New Developments in Aluminum Shipbuilding
Welded aluminum meets the needs of a changing military
BY TONY ANDERSON
The Incat USA
This article was inspired by current activities within the U.S. 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.
A recent visit to Incat Tasmania Pty. Ltd. (shipbuilding), off the southeast coast of Australia, revealed a manufacturer that has taken aluminum shipbuilding to exciting new levels. In 1977, it launched its first high-speed catamaran, and today it is manufacturing the new generation of 98-meter (322-ft) wave-piercers, which are being evaluated by the United States military. Incat has constructed more than 50 vessels of various lengths. The company's first passenger/vehicle ferry was delivered in 1990, a 74-meter (243-ft) wave-piercing catamaran with a maximum deadweight capacity of 200 metric tons (440,000 lb). The more recent 98-meter Evolution 10B range has a deadweight four times that amount.
While Incat-built ferries initially revolutionized transport links around the U.K., today its ships operate in North and South America, Australasia, the Mediterranean, and throughout 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 in Tasmania.
Aluminum Welded Ships within the U.S. Military
In response to great interest from the U.S. military in high-speed craft, Incat, via its U.S. affiliate, Incat USA, formed a strategic alliance with an American shipyard to market and build innovative craft designs for 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 U.S. Navy and Coast Guard.
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 U.S. Army.
Fig. 1 The Joint Venture is being used by the U.S. military for evaluation and demonstration trials.
The U.S. government has awarded Bollinger/Incat USA, New Orleans, La., the charter for a high-speed craft (HSC) for a multiservice program operated by various arms of the military. The HSC vessel, now known as Joint Venture HSV-X1 (Fig. 1), is being used for evaluation and demonstration trials to assess the usefulness of such technology in 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. 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, troop facilities for 363 personnel, crew accommodation, storage facilities, medical facilities, and long-range fuel tanks.
Spearhead (Fig. 2) is the U.S. Army's first theater support vessel (TSV) and is part of the Advanced Concept Technology Demonstrator program by the office of the secretary of defense and the U.S. Army. Spearhead is being used to demonstrate and evaluate its ability to perform during certain mission scenarios, to assess its usefulness to the U.S. military, and to refine the requirements for the next generation of Army watercraft. The TSV is critical to the Army's ability to perform its Title 10, intratheater mission. Spearhead is 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. Theater support vessels promise to change the way the U.S. Army gets to the fight.
Fig. 2 The Spearhead, one of a new generation of 98-meter wave-piercers, has been deployed in the Persian Gulf.
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 onshore 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 U.S. military. Military Sealift Command is the contracting arm that has leased a 98-meter craft from Bollinger/Incat USA to support U.S. Navy Mine Warfare Command. The craft, HSV-X2 Swift, was constructed at the Hobart, Tasmania, shipyard and will be stationed in Ingleside, Tex. The ship is capable of maintaining an average speed of 35 knots or greater loaded with 500 short tons (453,600 kg) 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.
It will be fitted with a stern ramp capable of on/off loading directly astern or to the starboard quarter. The ramp is capable of loading/unloading a multitude of military vehicles up to and including battle tanks of up to 140,000 lb (63,500 kg). The ramp is also capable of launch and recovery of amphibious assault vehicles. To achieve this, the ramp tip end can be submerged, allowing the amphibious vehicles to drive on and off.
The ship is also capable of launch and recovery of small boats and unmanned vehicles up to 10,400 kg (23,000 lb) while underway.
The vessel is 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 is provided, as is a Carriage Stream Tow and Recovery System (CSTARS). This helo deck has the capacity to transfer equipment up to 2720 kg (6000 lb) to and from the vehicle deck.
New Developments in High-Strength Aluminum Alloys for Marine Applications
Until fairly recently, the most popular base metal used for aluminum shipbuilding, 5083, 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, it 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 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 oceangoing 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 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 postweld 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).
Fig. 3 GMAW being used on high-strength, large aluminum structural components at Incat's shipyard in Australia.
Welding the New Aluminum Alloys|
The welding procedures used for these high-strength alloys are very similar to the procedures used for welding the more traditional 5083 base metals. The 5183 filler metal and the 5556 filler metal are both suitable for welding 5383 and 5059 base metals. These alloys are predominantly welded with the gas metal arc welding (GMAW) process using both pure argon and a mixture of argon/helium shielding gas Fig. 3. 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 strengths of these alloys are available from the material manufacturers; however, there would appear to be few 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 metal rather than the 5183 filler metal can also help 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-
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.
TONY ANDERSON (firstname.lastname@example.org) is Technical Director of AlcoTec Wire Corp., Traverse City, Mich.; Chairman of the Aluminum Association Technical Advisory Committee for Welding and Joining; Chairman of the AWS D10H Subcommittee on Aluminum Piping; Chairman of the AWS D3A Subcommittee on Aluminum Hull Welding; Chairman of the AWS/SAE Subcommittee on Automotive Arc Welding of Aluminum; and Vice Chairman of AWS D1G Subcommittee 7 on Aluminum Structures.