Structural optimisation of ship hull using finite element method

Abstract

The design of ship structures is a multilayered process governed by numerous regulations and standards, demanding meticulous consideration of structural responses and production costs. This research presents a multi-objective structural optimisation methodology tailored for a multipurpose cargo ship. It addresses compliance with classification society regulations, cost effectiveness, assessment of structural responses under diverse loads and conducts a comprehensive buckling analysis. The initial phase involves transforming a 2D CAD design into a 2D model, facilitated by BV Mars 2000 software, followed by a comprehensive evaluation of the ship's scantling compliance with Bureau Veritas rules. Subsequently, the meticulous construction of a 3D cargo hold model featuring three cargo compartments sets the stage for a comprehensive analysis employing Femap-integrated NX Nastran finite element software. This analysis scrutinises the structural response of the ship's hull under the combined influence of bending and torsional loads, including a detailed buckling analysis. The study explores the ramifications of torsion for both open-deck and closed-deck ship configurations. Furthermore, the research rigorously validates the precision of the 3D finite element model by means of exhaustive assessments involving beam theory and direct calculations. A notable finding connected with this study is the prominence of hull girder normal stresses at midship, arising from still water and vertical wave bending moments, contributing to nearly 70% of the total stress when the ship is inclined. Horizontal wave bending moments account for approximately 10% of the stresses, whereas warping stresses contribute roughly 20% in open-deck ship designs. Additionally, the research demonstrates that torsion has minimal impact on closed-deck ship configurations. The investigation extends to the analysis of hull girder deflection, systematically examined using numerical techniques and Euler-Bernoulli beam theory, focusing on the significance of longitudinal deflection over transverse deflection. A novel approach is presented using Minitab software's Fractional Factorial Design technique as part of the Design of Experiments (DOE) framework. The strategy aims to identify the critical parameters affecting hull girder Von Mises stress, torsional stress, as well as production costs. iv Ship design optimisation is then carried out by incorporating regression equations for Von Mises stress and production costs from Minitab software into the Non-dominated Sorting Genetic Algorithm II (NSGA-II), managed using Python software. The optimally designed midship section underwent rigorous validation to ensure conformity with industry standards and classification society regulations. Essential modifications to inner bottom plates and double bottom side girders are made to meet these stringent requirements. This optimisation process results in a substantial 10% reduction in ship weight and production costs compared to the initial design. It achieves a peak design stress of 296.2 MPa below the limit through prudent adjustments in plate thickness, web frame positioning and stiffener arrangement. This research delivers a comprehensive framework for the structural optimisation of ship hulls, potentially enhancing safety, sustainability and competitiveness within the maritime engineering industry.