Validated Frequency-Domain Seakeeping Analysis of Coupled Heave–Pitch Response of an S-175 Container Ship for Structural Reliability Screening

Abstract

Coupled heave and pitch motions dominate the vertical-plane dynamic response of displacement ships in head and near-head seas and drive global hull-girder loads, yet frequency-domain analyses often treat RAOs, added mass, damping and excitation in isolation; this study therefore develops and experimentally validates a mesh-converged three-dimensional linear potential-flow boundary-element model of the S-175 containership (LWL = 170.566 m, ≈12 480 panels) first against published RAOs at Fr = 0.275 and then applies it at 12.5 knots (Fr ≈ 0.157) to compute coupled heave–pitch RAOs, frequency-dependent hydrodynamic coefficients and Froude–Krylov/diffraction forces while retaining rigid-body characteristic frequencies (0.0866, 0.1111 and 0.1221 Hz) only as independent references, thereby addressing the persistent gaps of incomplete pre-speed-change validation, circular modal severity indices and limited coupled-response corridor identification through a non-circular equal-weight (0.5) severity index that excludes modal data and reveals a clear critical frequency–heading corridor of elevated demand in which heave RAOs reach 1.4–1.5 m/m and pitch RAOs reach 2.2–2.5 °/m (heave added mass ≈ 5.3 × 10⁷ kg), with validation yielding MAPE of 8–12 % (heave) and 6–10 % (pitch) together with R² = 0.92–0.97, ultimately furnishing a validated rigid-body basis for objective structural screening.