Nonlinear Finite Element Analyses for Ultimate Strength Determination of Ships under combined Loads

Abstract

ABSTRACT

To determine the ultimate strength of a double hull VLCC under combined loads, different progressive collapse analyses were performed. Nonlinear finite element analyses using Newton-Raphson iterative scheme were performed to simulate the collapse behaviour of the double hull VLCC. This was done using ANSYS Mechanical APDL. The ultimate strength of the double hull VLCC was determined for the cases of vertical bending, horizontal bending and biaxial bending under hogging and sagging conditions. In the case of vertical bending, the analysis performed shows that the ultimate strength under hogging condition is higher than that under sagging condition. Also, the collapse behaviour of the structure under hogging condition indicates that initial collapse starts at the central part of the deck in tension. This is followed by the collapse of the double bottom in compression. Under the sagging condition, the deck buckles in compression. Due to symmetry, the results of the analysis carried out in the case of horizontal bending show that the ultimate strengths and collapse behaviour of the structure under hogging and sagging conditions are similar. In the biaxial case, the interaction relationship between vertical and horizontal bending is illustrated. Depending on the applied curvature ratio, the influence of one over the other dominates. The influence of different material models on the ultimate strength was also investigated. The analysis conducted indicates that the bilinear elastic plastic material model gives a higher value of the ultimate strength when compared with the ideal elastic plastic material model. The structural components of the double hull VLCC are welded, hence, initial imperfections due to welding are introduced to the structural components. Therefore, the influence of welding residual stresses on the ultimate strength of the structure was also examined. The examination shows that welding residual stresses have little/negligible influence on the ultimate strength of the double hull VLCC. Finally, analyses were also performed to determine the residual strength of the double hull VLCC under combined loads. Symmetric grounding damages were implemented by removing parts (elements) of the model. Expectedly, the results show that the ultimate strength of the structure decreases as the damage extent increases.