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.

Practice Design of Ship Thin Section Considering Prevention of Welding-Induced Buckling

“…A hybrid experimental and computational method was proposed to examine the generation mechanism of buckling behavior due to welding. Moreover, Huang et al [8][9] presented a holistic assessment to investigate the production processes of welded structures with thin steel plates, and evaluated their contributions to the eventual fabrication accuracy and distortion tolerance. In detail, an optical device and effective computation were used to examine the underlying mechanism of welding buckling and the critical parameters of procedures.…”

“…Fig 9. Distributions of the residual plastic strains on the middle cross section of butt welded joint after cooling down…”

Analytical treatment of distortion effects on fatigue behaviors of lightweight shipboard structures

“…In recent years, the demand for structural lightweighting for surface ships has become more intensified as the industry strives to carry more payload at a higher speed with further improved fuel economy and reduced environmental emissions [1]. As reported by Huang et al [2,3], plates with thicknesses equal to or less than 5mm have become increasingly dominant in lightweight shipboard structures in surface combatants, which have caused significant challenges in accuracy control in construction processes. Weldinginduced distortions have become a major issue in the construction of lightweight ship structures [4,5,6].…”

Investigation of Residual Stresses Distribution in Titanium Weldments

“…In contrast to conventional steel and aluminum alloys, there exist a series of design and fabrication issues in the manufacturing industry with welded structures made of titanium materials. As stated in [1], current weld sizing design rules require a minimum of fillet leg size of 5mm regardless of plate thickness, which can cause severe buckling distortions in construction of lightweight ship structures [2]. In terms of fabrication, titanium material processing (cutting, forging, forming, casting, rolling, polishing, etc.)…”