Welding distortion prediction with elastic FE analysis and mitigation practice in fabrication of cantilever beam component of jack-up drilling rig

How to predict and mitigate the welding distortion in fabrication of large offshore structures such as jack-up drilling rig, is always an essential engineering problem. In general, welding distortion can be estimated beforehand with experimental measurement of mock-up or numerical computation, and then practical techniques will be carried out to correct the possible welding distortion in the actual fabrication. In this paper, the possible welding distortion during the fabrication of I section welded structure in cantilever beam of jack-up drilling rig was studied with prediction and mitigation. First, an elastic FE analysis was employed, in which (a) the whole examined welded structure was meshed by coarse shell elements; (b) welding inherent deformations of typical welded joints were evaluated with thermal elastic-plastic FE analysis, and will be applied to the welding lines as load; (c) interface element was used to represent the fitting procedure before full welding. In order to eliminate these predicted welding distortions, some mitigation practices such as application of inverse distortion, optimization of welding sequence and constraint methods, were conducted and examined. In particular, welding sequence will intensively influence the final dimensional accuracy via straightening in fitting procedure and updated stiffness of current main welded structure.