A Parametric Analysis on Hull Penetration Position and Frame Spacing Leading to Minimal Welding Distortion

Abstract

Hull penetration and openings are governed by standards, which ensure the structural integrity of the assembly. However, a number of stiffeners are usually welded to the panel, giving rise to welding residual stresses and distortion. This study identifies the optimal position and shape of penetration relative to various stiffener spacing configurations, leading to the least welding distortion. A numerical parametric study is adopted where an uncoupled thermo-elastoplastic technique based on the inherent strain method is used. The results have shown that the final distortion is highly dependent on the buckling mode shape predicted by the numerical models that in turn is influenced by the structural stiffness and support position of the assembly. When the models follow a similar buckling mode, the results have shown that smaller stiffener spacing leads to less distortion. Centrally positioning penetrations results in symmetric panel stiffness leading to near symmetric out-of-plane distortion and ultimately minimal distortion, when compared to positioning penetrations next to the stiffener. The difference in the predicted distortion for square and circular penetrations is relatively small when placed in the middle of the assembly. Nonetheless, when penetrations are placed close to the stiffener, circular penetrations are preferred to reduce distortion. Small circular penetrations gave rise to less overall distortions but more penetration residual twisting and rotations, particularly when placed close to the stiffener. 1. Introduction Hull penetrations and openings are governed by standards such as DNV GL SE (2015) and ASTM F994-86 (2011). Their prime focus ensures that the panels under consideration have significant stiffness to withstand loads. These panels are generally stiffened by means of stiffeners that are welded to the plate at different positions and frame spacing. The inherent thermal strains developed during welding give rise to residual stresses, that coupled with the structural stiffness of the welded assembly will manifest themselves in welding distortions (Masubuchi 1980; Radaj 2003). If the assembly is significantly stiff to withstand the welding contraction forces, the developed distortion will be small. However, welding residual stresses and consequently distortion will inevitably still develop (Gray et al. 2014). A number of techniques are possible to reduce the welding contraction forces, such as thermal tensioning (Masubuchi 1980; Michaleris & Sun 1997) and cryogenic cooling (Gabzdyl et al. 2002; Camilleri et al. 2008). Another possible strategy that can be adopted to reduce welding distortions involves the optimal identification of assembly designs that possibly provide higher stiffness to counter the inherent welding stresses. Different panel assemblies and configurations will give rise to different magnitudes of welding distortion and thus the optimal welded assembly that gives rise to minimal distortion can be identified prior to fabrication and established during the design phase (Gray et al. 2014). This study focuses on investigating and identifying the optimal penetration position and frame spacing leading to minimal welding distortion and aims to answer the following research hypothesis:Does central positioning of penetrations lead to minimal distortion?Placing penetrations close to stiffeners gives rise to less welding distortion?Will a square penetration create more distortion than a circular penetration?Does the frame spacing have an effect on the final welding distortion?How does the size of the openings influence the out-of-plane distortion?