The present paper describes a study of the response of a moored vessel to beam waves. The nonlinear stiffness characteristics of the fender and mooring system are idealized and the sway and roll response of the vessel as well as the forces in the mooring lines and on the berth are simulated numerically. The hydrodynamic coefficients are calculated using a computer program based on linear diffraction theory and are applied in turn to a timestepping procedure. Comparison of the numerical results with available experimental data shows favourable agreement. The effects of the ratio of the fender and mooring line stiffnesses and of the pretension forces on the vessel motions and mooring forces are examined and discussed.
In the design of marine facilities for the berthing of large vessels, fendering systems are normally designed to withstand the effects of berthing loads while the mooring system is designed to withstand the static loads on the vessel due to the effects of currents and wind (e.g. Gaythwaite, 1990; Bruun; 1987). Examples of model tests have been reported by Ito et al. (1972), Gao (1975), Van Oortmerssen et al. (1986), Ueda (1987) and Huang(1988). However, model tests are generally quite expensive, and semi-empirical formulae have been developed for predicting the impact energy and force of a moored vessel under the action of regular waves (e.g. Li, 1982; Gao, et aI., 1990). Tests have also been carried out on full scale models (e.g. McGehee, 1992). Such tests have generally verified and quantified some expected results: tightening mooring lines reduces ship motions; tighter mooring lines react more strongly to higher frequency motions. A number of dynamic effects have also been identified such as wave-induced impact and subharmonic motions (e.g. Lean, 1971; Ueda, 1987).
