Long waves and their effects on vessels at the port of Ferrol

Résumé

Disturbances to load and unload operations caused by excessive vessel movements are a recurrent problem at the Exterior Port of Ferrol (NW Spain), where a new container terminal will soon be inaugurated. This work not only sheds light on the nature and behaviour of long waves at the port, but also investigates their influence on the vessel movements. In addition, a novel method to determinate their energy levels is proposed, validated and tested. For this purpose, the sea level oscillations in the port area and the movements of a bulk carrier at berth were measured during winter-spring 2011. The spectral analyses reveal the occurrence of frequent resonant episodes in the basin with a moderate to high amplification and a varying behaviour of the long wave energy across the spectrum. On these grounds, three frequency bands are proposed: LF (Low Frequency), VLF (Very Low Frequency) and ULF (Ultra Low Frequency). The LF band, which is associated with the so-called infragravity (IG) waves, exhibits a high correlation with the offshore swell energy. At the other end of the long wave spectrum, the ULF band is only weakly correlated with the swell and responds to a different forcing, which may be related to atmospheric disturbances. Finally, the intermediate VLF band presents a mixed character. On the other hand, the analysis of the vessel movements shows large rectilinear motions, whose values are close to the safety limits for cargo handling operations proposed in literature. The joint analysis of sea-level oscillation and vessel movements reveals that the movements in the horizontal plane (sway, surge and yaw) are strongly correlated with the total wave energy in the system and, more importantly, with the ratio of LF band to total energy. Proved the relevance of the long waves¿and, particularly, LF oscillations¿to cargo handling operations, Artificial Neural Networks (ANNs) were implemented for the first time to estimate the infragravity energy levels in a port basin. With this purpose, two models with the same input (i.e. the short wave parameters outside the harbour and the tidal level) were successfully implemented. The results obtained with the ANNs outperform those obtained using a widespread empirical formula and confirm the reliability of the methodology presented.