TS-103
Optimization of Thin Stiffened Plate Panels in Shipbuilding
ABSTRACT
During these last ten years, in order to reduce the ship structural weight, thin plate panels have been more and more employed to build up decks and bulkheads of superstructure of new large ships such as passenger ships, tankers, and at fabrication stage, the arc welding parameters such as heat input energy and single pass welding speed have been increased in order to get higher productivity.
After assembly, unfortunately it often occurs important distortion due to welding effects on these thin plate panels induced by the straightening operations, which are expensive in cost and time-consuming and must be executed to reduce the deflection level for respecting the limit tolerance of the Quality Standard. The global cost and delay of ship construction will then be much affected.
The paper describes, for any new shipbuilding project, a simple procedure permitting to combine and to optimize the design and fabrication parameters by using a numerical method developed by IRCN for evaluating the welding effects in order to minimize the distortion levels, so minimizing the straightening cost and the ship structural weight by means of the significant welding parameters and the optimum span between two stiffeners of thin stiffened plate panels.
Key Words: Design and Fabrication parameters, Welding effects, Ship construction cost, Quality Standard, Optimum, Distortion, Heat input Energy, Straightening, Productivity, IRCN numerical method.
1 .INTRODUCTION
Nowadays in the world, the shipbuilding industry is coming into the phase of the advanced techniques, thanks to the progress in computers and technical controls. Several big shipyards could now achieve higher productivity, shorter delay and lower final ship construction cost.
Since these last decades in shipyards, for any shipbuilding project, an usual construction planning was set up following two successive stages: design and fabrication stages.
At the design stage, all dimensions of the ship are defined, decks and bulkheads included, for satisfying all loading conditions prescribed as rules by the ship classification society. In order to reduce the ship structural weight, and as the ship-owner wishes, thin plates were more and more employed in the upper decks and bulkheads of large ships such as passenger ships and big tankers.
At the fabrication stage, following strictly the construction drawings issued from the design stage, successive assembly operations are executed from different elements such as plates, stiffeners... to stiffened panels, decks, bulkheads and then, blocks and so on through the whole ship. For this task, the Constructor-Engineer has one of the most rapid and economic tools at his disposal: arc welding.
In order to get higher productivity, the use of robot (automatic and semiautomatic controls) welding processes in automation production chain has increased significantly:
High-Technology has been quasi-obtained, thanks to the high efficiency of tools and of the high performance welding equipment. "Quasi-obtained" because of the distortion phenomena due to welding effects appeared in these thin welded plate assemblies, of which the Constructor-Engineer could not well control the levels.
Thus, when the distortion levels overpass the limit tolerance fixed by the Quality Standard, the straightening operations must be executed and these supplementary works are expensive in cost and time-consuming. As noted in the Welding Journal published in August 1995 by the Navy Joining Center (NJC), an additional cost was estimated to 3.4 million US$ per destroyer for the U.S. Navy [1].
This paper describes a simple procedure using the IRCN numerical method to evaluate, verify, predict and prevent the three dimensional welding effects (deflections and stresses) on thin welded assemblies, and to look forward to the optimization of the design and fabrication parameters at both stages for any new shipbuilding project.
2. SHORT PRESENTATION OF THE IRCN NUMERICAL METHOD FOR EVALUATING THE WELDING EFFECTS
The method has been presented for the first time in 1992 at the ATMA conference in Paris [2] for 2D welding effects and recently at the 3rd International Conference on Marine Technology ODRA'99 held at Szczecin in Poland (October 11-13, 1999) for 3D welding effects on thin stiffened plate panels in shipbuilding.
For further information about the method, its validation on experiment testings and its industrial applications, please refer to Ref. [3]. In this paper, some technical points of the method are briefly described.
