Prediction of mooring line loads on cape size vessels berthed at Cape Lambert

The deep-water port facility at Cape Lambert in northwest Australia, operated and maintained by Rio Tinto subsidiary Pilbara Iron, has the ability to berth a cape size vessel on each side of the wharf in a staggered configuration. The length of the wharf is insufficient to allow the vessels to berth without overlap, which prevents the loaders from operating simultaneously. Pilbara Iron’s Cape Lambert Marine division is investigating a proposed mooring arrangement to reduce the overlap, which involves moving one of the vessels aft of the existing location. The Australian Maritime College (AMC) has conducted a numerical investigation to predict the mooring line loads for the existing and proposed mooring configurations for a series of water depth to draught ratios, vessel beams, vessel locations, wharf mooring points, wind and current velocities. The forces and moments on the berthed vessel induced by wind and current were calculated using published empirical formulae. This data, along with parameters associated with the mooring systems, were used as input to a mathematical model that accounts for the dynamics of the vessel as well as fender and mooring line responses. The predicted line loads for the existing mooring configuration have been compared to those for the proposed mooring configuration to ascertain if the proposed configuration will be sufficient. A number of variations of the proposed mooring configuration have been analysed to determine an optimum mooring configuration. The study has provided Pilbara Iron with valuable information that can be used to identify the conditions under which lines are susceptible to failure. Hence precautions can be taken to ensure mooring arrangements are sufficient to prevent vessels breaking away from the berth due to strong offshore winds and currents.