Shipshape laser applications
International conference highlights progress and problems for lasers in shipbuilding
The International Conference on Shipyard Applications of Industrial Lasers, held June 2–4th in Williamsburg, VA, highlighted the impact of industrial lasers on shipyard applications. In brief, the applications described included laser welding in Europe, laser cutting in the U.S. and laser cladding or weld repair in U.S. naval depots. Only one of the 120 conference participants was from the Far East, although laser cutting applications in Japanese shipyards were discussed.
The highlight of the conference was a presentation by Ralf Moeller of Schuler-Held Lasertechnik GmbH (Heusenstamm, Germany) on the hybrid laser system installed at the Meyer shipyards in Papenburg, Germany. Two CO2 laser systems prepare panels by welding together sheets, in some cases of materials of different thickness. Two more laser systems weld stiffeners into the panels. In both cases hybrid welding, combining laser welding with a gas-metal arc torch, is used to weld from one side. Immediately prior to welding, both edges of the sheet are machined with a mill that moves up between the tooling and the clamps. The clamping system pre-bends the sheet prior to welding to counteract the distortion that would otherwise be introduced by the welding process. The hybrid process introduces more total heat input than experienced with laser-only welding. This system, described in the December 2002 issue of Industrial Laser Solutions, was not a case of incorporating a new welding system in an existing factory, but of building a new construction dock around the laser welding technique.
Meyer's first CO2 laser welding system, introduced in 1994 using a laser system from Convergent Energy (now Convergent Prima —Chicopee, MA), is still active, producing specialized panels such as those used in stairwells. Other German, Danish and Italian shipyards are also using either laser welding or laser hybrid welding with CO2 lasers. In 2002 the Kvaerner shipyard in Warnow, Germany, introduced hybrid welding using a fiber-delivered 4.4kW Nd:YAG laser from Rofin Sinar (Hamburg, Germany). Hybrid welding with Nd:YAG lasers is also being used in the Kvaerner shipyard in Finland and will be introduced at Italian shipyard Fincantieri as part of the European ShipYAG project. This is one of a number of R&D projects funded by the European Community intended to ensure the competitiveness of the European shipbuilding industry.
Despite the use of laser welding in European shipyards, its acceptance into U.S. shipbuilding practice has been slow. The U.S. Navy has been sponsoring evaluation of the Lascor panel since 1988. Produced at the Applied Research Laboratory (State College, PA), these panels were formed by laser welding a corrugated sheet of steel between two relatively thin flat sheets. The assembled panel has strength and structural characteristics that exceed that of a thicker and heavier sheet of solid material. Use of Lascor panels would save on the weight of vessels and consequently on the cost of fuel. Apart from the installation of a test Lascor panel on the USS Whitney in 1994, and despite innumerable feasibility studies, no incorporation of laser welding technology into U.S. shipbuilding has taken place. It appears that managers fear criticism for adopting controversial new technology, but not for ordering feasibility studies.
At the conference, contract work being done at Applied Thermal Science in Sanford, Maine, described laser welding that would produce more accurate girders and structural members as an input into the shipbuilding process. This system will use a 25-kW laser from Convergent Prima, combined with a gas metal arc torch in a hybrid-welding configuration, and with laser scanners both for seam tracking and for weld profile inspection. It is conjectured that the use of more accurately profiled structural beams will dramatically decrease the amount of fit-up and welding time required in ship assembly, and hence cost. Perhaps the Maine work will become more than a feasibility study or a demonstration project.
Patrick Cahill of the Bender Shipbuilding Repair Company (Mobile, AL) gave a presentation of his company's adoption of laser cutting of sheet steel (see ILS May 2001), expansion to higher production capability, and adoption of the Lasox process for thicker sheet cutting. Lasox is an oxygen-assisted cutting process co-developed by BOC and the University of Liverpool in the UK, and brought to the U.S. marketplace by Alabama Laser Systems. The process, described in the January 2003 issue of Industrial Laser Solutions, uses supersonic nozzles to produce cuts with a minimum taper and heat affected zone in thick section material, with less laser power than would otherwise be necessary.
The General Dynamics Electric Boat division (Quonset Point, RI) has also recently installed a laser cutting system (see ILS June 2002). Mr. Cahill listed the impediments to adoption of laser techniques in shipyards as not only the conservative nature of the shipbuilding industry but also misconceptions about systems performance, perhaps attributable to over-selling by laser vendors, and a lack of detailed application analysis. In addition, lack of process certification has been a problem; in this connection it was interesting to see personnel from the American Bureau of Shipping, the US Marine Administration, and NAVSEA at the conference. It is hoped that having these gentlemen become familiar with the worldwide acceptance of the laser process will ease the regulatory approval process. This will allow America's few remaining shipyards to adopt modern technology to regain competitiveness in the international marketplace.
One success story is the increasing acceptance of laser cladding, or weld repair, for refurbishment at U.S. naval depots. In presentations from the Naval Undersea Warfare Center (Keyport, WA) and the Puget Sound Naval Shipyard (Bremerton, WA), work developing procedures for repair of pump shafts, torpedo housings, and catapult launchers was described. Overheard at the conference was the news that Pearl Harbour Naval Depot (Hawaii), and possibly other locations, are in the process of adopting the technology. Cladding work at European research establishments BIAS (Bremer Institut fur AngeWandte Strahltechnik) in Bremen and Schweisstechnische Lehr und Versuchsanstalt in Rostok, Germany, appeared to be conducted on an as-needed basis, rather than developing procedure for specific pieces of hardware. The European work described cladding repair of "cross-head" pins of two-stroke engines, 1.5-ton piston rods, exhaust valves from large marine engines, and drive shafts that had been machined undersized.
There were a number of presentations at the conference on laser forming, a process which was first investigated by the Massachusetts Institute of Technology for shipyard applications in the early 1980s, and one on laser shot peening. In laser forming, a partly focused high-power beam is scanned over material inducing stresses sufficiently great that plastic deformation and hence permanent bending of the plates can occur. Flat sheet has been bent into a bowl shape using this process. It is commonly done with gas torches, but it is felt that the highly controlled laser process, with feedback from sensors on the sheet indicating the amount of bend, should result in more precise shapes. In laser shot peening, a series of energetic laser pulses induces compressive residual stresses in the material, resulting in an ability to resist externally applied stresses. Heavily stressed components such as gears and turbine blades have been shown to have an improvement in fatigue life, but the process has not penetrated the shipbuilding industry to the present time. A presentation described ongoing work on this process and progress towards this goal at the Applied Research Laboratory of Penn State and at the University of Liverpool.
In summary, the conference provided an overview of current laser applications in European and North American shipyards, with no significant input on shipyard applications in the Far East. Twenty years ago, with the installation of a high-power laser in the Marine division of Westinghouse, one hoped that lasers might be adopted into shipbuilding. But this installation was short-lived. The excellent presentations at this conference revive the hope of laser technology having a significant impact on U.S. shipbuilding.
V.E. (Vivian) Merchant is an occasional contributor to Industrial Laser Solutions, and can be contacted via telephone at (937) 279-0877 or via email at email@example.com.