An Improved Method for Calculating the Contribution of Hydrodynamic Chain Damping on Low-Frequency Vessel Motions
The computation of low frequency mooring forces on a tanker exposed to survival conditions requires knowledge of the hydrodynamic excitation and reaction forces. The low frequency excitation is caused by wave drift forces. The reaction forces on the tanker are a combination of added mass forces, wave drift damping forces, viscous forces and damping forces due to interaction with the mooring chains.Wichers showed that for a chain-turret moored tanker the interaction forces between the mooring system and the tanker contribute significantly to the low frequency motions of the tanker. Huijsmans presented an approximate method to include the interaction forces due to high and low frequency motions of the chain-turret moored tanker.In this paper a direct simulation method for a chain-turret moored tanker will be applied for the correct instantaneous low and high frequency motions of the tanker, including the dynamic behaviour of the mooring chains. The latter will be modelled by a lumped mass method.Comparison will be made of the direct simulation method and an approximate method using the average chain damping. An evaluation of the chain damping contribution will be made using model test results and computations.