Experimental study on taut and hybrid moorings damping and their relation with system dynamics

Abstract

A de-coupled technique is applied to the analysis of mooring damping of taut mooring systems and two types of hybrid mooring systems. The effects of mooring damping on low frequency motion responses as well as mooring tensions are also investigated. Besides, the effects of mooring pretensions on floating systems dynamics are discussed. Irregular wave model tests were carried out in beam and heading seas to simulate a semi-submersible operating in 200 m water depth in typical South China Sea wave conditions. First the characteristics of mooring line models were checked to determine if they satisfy model test requirements. Free decay model tests and numerical simulations are carried out to obtain the critical damping of different systems. The 6 DOF motion responses obtained from model tests are input to solve mooring dynamics with de-coupled method and the accuracy of the numerical methods is validated. Mooring tensions as well as mooring motions at fairleads in horizontal directions are obtained to study mooring damping. The damping of all mooring systems in real sea states are then obtained and compared, the relation between mooring damping with horizontal motion responses are studied.