Vol 77 No 4 77404

Abstract

Wave impact loads, such as green water loading, are a critical concern for the structural safety of ships operating in severe sea conditions. Wave impact loads are commonly described as loads that increase rapidly over a short time; however, the criteria used to define wave impact loading differ between hydrodynamic descriptions and structural response considerations, and the connection between these two viewpoints remains unclear. To address this limitation, the present study investigates wave impact loading from a structural response perspective, with a focus on the roles of load impulse and loading rate. A simplified single-degree-of-freedom (SDOF) model, representing the local bending behavior of hull plating, is used to evaluate displacement and strain rate responses under idealized wave impact loading. The results indicate that displacement is mainly governed by load impulse, whereas strain rate response is strongly affected by the rate of load application. Even for wave impact loads with the same impulse, short-duration loading induces pronounced dynamic responses with high strain rates, while longer-duration loading results in quasi-static-like behavior. Based on these findings, a loading-rate-based impact index, Ip, is applied to green water loading and interpreted from a structural response perspective. Analysis of measured green water pressures shows that impact-type and quasi-static loads can be distinguished at approximately Ip ≈ 500, which is proposed as a practical threshold for classifying wave impact loading.