded pipes, forgings, large radius induction bends, and various fittings were manufactured and tested. Corrosion behavl0r under hydrogen charging condition and chlorinated seawater were also studied. The paper discusses suitability of the alloy for conditions prevailing in the Snorre field of Saga Petroleum, at approximately 150 Km off the Norwegian coast.

INTRODUCTION

Materials used in the offshore industry encounter numerous corrosion and mechanical problems. They consist of submerged and atmospheric marine corrosion, mechanical stress (static and/or dynamic) due to wind and mechanical wave action and marine fouling. Unfortunately, many other factors, other than the mere considerations of corrosion resistance in so-called "normal seawater" have to be taken into consideration when making material selection. This applies in particular for piping systems.

These include:oxygen content (aerated/deaerated fluids)presence of wet CO2presence of H2S and other sulfur compoundschlorination used to control biofoulingcrevice corrosion conditionsstagnant conditionsgalvanic interaction in presence of dissimilar metalscavitation, erosion and abrasion due to velocity effectsenvironmental embrittlementtemperature effectsstress effectsproduced brine and seawater salinity The seawater temperature and salinity can vary widely from 2°C (36°F) in the arctic to 30°C (85°F) in the tropics to 46°C (115°F) and higher at the outlet of some condensers. The salinity can vary from dissolved salt content of 7 g/kg in Baltic to 43 g/kg in the Persian Gulf.

A seawater piping system consists not only of pipe (seamless and welded) but other accessories such as pipe spools, pipe bends, reducers, flanges, return bends, tees, elbows, pumps, valves and metering devices. The important properties governing proper material selection are:resistance to various forms of corrosion, especially localized corrosion and stress corrosion cracking.physical and mechanical properties (coefficient of thermal expansion, thermal conductivity, tensile properties, erosion resistance, low temp. toughness)galvanic compatibilitygood fabricability and weldabi1ityresistance to marine growthavailability of various product formssuccessful case historiescosts A careful study of the above factors must be conducted prior to making a material selection.

MATERIALS FOR OFFSHORE APPLICATION

A large amount of data concerning behavior of materials in seawater has been generated over the last few decades and is well publicized in technical literature. These include uniform corrosion, pitting/crevice corrosion, galvanic interaction, fatigue, stress corrosion cracking, erosion-corrosion and others which aid in intensifying the corrosion damage such as velocity effects, weld decay, fouling and heat transfer.

Even though the precise determination of all corrosion variables as relating to offshore marine corrosion is not available, there is ample laboratory, field and case history experience available to make cost effective and