TIME–FREQUENCY CHARACTERIZATION AND SEVERITY SCREENING OF COUPLED HEAVE–PITCH MOTIONS OF AN S-175 CONTAINER SHIP

This study characterizes coupled rigid-body heave and pitch motions of the S-175 U-Waters containership under irregular head-sea conditions and develops a non-modal severity measure for structural screening. A three-dimensional linear potential-flow model in ANSYS AQWA was used to simulate a long-crested JONSWAP sea state with Hs=1.2 m, peak frequency fp=0.23Hz, head waves, and a vessel speed of 12.5 knots over a 300 s record. Response statistics, moving RMS, continuous wavelet analysis, cross-correlation, and a normalized coupled-response severity index were applied. Heave varied from −0.01102 to 0.01230 m with an RMS of 0.00826 m, while pitch varied from −0.01349° to 0.01612° with an RMS of 0.01055°. Dominant response bands were approximately 0.08–0.11 Hz for heave and 0.10–0.13 Hz for pitch. The maximum heave–pitch cross-correlation was approximately 0.82 at a lag close to zero, indicating strong temporal coupling. The normalized severity index reached 1.00, with a mean of approximately 0.63; severity exceeded 0.50 for approximately 60% of the record and 0.75 for approximately 25%. The identified response bands encompass the rigid-body characteristic frequencies of 0.0866, 0.1111, and 0.1221 Hz, which are used only as independent reference markers. The results provide a practical basis for identifying elevated rigid-body response conditions for subsequent structural screening.