Abstract Summary
Dynamic properties of ships, especially damping, are important for predicting fatigue damage from wave-induced vibrations. Operational modal analysis (OMA) is useful for characterising the dynamics of structures through the identification of a modal model from vibration measurements. A modal model comprising five modes is obtained by performing OMA on full-scale measurements conducted during purposefully-executed test sequences of a slamming-prone polar vessel. Close inspection of features in the measurements indicate that the hydro-elastic responses of the ship are associated with periodic, non-stationary wave excitation, and strong fluid-structure interaction. This violates many fundamental assumptions of OMA and introduces both bias and random errors in the identified modal model. A harmonic removal technique is investigated to suppress the influence of periodic inputs in the vibration signals, while random errors are quantified through a first-order sensitivity analysis. Removal of periodic components result in improved identification of weakly excited lateral bending and torsional modes, and increases damping estimates of two-node and three-node vertical bending estimates by as much as 800 % and 400 %, respectively. Comparison of modal estimates from test sequences at varying speeds indicate speed dependency of natural frequency and damping. The natural frequency tends to decrease, and damping ratio increases. Both natural frequency and damping estimates are found to have lower variances at higher ship speeds, which corresponds to measurements that match the underlying OMA assumptions more closely. The relative variances of natural frequency estimates are generally all below 0.008, while damping estimates are more uncertain with relative variances as high as 1.5. Vertical bending modes, which are well-excited, have lower variances, while weakly-excited modes that are buried in noisy measurements have higher variances.
