Lean Duplex Materials for Line Pipe Applications in Sweet and Slightly Sour Environments

Abstract

1. Introduction For some years, lower alloyed austenitic-ferritic stainless steel grades have been developed, which are summarised in the „lean duplex" - family of alloys. So far, lean duplex materials have been mainly used as plate material for construction of containers and vessels in the pulp and paper business as well as in architecture and construction. As they are lower alloyed in nickel and molybdenum compared to standard duplex grades, lean duplex grades can offer economically interesting advantages combined with a higher degree of technical reliability compared to supermartensitic stainless steels. Aim of the present study was to figure out, whether lean duplex grades can close the gap between supermartensitic steels and standard duplex stainless steels with regards to weldability, structural stability, mechanical properties as well as corrosion behaviour in typical media for the oil and gas industry. Due to the fact, that there are different grades of „lean" duplex stainless steels, which significantly differ one from the other in terms of chemical composition, the investigation covered three different commercially available grades. Plasma/TIG-, Laser- and Electron Beam-Welding have been used for the industrial production of the longitudinal welded pipes made of Uranus 35N, Al 2003 and LDX 2101. For comparison the complete characterisation of longitudinally Plasma/TIG-welded pipes has been carried out according to the Shell and PDO specifications SIEP 97–5763, SP-1095, and SP-1189. The results have proven the general suitability of longitudinally welded pipes made from lean duplex grades for linepipes for sweet and slightly sour service.

Pitting Behaviour of Lean Duplex Stainless Steels in Marine and Oilfield Environments

“…Among these are the service temperature, pressure, chloride content of the fluid, contamination by oxygen, and the partial pressure of hydrogen sulphide and carbon dioxide (Oberndorfer et al, 1999). Operators of oil and gas installations continue with the application of plain carbon and low alloy steels with inhibitor dosing but often deep water well conditions are too adverse for plain carbon steel materials even with the best inhibitors (Roche, 2007;Singh et al, 2007;Rincon et al, 2008 ).…”

“…The major limitation to the use of these alloys has been the high cost of the materials. The majority of such alloys have been found to be 10 -15 times more expensive than carbon steels (Singh et al, 2007). The recent surge in the price of nickel and molybdenum has driven the cost of production of stainless steels higher.…”

“…Lean Duplex stainless steels have economic advantages over the other duplex families and some of the 300 series Austenitic stainless steels (Janardhan et al, 2011;Gudme and Nielsen, 2009;Rommerskirchen et al, 2008). More so, they have corrosion properties that compare and mechanical properties that are better than 300 series austenitic family (Alvarez-Armas, 2008;Iversen, 2006;Olsson and Snis, 2007 ;Wei et al, 2008;Gudme andNielsen,2009, Merello et al, 2003).…”

“…More so, they have corrosion properties that compare and mechanical properties that are better than 300 series austenitic family (Alvarez-Armas, 2008;Iversen, 2006;Olsson and Snis, 2007 ;Wei et al, 2008;Gudme andNielsen,2009, Merello et al, 2003). Lean Duplex stainless steels are now candidates for many oil and gas applications such as flowlines, carcass of flexible pipes, umbilical tubing, heat exchangers, separation units, cable trays and transportation vehicles (Gudme and Nielsen, 2009;Rommerskirchen et al, 2008;Jordan et al, 2006). Lean Duplex stainless steels are also structurally stable and the phases reform easily after welding as result of nitrogen addition.…”