Selecting subsea stainless steel

Rodrigo Signorelli, Outokumpu’s Lead Technical Manager for Marine & Energy, outlines the various stainless steel and alloy grades used to ensure safety and reliability in demanding subsea applications. He also introduces Sanicro 35, our new cost-competitive alternative to Alloy 625 for the most challenging environments.

Oil and gas applications begin their journey below the surface with control lines and chemical injection lines. Control lines, for example, provide hydraulic services that operate surface-controlled subsurface safety valves (SCSSVs). These are used as a safety measure to shut down production if the line pressure drops. Chemical injection lines enable wellhead engineers to dose wells with fluids that enhance separation of sand from oil and gas, reduce foaming, enhance flow rates or inhibit corrosion.

It is vital for both applications to provide complete reliability in subsea environments up to 10,000 m below the surface. That is why the lines are manufactured from coiled tubing in corrosion resistant alloy (CRA) that can withstand the corrosive effects of chemicals and sour environments as well as providing the mechanical strength to withstand high pressure environments. Fatigue strength is also a vital factor to prevent failure under cyclic loading. 

Our nickel-based Ultra Alloy 825 is the standard choice for control lines and chemical injection lines due to its resistance to stress corrosion cracking (SCC) and sour media.   For less demanding applications, Supra 316L, our molybdenum-alloyed austenitic stainless grade, can also be a good choice.

 

Longitudinally-welded umbilicals

Umbilicals carry multiple subsea services bundled within a single line to provide the vital link between remote subsea wells and the oil & gas platform. They deliver control, power and communication services to exploration activities and CO2 storage fields. These services can include a variety of different applications including flow lines, power and data cables, water, chemical feeds, hydraulic fluid or high-pressure air for pneumatic control. 

They need to span long distances in deep water, this calls for construction materials that can withstand high pressures. That is why operators have traditionally used seamless stainless steel tube for umbilicals. The theory is that because there is no seam, weak points are eliminated. There are downsides to seamless tube - it is costly to produce and the maximum length of individual tubes is limited, requiring tubes need to be butt welded together to achieve the required length. These butt welds can themselves be potential weak points, so they call for particular care in testing and qualification that adds to manufacturing complexity and cost. 

In recent years Outokumpu has seen growing interest from design engineers who are adopting longitudinally welded tube as an alternative to seamless tube. Consistent process control ensures fewer weak points, making it possible to achieve higher mechanical strength for the same dimensions. In some cases this could reduce the thickness of tube needed and save weight. 

When it comes to the ideal grade for umbilicals, our Forta SDX 2507 is a popular choice. It is a super duplex stainless steel, with extremely high corrosion resistance and mechanical strength thanks to its chemical composition and low level of inclusions. 

 

Stainless steel rises to the challenge for flexible pipes

Flexible pipes are used as risers to transport oil and gas products from the seabed to the surface, as well as flowlines on the seabed. They need to be able to handle the dynamic loadings that result from tides and waves. 

In their construction, flexible pipes feature a number of layers including leakproof thermoplastic barriers, armor wires for tension and pressure resistance and a stainless steel carcass for collapse resistance.

For more than 20 years Outokumpu alloys have been used to provide the vital combination of strength and corrosion resistance for the carcass. We provide material in the form of strips ranging in width from 25 mm to 200 mm and up to 4 mm thick. Our customers form the strip into profiles and interlock them to create long flexible tubes. 

Typically, austenitic grades such as Supra 316L are used for projects in shallow seas, while some customers specify alloys with higher corrosion resistance such as Ultra 254 SMO and Ultra 6XN for installations where the water has a high salt content. 

In deeper waters there is a need for materials with additional strength that can withstand the increased pressure. The answer is to use duplex grades such as Forta LDX 2101, Forta DX 2205 and Forta SDX 2507. 

 

Clad and lined pipes for a cost-effective solution

Subsea pipelines  are essential to transport oil and gas from the production site to the refinery. At first sight, manufacturing them  completely from a corrosion resistant alloy (CRA) might seem the best option. However, this can be prohibitively expensive. A much more cost-effective approach is to use mechanically lined pipe (MLP) or clad pipe. In this case, Outokumpu alloys provide a thin corrosion-resistant layer on the inside diameter, while mechanical strength is provided by an outer shell made of carbon steel.

