Computation Model on a Chain-Turret-Moored Tanker in Irregular Seas

To compute the low frequency mooring forces of, for instance, a turret moored storage/production tanker exposed to survival conditions the hydrodynamic excitation and reaction forces have to be known. The low frequency excitation is assumed to be caused by the velocity dependent wave drift forces. The hydrodynamic reaction forces consist of the added mass forces, the low frequency viscous forces and the wave drift damping on the tanker and the damping forces acting on the chain legs.
Although in the past several investigations on chain damping have been carried out, data on low frequency chain damping, however, is scarce. In order to make a proper account of the effect of the mooring reaction force on the low frequency motions of the vessel, interaction effects have been established between the mooring system and the vessel hydrodynamics.
To evaluate these interaction effects model tests have been performed on a 200 kDWT tanker moored by means of a 6 chain leg turret.
A mathematical model describing the interaction effects between the mooring system and the vessel hydrodynamics is presented. Based on an analytical description of the chain dynamics the complete low frequency vessel motions with correct high frequency mooring loading have been established.

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