Objective To promote lightweight design in Very Large Crude Carriers (VLCCs) with double longitudinal bulkheads, this study conducts a detailed investigation of the topological configuration of their transverse frames and the factors influencing it. Rather than presenting only the final optimized structures, the research systematically elucidates the complete formation process and evolutionary patterns of transverse frame topology optimization.

Method The research adopts the variable density method. The optimization model constrains the structural volume fraction, with the objective of minimizing structural strain energy. For multi-load case analysis, the analytic hierarchy process (AHP) is applied 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 is conducted into the influence of mesh size, compartment length, and longitudinal bulkhead position on the evolutionary patterns of the topological configuration under multiple load cases. To ensure objective and reliable evaluation, the intersection over union (IoU) metric is employed to quantitatively measure the similarity between configurations.

Results Liquid cargo pressure is identified as the primary factor driving the formation of horizontal struts between the double longitudinal bulkheads, thereby enhancing structural stability and load-bearing capacity. Mesh size mainly influences the finer details of the optimized configuration; a mesh size equivalent to the longitudinal spacing achieves a balance between computational efficiency and structural clarity, yielding practical designs. Compartment length significantly influences the priority of horizontal strut formation. When the length reaches three times the frame spacing, the horizontal struts form preferentially even at low volume fractions, becoming the main load-bearing path and resulting in a configuration that is both stable and representative of the full compartment model. The position of the longitudinal bulkheads governs the significance and timing of bracket formation. When the central tank width ratio is below 37%, the structure prioritizes horizontal strut reinforcement, with brackets emerging as auxiliary supports at a volume fraction of 0.10. Beyond this threshold, however, brackets appear at a lower volume fraction of 0.05 and serve as essential load-bearing components in conjunction with the horizontal struts.

Conclusion This study clarifies the fundamental load-bearing paths and their evolutionary patterns in the transverse frames of double longitudinal bulkhead VLCCs, and provides practical guidance for both 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 to balance accuracy and efficiency. For design, a central tank width ratio of 37% is identified as a critical threshold that dictates the structural role of brackets. These findings provide both a solid theoretical foundation and practical reference for the optimal and lightweight design of VLCC transverse frames.