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New Developments In Aluminum Shipbuilding
The creation of this article was promoted by current activities within the United States military, the extraordinary developments in aluminum shipbuilding that have been taking place in Australia, and the creation of new high strength aluminum alloys, primarily in Europe, for the shipbuilding industry.
I was fortunate enough recently to visit Incat Tasmania Pty Ltd (Shipbuilding) off the southeast coast of Australia. This manufacturer has over the years taken aluminum shipbuilding to exiting new levels. In 1977 they launched their first high-speed catamaran, and today they are manufacturing the new generation of 98-metre (100 plus yards) wavepiercers that are being evaluated by the United States military (see Fig 1). Incat has constructed more than 50 vessels of varying lengths. The company’s first passenger/vehicle ferry was delivered in 1990, a 74-metre Wave Piercing Catamaran with a maximum deadweight capacity of 200 tonnes. The more recent 98m Evolution 10B range has a deadweight four times that amount. While Incat-built ferries initially revolutionized transport links around the United Kingdom, today its ships operate in North and South America, Australasia, the Mediterranean, and throughout greater Europe. Incat’s extensive shipbuilding activity is conducted from a modern facility with over 32,000 m² under cover, located at Hobart’s Prince of Wales Bay Tasmania.
Aluminum Welded Ships Within The United States Military
In response to great interest from the US military in high-speed craft, Incat, via its United States affiliate Incat USA, formed a strategic alliance with an American shipyard to market and build innovative craft designs for the US military and commercial markets. The Bollinger/Incat USA, strategic alliance combines Incat, the premier builder of the world’s fastest vehicle/passenger ferries with Bollinger, a proven builder of a variety of high speed, reliable and efficient patrol boats for the US Navy and Coast Guard.
The US government has awarded Bollinger / Incat USA, the charter for a High Speed Craft (HSC) for a multi-service program operated by various arms of the US military. The HSC Vessel, now known as Joint Venture HSV-X1 (see fig 1) is being used for evaluation and demonstration trials in order to assess the usefulness of such technology in US military and Coast Guard applications. The Joint Venture was selected as the optimum vessel to deliver the best performance for the scope of work required by the military. The scope of work listed minimum essential capabilities necessary to meet military operational needs. Undergoing a major refit with innovative design and construction, the craft was upgraded and fitted with military enhancements such as a helicopter deck, stern quarter ramp, rigid hulled inflatable boat, deployment gantry crane, and troop facilities for 363 persons, crew accommodation, storage facilities, medical facilities and long-range fuel tanks. As the Joint Venture continues to excel in her experimental role, Incat’s intention to see the military potential of such craft realized took another step forward in September 2002 with the sale of USAV TSV-1X Spearhead to Bollinger/Incat USA for charter to the US Army.
Spearhead (see fig 2) is the US Army’s first Theater Support Vessel (TSV) and is part of the Advanced Concept Technology Demonstrator (ACTD) program by the Office of the Secretary of Defense and the US Army. Spearhead will be used to demonstrate and evaluate its ability to perform during certain mission scenarios, assess its usefulness to the US military and refine the requirements for the next generation of Army watercraft. The TSV is critical to the Army's ability to perform its Title 10, intra-theater mission. Spearhead will be utilized on missions to maximize its speed and flexibility and is needed for both sustainment deliveries and the movement of Army prepositioned stocks and troop units. TSVs promise to change the way the US Army gets to the fight. They will allow the Army to quickly deliver intact packages of combat-ready soldiers and leaders with their equipment and supplies, enabling them to “fight off the ramp” if necessary. Delivering intact units within a theater also will reduce the need for a large-scale on-shore reception, staging, onward movement and integration of soldiers, vehicles and equipment within the battle space. The future vessels promise to transport units within a theater of operation in hours instead of days. The TSV will support the Army Transformation goal of deploying a combat-ready brigade anywhere in the world within 96 hours, a division in 120 hours and five divisions within 30 days. Speed, coupled with a large cargo capacity, will provide greater payload throughput at long ranges as well as the ability to rapidly reposition and mass assets within a theater of operations.
Just three weeks after the awarding of the contract for Spearhead came another, separate, order from the US military. Military Sealift Command, Washington, D.C., is the contracting arm that will lease a 98m craft from Bollinger/Incat USA, LLC, Lockport, La., to support US Navy Mine Warfare Command. The craft, HSV-X2 is currently under construction at the Hobart, Tasmania shipyard with delivery to Ingleside, Texas scheduled for June 2003. The ship will be capable of maintaining an average speed of 35 knots or greater, loaded with 500 short tons, consisting of 350 personnel and military equipment. A minimum operating range of 1100 nautical miles at 35 knots is required by the contract, as is a minimum transit range of 4000 nautical miles at an average speed of 20 knots. Furthermore, the craft must be capable of 24-hour operations at slow speeds (3-10 knots) for small boat and helicopter operations.
