Demand for offshore vessels is running at a low level currently, but when the market picks up again and new designs come to market, greater use of aluminium could have a number of advantages
The benefits of using aluminium in fast craft such as crewboats and crew transfer vessels are well known, but aluminium extrusion specialist Hydro believes that extruded aluminium has a lot to offer the market for conventional offshore vessels.
Prior to the downturn and the ‘new reality’ in the offshore vessel sector, Hydro completed a number of technical and cost studies with leading offshore support vessel builders that suggested aluminium could reduce weight significantly and be cost-competitive with steel.
As Hydro’s marine and offshore market manager Chris Moyle explained, aluminium is well suited to use in the offshore environment. One-third of the weight of steel, with lower maintenance costs, it is durable, strong and highly corrosion resistant.
“By reducing weight, aluminium reduces costs,” said Mr Moyle. “It also saves time and cost in installation. Further savings come because of the metal’s low maintenance qualities and its resistance to corrosion and because it has a considerably longer lifetime than steel applications. Extremely versatile, aluminium adapts easily into any design and offers a smooth surface finish, so it’s well suited to marine applications.”
Mr Moyle told OSJ that the aluminium Hydro extrudes is probably best known in the offshore vessel industry from helicopter decks. It also supplied extruded panels for yards that specialise in building fast craft for the offshore wind industry. However, a study with a leading Norwegian shipyard that wanted to reduce weight on an anchor-handling tug/supply (AHTS) vessel it was designing also showed that it could be used to provide weight savings on much larger units. Weight savings of 40–50% are possible compared with steel when used in areas of a vessel such as accommodation blocks.
“Aluminium can be used in any part of a ship’s superstructure,” said Mr Moyle. “It is suitable for use anywhere except where there are specific loadbearing requirements to bear in mind, where steel would still be required. Our study showed that, for the AHTS design in question, a weight saving of 140 tonnes was possible. A follow-up study of the economics of using aluminium showed that using friction stir welded extruded panels was also cost-competitive.
“Our message is that, in many applications, aluminium is every bit as suitable as steel and it is as cost-effective too. It can also meet all of the necessary fire protection requirements involved in building a vessel.” To provide fire protection in areas where special fire classes need to be met (such as A60), Hydro applies protective materials, including mineral wool or ceramic fibres. Even with the additional weight of the protective insulation and any active fire suppression system that may be required, the aluminium structure is still far lighter than steel.
To meet the demand of the use of aluminium in the marine environment, Hydro has developed new alloys. The latest, the 5083 series, is highly resistant to corrosion in salt water, so it is particularly suited to applications in the waterline area. It also offers higher weldability and predictable post-weld strength, and extruded sections maintain their flatness. It meets the stringent technical requirements for hull structure applications as required by DNV GL and ABS.
For the production of the panels, Hydro uses friction stir welding (FSW). Compared with melt welding, FSW provides greater strength and less deformation. Size is not an issue either, Mr Moyle said. Hydro can produce large profiles and – through FSW – extruded panels such as extruded decking profiles up to 18,000 mm long and 3,500 mm wide.