THE INFLUENCE OF A FREE SURFACE ON PASSING SHIP EFFECTS

Ships moored in harbours are subjected to hydrodynamic forces due to other ships passing nearby. These hydrodynamic forces induce motions of the moored ship which may hinder loading/discharging operations or cause damage to the mooring system. These low frequency forces, sometimes known as suction forces, are associated with the primary pressure system around the sailing vessel which acts upon the moored ship. These effects are not to be confused with forces due to the secondary wave system or wash waves, of the passing ship. The latter are a relatively high frequency phenomenon and will not be treated here. This paper adresses two methods to predict the low frequency forces due to a passing ship, the first of which is based on implementation of the so-called 'double-body' flow method introduced by other authors. Double-body flow methods have been shown to be effective in predicting passing ship forces on a ship moored in open water. However, ships are rarely moored in open water but rather in harbours with a more or less complex geometry which is orders larger that the ships involved. Ships passing through harbours besides generating forces on moored ships also generate long-wave activity or seiches in harbours. These are due to the discontinuities in the harbour geometry which are almost always present. The second method treated in this paper is concerned with prediction of passing ship effects which take into account the generation of seiches due to the passing ship and the forces these exert on the moored ship. Results of both methods are compared with results of model tests for the case of a ship moored in open water. Both methods are subsequently applied to cases involving a complex harbour geometry. Results of comparisons of predicted horizontal forces on a moored ship show the additional effects due to such seiches.