A hybrid Boussinesq-panel method for predicting the motion of a moored ship

This paper describes a new computational technique for predicting the wave-induced motion of a restrained floating body in restricted water. A combination of established methods is used in an attempt to account for the most important physical processes involved in this complicated problem, while keeping the computational burden modest. Potential theory is invoked to describe both the wave transformation over the bathymetry of the harbour, and the hydrodynamic interaction between the waves and the floating structure. Modified Boussinesq theory is used to predict the transformation of the waves as they propagate from deep water into to the harbour or bay where the body is moored. This model includes the effects of shoaling, refraction and non-linear wave-wave interaction, and most importantly sub-harmonic generation. This Row is then linearised locally, at the structure, to provide the incident wave forcing term in the equation of motion which is solved in the time-domain. Linear wave radiation and diffraction forces are computed using a constant-strength panel method, while the instantaneous, non-linear, point mooring forces are included exactly. The model is validated for the linear problem, and non-linear calculations are compared with experimental measurements for a ship moored in an L-shaped harbour. Qualitatively, the non-linear features of the dynamical system are successfully captured by the model. Some tuning, in the form of empirically obtained damping coefficients, is, however, necessary in order to get a reasonable prediction of the response amplitudes near resonance, when the linear hydrodynamic damping is very small. (C) 2000 Elsevier Science B.V. All rights reserved.