A stern ramp capable of on/off loading directly astern or to the starboard quarter will be fitted. The ramp will be capable of loading/unloading a multitude of military vehicles up to and including battle tanks of up to 140,000 lbs. The ramp will also be capable of launch and recovery of amphibious assault vehicles. To achieve this, the ramp tip end will be submerged, allowing the amphibious vehicles to drive on and off.
The ship will also be capable of launch and recovery of small boats and unmanned vehicles up to 23,000 lbs. while underway.
The vessel will also be fitted with a NAVAIR certified helicopter deck for operation of MH-60S, CH-46, UH-1 and AH-1 helicopters. An area protected from the weather for storage and maintenance of two MH-60S helicopters will be provided, as will a Carriage Stream Tow and Recovery System (CSTARS). This helo deck will have the capacity to transfer equipment up to 6000 lbs. to and from the vehicle deck.
New Developments in High Strength Aluminum Alloys for Marine Applications
Until fairly recently the most popular base material used for aluminum shipbuilding, 5083, has had very little rivalry from other alloys. The 5083 base alloy was first registered with the Aluminum Association in 1954, and while often referred to as a marine aluminum alloy, has been used for many applications other than shipbuilding. The popularity of the 5083 alloy within the shipbuilding industry has been largely based on its availability and also its ability to provide an aluminum alloy with excellent strength, corrosion resistance, formability and weldability characteristics. Other lower strength alloys such as 5052 and 5086 have been used for the manufacture of usually smaller, lower stressed and typically inland lake boats, but 5083 has been predominant in the manufacture of ocean-going vessels.
In recent years progress has been achieved by aluminum producers in the development of improved aluminum alloys specifically targeted at the shipbuilding industry. In 1995 the aluminum manufacturer Pechiney of France registered the aluminum alloy 5383 and promoted this material to the shipbuilding industry as having improvements over the 5083 alloy. These improvements provided potential for significant weight savings in the design of aluminum vessels and included a minimum of 15% increase in the post-weld yield strength, improvements in corrosion properties and a 10% increase in fatigue strength. These developments, coupled with formability, bending, cutting, and weldability characteristics, at least equal to that of 5083, made the 5383 alloy very attractive to designers and manufacturers who were pushing the limits to produce bigger and faster aluminum ships.
More recently in 1999 the aluminum manufacturer Corus Aluminum Walzprodukte GmbH in Koblenz (Germany) registered the aluminum base alloy 5059 (Alustar) with the American Aluminum Association. This alloy was also developed as an advanced material for the shipbuilding industry providing significant improvements in strength over the traditional 5083 alloy. The 5059 alloy is promoted by Corus as providing improvements in minimum mechanical properties over alloy 5083. These improvements are referenced as being a 26% increase in yield strength before welding and a 28% increase in yield strength (with respect to alloy 5083) after welding of H321/H116 temper plates of the AA5059 (Alustar alloy).
Welding the New Aluminum Alloys
The welding procedures used for these high strength alloys are very similar to the procedures used when welding the more traditional 5083 base alloys. The 5183 filler alloy and the 5556 filler alloy are both suitable for welding 5383 and 5059 base alloys. These alloys are predominantly welded with the GMAW process using both pure argon and a mixture of argon/helium shielding gas. The addition of helium of up to 75% is not uncommon and is useful when welding thicker sections. The helium content provides higher heat during the welding operations, which assists in combating the excessive heat sink when welding thick plate. The extra heat associated with the helium shielding gas also helps to reduce porosity levels, this is very useful when welding the more critical joints such as hull plates that are often subjected to radiographic inspection.
The design strength of these alloys are available from the material manufacturers, however there would appear to be little as-welded strength values incorporated in current welding specifications. Certainly these relatively new base alloys are not listed materials within the AWS D1.2 Structural Welding Code – Aluminum, and consequently no minimum tensile strength requirements are included in this code. If this material continues to be used for welded structures there will be a need to address this situation by establishing appropriate tensile strength values and including them in the appropriate welding codes.
Early testing on the 5059 (Alustar) base alloy indicated that problems could be encountered relating to the weld metal not being capable of obtaining the minimum tensile strength of the base material heat affected zone. One method used to improve the weld tensile strength was to increase the amount of alloying elements drawn from the plate material into the weld. This was assisted by the use of helium additions to the shielding gas, which produces a broader penetration profile that incorporates more of the base material. The use of 5556 filler alloy rather than the 5183 filler can also help to increase the strength of the deposited weld material.
Obviously these high performance vessels require high quality welding. The training of welders, development of appropriate welding procedures, and implementation of suitable testing techniques are essential in producing such a high performance product.
With the increasing demand to create larger and faster ships, particularly for military service, and the development of new improved, high-performance aluminum base materials, it is apparent that aluminum welding has acquired an interesting and important place within the shipbuilding industry. Also, with the pending introduction of this unique technology into the United States, it is important that designers, manufacturers, and particularly welders and welding engineers are adequately trained and familiar with this new technology.
Fig 1 The Joint Venture being used by the US Military for evaluation and demonstration trials.
Fig 2 The Spearhead - new generation of 98-metre wavepiercers.
Fig 3 GMAW being used on high strength large aluminum structural components at Incat’s Shipyard in Australia.