Professor Jeom Kee Paik FREng, UCL Mechanical Engineering’s Professor of Marine Technology, has won the Royal Institution of Naval Architects’ (RINA) Medal of Distinction by his paper “Full-scale measurements of welding-induced initial deflections and residual stresses in steel-stiffened plate structures” published in International Journal of Maritime Engineering, RINA Transactions. The RINA’s Medal of Distinction is awarded to the authors of papers of merit published in the Transactions.
Stiffened plate panels are used in naval, offshore, mechanical and civil engineering structures such as ships, offshore structures and bridges. They are constructed using welding techniques that attach support members (or stiffeners) to the plating. During this process, initial imperfections develop in the form of initial deformations (deflections or distortions) and residual stresses.
Fig.1. Living quarter structures in a 174k m3 class LNG carrier and a VLCC class FPSO constructed at the yard of Samsung Heavy Industries.
For example, deck houses or living quarters of ships and offshore structures as shown in Figure 1 are made of thin plate panels with a plate thickness of 6-8 mm. During construction by welding, thermal buckling always happens causing significant amount of distortions in the plate panels as shown in Figure 2 where door frames are seriously twisted and walls deflect significantly. In this case, it is inevitably required to perform fairing works as shown in Figure 3 which are costly and are delaying the building schedule. The fairing works and penalties due to the delay of building schedule cost 0.5-2 million USD per vessel.
In the paper, a full-scale measurement database of welding-induced initial imperfections in steel-stiffened plate structures was developed. For parametric study purposes, four test structures with varying plate thickness were measured where the stiffener types and weld bead length were fixed. Modern technologies such as 3D scanner and X-ray diffraction tool were used to measure the initial imperfections.
Fig.2. Welding-induced distortions occurred during construction by welding
The developed database was applied to prevent the thermal buckling of thin-walled structures during welding fabrication, where an innovative procedure for thermal plate buckling control was developed as shown in Figure 4. Plate panels buckle under biaxial compressive residual stresses during welding process, as shown in Figure 5, where support members are attached in both longitudinal and transverse directions. To prevent the thermal plate buckling, two ways can be considered, one by minimizing the magnitude of the welding compressive residual stresses and/or the other by increasing the plate buckling strength. The former can be achieved by reducing weld heat source to minimize weld bead length at the intersections of fillet welds. In shipbuilding industry practice, however, this way is not always acceptable to secure continuous welding quality. The other way can be applied to increase the plate thickness, but
Fig.3. Fairing works to fix the distorted plate panels.
with the increase of structural weight. It is not desirable to increase the structural weight of living quarters, and thus an innovative method is required to improve plate buckling strength without increasing the plate thickness. The plate buckling strength is governed by torsional rigidities of support members as well as plate thickness, among other factors. This means that plate buckling strength can be increased by changing types and dimensions of support members which provide greater torsional rigidities.
The developed technology is now being applied to the construction of all ships and offshore platforms at the shipyard of Samsung Heavy Industries. At least 0.5-2 million USD are saved per ship. The developed technology provides great contributions to the construction of ships and offshore structures in terms of building cost saving and quality control. Below table is track record that the present technology has been or will be applied.
Fig.4. An innovative procedure for thermal plate buckling prevention.
Fig.5. Distribution of welding-induced residual stresses in a plate element in both longitudinal and transverse directions.