Safety as Objective in Multicriterial Structural Optimization

Abstract

Multicriterial design methodology with safety as one of the design objectives is presented. The aim of the paper is to analyze the influence of safety based design objectives on generated nondominated designs on the Pareto frontier. Possible improvements in nondominated designs are investigated by comparison to ones obtained with the standard design procedure when safety criteria are used as design constraints only. It is assumed that safety based objectives and targets act as attractors, driving nondominated designs along the constant cost/weight contours in design space towards its safer regions. Global safety objectives (for hogging/sagging modes), are based on the maximization of ultimate longitudinal strength in vertical bending calculated via the extended IACS incremental-iterative method. Applied compound safety measures for gross-panel (stiffened panel with associated girders) are based upon 34 failure modes, belonging to serviceability/collapse subsets. Objectives based on the maximization of safety measures are applied together with standard design objectives such as minimization of initial cost and weight. The following problems were solved with different sets of objectives: (a) minimize cost and weight objectives subject to safety constraints (used for reference), (b) only the maximization of local safety measures is added to (a) as additional objectives, (c) only the maximization of global safety measures is added to (a) as additional objectives, (d) maximization of safety measures ad (b) and (c) are added to (a) as additional objectives. For each of the problems (a–d) the developed design procedure is executed. It contains two basic tasks for structural design of realistic (non-academic) problems: (1) multicriterial optimization with topology / geometry design variables; (2) multicriterial optimization of gross-panels with scantling / material design variables. Design procedure steps are executed using a fast and balanced collection of analysis and synthesis modules/methods of the OCTOPUS design system: • Determination of design load sets; • MOGA / MOPSO based generation of nondominated designs for the selected ship structure; • For each design the following analysis blocks are executed: – calculation of ship’s primary and racking response fields, – calculation of ship’s ultimate longitudinal strength, – calculation of serviceability and collapse safety criteria on the gross-panel level. Comparisons of results, based on generated Pareto hyper-surfaces and on subset of preferred designs, are given for problems (a–d). Insights into the results of optimization process, using 5-D graphics for design and attribute spaces, are also presented. Design problems of modern RoPax and SWATH structures are used in case studies.

Methods and concepts for the multi-criteria synthesis of ship structures

“…The influence of safety as an objective is further discussed in the papers Zanic et al (2010b) and Zanic et al (2013).…”

“…This subsection deals with the demanding and most general definition of design criteria (objectives and constraints) regarding the owner and society in the attempt to show the advantages of including or inserting safety considerations into the DP identification as design drivers/objectives. Simplification and practical formulation are extensively presented in Zanic et al (2010b), while another example of practical application is given as a part of the proposed DeSS-based design procedure in Zanic et al (2013).…”

Design synthesis of complex ship structures

“…Sensitivity analysis of the influence of safety taken as one of the objectives (not only as a constraint) on structural design shows that, for the same owner's gain, it is possible to increase design safety by means of the appropriate procedure, and satisfy both the owner's and societal constraints as formulated in references ISSC (2009) and Zanic, Kitarovic, et al (2010) and summarised in the Editorial of this special issue of the Journal (Zanic 2013).…”

“…DP formulations regarding safety as an objective (left column) and achieved results (right column) are presented in Figure 12. Those designs with their problem formulation are discussed in more details in Zanic, Kitarovic, et al (2010) and Zanic (2013).…”

“…DeSS is applied to the benchmark example of the RoPax ship Zanic, Kitarovic, et al 2010). CDP level calculation was performed using structural weight and simplified safety measures to improve given preferred design obtained in topology/geometry optimisation of the CDP.…”