High Alloy Duplex Stainless Steel For Cost-Effective Subsea Developments
Authors: Estberger Fredrik , Ostlund Lars , Nystrom Goran 
Published: October 2008

During the last decade the duplex stainless steel have gained more and moreexperience in the oil and gas industry. Due to the unique combination ofmechanical properties and the corrosion resistance inC02/H2S and Cl− systems the duplex stainlesssteel have proven to be one of the steel grades for off-shore use. The modernduplex stainless steel have, in many cases today, also offered the bestoptimised cost effective solution for off-shore use.

Oil and gas production off-shore is performed in hostile environment andtherefore safety and maintenance aspects must strongly be considered, The useof duplex stainless steel can reduce the cost of maintenance, give greatersafety and increase the lifetime of the system. Emphasis is placed within thesystems use in subsea production, hydraulic control, methanol transpiration andflowlines.

References for cost-effective subsea use of duplex stainless steel arepresented. Important concerns for subsea umbilicals and other applications arediscussed.

It is concluded that duplex stainless steel in many subsea applications willoffer an optimum combination of cost effectiveness and performance.

1.INTRODUCTION

The development of stainless steels over the last two decades has been veryintense, and governed partly by demands from the Oil and Gas (O&G)industry. To some extent this means that the course of development has followeddemands valid for the O&G industry in general. Initially, the main concernswere to develop higher performance grades for improved reliability and servicein increasingly hostile environments. In parallel with this, and of increasinginterest, have been concerns with the overall cost-effectiveness over the totallife cycle. These concerns have led to a great interest in Duplex StainlessSteels (DSS) which today very often offer the optimum cost-effectivecombination of attractive properties.

Similarly, for O&G developments, subsea installations have proven, as aresult of intense technical development, to become an increasingly importantcost-effective alternative.

The purpose of this paper is to indicate how DSS can be of importance in subseadevelopments, i.e. in many cases a combination of the most cost-effectivematerial for the most cost-effective production method.

2. THE HISTORY OF DUPLEX STAINLESS STEEL IN SUBSEA DEVELOPMENTS

DSS have been used extensively for subsea flowlines since the late 1970s (1). Table 1 summarizes some of that experience.

The duplex grade used has predominantly been the standard 22Cr (UNS S3 1803)DSS, even though more recently the "super-duplex" grades have become ofinterest.

For actual subsea manifold installations, a more integral part are thehydraulic and chemical injection systems, for which DSS have also proven to bea cost-effective alternative.

An early example comes from the subsea development of North-East Frigg by ElfAquitaine, where DSS was used for the umbilical hydraulic control lines andmethanol injection (2). The tubing for this application was supplied in 22CrDSS by Sandvik Steel in continuous lengths on spools.