Reliability-Based Design Optimization for a Vertical-Type Breakwater with Multiple Limit State Equations under Korean Marine Environments varying from Sea to Sea

Abstract

In this study, as part of basic research aimed at enhancing the accuracy of load- and resistance coefficients to ensure their suitability for practical design and promoting the application of underutilized reliability-based design in Korea, the author conducts optimal design based on the reliability analysis of a vertical-type breakwater in the seas off Haeundae, Yeosu, Mokpo, Gunsan, and Incheon – representative ports in Korea. In doing so, the author utilized the double-loop approach that solves simultaneously a reliability problem nested in an optimization process. To alleviate the extremely large numerical efforts demanded in the double-loop approach, the Subset Simulation method was employed. In this process, the author deliberately refrained from using design waves of a specific return period and linear probabilistic models such as Gaussian distribution, especially concerning wave- and lift forces, often viewed as barriers to the widespread application of reliability-based design in Korea. Instead, the author focused on characterizing the uncertainties associated with wave force, lift force, and overturning moment —variables that significantly impact the integrity of vertical-type breakwaters- by developing probabilistic models for these random variables directly from long-term in-situ wave data, which captures the varied characteristics of the Korean marine environment from sea to sea. The limit state of the vertical-type breakwater was defined to encompass sliding, overturning, and collapse failures, and the strong interrelation between wave force, lift force, and overturning moment was described using the Nataf joint probability distribution. As anticipated, simulation results show that solely considering sliding failure, as in the current reliability-based design platform in Korea, leads to an underestimated failure probability. Furthermore, it becomes evident that ensuring a consistent failure probability for vertical-type breakwaters using design waves with a specific return period, as in past studies, is not feasible. In contrast, this study demonstrates that breakwaters optimally designed to meet the reliability index requirement of β-3.5 to 4 consistently maintain a target failure probability in all sea areas.