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Moly-grade stainless steel makes waves at Middle Eastern airport

The world’s largest stainless steel roof is being built at the New Doha International Airport in Qatar. Shaped in a billowing wave-like form, it symbolizes the Gulf emirate’s aspiration to be a gateway to the world and sets new standards of elegance, comfort, and convenience in air travel. The roof of the passenger terminal is made of a moly-grade lean duplex stainless steel.

Doha International Airport

New Doha International Airport passenger terminal approach.
Image: Qatar Airways.

With a massive, boldly innovative expansion of its airport facilities, Qatar is literally reaching for the skies. New Doha International Airport (NDIA) promises to be an international gateway to the region that will showcase the cultural, technical, and environmental achievements of this small but wealthy Gulf state. Located about 4 km east of the existing airport (which it will eventually replace), NDIA will comprise a new passenger terminal, an Emiri (royal) terminal complex, Qatar Airways’ new headquarters, a public mosque, an 85-metre high control tower, cargo terminal buildings, hangars, and other buildings.

New Doha International Airport passenger terminal

New Doha International Airport passenger terminal – interior view.
Image: Qatar Airways.

With two of the longest commercial runways in the world, the new airport will be among the first to accommodate the Airbus A380 double-decker “super-jumbo,” the largest commercial aircraft ever built. When finished, NDIA will be about two-thirds the size of Doha and 12 times larger than the current Doha international airport. It is expected that the new airport will generate additional commercial activity. For this reason, a huge complex will be built alongside the airport: an area of over 100 hectares (about 10.8 million square feet) have been reserved for a hotel, an office and business park and a retail mall.

The airport is being built on 22 square kilometres (about 8.5 square miles) of land, nearly half of which was reclaimed from the sea. 50 million cubic metres (almost 1.8 billion cubic feet) of landfill and four large dredgers were used for the reclamation, which was completed in 2005. A reinforced seawall 13 km (8.1 miles) long was built to protect the area. A large-scale environmental cleanup was also carried out, with 6.5 million cubic metres (about 229.5 million cubic feet) of waste being moved to a new purpose-built facility.

Surrounded by sea, the airport incorporates water motifs in its design. The approach road for arriving passengers is flanked by water; the Emiri terminal is shaped like sailboats; undulating metal roof structures can be found in support facilities and on the stainless steel passenger terminal roof. When it reaches its definitive form after 2015, the entire passenger terminal complex will be the largest building in Doha. Designed by US architects HOK (Hellmuth, Obata & Kassabaum), it sets new standards in elegance, comfort, smart technology, and environmental consciousness. Moving walkways will help passengers move around the building, and CO2 and heat occupancy sensors will regulate air intake.

New Doha International Airport from the air

New Doha International Airport from the air.
Image: Qatar Airways.

But the terminal’s most striking feature is its undulating roof, said to be the largest stainless steel roof in the world. Several factors had to be taken into account when selecting the stainless steel grade. The most important of these was the airport’s closeness to the sea. The roof had to both resist the heat and humidity found everywhere in the Middle East, and withstand salt corrosion on top of these other problems. Other factors included cost and a favourable strength-to-weight ratio.

Another important consideration was that the roof should not reflect too much sunlight, which would not only cause discomfort on the ground but also distract pilots. Texture and appearance were therefore vital not only for aesthetic but also for practical reasons. To get these two factors right, Contrarian Metal Resources applied its proprietary InvariMatte® finish to the duplex stainless steel sheets. This non-directional, low-gloss, uniformly textured stainless steel finish is especially suited to surfaces where low reflectivity is required. The end result is extremely consistent, like paint. However, as there are no coatings to deteriorate, it can last much longer, without much maintenance.

Lean duplex stainless steel roof

Although this type of matte finish can be applied to stainless steel Types 304(L) and 316(L), it was a duplex stainless steel that was specified on this occasion. Contrarian Metal Resources turned to a local supplier of global renown, Pennsylvania-based ATI Allegheny Ludlum. The company’s AL 2003™ lean duplex stainless steel (1.8% Mo, 21.5% Cr, 0.17% N, 3.7% Ni) turned out to have exactly the right properties for the task in hand. The manufacturers have stated that this grade has a substantial strength advantage over Type 316L, allowing thickness reductions and therefore lighter weight; this translates into significant savings in raw material costs. Also, the corrosion resistance of this grade can be superior to 316L, even in the as-welded condition. Especially where weight-to-strength ratio and chloride pitting resistance are important, certain grades of duplex stainless steel can be more suitable than 316L. AL 2003™ has in fact replaced 316L in some applications: for example, the pipes supporting the canopy for a Washington DC metro station, flexible flowlines for the Kikeh oil and gas project offshore Malaysia, and laser-welded tubes for a geothermal condenser.1

AL 2003™ is the only lean duplex grade that contains moly in significant amounts. In this grade, moly is an indispensable ingredient in combating corrosion. This is vital in a region notorious for its hot, humid and saline atmosphere – especially as the airport site is virtually surrounded by sea.

1 John J. Dunn and David S. Bergstrom (ATI Allegheny Ludlum), “AL 2003™ LDSS (UNS S32003) as a substitute for Type 316L”, Stainless Steel World 2006 Conference, Texas, 14 November 2006.