Modelling and measurement of low-frequency surge motion associated with extreme storm conditions in the Port of Cape Town

Abstract

Infra-gravity wave propagation into the Port of Cape Town (South Africa) was modelled with Delft3D-Surfbeat. The model was calibrated and verified against wave measurements outside the port and inside one of the main port basins. The numerical model results corresponded well to the measured data and theory on port basin oscillations.The calibrated model was used to develop a long wave forecasting tool, by parametrising a framework of simulations that covers the offshore wave climate. The forecasting tool relies only on the NCEP offshore wave forecasts as input and was shown to give reliable results 3 days in advance.Two storm events were analysed in detail and presented as case studies. The first event was an extreme storm with a measured significant swell wave height of 11.5 m offshore and a measured significant long wave height of 0.7 m inside the port. The second event was a significant storm with a measured significant swell wave height of 6.2 m offshore and a measured significant long wave height of 0.3 m inside the port basin.The surge motion of moored container vessels was measured during each of the two events. A tool was developed to detect and track user-defined objects on a vessel in a sequence of recorded images and then transform their positions to surge motion. The maximum surge motion measured during the first (extreme) event was 12.51 m and 3.14 m during the second (significant) event.The low-frequency motions of the vessels during the two storm events were modelled in the time domain with the moored ship motion model, Quaysim. The Delft3D-Surfbeat model output was used to determine the wave forces acting on the vessels. During the first (extreme) event, the moored vessel was under tug assistance and a line parting was observed. Several likely mooring scenarios were modelled to account for any uncertainty in the actual mooring arrangements.The modelled and measured surge motion was in good agreement, especially during the second event where there was less uncertainty about the mooring arrangement. Alternative mooring arrangements, including a dynamic tensioning system, were also modelled to determine whether vessels can be moored more effectively in such conditions.