Time Domain Simulation of Large Amplitude Motions in Shallow Water
Amitava Guha, Abhilash Somayajula and Jeffrey Falzarano
A finite depth 3D Green function has been developed to estimate wave loads on the floating body in frequency domain which is then coupled with a time domain motion simulation tool. Nonlinear hydrostatic and Froude-Krylov forces have been incorporated considering the instantaneous wetted surface and the nonlinear equation of motion is then solved considering the large-amplitude rotations of the body. However, the option of using a consistent second order forcing model is also retained in the code where the linear diffraction forces and moments are augmented with the drift forces calculated from Newman approximation of difference frequency quadratic transfer functions (QTF).
The main advantage of the method lies in its capability to simulate large-amplitude motions of ship shaped and non-ship shaped vessels with or without mooring and with or without forward speed in deep or shallow water depths. This allows investigation of various nonlinear effects and ensures safe operation. It is particularly useful in modelling the motions of a ship in a port channel or towing of offshore platforms from fabrication yard to deep water installation site.
In this article, a methodical approach in validating the numerical results against published and other established numerical programs is presented for the developed finite depth Green function and hydrodynamic coefficient predictions. Also the drift forces and moments obtained by different forms of the Newman approximation are compared and validated against commercially available software. The ability of the time domain simulation program to capture nonlinearities is also shown by simulating the Mathieu type instability in heave and pitch modes of a Single Column Floater.
Keywords: Time domain; Shallow Water; Forward Speed; Large Amplitude; Hydrodynamics; Motion Simulation; Parametric Excitation; Single Column Floater.