High Strength Steels - 'Super' steels could lighten load imposed by offshore living quarters
High-strength steels have much to offer the offshore sector, particularly in weight savings on accommodation modules. According to Eero Heikin-pieti, LuCoil's manager of marketing/ product development, offshore accommo-dation modules are normally built from steel with a tensile strength of 250-450 MPa.
Switching to a high-strength steel offering double the tensile strength would bring a 33% weight reduction, he says, and if the tensile strength were quadrupled, a 55% reduction would ensue. So if a 600 MPa material were used instead of 300 MPa steel, the weight would be cut by one-third, and if a 1,200 MPa steel were applied, the weight could be halved. This would generate substantial cost savings in the context of the overall platform investment.
High-strength steels are also stronger, lighter and cheaper than aluminum, which is a preferred material of some quarters fabricators, says Heikinpieti. He illustrates the benefits using calculations prepared by an outside consultant for a theoretical accommodation unit made either from aluminum 6063-T6 grade or LuCoil's Hiloc Aluzink. The aluminum grade has a tensile strength of 170 MPa, compared with 700 MPa for the Hiloc Azulink.
Although the density of steel is three times that of aluminum, the wall thickness in aluminum has to be more than three times greater to achieve equivalent strength – 2 mm compared with 0.6 mm for Hiloc Azulink. The weight of the aluminum per sq meter is therefore greater than that of the steel – 5.4 kg against 4.68 kg. Factoring in a cost of SKr 30/kg for aluminum and SKr 18/kg for the steel, the cost per square meter comes out twice as high for the aluminum as for the steel – SKr 162 against SKr 84.
30% lighter containers
The products in LuCoil's Superstål range – Hiloc SH, Hiloc EH and Hiloc Stabil - have a tensile strength ranging from 900 to 1,700 MPa. The Luleå, Sweden-based company is part of the SSAB Group, one of the world's leading producer of high-strength steel.
The crossover benefits to the offshore sector of high-strength steels are also evident in other recent applications. For the manufacture of a 20-ft container, LuCoil supplied Hiloc SH grade steel with a tensile strength of 1,600 MPa. The container's wall thickness is 1.25mm, which compares with 2.5 mm for a unit made from conventional steel, while the bottom side rails, which use 1,000 MPa steel, are 3 mm thick, compared with 4-5 mm using a conventional steel. The consequent weight saving is 1.5 tonnes, which means that each container load can be increased by 1.5 tonnes. The container is also stronger – the walls, for example, will not be penetrated by badly aimed forks of a forklift truck – and has a longer life.
In another demonstration project, LuCoil supplied Hiloc EH and SH for three open-top scrap containers for Stena in Sweden. The weight of each unit was reduced by 700 kg, giving a total saving – or extra allowable payload – for the three-unit train of 2.1 tonnes. The customer can take the benefit either in transporting the same volume in fewer trips, or a higher volume in the same number of trips. Being high-strength, the containers are more resistant to tough handling and are expected to have a longer life.
To protect against corrosion in the salty sea air, LuCoil's steels can be coated with a layer of zinc or aluzinc – a mixture of aluminum and zinc. They are also easy to weld in situ, having been developed with low alloy contents in relation to the strength. All common welding methods can be applied. Although the Superstål range has not yet been qualified for offshore use, Heikinpieti sees no great barrier to achieving this status.
As regards offshore accommodation units, LuCoil is in contact with relevant manufacturers. "The volumes of steel used offshore are interesting for us, and the offer of stronger and lighter steels is interesting for them," says Heikinpieti. For instance, Hiloc HH is well suited for the walls and floors of such modules.
Deepwater pipelines
Another possible application identified by LuCoil is in deepwater pipelines. Here it could be used in conjunction with composite materials to provide reinforcement, with a steel strip wound round an inner layer of composite, and covered by an outer layer of composite. The company is exploring such possibilities with pipeline engineering companies.
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LuCoil produces its high-strength steels at its plant in Luleå, where two years ago it installed what it claims is the most modern and energy efficient blast furnace in Europe. It produces steel, which is sent to SSAB's Borlänge mill in mid-Sweden for rolling into coil and plate. Part of the production returns to LuCoil for further treatment. In the case of the high-strength steels, coils undergo cold rolling in a Sendzimir mill, a process that builds up high pressure on the roll and reduces the thickness of the steel. Such cold deformation increases all the mechanical properties of the steel, including its tensile strength.
Some of the cold rolled coils then undergo annealing in bell furnaces to reduce the brittleness of the steel. By placing twisted wire between the coil layers to allow the passage of hot air, LuCoil has reduced the time for annealing a 20-tonne coil from one week to 10 hours. The process creates very even properties. The coils can then undergo skin passing, a form of light rolling, and can be slit into strips if required. They are supplied in widths varying from 30 mm to 1,080 mm.
For more information, contact Eero Heikinpieti, LuCoil Steel. Tel: +46 920 928 76, fax: +46 920 25 58 56. [email protected]. www.lucoil.com