Clad pipe achieves a metallurgical bond between the CRA and the outer pipe. This means that there is no void between the two layers that the product could potentially seep into. As a result, clad pipe helps to control risk in high pressure and high temperature applications. 

One production route is through specialist rolling mills in which a sandwich of CRA and carbon steel is passed multiple times to create composite plates. Some manufacturers use explosion bonding to produce clad plate. This offers them wide flexibility when combining materials and is mainly used for thick plates and small batch sizes. A third technique for making clad pipe is weld overlay, where CRA welding wire or welding strip is bonded to the inner surface of a carbon steel pipe or fitting through welding. 

MLP is produced by lining lengths of carbon steel pipe with CRA. There are several options available to create a mechanical bond between the two layers. The most common process involves inserting a CRA liner pipe into a carbon steel shell and applying pressure with hydraulic fluid or mechanical force from a spinning mandrel to push the lining tightly into the pipe body. Some manufacturers also make use of heat on the outer pipe or adhesives to strengthen the bond between the two layers. 

These techniques do not need the intense energy and manufacturing infrastructure that is needed for clad pipes. As a result, they are less costly, but the mechanical bond is usually not as strong as the metallurgical bond in clad pipe. 

Popular grades for clad and lined pipes include austenitic Supra 316L and its high-molybdenum variant Supra 316L HiMo. For more aggressive environments, higher-alloyed stainless steels are specified such as Ultra 317L and Ultra 904L. In some cases, 254 SMO is used. This is a 6 % molybdenum and nitrogen-alloyed austenitic stainless steel with extremely high resistance to both uniform and localized corrosion. Where sour gas is a challenge, then Outokumpu can also supply Ultra Alloy 825, which is a titanium-stabilized austenitic nickel-base alloy used widely in the oil and gas industry for piping systems.

 

Sanicro® 35 – the cost-competitive alternative to Alloy 625

Traditionally, engineers have used nickel-based Alloy 625 as their go-to grade for equipment in the most challenging corrosive subsea environments. However, its price is high and variable due to its alloying content, which includes 58% nickel and up to 10% molybdenum. Both of these elements have recently experienced price volatility, making the cost of Alloy 625 unpredictable. In turn, this creates challenges in project planning and delivery.

Now Outokumpu is able to offer Sanicro® 35, a cost-competitive alternative to nickel-based Alloy 625 that provides excellent corrosion resistance to ensure a long service life in demanding environments. Sanicro® 35 provides similar performance while reducing exposure to market volatility as it contains only 35% nickel and 6.4% molybdenum.

Sanicro® 35’s good balance of alloying elements and properties enables it to fill the gap in the market between high-performance stainless steels and nickel-based alloys for the harshest environments. It can withstand a wide range of corrosive conditions, including seawater, acid and alkali, erosion-corrosion, pitting and crevice corrosion and stress corrosion cracking (SCC). In addition, it has high mechanical strength together with excellent formability, weldability and machinability. 

 

Accurate sustainability data is critical

At Outokumpu we understand that in addition to ensuring the performance of our materials in subsea applications, carbon footprint is becoming increasingly important as oil and gas operators face growing pressure to measure and report on sustainability. 
That is why we have taken the initiative to provide our customers with more accurate data than is available from other sources. So, in November 2022, we started publishing the product-specific carbon footprint on the product certificates that we supply when making deliveries. This is based on a rolling average that takes account of all the variations in production that influence carbon footprint and has been verified by engineering consultancy WSP. 

The result is that our customers can get accurate carbon footprint data so that they can have greater confidence in the accuracy of their own carbon footprint calculations. The tangible benefit is that until now, OEMs have often used an average carbon footprint, such as the European average of 2.8 kg of CO2 per kg of stainless steel. Now they can refer to an accurate figure, which in some cases will be lower.

Learn more

Oil and gas
Product specific footprint
Ultra
Forta
Sanicro® 35 product page

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