Swell and wave-groups at Saldanha bay

Abstract

Mooring problems are one of the most important issues in port construction and port operation when harbours are facing the open sea. Operational standards usually define that cargo handling for large vessels is possible when significant wave height is below some level, i.e., 0.5 [m]. However, several ports have reported mooring problems when the wave condition is below the operational level. Many authors have related this problem with the action of long waves inside the port areas. Long waves are known to be the cause of mooring problems in the Saldanha Bay. These waves have periods between 50 and 500 s. This is a range of natural oscillation for the horizontal motions of the moored ships. Therefore, moored vessels at the jetty experience conditions of resonance for these motions during the occurrence of these long waves. Such ship motions can be up to several meters and cause adverse conditions, both for vessels and the fender system. Long waves have been noticed to occur usually together with the storm conditions and the passing of weather fronts. If the occurrence and magnitude of these long waves could be predicted, measures can be taken to mitigate or prevent conditions of large moored ship motions at the jetty. This specific study focuses on the generation of long waves generated by wave grouping inside Saldanha Bay (South Africa).