Duplex stainless steel makes offshore oil and gas platform lighter and safer

The environment of offshore oil and gas platform represents some of the most severe environmental conditions for its construction materials. Offshore platforms must be able to operate safely for decades under the impact of extreme weather and big waves. The most challenging sites are the Arctic Ocean and its surrounding waters, where frequent storms further aggravate the extreme corrosivity of the environment. Under such harsh conditions, the service life of molybdenum alloy materials, especially duplex stainless steel and super duplex stainless steel, is longer than that of any other materials. A small and critical component is the super duplex stainless steel bolt.

What is an offshore platform?

Offshore platforms are used to extract and process the oil and gas resources on the seafloor. They are equivalent to an offshore chemical plant. The offshore platform is equipped with a complete set of self-sufficient equipment including power generation, seawater desalination, living facilities, etc., and the volume of all equipment should be compressed as much as possible, occupying the smallest space. In order to reduce the size of supporting and anchoring devices, the weight of the platform should be minimized. In addition to oil and gas production and processing equipment, several miles of pipelines are needed on the platform to transport oil and gas. The pipeline is connected by flange, which needs a lot of bolts. In the past, the bolts of hot-dip galvanized steel were used.

Material selection of oil platform

Statoil, the Norwegian oil company, is developing the Johan Sverdrup field on the Norwegian continental shelf. It is one of the largest oil fields discovered in this area in recent years. By 2022, the second phase project will be fully completed, and the oilfield will become the largest oil production base in the North Sea. The expected life of the oilfield is 50 years. The longer life expectancy and the requirement of light and compact platform set a high standard for the selection of platform components, equipment and pipelines.

Johan sverdroub oil platform starts production in 2019 © Statoil

The company's experience in other oil fields in the North Sea shows that the corrosion of traditional bolts is very serious and the service life of hot-dip galvanized steel fasteners is limited. The service life of galvanized protective coating is generally 8-10 years. Beyond this time, the steel fastener itself will be corroded. Therefore, the total life of these fasteners in marine environment is only 15 years. Only intact fasteners can ensure the safe and reliable operation of large and complex pipeline system of offshore platform, and ensure the safety of marine environment. The importance of fasteners is beyond doubt.

In the harsh marine environment, the corrosion of hot-dip galvanized bolts on the platform is serious © Statoil

Due to the extreme importance of fasteners and the extension of the effective life of old offshore platforms with technological progress and the improvement of oil production level, thousands of fasteners have to be replaced when they reach their service life.

The replacement task is very heavy, the investment is huge and affects the normal operation of the platform. In order to avoid similar problems in the future, Statoil replaced all the hot-dip galvanized bolts on the old platform with super duplex stainless steel bolts.

The design life of the Johan Sverdrup project is 50 years and duplex stainless steel was designated from the beginning. The economical duplex stainless steel is used for various structural components such as cable tray, pipe support device and other secondary components in non high temperature environment. Its advantages are long life, less maintenance and light weight. Lightweight is the key to control the construction cost of the platform. The heavier the upper platform is, the higher the requirements for the lifting capacity of the crane ship are, so the construction cost of the platform is greatly increased. On the contrary, by reducing the upper weight, the lower supporting structure can be smaller and lighter, and the construction cost will also be reduced.

Jacket installation of Johan Sverdrup platform © Jan Arne Wold/Statoil

Standard 2205 duplex stainless steel is used for condensing pipe and oil and gas pipe, and super duplex alloy is used for caisson, sea water pipeline and umbilical pipe. This project uses a variety of duplex stainless steel pipes with diameters ranging from 50 mm to 500 mm. The diameter of super duplex steel fastener for flange joint is 12-36mm.

Super duplex steel bolts and nuts of different specifications © Bumax

Super duplex stainless steel

Super dual phase alloy has high strength and good corrosion resistance to various forms of corrosion. The excellent corrosion resistance of super duplex stainless steel is due to its 3.5% molybdenum. Therefore, super duplex stainless steel is suitable for processing liquid and acid chemicals with high chloride content in seawater environment. They are increasingly used in oil and gas, seawater desalination, power generation, marine engineering and other corrosive environments.

The yield strength of duplex stainless steel and super duplex stainless steel is twice as much as that of solution annealed austenitic stainless steel 304 or 316, which is higher than that of 304 and 316 commonly used on offshore platforms after work hardening treatment. And they have good ductility and toughness at - 80 ℃, so they are important materials for Arctic Ocean or Arctic polar application.

Beyond oil field to the future

In addition to oil and gas fields, super duplex stainless steel fasteners have recently been used in energy projects such as offshore wind power generation. The greater Gabbard wind farm in the North Sea, 23 km from the coast of England, has 140 offshore wind turbines using UNS s32760 super duplex stainless steel fasteners. The Hummer gateway wind farm, also located in the North Sea, uses more than 50000 super duplex stainless steel bolts to secure turbine components. The problems encountered in the operation of offshore wind farms are the same as those of offshore oil fields: corrosive marine environment requires high strength alloy to reduce weight, maximize efficiency and prevent environmental disasters.

Super duplex steel bolts are also used in offshore wind turbines © West Special Fasteners

Marine technology is constantly developing to meet the world's energy needs in a more efficient, safer and more environmentally friendly way. At the same time, various types of duplex stainless steel have become more and more important factors in design decision. Duplex stainless steel has excellent corrosion resistance, high strength, good ductility and toughness. Therefore, they have a long service life and require little maintenance. They can realize lightweight and cost-effective design. From the perspective of investment return, they can not only recover higher initial investment, but also obtain excess return. The duplex stainless steel contains molybdenum. Molybdenum element endows the material with unique properties, which makes these high-end stainless steel materials widely favored.

Source: China DSS Pipe Fittings Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)

Next: Selection of nickel base alloy welding materials
Prev: List of Chemical Elements