Objective To achieve the lightweight design of Very Large Crude Carriers (VLCCs) with double longitudinal bulkheads, a detailed investigation into the topological configuration of their transverse frames and its influencing factors is presented. The research systematically reveals the complete formation process and evolution patterns of the transverse frame's topological optimization, offering more than just the final optimized structures.

Method The research is based on the variable density method. The mathematical model for optimization sets the structural volume fraction as a constraint, with the objective of minimizing the structural strain energy. For multi-load case analysis, the Analytic Hierarchy Process (AHP) is employed to determine the weight coefficients of each load case based on its initial strain energy. Load cases include combinations of deck loads, external water pressure, and liquid cargo pressure. A systematic investigation into how mesh size, compartment length, and longitudinal bulkhead position affect the topological configuration's evolution under multiple load cases is conducted. The Intersection over Union (IoU) metric is used to quantitatively assess the similarity of configurations, ensuring objective and reliable results.

Results Liquid cargo pressure is identified as the primary driver for the formation of the horizontal strut between the double longitudinal bulkheads, which enhances structural stability and load-bearing capacity. Mesh size primarily affects the detailed features of the configuration; a mesh size equivalent to the longitudinal spacing offers a good balance between computational efficiency and clarity, producing practical designs. Compartment length significantly influences the priority of horizontal strut formation. When the length reaches three times the frame spacing, the horizontal strut forms preferentially even at low volume fractions, becoming the main load-bearing path, with the configuration becoming stable and representative of the full compartment model. The longitudinal bulkhead position determines the importance and timing of bracket formation. When the central tank width ratio is below 37%, the structure prioritizes strengthening horizontal struts, with brackets appearing as auxiliary supports at a volume fraction of 0.10. Above this threshold, brackets form at a lower volume fraction of 0.05 and function as key load-bearing components alongside the horizontal struts.

Conclusion This study provides a clear understanding of the core load-bearing paths and their evolution in double longitudinal bulkhead VLCC transverse frames. It offers practical recommendations for modeling and design. For modeling, a computational model with a mesh size equal to the longitudinal spacing and a compartment length of three frame spaces is recommended for balancing accuracy and efficiency. For design, the central tank width ratio of 37% is identified as a critical threshold that determines the significance of bracket structures. These findings offer an essential theoretical foundation and practical reference for the optimal and lightweight design of VLCC transverse frames.