Are you looking to choose the right material for your offshore and subsea piping?
While there are a lot of options available in the market, both prevalent options– carbon steel and stainless steel have limitations when it comes to seawater.
To know what the best choice is based on your preferences, here’s an article:
Carbon steel depends on coatings, corrosion inhibitors, and corrosion margins. These measures can work well, but each has practical limitations.
Coatings deteriorate through cyclic wetting and mechanical wear. Inhibitor dosing can be inconsistent in long subsea flowlines, while corrosion allowance is gradually consumed. Once the remaining wall thickness becomes too low, exposure can quickly become an integrity risk.
According to Continental Steel and Tube, both carbon steel and stainless steel are affected by saltwater corrosion. Subsea equipment must also withstand abrasion and other mechanical loads, making material selection particularly important.
Stainless steel performs well under typical chloride exposure. Its passive oxide layer can nevertheless break down at high temperatures, high chloride concentrations, low pH levels, or inside crevices with limited oxygen.
Local breakdown of this layer can lead to pitting and crevice corrosion. Austenitic and duplex grades may also be vulnerable to stress corrosion cracking in sour service, especially around welds and heat-affected zones. Super duplex extends the operating range but is not immune under hot, chloride-rich, H2S-bearing areas.
Material risks are not distributed uniformly across a subsea system. Some locations are more difficult to inspect or repair and therefore require a greater level of corrosion resistance.
These include:
A material upgrade in these areas can be relatively minor compared with the cost of an offshore intervention or subsea repair.
Nickel alloys withstand several of the corrosion mechanisms that limit carbon steel and stainless steel.
High nickel content strengthens the austenitic structure and improves resistance to hydrogen embrittlement and sulfide stress cracking. This makes certain nickel alloys suitable for sour service conditions in which stainless grades may crack under sustained load.
Nickel, chromium, and molybdenum also provide enhanced resistance to H2S, CO2, chlorides, and organic acids. This broad corrosion resistance is valuable when multiple aggressive substances are present in the same process fluid.
Continental Steel and Tube suggests alloys such as Monel and Inconel for subsea applications requiring both corrosion and abrasion resistance. Nickel alloys can also retain their mechanical properties under repeated pressure and temperature cycles, which is relevant for risers and production flowlines.
Nickel alloys are available in more configurations than solid pipe. The Special Metals Corporation handbook lists sheet, strip, plate, pipe, and tube as common product forms for high-performance nickel alloys.
This allows engineers to use nickel alloys only where their properties are required.
In offshore piping systems, nickel alloy sheet and plate can be used for cladding or lining carbon steel pipe, bends, and fittings. The nickel alloy protects the wetted surface, while the carbon steel substrate provides structural strength.
PCC Energy describes the use of weld overlaying and co-extrusion to create corrosion-resistant surfaces in oil and gas piping. This approach combines a carbon steel base with a nickel alloy interior and can be more economical than using solid nickel alloy throughout a long flowline.
Sheet material can also be fabricated into flanges, transition spools, and connector components. Nickel Suppliers notes that nickel sheet provides resistance to seawater, acids, alkalis, and saltwater environments.
Nickel alloys have a higher initial price than carbon steel and standard stainless steel. The difference becomes less significant when inspection, maintenance, production downtime, and repair costs are included.
Carbon steel in aggressive service requires corrosion monitoring, inhibitor management, and eventual replacement when its inspection allowance has been consumed. Stainless steel may carry a risk of stress corrosion cracking that is difficult to detect during routine inspections.
Using nickel alloys in selected high-risk areas can reduce inhibitor reliance and extend the period between interventions. This is particularly relevant in deepwater and remote locations, where access is limited and repair costs are high.
The relevant comparison is therefore not only the price per tonne. It is the total cost of maintaining piping integrity throughout the intended service life.
Carbon steel remains suitable for topside systems in managed environments, non-corrosive gas service, and accessible applications where coatings and cathodic protection can be maintained.
Duplex and super duplex stainless steels can be used for typical chloride exposure, seawater injection systems, and less severe sour service conditions.
Nickel alloy sheets become more beneficial when several demanding conditions occur together, such as:
Material selection should be based on the conditions at each part of the system rather than using one alloy across the entire project.
Maass Special Alloys supplies nickel alloy sheet and plate for cladding, lining, and fabricated assemblies used in subsea and marine environments. A practical material review starts by assessing fluid composition, temperature, pressure, mechanical loading, and accessibility at each critical location.
Material selection is an important step when it comes to subsea piping. This is why, rather than going by market trends, it is important to follow an accurate procedure for material selection. This is where this blog will help you in decision-making.
Ans: It requires pre-treatment and then transportation of the pipelines for installation. The construction process is also complicated.
Ans: The material selection process is defined as the systematic approach to choosing appropriate materials for product design.
Ans: Steel is widely used in the construction of offshore oil rigs, the offshore wind industry, and many types of marine applications due to its strength, durability, and ability to withstand the harsh marine environment.
Ans: Offshore pipelines refer to the unique means of efficiently transporting fluids such as oil, gas, and water from subsea wells to production facilities and ultimately to shore, adapting to various environments.
