Practical Welding Techniques to Minimize Distortion in Lightweight Ship Structures

The trend in both military and commercial shipbuilding is the increased use of thin steel to reduce weight and improve performance. Complex panel structures have thickness transitions for weight and structural optimization with multiple inserts ranging from 5 to 45 mm. Welding practices developed for thicker plate can result in significant out-of-plane distortion when applied to thin-plate structures. Buckling distortion of complex lightweight panels has resulted in a significant negative effect on manufacturing cost and production throughput, limiting the shipbuilders' ability to produce innovative ship designs. High fitting and welding costs are the consequence of this large welding distortion. This problem is exacerbated as the fairness requirements are tightened. New methods are needed to control distortion when welding thinner materials. To tackle the distortion problems, in 2002 Northrop Grumman Ship Systems initiated a multiyear program to develop distortion-control technology for complex panels. This paper reports the results of a study to develop "best practices" for welding of lightweight structures. Control of welding distortion for thin structures requires control of each welding operation from butt welding of plates through to unit assembly. A general philosophy was applied to minimize welding heat input while maximizing restraint during unit construction. To achieve this, the following techniques were evaluated: increasing restraint during each welding operation, improving fitting practice, weld sequencing, and minimizing welding heat input. Additionally, an active distortion mitigation approach, known as transient thermal tensioning, was investigated for reduction of buckling distortion during thin-panel longitudinal stiffener welding. A series of tests were performed to evaluate various distortion control approaches and to optimize production processes. The culmination of the project will involve demonstrating best practices in the production of thin-steel structures. A plan is also being developed for implementing the most advantageous approaches into production.