Duplex Stainless Steels - Oil & Gas applications

Duplex and superduplex stainless steels have been used extensively in the onshore and offshore sectors of the oil and gas industry for over a decade, during which time an extensive project reference has been established. Duplex grades have been utilised primarily for two reasons: their corrosive resistance to the various corrosive media found in onshore/offshore environments, e.g. CO2, H2S, chlorides, low pH etcetera and secondly their increased strength levels. Typical applications are production tubing and X-mas tree systems, subsea manifolds and flowlines, subsea pipeline systems and topside process systems. The following articles are generally available through the trade press, we regret we cannot supply them in their through our site.

A guideline to the successful use of duplex stainless steels for flowlines

Authors: Dr. Liane Smith - Intetech ltd (Consultant to the International Molybdenum Association and to the Nickel Development Institute) Mario Celant - Mac, Antoine Pourbaix - Cebelcor, originally presented at the Duplex America 2000 Conference, Houston, USA, paper number DA_2-102

Abstract

Duplex stainless steels have been widely used for flowlines carrying oil and gas, with more than 845km now in service. Successful application requires selection of the correct steel grade, use of the material in the correct heat treatment condition, attention to specific procedures when welding, correct design of the cathodic protection system and control during the commissioning period. This paper details correct practice in all these areas, providing sound guidelines for the successful application of duplex stainless steels for flowlines.

Superduplex stainless steel grades for seawater application and organic and inorganic acidic environments

Authors: L. Scoppio (Centro Sviluppo Materiali SpA, Roma-Eur, Italy) and I. Nembrini (DMV Stainless Italia srl, Costa Volpino (BG), Italy). Based on a paper presented at the 6th World Duplex Conference organised by Associazione Italiana di Metallurgia (AIM) in Venice, Italy, 18-20 October 2000.

Abstract

Application of superduplex steels has considerably increased in the last decade. The advantages of superduplex stainless steels over conventional austenitic steels, in a wide spectrum of service environments, such as in chemical industry, have been experienced successfully. Most of the superduplex grades are characterised by similar chemical composition (Cr, Ni, Mo and N contents) with minor additions of Cu and W.
Two commercial superduplex stainless steels, namely DMV 25.7NCu (S39277) and DMV 25.7N (UNS S32760), were utilised. Seamless cold finished tubes in solution annealed condition characterised by different external diameter (< 30mm) and thickness were investigated.
Actual and potential applications of the selected superduplex stainless steels in a range of different organic and inorganic acidic environments are presented.
In addition, the stress corrosion cracking resistance of superduplex steels for umbilical application is discussed. The localised corrosion resistance of the two superduplex steel grades was extensively studied in seawater environment via long-term exposure tests.
Susceptibility to hydrogen embrittlement under cathodic protection was evaluated.
Superduplex stainless steels DMV 25.7NCu and DMV 25.7N both exhibit the same trend in behaviour in various acidic environments; they showed good corrosion resistance, hence, making them a valid alternative to superaustenitic alloys for critical industrial application.
The higher Cu content (1.85%) of DMV 25.7NCu makes it more resistant than DMV 25.7N to uniform corrosion in organic mixture of acetic + formic acids and in inorganic sulphuric acid.
Cracking resistance of superduplex steels umbilicals under cathodic protection, can be considered satisfactory, due to the suitable microstructure and to the small amount of residual stresses.

Corrosion, galling and wear testing of duplex and austenitic stainless steels for seawater valve applications

Authors: R.M. Kain, L.S. Marshall, D. G. Melton (LaQue Center for Corrosion Technology, Inc., Wrightsville Beach, NC, USA), D.M. Aylor, R.A. Hays and R.J. Ferrara (Naval Surface Warfare Center - Carderock Division, West Bethesda, MD, USA). Based on paper #2019, presented at Duplex America 2000, 29 February - 1 March 2000, Houston, TX, USA.

Abstract

As part of a larger programme, resistance to various forms of degradation affecting seawater valves has been investigated for 37 different alloys. Among these were 12 cast and wrought stainless steels. This paper reports on the performance of seven wrought stainless steels, UNS S31603, S21000, S31254, S32654, N08367, S32750 and S32760, that have been identified as candidate trim materials for a new generation of seawater valves. In addition to galling and wear tests, a number of seawater corrosion tests were conducted to assess these stainless steels' resistance to general and localised pitting and crevice corrosion, as well as to galvanic interactions with dissimilar metals. Also investigated was their resistance to erosion-corrosion caused by cavitation and direct impingement. Except for the cavitation test which utilised ASTM substitute ocean water, all of the other tests, including the galling and wear tests, were conducted in or with natural seawater. A brief description of each test is provided along with a summary of results. Overall, S31603 was the least resistant stainless steel tested. Alloy S32654 was the only stainless steel found to be resistant to crevice corrosion in a 6 month quiescent exposure at 30°C. For the most part, comparable behaviour was exhibited by the other superaustenitics and superduplexes tested. These materials exhibited erosion-corrosion resistance on par with titanium, and were found to offer enhanced wear resistance over titanium. Although inferior to the galling resistance identified for Co-based alloys and M (Pb-Sn) bronze, all of the stainless steels benefited from pre-exposure to seawater and the formation of biofilms which greatly elevated their galling threshold stress levels.

Application of duplex stainless steel in the offshore industry

Author: Mrs Margun Tystad, Norsk Hydro, Norway, originally presented at the 97 Duplex Conference in Maastricht, the Netherlands, paper number D97-208.

Abstract

This paper gives a summary of the most important offshore applications of duplex stainless steels, criteria used for their selection, requirements to materials and fabrication and existing experience gained during fabrication and operation. There is a focus on the North Sea, but world wide applications and experience are also included. 22Cr duplex and increasingly also 25Cr duplex stainless steels are today widely used in the offshore industry. The main applications are for hydrocarbon process piping systems and equipment like separators, scrubbers and pumps. Subsea the materials are used in down hole production tubing, piping and manifolds, X-mas tree components, flowlines and pipelines transporting corrosive oil and gas. Further, 25Cr duplex is now being used also for sea water service.