Engineering and Production Technology for Lightweight Ship Structures, Part II: Distortion Mitigation Technique and Implementation

Abstract

Shipboard applications of lightweight structures have increased over recent years in both military and commercial vessels. Thin steel reduces topside weight, enhances mission capability, and improves performance and vessel stability, but the propensity of buckling distortion has increased significantly. At present, several US Navy construction programs are experiencing high rates of buckling distortion on thin steel structures. The standard shipyard practice of fabricating stiffened steel panels by arc welding is one of the major contributors to this distortion. Correcting the distortion is a necessary but time-consuming operation that adds no value and ultimately tends to degrade the quality of the ship structure. With a major initiative funded by the US Navy, Northrop Grumman Ship Systems (NGSS) has undertaken a comprehensive assessment of lightweight structure fabrication technology since 2002. Through collaborative research, significant progress has been achieved in the development of distortion-control techniques. Reverse arching, transient thermal tensioning (TTT), stiffener assembly sequencing, and other preferred manufacturing techniques were developed at NGSS to reduce distortion and eliminate the high rework costs associated with correcting that distortion. Complex lightweight panel structures, which are reinforced by long slender stiffeners along with numerous cutouts and inserts, pose a major challenge for distortion control. The geometric complexity yields a more complicated buckling behavior, which drives the need to develop a more fine-tuned finite element model to determine critical parameters and heating patterns for the TTT process. NGSS has recently teamed with Edison Welding Institute (EWI), Battelle Memorial Institute, and the University of New Orleans on a Navy project to further refine TTT procedures for complex lightweight ship structures. In this paper, functional requirements and the design of TTT process and production equipment are discussed. The refined TTT process will be benchmarked by the test panel observations, and a laser scanning device, LIDAR, will be used to analyze panel distortion topography.