Ultimate limit state-based multi-objective optimum design technology for hull structural scantlings of merchant cargo ships

As the ultimate limit state is now a primary criterion for ship structural design, multi-objective optimization techniques for both minimizing weight and maximizing safety are routine practices in the structural design of naval ships as such techniques are critical for determining the structural weight needed to meet the functional requirements associated with naval armaments. However, a comparative approach is still used in the structural design of merchant cargo ships, based on measurements from existing as-built reference ships. This comparative approach obviously needs to be upgraded, because it requires more man-hours and causes more design errors than a fully automated design procedure. The comparative approach may also lead to inadequate design results, in which some structural members are too strong, and others barely satisfy the strength criteria, which can lead to catastrophic failures in some cases. The aim of this paper is to develop a fully automated methodology for the optimum design of hull structural scantlings for merchant cargo ships that are modelled by plate-shell finite elements. A full optimization technique with multi-objectives is applied for minimizing structural weight and maximizing structural safety, as per design constraints associated with the ultimate limit states of the plate panels, support members and hull girders. The developed procedure is applied to the hull structural scantlings of a very large crude oil carrier (VLCC), and this test demonstrates the procedure's capacity to meet the strength requirements of common structural rules. A comparison between the new design and an as built reference ship is made, confirming that the proposed procedure reduces the number of man-hours required by about 20%, lightens the structural weight by 3% and improves the safety factors for the critical members.