Original Date: 04/24/2007
Revision Date: / /
Best Practice : Shipboard Application of Lightweight Steel Structures
The University of New Orleans, College of Engineering, in conjunction with Northrop Grumman Ship Systems, the Edison Welding Institute, and the Battelle Memorial Institute, has developed a process that mitigates buckling and distortion issues associated with thin steel panel fabrication. The process has identified new cutting patterns, improved handling, and modified construction sequencing for fabricating thin steel structures used in low-weight ship structures. When implemented, the new manufacturing plan produced thin conventional panels with no buckling distortion and complex panels with some buckling near manually welded inserts. The successful implementation of the process techniques recommended from this program will lead to higher-quality ships for the U.S. Navy while realizing savings from reduced rework.
Reduction of topside weight is imperative for shipbuilders, providing improved ship stability by increasing both metacentric height and ship range. In addition to topside weight reduction, both commercial and military ship acquisition officials have increased strength and stiffness requirements for these same topside structures to improve performance. Buckling distortion of complex lightweight panels has historically had a significant negative effect on manufacturing cost and production throughput. This multiyear project was a collaboration between the University of New Orleans, College of Engineering (UNO COE), the Northrop Grumman Ship Systems (NGSS) Avondale Shipyard, the Edison Welding Institute, and the Battelle Memorial Institute. NGSS has increased the use of thin steel structures from less than 10% to more than 90% per vessel during the past two decades. With the increased use of thin steel, it has become evident that current infrastructure, design methodologies, and construction techniques are inadequate. Radar cross-section minimizing hull designs cannot accept distortion or deformation that often results. Structural distortion of thin- steel superstructure assemblies and its associated rework costs have emerged as a significant problem in ship construction. In response to this issue, a series of initiatives has been accomplished to understand the engineering issues and control production in thin-steel structural construction, with a goal to reduce overall cost. The objectives focus on detailed solutions for numerical finite element modeling, preferred cutting, welding and fabrication processes, and optimal assembly methods for distortion control.
The use of Light Detection and Ranging (LIDAR) mapping was developed as a technique to produce three- dimensional scans of shipboard assemblies for the purpose of quantifying the distortion phenomenon. The UNO COE investigated this problem by exploring all potential sources, from mill processes to assembly and rework. Sources of distortion include residual stress and condition of the incoming material, material-handling damage, cutting accuracy, intrinsic deflection in tool and foundations, fit-up accuracy, overwelding, assembly sequence, panel design complexity, and excessive rework.
An experiment was designed and carried out to investigate fabrication issues for lightweight test panel designs. Each panel was unique and incorporated different characteristics of this type of structure and used different techniques and processes during its fabrication. Out-of-plane dimensional variations were tracked during fabrication using LIDAR techniques previously developed in this program. Panel buckling, distortions, and anomalies discovered during this experiment were analyzed and characterized.
The work of the UNO COE resulted in the development of a preferred manufacturing plan for thin plate steel. The recommendations developed by the UNO COE were designed to mitigate the forces that cause the distortions and buckling inherent in thin steel fabrications. Recommendations of the preferred manufacturing plan include: Modify incoming material-handling and storage processes to prevent permanent deformation
Precision mill or laser cut panel pieces to control accuracy and distortion before assembly
Design and deploy an effective panel-handling and processing system
Build a hard deck foundation and provide a material-clamping capability for panel fabrication
Develop narrow-grove SAW or hybrid seam-welding procedures for panel blanket assembly
Prefit and tack all stiffeners before panel fillet welding to improve restraint and fit-up
Optimize tack weld size or grind to ensure blending into stiffener fillet welds
Deploy precision high-speed fillet-welding parameters and procedures with through-the-arc or laser seam tracking
Develop and deploy Transient Thermal Tensioning (TTT) distortion-prediction computer-aided engineering tools and production hardware for panel-welding systems
Develop best practices for manual welding of inserts and transverse stiffeners to minimize overwelding
Work is in progress to deploy this technology on full-size production panels. When implemented, the new manufacturing plan produced thin conventional panels with no buckling distortion and complex panels with some buckling near manually welded inserts. New panel manufacturing lines are being procured to replace the current production lines at NGSS. Downstream operations receiving flatter material should see a reduction in the number of hours needed for fitting and welding assembly of panel structures. Ship-fitting costs for unit assembly should also see a significant improvement.
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