The use of duplex stainless steel on oil platforms has become very common lately. Its application is concentrated on the topside, the upper part outside the water.
Stainless steel is used in several different forms: as plates, as supporting structural bars, as pipes, as grating and much more. The Norwegian national Oil Company Statoil is particularly conscious about lifetime costs and sustainability. Upon their calculations, even with a higher initial investment cost the use of stainless steel is justified.
Salty water and wind attack the exposed equipment on oil platforms as well as the platform itself constantly. Therefore a protection against corrosion is mandatory. Coating and painting steel parts are the most common ways to protect the material. This implies a constant maintenance due to repainting operations.
Another possibility is to increase material thickness. Even with corrosion, this is so oversized that there is no risk for failure, even though it is not so much common practice anymore.
The last possibility is to use stainless steel. Generally, the austenitic stainless steel grade 316L (1.4404) is used for common corrosion protection. Pipe racks and pipe supports, structural stiffeners and grating are often made of stainless steel. Also cladding or entire walls are made with stainless steel plates.
The major part of structural sections in stainless steel that engineers plan for the topside of off-shore rigs are square and rectangular hollow sections. They are followed by U channels and angle bars, generally equal leg angles. And last there are standard structural beams like HEA, HEB or IPE.
Due to weight, only seldom the heavy wide flange beams HEM in stainless are in the blueprints.
The heavier the topside of the oil drilling rig is (also of all accessory platforms like the living platform and the service platform), the stronger the steel legs or steel jackets must be, or the bigger the floating steel cylinder of spar platforms will have to be.
The goal of reducing weight makes special stainless steel grades like duplex steels very attractive for the topside.
Duplex and lean duplex steel grades grant high corrosion resistant properties. In addition, their mechanical performance as far as the material’s yield is concerned, enables designers and engineers to plan lighter sections. These will carry the same load within the stainless steel construction.
This is the case for the Johan Sverdrup oil field, one of the recent projects of Statoil. The Norwegian engineering company Aker Solutions won the contract for the design of the necessary structures and chose to go for a special lean duplex stainless steel grade.
This oilfield was discovered in 2010 and is located less than 160 km west from Norwegian City Stavanger. The sea is about 120 meters deep there. The subsea wells will be till 1900 meters below sea level.
The infrastructure consists in several oil platforms connected with bridges. The power supply will be from onshore. Two separate pipelines will bring the gas to Kårstø (165 kilometers) and the oil to Mongstad oil terminal (274 kilometers).
At peak, production will reach 550.000 to 650.000 barrels daily. In 2019 the oil production will start and go on for the following 50 years.
This time schedule allows thinking out of the box and evaluating several kind of options and material solutions for special applications like this one. And the choice is for duplex stainless steel.