A leader in materials

By Outokumpu

Why choose Outokumpu? We have the material, the know-how and the resources. As a global leader in stainless steel, Outokumpu serves customers in the oil and gas industry across every continent.

Decades of know-how
Though oil is not corrosive in itself, production takes place in highly corrosive environments. Over the years we have refined our grades to handle these conditions. Today, stainless steel, especially Duplex, has all but replaced traditional carbon steels. The benefits include superior corrosion resistance, excellent mechanical properties, greater ecological sensitivity and recompelling, both for topside construction and oil flow applications.

We don’t buy stainless steel – we make it
When you deal with Outokumpu you go straight to the source. We make the material that many others market. How can you benefit from this? For a start, you can be sure that we have full control over the entire production process from melt to delivery. Furthermore, as we make the steel you need never worry about availability. You will get the quantities you need when you need them. And because we handle all research and development (R&D), nobody is better able to answer any questions you may have about material performance.

Flowlines
Due to the presence of CO2, chlorides, hydrogen sulphide (H2S) and the formation of water condensate caused by reduced pressure, corrosion is a very real risk. If you then factor in high flow rates and temperatures, it is easy to see why the material used for flow lines and piping systems, has to be very tough indeed. Thanks to its superior mechanical strength and excellent corrosion resistance, grades LDX 2101 and 2205 are excellent choices. They are economical too when you consider that for some dimensions installation costs for Duplex are lower than for clad steel. Furthermore, grades 254 SMO, 2205 and SAF 2507 are approved by the National Association of Corrosion Engineers (NACE) for sour service, based on their proven corrosion resistance properties.

Topside construction requires top performance materials
Stainless steels lend themselves superbly to topside construction both technically and economically. In these applications the life-cycle benefits are significant while the dramatic weight reductions provided by stainless steel (especially Duplex) help to reduce the overall cost of the project.

Main process piping
In offshore wells production can be complicated. In addition to the incoming mixtures of oil, gas and sand, the main process piping has to withstand seawater, H2S and CO2. In extreme conditions such as at elevated temperatures and under high pressure, process fluids are often highly corrosive and require extreme measures. Grades 2205, SAF 2507 and 254 SMO lend themselves superbly to this type of piping.

Seawater piping
The up- and downside of chlorine is well known when it comes to seawater piping. Without it, heat exchangers lose their effectiveness. With it, corrosiveness increases dramatically. Outokumpu’s 254 SMO and 4529 have enjoyed widespread success in systems operating at up to 30ºC. Furthermore, if you complement 254 SMO or 4529 piping with 4565 for flanges or other parts with crevices, service temperatures can be increased.

Corrosion in marine environments
Practical experience and extensive testing have shown that conventional stainless steels are prone to localized corrosion in seawater. The key alloying elements for improving resistance are chromium, molybdenum and nitrogen. It is important to remember that pitting resistance equivalent (PRE) expressions are based on accelerated laboratory tests performed on perfectly heat-treated base material. In fact, the effect of various production processes and welding may completely overshadow small differences in steel composition.

Chlorinated seawater
Chlorinated water features higher corrosion potential than unchlorinated water. However, chlorination prevents the formation of an active biofilm, resulting in considerably less cathodic efficiency than in unchlorinated water. This means the propagation rate of crevice attack and the rate of galvanic corrosion are significantly lower than in unchlorinated water.

Corrosion in oil and gas production
Although oil is not corrosive in itself, production of this substance can involve highly corrosive environments due to the simultaneous presence of water and dissolved aggressive species such as O2, chlorides, CO2 and H2S. As there is no better way to combat the increasing amounts of corrosive agents, stainless steels are rapidly becoming the material of choice. Environmental considerations have also paved the way. After all, carbon steel demands the addition of vast amounts of artificial corrosion inhibitors that are more or less hazardous to the environment. Another key consideration is the bottom line. With stainless steels you stand to save money through low maintenance costs, weight savings and a service life that extends way beyond that of carbon steels.

‘Sweet’ and ‘sour’ environments
Sweet environments contain no amount or negligible amounts of H2S. High amounts of CO2 may be present in sweet environments, causing accelerated corrosion on carbon steel. Sour environments contain significant amounts of H2S, causing sulphide stress cracking under certain conditions.

Hydrogen sulphide
Hydrogen sulphide is a key consideration when selecting construction material for oil and gas production equipment. Sulphides, S2, HS and H2S, catalyse the absorption of hydrogen atoms by the metal, which may subsequently crack due to hydrogen-induced cracking, so-called sulphide stress cracking. Hydrogen embrittlement is the most active cracking mode at low temperatures. Chloride stress corrosion cracking predominates at high temperatures. Therefore, the combined risk of cracking due to H2S and chlorides tends to be most severe in non-ferritic stainless steels at intermediate temperatures (80–100ºC). H2S increases the risk of cracking. However, very small amounts of H2S are not necessarily a negative factor as they can ensure a low redox potential in the environment. This reduces the risk of stress corrosion cracking (SCC), pitting and crevice corrosion.

Cathodic protection and hydrogen: embrittlement
In cathodic protection significant hydrogen evolution may take place. If that is the case, certain alloys may suffer embrittlement and subsequent cracking. The risk is especially high if the material is cold worked and subjected to great mechanical stress and fatigue loading. However, austenitic stainless steels are far less prone to hydrogen embrittlement (HE) than ferritic and martensitic grades. And even though Duplex grades have an excellent track record in oil and gas production, tests have revealed a loss in ductility due to hydrogen pickup during cathodic polarisation. The good news is the recent development of lower alloyed Duplex stainless steel grades such as LDX 2101. Compared with super martensitic stainless steels (SMSS), leaner grades (LDSS) not only offer economic benefits but also greater technical reliability.

Chlorides
Chlorides are often present in high concentrations in water associated with oil and gas formations. Localized corrosion on stainless steel is induced by chlorides. However, the oxygen content of these fluids is for the most part, extremely low, which means significantly higher chloride levels are more acceptable than under oxygen-saturated conditions.

Solutions for the offshore oil and gas industry
The following stainless steel grades have been specially selected to meet the rigorous demands of the oil and gas industry. Each and every product has undergone extensive testing – both in the laboratory and in the field.

(For more, please visit http://www.outokumpu.com.)